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FUNDAMENTALS OF THERMAL RADIATION, image image, image image, 1)…
FUNDAMENTALS OF THERMAL RADIATION
BLACKBODY RADIATION
A body at a thermodynamic (or absolute) temperature above zero emits radiation in all directions over a wide range of wavelengths.
A blackbody:
The maximum amount of radiation that can be emitted by a surface at a given temperature
It absorbs all incident radiation, regardless of wavelength and direction
Emits radiation energy uniformly in all directions per unit are normal to direction of emission.
Stefan-Boltzmann Law
Wien's displacement law
RADIATIVE PROPERTIES
In these materials thermal radiation is emitted or absorbed within the first few microns of the surface.
Some materials like glass and water exhibit different behavior at different wavelengths
EMISSIVITY
Emissivity of a surface
─ the ratio of the radiation emitted by the surface at a given temperature to the radiation emitted by a blackbody at the same temperature.
The emissivity of real surfaces varies with:
Spectral directional emissivity
─ the most elemental emissivity of a surface at a given temperature.
Gray and diffuses surfaces
Diffuse surface
A surface which properties are independent of direction.
Gray surface
- surface properties are independent of wavelength.
1) visible spectrum- semitransparent
2) infrared spectrum- opaque
the temperature of the surface
the wavelength
the direction of the emitted radiation
