Direct Electron Imaging Using Back Thinned CCD's . ... where, Vim is the maximum radial emission energy of photoelectrons from the cathode (eV), Vs is the cathode to anode voltage (V), k is the spatial frequency (2 p lp/mm), L is the cathode to anode spacing of the proximity focused diode (mm), d is the field-free region thickness(mm), h is the backscattering coefficient, (G/G 0 .) is the ratio of the average EBS gain of the secondary electrons to the primary gain, and W is the CCD pixel pitch (mm). ...