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precisely to the monospectral sources as “pure.” Our understanding of the cones
can tell us why.
The sensitivities of the three cones to various wavelengths of light mean that
a monospectral light arriving at the eye generates a signal to the brain consisting
of the outputs of the three kinds of sensors, which can be read off the response
chart. In Figure 28.11 you can see how light of 440 nm generates lots of blue-
cone (i.e., short-wavelength) response, less green (i.e., medium-wavelength), and
even less red (i.e., long-wavelength). Similarly, at 570 nm, both red and green
cones produce large responses, while the blue cones generate almost none. One
can make a three-dimensional coordinate system labeled with S, M, and L, and
plot the curve defined by such responses (see Figure 28.10).
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Light that is a mix of these monospectral lights will (approximately, and within
certain bounds) provoke a response that is a linear combination (with positive
coefficients) of the responses to the monospectral lights, that is, the set of all
responses will form a
generalized cone
in this space of possible cone responses
(see Figure 28.12). The responses to monospectral lights are the points on the
boundary of this cone, as predicted, in the sense that each of them cannot be pro-
duced as a combination of other responses. There's one exception: The start and
end of the monospectral response curve are points representing pure red and pure
violet. Combinations of these form a line; the collection of rays from the origin
through this line is a planar region constituting a part of the cone's boundary.
Points on this part of the boundary
are
representable as combinations of other
response points; they are the “purples,” and are not “pure” colors. (Note that the
geometry of this response cone—the mostly convex shape of its cross section, in
particular—is a consequence of the shapes of the response curves for the three
types of cones in the eye. In the exercises in this chapter, you'll study what the
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Figure 28.10: The response
curve for monospectral visible
light. Note that the short-
wavelength-response axis has
a different scale. We show the
curve from two different views.
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L
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Figure 28.11: The response of the three cone types to a monospectral light can be read
from the plot of their sensitivities. Light at 450 nm, for instance, generates about equal
short- and medium-wavelength responses (shown in blue and green and labeled “S” and
“M”), but a slightly smaller long-wavelength response (shown in red and labeled “L”).
Light at 640 nm generates a large red response, a small green response, and almost no
blue response.
