Abstract
Chromatic confocal microscopy is a widely used method to measure the thickness of transparent specimens. In conventional configurations, both the illumination and imaging axes are perpendicular to the test specimen. The reflection will be very weak when measuring high-transparency specimens. In order to overcome this limitation, a special chromatic confocal measuring system was developed based on inclined illumination. This design was able to significantly improve the signal-to-noise ratio. Compared with conventional designs, the proposed system was also featured by its biaxial optical scheme, instead of a coaxial one. This biaxial design improved the flexibility of the system and also increased the energy efficiency by avoiding light beam splitting. Based on this design, a prototype was built by the authors’ team. In this paper, the theoretical model of this specially designed chromatic confocal system is analyzed, and the calculating formula for the thickness of transparent specimen is provided accordingly. In order to verify its measurement performance, two experimental methodology and results are presented. The experimental results show that the repeatability is better than 0.54 μm, and the axial measurement accuracy of the system could reach the micron level.
Highlights
Chromatic confocal measurement technology is an optical approach, which is widely used in surface 3D reconstruction [1,2], defect detection [3,4], roughness quantification [5,6], displacement sensing [7,8], and thickness measurement [9]
Chromatic confocal measurement technology evolved from traditional laser confocal measurement technology [10]
Instead of physically scanning along the optical axis and detecting the focal point, the chromatic confocal measurement system measures the surface topography using a series of dispersed light components with different wavelengths
Summary
Chromatic confocal measurement technology is an optical approach, which is widely used in surface 3D reconstruction [1,2], defect detection [3,4], roughness quantification [5,6], displacement sensing [7,8], and thickness measurement [9]. Instead of physically scanning along the optical axis and detecting the focal point, the chromatic confocal measurement system measures the surface topography using a series of dispersed light components with different wavelengths. Hillenbrand M et al [14] presented a novel mathematical function, called the intensity point-spread function, to reshape the characteristic curve of the chromatic confocal system Using this method, the measurement repeatability and computation efficiency could be improved. The results demonstrated that the correction significantly improved the robustness and accuracy against different light sources and specimen surfaces in chromatic confocal displacement measurement. The application of a beam splitter makes the receiver have only 1/4 of the energy of the light source To solve these limitations, a novel chromatic confocal system with inclined illumination to measure the thickness of transparent specimen was developed by the authors’ team and will be presented in this paper. Tnihnegp. olyThe scchhroemmaattiicc dligiahgtrbameamofecmhriottmedatfircocmonthfoecwalhtietcehlnigohlot gsyouisrcsehpoawsnseisnthFrigouurgeh1a.nThilelupmoilnya- tion chropminathicolleig(hptinbheaomle 1eminitFteigdufrreom1),twhehwichhimteolidguhltastoesurthcee pliagshstesbethamrouingthoaanpiolliunmt liingahttio. nThen, pinhtohlee (lpigihnthobleea1minwFitighudreiff1e)r,ewnthiwchavmeloednugltahtsesisthdeislitgrihbtubteeadmalionntog athpeoionpttliicgahl ta. xTihsebny, the the lidgihspt ebresaiomnwtuitbhe dleinffse.reTnhtewdaisvpeelersnegdthlisgihstds iasrteritbhuetnedfoacluosnegdtahtetohpetdiciafflearxenistbayxitahlepdoissi-tions persiboyntthuebeoblejencst.ivTeh.eBdeiisnpgerrseefldelcitgehdtsbayrethtehesnpefcoicmuesendsautrftahceed, iofnfelryenthteaxliigahl tpobseiatmionfsrobmy the the ofbojceacltipvoes.iBtieoinngcarnefpleacstsedthbroyutghhe tshpeecdiemteecntisounrpfaicneh,oolnel(yptinhheolilgeh2tibneFaimgufreom1) athnedfaorcrailve at positihoendceatnecptoarsswtihtrhoaugpheatkheindtentescityio. nBypidnehtoecletin(pginthheolwe a2vienleFnigtuhrew1it)haandpeaarkrivineteant stihtye, the detechteoirghwtiothf tahepceuarkreintemnseiatsyu. rBeymednetepcotingt ctahne bweadveetleernmgitnhedw.ith a peak intensity, the height of the current measurement point can be determined
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