Abstract: A light source for illuminating film transparencies is in the form of a cathode-ray tube provided with electronic circuitry which causes the beam of the tube to trace a raster on the screen. The circuitry is such that the area of the raster and its position on the screen can be selectively controlled by the observer for illuminating predetermined portions of the screen. The observer can thus illuminate only that portion of the film transparency of particular interest to him, and can select the intensity of illumination by controlling the electron beam intensity. The raster is developed by sweeping the beam in the horizontal and vertical directions at different speeds using a sweep wave form that is of a symmetrical triangular shape thus eliminating the necessity for blanking the beam as the raster is developed.