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Research on the Parallel Light Source in Laser Parallel Confocal Measurement System

Author: YuQing
Tutor: YuXiaoFen
School: Hefei University of Technology
Course: Measuring Technology and Instruments
Keywords: Laser parallel light source Confocal measurement Talbot effect Flexibledigital light source
CLC: TN247
Type: PhD thesis
Year: 2011
Downloads: 58
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Abstract

Laser confocal microscope is one of the important technologies to measure the3Dsurface topography of specimens in modern manufacturing and biomedical engineering. Thisdissertation was based on the project of “Research on Measuring3D Surface Topographyof Microstructure Based on the Principle of Parallel Astigmatism Confocal Detection”sponsored by the National Natural Science Foundation, and it focused on laser parallelsource to overcome the influence of Talbot effect in the laser parallel confocalmeasurement. A flexible digital parallel light source has been developed to improve thespeed and to extend the use of confocal measurement.When laser is used in parallel confocal measurement system, the resolution could beimproved, but the Talbot effect which causes lots of confocal images will appear. However,only one of those images could be called in-focus image. If the in-focus image cannot berecognized, the confocal measurement will not be realized. Many optical divide devices(ODDs), like pinhole array and microlens array, can divide a light beam into many finebeams, but those ODDs have poor adaptability to different measured object because theirparameters could not be changed, and in addition, they are difficult to manufacture, andthe prices are too high.In this dissertation, the parallel light source used in laser parallel confocalmeasurement has been studied systematically, and the main researches and innovations ofthe dissertation can be summarized as follows:(1) Established the Talbot distance model in laser parallel confocal measurementThe Talbot distance model in laser parallel confocal measurement based on FresnelOptics has been established, and the formula of Talbot distance in object space has beenderivated. The formula built the relationship among Talbot distance in object space,wavelength of laser, period of ODD, and visual magnification of telescope: zT=(dГ)2/λ.It will provide the theoretical basis to overcome the influence of Talbot effect in the laserparallel confocal measurement.(2) Developed some methods to overcome the influence of Talbot effectThe formula of Talbot distance in object space indicated that the Talbot distancecould be expanded by reducing the wavelength of laser, expanding the period of ODD, andincreasing the visual magnification of telescope. The thesis developed some projects, suchas Multi-source system, Dual-source system, and synthesis ODD based on pinhole arrayand microlens array, to suppress the impact of Talbot effect in a certain extent. One invention patent has been applied.(3) Established the spatial light modulation model of flexible parallel light sourcebased on Digital Micromirror DeviceThe reflection and diffraction models of Digital Micromirror Device (DMD) havebeen established by illuminating by coherent light, and the results indicated that the lightwave field after modulated was a complicated2D periodic light wave field, whichcontained a diffraction image of DMD itself, and real and virtual images of displayedpicture on DMD. Furthermore, the laser parallel confocal measurement would not beinfluenced by the Talbot effect when only the real image of displayed picture on DMD wasused, and a project of laser parallel confocal measurement system based on “single sourceand dual beam paths” was developed. One invention patent has been applied.(4) Constructed a laser parallel confocal measurement system based on DMDThe flexible digital light source based on DMD was studied, which could producepoint light array with different size of point and different distance between points, and alsocould produce grid light source. This flexible light source could help saving cost,developing different measurement strategies to different objects, and expanding the use ofparallel confocal measurement. One invention patent has been applied, and one utilitymodel patent has been obtained.(5) Developed a laser parallel astigmatism confocal measurement system based on theparallel light source with single source and dual beam paths, and completed relatedexperimentsThe influence on measurement from laser, ODD, CCD and other optical devices wasanalyzed systematically, and a laser parallel astigmatism confocal measurement systembased on the parallel light source with single source and dual beam paths was constructed.A method of image processing to inhibit drifting of measured spots, which is caused by theerror of system construction, was investigated, and this method could help improve theprecision and reliability of data processing. The experiments results indicated that theposition of in-focus image could be distinguished with the parallel light source of DMD,and high precision measurement could be completed with the parallel light source ofmicrolens array.

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