Key Engineering Materials Vols. 381-382

Abstract: A wide-range laser auto-collimation method to measure the surface profiles of targets with steep slopes has been proposed. This method employs a deflecting optical system to enlarge the measurement range of the angle sensor. However, this optical system also changes the sensitivity property of the angle sensor. Therefore, we propose a new in-situ self-calibration method that includes the geometrical designed data of the target and can be applied to wide-range laser auto-collimation method. In the proposed method, the calibration sensitivities at each measurement point are obtained with two sets of measured data, which are acquired before and after performing minute rotation or translation with respect to the measured target, and the simulated shift of the normal angle at each point. The present paper describes the calibration procedure and specific calculation method used in the proposed method.

Abstract: The phase-shifting method is widely used in 3-D profile measurement with optoelectronic technology and phase unwrapping is an important link where in. The traditional methods are too complex and have low efficiency. A novel method of evaluating absolute phase is presented in this paper. The technique combined the grey code fringe with phase shifting technique and colored encoded grating pattern. The order of the projecting fringes ) , ( y x n can be obtained from only one distortional colored image. The procedure is relatively simple in experimental setup and algorithm. In addition, the speed of measurement is faster than current phase unwrapping algorithm. The experimental results have proved the validity of this method.

Abstract: In this article, a new interferometer which accuracy does not depend on the accuracy of the reference is proposed. This interferometer calculates the surface profile of the measured mirror surface using two steps. At the first step, the interferometer obtains the data concerning the reference and a polarized beam splitter (PBS). The interferometer works as a Mach-Zehnder interferometer. At the second step, since the polarized beam splitter is rotated 90 degree, the measured mirror is lightened by a laser beam. In this step, the interferometer obtains the data concerning the reference, the PBS and the measured mirror. Therefore, the difference between these two results corresponds to the surface profile of the measured mirror and it does not include the data of the reference and the PBS. The principle and the experimental results were described.

Abstract: The precision positioning device becomes significant requirement in scientific instruments used for the applications of nanotechnology for a few decades. To achieve the high resolution positioning, the sensing methods of displacement sensor become more important in positioning device. In this paper, a novel method is presented for measuring both in-plane/out-plane displacements with the single heterodyne grating interferometry (HGI). We demonstrated the 1D, 2D, and 3D measurement results respectively and the smallest displacement can be detected was better than 6 pm. Furthermore, the nanometer resolution can be ensured within 20 µm displacement. Hence, the in-plane/out-plane measurements with single apparatus can be realized by our method and might be a displacement sensor using in the motorized stage with suitable opto-mechanics structure minimization.

Abstract: The implementation of the basic physical principle of Chromatic Confocal Microscopy in the field of Phase stepping interferometry (PSI) opens new opportunities for the development of an innovative surface metrology method specially dedicated to 3D nanotopography with subnanometric z axis resolution altogether with a very large measuring range: typically up to one hundred micrometers. The basic property of optical sectioning inherent to (chromatic) Confocal imaging is particularly well adapted to Phase stepping Interferometry since it automatically solves the critical and time consuming problem of phase unwrapping computation. The axial chromatic extension of the chromatic confocal setup offers a very fast and easy way to determine the height of the different elementary surfaces forming the measured object. It is then easy to carry out, for each one of those elementary surfaces, a measurement in phase shifting interferometry, at the wavelength corresponding to the altitude indicated by the confocal chromatic, in order to reach subnanometric axial resolutions. The four phases needed for implementing the phase stepping interferometric measuring procedure can be successively realized by adequate spectral shifts instead of the classical axial displacements of the reference mirror which then stands in a fixed position. Consequently this chromatic confocal phase stepping interferometer (CCPSI) has definitely no moving part, the spectral shifts being done by electrooptical means. Typical applications are MEMS and microoptics surface topography and/or roughness metrology. For this purpose we designed a new system incorporating confocal chromatic imaging and phase stepping interferometry. As a direct consequence of the optical sectioning property, this system allows measuring through any type of optical window (for example a cover glass).

Abstract: As an alternative for producing the meter-size diffraction grating demanded in many technical field, grating mosaic requires positional detection with accuracy of sub-microns. In this paper, we propose a positional detection method based on image processing of three far-field diffraction intensity patterns in two wavelengths. With a set of detailed adjustment steps deduced from theoretical analysis, we successfully detected and separated the two positional errors. Moreover, with the three patterns we enlarged the target range of coarse adjustment required for further fine adjustment in longitudinal position. We achieved positional detection sensitivity of less than 14 nm, and diagnosed the alignment with the far-field pattern in a third wavelength.

Abstract: In this study, registration methods used to estimate both position and orientation differences between two images had been evaluated. This is an important issue since that there are always some position and orientation differences when loading test samples on the inspection machine. These differences should be calculated and compensated before further analysis. Registration methods tested including one area method and three feature based method. It was shown that the area method had better performance than other feature based method in these cases studied. And it is shown that it is much easy to detect defect by analyzing the subtracted image with position and orientation compensation instead of those without compensation.

Abstract: Measuring grating profiles is very helpful for the analysis of specifications of gratings and improvement of grating fabrication techniques. We analyzed grating grooves by digitizing the scanning-electron microscope (SEM) images. Some kinds of filter and arithmetic were developed to extract the contour line of grating profile. In order to analyze the diffraction efficiency affected by the shape of grating profile, the calculated diffraction efficiency based on the SEM image and measured diffraction efficiency based on experiment was compared and analyzed.

Abstract: The dynamic behavior of a ball-beam system is highly nonlinear and its characteristic is difficult to define. In this paper we present a new ball-beam balancing control system using machine vision to feedback the beam angle and ball position on the beam. Adaptive threshold based continuously mean shift vision tracking algorithm is applied to record the ball position and the beam angle with highly captured frame-rate. The proposed vision tracking algorithm is tolerant to lighting influence, highly computing efficiency and more robust than traditional template pattern matching or edge detection algorithm under non-ideal environment. The vision tracking performance is experimentally tested on a ball-beam benchmark system, where a PD controller is applied to control the motion of the ball to maintain balance. Experimental result shows that the beam angle measurement, ball tracking and balancing control of the vision feedback system are robust, accurate and highly efficient.

Abstract: Many vision tasks such as 3D measurement, scene reconstruction, object recognition, etc., rely on feature correspondence among images. This paper presents a point matching method for 3D surface measurement. The procedure of the method is as follows: (1) rectification for stereo image pairs; (2) computation of epipolar lines; (3) sequential matching in vertical direction; (4) sequential matching in horizontal direction. The fourth step is performed to deal with the ambiguity in dense areas where points have closer vertical coordinates. In the fourth step a threshold limit of vertical coordinate difference is designed to determine those points potential to cause ambiguity. This method was applied to the 3D surface measurement for an inflatable parabolic reflector with validity of point matching up to 100%. Experiment results show that this method is feasible in application of sparse point matching for continuous surface measurements.