The technique of surface profile measurement utilizing white-light interferometry is widely used in industry. However, it has certain shortcomings, such as slow measurement speed and the possibility of error caused by a transparent film on the surface. This paper introduces four of our recent developments in white-light interferometry: 1) speed improvement by sub-Nyquist sampling, 2) accuracy improvement through the use of phase information, 3) profiling of a thick transparent film, and 4) profiling of a thin transparent film.

On-line non-contact roughness metrology is still an open problem. Usual methods involve either contact (stylus-type devices) or perform indirect evaluations of some roughness parameters, such as R_a, with light scattering techniques or speckle measurement (among the most common optical techniques), inductance (only for magnetic materials) or ultrasound methods. However, a generic method able to obtain every roughness parameter (what means recording the real distance profile), able to work with a variety of surface types, and able to be installed in production lines is still to be developed. In this article, the ongoing work towards the construction of a non-contact optical profile measuring sensor that could be used for roughness measurements is presented. Our approach is based on Conoscopic holography, a common-path interferometric technique which is a good candidate for industrial applications. Current research effort is focused in enhancing accuracy in these systems, by both reducing the coherence of the illuminating source (laser) and changing the hardware and software setup, with the aim of building a sensor able to capture a profile of an object's surface in a single shot with high precision from a relatively long standoff (several cm).

We propose a white light displacement sensor using a novel spectro-polarization modulator which generates spiral liner polarized light sorted along wavelength concentrically. It consists of a polarizer, a retarder with high order retardation, and a quarter wave plate. If we set the spiral polarizer after the spectro-polarization modulator, we can observe spectroscopic color concentrically. A displacement measurement method is proposed using chromatic aberration method and white light interferometer.

One of the most important monitoring tasks of tunnel inspection is the observation of cracks. This paper describes an approach for crack following using mid-resolution (2-5mm per pixel) images of the tunnel surface. A mosaic on the basis of the tunnel design surface is built from images taken with a mobile platform. On this image representing the unwrapped tunnel surface texture the starting points of each crack are found semiautomatically using a modified Hough transform. Crack following takes place on the basis of local line fitting and exhaustive search in both directions of the crack, taking into account several restrictions, rules and optimization criteria to find the correct crack trajectory. A practical implementation polygonizes the extracted cracks and feeds them into a tunnel inspection data base. The method is applicable to various types of background texture as expected in the tunnel environment.

The need for improved thermal efficiency of jet engines has led to changes in the design of combustor turbine blades. Modern turbine stage inlet temperatures now exceed the melting point temperatures of turbine blade materials. Super alloys, based on nickel, have been developed for use as blades, guide vanes, afterburners etc. To combat and avert blade failure caused by excessive operating temperatures, film cooling has been incorporated into blade design. In film cooling, cool air is bled from the compressor stage, ducted into internal chambers of the turbine blades, and discharged through small holes in the blade walls. This provides a thin, cool, insulating blanket along the external surface of the turbine blade, and large numbers of shaped holes have allowed designers to maximize the cooling effect. This paper explores a new design for measuring the presence and depth of blind holes in turbine blade. In the paper, we examine the inspection techniques currently in use and present a novel optical technique as an alternative. To precisely locate and measure the holes on the turbine blade, an XYZ translation stage is employed. Using a small collimating tube, a micro-beam illuminates each hole in a pre-programmed fashion. Depending on the level of reflected intensity and when it occurs, the presence of a hole bottom is determined. The optical inspection system consists of a laser, motorized micro-positioning stage, collimating tube, optical detector/amplifier, data acquisition software and a customized fixture for manipulating the samples.

The paper provides considerations relative to the application of 3D vision methods and presents some lessons learnt in this respect by presenting four 3D vision tasks and discussing the selection of vision sensing devices meant to solving the task. After a short reminder of 3D vision methods of interest for optical range imaging for microvision and macrovision applications, the paper enumerates and comments some aspects which contribute to find a good solution. Then, it presents and discusses the four following tasks: 3D sensing for people surveillance, measurement of stamping burrs, sorting burred stamping parts and finally, hole filling algorithm.

Through the introduction of a simply designed phase mask at the pupil of a conventional imaging system, the range to objects in view can be precisely determined. The mask allows for roughly 50% light transmission, and is segmented to introduce a 1/4 wave phase shift into half of the transmitted light. The resulting point spread function is highly sensitive to object range and is incorporated into the resultant image. Through comparison of the wavefront coded image with a conventional image of the same scene, the precise range to the object can be determined. The range measurement precision obtained is directly related to image contrast. For objects having a simple linear edge possessing a contrast of 32 gray levels above noise, range can be measured to 1% precision. While based on an interference effect, wide-band polychromatic light can be used to determine object range. No actively moving components in the optical system are required for operation. The resultant approach allows for imaging and range determination simultaneously. Based on a fundamental interference phenomenon, this approach is applicable to all passive optical imaging systems ranging from the UV to the infrared. Both a theoretical analysis and an experimental verification of the approach showing the expected performance is provided.

