The term "reflection coefficient" (or "reflectance") is widely employed in radiation and combined heat transfer. In most cases, it is the absolute analog of the term "reflectivity" and it is characterized by the ratio of radiation flux reflected by a sample surface to the incident radiation flux. If "reflection coefficient" is used in this sense the term "reflectivity" may be replaced with supplemental terms describing the directions of the incident and reflected radiation.
Sometimes the term "reflection coefficient" is used to characterize not surface reflection, but the reflection of the whole sample in specific conditions. For example, if a given sample is semi-transparent for thermal radiation, then the reflection coefficient depends not only on the properties of irradiated surface, but also on volumetric optical properties (absorption coefficient, refractive index, scattering coefficient and phase function of volumetric scattering), on temperature distribution inside the sample and sometimes (when the transmission of the sample is appreciable) even on the properties of other nonirradiated surfaces. In this case, the use of the term "reflection coefficient" or "reflectance" is possible and even desirable, but there is a need to define precisely the conditions of the reflection.
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