Triangulating laser profilometer as a navigational aid for the blind: optical aspects

We propose a navigational aid approach for the blind that relies on active optical profilometry with real-time electrotactile interfacing on the skin. Here we are concerned with the optical parts of this system. We point out the particular requirements the profilometer must meet to meet the needs of blind people. We show experimentally that an adequate compromise is possible that consists of a compact class I IR laser-diode triangulation profilometer with the following characteristics: 30-cm to 30-m range, 1-deg angular resolution, 20-ms acquisition time per measure of distance, 60° angular scanning field.     

Acoustic grating fringe projector for high-speed and high-precision three-dimensional shape measurements

A new acoustic grating fringe projector (AGFP) was developed for high-speed and high-precision 3D measurement. A new acoustic grating fringe projection theory is also proposed to describe the optical system. The AGFP instrument can adjust the spatial phase and period of fringes with unprecedented speed and accuracy. Using rf power proportional-integral-derivative (PID) control and CCD synchronous control, we obtain fringes with fine sinusoidal characteristics and realize high-speed acquisition of image data. Using the device, we obtained a precise phase map for a 3D profile. In addition, the AGFP can work in running fringe mode, which could be applied in other measurement fields.     

Suppression of phase ambiguity in digital holography by using partial coherence or specimen rotation

In this paper we present two approaches for extracting the surface profile as well as obtaining 3D imaging of near field objects by usage of partial coherence and digital holography. In the first approach a light source with given temporal partial coherence is used to illuminate a near field object. The reflected light is interfered with the reference source. By computing the local contrast of the generated fringes one may estimate the 3D topography and the profile of the object. This approach extracts the 3D information from a single image, and its accuracy does not depend on triangulation angle like in fringe projection methods. The second approach is tomography based. There we illuminate the object from several slightly different angles, and for each we compute the wrapped phase using digital holography techniques. Combining the wrapped phase estimation from several points of projection allows calculating the unwrapped phase and therefore the true profile of even a phase-only object. Increasing the number of points of view decreases the relative error of the estimated profile.     

Three-dimensional profiling using the Fourier transform method with a hexagonal grating projection

We present three-dimensional profilometry based on triangulation in which a hexagonal pattern is projected on the object. To obtain an accurate result with a one-shot photographic image, the Fourier transform method and method of excess fraction are adopted. The three grating components of the hexagonal pattern are used. For compactness a new pattern projection scheme is introduced. The experimental results show that the constructed optical system works well for measuring the profile of a mannequin with a height resolution of ~ +/- 1 mm.     

Multiperiod fringe projection interferometry using a backpropagation method for surface profile measurement

Interference fringes with different periods are projected on an object surface. There is a constant phase point where the phase of the fringe is kept at a constant value while the period is scanning. Multiple optical fields with different periods on the object surface are made from detected phases of the fringes. The multiple optical fields are backpropagated to the constant phase point of the phase where all of the phases of the multiple backpropagated fields become the same value and the amplitude of the sum of the multiple backpropagated fields becomes maximum. The distance of the backpropagation provides the position of the object surface. Some experiments show that this method can measure an object surface with discontinuities of several millimeters with high accuracy of several micrometers.     

大量程纳米级光栅干涉位移测量

针对传统光栅干涉仪中测量范围和分辨率难以同时提高的问题,提出利用单根大长度、低线数光栅实现大量程、高分辨率位移测量的方法.首先利用长度400mm,栅距10μm计量光栅的±5级衍射光生成条纹图,实现了条纹的10倍光学细分.然后提出一种基于傅里叶变换时移特性的条纹细分新方法,利用相邻两帧条纹图同一位置处相位的变化实现了高达1000倍的条纹电子细分.在此过程中,针对能量泄漏对傅里叶变换法相位提取精度的影响,提出条纹图整周期裁剪的方法,使条纹细分精度至少可达到1/1000条纹周期.仿真和实验结果表明,系统具有纳米级的分辨率和优于10nm的测量精度.     

