Abstract: An improved device for controlled illumination of a cavity such as in endoscopy or inspection of machinery within a housing; the device comprises a light source and a fiber optics for feeding light into the cavity, and a control means arranged between the light source and the entrance plane of the fiber optics for controlling the luminous density at the exit plane of the fiber optics; the control means is in the form of a diaphragm disc arranged between the light source and the entrance plane of the fiber optics and is capable of linear or rotational displacement or motion; the disc has an opening having a width that varies cross-wise to the direction of displacement or motion of the disc-shaped diaphragm so as to provide a variable aperture; the luminous density at the exit plane will, in general, be determined by the light flux that impinges upon the entry plane, and that flux is controlled by the variable aperture.