can reach 10³⁰ K. For the radiation of a helium-neon laser with a power of 1 mW, a frequency spread ?f = 1 GHz, an output aperture of 1 mm, and a beam dispersion half-angle of 0.56 mrad, the brightness temperature would be .

For a black body, Planck's law gives:

where (the Intensity or Brightness) is the amount of energy emitted

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