Abstract:
The invention relates to a method characterized in that it comprises—a step (E 1 ) of calibrating a device, whereby—a galvanometric head illuminates along two axes a calibration plate, situated at a depth, in order to illuminate a plurality of determined points of the calibration plate, while a camera observes said calibration plate, a control unit establishing a relationship between, on the one hand, the position of illumination of each of the illuminated points of the calibration plate at the depth, and, on the other hand, the position observed by the camera of the illuminated points; the calibration plate being successively positioned at a plurality of depths during the calibration step so as to allow a plurality of illuminations by the head, of observations by the camera and of relationships to he established by the control unit; the control unit establishes a correspondence relationship,—a step (E 2 ) of determining the three-dimensional shape of the surface that is to be ablated, from the calibration step (E 1 ), by triangulation, and—a step (ES) of ablating the three-dimensional surface whereby the control milt controls the galvanometric head as a function of the determined shape of the surface in order to focus and to direct, along axes that define a plane and to a depth, the beam onto the surface that is to be ablated. The invention also relates to a device for implementing an aforementioned method,

Description:
GENERAL TECHNICAL FIELD 
       [0001]    The present invention relates to a process for ablation of a three-dimensional surface by means of an ablation device, the device comprising:
       a laser source for generating a pulsed laser beam;   a lens for varying the focal point of the laser beam according to a depth z;   a galvanometric head for directing, according to axes defining a plane (X, Y), the beam on the surface to be ablated;   an f-theta lens for displaying the laser beam on a plane surface instead of a spherical surface;   at least one observation camera of the surface to be ablated; and   a control unit connected at least to the galvanometric head, to the lens and to the camera.       
 
         [0008]    The invention also relates to a device for executing the above process. 
       PRIOR ART 
       [0009]    As shown in  FIG. 1 , a solution known for effecting ablation of a surface  1 , for example for the restoration of building facades or for decontamination of nuclear installations, consists of using laser ablation. 
         [0010]    Laser ablation consists of removing a layer of reduced thickness of the material to be removed (dust, paint, or contaminant for example), via the interaction of light, coherent, focussed and originating from a pulsed laser, with this material. 
         [0011]    Rapid heating of the surface of this layer causes vaporisation then ejection of the first strata of the material. 
         [0012]    Known laser ablation devices  2  typically comprise a laser source  3  provided for generating a pulsed laser beam  4  and transport means of this beam to an optical module  6  located downstream of the laser source  3 , and which is provided with a lens  5 , a galvanometric head  7  and an f-theta lens  8  for focussing and directing according to the axes X and Y the pulsed beam onto the surface  1  to be ablated. The device also comprises  2  an outlet  13  and a discharge tube  12  for the evacuation of ablated materials. 
         [0013]    At the output of the lens  5  and of the galvanometric head  7 , the coordinates of the focal point are located on a spherical surface, which may complicate control of the beam. To overcome this difficulty the f-theta lens 8 is arranged on the path of the laser beam so as to locate the focal point on a plane surface. 
         [0014]    This means contributing a sufficient quantity of energy to the layer for attaining the ablation threshold of the latter. Yet this is not the only effect made by a laser beam on the layer. In fact, at the moment of laser impact a shockwave is created and contributes to separating the material of the surface  1  to which it is applied. 
         [0015]      FIG. 2  schematically illustrates the classic form of a beam  4 . 
         [0016]    The flow, or energy density (J/m 2 ), necessary for triggering ablation of the material depends on the nature of the latter, the thickness to be ablated and the composition of the surface. 
         [0017]    Tests show that a flow of 1 to 50 J/cm 2  is required. Consequently, the quantity of energy transmitted depends on the quantity of energy transported by the beam  4  and the section of this beam interacting with the material to be processed. The smallest section of the beam is located at the focussing distance L, a distance L at which the preferred point of ablation is located (see  FIG. 1  also). It is of the order of 50 cm from the lens  8 , for example. 
         [0018]    As shown in  FIG. 2 , the beam  4  has a considerable depth I of field, corresponding to the Rayleigh distance, that is to say around 1 cm, for working on surfaces while freeing one self from planeity defects of the latter. The device is therefore well adapted for two-dimensional surfaces. 
         [0019]    When the aim is to carry out ablation on a three-dimensional surface  1 , as shown in  FIG. 3 , a variable-focus lens  5  must be linked to the laser source  3  to correct the focussing distance L. 
         [0020]    As shown in  FIG. 4  also, dynamically modifying the focussing distance L by means of a control unit  9  enables controlling the ablation distance on a three-dimensional surface  1 . 
         [0021]    The known preceding techniques have disadvantages, however. 
         [0022]    The three-dimensional surface must be previously stored in the control unit  9  so it can be taken into account by the device. 
         [0023]    It is not possible to perform ablation on a surface not known previously and determined previously by additional devices for determining the surface, such as theodolites for example, interferometers, conoscopic sensors, etc. 
         [0024]    Also, additional devices for determining a three-dimensional surface of the prior art, necessary for prior determination of the surface, are expensive and bulky, and do not suit surface-ablation applications. 
         [0025]    Finally, additional devices do not work in the same framework as the ablation device, which generates distortions, since the additional device does not have the same vision of the surface to be ablated as the ablation device, and can generate errors in positioning the laser beam. 
         [0026]    US 2007/173792 discloses a technique for qualification and calibration of a laser system, according to which the laser system is qualified and/or calibrated as a function of deviation in a plane of a laser beam, relative to a preferred direction, observed by an imaging system. 
         [0027]    US 2004/144760 discloses a calibrating technique of laser marking on a face opposite a face observed by an imaging system. 
         [0028]    US 2009/220349 discloses a technique for triangulation of a three-dimensional surface by an imaging system also illuminating the surface, the system being distinct from a surface-ablation device. 
       Presentation of the Invention 
       [0029]    It is proposed to eliminate at least one of these disadvantages according to the invention. 
         [0030]    For this purpose, an ablation process as claimed in Claim  1  is proposed according to the invention. 
         [0031]    The invention is advantageously completed by the characteristics of Claims  2  to  9 , taken singly or in any technically possible combination. 
         [0032]    The invention also relates to a device for performing the above process. 
         [0033]    The invention has numerous advantages. 
         [0034]    The three-dimensional surface does not have to be previously known to be able to be taken into account by the control unit: the invention performs ablation on a surface not known previously. 
         [0035]    Also, the invention does not require the use of additional devices for determining a three-dimensional surface. The invention utilises only those elements of the ablation device which consequently have the same framework and do not generate distortion. 
         [0036]    Because of this, the device is less expensive and less bulky, which suits surface-ablation applications. 
     
