The term reflectance is defined as the ratio of reflected optical power to the incident optical power at a reflecting object – for example, an optical system. For reflections at flat unstructured surfaces, it is the same as the reflectivity. However, the reflectance is a more general term and can be specified in a wider range of situations:
- Reflections can occur on rough surfaces, where light is scattered. One may then specify the hemispherical reflectance, which is based on the total reflected radiant flux. Also, there is the directional reflectance, defined as the ratio of reflected 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 reflectance simply quantifies the amount of light getting back into the half space of the incoming light.
- When light is incident on a transparent plate with parallel surfaces, for example, Fresnel reflections occur on both surfaces. The reflected power can be affected by interference effects, making the reflectance strongly wavelength-dependent.
The results of reflection measurements with spectrophotometers, for example, are usually called reflectance rather than reflectivity, since the reflections do not need to occur on a single surface.