An experimental setup has been developed at the Istituto Elettrotecnico Nazionale G. Ferraris (IEN) for the absolute calibration of photodetectors in the photon-counting regime by means of correlated photons in parametric fluorescence. 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-state, 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 radiometry. The advantages of this as opposed to a conventional method at low power levels are shown.