Optical scanner for dental impressions, digitization method and system for dental models

The present invention pertains to an optical scanner for dental impressions, a method for digitization of dental impressions and a system for digitizing a dental impression.

Odontology, dental prosthetics and orthodontics often require an accurate mold and three- dimensional digital model of a patients teeth and intraoral structures including the gingiva. Commonly, a dentist, an orthodontist or a medical assistant prepares a dental impression using a dental tray filled with an elastomeric impression material, such as sodium alginate, polyether or silicone. In order to prepare a cast or mold the dental impression after having been partly or fully cured and removed from the oral cavity is filled with liquid gypsum plaster. Depending on individual practice and workflow the time between preparation of a dental impression and casting of a corresponding gypsum mold may vary between 30 min and two weeks. Subsequently, a three-dimensional digital model of a gypsum mold may be prepared using a conventional dental scanner. Common dental scanners are based on structured light triangulation technology. Dental scanner manufacturers typically specify a dimensional accuracy in the range of 150 to 300 μπι, i.e. the maximal deviation between a gypsum mold and a three-dimensional digital model thereof shall not exceed 300 μm in any spatial direction. The specified precision does not account for deviations of a dental impression and a thereof cast gypsum mold from a patients intraoral structures due to shrinkage of either the dental impression material and the gypsum plaster upon curing, and respectively drying. Many dental impression materials, particularly alginate exhibit considerable volume shrinkage upon curing due to H ₂ 0-cleavage and evaporation. Similarly, gypsum plaster continuously releases water over time and thus also shrinks to a measurable extent.

The present invention aims at remedying the above described problems by preparing a highly accurate dental impression using a silicone-based impression material and by directly capturing a three-dimensional digital image of the cured silicone-based dental impression. Typically the surface of a silicone-based dental impression is very smooth and reflects visible light quite well. Furthermore, depending on whether or not the silicone-based impression material comprises colorants or particulate fillers, e.g. silica, its surface is translucent or transparent. Conventional optical scanners including known dental scanners are based on structured light triangulation technology and are not suited for capturing objects having a specular, translucent and/or transparent surface. Structured light triangulation is widely used for shape measurement in industrial automation, graphics, human-computer interaction and surgery. The majority of structured light triangulation techniques is based on projecting a pattern of parallel light stripes onto an object, capturing the stripe image deformed due to the surface topology of the object with a digital camera and computing the coordinates of the object surface based on optical perspective i.e. triangulation. The two main types of structured light triangulation employ two-dimensional patterns of parallel light stripes or a single light stripe. In the latter case the single light stripe is scanned across the object either by optical beam steering or by mechanical translation of the object while a digital camera captures an entire image frame at each stripe position. Contour lines of the objects surface topology are derived from the deformed shapes of the projected stripes. In two- dimensional structured light triangulation various patterns such as phase shifting and maximum min-SW gray codes (maximum minimum stripe width gray code) are used. The spatial resolution of structured light triangulation techniques is limited by both the camera and projector resolution. Furthermore, structured light triangulation techniques are based on the assumption that each lighted point on the object surface is directly illuminated by the structured light source. However, for objects having a specular, translucent and/or transparent surface - with or without concave portions - this assumption is incorrect. For such objects certain regions of the object surface are indirectly illuminated by internal reflections and subsurface or volumetric light scattering. These effects which are usually subsumed under the term "global or indirect illumination" strongly depend on the shape and optical properties of the object. In specific regions of the object surface indirect illumination often dominates and results in large errors in the three-dimensional image reconstructed through structured light triangulation.

Thus, the present invention has the objective to provide an optical scanner that enables imaging of silicone-based dental impressions having a specular, translucent and/or transparent surface.

This objective is achieved through an optical scanner for dental impressions comprising a structured light projector, a digital camera, an image processing unit, an electronic controller and a mechanical stage, wherein the mechanical stage comprises an adapter for a dental tray.

In an alternative equally expedient embodiment of the invention this objective is achieved through an optical scanner for dental impressions comprising a conoscopic holography sensor, an image processing unit, an electronic controller and a mechanical stage, wherein the mechanical stage comprises an adapter for a dental tray.

