There is a well-known technique that uses oblique illumination to make indented markings more conspicuous. A light source directed at a low angle onto a surface will produce both a shadow thrown by one edge of the indentation and a slightly enhanced brightness on the opposite edge. One particular problem in reading indented writing is more difficult to solve by the oblique illumination method. Where indentations are present in a surface that already bears markings of varying tone, such as on a printed map, the subtle variations in shadows produced by the indentations can sometimes be obscured by the original background detail. The proposed technique is designed to reduce the background information. It uses digital image processing applied to images that have been captured with a video camera. Results are presented to show the variety of conditions for which the technique might be effective. All the illustrations have been obtained by photographing the display on a TV monitor and printed to the same contrast. In principle, the subtractive oblique illumination method is no different from the conventional oblique illumination technique and, as a result, will not reveal indentations that are at a more subtle level than can be seen on a plain surface. It is, however, able to differentiate between indented information and background matter for several different materials and in conjunction with extreme contrast variations. 6 figures and 3 references