Violations of the Inverse Square Law

The Inverse Square Law is not an innate property of light. The ISL is a property of expanding spheres. When light does not expand spherically (as in a collimated beam) or when the source cannot be approximated by a point, then light seems to 'violate' the Inverse Square Law.

Cases where the ISL does not hold.

Umbrellas and softboxes will follow the law quite closely for distances greater than twice their diameter.

To see the Inverse Square Law in action, see Depth of Light