Purity of partial polarization in the frequency and time domains

We discuss the properties of fields whose polarization states remain invariant in diffractive devices; in particular, in Young's interferometer. We investigate the conditions for such fields, called pure partially polarized fields, in both the space-time and space-frequency domains and show that the frequency-domain condition is a more general one. We also discuss the relationships of polarization purity and nonquantum entanglement between polarization and spatial (or temporal) correlation.