Allard's law

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
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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.
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