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Brightness temperature or radiance temperature is a measure of the intensity of electromagnetic energy coming from a source. [1] In particular, it is the temperature at which a black body would have to be in order to duplicate the observed intensity of a grey body object at a frequency ν {\displaystyle \nu } . [ 2 ]
According to Planck's distribution law, the spectral energy density (energy per unit volume per unit frequency) at given temperature is given by: [4] [5] (,) = alternatively, the law can be expressed for the spectral radiance of a body for frequency ν at absolute temperature T given as: [6] [7] [8] (,) = where k B is the Boltzmann ...
Radiance is the integral of the spectral radiance over all frequencies or wavelengths. For radiation emitted by the surface of an ideal black body at a given temperature, spectral radiance is governed by Planck's law, while the integral of its radiance, over the hemisphere into which its surface radiates, is given by the Stefan–Boltzmann law.
Comparison of Rayleigh–Jeans law with Wien approximation and Planck's law, for a body of 5800 K temperature.. In physics, the Rayleigh–Jeans law is an approximation to the spectral radiance of electromagnetic radiation as a function of wavelength from a black body at a given temperature through classical arguments.
However, because black-body radiation increases rapidly with temperature (as the fourth power of temperature, given by the Stefan–Boltzmann law), radiation pressure due to the temperature of a very hot object (or due to incoming black-body radiation from similarly hot surroundings) can become significant. This is important in stellar interiors.
When the body is black, the absorption is obvious: the amount of light absorbed is all the light that hits the surface. For a black body much bigger than the wavelength, the light energy absorbed at any wavelength λ per unit time is strictly proportional to the blackbody curve. This means that the blackbody curve is the amount of light energy ...