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The brightness temperature (TB) is one means of characterizing the brightness of an electromagnetic radiation source at some specific wavelength or band. It is the temperature of a black body that would produce an observed spectral radiance (specific intensity) at the given wavelength. For a given wavelength, the spectral radiance of black-body radiation increases with temperature, which is true even given the shifting peak of the black-body spectrum: higher temperature produces more EMR at each wavelength. Equipment to determine brightness is always sensitive to some range of wavelengths and a quoted brightness temperature is effectively based upon a (possibly very narrow) band. Brightness temperature is commonly used within radio astronomy where it can be usefully computed using the Rayleigh-Jeans law rather than the more complex Planck function. Determining the spectral radiance requires an extended source.
If the EMR is not thermal radiation, the brightness temperature is not a reliable temperature determination, but is still often quoted, only serving as a means of describing and comparing sources, quantifying brightness at some specific wavelength. It is often cited for a prominent emission line such as that of a maser's wavelength. Such brightness temperatures for non-thermal radiation can be extremely high, much higher than the source's actual temperature. In such cases, it serves to show how bright the spectral feature is compared to basic thermal radiation.