Abstract: An automated system for examining a workpiece 6 treated so that flaws are revealed by concentrations of indicator on its surface scans the workpiece surface to provide an output signal representative of the luminance of surface regions. These are stored in a memory 12 and first and second signals representative of the average luminance of a number of regions A, B, C, and E, F, G on opposite sides of a central region D are formed. The first and second average signals are compared to determine which one represents the greatest concentration of indicator and the selected signal is used to derive a threshold value which is compared with the output signal representative of the luminance of the central region D. A flaw is indicated when the central region luminance signal is greater than the threshold value.