In order to establish the luminous-efficiency function for mesopic vision of the standard observer, luminous efficiency functions were measured with the direct heterochromatic brightness-matching method at retinal illuminance in the range of −2–2 log photopic trolands in order to cover the scotopic, mesopic, and photopic conditions. A steady visual field of 10° arc was presented foveally. The functions underwent the usual complicated change from a rod type of luminous-efficiency function close to V′(λ) to a cone function that had a wide and almost double-peaked shape. A simple formula to represent the mesopic luminous-efficiency functions was derived in which log sensitivities of scotopic and photopic vision were linearly added after being multiplied by coefficients that were dependent on the luminance level. Saturation functions were also obtained at the various luminance levels for which luminous efficiency was investigated. In spite of a great variation of luminance level, the saturation function remained more or less the same, which indicates that the chromatic channels retain their contributions to the brightness sensation at low retinal illumination.
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