Quantitative defects detection has always been the one of the difficulties in optical element surface quality evaluation. In order to solve this problem, the optical element surface defects detection based on dark-field imaging system, which has been researched by our group team for nearly twenty years, has been summarized. The plane and sphere optical element surface defects detection details are introduced. Specifically, it involves plane optical element surface leveling, sphere optical element spherical center alignment, low magnification image acquisition, low magnification image stitching, feature extraction, high magnification defects detection and report output based on the form of specific standard (Such as America Military Standard MIL-PRF-13830B or China National Standard GB/T 1185-2006). Besides, a China National Standard about digitized quantitative measurement of the defect, which is proposed by our group (now is in the stage of request for public advice), is also introduced.
Surface defects inspection is a critical part in manufacturing of mobile phone cover glass. Considering the defects in the optical elements surface caused by the imperfection of manufacturing technique, the classification and the position information should be carried out for the necessary repairing process. The traditional manual inspection method is always labor-consuming and inefficient. Surface defects digital evaluation system based on machine vision draws much attention in the recent years. This paper proposed algorithms and applications for the detection task with higher efficiency and reliability comparing to the manual inspection. The detection method is based on machine vision and machine learning techniques. The images of optical elements surface are captured by line-scanning cameras, with the imaging systems of dark-field, bright-field and transmission-field. Only one image system is not enough to detect all kind of defects like scratch, bubble, crack of glass and edge chipping etc. The position information and category of defects are obtained based on image processing technique. The defective area was calculated by image filtering algorithm, the feature selection techniques based on segmentation methods are explored and the feature vector can be extracted before the next step of classification with Support Vector Machine (SVM) technique. Verified by the experiments, the results reveal this method has good performance and is very suitable for recognition and classification of glass defects.
Aimed at the problem of strong background interference introduced in digital image processing from complex surfaces under industrial defect detection, a method for complex surface defect detection based on human visual characteristics and feature extracting is proposed. Inspired by the visual attention mechanism, defect areas can be identified from the background noise conveniently by human eyes. We introduce the improved grayscale adjustment and frequency-tuned saliency algorithm combined with the salient region mask obtained by dilation and differential operation to eliminate the background noise and extract defect areas. Meanwhile the directional feature matching and merging algorithm is applied to enhance directional features and retain details of defects. Testing images are captured by our established detecting system. Experimental results show that our method can retain defect information completely and achieve considerable extracting efficiency and detecting accuracy.
In inertial confinement fusion system, the intermittent scratches on the polished surface of single-sided polished and bottom surface frosted optical components are complex, and it’s of great difficulty to extract them completely. In order to solve this problem, established in the light-field surface detection system, this paper brings forward a novel intermittent scratch detection method based on adaptive sector scanning algorithm (ASC) cascading mean variance threshold algorithm (MVTH). In the preprocessing step, dividing the original image into subimages with a number of integer multiple of cpu cores so as to fully compress image processing time utilizing parallel processing, using mean filter to balance background and then obtaining binary subimages utilizing morphology and threshold operations, finally, utilizing Two-pass algorithm to label the connected domains of binary subimages. In the detection step, considering the complexity of the pattern of intermittent scratches, ASC is first used for routine intermittent scratches stitching and then supplemented by MVTH. In the verification step, in order to prove that the detected intermittent scratches satisfy the criteria for scratches in human eyes, the method of support vector machine (SVM) pattern recognition is utilized to compare the detected results with the continuous scratch samples detected by human eyes. This algorithm has high degree of parallelism, high speed and strong robustness. The experimental results illustrate that the complete extraction rate of intermittent scratches is 93.59% , the average processing time of single image is merely 0.029 second and the accuracy rate of detection is up to 98.72% by SVM verification.
A lateral shearing interferometer based on randomly encoded hybrid grating (REHG) is proposed to measure the optical system aberrations. According to the theory of Fraunhofer diffraction, the REHG is designed to be a combination of a randomly encoded binary amplitude grating and a phase chessboard. Compared with the conventional cross-grating lateral shearing interferometer, the REHG is more suitable for the general aberration testing since no order selection mask is needed. Collimated beam for aberration measurement will converge after passing through the optics system under test. Then the quadriwave lateral shearing interferogram containing the wave-front aberration information is then recorded by the CCD. By selecting its +1 order of the Fourier spectrum in both X and Y directions, the shearing wavefronts in both two orthogonal directions can be obtained employing phase unwarping algorithm. Zernike polynomials are used as basic functions for the original wave-front, and the coefficients of Zernike polynomials can be obtained with shearing wave-fronts. In the experiment, we employed a REHG with a grating pitch of 240μm to test a cemented doublet optics with an aperture of 50mm and a focal lengths of 90mm. The test results showed the peak-to-valley (PTV) aberration is 0.242λ while the root-mean-square (RMS) is 0.064λ. The test results by the REHG are very close to the results by the ZYGO GPI interferometer while the error of PTV is 0.003λ and the error of RMS is 0.007λ. The measurement of optical system aberrations by REHG can reach high precision and exhibit good immunity to environmental disturbance. The REHG can be applied to the optical testing of beam quality, optical system aberration and biomedical research.
