Abstract:
An instrument has a light-emitting and light-receiving assembly including an image capture unit and image processing unit; a backscatterer and an opening provided therein; a support for receiving an ophthalmic lens between the assembly and backscatterer, the assembly, support and backscatterer placed so that an incident light beam traverses the lens, strikes the backscatterer, returns and re-traverses the lens to arrive at the capture unit; the light-receiving assembly, the support, backscatterer and opening configured so that the assembly receives light from the beam; and the opening and a drive device for cyclically driving and making the backscatterer perform an identical movement in each cycle, configured so that a fixed zone opposite the backscatterer includes at least one part of which, over the course of a cycle, every point is at times perpendicular to the opening and at times perpendicular to a solid portion part of the backscatterer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention deals with optical instruments for charting at least one characteristic point of an ophthalmic lens. 
       PRIOR ART 
       [0002]    It is known that in order to undertake the edging of an ophthalmic lens, that is to say to trim the edges of the ophthalmic lens to the shape of the frame into which it is to be placed, it is necessary to ascertain the location of one or more characteristic point(s) of the ophthalmic lens, for example its Prism Reference Point (PRP), its reference point for Far Vision (FV) or its reference point for Near Vision (NV). 
         [0003]    To ascertain the location of the characteristic point(s), use is generally made of indications placed on the ophthalmic lens by its manufacturer, for example in the form of ink markings and/or of micro-engravings. 
         [0004]    It is also possible to chart the characteristic point(s) directly on the basis of the optical properties of the ophthalmic lens, for example with a focimeter or with an optical deflectometry instrument employing the Hartmann or Shack-Hartmann method in which light is made to pass through the ophthalmic lens and thereafter through an opaque plate drilled with holes at regular intervals and then the light that has passed through the opaque plate is analysed. 
         [0005]    In general, instruments for identifying and locating the indications placed on the ophthalmic lens by its manufacturer are distinct from the instruments for charting the characteristic point(s) on the basis of the optical properties of the ophthalmic lens. 
         [0006]    European patent application EP 1 997 585 proposes an optical instrument designed to identify and locate markings of an ophthalmic lens or to chart the optical center of an ophthalmic lens on the basis of its optical properties. The instrument described by this document comprises:
       a light emission and reception assembly, comprising an image capture unit and a display unit linked to the image capture unit;   a retroreflector in two parts, one of which, at the center, is fixed, and the other, annular with the same center, is rotary;   a support configured to receive the ophthalmic lens between the light emission and reception assembly and the retroreflector, with the light emission and reception assembly, the support and the retroreflector which are disposed so that an incident light beam issuing from the light emission and reception assembly passes through the ophthalmic lens, encounters the retroreflector, returns towards the ophthalmic lens, passes once again through the ophthalmic lens and returns to the light emission and reception assembly so as to reach the image capture unit, which is configured to provide the display unit with images to identify and to locate the markings liable to be present on the ophthalmic lens; and   a light reception and analysis assembly, whose vertex is the fixed central part of the retroreflector, with the support and the retroreflector which are configured so that the light reception and analysis assembly receives light, of the incident beam, that has passed through the ophthalmic lens, through the aperture formed by the center of the annular part of the retroreflector and through the fixed central part of the retroreflector, the light reception and analysis assembly being configured to chart the optical center of the ophthalmic lens on the basis of the light received.       
 
         [0011]    SUBJECT OF THE INVENTION 
         [0012]    The invention is aimed at providing an optical instrument of the same kind but more efficacious. 
         [0013]    The invention proposes for this purpose an optical instrument for charting at least one characteristic point of an ophthalmic lens, comprising:
       a light emission and reception assembly, comprising an image capture unit and an image utilization unit linked to the image capture unit;   a light return unit;   a support configured to receive said ophthalmic lens between said light emission and reception assembly and said light return unit, with said light emission and reception assembly, said support and said light return unit which are disposed so that an incident light beam issuing from said light emission and reception assembly passes through said ophthalmic lens, encounters said light return unit, returns towards said ophthalmic lens, passes once again through said ophthalmic lens and returns to said light emission and reception assembly so as to reach the image capture unit, which is configured to provide the image utilization unit with images so as to identify and locate predetermined indications liable to be present on the ophthalmic lens so as to give the location of said at least one characteristic point;   an aperture made in said light return unit; and   a light reception and analysis assembly, situated on the side of the light return unit opposite to said support, with said support, said light return unit and said aperture which are configured so that said light reception and analysis assembly receives light, from said incident beam, that has passed through said ophthalmic lens and through said aperture, said light reception and analysis assembly being configured to chart said at least one characteristic point of the ophthalmic lens on the basis of the light received;       
 
         [0019]    characterized in that said aperture and a device for cyclic driving of the light return unit, making the light return unit make one and the same motion at each cycle, are configured so that a fixed zone facing the light return unit comprises at least one part, each site of which, in the course of a cycle, is at times in line with said aperture and at times in line with a solid part of the light return unit. 
         [0020]    Given that the light return unit makes one and the same motion at each cycle, the light return unit returns to the same location at the end of a cycle. 
         [0021]    If the cyclic driving device translates the light return unit to-and-fro, the same motion performed at each cycle is for example an outward-return movement of a certain length along the predetermined direction. 
         [0022]    If the cyclic driving device makes the light return unit rotate about a predetermined center of rotation, the same motion performed at each cycle is for example an outward-return movement of a certain angular extent about the center of rotation (alternating rotation motion) or a complete revolution about the center of rotation (continuous rotation motion). 
         [0023]    It will be noted that in the optical instrument described by document EP 1 997 585, where the light retroreflector is in two parts, one of which, at the center, is fixed, and the other, annular with the same center, performs a continuous rotation motion; any fixed zone facing the retroreflector comprises exclusively, on the one hand, a part which is permanently in line with the aperture formed by the center of the annular part of the retroreflector and, on the other hand, a part which is permanently in line with the annular part of the retroreflector. There is no part each site of which, in the course of a cycle, is at times in line with the aperture and at times in line with a solid part of the retroreflector. The images displayed by the display unit exhibit a central void given by the aperture and this void hinders the visibility of the markings situated at the center of the ophthalmic lens (see in particular FIG. 12 of document EP 1 997 585). 
         [0024]    On the contrary, in the instrument according to the invention, as explained hereinafter, the images taken by the image sensor comprise at least one part whose brightness is intermediate between the brightness given by a solid part of the light return unit and the brightness given by the aperture. 
         [0025]    The at least one part of the image with intermediate brightness is not dark and does not therefore form a void which hinders the identifying and locating of the predetermined indications liable to be present on the ophthalmic lens so as to give the location of said at least one characteristic point. 