    
     
       PRESENTATION OF THE FIGURES 
         [0037]    Other characteristics, aims and advantages of the invention will emerge from the following description which is purely illustrative and non-limiting and which must be considered with respect to the attached diagrams, in which: 
           [0038]      FIG. 1 , already commented on, schematically illustrates a known ablation device; 
           [0039]      FIG. 2 , already commented on, schematically illustrates a known ablation laser beam; 
           [0040]      FIGS. 3 and 4 , already commented on, schematically illustrate ablation of a three-dimensional surface; 
           [0041]      FIG. 5  schematically illustrates the principal steps of a process according to the invention; 
           [0042]      FIG. 6  schematically illustrates a possible example of a device for performing a process according to the invention; 
           [0043]      FIG. 7  schematically illustrates the principle of triangulation; 
           [0044]      FIG. 8  schematically illustrates determination of coordinates sx and sy in a reference plane of a head, by a control unit according to the invention; 
           [0045]      FIG. 9  schematically illustrates determination of coordinates px and py in a reference plane of a matrix of a camera, by a control unit according to the invention; 
           [0046]      FIG. 10  schematically illustrates the principal steps of a calibration step according to the invention; 
           [0047]      FIG. 11  schematically illustrates a succession of plates viewed in plan view, with a transverse plate (in dotted lines) to show admissible tolerances; 
           [0048]      FIG. 12  schematically illustrates correspondence curves of px as a function of sx, for different depths z; 
           [0049]      FIG. 13  schematically illustrates respectively correspondence curves of a, b and c as a function of px; 
           [0050]      FIG. 14  schematically illustrates a relation curve of z as a function of c; 
           [0051]      FIGS. 15 and 16  schematically illustrate the principal steps of a step for determining the three-dimensional form of the surface to be ablated; 
           [0052]      FIG. 17  schematically illustrates the principal alignment steps of the laser beam of the head  7  on the optical axis of the head; 
           [0053]      FIG. 18  schematically illustrates the principal steps for orthogonality of the matrix relative to the plane (xOz); 
           [0054]      FIG. 19  schematically illustrates an interpolation step by the control unit during the step for determining the three-dimensional form of the surface to be ablated; 
           [0055]      FIG. 20  schematically illustrates a device comprising two cameras. 
       
