An interactive display device (100) presents to a user (113) a visible image on an image layer (102) through a touching surface (101). The display device (100) has an illumination unit (107) that transmits infrared light and captures through the touching surface sets of infrared images of objects that are in the proximity of the touching surface. The display device (100) detects from the infrared images contact areas where a pointing object (112) is located with respect to the image layer (102) and casts in the infrared image a corresponding shadow. The illumination unit (107) alternates the infrared illumination so that each infrared image set has images taken with differing infrared illumination. Resulting variance in the shadows is used by the interactive display device (100) to detect whether the pointing object (112) contacts the touching surface (101) or not.