A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

A new computer-based technique for automatic 3-D shape measurement is proposed and verified by experiments. In contrast to the moire contouring technique, a grating pattern projected onto the object surface is Fourier-transformed and processed in its spatial frequency domain as well as in its space-signal domain. This technique has a much higher sensitivity than the conventional moire technique and is capable of fully automatic distinction between a depression and an elevation on the object surface. There is no requirement for assigning fringe orders and interpolating data in the regions between contour fringes. The technique is free from errors caused by spurious moire fringes generated by the higher harmonic components of the grating pattern.

Fourier transform profilometry for the automatic measurement of 3-D object shapes

The high resolution surface profile of a 3-D diffuse object is obtained by measurement of the phase distribution across the image of a projected sinusoidal grating deformed by the surface. A shearing polarization interferometer is used for projection. Deformed grating images are detected by an array camera and processed by a microcomputer. Surface height resolutions of better than 10 μm have been attained, limited essentially by electronic quantization noise. In contrast to direct deformed grating analysis, this method, based on phase-shifting interferometric techniques, is capable of accurate measurement even with coarse projected gratings and low density image sensing arrays. Areas of application include industrial quality control, biostereometrics, robotics, and solid modeling for computer graphics.

Automated phase-measuring profilometry of 3-D diffuse objects

There are some difficulties in applying 3-dimensional computer vision to industry. One of them is that a vast amount of computation is required for low level processing. Some special hardware systems are described and one device is shown in more detail. Applications of the hardware are discussed in three examples. Two methods for range data acquisition employing special processors are described. Then our studies for range data processing are introduced. Finally, a complete production system is proposed which makes use of unified geometric models to be shared by CAD, CAM, and visual processing. This approach can solve a second difficulty in applying 3-dimensional computer vision to industry, namely the problem of the extensive programming effort that is required.

Three-Dimensional Computer Vision

For a number of years, observations have been made of contour-like patterns created by the moire interference of a grid with its shadow cast onto a surface. The conditions under which these patterns correspond to planar contours are derived, and systems to generate these contours are suggested. It is shown that moire contour patterns provide a useful and flexible means of contouring surfaces. Experimental results are presented.

J B Allen

Papers

Generation of surface contours by moiré patterns.