First radiation constant

Planck's law is usually written with two lumped constants so the exponentials stay readable: the spectral exitance of a blackbody is M_λ = c₁ / [λ⁵ (e^(c₂/λT) − 1)]. The first radiation constant c₁ = 2πhc² sets the overall scale of the emitted power; integrating the whole curve over wavelength returns σT⁴, so c₁ and σ are two faces of the same physics.

Watch the variant: the constant for spectral radiance (per steradian), c_1L = 2hc², is smaller by a factor of π and is the one radiometry handbooks and pyrometer manuals usually tabulate. Both are exact in the post-2019 SI, since h and c are defined values.