 One of the most common industrial applications of optical interferometry is as a versatile measurement tool for the high precision examination of surface topography . Popular interferometric measurement techniques include Phase Shifting Interferometry ( PSI ) , and Vertical Scanning Interferometry ( VSI ) , also known as scanning white light interferometry ( SWLI ) or by the ISO term Coherence Scanning Interferometry ( CSI ) , CSI exploits coherence to extend the range of capabilities for interference microscopy . These techniques are widely used in micro @-@ electronic and micro @-@ optic fabrication . PSI uses monochromatic light and provides very precise measurements ; however it is only usable for surfaces that are very smooth . CSI often uses white light and high numerical apertures , and rather than looking at the phase of the fringes , as does PSI , looks for best position of maximum fringe contrast or some other feature of the overall fringe pattern . In its simplest form , CSI provides less precise measurements than PSI but can be used on rough surfaces . Some configurations of CSI , variously known as and Enhanced VSI ( EVSI ) , high @-@ resolution SWLI or Frequency Domain Analysis ( FDA ) , use coherence effects in combination with interference phase to enhance precision . 
