Patent Publication Number: US-2020296352-A1

Title: Three dimensional imaging apparatus with color sensor

Description:
RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 15/953,268, filed Apr. 13, 2018, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 62/520,417, filed Jun. 15, 2017, both of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention relate to the field of imaging and, in particular, to a system and method for performing imaging of a three dimensional surface and for accurately determining colors of locations on the three dimensional surface. 
     BACKGROUND 
     Intraoral scanners have been developed for direct optical measurement of teeth and the subsequent automatic manufacture of dental appliances such as aligners, bridges, crowns, and so on. The term “direct optical measurement” signifies surveying of teeth in the oral cavity of a patient. Intraoral scanners typically include an optical probe coupled to an optical scanning system, which may include optics as well as an optical pick-up or receiver such as charge coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) sensor. The optical scanning systems of intraoral scanners are generally able to generate three dimensional images with accurate shape. However, the optical scanning systems of Intraoral scanners generally have inaccurate color sensing capabilities. 
     Due to the inability of intraoral scanners to accurately generate color data for a patient&#39;s teeth, tooth coloring for dental prosthetics such as crowns and bridges are primarily performed manually by eye. However, the current practice of manually coloring dental prosthetics by eye is time consuming and inefficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
         FIG. 1A  illustrates a functional block diagram of an imaging apparatus according to one embodiment. 
         FIG. 1B  illustrates a block diagram of a computing device that connects to an imaging apparatus, in accordance with one embodiment. 
         FIG. 2A  illustrates optics of an optical scanning system of an imaging apparatus, in accordance with one embodiment. 
         FIG. 2B  illustrates optics of a color sensor for an imaging apparatus, in accordance with one embodiment. 
         FIG. 2C  illustrates optics of a color sensor for an imaging apparatus, in accordance with another embodiment. 
         FIG. 3  is a flow chart showing one embodiment of a method for generating three dimensional image data and color data by an imaging apparatus. 
         FIG. 4  is a flow chart showing another embodiment of a method for generating three dimensional image data and color data by an imaging apparatus. 
         FIG. 5  is a flow chart showing one embodiment of a method for calibrating a color sensor to a detector of an imaging apparatus. 
         FIGS. 6A-E  are figures showing calibration of a color sensor to a detector of an imaging apparatus. 
         FIGS. 7A-C  are color spectrum graphs of various regions of a calibration target. 
         FIG. 7D  is a graph of alpha value verses position of a calibration target. 
         FIG. 7E  is a graph of a receptive field position of a color sensor as a function of depth. 
         FIG. 7F  is a graph of receptive field size of a color sensor as a function of depth. 
         FIG. 8  is a diagram of tooth color zones. 
         FIG. 9  illustrates a block diagram of an example computing device, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is an imaging apparatus that includes both an optical scanning system capable of generating three dimensional images of three dimensional objects and a color sensor capable of taking highly accurate color measurements of locations (e.g., spots) on the three dimensional objects. For many dental procedures it is important to know both the three dimensional shape of one or more teeth as well as to know the colors of the one or more teeth. For example, a tooth crown should be the same shape as an original tooth or neighboring teeth and should have the same shading as that original tooth. If the shape is incorrect, then the crown may not fit in a patient&#39;s mouth. If the color of any portion of the crown is incorrect, then the crown shading will not match the shading of the patient&#39;s other teeth and the crown will be highly noticeable to viewers. Accordingly, dental practitioners and dental technicians often need accurate color data of multiple different portions of a patient&#39;s tooth in addition to accurate three dimensional shape data of the tooth. 
     In one embodiment, an imaging apparatus includes a first light source (e.g., a laser and illumination module) to generate an array of light beams. The imaging apparatus additionally includes a probe and focusing optics along an optical path of the array of light beams to direct the array of light beams through the probe. The probe directs the array of light beams toward a three dimensional object to be imaged. The array of light beams reflect off of the three dimensional object, and an array of returning light beams are reflected back into the probe and through the focusing optics. A detector detects the array of returning light beams and generates measurements of the array of returning light beams. The measurements of the array of returning light beams indicate a shape of the three dimensional object. 
     The imaging apparatus additionally includes a second light source to generate multi-chromatic light that is to illuminate the three dimensional object. Rays of the multi-chromatic light is reflected off of the three dimensional object and into the probe. One or more of the rays are collected by an optical transmission medium that is outside of the optical path of the focusing optics in one embodiment. The one or more rays are rays that have been reflected off of a spot on the three dimensional object, where the spot is within the receptive view of a color sensor. The optical transmission medium guides the one or more rays to color sensor, which may be a spectrometer, colorimeter or hyper spectral sensor that is optically coupled to an end of the optical transmission medium. The spectrometer, colorimeter or hyper spectral sensor receives the one or more rays of the multi-chromatic light and determines a color of the spot on the three dimensional object based on an analysis of the one or more rays. The color information of the spot and the three dimensional shape information of the three dimensional object may be transmitted to a computing device connected to the imaging apparatus. 
     In an alternative embodiment, the optical transmission medium may be omitted. In such an embodiment, the color sensor may be placed near to, and outside of, the optical path of the focusing optics. The color sensor may be oriented at an oblique angle to the optical path. One or more rays of the multi-chromatic light that reflected off of the three dimensional object and into the probe will also have an oblique angle to the optical path of the focusing optics. These one or more rays will be collected by the color sensor. The one or more rays are rays that have been reflected off of a spot on the three dimensional object, where the spot is within the receptive view of the color sensor. It should be understood that any of the embodiments discussed herein with reference to inclusion of an optical transmission medium may also be implemented without the optical transmission medium. 
     The three dimensional shape information generated by the detector is very accurate, and is usable to construct a three dimensional model of the object that is imaged (e.g., of a tooth or entire jaw of a patient). The color data is for small spots on the imaged object, but is highly accurate. The color data can be combined with the three dimensional shape data by the computing device to generate a three dimensional model that includes both accurate shape information and accurate color information. The three dimensional model can facilitate manufacture of dental prosthetics that accurately reflect the shape and color of a tooth or teeth to be replaced by providing improved color description. 
     Embodiments are discussed herein with reference to a confocal imaging apparatus that includes an accurate color sensor. For example, embodiments discuss a confocal imaging apparatus with confocal focusing optics that direct an array of light beams through a probe onto a three dimensional object. The confocal imaging apparatus additionally includes an optical transmission medium that collects multi-chromatic light rays outside of an optical path of the confocal focusing optics and a color sensor outside of the optical path of the confocal focusing optics that receives the multi-chromatic light rays from the optical transmission medium. However, it should be understood that embodiments also apply to other types of three dimensional imaging apparatuses that include an accurate color sensor. Examples of other types of 3D imaging apparatuses include apparatuses that measure time of flight of laser light, apparatuses that use triangulation from multiple lasers and detectors at different positions and orientations relative to an imaged object, apparatuses that project structured light onto an object and measure how the structured light pattern changes to determine a 3D shape, apparatuses that use modulated light, apparatuses that use multiple detectors for stereoscopic imaging, apparatuses that include photometric systems that use a single detector to generate multiple images under varying light conditions to determine a 3D shape, and so on. The embodiments discussed herein with reference to a confocal imaging apparatus may apply equally well to any of the other types of 3D imagers, such as those provided above. In such alternative embodiments, an optical transmission medium and color sensor (e.g., colorimeter, spectrometer or hyper spectral sensor) may be positioned outside of the optical path of the detectors and/or 3D scanning optics that are used to generate 3D images of an object. The optical transmission medium collects multi-chromatic light that is outside of the optical path of the 3D scanning optics, and directs the multi-chromatic light to the color sensor, as is described in embodiments herein below. 
       FIG. 1A  illustrates a functional block diagram of an imaging apparatus  20  according to one embodiment. In one embodiment, the imaging apparatus  20  is a confocal imaging apparatus.  FIG. 1B  illustrates a block diagram of a computing device  24  that connects to the imaging apparatus  20 . Together, the imaging apparatus  20  and computing device  24  may form a system for generating three dimensional images of scanned objects and for generating color data of spots on the scanned objects. The computing device  24  may be connected to the imaging apparatus  20  directly or indirectly and via a wired or wireless connection. For example, the imaging apparatus  20  may include a network interface controller (NIC) capable of communicating via Wi-Fi, via third generation (3G) or fourth generation (4G) telecommunications protocols (e.g., global system for mobile communications (GSM), long term evolution (LTE), Wi-Max, code division multiple access (CDMA), etc.), via Bluetooth, via Zigbee, or via other wireless protocols. Alternatively, or additionally, imaging apparatus  20  may include an Ethernet network interface controller (NIC), a universal serial bus (USB) port, or other wired port. The NIC or port may connect the imaging apparatus to the computing device via a local area network (LAN). Alternatively, the imaging apparatus  20  may connect to a wide area network (WAN) such as the Internet, and may connect to the computing device  24  via the WAN. In an alternative embodiment, imaging apparatus  20  is connected directly to the computing device (e.g., via a direct wired or wireless connection). In one embodiment, the computing device  24  is a component of the imaging apparatus  20 . 