         [0026]    The aperture of the light return unit can thus be relatively large, and consequently more light can be transmitted towards the light reception and analysis assembly. 
         [0027]    The optical instrument according to the invention therefore offers both easier identification and easier locating of the predetermined indications liable to be present on the ophthalmic lens and better capabilities for light transmission towards the light reception and analysis assembly, to the benefit of this assembly&#39;s analysis capabilities. 
         [0028]    It will now be explained why the images taken by the image sensor comprise at least one part whose brightness is intermediate between the brightness given by a solid part of the light return unit and the brightness given by the aperture. 
         [0029]    In the fixed zone, the at least one part, each site of which, in the course of a cycle, is at times in line with the aperture and at times in line with a solid part of the light return unit, if this part is situated on the side of the support, at any instant:
       the sub-part which is in line with a solid part of the light return unit is traversed by light heading towards the light emission and reception assembly, and   the sub-part which is in line with the aperture is not traversed by light heading towards the light emission and reception assembly, or in any event is traversed by such light with low intensity.       
 
         [0032]    Consequently, the light passing through the at least one part of the fixed zone in the direction of the light emission and reception assembly has an average intensity intermediate between:
       the intensity of light passing through a part permanently in line with a solid part of the light return unit; and   the intensity of light (low or zero) passing through a part permanently in line with the aperture of the light return unit.       
 
         [0035]    Because the light received by the image sensor originates from the light return unit, and because the image sensor is fixed, the images taken by the image sensor comprise, in view of the average intensity of light passing through the at least one part of the fixed zone, at least one part whose brightness is intermediate between the brightness given by a solid part of the light return unit and the brightness given by the aperture. 
         [0036]    According to advantageous characteristics, said aperture and said device for cyclic driving of the light return unit are configured so that said fixed zone facing the light return unit does not comprise any part which, in the course of a cycle, is permanently in line with said aperture. 
         [0037]    Thus the images taken by the image sensor do not exhibit any void given by the aperture of the light return unit. 
         [0038]    In a first advantageous embodiment, the device for cyclic driving of the light return unit is configured to rotate the light return unit about a predetermined center of rotation; and said aperture is configured so that the center of rotation ( 40 ) of the light return unit is elsewhere than in the aperture. 
         [0039]    According to advantageous characteristics of this first embodiment:
       said light return unit has a circular contour and said aperture is angular-sector-shaped; or   said light return unit has a circular contour and said aperture is spiral-shaped.       
 
         [0042]    In a second advantageous embodiment, the device for cyclic driving of the light return unit is configured to rotate the light return unit about a predetermined center of rotation; and said aperture is configured so that the center of rotation of the light return unit is in the aperture whilst the contour of the aperture is other than axisymmetric about the center of rotation. 
         [0043]    In a third advantageous embodiment, the device for cyclic driving of the light return unit is configured to translate the light return unit to-and-fro along a predetermined direction. 
         [0044]    According to advantageous characteristics of this third embodiment, the light return unit has rectangular contour and said aperture is rectangle-shaped. 
         [0045]    According to advantageous characteristics of implementation of the instrument according to the invention:
       the device for cyclic driving of the light return unit and the image capture unit comprised by said light emission and reception assembly, are linked to a control device configured so that each time interval for which the image capture unit takes an image has a duration for which the light return unit makes an integer number of cycle(s);   the device for cyclic driving of the light return unit and a light source comprised by said light emission and reception assembly, are linked to a control device configured to make said light source emit flashes each having a duration for which the light return unit makes an integer number of cycle(s);   said light reception and analysis assembly comprises a device for measuring deflection of said light, of the incident beam, that has passed through said ophthalmic lens and through said aperture;   said light reception and analysis assembly comprises a matrix of patterns that is illuminated by said light, of the incident beam, that has passed through said ophthalmic lens and through said aperture, another image capture unit illuminated by the light that has passed through the matrix of patterns, and an analysis device for analysing the images captured by said other image capture unit so as to chart said at least one characteristic point of the ophthalmic lens;   said analysis device for analysing the images captured by said other image capture unit is linked to a display unit for displaying said at least one characteristic point of the ophthalmic lens;   said analysis device for analysing the images captured by said other image capture unit is linked to a device for automatic positioning of a centering pin on said ophthalmic lens; and/or   said light reception and analysis assembly furthermore comprises a light source configured to emit light which passes through said aperture and is received by said light emission and reception assembly; and said light reception and analysis assembly comprises a mask exhibiting holes forming at least one predetermined pattern, holes through which said light emitted by said light source comprised by said light reception and analysis assembly passes.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The disclosure of the invention will now continue with the detailed description, given by way of nonlimiting illustration, of exemplary embodiments, with reference to the appended drawings. In the latter: 
           [0054]      FIG. 1  is a very schematic view showing a light emission and reception assembly as well as a support comprised by a first embodiment of an optical instrument useful for the comprehension of the invention, with an ophthalmic lens exhibiting micro-engravings on one of its faces, this ophthalmic lens being received on the support in a position where the face on which the micro-engravings are present is facing the light emission and reception assembly; 
           [0055]      FIG. 2  is a very schematic view showing a backscatterer that this optical instrument includes and the ophthalmic lens received on the support; 
           [0056]      FIG. 3  is a very schematic view showing the light emission and reception assembly, the ophthalmic lens and the backscatterer and the path of the light from the backscatterer to the light emission and reception assembly; 
           [0057]      FIG. 4  is a schematic view of the first embodiment of the instrument useful for the comprehension of the invention, showing in a detailed manner the light emission and reception assembly; 
           [0058]      FIG. 5  is a schematic view illustrating certain geometric characteristics of the point light source and of the collimation unit; 
           [0059]      FIG. 6  is a block diagram showing elements linked to the image capture unit to allow the automatic positioning of a centering pin on the ophthalmic lens; 
           [0060]      FIG. 7  is a plan view of the backscatterer; 
           [0061]      FIG. 8  is a view similar to  FIG. 7 , but for a second embodiment of the optical instrument according to the invention; 
           [0062]      FIG. 9  is a schematic view similar to the bottom of  FIG. 4  but for the second embodiment of the optical instrument according to the invention; 
           [0063]      FIG. 10  is a view similar to  FIG. 8  but for a first variant of the second embodiment of the optical instrument according to the invention; 
           [0064]      FIG. 11  is a view similar to  FIG. 9  but more detailed and corresponding to the first variant illustrated in  FIG. 10 ; 
           [0065]      FIG. 12  is a view similar to  FIGS. 8 and 10  but for a second variant of the second embodiment of the optical instrument according to the invention; 
           [0066]      FIG. 13  is a block diagram showing elements linked to the image capture units of the second embodiment of the optical instrument according to the invention; 
           [0067]      FIG. 14  is a block diagram showing elements linked to the light source in a version where it emits flashes; 
           [0068]      FIG. 15  is a view similar to  FIGS. 8, 10 and 12  but for a third variant of the second embodiment of the optical instrument; 
           [0069]      FIG. 16  shows the various parts comprised by a fixed zone facing the backscatterer illustrated in  FIG. 15 ; 
           [0070]      FIG. 17  is a view similar to  FIGS. 8, 10, 12 and 15 , but for a fourth variant of the second embodiment of the optical instrument; 
           [0071]      FIG. 18  is a view similar to  FIG. 17  but also showing another position taken by the backscatterer in the course of its rotation; 
           [0072]      FIG. 19  is a view similar to  FIG. 16  but for the backscatterer of  FIG. 17 ; 
           [0073]      FIG. 20  is a view similar to  FIG. 7  but for a third embodiment of the optical instrument, which is according to the invention; 
           [0074]      FIG. 21  is a view similar to  FIG. 20  but simultaneously showing the two extreme positions taken by the backscatterer in the course of a to-and-fro movement; 
           [0075]      FIG. 22  is a view similar to  FIGS. 16 and 19 , but for the backscatterer of  FIG. 20 ; 
           [0076]      FIG. 23  is a schematic sectional view in elevation, showing the backscatterer illustrated in  FIGS. 10 and 11  as well as a light source configured to emit light destined for the light reception and emission assembly and a mask exhibiting holes through which this light passes; 
           [0077]      FIG. 24  is a plan view showing a variant of the mask illustrated in  FIG. 23 ; and 
           [0078]      FIG. 25  is a plan view showing a variant of the organization of the holes of the matrix of patterns of the light reception and analysis assembly. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0079]    The optical instrument  10  illustrated in  FIGS. 1 to 7  conforms to a first embodiment of an optical instrument useful for the comprehension of the invention. 
