## Transmittance

Transmittance is the fraction of electromagnetic radiation that passes through a medium. Transmitted energy is not lost or attenuated as it passes through a medium, such as the earth's atmosphere. The term extinction refers to the loss or attenuation of radiant energy as it passes through a medium and includes the combined processes of scattering and absorption. Transmittance is inversely proportional to extinction and is also dependent on the type and concentration of gases present in the medium, the path length through the medium, and the wavelenth of the incident radiation. For example, in the visible portion of the electromagnetic spectrum (0.4 to 0.7 µm), the earth's atmosphere is highly transmissive of radiation. Portions of the spectrum that transmit radiant energy effectively are called atmospheric windows.

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