Emissivity

Emissivity is the ratio of radiation emitted by a surface to that emitted by a black body at the same temperature. It varies with wavelength, direction, temperature, and surface condition.

Black body: An idealized object that absorbs all incident radiation and emits the maximum possible thermal radiation at each wavelength and temperature. It provides the reference emission against which emissivity is defined.
Infrared thermography: A technique that maps surface temperatures by measuring infrared radiation with a thermal camera. Incorrect emissivity settings can make camera temperatures substantially inaccurate.
Thermal radiation: Electromagnetic radiation emitted by matter because of its temperature. Emissivity quantifies how strongly a surface emits this radiation relative to an ideal reference.
Total emissivity: Emissivity integrated over all emitted wavelengths at a specified temperature and surface condition. It differs from spectral emissivity because the weighting depends on the full thermal spectrum.
Planck's law: A law giving the spectral radiance of thermal radiation emitted by a black body as a function of wavelength and temperature. Its black-body spectrum supplies the wavelength-specific denominator in spectral emissivity.
Infrared thermometer: A device that estimates an object's temperature from its infrared radiation without physical contact. Its temperature estimate depends on the target's emissivity and reflected surroundings.
Absorptivity: The fraction of incident radiation absorbed by a material, specified for conditions such as wavelength and direction. At thermal equilibrium, absorptivity and emissivity match under corresponding conditions.
Hemispherical emissivity: Emissivity integrated over all outward directions from a surface, for a specified wavelength or spectrum. It contrasts with directional emissivity, which describes emission along a selected direction.
Kirchhoff's law of thermal radiation: At thermal equilibrium, a body's spectral emissivity equals its spectral absorptivity for the same wavelength and direction. It connects emission to absorption, allowing emissivity to be inferred from absorptivity.
Pyrometry: The measurement of temperature from the thermal radiation emitted by an object. Radiation pyrometers require an emissivity assumption or correction to infer temperature.

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