The inspection of surface defects is one of significant sections of optical surface quality evaluation. Based on microscopic scattering dark-field imaging, sub-aperture scanning and stitching, the Surface Defects Evaluating System (SDES) can acquire full-aperture image of defects on optical elements surface and then extract geometric size and position information of defects with image processing such as feature recognization. However, optical distortion existing in the SDES badly affects the inspection precision of surface defects. In this paper, a distortion correction algorithm based on standard lattice pattern is proposed. Feature extraction, polynomial fitting and bilinear interpolation techniques in combination with adjacent sub-aperture stitching are employed to correct the optical distortion of the SDES automatically in high accuracy. Subsequently, in order to digitally evaluate surface defects with American standard by using American military standards MIL-PRF-13830B to judge the surface defects information obtained from the SDES, an American standard-based digital evaluation algorithm is proposed, which mainly includes a judgment method of surface defects concentration. The judgment method establishes weight region for each defect and adopts the method of overlap of weight region to calculate defects concentration. This algorithm takes full advantage of convenience of matrix operations and has merits of low complexity and fast in running, which makes itself suitable very well for highefficiency inspection of surface defects. Finally, various experiments are conducted and the correctness of these algorithms are verified. At present, these algorithms have been used in SDES.
The principle of microscopic scattering dark-field imaging is adopted in surface defects evaluation system (SDES) for large fine optics. However, since defects are of micron or submicron scale, scattering imaging cannot be described simply by geometrical imaging. In this paper, the simulation model of the electromagnetic field in defect scattering imaging is established on the basis of Finite-Difference Time-Domain (FDTD) method to study the scattering imaging properties of rectangular and triangular defects with different sizes by simulation. The criterion board with scribed lines and dots on it is used to carry out experiments scattering imaging and obtain grayscale value distributions of scattering dark-field images of scribed lines. The experiment results are in good agreement with the simulation results. Based on the above analysis, defect width extraction width is preliminary discussed. Findings in this paper could provide theoretical references for defect calibration in optical fabrication and inspection.
For the Spherical Surface Defects Evaluation System (SSDES), lens centering is essential to obtain the precise scanning trace and defect features without mismatch. Based on a combination of auto-collimating microscopy and Computer-Aided Alignment (CAA), an auto-centering system that can measure the deviation of large spherical center with respect to a reference rotation axis rapidly and accurately is established in this paper. The auto-centering system allows the closedloop feedback control of spherical center according to the different images of the crosshair reticle on CCD. Image entropy algorithm is employed to evaluate image clarity determined by the auto-focus experiment of 50μm step-length. Subsequently, an improved algorithm that can search the crosshair center automatically is proposed to make the trajectory of crosshair images and the position of rotation axis more reliable based on original circle fitting algorithm by the least square method (LSM). The comparison results indicates to show the high accuracy and efficiency of the proposed fitting method with LSM.
Digital elevation models (DEMs) of buildings in urban areas are becoming increasingly important for a large range of applications, whereas extracting building DEMs over a city is still a complicated and expensive task. In this paper, we present and evaluate two approaches for fast and handy extraction of building DEMs from ALOS PRISM stereo pairs. A Digital Surface Model (DSM) is firstly generated based on the stereo data. Approach 1 uses a digital terrain model (DTM) derived from topographic maps, and Approach 2 creates another DTM by removing buildings on the DSM through a filtering approach. Building DEMs are then acquired by subtracting the two sets of DTMs from the DSM, respectively. The results received with the two approaches are promising. A mean absolute error of 7.07 m and 7.37 m respectively is achieved. Approach 1 has better accuracy in areas with high-density buildings, while Approach 2 has better one in areas with low-density buildings. Combination of the two approaches would be a useful tool for extraction of building DEMs over a city.
Axial resolution is a key factor in optical coherence tomography (OCT). Biomedical applications will benefit from
improved resolution and quality that ultrahigh resolution OCT can provide. Existing approaches to improve axial
resolution of OCT mostly depend on new broadband light sources, which are always costly and inconvenient in
instrumentation. In this paper we adopt an alternative method to enhance the axial resolution of OCT by combining
coherence gate with optical superresolution. A three-zone phase pupil filter is designed and inserted into the sample
arm of OCT. The depth responses measured demonstrate that an improvement of more than 15% in axial resolution is
achieved in the proposed OCT system.
In this paper we introduce a method to increase the axial resolution in optical coherence tomography (OCT) by combining coherence gating with apodization through an appropriate pupil filter. In the proposed probe of our OCT system, the width of the central lobe of the axial point spread function is apodized to be within the coherence length of the light source, while its side-lobes are lying outside without contributing to coherence imaging. It comes to a conclusion that we can obtain improved resolution in OCT system without recurring to broadening the bandwidth of light source, which is nevertheless costing and inconvenient in implementation.