Characterisation of the signal and noise transfer of CCD cameras for electron detection

Methods to characterise the performance of CCD cameras for electron detecti on are investigated with particular emphasis on the difference between the transfer of signal and noise. Similar to the Modulation Transfer Function M TF, which describes the spatial frequency dependent attenuation of contrast in the image, we introduce a Noise Transfer Function NTF that describes th e transfer of the Poisson noise that is inevitably present in any electron image. A general model for signal and noise transfer by an image converter is provided. This allows the calculation of MTF and NTF from Monte-Carlo si mulations of the trajectories of electrons and photons in the scintillator and the optical coupling of the camera. Furthermore, accurate methods to me asure the modulation and noise transfer functions experimentally are presen ted. The spatial-frequency dependent; Detection Quantum Efficiency DQE, an important figure of merit of the camera which has so far not been measured experimentally, can be obtained from the measured MTF and NTF. The experime ntal results are in good agreement with the simulations and show that the N TF at high spatial frequencies is in some cases by a factor of four higher than the MTF. This implies that the noise method, which is frequently used to measure the MTF, but in fact measures the NTF, gives over-optimistic res ults. Furthermore, the spatial frequency dependent DQE is lower than previo usly assumed. Microsc. Res. Tech. 49:269-280, 2000. (C) 2000 Wiley-Liss, In c.