One of the original touch screens was a frame around the crt screen that had across the left side ir led's and across the top ir led's and across the right side and bottom photo diode that were sensitive to the ir led's. THey were matrixed so that if you broke the light path it was read as a position and integrated into the software as an input of position. YOu could do the same thing. The more leds you have the better the resolution. with todays faster computers you should be able to get hight resolution and quick responce. This system was even marketed as an add on. You basically stuck the frame on a crt screen and you had a touch screen.