     Referring now to  FIG. 1A , in one embodiment imaging apparatus  20  includes a first light source  28 , which may be a semiconductor laser unit that emits a focused light beam  30 , as represented by an arrow. The focused light beam  30  may be a focused light beam of coherent light (e.g., laser light having a wavelength of 680 nm in an embodiment). The light beam  30  passes through a polarizer  32 , which polarizes the light beam  30 . Alternatively, polarizer  32  may be omitted in some embodiments. The light beam  30  may then enter into an optic expander  34  that improves a numerical aperture of the light beam  30 . The light beam  30  may then pass through an illumination module  38  such as a beam splitter, which splits the light beam  30  into an array of light beams  36 , represented here, for ease of illustration, by a single line. The illumination module  38  may be, for example, a grating or a micro lens array that splits, the light beam  30  into an array of light beams  36 . In one embodiment, the array of light beams  36  is an array of telecentric light beams. Alternatively, the array of light beams may not be telecentric. 
     The imaging apparatus  20  further includes a unidirectional mirror or beam splitter (e.g., a polarizing beam splitter)  40  that passes the array of light beams  36 . A unidirectional mirror  40  allows transfer of light from the semiconductor laser  28  through to downstream optics, but reflects light travelling in the opposite direction. A polarizing beam splitter allows transfer of light beams having a particular polarization and reflects light beams having a different (e.g., opposite) polarization. In one embodiment, the unidirectional mirror or beam splitter  40  has a small central aperture. The small central aperture may improve a measurement accuracy of the imaging apparatus  20 . In one embodiment, as a result of a structure of the unidirectional mirror or beam splitter  40 , the array of light beams will yield a light annulus on an illuminated area of an imaged object as long as the area is not in focus. Moreover, the annulus will become a completely illuminated spot once in focus. This ensures that a difference between measured intensities of out-of-focus points and in focus points will be larger. 
     In one embodiment, along an optical path of the array of light beams after the unidirectional mirror or beam splitter  40  are confocal focusing optics  42 , and a probe  46  (e.g., such as an endoscope or folding prism). Additionally, a quarter wave plate may be disposed along the optical path after the unidirectional mirror or beam splitter  40  to introduce a certain polarization to the array of light beams. In some embodiments this may ensure that reflected light beams will not be passed through the unidirectional mirror or beam splitter  40 . Confocal focusing optics  42  may additionally include relay optics (not shown). Confocal focusing optics  42  may or may not maintain the same magnification of an image over a wide range of distances in the Z direction, wherein the Z direction is a direction of beam propagation (e.g., the Z direction corresponds to an imaging axis that is aligned with an optical path of the array of light beams  36 ). The relay optics enable the imaging apparatus  20  to maintain a certain numerical aperture for propagation of the array of light beams  36 . 
     The probe  46  may include a rigid, light-transmitting medium, which may be a hollow object defining within it a light transmission path or an object made of a light transmitting material, e.g. a glass body or tube. In one embodiment, the probe  46  includes a prism such as a folding prism. At its end, the probe  46  may include a mirror of the kind ensuring a total internal reflection. Thus, the mirror may direct the array of light beams towards a tooth  26  or other object. The probe  46  thus emits array of light beams  48 , which impinge on to surfaces of the tooth  26 . 
     The array of light beams  48  are arranged in an X-Y plane, in the Cartesian frame  50 , propagating along the 7 axis. As the surface on which the incident light beams hits is an uneven surface, illuminated spots  52  are displaced from one another along the Z axis, at different (X i , Y i ) locations. Thus, while a spot at one location may be in focus of the confocal focusing optics  42 , spots at other locations may be out-of-focus. Therefore, the light intensity of returned light beams of the focused spots will be at its peak, while the light intensity at other spots will be off peak. Thus, for each illuminated spot, multiple measurements of light Intensity are made at different positions along the Z-axis. For each of such (X i , Y i ) location, the derivative of the intensity over distance (Z) may be made, with the Z i  yielding maximum derivative, Z 0 , being the in-focus distance. As pointed out above, the incident light from the array of light beams  48  forms a light disk on the surface when out of focus and a complete light spot when in focus. Thus, the distance derivative will be larger when approaching in-focus position, increasing accuracy of the measurement. 
     The light scattered from each of the light spots includes a beam travelling initially in the Z axis along the opposite direction of the optical path traveled by the array of light beams  48 . Each returned light beam in an array of returning light beams  54  corresponds to one of the incident light beams in array of light beams  36 . Given the asymmetrical properties of unidirectional mirror or beam splitter  40 , the returned light beams are reflected in the direction of detection optics  60 . 
     The detection optics  60  may include a polarizer  62  that has a plane of preferred polarization oriented normal to the plane polarization of polarizer  32 . Alternatively, polarizer  32  and polarizer  62  may be omitted in some embodiments. The array of returning light beams  54  may pass through imaging optics  64  in one embodiment. The imaging optics  64  may be one or more lenses. Alternatively, the detection optics  60  may not include imaging optics  64 . In one embodiment, the array of returning light beams  54  further passes through a matrix  66 , which may be an array of pinholes. Alternatively, no matrix  66  is used in some embodiments. The array of returning light beams  54  are then directed onto a detector  68 . 
     The detector  68  is an image sensor having a matrix of sensing elements each representing a pixel of the image. If matrix  66  is used, then each pixel further corresponds to one pinhole of matrix  66 . In one embodiment, the detector is a charge coupled device (CCD) sensor. In one embodiment, the detector is a complementary metal-oxide semiconductor (CMOS) type image sensor. Other types of image sensors may also be used for detector  68 . The detector  68  detects light intensity at each pixel. 
     In one embodiment, detector  68  provides data to computing device  24 . Thus, each light intensity measured in each of the sensing elements of the detector  68 , is then captured and analyzed, in a manner to be described below, by processor  24 . 
     Confocal imaging apparatus  20  further includes a control module  70  connected both to first light source  28  and a motor  72 , voice coil or other translation mechanism. In one embodiment, control module  70  is or includes a field programmable gate array (FPGA) configured to perform control operations. Motor  72  is linked to confocal focusing optics  42  for changing a focusing setting of confocal focusing optics  42 . This may adjust the relative location of an imaginary flat or non-flat focal surface of confocal focusing optics  42  along the Z-axis (e.g., in the imaging axis). Control module  70  may induce motor  72  to axially displace (change a location of) one or more lenses of the confocal focusing optics  42  to change the focal depth of the imaginary flat or non-flat focal surface. In one embodiment, motor  72  or imaging apparatus  20  includes an encoder (not shown) that accurately measures a position of one or more lenses of the confocal focusing optics  42 . The encoder may include a sensor paired to a scale that encodes a linear position. The encoder may output a linear position of the one or more lenses of the confocal focusing optics  42 . The encoder may be an optical encoder, a magnetic encoder, an inductive encoder, a capacitive encoder, an eddy current encoder, and so on. After receipt of feedback that the location of the one or more lenses has changed, control module  70  may induce first light source  28  to generate a light pulse. Control unit  70  may additionally synchronize three dimensional (3D) image-capturing module  80  from  FIG. 1B  to receive and/or store data representative of the light intensity from each of the sensing elements at the particular location of the one or more lenses (and thus of the focal depth of the imaginary flat or non-flat focal surface). In subsequent sequences, the location of the one or more lenses (and thus the focal depth) will change in the same manner and the data capturing will continue over a wide focal range of confocal focusing optics  42 . Since the first light source  28  is a coherent light source, the 3D image data generated by detector  68  based on the light beam  30  is a monochrome image. 
