This is a great Digital Photo Tachometer. It measures the rotational speed with a visible red light beam from a powerful LED.
Just attach a reflective sticker to the rotating part and point the beam on the mark from a safe distance.
It is a great tool to measure the RPM of motors, engines and machine parts.
In wood and metal working it can be used to accurately measure the speed of a lathe or milling machine.
I have many customers who use it to measure fan speeds in HVAC applications.