Parallel processing for multiplication modulo by means of phase modulation

We apply optical parallel processing to operations for multiplication modulo, which is one of the key components of a factorization algorithm. With this method, optical phase modulation provides the results of modulo operations. We construct a prototype system based on a Michelson interferometer with a photodetector array. Mirrors are set at both object and reference arms to generate interference fringes. A mirror in the object arm is tilted slightly, whereas the reference arm is set perpendicular to the optical axis. The tilt angle is determined by parameters for the target modulo operations. The presented system can achieve massive data processing in parallel with only simple implementation. We present our experimental results to verify the usefulness of our method.     

Lensacon

A diffractive optical element has been synthesized with laser phototechnology. It can transform a point source into a bright caustic (a light line) stretched along the optical axis. The element transforms a spherical wave into a conic and a point source in the object space to a ring structure in the image space. We investigated how the parameters of the light line (the length and the effective diameter) depend on the wavelength of the light source and the movement of the source along the optical axis. Also the interference field outside the caustic (Fresnel and Poisson fringes) was investigated. For an explanation of experimental data the geometrical theory of interferometers was used. The conditions for the appearance of fringes in white light are given. A device for rectilinear control of large tool beds and for centering machine components was manufactured. Experiments on determining the macroshape and the microshape of objects were performed.     

显微栅线投影相关法及其在三维形貌测试中的应用

基于数字图像相关法解调被物体高度调制的投影栅线平移量的原理，提出了显微栅线投影相关法的概念。该方法是栅线投影法和数字图像相关方法相结合的产物，用于测量微结构的三维形貌和离面变形。系统阐述了该方法的测量原理和详细的标定过程，研究了该方法在微薄膜挠度测量中的应用。测量结果表明，显微栅线投影相关法简单易行，测试结果可靠，配合高速图像采集系统，可用于微结构的动态测试，从而为实现微结构动、静态测试展现了新的应用前景。     

Computed tomography postacquisition data correction for systemalignment errors

A critical issue in obtaining high-quality images from a computed tomography system is the mechanical alignment of the data acquisition system. This paper will compare two methods of measuring and correcting errors introduced into second-generation computed tomography data sets from misalignment of the system. Both of these methods use a projection data set from a complete scan of an object as a starting point. One method is an iterative method that looks at residual errors in selected portions of the reconstructed image. The other method requires that the centroids of the projection data coincide with the centroid of the object. It will be shown that the centroid registration algorithm can be used to accurately measure a number of important computed tomography system alignment parameters and is mathematically efficient. The residual error algorithm will be shown to give less satisfactory results     

Real-Time Phase-Stamp Range Finder Using Correlation Image Sensor

This paper proposes the phase-stamp range finder (PSRF) for real-time three-dimensional (3-D) imaging. The PSRF consists of a time-domain correlation image sensor (CIS) proposed by us, a scanning sheet of light (SOL), and three-phase reference signals supplied to the CIS. After the SOL was scanned over the object during a frame period, the CIS records the “phase stamp” of the reference signals at the moment when the SOL reflected on the object arrived at each pixel. The range image is then reconstructed at the frame rate by converting the phase stamp image from the CIS pixel-wise to the projection angle of the SOL and then applying triangulation. An experimental system is constructed with a 200 $times$ 200-pixel CIS camera and a light-sectioning laser projection system. Experimental results confirm that the constructed system outputs range images at 12.5 frames/s, achieves measurement accuracy of less than 1 mm in standard deviation for imaging distance of 300–500 mm, and is robust to time-varying, spatially nonuniform ambient illumination and fine surface texture of objects.      

动态傅里叶变换轮廓术实现路面形变快速检测

公路表面的三维结构对于判断公路路面破损状况具有重要的意义。基于光栅投影的傅里叶变换轮廓术将物体的高度信息编码在投影到待测物面上的正弦条纹的形变信息中,可以实现对物体的垂直测量。探讨了傅里叶变换轮廓术在高速公路路面面形测量-动态实时系统中的应用,提出了利用自适应标准模版匹配,基于特征图像识别的方法,分选破损的公路路面,从而实现对高速公路路面形变的快速实时检测。     