     Confocal imaging apparatus  20  additionally includes one or more second light source  80 . In some embodiments the second light source  80  is actually multiple light sources arranged at various positions on the probe  46 . The multiple different light sources may provide the same type of light, but may each provide the light from different directions and/or positions. The second light source  80  may be a multi-chromatic light source such as a white light source. The second light source  80  may be, for example, one or more incandescent light, one or more light emitting diodes (LEDs), one or more halogen lights, or other types of light sources. In one embodiment, the second light source  80  emits visible light at wavelengths of about 400-650 nm. The second light source  80  may be positioned internally or externally to the endoscope to shine light directly on the tooth  26 . In such an embodiment, the second light source  80  may not shine light through the probe  46  onto the teeth. In an alternative embodiment, the second light source  80  may be internal to the imaging apparatus  20 , and may shine light along the optical path of the confocal focusing optics  42  and/or into the probe  46 . The probe  46  may then emit the multi-chromatic light to illuminate the tooth  26 . 
     The multi-chromatic light is reflected off of the tooth  26 , and a plurality of reflected rays of the multi-chromatic light enter the probe  46  and travel through the confocal focusing optics  42  and into the detection optics  60 . The multi-chromatic light received by the detection optics  60  may not be focused light (e.g., may not be laser light), and may not be used to detect a 3D shape of the tooth  26 . Instead, the multi-chromatic light may enter the sensing elements of the detector  68  to generate a 2D image of the tooth. 
     Detecting optics  60  may include a set of color filters, which may include a red color filter, a blue color filter and a green color filter. In one embodiment, detecting optics  60  include a Bayer-pattern color filter. The Bayer-pattern color filter may include a set of 4-pixel RGGB (red, green, green, blue) groups, where each RGGB group determines a color for four adjacent pixels. The color filters filter out the multi-chromatic light rays impinging on particular sensors of the detector  68 , and are usable to generate a color 2D image of the tooth. These color filters may be low accuracy pigment based absorption color filters. Each of the color filters may have a relatively wide bandwidth. The spectral overlap between the color filters and the use of only three basic colors results in an inaccurate color separation ability. As a result, the green color filter may pass some blues and greens, the blue color filter may pass some greens and reds, and so on. Accordingly, the color 2D image of the tooth has a low color accuracy. 
     In addition to generating image data (e.g., a collection of 2D images with varying focus settings) usable to generate a highly accurate 3D monochrome image of the tooth  26 , detection optics  60  and/or detector  68  may be usable to generate a 2D color image of the tooth  26 . The 2D color image data may be sent to a 2D image capturing module  83  of computing device  24  of  FIG. 1B . Confocal imaging apparatus  20  may alternate between use of the first light source  28  to generate first image data for a 3D image of the tooth  26  and second light source  80  to generate second image data for a color 2D image of the tooth  26 . Confocal imaging apparatus  20  may rapidly alternate between use of the first and second light sources  28 ,  80 . A scan rate of the detector  68  for the 3D image data and for the color 2D image data may be about 20 scans per second. The color 2D image data may be used to present a view finder image to a user during use of the imaging apparatus  20 . This may facilitate accurate placement of the imaging apparatus  20  in a patient&#39;s mouth and scanning of desired dental regions. 
     The color accuracy of the detector  68  is insufficient for some applications, such as estimating the shade (e.g., coloring) of prosthetic teeth. Accordingly, imaging apparatus  20  includes a very accurate color sensor  84  that determines a color of spots in a small receptive field (also referred to as a field of view (FOV)) of the color sensor  84 . The receptive field is the region in space that the color sensor is sensitive to. The level of sensitivity is not uniform in the receptive field, and may have an approximately circular shape with smooth edges. As shown, one or more rays of multi-chromatic light  77  may reflect off of a small spot  79  on tooth  26  at an angle that is oblique to the imaging axis (e.g., that is oblique to the z axis in Cartesian frame  50 ). One or more rays  13  of multi-chromatic light may reflect off of the tooth  26  and enter probe  46  at an oblique angle to the imaging axis, and then exit the probe  46  at an oblique angle to the imaging axis. These one or more reflected rays  13  of multi-chromatic light then enter an optical transmission medium  82  that is oriented to receive the one or more rays  13  at a specific oblique angle to the imaging axis of the optical path for the confocal focusing optics  42 . The oblique angle may be an angle of 2-60 degrees in one embodiment. In one embodiment, the oblique angle is an angle of 5-45 degrees. In one embodiment, the oblique angle is an angle of 5-30 degrees. Some exemplary angle ranges for the oblique angle include 5-10 degrees, 10-15 degrees, 15-20 degrees, 20-25 degrees and 25-30 degrees. Smaller angles may result in improved accuracy for determination of the location of the receptive field for the color sensor. Accordingly, in one embodiment, the oblique angle is 5-15 degrees. The optical transmission medium  82  is positioned outside of the optical path of the confocal focusing optics  42  so as not to occlude any returning light beams during 3D imaging. The optical transmission medium  82  may be a light pipe, optical fiber, light guide, and so on. In some embodiments, the optical transmission medium  82  is a flexible or semi-flexible optical fiber. Alternatively, the optical transmission medium  82  may be rigid. The optical transmission medium  82  may be, for example, an optical fiber that includes a transparent core surrounded by a transparent cladding material with a low index of refraction. The optical fiber may be made from silica, fluoride glass, phosphate glass, fluorozirconate, fluoroaluminate, chalcogenide glass, sapphire, plastic, or other materials. Plastic or polymer optical fibers may have a fiber core formed from Poly(methyl methacrylate) (PMMA) or Polystyrene, and may have a fiber cladding of, for example, silicone resin. Silica (glass) based optical fibers have less internal scattering and absorption than plastic based optical fibers. However, glass based optical fibers have a limited bending radius, while plastic based optical fibers have a larger bending radius. 
     In an alternative embodiment, the multi-chromatic light rays may reflect off of the spot  79  at an angle that is parallel to the imaging axis. In such an embodiment, a beam splitter (not shown) may be positioned between the probe  46  and the confocal focusing optics  42  along the optical path of the confocal imaging optics  42 . The beam splitter may reflect some portion of the multi-chromatic light rays into the optical transmission medium  82 . 
     The optical transmission medium  82  may direct the one or more rays  13  into the color sensor  84 . The color sensor  84  may be a colorimeter, spectrometer or hyper spectral sensor (also referred to as a multi-spectral sensor). A spectrometer is a special case of a hyper spectral sensor. The hyper spectral sensor (or spectrometer) is able to determine the spectral content of light with a high degree of accuracy. The color sensor  82  is able to determine with a high degree of accuracy a color spectrum of the small spot  79  by determining, for example, intensities of the light as reflected off of the spot  79  at many different light wavelengths. The relative intensities of the different wavelengths provide a highly accurate color measurement of the spot  79 . The wavelength separation achievable by the hyper spectral sensor can be as good as a few nanometers or tens of nanometers. Additionally, in hyper spectral sensors the use of interference filters allows for an overlap between various colors. Color sensor  84  may alternatively be a colorimeter. A colorimeter is a device that mimics the human color response, and can be used for exact color definition. Color sensor  84  may additionally send the color measurement to a color capturing module  85  of the computing device  24  of  FIG. 1B . 
     In an alternative embodiment, the optical transmission  82  medium may be omitted. In such an embodiment, the color sensor  84  may be placed near to, and outside of, the optical path of the confocal imaging optics  42 . The color sensor  84  may be oriented at an oblique angle to the optical path. One or more rays of the multi-chromatic light that are reflected off of the tooth  26  and into the probe  46  will also have an oblique angle to the optical path of the confocal imaging optics  42 . The oblique angle may be an angle of 2-60 degrees in one embodiment. In one embodiment, the oblique angle is an angle of 5-45 degrees. In one embodiment, the oblique angle is an angle of 5-30 degrees. Some exemplary angle ranges for the oblique angle include 5-10 degrees, 10-15 degrees, 15-20 degrees, 20-25 degrees and 25-30 degrees. Smaller angles may result in improved accuracy for determination of the location of the receptive field for the color sensor. Accordingly, in one embodiment, the oblique angle is 5-15 degrees. These one or more rays will be collected by the color sensor  84 . The one or more rays are rays that have been reflected off of a spot on the tooth  26 , where the spot is within the receptive view of a color sensor  84 . 
     In some embodiments the second light source  80  emits light at approximately 405 nm. Alternatively, a third light source may be included that emits the light at approximately 405 nm. Light at this wavelength causes the tooth  26  to fluoresce when the light impacts the tooth  26 . The color sensor  84  may measure the magnitude of fluorescence of the tooth  26  at the spot  79 . The level of tooth fluorescence may indicate a health of the tooth  26 . In one embodiment, a filter which rejects the light source  80  from reaching the color sensor  84  may be included. The filter can prevent over-saturation of the hyper spectral sensor. 
