Measuring watches with an optical detector
It’s not easy to apply the benefits of an optical sensor to watch measurement. The watch escapement is so small that it’s very difficult to get the sensor components where they need to be. However, a man named John Frieman developed a sensor for watches that uses an optical fiber to place a tiny spot of light on the arm of the balance wheel and another optical fiber to read this spot and direct the light back to a detector. In this way he’s able to greatly reduce the jitter inherent in acoustic measurements of precision watches.

We also use a specially constructed optical sensor to “vibrate” a hairspring. This refers to the process of finding the correct termination point of a hairspring before it’s installed in the watch. Traditional hairspring vibrating tools compare the hairspring under test to a master balance of correct beat to gage similarity. The specially constructed optical sensor can be used to measure the period of a temporarily mounted hairspring before installation.