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What is the Second Radiation Constant?

illustration of a pyrometer

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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