Patent Publication Number: US-11638525-B2

Title: Intraoral scanning apparatus with decreased ray footprint

Description:
CROSS REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 17/037,398 filed Sep. 29, 2020, which is a continuation of U.S. patent application Ser. No. 16/820,505 filed Mar. 16, 2020, which is a continuation of U.S. patent application Ser. No. 14/741,172, filed Jun. 16, 2015, which claims the benefit of U.S. Provisional Application No. 62/021,608, filed Jul. 7, 2014, the contents of each of which are hereby incorporated by reference in their entirety. This application is further related to U.S. patent application Ser. No. 17/191,579 filed Mar. 3, 2021. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the present invention are directed to methods and apparatus for dental imaging, such as for measuring surface topology of teeth. 
     BACKGROUND 
     In the fields of orthodontics and prosthodontics, different methods are known to determine a current teeth topology in a patient&#39;s mouth. One of the methods involves taking an impression of a patient&#39;s dentition. Using this impression, a plaster cast is made, representing a (positive) physical teeth model. This physical teeth model may then be used for a subsequent treatment plan. 
     If CAD (computer-aided design) and/or CAM (computer-aided manufacturing) techniques are to be employed, a digital dataset corresponding to the teeth may be obtained by scanning. For example, the (positive) physical teeth model in form of the plaster cast or a (negative) physical teeth model in form of the impression may be scanned or imaged using x-rays, computed tomography, magnetic resonance imaging, or laser scanning apparatuses. With the thus obtained image data, a computer model of the teeth or a part thereof may be established. However, such methods and apparatus can be somewhat time consuming and more expensive than would be ideal. 
     As an alternative, teeth in a patient&#39;s mouth may be imaged directly. For this purpose, different imaging apparatuses are known. 
     The prior apparatus for non-contact imaging with a probe having a sensing face have been less than ideal in at least some respects. The prior probe devices can be somewhat larger than would be ideal, and may have a large intraoral front tip which can make the prior devices somewhat cumbersome to use in at least some instances. Although an array of incident light beams passing through focusing optics can be used, the larger than ideal probe heads of such devices can provide less than ideal measurements of the oral cavity of a patient. Also, the prior devices that rely on beams to generate illuminated spots on the structure and the intensity of returning light rays propagating along an optical path can be somewhat cumbersome to use and maintain and can be somewhat more costly to manufacture than would be ideal. 
     Although three-dimensional (3D) data acquisition using triangulation has been proposed, such devices can be less compact than would be ideal and can be somewhat difficult to place in the mouth of the patient. Also, such devices can require alignment and can be less accurate and reliable than would be ideal in at least some instances. 
     In light of the above, improved methods and apparatus for measuring surfaces such as the intraoral cavity are needed. Ideally such methods and apparatus will overcome at least some of the deficiencies of the prior methods and apparatus and be more accurate, reliable, compact, easier to use with the patient&#39;s mouth and less costly than the prior devices. 
     SUMMARY 
     In accordance with embodiments, an apparatus for confocal imaging is provided, which may comprise an illumination module for generating an array of light beams, an optics system for confocal focusing of the array of light beams, and a probe head with a light-guiding part having an entrance face and an exit face. The illumination module, the optics system, and the probe head can be arranged such that the array of light beams from the illumination module passes through the optics system, enters the light-guiding part via the entrance face, and exits the light-guiding part via the exit face. Embodiments disclosed herein provide probes having decreased size which can facilitate measurement of surfaces which can be difficult to reach with prior devices such as an oral cavity of a patient. The embodiments disclosed herein also have the advantage of providing improved accuracy and reliability with decreased manufacturing costs. In many embodiments, a plurality of beams is directed toward a measurement surface in which each of the plurality of beams extends to a focal point and comprises a chief ray. The chief rays of the plurality of beams can diverge from each other between the probe and the focal points in order to decrease the size of the probe and inhibit spread of the outermost marginal rays. In many embodiments, the confocal imaging system comprises a non-telecentric configuration such that the off-axis chief ray angles of the light beams exiting the probe have an opposite orientation with respect to angles of the marginal rays entering the probe, such that a substantially decreased cross-sectional size of the probe can be provided. In many embodiments, laterally outermost marginal rays of each of the outer beams on either side of the optical axis extend along an optical path substantially parallel to the optical axis and substantially parallel to the optical axis, or divergent from the optical axis. In many embodiments, lateral spread of the array of beams is inhibited as each individual beam converges toward the focal point, and this spread can be inhibited when the focal point is shifted. 