         [0080]    This optical instrument  10  comprises a light emission and reception assembly  11 , a backscatterer  12  and a support  13  ( FIG. 1 ) which is configured to receive an ophthalmic lens  14  between the assembly  11  and the backscatterer  12  in a position where its face  15  on which micro-engravings  16  are present is facing the light emission and reception assembly  11 . 
         [0081]    The micro-engravings  16  are of small local variations of thickness of the lens or of small local variations of the optical index. 
         [0082]    Diverse techniques make it possible to render micro-engravings  16  present on a face of an ophthalmic lens: slight thickenings when the micro-engravings are molded with the ophthalmic lens, slight hollows generated by laser or alterations of the material modifying its optical index locally. 
         [0083]    When a coherent light beam encounters a micro-engraving  16 , its phase is locally modified by the micro-engraving. 
         [0084]    This local variation of the phase causes the light beam to diffract. 
         [0085]    In the case of a spatially coherent light beam, the diffraction is rendered visible by a local modification of the intensity (Fresnel diffraction). 
         [0086]    The micro-engravings  16  serve to chart characteristic points of the ophthalmic lens  14 , for example its Prism Reference Point (PRP). 
         [0087]    The light emission and reception assembly  11  emits a beam  20  ( FIG. 1 ) of spatially coherent collimated light. 
         [0088]    As shown on the left of  FIG. 2 , when the beam  20  encounters a micro-engraving  16 , the light is diffracted locally. 
         [0089]    The projection of the light that has passed through the lens  14  on the backscatterer  12  exhibits intensity variations due to the diffraction of the light caused by the micro-engravings  16 . 
         [0090]    The beam  20  image projected on the backscatterer  12  therefore exhibits variations of intensity of similar forms to those of the micro-engravings  16 . 
         [0091]    As shown on the right of  FIG. 2 , the backscatterer  12  returns the light which has reached it in the same direction with a slight scatter. 
         [0092]    The light beam  21  returned by the backscatterer is spatially incoherent because of this slight scattering. 
         [0093]    As may be seen in  FIG. 3 , the light beam  21  thus emitted by the backscatterer  12  passes through the ophthalmic lens  14  without or almost without being modified by the micro-engravings  16 , and reaches the light emission and reception assembly  11 . 
         [0094]    In the preceding description, no mention was made of the prismatic deviation that the incident beam  20  undergoes when it passes through the lens and that the beam  21  emitted by the backscatterer  12  undergoes when it once again passes through the lens. 
         [0095]    This is because these two successive prismatic deviations compensate one another perfectly. 
         [0096]    Thus, whereas the image of the micro-engravings  16  formed on the backscatterer  12  is deformed by the prismatic deviation, the image of the backscatterer  12  seen through the ophthalmic lens  14  by the assembly  11  is deformed in exactly the inverse way. 
         [0097]    Therefore, the image of the backscatterer  12  seen by the assembly  11  contains an exact representation of the micro-engravings  16 . 
         [0098]    The fact that the optical beam passing through the ophthalmic lens  14  is collimated and spatially coherent makes it possible to obtain a very contrasted projection of the micro-engravings  16  on the backscatterer  12 . 
         [0099]    The support  13  is positioned so that the projection of the micro-engravings  16  on the backscatterer  12  is easy to observe: the distance between the ophthalmic lens  14  and the backscatterer  12  is sufficiently small for the projection on the backscatterer  12  to remain sharp (if this distance is too large the image would be blurred because of the diffraction due to the micro-engravings  16 ) and sufficiently large for the projection of the micro-engravings  16  to be large enough to be observed. 
         [0100]    The light emission and reception assembly  11  will now be described in detail with reference to  FIG. 4 . 
         [0101]    In the illustrated example, the assembly  11  includes: an extended light source  25 ; a diaphragm  26  containing a pinhole  27 ; a semi-reflecting plate  28 ; a return mirror  29 ; a collimating lens  30 ; a video camera  31 ; and a display unit  32  linked to the video camera  31 . 
         [0102]    The diaphragm  26  and the objective  35  of the video camera  31  are located on either side of the semi-reflecting plate  28 , in conjugate places, i.e. they are seen from the point of view of the return mirror  29  as being located in one and the same place. 
         [0103]    This place is chosen to be the focus of the collimating lens  30 . Thus, each of the two conjugate places corresponds to the focus of the collimating lens  30 . 
         [0104]    The pinhole  27  of the diaphragm  26  may therefore be considered to be placed at a first focus of the collimating lens  30  and the objective  35  of the video camera  31  may be considered to be placed at a second focus of the collimating lens  30 . 
         [0105]    The extended light source  25  is placed as close as possible to the diaphragm  26 , for example less than 0.5 mm away, so that the pinhole  27  of the diaphragm  26  forms a point light source. 
         [0106]    The light emitted by this point source reflects from the semi-reflecting plate  28  then from the return mirror  29  and passes through the collimating lens  30 . 
