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

of perceptually equal sizes. Unfortunately, this is not the case in XY Z space.
Figure 3.25 shows the nonlinearity of the Chromaticity diagram. In psy-
chophysics, the smallest difference in a specified modality of sensory input that
is detectable by a human being is known as a just noticeable difference (jnd). The
average user’s jnd’s are used to compare sets of colors in an attempt to distinguish
the threshold of visual color differences. The region on a chromaticity diagram
which contains all colors which are indistinguishable, to the average human eye,
from the color at the center of the ellipse is known as a MacAdam ellipse.
Figure 3.25: The MacAdam ellipses. Adapted from [G95a].
The important observation is that the MacAdam ellipses are not the same size
nor in the same orientation. Thus, a particular magnitude of shift in color space
may be undetectable at one point, but the same shift applied to a different color
would be quite visible. For example, our acuity in distinguishing between nearby
green colors is much poorer than in nearby reds, and poorer still than in the blue
rangeoftheXY Z chromaticity diagram.
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