# Allard's Law

**2-1-265**

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 the 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)*