Hue Angles

Henry Hemmendinger Contemplating a Print of M. C. Escher

2011-11-26T08:11:00.001-05:00

We know Rembrandt’s painting, Aristotle contemplating the bust of Homer. Now behold Hugh Fairman’s essay…

In the early 1990s, Henry Hemmendinger’s son got married in San Francisco. Henry was walking the streets there one afternoon when he chanced upon an art gallery that was exhibiting M. C. Escher prints. There, in a well-lit (with daylight) stair-well, he found an Escher print that he interpreted to be a daylight scene of the sun reflecting from a puddle of water. That evening he took his wife to see the exhibit. This time the daylight was missing, and the puddle print was illuminated with incandescent light. Henry was convinced that the print now depicted the moon reflecting from the puddle.

For years thereafter, Henry wondered whether Escher knew enough about spectral interaction of light with matter to be able to create something resembling a metamer between the daylight illumination of the print and the incandescent illumination of it. In the early 2000s, he became aware that John Horton Conway, an esteemed mathematician and fellow resident of Princeton, N.J., had studied Escher’s tilings of the plane from a purely mathematical standpoint and had published extensively on that subject. Henry contacted Conway with his thoughts on the puddle print. Conway lent Henry all his books on Escher, and in one of those Henry identified the print he thought he had seen in San Francisco.

The print Henry identified was Escher’s Puddle, a 1951 lithograph which was in two colors on white paper (called three-color print in the art world). It may be viewed at www.globalgallery.com. First find M. C. Escher in left-hand column (perhaps under “All artists”), then click on the Puddle print. . The original is 9½ by 12½ inches in size. It carries Escher’s Catalogue Number 175.

One can be pretty sure that Escher intended the whitish disk to be interpreted ambiguously by the viewer either as the moon or as the sun. I offer as evidence of this the following items taken as a whole:

1) On the 29^th of October in 1963, Escher gave a lecture in Amsterdam in which he said:

“If you want to focus the attention on something non-existent, then you have to try to fool yourself first and then your audience, by presenting your story in such a way that the element of impossibility is veiled, so that the superficial listener doesn’t even notice it. There has to be a certain enigma in it, which does not immediately catch the eye.”

2) A series of elements in the print all appear in pairs.

a) There are two bicycle tracks in the mud. One rear-tire track crosses the front track in the mud; the other rear-tire track crosses the front under the puddle in the water. One bicycle track overlaps no other object in the print; the other bicycle track intercepts a footprint.

b) There are two truck, or tractor, tire tracks in the mud. The two tracks were made at a different time from each other ; they overlap. That is, one of the truck’s right tire track is inside the other’s left tire track. I presume that Escher would never, under these conditions, allow both trucks to be going in the same direction. One tire tread consists of two zig-zags and two straight beads; the other of two deep, parallel treads and a single bead, imparting a duality to even the tire treads.

c) There are two human foot-print tracks in the mud, going in opposite directions. One walker wore hob-nail boots; the other wore plain soled shoes. The right footprints of both tracks contain two prints, both dry. The left footprint in each case is singular and it is water-filled in both cases.

d) There are two large trees in the foreground of the reflection and two small trees in the background of the reflection. For these trees, Escher reused trees he had drawn in 1933 in a woodcut called Calvi, Corsica (Escher Catalog Number 56). If the reader believes it is a stretch to cite the use of two trees each here, be informed that the 1933 print had four large trees and eight small trees. Escher must, therefore, have carefully chosen a sub-section, and the appearance of duality here must have been conscious.

e) The season is Spring or Summer; but not Fall or Winter. There are leaves on the trees.

f) Of course, mud and puddle is the ultimate duality of the print. Sometimes duality is achieved by sameness; sometimes by differentness. That is how we know that Escher intended to communicate duality rather than differentness, or sameness.

There are, therefore, enough occurrences of duality in the print that it is almost certain that Escher, in accordance with his lecture precept. was leading the observer to the duality, or ambiguity, of whether the orbital object was the sun or the moon. It is highly unlikely, then, that the interaction of lighting quality is causing the ambiguity. Escher has put duality in our head, and we can interpret the orb as we wish as sun or as moon.

Hugh S. Fairman

[Editor’s note: I think the light helps with the duality. The sky around the orb is greenish, and will be darker (relative to the white orb) under tungsten light than under daylight. A dark sky implies night rather than day, and Moon rather than Sun. Perhaps readers can also think of other mechanisms. MHB]

Dances with spectra ... and some famous historical figures from color photography

2011-09-28T12:28:00.004-04:00

Sometimes an out-of-towner can encourage you to explore your own neighborhood. Here’s how Mark Fairchild acted on such encouragement.

Through a sequence of fortunate events instigated by the editor of Hue Angles, Michael Brill, I recently found myself deep in the archives of the George Eastman House International Museum of Photography and Film. Michael asked me to poke around and write something for Hue Angles on what I found.

Prior to my visit, I had to narrow the topic to provide some focus for the archivist. We decided that Lippmann photography would be of interest to many in the ISCC and I was aware that the Eastman House had several very special examples of Lippmann plates. This is where the dance with spectra began.

I was hosted by the Photo Collection Archivist, Joe Struble, who took me underneath the museum, provided wonderful conversations about old color technologies, instructed me on how to properly handle priceless photographic artifacts, and fetched the sought-after Lippmann plates from the vast underground archives. We were also joined by Mark Osterman, the Eastman House's resident guru on early photographic materials and processes. Mark discussed the Lippmann technology and shared tips for best viewing the images. One key to his instruction was that I would need to “dance with” the lighting and plates in order to line everything up just right to see some amazing spectral images. That dance also led me down a path of intersections with several historical greats in the color photography universe.

It starts at the Palais de Versailles where Gabriel Lippmann once stood with a complex camera system collecting one of the earliest spectral images ever made. I was holding the very same photographic plate that Lippmann placed in his camera doing that dance to allow essentially the same spectra present in France on that 19th-century day to fall upon my eyes. Pretty cool stuff.

The Lippmann process used an extremely fine-grain panchromatic emulsion (a black and white emulsion sensitive to all visible wavelengths) very similar to those used for holography today. The plate was placed in the camera with glass side toward the lens and then a layer of mercury was placed behind the plate to form a very good mirror in contact with the emulsion. This arrangement sets up standing light waves in the emulsion. The interference patterns were recorded as layers of exposed silver. This means the exposures essentially created interference filters at each location across the image. The plate is then viewed in white light and only the appropriate wavelengths are reflected from the stacked layers of silver in the emulsion. The result is a nearly perfect spectral reproduction of the scene. Interestingly, Lippmann won the Nobel Prize in Physics in 1908 for this invention.

The accompanying image illustrates the capture and display processes schematically. In part b, the diagonal line represents a half-silvered mirror, and the viewer is supposed to look at the leftward-going rays, which may be seen either through the depicted lens or through a prism.

The Lippmann Process

Figure reproduced from The Reproduction of Colour by

R.W.G.Hunt, 6^th edition, published by Wiley, Chichester, 2004, figure 1.3, p. 6.

The color appearance was stunning. Dancing was indeed required to get the light, the plate, and my eyes all in the proper position, but when that tango was just right, the images were fantastic. While more light than one typically finds in a museum would have helped, it was easy to see vivid greens of foliage, purples and oranges of flowers, and a blue sky that looked accurate and not the over-saturated color we have come to expect from consumer imaging systems. Well worth the dance!

Dancing continued when we discussed the provenance of that plate and others. The plate was given to Josef Eder by Lippmann. Eder wrote the classic treatise, History of Photography, (translated into English by Edward Epstean) that includes a wonderful contemporaneous discussion of the Lippmann process. As I was shown the Eastman House copy of the book, I realized that I had that same edition on my shelf at home (passed to me by a retiring scientist). The dance continued when Eder gave his photography collection, including Lippmann’s Versailles plate, to none other than George Eastman, beneath whose wonderful home on East Avenue in Rochester the archives sit. The plate was displayed at Kodak until the museum's creation in 1949.

Other dancers in this story included Frederick Ives, his son Herbert, and Howard Wood. The Iveses had also been experimenting with the Lippmann process and a similarly interesting process based on diffraction rather than interference. I also saw some of these interference color photographs (another potentially spectral imaging system although those were trichromatic) that Ives, Ives, and Wood had perfected and patented. Several of the Lippmann plates I viewed came from the Ives family, and there were also diffractive plates that came from Wood. Frederick Ives is well known for inventing systems of trichromatic color photography, while his son Herbert is noted for developing early facsimile and television systems. Wood, their colleague at Johns Hopkins, was known for exploring fluorescence and discovering the “blacklight effect” as well as developing IR and UV photography. I will have to relive that particular dance another day. However it wrapped up with me discovering a new-to-me type of photographic process.

I will most certainly return to the archives one day--perhaps to learn more about diffractive color imaging or to explore original Kodachrome plates that were actually a two-color system (long before the time of Edwin Land). Thank you, Michael, for turning on the music for this particular dance and helping me find some new treasures in my own back yard!

-Mark D. Fairchild
Munsell Color Science Laboratory
Rochester Institute of Technology

[I wonder if Mark’s dance trope emerged from a Ph.D. trauma: Emil Wolf, who chaired his dissertation committee, queried him about spectra during his defense. The “dances with” trope and a certain movie title would be a standing wave, if not a standing ovation, in Prof. Wolf’s direction. MHB]

My First Experiment in Jerry Lettvin’s Lab

2011-08-05T11:42:00.000-04:00

Dr. Jerome Y. Lettvin, my de facto Ph.D. advisor, passed away on April 23. Many impressive obituaries have been written, but here is a reminiscence…

Dr. Jerome Y. Lettvin (Feb. 23, 1920 – Apr. 23, 2011) was a professor of Electrical Engineering and of Biology in the Research Laboratory of Electronics at MIT. He is best known for the 1959 Proc. IRE article, “What the frog’s eye tells the frog’s brain,” which he wrote with H. Maturana and W. Pitts. He is also known for his televised debate with Timothy Leary in 1967, in which he used the uncensored word “bullshit” to describe Leary’s rationale for endorsing drug-induced euphorias. To color science he gave a Scientific American article, and perhaps more significantly, “The colors of colored things” [1], which was formative to all who studied color with him (see the only English title in [2]).

Obituaries for Jerry abound (e.g., [3], [4]). His sons David and Jonathan have both created Web postings containing information and memorabilia (see [5], [6]). So rather than another obituary, I offer here a story from personal experience.

Having read “The colors of colored things” and heard Jerry’s intriguing (to me spellbinding) lecture, I obtained permission to write my PhD. dissertation under Jerry in absentia from Syracuse University, I reported to Jerry’s lab ready to create profound theories. Jerry had other ideas. He declared that I must first do a few experiments. We started talking about color effects, and he mentioned Abney’s effect. I was eager to show off, so I said the effect was that most monochromatic lights shift toward yellow when mixed with white light. Then the conversation went something like this:

Jerry: No. All lights get yellower when mixed with white light.

Me: Surely not all lights, Jerry. Surely the yellow lights near the spectrum locus don’t get yellower.

Jerry: Yes, they do. In fact, that is the first experiment I want you to do: Show that the yellows get yellower. You can use the materials around the lab.

Me: Surely there’s some sort of trick. Can you give me a hint?

Jerry: Just remember what I said in “The colors of colored things.” Pay attention to spatial boundaries.

Well, I found a 35-mm Wratten 15 filter, several lenses, and two projectors, and thought I would just project a spot of white light onto a diffuse yellow field created by the other projector. But I couldn’t get the effect. I had to make the white spot have a very sharp edge, and to do this I directed a lot of light through a diaphragm aperture at the end of a collimating lens. That made the white spot too bright.

So I had to dim the white spot. I couldn’t do it by decreasing the power to the projector, because that would make the light redder and it wouldn’t be the same white that referenced the other projector. I would simply be adding one yellow to the other, and that wouldn’t be fair. So I needed neutral-density filters---a lot of them.

Jerry suggested I enlist the help of John McCann (then at Polaroid, a short walking distance away). After some cajoling from Jerry, John offered his facilities to me. He had great projectors and as many neutral-density filters as I needed. John was very gracious. What harm could it do?

So I took my lens and diaphragm over to John’s lab, set up the experiment, and started putting neutral-density filters one after the other in the various slots in his projector until the white spot had dimmed a lot. Then I noticed the effect. What Jerry had said was true! On the edge of the white spot, on the white side, a band of yellow appeared, which was much more saturated than the yellow in the dim diffuse field. Evidently the jitter of my eye was causing the edge to induce yellow color into the white field. Even now I am not sure of the exact mechanism, but it does work.

With great excitement I rushed down the hall to summon John McCann to be a witness, so I could put a checkmark “done” in the box that would bring me closer to theory and my Ph.D. dissertation. John came quickly, but not quickly enough. By the time I re-entered the room with the apparatus, smoke was streaming out of the white-light projector. The neutral-density filters were burning!

Fortunately John and I are still friends. And fortunately Jerry took my word for having achieved the effect, perhaps fearful that my re-creating it in his lab would imperil the far more flammable Building 20. Jerry never told me how he himself had achieved the effect.

You can try the yellow-light Abney effect at home. But perhaps you’d better use a calibrated monitor and not my dangerous projectors!

Michael H. Brill

Datacolor

[1] J. Y. Lettvin, “The colors of colored things,” MIT RLE Quarterly Progress Report No. 85, 15 Oct. 1967, pp. 193-229.

