Abstract:
An optical instrument includes: a collimation element ( 30 ) having a focal distance; a point light source ( 25 - 27 ) with a wavelength of between 700 and 1000 nm and a diameter less than or equal to a fiftieth of the focal distance, placed at a first focus of the collimation element, so that the light becomes a beam ( 20 ) of collimated light; a backscatterer ( 12 ); a support for receiving an ophthalmic lens ( 14 ), with the collimation element, support and backscatterer being placed so that the beam of collimated light encounters the lens location ( 15 ) where micro-etching is present; an image analyzing element ( 32 ) and an image capture element ( 31 ) linked to the analyzing element and including an objective lens ( 35 ) placed at a second focus of the collimation element, which objective lens is developed to provide the analyzing element with images of the backscatterer in order to identify and locate the micro-etching.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to optical instruments for identifying and localizing micro-engravings present on an ophthalmic lens. 
       PRIOR ART 
       [0002]    As is known, micro-engravings are small local variations in lens thickness or small local variations in refractive index that serve to pinpoint characteristic points of an ophthalmic lens, for example its prism reference point (PRP). 
         [0003]    When a light beam encounters a micro-engraving, its phase is modified locally by the micro-engraving. 
         [0004]    One of the techniques allowing micro-engravings to be viewed is the phase-contrast technique, which is based on diffraction of the light beam, which diffraction is caused by the local phase variation that occurs when a coherent light beam encounters a micro-engraving. More precisely, the light beam employed is spatially coherent. The diffraction is made visible by a local intensity modification (Fresnel diffraction). 
         [0005]    From European patent applications EP 1 613 450 and EP 1 739 472, an optical instrument is already known including, in order to allow micro-engravings present on a face of an ophthalmic lens to be viewed:
       a point light source;   a collimating member, said point light source being placed at a first focal point of this collimating member so that light issued from the point light source becomes, after having passed through the collimating member, a beam of collimated light;   a reflector of light;   a holder configured to hold the ophthalmic lens between the collimating member and the reflector of light in a position in which that face of the ophthalmic lens on which the micro-engravings are present is facing the collimating member; the collimating member, the holder and the reflector being placed so that the beam of collimated light issued from the collimating member encounters that face of the ophthalmic lens on which the micro-engravings are present, passes through the ophthalmic lens, encounters the reflector, returns toward the ophthalmic lens and once again passes through the ophthalmic lens then the collimating member;   a displaying unit; and   an image-capturing unit connected to the displaying unit, which image-capturing unit includes an objective placed at a second focal point of the collimating member, which objective is focused on that face of the ophthalmic lens on which the micro-engravings are present so that the images displayed by the displaying unit connected to the image-capturing unit contain a representation of the micro-engravings allowing the micro-engravings to be identified and localized.       
 
       SUBJECT OF THE INVENTION 
       [0012]    The invention aims to provide an instrument of the same type having a better performance. 
         [0013]    To this end, it proposes an optical instrument for identifying and localizing micro-engravings present on an ophthalmic lens, including:
       a point light source;   a collimating member, said point light source being placed at a first focal point of said collimating member so that light issued from said point light source becomes, after having passed through said collimating member, a beam of collimated light;   a member for sending back light;   a holder configured to hold said ophthalmic lens between said collimating member and said member for sending back light; said collimating member, said holder and said member for sending back light are placed so that the beam of collimated light issued from said collimating member encounters the location on the ophthalmic lens where micro-engravings are present, passes through said ophthalmic lens, encounters said member for sending back light, returns toward said ophthalmic lens and once again passes through said ophthalmic lens then said collimating member;   an image-exploiting unit; and   an image-capturing unit connected to said image-exploiting unit, which image-capturing unit includes an objective placed at a second focal point of said collimating member, which objective is focused so that the images delivered to the image-exploiting unit, which is connected to said image-capturing unit, contain a representation of said micro-engravings allowing said micro-engravings to be identified and localized;       
 
         [0020]    characterized in that:
       said member for sending back light is a backscatterer;   said objective of said image-capturing unit is focused in order to deliver to the image-exploiting unit images of the backscatterer;   said point light source has a wavelength A comprised between 700 and 1000 nm; and   said collimating member has a focal length F and said point light source has a diameter D smaller than or equal to one fiftieth of the focal length F.       
 
         [0025]    Thus, in the optical instrument according to the invention, the member for sending back light is not a reflector but a backscatterer, so that the light sent back toward the ophthalmic lens is no longer spatially coherent. 
