Abstract:
A method and device for detecting the contour data and/or optical characteristics of an object, such as a tooth or a tooth restoration, based on an interference and/or autocorrelation measurement using an image sensor. To permit an exact surface detection in addition to a determination of the optical characteristics of the object, individual light beams strike the object, which are located at a distance from one another in such a way that no impact of reflected individual light beams takes place on immediately adjacent pixels of the image sensor.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for recording contour data and/or optical properties of a three-dimensional semi-transparent object, especially a semi-transparent object in the dental area, such as a tooth or tooth restoration, on the basis of an interference and/or autocorrelation measurement, whereby
         a bundle of rays from at least one light source of short coherence length is generated,   the bundle of rays is passed through a beam splitter and is preferably guided to the object through a focusing optical system,   a reference beam is split off in the beam splitter from the bundle of rays and is reflected by a reference mirror movable along the reference beam, whereby by moving the reference mirror, a position relative to a signal gaining surface is fixed relative to the object, and   the beam reflected from the object and from the reference mirror are brought together in the beam splitter and transferred into an image sensor having pixels, whereby temporally and/or spatially altered signal patterns can be recorded upon passing through the signal recovering surface.       

     Furthermore, the invention makes reference to a device for recording contour data and/or optical properties of a three-dimensional semi-transparent object, especially a semi-transparent object in the dental area such as a tooth or tooth restoration, including at least one light source of short coherence length for generating a bundle of rays, a radiation component guiding the bundle of rays to the object through a focusing optical system on the one hand, and on the other into a beam splitter splitting up into a beam component leading to an adjustable reference mirror as well as an image sensor having pixels, which can be acted upon by the object and radiation reflected from the reference mirror and the beam splitter. 
     A method of the aforementioned type is described in German Patent DE-A-43 09 056, for example. With the known process, it is a matter of an interferometric method for ascertaining the distance and the scattering intensity of scattering points. These are illuminated by a broad band, spatially partially coherent light source and are located in one arm of an interferometer. An incandescent lamp or a super luminescence diode are indicated as light source. The light is separated into a spectrum and the output of the interferometer and information on the distance and the scattering intensity is ascertained on the basis of the brightness distribution in the spectrum. The disadvantage with the described method is that the resolution in the z direction, that is, into the depth of the object, is restricted. 
     In the article by Prof. G. Häsler: ““COHERENCE RADAR”—an Optical 3D Sensor with an Exactitude of 1 μm,” LASER INFO EXCHANGE, No. 36/April 1999, Association of German Engineers Technology Center, a method and a device for measuring a surface are described. The measuring principle rests upon white light interferometry, whereby local speckles are generated by a particular illumination selectively so that even the most distinct optically raw objects, such as milled surfaces or rubber, can be measured interferometrically. According to the method, one basically compares the length of the path of light for each object point with the length of the corresponding reference path of the interferometer. Only when the path lengths are approximately equal does an information contrast arise in the corresponding image point. While the sensor is moving toward the object, the point in time of the maximal interference contrast is determined individually for each image point and the respective sensor position is stored in memory. 
     A method and a device are known from German Patent DE-A-40 34 007, whereby the coating of the object is provided for obtaining three-dimensional data to avoid disturbing scattered radiation from the depth of a semi-transparent object such as, for example, a tooth or a dental filling which prevents this scattered radiation. This layer must nonetheless be applied by the dentist. This is thus an additional operation, which moreover can lead to irritations of the patient&#39;s respiratory passages due to the aspiration of dust articles in the event large areas of the dental corona are powdered. 
     In U.S. Pat. No. 6,697,164, a method and device are described, whereby the influence of a scatter beam is reduced through a confocal optical system. With this method, an array of incident light rays is guided into an optical beam path which is guided though a focusing optical system to a test surface. The focusing optical system defines one or more focal planes in front of the test surface in a position which can be changed by the optical system, whereby each light ray has its focus in reference to one or more focal planes. The rays generate a manifold of light spots on the contour. The intensity of each of these light spots is recorded. The steps mentioned above are repeated several times, whereby each time the position of the focal plane is altered relative to the contour. A light-point specific position is determined for each of the light spots which corresponds to a position of the respective focal plane which leads to a maximal measured intensity of a respective reflected light ray. Data are generated on the basis of the light spot-specific positions which represent the topology of the contour. 