[2] http://www.iscc.org/resources/translations.php

[3] http://www.sfn.org/index.aspx?pagename=memberObituaries_Lettvin

[4] http://web.mit.edu/newsoffice/2011/obit-lettvin-0429.html

[5] http://jerrylettvin.blogspot.com/2011/04/jerry.html

[6] http://en.wikipedia.org/wiki/Jerome_Lettvin and http://jerome.lettvin.com

Health effects of blue light

2011-05-18T14:50:00.004-04:00

What a difference a century makes, and 134 years even more so...

The year 1877 marked the peak of a craze to use light transmitted through blue glass to enhance crop growth and to heal animals and humans of all kinds of ills. The originator of the craze was Augustus J. Pleasonton---a Civil War general, amateur naturalist, and arguably the father of the color-healing movement that survives today. Pleasonton is featured in Chapter 11 of Paul Collins’s book, Banvard’s Folly: Thirteen Tales of Renowned Obscurity, Famous Anonymity, and Rotten Luck (New York: Picador, 2001). Collins compassionately chronicles several “brilliant but fatally flawed thinkers,” most from the late 19th century in the U.S. The frontispiece by Walt Whitman summarizes Collins’s sympathy: “Battles are lost in the same spirit in which they are won.”

Unlike some of his later imitators, Pleasonton strongly believed in his cause. His intellectual inspiration came from Robert Hunt’s 1844 Researches on Light. (No known relation to our Robert Hunt.) Pleasonton conducted several well-intentioned but ill-controlled experiments with grapevines, pigs, and even human subjects. He also published a book (on blue paper, of course) that purported some theoretical underpinnings. One passage quoted by Collins begins, “Our sun is simply a huge reflector of light,” and goes downhill from there. In 1871, after a home visit from a patent examiner and a month of waiting, Pleasonton obtained a US Patent (US 119,242). The first claim was as follows: “The method herein described for utilizing the natural light of the sun transmitted through clear glass, and the blue or electric solar rays transmitted through blue, purple, or violet-colored glass, or its equivalent, in the propagation and growth of plants and animals, substantially as herein set forth.” After a few years, Pleasonton became shocked at the quackery and nonsense whose spread he had begun, and tried but failed to enforce his patent. Modern patent attorneys would cite "laches,"* but I would make the broader attribution to the whole barn door being open…

Of course, not everyone was buying blue glass as a cure-all. Satire abounded. For example, John Carboy's book satirizing the craze gave the helpful hint: "Square pieces of blue glass weighing six pounds each may be used for dispersing a cluster of tom cats."

At the peak of the craze, the Scientific American finally cried foul and, in 1877, published many articles (sometimes more than one in the same issue) to discredit Pleasonton. The most cogent comment was that sunlight through blue glass actually contains less blue light than unfiltered sunlight, and so blue light itself could not be the agent of the observed cures.

Fast-forward 134 years. How much has changed! Patent office actions don't take just a month or two, but five years as we grow old and companies rise and fall. Not only is blue light not a healer, but it actually can cause injury to the eye’s photopigment [1]. If it betokens ultraviolet components, these components pose an injury threat to the cornea and lens of the eye [2]. In lesser doses, a blue light can also produce a dissonance between the eye’s papillary and focus reactions, resulting in eyestrain. Blue light can also encourage wakefulness, because the eye has a special melatonin receptor in the blue region of the spectrum [3]. This last may or may not be healthy depending on how late you stay up in front of your blue computer screen.

But one thing that hasn’t changed is the rule of logic. In view of the new knowledge, one finds that the 1877 Scientific American argument is not as effective as it first appears. Certainly absolute blue-light doses may be quantitatively associated with eye injuries, but the ergonomic problems of eyestrain and wakefulness rely more on imbalances of receptor inputs, which also manifest in---you guessed it---color perception. Accordingly, finer critical tools are needed to assess cause and effect than the notion of a “dose” of light. Besides mentioning the need for controlled experiments, I will not try to teach any of these arguments, but will defer to the work of, e.g., ISCC member George Brainard.

In any event, I recommend Collins’s book for much more than just the Pleasonton article. Try the article that gave its title to the book: Banvard’s folly, a three-mile long painting of the shore of the Mississippi River, scrolled past viewers between two rollers. It was world-famous in its own time, unknown in ours. How many of our own names or works will survive 134 years?

References

1. International Non-Ionizing Radiation Committee of the International Radiation Protection Association, Guidelines on limits of exposure to ultraviolet radiation of wavelengths between 180 nm and 400 nm (incoherent optical radiation), Health Physics 87 (2), 177-186 (2004).

2. Information Page for Therapists: The Risk of Eye Damage from Bright and Blue Light Therapy and see primary research references therein.

3. S. W. Lockley, Spectral sensitivity of circadian, neuroendocrine, and neurobehavioral effects of light, J. Human-Environmental System 11, 43-49 (2008)

*Note: "Laches" means undue delay in asserting a legal right; "latches" are part of a barn door.

Michael H. Brill
Datacolor

From Color Science to Wall Street

2011-03-16T16:50:00.001-04:00

You may remember from the 1980s a vector model of color by Guth, Massof, and Benzschawel. The last author is Terry Benzschawel, a noted color-vision psychophysicist at Indiana University, Berkeley, and Johns Hopkins. But Terry has spent the past two decades as a Wall-Street “quant.” Here Terry writes of his journey:

For much of my life, I have wondered how and why I perceive myself as separate from my environment and other people. Studying the human visual system provided a perfect opportunity to think deeply about the relationship between mind and body.

My post-doc trail took me through psychology, optometry, ophthalmology and engineering. But when I failed to secure a faculty position by my third post-doc, I became so despondent that I quit my last position and remained unemployed for nearly a year. Finally, I answered a New York Times ad, “Scientists – Earn Big $$$ on Wall Street.” Upon meeting me, the recruiter told me that I was “totally unsuited for a career in finance.” To her surprise, a mathematician consulting for a prominent bank picked my resume out of a stack, interviewed me, and offered me a job. Thus, my career in finance was launched.

I was unprepared for the financial world. Although I was expected to master the financial literature and terminology, my environment didn’t support that effort. I had to compete with people ten years my junior who had been preparing for finance for their entire career. Most of my immediate superiors had less education than I did. I had to overcome my Ph.D. arrogance and acknowledge that there are many very intelligent people in the world without Ph.D.s.[1] Also, I had to give up publishing my research. Models similar to the proprietary ones I developed were published independently by academics several years after mine were in use.

My first job in finance was on the ill-fated 78th floor of the World Trade Center. My boss had a genetic algorithm to predict the likelihood of corporate bankruptcy. I was given information about the model only on a “need-to-know” basis. This was frustrating to me, but guarding information is common in the business world.

After about a year, my boss’s contract was terminated and I faced unemployment. However, my original recruiter quickly found me a position building neural networks to detect fraud on credit card transactions. Having built non-linear models of the visual system, I was able to build a successful network model that was used in the company’s fraud early-warning call center. Still, salaries and promotions were frozen, so I was dissatisfied. While on vacation in 1992, I met a managing director at a bond trading house. He passed my resume to their Fixed Income Arbitrage Group, featured in Michael Lewis’s book Liar’s Poker. I interviewed, was offered a job, and gleefully accepted.

With prospects of wealth and glamour in the famous “Arb group,” I began the Associate Training program. The Arb group was engaged in “proprietary trading”, risking the firm’s money, in contrast to their larger broker/dealer “sell side” business. In my second year, my direct supervisor resigned and my job suddenly worsened. Things got better after I built several successful models for pricing risky debt in emerging markets and we traded on those models. In 1998, after a corporate takeover, the Arb group was disbanded and we were all fired. The firm found me a job as a trader/strategist. I built neural network models and traded U.S. Treasury securities and the Mexican Peso while applying my credit models to help our customers manage their credit portfolios.

By 2002 I had gained some notoriety and began to travel the world visiting clients while building a research group. The great liquidity boom of the new century was on and I was riding high, helping clients manage their risk. Unfortunately, my firm didn’t apply my methods to manage our own risk, but instead offered my wares to induce clients to buy our products. One advantage of working on “the customer side” is that I was encouraged to publish my work for clients and, at last, in journals and at conferences.

During the past decade, I have coordinated the recruitment and training of Ph.D.s for the firm’s “quant” groups. In that role I travel to major universities and give talks about our firm. I speak with hundreds of talented young prospects each year and review resumes of several times that. Supervising young staff, both interns and full-time hires, has been a satisfying aspect of my job. Having temporary help, such as interns, has allowed me to do more speculative work I do not have to justify to the trading desks. I also coordinate a weekly seminar series featuring speakers from our firm and faculty at major universities.

In late 2008 I become a partner, called a “managing director”---no small achievement for someone of my temperament. There were challenges. With the financial crisis, much credit business was lost or curtailed. During this period, I’ve made myself useful by applying my methods to help manage risk within the firm. Only recently, as market activity returns, I’m back helping clients manage their credit portfolios.

The markets are relentless. They open every business day and proceed regardless of one’s mood or personal problems. Workdays are consistently long. Personally, I have had a lot to learn both about finance and life. I still do.

Even after 20 years, I sometimes view myself as an academic “spy,” probably because my ambitions are atypical for this business. My interests are not always on the direct track to short-term corporate revenue, so the road to partner was longer than typical. But because I established a pipeline of speculative projects that have come to fruition, I have bought the freedom to explore issues not directly related to our trading business.

I am grateful for my past and present opportunities. Much of my time now is spent on innovation and my mental life is as stimulating as it was when I was a vision scientist. I am certain that this is rare for someone in finance. I continue to have a passion for learning and enjoy collaborating with talented younger people.

Terry L. Benzschawel

[1] See Emmanuel Derman’s book My Life as a Quant. My experience resonates with what is written there and I found the book entertaining.

The lipstick smudge that betrays color infidelity

2011-01-20T18:40:00.003-05:00

Have you ever been a subject in a color-matching experiment? If so, you may have encountered…

by Michael H. Brill, Datacolor

The Maxwell spot is an entoptic image of the eye's macula, a yellow-pigmented retinal area extending 3 or so degrees about the center of fixation. Until this year I regarded the Maxwell spot as an arcane effect that I would never see. Reportedly the spot is inconspicuous because it is fixed to the retina and hence the retinal receptors adapt to it. But even with rapid fading of the spot, I still should have seen it transiently in moving my gaze, say, from a blue sky to a white sheet of paper. But that didn’t happen. The paper showed me yellow journalism, but never a yellow spot. Ethan Montag [1] gave a demo (alternating blue and yellow field) to show the Maxwell spot---but no guarantees. (Evidently Montag also found it hard to see.) Also, Montag's demo shows the spot as a dark smudge on the blue field or a light smudge on the yellow field. It's still not yellow.

Then, twice in the past year I saw the Maxwell spot, both times in the context of a white light created by three narrowband LEDs. In neither case was the spot yellow. It was rather like a pink lipstick smudge on a white collar---betraying color infidelity by interfering with my ability to match colors. What a nuisance!

I first saw it when looking at a broad white surface in a light box that simulated daylight by mixing LED illumination. Several light mixtures flashed on and off in sequence, and curiously the “three-band lamp” always revealed a pink smudge for a few seconds. Could it be spatial inhomogeneity of the three-band lamp? No, the smudge covered less area when I got closer, and it always was centered about the direction of my gaze.

I saw it again at the latest IS&T/SID Color Imaging Conference. Abhijit Sarkar (a PhD student at Technicolor Research in Rennes, France and University of Nantes) gave what was judged to be the best student paper at the conference, on devising observer categories to reduce observer metamerism. He performed abbreviated color-matching experiments on multiple observers, using two 3-primary displays powered by different primaries. The observer categories he found did not agree well with the age dependency found by earlier investigators. As an on-site demonstration, Sarkar brought a 10-degree matching setup powered by a pair of LED triads, with wavelength peaks (452, 508, 642) nm and (462, 522, 592) nm. I was amazed how difficult it was for me to make the match, because the left-hand semicircle always had a fuzzy pink spot that faded away when I attended to the right-hand semicircle. When I backed away from the apparatus, the left-hand side of the match appeared uniformly purplish-pink. This latter effect had been noted by Sarkar. I thought we were seeing the Maxwell spot, and Mark Fairchild agreed.

Why is the spot called yellow and yet looks pink? Because the macular pigment absorbs strongly in a broad band about 450 nm [1], it would appear yellow when transilluminated by a full-spectrum daylight. When there are gaps in the light spectrum (as with 3-band lamps), attenuation of the green band can enhance the relative weight of the red, hence we see pink.

Not all three-band lamps show the effect, but Sarkar’s left-side green wavelength (508 nm) is low enough to be highly absorbed by the macula, leaving the 642-nm red primary to predominate. Because the G primary carries a lot of luminance, lack of that luminance in the Maxwell spot makes the pink darker and enhances my perception of it (relative to the yellow I'd managed to escape all the rest of my life).

Jack Moreland [2] describes a related way to reveal the Maxwell spot: “A large bipartite field (14 deg square) is presented. The two half-fields are approximately matched in colour: the appearance being a near-white. The mixtures are cyan and reddish-orange (490 + 610 nm) on the left, and blue and yellowish-green (460 + 470 nm) on the right […] An observer sees [a] patch about 3 or 4 deg in diameter [that] changes from ‘pink on green’ (left) to ‘green on pink’ (right) on switching gaze between the two half-fields.” So the Maxwell spot has shown itself to be pink to other eyes before mine.

