Use of two-photon light for absolute calibration of photoelectric detectors

A description is given of a method for calibrating photodetectors based on some characteristics of the statistics of the field of spontaneous parametric light scattering. A phenomenological theory of parametric scattering is used to demonstrate that photons are only emitted in pairs. It is shown that, in principle, a priori information on the two-photon character and directionality of parametrically scattered light can be used for absolute calibration of photodetectors operated both in the analog linear regime and in the discrete photoelectron pulse counting regime.