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

Note that the sample can be a mixture of pigments, in which case Duncan’s K-M
linear <strong>pigmented</strong> mixing from Equation A.16 would be substituted with respective
weighting.
In order to analyze the data in terms of hue shift in a color-matching program,
Johnston suggests calibration with at least a three color pigment basis plus white,
and the use of Duncan’s <strong>pigmented</strong> mixing equation. One selects the hues of the
pigments to ensure coverage over the entire gamut of colors (typically this selection
mimics the opponent color channels, choosing white, black, red or green, and yellow
or blue). In this way, widely varying temporal changes (such as fading, darkening,
and hue changes) can all be detected and recorded effectively [JFFBC84]. By
means of Munsell notation and color difference calculations, the work found that
orderly changes in concentration related to a nonlinear change in the perceived
color of a paint.
Whitmore and Bailie presented a model predicting and verifying experimen-
tally the loss from fading from two perspectives: in terms of colorant loss from
photochemical reaction and the resulting perceptual color changes [WB97]. The
work determined that colorant loss takes place at three distinct stages. The dark-
est (or most concentrated) glazes lose colorant at the maximum (linear) rate, yet
the color change is slight because there is little spectral change. Usually, ‘fading’
for these glazes includes a shift in hue, followed by an increase in chroma. This is
due to reflectance changes away from the absorption peak.
As colorant is further lost, the absorbed wavelengths are less highly absorbent,
hence the colorant loss slows. However, the magnitude of the perceptual color
change increases (this is what is known as typical fading, with the chroma decreas-
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ing and value increasing). Glazes having an intermediate reflectance (20 − 80%