Hi I'm trying to follow a derivation, I don't think I'm allowed to post the content (copyright) but the paper is here: http://www.sciencedirect.com/science/article/pii/002240739290117M. Does anyone have access to this and know a little bit about integrating over solid angles? I've spend half a day pondering over the logic linking equations 20-23. It seems they're defining a unit sphere and taking the intensity change over the surface to equal the change in number of photons inside the sphere. What I don't understand is: 1. where the log goes between eq. 20 and 21 2. where dL is relative to this sphere? Thanks for any help. Very stuck.