Together with the best-paper prize, Sarkar now has a new factor to consider in selecting LED primaries. Also, I begin to understand how color-matching subjects must feel when told to "ignore the Maxwell spot." When the spot is lipstick-pink, that task is hard enough to make one consider “cosmetic” surgery.

[1] Ethan Montag, JIMG 774: Vision & Psychophysics, Chapter 8, Part 3: Parts of the eye.

[2] Jack D. Morehead, Entoptic visualization of macular pigment, J. Physiol. 485, 4P-5P (1995).

Afterthought on afterimages: Green flashes and green lights

2010-11-22T09:05:00.006-05:00

Some experiments are fun and not painstaking. Here is one from my student days ...

by Michael H. Brill, Datacolor

Afterimages from bright lights are usually undesired and hardly ever helpful, but they can tell you something about the visual system. Here’s an experiment you can try at home, that shows a remarkable interaction of your two eyes during an afterimage. You will need a strong pen-light overlaid by a green filter, a strong, directed white light (such as a 500-watt fiber-optic projector), a magnifying glass lens, half a ping-pong ball (placed over the right eye), and a white wall under artificial light (incandescent will do). The problem, as you do the experiment, is to explain the afterimage effect.

First of all, hold the green pen-light at arm’s length and flash it into your left eye. Then look away at the white wall. The long-lasting dark (negative) afterimage will look magenta at first and then turn bluish purple. This happens whether or not your right eye is open. Closing the left eye or dimming the light on the white paper makes the afterimage turn bright green (positive).

Now repeat, but when you look at the wall with your left eye, shine the projector into your right eye through the half ping-pong ball, producing a uniform field that won’t distract you from the left-eye’s afterimage. I think you will see the afterimage in the left eye flash bright green for a second and then return to a dark purple appearance. By the way, the projector should be about 3 feet away from your right eye, and you should look at it only through the ping-pong ball.

As long as the negative afterimage persists, the green flash can be elicited repeatedly by turning the projector light on and off. The flash is the same color as the light occasioning the afterimage (in this case green). The green flash cannot be attributed to stray light entering the left eye from the light producing the uniform field on the right eye: More light in the left eye just makes the afterimage appear darker and more purple. On the contrary, the flash effect is similar to the polarity reversal that happens when light is dimmed in the left eye.

So why is this happening, and how can you prove it?

Give up? Well, it seems that when you turn on the light in the right eye, the right-hand iris contracts, and that causes the left-hand iris to contract as well. That dims the light from the white wall by decreasing the pupil diameter. (This effect will be strongest in young people who still have some action in their irises.)

How can I show this? Repeat the above experiment, instead of looking at the white wall, look at a distant white light source through the magnifying glass. Position the lens so its near focal point is in the plane of the pupil, sends light through the middle of the pupil without being affected by the iris. You’ll know you have the right distance when the image of the light source floods your whole retina. Now turn on the projector to the right eye, and lo! The green flash will not appear.

It might appear that you need about eight arms to do this, so it is not as casual an experiment as I have misled you to believe. But it is not quantitatively demanding, and I published it for a small audience at MIT without incident [1]. As of this year, you can read any of the progress reports on the Web. It costs no money, and the complete obscurity of this unrefereed publication is balanced by its refreshing availability to all, without passing a toll gate. Academic freedom has turned inside out, and we have found an unexpected place where the green light is flashing.

M. H. Brill, Binocular afterimage effect, MIT Research Lab. of Electronics Progress Report, PR 120, pp. 168-169 (1978). http://dspace.mit.edu/bitstream/handle/1721.1/56698/RLE_PR_120_XXVI.pdf?sequence=1.

Thoughts on Annotated Webliography and its Relatives

2010-09-20T13:25:00.004-04:00

[ Once in a while it is good to revisit an old idea we decided not to pursue, to see what has emerged instead…MHB]

Ten years ago the ISCC approved a Project Committee to establish an Annotated Webliography of Color. The goal was to point to excellent Web sources of color information. The scope was daunting: Criteria to select a website for annotation/citation, a first selection of websites for citation, rules for updating the resulting Annotated Webliography, and transition of the effort to a standing committee that (ever vigilant) would update the webliography.

As the chair, I started to compile a list of websites, and several people added to it. But soon enthusiasm waned. Also, some URLs on the list began to fail. I resigned as committee chair in 2002 because the Web medium seemed too transitory to justify ever-vigilant ISCC checking. Ironically, the Project Committee description is fossilized at http://www.iscc.org/functions/pc.php#proj53. If nothing else, it proves we were among the first to coin the term “webliography”!

Where has the Web gone since 2002? Aside from installing more toll gates so you have to pay for information (in my opinion a bad thing), it has moved in two directions, attempts at objective truth (often consensus-driven, exemplified by Wikipedia) and undisguised opinion (blogs). The goal of our Webliography was more like Wikipedia’s model. I will now discuss two excellent websites as examples and try to classify them. My classification doesn’t follow ISCC interest-group stereotypes.

First consider Jill Morton’s “Color Matters”. It addresses artists and designers, but there are also stories relating to color science. My favorite from “Color Matters” is about a red-tailed hawk named Windwalker who, from a perch on the author’s arm, removed and discarded all the strawberries from her strawberry short cake, presumably because “Windwalker had never seen me eat anything as bloody red as the meat that he himself consumes on a daily basis. He was using a delightful combination of memory, loyalty and his ability to discern colors to intelligently correct a situation that in his mind was not normal.”

Is “Color Matters” more like Wikipedia or more like a blog? Most of its content makes it more like Wikipedia, even though it has a small, carefully labeled blog section and a forum called “Color Tales.” Outside these sections, I don’t see much editorializing, but lots of information gathering. In one way it is not like Wikipedia: Its structure makes it hard to locate particular material. If you want to find the article on Windwalker, it’s best to type “Color Matters” “Windwalker” into Google.

Now consider the “Mostly Color Channel”---which I abbreviate as MCC. MCC is the vision of Giordano Beretta (an innovator in color printing technology), but with contributions by Nathan Moroney and three others. MCC cross-references to many sites, including CIE activity reports, the ISCC historical translations and Hue Angles. But it offers much more, including videos, slide presentations, historical essays and blog entries---a huge amount of work! One of the historical essays quotes a letter to Science from toymaker Milton Bradley describing a telephone-controlled color wheel to “telephone a color”---an 1892 ancestor of Ralph Stanziola’s VCS-10! There’s also a video showing a display comprising a 3D swarm of individually controllable lights. Among the blog entries, Giordano offers book reviews---some in Italian. Try his review of Snakes in Suits on for size!

The site originated under the auspices of Hewlett-Packard, but now is independent of HP and represents the untrammeled opinions of its authors. You will see lots of color technology, as well as cultural artifacts that involve color. But also you will see intellectual tangents and corners of knowledge---interspersed with passionate editorials. The site is not organized for easy access to targeted information, but if you just click on anything you will be fascinated, as I was. (Again, Google will help you find a particular item.)

I think MCC is the Whole Earth Catalog of color---but with no price-tags. Interestingly, Steve Jobs called the Whole Earth Catalog the forerunner of the World-Wide Web---a sort of “Google in paperback form”. I disagree, because unlike Google, the Whole Earth Catalog purveyed a vision---to enable people to develop a self-sustainable lifestyle.

MCC also has a vision, conveyed in the footer of Giordano’s blog: “The Internet is an amalgam of forms blurred under epistemological pressures. In Søren Kierkegaard’s words, under this flat shower of leveled information, where everybody is interested in everything and nothing is too trivial or too important, people just accumulate information and postpone decisions indefinitely, i.e., nobody takes action and nobody is responsible for truth — there is no mastery, just gossip. He called this the æsthetic sphere of existence, exhorting us to evolve to the ethical sphere, where we do not just accumulate information but take action and make commitments. Blogs are instruments to overcome flatness by creating opportunities for vertical activities. In this sense this blog is a view from my window — a collection of tidbits I judged relevant to computational color science and in general to the promotion of scientific excellence in areas of strategic importance for the future of research, economy and society.”

That’s a hard act to follow, Giordano. Bravo!

Notes from the NPA: New platonic solids, new visions of Doppler shift

2010-07-20T09:17:00.004-04:00

Michael H. Brill
[Here are some notes from a most unconventional convention. Perhaps they will make the ISCC blog light up… …MHB]

The Natural Philosophy Alliance is even more diverse than the ISCC. Its members--- artists, lawyers, physicians, chemists, physicists and mathematicians---all give voice via their own expertise to challenge current scientific theories. They agree with each other less often than ISCC members. (Yes, that’s possible.) I attended their meeting (NPA17) [1] in Long Beach, CA the week after ISCC met in Princeton. Two NPA nuggets might intrigue ISCC members: a geometrical shape with interesting optical properties, and a thought experiment to clarify the Doppler effect.

Artist Michael R. Evans [2] dubs the “atom” out of which he builds his creations the Trion-Re’. Realizable in paper or clear acrylic plastic, the Trion-Re’ looks like a shortened weaving-shuttle. You can make one by cutting out the 60°-arc-limited parts in the figure below. Assemble the parts so A is preserved, the three B vertices coincide, edge 1 meets 1’, 2 meets 2’, and 3 meets 3’. [Exercise for the reader: Is this construction mathematically possible or must it be forced? See “Trion Re’dux” below.]

On a mathematical note, the Trion-Re’ has the minimum number of faces (F), vertices (V) and edges (E) that satisfies Euler’s formula V + F - E = 2 for polyhedra. Although the Trion-Re’s faces are not flat when assembled, given the above construction they start out flat and are never stretched (i.e., have zero Gaussian curvature).

Another NPA nugget was Physicist Francisco J. Műller’s paper [3] asking, if we think we understand Doppler red-shifts of light, then how do shifts of non-light happen (e.g, for a Fraunhofer absorption line incurred by an interstellar cloud)? Let a stationary Earth E look at a star S (possibly receding) through a cloud C (possibly receding). Say the recession red-shifts 656 nm to 670 nm. (I will speak of shift rather than scaling because the wavelengths here are not very different from each other.) There are three cases.

Case I: If S and C recede together from E, then the S’s spectrum and C’s absorption line are red-shifted together (the latter to 670 nm). C receives 656 nm light from S with no Doppler shift (because C and S are not in relative motion). The cloud absorber stops that radiation, and the rest of the light is passed Doppler-free. The lengthening path from C to E then shifts the entire spectrum including the absorption line.

Case II: If S recedes while C and E are stationary, then S’s spectrum is red-shifted but not C’s absorption line. Now C is in the same frame as E. S’s light is already red-shifted upon reaching C, and in particular 642 nm light is shifted to 656 nm and is stopped by the absorber. The absorption is at 656nm, but the rest of the spectrum is shifted 14 nm higher.

Case III: If S is stationary relative to E and C recedes, then C’s absorption line is red-shifted but S’s spectrum is not. Light received through C outside C’s absorption band has no interaction with C. For these wavelengths, the cloud does not exist, so S and E are static and have a vacant path between them—incurring no Doppler shift. But light emitted at 670 nm is blocked by C (at 656 nm), and that line is Doppler-shifted back to 670 nm due to the recession C from E.

Műller and I seem to agree on the results of these three cases, but whereas I model Doppler shifts based on how the whole optical path stretches in time [4], Műller has a different view. I thought this problem would be interesting for ISCC thinkers. In a sense the assumptions are simpler than the ones we take for granted in color science. At first I saw the cloud as a filter, but Műller correctly noted that even a transparent filter interacts with the light at all wavelengths, unlike in parts of the Doppler example.

Oh, by the way, much is said in NPA about Einstein’s relativity theories, and I added to that this year [5]. Back to normal stuff next issue!

1. http://conf17.worldnpa.org/ and click Abstracts to read any paper.
2. M. R. Evans, The geometry of light, Proc Nat. Philos.Alliance, 7, 149-153 (2010).
3. F. J. Műller, The Doppler effect of absorption spectral lines in moving astronomic bodies (How can it happen?) Proc Nat. Philos.Alliance, 7, 336-342 (2010).
4. M. H. Brill, Doppler effect: surprises from the time domain, J. Nanophotonics 4, 041520 (4 Feb 2010).
5. M. H. Brill, Cochetkov’s speeding bola---yet another entanglement for special relativity, Proc Nat. Philos.Alliance, 7, 62-63 (2010).

P. S. Trion Re’dux

My Trion Re’ puzzle (above) has several levels of answer---so I won’t wait for the next issue to clue you in. Can the three flat leaves be rolled up into a 3D convex figure with three-fold symmetry about the axis AB? Yes, but….

Examined along the AB axis, any cross-section of the Trion Re’ is an equilateral triangle, and the triangles are all centered on the axis and have the same orientation. Any progression of triangle sizes (as a function of position on the AB axis) is enforced by the shape of the unrolled leaf (whether or not limited by 60° arcs). Since the corresponding triangle sides are straight and parallel, each face rolls and unrolls between 3D and flat. So the answer to my puzzle is “Yes.”

A this point, it bothered me that the lining-up of the triangles implies that the edges of the Trion-Re’ are plane curves in 3D. They are also plane curves when unrolled flat. So how do you roll a planar 60° arc out of its plane so it becomes again a plane curve? I was able to show this is not possible if you use a circular cylinder as a “curling iron”. But I still don’t know the cross-sectional shape of the cylinder that makes it work. Maybe some ISCC geometer can find it.

