Thursday, December 18, 2008

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.