Graphics Reference
In-Depth Information
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Figure 28.21: Two metameric light spectra (top) are each multiplied (wavelength by
wavelength) by the reflectance spectrum (middle). The resultant spectra are no longer
metameric. (Next to the spectrum for each light are its corresponding RGB response
values.)
(at least for not-too-bright lights, where saturation comes into play, and not-too-
dim lights, where photopic/scotopic differences enter); this means that it's nec-
essarily many-to-one. Different spectra that generate the same response values
are called
metamers;
metameric lights are interesting because, upon reflection by
a surface, they can become nonmetamers (see Figure 28.21). In practice, most
reflectance functions are nonspiky enough that metameric effects like this are
not significant, although with LED lamps, which tend to have spikier spectra, the
problem may be more serious.
The colors in the
x
+
y
+
z
=
1 plane of the CIE XYZ space are
not
all possible
colors. As the sum
x
+
y
+
z
varies, other colors appear (such as maroon). Fur-
thermore, colors like brown, which are generally used to describe reflective color
rather than emissive color, tend not to appear at all.
The colors purple and violet are often considered to be synonymous, but violet
is the name for a pure spectral color (at about 380 nm, just on the edge of percep-
tibility), while as we said, purple is the name for points on or near the straight
bottom edge of the CIE horseshoe.
Let's pause and note the important points so far. First, color is a three-dimensional
perceptual phenomenon evoked by the arrival of different spectral power distribu-
tions at the eye. Any color percept can be generated by a combination of the CIE