Then I heard from artist Michael Evans (through physicist Greg Volk): The faces of the acrylic Trion Re’ are not Gaussian-flat, but are closer to being parts of spheres! So we have two distinct constructions, folks, the paper-folding one (with its neat math problem) and the acryllic one (with its neat optical property). They may be “artistically equivalent,” a matter to be decided by Interest Group 3!

The Peculiar Distribution of Last Color Names

2010-05-17T09:33:00.002-04:00

Michael H. Brill and Karen E. Linder

This column might seem an excuse for a tangent on mathematics, but be patient---the color-related topic will re-appear…MHB]

Here’s a color-related experiment you can do with the phone book: Record the number of people whose last names are colors. The distribution is far from uniform. The 2010 Residential White Pages for Princeton/Suburban-Trenton NJ shows the following incidence of last names that are color names: Brown (555), White (228), Green (154), Gray (73) [Grey (3)], Black (47), Blue (14). As a check on the geographic specificity of this result, we tried Marquis Who’s Who in the World 2001 and got a similar ranking: Brown (140), White (51), Green (30), Black (25), Gray (23) [Grey (2)], Blue (4). Neither source has any last names Red, Orange, Yellow, Indigo, Violet, We didn’t try any other color names. Why does this regularity exist? Mathematicians have taken such observations as points of departure for century-long theorizing. For example, in 1881, Simon Newcomb [1] noted, “that the ten digits do not occur with equal frequency must be evident to any one making use of logarithmic tables, and noticing how much faster the first pages wear out than the last ones. The first digit is oftener 1 than any other digit, and the frequency diminishes up to 9.” Benford rediscovered the tendency (henceforth called Benford’s law) in 1938. Others [2] have given theoretical explanations, and it is discussed in the context of the mathematics of fractals [3].

The probability of first digit d works out to log₁₀(1 + 1/d), which summed over d gives 1. Proof of plausibility: Require the probability density function of a continuous number to be scale-invariant [constant in log₁₀(x)]; note that implies a/x is the density function in x; note the integral of a/x diverges, so consider first only over one decade, from 10^m to 10^m+1\, and then realize the numbers don’t change when 1 decade is expanded to n decades. Over one decade of x (10^m to 10^m+1), the probability density is f(x) = 1/x, and the probability of first digit d is log₁₀(d+1) – log₁₀(d) = log₁₀(1 + 1/d); Finally, realize that, although starting or stopping in the middle of a decade produces an artifact, the artifact gets vanishingly small when the number of decades n gets larger and larger. Another view of this argument and its limitations appears in [4].

You can confirm Newcomb’s observation by an experiment with a phone book: Tally the first digits of street-address numbers, and observe that Newcomb, Raimi, et al. were right. You can stop after one or two pages…

By the way, Benford’s law is not just a curiosity, but is now used in detecting fraudulent random-guess data in income tax returns and other financial reports (http://www.rexswain.com/benford.html, and also [5]).

A related effect is called Zipf’s law (http://en.wikipedia.org/wiki/Zipf's_law), which has the same form in a variety of venues: The most frequent word in a natural language occurs about twice as often as the second-most-frequent word, about three times as often as the third-most-frequent word, etc. The same kind of relationship applies to the populations of cities in a country versus their population ranking.

Can such roots be found in the peculiar distribution of last color names? The last-color-name distribution is similar to Zipf’s law, but the decrease is too steep. Have we forgotten some common last-color-names that would fill in the gaps? Perhaps a cross-cultural study (e.g., performed by previous authors of this column) might fill in some of the gaps or give a key insight. We hope any such insights might find happier uses than detecting tax evasion.

1. S. Newcomb, Note on the frequency of use of the different digits in natural numbers, Amer. J. Math., 4, 39-40 (1881).
2. R. A. Raimi, The peculiar distribution of first significant digits, Sci. Amer. 221, 109-120 (December, 1969).
3. B. Mandelbrot, Fractals: Form, Chance, and Dimension, W. H. Freeman, 1977.
4. http://mathworld.wolfram.com/BenfordsLaw.html
5. T. P. Hill. "The First-Digit Phenomenon", American Scientist< 86, p. 358 (July-August 1998).

Disk Color Mixture and Beyond

2010-04-07T14:12:00.007-04:00

Don Hall, Former President of Applied Color Systems, Inc. (ACS) and ACS-Datacolor

[Rotating a disk for fun, knowledge, dizziness, and profit. This month we’ll hear from Don Hall, who has engaged in all of the above and even patented some of it. Don starts by reviewing a recent paper by Rolf Kuehni, and then reaches farther… - MHB]

Rolf Kuehni [1] recently published a comprehensive and well researched paper on the historic use of a modified child’s toy, a spinning top, to investigate the mysteries of color.

Kuehni starts with Ptolemy’s second-century, first-recorded, observation that a fusion of color occurs when a spinning multi-colored potter’s wheel reaches a certain speed. Eight hundred years later Alhazen, a Persian natural scientist, made a similar observation. After another 700 years, experimenters tried to understand why disk mixtures don’t ‘properly’ correlate with pigment mixtures. After this was understood, disk color mixture found practical use. In 1763, Antonio Scopoli, an Austrian physician and natural scientist, used disk mixture in classifying insect colors. Two years later, Chevalier D’Arcy, a physicist, measured the persistence of a rotating visual image using a glowing piece of coal. He needed at least eight revolutions per second for the image to fuse. (See [2] for more on the time resolution of vision.)

Over the next hundred years, Kuehni continues, disk-mixture studies revealed “accidental colors”, afterimages and complementary colors. During this period attention gradually shifted to empirically based color order systems.In 1810, the artist Philipp Otto Runge proposed such a system but had difficulty correlating Newton’s colors with pigment mixtures and decided to experiment with disk mixtures. In the process he lightness-matched the chromatic segments against the black & white segments.

Several years later, in 1855, James C. Maxwell reported using the famous disk-mixture device of his own design to advance color theory. By 1860 he switched from a spinning
disk to a visual colorimeter to match spectral colors with spectral primaries. A few years later O. N. Rood used disk mixture to reconstruct or “correct” Maxwell’s diagram.

In 1900 A.H. Munsell patented his Color Sphere which, when spun, produced neutral grays of decreasing Value from the top to bottom of the sphere. Later Munsell used disk mixture to create the color panels that populated his 3D Color Tree. In subsequent years the Munsell Company offered his color chips in a circular form with a center hole and slitted radius for mounting on a spinning disk device.

By the 1920’s and 30’s, visual photometers, tristimulus colorimeters and spectrometers obviated the need for spinning-disk mixture. However, as Kuehni reports, there was one final gasp for that old technology. In 1977, Applied Color Systems Inc. (Princeton, NJ) began to develop an instrument for measuring color materials that were not readily measured on a reflectance spectrophotometer because of texture, pattern, size or geometry. After nearly two years of unsuccessfully exploring color CRT and color projection systems, Ralph Stanziola, ACS’s Executive Vice President and Technical Director, decided to take a page out of the past, to the amazement of his associates, by developing a “Maxwell Disc” connected to a computer to match colors. Although the Visual Color Simulator (VCS-10) took three years to develop, it was technically successful as a visual color measuring input device for computer color matching and also as an accurate color simulator that could rapidly transmit visually simulated colors to remote locations. As Kuehni correctly points out, it may have worked well but it was too expensive to be widely accepted.

During the development of the VCS-10, some visual anomalies were encountered, particularly during start-up while the color disk pack accelerated to the flicker-fusion rate. Some observers complained of nausea and vertigo, and others saw different colors in their peripheral vision. To avoid these perturbations, the power to the controlled illumination lamps was not activated until the rotation was fast enough for flicker fusion.

The effect we were avoiding by turning the lamps off is related to a much weaker disk-color effect that is not properly disk color mixture (and which Kuehni doesn’t mention). Produced in 1894 by toy maker C.E. Benham, the “Artificial Spectrum Top” [3] was a disk that was one half black and the other half a white background overprinted with four areas having a series of three concentric black arcs arranged in a step-wise fashion. When this disk was spun below the flicker-fusion rate, concentric circles of weak colors appeared. When rotated the opposite way the illusionary colors reversed order.

Benham’s top is still a subject of scientific investigation [4, 5] and disk color mixture may be a matter for history. Nevertheless, I still have a Swiss Made “Optischer Farbmischer” [www.swissmade.com] on my desk as a reminder of the pleasure of working with Ralph Stanziola on the VCS-10 and in a sense reliving some experiences of the scientific pioneers that Kuehni documents so well.

REFERENCES:
1. Kuehni, RG. A brief history of disk color mixture, Col. Res. Appl. 35, 110-121 (2010).
2. Morgan Eye Center, Univ. of Utah, Temporal Resolution http://webvision.med.utah.edu/temporal.html
3. Benham C.E. The artificial spectrum top, Nature 51, 113-114 (1894).
4. LeRohellec J, Vienot, F. Interaction of luminance and spectral adaptation upon Benham subjective colors, Col. Res. Appl. 26, S174-S179 (2001).
5. Kenyon,G, Hill, D, et al. A theory of the Benham top based on center-surround interactions in the parvocellular pathway, Neural Networks 17, 773-786 (2004).

Engineering, Insanity, and 999 Years of Optics

2010-01-27T11:29:00.007-05:00

Michael H. Brill, Datacolor

We’re not quite at the millennium mark for Alhazen’s Book of Optics. It brings to mind a story of how Alhazen got started in his “second career” that should inspire us all…

A recent Scientific American article [1] prefaced the results of a photo contest by noting that we are approaching the millennial anniversary (2011) of Persian scientist Ibn al-Haytham’s (Alhazen’s) starting to write his Book of Optics. The article used Alhazen’s discovery of the properties of a magnifying glass as a segue to the wonders of photography through microscopes---and hence to the contest.

Quite apart from the science, one story of that beginning [2, 3, but not 4] has a message to us today. As a civil servant at age 45, Alhazen was called to Egypt by Al-Hakim bi-Amr Allah, the sixth ruler of the Fatimid caliphate, to regulate the flood waters of the Nile. At first Alhazen envisioned a dam where the Aswan now stands, but upon reconnoitering on the south border of Egypt, saw no way to bring that plan to fruition. Quickly realizing his inability, he retired from engineering. In that regime a “severance package” likely would mean your head cut off in a basket. To save his life, Alhazen feigned insanity, and was forthwith held in house arrest from 1011 until al-Hakim died in 1021. Under house arrest, he immediately began his Book of Optics, and thereafter generated many scientific works for the rest of his life (i.e., through 1039).

In the Book of Optics one finds the first modern scientific treatment of optics and vision. Alhazen methodically investigated the magnifying properties of a lens, and also its property as a “burning glass” when focusing the sun’s radiation. He also was the first to project an entire image from outdoors onto a screen indoors through a small hole, and thereby to demonstrate and explain the action of a camera obscura. He had a theory of the Moon illusion (one of many such theories that still mark our landscape). He might have attributed the focusing aspect of a glass lens to the function of the lens of the eye, but the lens’s inversion of the image (which is not evident in vision) was a stumbling block in those days.

One could go on: In the remaining decades of his life, Alhazen contributed to many more areas of science. A geometry problem named after him has stimulated recent publications, and his challenge to Ptolemy’s cosmology was a fitting precursor of the Copernican revolution. Alhazen also described the modern scientific method (usually credited to Francis Bacon). He even noted that a prism splits light into colored components that have different refrangibility---a creditable forerunner of Newton’s work more than 600 years later.

So what encouragement can we take from the example of Alhazen? When confronted by mid-life requirements to reinvent ourselves, we can remember Alhazen’s “retirement” from engineering. Even after a good career goes awry, even after ten years of putative insanity, it is possible to propagate 1000 years of heritage in one’s chosen field.

REFERENCES:

1. G. Stix, “Illuminating the Lilliputian: 10 Bioscapes Photo Contest winners revealed,” Scientific American, Dec. 2009.

2. Alhazen, Wikipedia.

3. Lorch, Richard (2008), "Ibn al-Haytham", Encyclopædia Britannica.

4. Roshdi Rashed, “A polymath in the 10th century”, Science 297, p. 773 (2002).

Color Creeks Ray Winey Has Found People Up

2009-11-16T09:53:00.004-05:00

Bill Longley, Adjunct Professor, Eastern Michigan University
Retired, Ford Motor Co.

[Who was the first to perform colorant formulation on a digital computer? Bill Longley here makes the case that---at least in the U.S.---it was his mentor, Ray K. Winey. Winey is also known for the metameric specimens he made, which screamed, “Matching under two lights is not enough!” It is hard to make metamers, and also to convince people to avoid mistakes in color science, yet Winey did both. - MHB]

I had the rare good luck after graduating from college in 1958 to work under direction of Ray K. Winey at US Rubber Co (later Uniroyal) in coloring of Naugahyde upholstery material. Ray had majored in chemistry at Notre Dame and spent his career in Mishawaka, Indiana, never wanting to wander very far. When he spotted an interesting conference he would send one of us and then eagerly debrief us upon our return. Ray, now deceased, was a genius with an intriguing grasp of poetry as well as battlefield war strategy. His recall and passion for all aspects of color science was overwhelming to color chemists who came under his direction.

Davidson & Hemmendinger had introduced the COMIC, an analog computer with wavelength/reflectance scope which aided the colorist in developing initial colorant formulations. I persuaded Ray to make the short drive to Chicago to see COMIC at an equipment show. Here we met Henry Hemmendinger, who patiently demonstrated COMIC for the umpteenth time that day. Ray folded his arms and commented, “Very interesting, but I prefer the digital approach.” Henry wanted more discussion, so he called for assistance with the exhibit and then he and Ray moved to a nearby table and launched into an unforgettable session.

