In order to examine the utility of the double diffraction interferometer (refer to Trans. SICE, Vol.8, No.2, 125/131, 1972), the wave aberrations arising from the longitudinal shift of the focus and/or from the third order spherical aberration at the zonular spatial frequency filter ware studied. The effect of these aberrations on the amplitude and phase of the reference light in the image plane were numerically obtained. The numerical results brought practically meaningful information, e.g.;
(1) For a relatively large area to be apportioned to the object in a particular field of view and for high contrast of interferential fringes, the product of the propagation constant, the semiapex angle of the condensing flux, and the width of the filter should be from 3 to 8 (for zero spherical aberration) or slightly larger (for non-zero spherical aberration).
(2) When the width and hight of an object were respectively a fifth and a half of the diameter of the field, spherical wave aberration is allowed up to about four wave lengths at the edge of the condenser lens.
(3) By the spherical aberration, the optimum position of the filter is slightly shifted from the Gaussian plane toward the circle of minimum confusion.
It was found also that the phase of the reference light can be easily and accurately adjusted by longitudinal displacement of the filter. This seemes to be an important characteristic of the double diffraction interferometer.
Experimental work was carried out for verification of the above.