I'm looking through some of my physics notes where we derived radiancy equations for black body radiation (Rayleigh-Jeans and Planck), and I have a quick question. In class, we went through the derivation for the energy density as a function of frequency and temperature. However, I can't find anything in my notes about how radiancy is related to energy density. I tried to look online, and didn't find anything helpful, other than the equation that relates them: [tex] R(\nu ,T) = c/4 u(\nu ,T) [/tex], where u is the energy density and R is the radiancy. From the units it looks like radiancy is the power per area per time, but I am totally mystified by the factor c/4. Can anyone quickly go over where this comes from?