         [0107]    Because the pinhole  27  is located at the focus of the collimating lens  30 , the beam  20  emerging from the lens  30  is collimated, i.e. all its rays are oriented parallel to one another. 
         [0108]      FIG. 5  schematically shows the focal length F of the collimating lens  30  and the diameter D of the pinhole  27 , i.e. the diameter of the point light source formed by the extended light source  25  and by the diaphragm  26 . 
         [0109]    In practice, the point light source may be formed by a commercially available component combining an LED forming the extended light source  25  and a diaphragm  26 . 
         [0110]    For the light flux to be sufficient, the diameter D is at least 10 μm or even at least 20 μm. The maximum values provided for the diameter D are described below. 
         [0111]    After it has passed through the ophthalmic lens  14 , the light of the beam  21  issuing from the backscatterer  12  passes through the collimating lens  30 , is reflected by the return mirror  29 , passes through the semi-reflecting plate  28  and reaches the objective  35  of the video camera  31 . 
         [0112]    This objective is focused so that the sensor  36  of the video camera  31  takes images of the backscatterer  12 . 
         [0113]    These images are displayed on the display unit  32 , which is linked to the video camera  31 . 
         [0114]    Thus, an observer looking at the display unit  32  sees images allowing the micro-engravings  16  present on the face  15  of the ophthalmic lens  14  to be identified and located. 
         [0115]    This makes it possible for an operator to determine where the micro-engravings  16  are on the ophthalmic lens  14  and therefore where the optical center and the axis of the spherical power of this ophthalmic lens are located, these parameters for example being useful to the user when he wants to edge the ophthalmic lens  14 , i.e. trim the edges of the ophthalmic lens  14  to the shape of the frame in which it is to be fitted. 
         [0116]    In practice, the centering pin used to fix the ophthalmic lens  15  to the edging machine is placed in the instrument  10  by virtue of the micro-engravings  16  thus viewed. The centering pin may be placed manually by the operator. 
         [0117]      FIG. 6  shows elements allowing the centering pin to be placed automatically. 
         [0118]    In addition to being linked to the display unit  32 , the video camera  31  is linked to an image analysis device  37  that is capable of identifying and locating the micro-engravings  16 . A device  38  for automatically positioning a centering pin is linked to the device  37 , which delivers to the device  38  the coordinates of that place on the face  15  of the ophthalmic lens  14  on which the centering pin must be placed. 
         [0119]    The device  38  for automatically positioning a centering pin is for example such as described in French patent application 2 825 466, which corresponds to U.S. Pat. No. 6,888,626. 
         [0120]    Generally, it is advantageous for the extended light source  25  and therefore the point light source that it forms with the diaphragm  26 , to have a wavelength of between 700 nm and 1000 nm, i.e. in the infrared near the spectrum of visible light. 
         [0121]    Thus, the attenuation of the light on its path between the pinhole  27  and the sensor  36  of the video camera  31  is moderated, including when the ophthalmic lens  14  is tinted. 
         [0122]    Of course, the sensor  36  of the video camera  31  is chosen to be sensitive in this wavelength range. 
         [0123]    Generally, the instrument  10  is here configured for micro-engravings  16  the width of which is between 10 and 80 μm. 
         [0124]    It is important that image of the micro-engravings  16  that is projected onto the backscatterer  12  be contrasted. Specifically, this makes it possible to use a video camera  31  with a relatively sizeable aperture of the objective  35 . Such an aperture limits the loss of light flux en route to the sensor  36  of the video camera  31 . 
         [0125]    Thus, enough light flux is received by the sensor  36  of the video camera  31  to allow fluid observation of the micro-engravings  16 , i.e. the user may move the ophthalmic lens  14  over the support  13  with the display unit  32  which is refreshed in real time (in practice, at a frequency at least equal to 15 Hz). 
         [0126]    It has been observed that with the aforementioned range of wavelengths, a pinhole  27  with a diameter D less than or equal to 1 fiftieth of the focal length F of the collimating lens  30  (distance between the lens  30  and its focus) makes it possible to ensure that the image of the micro-engravings  16  projected onto the backscatterer  12  is contrasted. 
         [0127]    It is believed that this results from a good match between the spatial coherence width of the beam  20  and the width of the micro-engravings  16 . 
         [0128]    Generally, given the aforementioned lower limit of 10 μm for the width of the micro-engravings, it is advantageous for the spatial coherence width of the beam  20  to be larger than or equal to 5 times the width of the micro-engravings  16 . 
         [0129]    By definition, the spatial coherence width is equal to Fλ/D, where λ is the wavelength of the light flux. 
         [0130]    If the width of the micro-engravings is denoted a, the following relation is obtained: D≦Fλ/5a 
         [0131]    For example, if:
       the width of the micro-engravings is 50 μm;   the wavelength of the light flux is 850 nm; and   the focal length of the collimating lens  30  is 200 mm, then the diameter D of the pinhole  27  is less than or equal to 680 μm.       
 
         [0135]    It has been observed that excellent results are obtained for micro-engravings  16  having a width a of between 30 μm and 60 μm when the wavelength of the light flux λ is between 800 and 900 μm and the focal length F is between 150 and 300 mm. 
         [0136]    As indicated above, with a light source  25  emitting at a wavelength of between 700 nm and 1000 nm, and a light-source diameter D of less than or equal to a fiftieth of the focal length F of the collimating lens  30 , the image of the micro-engravings  16  projected onto the backscatterer  12  is well contrasted. 
         [0137]    Depending on the circumstances, the diameter D is selected to be less than or equal to a hundredth, a hundred and fiftieth, a two hundredth or two hundred and fiftieths of the focal length F. 
         [0138]    It has also been observed that parameters favorable for rendering the light flux received by the sensor  36  of the video camera  31  sufficient are:
       a high intensity of the light flux emitted through the pinhole  27  of the diaphragm  26 ; and/or   an aperture of the objective  35  in accordance with the scattering angle of the light scattered by the backscatterer  12  (see  FIG. 3  and the right-hand part of  FIG. 2 ).       
 
         [0141]    As shown in  FIG. 7  by the arrow  39 , to average graininess, the backscatterer  12  is rotated during use of the instrument  10 . 
         [0142]    It is also possible to place calibrating patterns on the backscatterer  12  (it will be recalled here that the objective  35  of the video camera  31  is focused on the backscatterer  12  and that it is therefore the backscatterer  12  that is seen, moreover whether the ophthalmic lens  14  is present or not), such calibrating patterns no longer being perceptible when the backscatterer is rapidly rotating. 
         [0143]    It will be observed that the micro-engravings such as  16  are more precise than the markings carried by the ophthalmic lenses originating from their manufacturers; and that the instrument useful for the comprehension of the invention makes it possible to use the micro-engravings directly, to the benefit of precision. 
