Transmittance

The transmittance of a material and any surfaces is its effectiveness in transmitting radiant energy; the fraction of the initial (incident) radiation which propagates to a location of interest (often an observation location).

Spectral hemispherical transmittance in frequency and spectral hemispherical transmittance in wavelength of a surface, denoted Tν and Tλ respectively, are defined as[2] where Directional transmittance of a surface, denoted TΩ, is defined as[2] 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[2] where In the field of photometry (optics), the luminous transmittance of a filter is a measure of the amount of luminous flux or intensity transmitted by an optical filter.

The luminous transmittance with respect to the standard illuminant is defined as: where: The luminous transmittance is independent of the magnitude of the flux or intensity of the standard illuminant used to measure it, and is a dimensionless quantity.

By definition, internal 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, 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[3] Cases of non-uniform attenuation occur in atmospheric science applications and radiation shielding theory for instance.

Earth's atmospheric transmittance over 1 nautical mile sea level path (infrared region [ 1 ] ). Because of the natural radiation of the hot atmosphere, the intensity of radiation is different from the transmitted part.
Transmittance of ruby in optical and near-IR spectra. Note the two broad blue and green absorption bands and one narrow absorption band on the wavelength of 694 nm, which is the wavelength of the ruby laser .