Advantageous implementations of the inventive optical scanner are characterized in that: the adapter comprises one or more mechanical fixtures that are form-fit to a predetermined outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter; the adapter comprises one or more mechanical fixtures that are form-fit to a predetermined outer surface of a dental tray such that the dental tray can be inserted in a force-fit manner into the adapter and removed therefrom; the at least one mechanical fixture is form-fit to a predetermined outer surface of a dental tray with an accuracy of better than 100 μηι, such that the misalignment of an identical dental tray from its average insertion position is smaller than 100 μm; the adapter comprises one or more leaf springs arranged in such manner as to bound a space for insertion of a dental tray with the space comprising at least one section having a predetermined width form-fit to an outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter; the adapter comprises one or more notches form-fit to one or more corresponding tongues arranged on an outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter; the adapter comprises one or more tongues form-fit to one or more corresponding notches arranged in an outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter; the adapter comprises one or more trunnions form-fit to one or more corresponding bores arranged in an outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter; the adapter comprises one or more bores form-fit to one or more corresponding trunnions arranged on an outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter; the structured light projector comprises one or more light sources having an emission wavelength in the range of 380 to 780 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 380 to 780 nm; the structured light projector comprises one or more light emitting diodes (LED) or one or more laser diodes having an emission wavelength in the range of 380 to 780 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 380 to 780 nm; the structured light projector comprises one or more light sources having an emission wavelength in the range of 1000 to 3300 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 1000 to 3300 nm; the structured light projector comprises one or more infrared laser diodes having an emission wavelength in the range of 1000 to 3300 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 1000 to 3300 nm; the structured light projector comprises one or more infrared light emitting diodes (LED) having an emission wavelength in the range of 1000 to 3300 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 1000 to 3300 nm; the structured light projector comprises one or more light sources having an emission wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm; the structured light projector comprises one or more infrared laser diodes having an emission wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm; the structured light projector comprises one or more infrared light emitting diodes (LED) having an emission wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm; the scanner comprises one or more light sources having at least one emission wavelength in the range of 150 to 280 nm; the scanner comprises one or more light sources having at least one emission wavelength in the range of 150 to 280 nm with a total emitted light power of 10 to 300 mW in the wavelength range of 150 to 280 nm; the structured light projector comprises one or more light sources having an emission wavelength in the range of 150 to 380 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 150 to 380 nm; the structured light projector comprises one or more ultra-violet laser diodes having an emission wavelength in the range of 150 to 380 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 150 to 380 nm; the structured light projector comprises one or more ultra-violet light emitting diodes (LED) having an emission wavelength in the range of 150 to 380 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 150 to 380 nm; the structured light projector comprises one or more light sources having an emission wavelength in the range of 150 to 300 nm or 200 to 350 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 150 to 300 nm or 200 to 350 nm; the structured light projector comprises one or more ultra-violet laser diodes having an emission wavelength in the range of 150 to 300 nm or 200 to 350 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 150 to 300 nm or 200 to 350 nm; the structured light projector comprises one or more ultra-violet light emitting diodes (LED) having an emission wavelength in the range of 150 to 300 nm or 200 to 350 nm and the digital camera comprises an objective and one or more image sensors configured for imaging at a wavelength in the range of 150 to 300 nm or 200 to 350 nm; the mechanical stage comprises one or more optical alignment marks; the mechanical stage comprises three or more non-coplanar optical alignment marks; the mechanical stage comprises one or more optical alignment marks arranged proximal to a dental tray mounted to the mechanical stage; the mechanical stage comprises one or more optical alignment marks arranged proximal to a dental tray mounted to the mechanical stage such that a shortest distance between the alignment mark and the dental tray is less than 20 mm; the at least one optical alignment mark is shaped as polyhedron; the at least one optical alignment mark is shaped as polyhedron having edges with lengths from 0.1 mm to 10 mm; the at least one optical alignment mark is shaped as polyhedron having edges with lengths from 2 mm to 5 mm; the at least one optical alignment mark is shaped as tetrahedron; the at least one optical alignment mark is shaped as tetrahedron having edges with lengths from 0.1 mm to 10 mm; the at least one optical alignment mark is shaped as tetrahedron having edges with lengths from 2 mm to 5 mm; the at least one optical alignment mark comprises light diffusing or light scattering elements; the at least one optical alignment mark comprises light diffusing or light scattering elements made from a ceramic or polymeric material; the at least one optical alignment mark comprises a fluorescent element; the at least one optical alignment mark comprises a fluorescent element containing phosphors for converting infrared radiation into visible light, so-called infrared-to-visible upconversion phosphors (IUP); the at least one optical alignment mark comprises a fluorescent element containing infrared- to-visible upconversion phosphors (IUP), e.g. Er -activated yttrium fluoride of general formula Y _1-x Er _x F3, wherein x is in the range of 0.05 to 0.3; the mechanical stage comprises a first rotary stage having a first rotation axis and an angle between the first rotation axis and a vertical direction is from 50 to 130 degree; the mechanical stage comprises a second rotary stage having a second rotation axis and an angle between the second rotation axis and a vertical direction is from 0 to 40 degree; the mechanical stage comprises a first and second rotary stage and the second rotary stage is mounted to the first rotary stage; the mechanical stage comprises a first and second rotary stage and the first rotary stage is mounted to the second rotary stage; the mechanical stage comprises a linear stage and the first and/or second rotary stage is mounted to the linear stage; the mechanical stage comprises a linear stage and the second rotary stage is mounted to the linear stage; the mechanical stage comprises a linear stage, the second rotary stage is mounted to the linear stage and the first rotary stage is mounted to the second rotary stage; the mechanical stage comprises a linear stage and the first rotary stage is mounted to the linear stage; the mechanical stage comprises a linear stage, the first rotary stage is mounted to the linear stage and the second rotary stage is mounted to the first rotary stage; the first rotary stage comprises an electronic actuator; the first rotary stage comprises a magnetic, electronic or optoelectronic switch for referencing its absolute angular position; the first rotary stage comprises a magnetic, electronic or optoelectronic encoder for referencing its relative angular position; the second rotary stage comprises an electronic actuator; the second rotary stage comprises a magnetic, electronic or optoelectronic switch for referencing its absolute angular position; the second rotary stage comprises a magnetic, electronic or optoelectronic encoder for referencing its relative angular position; the linear stage comprises an electronic actuator; the linear stage comprises a magnetic, electronic or optoelectronic switch for referencing its absolute position; the linear stage comprises a magnetic, electronic or optoelectronic encoder for referencing its relative position; the mechanical stage comprises one or more optical alignment marks fixedly mounted to the first or second rotary stage; the mechanical stage comprises one or more optical alignment marks fixedly mounted to the adapter for a dental tray; the mechanical stage comprises one or more optical alignment marks arranged proximal to a space designated for a dental tray such that a shortest distance between the alignment mark and said space is less than 20 mm; the mechanical stage comprises one or more optical alignment marks arranged proximal to a space designated for a dental tray and distanced from line of sights from said space to the structured light projector or to the digital camera; the structured light projector is configured to project parallel stripe patterns with variable stripe width of 10 μηι to 10 mm; the structured light projector is configured to project micro phase shifting patterns; the structured light projector is configured to project micro phase shifting patterns having intensity distributions I _n (x ) , I _j (x ) according to the formulas