     In one embodiment, the first light source  28  and second light source  80  are used one at a time. For example, first light source  28  is activated and detector  68  generates one or more images, then first light source  28  is deactivated and second light source  80  is activated and detector  68  generates one or more additional images. While the second light source  80  is active, color sensor  84  additionally generates a color measurement. The second light source  80  is then deactivated and the first light source  28  is reactivated and the detector  68  generates one or more additional images, and so on. 
     In one embodiment, detector  68  is not used to generate 2D images. In such an embodiment, color sensor  84  may generate color measurements at the same time as detector  68  generates 3D image data. For example, a filter (not shown) may be disposed between probe  46  and confocal focusing optics  42 . The filter may filter out the multi-chromatic light from the second light source  80  and may pass the light from the first light source  28 . Accordingly, the first light source  28  and second light source  80  may be activated in parallel, and color measurements and 3D measurements may be taken in parallel. 
     Referring now to  FIG. 1B , 3D image capturing module  81  may capture images for 3D imaging responsive to receiving first image capture commands from the control unit  70 . The captured images may be associated with a particular focusing setting (e.g., a particular location of one or more lenses in the confocal focusing optics as output by the encoder). 3D Image processing module  82  then processes captured images captured over multiple different focusing settings. 3D image processing module  15  includes a depth determiner  90  and may include a field compensator  92  for processing image data. 
     Depth determiner  90  determines the relative intensity in each pixel over the entire range of focal settings of confocal focusing optics  42  from received image data. Once a certain light spot associated with a particular pixel is in focus, the measured intensity will be maximal for that pixel. Thus, by determining the Z i  corresponding to the maximal light intensity or by determining the maximum displacement derivative of the light intensity, for each pixel, the relative position of each light spot along the Z axis can be determined for each pixel. Thus, data representative of the three-dimensional pattern of a surface in the teeth segment  26  or other three dimensional object can be obtained. 
     In some embodiments where the imaging apparatus has a curved field, field compensator  92  may compensate for the curved field caused by the lack of a field lens 2D image capturing module  83  may receive color 2D image data from the imaging apparatus  20 . The color 2D image data may then be used to output a real time image of a FOV of the imaging apparatus  20 . The real time image may be output to a view finder  98  via a user interface  97  of the computing device  24 . A user may view the view finder in a display to determine how the imaging apparatus is positioned in a patient&#39;s mouth. 
     As mentioned, the imaging apparatus  20  may alternate between use of detector  68  to generate 3D monochrome image data and use of detector  68  to generate color 2D image data. Accordingly, the computing device  24  may alternately receive image data usable by 3D image capturing module  82  and 3D image processing module  15  to generate a 3D image of an object and receive image data usable by 2D image capturing module  83  to generate a color 2D image of the object. 
     As 3D images are generated by 3D image processing module  15 , 3D image processing module  15  may stitch those 3D images together to form a virtual 3D model of an imaged object (e.g., of a patient&#39;s tooth and/or dental arch). The user interface  97  may be a graphical user interface that includes controls for manipulating the virtual 3D model (e.g., viewing from different angles, zooming-in or out, etc.). In addition, data representative of the surface topology of the scanned object may be transmitted to remote devices by a communication module  88  for further processing or use. 
     By capturing, in this manner, an image from two or more angular locations around the structure, e.g. in the case of a teeth segment from the buccal direction, from the lingual direction and optionally from an occlusal portion of the teeth, an accurate three-dimensional representation of a tooth may be reconstructed. This may allow a virtual reconstruction of the three-dimensional structure in a computerized environment or a physical reconstruction in a CAD/CAM apparatus. For example, a particular application is imaging of a segment of teeth having at least one missing tooth or a portion of a tooth. In such an instance, the image can then be used for the design and subsequent manufacture of a crown or any other prosthesis to be fitted onto a dental arch of a patient. 
     Color capturing module  85  receives color measurements of spots on an imaged object. The color measurements may be generated in parallel to 2D color images in embodiments. The computing device may alternately receive color data and 3D imaging data in some embodiments. In other embodiments, the color measurements may be generated in parallel to 3D monochrome images. 
     Color image processing module  87  processes color measurement data to determine a location on an object that has a particular color and to determine what that particular color is. In one embodiment, color image processing module  87  includes a spot location determiner  94  and a normalizer  96 . 
     When color capturing module  85  receives color measurement data, spot location determiner  94  is responsible for determining a location on an imaged object to associate with the color measurement data. In one embodiment, the color sensor  84  has a small receptive field at a known position within a larger FOV of the confocal focusing optics  42  and detector  68 . The receptive field is small relative to the object that is being scanned and/or to the size of a constant color region of the object being scanned. Color capturing module  85  may receive a color measurement commensurate with 2D image capturing module  83  receiving a color 2D image. The position of the receptive field of the color sensor in the larger FOV may be used to determine a spot on the color 2D image that has the color of the received color measurement in an embodiment. Alternatively, color capturing module  85  may receive a color measurement commensurate with 3D image capturing module  81  receiving a 3D image. The position of the receptive field of the color sensor in the larger FOV may be used to determine a spot on the 3D image that has the color of the received color measurement in an embodiment. The color measurement data may be synchronized with the 3D image data or the 2D image data so that they are generated at a constant interval from one another. Alternatively the color measurement data may be unsynchronized with the 3D or 2D data. In such a case, any of the data types may include a timestamp which indicates when they were taken. The timestamp may be used to decide the timing relation between them. 
     In another embodiment, a 3D location of a spot to associate with the color measurement data may be determined using multiple 3D images. A first 3D image may be generated shortly before a color measurement is made, and a second 3D image may be generated shortly after the color measurement is made. A location of the receptive field of the color sensor within the larger FOV of the detector may be determined for the first 3D image and the second 3D image, though no color measurements were taken at the times that these two images were generated. A location of a spot on an object for which the color measurement was taken may then be determined by interpolating between the location of the receptive field of the color sensor in the first 3D image and the location of the receptive field of the color sensor in the second 3D image. For example, an average of the locations of the receptive field of the color sensor in the first and second 3D images may be computed. The 3D images and the color measurements may each be generated at a scan rate of up to 20 measurements/images per second. Accordingly, the imaging apparatus will have moved at most only a small distance between taking of the first and second 3D images. As a result, the interpolated position of the spot measured by the color sensor is highly accurate in embodiments. 
     An intensity of the color measurement received by color capturing module  85  may vary depending on variables such as angle of incidence and distance of the probe  46  from a measured object. In particular, characteristics of light impacting an object include a) the light source spectrum and angles, b) angle of incidence from the light source to the object and c) strength or intensity of light impacting the object, which depends on a distance between the object and the light source. Additionally, the color measurement may also be based on object color and reflectance properties, such as what is absorbed (e.g., what wavelengths of light are absorbed and at what levels) at particular angles, what is reflected (e.g., what wavelengths of light are reflected and at what levels) at particular angles and what is diffused (e.g., what wavelengths of light are absorbed and at what levels) at particular angles. These values may vary depending on angle and wavelength. Additionally, characteristics of the image capturing may also affect the color measurement, such as angles of the rays that would be collected and the distance of the object from the collecting device. 
     Accordingly, normalizer  96  is responsible for normalizing color measurement data so that the final measurement does not depend on such variables as those outlined above. Once the spot location determiner  94  determines a location of a spot on the 3D object that has been measured for color, an angle of incidence of a ray of light that reflected off of that spot and was received by the color sensor may be determined. The 3D image provides a 3D surface of the spot. An angle of rays on the spot (e.g., in the receptive field) that are detected by the color sensor may be known with respect to an imaging axis. Accordingly, the known angle of the ray with respect to the imaging axis and the 3D surface of the object may be used to compute an angle of incidence of the ray with the spot on the 3D object. The intensity of the color measurement data may then be adjusted based on the determined angle of incidence. In one embodiment, a normalization table is used to adjust the intensity, where the normalization table indicates a multiplier to multiply by the intensity values of the color measurement based on the angle of incidence. Additionally, the other parameters set forth above may be known or computed, and these known or computed parameters may be used to further normalize the intensity values for the various wavelengths. 
