Optics and Lasers in Engineering 54 (2014) 203–221
`
`Contents lists available at ScienceDirect
`
`Optics and Lasers in Engineering
`
`journal homepage: www.elsevier.com/locate/optlaseng
`
`Recent advances in dental optics – Part I: 3D intraoral scanners
`for restorative dentistry
`Silvia Logozzo a,n, Elisabetta M. Zanetti a, Giordano Franceschini a, Ari Kilpelä b,
`Anssi Mäkynen b
`a Department of Industrial Engineering, University of Perugia, Via Duranti, 93 06125 Perugia, Italy
`b Department of Electrical Engineering, University of Oulu, P.O.BOX 8000, FI-90014 Oulu, Finland
`
`a r t i c l e i n f o
`
`a b s t r a c t
`
`Available online 17 September 2013
`
`Keywords:
`Intraoral scanner
`Digital dental impressions
`Triangulation
`Confocal microscopy
`Optical coherence tomography
`
`Intra-oral scanning technology is a very fast-growing field in dentistry since it responds to the need of an
`accurate three-dimensional mapping of the mouth, as required in a large number of procedures such
`as restorative dentistry and orthodontics. Nowadays, more than 10 intra-oral scanning devices for
`restorative dentistry have been developed all over the world even if only some of those devices are
`currently available on the market. All the existing intraoral scanners try to face with problems and
`disadvantages of traditional impression fabrication process and are based on different non-contact
`optical technologies and principles. The aim of this publication is to provide an extensive review of
`existing intraoral scanners for restorative dentistry evaluating their working principles, features and
`performances.
`
`& 2013 Elsevier Ltd. All rights reserved.
`
`1. Background
`
`Three-dimensional scanning of the mouth is required in a large
`number of procedures in dentistry such as restorative dentistry
`and orthodontics. The aim of the 3D mapping of the oral cavity is
`to create digital impressions.
`Restorative dentistry is of course the main field that require the
`application of very accurate 3D intraoral scanners. For the realiza-
`tion of any dental prosthesis it is necessary to realize three-
`dimensional mathematical models of the dentition, performing a
`reverse engineering procedure. Then the prosthesis can be realized
`by means of CAD/CAM systems.
`At present, according to the traditional work flow, this procedure
`starts at the dentist's office, and the steps leading to prosthesis's
`creation are as follows:
`
`Abbreviations: AFI, accordion fringe interferometry; AWS, active wave-front
`sampling; CAD/CAM, computer aided design/computer aided manufacturing;
`CLSM or LSCM, confocal laser scanning microscopy; HIPAA, health insurance
`portability and accountability act; LASER, light amplification by stimulated
`emission of radiation; LED, light emitting diode; MEMS, micro electro-mechanical
`system; NA, numerical aperture; OCT, optical coherence tomography; OBJ, alias
`wavefront technologies file format; PLY, polygon file format or Stanford triangle
`format; PMT, photo-multiplier tube; SLA, stereo-lithography; S/N or SNR,
`signal-to-noise ratio; USB, universal serial bus
`n Corresponding author. Tel.: +39 348 7142939.
`E-mail addresses: sililog@hotmail.com (S. Logozzo),
`elisabetta.zanetti@unipg.it (E.M. Zanetti), giordano.franceschini@unipg.it
`(G. Franceschini), arik@ee.oulu.fi (A. Kilpelä), anssi.makynen@ee.oulu.fi
`(A. Mäkynen).
`
`0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
`http://dx.doi.org/10.1016/j.optlaseng.2013.07.017
`
` the dentist captures the traditional impression by means of
`impression trays and impression materials;
` the dentist sends the impression tray to the dental laboratory;
` the laboratory's technician pours plaster inside the tray;
` after the hardening he scans the plaster model to have the 3D
`virtual digital model of the full arch;
` the technician can design the prosthesis by means of CAD/CAM
`systems and send the file to a milling machine;
` the milling machine produces the prosthesis;
` the prosthesis is applied by the dentist and refined inside the
`patient's mouth to verify and adjust the occlusion.
`
`Basically, the 3D digital model is used to design the prosthesis
`and as an input to the program of the milling machine referring to
`CAD/CAM systems. It can also be used to perform surgery simula-
`tions or to build plastic models of the teeth by means of rapid
`prototyping techniques.