wherein A and B are constants, J is the width of the stripe pattern in a projection plane perpendicular to the optical axis of the structured light projector, CFo is a real number from 64 to 128, Xp is a coordinate in said projection plane in a direction perpendicular to the longitudinal stripe axis with Nis equal to 3 or 5, n = 0, 1, ..., (N-l), and CF _j are real numbers with such that form a progression of evenly spaced

and monotonically increasing numbers; the structured light projector is configured to project maximum min-SW gray code patterns (maximum minimum stripe width gray code patterns); the structured light projector is configured to project maximum min-SW gray code patterns (maximum minimum stripe width gray code patterns), wherein the ratio of maximum to minimum stripe width is from 16:1 to 64:1 ; the structured light projector is configured to project a single light stripe having a width of 2 to 100 μηι; the structured light projector is configured to project a single light stripe having a width of 2 to 100 μηι over a depth of field of 5 to 20 mm; the structured light projector is configured to project a single light stripe having a width of 2 to 20 μηι over a depth of field of 5 to 20 mm; the structured light projector comprises a spatial light modulator (SLM), such as a liquid crystal array, a liquid crystal on silicon (LCOS) modulator or a microelectromechanical (MEMS) mirror deflector; the structured light projector comprises a spatial light modulator (SLM) configured to

1 1 12 13 generate a stripe pattern having a stripe width resolution of < W/2 , < W/2 , < W/2 , < W/(10-2 ¹⁰ ) or < W/(12-2 ¹⁰ ), wherein W is the full width of the SLM active area; the structured light projector comprises an optical beam scanner, such as a rotating polygon mirror scanner, a galvanometer mirror scanner, an acousto-optic modulator or a microelectromechanical (MEMS) mirror deflector; the electronic controller comprises a control program for automated image capture of a dental impression at different spatial orientations of the dental impression relative to the digital camera; the structured light projector comprises one or more laser diodes; the structured light projector comprises one or more laser diodes having a polarization ratio of≥10:l,≥50:l or≥100:l; the structured light projector comprises two light sources with mutually orthogonal main polarizations; the structured light projector comprises a first and second light source having mutually orthogonal main polarizations and a beam combiner for transmission of light from either the first and/or second light source; the structured light projector comprises two laser diodes with mutually orthogonal main polarizations; the structured light projector comprises a first and second laser diode having mutually orthogonal main polarizations and a beam combiner for transmission of light from either the first and/or second laser diode; the structured light projector comprises one or more optical polarizers; the digital camera comprises a polarizing beam splitter and a first and second image sensor, the polarizing beam splitter and the first and second image sensor are arranged behind an objective of the digital camera in such manner that light entering the camera through the objective is split into a first and second light component, the first and second light component have mutually orthogonal polarizations and the first and second light component are directed onto the first, and respectively the second image sensor; the conoscopic holography sensor comprises a laser diode and an optical objective with a beam splitter; the conoscopic holography sensor comprises a laser diode and an optical objective with a beam splitter configured in such manner that laser light emitted by the laser diode is coupled into the optical objective and directed parallel to an optical axis of the optical objective; the conoscopic holography sensor comprises a birefringent crystal; the conoscopic holography sensor comprises a birefringent crystal arranged in such manner that an optical axis of the birefringent crystal is aligned parallel to the optical axis of the optical objective; the conoscopic holography sensor comprises a first and second polarizer; the conoscopic holography sensor comprises a first and second polarizer, wherein the first polarizer is arranged before the birefringnet crystal and the second polarizer is arranged behind the birefringent crystal in a direction parallel to the optical axis of the optical objective; the conoscopic holography sensor comprises an electronic image sensor, such as a charge- coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) image sensor; the conoscopic holography sensor comprises an electronic image sensor arranged in such manner that the optical axis of the optical objective traverses a central portion of the electronic image sensor; the conoscopic holography sensor comprises an electronic image sensor and the birefringent crystal and the first and second polarizer are arranged between the optical objective and the electronic image sensor; - the conoscopic holography sensor comprises one or more laser diodes having an emission wavelength in the range of 380 to 780 nm and an optical objective and an electronic image sensor configured for imaging at a wavelength in the range of 380 to 780 nm;

- the conoscopic holography sensor comprises one or more infrared laser diodes having an emission wavelength in the range of 1000 to 3300 nm and an optical objective and an electronic image sensor configured for imaging at a wavelength in the range of 1000 to 3300 nm;

- the conoscopic holography sensor comprises one or more infrared laser diodes having an emission wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm and an optical objective and an electronic image sensor configured for imaging at a wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm;

- the conoscopic holography sensor comprises one or more ultra-violet laser diodes having an emission wavelength in the range of 150 to 380 nm and an optical objective and an electronic image sensor configured for imaging at a wavelength in the range of 150 to 380 nm;

- the conoscopic holography sensor comprises one or more ultra-violet laser diodes having an emission wavelength in the range of 150 to 300 nm or 200 to 350 nm and an optical objective and an electronic image sensor configured for imaging at a wavelength in the range of 150 to 300 nm or 200 to 350 nm; and/or

- the image processing unit comprises software for automated alignment, merging and/or

interpolation of two or more 3 -dimensional image data sets of a dental impression, wherein the two or more 3 -dimensional image data sets are captured at different spatial orientations of the dental impression relative to the digital camera or conoscopic holography sensor.

The present invention has the further objective to provide a method for preparation of highly accurate three-dimensional digital dental models. This objective is achieved by a method comprising the steps of

- providing a dental impression contained in a dental tray;

- providing an optical scanner comprising a structured light projector, a digital camera, an image processing unit, an electronic controller and a mechanical stage; - mounting the dental tray with the therein contained dental impression on the mechanical stage;

- capturing two or more 3-dimensional image data sets of the dental impression at different spatial orientations of the dental impression relative to the digital camera;

- aligning, merging and/or interpolating the two or more 3-dimensional image data sets into a digital model of the dental impression.

