A look at photon detectors
In the best of all possible worlds, we would have the ideal photon detector, a device that caught a photon, gave an unambiguous meter reading and kept count of the number of events. In the real world, these ideal devices do not exist; competing events both outside and inside the detector confuse the true measure of the photons that we are trying to monitor. Phenomena such as quantum noise, “dark” current and background radiation interfere to a degree that depends on the intensity of the signal being measured and on the photon frequency, to mention only a few of the experimental parameters.