The scotopic sensitivity function describes the response of the human visual system to luminances corresponding to scotopic adaptation. The upper limit for scotopic adaptation has been changing over recent years, with the older value of 0.034 cd/m2 dropping to 0.001 cd/m2. In any case, the level of brightness under full moonlight may be considered an approximation of the upper limit of scotopic adaptation. Scotopic sensitivity applies under very low light conditions, when the only photoreceptors responding are rods, so there is no color perception. The CIE publishes the specific data for the scotopic sensitivity function called "vprime".
The following diagram shows the different visual sensitivity functions: the scotopic as a black line and the photopic sensitivity functions as white and gray lines.
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