An advantageous alternative embodiment of the inventive method comprises the steps of

- providing a dental impression contained in a dental tray;

- providing an optical scanner comprising a consocopic holography sensor, an image

processing unit, an electronic controller and a mechanical stage; - mounting the dental tray with the therein contained dental impression on the mechanical stage;

- capturing two or more 3-dimensional image data sets of the dental impression at different spatial orientations of the dental impression relative to the conoscopic holography sensor;

- aligning, merging and/or interpolating the two or more 3-dimensional image data sets into a digital model of the dental impression.

A further advantageous alternative of the inventive method comprises the steps of

- providing a silicone-based dental impression contained in a dental tray;

- providing an optical scanner comprising a structured light projector, a digital camera, an image processing unit, an electronic controller and a mechanical stage, wherein the mechanical stage comprises an adapter for the dental tray;

- inserting the dental tray with the therein contained dental impression into the adapter;

- capturing two or more 3 -dimensional image data sets of the dental impression at different spatial orientations of the dental impression relative to the digital camera;

- aligning, merging and/or interpolating the two or more 3 -dimensional image data sets into a digital model of the dental impression.

A further advantageous alternative of the inventive method comprises the steps of

- providing a silicone-based dental impression contained in a dental tray;

- providing an optical scanner comprising a conoscopic holography sensor, an image

processing unit, an electronic controller and a mechanical stage, wherein the mechanical stage comprises an adapter for the dental tray;

- inserting the dental tray with the therein contained dental impression into the adapter;

- capturing two or more 3 -dimensional image data sets of the dental impression at different spatial orientations of the dental impression relative to the conoscopic holography sensor;

- aligning, merging and/or interpolating the two or more 3 -dimensional image data sets into a digital model of the dental impression.

Expedient embodiments of the inventive method are characterized in that

- the dental impression is prepared using a curable elastomeric impression material contained in a dental tray;

- the dental impression is prepared using a curable silicone-based impression material

contained in a dental tray; a first and second 3-dimensional image data set of the dental impression are aligned through a first rotation R(r \ g,d<p) = p , a. second rotation R'(p \ h,A0) = q and a translation T(q \ t ) = q + t of image coordinates r of the second 3-dimensional image data set in consecutive order, wherein

- g is the rotation axis of the first or second rotary stage, and Αφ = φ _ι - φ ₂ is the difference between the angular settings φ _ι , φ ₂ of the first or second rotary stage while capturing the first, and respectively second 3-dimensional image data set;

- h is the rotation axis of the second or first rotary stage, and Αθ = θ _ι - Θ ₂ is the difference between the angular settings θ _γ , θ ₂ of the second or first rotary stage while capturing the first, and respectively second 3-dimensional image data set;

- translation vector t is determined through comparison and alignment of three- dimensional image data sets of a calibration target captured at angular settings φ _ι , φ ₂ , θ _ι , θ ₂ of the first and second rotary stages; or

- translation vector t is determined through image comparison or image correlation of the transformed second 3-dimensional image data set with image coordinates q + 1 and the first 3-dimensional image data set; the two or more 3-dimensional image data sets are captured with light having a wavelength in the range of 380 to 780 ran; the two or more 3-dimensional image data sets are captured with light having a wavelength in the range of 1000 to 3300 nm; the two or more 3 -dimensional image data sets are captured with light having a wavelength in the range of 1100 to 1500 nm, 1600 to 1900 ran, 2200 to 2900 ran or 3000 to 3300 nm; the dental impression has at least one absorption maximum for light with a wavelength in the range of 1000 to 3300 nm; the dental impression has at least one absorption maximum for light with a wavelength in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm; the two or more 3-dimensional image data sets are captured with light having a wavelength in the range of 150 to 380 nm; the two or more 3-dimensional image data sets are captured with light having a wavelength in the range of 150 to 300 nm or 200 to 350 nm; the dental impression has at least one absorption maximum for light with a wavelength in the range of 150 to 380 nm; the dental impression has at least one absorption maximum for light with a wavelength in the range of 150 to 300 nm or 200 to 350 nm; the dental impression fluoresces when irradiated with light having a wavelength in the range of 150 to 3300 nm; the mechanical stage comprises one or more optical alignment marks;

- the one or more optical alignment marks are imaged jointly with the dental impression during capture of 3-dimensional image data sets; and

- the images of the one or more optical alignment marks are employed for alignment of the two or more 3-dimensional image data sets; a first and second 3-dimensional image data set of the dental impression are aligned using images of three non-colinear optical alignment marks A, B, C through a first translation T(f \ a) = f - a = u , a first rotation R(u | ή,α) = v , a second rotation R'(v \ m,P)— w and a second translation T(w \ A) = w + A of image coordinates r of the second 3-dimensional image data set in consecutive order, wherein

- images of optical alignment marks, A, B, C are located in the first 3-dimensional image data set at image coordinates A , B , and respectively C ;

- images of optical alignment marks A, B, C are located in the second 3-dimensional image data set at image coordinates a , b , and respectively c ;

- a distortion correction transform that is stored in the image processing unit of the optical scanner is employed for image correction during capture of images of dental impressions;

- images of a dental impression are captured using light of a first and second light source having mutually orthogonal first and second main polarization, and image contrast is enhanced by comparing an image captured with light from the first light source with an image captured with light from the second light source; - images of a dental impression are captured using a digital camera comprising a polarizing beam splitter and a first and second image sensor, wherein the first and second image sensor register a first, and respectively second light component having mutually orthogonal main polarizations and image contrast is enhanced by comparing images of the first and second image sensor; - the dental impression is coated with a light diffusing agent prior to capture of the two or more 3 -dimensional image data sets;

