Transmittance

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 Φet is the radiant flux transmitted by that surface; Φei is the radiant flux received by that surface. Spectral hemispherical transmittance in frequency and spectral hemispherical transmittance in wavelength of a surface, denoted Tν and Tλ respectively, are defined as where Φe,νt is the spectral radiant flux in frequency transmitted by that surface; Φe,νi is the spectral radiant flux in frequency received by that surface; Φe,λt is the spectral radiant flux in wavelength transmitted by that surface; Φe,λi is the spectral radiant flux in wavelength received by that surface. Directional transmittance of a surface, denoted TΩ, is defined as where Le,Ωt is the radiance transmitted by that surface; Le,Ωi is the radiance received by that surface. 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 Le,Ω,νt is the spectral radiance in frequency transmitted by that surface; Le,Ω,νi is the spectral radiance received by that surface; Le,Ω,λt is the spectral radiance in wavelength transmitted by that surface; Le,Ω,λi is the spectral radiance in wavelength received by that surface. Beer–Lambert law By definition, internal transmittance is related to optical depth and to absorbance as where τ is the optical depth; A is the absorbance.