Abstract:
The invention concerns a device for acquiring the three-dimensional shape of an object ( 10 ) by opto-electronic process, comprising a chromatic system ( 18 ) for illuminating the object ( 10 ) and for picking up the light reflected or backscattered by the object ( 10 ), and a reflecting mirror ( 26 ) placed on the optical axis between the optical system ( 18 ) and an illumination slot ( 16 ) for deflecting the light reflected by the object towards a spectral analysis means ( 30 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The application claims priority of international application 00/04954 filed Apr. 14, 2000. 
   FIELD OF THE INVENTION 
   The invention relates to an apparatus for opto-electronically acquiring a three-dimensional shape, of the type described in international application WO99/64816. 
   BACKGROUND OF THE INVENTION 
   International application WO99/64816 describes an apparatus for acquiring shapes comprising lighting means, the lighting means comprising a luminous polychromatic source and a chromatic lens, the lighting means coupled to an optical system for magnification, means for sensing light reflected or backscattered by an illuminated object, and spectral analysis means for analyzing sensed light, coupled to data processing means. The spectral analysis means are on the optical axis of the light sensing means, while the lighting means are offset angularly and illuminate the object by means of a semi-transparent blade placed on the optical axis of the light sensing means. 
   This known apparatus performs to a satisfying degree, with a measurement depth that is relatively high and notably superior to other apparatus in the state of the art. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to perfect the apparatus described above and improve its performance. 
   According to a first broad aspect of the present invention, there is provided an apparatus for opto-electronically acquiring three dimensional shapes, the apparatus comprising a luminous source with a continuous spectrum, an illumination slot provided on the optical axis in front of the source, an optical image forming system, an optical light sensing system for sensing light reflected by the object, spectral analysis means for analyzing the spectrum of the light sensed by the light sensing means, and data processing means coupled to the spectral analysis means, wherein the image formation system and the image sensing system are comprised within one optical chromatic system and wherein light reflected by the object and being output by the optical chromatic system is reflected off a mirror placed on the optical axis between the illumination slot and the optical system and is deflected towards an analysis slot placed on the optical axis of the spectral analysis means. 
   The apparatus, according to the present invention, provides several advantages with respect to other known techniques. The depth of the field and the measurement depth are increased. The apparatus is less sensitive to parasitic wavelengths and the measurements are more precise. 
   Alternatively, the mirror is a mask that intersects the light rays output from the illumination slot on the optical axis towards the object. This mask avoids illuminating a point on the surface of the object with a set of rays of different wavelengths and facilitates locating the point on the optical axis by illuminating with only one wavelength. 
   Preferably, the optical chromatic system is afocal and an optical magnification system, preferably afocal, is mounted on the optical axis between the optical chromatic system and the illuminated object. 
   In order to obtain a telecentric path of rays and to reduce shadow zones on the illuminated object, a mask comprising a circular hole can be placed on the optical axis to allow reflected rays to pass through, the mask being placed in the centre of the optical chromatic system and comprising at least two lateral orifices for the passage of illumination light for the object. 
   In order to increase the signal to noise ratio and therefore increase the precision of the measurement (or the gain of the signal when it is weak), the spectral analysis means can comprise a matrix of cameras of type CCD or analog, wherein the outputs of the cameras are coupled to means for analog to digital conversion via low pass filters that eliminate noise due to high frequencies on analog signals at the output of the sensing means. 
   According to another alternative embodiment, the apparatus also comprises optical image rotation means placed on the optical axis in an area intersected by the illuminating light of the object and by the light reflected by the object, the image rotation means comprising, for example, a DOVE (Distributed Object Visualization Environment) prism that can be placed in the optical chromatic system. 
