Method and apparatus for measuring the shape and thickness variation of polished opaque plates

An apparatus for measuring the thickness variation and shape of polished opaque plates including in combination:

• first and second spaced apart reference flats forming a cavity therebetween for placement of a polished opaque plate wherein the first and second reference flats have dimensions such that an oversized field of view is provided for the opaque plate;
• first and second interferometer devices located on diametrically opposite sides of the cavity to map the opposite first and second surfaces of the plate;
• a light source;
• means for synchronizing the phase shifting of interferograms in the two interferometer devices;
• first and second interferogram detectors;
• and
• at least one computer coupled to receive the outputs of the first and second interferogram detectors for determining thickness variations of the plate.