`The whole traditional process is often slow and affected by
`errors. Furthermore, although the traditional impression taking
`process is very cheap, it is certainly bothering for the patient and,
`at the present state of the art, definitively obsolete.
`By means of devices here described, the dentist can scan the
`teeth in vivo and he can directly create the virtual 3D model of the
`dentition. This allows bypassing the dental laboratory for a lot
`of steps.
`According to the state of the art, there are three kinds of
`workflows in restorative dentistry. The traditional workflow has
`been described above; it is the oldest and is illustrated in Fig. 1.
`
`Align Ex. 1028
`U.S. Patent No. 9,962,244
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`Fig. 1. Traditional workflow for dental impressions.
`
`Fig. 2. Former digital workflow for dental impressions.
`
`Sometimes the plaster pouring can be skipped because the
`impression tray is directly scanned.
`The second kind of workflow is the ‘former digital workflow’.
`The term ‘former’ is used to distinguish this method from the
`newest one, that is mentioned here as ‘rapid digital workflow’. The
`‘former’ digital workflow can be followed by a clinician who owns
`a standalone intraoral scanner, which is not equipped with a
`milling unit. The former digital workflow is reported in Fig. 2.
`According to the former digital workflow, the steps for pros-
`thesis creation are as follows:
` the dentist captures the digital impression by means of an
`intraoral scanning device;
` the dentist sends the digital prescription to a laboratory;
` the lab downloads the digital file and uses customized software
`to digitally cut the die and mark the margins;
` the SLA model is generated by using CAD/CAM systems;
` the technician can proceed with his preferred finishing
`technique:
`○ hand layered porcelain,
`
`○ pressing with wax patterns,
`○ digitally designed and milled full contour glass ceramic
`restoration by means of CAD/CAM systems (the technician
`must also design the program for the milling machine by
`means of CAM systems);
` the final restoration is then sent to the doctor for seating.
`
`The third kind of workflow is the rapid digital workflow. This
`workflow can be followed when the clinician owns an intraoral
`scanner equipped with an in-office milling unit. The rapid digital
`workflow is shown in Fig. 3.
`According to the rapid digital workflow, the steps for prosthesis
`creation are as follows:
` the dentist captures the digital impression by means of an
`intraoral scanning device;
` the dentist designs the restoration and the software automa-
`tically generates the program for the milling unit;
` the final restoration is milled in a few minutes;
` the doctor applies the restoration.
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`205
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`Fig. 3. Rapid digital workflow for dental impressions.
`
`As a result of the application of 3D intraoral scanners, some
`of the disadvantages related to the traditional workflow can be
`overcome, as:
` Mould instability;
` Mould transport and packaging;
` Plaster pouring and solidification;
` Delamination;
` Lacerations on margins;
` Contact between the tray and the teeth;
` Geometrical and dimensional
`inconsistencies between the
`plaster model and the real teeth.
`
`the following
`
`3D Intraoral scanners can therefore entail
`advantages:
` Implementation of highly accurate models;
` Traditional workflow simplification;
` Possibility to create and periodically update a database of
`dentitions for future interventions;
` Possibility to simulate surgery interventions on the digital model;
`
`Overcoming all the prior disadvantages.
`
`2.
`
`Introduction
`
`The application of CAD/CAM methodologies to the dental field
`was the brainchild of Dr. Francois Duret in his thesis, presented at
`the Université Claude Bernard, Faculté d′Odontologie, in Lyon,
`France in 1973, and entitled ‘Empreinte Optique’ (Optical Impres-
`sion). In detail, he developed and patented a CAD/CAM device in
`1984. The developed system was presented at the Chicago Mid-
`winter Meeting in 1989, and was able to fabricate a dental crown
`in 4 h [2,3].
`Digital
`impressions have been introduced, and successfully
`used, for a number of years, in orthodontics, as well, including
`Cadent's IOC/OrthoCad, DENTSPLY/GAC's OrthoPlex, Stratos/Ora-
`metrix's SureSmile, and EMS’RapidForm, but the introduction of
`the first digital intraoral scanner for restorative dentistry was
`in the 1980s by a Swiss dentist, Dr. Werner Mörmann, and an
`Italian electrical engineer, Marco Brandestini, who developed the
`
`fundamentals for CERECs by Sirona Dental Systems LLC (Charlotte,
`NC), introduced in 1987, as the first commercially available CAD/
`CAM system for dental restorations [2,4]. Ever since, research and
`development by a lot of companies have improved the technolo-
`gies and created in-office intraoral scanners, which are increas-
`ingly user-friendly
`and produce precisely fitting dental
`restorations. These systems are capable of capturing three-
`dimensional virtual
`images of tooth preparations; restorations
`may be directly fabricated from such images (using CAD/CAM
`systems) or the same images can be used to create accurate master
`models, for the restorations in a dental laboratory [2].