     Note that calibration and normalization techniques are described herein with regards to calibrating and normalizing for color measurements of a color sensor such as a hyper spectral sensor, spectrometer or colorimeter. However, in embodiments the calibration and normalization techniques described herein may also be used to calibrate and normalize the detector for generating a 2D color image. For example, a spectral reflection of a region of the target may be measured by the detector at a known distance of the target from the scanner. The color sensitivity of the detector may then be calibrated and normalized as described herein with reference to calibration and normalization of the hyper spectral sensor. For example, the color sensitivity of the detector may be calibrated and normalized based at least in part on the first spectral reflection as measured by the detector, an angle of incidence from a light source to the first region of the target and the first distance. Other parameters that may also be used for the calibration and normalization include a) light source spectrum and angles, b) angle of incidence from the light source to the object and c) strength or intensity of light impacting the object, which depends on a distance between the object and the light source. Additionally, the color measurement may also be based on object color and reflectance properties, such as what is absorbed (e.g., what wavelengths of light are absorbed and at what levels) at particular angles, what is reflected (e.g., what wavelengths of light are reflected and at what levels) at particular angles and what is diffused (e.g., what wavelengths of light are absorbed and at what levels) at particular angles. These values may vary depending on angle and wavelength. Additionally, characteristics of the image capturing may also affect the color measurement, such as angles of the rays that would be collected and the distance of the object from the collecting device. All of these parameters may be known or computed, and these known or computed parameters may be used to calibrate and normalize the intensity values for the various wavelengths measured by the detector. 
     Intensity may also be affected by distance of the probe from the imaged object. A field of view depth of the confocal focusing optics is known, and the distance can be determined based on the 3D images and determined position of the spot in the 3D images. Once the distance is determined, another multiplier may be applied to the intensity based on the determined distance. The distance multiplier may be determined from a table that relates distances to multiplier values, for example. 
     Normalizer  96  may additionally apply other normalization factors in addition to the normalization factors for distance and angle of incidence. For example, normalizer  96  may automatically subtract known color sensor offsets of the color sensor from the color measurement. The color sensor offsets may have been determined during calibration of the imaging apparatus. After normalization, the detected color should be a true color, and two different imaging devices should measure approximately the same color regardless of distance and/or angle of incidence. Normalizer  96  may also take into account a specific spectrum of the multi-chromatic light source (e.g., a specific white illumination source). Normalizer  96  may calibrate both for specific wavelength responses of the color sensor as well as the light source spectrum. 
     Color capture of spots on an imaged object may be performed in two modes of operation. In a first mode of operation, colors of spots on an imaged object are determined during 3D scanning of the object. In such an embodiment, color spectrum measurements may be automatically generated while 3D scanning is performed. The detector and the color sensor of the image capture device may have a high scanning rate (e.g., of 10-30 images per second). Accordingly, color measurements of many spots of an object (e.g., of a tooth) may be generated during 3D scanning. 
     In a second mode of operation, color navigator  99  provides a graphical interface that identifies color zones of a tooth and indicates which color zones still need one or more color measurements. A tooth is divided into multiple color zones, where each color zone indicates a separate region of the tooth that should have a relatively uniform color. However, color of the tooth may vary between color zones.  FIG. 8  is a diagram of tooth color zones. As shown, a tooth may be divided into a cervical zone  805 , interproximal zones  810 ,  815  on either side of the tooth, a body zone  825  and an incisal zone  820 . In order to generate a prosthetic tooth that will blend in with other teeth in a patient&#39;s mouth, separate color measurements should be made of each of these color zones. 
     In some instances, the second mode of operation may be invoked after a 3D scan has been completed. For example, the 3D scan may lack color measurements for one or more color zone of a tooth. When the second mode is invoked, an image of a scanned tooth may be presented, with an overlay that indicates which color zones of the tooth have not yet been measured for color. If color measurements were generated during 3D scanning, then at least some of the color zones should be populated with color measurement information from one or multiple color measurements. 
     Returning to  FIG. 1B , while in the second mode of operation, a user may move an imaging device (e.g., an intraoral scanner) to point the probe of the imaging device at a region of a tooth associated with a particular color zone for which color information is lacking. The user interface may display a cross hairs, circle, or other indicator or marker that identifies a location of a receptive field of the color sensor in the view finder  98 , Accordingly, a user may move the imaging device until the cross hairs are pointed at a region of a tooth associated with a color zone that lacks color data or that has insufficient color data. The user interface  97  may provide a visual and/or audible signal when the probe is positioned for measurement of a region associated with such a color zone. The user may then initiate a color measurement (e.g., by pressing a button on the imaging device), and color capturing module  85  may receive the new color spectrum measurement. Color navigator  99  may then update the display to show that color information has been received for a particular tooth color zone (e.g., by coloring the tooth color zone green). In one embodiment, color navigator  99  updates the display once a threshold number of color measurements are generated for a color zone. The threshold may be, for example, 3, 5, or another number. Additionally, the image of the tooth may be updated using the received color information. This process may continue until color data is received for all tooth color zones. In some embodiments, the color image processing module  87  categorizes the colors of the different tooth color zones according to a color pallet, which may be a standard color pallet used for dental prosthetics. The color information may be recorded in an appropriate format along with the 3D shape information for use by a dental lab. 
     In some embodiments, color navigator  99  determines when an angle of incidence of light rays that reflect off of a spot on the object in the receptive field of the color sensor and are collected into the color sensor are too high or are within a spectral reflection angle. Color navigator  99  may additionally detect specular reflections. Color navigator  99  may provide feedback instructing a user to change the angle or orientation of the imaging apparatus to move outside of the spectral or specular reflection angles and/or to reduce the angle of incidence. 
       FIG. 2A  illustrates light rays entering an optical scanning system  230  of an imaging apparatus  197 , in accordance with one embodiment.  FIG. 2B  illustrates light rays entering a color sensor  275  for the imaging apparatus  197  of  FIG. 2A , in accordance with one embodiment.  FIG. 2C  illustrates light rays entering a color sensor  275  for an imaging apparatus  199 , in accordance with another embodiment. Imaging apparatus  199  may be identical to imaging apparatus  197  in embodiments except for the addition of a beam splitter  276  and the positioning of an optical transmission medium  270  and lens  264 . 
     Each of  FIGS. 2A-B  illustrate a probe  200  that directs light rays towards an optical scanning system  230  and/or toward a color sensor  275 . The probe  200  is made of a light transmissive material such as glass. In one embodiment, the probe  200  acts as a prism (e.g., as a folding prism). Probe  200  may include an anterior segment  201  and a posterior segment  202 , tightly bonded (e.g., glued) in an optically transmissive manner at  203 . Probe  200  may additionally include a slanted face  204  covered by a reflective mirror layer  205 . A window  206  defining a sensing surface  207  may be disposed at a bottom end of the anterior segment  201  in a manner leaving an air gap  208 . The window  206  may be fixed in position by a holding structure which is not shown. 
     Referring to  FIG. 2A , an array of light rays or beams  209  are represented schematically. As can be seen, the array of light beams  209  are reflected at the walls of the probe  200  at an angle in which the walls are totally reflective and finally reflect on mirror layer  205  out through the sensing face  207 . The array of light beams  209  impact tooth  210 , and the array of light beams  209  are directed back through the probe, along an imaging axis  250  of the optical scanning system  230  on an optical path for the optical scanning system  230 , and into optical scanning system  230 . The term optical scanning system  230  is used herein as a short hand to refer to the optics, additional components and detector (e.g., confocal focusing optics  12 , detecting optics  60  and detector  68  of  FIG. 1A ) that are usable to generate a 3D image of the tooth  210 . If the imaging apparatus  197  is a confocal imaging apparatus, then the optical scanning system  230  includes confocal focusing optics. However, the imaging apparatus  197  may use any 3D imaging techniques, and is not limited to a confocal imaging apparatus. 
     As shown, the array of light beams  209  remain within an optical path of the optical scanning system  230  so that all of the light beams in the array of light beams  209  enter the optical scanning system  230 . For example, as shown all of the rays in the array of light beams  209  are parallel to the imaging axis  250 . 
     The imaging apparatus  197  includes a color sensor  275  as described herein above. An optical transmission medium  270  optically connects the color sensor  275  to a lens  264  or other optical collector. The lens  264  is oriented so as to collect light beams or rays that exit the optical path of the optical scanning system  230  and that have a particular oblique angle to the imaging axis  250 . As shown, none of the light beams in the array of light beams  209  enter lens  264 , optical transmission medium  270  or color sensor  275  of the imaging apparatus  197 . 
