Optical Imaging Touchscreen
Optical Imaging is a relatively modern development in touchscreen technology, in which two or more image sensors are placed around the edges (mostly the corners) of the screen. Infrared back lights are placed in the camera's field of view on the other side of the screen. A touch shows up as a shadow and each pair of cameras can then be triangulated to locate the touch or even measure the size of the touching object.
This technology is growing in popularity due to its scalability, versatility and affordability, especially for larger units.