${\displaystyle E=\frac{I\cos \theta }{d^2}}$

where θ is the angle between the normal to the surface and the line between the surface and the source and d is the distance between the source and the surface

Note 1 to entry: The photometric distance law strictly only applies for a point source. However, it can also be applied for non-point sources at sufficiently large distances within acceptable limits of error or measurement uncertainty; in this case the degree of approximation at the distance used should be confirmed by measurement.

Note 2 to entry: For radiometric sources the equivalent term is "radiometric distance law".