     The described device for recording a surface topology of a region of a three-dimensional structure includes a probe with a contour to be scanned. Furthermore, a light source for generating an array of incident light rays which is transferred to the structure along an optical pathway is provided in order to generate light spots on the region. A light-focusing optical system defines one or more focal planes before the sample structure in a position which can be altered by the optical system. Each light ray has its focus on one or more focal planes. Furthermore, a displacement mechanism is linked with the focusing optical system in order to move this relative to the structure along the axis which is defined by the incident light rays. Moreover, a detector is provided with an array of sensor elements for measurement of the intensity of each of a large number of light rays which are reflected from the light spots opposite to the incident light. A processor is linked with the detector in order to ascertain a light spot-specific position for each light ray. Since a reflected light ray reaches the maximal intensity when the reflection position is situated in the focal plane, its specific position can be ascertained therewith. Data on the topology of the region are generated on the basis of the ascertained light spot-specific positions. 
     The influence of scattered radiation can be significantly reduced by using a confocal optical system from the aforementioned U.S. Pat. No. 6,697,194 B1. 
     A process and a device for measuring the contour data of an object can be derived from WO-A-95/33971. Here the interferometer principle is used, whereby a light source of coherence length is used. In order to subject the object to the action of light in sufficient spatial extension, there exists the possibility of expanding the light originating from a light source. The light rays running in the bundle of rays are nonetheless not separated from one another, but in part overlap one another. 
     A method for measuring dimensionings or optical properties of biological samples is known from U.S. Pat. No. 5,321,501, whereby the interferometer principle is likewise used. According to one embodiment, rapidly changing biological samples can be acted upon with radiation from different optical sources at the same time. Each ray source is allocated a detector. Several regions of the sample can be scanned parallel and simultaneously. 
     SUMMARY OF THE INVENTION 
     The present invention is based upon the objective of further developing a method and a device of the type mentioned in the beginning, such that an exact surface recording and an ascertainment of the optical properties in the desired extent can take place. At the same time, an adaptation to the conditions of the shape of the object and the optical properties should take place in so far as the rays can be adjusted to the desired extent. 
     As regards the method, the objective is accomplished in that the bundle of rays is being or is divided before impingement upon the beam splitter into spatially distanced parallel individual light rays, whereby the individual light rays have a spacing from one another such that an impingement of reflected individual light rays on immediately adjacent pixels of the image sensor does not occur. The scattered radiation generated by a semi-transparent object is detected with the remaining pixels which are not illuminated by individual rays. 
     While according to German Patent DE-A-43 09 056, the object is irradiated with a continuous illumination that is irradiated with a non-interrupted, uniform bundle of rays, owing to which an expensive evaluation is necessary, according to the invention, operations take place with a raster of spaced light rays. That is, with a bundle of rays of parallel light rays so that scatterings inside the object to be measured are detectable between the reflecting rays and the adjacent rays are not influenced. Here is it especially proved that the spacing of the individual light rays is adjusted such that two immediately adjacent individual light rays impinge upon pixels or pixel regions between which at least one pixel, preferably at least two to five pixels, are not acted upon directly by a reflected light ray. 
     Consequently, it is provided in accordance with the invention that each pixel illuminated by a reflected individual light ray or each correspondingly illuminated pixel group is surrounded by a primarily non-illuminated region, which once again can be illuminated in the event of scattered light. Regardless of this, this region acted upon by scattered light is designated as a pixel or pixel region not acted upon by an individual light ray. 
     Consequently, an exceedingly precise recording of the contour of the object to be measured is possible on the basis of the theory of the invention, whereby it can not only be a question of solid object, but can also be a matter of flexible objects, such as, for example, the mucosa. 
