Simple mod turns diode into photon counter

I remember reading about these in infra-red tacheometers (or surveyor’s “total stations”) which time the arrival of photons from a corner cube prism use adjustments for the speed of light (altitude and temperature) and with the recorded angles from the circles in the theodolite portion calculate x,y,z coordinates of the target prism, today without a target or from reflective tape. I think of it as more like a “night vision” collector than what they were being called “lasers” though the LED diode was restricted to a specific frequency in infra-red sent through the telescope to the prism and back. the physics arXiv blog » Blog Archive » Simple mod turns diode into photon counter