Colors in Dim Illumination and Candlelight

A variety of papers have studied color 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 experimental results agree with and extend previous results. Since these experiments use illuminates more appropriate for rod-cone interactions they measure a much greater range o f colors. They also provide new data that clarifies how the rod information interacts with the cone-cone color channels. Color appearances indicate rods share Mand S-color channels. Introduction A recent paper by Shin et al. reported the color observed in Photopic, Mesopic, and Scotopic conditions [1], extending earlier work. [2] These 2004 experiments used D65 fluorescent lamps illuminating 45 colored and 3 achromatic square papers (JIS equivalent to Munsell) subtending 10°. They matched these color appearances with a color CRT screen. They matched each paper individually in a middle-gray N/5 viewing booth environment. They repeated the experiment in six different illuminances (1000, 100, 10, 1, 0.1, 0.01 lux). The color matches at 1000 lux included many colorful objects. The matches at 0.01 lux cluster near gray, covering only the s small range in L*a*b* units [a*=3 to 6, b*= -7 to 1]. The intermediate illumination show a systematic change from colorful to achromatic consistent with Max Schultze’s Duplicity Theory [3] This theory describes two visual mechanisms: a colorful Photopic and an achromatic Scotopic system. Intermediate illuminants are thought of as generating additive mixtures of colorful and colorless images from Scotopic vision. Pokorny et al. [4] used color-naming experiments to describe color appearances in dim light. They studied 24 OSA Uniform Color Scale chips in 5000°K fluorescent illumination. Their experiments covered the illumination range of 10 to 0.0003 lux. They viewed the 24 square samples (8° to 10° visual angle) in a black matte surround on a table. They reported a general loss of colorfulness, yet reported seeing color generated by rod and L-cone interactions. Over the years many authors have reported colors from rod and L-cones (See Stabell and Stabell [5], Buck, [6] and McCann, Benton, and McKee [7] for reviews). In 1969, McCann and Benton used narrow-band illumination on a Mondrian display of ColorAid papers [8]. After, total dark adaptation they asked observers to increase the amount of 546 nm light until they saw a variety forms and shapes. They reported a range of lighter and darker achromatic areas, one log unit above absolute rod threshold (measured by dark adaptation threshold vs. time). Then observers adjusted 656-nm light alone until they saw forms. At .7 log unit above L-cone threshold they saw light and dark areas in a uniform red wash. No variegated color was seen. When these 546and 656-nm lights were combined, observers reported a wide range of colors. The 546-nm light was nearly 2 log units below M-cone threshold, showing that these colors were from rod and L-cone interactions. Additionally, observers showed they needed considerably more 656-nm light than 546nm light for these color interactions. McCann and Benton also used dual-image monochromators to illuminate black and white film separations transparencies of a complex image. They changed the monochromator wavelength from 400 to 600nm illuminating a black and white (Wratten 58) green record of the scene. At high luminance levels the image had no variegated color, but the hue of the color wash changed from violet, blue, green, yellow, to red with changing wavelengths of illumination. Repeating the experiment, after dark adaptation, just above absolute threshold, the color wash was gone for all wavelengths below 600 nm. Observers reported that that the achromatic images were brightest at 500 nm and decreased with longer and shorter wavelengths. When experimenters added a black and white (Wratten 24) red record in 656-nm light to the middle-wave record observers reported a variety of different colors. Observers were asked to change the wavelength illuminating the W58 record while adjusting the radiance for best color. They reported the colors in the scene were constant. McCann and Benton asked observers to match the colors seen in rod-Lcone interactions (in the left eye) to cone-cone colors of the same scene at high radiance in a second image monochromator (right eye). They reported that rod-Lcone colors are best matched with 656 nm and 495 nm light. They suggested that the rod information was shared with both Mand S-color channels. [8] The combination of these recent and older experiments still leave a number of important questions unanswered. There are conflicting claims and interpretations. Shin’s D65 matches support the traditional additive mixture of colorful cone and achromatic rod images. Pokorny’s 5000° K color naming experiments report color names even in rod only conditions. McCann and colleagues report much more colorful images using illuminants with 100 to 1000 times more 656nm light than 500nm light. McCann measured the exitance of wood-fire to be equivalent to 1700°K and candlelight to be 2000°K. [9] These spectral emissions are well suited to generating supra-threshold response for both rod and L-cones. Figure 1 plots the relative rod and cone thresholds (amount of light at threshold vs. wavelength). Figure 1. Plots of the amount of light necessary for a threshold response to light as a function of wavelength for rods and L-, M-, S-cones. The data is the Photopic sensitivity function and cone fundamentals from Stockman