A formula relating the illuminance produced on a normal surface at a given distance from a point source of light, the intensity of the light and the degree of transparency of the atmosphere (assumed uniform).
The law may be expressed as follows
where E is the resulting illuminance on this surface
I is the intensity of the incident light in the direction of measurement.
T is the atmospheric transmission factor (assumed uniform) for specified Unit: distance.
x is the distance of the surface from the light source, measured in the specified units of distance.
Note: Another form of Allard's Law is
where V = meteorological visibility (2-1-280 and 2-1-290, Note:).
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM