To test a flat surface, mirror M₂ is replaced by the flat surface and the alignment is done to obtain a fringe pattern as shown in (Fig. 11.4.3(b.1)), the flatness of the surface is given by . If the surface is irregular, the fringe shape is also irregular as these fringes are loci of constant thickness.

A beam splitter is to be tested for the angle of it as well as to check if it introduces any deformation of wave front due to surface imperfection or refractive index inhomogeneities, First the interferometer is set for infinite fringe setting (ie when both the plane waves propagate along the same direction). Now the beam splitter is introduced in front of a mirror, normally. If there is no distortions of wave front due to beam splitter straight line fringes will be observed with fringe width given by where is angle of the wedge (Fig.11.4.3(b)).

A prism is to be tested for the collimation. Any incident collimated beam should emerge out as collimated to test the prism. The testing configuration should be such that it retro reflect the beam to compare it with the reference beam. The experimental arrangement is shown in (Fig.11.4.3(b.3)) The wave front distortions could be due to surface imperfections of both the surfaces (entrance and exit surfaces) and refraction index inhomogeneities. A good prism should provide straight line fringes.

A lens is to be tested for aberrations. It means that a collimated beam incident on lens will not focus on one point but will have finite distribution. Also a point source at the focus of lens will not give a collimated beam. To check for the chromatic aberrations, the test lens is placed such that beam is retro reflected. The plane mirror in interferometer is replaced by a convex mirror and experimental arrangement is shown in the (Fig.11.4.3(b.4)). The interferogram is analysed to obtain the aberration coefficients.