     It can be a matter of white light with the light source. Alternatively, the light can also be generated from at least one super luminescent diode or an array of single or several super luminescent diodes or at least one broad band high performance light diode or an array of single or several relatively broad banded high performance light diodes. Likewise, the combination of several laser diodes with central wave lengths offset in relation to one another is possible, whereby a wave length shift can lie in the range of 5 nm to 150 nm, preferably in the range from 10 nm to 50 nm. 
     Preferably the coherence length l c  of the light source used lies in the range of 2≦l c ≦20 μm, with an emission output P E  of the light sources in the range of 1≦P E ≦100 mW, preferably 3≦P E ≦50 mW. 
     In deviation from the state of the art, according to the method of the invention, surface as well as depth information can be obtained, and to be sure through selection of the suitable wave length in which the scattering coefficient of the object is correspondingly high or low. With measurements with light in near infrared, the scattering becomes less, since this decreases with increasing wave length. 
     It is provided in accordance with a preferred procedure that light of short coherence length of a single or several light sources be expanded though a beam expander and projected onto an array of lenses which generates a large number of parallel individual rays. The lens array can have a large number of lenses arranged like a honeycomb, through which the impinging bundle of rays is subdivided into the desired individual light rays running spaced and parallel to one another. The parallel light rays run through a beam splitter, a beam shifter and through an axially displaceable focusing optical system to the object whose geometrical data are to be measured. The beam shifter serves to shift the bundle of rays by fractions of the distance between the individual rays in order thus to heighten the resolution. 
     In a preferred embodiment, the beam shifter is constructed as a planar plate, which can be slightly tilted perpendicular to the ray direction in the x and y direction. A reference beam is split up in the beam splitter and reflected on a reference mirror. The reference mirror is arranged displaceable in the direction of the ray, thus in the axial direction, and establishes a signal recovering surface with the length of a reference arm, ideally a plane in one measuring arm of the interferometer. This can be identical with the focusing plane of the focusing optical system, but can also be different from this in order to obtain further information on the scattering behavior of the semi-transparent object for subsequent image processing. 
     The individual light rays reflected by the object and the reference mirror are brought together in the beam splitter and overlapped in the detector. If the path length differences of the reference arm and the measuring arm lie in the region of the coherence length of the light source used, maxima and minima are shown during axial motion of the reference arm mirror. 
     The same technical possibilities exist with a multiplanar wave guiding element as beam splitter. 
     In accordance with an alternative embodiment, the large number of individual rays can be generated directly in a VCSEL array (vertical cavity surface emitting laser). This has the advantage of individual addressability of the individual rays. 
     An extension of the method of the invention provides that the ray guidance can also be realized by dispersion-poor monomodal fiber bundle. Here the light source is launched into a large number of parallel guided fibers following expansion. The decoupling likewise takes place through a focusing optical system. The functions of the beam splitter take over the fiber coupler in this case. 
     A differential measurement can be conducted with at least two different wave lengths. These measurements can be conducted in accordance with the invention with wave lengths in which the semi-transparent object in each case has a very different scatter and absorption coefficient in order to compile a differential image on the basis of it. 
     To improve the signal-disturb signal ratio, it is provided that a large number of frames, that is, the overall image content of the image sensor of a defined period of time (sampling time), is filed in the memory of an image processing computer attached to the image sensor and cleared with one another during the traverse of the reference arm. 
     A device of the type mentioned at the beginning, through which the objective underlying the invention is accomplished is distinguished in that an optical element subdividing the bundle of rays into spatially distanced parallel single light rays is arranged between the light source and the beam splitter or the light source of the bundle of rays consisting of a large number of parallel individual light rays is constructed, whereby the individual light rays have a spacing from one another such that the impingement of reflected individual light rays on pixels of the image sensor directly bordering upon one another does not occur. 