         [0144]    Such precision, for example for the centering, is important since lenses are becoming ever more personalized. 
         [0145]    It will be observed that the instrument  10  can be easily integrated into an already existing instrument, for example a tracer/blocker or a grinder. 
         [0146]    It will further be observed that a possible use of the instrument useful for the comprehension of the invention is to measure a possible shift between a reference given by the micro-engravings and other marks present on the lens for example markings with which the lens is delivered; and/or that another possible use of the instrument useful for the comprehension of the invention is to make the markings very precisely with respect to the micro-engravings aided by tracing with the instrument  10 . 
         [0147]    In the embodiment of the instrument  10  which has just been described, the backscatterer  12  is made up of a solid rotary platen, that is to say one not exhibiting any aperture. 
         [0148]    A second embodiment of the optical instrument according to the invention will now be described in support of  FIGS. 8 to 13 , in which the backscatterer  12  is replaced with a backscatterer in which an aperture is made while a light reception and analysis assembly is disposed under this backscatterer, that is to say on the side of the backscatterer which is opposite to the support  13  designed to receive the ophthalmic lens  14 . 
         [0149]    The support  13 , the aperture in the backscatterer and the light reception and analysis assembly are configured so that the latter receives light from the beam  20  of collimated light after said light has passed through the ophthalmic lens  14  and through the aperture in the backscatterer. 
         [0150]    The received light is analyzed to determine certain optical data of the ophthalmic lens  14 , in particular the optical center and axis of cylindrical power, which data are of use if the ophthalmic lens  14  is to be edged, or even to determine other data such as the spherical power and cylindrical power of the lens. 
         [0151]    A first version of the optical instrument according to the second embodiment is illustrated in  FIGS. 8 and 9 , a second version in  FIGS. 10 and 11  and a third version in  FIG. 12 . 
         [0152]    Elements common to the three versions are illustrated in  FIGS. 11, 13 and 14 . 
         [0153]    In each of the first, second and third versions, the backscatterer  12  is replaced by a backscatterer  112 . 
         [0154]    Just like the backscatterer  12 , the backscatterer  112  has a circular contour centered on the center of rotation  40 , but it has an aperture  41 , an aperture  141  and an aperture  241 , respectively. 
         [0155]    In the first version illustrated in  FIGS. 8 and 9 , the aperture  41  takes the form of an angular sector. It extends from the center of rotation  40  to the periphery of the backscatterer  112 . 
         [0156]    As may be seen in  FIG. 8 , the light reception and analysis assembly  42  is placed centered on the center of rotation  40 . 
         [0157]    As may be seen in  FIG. 9 , the support  13  for the ophthalmic lens  14  is placed centered with respect to the center of rotation  40 . 
         [0158]    After light from the beam  20  of collimated light has passed through the ophthalmic lens  14  and through the aperture  41 , it reaches the light reception and analysis assembly  42 . 
         [0159]    As will be understood in light of  FIG. 8 , at any instant a portion of the light reception and analysis assembly  42  is in line with the aperture  41 . 
         [0160]    Thus, at any instant, a portion of the light reception and analysis assembly  42  receives light that has passed through the aperture  41 . 
         [0161]    Because of the rotary movement of the backscatterer  112 , each portion of the light reception and analysis assembly  42  is, at a certain moment, in line with the aperture  41  when the backscatterer  112  makes one revolution. 
         [0162]    Therefore, at each revolution of the backscatterer  112 , the entirety of the light reception and analysis assembly  42  receives light that has passed through the ophthalmic lens  14  and through the aperture  41 . 
         [0163]    Thus, at each revolution of the backscatterer  112 , the light reception and analysis assembly  42  receives light that has passed through the entirety of the corresponding zone of the ophthalmic lens  14 . 
         [0164]    By analyzing the light received during at least one revolution of the backscatterer  112 , the light reception and analysis assembly  42  is able to determine optical data of the lens  14 , and more precisely of the zone through which the light passed before reaching the light reception and analysis assembly  42 . 
         [0165]    Because the center of rotation  40  is not located in the aperture  41  (the center  40  is here on the perimeter of the aperture  41 ), no part of the aperture  41  is centered on the center of rotation  40 . 
         [0166]    Therefore, any stationary point facing the backscatterer  112  between its center of rotation  41  and its periphery is in line, during part of each revolution, with a portion of the backscatterer  112  not forming part of the aperture  41 , i.e. a solid part. 
         [0167]    Here, where the aperture  41  takes the form of an angular sector having its vertex at the center of rotation  40  and an angular aperture of 30°, each stationary point is in line with the aperture  41  during 1/12 ( 30/360) of a revolution and therefore in line with a portion of the backscatterer  112  not forming part of the aperture  41  during 11/12 of a revolution. 
         [0168]    The images of the backscatterer  112  displayed by the display unit  32  therefore do not contain a void corresponding to the aperture  41  and therefore allow the micro-engravings  16  to be seen in their entirety. 
         [0169]    Measure that make it possible to prevent the aperture  41  from appearing at all on the display unit  32  will be described below with reference to  FIG. 11  and with reference to  FIG. 14 . 
         [0170]    The second version of the optical instrument according to the second embodiment illustrated in  FIGS. 9 and 10  is a variant of the first version in which the aperture  41  is replaced by an aperture  141  that also takes the form of an angular sector, but the vertex of which is a distance away from the center of rotation  40 ; furthermore, the light reception and analysis assembly  42  is off-center with respect to the center of rotation  40 , as is the support  13  provided to receive the ophthalmic lens  14 ; the support  13  and the light reception and analysis assembly  42  are centered relative to one another. 
         [0171]    After light from the beam  20  of collimated light has passed through the ophthalmic lens  14  and through the aperture  141 , it reaches the light reception and analysis assembly  42 . 
         [0172]    As will be understood in light of  FIG. 10 , at any instant a portion of the light reception and analysis assembly  42  is in line with the aperture  141 . 
         [0173]    Thus, at any instant, a portion of the light reception and analysis assembly  42  receives light that has passed through the aperture  141 . 
         [0174]    Because of the rotary movement of the backscatterer  112 , each portion of the light reception and analysis assembly  42  is, at a certain moment, in line with the aperture  141  when the backscatterer  112  makes one revolution. 
         [0175]    Therefore, at each revolution of the backscatterer  112 , the entirety of the light reception and analysis assembly  42  receives light that has passed through the ophthalmic lens  14  and through the aperture  141 . 
         [0176]    Thus, at each revolution of the backscatterer  112 , the light reception and analysis assembly  42  receives light that has passed through the entirety of the corresponding zone of the ophthalmic lens  14 . 
         [0177]    Because the center of rotation  40  is not located in the aperture  141 , no part of the aperture  141  is centered on the center of rotation  40 . 
