Inverse Square Law, Light

As one of the fields which obey the general inverse square law, the light from a point source can be put in the form

where E is called illuminance and I is called pointance.

The source is described by a general "source strength" S because there are many ways to characterize a light source - by power in watts, power in the visible range, power factored by the eye's sensitivity, etc. For any such description of the source, if you have determined the amount of light per unit area reaching 1 meter, then it will be one fourth as much at 2 meters.