[rosser]

Lasers or light bulbs, photon density (flux) follows the inverse square law.

EDIT: For the follow-up questioning the correctness of my comment:

https://en.wikipedia.org/wiki/Inverse_square_law#Light_and_o... https://en.wikipedia.org/wiki/Radiative_flux

[cjfont]

I believe this is incorrect, anyone can confirm?