The first radiation constant, c1, is used in the Planck's law. It has the SI units of watts square metres. It is given by the equation: c1= 2π.hc2 where h is the Planck constant and c is the speed of light. Its value is 3.741771x10-16Wm2. Planck's law gives the radiation intensity emitted by a black body for a given frequency and temperature.

Fundamental physical constants characterizing black body radiation. The first radiation constant is c1 = 2π.h.c02 = 3.7417749(22) x 10E-16 W m2, the second is c2 = h.c0/k = 1.438769(12) x 10E-2 m K, where h is the Planck constant c0 the speed of light and k the Boltzmann constant.

The first radiation constant is also seen defined as: c1 = 8πhc c 1 = 8 π h c where: h h denotes the Planck's constant c c denotes the speed of light. This can be obtained from the conventional definition of first radiation constant by multiplying it by 4π c 4 π c.

c1 = 1.19104·108 W μm4 m-2 sr-1 c2 = 1.43877·104 μm K, are the first radiation constant (for spectral radiance) and second radiation constant, respectively, B (λ, Ts) is given in W m-2 sr-1 μm-1 if λ the wavelength is given in μm.

(statistical mechanics) A constant appearing in the Planck radiation formula; its value depends on the form of the formula used; in the formula for power emitted by a blackbody per unit area per unit wavelength interval, it is 2π times Planck's constant, times the square of the speed of light, or approximately 3.74177 × 10 -16 watt (meter) 2.