First radiation constant

c_1 = 3.741771852 \times 10^{-16}\ \text{W}{\cdot}\text{m}^{2}

Value	3.741771852e-16 W·m²
Status	Exact by definition — no uncertainty
Source	CODATA 2022 (exact in the 2019 SI)
Categories	Thermodynamic Universal & Atomic

Learning zone

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.

Used by these solvers

Stefan-Boltzmann Law

P = \varepsilon \sigma A T^4

Photon Energy from Wavelength (E = hc/λ)

E = \frac{h c}{\lambda}