From Glossary of Meteorology
An equation used in predicting the night visual range of self-luminous targets.
Assume a light source at distance x from an observer and having (monochromatic) luminous power Pv in that person's direction. Then for uniform extinction coefficient σ, the illuminance Ev reaching the observer is
When Ev equals the threshold illuminance for a given background luminance and detection probability, x is the night visual range, or maximum distance for detecting the light source.