Abstract
The utility of the double diffraction interferometer having an extended light source was examined theoretically and experimentally. In the theoretical development on the basis of the scalar diffraction theory, the source was assumed to be a set of point sources and the followings were shown.
(1) It is possible to construct a practical interferometer using an extended source if a zonular spatial frequency filter of an adequate width (refer to Trans. SICE 11-2, 160/167 (1975)) is used and if the source is so thin that the width of its geometrical image on the filter plane is not much larger than that of the filter.
(2) Spatial coherency of the source has no effect on the phase of interferential fringes. It has so little effect on the visibility of the fringes that it has no substantial meaning in the practical use of the interferometer.
Experiments were carried out by using a variable slit illuminated by a sodium lamp or an ultra-high pressure mercury lamp. Experimental results supported theoretical ones. It was shown also that even when the main beam was partially affected by the filter after passing through the object, the practical utility of the interferometer was not lost although the maximum allowable width of the slit was reduced.