Ray summarized his work on digital computer matching in a company report [1] dated 15 August 1962, complete with color standards and resulting swatch matches. The report discusses limitations of analog computing, especially assumption of zero scattering. Rather than requiring the colorist to select pigments for the match, Ray directed the program to consider all possibilities of 3 pigments plus white, and then report the matches with least spectral differences. If dissatisfied with results, he could direct the program to add a fourth pigment. Ray’s report cites Kubelka-Munk and computes relative K and S coefficients, 10 parts TiO2 to one part color pigment. He relates reflection to K/S ratio using the Saunderson surface correction. Computing was slow so he loaded data into the Bendix G20 at close of office hours and collected results the next morning. Contemporaries in the field who have seen a copy of the report have marveled at digitally calculated two-constant theory at that early date. It wasn’t until five years later that D&H offered the COMIC II digital computation unit.

Details and comments on the report are offered by another Winey protégé, Ron Penrod, at www.rpdms.com. Site viewers will also find there an excellent color matching program, workable in four modes with conversions. [Editor’s note: Do any readers know if E. Atherton’s program from the UK (reported in 1961 J. Soc. Dyers Col., and reputed to have run since 1956) had the capabilities described here?]

Ralph Stanziola and Max Saltzman were among many who visited Ray in Mishawaka. He enjoyed showing visitors his “dilemma” samples (two metameric swatches), asking how the visitor would add a correction to the metameric mismatch that was redder than the standard in daylight and greener in tungsten. Told this was impossible, he then presented another swatch containing pigments that flared green to the standard in daylight and red in tungsten. In fact this is how the standard had been made.

Ray had some classic correspondence with Norman Macbeth and Warren Reese concerning Macbeth claims for their industrial light-source unit, that samples matching in D7500 daylight (blue end) and D2300 tungsten (yellow end) would match anywhere. Ray produced numerous samples to disprove the claim. Eventually Macbeth added cool white fluorescent as a third source for matching, also standardizing on D6500 daylight. I like to think that Ray provided the impetus.

I offer these notes here to credit Ray for his pioneering work in color science. I inherited Ray’s “golden” files and sometimes wonder what to do with the intriguing letters and metameric specimens. I have his hand-written note saying he wanted to write a book Color Creeks I Have Found People Up. He never wrote the book, but certainly had enough material for it.

1. D. F. Larimore and R. K. Winey, "Color Matching with the Aid of a Digital Computer," Report No. 62-L3-35, US Rubber Co., Divisional Laboratories, Consumer and Industrial Products Division, Mishawaka, Indiana, 15 August 1962.

C. V. Raman’s Explorations in Color Science

2009-09-25T15:54:00.003-04:00

by Michael H. Brill, Datacolor

This year (and hopefully this month) will mark the end of my ten-year chairmanship of CIE TC1-56, during which I found that the laws of color matching are not so simple as Hermann Grassmann thought. Is color science possible without Grassmann’s underpinnings? To find out, I look at a little-known corner of history….

Chandrasekhar Venkata (C. V.) Raman (1888-1970) was always interested in the science of color. Indeed, that interest seemed to motivate his Nobel-Prize-winning work in spectroscopy. A 1921 trip returning to India from England made him marvel at the blue of the ocean, and to posit that blue as arising from molecular scattering of light by water molecules, not just reflection of the blue of the sky (as Lord Rayleigh supposed). By 1928, Raman found that blue light through glycerin had a shift to the green, and that the shift was due to quantum transitions in molecular electron states. Raman spectroscopy was soon born, quantum mechanics and the photon theory of light were vindicated, and the Nobel Prize followed two years later.

In 1959, after a fruitful career in optics, acoustics, and the interaction of light with sound waves, Raman turned his attention exclusively to color, including the colors of plants and minerals and color blindness. He used only a pocket spectroscope, some black-and-white photographic film, and a few functional human visual systems. The color-science period of his life was to last more then ten years, and gave rise to many publications in the annals of the Indian Academy of Sciences in Bangalore. I found one book of this work [1] on a shelf in Datacolor earlier this year, and a Google search revealed this work and a lot more, on individual pdf pages from the Indian Academy of Sciences. Even partial retrieval of the work was painful, but it was worthwhile and timely for me.

Here is a quote from a 1966 work [2]:

“It is a remarkable fact that a person endowed with normal vision is capable of recognizing quite small differences in colour if these are presented to him in an appropriate fashion. For example, the two yellow lines in the spectrum of a mercury lamp, whose wavelengths are respectively 5770 Å and 5790 Å and which are of equal intensity when seen simultaneously through the eye-piece of a spectrometer exhibit an observable difference in colour, the former line appearing of a greenish hue while the latter is a pure yellow. This fact suggested to the author that an arrangement by which the entire continuous spectrum is presented as a series of discrete lines would be a useful device for the study of the spectrum colours and especially for exhibiting the differences in the rate of progression of colour in different parts of the spectrum.

“The idea indicated above can be realized in practice by setting two half-silvered plates of glass in parallel positions before the slit of a wave-length spectrometer and viewing the spectrum of a brilliant source of white light of restricted area normally through the combination. The entire spectrum is then seen as an array of discrete lines or bands in a dark field, their number and spacing being determined by the separation between the plates. By making one of the plates movable with respect to the other, the number of lines or bands seen in the spectrum can be varied within wide limits. […] A channeled spectrum of 100 bands […] was presented.” (p. 269)

“ A remarkable and convincing demonstration that Daltonian vision arises by reason of an abnormal enhancement of the sensation of yellow in relation to other colours in the spectrum…” (p. 270)

This work is experimentally ingenious. I’m especially impressed with Raman’s recognition of (and use of) the subtle property of human vision that enhances the discrimination of colors when they are separated by a dark boundary. But something is missing in the description. There’s no mention of trichromacy, none of the heritage of Newton, Young, Helmholtz, or Hering. That seems to be true of all of Raman’s work. And of course, Grassmann’s laws are also absent from the discussion

How could a 20th century physics Nobelist devote ten years to color research and write copious articles without once referring to the trichromacy of color vision? Perhaps the answer can be found in Raman’s famous 1968 lecture, “Why the sky is blue” [3]. Here, Raman recounts the familiar Rayleigh-scattering story, but adds much more. Why don’t we see the stars during the day? Because the atmosphere of the earth is a veil that hides them by scattering the Sun’s light. Why isn’t the sky blue on a moonlit night (for which the same spectrum acts in an attenuated form)? Well, here Raman isn’t so sure. He says it’s a difficult question why we don’t see colors at night. He never mentions rods and cones, or of the body of literature that culminated in the same year with Yves LeGrand’s second edition of Light, Color, and Vision. And yet Raman espouses a holistic philosophy of science: “Ultimately you find that you have to travel the whole field of science before you get the answer to the question: Why the sky is blue?” Another quote reveals how exploratory he is willing to be: “To get any colour, red, green, or blue, you have to take out the yellow. Yellow is the deadly enemy of colour.” Later, he seems to be getting closer to opponent-color theory: “It is the reduction of the yellow of the spectrum that is to say the predominance of the blue which is responsible for the blue light of the sky.” But here’s the final take-home lesson: “I want you to realize that the spirit of science is not finding short and quick answers. The spirit of science is to delve deeper.” I will guess that Raman was well aware of rods, cones, and trichromatic theory, but felt he had not been able to delve deeper, to add as much to the vision explanation as he had to the simple one-sentence “Rayleigh scattering” answer to “Why is the sky blue?”

Well, Dr. Raman, I am ready to delve deeper now that TC1-56 is at its end. Simple sound bites such as “linearity” and “trichromacy” aren’t going to cut it anymore.

1. “Memoirs of the Raman Research Institute No. 137: Floral Colours and the Physiology of Vision”, by Sir C. V. Raman (Bangalore, 1963); pp. 57-108.

2. C. V. Raman, The New Physiology of Vision, Chapter XXXIX. Daltonian Colour Vision, J. Indian Academy of Sciences, pp. 267-274 (1966). http://www.ias.ac.in/jarch/proca/63/00000280.pdf and subsequent pages (last 3 digits in filename).

3. C. V. Raman, “Why the Sky is Blue,” Lecture Dec. 22, 1968 at Ahmedabad. http://dspace.rri.res.in/bitstream/2289/1509/1/1968%20(Dec.%2022)%20Raman's%20Lecture%20-%20Ahmedabad.pdf

Color and Thesauruses

2009-07-21T10:13:00.013-04:00

by Nathan R. Moroney, HP Labs

At the recent MCSL 25th Anniversary Symposium, Nathan Moroney showed us a “book with 5000 authors” comprising color names printed in colors that denote their synonyms. The 5000 authors were contributors to his on-line color thesaurus. Here are more details from Nathan ...

Is the color “zaofulvin” synonymous with “orpiment” or “smalt”?
The 1911 edition of Roget’s Thesaurus1, which early-on attempted to cluster color names, has an answer.

As you can see from Table 1, “zaofulvin” and “orpiment” are synonyms.

White	Niveous Canescent Lactescent
Black	Atramentous Fulginous
Gray	Favillous Cinereous
Brown	Casteneous Fuscous
Red	Anotto Realgar Minium
Green	Verdine Copperas
Yellow	Orpiment Zaofulvin Luteous
Purple	Gridelin Heliotrope
Blue	Bice Zaffer Smalt
Orange	Gild Ocherous

But if a thesaurus is “a resource to group words according to similarity”² then how are we to judge the similarity of words, and particularly of color names? Kilgarriff2 summarizes the contrasting methods of manual creation (e.g., for Roget’s Thesaurus) with the automatic extraction from corpora or large collections of machine-encoded text. He also emphasizes that, besides grouping words according to similarity, a thesaurus should also indicate how frequently each word is used.

The ISCC-NBS color dictionary³ significantly advanced the grouping of color names by providing about 300 name categories for over 7,500 color names taken from 13 different earlier color dictionaries and vocabularies. This work partitions Munsell color space for the core vocabulary and maps it to the larger collection of earlier color-name collections. This is a hybrid approach that merges multiple manual efforts into a single manual or expert framework.

Modern efforts at thesaurus creation through automatic extraction are making progress, but experiments with nine similarity metrics show⁴ that much more work is needed. Some of the challenge is to have a large enough collection of text for analysis. However, size alone is not the solution. For instance, the very large Moby Thesaurus⁵ returns “pineapple” and “pear” as synonyms for “orange”, thereby apparently including the fruit meaning with the color-name meaning.

An alternative approach is the direction construction of a specific color naming corpus using the World Wide Web. For the past eight years, I collected over 35,000 color names from over 5,000 online volunteers. Each volunteer named seven randomly generated colored patches displayed on a white background. The colors were selected from a uniform red, green and blue sampling of what was at the time the “web-safe” palette for lower bit-depth displays. Of these 35,000 color names, many are used repeatedly. Assuming an imposed minimum of three participants to provide a specific color name, the program derives over 600 color names. Using these names as an initial collection, it computes synonyms by finding the closest color names in a corresponding color space. It also finds the color names that are closest to the inverses in hue and lightness as possible color antonyms. Finally, because the data are collected from thousands of participants, the program infers the relative frequency of use. In this way, we created a color thesaurus that is closest to an aggregate or collective clustering of colors across a large number of English speakers.

This data is formatted as a web-based color thesaurus,⁶ which has been well received by online users. It has served almost 200,000 color names to date --- although zaofulvin and orpiment are not included.

(1) Peter Mark Roget, Roget's Thesaurus (1911), Project Gutenberg edition, http://www.gutenberg.org/etext/10681, retrieved May 2009.

(2) Adam Kilgarriff, "Thesauruses for Natural Language Processing", Proc. Natural Language Processing and Knowledge Engineering, p. 5-13, (2003)

(3) Kenneth L. Kelly and Deane B. Judd, The ISCC-NBS Method of Designating Colors and a Dictionary of Color Names, National Bureau of Standards Circular 553, (1965).

(4) James R. Curran and Marc Moens, "Improvements in Automatic Thesaurus Extraction", Unsupervised Lexical Aquisition: Proceedings of the Workshop of the ACL Special Interest Group on the Lexicon (SIGLEX), Philadelphia, Association for Computational Linguistics, pp. 59-116, (July 2002).

(5) Moby Thesaurus – online interactive version from dict.org, http://www.dict.org/bin/Dict?Form=Dict3&Database=moby-thes , results retrieved May 2009.

(6) The Online Color Thesaurus - http://www.hpl.hp.com/personal/Nathan_Moroney/color-thesaurus.html.

Lessons from Eastman Kodak in the Great Depression

2009-05-26T10:34:00.003-04:00

by Michael H. Brill, Datacolor

We know 1931 as the birth-year of the ISCC and of the CIE Standard Observer, but others associate that year with the Great Depression. Color science seems to have matured and thrived during the Depression years, and in particular we can learn many things….

Great wine thrives in a dry season, and great color science has thrived in times of economic downturn. Eastman Kodak thrived twice in this way before most of us were born. The first time was in the 1890s, when the panic was about redeeming securities for gold (of which there was not enough). In Rochester, a backwater with no particular natural resources, Kodak produced its Brownie and Folding Pocket cameras. Photography took off.

