Abstract: A touch screen apparatus 1, having a screen 2 which is adapted to be touched by one or more users to operate a computer system according to command steps which appear on the screen for displaying information thereon. There is sensor means 3 in the form of an array of sensor elements or touch points, which are each indicated by a ‘X’ in the Figure but which are not normally visible to a user. However, they are able to detect when a wiping element is wiped across an area of the screen 2, which it is desired to clean, usually but not always the whole surface area. The number, position and disposition of the touch points ‘X’ is controlled by computer software, which is able to vary the number, location and distribution of the points so that a user is not able to learn the characteristics of a particular array 3. This ensures that a complete wiping action is effected between uses of the screen.