1. The problem statement, all variables and given/known data I am checking some equations for my simulation and are looking at the Planck function. My question involves the constant used for the Planck function expressed in Wavenumbers. I have found this expression for the function (http://pds-atmospheres.nmsu.edu/education_and_outreach/encyclopedia/planck_function.htm) but I don't understand where the unit comes from for the first Planck constant alpha1 = 2 h c^2 = 1.191066 · 10^-5 mW · m-2 · steradian-1/cm-4? I would like to understand this before I use it. There is another expression for the first Planck constant including pi but has a different unit. I guess I am looking for a Planck function expression which uses wavenumbers, but it would be nice to see how alpha 1 is obtained. 2. Relevant equations Planck function dependent on wavenumber w (cm-1) B(T,w) = (alpha1 w^3)/[exp(alpha2 w/T) - 1] alpha1 = 2 h c^2 = 1.191066 · 10^-5 mW · m-2 · steradian-1/cm-4 <-- how? when h is Planck's constant (6.62620 · 10^-34 Joule second) c is the speed of light (2.99793 · 10^8 m/second) I have found another source that alpha1 = 2 pi h c^2 = 3.741 771 18(19) × 10−16 W·m² 3. The attempt at a solution Putting in the numbers I obtain: alpha1 = 2 h c^2 = 1.19107 * 10 ^-12 W/cm^2 = 1.19107 * 10 ^-16 W/m^2 This is not, 1.191066 · 10^-5 mW · m-2 · steradian-1/cm-4 .. where does the unit steradian-1/cm-4 come from? I might add that I did find a few papers using this number, C1 = 1.191062£10^-12 W cm^2 /sr, but it still does not explain the origin of the two unit values. (http://www.iop.org/EJ/article/1674-...quest-id=2a611551-3e23-4cae-9cb4-58c619d41cbe) I couldn't find the original when looking at references..