An experimental setup has been developed at the Istituto Elettrotecnico Naz
ionale G. Ferraris (IEN) for the absolute calibration of photodetectors in
the photon-counting regime by means of correlated photons in parametric flu
orescence. A nonlinear crystal of lithium iodate is pumped by an argon-ion
laser at 351.2 nm, generating a broadband source of correlated photons from
the incoming wavelength to the far-infrared. Photomultipliers and solid-st
ate, single-photon detectors are employed, operating in the photon-counting
regime with very low dark counts. Quantum efficiency is measured at 633 nm
. The results show the correlated photons to be a promising technique for r
adiometry. The advantages of this as opposed to a conventional method at lo
w power levels are shown.