 Phase shifting interferometry overcomes these limitations by not relying on finding fringe centers , but rather by collecting intensity data from every point of the CCD image sensor . As seen in Fig . 17 , multiple interferograms ( at least three ) are analyzed with the reference optical surface shifted by a precise fraction of a wavelength between each exposure using a piezoelectric transducer ( PZT ) . Alternatively , precise phase shifts can be introduced by modulating the laser frequency . The captured images are processed by a computer to calculate the optical wavefront errors . The precision and reproducibility of PSI is far greater than possible in static interferogram analysis , with measurement repeatabilities of a hundredth of a wavelength being routine . Phase shifting technology has been adapted to a variety of interferometer types such as Twyman @-@ Green , Mach – Zehnder , laser Fizeau , and even common path configurations such as point diffraction and lateral shearing interferometers . More generally , phase shifting techniques can be adapted to almost any system that uses fringes for measurement , such as holographic and speckle interferometry . 
