Abstract:
The disclosure relates to a device and a method for the space-colorimetric measurement of a three-dimensional object, in order to digitally model the low-relief and the colorimetric coordinates of this object according to multiple analysis points. In order to do so, the measuring device of the disclosure combines a lighting means with at least four optical detection means, including at least two twin detection means sensitive to substantially identical light wavelength ranges, in order to determine by stereoscopic effect the low-relief of the object analysed. The disclosure thus proposes a device for the space-colorimetric measurement of a three-dimensional object, that comprises a detection head including a lighting means for the object and at least four detection means for detecting the light reflected by the object, wherein said device further includes a unit for processing the information received by the detection means. At least two twin detection means are sensitive to substantially identical light wavelength ranges.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Phase Entry of International Application No. PCT/FR2008/000081, filed on Jan. 23, 2008, which is incorporated by reference herein. 
     
    
     BACKGROUND AND SUMMARY 
       [0002]    The invention relates to a device and a method for a non-invasive, space-colorimetric measurement of a low-relief three-dimensional object. In this regard, the present invention relates to the space-colorimetric diagnostic field, also called colorimetric metrology field. 
         [0003]    There has been known from the prior art various solutions allowing to analyze the colorimetric characteristics of a two-dimensional object. For example, dental surgeons and prosthetists use apparatuses that make it possible to precisely define a colorimetric cartography of teeth in order to produce prostheses exhibiting substantially the same colorimetric characteristics as the original teeth. In order to do this, it is essential to precisely pick the colorimetry of a tooth according to a plurality of analysis points. 
         [0004]    In this regard, document No. WO 05/080929 shows a device for measuring the colorimetric characteristics of a tooth in a plurality of points and in a two-dimensional space. In the same manner, document No. WO 06/002703 discloses a device comprised of several light-emitting diodes that emit differently-colored light beams on an object. The light beams are then reflected by the object and then received by a detection system and a central image processing unit. Thus, each point of the image is analyzed in such a way as to determine the spectrum constituted of various levels of colors corresponding to the different wavelengths emitted from the light-emitting diodes. Based on these color levels, the central processing unit calculates, for each point of the analyzed surface, the corresponding colorimetric coordinates. Thus, the device described in this document makes it possible to determine a two-dimensional colorimetric cartography of the analyzed object. 
         [0005]    However, these solutions are not satisfactory in as far as the relief, or topology of the analyzed objects, is not taken into account during the measurements. Because of this, the devices of the prior art calculate the colorimetric coordinates of objects considered as planes by approximation. Yet, the combination of spatial and colorimetric measurements for three-dimensional objects exhibits a wide range of applications in the dental field, biometry, industrial or artistic metrology, etc. More specifically, the fact of simultaneously measuring spatial and colorimetric coordinates according to three directions in space of the analyzed object allows to substantially improving the quality of the results. 
         [0006]    In fact, the value of the colorimetric coordinates directly depends on the position of the measured object with respect to lighting means since the quantity of light received by an object decreases proportionally to the square of the distance which separates it from these lighting means. Likewise, the higher the angle between the ray of light and the normal to the object is, the lower the quantity of reflected light, within the scope of a scattered reflection, is. Because of this, the approximations carried out by the devices of the prior art generate significant errors which denature at least partially the quality of the colorimetric analysis carried out. 
         [0007]    Another difficulty pertaining to the colorimetric measurement comes from the choice of lighting of the object to be analyzed. In fact, it is preferable to use lighting means that allow performing a splitting of the light flux according to predetermined geometric and chromatic criteria. However, this choice depends on criteria of quality, expense, encumbrance and life span. 
         [0008]    The present invention aims to overcome the aforementioned drawbacks of the prior art by providing a device and a method for the space colorimetric measurement of a three-dimensional object allowing to digitally model the low-reliefs and the colorimetric coordinates of this object based on a multitude of analysis points. Also, the aim of the invention is to provide a method for calculating the colorimetric cartography of a three-dimensional object by taking into account parameters of the measurement device. In order to do this, the measurement device according to the invention provides the combination of lighting means with monochromatic detection means, of which at least two twin detection means sensitive to substantially identical light wavelength ranges, in order to determine by stereoscopic effect the low-relief of the object analyzed. 
