Henry Hemmendinger Contemplating a Print of M. C. Escher

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

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

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

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 pupillary and focus reactions, resulting in eyestrain. Blue light can also encourage wakefulness, because the eye has a special melanopsin 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

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

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).