`Nowadays, more than 10 intra-oral scanning devices for restora-
`tive dentistry exist all over the world. In this paper all these devices
`are mentioned and 11 are described and analysed. Existing devices
`are based on different non-contact optical technologies such as
`confocal microscopy, optical coherence tomography, active and
`passive stereovision and triangulation, interferometry and phase
`shift principles. Basically, all these devices combine more than one of
`the cited imaging techniques to minimize the noise arising when
`scanning inside an oral cavity as, for example: noise related to the
`optical features of the target surfaces (translucency and the different
`reflectivity of target materials such as teeth, gums, preparations,
`resins, etc.), to wetness and to random relative motions. Also several
`typologies of structured light sources and optical components are
`employed. The analysed intra-oral scanning devices for restorative
`dentistry are listed below:
`
`(1) CERECs – by Sirona Dental System GmbH (Germany)
`(2) iTero – by CADENT Ltd (Israel)
`(3) E4D – by D4D TECHNOLOGIES, Llc (USA)
`(4) Lava™C.O.S. – by 3M ESPE (USA)
`(5) IOS FastScan – by IOS TECHNOLOGIES, Inc. (USA)
`(6) MIA3d™ – by Densys3D Ltd (Israel)
`(7) DPI-3D – by DIMENSIONAL PHOTONICS INTERNATIONAL,
`Inc. (USA)
`(8) 3D Progress – by MHT S.p.A. (Italy) and MHT Optic Research
`AG (Switzerland)
`(9) directScan – by HINT – ELS GmbH (Germany)
`(10) trios – by 3SHAPE A/S (Denmark)
`(11) Bluescans-I – ATRON3Ds GmbH (Austria)
`Only some of these are already commercially available. As
`already mentioned, even if a lot of advantages in taking
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`Fig. 4. Confocal microscopy principle [6].
`
`digital impressions are achievable, also some disadvantages
`subsist. For example, it is often necessary to apply some
`coatings on the teeth (to minimize the noise of the measure-
`ment) and to rest the camera wand on a tooth to get a steady
`focus. Moreover, the 3D virtual model is often reconstructed
`by post-processing single images (acquired from a single
`perspective); accordingly the reconstruction is not performed
`in real time with a continuous data capture. Furthermore,
`data concerning the accuracy of the available instruments are
`often missing [1].
`Some other new intra-oral scanners have been recently
`presented at International Dental Show 2013 in Cologne:
`(12) Planscan – Planmeca Oy (Finland)
`(13) Condor – Remedent Inc. (Belgium)
`(14) CS 3500 – Carestream Health, Inc. (USA)
`(15) DigImprint – Steinbichler Optotechnik GmbH (Germany)
`
`3. Confocal laser scanner microscopy and devices
`
`Confocal laser scanning microscopy (CLSM or LSCM) is a technique
`to acquire in-focus images from selected depths, a process known as
`optical sectioning (high-resolution optical images with depth selecti-
`vity) [5]. Images are acquired point-by-point and reconstructed by a
`computer. By using this technique, one can reconstruct the surface
`profile of opaque specimens and obtain the interior imaging of non-
`opaque specimens.
`A conventional microscope sees as far into the specimen as the
`light can penetrate, whereas a confocal microscope only images
`one depth level at a time.
`The CLSM achieves a controlled and highly limited depth
`of focus.
`The principle of confocal microscopy was originally patented by
`Marvin Minsky in 1961 [6], but it took another 30 years and the
`development of lasers for CLSM to become a standard technique,
`toward the end of the 1980s.
`In a CLSM technique a laser beam passes through an aperture
`(14 in Fig. 4) and then is focused by an objective lens (11 in Fig. 4)
`into a small focal volume, within or on the surface of a specimen;
`in biological applications the specimen may be fluorescent. Scat-
`tered and reflected laser light, as well as any fluorescent light from
`the illuminated spot, is then re-collected by the objective lens.