    
    
       [0056]    In all the figures similar elements bear identical reference numerals. 
       DETAILED DESCRIPTION 
       [0057]      FIGS. 5 and 6  schematically illustrate the principal steps of an ablation process of a three-dimensional surface  1 , performed on an ablation device  2 . 
         [0058]    The device  2  conventionally comprises:
       a laser source  3  for generating a pulsed laser beam  4 ;   a lens  5  for adjusting the focal point of the laser beam  4  according to a depth z;   a galvanometric head  7  for directing, according to axes defining a plane (X, Y), the beam  4  on the surface  1  to be ablated;   an f-theta lens  8  which locates the focal point on a plane surface;   at least one observation camera  10  of the surface to be ablated.       
 
         [0064]    The source  3  is for example a low-power pulsed fibre laser, (for example 20 W average power at a rate of 20 kHz, or 1 mJ by impulsion laser), with good beam quality (M 2 =1.5), signifying that the interaction beam/material section is sufficiently small to attain the above mentioned ablation flow. The point of impact has a diameter of 30 to 200 μm. 
         [0065]    The lens  5 , the head  7  and the lens  8  form an optical module referenced by  6  in  FIG. 6 . The lens  8  can also be independent of the module  6 . 
         [0066]    The head  7  conventionally comprises a set of two mirrors with motorised rotation. Each of these mirrors deviates the laser beam along the two axes X and Y with very rapid movement of the beam (up to 7 m/s at a focal distance of 160 mm). 
         [0067]    The f-theta lens  8  is arranged downstream (in the direction of propagation of the laser beam) of the head  7  so as to locate the focal point of the laser beam on a plane surface. This f-theta lens  8  fixes the initial focal point of the laser beam in the absence of any command. 
         [0068]    As shown in  FIG. 7 , the camera  10  is for example a low-definition camera comprising a matrix  100  of CCD type (512×512 with pixels of around 8 μm per side). This pixel size is largely sufficient for the preferred application: uncertainty of 1 pixel causes an error at the depth z of the surface  1  of the order of one hundredth of a mm. For a lens  101  of the camera  10  of 8 mm, the above uncertainty causes an angular error of the order of six one hundredths of a degree. For a lens  101  of 16 mm, the above measured angle error is of the order of three one hundredths of a degree. 
         [0069]    The device  2  also comprises a control unit  9  attached on the one hand to the module  6 , that is at least to the lens  5  and to the galvanometric head  7 , and on the other hand to the camera  10 . The lens  5  and the head  7  are fully controlled by the control unit  9 . To this effect, the control unit  9  comprises all conventional memory, control unit and data-processing means. 
         [0070]      FIG. 5  schematically shows that the process mainly comprises:
       a calibration step E 1  of the device  2 ,   a step E 2  for determining the three-dimensional form of the surface  1  to be ablated, from the calibration step E 1 , by triangulation, and   an ablation step E 3  of the three-dimensional surface, according to which the control unit  9  controls the module  6 , specifically at least the lens  5  and the galvanometric head  7 , as a function of the determined form of the surface, for focussing and directing according to axes defining a plane (X, Y) and according to a depth z the beam  4  on the surface  1  to be ablated.       
 