The high precision ray tracing inside inertial confinement fusion (ICF) cryogenic targets plays an important role in the reconstruction of the three-dimensional density distribution by algebraic reconstruction technique (ART) algorithm. The traditional Runge-Kutta methods, which is restricted by the precision of the grid division and the step size of ray tracing, cannot make an accurate calculation in the case of refractive index saltation. In this paper, we propose an improved algorithm of ray tracing based on the Runge-Kutta methods and Snell's law of refraction to achieve high tracing precision. On the boundary of refractive index, we apply Snell's law of refraction and contact point search algorithm to ensure accuracy of the simulation. Inside the cryogenic target, the combination of the Runge-Kutta methods and self-adaptive step algorithm are employed for computation. The original refractive index data, which is used to mesh the target, can be obtained by experimental measurement or priori refractive index distribution function. A finite differential method is performed to calculate the refractive index gradient of mesh nodes, and the distance weighted average interpolation methods is utilized to obtain refractive index and gradient of each point in space. In the simulation, we take ideal ICF target, Luneberg lens and Graded index rod as simulation model to calculate the spot diagram and wavefront map. Compared the simulation results to Zemax, it manifests that the improved algorithm of ray tracing based on the fourth-order Runge-Kutta methods and Snell's law of refraction exhibits high accuracy. The relative error of the spot diagram is 0.2%, and the peak-to-valley (PV) error and the root-mean-square (RMS) error of the wavefront map is less than λ/35 and λ/100, correspondingly.
A wavefront retrieval method for the quadriwave lateral shearing interferogram obtained by randomly encoded hybrid grating (REHG) is proposed. The REHG consists of a binary amplitude grating and a phase chessboard, and the Faunhofer diffractions of this grating only contain the ±1 orders in two orthogonal directions. As a result, no order selection mask is ever needed by the REHG for quadriwave lateral shearing interference. To retrieve the phase distributions from the REHG interferograms, fast Fourier transform (FFT) technique is employed at first to get the frequency spectrum. By performing inverse fast Fourier transform (IFFT) of the +1 order spectrum in the x and y directions, it is possible to extract shearing wavefronts from the interferogram in both two orthogonal directions. Using the translation property of Fourier transform, the relationship between the Fourier spectrum of the shearing wavefronts and the Fourier spectrum of the wavefront under test is deduced. The wavefront under test is retrieved by establishing an evaluation function firstly and finding the minimum value with least-square-solution. Analysis and compensations are made to reduce the errors in the testing results. Simulation experiments have shown that this method can retrieve different phase distributions without losing high-frequency information.
A compact wavefront diagnosis system with nice repeatability based on the randomly encoded hybrid grating (REHG) is proposed. The REHG comes from the ideally calculated grating for quadriwave lateral shearing interference, and it consists of a binary amplitude grating and a phase chessboard. The phase chessboard simulates the phase modulation of the ideally calculated grating, while the binary amplitude grating is designed and fabricated based on the randomly encoding method. In this method, the amplitude distribution on the ideally calculated grating is firstly divided into discrete grids. And the radiant flux in each grid is quantized into several quantization levels. The binary amplitude grating is then generated by encoding the pixels in the grids with 1 and 0, which stands for whether the light can pass through or not, so that the total radiant flux in each grid on this grating approximate to the flux in the corresponding grid on the ideally calculated quadriwave grating. In addition, random pattern is employed in the encoding process to avoid introducing extra diffraction orders. The far-field diffractions of the REHG only contain the ±1 orders in two orthogonal directions, and no order selection mask is needed for quadriwave lateral shearing interference. Due to the common-path configuration, the wavefront testing results obtained by the REHG lateral shearing interferometer exhibits nice repeatability and good suppression over environmental noise and vibration.
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