         [0178]    For the same reasons as those explained above for the aperture  41 , the images of the backscatterer  112  containing the aperture  141  and displayed by the display unit  32  do not contain a void corresponding to the aperture  141  and therefore allow the micro-engravings  16  of the ophthalmic lens  14  to be seen in their entirety. 
         [0179]    The third version of the optical instrument according to the second embodiment, which version is illustrated in  FIG. 12 , is a variant of the first version in which the aperture  41  is replaced by an aperture  241  that is spiral shaped; furthermore, the light reception and analysis assembly  42  is off-center with respect to the center of rotation  40 , as is the support  13  provided to receive the ophthalmic lens  14 ; the support  13  and the light reception and analysis assembly  42  are centered relative to one another. 
         [0180]    After light from the beam  20  of collimated light has passed through the ophthalmic lens  14  and through the aperture  241 , it reaches the light reception and analysis assembly  42 . 
         [0181]    As will be understood in view of  FIG. 12 , at any instant a portion of the light reception and analysis assembly  42  is in line with the aperture  241 . 
         [0182]    Thus, at any instant, a portion of the light reception and analysis assembly  42  receives light that has passed through the aperture  241 . 
         [0183]    Because of the rotary movement of the backscatterer  112 , each portion of the light reception and analysis assembly  42  is, at a certain moment, in line with the aperture  241  when the backscatterer  112  makes one revolution. 
         [0184]    Therefore, at each revolution of the backscatterer  112 , the entirety of the light reception and analysis assembly  42  receives light that has passed through the ophthalmic lens  14  and through the aperture  241 . 
         [0185]    Thus, at each revolution of the backscatterer  112 , the light reception and analysis assembly  42  receives light that has passed through the entirety of the corresponding zone of the ophthalmic lens  14 . 
         [0186]    Because the center of rotation  40  is not located in the aperture  241 , no part of the aperture  141  is centered on the center of rotation  40 . 
         [0187]    For the same reasons as those explained above for the aperture  41 , the images of the backscatterer  112  containing the aperture  241  and displayed by the display unit  32  do not contain a void corresponding to the aperture  241  and therefore allow the micro-engravings  16  of the ophthalmic lens  14  to be seen in their entirety. 
         [0188]    As may be seen in  FIGS. 9 and 11 , the light reception and analysis assembly  42  includes a Hartmann matrix  45  and an image sensor  46  placed at a predetermined distance e ( FIG. 11 ) from the Hartmann matrix  45 . 
         [0189]    The Hartmann matrix  45  is an opaque plate drilled with holes at regular intervals, so that the image captured by the sensor  46  is a matrix of luminous dots each corresponding to one hole of the matrix  45 , the locations of these luminous dots depend on the optical characteristics of the ophthalmic lens  14 . 
         [0190]    Analysis of the image captured by the image sensor  46  therefore makes it possible, for example via the Hartmann or Shack-Hartmann deflectometry method, to determine optical characteristics of the ophthalmic lens  14 , especially its optical center, the axis of its cylindrical power, its spherical power, its cylindrical power and other characteristics. For more details, reference may be made for example to French patent application, 2 825 466 which corresponds to the U.S. Pat. No. 6,888,626, or to the international patent application WO 95/34800. 
         [0191]    As may be seen in  FIG. 11 , the image sensor  46  is linked to an image analysis device  47  that is capable of determining the optical characteristics of the ophthalmic lens  14 . 
         [0192]    The image analysis device  47  is linked to the display unit  32  that may thus display the optical characteristics of the ophthalmic lens  14  as determined by the image analysis device  47 . 
         [0193]    Thus, as shown in  FIG. 13 , in the second embodiment of the optical instrument according to the invention, the device  38  for positioning a centering pin is linked not only to the image analysis device  37 , but also to the image analysis device  47 , this meaning that the device  38  for automatically positioning a centering pin may ascertain the coordinates of the place on the face  15  of the ophthalmic lens  14  on which the centering pin must be placed not only by way of the micro-engravings  16  but also by way of the optical characteristics of the lens  14 . 
         [0194]      FIG. 11  schematically shows the device  50  for driving the backscatterer  122  to rotate, which device is configured to make the backscatterer  122  turn about the center of rotation  40 . 
         [0195]    With reference to  FIG. 11 , it will now be explained how it is possible to prevent a void corresponding to an aperture such as  41 ,  141  or  241  from appearing on the display unit  32 . 
         [0196]    The video camera  31  and the device  50  for driving the backscatterer  112  in rotation are linked to a control device  51  configured so that each time interval during which the video camera  31  takes an image corresponds to an integer number of revolution(s) of the backscatterer  112 . 
         [0197]    In the illustrated example, each image is taken during 1/20 s and the backscatterer  112  makes one revolution in 1/20 s, i.e. a speed of 1200 revolutions/min. 
         [0198]    As explained above, because the center of rotation  40  is not located in an aperture such as  41 ,  141  or  241 , no part of these apertures is centered on the center of rotation  40 . 
         [0199]    Therefore, any stationary point facing the backscatterer  112  between its center of rotation  40  and its periphery is in line, during part of each revolution, with a portion of the backscatterer  112  not forming part of the aperture (solid portion). 
         [0200]    Likewise, each pixel of the sensor  36  of the video camera  31  receives, during part of each revolution, light from a portion of the backscatterer  112  not forming part of the aperture. 
         [0201]    As each image is taken during one or more entire revolutions of the backscatterer  112 , each pixel receives the same proportion of light coming from the portion of the backscatterer  112  not forming part of the aperture such as  41 ,  141  or  241 . 
         [0202]    Each image taken by the video camera  31  therefore does not include a void corresponding to an aperture such as  41 ,  141  or  241 . 
         [0203]    The operator looking at the display unit  32  perceives the same images as with the solid backscatterer  12 , except that all or part of the backscatterer  112  appears less bright. 
         [0204]    For example, for the backscatterer  112  having the aperture  41  shown in  FIGS. 8 and 9 , the perceived brightness for the backscatterer  112  in its entirety is 11/12 of the perceived brightness for the backscatterer  12 , all other things being equal. 
         [0205]    The backscatterer  112  having the aperture  41  shown in  FIGS. 8 and 9  is thus perceived as an entirely solid backscatterer (just like the backscatterer  12 ) while still letting light pass to the light reception and analysis unit  42 . 
         [0206]    For the backscatterer  112  having the aperture  141  shown in  FIGS. 10 and 11 , a central portion, the radius of which corresponds to the distance between the center of rotation  40  and the vertex of the aperture  141 , is perceived with the same brightness as the backscatterer  12  whereas the rest of the backscatterer  112  is perceived to be less bright. 
         [0207]    In all cases, the image of the backscatterer  112  allows the micro-engravings  16  to be seen in their entirety. 
