Optical thickness
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Optical thickness 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 ⅔ 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:
where σλ is the optical thickness for the material at wavelength λ.
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.