Unfortunatly, the TNC tach will not work in total darkness. They will however work in a lower light situation than what the cheap tachs do.

A TNC Tach when pointed at a light bulb or fluorescent lamp will always show a very definite number. I cannot remember what it is on mine but I think it is 100. That is how the book says to check the calibration on them. You can use a regular flash light to check the rpm when using a TNC tach. The TNC tach has an amplified detector in it and works just fine after the sun gets behind a cloud or in the shade etc. But you must have a little non-pulsating ambient light. When I bench run my engines, I do it in the garage with the prop blast going out the opened garage door. The TNC tach works fine just by using the light relecting off the back side of the prop blades. It will work after the day begins to fade but it doesn't have an internal light and it will not work in total darkness.

When you use a flashlight, you do not have to let the tach see it through the prop blades. You can hold the flashlight and the tach together and let the relected light off the prop blades do the job.

Good luck

Jim