Non Initialized systems
Gray code strip or wheel- a number of photodetectors can sense a pattern, creating a binary number. The gray code is a mutated pattern that ensures that only one bit of information changes with each measured step, thus avoiding ambiguities.
Initialized systems
These require starting from a known distance and accumulate incremental changes in measurements.
Quadrature wheel- A disk-shaped optical mask is driven by a gear train. Two photocells detecting light passing through the mask can determine a partial revolution of the mask and the direction of that rotation.
whisker sensor- A type of touch sensor and proximity sensor.