The invention relates to a process and device for exposing imaging material. A light beam (2) generated in a light source (1) is modulated linewise during an exposure time in a video modulator (5) and deflected over the imaging material (9) by means of a light beam deflector (7). During a returm time following the exposure time the video modulator (5) is deactivated. To present back-reflections, between the light source (1) and the video modulator (5) there is an optical isolator (4) which is switched on during the exposure time by gate pulses. In oder to maintain a constant operating temperature of the optical isolator (4) during the exposure and return times, the optical isolator (4) is swiched on linewise by additional gate pulses (GI) during a compensation time within the return time, during which the optical isolator (4) heats up. The optical isloator (4) is controlled in such a way that the sum of the lenghts of the exposure time and the compensation time is fairly constant within each line.