     Referring to  FIG. 2B , one or more multi-chromatic light rays  260  are incident on a spot  262  on the tooth  210 . The one or more multi-chromatic light rays  260  are reflected through window  206  into probe  200 , and exit the probe  200  at an angle that is oblique to the imaging axis  250 . The multi-chromatic light rays  260  exiting the probe  200  are parallel to a color sensing axis  255 . The color sensing axis  255  has an oblique angle to the imaging axis  250 . The lens  264 , which may be a collection lens, collects the multi-chromatic light rays  260  that have the angle to the imaging axis  250  (e.g., that are parallel to the color sensing axis  255 ), and focuses the light rays  260  into optical transmission medium  270 . In one embodiment, the color sensor has a receptive field of 2 mm round, the optical transmission medium  270  has a fiber core with a diameter of 100 μm, and the lens  264  has a 1/20 magnification. In one example embodiment, the optical path from the lens to the imaged object is around 80 mm. Accordingly, in such an example the focal length of the lens is about F=4 mm. The lens diameter will determine the amount of collected light. In one embodiment, the lens has an aperture of about 2 mm to 3 mm. The optical transmission medium  270  then directs the light rays  260  into color sensor  275 . The optical transmission medium  270  with the lens  264  allows the passing of light rays from the optical path of the optical scanning system  230  into the color sensor  275 , which may be positioned at any convenient location in the imaging apparatus  197 . 
     Spectrometers are usually large devices that cannot be fit within tight spatial constraints. Use of the optical transmission medium  270  may alleviate size constraints of the color sensor by enabling it to be placed away from the probe  200 . One end of the optical transmission medium (e.g., the end that includes lens  264 ) may be placed near the optical path of the optical scanning system  230 , but the color sensor  275  may be placed distant from the optical path of the optical scanning system  230 . Additionally, use of the optical transmission medium  270  may allow for a less constrained optical design of a lens and optics of the color sensor  275 . Notably, the lens  264 , optical transmission medium  270  and color sensor  275  are outside of the optical path of the optical scanning system  230  (e.g., outside of the optical path of a detector that generates 3D images of the tooth  210 ). Additionally, no beam splitter or mirror is used to direct the light rays  260  into the lens  264 . Additionally, in some embodiments the color sensor  275  is positioned near the optical path of the detector and directly receives rays of multi-chromatic light. In such embodiments, the optical transmission medium or other optical transmission medium may be omitted. 
     By knowing the exact relation between optical scanning system (scanning optics) and the color sensor&#39;s  275  optics, it is possible to accurately determine the path of the light entering the color sensor  275  in relation to the light entering the optical scanning system  230 . The imaging device  197  can determine the 3D shape of the tooth  210  (or other scanned object) using the optical scanning system  230 . By combining the 3D shape and the known relation between the optics, it is possible to determine the exact position in space which is sampled by the color sensor  275 . The optical path of the optical scanning system  230  and the light path of the color sensor need not be the same as long as they overlap over the scanned object. 
     In one embodiment, the optical transmission medium  270  may be omitted. In such an embodiment, the color sensor  275  may be placed near to, and outside of, the optical path of the optical scanning system  230 . The lens  264  may be placed in front of the color sensor  275 , and may focus rays into the color sensor  275 . The lens  264  and/or color sensor  275  may be oriented at an oblique angle to the imaging axis  250  such that the color sensing axis  255  is at the oblique angle to the imaging axis  250 . Notably, no beam splitter or mirror is used to direct the light rays  260  into the color sensor  275  in this embodiment. 
       FIG. 2C  illustrates an alternative design for an imaging apparatus  199  with a color sensor  275 . In some embodiments, as shown in  FIG. 2C , the color sensing axis  255  (e.g., the optical path of the color sensor&#39;s optics) is parallel to the imaging axis  250  (e.g., the optical path of the optical scanning system  230 ). To enable the color sensing axis  255  to be parallel to the imaging axis  250 , a beam splitter  276  is placed in the optical path of the optical scanning system  230 . The beam splitter  276  may be a relatively small beam splitter that only occupies a small portion of the FOV of the optical scanning system  230 . For example, the beam splitter may be a few square millimeters. The beam splitter  276  may pass 100% of light rays at the wavelength (or wavelengths) used by optical scanning system  230 , but may reflect up to 50% of the energy of wavelengths of light used by color sensor  275 . In one embodiment, the beam splitter reflects 10-30% of the energy of the wavelengths used by color sensor (e.g., wavelengths at the visible spectrum) and passes 70-90% of the energy. The passed light rays may be received by optical scanning system  230  to generate a color 2D image. 
       FIG. 3  is a flow chart showing one embodiment of a method  300  for generating three dimensional image data and color data by an imaging apparatus. Method  300  may be performed by processing logic that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, or a combination thereof. In one embodiment, at least some operations of method  300  are performed by a computing device (e.g., computing device  24  of  FIG. 1B ). In one embodiment, at least some operations of method  300  are performed by an imaging apparatus (e.g., imaging apparatus  20  of  FIG. 1A ). 
     At block  305  of method  300 , processing logic activates a first light source. The first light source may be a laser that generates coherent light. In one embodiment, the first light source emits a coherent light beam that is split into an array of light rays. At block  310 , a detector of an imaging apparatus receives rays of the first light source and 2D image data of an object that the rays have reflected off of at multiple different focus settings. This 2D image data is usable to generate a 3D image of the object. 
     At block  315 , processing logic deactivates the first light source and activates a second light source. The second light source may be a multi-chromatic light source (e.g., a light source that generates white light). The second light source may generate incoherent light. At block  320 , the detector of the imaging apparatus receives rays of the second light source that have been scattered off of the object and generates a 2D color image of the object. At block  325 , a color sensor (e.g., hyper spectral sensor or colorimeter) of the imaging apparatus receives one or more rays of the second light source and generates color data for a spot on the object that the one or more rays have been scattered from. 
     At block  330 , processing logic deactivates the second light source and reactivates the first light source. At block  335 , the detector receives additional rays of the first light source that have reflected off of the object and generates new 3D image data of the object. 
     At block  340 , processing logic determines a location of the spot on the object for which color data was generated using the 3D image data generated at block  310  and the additional 3D image data generated at block  325 . The location of a receptive field of the color sensor within a larger FOV of the detector may be known. Accordingly, the location of the receptive field of the color sensor within the first 3D image data and the location of the receptive field of the color sensor within the additional 3D image data may be determined. The location of the spot on the object may then be determined by interpolating between the location of the receptive field of the color sensor within the first 3D image data and the location of the receptive field of the color sensor within the additional 3D image data. 
     In one embodiment the location of the receptive field of the color sensor is interpolated using image data from two or more consecutive 3D scans. These 3D scans are registered with one another (stitched together) using, for example, an iterative closest point (ICP) algorithm. This provides the relative probe position as a function of time. A motion trajectory can be estimated from the changing relative position of the probe as a function of time. The time at which the spectral measurement (color measurement) was generated between two consecutive 3D scans may be determined or estimated (e.g., estimated as a time equidistant from the times of the first 3D scan taken prior to the spectral measurement and the second 3D scan taken after the spectral measurement). The relative position in the time of the spectral measurement is computed from the trajectory. The position of the receptive field position and direction vector in the time of the spectral measurement is computed. The point on the 3D object that intersects with the direction vector that represents the direction of rays of light in the receptive field may then be computed. 
     At block  345 , an angle of incidence of the received rays of the second light source on the spot are determined based on a known trajectory of the rays and the 3D shape of the object at the spot. At block  350 , a distance of the spot from a probe of the imaging apparatus is determined. At block  355 , an intensity of the color data is adjusted based on the angle of incidence and the distance. The intensity and/or other parameters of the color data may also be adjusted based on other normalization factors as discussed above. 
     Repeated measurements of the same region will be (a) normalized and then (b) weighted and normalized according to the weighting. As set forth above, different angles have different intensity. For small angles, the relationship of diffused model will be used for normalization. Small angles are preferred over large angles. Any angles of incidence that are larger than a threshold angle will be discarded. Otherwise, the intensity will be weighted according to angle. Different regions of the FOV and different distances offer different illumination power. These parameters are used for normalization as well. Additionally, there is a range of angles for which spectral reflection will occur. Spectral reflectance occurs when the angle from the surface normal to an illumination source (e.g., to second light source) and the angle of the incidence at the spot (receptive field of the color sensor) are identical and opposite. Color measurements at these angles may also be discarded. 
     At block  360 , processing logic determines whether a scan is complete. If the scan is not complete, then the process returns to block  315 , and the first light source is again deactivated and the second light source is again activated. The method then repeats blocks  320 - 355 . At the reiteration of block  340 , a new spot on the object may be determined using the 3D image data generated at the first iteration of block  335  and the additional 3D image data generated at the second iteration of block  335 . If at block  360  a determination is made that the scan is complete, then the method ends. 
