The photon intensity or flux of photons changes as photons move depeer into the material. The flux is indicated by I(x) which has units of energy/cm2-s.

Writing an equation for the above figure relates the rate of change of photon intensity to the photon absorption coefficient.

Plotting I(x) shows how higher energy photons with large absportion coefficients are absorbed in a shorter distance. Namely, nearly all photons are absorbed within a distance of x = 5/α.