The duplicity theory states that cones produce photopic or color vision, whereas the rods produce scotopic or colorless night vision. This paper reports experimental findings which demonstrate the capacity of the rods to interact with the long-wave cones to produce color sensations. Radiances of 546 and 450 nm that excited only the rods, and radiances of 656 nm that excited only the long-wave cones were determined. When the rods and long-wave cones were selectively excited with the minimum radiance necessary to see form, the observers reported seeing a large variety of color sensations. These observers also reported the same variety of color sensations at greater radiances when the rods and long-wave cones were selectively excited. Color sensations produced by the excitation of rods and long-wave cones were independent of the wavelength used to excite the rods. Color sensations produced by rods and long-wave cones were identical, except for slight differences of brightness and sharpness, to the color sensations produced by 656 and 495±5 nm light when both were above cone threshold. Therefore, under the described conditions, the rods can be as much a part of the human color-producing system as the cones. All of the above results can be explained by Land’s retinex theory of color vision.
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