The fringe patterns of a measured mirror are captured by Twyman-Green interferometer

the sampling height of the mirror surface is obtained by image processing of fringe patterns

Zernike polynomial is adopted to fit the function of the mirror curved surface and get the 3D measurement of the mirror surface height. The distances between two adjacent fringes to reference mirror space is a standard light source wavelength apart

so the key of this measurement method is to locate the wave crest or wave hollow of fringe pattern accurately

thus the task of sampling height of mirror surface is completed. A Butterworth low-pass filter is used to remove the noise of fringe image

then the bright fringe is segmented by binary conversion and is thinned into its centerline by morphologic method. The thinning result indicates that low-pass filter could reduce the amount of branches and breaks of center lines. Finally

giving the relevant height of every center lines and using the pre-39 items of Zernike polynomial to fit the mirror surface height by least square solution

the 3D data of sample surface of mirror is calcalated. This method can be use to measure the laser-induced deformation of mirror.