17-1313

threshold of illuminance

smallest illuminance (point brilliance), produced at the eye of an observer by a light source seen in point vision, which renders the source perceptible against a background of given luminance, where the illuminance is considered on a surface element that is normal to the incident rays at the eye

Equivalent term: "visual threshold"

NOTE For visual signalling, the light source must be rendered recognizable, and hence a higher threshold of illuminance is to be expected.

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