Abstract: A method for testing a semiconductor device wherein an image produced from secondary electron emissions generated by exposing a cross section of the device to a beam of electrons with such device being in a first biased condition, preferably an unbiased condition, is digitally recorded in a memory of a digital computer. An image is produced from secondary electron emissions generated by exposing a cross section of the device to a scanning beam of electrons with such device being in a second biased condition is digitally recorded in the memory of the digital computer. The recorded images are digitally subtracted one from the other in the digital computer to produce a difference image. The digital computer digitally calculates a quantitative measure of voltage distribution across the exposed cross section from two regions in the difference image having different degrees of secondary electron emissions. The digital computer displays one of the digitally recorded images on a video display.