Multiple-beam localized fringes: part II. - Conditions of observation and formation of ghosts

Factors which tend to reduce the perfection of the fringes exist in the optical systems associated with a Fizeau multiple-beam interferometer and in its complexity of surface detail. A parallel is drawn between the mechanism of image formation of these fringes and the theory of microscope image-formation due to Abbe. Experiments are described showing the effect of the presence of diaphragms in the second focal plane of the objective and also in contact with the objective The conclusion reached experimentally is that if the objective has insufficient aperture the fringes broaden and secondary maxima appear. At the same time, the resultant of a non-infinite series of geometrically decreasing waves is studied. The results obtained are also valid for the Lummer-plate interferometer. Detailed attention is given to the parasitic reflected light from the many glass-air surfaces of the optical train used, and the resulting ghosts are described. It is shown that the interferometer gap must be of only a few wave-lengths if the localized fringes are to follow the contour lines correctly over the smallest surface details.