PIXEL-BASED ABSOLUTE TOPOGRAPHY TEST FOR 3 FLATS

We demonstrate a method of performing the absolute three-flat test by using reflection symmetries of the surfaces and an algorithm for gener ating the rotation of arrays of pixel data. Most of the operations inv olve left/right and top/bottom flips of data arrays, operations that a re very fast on most frame grabbers and are available on most commerci al phase-measuring interferometers. We demonstrate the method with sim ulated data as well as with actual data from 150-mm-diameter surfaces that are flat to less than 25 nm peak to valley. (C) 1998 Optical Soci ety of America.