An experimental study of the spectra of a partially coherent field, pr
opagating beyond an optical system (a lens), reveals that no spectral
shift takes place on the geometrical-image plane, while large blue shi
ft occurs at the back focal plane of the optical system for on-axis po
ints of observation. Different light sources taken for the study are (
i) partially coherent fields produced by a multimode laser and (ii) a
quasi-homogeneous source with different degrees of spatial coherence.
These results are consistent with the theory and might find applicatio
ns in metrology. (C) 1998 Elsevier Science B.V.