Dodd J G
Appl Opt. 1977 Feb 1;16(2):470-2. doi: 10.1364/AO.16.000470.
It is demonstrated theoretically that if the stop in a schlieren system is no larger than the central maximum of the empty field Airy disk at the stop location, the system will behave as an interferometer for objects much smaller than the empty field. Interferograms produced by such a system are shown.