         [0009]    More specifically, the object of the invention is a device for the space-colorimetric measurement of a three-dimensional object comprising a detection head made up of an object lighting means and at least four detection means for detecting the light reflected by the object, the device further comprising a unit for processing the data received by the detection means, wherein at least two twin detection means are sensitive to substantially identical light wavelength ranges. The use of at least two twin detection means sensitive to substantially identical light wavelength ranges allows calculating, by stereoscopy, the distance of the analyzed points with respect to the detection means. Thus, the spatial coordinates of the object may be determined according to three directions in space and the colorimetric coordinates may be corrected according to the position of the analysis points with respect to the detection head (distance and normal to the surface). Moreover, the simultaneous implementation of several monochromatic detection means, each sensitive in a complementary manner to a part of the visible wavelength field, allows, by means of a calculation algorithm, composing a digital color image of the analyzed objects. This method brings a better precision than color matrix photonic sensors and an acquisition speed lower than sequential multi-specter monochromatic photonic detection systems. 
         [0010]    According to specific embodiments: 
         [0011]    the two twin detection means comprise twin filtration members associated to at least one matrix photonic sensor; 
         [0012]    the matrix photonic sensor is divided into several areas that respectively receive rays of light coming from each of the twin filtration members. 
         [0000]    Thus, the photonic sensor areas do not need to be synchronized with respect to each other; 
         [0013]    the matrix photonic sensors are CMOS sensors such that even if a pixel is severely saturated with photons, it has hardly any effect on the neighboring pixels. A bilinear interpolation taking into account the colorimetric values of the pixels surrounding an analysis point is nevertheless provided such as to smooth the results obtained; 
         [0014]    the two twin detection means are sensitive to a wavelength range substantially equal to the field of wavelengths of green, which allows to obtain particularly relevant results as to the topology of the objects analyzed; 
         [0015]    the two primary detection means are sensitive, one to a range of the wavelength field of blue and the other to a range of the wavelength field of red; 
         [0016]    the lighting means are constituted of a central lighting source around which the detection means are arranged; 
         [0017]    the lighting means are constituted of an annular lighting source arranged around detection means, which is advantageous as the lighting is thus, substantially homogenous for the set of analysis points; 
         [0018]    the detection head is topped with an end cover of predetermined depth such as to reduce the calculation time of the method. In fact, the iterative calculation is thus achieved between a minimum distance substantially corresponding to the depth of the end cover and a maximum distance corresponding to the depth of the observation range. 
         [0019]    According to another aspect, the invention also relates to a method for the space-colorimetric measurement of a three-dimensional object comprising the following steps: emitting at least a light to light the object to be analyzed, receiving the rays of light reflected by the object on at least four detection means, and transferring the light information gathered by the detection means towards a processing unit. The rays of light reflected by the object are detected by at least two twin detection means sensitive to substantially identical light wavelength ranges. 
         [0020]    According to specific embodiments: 
         [0021]    the method comprises a step of calibrating the detection means beforehand; 
         [0022]    the processing unit determines, by iterative calculation, the relative position of a plurality of analysis points with respect to the detection head in order to take into account the position of these points with respect to the light source and the detection means to adjust the colorimetric coordinates of the analyzed object. 
         [0023]    the processing unit determines, by stereoscopy, the distance of a plurality of analysis points with respect to the detection means; 
         [0024]    the processing unit determines the coordinates of the normal to the surface of the object, in a plurality of analysis points; 
         [0025]    the iterative calculation of the depth is carried out between a minimum depth, corresponding to the distance between the detection means and the end of an end cover, and a determined maximum depth; 
         [0026]    the iterative pitch is substantially equal to the size of the range corresponding to a pixel for the predetermined minimum depth. The measured value is thus substantially isotropic; 
         [0027]    the processing unit discards analysis points the intensity of the colorimetric values of which exceeds a value predetermined by calibration such that the errors due to specular reflection be identified; 
         [0028]    the method comprises a step of calculating the colorimetric coordinates of a plurality of weighted analysis points according to the position of said analysis points; 
         [0029]    the colorimetric coordinates of each point are adjusted by bilinear interpolation such as to respect the linearity of the colorimetry of the analyzed object. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Other characteristics and advantages of the invention will become apparent upon reading the following detailed embodiments, with reference to the figs. which respectively illustrate: 