         [0074]    The following developments show that calibration step E 1  enables triangulation step E 2 . 
         [0075]    As shown in  FIG. 7 , the device  2  utilises the principle of triangulation to measure the depth z of a point P 1  to be processed on the surface  1  to be ablated. 
         [0076]    By way of triangulation in the triangle (PO, P 1 , P 2 ), knowing by construction the distance D between the camera  10  (P 2 ) and the head  7  (PO), and by measuring the two angles beta and theta, the control unit  9  can return to the depth at z of the illuminated point P 1  by means of the relationship known to the expert: 
         [0000]    
       
         
           
             z 
             = 
             
               D 
               
                 
                   1 
                   
                     tan 
                      
                     
                       ( 
                       
                         
                           π 
                           2 
                         
                         - 
                         beta 
                       
                       ) 
                     
                   
                 
                 + 
                 
                   1 
                   
                     tan 
                      
                     
                       ( 
                       
                         
                           π 
                           4 
                         
                         + 
                         theta 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
         [0077]    The matrix  100 /lens  101  assembly enables measurement of the angle theta which the beam image of the illuminated point P 1  makes with the optical axis  102  of the lens  101 . 
         [0078]    Knowing the distance of between the optical centre P 2  (given that the lens  101  can be approximated on a thin lens) of the lens  101  and the centre c′ of the matrix  100  of the camera, as well as the distance (p-c′) separating the illuminated pixel p and the central pixel of the matrix the control unit  9  calculates the angle theta via the following trigonometry formula: 
         [0000]    
       
         
           
             theta 
             = 
             
               
                 arctan 
                  
                 
                   ( 
                   
                     
                       p 
                       - 
                       
                         c 
                         ′ 
                       
                     
                     
                       d 
                       ′ 
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
         [0079]    By using the galvanometric head  7 , the control unit  9  can return to the angle beta which the laser beam object with the optical axis  70  of the galvanometric head  7 , by the following trigonometry formula: 
         [0000]    
       
         
           
             beta 
             = 
             
               arctan 
                
               
                 ( 
                 
                   
                     x 
                      
                     
                         
                     
                      
                     1 
                   
                   
                     z 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 ) 
               
             
           
         
       
     
         [0080]    As shown on  FIG. 7 , in the same plane of depth focussing z, the points of a vertical line (according to the axis Y) have the same triangulation parameters (D, beta and theta) if and only if the two reference points of the triangulation P 0  (head  7 ) and P 2  (camera  10 ) are in the plane containing the optical axis of the head  7  (xOz). In the depth plane z, two points according to the axis Y are differentiated only by their respective coordinates on the axis Y, and P 1  is the orthogonal projection of the point which the control unit wants to measure along the axis Y on the plane (xOz). 
         [0081]    Therefore it is only the position according to the axis X of the sought-after point which will allow the control unit  9  to determine the depth z by triangulation. 
         [0082]    In conclusion, if by construction of the device  2  the triangle (PO, P 1 , P 2 ) is in the plane (xOz) (that is to say the camera  10  is correctly positioned relative to the head  7 ), the variations in the triangulation system caused by variation in position according to the axis Y of the measured point are eliminated. 
         [0083]    Also, as shown by  FIG. 8 , the trajectory of the laser beam  4  passing through the galvanometric head  7  is completely defined by the two coordinates (sx, sy) of the point of intersection P of the beam  4  with a reference plane R of the head  7 . The reference plane R of the head  7  is the plane orthogonal to the optical axis  70  in which all the points have their known coordinates of the head  7  (in a square the size of which is limited by the characteristics of the f-theta optical system  8 ). 
         [0084]    Also, and as shown in  FIG. 9 , px is called the really observed position measured according to the axis X on the matrix  100 , for example CCD, of the camera  10 , and corresponding to the position x where it is located effectively on a plane  11  of focussing z which intercepts a laser beam  4 ′. 
         [0085]    In reference to  FIG. 7 , a measured point is defined by D, beta and theta. The parameters which vary in the measuring system when the point P 1  (orthogonal projection of the point measured on the plane (xOz)) moves are:
       the coordinate sx according to the axis X of the point to be measured in the reference plane R of the galvanometric head  7 ;   the depth z of the focussing plane containing the point P 1 ;   the coordinate px according to the axis X of the image of this point on the matrix  100  of the camera  10 .       
 
