To answer my own question. From the point source to the nearest point on the screen is distance r1. This is a vector lying on a sphere with area (4 pi r1^2). The density of vectors on this sphere is N/(4 pi r1^2). For a point further on the screen, the vector from the point source is distance r2. The density of vectors on this sphere is N/(4 pi r2^2). Density_2 / Density_1 = r1^2 / r2^2 < 1. This will give you the density of the flux on any point of the screen.
On Tuesday, November 20, 2012 4:21:09 PM UTC+1, machiel wrote: > Hi, > > > > When I have an isotropic point-like source that radiates isotropically in 3-D space, how can I calculate the number of events that reach a screen a > > certain distance away, with the probability as a function of (transversal) x and/or y on that screen. > > Hence, I want to know how to derive a function for the amount of events that reach a certain region on a flat screen, when the origin of these events is some isotropic point-like source. > > Any help would very much be appreciated. > > Best regards, > > > > Machiel