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Blackbody ○꠹｜Definition｜1st｜20251119205401-00-⌔
Black body
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is called black-body radiation. The name “black body” is given because it absorbs all colors of light. In contrast, a white body is one with a “rough surface that reflects all incident rays completely and uniformly in all directions.”1
A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation is emitted according to Planck’s law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body’s shape or composition.
An ideal black body in thermal equilibrium has two main properties:2
- It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.
- It is a diffuse emitter: measured per unit area perpendicular to the direction, the energy is radiated isotropically, independent of direction.
Real materials emit energy at a fraction—called the emissivity —of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity ε = 1. A source with a lower emissivity, independent of frequency, is often referred to as a gray body.34 Constructing black bodies with an emissivity as close to 1 as possible remains a topic of current interest.5
In astronomy, the radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a black body that would emit the same total flux of electromagnetic energy.
Printed 2026-06-28.
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Link to original Footnotes
Planck 1914, pp. 9–10 ↩
Mahmoud Massoud (2005). “§2.1 Blackbody radiation”. Engineering thermofluids: thermodynamics, fluid mechanics, and heat transfer. Springer. p. 568. ISBN 978-3-540-22292-7. ↩
The emissivity of a surface in principle depends upon frequency, angle of view, and temperature. However, by definition, the radiation from a gray body is simply proportional to that of a black body at the same temperature, so its emissivity does not depend upon frequency (or, equivalently, wavelength). See Massoud Kaviany (2002). “Figure 4.3(b): Behaviors of a gray (no wavelength dependence), diffuse (no directional dependence) and opaque (no transmission) surface”. Principles of heat transfer. Wiley-IEEE. p. 381. ISBN 978-0-471-43463-4. and Ronald G. Driggers (2003). Encyclopedia of optical engineering, Volume 3. CRC Press. p. 2303. ISBN 978-0-8247-4252-2. ↩
Some authors describe sources of infrared radiation with emissivity greater than approximately 0.99 as a black body. See “What is a Blackbody and Infrared Radiation?”. Education/Reference tab. Electro Optical Industries, Inc. 2008. Archived from the original on 7 March 2016. Retrieved 10 June 2019. ↩
Chun, Ai Lin (2008). “Blacker than black”. Nature Nanotechnology. doi:10.1038/nnano.2008.29. ↩
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