         [0208]    The implementation of the control device  51  for example involves a common time-base for controlling the video camera  31  and the rotation-driving device  50 . 
         [0209]    In the example illustrated in  FIG. 11 , provision is also made for a detector  52  for ascertaining the position of the backscatterer  112  in order to allow its speed of rotation to be automatically controlled. 
         [0210]    As a variant, as illustrated in  FIG. 14 , rather than providing a control device such as  51  to control the video camera  31  and the device  50  for driving the backscatterer  12  in rotation, provision is made for a device  55  linked to the device  50  for driving the backscatterer  112  in rotation and to the extended light source  25  of the point light source formed by the extended light source  25  and by the diaphragm  26 . 
         [0211]    The control device  55  is configured to make the light source  25  emit flashes, each time interval during which the light source  25  emits a flash corresponding to an integer number of revolution(s) of the backscatterer  112 . 
         [0212]    For example, if the backscatterer  112  turns at a speed of 1200 revolutions/min, and therefore makes one revolution every 1/20 s, each flash has a duration of 1/20 s or of a multiple of 1/20 s. 
         [0213]    Since no light is emitted between the flashes, only the moments when a flash is emitted appear on the display unit  32 . 
         [0214]    As each moment in which an image appears on the display unit  32  lasts one or more entire revolutions of the backscatterer  112 , each displayed image contains no void corresponding to an aperture such as  41 ,  141  or  241 . 
         [0215]    The operator looking at the display unit  32  sees the same images as with the control device such as  51 , except that the images have a brightness that oscillates in tempo with the flashes emitted by the light source  25 . 
         [0216]    The implementation of the control device  55  for example involves a common time-base for controlling the flashes of the light source  25  and the rotation-driving device  50 . 
         [0217]    Generally, a suitable range of rotation for the backscatterer  12  or the backscatterer  112  is between 600 and 5000 revolutions/min. 
         [0218]    The fourth version, illustrated in  FIG. 15 , of the optical instrument according to the second embodiment is a variant of the second version ( FIGS. 10 and 11 ) in which the aperture  141  is replaced with an aperture  341  not going as far as the periphery of the backscatterer  112 . 
         [0219]    More precisely, the aperture  341  is in the shape of an angular sector whose vertex is offset from the center of rotation  40  and whose circular-arc-shaped side is offset from the periphery of the backscatterer  112 . 
         [0220]      FIG. 16  shows the various parts comprised by a fixed zone facing the backscatterer  112 . 
         [0221]    This entails an annular part  60 , a disc-shaped part  61  which is surrounded by the part  61  and an annular part  62  which surrounds the part  60 . 
         [0222]    Each site of the part  60  is situated in the course of a revolution of the backscatterer  112 , at times in line with the aperture  341  and at times in line with a solid part of the backscatterer  112 . 
         [0223]    Each site of the part  61  and each site of the part  62  is permanently in line with a solid part of the backscatterer  112 . 
         [0224]    The circle-shaped contour of the part  61  has a radius corresponding to the offset between the center of rotation  40  and the vertex of the aperture  341 . 
         [0225]    The inner contour of the part  62  has a radius which is that of the circular-arc-shaped side of the aperture  341 . The outer contour of the part  62  has a radius which is that of the backscatterer  112 . 
         [0226]    The inner contour of the part  60  corresponds to the contour of the part  61 . The outer contour of the part  60  corresponds to the inner contour of the part  62 . 
         [0227]    If the fixed zone facing the backscatterer  112  of  FIG. 15  is on the side of the light reception and analysis assembly  42 :
       neither the part  61  nor the part  62  are traversed by light heading towards the light reception and analysis assembly  42 ; and   the part  60  comprises at any instant a sub-part which is in line with a solid part of the backscatterer  112  and a sub-part which is in line with the aperture  341 .       
 
         [0230]    The sub-part which is in line with a solid part of the backscatterer  112  is not traversed by light heading towards the light reception and analysis assembly  42 . 
         [0231]    The sub-part which is in line with the aperture  341  is traversed by light heading towards the light reception and analysis assembly  42 . 
         [0232]    If the light reception and analysis assembly  42  is disposed in the part  60 , at each revolution of the backscatterer  112 , the entirety of the light reception and analysis assembly  42  receives light that has passed through the ophthalmic lens  14  and through the aperture  341 . 
         [0233]    Thus, at each revolution of the backscatterer  112 , the light reception and analysis assembly  42  receives light that has passed through the entirety of the corresponding zone of the ophthalmic lens  14 . 
         [0234]    If the fixed zone facing the backscatterer  112  of  FIG. 15  is on the side of the support  13 :
       the part  61  and the part  62  are traversed permanently by light heading towards the light emission and reception assembly  11 ; and   the part  60  comprises at any instant a sub-part which is in line with a solid part of the backscatterer  112  and a sub-part which is in line with the aperture  340 .       
 
         [0237]    The sub-part which is in line with a solid part of the backscatterer  112  is traversed by light heading towards the light emission and reception assembly  11 . 
         [0238]    The sub-part which is in line with the aperture  341  is not traversed by light heading towards the light emission and reception assembly  11 , or else by light of low intensity. 
         [0239]    The images of the backscatterer  112  which are taken by the sensor  36  of the video camera  31  have, as regards brightness, the same aspect as  FIG. 16 , with at the center and at the periphery more luminous zones corresponding to the part  61  and to the part  62  and, between these parts, a slightly less luminous part corresponding to the part  60 . 
         [0240]    It will be observed that for the rear view mirror  112  having the aperture  41  ( FIG. 8 ), there exists only a part such as  60  since the vertex of the aperture  41  is on the center of rotation  40  (there is therefore no part such as  61 ) and since the aperture  41  goes as far as the edge of the backscatterer  112  (there is therefore no part such as  62 ). 
         [0241]    Likewise, for the backscatterer  112  having the aperture  141  ( FIG. 10 , there is no part such as  62  but only a part such as  61  and a part such as  60 . 
         [0242]    The fifth version, illustrated in  FIG. 17 , of the optical instrument according to the second embodiment is a variant of the first version ( FIG. 8 ) in which the aperture  41  is replaced with a rectangle-shaped aperture  441  with the same center as the center of rotation  40 , whose sides are not equal: the sides oriented vertically in  FIG. 17  are larger than the sides oriented horizontally in  FIG. 17 . 
         [0243]      FIG. 19  shows the various parts comprised by a fixed zone facing the backscatterer  112  of  FIG. 17 . 
         [0244]    This entails an annular part  63 , a disc-shaped part  64  which is surrounded by the part  63  and an annular part  65  which surrounds the part  63 . 
         [0245]    Each site of the part  63  is situated in the course of a revolution of the backscatterer  112 , at times in line with the aperture  441  and at times in line with a solid part of the backscatterer  112 . 
