Electromagnetic radiation can be affected in several ways by the medium in which it propagates.  It can be scattered, absorbed, and reflected and refracted at discontinuities in the medium.  This page is an overview of the last 3. 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). This may be described by the transmission coefficient.

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

where

Hemispheric transmittance may be calculated as an integral over the directional transmittance described below.

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

where

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

where