           [0031]      FIG. 1 , a schematic representation of a measurement device according to the invention; 
           [0032]      FIGS. 2   a  and  2   b , schematic representations of a first embodiment of a detection head according to the invention comprising annular lighting means; 
           [0033]      FIGS. 3   a  and  3   b , schematic representations of a second embodiment of a detection head according to the invention comprising central lighting means; and 
           [0034]      FIG. 4 , a schematic representation of the operating of the twin detection means. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    It should be noted that the term isotropic measurement means that the resolution of the measurement is substantially the same according to the three directions in space. An embodiment of a measurement device according to the invention will now be described with reference to  FIG. 1 . In this embodiment, the device allows to achieve a space-colorimetric measurement of a three-dimensional object  2 , in this case a tooth. Of course, any other low-relief three-dimensional object  2 , i.e., whereof the topology does not exhibit any clearance, could also be the subject of such a space-colorimetric measurement. For example, the three-dimensional object  2  measured could be a painting, an industrially-produced piece, a ticket, etc. 
         [0036]    The device according to the invention preferably comprises a detection head  4  and a support housing  6  connected to a unit for processing  8  data coming from the detection head  4 . The processing unit  8  is separated from the support housing  6  and connected thereto by means of communication means  10 . This configuration also allows reducing the size of the support housing  6  as well as the production cost of the measurement device. The device is thus compact such as to be able to be easily handled with one hand by an operator. The processing unit  8  may also be integrated within a more stable support  8  in order to improve the precision of results and to measure more voluminous objects  2 . 
         [0037]    Advantageously, the digital data gathered by the detection head  4  are transmitted, by means of communication means  10 , to a processing unit  8  allowing reconstituting by iterative calculation the space-colorimetric coordinates of the analyzed object. These communication means  10  may alternatively have a wire or be wireless. It should be noted that the detection head  4  exhibits dimensions adapted to the size of the three-dimensional object  2  measured in such a way as to reduce the time for processing data provided by the detection head  4  to the processing unit  8 . 
         [0038]      FIGS. 2   a  and  2   b  are schematic representations of a first embodiment of a detection head  4  according to the invention. In this example, the detection head  4  comprises central lighting means  14  and four optical detection means  16  arranged around and at equal distance from the central lighting means  14 . The annular lighting means  14  comprise wide specter light sources  14   a  in the visible field. One could consider the possibility to use more or less light sources  14   a . Nevertheless the experimental results have shown that starting from eight light sources  14   a , the resolution at each analysis point is relatively constant. The lighting provided by the annular lighting means  14  is thus continuous and the power is liable to be adjusted to the measurement requirements. The annular lighting means  14  also advantageously comprise a ground or holographic glass  14   b , located downstream from the light source  14   a  in order to improve the lighting homogeneity. According to an alternative embodiment, the annular light source  14  may be constituted of a circular neon tube. 
         [0039]    Advantageously, the optical detection means  16  are constituted of an infrared filter  16   a  eliminating infrared disturbances which the CMOS-type photonic sensors are sensitive to (presented hereafter). According to an embodiment the infrared filter  16   a  is a filter BG40 from SCOTT company. The detection means  16  further comprising four filtration members  16   b ,  16   c  arranged behind the optical members  16   a  and at the centre of the annular lighting means  14 . Preferably, the filtration members  16   b ,  16   c  are lenses allowing at the same time to filter and focus on the rays of light coming from the analyzed object towards the photonic sensors (presented hereafter). 
         [0040]    The optical axes of these four filtration members  16   b ,  16   c  are substantially parallel to each other and substantially according to the same direction as the propagation axis of the annular lighting means  14 . Meanwhile, according to various alternatives, the filtration members  16   b ,  16   c  may also exhibit optical axes convergent towards a same point, or towards different points, or even a composition of these various possibilities. One first pair of primary filtration members  16   b  is constituted of a blue filtration lens, of reference B440 from the HOYA company, and of a red filtration lens of reference DG570 from SCHOTT company. Preferably, this pair of filtration members is arranged symmetrically with respect to the central axis of the annular lighting means  14 . 
