spectral optical depth (of a medium, for a given length) [*δ*(*λ*)]

quantity used in atmospheric physics and physical oceanography: for a monochromatic component of wavelength, *λ*, of the radiation of a collimated beam propagating along the given length from the point, *x*_{1}, to the point, *x*_{2}, on its path through a homogeneous or non-homogeneous diffusing medium, the spectral optical depth, *δ*(*λ*), of the medium between *x*_{1} and *x*_{2} is defined by the formula:

where *µ*(*x*, *λ*) is the spectral linear attenuation coefficient at the location of d*x*

Unit: 1

Equivalent term: "spectral optical thickness"

NOTE 1 The spectral radiant flux, *Φ*_{e,λ}(*x*_{1}, *λ*), of the beam at the point *x*_{1} is reduced to the value of *Φ*_{e,λ}(*x*_{2}, *λ*), at the point *x*_{2} according to the formula:

so that

.

NOTE 2 For a homogeneous non-diffusing layer, *δ*(*λ*) is the Napierian spectral internal transmittance density.

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