     In a first aspect, embodiments provide an apparatus for confocal imaging, comprising an illumination module for generating an array of light beams, an optics system for confocal focusing of the array of light beams and a probe head with a light-guiding part having an entrance face and an exit face. The illumination module, the optics system and the probe head are arranged such that the array of light beams from the illumination module passes through the optics system, enters the light-guiding part via the entrance face and exits the light-guiding part via the exit face. The optics system is configured such that, after having passed through the optics system, the outermost marginal rays of the outermost light beams with respect to the optical axis of the optics system are parallel or divergent to the optical axis. 
     In another aspect, embodiments provide an apparatus for confocal imaging comprising an illumination module for generating an array of light beams, an optics system for confocal focusing of the array of light beams and a probe head with a light-guiding part having an entrance face and an exit face. The illumination module, the optics system and the probe head are arranged such that chief rays of the array of light beams are divergent to each other. 
     In another aspect, embodiments provide a method for confocal imaging. An illumination module is provided for generating an array of light beams. An optics system is provided for confocal focusing of the array of light beams and a probe head with a light-guiding part having an optical axis and an entrance face and an exit face. The optics system is configured such that, after having passed through the optics system, outermost marginal rays of outermost light beams with respect to the optical axis are parallel or divergent with respect to the optical axis between the probe and focal points of the light beams. 
     INCORPORATION BY REFERENCE 
     All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIG.  1    is a schematic view of a confocal imaging apparatus, in accordance with embodiments; 
         FIG.  2 A  is a schematic view of the optical path in an optics system of a confocal imaging apparatus in a paraxial design, in accordance with embodiments; 
         FIG.  2 B  is a schematic view of the optical path in an optics system of a confocal imaging apparatus in a thick lens design, in accordance with embodiments; 
         FIG.  3    is a top view of a schematically illustrated probe head, in accordance with embodiments; 
         FIG.  4    is a longitudinal cross-section through line A-A in  FIG.  3   ; and 
         FIG.  5    is a longitudinal cross-section through a schematically illustrated probe head in accordance with embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The methods and apparatus disclosed herein can be combined in one or more of many ways and are well suited for combination with many devices related to surface topology such as the measurement of tissue surfaces such as surfaces of the oral cavity. The tissue surfaces may comprise one or more surfaces of teeth of the mouth, for example. The measured surfaces can be used by health care providers such as orthodontists and dentists. 
     In many embodiments, the optics system can be configured such that, after having passed through the optics system, the outermost marginal rays of the outermost light beams with respect to the optical axis of the optics system are parallel or divergent to the optical axis. In many embodiments, a plurality of light beams exits the probe head with divergent angles. Each of the plurality of light beams may comprise a chief ray divergent from an optical axis of the optics system. 
     In many embodiments, the outermost marginal rays of the outermost light beams, after exiting the optics system, do not converge towards the optical axis, in order to avoid a lateral spread of the array of beams even when shifting the focal plane of the focusing optics. In many embodiments, the term “outermost” refers to a distance transverse to the optical axis, such as a distance perpendicular to the optical axis. In many embodiments, an outermost light beam or most off-axis light beam comprises a beam having a largest distance to the optical axis in a direction perpendicular thereto. 
     The apparatus may be configured for intraoral confocal imaging, such as an apparatus for intraoral confocal imaging, which can be used for confocal imaging of positive and/or negative physical teeth models. The apparatus may comprise a scanning apparatus. The focusing optics of the apparatus may comprise non-telecentric optics, such that a reduced ray footprint at the probe head entrance face can be provided. 
