A rear-facing pupil diameter sensing system for an ophthalmic lens comprising an electronic system is described herein. The rear-facing pupil diameter sensing system is part of an electronic system incorporated into the ophthalmic lens. The electronic system includes one or more batteries or other power sources, power management circuitry, one or more sensors, clock generation circuitry, control algorithms and circuitry, and lens driver circuitry. The rear-facing pupil diameter sensing system is utilized to determine pupil position and use this information to control various aspects of the ophthalmic lens.
