Jeffery A. Hooker

Adaptation of video moiré techniques to undersea mapping and surface shape determination

Abstract Fixed and variable resolution video moire techniques have been used to project structured illumination in a model undersea environment and a prototype system has been developed which generates equal depth contours of undersea objects and has applications in sizing, orientation and ranging. An advantage of this system is that the entire field is continously illuminated, and the moire contours and images are formed at video rates. The spatial frequency of the structured illumination can be continously varied, providing optimal contours for a variety of object sizes. The data can be easily interpreted by eye or processed by computer to obtain surface shape, range and orientation of a known structure.

Generation of surface shape from variable-resolution video moire contours

Several methods for generation of three dimensional surface shapes from variable resolution video moire contours are described. In a classical moire system, a physical grating is projected on a target and also used to view the target. The moire contours are generated in the plane of the viewing grating. An unambiguous surface shape can then be computed by processing a set of moire images where the grating, target, or both are moved. By using an interferometer to generate and project variable pitch gratings and video technology to generate the moire contours, a 3-D surface can be scanned at different resolutions and used on a wide range of object sizes. The elimination of the physical grating also leads to surface generation techniques that do not use moving parts, increasing reliability. From these video moire contours, it is possible to uniquely reconstruct the 3-D surface, making the distinction between concave and convex surfaces. In one technique, a computer is used to mix digitized images of distorted gratings projected on the object with computer generated gratings, creating the moire patterns. By shifting one grating, it is possible to reconstruct the surface without having to move the object being scanned.

Video Applications to Moiré Metrology

Video technology is applied to the problem of moire metrology. In the past, moire metrology seemed a promising yet limited method in the measurement and comparison of surface shape. The use of video technology has widened the area of application of moire metrology by reducing the complexity of the optical set up and providing real time information on surface shape and deformation. A continuously variable grating projector and analog video circuitry are used to generate real time additive (bright line) and transmissive (dark line) moire patterns. These patterns are used to compare a test object against a “perfect” reference object. This is done in both real time and through the use of computer image processing. Depth resolutions on the order of 0.3 mm are obtained on a cone 25.4 mm high and 50.8 mm wide. The projection system allows easy expansion to large objects. Because of the use of video technology moire metrology can now be more readily applied to robotic vision and factory assembly line quality cont...

Variable-resolution video moire error map system for inspection of continuously manufactured objects

Moire techniques can be a powerful tool to determine deviation of a manufactured shape from a desired shape. In a traditional moire system, distorted gratings on an object are viewed through an undistorted grating. The moire contours that result represent equal depth contours over the entire viewed surface. By generating the moire patterns in video, it is possible to view the distorted gratings on a test object through a set of gratings that has been distorted by a similar but perfect object. The output is then a set of moire contours that corresponds to the differences between the two surfaces. This difference or error map eliminates much of the unnecessary information generated in traditional moire inspection and thus becomes a valuable tool for comparisons between an imperfect test object and a manufacturing standard. We have developed a variable resolution video system for creating this error map using a Michelson interferometer to generate the gratings. We have successfully applied this system to damage detection on a long, continuous lengths of pipe by having two side-by-side cameras looking at different sections of pipe and also by having one cameras view filtered with a video-taped recording of an undamaged section of the pipe.

Application of acousto-optic cells and video processing to achieve signal-to-noise improvements in variable resolution moire profilometry

Moire techniques can be a powerful tool to determine surface shape or deviation of a shape in progress from a final or desired shape. The presence of the high-contrast viewing grating and the distorted grating in the final image plane makes the moire pattern hard to see. Moving grating techniques have been developed to improve the visibility of the moire pattern, but at the expense of complex moving parts. Several variable resolution projection moire techniques have been developed that either move the grating or eliminate its presence electronically, and have neither mechanical moving parts nor any physical gratings. One system uses an acousto-optics cell to generate, project, and move the gratings, while the moire is viewed through a second synchronized A-O cell. The second system uses an interferometer to generate and project variable spacing gratings that are made to move across the target and across a reference surface by an A-O beam deflector. Video processing of the reference image generates the transmissive filter that produces the moire pattern. A third system removes the grating presence electronically but retains high-contrast moire contours. Noise reduction is shown in a series of moire images of targets.

3-D Inspection of Large Objects by Moire Profilometry

A moire profilometry method has been developed that is capable of inspecting objects whose size is limited only by the available laser power. The moire contours are generated in real time by projecting variable spacing gratings upon the target and filtering the image either optically in the video signal or in the computer. The system is designed to compare the surface shape of a test object with a previously stored image of a perfect object and to display the surface errors superimposed on a video image of the test object. A unique feature of the system is the continuously variable depth resolution which makes it usable either for human or robot adjustment of the test object or for automated pass/fail inspection.

Real-time generation of intersection of surfaces for welding by video moire

A common problem in fabrication and welding of complex structures is that there is no simple way to determine where to cut one part so that it will fit another part unless both parts designed and built on a CAD/CAM system. Particularly in prototype or retrofit work, cutting and fitting parts for welding is more of an art than a science. We have developed a unique video moire system that generates the intersection contour in near real time with the contour superimposed on a video image of the part, allowing the cut line to be marked while following the contour on the video monitor.

Detection, location, and quantification of structural damage by neural-net-processed moiré profilometry

The development of efficient high speed techniques to recognize, locate, and quantify damage is vitally important for successful automated inspection systems such as ones used for the inspection of undersea pipelines. Two critical problems must be solved to achieve these goals: the reduction of nonuseful information present in the video image and automatic recognition and quantification of extent and location of damage. Artificial neural network processed moire profilometry appears to be a promising technique to accomplish this. Real time video moire techniques have been developed which clearly distinguish damaged and undamaged areas on structures, thus reducing the amount of extraneous information input into an inspection system. Artificial neural networks have demonstrated advantages for image processing, since they can learn the desired response to a given input and are inherently fast when implemented in hardware due to their parallel computing architecture. Video moire images of pipes with dents of different depths were used to train a neural network, with the desired output being the location and severity of the damage. The system was then successfully tested with a second series of moire images. The techniques employed and the results obtained are discussed.

Signal-to-noise improvements in moiré profilometry by acousto-optic scanned gratings and video processing

Moire techniques can be a powerful tool to determine surface shape or deviation of a shape in progress from a final or desired shape. The presence of the high contrast viewing grating and the distorted grating of the final image plane makes the moire pattern hard to see. Moving grating techniques have been developed to improve the visibility of the moire pattern, but at the expense of complex moving parts. We have developed several variable resolution projection moire techniques that either move the grating or eliminate its presence electronically, and have neither mechanical moving parts nor any physical gratings. One system uses an acousto-optics cell to generate, project, and move the gratings, while the moire is viewed through a second synchronized A-O cell. The second system uses an interferometer to generate and project variable spacing gratings which are made to move across the target and across a reference surface by an A-O beam deflector. Video processing of the reference image generates the transmissive filter which produces the moire pattern. A third system removes the grating presence electronically but retains high contrast moire contours. Noise reduction is shown in moire images of targets ranging in size from 1 to 700 cm.