Then, in the Great Depression of the 1930s (whose circumstances are more familiar to us), color photography took off. The ingredients were present well in advance: Invention of the subtractive technique by Cros and Ducos du Hauron (1869), and CMY color separations that required three separate shots for one picture (or three beam-split images in register). Rudolf Fischer’s discovery of dye-coupling (1912) was critical, but lay in hibernation until, in 1935, Mannes and Godowsky at Kodak (locally known as “Man and God”) invented three-layer subtractive-color film that enabled a full color photo in a single shot. At that point Kodachrome film was born [1]. There followed in 1937 and 1938 a number of top-notch color-photography articles [e.g., J. A. C. Yule, J. Opt. Soc. Am. 28, 419-426 (1938); D. L. MacAdam, J. Opt. Soc. Am. 28, 466-480 (1938); D. L. MacAdam, J. Opt. Soc. Am. 28, 399-418 (1938); A.C. Hardy and F. L. Wurzburg, J. Opt. Soc Am. 27, 227-240 (1937).]

Credit is of course due to the inventors of Kodachrome and to the authors of these articles, but also credit belongs to the direction of the Kodak laboratories by C. E. K. Mees, author of many books on organization management as well as on photography. Much constructive incubation was happening during the bad years. Together with a keen sense of what is needed to “prime the pump” during such years, Mees had a perspective of science and technology as being in a reciprocal relationship. Martin Scott [2] has said, “Paraphrasing Mees: ‘Science has been good to Photography, and Photography should be good to Science.’ Guided by that motto, he made many special films and emulsions for scientists with no regard for profitability.”

Credit for Kodak’s success accrues to an even higher managerial level. It must not be forgotten that, in 1931, Kodak and 13 other companies fostered what was known as the Rochester plan [3]---a system of unemployment insurance that came some years before Roosevelt’s national plan. Because only 14 companies participated, the plan lasted only a few years, but it managed to cushion the blow to unemployed workers from 1933 to 1935, and Kodak didn’t need it by 1936. (Presumably Kodachrome helped here.) Notable in the implementation of the Rochester plan was a curious confluence of pragmatism and compassion: “Just before implementation of the Rochester Plan, corporate executives at Kodak let plant managers know that they would be held accountable for future unemployment and that it would not reflect favorably upon them if benefit payments were too high.“ [3] Big bonuses were not linked with layoffs...then. Another difference from our recent experiences nationally: Kodak stockpiled inventory and redirected people to increase this inventory during lean times. This is the exact opposite of “just-in-time” delivery, and helped decrease the need for seasonal layoffs (and perhaps longer-term effects as well).

Of course, part of the credit for Kodak’s success during the 1930s belongs to circumstance. During other economic downturns, Kodak learned how to stockpile inventory, respect research, and reward employee retention. Part of the circumstance was the extreme profitability of photographic film---which has lately become somewhat obsolete.

What can we learn for today from Kodak’s Depression experience? Do our lean inventories and draconian logistics destabilize our corporate survivability? Do AIG-style bonuses de-incentivize economic prudence? Does “just-in-time” research undermine longer-term goals? Maybe we should just leave this alone and paraphrase Freud: Sometimes a roll of film is just a roll of film.

I would like to adopt Martin Scott’s positive note that may inform other answers: “Five hundred years of letterpress; fifty years of lithography; and now, how many years of the new technologies? If I've learned anything, it is not to predict, and especially don't presume to know the rate of change.”

Michael H. Brill

[1] Beaumont Newhall, The History of Photography,” Museum of Modern Art, New York, 1964, p. 193.

[2] Martin L Scott, “Introduction: Images for Science,” Images from Science 2, June 2008, http://www.rit.edu/cias/ritphoto/ifs-2008/about_IFS.html

[3] Richard E. Noll, “Marion B. Folsom and the Rochester Plan of 1931,” Rochester History (Vol. 61, 1999), Ed. Ruth Rosenberg Naparsteck.

Language and color perception

2009-04-10T11:24:00.002-04:00

by Terry Regier, University of Chicago and Paul Kay, University of California at Berkeley

[At the 2007 IS&T/SID Color Imaging Conference, Terry Regier gave a keynote address on how color discrimination is influenced by linguistic categories in the right but not in the left visual field. Now he revisits the topic with noted color/language expert Paul Kay. While reading, you might contemplate, how do I try this at home? - MHB ]

Does language affect perception, or not? The yes-or-no framing of this question obscures an interesting possibility. Several recent studies on color suggest that language does indeed affect perception (or at least perceptual discrimination) – but it does so primarily in the right visual field (RVF), and much less if at all in the left visual field (LVF), a pattern suggested by the functional organization of the brain. Thus, half of our perceptual world is viewed through our native language, and half is viewed without a linguistic filter.

This pattern was first shown in a study [1] that probed the discrimination of colors straddling the boundary between green and blue, a boundary present in English but absent in many other languages. The study found evidence for categorical perception of color – faster discrimination of colors from different categories – but only in the RVF, not in the LVF. This lateralization was disrupted by a concurrent task that interfered with verbal processing, but not by a concurrent task of comparable difficulty that interfered only with non-verbal processing – suggesting that the pattern is verbal in origin. Other studies replicated and extended this finding, exploring the cross-cultural and developmental origins of our tendency to view half of our visual world through language, and half of it less so if at all.

If color categories affect perception, at least in half the visual field, where do those categories come from? Why do languages have the color categories they do? An influential universalist view of color naming holds that color categories across languages are organized around the universal focal colors black, white, red, green, yellow, and blue. A recent relativist challenge holds in contrast that there are no such universal foci, and that color categories are defined at their boundaries by largely arbitrary linguistic convention. Both of these views are partly supported and partly challenged by existing data, which show universal tendencies in color naming, coupled with interesting cross-language variation of category boundaries.

This complex picture can be accounted for starting with the observation that perceptual color space is irregular, in the sense that the maximum possible saturation varies unevenly across hue/lightness combinations. One proposal [2] is that color naming reflects optimal or near-optimal partitions of this irregular space. Recently, this idea was formalized and tested against empirical data [3]. A well-formedness measure was defined that captures the extent to which a given categorical partition of color space maximizes perceptual similarity within color categories and minimizes it across categories. Across the 110 languages of the World Color Survey – a database of color naming from non-industrialized societies worldwide – color naming tended to be near-optimal in well-formedness. At the same time, linguistic convention may get some wiggle room: Often, similar but different partitions are roughly equally well-formed, suggesting a middle ground between “nature” and “nurture” in color naming across languages.

Neither of these findings – that language affects color perception primarily in the right visual field, or that color naming is near-optimal – is anticipated by the traditional universalist-versus-relativist debate over language and perception. Instead, these findings suggest novel perspectives on the relation of language and perception.

[1] A. Gilbert et al. (2006). Whorf hypothesis is supported in the right visual field but not the left. PNAS 103, 489-494.
[2] K. Jameson & R. D’Andrade (1997). “It’s not really red, green, yellow and blue: an inquiry into perceptual color space,” in Color Categories in Thought and Language, C. L. Hardin and L. Maffi (eds.), Cambridge University Press, 295-319.
[3] T. Regier et al. (2007). Color naming reflects optimal partitions of color space. PNAS 104, 1436-1441.

2009-02-13T19:41:00.002-05:00

by Michael H. Brill, Datacolor

Physics Nobel Laureate Richard Feynman not only played bongo drums in nightclubs, but also wrote two chapters on color and vision in his Lectures on Physics. And that’s not all: There’s also…

Feynman’s Paint-Mixing Problem

Richard Feynman tells an interesting story [1] about revealing a painter's trick in mixing red and white paint to get yellow. Here's how it goes:

Feynman: "I don't know how you get yellow without using yellow."
Painter: "Well, if you mix red and white, you'll get yellow."
Feynman: "Are you sure you don't mean pink?"
Painter: "No, you'll get yellow."
Feynman: "It must be some kind of chemical change. Were you using some special pigments that make a chemical change?"
Painter: "No. Any old pigments will work."

So Feynman got a can of red and a can of white paint, and the painter began to mix them. It kept looking pink to Feynman. But then:

Painter: "I used to have a little tube of yellow here, to sharpen it up a bit---then this'll be yellow."
Feynman: "Oh! Of course! You add yellow, and you can get yellow, but you couldn't do it without the yellow."

Touché. Feynman wins.

But did he really? I remember looking at a white wall through a vial of yellow food-coloring liquid, and seeing it as red. That’s because the transmission spectrum goes from very low at the short-wavelength (blue) end of the spectrum to nearly 1 at the long-wavelength (red) end of the spectrum. As one piles on more layers of the same fluid, the transmission spectrum multiplies by itself wavelength-by-wavelength (an action known as Beer’s law, which by coincidence also happens when you look through beer). Therefore, at the wavelength where one ply of the liquid transmits half the incident energy, two ply of the liquid transmits only 1/4 of the energy. On the other hand, at wavelengths where one ply transmits all the energy, two ply will transmit all the energy as well. For a transmission coefficient that increases monotonically in wavelength (such as most yellows), the transmitted-light spectrum becomes biased toward longer wavelengths (i.e., is redder) when the layer is thicker.

So there’s at least one way red and white can will mix to give yellow: a clear diluting vehicle for the white and a red Beer's-law ink that transmits enough light at medium wavelengths so it yellows up when you see through less of it. Of course, you must have a reflecting background---let's make it matte white. As a numerical example, suppose a unit optical thickness of the red ink has transmittance zero for wavelengths below 540 nm, t for wavelengths between 540 and 640 nm, and 1 for wavelengths above 640 nm. The light reflected from the background through a unit thickness of ink can then be represented as the triplet (0, t², 1). That triplet will change to (0, t^2x, 1) when the optical thickness is changed to x. A deep red ink will have, say, t² = 0.1, whereupon ten-fold dilution of the ink (x = 0.1) will produce t^2x = 0.7943. The layer will therefore be substantially yellow.

You can also do this exercise (at least theoretically) with opaque red and white paints that obey Kubelka-Munk mixture algebra. [2]. I’ll elaborate about that in a future publication.

It seems, then, that the painter could have made a yellow by mixing particular red and white paints, contrary to Feynman’s intuition. But it certainly couldn’t be expected for all reds and whites as asserted by the painter. For example, drinkers of red wine (instead of beer) won’t see the yellowing effect---diluted red wine looks pink, not yellowish. Why should wine obey Feynman’s intuition where beer does not? The subject is worth much experimentation. Care to join me?

[1] R. P. Feynman and R. Leighton, Surely You're Joking, Mr. Feynman (Norton, New York, 1997), pp. 82-83.
[2] G. Wyszecki and W. S. Stiles, Color Science, (2nd Ed., Wiley, New York, 1982), p. 785.

Green Technology and Yellow Afterimages

2008-12-18T14:41:00.007-05:00

By Michael H. Brill, Datacolor
A logo shown at the last ISCC meeting evoked a memory from graduate school….

At the recent Baltimore ISCC meeting, David Oakey gave a talk on “Respect for the future through the use of color.” One of his visual aids was the new British Petroleum logo (see below, or search “BP logo” and click on “Image Results”), which spoke of solar power and green energy through its sun-like white center with yellow-bordered rays, surrounded by green leaf-like structures. Staring at the pattern on a large screen, and then at a piece of white paper, I saw a quite distinctive afterimage: bright yellow in the center of the pattern, surrounded by nothing very distinctive. I was surprised that the afterimage was brighter than the paper (the white center should have evoked a dark afterimage), and also by the yellow color (as opposed to blue, induced by the yellow border in the logo). For a smaller image of the logo, I saw something more like what I had expected: a faintly yellowish center with a diffuse purple surround.

This was reminiscent of two effects I found [1] in exploratory efforts as a graduate student under the direction of Jerome Y. Lettvin (MIT).

(1) Extending Abney’s finding [2] that all colors seem to shift toward yellow when mixed with white light, Lettvin [3] proposed that even yellows should get yellower: i.e., a yellow light should become more saturated when mixed with white. Accordingly, I projected sharply focused white spot on the diffuse yellow background produced by shining a white light through a Wratten 15 filter. The apparatus consisted of two quarter-inch light pipes, two American Optical fiber-optic illuminators, two rotary neutral-density filter wedge assemblies, and a focusing lens and diaphragm for the white spot. The white spot indeed seemed a more saturated yellow than the surround when it was not too bright.

(2) When a diffuse, barely discernible blue light (e.g., through a Wratten 98 filter) is shone (e.g., by a projector with no lens) on a white screen in a generally lit room, the shadow cast by an interposed object appears startlingly yellow, and the edge of the shadow appears diffuse no matter how sharp it looked using another light. The shadow can look brighter than the rest of the wall (despite reflecting less light). Furthermore, if the object casting the shadow is a pendulum in motion, the shadow lags the pendulum at the ends of its trajectory (where the acceleration is greatest), in a manner reminiscent of the Pulfrich effect (whereby a pendulum seen binocularly with one eye filter-covered appears to move in 3 dimensions due to the receptor-response lag in the filtered eye). I called the yellow-shadow version a “monocular Pulfrich effect.”

How can all this be explained? One clue is to realize that blue contributes very little to the luminance channel in vision, hence bright yellow has almost the same luminance as white (which matches yellow + blue). Since the luminance channel has much higher resolution both in space and time, it is clear that a border between yellow and white will look blurrier than a border between colors of appreciably different luminance, and will also evoke a time-lagged visual response. That explains the blurriness and time lag of the yellow shadow edge in the “monocular Pulfrich effect.”

