A body-mountable device equipped with a light sensor is disclosed. The light sensor includes a photodiode that operates in either a photoconductive mode or a photovoltaic mode depending on whether the photodiode is reverse biased. A group of switches are arranged to selectively couple the photodiode to first and second voltage sources to configure the photodiode to operate in either mode. A controller selects a mode of operation of the photodiode and controls the switches to cause the photodiode to respond to incident light while operating in the selected mode. The controller then obtains a measurement from the photodiode indicative of the intensity of light received during an exposure interval. The light sensor can thus include a single photodiode and yet be used to alternately obtain measurements based on the photoconductive response of the photodiode or based on the photovoltaic response of the photodiode.