wavefronts reflected from reference surface 141 in some embodiments auxiliary reference mirror 150 is oriented so that the carrier fringes have a period corresponding to about three or more detector pixels at detector 160 e g about five or more about eight or more about 10 or more about 20 or more detector pixels auxiliary reference mirror 150 can be mounted on an adjustable mount to facilitate easy adjustment of the auxiliary mirror 39 s orientation determining a surface profile of test surface 191 using interferometry system 100 involves a two step process one interferometer measurement without the test object 191 and one measurement with the test object for each measurement electronic controller 180 determines an interference phase u03b8 and a signal modulation m for the image data at each detector pixel one way to extract phase information from each measurement is to perform a spatial fourier transform of the interference pattern and subsequently identify the carrier fringe frequency with a digital filter in the frequency domain an inverse transform of the filtered spectrum provides the phase u03b8 and signal modulation m information at each pixel phase extraction methods using fourier transforms are described for example by w macy in u201c two dimensional fringe pattern analysis u201d appl opt 22 pp 3898 3901 1983 the entire contents.