         [0041]    Moreover, the detection means  16  also comprise one pair of twin filtration members  16   c  exhibiting a substantially identical bandwidth. Advantageously, these twin filtration members  16   c  are green colors lenses, for example lenses of reference G550 from HOYA company. These twin filtration members  16   c  are advantageously arranged such as to form a rotational symmetry around the central axis of the annular lighting means  14 . 
         [0042]    A photonic sensor  16   e , subdivided into four quadrants respectively in correspondence with the four filtration members,  16   b ,  16   c , is arranged behind the filtration members  16   b ,  16   c  such as to receive the rays of light propagated through these filtration members  16   b ,  16   c . This photonic sensor  16   e , is preferably a CMOS sensor. The combination of the twin filtration members  16   c  with the corresponding photonic sensor zone  16   e  forms twin detection means ( 16   b ,  16   c ). Likewise, the combination of primary filtration members  16   b  with the corresponding photonic sensor zone  16   e  forms the primary detection means ( 16   b ,  16   e ). 
         [0043]    According to a second embodiment described with reference to  FIGS. 3   a  and  3   b , the detection head  4  is constituted of detection means  16  exhibiting four filtration members  16   b ,  16   c  arranged around central lighting means  14 , preferably positioned behind a holographic type diffuser filter  14   b . According to this embodiment, the detection means  16  exhibiting two primary filtration members  16   b , respectively of red and blue colors, and two twin filtration members  16   c , of green color  16   c . Advantageously, these two twin filtration members  16   c , are interposed between the two primary filtration members  16  such as to maintain a symmetry with respect to the rotational axis of the detection head  4 . Meanwhile, the twin filtration members  16   c  may also be arranged side by side. 
         [0044]    In this embodiment, the detection head  4  comprises four independent and synchronized photonic sensors  16   e , also arranged behind the filtration members  16   b ,  16   c  such as to receive the light rays propagated through these filtration members  16   b ,  16   c . The detection head  4  is preferably topped with an end cover  20  of predetermined depth allowing defining a chamber wherein the analyzed object is not disturbed by external light. The depth of the end cover  20  defines the minimum observation depth. In fact, the analyzed object  2  cannot be located at a variable distance with respect to the detection means  16  that correspond to a predetermined tolerated distance in front or behind the nominal distance of the end cover  20 . 
         [0045]    This end cover  20  exhibits a depth of a few centimeters within the scope of a hand-held measurement device or a few meters within the scope of a device mounted on a support. Advantageously, the depth of the end cover  20  is five times higher than the depth of the object  2  to be measured. Likewise, the width and height of the analyzed object  2  are, preferably, around three times higher than the depth of the object to be measured. In the utilization phase, the device according to the invention may be maintained by means of a support housing  6  and activated thanks to the controlling circuit  9  of the device. 
         [0046]    Preferably, the operator first achieves a calibration of the measurement device by placing a white surface against the end cover  20 . The duration of the measurement is determined such that the maximum intensity of the photonic sensor(s) does not exceed around 85% of the acceptable maximum intensity. Thus, when taking the measurement, the possible specular effects will be translated by intensity equal to the acceptable maximum intensity and will thus be detectable. 
         [0047]    The end cover  20  of the measurement device according to the invention is then placed against the object  2  to be analyzed, such that the object is at least partially protected from outside light. The method according to the invention then consists, in achieving at least one measurement, or non invasive digitizing of a very short period. In fact, this measurement is achieved without contact and by using lighting means  14  of perfect innocuousness. On the other hand, the measurement time may be less than a tenth of a second. 
         [0048]    During this second measurement, the rays of light, emanating from the lighting means  14 , propagate towards the analyzed object  2  before being reflected towards the detection means  16 . Thus, these reflected light beams successively pass through the optical member  16   a , then through the filtration members  16   b ,  16   c  before reaching the matrix photonic sensors  16   e . An optical data corresponding to an analysis point is thus gathered by each of the pixels constituting the matrix photonic sensors  16   f . These data are then transmitted towards a processing unit  8 , by means of communication means  10  with a central unit  12 , in order to deduce the space-colorimetric cartography of the analyzed object  2 . 
         [0049]    The twin detection means, constituted of the twin filtration members  16   c  and the corresponding matrix sensors  16   e , by stereoscopic calculation in the processing unit  8 , allow to determine the space coordinates of each of the analyzed points. In fact, the twin detection means receive the light reflected on the object in the same spectral conditions. In these cases, the values obtained by the twin detection means should be equal. 
         [0050]    Within the scope of a measurement device according to the invention exhibiting characteristics represented on  FIG. 4 , the value of light intensity remitted by a point of the analyzed object may be expressed by the following relations: 
         [0000]    
       