   By rotating the prism around the optical axis, we can rotate around the optical axis the measurement profile on the illuminated object. This allows a rotating sweep of the illuminated surface of the object without relative displacement between the acquiring means and the illuminated object in order to acquire the three-dimensional shape in a cylindrical volume of measurement that is defined by the rotation of the measurement profile around the optical axis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings wherein: 
       FIG. 1  is a schematic of the apparatus according to the invention; 
       FIG. 2  is a simplified partial view illustrating the operation of the apparatus; 
       FIG. 3  is a frontal view of a mask used for obtaining a telecentric path of rays; 
       FIG. 4  is a frontal view of an alternative embodiment for the mask of  FIG. 3 ; 
       FIG. 5  is a schematic showing the optical image rotation means within the apparatus according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIG. 1 , reference numeral  10  designates an object for which we want to acquire a three dimensional shape, the object being on the optical axis  12  of an apparatus according to the invention, the apparatus comprising an illuminating source  14  with a continuous spectrum, connected by, for example, optical fibers to the focal point of an optical system  15  which is a condenser that focalizes light emitted by the source  14  through an illumination slot  16  placed on the optical axis  12 . 
   The slot  16  is followed by an optical chromatic system  18  which in this case is an afocal system, comprising two identical chromatic lenses  20 , the slot  16  being at the focal point of the first lens  20 . 
   The optical chromatic system is followed by an optical magnifying system  22 , preferably afocal. 
   The illuminated object  10  is placed approximately at the focal point of the last lens  24  of the optical magnifying system. 
   The optical chromatic system  18  comprises the light sensing system to sense light reflected or backscattered by the surface of the object  10 , and a reflecting mirror  26  is placed on the optical axis between the illuminating slot  16  and the first lens  20  of the chromatic system  18  in order to deflect, preferably perpendicularly to the optical axis  12 , the light sensed, in the direction of the analysis slot  28  placed on the optical axis of the spectral analysis means  30  that is coupled to data processing means  32 . 
   The spectral analysis means  30  are, for example, such as those described in international patent application WO99/64816, hereby incorporated by reference. 
   The apparatus described according to the present invention provides the advantage that, with respect to the apparatus described in the document cited above and incorporated by reference, it does not use semi-transparent blades placed on the optical axis to illuminate the object and sense the luminous flux reflected or backscattered by the illuminated object. The use of a semi-transparent blade necessarily implies that a major portion of the luminous flux reflected or backscattered by the illuminated object is lost. In the apparatus as shown in  FIG. 1 , the gain of the luminous flux reflected or backscattered by the object  10  is within a range of 2 to 4, approximately, with respect to the embodiments described in previous documents cited above. 
   Another advantage of the apparatus according to the invention will now be described in reference to  FIG. 2 . 
   In this figure, in order to simplify it, the optical chromatic system  18  is represented by a single lens and the mirror  26  is represented by a mask that is placed on the optical axis  12  and that intersects the central polychromatic light rays exiting the illumination slot  16 . 
   Since the optical system  18  is a chromatic system with a focal length that varies continuously with wavelength, the different wavelengths exiting the illumination slot  16  are focalized at different points on the optical axis  12 . For example, a luminous ray R 1  having wavelength λ 1  is focalized at point P 1 , and a luminous ray R 2  having wavelength λ 2  greater than λ 1  is focalized on the axis  12  at a point P 2  which is further from the chromatic system  18  than point P 1 . 
   If the wavelengths of rays R 1  and R 2  correspond to the extremities of the wavelength illumination band, than the distance P 1 −P 2  on the optical axis  12  represents the measurement depth. 
   Since the central luminous rays exiting the illumination slot  16  are intersected by the mask  26 , we can say that point P 1  on the axis  12  is illuminated uniquely or almost uniquely by a light of wavelength λ 1 , point P 2  will be illuminated by a light of wavelength λ 2 , and a point P 1  in between P 1  and P 2  will be illuminated by a light of wavelength λ 1  in a range between λ 1  and λ 2 . 
