Predictive algorithms in the CPU of the mouse calculated the speed and direction over the grid. One of these, invented by Steve Kirsch of MIT and Mouse Systems Corporation, used an infrared LED and a four-quadrant infrared sensor to detect grid lines printed with infrared absorbing ink on a special metallic surface. The first two optical mice, first demonstrated by two independent inventors in December 1980, had different basic designs:
An early Xerox optical mouse chip, before the development of the inverted packaging design of Williams and Cherry