`A beam-splitter (17 in Fig. 4) separates off some portion of the
`light into the detection apparatus (28 in Fig. 4).
`This apparatus, in fluorescence confocal microscopy, has also a
`filter, which selectively passes the fluorescent wavelengths while
`blocking the original excitation wavelength. After passing a pin-
`hole (24 in Fig. 4), the light intensity is detected by a photo-
`detection device (usually a photomultiplier
`tube (PMT) or
`
`Fig. 5. iTero digital impression system [2].
`
`avalanche photodiode (APD)), transforming the light signal into
`an electrical one which is recorded by a computer [7].
`The limited detector aperture obstructs the light which is not
`coming from the focal point. The out-of-focus light is suppressed:
`most of the returning light is blocked by the pinhole, which results
`in sharper images than those from conventional fluorescence
`microscopy techniques and permits to obtain images of planes
`located at various depths within the sample (sets of such images
`are also known as ‘z stacks’) [5].
`The detected light, originating from an illuminated volume
`element within the specimen, represents one pixel in the resulting
`image. The brightness of a resulting image pixel corresponds to the
`relative intensity of the respective detected light. The beam is
`scanned across the sample in the horizontal plane by using one or
`more servo-controlled oscillating mirrors.
`Slower scans provide a better signal-to-noise ratio, resulting in
`better contrast and higher resolution. Information can be collected
`from different focal planes by raising or lowering the microscope
`stage or objective lens. The computer can generate a three-
`dimensional picture of a specimen by assembling a stack of these
`two-dimensional images, from successive focal planes [5].
`
`3.1.
`
`iTero by CADENT LTD (IL)
`
`The Cadent iTero digital impression system by Cadent LTD, IL
`(Fig. 5) came into the market in early 2007. iTero system employs a
`parallel confocal imaging technique (Fig. 6) [8]. As shown in Fig. 7,
`an array of incident red laser light beams (36), passing through a
`focusing optics (42) and a probing face, is projected onto the teeth.
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`
`Fig. 6. iTero's wand [2,9].
`
`Fig. 7. iTero scanning system [8].
`
`This technique allows iTero capturing all structures and mate-
`rials in the mouth, without the need to apply any coating to the
`patient's teeth [9]. The complete three-dimensional representation
`of the entire structure can be obtained assembling surface topol-
`ogies of adjacent portions, taken at two or more different angular
`perspectives [10]. The iTero camera capability of scanning without
`the need of coating powders is advantageous, however it requires
`the addition of a colour wheel to the acquisition unit itself (Fig. 8),
`resulting in a camera with a larger scanner head, compared to the
`other systems [2]. In fact, also a two-dimensional (2D) colour
`image of the 3D structure of teeth is taken at the same angle and
`orientation with respect to the structure. As a consequence, each
`X–Y point on the 2D coloured image corresponds to one point on
`the 3D scan, having the same relative X–Y values. The coloured
`image (Fig. 8) is obtained illuminating the target surface with
`three beams having three complementary colours (red, green or
`blue light), and combining the respective monochromatic images
`to create a full colour image. The three beams are obtained from
`the same white light source, with colour filters. The filters are
`arranged as sectors of a rotatable disc, coupled to a motor [11].
`Capturing the digital impression requires following a series of
`steps for every impression which the operator is guided through.
`These include five scans of the prepared area: occlusal, lingual,
`buccal, and interproximal contacts of the adjacent teeth [2], and
`require approximately 15 or 20 s per prepared tooth. Then buccal
`and lingual 451 angle views of the remaining teeth in the quadrant
`or arch and opposing arch are obtained. When these scans (at least
`
`Fig. 8. iTero colour imaging system [11].
`
`The focusing optics defines one or more focal planes beyond the
`probing face, in a position which can be tuned by a motor (72).
`The beams generate illuminated spots on the structure and the
`intensity of returning light rays is measured for various positions
`of the focal plane. The topology of the three dimensional structure
`of the teeth is reconstructed on the basis of spot-specific positions
`yielding a maximum intensity of the reflected light beams (Fig. 7)
`[8,10].
