Wednesday, December 16, 2009

Connecting the Dots between Color Vision Experiments

John J. McCann

Thomas Young, James Clerk Maxwell and Herman von Helmholtz describe human color as the trichromatic response to the quanta catch of three retinal receptors, at a pixel.  In the 204 years since Young’s trichromatic suggestion, there have been many hundreds, if not thousands, of vision experiments that demonstrate that two patches of retina stimulated by identical spectral radiances do not generate identical appearances.  The usual description of these experiments is they are illusions


There is an alternative way to think about color vision, as succinctly put by Leonardo da Vinci: “Colors are most beautiful in the presence of their opposites”.  In this alternative construct, color illusions are simply color appearances synthesized from spatial comparisons.  For example, color constancy experiments show that objects have nearly constant appearance, despite significant changes in the spectral illumination.  Color appearance correlates with the spatial computation the object’s reflectance, using cone sensitivity functions.

This talk will discuss a wide variety of dramatic color experiments, including: constancy, adaptation, assimilation, Maximov’s shoe box, Land’s red and white photography, color-gamut mapping and color from rod/Lcone interactions.  All these experiments make a convincing argument that human vision differs from film and electronic sensors.  Vision works by making spatial comparisons.  We will discuss whether these experiments are in fact illusions, or the signature of vision’s underlying mechanisms.  As well, we will discuss whether many small modifications of trichromatic theory can account for illusions, or if this possibility is a delusion.


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