 Phase Shifting Interferometry addresses several issues associated with the classical analysis of static interferograms . Classically , one measures the positions of the fringe centers . As seen in Fig . 13 , fringe deviations from straightness and equal spacing provide a measure of the aberration . Errors in determining the location of the fringe centers provide the inherent limit to precision of the classical analysis , and any intensity variations across the interferogram will also introduce error . There is a trade @-@ off between precision and number of data points : closely spaced fringes provide many data points of low precision , while widely spaced fringes provide a low number of high precision data points . Since fringe center data is all that one uses in the classical analysis , all of the other information that might theoretically be obtained by detailed analysis of the intensity variations in an interferogram is thrown away . Finally , with static interferograms , additional information is needed to determine the polarity of the wavefront : In Fig . 13 , one can see that the tested surface on the right deviates from flatness , but one cannot tell from this single image whether this deviation from flatness is concave or convex . Traditionally , this information would be obtained using non @-@ automated means , such as by observing the direction that the fringes move when the reference surface is pushed . 
