Optical touchscreens! Underutilized technology, IMO. I had a very small part to play in the development of the FlatFrog display system which uses the same approach but inside the glass, using the fact that touching the screen alters the internal reflection properties. https://www.flatfrog.com/inglass

I was one of the contract IC designers involved in the custom chip which is deployed all round the display to illuminate/measure. A little single-purpose DSP with an ARM on the side.