Pengcheng Yang

Quality-guided phase unwrapping algorithm based on reliability evaluation

For optical interferometry, a new quality-guided phase unwrapping algorithm based on the reliability evaluation of each pixel of the wrapped phase is proposed. First, the parameters used as quality measures in the past quality-guided algorithms are classified into the reliability measure and the quality measure, and the intensity of the object image belongs to the reliability measure. Then, by computing and applying a threshold to the intensity of the object image, the valid region (i.e., the interference region) is distinguished into the reliable region and the doubtful region. The wrapped phase in the reliable region is subsequently unwrapped by the way of multipaths integration, and different paths are guided by separate quality measures. Finally, starting from the reliable region, the doubtful region is unwrapped by the way that each path takes in the reliable region. Experimental results have shown that the proposed algorithm not only performs well, but also computes efficiently.

Object-image-based method to construct an unweighted quality map for phase extraction and phase unwrapping

A method to construct an unweighted quality map for phase extraction and phase unwrapping is proposed, based on an object image pattern. The object image pattern must be recorded under the same conditions as that of the corresponding interference patterns, except that the lights coming from the reference arm of the interferometer are hidden. An unweighted quality map that can represent the valid and invalid regions in the interference patterns is completed successfully, based on two factors: the fact that the object region in the object image pattern is homologous with the valid region (i.e., the interference region) in the interference patterns, and on distinguishing between the object and background regions in the object image pattern using neighbor window threshold filtering and fitting the boundary of the object image. The application of the proposed method to the real measurement shows its feasibility and correctness. This paper might provide an alternative method for constructing an unweighted quality map for phase extraction and phase unwrapping.

Image-inpainting and quality-guided phase unwrapping algorithm

For the wrapped phase map with regional abnormal fringes, a new phase unwrapping algorithm that combines the image-inpainting theory and the quality-guided phase unwrapping algorithm is proposed. First, by applying a threshold to the modulation map, the valid region (i.e., the interference region) is divided into the doubtful region (called the target region during the inpainting period) and the reasonable one (the source region). The wrapped phase of the doubtful region is thought to be unreliable, and the data are abandoned temporarily. Using the region-filling image-inpainting method, the blank target region is filled with new data, while nothing is changed in the source region. A new wrapped phase map is generated, and then it is unwrapped with the quality-guided phase unwrapping algorithm. Finally, a postprocessing operation is proposed for the final result. Experimental results have shown that the performance of the proposed algorithm is effective.

Improvement of the oblique-incidence optical interferometric system to measure tooth flanks of involute helical gears.

We put forward a plan of improving the oblique-incidence optical interferometric system applied in the measurement of tooth flanks of an involute spur gear in order to expand its capability to measure an involute helical gear. On the basis of the features of an involute helical tooth flank, we discuss how to realize the parallelism between the optical axis of the object arm of the optical system and the straight lines constructing the involute helical tooth flank. This parallelism helps the optical system produce an interference fringe pattern as clear as the one of an involute spur gear [Appl. Opt.49, 6409 (2010).]. A numerical simulation is then performed to examine the correctness of the improvement. During simulating, we unify the equation of difference tooth flanks by means of importing two parameters in relation to the left or right side of a tooth flank and the helical direction of teeth, respectively. Finally, the actual experiment is fulfilled through the real optical system built on an optical table. The simulation and experiment results verify the correctness and feasibility of the proposed improvement.

Correction method for segmenting valid measuring region of interference fringe patterns

Segmenting the valid measuring region from an interferogram is an important step in the grazing incidence interferometry, which is related to the accuracy of the measurement result. For the spurious fringes which are generated by the diffuse reflection of the chamfering in the edge of the object surface, we propose a method-spin segment to correct the result of the image segment. First, a phase schematic diagram whose stripe feature is clearer than the original interference fringe patterns is generated. Next, the difference of the gray gradient between the fringes local normal direction and local tangent direction is calculated to judge the valid measurement region. The principles of selecting some parameters are summarized and the calculation steps are shown. To examine the correction method, the results of an experimental application of spin segment are given to verify the feasibility and accuracy.

Construction of virtual measurement datum in measuring gear tooth flank by laser interferometry

Abstract. A measurement datum is crucial in measuring a gear tooth flank by laser interferometry. Manufacturing a physical reference tooth flank as a measurement datum has many drawbacks: many different reference tooth flanks are needed with the changing of the measured gear’s parameters; the reference tooth flank’s manufacturing error lowers the measurement precision; the installation error between the measured and referenced tooth flank lowers the measurement precision also. The drawbacks restrain the practical use of measuring gear by laser interferometry. A construction method of a virtual measurement datum has been proposed to solve the problem. First, a virtual error-free model of the measured tooth flank is built according to its parameters and actual measurement position. Subsequently, the interferogram of the virtual tooth flank is simulated as a measurement datum through the ray tracing. To harvest the actual measurement position of the measured tooth flank, a compensation approach for optical system error and a tracing algorithm for actual installation position are developed. Experimental results prove that a virtual measurement datum can be constructed and used in measuring a gear tooth flank. Furthermore, this research verifies that ray tracing can be used to construct a virtual measurement datum in the relevant comparative measurements processes by laser interferometry.

Analysis and compensation method for installation error in measuring gear tooth flank with laser interferometry

The installation position errors of a measured gear are unavoidable when measuring gear tooth flank form deviation with laser interferometry, causing mistakes when using the simulated phase difference as a measurement reference to calculate the form deviation. In order to avoid manufacturing many standard gears as measurement references, a compensation method for the installation errors of the measured gear has been developed. In this process, the actual installation position of the measured gear can be traced by modifying the theoretical position parameters using the ray-tracing method. Experimental results show that this method is feasible. Thus, the simulated phase difference that compensate for installation errors can be utilized as a measurement reference in calculating the form deviation of the measured gear.

Autofocusing algorithm of interferogram based on object image and registration technology

Because the depth of field (DOF) of lenses is limited, a captured interferogram usually includes a focused region and defocused region simultaneously when the form deviation of the helical tooth flank is measured with laser interferometry. Because of the impact of the interference fringe, the existing autofocusing algorithms are difficult for identifying the focused region of the interferogram directly. This paper proposes a new autofocusing algorithm based on the object image and registration technology. First, the reliability region is evaluated according to the gray level. Then the corresponding pixels in the image sequence are determined with the registration technology. Next, the focused region is judged by comparing the average gray gradients in object images. Based on the characteristics of measured gear and the DOF of lens, an experiment is designed. The experimental results are given to verify the feasibility and accuracy of the proposed autofocusing algorithm.