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Transmitted light


Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is the ratio of the transmitted to incident electric field.

Internal transmittance refers to energy loss by absorption, whereas (total) transmittance is that due to absorption, scattering, reflection, etc.

Hemispherical transmittance of a surface, denoted T, is defined as

where

Spectral hemispherical transmittance in frequency and spectral hemispherical transmittance in wavelength of a surface, denoted Tν and Tλ respectively, are defined as

where

Directional transmittance of a surface, denoted TΩ, is defined as

where

Spectral directional transmittance in frequency and spectral directional transmittance in wavelength of a surface, denoted Tν,Ω and Tλ,Ω respectively, are defined as

where

By definition, transmittance is related to optical depth and to absorbance as

where

The Beer–Lambert law states that, for N attenuating species in the material sample,

or equivalently that

where

Attenuation cross section and molar attenuation coefficient are related by

and number density and amount concentration by

where NA is the Avogadro constant.

In case of uniform attenuation, these relations become

or equivalently

Cases of non-uniform attenuation occur in atmospheric science applications and radiation shielding theory for instance.


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