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`21) are complete, the patient is asked to close into centric
`occlusion and a virtual registration is scanned. Overall, complete
`upper and lower quadrant scans and the virtual bite registration
`can take less than 3 min time [2]. When the digital impression has
`been completed, the clinician can have a series of diagnostic tools
`to evaluate the preparation and to complete the impression itself.
`For example, a margin line tool
`is available to assist in the
`identification of clearly defined margin [12]. The completed digital
`impression is sent via a HIPAA-compliant wireless system to the
`Cadent facility and to the dental laboratory. Upon ratification of
`the laboratory, the digital file is output to a model by Cadent.
`Finally, the model is milled from a proprietary blended resin [2].
`
`3.2. 3D Progress by MHT S.p.A. (IT) & MHT optic research AG (CH)
`
`3D Progress, produced by MHT (Medical High Technologies) S.p.A
`(IT) and created by MHT Optic Research AG (CH), is a light-weight,
`portable, digital impression system which can be interfaced to a PC via
`USB 2.0 cable (Fig. 9). MHT Optic Research AG and MHT S.p.A were
`founded in 1995 by Markus Berner, a Swiss engineer, and Carlo
`Gobbetti, an Italian businessman and entrepreneur.
`Besides being available for purchasing, in North America it will
`be also made available for a low monthly rental fee and commer-
`cialized by Clōn 3D Employee, as ‘Progress IODIS (an acronym
`of
`Intra Oral Digital
`Impression System)’. Another authorized
`distributor is Oratio BV company, from Netherlands, which will
`commercialize this device as ‘CYRTINAs Intraoral Scanner’. 3D
`Progress performs the digital impression, taking less than 1/10th
`of a second for a single scan, with an average scanning speed equal
`to 14 scan/second (depending on the PC); therefore, it can scan a
`
`Fig. 9. 3D Progress portable system [13].
`
`full-arch in under 3 min. The scanner will not usually require
`powdering of the translucent surfaces, with the exception of
`highly reflective surfaces as, for example, implant scan abutments
`and markers. Scans are output first as a cloud of points and then,
`as a final output, as a usual STL format surface file, compatible
`with most CAD platforms. The main technical features of the 3D
`Progress components are as follows: a smart Pixel Sensor (which
`enables fast and accurate scanning), an automatic real time
`stitching of each single scan, the possibility to pause/stop the scan
`whenever required, automatic (or semi-automatic) margin line
`detection, a USB 2.0 PC connection. 3D Progress works as a
`confocal microscope combined with Moireé effect detector
`[14,1]. The focal plane is shifted translating a movable lens, located
`as far distal as possible to maximize the miniaturization of the
`optical system. Unlike the prior optical system, based on confocal
`microscope shown in Fig. 10, in 3D Progress (Fig. 11) it is the first
`lens (4), distal from the object, to be moved in three different
`positions (each identified as 4a, 4b and 4c) in order to shift the
`focal plane (7) on the object (6), to positions identified as (7a), (7b)
`and (7c), respectively. The light rays, generated by the illumination
`pattern (1) and reflected at each focal plane (7a), (7b) and (7c),
`pass through the lens assembly (4), (5) and the beam guidance
`means (8) and are deflected by the beam splitter (2), in the
`direction of the detector (3), where the image of the object 6 is
`detected in the focal plane 7.The movable lens is aspherical in
`order to guarantee the necessary imaging quality for all focal
`planes (7a), (7b) and (7c), thus the focal plane is not actually a
`plane, but a curved surface and the scanned surfaces appear
`distorted: flat surfaces and straight lines appear curved, and the
`magnification and curvatures are different for different positions
`in the image. These distortions can be compensated because the
`theoretical distortions are known, having been computed on a
`reference image. The curvatures can be well approximated by an
`analytical function, such as for example, a polynomial [14].
`
`3.3. TRIOS™ by 3Shape A/S (DK)
`
`In December 2010, 3Shape announced the launch of a new
`patient-friendly and high-performance intraoral scanning solu-
`tion, named TRIOS™ (Fig. 12).
`The TRIOS™ system works according to the principle of con-
`focal microscopy, with a fast scanning time. The light source
`provides an illumination pattern producing a light oscillation on
`the object. This variation/oscillation may be spatial and/or it may
`be time varying. The system produces a variation of the focus
`plane of the pattern as well, over a range of focus plane positions,
`while maintaining a fixed spatial relation of the scanner to the
`
`Fig. 10. Prior optical system based on confocal microscope [14].