         [0246]    Each site of the part  64  is permanently in line with the aperture  441 . 
         [0247]    Each site of the part  65  is permanently in line with a solid part of the backscatterer  112 . 
         [0248]    As is well understood in view of  FIG. 18 , the circle-shaped contour of the part  64  has a diameter which is the length of one of the small sides of the aperture  441 ; the inner contour of the part  65  has a diameter which is on the diagonal of the aperture  441 ; and the outer contour of the part  65  has a diameter which is that of the backscatterer  112 . 
         [0249]    It will be noted that the part  63  exists because the aperture  441  is not asymmetric about the center of rotation  40 . 
         [0250]    In relation to the light reception and analysis assembly  42  and in relation to the light emission and reception assembly  11 , the parts  63  and  65  behave like respectively the part  60  and the part  61  or  62  of the fixed zone shown in  FIG. 16 . 
         [0251]    On the other hand, as each site of the part  64  is permanently in line with the aperture  441 :
       if the fixed zone is on the side of the light reception and analysis assembly  42 , the part  64  is traversed permanently by light heading towards the light reception and analysis assembly  42 ; and   if the fixed zone is on the side of the support  13 , the part  64  is not traversed by light heading towards the light emission and reception assembly  11 , or else is traversed by light of low intensity.       
 
         [0254]    The images of the backscatterer  112  taken by the sensor  36  of the video camera  31  have, as regards brightness, the same aspect as  FIG. 19 , with at the center a dark zone corresponding to the part  64 , a more luminous zone at the periphery corresponding to the part  65  and, between these parts, a slightly less luminous zone corresponding to the part  63 . 
         [0255]    In a general manner, to allow good tracing of the micro-engravings  16 , or anyway of the predetermined indications of some other nature liable to be present on the ophthalmic lens so as to give the location of at least one characteristic point, it is desirable that the part such as  64  be non-existent or as small as possible, for example no larger than 100 mm 2 . 
         [0256]    It will be noted that the part  64  is present because the center of rotation  40  is in the aperture  41 . 
         [0257]    In all the exemplary embodiments illustrated and described hereinabove, the backscatterer  112  can rotate continuously about the center of rotation  40 . 
         [0258]    It will be observed that the rotation-driving device  50  is a cyclic driving device, making the backscatterer  112  make one and the same motion at each cycle, that is to say a complete revolution about the center of rotation  40 . 
         [0259]    In the third embodiment of the optical instrument illustrated in  FIG. 20 , the backscatterer  112  driven in rotation in a continuous manner about the center of rotation  40  is replaced with a backscatterer  112  which is translated in a to-and-fro movement along the direction  70 . 
         [0260]    This device for driving in a to-and-fro movement is symbolized in  FIG. 21  by the arrows  71 . 
         [0261]      FIG. 21  shows as a solid line one of the extreme positions of the to-and-fro motion and, as a dashed line, the other extreme position. 
         [0262]    Here, the backscatterer  112  has a rectangular contour and exhibits an aperture  541  which has the same center as the backscatterer  112 , and is relatively narrow. 
         [0263]      FIG. 22  shows like  FIGS. 16 and 19  the various parts comprised by a fixed zone facing the backscatterer  112  of  FIG. 20 . 
         [0264]    This entails a rectangular part  75  and a part  76  whose inner contour corresponds to that of the part  75  and whose outer contour is rectangular. 
         [0265]    In the course of an outward-return movement of the backscatterer  112 , each site of the part  75  is situated at times in line with the aperture  541  and at times in line with a solid part of the backscatterer  112 . 
         [0266]    Each site of the part  76  is permanently in line with a solid part of the backscatterer  112 . 
         [0267]    Thus, the part  75  behaves in a similar manner to the part  60  ( FIG. 16 ) or to the part  63  ( FIG. 19 ) while the part  76  behaves in a similar manner to the part  61  or  62  ( FIG. 16 ) and to the part  65  ( FIG. 19 ). 
         [0268]    In a variant, not illustrated, the device for cyclic driving of the backscatterer is configured to translate it along a predetermined direction but in a continuous manner rather than in an alternating manner (to-and-fro movement). For example, the backscatterer is carried by an endless belt running around several rollers. 
         [0269]    In another variant not illustrated, the backscatterer is driven in rotation but in an alternating rather than continuous manner, by performing to-and-fro movements of a predetermined angular amplitude. 
         [0270]    In variants, not illustrated, the light backscatterer is replaced with a light unit of another nature, for example a retroreflector. 
         [0271]    In this case, the objective such as  35  of the camera such as  31  is focused on the lens  14  rather than on the light return unit. 
         [0272]    In variants, not illustrated, the micro-engravings  16  are replaced with other predetermined indications present on the lens  14  so as to give the location of at least one characteristic point such as the Prism Reference Point (PRP), for example erasable ink markings. 
         [0273]    In variants, not illustrated, the light emission and reception assembly  11  is different, with for example with the incident beam  20  which is not a collimated light beam and/or the light source which is different from the point source formed by the extended source  25  and by the diaphragm  26 , for example directly an extended light source. 
         [0274]      FIG. 23  shows a variant of the light reception and analysis assembly  42  illustrated in  FIG. 11  in which the HARTMANN matrix  45  is not isolated but forms part of a more extended mask  80  while the assembly  42  also comprises a light source  81  disposed facing holes  82  in the mask  80 . 
         [0275]    The light emitted by the light source  81  passes through the holes  82 , the aperture  141  and heads towards the light emission and reception assembly  11  where it is received by the sensor  35  of the video camera  31 . 
         [0276]    The holes  82  can for example form predetermined patterns  83 , as shown in  FIG. 24 , here a cross-grid of three holes by three holes, serving to perform a calibration of the optical instrument. 
         [0277]    It will be noted that in  FIG. 24 , to simplify the drawing, the HARTMANN matrix  45  has not been represented. 
         [0278]      FIG. 25  shows a variant of the organization of the holes of the HARTMANN matrix  45 , in which certain holes  85  and  86  are bigger so as to improve the analysis capabilities in respect of the light passing through this HARTMANN matrix  45 . 
         [0279]    In a variant, not illustrated, the HARTMANN matrix  45  is replaced with another matrix of patterns making it possible to determine the characteristic points of the ophthalmic lens  14 , and more generally to determine other characteristics of this lens such as its spherical power, its cylindrical power and its cylindrical power axis. 
         [0280]    In variants, not illustrated, the light reception and analysis assembly  42  is capable of determining other optical characteristics, for example a polarization axis of the lens  14 . 
         [0281]    Many other variants are possible depending on the circumstances and it will be recalled, in this respect, that the invention is not limited to the examples described and represented.