Reconstructing a specular surface from observation of structured illumination is mathematically a severely illposed problem. This contribution discusses the close relationship of Shape-from-Shading and the deflectometry problem. Consequently, a novel regularization approach for partially diffuse objects is proposed in this paper. By considering different characteristic curves on the unknown surface, we estimate a set of points that the reconstruction must embed. A solution satisfying the reflection conditions is afterwards fitted to the obtained points with the help of a level set iteration.

Three-dimensional edge measurement is critical in many applications such as blade manufacturing due to the stringent requirements on aerodynamic performance of blades. Optical metrology techniques provide very good tools in terms of speed and accuracy, but face some challenges as well, such as discontinuity, irregularity, and size variation in the edge shape. This paper presents several methods that include stereovision techniques to capture a sharp edge and phase-shifting to capture an edge profile of a blunt edge. For sharp edge measurement with stereovision, multiple cameras are used to view the edge from different directions. Then the captured images are processed to obtain the point cloud of the edge. For blunt edge measurement, a phase-shifting method is applied. After the edge profile is obtained, all edge information can be extracted. In this paper, different illumination methods are discussed and different edges are measured. Experimental data shows that these methods are practical in obtaining accurate edge or edge profile.

Many trials have been proposed to measure inner diameter of pipes and/or holes. However most of them are classified into contact methods with any kind of stylus. Here we propose to measure inner diameter and profile of pipes using a ring beam device which consists of a conical mirror. and LD. The beam from the LD is directed to the top of the conical mirror which is precisely fabricated in angle and polished so as to form a ring beam for optical sections of the inner wall. This optically sectioned profile is analyzed to calculate the inner diameter and/or profile. In addition to pipes such as water mains and sewers, engine blocks for automobiles are tested to measure the inner size of cylinders and to find defects of inner surfaces.

A new spatial-domain Blur Equalization Technique (BET) is presented. BET is based on Depth-from-Defocus (DFD) technique. It relies on equalizing the blur or defocus of two different images recorded with different camera parameters. Also, BET facilitates modeling of images locally by higher order polynomials with lower series truncation errors. The accuracy of BET is further enhanced by discarding pixels with low Signal-to-Noise ratio by thresholding image Laplacians, and relying more on sharper of the two blurred images in estimating the blur parameters. BET is found to be superior to some of the best comparable DFD techniques in a large number of both simulation and actual experiments. Actual experiments used a large variety of objects including very low contrast digital camera test charts located at many different distances. In autofocusing experiments, BET gave an RMS error of 1.2% in lens position.

Booming demand for high speed and high accuracy 3D inspection, especially in the area of manufacturing of electronic devices and components, causes fast progress in research related to this area and significant improvement in these systems capabilities, specification and features. Accuracy can be considered as one of the most important features for 3D machine vision inspection systems, probably along with speed and robustness. Phase shift based systems are believed to be among the most accurate 3D measurement systems with resolution at sub-micron level, though to achieve such a high resolution and accuracy considerable efforts should be made and various physical effects should be taken into consideration. 3D accuracy problems comprise of 2D accuracy problems, such as camera radial and perspective distortions, subpixel measurement problems, etc. combined with special issues related specifically to the phase-shift measurement profilometry. Among them there are distortions of the projected light pattern, including projector radial, perspective distortions and non-sinusoidality of the projected pattern (harmonic distortion) and dynamic range problems caused by camera's signal to noise ratio and by constraints of limited digitized signal bit width. To eliminate or minimize negative influence of the above mentioned factors, a number of measures should be carried out when making 3D measurement head design followed by compensation and calibration procedures. Experimental results on various accuracy problems based on real 3D measurement system developed by authors as well as simulation results are presented in this paper.

