Jim,

Maybe your are correct about "Total" darkness. However, I mount the sensor for mine in the ductwork of my ducted fans. I have tached them in the late evening hours with no additional light. That is as close to total darkness as you can get and still start the engine. I must say, this is a great tool. Maybe, the fan version of the tach is different than the others. I got mine so long ago, I don't remember.

Mark M.