         [0089]    For a point of depth z there is one and one only couple (sx, px), and calibration step E 1  consists for the control unit  9  of finding the correspondences which connect each point couple (sx, px) at z. Since the depth z is the magnitude which the control unit  9  aims to find, the control unit  9  must determine the relationship which, from sx, enables illumination of the point x of the object to be measured, and of the measurement px made by the camera  10  of the illuminated point, allows the control unit  9  to return to z. 
         [0090]    So, for a focussing plane of known depth z, the control unit measures a plurality of couples of points (sx, px), and illustrates the curves px(sx) for different focussing planes with several z. 
         [0091]    For this purpose, and as shown in  FIG. 10  in combination with  FIG. 9 , calibration step E 1  comprises a step S 1  according to which the module  6 , more precisely the galvanometric head  7 , illuminates a point  111  of a calibration plate  11 , located at a depth z, by means of a beam  4 ′. The coordinate sx is therefore known to the control unit  9 , by means of the galvanometric head  7 . This depth z must be known with precision better than the Rayleigh distance I, preferably under a tenth of this distance. In the same way, the parallelism error ε of this calibration plate  11  with the plane (xOy) must not exceed a tenth of the Rayleigh distance I, as shown in  FIG. 11 . 
         [0092]    Under these conditions, the plates  11  must be sufficiently plane and sufficiently large to intercept the laser beam  4 ′ on the entire field reachable by the latter, irrespective of the depth z of the plate  11 . The calibration plate  11  is illustrated schematically in  FIGS. 11 and 17 . 
         [0093]    The beam  4 ′ is the laser ablation beam used at reduced power, or an auxiliary alignment beam (for example a HeNe laser) of the source  3 , available, by construction, on the source  3 , and colinear to the source  3 . The power of the beam  4 ′ is reduced as it is not necessary to perform ablation of the calibration plate  11 , but only illumination which can illuminate each point  111 , such that each point  111  may be observed by the camera  10 , as explained hereinbelow. During a step S 2 , the camera  10  observes said calibration plate  11  and the illuminated point  111 . The control unit  9  then determines the coordinate px observed on the matrix  100  of the camera  10 . 
         [0094]    During a step S 2 , according to said axes X, Y the head  7  of the module  6  shifts the beam  4 ′ on the sight  11  for illuminating a plurality of determined points  111  of the calibration plate  11 . 
         [0095]    The determined plurality of points  111  of the calibration plate  11  is distributed according to continuous or dotted lines, and/or continuous or dotted columns. 
         [0096]    When the number of points  111  is sufficient (of the order of 5 for example), the control unit moves to step S 3 , 
         [0097]    During step S 3 , the control unit  9  sets up correspondence between:
       on the one hand the illumination position sx of each of the illuminated points  111  of the calibration plate  11  at the depth z, and   on the other hand the position px observed by the camera  10  of the illuminated points.       
 
         [0100]    For this purpose, the control unit  9  traces the curve during step S 31  included in S 3 : 
         [0000]        px=f ( sx ). 
         [0101]    Examples of these curves are illustrated in  FIG. 12  (curves with crosses). It is noticed that these curves can be approximated by a polynomial of the second degree. 
         [0102]    During a step S 32 , from the curve of the step S 31  the control unit  9  then determines the coefficients a, b and c linking px and sx in the form of a polynomial of the second degree such that: 
         [0000]        px ( sx )=α·sx 2   +b·sx+c    (EQ1).
 
         [0103]    As shown on  FIG. 13 , this approximation is possible since a, b and c are substantially linear functions of sx. 
         [0104]    During a step S 33 , the control unit  9  traces the corresponding curve (in full lines in  FIG. 12 ). 
         [0105]    During a step S 4 , steps S 1 , S 2 , S 2 ′ and S 3  described previously are repeated for another depth z and the calibration plate  11  is therefore placed at another depth z. Repeating steps S 1 , S 2 , S 2 ′ and S 3  allows a plurality of illuminations S 1  by the module  6 , a plurality of observations S 2  by the camera  10  and a plurality of setting up correspondences S 3  by the control unit  9 . 
         [0106]    The control unit  9  therefore has a network of curves as illustrated in  FIG. 12 , each curve corresponding to a given depth z. 
         [0107]    When the number of depths z is sufficient (of the order of 5, for example), the control unit c moves to step S 5 . 
         [0108]    A reminder that to trace one of the curves of  FIG. 12 , five couples of points (sx, px) for example are placed in correspondence, for each depth z of the calibration plate  11 . The beam  4 ′ is for example projected in (coordinates in sx in mm):
       −120, −60, 0, 60 and 120,
 
on each of the depths z of the calibration plate  11 .
       
