In a display, an optical sensor array comprises a photo - sensitive area formed by an array of chiplets having individual light-sensitive elements, each element configured to produce a signal or signals in response to incident light. In one embodiment, the displacement of a chiplet from a predetermined position is derivable from the output signal or signals of the photo - sensitive element or elements of the chiplet, the signals being output as response to the light emitted by the pixels of the display which surround the photo - sensitive area of the chiplet. The arrangement provides a method of measuring the displacement of at least one chiplet in an active display.
