1. A device, comprising: an image sensor; and a circuit configured to control the image sensor, said circuit configured, in a first operating mode, to output an image acquired by the image sensor and further configured, in a second operating mode where the image sensor provides no image, to: select a subassembly of pixels of the image sensor; for each pixel of the subassembly, read an output value of the pixel independent from the output values of other pixels; and provide an ambient luminosity value based on the output values of the selected pixels of the subassembly.