Abstract: A touch sensitive screen arrangement includes an optically dispersive base plate and structures for transmitting light onto the base plate in response to an external body touching the screen at a touch point such that the location of incident light on the surface of the base plate is representative of the relative position of the touch point on the screen. The base plate captures and disperses light that is incident on it. The decrease in incident light intensity with distance from the location of incidence on the base plate approximates a substantially exponential function. A sensor detects intensity of light at a respective edge of the base plate. The arrangement calculates the relative position of the touch point on the screen from the detected light intensity and the exponential function and generates a control signal representative of a control input derived from the position of the touch point.