ABSTRACT
Test surfaces used for the measurement are symmetrical with respect to an optical axis z. A zoneplate is a circular grating such as a Newton ring, as shown in Figure 30.1. The zone-plate must be arranged perpendicularly with respect to the optical axis. Figure 30.2 shows that illumination beams
are transmitted and diffracted through the zone-plate [5]. The beams transmitted toward the test surface focus at the center 0 of the test surface. Part of the beams diffracted to many directions is used for the zone-plate interferometer. The direction of the beams used for the interferometer is perpendicular to the test surface. The measurement of an arbitrary point C on the test surface is performed as follows. Points C and D are symmetrical with respect to the optical axis. Points A and B on the zone-plate are symmetrical with respect to the optical axis. The arrow in the solid line passes through the zone-plate at A, refl ects at the center 0 of the test surface, diffracts at B to the direction E. We denote the diffracted diverging light beam and the diffracted converging light beam of the fi rst order as +1 and −1, respectively. The light beam of (0, +1), of which a typical path is indicated as A-O-B-E in Figure 30.2, generates a wavefront, does not involve the shape error, and acts as a reference beam.
