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rd - set the diffuse reflectance for the current material
Set the diffuse reflectance for the current material to
using the current color to determine the spectral characteristics.
This is the fraction of visible light that is reflected from a
surface equally in all directions according to Lambert's law, and is
often called the "Lambertian component."
Photometric reflectance is measured according to v(lambda)
response function of the 1931 CIE standard 2
degree observer, and assumes an equal-energy white light source.
The value must be between zero and one, and may be further
restricted by the luminosity of the selected color.
(I.e. it is impossible to have a violet material with a photometric
reflectance close to one since the eye is less sensitive in this part
of the spectrum.)
The default diffuse reflectance is zero.
# An off-white paint with 70% reflectance
m flat_white70 =
cxy .3632 .3420