Optics: measuring and testing – Shape or surface configuration – By focus detection
Reexamination Certificate
1999-05-17
2001-07-24
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
By focus detection
Reexamination Certificate
active
06266148
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a process for surface measurement using confocal microscopy in a reflection process, especially for measuring the surface profile of teeth in the untreated and in the treated or drilled conditions.
This is essentially a process for measurement of surfaces of any type and any contour. Various processes for surface measurement are already known in practice.
For instance, a line of light can be projected on the object with a light sectioning sensor, and observed at an angle with a CCD camera. The geometric deformation of the line is measured. The height differences on the object are computed from this deformation. By moving the object under the sensor—perpendicularly to the light line—and by repeated measurement, a surface form can be measured or determined from a series of profiles.
The light-sectioning sensor is indeed a simply designed and thus a robust sensor, but the oblique lighting which it requires causes unilateral shading of steep surfaces. That causes asymmetries in the imaging, or inaccuracies. Furthermore, error can be introduced into the measurements because of scattering of light from various depths of, for instance, an at least partially transparent tooth material.
Furthermore, it is already known in practice that surfaces can be scanned with confocal microscopy so as to generate three-dimensional pictures of the surface. In this respect, it is only necessary to refer to an article by J. Engelhardt and W. Knebel in Physik in unserer Zeit′ [Physics in Our Time], Vol. 24, 1993, No. 3, titled “Confokale Laserscanning-Mikroskopie” [Confocal Laser Scanning Microscopy], and one by D. K Hamilton and T. Wilson in Appl. Phys., B27, 211-213, 1982, titled “Three-dimensional Surface Measurement Using the Confocal Scanning Microscope”. Confocal microscopy is very specially suited to surface measurements of tooth surfaces, because this process images only those structures which are exactly in the focal plane of the microscope objective. Thus measurement errors due to the partially transparent tooth material are effectively avoided. To be sure, the method of reflection measurement with the usual confocal microscope fails at steep transitions or flanks if their angle is greater than the aperture angle of the objective, because then the reflection no longer enters the objective, and is lost for evaluation. (See P. C. Cheng and R. G. Summers in: “Handbook of Biological Confocal Microscopy”, Plenum Press, New York, 1989, Chapter 17.)
SUMMARY OF THE INVENTION
Therefore this invention is based on the objective of providing a process for surface measurement with which it is possible to measure surfaces of partially trasparent materials, and surface profiles with steep flanks, without problems. This process should be very particularly suitable for use in dentistry, that is, to measure the surface profile of teeth in the untreated and in the treated or drilled conditions.
The surface measurement process according to the invention attains the objective stated above by a process for surface measurement using confocal microscopy in a reflection mode, especially to measure the surface profile of treated or drilled teeth, which is characterized by confocal imaging with high dynamic range (relative sensitivity) so as to image reflections and also weak scattered light or fluorescent light from the particular focal plane.
It is learned here for the first time, according to the invention, that confocal microscopy is quite particularly suited to surface measurement of partially transparent materials, because in confocal microscopy only those structures which are exactly in the particular focal plane of the microscope objective are imaged. It is also learned that the disadvantage of ordinary reflection confocal microscopy with respect to the aperture problem mentioned above can be eliminated by utilizing scattered light or fluorescent light from the particular focal plane for the usual evaluation of the reflection.
The evaluation of the scattered or fluorescent light can be accomplished in a further manner according to the invention through confocal imaging with a high dynamic range, i.e., with high relative sensitivity, so that it is possible to image both strongly reflecting flat surfaces and also to show the scattered or fluorescent light even on steep flanks. Accordingly, imaging is possible by the process according to the invention even if the light reflected from steep flanks misses the objective so that, in the usual reflection process, no profilometry can be done. Finally, the scattered light is always used for evaluation if imaging is no longer possible in the absence of specular reflections by the usual confocal microscopy.
As already mentioned above, the detector signal is digitized at high resolution, particularly advantageously with a dynamic range substantially greater than 8 bits. The relative sensitivity, or dynamic range, of the confocal imaging can be 16 bits for very particularly effective utilization of the weak scattered light or fluorescent light in the vicinity of steep surface slopes.
An algorithm is provided to evaluate elevations, or to produce the surface profile, using weak scattered light. It takes into consideration, or tolerates, the high dynamic range of the system. This algorithm takes the nearest, or indirectly adjacent focal planes into consideration by interpolating, with the higher intensities in the local region being relatively over-weighted so as to reduce the dependence on the background signals. Finally, a suitable algorithm is provided, so that, after detection of the scattered light signal and after high-resolution digitizing, an adequate height evaluation can be made from the digitized signal.
It must be emphasized here that the surfaces can also be scanned with a dark-field system. Either a point light source or a light source appropriately diaphramged can be provided.
In the area of application to dentistry, and particularly to producing exactly fitted inlays instead of the usual amalgam fillings, it is very specially advantageous first to scan the surface of the untreated tooth and to store the detected values, preferably digitized and already processed to give the height profile. In the next step the tooth is treated or drilled. Then the treated or drilled tooth is scanned again, again with storage of the values giving the surface profile of the treated tooth. From the difference between the two surface profiles, or from the values across the surface profile, the surface, or the exact measurements, are calculated for the inlay required so as to give optimal occlusion of the treated tooth.
It is highly advantageous, to get particularly high precision in processing the inlay, if the inlay being produced is scanned after an initial processing, and if the further processing is done by means of correction values obtained by a comparison of the actual and desired values. Correction to verify the inlay shape is possible to the extent that, with repetition of this process, high precision is possible in producing the inlay and optimal occlusion is possible. The measures described above also allow consideration of inaccuracies caused by the equipment or the tools, such as tool wear, to be taken into consideration so that optimal fitting of the inlay and thus optimal occlusion are possible even with a tolerance range at the processing station.
It is also possible that, in a subsequent step, the cavity produced in the tooth may be filled with a plastic composition so that, when the patient bites on it, the contact points with the opposing teeth are marked in the plastic mass. Then the surface profile generated in that way is scanned, the measurements obtained with respect to the surface profile are stored, and they are taken into consideration in calculating the surface or dimensions of the inlay to be produced.
REFERENCES:
patent: 4732473 (1988-03-01), Bille et al.
patent: 5120953 (1992-06-01), Harris
patent: 5448359 (1995-09-01), Schick et al.
patent: 5532873 (1996-07-01), Dixon
pa
Engelhardt Johann
Zapf Thomas
Leica Microsystems Heidelberg GmbH
Rosenberger Richard A.
Simpson, Simpson & Snyder, L.L.P.
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