1. A head mountable display (HMD) comprising: an infrared light source configured to illuminate foreground objects but not background objects greater than a threshold distance from the HMD, the infrared light source being configured to provide illumination for successive intervals of time separated by intervals during which the infrared light source does not provide illumination; one or more cameras configured to capture infrared illuminated images and visible light illuminated images, the one or more cameras being configured to capture the infrared illuminated images during the intervals in which the infrared light source provides illumination and configured to capture the visible light illuminated images during the intervals in which the infrared light source does not provide illumination; and an image processor configured to detect, from the infrared illuminated images, foreground objects in the visible light illuminated images; in which the one or more cameras comprise a single camera and a light-directing arrangement for directing infrared and visible light to different image regions of the single camera.