[Al Skierkiewicz]

The tape is placed on a rotating object in segments. This allows a photo sensor aimed at the tape to alternately turn on and off at the segment rate effectively counting segments. Say there are eight transitions on a 6" diameter wheel, you might be able to resolve horizontal movement of the robot on the order of less than one inch per segment. More transitions gives you better accuracy. If you use two sensors you can actually determine direction as well as rotation distance.