Abstract: In a Scanning Electron Microscope (SEM) a spot made from a very narrow beam of electrons (11) is scanned in fine strips across the specimen (18), and an image is built up from the electrons (e) that backscattered from area of specimen being scanned. To gather these back-scattered electrons it is presently common to employ a scintillator layer on the front face of a light guide that directs the generated light to a photomultiplier tube (23). However, having the scintillator layer on the front face means that the light has to pass through the layer to enter the guide, which necessarily wastes light by absorption within the thickness of layer material; the invention proposes instead that the light guide (22) have on its back side a scintillator layer (34) angled as though to reflect received electrons along the guide to the PMT (23).