         [0026]    As a result, the beam sent back toward the ophthalmic lens, when it passes through the ophthalmic lens, is not affected by the micro-engravings. 
         [0027]    Because, in the optical instrument according to the invention, the point light source has a wavelength comprised between 700 nm and 1000 nm, i.e. in the infrared near the spectrum of visible light, the attenuation of the light on its path between the point light source and the image-capturing unit is moderated, including when the ophthalmic lens is tinted. 
         [0028]    Of course, the image-capturing unit is chosen to be sensitive in this wavelength range. 
         [0029]    In addition to implementing a wavelength comprised in this range, the point light source of the optical instrument according to the invention has a diameter D smaller than or equal to one fiftieth of the focal length F of the collimating member. 
         [0030]    Specifically, it has been observed that, with this wavelength range and with such a diameter D of the point light source, it is possible for a contrasted image of the micro-engravings to be projected onto the backscatterer. 
         [0031]    It is believed that the contrasted character of the image is due to the fact that the transverse spatial coherence width of the beam of collimated light is a good match to the width of the micro-engravings. 
         [0032]    Because of the contrasted character of the image, the aperture of the objective of the image-capturing unit may be relatively large. 
         [0033]    Such an aperture limits the loss of light flux in the image-capturing unit. 
         [0034]    Thus, the instrument according to the invention may allow a fluid exploitation of the micro-engravings. For example, if the image-exploiting unit is a displaying unit, the micro-engravings may be observed on the displaying unit while the user positions the ophthalmic lens on the holder. 
         [0035]    It will be noted that, in contrast, in the prior-art optical instruments such as described in the aforementioned European patent applications EP 1 613 450 and EP 1 739 472, when the objective is focused on that face of the ophthalmic lens on which the micro-engravings are present, the image-capturing unit sees on this face the effect of light coming back from the reflector, this meaning that the captured images have a tendency to be blurry and that therefore the aperture of the objective of the image-capturing unit must be relatively small, this generating a substantial loss of light flux in the image-capturing unit. 
         [0036]    Moreover, it will be noted that a method and an optical device for detecting defects in objects such as sheets of transparent material, and in which light is returned back toward the object by a backscatterer, are already known from European patent application EP 0 856 728. 
         [0037]    The method and optical device described in this document make it possible to easily identify gross defects; hence they bear no relation to the identification and localization of micro-engravings present on an ophthalmic lens. 
         [0038]    It will also be noted that European patent application EP 1 093 907 proposes to use the optical device of EP 0 856 728 to detect characteristics of an ophthalmic lens taking the form of printed marks or hidden marks. For the reasons indicated above, these hidden marks cannot be micro-engravings. 
         [0039]    According to features that are advantageous, especially because of the micro-engraving identification and localization accuracy that they ensure, the optical instrument according to the invention is configured for micro-engravings having a width a comprised between 10 μm and 80 μm, the diameter D of said point light source and said focal length F of said collimating member respecting the relationship: 
         [0000]        D≦Fλ/ 5 a    
         [0040]    where λ is the wavelength of said point light source. 
         [0041]    According to more particular advantageous features, ensuring an excellent micro-engraving identification and localization accuracy, the optical instrument according to the invention is configured for micro-engravings having a width a comprised between 30 μm and 60 μ, λ being comprised between 800 and 900 nm while F is comprised between 150 and 300 mm. 
         [0042]    According to other advantageous features:
       said point light source is formed by an extended light source and by a diaphragm containing a pinhole placed facing said extended light source, the diameter of said pinhole thus forming the diameter D of said point light source;   said optical instrument includes a half-silvered mirror on either side of which said point light source and said objective of said image-capturing unit are placed;   said backscatterer is rotatable; and/or   said optical instrument includes an image-analyzing device configured to identify and localize said micro-engravings, and a device for automatically positioning a centering pin on a face of said ophthalmic lens, which device is connected to said image-analyzing device.       
 
         [0047]    According to advantageous features, the optical instrument furthermore includes:
       a device for driving the backscatterer to rotate, which is configured to make the backscatterer turn about a predetermined center of rotation;   an aperture in said backscatterer, said aperture being configured so that the center of rotation of the backscatterer is elsewhere than in the aperture; and       
 
         [0050]    an assembly for receiving and analyzing light, which assembly is located on that side of the backscatterer which is opposite said holder, said holder, said backscatterer and said aperture being configured so that said assembly for receiving and analyzing light receives light from said beam of collimated light after it has passed through said ophthalmic lens and through said aperture, said assembly for receiving and analyzing light being configured to determine at least one optical characteristic of said ophthalmic lens. 
