US Patent 7995039 Touch pad system

Radiation guided by Total Internal Reflection inside a solid element from a point of engagement of a reflecting/emitting/scattering object and toward a detector reaches an area of the solid element on which the surface has areas preventing reflection of the radiation back into the element. This provides a spatial modulation of the radiation reaching the detector through one of the surfaces of the solid element. From this spatial modulation, the position of engagement may be determined.