the invention contemplates dose - integrating instrumentation which relies upon continuously monitoring the illuminance level of a small sampling fraction of the total projected - light flux of the field - illuminating system of an operating microscope , the sampling being optionally within the projection system , or taken elsewhere . the sampled illuminance is in the form of an electrical signal which is continuously time - integrated to provide a continuous display of the dosage as it builds in the course of an operative procedure . provision is made for the surgeon to preset what he chooses to be the safe upper limit of integrated exposure , beyond which he does not wish to go for fear of possible jeopardy to the retina of his patient , and the instrument continuously indicates the remaining time for him to complete his operation , should he continue at the current level of illumination . the instrument also provides a succession of different indications throughout the progress of the operation , as successive predetermined thresholds of building dosage are reached .