A touch system for sensing a touch event that includes a transparent sheet having opposite upper and lower surfaces and an index of refraction n2. The system also has at least one light source that emits light. The light source is arranged in optical communication with the transparent sheet to cause the light to travel within the transparent sheet by total-internal reflection (TIR). At least one detector is arranged to detect the TIR-traveling light and to detect an amount of attenuation in the TIR-traveling light caused by the touch event. An interface layer is disposed on the lower surface of the transparent sheet. The interface layer has a refractive index n1, wherein n1