ORIGINAL: jester_s1
The handhelds don't read air. They read the flashes caused by the prop going by. They work just like the chronographs used for measuring bullet speed. A big factor in their accuracy is the background in front of the plane. A plain white board or building gives lots of contrast for the little electric eye to read the prop's movement against.
And they go nuts under house lighting. Point one at a light bulb or a florescent light and you have 3600RPM on the screen. They just count the light/dark transitions and divide by two for a two blade prop or by three for a three blade prop. It's one way to verify your tach is counting OK.
Don