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 d0 is the length specified in the definition of T

NOTE 1 The above formula is sometimes written:


where the exponent, δ, in Tδ is the numerical value of the distance, d, measured with d0 as "unit".

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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