   Since the optical chromatic system  18  comprises the optical sensing system that senses light reflected or backscattered by the object, the light rays of wavelength λ 1  reflected or backscattered by the point P 1  are focalized on the optical axis  12  at the illumination slot  16 , the light rays of wavelength λ 2  reflected or backscattered by the point P 2  are also focalized on the illumination slot  16 , and the light rays of wavelength λi reflected by the intermediate point Pi are focalized on the illumination slot  16 . 
   This shows that a clear image can be formed from any point on the axis between points P 1  and P 2  by the chromatic system  18  on the spectral analysis means, without adjustment to the apparatus. 
   This also shows that if a parasitic light ray having a wavelength different from λ 1  is reflected by point P 1 , this parasitic light ray will not be focalized at the illumination slot  16  and a clear image of point P 1  at the wavelength(s) of the parasitic light ray will not be formed on the spectral analysis means. 
   The optical chromatic means  18  therefore allow a spectral filtering of the sensed light to occur, while ensuring the clearness of the image formed on the spectral analysis means on the entire range of measurement depth useful along the optical axis  12 , and the measurements are more precise. 
   The analysis slot provides a spectral filtering and a spatial filtering of the sensed light. 
   To improve the performance of the apparatus, a telecentric analysis channel that avoids or reduces the shadow zones on the illuminated object  10  is provided via a mask  34  shown in  FIG. 3  and placed at the center of the optical chromatic system  18 , the mask  34  comprising a central zone  36  of intersection of the light rays, provided by a circular axial hole  38  that lets reflected light through, that is, light reflected or backscattered by the object  10 . 
   The illumination light exiting the slot  16  passes on each side of the central section, as shown by reference numeral  40 . 
   Alternatively, and as shown in  FIG. 4 , the sections  40  for the illumination light to pass through can be reduced to circular orifices formed within a disc  34  comprising a central hole  38  for reflected light to pass through. 
   With mask  34  of  FIG. 4 , a vertical telecentric illumination path is provided. 
   As already stated in prior art reference WO99/64816, hereby incorporated by reference, a profile image of the illuminated surface of the object  10  can be formed on spectral analysis means  30 , which comprise a matrix of cameras, such as CCD or analog cameras. A rotational sweep of the illuminated surface of the object  10  can be done using an optical image rotation system, such as a DOVE prism placed in the apparatus according to the invention on the optical axis  12 , in a zone that is intersected by the illumination light and by the sensed light. For example and as shown in  FIG. 5 , the DOVE prism  42  can be placed between two lenses  20  of the chromatic system  18 . The rotation of the DOVE prism  42  around the optical axis  12  produces the result of rotating around the axis the measurement profile on the illuminated surface of the object  12 , the rotation of the profile being twice as good as that of the prism. We can then obtain, without any other displacements of the apparatus according to the invention or of the object  10 , a three dimensional shape of the surface of the object  10  illuminated by a luminous cylindrical axis  12  which has a diameter equal to the length of the measurement profile. 
   To increase the signal to noise ratio of the signals exiting the CCD sensors of the camera matrix, a low pass filter is used to connect the sensor outputs to means for analog to digital conversion that connect the spectral analysis means  30  to signal processing means  32 . Typically, the cut-off frequency of the low pass filter is approximately 1 MHz. 
   In the embodiment of the apparatus shown in  FIG. 1 , the lenses  20  of the chromatic system  18  have a focal length of 60 mm, the lens  24  of the optical magnifying system has a focal length of 150 mm, the transversal magnifying ratio of the system  22  is 3, and the measurement depth on the optical axis  12  is approximately 40 mm. The image formed on the analysis means is clear without any adjustments on the entire measurement depth (in prior art techniques, the range of clearness of the image is approximately ±4 mm from a central position according to a setting). Furthermore, the spectral width of the signal is approximately two times smaller in the apparatus according to the invention than in the prior art techniques, and the precision is uniform on the entire measurement depth (0.01%) instead of having a bell curve-type variation with a progressive degradation when moving away from the central position.