The term transmittance is defined as the ratio of transmitted optical power to the incident optical power for some object, for example an optical system. For transmission through flat unstructured surfaces, it is the same as the transmissivity. However, the transmittance is a more general term and can be specified in a wider range of situations:
- Some objects can cause scattering of light. One may then specify the hemispherical transmittance, which is based on the total transmitted radiant flux. Also, there is the directional transmittance, defined as the ratio of transmitted and incident radiance; it is a function of observation angle.
- There are extended objects, where light can penetrate, is internally scattered and thus partially transmitted and partially reflected. The transmittance simply quantifies the amount of light getting through the object, on whatever ways.
- When light is incident on a transparent plate with parallel surfaces, for example, Fresnel reflections occur on both surfaces. The transmitted power can be affected by interference effects, making the transmittance strongly wavelength-dependent.