一种激光探测与波长测定装置的研究

介绍了一种主要用于夜间的激光探测与波长测定装置，它是基于激光在光学劈尖上产生干涉条纹的特点、通过傅里叶变换测定条纹间距来计算激光波长的，测试结果表明该装置在背景光微弱的情况下，性能令人满意，具有一定的实用性。     

采用标记拼接的相位测量轮廓术

提出一种采用标记拼接的新的光学三维传感方法。在采用结构照明的光学三维传感中,阴影及条纹错位关系到被测量物体三维面形重建的质量。在处理阴影时,传统方法是采用插值方法,在处理条纹错位时,通常采用增大条纹周期或采用几组条纹周期不同的光波进行投影,这样通常降低了测量精度。为了提高测量精度,本文提出一种在同一参考面内,采用单组相移正弦条纹进行投影,通过旋转被测物体,并对旋转前后两次重建物体在信息可靠区域内进行标记拼接,恢复物体表面完整三维信息的方法。实验证实了此方法的有效性及实用性。     

Three-dimensional imaging based on Gray-code light projection: characterization of the measuring algorithm and development of a measuring system for industrial applications

A three-dimensional (3-D) imaging system based on Gray-code projection is described; it is thought to be used as an integration to the already developed profilometer based on the projection of multifrequency gratings. The Gray-code method allows us to evaluate the 3-D profile of objects that present even marked discontinuities of the surface, thus increasing the flexibility of the measuring system as to the topology of the objects that can be measured. The basic aspects of Gray-code projection for 3-D imaging and profiling are discussed, with particular emphasis devoted to the study of the resolution of the method and to the analysis of the systematic errors. The results of this study allow us to determine the optimal setting of the parameters of the measurement and to develop a suitable calibration procedure. The procedures for implementing the Gray-code method are presented, and some interesting experimental results are reported. Calibration of the system reveals an accuracy of 0.2 mm, corresponding to 0.1% of the field of view.     

干涉条纹快速预处理的新方法

介绍一种基于数学形态学的新方法以实现干涉条纹的快速预处理（去噪声、细化条纹等），该方法所采用的光学系统结构简单，具有图像处理的快速并行性。用它处理了三种干涉条纹，获得了很好的处理效果。     

Flying triangulation--an optical 3D sensor for the motion-robust acquisition of complex objects

Three-dimensional (3D) shape acquisition is difficult if an all-around measurement of an object is desired or if a relative motion between object and sensor is unavoidable. An optical sensor principle is presented-we call it "flying triangulation"-that enables a motion-robust acquisition of 3D surface topography. It combines a simple handheld sensor with sophisticated registration algorithms. An easy acquisition of complex objects is possible-just by freely hand-guiding the sensor around the object. Real-time feedback of the sequential measurement results enables a comfortable handling for the user. No tracking is necessary. In contrast to most other eligible sensors, the presented sensor generates 3D data from each single camera image.     

Surface contouring by optical edge projection based on a continuous wavelet transform

A novel optical edge projection method for surface contouring of an object with low reflectivity is presented. A structured light edge is projected onto a dark surface, and the image is captured by a CCD camera. The surface profile of the object is then evaluated by an active triangular projection technique, and a whole-field three-dimensional contour of the object is obtained by scanning the optical edge over the entire object surface. An edge detection method based on a continuous wavelet transform (CWT) is employed to determine the location of the optical edge. The method of optical edge detection is described, and characteristic details of gray-level distribution along the edge are analyzed. It is shown that the proposed wavelet edge detection method is not dependent on any threshold values; hence the true edge position can be determined without subjective selection. A black low-reflectivity object surface made from woven carbon fiber is measured, and the experimental results show that the profile of a woven carbon fiber can be obtained by the proposed method.     

Study of transient deformations with pulsed TV holography: application to crack detection

We report the measurement of reproducible transient deformations with single-pulsed-subtraction and double-pulsed-addition TV holography with both in-plane and out-of-plane optical configurations. An original synchronization system (controlling mechanical excitation of the object, firing the laser pulses, and image acquisition) used to obtain the subtraction and the addition correlation fringes is described. Experimental results are presented for the propagation of mechanical shock waves in a cantilever specimen and their subsequent diffraction by a crack in the specimen. These preliminary results show that the technique may be used for crack detection.