     In many embodiments, the outermost marginal rays of the outermost light beams, after having passed through the optics system, are divergent with respect to the optical axis, and the divergence angle between the outermost marginal rays and the optical axis may be at most 10°, in some embodiments at most 8°, and in specific embodiments at most 5°. 
     In many embodiments, the light-guiding part comprises a transparent body. The transparent body may comprise one or more of glass or plastics, and may comprise a stiff, solid body, such as a rigid body. 
     The light-guiding part may be arranged such that the array of light beams enters the light-guiding part at an angle of approximately 90° with respect to the entrance face. In some embodiments, the light guiding part may be arranged such that the array of light beams enters via the entrance face at an angle of 90°±10°, such as 90°±8°, and more particularly of 90°±5°. 
     The light-guiding part may be bounded by sidewalls, and the sidewalls of the light-guiding part and entrance face may be arranged such that each light beam entering the light-guiding part via the entrance face is reflected at the sidewalls an odd number of times before exiting via the exit face. In many embodiments, each light beam may be reflected at the sidewalls three or five times before exiting via the exit face. Each of the sidewalls and/or the exit face and/or the entrance face may be planar. 
     The light-guiding part may be configured with an index of refraction and arrangement of the entrance face and sidewalls such that each light beam entering via the entrance face is reflected from at least one of the sidewalls of the light-guiding part by way of internal reflection. In many embodiments, at least some of the reflections at the sidewalls result from internal reflection such as one or more of total internal reflection, attenuated total internal reflection, or frustrated total internal reflection. Alternatively or additionally, at least some reflections may be provided with a mirror coating on a sidewall or part of a sidewall. 
     In many embodiments, the light-guiding part comprises a one-piece body. 
     In many embodiments, the light-guiding part comprises an upper sidewall being arranged at an acute angle with respect to the entrance face, a lower sidewall being arranged at an obtuse angle with respect to the entrance face, and an end sidewall being arranged at an acute angle with respect to the entrance face and/or the exit face. The upper sidewall may adjoin the entrance face; the lower sidewall may adjoin the entrance face and/or the end sidewall may adjoin the upper sidewall and/or the lower sidewall. The angle between the end sidewall and the upper sidewall may be an obtuse angle. The lower sidewall may comprise the exit face. 
     In many embodiments, the end sidewall may comprise a mirror. For example, the end sidewall may comprise a mirror coating. In such embodiments, the light-guiding part may be arranged and/or configured such that each light beam entering via the entrance face is reflected at all (remaining) sidewalls except for the end sidewall by way of internal reflection. 
     The angle between the entrance face and the lower sidewall may lie between 90° and 125°, in particular, between 90° and 115°. The angle between the exit face or the lower sidewall and the end sidewall may lie between 20° and 45°, in particular, between 25° and 35°. The angle between the entrance face and the upper sidewall may lie between 90° and 65°, in particular, between 90° and 80°. 
     The exit face may be covered by a transparent cover plate. The transparent cover plate may be a glass or plastics plate. The cover plate may be arranged at a distance from the exit face. 
     In many embodiments, the illumination module comprises a single light emitter or a plurality of light emitters. The one or more light emitters may emit coherent light. The light emitter may comprise one or a plurality of laser emitters. In embodiments comprising a single light emitter, the illumination module may further comprise a beam-expander element and/or a beam-splitter element for splitting a light beam from the light emitter into a plurality of light beams and/or an array of light beams. The beam-splitter element may comprise diffractive optics or a refractive optics, such as a grating or a microlens array. 
     Any of the above-described apparatuses may comprise a polarizer for linearly polarizing a light beam, wherein the polarizer is arranged along the optical path between the illumination module and the probe head, in particular, between the illumination module and the optics system. 
     Any of the above-described apparatuses may comprise a beam splitter being arranged along the optical path between the illumination module and the optics system such that the array of light beams from the illumination module passes through the beam splitter and an array of returning light beams from the optics module is reflected, in particular, towards a detector. The beam splitter may comprise a semi-transparent mirror. The light beams from the illumination module passing through the optics system may comprise incident light beams, and light beams following the optical path in an opposite direction through the optics system may comprise returning light beams. The array of returning light beams may comprise an array of light beams having been reflected by an object to be imaged, such as teeth portions, for example. 
