Use of an optical parametric converter in determination of the brightness of radiation emitted by a thermal source

A new method was developed for the absolute determination of the brightness of thermal radiation with the aid of a parametric converter (Klyshko method). The brightness of zero fluctuations of electromagnetic vacuum was used as a standard. The brightness of 3900 nm radiation emitted by cw thermal sources, heated to $\thicksim1000$K, was determined. An analysis was made of a photon meter prototype system for the determination of the brightness of light fluxes by the Klyshko method.