An optical sensor array includes an array of pixel circuits. Each pixel circuit includes a photo detector and a voltage supply line. The voltage supply line from each pixel circuit is connected to a common node. A voltage supply input configured to be coupled to a voltage supply and to the common node supplies a voltage to each pixel circuit. A sensing circuit coupled to the common node senses signals from the common node and outputs at least one signal representative of an average intensity of light directed onto the array of pixel circuits.