 In coherence scanning interferometry , interference is only achieved when the path length delays of the interferometer are matched within the coherence time of the light source . CSI monitors the fringe contrast rather than the phase of the fringes . Fig . 17 illustrates a CSI microscope using a Mirau interferometer in the objective ; other forms of interferometer used with white light include the Michelson interferometer ( for low magnification objectives , where the reference mirror in a Mirau objective would interrupt too much of the aperture ) and the Linnik interferometer ( for high magnification objectives with limited working distance ) . The sample ( or alternatively , the objective ) is moved vertically over the full height range of the sample , and the position of maximum fringe contrast is found for each pixel . The chief benefit of coherence scanning interferometry is that systems can be designed that do not suffer from the 2 pi ambiguity of coherent interferometry , and as seen in Fig . 18 , which scans a 180μm x 140μm x 10μm volume , it is well suited to profiling steps and rough surfaces . The axial resolution of the system is determined in part by the coherence length of the light source . Industrial applications include in @-@ process surface metrology , roughness measurement , 3D surface metrology in hard @-@ to @-@ reach spaces and in hostile environments , profilometry of surfaces with high aspect ratio features ( grooves , channels , holes ) , and film thickness measurement ( semi @-@ conductor and optical industries , etc . ) . 
