**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.^{[2]}

**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^{[3]}

where

- Φ
_{e}^{t}is the radiant flux*transmitted*by that surface; - Φ
_{e}^{i}is the radiant flux received by that surface.

**Spectral hemispherical transmittance in frequency** and **spectral hemispherical transmittance in wavelength** of a surface, denoted *T*_{ν} and *T*_{λ} respectively, are defined as^{[3]}

- scattering, reflection, etc.
**Hemispherical transmittance**of a surface, denoted*T*, is defined as^{[3]}where

- Φ
_{e}^{t}is the radiant flux*transmitted*by that surface; - Φ
_{e}^{i}is the radiant flux received by that surface.

### Spectral hemispherical transmittance

**Spectral hemispherical transmittance in frequency**and**spectral hemispherical transmittance in wavelength**of a surface, denoted*T*_{ν}and*T*_{λ}respectively, are defined as^{[3]}