An estimate of the distribution of electrons striking the specimen in a Philips EM200 electron microscope

Constants have been measured for the Philips EM200 which enable the total current striking the specimen to be determined from a single reading of the exposure meter. In addition an approximate measure of the distribution of electrons striking the photographic plate can be obtained simply by photographically recording an image of the beam and measuring the radius of the blackened area. Together with the exposure meter reading this is sufficient to give the approximate Gaussian distribution at the photographic plate. It is related to the distribution at the specimen level by assuming that electrons are lost from the beam by collision with molecules of the residual gas.