A few years ago I created a macro that returns reflective spectral power
distribution (SPD) curves, but I hesitated to publish it due to its poor
correspondence with RGB colors.  But over the past couple of months, I
developed a filter that solves this problem: you submit an RGB spec, and
it figures out what to tell the SPD generator.  For good measure, I also
threw in a true RGB generator.

  https://github.com/CousinRicky/POV-MakeSpectrum

The illustrations show three examples of X11/SVG colors passed through
my library.  For each illustration, the large spheres were pigmented
with SPDs converted to colors using Lightsys IV.  The left SPD was built
from individual red, green, and blue curves, and the right SPD is from
my original SPD generator (albeit tweaked over the years), which was
designed to generate reflective SPDs resembling those bundled with
Lightsys IV and SpectralRender.  The small sphere at the center was
simply pigmented with the straight sRGB color.

The sphere colors are all reduced to 62.5% value, corresponding to a
finish diffuse of 0.625.  The scene has the recommended assumed_gamma 1:
the library handles sRGB and gamma issues.

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Preview of image 'test_makespectrum_x11-pbi003.jpg'

Preview of image 'test_makespectrum_x11-pbi040.jpg'

Preview of image 'test_makespectrum_x11-pbi100.jpg'