I have a 0.05 mm radius sperical source of Photons, and a 10 mm X 10 mm detector aligned to be orthogonal to their distance vector. Distance is D $\approx$ mm.
I want to know how good the point source approximation for my source is, given distance D. In particular, I suspect that if $D\approx 4\, mm$, the point source approximation is not very good.
Characteristic lengths of the problem are Distance $D$ and radius $r$. I can calculate the solid angle $\theta$ from which the detector looks at the source, but how should I relate those quantities?