 
         [0110]    In the same way, measurements are taken for five depths z, specifically for example (coordinates in z en mm, measured by means of a graduated rule, for example):
       −100, −50, 0, 50 and 100.       
 
         [0112]    The coordinate focussing plane at z 0 mm is the reference plane of the galvanometric head  7 . 
         [0113]    During a step S 5 , the control unit  9  determines a relationship between the correspondences. 
         [0114]    For this purpose, during step S 51  included in step S 5  the control unit  9  traces the curve: 
         [0000]        z=g ( c ). 
         [0115]    An example of such a curve is illustrated in  FIG. 14  (curve with crosses). 
         [0116]    It is noted that this curve can be approximated by a polynomial of the second degree. 
         [0117]    During a step S 52 , the control unit determines the parameters α, β and γ linking all the couples z and c determined previously in the form of a polynomial of the second degree such that: 
         [0000]        z ( c )=α· c   3   +β·c+γ   (EQ2).
 
         [0118]    The control unit  9  can therefore trace the corresponding curve (in solid lines in  FIG. 14 ). 
         [0119]    Calibration step E 1  is terminated by means of the calibration plate  11 . 
         [0120]    In reference to  FIG. 5 , the process also comprises a step E 2  for determining the three-dimensional form of the surface  1  to be ablated by triangulation from calibration step E 1 . 
         [0121]    The following developments concern step E 2 . 
         [0122]    In reference to  FIGS. 15 and 16 , for step E 2  for determining the three-dimensional form of the surface  1 , the galvanometric head  7 , that is, the module  6 , illuminates during step S 6  a point e of the surface  1  to be ablated. Illumination is effected the same way as for step E 1 , by a beam  4 ′ of reduced power. 
         [0123]    During step S 6 , the control unit  9  therefore determines the coordinate sxe according to the axis X, by means of the galvanometric head  7 . 
         [0124]    During a step S 7 , the camera  10  observes the surface  1 . 
         [0125]    The control unit  9  then determines the coordinate pxe observed on the matrix  100  of the camera  10 . 
         [0126]    During a step S 8 , the control unit  9  determines the three-dimensional form of the surface  1  by means of the correspondences set up by the control unit  9  during calibration step E 1 . 
         [0127]    So, during step S 81 , included in step  38 , the control unit  9  determines the value ce by means of the values pxe and sxe by the formula: 
         [0000]        ce=pse−α+sxe   3   −b·sxe    (EQ3)
 
         [0000]    by using the coefficients a and b determined by the control unit  9  during calibration step E 1 . 
         [0128]    The coefficient a and b are selected by the control unit  9  as indicated hereinbelow. 
         [0129]    As shown in  FIG. 19 , on completion of step S 4  of step E 1 , the control unit  9  has a network of curves C illustrated in solid lines in  FIG. 19 . Each curve C corresponds to the different values of coefficients a, b and c. 
         [0130]    Yet, during steps S 6  and S 7  of E 2 , the control unit  9  determines sxe and pxe (illustrated by a cross  1000  in  FIG. 19 ). 
         [0131]    To select the good values of a and b in (EQ3), the control unit  9  performs interpolation Δ. 
         [0132]    The control unit  9  calculates and stores the coordinate px of the points belonging to the curves C of the calibration network of  FIG. 19  and having their coordinate sx identical to that of sxe of the sought-after point. Secondly, via a series of successive tests the control unit  9  will determine the curve C 1  of the calibration network located just above the point to be measured. If there is no curve above this point, the control unit  9  takes the curve C 2  located just below. Finally the control unit  9  utilises the relationship (EQ3) corresponding to the determined curve, with the corresponding values of a and b. 
         [0133]    Also, during step S 82 , also included in step S 8 , the control unit  9  determines the depth ze by the formula: 
         [0000]        ze=α·ce   3 +β·ce+γ  (EQ4)
 
