A global direction metric method based on Fourier-polar transform is proposed in this paper, which calculates the global fringe direction according to the directional distribution of intensity from the power spectrum of fringe pattern. By introducing polar coordinate transform, the rotation of power spectrum is transformed into translation component, which can make the calculation process simple and fast. Then the original image is projected along the global fringe direction and the mean value of pixel gray is calculated. Also, the fringe pitch can be calculated from the projection curve close to cosine distribution. This detection method of optical fringe parameters uses overall information of the image, and holds good adaptability and robustness to noise and degraded image. Moreover, without any pre-processing operations such as smoothing filter and threshold segmentation is required in this method. It can directly detect two parameters of global fringe direction and fringe spacing, which is of great significance for quantitative analysis of fringe image.
In visual detection fields based on line-structured light, the analysis of optical stripe image is a key problem. For the cross-line target image, through measuring the angle between two linear optical stripes the target position or some system’s parameters can be obtained. The traditional technique usually needs many preprocessing steps including image filtering, threshold segmentation, thinning processing and so on. For the images with low signal noise ratio or non-uniform intensity distribution, their application performance will be challenged. Based on the characteristic of translation invariance and rotation synchronization of two-dimensional Fourier transform, the paper combines Fourier transform with polar transform to form new Fourier-polar transform algorithm. It implements the angle measurement in the frequency-domain replaced in the spatial domain. At the same time, to improve the convenient of compute, the polar transform is adopted to calculate the distribution direction of amplitude spectrum energy. The proposed Fourier-polar transform algorithm uses the overall information of the image, and the calculating process is simple and no requirement of image preprocessing. Therefore, it can be applied to measure the angle of cross-line target image in low quality image such as low signal-to-noise ratio or with noise.
Optical fringe is one of important output information from the optical systems. Some important optical or system parameters can be obtained by analyzing the fringe information from optical system such as interferometer system or diffraction setup. The straight fringe is a kind of optical fringes frequently appearing in Young’s double slit interference and single-slit diffraction and other optical structures. For the information extraction of straight fringes, it is often necessary to calculate the fringe spacing parameters. Popular straight fringes analysis methods include the fringe center method and the Fourier transform method. In addition, some image processing methods realized line detection can also be used to analyze this straight fringes image, which include Hough transform and Radon transform. In this paper, four algorithms for fringe analysis are discussed and compared, which focus on method’s principle, algorithm’s simulation and performance when they be applied to detect the fringes spacing. At the same time, the anti-noise performance of two image processing algorithms including Hough transform and Radon transform are analyzed.
A surface roughness measurement method is introduced in the paper, which is based on laser triangulation and digital image processing technique. In the measuring system, we use the line-structured light as light source, microscope lens and high-accuracy CCD sensor as displacement sensor as well. In addition, the working angle corresponding to the optimal sensitivity is considered in the optical structure design to improve the measuring accuracy. Through necessary image processing operation for the light strip image, such as center-line extraction with the barycenter algorithm, Gaussian filtering, the value of roughness is calculated. A standard planing surface is measured experimentally with the proposed method and the stylus method (Mitutoyo SJ-410) respectively. The profilograms of surface appearance are greatly similar in the shape and the amplitude to two methods. Also, the roughness statistics values are close. The results indicate that the laser triangulation with the line-structured light can be applied to measure the surface roughness with the advantages of rapid measurement and visualized display of surface roughness profile.
Pyramid wavefront sensor (PWFS) without modulation is prevailing over one with modulation. So far how to describe
measured signals of non-modulation PWFS needs deeply research. In this paper, the theory of the non-modulation PWFS
is briefly presented according to wave optics. This paper analyses the existing four approaches in theory. By numerical
simulation this paper further verifies the performance of four approaches under the experiment condition. The result
shows that the approach with total intensity of pixels conjugate to the same spot in the pupil as signal denominator is the
best choice for the non-modulation PWFS in closed-loop correction.
To overcome the difficulty in the non-contact angle measurement especially for large-scaled workpieces, the paper presents a new angle measurement method based on Hough transform. The realization process of the method consists of workpiece image acquisition & preprocessing and angle measurement. With the preprocessing one can obtain an ideal binarized edge image of the workpiece image. The angle measurement is then carried out by employing the classical Hough transform. Except the discussion of measurement principle, both practical examples and simulated experiments are introduced to demonstrate the performance of the method proposed in the paper. The experiment results indicate that this method can be applied to measure the workpiece angle under many occasions and can achieve the aim of the non-contact angle measurement. At the same time, it does still possess great advantages in terms of reliability and noise rejection capability.
The paper firstly discusses the derivation of interference fringes and FFT method that is a conventional method for measuring fringes with equal spacing. And then, a new method for measuring the interference line fringes is presented. On the basis of Hough transform and digital image processing technology, this method accomplishes fringes image preprocessing (i.e., fringes skeletonizing) and fringes spacing measurement. It has been shown, with both theoretical analysis and experimental measurements, that this method is a universal method for fringes measurement and has more remarkable advantage than the FFT method because of the application of the former not being limited by some regulations such as equidistant fringes and sampling length, and so on. In addition, it also holds a good noise rejection capability and great reliability.