visual threshold in point vision

[5.11.2] The weakest illuminance at the eye that makes it possible to see a small source against a given background. The psychophysical definition is the point illuminance that gives 50 percent probability of detection. A widely accepted practical value for reliable vision is 2 x 10-7 lux, or 0.686 nautical-mile-candelas, for a steady light against a dark background.

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