- the dental impression is powder coated with electrically charged particles prior to capture of the two or more 3-dimensional image data sets; - the dental impression is powder coated with particles electrically charged using a corona discharge electrode or a triboelectric nozzle prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is immersed in an electric field generated by electrodes and powder coated with electrically charged particles prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is powder coated with electrically charged particles having an

equivalent diameter of 1 to 10 μηι prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is coated with a liquid suspension of particles prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is immersed in an electric field generated by electrodes and coated by electrophoresis with a liquid suspension of particles prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is coated with a liquid suspension of particles having an equivalent diameter of 1 to 10 μm prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is coated with an aerosol using a nebulizer prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is coated with an aerosol using a nebulizer comprising a vibrating

membrane nozzle prior to capture of the two or more 3-dimensional image data sets; - the dental impression is coated with an aerosol comprised of droplets having an equivalent diameter of 1 to 10 μπι prior to capture of the two or more 3-dimensional image data sets;

- the dental impression is coated with a water-based aerosol comprised of droplets having an equivalent diameter of 1 to 10 μm prior to capture of the two or more 3-dimensional image data sets; - the dental impression is coated with an aerosol comprised of water droplets having an equivalent diameter of 1 to 10 μπι prior to capture of the two or more 3 -dimensional image data sets;

- the dental impression is coated with a light absorbing agent prior to capture of the two or more 3 -dimensional image data sets; and/or

- the dental impression is coated with a fluorescent agent prior to capture of the two or more 3 -dimensional image data sets.

The present invention has the further objective to provide a system for preparation of highly accurate three-dimensional digital dental models. This objective is achieved by a system comprising

- an optical scanner according to any one of the above described embodiments;

- a dental tray; and

- an elastomeric impression material.

Advantageous implementations of the inventive system are characterized in that: - the system comprises a dental impression material having one or more optical absorption maxima for one or more wavelengths in the range of 1000 to 3300 nm;

- the system comprises a dental impression material having one or more optical absorption maxima for one or more wavelengths in the range of 1100 to 1500 nm, 1600 to 1900 nm, 2200 to 2900 nm or 3000 to 3300 nm; - the system comprises a dental impression material having one or more optical absorption maxima for one or more wavelengths in the range of 150 to 380 nm;

- the system comprises a dental impression material having one or more optical absorption maxima for one or more wavelengths in the range of 150 to 300 nm or 200 to 350 nm;

- the system comprises a dental impression material that fluoresces when irradiated with light having a wavelength in the range of 150 to 3300 nm;

- the system comprises a dental impression material based on silicone;

- the system comprises a dental impression material based on a room temperature vulcanizable or curable silicone, designated as RTV-silicone; - the system comprises a dental impression material based on a transparent RTV-silicone having an optical transmission of >50 % for wavelengths in the range of 380 to 780 nm;

- the elastomeric impression material comprises a light diffusing agent;

- the system comprises a powder coating device configured for deposition of electrically charged particles;

- the system comprises a powder coating device configured for deposition of electrically charged particles having an equivalent diameter of 1 to 10 μm;

- the the system comprises a powder coating device equipped with a corona discharge

electrode or a triboelectric nozzle;

- the system comprises electrodes for immersing a dental impression in an electric field;

- the system comprises a nebulizer configured for generation of an aerosol comprised of droplets having an equivalent diameter of 1 to 10 μm;

- the system comprises a nebulizer equipped with a vibrating membrane nozzle;

- the system comprises a light diffusing agent for coating of a dental impression;

- the system comprises a fluorescent agent for coating of a dental impression;

- the system comprises a liquid suspension of particles having an equivalent diameter of 1 to 10 μιτι; and/or

- the system comprises a powder consisting of particles having an equivalent diameter of 1 to 10 μηι.

In the present invention the term "image" pertains to regular two-dimensional digital image data either directly captured with a digital camera or conoscopic holography sensor or after digital processing such as polarization contrast enhancement or distortion correction.

Polarization contrast enhancement is achieved by

- projecting a light stripe pattern onto the dental impression with light that is mainly linearly polarized in a first polarization plane and capturing a first image;

- projecting an identical light stripe pattern onto the dental impression with light that is mainly linearly polarized in a second polarization plane that is orthogonal to the first polarization plane and capturing a second image; - calculating a normalized contras ) in the first image according to

the formul denotes the intensity of pixel (i, j) and I ₁ \ is the average intensity of all pixels in the first image;

- calculating a normalized contras for each pixel (/, j) in the second image according

to the formula wherein I ₂ (i,j) denotes the intensity of pixel and

I ₂ is the average intensity of all pixels in the second image;

determining pixels (p, q) in the first image with , wherein t

designates a suitably chosen threshold value with 1.5 < t < 10 and substituting the intensity I, (p, q) of pixels (p, q) in the first image with

leaving the intensit of pixels in the first image that do not exceed the contrast ratio threshold unchanged, i.e.

determining pixels (r, s) in the second image with

substituting the intensity

leaving the intensity of pixels (i, j) in the second image that do not exceed the contrast ratio threshold unchanged, i.e.

constructing a polarization contrast enhanced image wherein each pixel (i, j) is assigned an intensity value that corresponds to the average of the corrected intensity values of the first and second image, i.e.

