threshold of illuminance
visual threshold, <in point vision>
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
Note 1 to entry: For visual signalling, the light source has to be rendered recognizable, and hence a higher threshold of illuminance is to be expected.
Note 2 to entry: This entry was numbered 845-11-26 in IEC 60050-845:1987.
Note 3 to entry: This entry was numbered 17-1313 in CIE S 017:2011.