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Question on a paper (about simulating camera optics)

Hi all. Not sure if this is the right forum, so please move as necessary.

I was trying to understand a research paper, and some of the math went over my head. The paper is public, but in .ps form, so I've attached a converted PDF version.

Specifically, this section eludes me:

http://img202.imageshack.us/img202/4330/ctpj.png

Firstly, what is meant by form factor here, and what is a differential form factor? I tried looking on Wikipedia, but I see a variety of defintions and it's not clear which one is relevant here.

Secondly, it's not clear where that formula came from. Is it just some well-known formula? Does it have a name?