The optical MPG has many similar names, like handwheel encoder, hand held encoders. It is basically a type of rotary encoder. In electrical, the theory is similar to optical encoders. MPG also output A,B signals. The output format are TTL, Line driver, which are mostly required. The disc of handwheel encoder is made of steel. As for the resolution is low: 25ppr and 100ppr, the steel disc could meet the resolution requirement completely. When handwheel as a seperate part for CNC machine tools, it is called hand held encoder. That means you can hold the encoder in hand to operate the tool’s moving.