     In accordance with the invention, a method or a device for recording optical and geometrical properties of three-dimensional, semi-transparent objects, especially of the dental region, such as teeth, composite materials, veneer ceramics through the use of an interference and/or auto-correlation measurement with at least one light source of short coherence length is proposed, which is distinguished by the following features or groups of features:
         A raster of defined spaced light rays, that is, a bundle of rays of parallel light rays, is used so that scatterings inside the object to be measured are detectable between the reflected rays, and the neighboring rays are not influenced,   At the same time, at the image sensor, only one subset of available pixels is directly illuminated by a corresponding light ray (at least one un-illuminated pixel interval between the illuminated pixels). Around each illuminated pixel or each pixel group (in the event that a light spot simultaneously illuminates several pixels lying alongside one another), there is located a primarily non-illuminated region which nonetheless in the event of the impingement of scattered light (in the semi-transparent object) is also illuminated,   A beam shifter is used to shift the bundle of rays by fractions of the spacing of the individual rays in order to increase the exactitude of the method by a large number of measurements displaced in relation to one another. Preferred embodiment of the beam shifter: plane parallel plate which is easily tilted. A displacement takes place between the frames by fractions of the distance of the illumination rays,   A combination of various laser diodes with offset central wave length is used, whereby a wave length offset can lie from 5 nm to 150 nm, preferably in the range from 10 to 50 nm,   Through suitable choice of the wave length at which the scattering coefficient of the object is correspondingly high or low, depth or surface information can be obtained,   A differential measurement of at least two different wavelengths can be conducted in which the semi-transparent object in each case has a very different scatter and absorption behavior in order to generate a differential image,   When using the spectral range in the near-infrared, for example 750 nm to 1000 nm, available depth information can then also be used for caries diagnosis,   In the event that two or more wave lengths are used simultaneously, an RGB variant of a CMOS sensor can be used which has sensitivity maxima in the red, green and blue spectral region,   The signal recovering surface (plane with maximal interference contrast) can be identical with the focal plane, but can also be different from this in order to obtain further information on the scatter behavior of the semi-transparent object for subsequent image processing,   In accordance with an alternative embodiment, a large number of individual rays can be generated directly in a VCSEL array. This has the advantage of individual addressability of the individual rays,   A launching of light into a large number of individual fibers can assume the function of the beam splitter.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further particularities, advantages and features of the invention become apparent not only from the claims, the features to be inferred from these—by themselves and/or in combination—but also from the subsequent description of the embodiments to be derived from the drawings. Features and feature combinations of the subsequently described embodiments are also expressly maintained in this connection, wherein: 
         FIG. 1  Illustrates a basic construction of a device for recording contour data of an object, 
         FIG. 2  Is a schematic diagram for clarification of the measuring principle, 
         FIG. 3  Illustrates coherence length of the light source as a function of its central wave length, 
         FIG. 4  Illustrates the scattering coefficient of enamel and dentine as a function of wave length, 
         FIG. 5  Illustrates the absorption coefficient of enamel and dentine as a function of wave length, 
         FIG. 6  Is a schematic diagram of a further arrangement for recording contour data of an object, 
         FIG. 7  Is a schematic diagram of a sensor housing and 
         FIG. 8  Illustrates the underside of the sensor housing in accordance with  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a schematic construction of a device  10  for recording contour data of a free form surface  12  of a semi-transparent object  14 . 
     The light of a light surface  16  of short coherence length is expanded through a beam expander  18  and projected on a lens array  20  which generates from this a bundle of rays  22  of a large number of individual rays. These run through a beam shifter  26 , a beam splitter  24  and through an axially displaceable focusing optical system  28  to the object  14 , such as a tooth, whose geometrical data are to be measured. 
     The beam shifter  26  serves to shift the bundle of rays  22  by fractions of the distance between the individual rays in order to increase the resolution. In this way, there exists the possibility of shifting the bundle of rays  22 , that is its individual light rays overall, with respect to place in order consequently to be able to measure the regions of the object  14  as well which possibly could not be recorded by other impingement points of individual light rays. 
     The beam shifter  26  can, for example, be constructed as a plane parallel plate which can be slightly tilted perpendicular to the ray direction in x and y. 
