I've put together a short wiki page on some of the basic radiometric terms, as well as an example of working with BRDFs. I found it a hard area to get enough information on, and often very confusing. Hopefully everything on the page is consistent and will serve as a grounding for anyone else that wants to tweak BxDFs.
http://www.luxrender.net/wiki/index.php ... Radiometry
If anyone with expertise in this area has a chance, i'd appreciate it if they could look over some of the information and see if i've messed anything up.
Also if anyone is trying to understand the maths behind it all, and finds things unclear or confusing please let me know and i'll do my best to fix it.