Optical3D Shape Measurement Of Ultra-precision Specular Surface

Ultra-precision machining technology of specular surfaces has become one of theleading research fields in defense and high technology. Precisely measuring andevaluating the profile of the specular surfaces, studying the relationship between thesurface geometry characteristics and the usability have important significance inincreasing the quality of the surface processing and product performance. For3Dshape measurement of the ultra-precision specular surfaces, the contact measuringmethods have the disadvantages of low measurement speed and damage for thesurface while it is hard to finish high precision measurement by just one of the opticalnon-contact methods when the parts are large or have high curvature. To solve theseproblems, this paper proposed a novel method which combines the phasedeflectormetry technology and the white light scanning interferometry technology.The main work is as follows:1. A novel method that fuses the phase deflectormetry technology and the whitelight scanning interferometry technology for the measurement of ultra-precisionspecular surfaces was proposed. The whole profile of the measured parts could beobtained by the application of the phase deflectormetry technology with measurementaccuracy50μm~100μm. Then path planning was performed to guide the white-lightinterferometry detector finishing the high precision scanning measurement of the localregion of the measured parts. The measurement range of the white-lightinterferometry system is847μm×710μm×100μm while the lateral and verticalresolution are1.047μm and0.126nm respectively.2. Based on thorough research on the fringe coded scheme and the phaseextraction algorithm, a fast phase retrival algorithm by combining the compositefringe pattern and perturbation phase unwrapping method was proposed. Byprojecting only one composite fringe pattern onto the surface of the measured parts,the unwrapping phase distribution could be retrivaled by the perturbation informationmodulated in the composite fringe pattern. The dependability and speed of the newalgorithm were much better than those of the traditional methods.3. High precision calibration of the camera intrinsic parameters was finished byuse of the movement target calibration method. The non-linear error calibration of the piezoelectric ceramic was completed by the Millitron precision gauge. The standardcheckerboard flat and gauge blocks were used to put forward the whole calibrationscheme for the measurement system of the ultra-precision specular surfaces.4. A novel method of virtual reference surface was proposed. A special codedpattern was displayed on the LCD screen at the stage of system calibration and thevirtual image plane appeared on the other side of the mirror was defined as the virtualreference surface. The reference phase distribution can be calculated directly throughthe corresponding relationship between the points on the virtual reference surface andthe phase distribution on the encoded fringe patterns. Thus the procedures of fringeprojection and phase unwrapping on the real reference surface can be avoided beforethe measurement of the specular parts. With this method the three dimensional shapeof the specular surfaces could be obtained fast and precisely. The detection efficiencyhas been improved and the measurement complexity can be reduced.5. The3D shape measurement model of specular surfaces based on phasedeflectormetry method was proposed. The corresponding relationship between thephase deflectormetry information and the slope and height of the measured parts wasestablished based on iterative strategy. The3D profile of the measured parts could berecovered by the slope information. A3D shape reconstruction method combined thepath-integration and the zonal wave-front reconstruction algorithm was presented.The initial profile of the measured parts was reconstructed by the path-integrationmethod, and then the zonal wave-front reconstruction algorithm was performed tooptimize the profile. Contour extraction method based on phase shift characteristics ofthe encoded grating stripe was proposed, the whole process of reconstruction is onlyconducted in the extraction area.6. The path planning scheme was presented to guide the high precison scanningmeasurement of the local area of the measured parts by use of the white lightmicroscopic interference lens. The theoretical derivation and algorithmimplementation for the peak extraction of the white light interferometry patterns werecarried out. The system errors and noise influenced the measurement accuracy wereanalysized. The3D shape reconstruction of a step with the height variation0.5μm,10μm,40μm was completed.7. The3D shape measurement of steps, an ultra precision manufactured flatmirror and an optical hypoid surface were measured based on phase deflectormetrysub-system. The measurement accuracy can reach approximately50μm which is in the measurement range of the white light scanning interferometry system. Themeasurement results of the phase deflectormetry sub-system lay a foundation for thehigh accuracy scanning measurement of the local area of the parts by use of the whitelight interferometry lens.