         [0000]    by using the parameters α, β and γ determined by the control unit  9  during calibration step E 1 . 
         [0134]    As shown in  FIG. 15 , for each point e 1  illuminated by the beam  4 ′ on the surface  1  the control unit  9  can determine the associated depth ze 1 . 
         [0135]    As indicated by step S 9  of  FIG. 16 , steps S 6 . S 7  and S 8  described previously are repeated for as many points of the surface as wanted, as a function of the preferred precision for determining the surface  1 . The maximal spread according to the axes X and Y between two successive measuring points depends on the Rayleigh distance of the laser beam used and on the maximal variation according to the depth z observed on the surface  1  to be ablated. 
         [0136]      FIG. 15  illustrates only two examples e 1  and e 2 , for ze 1  and ze 2 . The control unit  9  determines the three-dimensional form of the surface  1  by means of the correspondences set up by the control unit  9  during the calibration step. 
         [0137]    Step E 2  for determining the three-dimensional form of the surface  1  to be ablated is therefore terminated. 
         [0138]    In reference to  FIG. 5 , the process also comprises an ablation step E 3  of the three-dimensional surface, according to which the control unit  9  controls the module  6  as a function of the determined form of the surface, for focussing and directing, according to axes defining a plane (X, Y) and according to a depth z, the beam  4  on the surface  1  to be ablated. 
         [0139]    This touches on ablation as known to the expert, and this step is no longer detailed throughout the present description. 
         [0140]    However, for ablation step E 3  the control unit  9  advantageously controls the module  6  for focussing and directing the beam  4  onto all the points of the surface  1  to be ablated, according to successive depths z. In this way, the control unit  9  controls the module  6  a lesser number of times and can therefore gain time for ablation. The control unit  9  controls the module  6  such that all the points to be calibrated located at the same depth z are processed prior to processing the points of the surface  1  located at another depth. 
         [0141]    As shown in  FIG. 20 , the device advantageously comprises two cameras  10 , which gives better knowledge of the surface  1  to be ablated, and if needed giving better ablation if the beam  4  can access the zones observed by the cameras (ablation of zones not observed in the case of a single camera). 
         [0142]    The following developments concern adjustments to be made for better precision for calibration step E 1  of the device  2 . 
         [0143]    The head  7  and the camera  10  must be in the same plane (xOz) (see  FIG. 7 ). The calibration plate  11  must be parallel to the plane (xOy) with maximal admissible tolerance less than the Rayleigh distance I of the laser beam used, preferably better than a tenth of this distance. If, however, such tolerance were exceeded, the control unit  9  could perform correction of the position observed px by the camera  10  of the illuminated points to compensate the effects of distortion. 
         [0144]    The head  7  is parameterised such that it points the laser beam to the centre of its reference plane R to define its optical axis  70 . The beam of the head  7  must be in a plane (xOz). A simple actuator known to the expert shifts the reference calibration plate  11  along the optical axis of the head. As shown in  FIG. 17 , the laser beam  4 ′ of the head  7  is aligned with the optical axis  70  if it intersects the two planes z and z′ at the same point O. 
         [0145]    Once the optical axis  70  is in the plane (xOz), the points P 0  and P 1  being located on the axis  70 , with the parameterising hereinabove (point P 1  is the intersection of the optical axis  70  with the calibration plate  11 ), the two points P 0  and P 1  are therefore located in the same plane (xOz). 
         [0146]    Also, the centre of the matrix  100  and the centre P 2  of the lens  101  are substantially placed on the same straight line as P 1 . The triangle defined by points (P 0 , P 1 , P 2 ) is now placed in the same plane (xOz). 
         [0147]    To ensure that the matrix  100  is orthogonal to the plane (xOz), and as shown in  FIGS. 17 and 18 , the head  7  illuminates a given point of coordinate y 1  according to the axis Y on the calibration plate  11 . By way of image processing, the control unit  9  measures the pixelic coordinate py 1  according to the axis Y of the point image on the matrix  100  of the camera  10 . Still on the calibration plate  11  at the same depth z, the head  7  illuminates a second point of same coordinate at x, but coordinate y 2  opposite the preceding one at y. In the same way, the control unit  9  measures the pixelic coordinate py 2  at y on the matrix  100 . If this coordinate is the opposite to that of the first point relative to the central point of the  100 , then the matrix  100  is orthogonal to the plane (xOz).