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

visible spectrum are increasing at different rates. Yet, understanding the effect on
perception of nonuniform changes in the spectral distribution is more complicated
than with uniform changes.
Hue shifts occur when spectral distribution reflectance curves change unequally
with respect to wavelength. The color of a material depends on both the light that
is reflected and light that is absorbed. First, a nonlinear increase in a material’s
reflectance affects the appearance. For instance, increasing a material’s reflectance
between 565 − 590nm will in turn reflect more yellow light back into the environ-
ment. As a result, the viewer will see the material as being yellower.
The second case is slightly less intuitive. For instance, oil paints typically
experience a slight yellowing of the binding material due to oxidation. This equates
to a drop in the lower regions of the spectral reflectance curve. Then, the material
is absorbing more blue light than previously. In other words, the material is
thus reflecting less blue light back toward the viewer. Therefore, the material is
reflecting relatively more yellow light and hence appears to have yellowed.
In both cases, a hue change is dependent on the relative balance between light
reflected and absorbed. A nonuniform change in either can affect the overall per-
ception of a hue.
It is worth mentioning that in paint, what we perceive as hue shifts typically
occur when a nonlinear change occurs away from the central tendency of the ma-
terial’s reflectance. For instance, we typically state that a blue has yellowed, or
a purple shifted toward red, etc. However, when these changes occur near the
central tendency of the reflectance, the perceptual change is not that of hue, but
of chroma.
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Consider the previous case of Lapis Lazuli in gouache. As the paint evolves,