Color gamuts in dim illumination

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.