Green Technology and Yellow Afterimages

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

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

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

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

(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

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

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.