For coiTection of optical image distortion several methods are employed in the present invention. These methods are based on the electronic compensation of optical image distortion via a suitable distortion correction transform and comprise the steps of providing a calibration object with a highly accurate geometric pattern, such as a grid or dot pattern. The calibration pattern may be periodic in one or two spatial directions or irregular. Preferably, the calibration pattern is formed with sub-micron precision by photolithography and etching of a metallic or ceramic thin film disposed on a silicon or glass wafer using semiconductor manufacturing methods; constructing digital reference images of the calibration object in select perspectives using image simulation or ray tracing software such as Blender (https://www.blender.org/) or POV- Ray (http://www.povray.org/); capturing images of the calibration object with a digital camera of an optical scanner in perspectives equivalent to those of the reference images; devising a mathematical procedure for calculation of the degree of deviation or the degree of accordance of a captured image and its equi-perspective reference image. Suitable mathematical procedures are known in the art of digital image processing and are based on comparision or correlation. The term "comparison" pertains to pixel-based mathematical operations, wherein the difference or the ratio of the intensity or gray-scale values of image pixels at equivalent positions in a captured image and an equi-perspective reference image are calculated as sum of squares or as average. Such comparison may be carried out for all image pixels or for select image regions containing distinctive features such as alignment crosshairs. The term "correlation" refers to the cross-correlation or digital image correlation (DIC) of a captured image and its equi-perspective reference image. Digital image con-elation (DIC) may be calculated according to various mathematical formulas, such as

wherein p _c (i,j) ® p _r (i,j) denotes the image correlation at position , p _c (m, n) , p _c (m + i,n + j) and p _c are the gray-scale value at position (m, n) , and respectively

(m + i,n + j) of the captured image and the average thereof and p _r (m,n) and ]?,, designate the gray-scale value at position (m,n) of the reference image and the average thereof. Digital image correlation (DIC) may be calculated via convolution or Fourier transform, in particular fast Fourier transform (FFT). Both image comparison and image correlation yield a measure for the degree of deviation or the degree of accordance between a captured image and its equi-perspective reference image; - devising a discrete or continuous distortion correction transform. Discrete distortion correction transforms encompass e.g. a grid or matrix of image coordinate shifts at discrete image coordinates in conjunction with a suitable interpolation method (e.g. spline interpolation) in order to calculate image coordinate shifts for intermediate image coordinates. The image coordinate shifts at discrete grid coordinates represent adjustable parameters of the discrete distortion correction transform. Continuous (function based) distortion correction transforms are e.g. based on the physics of optical distortion and may comprise parameterized terms for radial distortion (caused by non-paraxial light rays), decentering distortion (caused by non-coaxial lens alignment) and prism distortion (caused by lens tilt). Alternatively, a suitable function for a continuous distortion correction transform may be chosen on a heuristic basis. Continuous distortion correction transforms may be formulated with a limited number of parameters and are preferably employed in the present invention;

- determining the parameters of the discrete or continuous distortion correction transform based on numerical optimization methods such as the Newton-Raphson algorithm or the Levenberg-Marquardt method (damped least-squares method) in such manner that the degree of deviation or the degree of accordance of captured images and equi-perspective reference images is minimized, or respectively maximized;

- storing each parameterized distortion correction transfomi in an image processing unit of the optical scanner and applying it to a digital image of a dental impression captured in a perspective equivalent to that of the reference image used for derivation of the distortion correction transform.

A continuous distortion correction transfomi may e.g. have the form

wherein (x,y) and designate raw image coordinates and, respectively, distortion corrected image coordinate denote corrections for radial

distortion, decentering distortion and prism (tilt) distortion, respectively. The above stated distortion correction transform comprises five parameters which are determined

by the afore described procedure using a calibration object.

Triangulation refers to a simple geometrical transform whereby a perspective image of a light stripe projected onto a contoured surface of an object is converted into a three-dimensional profile. Therein, the light stripe may be envisaged as the edge of a thin slice cut from the object.

In the present invention the term "3-dimensional image data set of a dental impression" pertains to digital image data describing a contoured surface of a dental impression as a set

of N points in three-dimensional space, wherein each point r' is

represented as a vector comprising three coordinates

wherein vectors designate three linearly independent basis vectors. Typically, basis

vectors are orthonormal with wherein "· " and denote the vectorial

scalar product, and respectively the Kronecker delta product, which is 1 for i = j and zero for

According to the invention 3-dimensional image data sets of a dental impression are captured at different spatial orientations of the dental impression relative to the digital camera or conoscopic holography sensor of an optical scanner. The spatial orientation of a rigid object, such as a dental impression in three-dimensional space can generally be described as a combination of at least one translation and at least one rotation about an axis of unit

length by an angle χ . Rotation may be described in various forms, such as

- vector notation: wherein

"x" denotes the vectorial cross product; or

- matrix notation: with , wherein

denotes the Levi-Civita tensor, which is zero if two of indices i, j, k are identical and +1

or -1 for even, respectively odd permutations of indices i, j, k.

A 3-dimensional image data set may be stored in the customary STL-format (wherein STL is an acronym for either "stereo lithography" or "standard tessellation language").

