Scotopic vision is vision at low illumination dominated by rod cells. It provides high sensitivity to weak light but little of the color discrimination associated with cone cells. Consequently equal radiant powers at different wavelengths need not appear equally bright, and brightness comparisons change with illumination level.
A simple observation compares colored patches as illumination is reduced after adaptation: their relative brightness can change even though the illuminating optical spectrum is held approximately fixed. Controlled measurements must allow adaptation and avoid stray illumination. A photopic luminous efficiency function is appropriate to a different viewing regime and cannot be used indiscriminately for scotopic vision.
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