The radio modelers have developed a useful market for gadgets.
I bought a tach a while ago, and I keep meaning to mention it
here.

The price was right - around $25.
It's intended for measuring off two or three blade props.
In other words, it counts either two flashes or three as a revolution.

When I point it at this screen (Free Agent News Reader) it registers
1800 rpm i.e 60 flashes / sec if I select two blade, or
1200 rpm if I select 3 blade. If I point it at a compact fluorescent,
it indicates 3600 rpm (120 flashes/sec)

People apparently use it with reflective patches stuck on their props
Half inch numerals on an LCD display with a prop image (2 or 3 blade).
Battery lasts a long long time.

"Tower Hobbies Digital Mini-Tach"

Brian W