       FIG. 4  is a flow chart showing another embodiment of a method  400  for generating three dimensional image data and color data by an imaging apparatus. Method  400  may be performed by processing logic that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, or a combination thereof. In one embodiment, at least some operations of method  400  are performed by a computing device (e.g., computing device  24  of  FIG. 1B ). In one embodiment, at least some operations of method  400  are performed by an imaging apparatus (e.g., imaging apparatus  20  of  FIG. 1A ). 
     At block  405  of method  400 , processing logic activates a first light source and a second light source. The first light source may be a laser that generates coherent light. In one embodiment, the first light source emits a coherent light beam that is split into an array of light rays. The second light source may be a multi-chromatic light source (e.g., a light source that generates white light). The second light source may generate incoherent light. 
     At block  410 , a detector of an imaging apparatus receives rays of the first light source and generates 3D image data of an object that the rays have reflected off of. A filter in an optical path of the detector may filter out rays of the second light source so that the rays of the second light source do not reach the detector. 
     At block  415 , a color sensor (e.g., hyper spectral sensor or colorimeter) of the imaging apparatus receives one or more rays of the second light source and generates color data for a spot on the object that the one or more rays have been reflected from. The rays of the second light source may have an oblique angle to the optical path of the detector and to an imaging axis of the detector. Accordingly, the rays of the second light source may exit the optical path of the detector without any additional optics to redirect the rays of the second light source. 
     At block  420 , processing logic determines a location of the spot on the object for which color data was generated using the 3D image data generated at block  410 . The location of a receptive field of the color sensor within a larger FOV of the detector may be known. Accordingly, the location of the receptive field of the color sensor within the 3D image data may be determined since the 3D image data and the color data is generated in parallel. 
     At block  425 , an angle of incidence of the received rays of the second light source on the spot are determined based on a known trajectory of the rays and the 3D shape of the object at the spot. At block  430 , a distance of the spot from a probe of the imaging apparatus is determined. At block  435 , an intensity of the color data is adjusted based on the angle of incidence and the distance. The intensity and/or other parameters of the color data may also be adjusted based on other normalization factors as discussed above. Accordingly, when a uniform region is captured (e.g., a receptive field or spot of the color sensor is fully on a single tooth), the surface captured may be connected to a spectrum at a specific distance and angle. 
     Repeated measurements of the same region will be (a) normalized and then (b) weighted and/or summed (e.g., with a weighted sum) and normalized according to the weighting. As set forth above, different angles have different intensity. For small angles, the relationship of diffused model will be used for normalization. Small angles are preferred over large angles. Any angles of incidence that are larger than a threshold angle will be discarded. Otherwise, the intensity will be weighted according to angle. Different regions of the FOV and different distances offer different illumination power. These parameters are used for normalization as well. Additionally, there is a range of angles for which spectral reflection will occur. Spectral reflectance occurs when the angle from the surface normal to an illumination source (e.g., to second light source) and the angle of the incidence at the spot (receptive field of the color sensor) are identical and opposite. Color measurements at these angles may also be discarded. 
     At block  440 , processing logic determines whether a scan is complete. If the scan is not complete, then the process returns to block  410 . If at block  440  a determination is made that the scan is complete, then the method ends. 
       FIG. 5  is a flow chart showing one embodiment of a method  500  for calibrating a color sensor to a detector of an imaging apparatus. Method  500  may be performed by processing logic that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, or a combination thereof. In one embodiment, at least some operations of method  500  are performed by a computing device (e.g., computing device  24  of  FIG. 1B ). In one embodiment, at least some operations of method  500  are performed by an imaging apparatus (e.g., imaging apparatus  20  of  FIG. 1A ). Method  500  may be performed after color 2D images (e.g., view finder images) have been calibrated with 3D images, where the color 2D images and 3D images may be generated using the same detector or using different detectors. These images may be calibrated by generating a color 2D image of a target at a relative position of a probe of the imaging apparatus to the target and generating a 3D image of the target at the relative position of the probe to the target. The two images may then be registered, and an offset (if any) may be determined between the two images. Calibration of the color sensor to the imaging apparatus may include determining a size, position, shape and/or direction of a receptive field of the color sensor within a larger FOV of the detector. The receptive field of the color sensor is essentially a spot. The color sensor is not a camera imaging system, and so it is not possible to calibrate the color sensor by finding an image to register to. 
     At block  505  of method  500 , multiple regions on a specialized target are measured for spectral reflectance (e.g., for color). The target may include a first region having a first spectral reflectance, a second region having a second spectral reflectance and a third region having a third spectral reflectance. The first region may be a background region, and may have a spectral reflection for black, white, or a well known color. In one embodiment, the second spectral reflectance is for green and the third spectral reflectance is for blue. The second region may be offset from the first region in a first direction. The third region may be offset from the first region in a second direction that is orthogonal to the first direction. For example, the target may have a background color, at least one vertical patch of a first color, and at least one horizontal patch of a second color. The spectral reflectances of the various regions on the target may be measured using a color sensor (e.g., a hyper spectral sensor or colorimeter) of an imaging apparatus (e.g., of an intraoral scanner). 
     At block  510 , the target is positioned in front of a probe of the imaging apparatus at a first distance from the probe and at a first relative position (e.g., x and y position) of the target to the probe so that the first region of the target is in a receptive field of the color sensor. The spectral reflectance at the starting relative position is measured to verify that the first region is in the receptive field of the color sensor. 
     At block  515 , the target is moved in the first direction (e.g., horizontally) until the color sensor measures the second spectral reflection at a first relative position of the target to the probe. At block  520 , a first image of the target is generated by the detector. In one embodiment, the detector generates both a 3D image of the target at the first relative position of the target to the probe and generates a 2D image (e.g., a view finder image) of the target at the first relative position of the target to the probe. 
     At block  525 , the target is moved in the second direction (e.g., vertically) until the color sensor measures the third spectral reflection at a second relative position of the target to the probe. At block  530 , a second image of the target is generated by the detector. In one embodiment, the detector generates both a second 3D image of the target at the second relative position of the target to the probe and generates a second 2D image (e.g., a view finder image) of the target at the second relative position of the target to the probe. 
       FIGS. 6A-E  are figures showing calibration of a color sensor to a detector of an imaging apparatus. Each of  FIGS. 6A-E  show a FOV  605  of the detector of the imaging apparatus and the receptive field  610  of the color sensor within the FOV  605  of the detector. The first FOV  605  may have a size of about 14×18 mm in some embodiments. The receptive field  610  may have a diameter of about 1-2 mm in some embodiments. Also shown are various relative positions of a target  612  within the FOV  605  of the detector. The target includes a first region  615  having a first spectral reflectance, a second region  620  having a second spectral reflectance, and a third region  625  having a third spectral reflectance. As shown, the first, second and third regions are considerably larger than the size of the receptive field  610  that has an unknown position. In one embodiment, it is assumed that a material type of the target is fully diffusive so that exact angles of view will not affect the spectral reflectance measurements. 
       FIG. 6A  corresponds to the starting relative position of the target to the probe at block  510 .  FIG. 6B  shows the first relative position of the target to the probe at block  520  after the target has been moved horizontally in the first direction and the color of the second region is detected. As shown, the receptive field  610  includes an edge of the second region  620 .  FIG. 6C  shows an additional relative position of the target to the probe after the target has been moved further in the first direction (e.g., in the horizontal direction). 
       FIG. 6D  shows the second relative position of the target to the probe at block  530  after the target has been moved vertically in the second direction and the color of the third region is detected. As shown, the receptive field  610  includes an edge of the third region  625 .  FIG. 6E  shows an additional relative position of the target to the probe after the target has been moved further in the second direction (e.g., in the vertical direction). 
       FIGS. 7A-C  are color spectrum graphs of various regions of a calibration target (e.g., of the target  612  of  FIGS. 6A-E ).  FIG. 7A  shows the spectral reflectance of the first region (in this case a white region) as measured at the initial relative position shown in  FIG. 6A .  FIG. 7B  shows the spectral reflectance measured at the second relative position shown in  FIG. 6B , where the receptive field  610  is partly on the second region  620  and partly on the first region  615 . A mixture of two spectrums can be seen in  FIG. 7B .  FIG. 7C  shows the spectral reflectance measured at the relative position shown in  FIG. 6C , where the receptive field  610  is completely on the second region  620 . 