     The described apparatuses may further comprise a detector for detecting an array of returning light beams. The detector may comprise an array of detector elements. The detector elements may comprise a CCD camera or a photodiode array. The detector may comprise a spectrophotometer. 
     The above-described apparatuses may comprise a focus-shifting mechanism for shifting the focal plane of the optics system. For example, the focus-shifting mechanism may be configured to shift one or more lenses of the optics system along the optical axis. In many embodiments, the focus-shifting mechanism may comprise a translation mechanism for translating the one or more lenses of the optics system. 
     In many embodiments, the probe heads comprise a housing, wherein the light guiding part and/or the optics system are provided within the housing. In embodiments where a focus-shifting mechanism is provided, the focus shifting mechanism may also be provided within the housing. 
     In many embodiments, the probe head comprises a part of a handheld device. The optics system and/or a focus-shifting mechanism may comprise part of the handheld device, for example. In particular, the handheld device may be defined by the housing described above. 
     Further features will be described with reference to the accompanying drawings, in accordance with embodiments. 
     As used herein, a chief ray encompasses a central ray of a beam of light. In many embodiments, a plurality of beams is directed onto the surface to be measured, in which each of the plurality of beams comprises a chief ray. 
       FIG.  1    schematically illustrates an example of an apparatus for dental confocal imagining of a teeth segment or teeth portion, in accordance with many embodiments. The teeth segment may comprise one tooth, a plurality of teeth, a tooth stump and/or a portion where one or more teeth are missing. The apparatus may be used, for example, for intraoral imaging of teeth. Alternatively, imaging of a positive or negative teeth model may be performed as well. 
     The illustrated apparatus comprises a light emitter  1  as a source of coherent light. As an example, the light emitter may be a laser source such as a semiconductor laser. 
     As indicated by the arrow shown, emitted light passes through a beam expander  2 , which may comprise a collimating lens so as to obtain a collimated light beam having a desired width or numerical aperture. 
     Along the optical path between the light emitter  1  and the beam expander  2 , optionally, a polarizer such as a polarization filter may be provided. 
     The beam expander  2  is followed by a spot array generator element  3  for splitting the beam into an array of light beams. The spot array generator element  3  in the form of diffraction or refraction optics may comprise a grating or a microlens array, for example. 
     In the illustrated example, the light emitter comprises a single light source from which the array of light beams is generated via the beam expander and the beam splitter element. As an alternative, the light emitter  1  may already comprise a plurality of light sources being arranged in form of an array. In this case, the array of light beams is generated directly at the light emitter  1  so that a beam expander and/or beam splitter may be avoided. As an example, the array of light sources may be provided in the form of an array of laser sources such as semiconductor lasers. 
     In this example, the light emitter  1 , the beam expander  2  and the spot array generator element  3  define an illumination module generating an array of light beams. 
     The array of light beams, represented here, for ease of illustration, by a single line, passes through a beam splitter  4  in form of a semi-transparent mirror and enters optics system  5 . The optics system  5  comprises a non-telecentric, confocal lens arrangement which will be described in more detail below. 
     The light beams coming from the light emitter  1  and propagating towards the sample to be imaged (e.g. a teeth segment) are called incident light beams, whereas light beams being reflected at the sample and propagating along the optical path of the incident light beams but in opposite direction are called returning light beams. 
     After the optics system  5 , the incident array of light beams enters a probe head  6 . In particular, the array of light beams is coupled into a light guiding part of the probe head via the light guiding part&#39;s entrance face. Within the light guiding part, each beam is reflected several times before it is coupled out via an exit face onto an object to be imaged, such as a teeth segment  7 . In this way, an incident array of light beams is emitted towards the teeth segment  7 , thus, resulting in an array of light spots on the teeth surface. 
