Also sometimes referred to as square law.
(Term of photometry/radiometry)
The measured luminous flux density (or radiant flux density) from a point light source decreases along any line from the source. It falls in inverse proportion to the square of the relative distance. An illuminance measurement 2 meters from the light source will be 1/4 of the measurement 1 meter from the source.
The law holds because the energy from the source radiates in all directions, in effect spreading itself over the surface of a sphere. At a greater distance, the same amount of energy is spread over a larger surface, and thus any one area of the surface will receive less. The increase in surface area is proportional to the square of the radius.
This model of propagation is based on a point source, radiating equally in all directions. Real light sources rarely behave exactly like point sources in this respect, especially for very short distances relative to the size of the source. With larger distances, however, the inverse square law gives a sufficiently accurate approximation for most practical purposes, at least within the bounds of a specified solid angle.
References: 
luminous flux density photometry radiant flux density radiometry solid angle 

 
English  German  
inverse square law  Das Gesetz des inversen Quadrates 