1. A dosimeter for use in limiting retinal exposure to field illumination in the course of an ophthalmic surgical procedure, said dosimeter comprising sampling means including a light detector producing an electrical-signal output responsive to a sampled fraction of the illumination, field-dosage integrating means including a clock timer and connected to the electrical-signal output of said detector, said integrating means integrating detector output as a function of time of illumination, presettable signal means for providing a predetermined reference signal value representing a predetermined maximum tolerable dosage level of integration value, and indicator means differentially responsive to said reference signal value and to the instantaneous integration value to produce a difference signal value which is indicative of remaining tolerable dosage.