Structured light is widely used for shape measurement of surfaces using the triangulation principle. To fulfill this task, precise information about the optical path is necessary. We propose a novel method for the overall calibration of a setup consisting of a structured light source, a projection screen and a camera. As computer controlled video projectors become cheaper and cheaper, it is reasonable to use these off-the-shelf devices for measurement applications. However, to achieve high accuracy with standard components, a precise calibration of the measurement system is indispensable. Absolute position and orientation of the camera, the projector and the projection screen has to be known. Furthermore, intrinsic calibration of both the camera and the projector is necessary. After acquiring a large set of data points using a versatile phase encoding technique, we estimate the optimal parameters using a bundle adjustment technique. We consider all extrinsic and intrinsic parameters for the optical mapping including a distortion model for the projector and for the camera, respectively. We propose a method which ensures complete knowledge about the optical mapping of each ray observed by the camera. The proposed metric calibration method has also importance for other measurement applications as e.g. shape reconstruction of specular surfaces. Hereby structured light patterns are projected on the screen, and their reflection on the specular surface is observed by the camera.

We propose a nonlinear calibration method for improving the accuracy of structured light systems that use cameras and projectors. Previously we have developed a systematic method for the calibration of projectors as well as structured light systems for 3-D shape measurement. However, we used only a linear model and did not consider lens distortions of the camera and the projector. As a result, measurement accuracy was limited. In this paper, we develop nonlinear models for both the camera and the projector and apply them to develop a nonlinear algorithm for 3-D shape measurement. The aim is to improve system accuracy by reducing the nonlinear error caused by lens distortion. Experimental results on nonlinear camera and projector calibration, comparison of 3-D measurement errors with linear and nonlinear camera and projector models, as well as 3-D shape measurement of some sample objects are presented.

One of the economically most important branches is the automotive industry with their component suppliers. The high degree of automation in manufacturing processes, requires automated control and quality assurance equally. In this scope, we present a complex 3D measuring device, consisting of multiple optical 3D sensors, which is designed to capture the geometry of wheel rims. The principal challenge for automated measurements is the variety of rims with respect to design, dimensions and the production flow. Together with connected conveyers, the system automatically sorts good rims without interrupting the manufacturing process. In this work we consider three major steps. At first we discuss the application of the used 3D sensors and the underlying measuring principles for the 3D geometry acquisition. Therefore, we examine the hardware architecture, which is needed to fulfill the requirements concerning to the variety of shapes and to the measuring conditions in industrial environments. In the second part we focus on the automated calibration procedure to integrate and combine the data from the set of sensors. Finally, we introduce the algorithms for the 3D geometry extraction and the mathematical methods which are used for the data preprocessing and interpretation.

Machine vision plays an important role in automated assembly. However, present vision systems are not adequate for robot control in an assembly environment where individual components have sizes in the range of 1 to 100 micrometers, since current systems do not provide sufficient resolution in the whole workspace when they are fixed, and they are too bulky to be brought close enough to the components. A small-size 3D vision system is expected to provide two decisive advantages: high accuracy and high flexibility. The presented work aims to develop a 3D vision sensor easily embedded in a micro-assembly robot. The paper starts by a screening of 3D sensing methods, performed in order to identify the best candidates for miniaturization, and that results in the selection of the multifocus principle (which elegantly avoids the depth of field problem encountered for example in stereo vision). Here, depth is measured by determination of sharpness maxima in a stack of images acquired at different elevations. Then, it presents a preliminary system configuration, that delivers images of a 1300×1000 micrometers field of view with lateral resolution better than 5 micrometers and vertical resolution better than 20 micrometers. Finally, future steps in development of a real-time embedded multifocus sensor are presented, with a discussion of the most critical tradeoffs.

Projected fringe methods have lead to a wide selection of commercial sensors for 3D measurement applications. The basis of these systems is a projector such as an LCD presentation projector that is used to generate a coarse pattern that is shifted across the part and viewed by a camera. Three or more images with a small pattern shift between each are sufficient to obtain a detailed 3D map using phase shift analysis methods. The limitation of these systems has been that to obtain high resolution the system is limited to viewing only a small field-of-view. Moire methods are a way to leverage this resolution, particularly on flat or only slightly contoured areas. The approach described here takes advantage of moire methods used in connection with the fringe projection method to provide high resolution over key reference areas, while still provided a less precision measurement over a larger region.

Polarization components can be used as nearly achromatic phase shifters in an interferometers. Here wepresent an improved achromatic half-wave plate (HWP) phase shifter that can be incorporated at the input end of a white light interferometer. A nearly achromatic HWP phase shifter that can be used at the input of an interferometer consists of a rotating HWP followed by a quarter wave plate (QWP) fixed at an azimuth of 45°. An improved achromatic phase shifter for the input end can be constructed using achromatic circular polarizers. The performance of this achromatic HWP phase shifter when used at the input end of a white light interferometer is studied using Jones calculus. The calculated values of the phase shifts between the interfering beams and their amplitudes are very nearly same for all the wavelengths. This phase shifter thus gives much improved performance.

