The appts. (300) includes a test pattern generator (200) to which it is connected and at least two data image receivers. The pattern generator applies test patterns to a test IC (DUT) while ion beams scanned across the surface of the IC. The resulting quantity of generated secondary electrons is measured to determine the potential distribution on the surface. The image data are acquired by the image data receivers depending on a pattern stop signal which is generated whenever two stop patterns occur. A computer (309) determines the difference between the image data received by the two or more receivers. The difference is displayed on a monitor (306).
