Radiation Terms
- Blackbody
- A hypothetical entity which absorbs all energy, reflects none and emits energy with
perfect efficiency. A blackbody is assumed to satisfy the following ideal
conditions;
- A blackbody absorbs all incident radiation regardless of wavelength and direction.
- For a prescribed temperature and wavelength, no surface can emit more energy than a
blackbody.
- Although the radiation emitted by a blackbody is a function of wavelength, it is
independent of direction. A blackbody is defined as a diffuse emitter.
Since a true blackbody is only a theoretical device, practical devices must have
smaller emissive powers. It is therefore necessary to identify a property called
emissivity that is defined as;
- Emissivity
- The ratio of emittance of a given object and a theoretical blackbody at the same
temperature. The emissivity of a theoretical blackbody is defined as 1.0
and that of a perfect reflector as 0.
(Thus emissivity = 1.0
- reflectivity.)
- In practice emissivity is a function of both wave length and temperature. However, for
most practical applications a single average value (which covers a selected wave band) can
usually be used.
- Radiant Flux
- Rate at which photons strike a surface measured in watts (amount of energy delivered per
unit of time).
- Irradiance
- Radiant flux per unit area (watts per square meter) incoming.
- Radiant Exitance
- The rate at which radiation is emitted from a unit area (watts per square meter).
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