         [0051]    Because the center of rotation of the backscatterer is not located in the aperture, no part of the aperture is centered on the center of rotation. 
         [0052]    Therefore, any stationary point facing the backscatterer between its center of rotation and its periphery is in line, during at least part of each turn, with an unapertured section of the backscatterer. 
         [0053]    The images of the backscatterer delivered to the image-exploiting unit therefore do not contain a void corresponding to the aperture and therefore allow the micro-engravings to be seen in their entirety. 
         [0054]    It will be noted that document EP 1 997 585 describes an optical instrument in which the reflector of light is made up of two portions one of which, at the center, is stationary and the other of which, which is annular and of same center, is rotatable, the stationary portion of the reflector at the center being the top of an assembly for receiving and analyzing light. 
         [0055]    In this optical instrument, the images displayed by the displaying unit contain a central void corresponding to the stationary portion of the reflector at the center. This void decreases the visibility of the marks located at the center of the ophthalmic lens. 
         [0056]    According to advantageous features, the device for driving the backscatterer to rotate and the image-capturing unit are connected to a controlling device configured so that each time interval during which the image-capturing unit takes an image has a duration during which the backscatterer makes an integer number of turns. 
         [0057]    According to alternative advantageous features, the device for driving the backscatterer to rotate and the point light source are connected to a controlling device configured so as to make said point light source emit flashes each having a duration during which the backscatterer makes an integer number of turns. 
         [0058]    According to other advantageous features:
       said assembly for receiving and analyzing light includes a Hartmann matrix illuminated by said light of said beam of collimated light after it has passed through said ophthalmic lens and through said aperture; an additional image-capturing unit illuminated by the light after it has passed through the Hartmann matrix; and an analyzing device for analyzing the images captured by the additional image-capturing unit in order to determine said at least one optical characteristic of the ophthalmic lens;   said analyzing device for analyzing the images captured by the additional image-capturing unit is connected to a displaying unit in order to allow it to display said at least one optical characteristic of the ophthalmic lens;   said analyzing device for analyzing the images captured by the additional image-capturing unit is connected to a device for automatically positioning a centering pin on a face of said ophthalmic lens;   said backscatterer has a circular outline and said aperture takes the form of an angular sector; and/or   said backscatterer has a circular outline and said aperture is spiral shaped.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0064]    The disclosure of the invention will now continue with a detailed description of embodiments thereof, given below by way of illustration and nonlimiting example, and with reference to the appended drawings. In said drawings: 
           [0065]      FIG. 1  is a very schematic view showing an assembly for emitting and receiving light and a holder that a first embodiment of an optical instrument according to the invention includes, an ophthalmic lens having micro-engravings on one of its faces being held on the holder in a position in which that face on which the micro-engravings are present is facing the assembly for emitting and receiving light; 
           [0066]      FIG. 2  is a very schematic view showing a backscatterer that this instrument includes and the ophthalmic lens held on the holder; 
           [0067]      FIG. 3  is a very schematic view showing the assembly for emitting and receiving light, the ophthalmic lens and the backscatterer and the path of the light from the backscatterer to the assembly for emitting and receiving light; 
           [0068]      FIG. 4  is a schematic view of the first embodiment of the instrument according to the invention, showing in detail the assembly for emitting and receiving light; 
           [0069]      FIG. 5  is a schematic view illustrating certain geometric features of the point light source and of the collimating member; 
           [0070]      FIG. 6  is a block diagram showing elements connected to the image-capturing unit in order to allow a centering pin to be automatically positioned on the ophthalmic lens; 
           [0071]      FIG. 7  is a plane view of the backscatterer; 
           [0072]      FIG. 8  is a view similar to that in  FIG. 7 , but for a second embodiment of the optical instrument according to the invention; 
           [0073]      FIG. 9  is a schematic view similar to the bottom part of  FIG. 4 , but for the second embodiment of the optical instrument according to the invention; 
           [0074]      FIG. 10  is a view similar to that in  FIG. 8 , but for a first variant of the second embodiment of the optical instrument according to the invention; 
           [0075]      FIG. 11  is a view similar to that in  FIG. 9  but showing more detail and corresponding to the first variant illustrated in  FIG. 10 ; 
           [0076]      FIG. 12  is a view similar to those in  FIGS. 8 and 10 , but for a second variant of the second embodiment of the optical instrument according to the invention; 
           [0077]      FIG. 13  is a block diagram showing elements connected to the image-capturing units of the second embodiment of the optical instrument according to the invention; and 
           [0078]      FIG. 14  is a block diagram showing elements connected to the light source in a version in which it emits flashes. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0079]    The optical instrument  10  illustrated in  FIGS. 1 to 4  includes an assembly  11  for emitting and receiving light, a backscatterer  12  and a holder  13  ( FIG. 1 ) that is configured to hold an ophthalmic lens  14  between the assembly  11  and the backscatterer  12  in a position in which its face  15  on which the micro-engravings  16  are present is facing the assembly  11  for emitting and receiving light. 
