This one should be a bit of a softball, but I can't find it explicitly stated anywhere on the internet, and my basic unit analysis doesn't seem to work.
Suppose you have a beam of synchrotron radiation with a brilliance $B$ at a given energy. (For instance, the brilliance of the Advanced Light Source at Berkeley is given in a graph form here). If I know the distance $d$ from the source and the area $A$ of the (for simplicity, unfocused) spot on my sample, how do I calculate the flux (units photons/sec) to my sample? Another way to phrase the same question is given the distance from the source and the brilliance, how do I calculate the intensity (units photons/sec/area) of the photon source at my sample? Am I missing some parameter?