Optics: measuring and testing – Inspection of flaws or impurities – Bore inspection
Reexamination Certificate
2000-05-05
2002-10-01
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Bore inspection
Reexamination Certificate
active
06459481
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
(Not Applicable)
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical metrology, and particularly to the problem of making accurate non-contact dimensional measurements of objects that are viewed through an endoscope.
2. Description of Related Art
A. Endoscopic Measurements
Making accurate dimensional measurements of objects viewed through endoscopes is important to aerospace as well as other industries in which expensive equipment must undergo periodic internal inspections to maintain safe operation. Such measurements also have medical applications, where the internal condition of a patient is evaluated prior to or during surgery by viewing through an endoscope.
The fundamental problems in making an accurate measurement through an endoscope are that the magnification of the image varies rapidly with the range of the object, and that objects of interest (defects) lie on surfaces which are curved in three dimensions; thus the magnification varies from one point on the object to another. What is needed is a fully three-dimensional measurement, that is, one which determines the depth, as well as the height and width, of an object.
Endoscopes are long and narrow optical systems, typically circular in cross-section, which can be inserted through a small opening in an enclosure to give a view of the interior. They almost always include a source of illumination which is conducted along the interior of the scope from the outside (proximal) end to the inside (distal) end, so that the interior of the chamber can be viewed even if it contains no illumination. Endoscopes are divided into two basic types: these are the rigid “borescopes” and the flexible “fiberscopes” or “videoscopes”.
Probably the simplest approach to obtaining quantitative object size information is to place a physical scale in contact with the object to be measured. U.S. Pat. No. 4,825,259 to Berry teaches this approach, as does Diener, in U.S. Pat. No. 5,803,680. Berry attaches the scale to the distal tip of an endoscope, while Diener attaches the scale to the distal end of a remote machining apparatus, where the apparatus includes an endoscope.
One problem with this approach is that it is sometimes not possible to insert the desired scale through the available access port. As an alternative, Watanabe in U.S. Pat. No. 4,721,098 teaches the construction and use of a measurement scale apparatus that has both a collapsed configuration and an expanded configuration, so that the scale can be passed through the access port when the apparatus is collapsed, and then the apparatus can be erected to an operating configuration once it is near the object of interest.
Other problems with such direct physical scale approaches are that the objects of interest are almost never flat and oriented in the correct plane so that the scale can lie against them, and that it is often not permissible to touch the objects of interest with a rigid scale. Even the seemingly simple task of bringing the scale into contact with the object so that the points of interest on the object are adjacent to the indicia of the scale is often difficult. In many cases, the precision with which the position of the scale can be manipulated is insufficient. In addition, it is difficult to determine that all of the desired indicia on the scale are in contact with the object rather than lying either in front of or behind the object.
Even when the object is suitable for measurement with a physical scale and the scale can be manipulated satisfactorily, there is an additional requirement to manipulate the end of the endoscope into the correct position to make the desired measurement accurately. Ideally, the scale is oriented perpendicular to the line of sight of the endoscope. If the scale is not so oriented, then determining when the points on the object are aligned with the indicia of the scale becomes more difficult and subject to error.
In the current marketplace, there are many existing so-called endoscopic “measurement” systems that simply make straightforward two-dimensional measurements based on applying a scale factor to the image viewed through the scope. Such systems are today almost always implemented with a video camera attached to the proximal end of the endoscope, and with a digital “frame grabber” being used to acquire and store the video images. The measurement is then made simply by counting “pixels” between features of interest in the digital video images, and a computing device multiplies this number by an appropriate scale factor, where the scale factor may or may not take into account the Seidel distortion of the optical system.
Such systems are sometimes useful, but they are severely limited by the following requirements for the two dimensional measurement to have a meaningful relationship to the true dimension of the object. First, the object being measured has to be oriented at a known angle with respect to the line of sight. Second, the distance of the object from the endoscope optical system has to be known in order to determine the correct scale factor. The angle of the object can sometimes be estimated from the known geometrical relationship of the various components of the device being inspected. The magnification of the object is sometimes estimated by incorporating an auxiliary object of known size, such as a wire, to be compared to the object of interest, or the distance is estimated by adding a physical projection to the endoscope. An example of the latter is the system of Krauter, U.S. Pat. No. 5,047,848, in which a flexible distance gauging element is attached to the distal tip of an endoscope.
Clearly there are many sources of error in such “measurements”—these are in fact crude estimates that are useful in only a limited set of circumstances. One cannot often depend on these for critical applications. Thus, much effort has gone into the development of non-contact, truly three dimensional, methods of measurement through endoscopes.
Many of the prior art approaches to three-dimensional, non-contact measurements involve adding optical projection apparatus to the distal tip of the endoscope. Besides having the problem that the measurement precision is poor, these approaches inherently involve adding apparatus to an area where space is already at a premium. The distal tip of the endoscope must be kept as small as possible in order to allow inspections in close quarters, and to allow the endoscope to be inserted through access ports which are as small as possible. Thus, ideally, one wants to be able to make the measurement without adding anything to the distal tip of an endoscope.
U.S. Pat. No. 4,895,431, “Method Of Processing Endoscopic Images”, to Tsujiuchi, et. al., describes a number of methods to mathematically process two images obtained from different camera positions to derive object surface contour information. The two images are obtained by bending the end of a fiberscope. The bending is achieved using the internal articulation capability of the scope. Their technique assumes a simple linear relationship between the bend angle of the fiberscope and the offset of the nodal point of the optical system to estimate the geometrical relationship of the images. The image processing begins by correcting for distortion, then doing correlations over a series of smaller and smaller sub-images. The patent teaches that one can thereby derive full three-dimensional position data in the overlap region between the images.
One problem with this is that the bending of the end of a fiberscope is subject to a number of difficult-to-correct mechanical errors that make the feasibility of this approach questionable.
U.S. Pat. No. 5,432,543, “Endoscopic Image Processing Device for Estimating Three-Dimensional Shape of Object Based on Detection of Same Point on a Plurality of Different Images”, to Hasegawa, et. al., is an improvement to U.S. Pat. No. 4,895,431, which avoids the problem of mechanical errors. The new approach is to estimate the relative po
Stafira Michael P.
Tavella Michael
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