     Referring back to  FIG. 5 , at block  535  a relative position of the receptive field  610  of the color sensor within the FOV  605  of the detector at the first distance is determined based on the first relative position of the target to the probe and the second relative position of the target to the probe. The spectral reflectance at the interface of the first region to the second region may be determined according to the equation: 
         S   mixed ( w )= S   color1 ( w )·α+ S   color1 ( w )·(1−α)
 
     Where S mixed (w) represents the mixed spectral reflectance, S color1 (w) represents the spectral reflectance from the first region, and the S color2 (w) represents the spectral reflectance of the second region, and a is the alpha value. Processing logic may minimize α so that Sum(|S mixed (w)−S measured |) is minimized, where S measured  is the spectral reflectance that is measured at a given relative position of the target to the probe. 
     Plotting α as a function of position will give the shape of the one dimensional integral of the receptive field  610  in the first direction. Similar computations may be made of the second direction. 
     The spectral reflectance at the interface of the first region to the third region (or the second region to the third region) may be determined according to the equation: 
         S   mixed ( p )= S   color1 ( p )·α+ S   color3 ( p )·(1−α)
 
     Where S mixed (p) represents the mixed spectral reflectance, S color1 (p) represents the spectral reflectance from the first region, and the S color3 (p) represents the spectral reflectance of the second region, and α is the alpha value. Processing logic may minimize α so that Sum(|S mixed (p)−S measured |) is minimized, where S measured  is the spectral reflectance that is measured at a given relative position of the target to the probe. 
     Again plotting α as a function of position will give the shape of the one dimensional integral of the receptive field  610  in the second direction. Assuming a separable function, FOV(x,y)=Fx(x)·Fy(y) can give a full distribution function for the receptive field  610  (also referred to a full spot distribution function), where FOV(x,y) is the receptive field sensitivity. The receptive field sensitivity may be used when doing a weighted sum when the receptive field is on the border of two regions of the target. 
       FIG. 7D  is a graph of alpha value verses position of a calibration target. The highest derivative position will point to the position of the center and identify the location of the receptive field  610  in the first direction and/or second direction. Accordingly, the x and y coordinates of the center of the receptive field  610  within the FOV  605  may be determined at block  535 . 
     Returning to  FIG. 5 , at block  540  processing logic determines whether additional distances of the target to the probe are to be tested. In some embodiments, the axis and optical path for the color sensor have an oblique angle to the imaging axis and optical path of the detector used to generate 3D images. Accordingly, the spot position (position of the receptive field) of the color sensor will change with changes in distance of an object to the probe. In such embodiments additional distances should be tested to generate a vector that is usable to determine the spot location at each object distance. If an additional distance is to be tested, the method proceeds to block  545 , and a distance of the target to the probe is adjusted. For example, the target may be moved 1-10 mm in depth (z direction). The operations of blocks  510 - 535  are then repeated at the new depth. The center of the receptive field of the color sensor will change linearly with changes in Z. Accordingly, two depths are sufficient to generate a three dimensional vector that indicates the x,y,z position of the center of the receptive field of the color sensor. Additional distances may be tested to increase accuracy however. 
     At block  550 , an orientation of a second axis of rays detected by the color sensor that is different from the imaging axis of the detector is determined based on the relative position of the receptive field within the FOV at the first distance and the relative position of the receptive field within the FOV at the second distance.  FIG. 7E  is a graph of a receptive field position of a color sensor as a function of depth. The angles of the x and y lines in  FIG. 7E  imply the angles of the 3D rays of the receptive field of the color sensor (the spot). As the Z position of the target and X,Y positions of the regions are also scanned (e.g., 2D and 3D images are generated), these angles will provide the spot direction relative to the 3D coordinate system of the imaging device. 
     The spot size (size of the receptive field) of the color sensor can also change with changes in depth. The best focus position is the position that has the smallest spot size. The nearest and furthest distances will have the largest spot sizes. Repeated spot size measurements in different depth settings of the target (different Z values) may create the chart shown in  FIG. 7F .  FIG. 7F  is a graph of receptive field size of a color sensor as a function of depth. 
     Accordingly, as a result of calibration the position and angles of the spot are known as a function of distance, and a size of the spot is also known as a function of distance. Additional calibration may also be performed of the color sensor, such as a calibration for stray light level and offsets. In one embodiment, a color measurement is performed by the color sensor with no illumination in a dark environment. This color measurement may then be subtracted from each future color measurement. Additionally, a measurement of air (no target) may be taken using each available illumination source (e.g., with the second light source turned on but no target or object in the FOV of the probe). This will measure a stray light level and offsets for each wavelength of the color sensor. These values may also be subtracted from each color measurement generated, depending on the light source used for the color measurement. For example, the measurement of air using the second light source may be subtracted from each future color measurement that also uses the second light source. Additionally, the spectral responses of each of the regions of the target may be measured for one or more illumination sources. By analyzing the spectral response of the different colored regions and different illumination sources, processing logic can determine the spectral intensity of each light source multiplied by the color sensor&#39;s gains per wavelength as well as color sensor cross talk levels. 
     At some angles of incidence spectral reflection and/or specular reflection will occur. In one embodiment, calibration is performed to identify the range of angles of incidence for which spectral and/or specular reflection occurs. Specular reflections may later be discarded. 
     In one embodiment, specular reflection angles are determined using a special target having a shape of a sphere or semi-sphere (e.g., a half ball). The target is composed of a reflective material. The target&#39;s three dimensional shape, including a diameter and position of the target, is measured using the detector. Then the multi-chromatic light source is enabled, and points of reflection can be identified in a color 2D image generated by the detector based on detection of the multi-chromatic light that reflects off of the target. From these points of reflection in the 2D image and the known 3D shape of the target, the position of the light sources for the multi-chromatic light in the FOV of the detector can be computed. The position of the light sources can then be used to determine specular reflections. 
     In an example, the detector may measure a three dimensional shape of a target, the target having a shape of a sphere or semi-sphere and being comprised of a reflective material. One or more multi-chromatic light sources may then be enabled. The detector may measure intensities of light reflected off of a plurality of points on the target. This measurement may be a 2D measurement in embodiments. Processing logic may then identify one or more points of reflection on the target based on the intensities of light reflected off of the plurality of points on the target, wherein those points of the plurality of points having a highest intensity are identified as the one or more points of reflection. Processing logic may then determine a position of each of the one or more multi-chromatic light sources based on the three dimensional shape of the target and the one or more points of reflection (e.g., by computing the angle of incidence at a point of reflection and tracing a ray back to a light source using the angle of incidence). The position of the light sources can then be used to determine specular reflections. 
       FIG. 9  illustrates a diagrammatic representation of a machine in the example form of a computing device  900  within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. In alternative embodiments, the machine may be connected (e.g., networked) to other machines in a Local Area Network (LAN), an intranet, an extranet, or the Internet. The machine may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet computer, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines (e.g., computers) that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. In one embodiment, computing device  900  corresponds to computing device  24  of  FIG. 1B . 
     The example computing device  900  includes a processing device  902 , a main memory  904  (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM), etc.), a static memory  906  (e.g., flash memory, static random access memory (SRAM), etc.), and a secondary memory (e.g., a data storage device  928 ), which communicate with each other via a bus  908 . 
     Processing device  902  represents one or more general-purpose processors such as a microprocessor, central processing unit, or the like. More particularly, the processing device  902  may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device  902  may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processing device  902  is configured to execute the processing logic (instructions  926 ) for performing operations and steps discussed herein. 
     The computing device  900  may further include a network interface device  922  for communicating with a network  964  or other device. The computing device  900  also may include a video display unit  910  (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device  912  (e.g., a keyboard), a cursor control device  914  (e.g., a mouse), and a signal generation device  920  (e.g., a speaker). 
     The data storage device  928  may include a machine-readable storage medium (or more specifically a non-transitory computer-readable storage medium)  924  on which is stored one or more sets of instructions  926  embodying any one or more of the methodologies or functions described herein. A non-transitory storage medium refers to a storage medium other than a carrier wave. The instructions  926  may also reside, completely or at least partially, within the main memory  904  and/or within the processing device  902  during execution thereof by the computer device  900 , the main memory  904  and the processing device  902  also constituting computer-readable storage media. 
     The computer-readable storage medium  924  may also be used to store a user interface  950 , color capturing module and/or color image processing module which may correspond similarly named modules of  FIG. 1B . The computer readable storage medium  924  may also store a software library containing methods that call the user interface  950 , color capturing module and/or color image processing module. While the computer-readable storage medium  924  is shown in an example embodiment to be a single medium, the term “computer-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “computer-readable storage medium” shall also be taken to include any medium that is capable of storing or encoding a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present invention. The term “computer-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, and optical and magnetic media. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent upon reading and understanding the above description. Although embodiments of the present invention have been described with reference to specific example embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.