Brightness temperature

Brightness temperature is the temperature a blackbody would need to produce the same spectral radiance as an observed source at a specified frequency or wavelength. It expresses radiance as an equivalent temperature, not necessarily the source’s physical temperature.

Planck's law: Planck’s law gives the spectral radiance emitted by a blackbody as a function of temperature and frequency or wavelength. Its inverse yields the blackbody temperature matching an observed radiance.
Radio astronomy: Radio astronomy studies celestial objects by detecting radio-frequency electromagnetic radiation. Radio observations commonly report intensity as an equivalent brightness temperature.
Blackbody: A blackbody is an idealized object that absorbs all incident radiation and emits a spectrum determined solely by its temperature. Brightness temperature is defined by comparison with this ideal emitter.
Color temperature: Color temperature is the blackbody temperature whose spectral shape or color most closely matches a source’s emitted light. It characterizes spectral shape, while brightness temperature matches radiance at a specified spectral point.
Spectral radiance: Spectral radiance is radiant power emitted or passing through a surface per unit area, solid angle, and frequency or wavelength interval. Brightness temperature is calculated from this measured quantity.
Cosmic microwave background: The cosmic microwave background is relic electromagnetic radiation from the early universe, now observed as an almost uniform microwave sky. Its measured spectrum is closely described by a blackbody brightness temperature near 2.7 kelvin.
Thermodynamic temperature: Thermodynamic temperature is a state variable that characterizes thermal equilibrium and determines the direction of heat transfer. It is a physical temperature, unlike an equivalent temperature inferred from radiance.
Kinetic temperature: Kinetic temperature measures the average translational kinetic energy of particles in a material. It describes particle motion rather than radiance at a selected frequency.
Rayleigh–Jeans law: The Rayleigh–Jeans law approximates blackbody spectral radiance at frequencies low enough that photon energy is small relative to thermal energy. In this regime, radiance is proportional to brightness temperature.
Antenna temperature: Antenna temperature expresses received radio power as the temperature of a matched thermal source that would deliver equivalent power. It is an instrument-related temperature measure that can be calibrated against source brightness temperature.

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