     A reference ray  30  is split in the beam splitter  24  and reflected on a reference mirror  32 . The reference mirror  32  is arranged displaceable in the direction of the reference ray  30 , thus in an axial direction in accordance with arrow  34  and establishes with the length L of a reference arm  36  a signal recovering surface  38 , ideally a plane, in a measuring arm  40  of an interferometer. 
     This can be identical with a focal plane of the focusing optical system  28 , but can also be different from this in order to obtain further information on the scatter behavior of the semi-transparent object  14  for subsequent image processing. On the return path of the object  14 , the individual light rays reflected from the object  14  and the individual light rays reflected from the reference mirror  32 , thus both light paths, are brought together in the beam splitter and overlapped in an image sensor  42 . 
     The signal recovering surface  38  is the plane with maximal interference contrast or should be this. 
     If the path length difference of the reference arm  36  and the measuring arm  40  lie in the range of the coherence length of the light source  16  used, minima and maxima are shown on the image sensor  42  upon axial movement of the reference arm mirror  32 . 
       FIG. 2  shows a raster-like distribution of the illumination points  44  on the measured object  14  as well as on the image sensor  42 . At the same time, only a subset  46 ,  47  of available pixels  48  is directly illuminated on the image sensor by a corresponding light ray  43 ,  45 . A shifting of the raster takes place between the frames by fractions of the spacing of the illumination rays. 
     A large number of parallel individual rays  43 ,  45  (illumination raster) are correspondingly thus illustrated centered on respectively one or few pixels  46 ,  47  of the image sensor  42 . Around each illuminated pixel  46 ,  47  or each pixel group, there is located a primary non-illuminated region (for example, pixel  49 ) which is nonetheless illuminated in the event of the impingement of scattered light which is the normal case with a semi-transparent object. The signal-disturb signal ratio is therewith worsened. 
     In order nonetheless to be able to obtain three-dimensional data on the surface  12  of object  14 , a large number of frames, that is, the entire image content of the image sensor  42  of a defined period of time (sampling time) are filed in the memory of an image processing computer connected to the image sensor  42  during the traverse of the reference arm  36 . 
     If one knows a position of the reference arm  36  in which definitively no measuring signal of the objects  14  can be present (for example, in the shortest position of the reference arm at which the measuring plane  38  lies close above the object  14 ), one can proceed from the assumption that residual signals, which nonetheless occur, are disturb signals and can be classified as such. If one moves the signal recovering surface  38  by displacing the reference mirror  32  further in the direction of the measured object  14 , at some time there arises a point of intersection or a line of intersection or if need be also a surface of intersection between the signal recovering surface  38  and the object contour  12 . Then characteristic intensity fluctuations which express changing image patterns from frame to frame on the corresponding pixels of the image sensor  42  occur in reference to the corresponding pixels of the image sensor. The rather static intensity distributions can in contrast be restricted in connection with the method. Hence a brightness pattern sensor can be built up when the signal recovering surface is passed through by the measured object by linking the temporo-spatial signal pattern of the consecutive frame. 
     A priori knowledge in the form of a data base is used for surface contour data extraction which contain typical combinations from scatter, absorption and anisotropy factors of the corresponding semi-transparent material. The scattered light distribution to be expected is calculated in this way. A calculation method which describes the optical properties of semi-transparent tissues is, for example, described in the dissertation of Weniger K., Free University of Berlin, 2004. 
     Likewise, to the extent that the contour to be expected is known, thus capable of being allocated to a specific contour class, this can be used to provide data with priorities. The sequence of measurement is guided by a microcontrol unit. This entails shifting the focusing optical system and the reference arm mirror as well as motion of the beam shifter. 
     3D contour data, which are received partially overlapping from various positions of the image sensor are combined using software toward an overall dataset. 
     An STL file is compiled on the basis of the extracted contour data of the measured object which can be further processed with suitable CAD/CAM systems. 
     The present invention describes a device and a method for recovery of 3 D data of semi-transparent objects by using interference measurements/auto-correlation measurements with light sources of short coherence length. This can be white light in the extreme case, but also originate from one superluminescent diode or an array of them, or from one relatively broad-banded high performance light diode or from an array of several of them. Likewise, a combination of several laser diodes with central wave lengths offset in relation to one another is possible. The wavelength offset can amount to 5-150 nm, preferably 10-50 nm. 