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`209
`
`Fig. 11. Optics for a confocal microscope like 3D Progress [14].
`
`Fig. 12. Trios’ wand and cart [15].
`
`When the focus plane coincides with the scanned surface at
`certain pixel position, the pattern is projected onto the surface
`point in-focus and it has high contrast, thereby giving rise to a
`large amplitude variation of the pixel values over time. It is thus
`possible to identify specific settings of the focusing plane, which
`make each pixel to be in focus. In other words, it is possible to
`transform the contrast information vs. position of the focus plane
`into a 3D surface information, pixel by pixel. The third dimension
`of the scanned object is determined by finding the plane corre-
`sponding to a peak in the correlation measured for each sensor
`belonging to the camera's sensor array.
`A peculiarity of this system is the variation of the focal plane
`position without moving the scanner in relation to the object
`being scanned. The focal plane should be continuously varied in a
`periodic fashion with a predefined frequency, while the pattern
`generation means, the camera, the optical system and the object
`being scanned fixed in relation to each other. Further, the 3D
`surface acquisition time should be small enough to reduce the bias
`produced by involuntary relative movements between the probe
`and the teeth [16]. The scanning system has the property of
`telecentricity in the space of the object being scanned and this
`property as well as magnification scale can be maintained while
`shifting the focal plane.
`
`Fig. 13. Trios scanning system [16].
`
`object (Fig. 13). When a time varying pattern is applied, a single
`sub-scan is actually the collection of a certain number of 2D
`images, corresponding to different positions of the focus plane and
`to their respective different time instances of the illumination
`pattern.
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`4. Triangulation techniques and devices
`
`Triangulation is a non-contact technique for digitally collecting
`data of the shape of a 3D object and constructing digital 3D
`models, for a wide variety of applications. Both passive and active
`triangulation techniques may be used.
`In active triangulation
`methods, a light radiation is projected onto the scene, and its
`reflection is acquired in order to calculate the position of the target
`object. In passive triangulation methods no kind of radiation is
`emitted by the scanning device itself and the system is based on
`detecting reflected ambient radiation.
`Passive triangulation is also called passive stereovision and uses
`photogrammetric algorithms. This technique is based on processing
`of two stereo images, obtained from two cameras, whose respective
`positions and angulations are known (Fig. 15). This information is
`needed in order to identify points with corresponding features on
`the two images and to apply triangulation, with respect to the same
`
`corresponding points on the epipolar line; the algorithms are based
`on the epipolar geometry (Fig. 14).
`Passive triangulation provides the highest accuracy among
`vision systems of this type. However, only high contrast targets
`and well defined edges can be measured with high accuracy. Using
`three cameras the ambiguity can be reduced.
`Untargeted, or featureless, surfaces may not be measured at all.
`In addition, the ambient light affects significantly the ability of the
`system to successfully extract all desired features, unless con-
`trolled lighting is used [17]. The main advantage is the low cost of
`these systems, made of few and cheap components; furthermore,
`also the working principle is simple, the same as in the human eye.
`In active triangulation, a light beam generated by a laser is
`deflected by a mirror and scanned on the target object. Fig. 16 shows
`a block diagram of a 2D active triangulation system. A camera,
`composed of a lens and a position sensitive photo-detector, mea-
`sures the location of the image of the illuminated point on the
`object. The laser dot appears at different places in the camera's field
`of view, depending on how far away the laser strikes the surface,
`(Fig. 17). This technique is called triangulation because the laser dot,
`the camera and the laser emitter form a triangle. The distance d
`between the camera and the laser emitter is known, it is called
`baseline distance and it corresponds with one side of the triangle.
`The angle θ of the laser emitter corner is also known. The angle Φ of
`the camera corner can be determined by looking at the location of
`the laser dot in the camera's field of view. These three data fully
`determine the shape and size of the triangle and gives the location
`(X, Y, Z coordinates) of the laser dot corner of the triangle, by simple
`trigonometric calculations [17]. In most cases, a laser stripe, a grid or
`
`Fig. 14. Passive triangulation method.
`
`Fig. 16. 2D active triangulation method [17].
`
`Fig. 15. Stereo images and cameras.
