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.wik

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