Basically the problem that I am facing is when I am getting the interferogram in W/m2, I need to calculate the shot noise in terms of photons. Now to do that I need an area that I should multiply with the intensity term that I get for each pixel on the FPA. If I multiply pixel area with it the photon count goes to signal. On the other hand multiplying by aperture area of the instrument does not make much sense either to me because that would amount to all photons concentrating at x=0 which is wrong.