A 3-dimensional image data set of a dental impression is captured by either of two methods la) to le), 2a) to 2e) or 3a) to 3d) comprising the steps of

1 a) projecting one or more multiple stripe light patterns onto the dental impression along a projection direction, the longitudinal axis of the pattern stripes and the projection direction spanning a projection plane; lb) capturing an image of at least one multiple stripe light pattern with a digital camera having an optical axis that is inclined relative to the projection plane under an observation angle; lc) storing the image of each light stripe or respectively of an edge of each light stripe as a set of stripe points;

Id) assigning each stripe point coordinates in three-dimensional space based on a triangulation algorithm, wherein the observation angle is accounted for; le) storing the thus computed coordinates as a 3-dimensional image data set; or a) projecting a single light stripe onto the dental impression along a projection direction, the longitudinal axis of the single light stripe and the projection direction spanning a projection plane; b) scanning the single light stripe across the dental impression along a scan axis either by optical beam steering or by translating the dental impression and simultaneously capturing images of the single light stripe with a digital camera having an optical axis that is inclined relative to the projection plane under an observation angle; c) storing images of the single light stripe or respectively of an edge of the single light stripe as a set of stripe points; d) assigning each stripe point coordinates in three-dimensional space based on a triangulation algorithm, wherein the observation angle is accounted for; 2e) storing the thus computed coordinates as a 3 -dimensional image data set; or

3a) directing a light beam of a conoscopic holography sensor onto the dental impression;

3b) scanning the light beam across the dental impression by translating the dental impression and simultaneously capturing images of the light beam at each scan position with the conoscopic holography sensor;

3c) assigning each scan position coordinates in three-dimensional space based on a

mathematical algorithm for determination of the distance between the dental impression and the conoscopic holography sensor from the Fresnel interference fringe pattern in the captured image of the light beam;

3d) storing the thus computed coordinates as a 3 -dimensional image data set.

When using an optical scanner comprising a conoscopic holography sensor the mathematical algorithm recited in step 3c) is based on the equation which relates the

distance D between the object point and the image plane to the average radius R _m of the interference fringe of m-th order in the Fresnel interference pattern captured by the electronic image sensor of the conoscopic holography sensor, wherein λ designates the wave length of the light emitted by the laser diode of the conoscopic holography sensor.

The present invention is further illustrated in Figures 1 to 6 showing

Fig. 1 schematic diagram of an optical scanner;

Fig. 2 a structured light projector generating a multiple stripe light pattern;

Fig. 3 a structured light projector generating a single stripe light pattern;

Fig. 4 a schematic illustration of the triangulation principle;

Fig. 5 a dental impression illuminated with a single stripe light pattern;

Fig. 6 a mechanical stage of an optical scanner; and

Fig. 7 multiple optical scans under different perspective settings.

Fig. 1 depicts an optical scanner 1 for dental impressions according to the invention. Optical scanner 1 comprises a structured light projector 2, that is configured to project a light stripe pattern 20 onto a dental impression contained in a dental tray 30 (the dental impression is not shown in Fig. 1). Images of the projected light stripe pattern 20 are captured with a digital camera 3. Digital camera 3 comprises an objective and an electronic light sensor such as a CCD- or CMOS -sensor. Preferably, the objective of digital camera 3 is telecentric and/or aberration corrected. The images captured by digital camera 3 are processed using an image processing unit 4, which preferably comprises one or more graphics processing units (GPUs). The various subunits and components of the optical scanner are connected to an electronic controller 5.

Electronic controller 5 is preferably configured as programmable logic controller (PLC) or as regular personal computer with a thereon installed PLC software. The latter PLC configuration is commonly termed as soft-PLC. Optical scanner 1 further comprises a mechanical stage 6 for mechanical fixture and spatial orientation of dental tray 30 and a therein contained dental impression relative to structured light projector 2 and digital camera 3. Mechanical stage 6 comprises a mechanical adapter for a dental tray. The adapter preferably comprises one or more mechanical fixtures that are form-fit to a predetermined outer surface of a dental tray such that the dental tray can be inserted in a force-fit and reversible manner into the adapter.

Fig. 2 schematically shows a dental tray 30 with a therein contained dental impression (omitted in Fig. 2) whereon a multiple stripe light pattern 20 is projected using a structured light projector 2 of an optical scanner. The projected multiple stripe light pattern 20 is captured with a digital camera 3. According to the invention at least two projected multiple stripe light patterns 20 are captured a different spatial orientations of dental tray 30 and the therein contained dental impression relative to structured light projector 2 and digital camera 3. This enables imaging of surface regions of a dental impression for which at a first spatial orientation the line of sight either from structured light projector 2 or to digital camera 3 is blocked. In the present invention such line of sight blocking is also designated as "optical occlusion". By capturing projected multiple stripe light patterns 20 at two or more spatial orientations of dental tray 30 and a therein contained dental impression relative to structured light projector 2 and digital camera 3 the problem of optical occlusion is overcome and the relevant surface of a dental impression is captured in its entirety. Aside from resolving problems due to optical occlusion, capture of projected multiple stripe light patterns 20 at two or more spatial orientations in conjunction with interpolation of corresponding 3 -dimensional image data sets of a dental impressions improves geometric accuracy (fidelity) relative to the concrete dental impression and helps eliminate imaging errors.

Fig. 3 schematically shows a dental tray 30 with a therein contained dental impression (omitted in Fig. 3) whereon a single stripe light pattern 20 is projected using a structured light projector 2 of an optical scanner. The projected single stripe light pattern 20 is captured with a digital camera 3. In order to capture projections of the single light stripe pattern 20 at a multitude of positions across the dental impression, dental tray 30 and the therein contained dental impression is translated (or scanned) along an axis 300 in a stepwise or continuous manner. The translation or scanning motion along axis 300 is effected with the mechanical stage of the optical scanner. For this purpose the mechanical stage is suitably equipped with a motorized linear stage, such as depicted in Fig. 6. While dental tray 30 and the therein contained dental impression are scanned along axis 300 images of the thereon projected single light stripe pattern are captured with digital camera 3 at a multitude of positions along axis 300. Preferably, the translation or scanning motion along axis 300 is performed in a continuous manner with little or no variation of scanning velocity. Images of the projected single stripe light pattern 20 are captured at regular time intervals which - for constant scanning velocity correspond to equidistant positions along axis 300. In an expedient embodiment of the invention, the motorized linear stage is equipped with an encoder thus enabling highly accurate electronic registration of the scanning position along axis 300. In a particularly advantageous embodiment of the invention the encoder signal is used to trigger digital camera 3 to capture images of the projected single stripe light pattern 20 at defined scanning positions along axis 300.

