Optical depth

Aerosol Optical Depth (AOD) at 830 nm measured with an LED sun photometer (1990–2016) at Geronimo Creek Observatory, Texas. Measurements were recorded at or near solar noon. Peaks indicate smoke, dust, and smog events.

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 fraction of transmitted radiant power through the medium. Spectral optical depth or spectral optical thickness is the natural logarithm of the ratio of incident to transmitted spectral radiant power through a material.

Optical depth is a dimensionless quantity. Although it is a monotonically increasing function of path length, it is not a physical distance itself, approaching zero as the path length vanishes. The use of the term "optical density" for optical depth is discouraged.

In chemistry, a closely related quantity called absorbance (or decadic absorbance) is used instead of optical depth. Absorbance uses the common logarithm (base 10) of the ratio of incident to transmitted radiant power. It relates to optical depth by a factor of loge(10).

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