1. A electronic device, comprising: a touch-sensitive display screen including an array of electromagnetic radiation emitters and electromagnetic radiation detectors, wherein the array of electromagnetic radiation detectors is configured to generate an image signal that represents an image of a user input object that is closely spaced to the display responsive to detected electromagnetic radiation that is reflected by the user input object; and a controller circuit configured to control activation of the array of electromagnetic radiation emitters so that at least a portion of the array of electromagnetic radiation emitters are inactive in an absence of detecting electromagnetic radiation reflected by the user input object, and to selectively activate ones of the array of electromagnetic radiation emitters responsive to the array of electromagnetic radiation detectors detecting electromagnetic radiation that is reflected by the user input object.