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`
`Fig. 17. Camera's field of view in active triangulation method.
`
`a series of patterns, instead of one single laser dot, are swept across
`the object to speed up the acquisition process.
`The measuring accuracy of this method depends on several
`parameters. The accuracy error in the estimation of the distance
`(Z) is inversely proportional to both the distance between the laser
`and the position detector (baseline d) and the distance between
`the effective position of the lens and the position detector (f0).
`Unfortunately, neither f0 nor d can be made large. Distance d is
`limited by the mechanical structure of the optical setup and by the
`arising of shadow effects [18]. The accuracy also improves increas-
`ing the number of pixels (photosensitive elements) of the camera
`and for shorter measuring distances [19]: the error of accuracy in
`the estimation of the distance Z is directly proportional to the
`square of the distance itself [18]. The total error of accuracy can
`be roughly estimated to be proportional to the working volume
`(a typical value is 1000:1).
`A major problem affecting all triangulation methods is occlu-
`sion; it takes place whenever an area of the target surface is
`invisible to both, or either of, the laser (laser occlusion) and the
`camera (camera occlusion). A theoretical solution is keeping the
`triangulation angle as small as possible, but the result would lose
`in terms of accuracy. So a good balance between minimal occlu-
`sion and good accuracy must be reached. The measure accuracy is
`also affected by the surface reflectivity of measured objects [19].
`
`4.1. CERECs by Sirona dental system GMBH (DE)
`
`CERECs (an acronym for Chairside Economical Restoration of
`Esthetic Ceramics) was introduced by Sirona Dental System GMBH
`(DE) in 1987 and it has undergone a series of technological improve-
`ments, culminating in the CEREC ACs, powered by BlueCams,
`launched in January 2009.
`The latest versions of the CERECs system (Fig. 18) are capable
`of producing inlays, onlays, crowns, laminate veneers, and even
`bridges and combine a 3D digital scanner with a milling unit, to
`create in-office dental restorations from commercially available blocks
`of ceramic or composite material, in one single appointment [2].
`The latest version of the milling centre, CEREC inLabs MC XL is
`capable of milling a crown in as short as 4 min. CERECs systems
`
`may be described as measurement devices which operate accord-
`ing to the basic principles of confocal microscopy [4,24], and
`according to the active triangulation technique [4,25] and [26]. A
`camera projects a changing pattern of blue light onto the object
`(Fig. 18), using projection grids, which have a transmittance
`random distribution, and which are formed by sub-regions, con-
`taining transparent and opaque structures [27].
`Moreover, it is possible, for each acquired profile, to define a
`specific relationships between the light characteristic and the optical
`distance of the image plane from the imaging optics [4,24], thanks to
`elements designed for varying the length of the optical path.
`A light source (3) (Fig. 19) produces an illumination beam (7.1,
`7.2, 7.3), that is focused onto the surface of the target object (2). An
`image sensor (6) receives the observation beam (9.1, 9.2, 9.3)
`reflected by the surface of the target object. A focusing system
`(5) focuses the observation beam onto the image sensor (6). The
`light source (3) is actually made of various units (3.1, 3.2, 3.3),
`which can be independently regulated in terms of light intensity
`[26]. Thus, the intensity of the light detected by each sensor
`element is a direct measure of the distance between the scan head
`and a corresponding point on the target object [4]. A critical aspect
`of the system is that the triangulation technique requires a
`uniform reflective surface, and different materials (as dentin,
`amalgam, resins, gums) reflect light differently. Therefore, it is
`necessary to coat the teeth with suitable powders, before the
`scanning stage, to make the reflectivity of the surfaces uniform.
`The earlier versions of CERECs employed an acquisition camera
`with an infrared laser light source. The latest version employs blue
`light-emitting diodes (LEDs) (Fig. 18); the shorter-wavelength,
`intense, blue light allows reaching a greater accuracy. The images
`are distortion-free, even at the periphery, so that multiple images
`(e.g. of a complete quadrant) can be stitched together with great
`accuracy. The CERECs AC Bluecam boasts an automatic shake
`detection system, which let the images to be acquired only if the
`camera is absolutely still. It is possible to capture a complete half
`arch in less than a minute. The new CERECs AC Bluecam offers
`image stabilization systems. It implies that the practitioner does
`not have to rest the camera wand on a tooth to get a steady fo