     The coherence length l c , which is determinative for the longitudinal resolution of an OCT (Optical Coherence Tomography) measurement, exists for a Gaussian spectral distribution by: 
     
       
         
           
             
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     The coherence length should lie preferably in the 2-20 μm range, with emission outputs in the range from 1 to 100 mW, preferably 3-50 mW. A connection between the central wave length and the band width (FWHM) of the light source is represented in  FIG. 3 . 
     Deviating from the state of the art, only surface information is to be recovered in the described method. Therefore light sources with wavelengths in which the scattering coefficient of the object is high can also be selected. In this way, a differential measurement of at least two different wave lengths is possible. 
     In accordance with the invention, these can also be two measurements with wave lengths at which the semi-transparent object  14  in each case has very different scatter and absorption coefficients to compile a differential image from them. 
     In the event of a high scattering coefficient, a small but bright scatter halo will then form in the near field of the illumination point. In the event of a small scattering coefficient, the scattered light will spread wide in the semi-transparent medium but will have a lower intensity in the near field. This opens extended evaluation possibilities through image processing software. 
     In the case of tooth hard substance, one wave length range with a high scattering coefficient lies in the blue and ultraviolet spectral range, whereby wave lengths under 350 nm should be avoided due to the danger of the induction of DNA strand breaks and radical formation. The scattering coefficient lies in the case of dental hard substance in the 8-90 l/mm range with absorption coefficients in the 0.1-1.5 l/mm range. With dental filler material, the scattering coefficient lies here in the 8-25 l/mm range and the absorption coefficient at 0.3-4 l/mm. 
     Wave length ranges of low scatter for semi-transparent objects in contrast lie in the red and infrared spectral ranges. For dental hard substance, the scattering coefficient lies in the 1-40 l/mm range for dental enamel at the lower boundary and for dentine in the upper range. Filler materials lie in the 3-20 l/mm range. Examples for scatter and absorption coefficients for enamel and dentine can be gathered from  FIGS. 4 and 5 . 
     The diminishing spectral sensitivity of the detector is limiting in the near infrared. In the case of a preferred embodiment, this falls at 1000 nm under 5% with a CMOS sensor. The use of a CCD sensor is likewise possible. Furthermore the wave length range can be extended into the infrared with suitable sensors. InAs or HgCd Te detectors, for example are suitable for this with which the 2.5-10 μm range can be covered. 
     In case the spectral range of the near infrared, for example 750 nm to 1000 nm, is being used, the depth information then available can also be used for caries diagnosis. In the event that two or more wave lengths are to be used simultaneously, an RGB variant of a CMOS sensor can be used, which has sensitivity maxima in the red, green and blue spectral range. 
       FIG. 6  shows a schematic diagram of an alternative embodiment of a device  50  for recording contour data of three-dimensional objects, whereby like elements are characterized with the same reference numbers. In extension of the device  10  in accordance with  FIG. 1 , a tracking device  52  is provided which makes possible an axial change in length of the reference arm  36  when shifting the focusing lens  28  along arrow  54 . Deflection mirrors  56  are arranged in the tracking device  52  for this purpose. A further deflection mirror  58  is arranged in lengthening the light ray exiting from the beam splitter  24  in order to attain a deflection of the light ray on the mirror arrangement  56  arranged in the tracking device. In order nonetheless with this device to make possible a separation of the focal plane of the signal recovering plane, the mirror  32  can preferably be shifted axially separated in the direction of arrow  60 . 
       FIG. 7  depicts an outer contour of a sensor housing  62  for use in dentistry for intraoral scanning of teeth. In order to make possible a comfortable operation in the month of a patient, the dimensions must be oriented around the anatomy of the patient. A wedge-shaped arrangement is a preferred embodiment. 
       FIG. 8  shows an underside  64  of the sensor housing  62  in which a scan window  66  is arranged. The length of the scan window  66  makes possible a simultaneous recording of a quadrant.