Allard's law

law relating the illuminance, *E*, produced on a surface by a light source to the luminous intensity, *I*, of the source in the direction of the surface, to the distance, *d*, between the surface and the source, and to the atmospheric transmissivity, *T*, which is assumed to be uniform; the surface is normal to the direction of the source and sufficiently distant for the source to be considered as a point source

where *d*_{0} is the length specified in the definition of *T*

NOTE 1 The above formula is sometimes written:

where the exponent,* δ*, in *T** ^{δ}* is the numerical value of the distance,

NOTE 2 Taking into account the relationship given in "meteorological optical range" between *T* and the meteorological optical range, *v*, this law can also be written as:

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