mice, nearly all mechanical mice tracked movement using LEDs and photodiodes to detect when beams of infrared light did and didn't pass through holes in a pair of incremental rotary encoder wheels (one for left/right, another for forward/back), driven by a rubberized ball. Thus, the primary distinction of "optical mice" is not their use of optics, but their complete lack of moving parts to track mouse movement, instead employing an entirely solid-state system.

Early optical mice