     As also illustrated by one of the arrows between a tooth and the probe head, reflected light re-enters the probe head  6 , particularly its light guiding part via the exit face. In this way, each reflected or returning light beam travels along the optical path in opposite direction as travelled by the incident light beams. Therefore, the returning light beams are also reflected several times within the light guiding part of probe head  6  and pass through optics system  5  in an inverse direction. At the semi-transparent mirror  4 , the returned light beams are reflected towards imaging optics  8  comprising one or more lenses, followed by a pinhole array  9 . 
     Then, the array of returning light beams impinges onto a detector  10 , comprising an array of detector elements. For example, the detector  10  may be a CCD camera or a photodiode array. Each detector element or sensing element corresponds to a pinhole in the array  9 . 
     The detector  10  is connected to a processing unit  11  where each light intensity measured in each of the detector elements is grabbed and analyzed. 
     The apparatus further comprises a control unit  12  being connected to the light emitter  1  as well as to a motor  13 . Motor  13  is an example of a focus-shifting mechanism for shifting the focal plane of the optics system  5 . In particular, motor  13  is coupled to the optics system  5  so as to shift or translate one or more lenses of the optics system along the optical axis. In this way, the focal plane location may be changed or shifted. 
     After receipt of a feedback signal that the location of the focal plane has changed (or that the one or more lenses have been shifted), control unit  12  triggers light emitter  1  to generate a light pulse. Processing unit  11  will grab data representative of the light intensity as detected by detector  10  corresponding to the light pulse which was reflected at the teeth portion  7 . This procedure will be repeated for a plurality of locations for the focal plane. 
     As outlined in detail in WO 00/08415, the entire disclosure of which is incorporated herein by reference, for example, the surface topology of the image object (e.g. a teeth segment) is determined by determining the focal plane location for which, for a particular pixel, the light intensity is maximal. In this way, a three-dimensional representation of the object, e.g., the teeth segment, may be obtained. It may be displayed and/or further processed. 
     The array of light beams may comprise light beams having different wavelengths. For this purpose, the light emitter  1  may comprise different light sources emitting light of different wavelengths. In case of an array with beams of different wavelengths, the detector may be a spectrophotometer with color resolution. Examples for spectrophotometers are a three-chip CCD camera or the use of a Bayer mask over a monochrome CCD or other light sensor. 
     By using light components or light beams with different wavelengths, each being focused simultaneously on a different focal plane, the time for imaging may be reduced as different focal plane ranges can be simultaneously measured. 
     The probe head  6  may comprise a housing. For example, both the optics system  5  and the light guiding part may be provided within such a housing. The housing may be configured as a handheld device so that the light guiding part and/or optics system  5  and/or motor  13  are included in the handheld device. 
       FIG.  2 A  schematically illustrates a paraxial design example of the optics system, showing the first order imaging configuration, in accordance with many embodiments. In this schematic view, probe  6  is illustrated as a single block. In this example, a central beam  14  and an outermost beam  15  (in a direction perpendicular to the optical axis) are emitted from a source plane  16 . By way of example, reference numerals  15 ′ and  15 ″ denote marginal rays of the outermost beam  15 , whereas reference numeral  14 ′″ denotes the chief ray of central beam  14 . Thus, the term “marginal rays” is used in the conventional sense denoting the rays (of a specific light beam) defining the beam&#39;s circumference or envelope; the “chief ray” corresponds to the central ray of a beam. 
     As can be seen in this example, the outermost marginal ray  15 ″ of outermost light beam  15 , after having passed through the optics system  5  and before entering the probe head  6 , is parallel to the optical axis  24 , and the marginal ray  15 ″ is parallel to chief ray  14 ′″ of the central beam that is on the optical axis  24 . 
     In case of the complete array of light beams, the outermost marginal rays of the outer beams of the array (on either side of the optical axis) after having passed through the optics system and before entering probe head, are all parallel to the optical axis. As a consequence of this configuration, there is little or no lateral spread of the array of beams even when shifting the focal plane  17  of the array. 
     Each of the plurality of light beams may comprise a chief ray. For example, the outer beam  15  may comprise a chief ray  15 ′ extending from an outer aperture at source plane  16  to the focus at focal plane  17 . 