Another clue is that the blue receptors also operate in low resolution both in space and time. That is another clue, which together with the first can help explain the BP-logo afterimage and yellow-spot effect. One must also remember that, when looking at the primary pattern, the eye is always moving in a jittering motion to refresh the image.

Anyone care to offer an explanation
for the BP-logo afterimage based
on these clues?

[1] M. H. Brill, Color Vision: An Evolutionary Approach, Ph.D. Dissertation, Syracuse University, 1976, pp. 57-58.
[2] Abney, W. de W. Researches in Normal and Defective Color vision and the Trichromatic Theory, London: Longman, Green and Co., 1913.
[3] J. Y. Lettvin, The Colors of Colored Things, Quarterly Progress Reports of the MIT Research Laboratory of Electronics 87 (1967), 193-225.

150th Anniversary of Albert Henry Munsell’s Year of Birth

2008-10-12T14:22:00.006-04:00

by Rolf G. Kuehni

[This issue we have the privilege of a column from Rolf G. Kuehni, author of many books on color order and on color technology (the latest two published in 2008, one of them reviewed in this issue). I believe Rolf’s topic is a sesquicentennial. - MHB]

The year 2008 should not pass without those of us in the color world remembering that 150 years ago A. H. Munsell (1858-1918) was born. Munsell was an artist, educator, and inventor, with five patents to his name. His color order system has proved to be enormously influential if, like all such efforts, less than perfect.

Born in Boston into comfortable circumstances, Munsell showed early interest in art as well as science related to art. In 1879, at age 21, he studied the newly published book by O. N. Rood, “Modern Chromatics,” a book that became important to French postimpressionist artists. In the later 1880s he spent time in Paris, studying painting and the color order systems of people like Chevreul. Toward the end of the 19th century Munsell was employed as art instructor at the Massachusetts Normal Art School. Belonging to Boston’s high society, he was widely acquainted with people in the arts and the region’s academic establishments. All this proved helpful when he decided to develop a systematic means of teaching color order to his students. His initial idea was to use a “balanced” form of Runge’s color sphere. When rapidly rotating the sphere, the colors on horizontal planes were to add up to neutral grays of various lightness levels. He already had twirled a multicolored double pyramid in 1878, observing the phenomenon. For this idea he obtained in 1900 his second patent. Munsell realized the importance of objectively defining the color chips he prepared and in the same year invented a visual photometer, the ‘Lumenometer,’ patented in 1901.

The sphere implies three dimensions and after much thinking and discussion Munsell settled on hue, value (lightness), and chroma, the last a concept that his physicist friends had to become used to. Working with “aniline colors,” he realized that different colorants have different maximal chroma levels and as a result the solid of his ordering system would have to be of irregular shape, a shape he came to call “Munsell tree.” Much thought was given to the system’s final design.

As his “Color Diaries” show, Munsell had numerous discussions with many academics on the subject of color order. In 1905 Wilhelm Ostwald visited him and declared his interest in the new approach. A patent for his version of a color chart was applied for and granted in June 1906, with (at the time) a division of the hue circle into seven categories.

Fig. 1 Basic design of the color chart from U. S. Patent 824,374 of 1906.

In 1906 Munsell got to think about the relationship of his psychological order system to a psychophysical one. As usual, he spent the summer in Europe and returned in September on the ‘Arquette’ from Antwerp to Boston. At the Captain’s Table he met several academics, among them “Dr. & Mrs. Franklin” (Christine Ladd-Franklin, famed psychologist, mathematician, and color scientist). They had extended discussions on the ship and during her visit to his office. She introduced him to König’s early version of the chromaticity diagram (Fig. 2) and encouraged him to consider the relationship between that and his own color order system.

Fig. 2 Tracing of Munsell’s sketch of König’s chromaticity diagram, Color Diary, page 232.

In 1907 the first version with eight charts of the ‘Atlas of the Color Solid’ of the Munsell Color System was published, and the second edition in 1915 had grown to 15 charts. In 1918, the year of Munsell’s death, the Munsell Color Company was founded and the rest, as they say, is history. Munsell’s landscapes and portraits are curiosities today; his color order system is a lasting contribution to our understanding of the world of color.

Black to the Future

2008-07-31T15:14:00.003-04:00

by Michael H. Brill, Alan Ingleson, Chuck McLellan
Datacolor

New technology prompts anticipatory thoughts about how we can use it. Now, some adaptations of carbon nanotubes seem ready to be great light absorbers and others should make great detectors. Let’s take a look, then…

Black to the Future

A bit over a year ago, one of us (MHB) discussed parallel development of similar innovations for power engineering and color-measurement technology (“Power to the pupil: color vision, cameras, and the energy crisis,” ISCC News Issue 427). Now another example of the phenomenon is emerging---the interaction of nanotubes with light. Carbon nanotubes are strong enough so that, even when very thin, they can be vertically grown to considerable length on a horizontal surface. Growing carbon nanotubes on a surface produces a region of very low refractive index at that surface, from which very little light is reflected over a wide range of angles. Depending on the dimensions of the nanotubes and on materials with which they might be coated, the light incident on the surface is efficiently absorbed and transduced into either heat or electricity.

Consider the low-reflectance objective [1]. To further darken a black carbon surface, investigators at RPI and Rice roughened the surface by a carpet-like arrangement of carbon nanotubes (.01" long, 1/30,000 as wide) standing on their ends. The result is a surface with a reflectance as low as 0.045 percent (three times darker than any previous material) and a refractive index that could theoretically be as low as 1.01. We’ll hear more about the nanotube absorber at the November ISCC topical meeting on black and white.

The efficient-electricity objective [2] uses carbon nanotubes in a different geometry and context to obtain an efficient solar-cell design. Jud Ready at Georgia Tech Research Institute (GTRI) developed photovoltaic cells that trap light between their tower structures, which are about 100 microns tall, 40 microns by 40 microns square, 10 microns apart-and grown from arrays containing millions of vertically aligned carbon nanotubes. Conventional flat solar cells reflect a significant portion of the light that strikes them, reducing the energy they absorb. Light incident in the new design is turned efficiently into electricity through semiconductor layers (cadmium telluride and cadmium sulfide) deposited on the nanotube “towers.” In a solar cell so designed, the carbon nanotubes serve not only to support the structure in three dimensions, but also to conduct the charge carriers quickly away from the absorption site before they can recombine and waste energy.

For spectrophotometry (a subject near and dear to many ISCC members), one is prompted by both these technologies to envision a spectrophotometer in which one or more of the following is incorporated:

Following [1], using vertically aligned nanotubes that are thousands of times longer than they are wide, one could comprise

1. Black surfaces for minimizing stray light in optical instruments.
2. Light traps for suppressing unwanted diffraction orders.
3. Gloss traps for removing specular reflection.
4. Black calibration standards.

Using nanotubes only a few times longer than they are wide (as suggested by [2]) it may be possible to comprise:

5. An array of nanotubes to guide light to a detector very efficiently. The longer the nanotubes, the more they can guide light in a collimated way and eliminate unwanted diffraction orders. If the nanotubes have different diameters (commensurate with wavelengths of light), they can wavelength-select, perhaps enough to make gratings unnecessary.

The originators of [1] have cross-claimed into the solar-power arena, saying “The observed reflectance from the nanotube arrays is the lowest-ever reported reflectance from any material and could have applications from solar energy conversion to pyroelectric detectors.” For such adaptations, however, there remains the daunting problem of transducing the absorbed light efficiently. For any applications, the long nanotubes also apparently pose health risks similar to those of asbestos once they get in our lungs [3]. The future is once again obscure, then, as perhaps it should be.

[1] Z-P Yang, L. Ci, JA Bur, S-Y Lin, PM Ajayan, Experimental observation of an extremely dark material made by a low-density nanotube array, Nano Letters 8, No. 2 (Feb. 2008), 446-451.
[2] J. Toon, NanoManhattan: 3-D solar cells boost efficiency while reducing size, weight, and complexity, Georgia Tech News, 11 April 2007. See this web link.
[3] C-C Chou, et al., Single-Walled Carbon nanotubes can induce pulmonary injury in mouse model, Nano Letters 8 No. 2 (Feb 2008) pp 437 – 445

Beer's-law dyes and the purple limit

2008-06-30T20:51:00.008-04:00

by Michael H. Brill, Datacolor

Everyone who takes a colorimetry course learns that, when you pile on layers of a light-transmitting material (multiplying the transmission spectrum by itself, in an action called Beer’s law), the transmitted light gets dimmer and more nearly monochromatic, and its chromaticity approaches the spectrum locus. The limiting wavelength on the spectrum locus is the maximum-transmission wavelength of the original (unit-thickness) transmission curve. Starting from that idea, how can we design a Beer's-law-unit-thickness transmission curve for a material such that greater and greater thickness of the material, trans-illuminated by the same light, would approach arbitrarily closely to a given chromaticity point on the line of purples?

Here's how. First, design a U-shaped transmission curve with its two maxima at the ends of the visible spectrum. As you multiply this spectrum by itself (i.e., increase its thickness), the bigger maximum will eventually dominate and the chromaticity will go to the blue or red end of the spectrum---the end with the global maximum of the U. So far we have reached (arbitrarily closely) only two points on the line of purples.

Now design a U-shaped transmission curve with equal maxima at the ends of the spectrum. As you multiply this spectrum by itself, neither maximum ever dominates, so the chromaticity must move to a mid value on the line of purples (whose location depends on the illuminant spectrum at the endpoint wavelengths of the visible spectrum). This is what I call the "Buridan's ass" (or BA) point, named after the donkey invented by Jean Buridan (1300-1358) that starved to death when placed exactly equidistant from two bales of hay. For the transmission curve, the analogue of starvation is balance between the spectrum-locus ends.

Finally, design a U-shaped transmission curve with maxima that are different by only, say, one part in 1010. As you multiply this spectrum by itself, initially neither maximum dominates, so the chromaticity moves toward the BA point. But eventually, that 1 part in 1010 breaks the symmetry, and the transmission rapidly moves, from (very) near the line of purples at the BA point, to one end of the spectrum. A single dye can be made, by self-multiplication, to come arbitrarily close to all the purple-line points on one side of the BA point. Another dye with a similar imbalance the other way will do the same for the other side of the BA point. QED.

Let’s simulate the filter algebra using Gaussians and inverse Gaussians, equal-energy illuminant, and 1931 CIE color-matching functions. Figure 1 shows the unit-thickness transmission spectra, and Fig. 2 shows the chromaticity trajectories of these spectra as thickness is increased. The BA point is given by

X_BA = [ X(λ_e) + X(λ_b)] / [X(λ_e) + X(λ_b) + Y(λ_e) + Y(λ_b) + Z(λ_e) + Z(λ_b)]
Y_BA = [ Y(λ_e) + Y(λ_b)] / [X(λ_e) + X(λ_b) + Y(λ_e) + Y(λ_b) + Z(λ_e) + Z(λ_b)].

Here, λ_b and λ_e are the wavelengths at the beginning and end of the visible spectrum. I chose λ_b = 380 nm and λ_e = 700 nm, so BA is at (0.5458, 0.1776).

Note that the BA point is sensitive to the choice of λ_b and λ_e. If I change λ_e to 720 nm, then BA moves to (0.3615, 0.0920). Amazingly, the chromaticity at λ_e stays the same to five decimal places!

Figure 1.

Figure 2.

After reading this essay, Jack Ladson asked me, “Did Buridan recognize that repeated multiplication of the spectrum ends in black?” I'm sure Buridan, like other philosophers, recognized analogously that all life ends in death, but it matters from which direction you enter that state. In both cases, given enough light, you can see the direction.

Question for discussion: For a real filter, does Beer’s law break down so the chromaticity of a light shone through increasing thickness of the filter fails to approach the spectrum locus?

It’s Not Easy Being Seen

2008-04-15T13:41:00.002-04:00

(April Fool! Color will return to the column next month---I promise.)

by Michael H. Brill, Datacolor

“Plagiosphere” is a term coined by Edward Tenner [1] to denote the fragile, finite volume of our creative phrases that now can be checked for plagiarism by Internet search. Tenner said poignantly, “Copernicus may have deprived us of our centrality in the cosmos, and Darwin of our uniqueness in the biosphere, but at least they left us the illusion of the originality of our words. Soon that, too, will be gone.”

On the anniversary of Margaret Walch’s article highlighting green in the fashion industry and harkening back to Kermit the Frog’s “It ain’t easy bein’ green,” I address this column to the plagiosphere, the bane of high-school term-paper writers and the planet where Kermit’s descendents and catchy titles seem to multiply without bound.

Consider this column’s title (a takeoff on Kermit’s complaint), which I found as the heading of an article in the February 2008 issue of Discover. The referent was a mutant frog (genetic-engineered by scientists in Hiroshima, Japan) with a transparent skin. The article explains that transparent frogs are useful in the lab because you can see their responses to stimuli in real time. There are also transparent frogs in the wild---in tropical rain forests---which don’t survive in more sun-exposed areas because of the vulnerability of internal organs to direct sunlight. Hence “It’s not easy being seen.” [By the way, the eyes of a transparent frog can’t really be transparent or there would be no retinal image.]

Another of Kermit’s descendents appeared recently under the same title, this one a new frog species that “leaped into view in Oklahoma” (Tulsa World, Jan. 12, 2008). This frog seemed really new, not just invisible. It is most remarkable for its mating call, with a sound like a finger run along a metal comb, increasing slowly in pitch and adding to the other Cajun frogs’ calls to make a deafening noise. It may not be so easy being seen, but easier to be heard.

