This paper reports on an experiment performed at the Istituto Elettrot
ecnico Nazionale G. Ferraris (IEN), aiming at the production of biphot
on fields by parametric scattering of light in a nonlinear crystal of
lithium iodate. The blue line at 457,9 nm from an argon laser is used
as pumping radiation, thereby resulting in a fluorescence in the visib
le region with degenerate pairs of photons at about 916 nm. These biph
oton fields are well suited to the measurement of the quantum efficien
cy of photodetectors in the photon-counting regime and to the realizat
ion of an absolute source of radiation.