Optics: measuring and testing – By light interference – Holography
Reexamination Certificate
2001-08-17
2004-01-06
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Holography
C356S458000, C356S035500
Reexamination Certificate
active
06674531
ABSTRACT:
BACKGROUND OF THE INVENTION
The option of using interferometric methods, such as the ESPI (electronic speckle pattern interferometry) or the shearing technique for determining areas of displacement or expansion on a test object with diffuse scattering surface is known. The desired result and the required resolution or rigidity of the text object relative to the magnitude of the applied forces determine which method can be used for a given test object. However, all test methods for determining specific areas have in common that a camera is used for imaging the object on an image sensor. Regardless of the measuring principle used, these camera-based methods consequently produce results in the form of images. If a CCD chip is used as image sensor, then the generated (resulting) images consist of an endless number of image points or pixels. The images from the test system camera and, in the final analysis, the intensity values stored in the individual image points represent the starting data for any further processing of the measuring results. Digital image processing systems are used almost without exception for the continued processing of the measuring results. With the above-mentioned test methods, two test object states are normally compared during the static test by taking pictures of the object in two different conditions of stress and by subtracting the interferograms of the two states. As a result, a difference interferogram is obtained, which represents either the displacement or the expansion of the object between the two states in the form of interference lines, depending on the measuring principle used. The amount of the displacement or expansion at one image point of the difference interferogram in that case can be determined by counting the interference lines, starting with one image point with known displacement or expansion and taking into consideration the light wavelength used. If the measuring head is equipped with a phase displacement unit, an additional evaluation based on the principle of the phase-shifting technique can be carried out (W. Osten, “DIGITALE VERARBEITUNG UND AUSWERTUNG VON INTERFERENZ-BILDERN” [Digital Processing and Evaluation of Interference Images], Chapter 6, Akademie Verlag [Publishing House] ISBN 3-05-501294-1). In the process, images are created that assign a specific phase angle to each image point. If the phase images are subtracted from two states of the object, a phase difference image is obtained. In contrast to the above-mentioned difference interferogram, the phase difference image does not show sine-shaped modulated interference lines, but shows directly the phase difference angle between the second and the first state. Another advantage of this representation is that owing to the computing rules used for the phase shifting technique, the phase angle is standardized, meaning the gray value that corresponds in a phase image with a phase angle is always constant, regardless of the image coordinate.
Depending on the shape and size of the test object, the interferometric measurement frequently must be taken successively or simultaneously over several sections. To be sure, the camera of the measuring system cannot detect and subsequently evaluate more than the camera can see of the test object in its actual position and in one view. In order to detect the test object completely, the test object must therefore be photographed from several different positions, wherein this action can occur parallel if several measuring heads are used simultaneously. The interferometric testing of tires, for example, represents a typical use of this type is. Thus, German Patent 42 31 578 C2 describes a method for detecting deformations on test objects, wherein the exemplary embodiment shows that a sector-by-sector testing of tires in particular was considered. U.S. Pat. No. 5,786,533 suggests a method for the purposeful localization of defects in tires, in particular in the belt area. In this case as well, the arrangement of the illustrated measuring head and test object permits an evaluation of the total belt region only if the testing occurs over several sectors. With the above-mentioned exemplary embodiments, a search is typically conducted for structural defects located under the surface, which cannot be detected from the outside. The camera of the interferometric measuring head delivers perspective images of the objects, which additionally show this object more or less distorted, depending on the projection lens used. If the defects displayed in the result images are to be localized on the test object, problems arise because the resulting image of the object is shown geometrically distorted and an exact coordination of image position and object coordinates is therefore not possible. If the defect is not visible from the outside and if the images otherwise do not contain visible features such as marks, edges, and the like that would characterize its position on or in the test object, then only a rough guess of its true position is possible with the aid of result images. A further difficulty is the unknown orientation of the result images from different surface sections, relative to each other, if the test object is measured section-by-section. The images of different surface sections in that case will overlap more or less or there will be gaps of an unknown size. A quantitative, computer-supported evaluation of the results is consequently more difficult, e.g. because an exact determination of the area of the detected defects is not possible. For example, if the overlapping areas are not determined, then defects located in these areas are counted several times and incorrect results are obtained. Thus, it is frequently requested that all measured sections of the surface be displayed in a joint coordinate system. Frequently, marks or numbered lines, visible on the camera images, are simply painted onto the test object as auxiliary aids. However, this course of action is very involved because the marks must be applied anew to each test object and may have to be removed following the measuring operation. It is also possible to use the edges of objects in the video image for the orientation and then attempt to rectify the images interactively via suitable image processing functions. This course of action, however, cannot be used with an automatic test run and without specially trained personnel. A method is described, among other things, in patent WO 97/05449 A1, for which the three-dimensional surface contour of an object is measured in sections. A surface model of the object is generated from the contour data. The measuring head for the three-dimensional detection of the object surface additionally comprises a device for detecting the color information on the object surface. By means of a so-called texture-mapping technique, the obtained color information is transferred point-by-point or section-by-section onto the generated surface model. Finally, a surface model is obtained that not only displays the shape of the object, but also its coloration true to the original.
SUMMARY OF THE INVENTION
Starting with this prior art, it is the object of the invention to provide a method and a device for exactly determining the spatial position of the interferometrically obtained measured values on the object and for spatially coordinating the individual interferometrically measured surface sections relative to each other. The process should run down automatically and should not require interactive steps by the user. Furthermore, the method should make it possible to transfer the interferometric measuring results to a joint coordinate system.
The above object generally achieved according to a first aspect of the invention by a method for testing the deformation of test objects with diffuse scattering surface that are subjected to different stresses, for which method the object surface is tested section-by-section with an interferometric measuring head with a camera that uses an interferometric technique for measuring an area, the areas of displac
Connolly Patrick
Smith Stuart I.
Turner Samuel A.
Venable LLP
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