John T. Atkinson

The use of carrier frequency shifting for the elimination of phase discontinuities in Fourier transform profilometry

This paper presents a novel application of frequency shifting in the Fourier transform fringe analysis technique applied to surface shape measurement. In recent years little use has been made of frequency shifting, although it was a feature of the method as originally proposed. The paper shows that if the angle between fringe projection and viewing is small, it is possible to employ a frequency shift to effectively eliminate the occurrence of any wraps in the resulting phase distribution. This is obviously of value in cases where the object to be measured contains several discontinuities which would lead to a highly complex, maybe unprocessable, wrapped phase distribution. The technique is presented and analysed, and its effects on measurement resolution discussed. It is illustrated with a measurement made on a populated surface mount technology printed circuit board.

Three-map absolute moiré contouring

In the shadow-moiré system, the period of the grating is varied by rotation of the grating, so the phase of the moiré pattern is changed as well. By the selection of suitable rotation angles, three images at different positions of the grating are acquired to obtain the absolute distance from the object to the grating. A theoretical analysis is presented for the method, and some experiments have been done to verify the theoretical analysis. The results show that the method is fast and the accuracy is better than 10 μm. The measurable range is directly proportional to the period of the grating and inversely proportional to the angles at which the grating is rotated.

The application of digital filtering to phase recovery when surface contouring using fringe projection techniques

Abstract If structured light consisting of parallel stripes, or fringes, is projected onto a surface, then the surface acts as a phase modulator, with the amount of modulation at any point depending upon the height of the surface at that point. In recent years considerable effort has been devoted to the problem of fringe demodulation, with prominence given to techniques using fringe-phase stepping and Fourier analysis. It has long been known that phase demodulation is possible using a system of filters, and the technique has been widely used in the related area of frequency demodulation in radio. In this paper the development of phase demodulation using a system of digital filters is considered. For the accurate recovery of image phase it is necessary for the filters to introduce zero phase shift, or to have a phase shift proportional to frequency. The design of the digital filters is considered and their performance is assessed using the signal from a real modulated fringe pattern and a simulated signal. It is shown that the demodulation technique works well, even with a poor signal-to-noise ratio.

Phase-measuring methods for measurement of three-dimensional shapes in automated inspection of manufactured electronic assemblies

This paper describes research into a noncontact system for the measurement of components and solder on printed circuit boards (PCBs). The objective of the system is the inspection of the PCB to identify faults in the manufacturing process. A fringe pattern is generated on the PCB, using either interferometric methods or projection of a grating, and the image of the fringe pattern is captured and stored on a frame store. The pattern is then processed to reconstruct the 3D shape of the PCB surface from which parameters which relate to PCB integrity can be determined. Results demonstrate the feasibility of the method.

Inspection of ceramic tableware for quality control using a neural network vision system

Automated ceramic tableware inspection displays many of the problems associated with automated inspection. A large number of possible faults must be considered, further complicated by the large variety of patterns with which the tableware is decorated. A hierarchical approach is taken in this work involving several levels of feature extraction and classification. Faults are detected, evaluated and the item as a whole is classified as a function of the combined fault measurements. Faults can be categorized as visual, surface, and structural. This paper concentrates on one visual fault known as shading variation, a result of color pigments registering as light, dark, or irregular. This fault is evaluated by analysis of the image intensity histogram. The histogram displays characteristics which vary with shading. Deviations from the standard are evaluated and the results passed onto a further classification stage which takes results from other feature extraction stages to give an overall evaluation of the item under inspection.

Effects on absolute moiré contouring by using gratings with different period and profile and rotating angles

Abstract Effects on the absolute moire contouring systems from using different period and profile gratings and rotating angles are described in this paper. It was found that by choosing a different period of the grating or the rotating angle, different resolution and accuracy of the measurement can be obtained. So, for objects with different height or depth, different period of grating or rotating angle should be chosen to ensure accuracy of the measurement. The square wave intensity grating with small period can be treated as a sinusoidal wave intensity grating in the absolute moire contouring system. A few objects with different shapes and dimensions have been used as samples to verify the above conclusion.

Diffraction pattern analysis for automatic defect classification in manufactured electronic assemblies

A technique for the quality assessment of fine-pitch electronic components is described. A system for the capture of Fraunhofer diffraction patterns reflected from the component is presented. The processing of the image to enable classification by a neural network is discussed. Simulation results are presented, showing the feasibility of the technique.

Optical measurement of solder bonds on printed circuit boards

This paper describes research into a non-contact system for the measurement of components and solder on printed circuit boards (PCBs). The objective of the system is the inspection of the PCB to identify faults in the manufacturing process. Two approaches are described: diffraction pattern analysis and fringe pattern analysis. For the diffraction pattern analysis, a diffraction pattern is formed by the lead array on a component. For fringe pattern analysis, a fringe pattern is generated on the PCB using either interferometric methods or projection of a grating. For both methods, an image of the pattern is captured and stored on a frame store. The pattern is then processed to provide data from which parameters which relate to solder joint integrity can be determined. Results demonstrate the feasibility of the methods.

Detection and measurement of surface defects by automatic fringe analysis

Abstract A novel measurement and inspection system for the detection and measurement of surface defects, such as chips and cracks, has been developed and applied to engineering components such as cylindrical rollers. The position, depth and volume of such a defect can be determined automatically. Measurement of such defects requires that three-dimensional information be made available for computer analysis. This is achieved by using a grating projection technique to provide contour fringes on the object, i.e. to give depth information. The contoured object is viewed by a charge coupled device (CCD) camera, the output of which is digitised and stored in a frame buffer from which it is available to a computer system for digital image processing, analysis and comparison. Calibration tests on defects of known size have confirmed that the system is fast and accurate.

Neural network classification of Fraunhofer diffraction patterns for inspection of fine pitch electronic components

This paper describes research into a non-contact system for the inspection of fine pitch electronic components on manufactured electronic assemblies. A far-field diffraction pattern from the leads of a tape automated bonding component is captured and stored on a frame store. The diffraction pattern is statistically represented and then classified using an artificial neural network. Results from simulation and experimentation show the feasibility of the technique.