Transmittance

The term transmittance refers to the ratio between the transmitted and incident radiant power for a specific object. When looking at flat, unstructured surfaces, the term has the same meaning as transmissivity. However, the term transmittance is more general.

Transmittance also takes into account radiation that does not pass through the object directly but through internal scattering or multiple reflections. The hemispherical transmittance considers the total transmitted radiant flux, which is important when objects cause light scattering. Directional transmittance, on the other hand, is defined as the ratio of transmitted to incident radiant power, making it a function of the angle of observation.