a method and apparatus for measuring an absolute profile of a flat using an interferometer system that includes an interferometer adapted to support two flats , a detection system , and a computer adapted to compute the opd between surface of the two flats , wherein a first flat a ! having a first surface and a second flat b ! having a second surface are supported in the interferometer , with the second surface facing the first surface . the interferometer system measures the opds between the first and second surfaces for each pixel . the first flat a ! then is rotated by a number of predetermined angles relative to its initial position and each time the opds are measured . the first flat a ! is rotated to its initial position or 180 ° therefrom . a third flat c ! having a third surface is substituted for the second flat . the opds between the first and third surfaces are measured . the first flat a ! is replaced by the second flat , with the second surface facing the third in an orientation mirror imaged to its original orientation . the interferometer system is operated to measure the opds . the computer solves first , second , and third equations to obtain the entire surface topographies of the first , second , and third surface , wherein each equation is expressed as a sum of even -- even , even - odd , odd - even , and odd -- odd parts so as to effectuate cancellation of terms , permitting solving of the equations for the surface topography .