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What is the Second Radiation Constant?

illustration of a pyrometer from an early 20th century textbook

The second radiation constant, c2, is used in Planck's law. It has the SI units of metres kelvin. It is given by the equation:

c2 = hc / k

where h is the Planck constant, c is the speed of light and k is the Boltzmann constant. Its value is 1.438775x10-2 m K.

Planck's law gives the radiation intensity emitted by a black body for a given frequency and temperature. It has the form:

u = c1-5 / (εc2/λT - 1)

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