A touch sensitive device is arranged with substantially parallel surfaces inside which light from a synchronized source, such as a CRT, can become trapped by total internal reflection. The edges of the device are fitted with photodetectors which respond to the entrapment of light within the surfaces. The top surface is arranged so that a touch of the device at a particular point will cause a medium change on the surface at that point. The medium change causes light to become entrapped within the surface by total internal reflection thus making it possible, by comparing the photodetector output with the CRT raster position, to determine the exact surface position of the touch. Using this arrangement, positional determination is uneffected by raster drift.