pigmented colorants: dependence on media and time - Cornell ...

This routinely happens when you view an object under different lighting conditions.
For instance, the spectral distribution of indoor tungsten lights is different to that
of natural sunlight. Yet, a sheet of notebook paper in each is still perceived as
white. The tendency of an object’s color to remain unchanged despite changes
in the light spectrum is called color constancy. To achieve color constancy, the
visual system must separate the surface’s reflection properties from the spectral
distribution of the emitted light. One way this is done is the visual system adjusts
its sensitivity to the overall level of illumination, called light adaptation. One
has experienced this by entering a very dark room, such as a movie theater–the
eyes gradually adjust to much lower light levels after a few minutes. Colored
afterimages, which are often experienced after long exposures to the same stimulus,
are attributed as an effect of adaptation. In this case, different regions of the eye
have adapted to different colors, not an overall luminance change.
Yet, adaptation is not a complete explanation for color constancy. Color con-
trast among objects and their surroundings is also important. Since contrast does
not change much when the illumination changes, the visual system exploits the
ratios between colors in a scene. This is evident in the work of Edwin Land in
his Retinex Theory of Color Vision [Lan77]. In Land’s experiment, he created
a painting with a collection of rectangular shapes of varying hues in the manner
of Mondrian, a famous abstract painter. For each of five trials, he illuminated a
different colored shape such that it exhibited an identical energy flux to the other
four colors. Hence, all of the energy reaching the viewer’s eye from the five shapes
is identical. The viewer then matched each shape’s color with the closest Munsell
color chip, illuminated under the same source. Even though the five samples all
sent the same physical color sensation to the eye, the viewer had different color
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