Optics: measuring and testing – By polarized light examination – With light attenuation
Patent
1997-01-21
1999-02-16
Font, Frank G.
Optics: measuring and testing
By polarized light examination
With light attenuation
G01B 1124
Patent
active
058726314
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for dimensional measuring, and an apparatus for carrying out the method;
The present invention can be used for generally determining the dimensions of objects of all sizes, but it is especially well suited for determining the dimensions of objects or parts of objects having typical dimensions in the decimeter-micrometer range. It can be of assistance in providing precise and absolute measurement details ranging from percentages to fractions of one tenth per thousand of the dimensions to be determined.
The invention is especially advantageous in cases where the dimensions of convex surfaces or convex parts of surfaces and protrusions, located approximately parallel to the direction of observation, are to be determined. Good examples of such surfaces are the interior walls of wire-drawing dies, nozzles, extruders, ferrules, or the exterior surfaces of wires, fibers, cutting tools, etc.
Objects to be dimensionally determined have in general a three-dimensional demarcation against the surrounding space. Dimensional measuring usually involves characterizing these demarcations by means of diverse distance and angular measures. Optical imaging techniques produce an image of the object. In that process much of the three-dimensional information is lost, and we obtain images which mainly reproduce two-dimensional sections. The parts of the object that lie out of focus will be blurred, and it will be difficult to determine the dimensions of these parts. Seen from an image-processing point of view, the more or less steep steps in the light intensity of the images are generally used as reference positions for the desired dimensional measurements.
In order to find an accurate measuring position, the steps in the images are data processed by means of threshold determination or other, more complicated, algorithms. For example, it is usual to inspect the change in light intensity by means of variations of mathematic derivation followed by center-of-gravity resolution.
These techniques have three important shortcomings: parallel to the direction of observation; approximately parallel to the direction of observation; in light intensity implies uncertainty with respect to geometric positions in the object.
The present invention is an optical imaging technique which, by means of mechanical movements, achieves advantages particularly with respect to the three points mentioned above.
At the present time there is a large selection of optical methods and techniques available for determining dimensions. We shall here briefly describe some of the most important principles and explain some of their advantages and disadvantages.
In general it can be said that optical measuring instruments are frequently very suitable for determining the dimensions of an object.
Lately, the use of relatively sophisticated arrangements has become common. Some of these consist or a more or less conventional microscope objective in combination with a TV camera, video digitalization electronics, and a computer. For somewhat larger objects the microscope objective is replaced by one form or other of a macro or zoom imaging objective. Depending upon the objects, the illumination optics is either arranged with direct lighting and measurement in reflected light or with back lighting and measurement in the shadow image of the object. This gives direct images of objects the dimensions of which are to be determined. More or less steep steps in the light intensity of these images will then be data-processed in order to provide precise positions which can be used as measuring positions for the desired dimensional measurements. These techniques function best when that part of the object of which the dimension is to be determined has a two-dimensional structure which simultaneously can be brought into focus. Thus, they measure mainly in two dimensions perpendicular to the direction of observation.
There are also optical measuring instruments which provide dimensional measurements in all three dimensions. These can be classifi
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Font Frank G.
Smith Zandra V.
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