Threshold illuminance

From Glossary of Meteorology

threshold illuminance

For a point light source at night, the smallest illuminance Ethresh at the observer that can be seen for a given surrounding luminance and state of dark adaptation.

Although the threshold illuminance is not a constant, a dark-adapted observer can usually see a nonflashing light that produces ∼1.5×10-7 lux at the eye.
See also Allard's law, night visual range.