Calibrating photon counts from a single image
  Current methods for detector gain calibration require acquisition of tens of special calibration images. Here we propose a method that obtains the gain from the actual image for which the photon count is desired by quantifying out-of-band information. We show on simulation and experimental data that our much simpler procedure, which can be retroactively applied to any image, is comparable in precision to traditional gain calibration procedures. 
Rainer Heintzmann, Peter K. Relich, Robert P.J. Nieuwenhuizen, Keith   A. Lidke, Bernd Rieger
physics.ins-det astro-ph.IM