Modern manufacturing requires continuous quality control to assure a constant quality. For highly automated processes, industrial 2D and 3D image processing methods are often used. In the recent years the need for 3D geometry measuring has increased significantly. Therefore, we present a photogrammetric stereo vision system, including hardware as well as software components, which is designed to measure the 3D shape of complex extruded plastic profiles online. Thus, the main challenge is to capture the profiles while they are moving and deforming at the same time. The profiles are measured at several defined points in time and the geometry of the front surfaces are extracted respectively. This enables us to reconstruct and evaluate the deformation and shaping process. Based on these results we are able to simulate the viscous flow of plastics in the extrusion process. The 3D geometry capturing and the simulation are repeated iteratively to generate reliable simulation input close to reality on the one hand and to optimize the extrusion tool and the involved mechanical parts on the other. Our work is primarily concerned with the 3D data acquisition. Thus, we particularly focus on the stereo matching procedures used and discuss the optimal, system specific configuration.

A dynamic 3-D nano-scale surface profilometer using stroboscopic white light interferometry with novel image deconvolution and automatic identification of structure resonant modes was successfully developed. As micro electromechanical systems (MEMS) increase rapidly towards industrial application, the needs of accurate dynamic characterization are extremely important to optimal design and fabrication. To meet the demands, an optical microscopy based on stroboscopic interferometry was developed to achieve full-field vibratory out-of-plane surface profilometry and system characterization. A novel deconvolution strategy with correction of the light response function was established to remove the potential image blurs caused by the unavoidable vibration of the tested parts. With this technical advance, the bandwidth of dynamic measurement can be significantly increased up to 10 MHz without sacrificing measurement accuracy. Meanwhile, an innovative detection algorithm based on image contrast measure was developed for automatic identification of accurate resonant modes. The detection method provides the simplest and most economic way to detect accurate resonant peaks without adding any significant hardware in a stroboscopic interferometric framework. To verify the effectiveness of the developed methodology, AFM cantilever beams were measured to analyze the full-field resonant vibratory modes and dynamic characteristics. The experimental results confirm that the resonant vibration behavior of the tested microcantilever beams can be accurately characterized and 5 nm of vertical measurement accuracy as well as tens micrometers of vertical measurement range can be achieved. The measured results were satisfactorily consistent with the theoretical simulation outcomes from ANSYS.

It is a new field for applying stereovision into guidance system. The objective was to find out a better correlation method and develop an algorithm for detecting vegetable rows for field robot. Several Area correlation methods were compared for obtaining disparities, such as sum of absolute differences and Mahalanobis Distance. A method to eliminate error matched results was also studied, which was comparing of minimum extremum and the second minimum extremum. The 3D data of fields were calculated based on the disparity images and they were matched with a trapezium model to detect vegetable rows. It shows that stereovision could obtain the landforms of fields, especially that of vegetable rows. In future, a real time tilt angle sensor might be added for more reliable 3D data.

The real-time on-line checkout systems are wildly required to assure the quality of products. A product optical on-line product inspection system is designed based on the principle of image cross-correlation technique. The principle of this optical product inspection method is comparing the image of products on-line with the image of references. To enhance the effectiveness of the cross correlation, a Gaussian filter is introduced to weaken the intensity of the autocorrelation peak. For angle and position detection, ten referential images under different rotation angles are put on a circle position while the image of the object is shown at the center of the circle. The information of the direction and position of the object can be obtained immediately.

Phase shift analysis sensors are popular in inspection and metrology applications. The sensor's captured image contains the region of interest of an object overlaid with projected fringes. These fringes bend according to the surface topography. 3D data is then calculated using phase shift analysis. The image profile perpendicular to the fringes is assumed to be sinusoidal. A particular version of phase shift analysis is the image spatial phase stepping approach that requires only a single image for analysis, but it is sensitive to noise. When noise, such as surface texture, appears in the image, the sinusoidal behavior is partially lost. This causes an inaccurate or noisy measurement. In this study, three digital de-noising filters are evaluated. The intent is to retrieve a smoother sine-like image profile while precisely retaining fringe boundary locations. Four different edge types are used as test objects. "Six Sigma" statistical analysis tools are used to implement screening, optimization, and validation. The most effective enhancement algorithms of the selection comprise (1) line shifting followed by horizontal Gabor filtration and vertical Gaussian filtering for chamfer edge measurement and (2) edge orientation detection followed by 2-D Gabor filter for round edges. These algorithms significantly improve the gauge repeatability.