Whenever electrical signals are not advisable
(eg measuring the flow of highly-volitile liquids), optical
sensing can offer a cheap, reliable alternative.
Optical sensors can measure a range of
parameters such as distance, optical density, turbidity of fluids,
and rotation without relying on mechanical parts and hence improve
above image shows a prototype low-cost optical rotation sensor
assembly. This type of sensor over comes the need for mechanical
bearing and slip-rings to accurately measure angular position"