Structured Light Technique Research Articles

TDI imaging and scanning moiré for online defect detection

This paper explains the application of scanning moiré in association with TDI (Time Delay and Integration) imaging for complete peripheral inspection of cylindrical objects. Based on the structured light technique, a grating pattern modulated by deformations, such as dents on the surface of a rotating cylindrical object, is recorded using a TDI camera. The illumination system consists of an intensity modulated, line generating laser diode aligned at an angle to the camera axis. By using the TDI option for recording every nth line, an online scanning moiré pattern is generated. After providing a simple explanation of the scanning moiré, the effects of object rotation speed and TDI scan speed on the fringe pattern are discussed and demonstrated. The techniques presented here offer low cost solutions to industrial machine vision tasks related to peripheral imaging and inspection of cylindrical objects.

Enhanced observation of number fluctuations by use of structured light

The point statistics of light scattered from structured, incoherent illumination of a random and fluctuating population of identical scatterers are derived. Theoretical predictions for a two-beam illumination pattern in which the relative beam intensity is modulated are compared with computer simulations and experiments performed by illumination of a rotating mask of identical but random apertures. The use of structured light is found to enhance the size of fluctuations, and quantitative comparisons between theory and practical measurement are made. The structured-light technique introduces flexibility in fluctuation measurement, permitting optimal control of the dynamic range, and is relevant to the development of instrumentation for particle and flow analysis.

Apple Stem and Calyx Identification with Machine Vision

This paper presents an image analysis technique for the identification of apple stems and calyxes. As apple stem and calyx areas appear as dark patches in images, the analysis is focused on the dark patches of fruit surfaces. The patches are first segmented out by a flooding algorithm. To distinguish stem and calyx areas from patch-like blemishes, the three-dimensional shape of an apple geometric surface is used, which is obtained by using a structured light technique. For each patch, the characteristic features are extracted from both the image under normal diffused light and the image with structured light. With these features, back-propagation neural networks are used to classify each patch as stem/calyx or patch-like blemish, to identify stems and calyxes. The proposed technique was tested with sample apples and an average identification accuracy of 95% was achieved.

Error analysis of 3D shape construction from structured lighting

In this paper, we present a detailed model and analysis of several error sources and thier effects on measuring three-dimensional (3D) surface properties using the structured lighting technique. The analysis is based on a general system configuration and identifies three types of error surces—system modeling error, image processing error and experimental error. Absolute and relative error bounds in obtaining 3D surface orientation and curvature measurements using structured lighting are derived in terms of the system parameters and likely error sources. In addition to the quantization error, other likely error sources in system modeling and experimental setup are also considered. Even though our analysis is on structured lighting, the results are readily applicable to other triangulation-based techniques such as stereopsis. Finally, our analysis focuses on error in inferring surface orientation and principal surface curvature. Such analyses, to our knowledge, have never been attempted before.

Optimized three-dimensional recovery from two-dimensional images by means of sine wave structured light illumination

A common problem in manufacturing design is the accurate rendering of a 3-D object into a digitized 3-D representation. Such a rendering can be used to duplicate or modify existing objects as well as perform quality control and object recognition in automated manufacturing operations. A structured light technique that uses a sinusoidal illumination pattern to retrieve 3-D information is presented. This technique requires only one image of a surface area to process the topology and can be accomplished with incoherent light. The sine section technique projects a sinusoidal pattern onto an object and recovers the local slopes (first order), which are then combined to reconstruct the surface topology. Surface variations act to create a wideband frequency modulation (zero to infinite frequency), so an optimum frequency demodulation technique is developed to yield average slopes in local regions of the illuminated area. A reconstruction algorithm is introduced that uses these slopes with minimum mean square error. The performance of this technique appears to be very robust and insensitive to intensity variations. Results are presented for recovery of synthesized surfaces corrupted in altitude by additive colored Gaussian spatial noise. The colored noise is generated with a fractal filter to best emulate real surface topologies.

Analysis technique for the measurement of a three-dimensional object shape

A new analysis technique for the automatic measurement of a three-dimensional object shape is proposed and verified experimentally. In this structured-light technique the process of projecting a grating pattern upon the object is considered for using carrier frequencies to modulate the object spectrum in the spatial frequency domain or to add a reference surface to the data in the space domain. The projected fringes are considered as interferometric fringes; therefore the interferogram analysis technique can be introduced into the measurement of a three-dimensional object shape.

Use of vision in dispensing control and inspection of adhesives

SUMMARYA prototype system utilizing a vision technique to monitor the parameters of adhesive beads or sealants is presented. The system uses a structured light technique to investigate adhesive beads and extract information about the shape of the bead from binary images. The geometrical model of the bead, the relationship between this model and the shape of a line of light on the image are investigated. The models for calculating the parameters of the bead and the height of the camera are analyzed. Control strategies are suggested.

Soil topography measurements using image processing techniques

A non-contact instrumentation system for measuring soil topography is described. This system is based on structured lighting techniques using image processing algorithms. Necessary equations are developed and implemented. The system was evaluated for accuracy using precisely constructed rigid objects. Measurement errors of less than 1 mm were obtained in all tests. The system was then evaluated for applicability to soils using profiles for two different soil types. Observed profiles were within the accuracy of a pin displacement measuring unit. The system proved to be an efficient and accurate technique for measuring surface profiles.

Advanced vision systems for computer-integrated manufacture Part I

As computer-integrated manufacturing facilities continue to grow in complexity, increasingly sophisticated computer based sensory systems will be required to carry out functions such as robot guidance, tool wear monitoring, and product inspection. This paper reviews progress in the development of advanced computer vision systems at the National Engineering Laboratory, UK and describes the design of hardware and software for two dimensional image analysis. A component recognition system for robotic applications is detailed and a novel structured light technique for three dimensional component analysis is described. Part II, which contains several case histories, will be published in the November issue.

Facial image reconstruction and manipulation from measurements obtained using a structured lighting technique

This paper presents a method of capturing three-dimensional data from live subjects and using that data together with photographic fragments of the subject to reconstruct and manipulate a computer-generated image. The results show that highly realistic images can be obtained and that animation of the images is possible.

A vision system for the on-line quality control of adhesive beads

Abstract A new application for a structured-light technique — to monitor adhesive beads — is presented. This sensing system, which is based mostly on software, is used to investigate adhesive or sealant beads, on line, while they are dispensed and achieves a very short cycle time. The programming concept, which provides a powerful tool for many process-control applications, is described here in the process of automatically dispensing adhesives and sealants. The performance of the system is analysed and suggestions for future development are offered. The image processing methodology will enable a variety of quality assurance problems to be addressed in the automatic extrusion of adhesives and sealants.

Real-time cooperative interaction between structured-light and reflectance ranging for robot guidance

SUMMARYWhen applied to rapidly moving objects with complex trajectories, the information-rate limitation imposed by video-camera frame rates impairs the effectiveness of structured-light techniques in real-time robot servoing. To improve the performance of such systems, the use of fast infra-red proximity detectors to augment visual guidance in the final phase of target acquisition was explored. It was found that this approach was limited by the necessity of employing a different range/intensity calibration curve for the proximity detectors for every object and for every angle of approach to complex objects. Consideration of the physics of the detector process suggested that a single log-linear parametric family could describe all such calibration curves, and this was confirmed by experiment. From this result, a technique was devised for cooperative interaction between modalities, in which the vision sense provided on-the-fly determination of calibration parameters for the proximity detectors, for every approach to a target, before passing control of the system to the other modality. This technique provided a three hundred percent increase in useful manipulator velocity, and improved performance during the transition of control from one modality to the other.