Abstract: A two-dimensional array of phase values is calculated from digitized phase-shifted shadow moire fringe patterns that are indicative of the flatness of the surface of an object. The light intensity "I.sub.1 " of a pixel in a first of such shadow moire fringe patterns is approximated by I.sub.0 +Acos.o slashed., the light intensity "I.sub.2 " of the pixel in a second of such shadow moire fringe patterns is approximated by I.sub.0 +Acos[.o slashed.+(.pi./2)], the light intensity "I.sub.3 " of the pixel in a third of such shadow moire fringe patterns is approximated by I.sub.0 +Acos[.o slashed.+.pi.], and the light intensity "I.sub.4 " of the pixel in a fourth of such shadow moire fringe patterns is approximated by Acos[.o slashed.+(3.pi.)/2)]. The phase values ".o slashed." are equal to arctan((I.sub.2 -I.sub.4)/(I.sub.1 -I.sub.3), and a first two-dimensional array of phase values are calculated.