Photoelectric detection of polychromatic light

The problem of photoelectric detection of light is reexamined when the light is polychromatic and not strictly stationary, and when the frequency response of the detector has to be taken into account. It is shown that, whereas the one-photoelectron detection probability can always be expressed in a familiar form involving the expected light intensity, the same is not true of the twofold detection probability. In general, the photodetector does not count photons in any precise sense.