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Rods act as color receptors in dim illumination. Several recent studies have measured the range of colors at low-light
levels in different illuminants. This paper reviews these results and adds new experiments using long-wave-rich
illumination, appropriate for rod and long-wave cone interactions. The experiment illuminates Munsell ColorChecker
papers with 546, and then with 455 nm narrowband lights at radiances below cone thresholds. The third illuminant is 625
nm light, above cone threshold. Observers make asymmetric matches of the ColorChecker using a digital computer
display. The observers make these matches while viewing the entire ColorChecker. Observers report a wide range of
colors from the combination of cone response to 625 nm plus rod response to 546 nm light. The same is true with the
combination of cone response to 625 nm plus rod response to 455 nm light. Although the color matches vary with the
ColorChecker's reflectances, the range of colors is the same. Since these experiments use illuminants more appropriate
for rod-cone interactions, they measure a much greater color gamut than photopic illuminants. They also provide new
data that clarifies how the rod information interacts with the cone-cone color channels. Color appearances indicate rods
share M- and S-color channels.
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