| law relating the illuminance E produced on a surface by a light source to the luminous intensity I of the source in the direction to 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
X
where d0 is the length specified in the definition of T
NOTE 1 – The above formula is sometimes written
X
where the exponent d in Td is the numerical value of the distance d measured with d0 as "unit".
NOTE 2 – Taking into account the relationship given in 845-11-20 between T and the meteorological optical range ν, this law can also be written as
X
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