Blackbody and Blackbody Radiation
Blackbody (specific terms) – Black body, Full radiator, Planckian radiator, Natural object
Blackbody (alt. terms) – Ideal black surface, Ideal emitter, Ideal thermal radiator
Blackbody radiation – Black-body radiation, Cavity radiation
Blackbody locus – Planckian locus
A blackbody is an ideal thermal radiator that absorbs completely all incident radiation whatever the wavelength, the direction of incidence, or the polarization . This radiator has, for any wavelength and any direction, the maximum spectral concentration of radiance for a thermal radiator in thermal equilibrium at a given temperature.
Blackbody radiation is radiant energy emitted by an ideal black surface (blackbody) whose spectral power distribution is only governed by its own temperature.
Blackbody color is the temperature of a true blackbody emitter which to the human eye is a close match to the color of an incandescent object, which is not a blackbody.
Blackbody locus is the locus of points in a chromaticity diagram that represents chromaticities of the radiation of Planckian radiators at different temperatures.
All bodies at nonzero temperature absorb and emit radiation. If a body is at thermal equilibrium, it is because the amount of radiation that is emitted per second is equal to the amount absorbed. A blackbody (or ideal Planckian or black radiator) is a body that can completely absorb radiation at any wavelength and completely emit radiation at any wavelength so that it emits more radiant power at each wavelength than any other object at the same temperature. A blackbody is not only an ideal emitter but also a diffuse emitter, which means that the emitted radiation is isotropic or independent of direction . It gets its name because of the fact that, at room temperature, it appears visibly black.
For colorimetric applications, the International Commission on Illumination (CIE) has defined a standard illuminant A, which is representative of illumination under incandescent lighting  and whose values of relative spectral power distribution are that of a Planckian radiator at a temperature of approximately 2856 K. It is important to note that while the original CIE standard illuminant A was defined in 1931 in terms of a relative spectral power distribution of a Planckian radiator at a temperature of 2848 K, the Planckian radiation constants, c1 and c2, have changed several times since then (in 1948, 1968, and 1990) with changes in the International Temperature Scale (ITS), causing a corresponding change in the associated temperature. For this reason, the CIE Standard which currently defines CIE standard illuminant A is no longer a function of temperature . However, for an artificial source that is representative of CIE standard illuminant A, the concept of correlated color temperature is used, which is the temperature of the Planckian radiator having a chromaticity along the Planckian locus nearest the chromaticity calculated for the spectral distribution of the test source. There are also many artificial white light sources whose spectral power distribution is very different from a blackbody source, such as fluorescent lamps and sources based on light-emitting diodes. However, the concept of correlated color temperature is still used to describe the colorimetric properties of these types of white light sources.
For astronomical applications, the electromagnetic radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a blackbody that would emit the same total flux of electromagnetic energy. However, stars are observed at a depth inside the atmosphere (the photosphere), so it is more typical to describe a star’s visual surface by its photospheric temperature (see entry on Apparent Magnitude, Astronomy).
There are several other important properties of blackbody sources that are given by the following three laws :
Since the last century, blackbody radiation has been of great interest for use as a primary (calculable) source because the properties are universal and independent of the particular material substance. A good laboratory implementation of a blackbody is a small hole in the wall of an otherwise opaque cavity . The light that enters the cavity through the hole will undergo so many reflections before it can escape from the hole that the light will be effectively absorbed and the hole will appear black. Until 1979, a blackbody like this, heated to the temperature of melting platinum, was the basis of the definition and realization of the SI base unit for photometry, the candela .
The earliest types of artificial light were based on heating an object until it was red hot or hotter and was able to emit visible light. These types of artificial light sources are referred to as incandescent. Examples of incandescent objects are fire, candles, carbon arcs, and tungsten-filament lamps. They emit radiation with approximately the same spectral power distribution as a blackbody but with an intensity reduced by a factor called the emissivity, ε. If the emissivity does not vary with wavelength, the source is called a gray body or nonselective radiator. An incandescent tungsten-filament lamp is close to being a gray body with an emissivity of approximately 0.40 . The temperature of a blackbody that is closest in color to this gray body source is often used as an index of its color. This is referred to as the correlated color temperature, Tc. An alternative index is the distribution temperature, which is the temperature of a blackbody whose relative spectral power distribution is closest to that of the source in question. However, all three terms should be used with caution as they do not give a full picture of the actual spectral distribution . Typical tungsten-filament lamps have correlated color temperatures and distribution temperatures in the range 2500–3000 K.
However, these incandescent lamps emit not only visible light but a significant amount of infrared radiation which makes them inefficient as light sources compared with more modern lighting technologies, such as solid-state lighting which can be designed to emit optical radiation only in the visible range.
There are many examples of everyday materials that behave like blackbodies but emit significant thermal radiation. The human body is one example of a blackbody radiator which emits at roughly 300 K. Another obvious example is the Sun, whose surface is roughly 6000 K. The Earth is also considered to be a blackbody radiator, but it is less than ideal.
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