I wanted to check the rpm on a new generator I bought as the exact rpm is needed to produce the rated capacity of the generator. I painted a white mark on the flywheel and cranked the generator. Aimed the little laser tachometer at it, and it showed perfect rpm for the engine. I even checked a fan to see if what they claimed the rpm were for different speeds were accurate, and those were pretty close.  It's a great no-touch tach. Just need a piece of chalk or a dot of white paint for it to see.