     As an alternative to or in combination with the above-described parallelism of the outermost marginal rays of the outermost beams and the chief ray of the central beam, the optics system may be configured such that the outermost marginal rays of the outermost light beams with respect to the optical axis of the optics system are divergent relative to the optical axis. In many embodiments, after having passed through the optics system, the outermost marginal ray of the outermost beams may show an opening angle with respect to the optical axis. This divergence angle or opening angle with respect to the optical axis may be at most 10°, preferably at most 8°. 
     In many embodiments, the confocal focusing system of the optics system comprises a non-telecentric optical system. The optics system can be configured such that the angle defined by the marginal rays of an outermost beam is complementary to the angle defined by the extreme off-axis chief rays with respect to the optical axis. In such embodiments, a decreased ray footprint at object to be imaged, e.g. the teeth segment, is provided. 
       FIG.  2 B  schematically illustrates a thick lens design example corresponding to the embodiments of  FIG.  2 A . Also in  FIG.  2 B , from the array of beams generated by the illumination module, only one central beam  14  and one outermost beam  15  are shown with dashed lines for the sake of simplicity, in accordance with embodiments. 
       FIGS.  3  and  4    schematically illustrate a top view and a cross-sectional view (through line A-A) of a probe head, respectively, in accordance with many embodiments. The probe head  6  has a light-guiding part with an entrance face  18  and an exit face  19 . Light coming from the light emitter and the optics system enters the light-guiding part via the entrance face  18  at an angle of about 90°, i.e., substantially normal to the entrance face. The entrance angle may be 90°±10°, preferably 90°±8°, for example. This particularly applies to the embodiments in which the outermost marginal rays of the outermost light beams are divergent with respect to the optical axis after having passed through the optics system. 
     As schematically illustrated in  FIG.  4   , the light-guiding part is arranged and light is coupled into the light guiding part in such a way that each light beam entering the light-guiding part via the entrance face  18  is reflected at the sidewalls. In embodiments according to  FIG.  4   , three reflections occur before a light beam exits the light-guiding part via the exit face  19 . A first reflection takes place at an upper sidewall  20 , adjoining the entrance face  18  and being arranged at an acute angle with respect to entrance face  18 . The second reflection occurs at lower sidewall  21 , also adjoining the entrance face  18  and being arranged at an obtuse angle with respect thereto. The last reflection occurs at end sidewall  22 , adjoining both the upper and the lower sidewalls. The exit face  19  is part of the lower sidewall  21 . 
     End sidewall  22  comprises a mirror, which may be obtained by a suitable coating of the corresponding surface of the light-guiding part. The light-guiding part as a whole, in these embodiments, may comprise a one-piece body, for example, comprising a glass body or a body composed of a transparent resin. The first and second reflections at upper sidewall  20  and lower sidewall  21  may result from internal reflection. 
     In view of these embodiments, the light-guiding part of the probe head, in accordance with its one-piece form, may be manufactured in a simple and economical way. 
     The specific configuration of the optics system as described above allows for a long light-guiding part and probe head while maintaining a small thickness resulting in an improved intraoral use. 
     In many embodiments, and as illustrated in  FIG.  5   , the exit face  19  may be covered by a transparent cover plate  23  which is arranged at some distance from the exit face  19 . This cover plate  23  may be exchangeable for hygienic reasons, and may comprise a single use component. 
     The embodiments illustrated in  FIG.  5    provides for an even longer light-guiding part along which the light beams are reflected five times before exiting via the exit face  19 . In principle, any odd number of reflections is possible. 
     In many embodiments, the chief rays of each of a plurality of light beams exits the exit face  19  with a divergent angle with respect to the optical axis, for example. This divergence of each of the chief rays of the light beams provides improved measurements and can decrease the overlap of spots imaged onto the oral cavity, for example. The decrease of spread of the marginal rays the light beams as described herein can be combined with the divergence of the chief rays of each of the light beams in order to provide improved the accuracy of measurements as described herein, for example. 
     While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.