When light or other radiation passes through a partially transparent medium, its intensity is diminished by scattering and/or absorption. The effect of the medium can be characterized by an attenuation coefficient α, defined as the fraction of intensity lost per unit distance traveled. The integration of this attenuation over the total path through the medium is called the optical depth:
where the attenuation in this expression is allowed to vary over the path (0, L). The intensity decreases with increasing optical depth; a medium with unit optical depth (Ï„ = 1) reduces the intensity of the original signal by a factor e (to about 37% of its original strength). A completely opaque medium would have infinite optical depth.
Optical depth is commonly used in describing aerosol effects on atmospheric clarity; high concentrations of aerosols increase the optical depth and reduce visibility. Optical depth has also been used to describe the transmission properties of planetary rings — higher...
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Simpson, R.A. (1997). Optical depth . In: Encyclopedia of Planetary Science. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4520-4_294
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