law relating the illuminance, Ev, produced on a surface (which is normal to the direction of the light source and sufficiently distant for the light source to be considered as a point source) by a light source to the luminous intensity, Iv, of the light source in the direction of the surface, to the distance, d, between the surface and the light source, and to the atmospheric transmissivity, T, which is assumed to be uniform
where d0 is the length specified in the definition of T
Note 1 to entry: to entry: The formula in the definition is sometimes written:
where the exponent, δ, in Tδ is the numerical value of the distance, d, measured with d0 as "unit".
Note 2 to entry: Taking into account the relationship between atmospheric transmissivity, T, and meteorological optical range, v, given in Note 2 to entry of the definition of "meteorological optical range", Allard's law can also be written as:
Note 3 to entry: This entry was numbered 845-11-27 in IEC 60050-845:1987.
Note 4 to entry: This entry was numbered 17-34 in CIE S 017:2011.