Optical thickness

Optical thickness, also called optical depth, is the depth of a material or medium in which the intensity of light (or other radiation) of a given frequency is reduced by a factor of $1/e$. In rough terms 1/3 of the light is absorbed within one optical thickness depth.

This describes how quickly light is absorbed within a material. The intensity of light remaining can be calculated by the formula:

:$I\_\{x,lambda\}\; =\; I\_\{0,lambda\}\; e^\{-sigma\_lambda\}\; ,$

where $sigma\_lambda$ is the optical thickness for the material at wavelength $lambda$.

The optical thickness of a material is different for each wavelength of light, and sometimes varies with other factors such as temperature.

Compare to mean free path.

ee also

*Beer's law
*Optical depth