Lλ = the spectral radiance
dIλ = the spectral radiant intensity
dA’ = the projected area (dA cosθ ) of the aperture of the blackbody
e = the base of natural logarithms ( approx. 2.71828)
Τ = absolute temperature c1L and c2 are constants designated as the first and second radiation constants.
Note: The designation c1L is used to indicate that the equation in the form given here refers to the radiance L, or to the intensity I per unit projected area A’, of the source. Numerical values are commonly given not for c1L but for c1, which applies to the total flux radiated from a blackbody aperture (that is, in a hemisphere [2π steradians]) so that, with the Lambert cosine law taken into account, c1 = πc1L.
The currently recommended value of c1 is 3.741832 x 10-16 W·m2, or 3.741832 x 10-12 W·cm2.
Then c1L is 1.191062 x 10-16 W·m2/sr, or 1.191062 x 10-12 W·cm2/sr.
If, as is more convenient, wavelengths are expressed in micrometers and area in square centimeters, c1L = 1.191062 x 104 W·μm4/(cm2·sr), with Lλ being given in W/(cm2·sr·μm).
The currently recommended value of c2 is 1.438786 x 10-2 m·K.[note]Values of radiation constants from NBS Special Publication 398.[/note]
The Planck law in the following form gives the energy radiated from the blackbody in a given wavelength interval (λ1 – λ2):