A parabolic light reflection device in a cathodoluminescence (CL) apparatus is integrated with a photosensitive solid state device mounted on a supporting plate, and supported in an electron microscope vacuum chamber specimen stage by an adaptor element. These components are optically aligned with respect to one another and a readily exchangeable unit is thus obtained. Designed for CL emission operation in an electron microscope, the parabolic light reflection device and the photosensitive solid state detection device are optically aligned and then mounted on a supporting plate of the photosensitive solid state device. The unit thus configured is supported by the specimen stage adaptor element to obtain a mechanical element which can be easily inserted in and removed from any standard electron microscope vacuum chamber stage as a single integrated unit.