According to the invention at least two scan series of projected single stripe light patterns 20 are captured a different spatial orientations of dental tray 30 and the therein contained dental impression relative to structured light projector 2 and digital camera 3. By capturing two or more scan series of projected single stripe light patterns 20 at two or more spatial orientations of dental tray 30 and a therein contained dental impression relative to structured light projector 2 and digital camera 3 the problem of optical occlusion is overcome and the relevant surface of a dental impression is captured in its entirety. Aside from resolving problems due to optical occlusion, capture of scan series of projected single stripe light patterns 20 at two or more spatial orientations in conjunction with interpolation of corresponding 3-dimensional image data sets of a dental impressions improves geometric accuracy (fidelity) relative to the concrete dental impression and helps eliminate imaging errors. Fig. 4 depicts the triangulation principle using a structured light projector 2 which projects a stripe pattern 20 onto an object. Stripe pattern 20 is captured with a digital camera 3. Structured light projector 2 and digital camera 3 are arranged relative to each other in a fixed manner, such that their optical axes are oriented under predetermined azimuthal and polar angle relative to a global coordinate system and two select points F2 and F3 on the optical axis of structured light projector 2 and digital camera 3, respectively, are spaced apart at a known distance B. The select points F2 and F3 on the optical axis of structured light projector 2 and digital camera 3, respectively, may be arbitrarily chosen or may e.g. correspond to a cardinal point, i.e. to a focal, principal or nodal point. In such predetermined geometrical arrangement of digital camera 3 relative to structured light projector 2 each point P on the surface of the object can be assigned a triangle with vertices F2, F3, P and angles a and β at vertices F2 and F3, respectively. A distance L between vertices P and F3 is then determined according to the formula

Based on distance L point P on the surface of the object can be assigned spatial coordinates. Fig. 5 depicts a schematic perspective view of a dental impression 31 and a thereon projected single stripe light pattern 32. Single stripe light pattern 32 is formed by illuminating dental impression 31 with a thin light sheet 200.

Fig. 6 shows perspective views of a mechanical stage 6 of an optical scanner in assembled state and as explosion drawing. Mechanical stage 6 comprises a linear stage 61, a first rotary stage 64 and a second rotary stage 66. The first rotary stage 64 is mounted to linear stage 61 via a first angle bracket 63. The second rotary stage 66 is mounted to the first rotary stage 64 via a second angle bracket 65. Linear stage 61 is equipped with an electronic motor 62. Preferably, electronic motor 62 is configured as DC-motor or as stepper-motor. In an expedient embodiment of the invention electronic motor 62 is equipped with a magnetic, electronic or electrooptic encoder for registration of the relative position of linear stage 61 along its motion axis 610. Similar to linear stage 61, the first and second rotary stage 64 and 66 are each equipped with an electronic motor (not shown in Fig. 6) such that the respective rotation angles can be electronically set. In a particularly advantageous embodiment of the invention all the electronic motors of mechanical stage 61 are each equipped with an encoder and one or two limit switches. A dental tray 30 is mounted to the second rotary stage 66 via a mechanical adapter (not shown). The first rotary stage 64 enables rotation of dental tray 30 about a first rotation axis 640 as indicated by arrow 640' and the second rotary stage 66 enables rotation of dental tray 30 about a second rotation axis 660 as indicated by arrow 660'. The first rotary stage 64 is mounted to linear stage 61 in such manner that an angle between the first rotation axis 640 and a vertical direction is from 50 to 130 degree, preferably from 80 to 100 degree and most preferably from 89.9 to 90.1 degree. The second rotary stage 66 is mounted to the first rotary stage 64 in such manner that an angle between the second rotation axis 660 and a vertical direction is from 0 to 40 degree, preferably from 0 to 10 degree and most preferably from 0 to 0.2 degree.

Fig. 7a) to 7f) depict a dental tray 30 and a therein contained dental impression (not shown in Fig. 7) that is scanned at six different spatial orientations relative to a structured light projector 2 and a digital camera 3 of an optical scanner. Structured light projector 2 and digital camera 3 are fixed and retain their positions. Structured light projector 2 is configured to project a single stripe light pattern onto the dental impression contained in dental tray 30. In Fig. 7 the single stripe light pattern projected by structured light projector 2 is visualized as light sheet 200. Similarly, the image of the projected single stripe light pattern captured by digital camera 3 is depicted as light sheet 300.

Dental tray 30 and a therein contained dental impression are mounted to a mechanical stage 6 of an optical scanner. Mechanical stage 6 comprises the same components and is configured as afore described in conjunction with Fig. 6. In particular, mechanical stage 6 comprises a linear stage 61 and a first and second rotary stage 64, and respectively 66. Linear stage 61 provides translation or scanning motion of dental tray 30 along axis 610. Using the first rotary stage 64 dental tray 30 can be rotated about a first rotation axis by a first rotation angle. Using the second rotary stage 66 dental tray 30 can be rotated about a second rotation axis by a second rotation angle. The first and second rotary stage 64 and 66 are preferably arranged in such manner that the first and second rotation axes are inclined relative to a vertical direction by an angle from 89.9 to 90.1 degree, and respectively from 0 to 0.2 degree. Six 3-dimensional image data sets of the dental impression are captured through six consecutive scans along axis 610 with six different spatial orientations of dental tray 30 relative to structured light projector 2 and digital camera 3, wherein the first and second rotation angle have values as e.g. listed beneath

The above stated first and second rotation angles are exemplary. The present invention encompasses therefrom differing spatial orientations, wherein the first and second rotation ang are selected from the range of -60 to 60 degree, and respectively 0 to 90 degree.