A handheld computing device comprises a display comprising an array of pixels illuminated by a plurality of visible light sources, and a plurality of infra-red light sources interleaved between the visible light sources, the IR light sources being actuable to emit diffuse IR light with a first intensity. A camera has an image sensor comprising an array of pixels responsive to infra-red light and a lens assembly with an optical axis extending from the image sensor through the surface of the display. A dedicated illumination source is located outside the display and is actuable to emit infra-red light with a second greater intensity. A processor is configured to switch between an iris region processing mode in which a subject is illuminated at least by the dedicated light source and a face region processing mode in which a subject is illuminated by the plurality of IR light sources.