Scale-tunable optical correlation with natural light

We describe two different scale-tunable optical correlators working under t otally incoherent light. They behave as spatially incoherent wavelength-ind ependent imaging systems with an achromatic point-spread function (PSF). ln both cases it is possible to adapt the scale of the achromatic PSF, i.e., to modify the scaling factor of the PSF and preserve the chromatic compensa tion, by one's shifting the input along the optical axis. The remarkable pr operties of these systems allow us to carry out a scale-tunable color patte rn-recognition experiment with natural light. (C) 2001 Optical Society of A merica.