Editing Optical depth (section)

{{Short description|Physics concept}}
{{Other uses|Optical depth (astrophysics)}}

[[File:Aerosol Optical Depth (haze) at Geronimo Creek Observatory, Texas (1990-2016).jpg|thumb|Aerosol Optical Depth (AOD) at 830 nm measured with the same LED sun photometer from 1990 to 2016 at Geronimo Creek Observatory, Texas. Measurements made at or near solar noon when the Sun is not obstructed by clouds. Peaks indicate smoke, dust and smog. Saharan dust events are measured each summer.]]

In [[physics]], '''optical depth''' or '''optical thickness''' is the [[natural logarithm]] of the ratio of incident to ''transmitted'' [[radiant power]] through a material.
Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through the material. 
'''Spectral optical depth''' or '''spectral optical thickness''' is the natural logarithm of the ratio of incident to transmitted [[Radiant power|spectral radiant power]] through a material.<ref name=GoldBook>{{GoldBookRef|title=Absorbance|file=A00028|accessdate=2015-03-15}}</ref> Optical depth is [[Dimensionless quantity|dimensionless]], and in particular is not a length, though it is a [[Monotonic function | monotonically]] increasing function of [[optical path length]], and approaches zero as the path length approaches zero. The use of the term "optical density" for optical depth is discouraged.<ref name=GoldBook />

In [[chemistry]], a closely related quantity called "[[absorbance]]" or "decadic absorbance" is used instead of optical depth: the [[common logarithm]] of the ratio of incident to transmitted radiant power through a material. It is the optical depth divided by {{math| ''log''<sub>''e''</sub>(10)}}, because of the different logarithm bases used.