Encouraged by these examples, I wondered how many Google hits would arise in a search of “It’s not easy being seen.” I got 25, including one on prominent economic pundits who bemoan the tendency of their gloomy predictions to become truths. And many on “coming out of the closet.”

One can scarcely coin a phrase anymore. Whole term papers may be matters of coincidence in the stifling plagiosphere. I am worried about creative works becoming matters of coincidence as we near the monkeys-on-typewriters limit. Whenever I think I’ve turned a clever phrase now, I look it up in Google before I take too much pride in it---but I don’t necessarily avoid using it. The context is worth something, and I believe we can become too prudish in our demands for originality. That especially applies to high-school curricula, whose well-worn paths are deep ruts in the plagiosphere that---I should think---would tend to entrain as well as to train students.

So as not to get trapped in the plagiosphere myself, I offer here a premise for an article under this column’s title that may be truly original. The Iranian New Year, celebrated at the Spring Equinox, is highlighted by the custom of gathering seven things whose names begin with the Iranian letter “Seen.” Success at gathering such things is a token of good luck in the coming year. [en.wikipedia.org/wiki/Norouz]. Given that premise, discuss among yourselves: “It’s not easy being Seen.”

[1] E. Tenner, The rise of the plagiosphere, Technology Review, June 2005.

Cows and Other Hazards of Color Science

2008-03-05T08:45:00.003-05:00

By Thor Olson
(Imaging scientist by day, Astrophotographer by night)

Editor's Note:
Thor Olson has captivated audiences at IS&T/SID Color Imaging Conferences with his creative astrophotography. In 1998 he showed stereo images with millions of miles between the “eyes,” in 2002 he showed colors of the deep sky, and last November he explored colorful high dynamic range. Here he describes an encounter with nearer planetary objects.

The Color Imaging Conference is always an inspiration for me, and the 2006 meeting in Scottsdale was especially so. Having just taken Greg Ward's conference tutorial on high dynamic range (HDR) imaging, I was excited about applying it to astrophotography.

A plan was made. Following the conference, I traveled to Monument Valley, a remote corner of Arizona where I could find desert skies to take exposures through my telescope for HDR image stacking.

The weather was clear, cool, and windy, and I found protection on the veranda of the Navaho Tribal Park's visitor center. To gain my night vision I temporarily unscrewed the security floodlights, and then spent a productive evening taking pictures of the night sky. I imaged some of my favorite astrophoto targets at the exposure times I would need to compute an HDR composite. The pleasant challenges of the evening came to a close as I realized how cold I had become. I packed up and started the drive back to my hotel in Kayenta, twenty miles away.

Driving home after a night's observing is always more challenging than the drive out. It is late, or rather, early morning, your blood sugar is at its diurnal lowest, and your usual bedtime was hours ago. If you are like me, you are pumped up from observing the sky on a clear night. Running the defroster at full strength to the windshield, you are in an odd mix of mental and physical states.

The roads are empty, and even though you are not looking directly at them, the blast of your headlights onto the pavement ahead obliterates the night vision you had so carefully cultivated and protected throughout the night. The world that you had so easily navigated with nothing more than starlight and a dim red lamp, now closes in to a narrow tunnel of visibility directly in front, and the best you can do is follow the reflective dotted line down the center of the road.

Signposts advised me to watch for animals and so I proceeded with vigilance, expecting rabbits or maybe coyotes. I made it all the way back to the town and a few blocks from my hotel, when I noticed cows grazing beside the road. No, they were ON the road.

This was odd, since fencing parcels all the grazing land. Where the road interrupts the barbed wire, a cattleguard is used---bars of steel, spaced to make it hard for a cow to cross (its foot slips into the gaps), but allowing tires to roll across. Somehow, these cows had ended up on the wrong side of the fence. And they probably couldn't get back!

I looked to the other side of the road. Cows were milling around there too. I was surrounded, and suddenly I was about to plow into one! What?! Huh? I slammed on the brakes but it was too late.

Just before the collision I felt the world in slow motion. I thought I would suffer the fate of drivers from my part of the country that encounter large moose and elk; the animal is gutted as it crashes through the windshield and its butchered parts are delivered into the driver's lap. Sometimes the driver survives. My mind raced in my time-altered world, but my body couldn't react.

The car smacked into the cow, which skidded up the hood. Before reaching me, however, it stopped, slid back, and flipped onto its other side, flat on the road. Then the poor animal somehow got to its feet and staggered off.

I survived too. My speed was low enough, and the cow soft enough, so even the airbags stayed stowed. With the cows watching carefully (one with tenderized ribs), I drove at snail's pace the last few blocks home.

All this is a lesson in the dangers of field work in color science. It’s like the picture of Dorian Gray: I program the camera for high dynamic range to look at the stars, but my own vision still suffers due to a bright light in the near field. That field had a few bovine visitors, hardly at the limits of human perception. Go figure.

Through a Pinhole Colorfully

2008-02-20T16:35:00.003-05:00

Contributed by Michael H. Brill, Datacolor

Here’s an illusion published by B. F. Skinner [1]. Just stare through a pinhole at a tangent point (where two circles meet) in the pattern below, and the circles will take on various pastel colors. Viewing distance should be about 18 inches. No preconditioning by a "Skinner box" is required. By the way, I can see colors without using a pinhole.

Click the image to download a full sized graphic.

What colors do you see? Can you find a presentation that accentuates the colors? Any explanation for the effect? Please post your thoughts.

[1] B. F. Skinner, A paradoxical color effect. Journal of General Psychology 1932; 7:481-82.

What color is your PET?

2007-12-21T18:25:00.000-05:00

Contributed by Michael H. Brill, Datacolor
Appears in ISCC News, No. 430 (Nov-Dec 2007)

Last September I was an “accompanying person” at a conference on molecular imaging. It might seem close to my area of expertise, but it is not. Accordingly, my color-scientist thinking led to a strange take on concepts in that field. Ostranenie (from the Russian остранение) is the literary device of forcing an audience to see something in an unfamiliar way, to enhance perception of the familiar. An example is to refer to driving a car as sitting on top of repeated gasoline explosions, or to refer to the human brain as electrified meat. For lots of ostranenie, read any book by Kurt Vonnegut---or be an accompanying person.

At the conference, I heard about positron-emission tomography1 (PET). The PET scan is a diagnostic tool whose cuddly name hides the fact, salient at the conference, that the imaging events are positron-electron (matter-antimatter) annihilations that are made to happen inside your body. Now, an electron is a sizeable particle whose complete annihilation (and also that of an injected positron with the same mass) produces a lot of energy. “Is it safe?” I asked, with none of the menace of Szell in Marathon Man. “It’s been known to be safe for three decades,” was the reply, replete with the condescension I’ve come to associate fondly with the medical profession.

Seeking confirmation from my own meager knowledge, I tried to cast this problem in a framework familiar to color science: Find the wavelength associated with the energy of a positron-electron annhilation. Does it convey warmth by the mechanism of a heat lamp, sunburn you, incur the dose-calibrated ionization damage of a dental X-ray, or more generally rock your world? (In the title to this column, I loosely call “PET color” the PET-induced-photon spectrum.)

Everyone I asked at the conference knew that a positron-electron annihilation liberates two photons, each with 511 KeV of energy, in opposite directions (to conserve momentum). But nobody had turned that number into wavelength, so I did a back-of-the-envelope calculation:

PET photon energy 511 KeV converts to energy E = 8.186 x 10^-14 Joules (J), via 1.602 x 10^-19 J/eV,. Planck's constant is h = 6.626 x 10^-34 J-sec, and c = 2.998 x 10⁸ m/sec. Hence the PET photon wavelength is h c/E = 0.002427 nm. At this wavelength, a photon from PET has far more energy than one from a dental X-ray (~0.06 nm) or chest X-Ray (~ 0.03 nm), and in fact is near the shortest wavelength attributed to X-Rays (0.001 nm).

Given that PET positrons produce hard X-rays in our fragile bodies, how can this be safe? Presumably, the total dosage of radiation in a PET scan is low, even though the energy per photon is high. Let’s check the plausibility of this assumption. One application of PET is to see brain metabolism through uptake of a radioactive glucose analogue that emits (you guessed it) positrons. The pseudocolors that are used to encode the metabolism level in the PET image1 are, of course, correlated with radiation emitted from the affected brain cells when the positrons annihilate with local electrons. One would think the radiation dosage to those brilliantly metabolizing brain cells would be quite high, even for relatively low average dosages in the whole brain.

Ultimately, the dose depends on how many photons are needed for a PET scan. Although tomography implies a volume scan and hence a lot of photons needed to light up the right pixels, a PET scan has a spatial resolution of 5-6 mm (much coarser than most other diagnostic images). Relative to diagnostic X-rays, PET scans need fewer photons per pixel to combat shot noise. But a lot of photons are wasted, because the image is captured only in a short cylinder around the affected area. And surprisingly, the photons do less damage to the local tissue than ionization due to the original positron.

As I found out, the computation of radiation dose is serious business in radiochemistry. Empirical studies (mostly on mice) make model fits to vulnerability as a function of such variables as organ type, metabolic rate, uptake rate, and geometry. The complication quickly exceeded the envelope I was writing on.

Back to color science, then. Can we tweak those PET pseudocolors so they’re more informative to doctors? Don’t even try: doctors are used to the present colors. Okay, back to literary devices then, such as ostranenie. That seems safe enough.

1. http://en.wikipedia.org/wiki/Positron_emission_tomography
2. http://www.pbs.org/wnet/brain/scanning/pet.html

Colors and contextual effects

2007-10-10T11:45:00.000-04:00

Osvaldo da Pos, University of Padua, Italy
Appears in ISCC News, No. 429 (Sept-Oct 2007)

There are two basic convictions about colors: (1) they carry information about the objects they belong to; and (2) their appearance depends on the context of the light, the spatial disposition of nearby objects, and their temporal changes. The two features sometimes strongly conflict. Constancy, whereby the color of an object tends to appear unchanged although environmental factors vary, vies with dependence on context. It is not rare for contextual effects to be considered illusory, although their occurrence obeys established rules.

The figures here exemplify how a visual illusion can be analyzed. Why does the central square, always the same, appear different when the surroundings are varied?

Striking changes occur when the lateral squares start in contact with the central one, and then a small misalignment or gap between them produces a completely different appearance. When there is no gap or misalignment, a cross is seen, composed of one strip transparent over the other: Two superimposed colors are seen in the grey square at the same time and in the same direction of sight, one in front and the other behind and through the first.
The two colors seen in the central grey square depend on the colors of the two adjacent squares. Already Helmholtz [1] tried to explain why those specific two colors were seen: it depended on the knowledge of the laws governing additive color mixtures, which, in the case of complementary colors, give an achromatic result. Therefore the colors of the adjacent squares are perceived in the central grey because their fusion precisely gives rise to that particular grey. Hering gave a radically different explanation involving no cognitive activity, but only physiological interactions. The two adjacent colors induce their complements in the grey area, so both the colors are visible in that grey square, although in different parts. Nevertheless those two weak colors can spread inside the square and completely characterize it, as only the sides can limit their spreading. Accordingly the central square appears transparent because in it both the colors of the back strip and of the front one are simultaneously perceivable (this would ultimately be the basic definition of transparency/translucency).

Although cognitive science still follows Helmholtz, the current understanding of the colored transparency (or translucency) effect does not resort to higher level "knowledge," but rather to physiological processes (based on cone excitation ratios [2], physical principles (based on Kubelka-Munk rules [3]; or on spectral filtration [4]), psycho-physical models (based on color convergence [5], [6]), phenomenological models (based on color similarity [7], [8], X junctions [9],[10]).

Even today we do not share a unique explanation of why the central grey square appears so different in different situations, so most people still speak of perceived translucency as an illusion, implicitly assuming that when a good explanation is achieved no illusion will exist anymore. A reasonable objection would be that, even when we reach a convincing explanation, still we would remain amused in seeing that the same grey square appears so different in different contextual conditions; the illusory aspect would remain intact, despite the scientific explanation.

[1] Helmholtz H. von, 1866 Handbuch der Physiologischen Optik, Leipzig: Voss
[2] Ripamonti C., Westland S., da Pos O. 2004 Conditions for perceptual transparency. Journal of Electronic Imaging. 13, pp. 29-35
[3] Brill M. H. 1976 Physical foundation of the perception of achromatic translucency. MIT Research Laboratory of Electronics Progress Reports No. 117 January, pp. 315-320
[4] Beck J. 1978 Additive and subtractive color mixture in color transparency, Perception & Psychophysics, 23, 256 - 267.
[5] Metelli F. 1974, The Perception of Transparency, Scientific American, 230, (16), pp. 90-96
[6] Chen, V. J., & D’Zmura, M. 1998 Test of a convergence model for color transparency perception. Perception, 27, 595-608.
[7] Hering E., Über die Theorie des simultanen Contrastes von Helmholtz. Vierte Mitteilung. Die subjective "Trennung der Lichtes in zwei complementare Portionen" In: Wissenschafliche Abhandlungen; hrsg. von der Sachsischen Akademie der Wissenschaften zu Leipzig, Leipzig : G. Thieme, n. [57], pp. 1- 11)
[8] Da Pos O. 1989-1991 Trasparenze. Transparency. Icone: Milano
[9] Watanabe T., Cavanagh P. 1993 Transparent surfaces defined by implicit X- junctions. Vision Research 33, 2339-2346
[10] Masin S.C. 2006 Test of models of achromatic transparency. Perception 35(12), 1611-1624.