Investigation On White-light Scanning Interferometry And Its Applications In Accurate Measurement Of Surface Profile

The white-light scanning interferometry is a surface profile scanning technology which has been developed fast in recent years.It has many advantages,such as non-contact,high accuracy,high speed and large range.With the increasing maturity of white-light interferometry,most researches concentrate on increasing the accuracy.This paper presents simulations and experiments on three kinds of errors in white-light interferometry.A new dispersion mode and a new method to eliminate dispersion,which had a great innovation,are proposed.There are three kinds of measurement values,which contains amplitude peak(za),zero phase position(zp)and the slope of phase in wavenumber domain(zs).Characteristics of those measurement values are analyzed.The measurement accuracy is improved.The main contents are as follows:(1)We investigated the influence of the piezoelectric ceramics motion error on the accuracy.It is found that the motion error not only deviates the measurement result from the accurate position,but also brings some distortion to the interference signal.In order to analyze the influence of the piezoelectric ceramics motion error,it is necessary to add the different motion curve to the phase distribution of the white-light interference signal.As the result shows,the amplitude of interference signal will have a flat peak if the motion error larger than the range we get.For elimination of the error,the linear fitting method is used on the phase distribution to decrease the non-linear component.It helps to raise the accuracy of measurement.(2)The influence of the dispersion effect on the measurement results is studied,and a new method is proposed to improve the detection accuracy.In the experiment,owning to the wide spectrum of white-light source,the interference signal is easy to be affected by dispersion.According to the research,the dispersion phase caused by the two unequal sides of beam-splitter will decrease the accuracy of measurement.Therefore,a new method was proposed.A spectral resolved interferometry is used to detect the dispersion phase.Then,it is simple to subtract the dispersion from the phase distribution of white-light interference signal.Compared with the original interference signal,the processed signal has a shorter FWHM(Full Wave at Half Maximum)and the amplitude peak position is closer to the corresponding position on which the phase is equal to zero.The change of amplitude and phase prove that the new method to eliminate the dispersion is useful to increase the accuracy.(3)Influence of the random noise in the interference signal and the filter range on the measurement results are studied.In the simulation,the White-light interferometry is sensitive to mechanical vibration which will bring the random noise to the interference signal.In order to analyze the influence caused by random noise,a processed Gaussian noise distribution is used to simulate as the random noise.The phase distribution of the interference signal in the wavenumber domain is represented by a polynomial superposition expression.To realize the function of phase deeply,the coefficients of polynomial is analyzed.The measurement accuracy will change when the different ranges of wavenumber are chosen.The sensitivity of each measurement values to the filter range is analyzed,and the details of the signal processing are summarized.This simulation summarizes the details of the white light interference signal processing,which is significant to the white-light interferometry.(4)According to the spectroscopic structure,a new dispersion model is established.A new method to eliminate the dispersion is used to process the scanning signal of surface profile.To verify the validity of the new method,we analyze the difference between processed signal and original signal.In the experiment,the dispersion phase equation is obtained by the spectral resolved interferometry.The dispersion phase is eliminated from the white-light interference signal by this equation.The surface profile is gained by three measurement values.The measurement value zp is better after the comparison of the roughness and repeatability of all the results and will be selected as the basis of the measurement.