An optical sensor includes pixels. Each pixel has a photodetector. A readout circuit performs a process over an exposure time where the photodetector is connected to a reverse bias voltage supply to reset a voltage across the photodetector, and the photodetector is disconnected from the reverse bias voltage supply until that the voltage across the photodetector decreases in response to received ambient light. An ambient light level is then determine an based on a number of times the voltage across the photodetector is reset over the exposure time.