         
           
             
               
                 Lo 
                 G 
               
               = 
               
                 
                   
                     
                       Lp 
                       G 
                     
                     
                       
                         cos 
                          
                         
                           ( 
                           
                             b 
                             G 
                           
                           ) 
                         
                       
                       × 
                       
                         d 
                         
                           2 
                            
                           G 
                         
                         2 
                       
                     
                   
                    
                   
                       
                   
                    
                   and 
                    
                   
                       
                   
                    
                   
                     Lo 
                     D 
                   
                 
                 = 
                 
                   
                     Lp 
                     D 
                   
                   
                     
                       cos 
                        
                       
                         ( 
                         
                           b 
                           D 
                         
                         ) 
                       
                     
                     × 
                     
                       d 
                       
                         2 
                          
                         D 
                       
                       2 
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where 
         [0051]    L oD  represents the value of light intensity re-emitted by an analysis point, determined based on a right sensor; 
         [0052]    L oG  the value of the light intensity re-emitted by an analysis point, determined based on a left sensor; 
         [0053]    L pD , the light energy received by the pixel of the right sensor after a diffuse reflection on the object; 
         [0054]    L pG , the light energy received by the pixel of the left sensor after a diffuse reflection on the object; 
         [0055]    b D , the angle between the normal of the right sensor at one pixel and the ray from the object; 
         [0056]    b G , the angle between the normal of the left sensor at one pixel and the beam from the object; 
         [0057]    d 2D , the length of the light path from the analysis point to the corresponding pixel on the right sensor; and 
         [0058]    d 2G  the length of the light path from the analysis point to the corresponding pixel on the left sensor. 
         [0059]    Consequently, the method according to the invention provides to calculate in an iterative manner, for each potential depth of an analysis point comprised between a predetermined minimum depth and a maximum depth, the depth for which the values of light intensity (L OG , L OD ) re-emitted by an analysis point and calculated based on twin detection means, are the closest. It is worth noting that the minimum depth advantageously corresponds to the depth of the end cover  20  whereas the maximum depth corresponds to the depth of the observation range. Preferably, the iteration pitch is substantially equal to the size of the range corresponding to a pixel for the predetermined minimum depth. 
         [0060]    The processing unit  8  determines at this stage a couple of data corresponding to the depth of a plurality of analysis points and the light intensity re-emitted by said analysis points corresponding to the length range of the twin detection means. The processing unit thus deduces, the coordinates (x, y, z) of each analysis point of the measured object. Based on these data, the processing unit  8  also determines the normal at each analysis point in order to be able to restore the colour at this analysis point. This operation is carried out by calculating the mid-plane passing through each analysis point. 
         [0061]    The processing unit  8  finally determines, based on values of light intensity gathered by primary and twin detection means, the colorimetric cartography of the analyzed object. This cartography is weighed according to the space position of the analysis points and particularly the distance of these analysis points with respect to the detection means  16  as well as the direction of the normal to the surface of the object at each of these analysis points. It may be possible to achieve several measurement sets such as to increase the precision of the results. 
         [0062]    The invention is not limited to the embodiments described and represented above. Particularly, the skilled person is able to achieve various alternatives of the abovementioned device and method within the framework of the invention. Particularly, although it is preferable to use monochromatic filtration lenses the filtration members  16   b ,  16   c  may be composed of lenses combined with colour filters. Furthermore, the device according to the invention may also be composed of four pairs of detection means  16  or more, in order to improve the quality of the results, specifically on the colorimetric plane. On the other hand, it would be also considered possible to replace the CMOS matrix photonic sensors by CDD sensors or any other type of photonic sensor.