         [0080]    The micro-engravings  16  are small local variations in lens thickness or small local variations in refractive index. 
         [0081]    Various techniques allow micro-engravings  16  to be formed on a face of an ophthalmic lens: the micro-engravings may take the form of small additional thicknesses when they are molded with the ophthalmic lens, small recesses may be generated by laser or a laser may be used to modify the refractive index of the lens material locally. 
         [0082]    When a coherent light beam encounters a micro-engraving  16 , its phase is modified locally by the micro-engraving. 
         [0083]    This local variation in phase causes the light beam to diffract. 
         [0084]    In the case of a spatially coherent light beam, the diffraction is made visible by a local intensity modification (Fresnel diffraction). 
         [0085]    The micro-engravings  16  serve to pinpoint characteristic points of the ophthalmic lens  14 , for example its prism reference point (PRP). 
         [0086]    The assembly  11  for emitting and receiving light emits a beam  20  ( FIG. 1 ) of spatially coherent collimated light. 
         [0087]    As shown on the left in  FIG. 2 , when the beam  20  encounters a micro-engraving  16 , the light is diffracted locally. 
         [0088]    After the light has passed through the lens  14 , its projection on the backscatterer  12  contains intensity variations due to the diffraction of the light caused by the micro-engravings  16 . 
         [0089]    The image of the beam  20  projected onto the backscatterer  12  therefore contains intensity variations of similar shapes to those of the micro-engravings  16 . 
         [0090]    As shown on the right in  FIG. 2 , the light that reaches the backscatterer  12  is sent back thereby slightly scattered in the same direction. 
         [0091]    The light beam  21  backscattered by the backscatterer is spatially incoherent because of this slight scattering. 
         [0092]    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 assembly  11  for emitting and receiving light. 
         [0093]    In the preceding description, 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 was not mentioned. 
         [0094]    This is because these two successive prismatic deviations cancel each other out perfectly. 
         [0095]    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. 
         [0096]    Therefore, the image of the backscatterer  12  seen by the assembly  11  contains an exact representation of the micro-engravings  16 . 
         [0097]    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 . 
         [0098]    The holder  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 clear (if this distance is too large the image would be blurry because of the diffraction at the micro-engravings  16 ) and sufficiently large for the projection of the micro-engravings  16  to be large enough to be observed. 
         [0099]    They assembly  11  for emitting and receiving light will now be described in detail with reference to  FIG. 4 . 
         [0100]    In the illustrated example, the assembly  11  includes: an extended light source  25 ; a diaphragm  26  containing a pinhole  27 ; a half-silvered mirror  28 ; a redirecting mirror  29 ; a collimating lens  30 ; a video camera  31 ; and a displaying unit  32  connected to the video camera  31 . 
         [0101]    The diaphragm  26  and the objective  35  of the video camera  31  are located on either side of the half-silvered mirror  28 , in conjugate places, i.e. they are seen from the point of view of the redirecting mirror  29  as being located in one and the same place. 
         [0102]    This place is chosen to be the focal point of the collimating lens  30 . Thus, each of the two conjugate places corresponds to the focal point of the collimating lens  30 . 
         [0103]    The pinhole  27  of the diaphragm  26  may therefore be considered to be placed at a first focal point of the collimating lens  30  and the objective  35  of the video camera  31  may be considered to be placed at a second focal point of the collimating lens  30 . 
         [0104]    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. 
         [0105]    The light emitted by this point source reflects from the half-silvered mirror  28  then from the redirecting mirror  29  and passes through the collimating lens  30 . 
         [0106]    Because the pinhole  27  is located at the focal point 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. 
         [0107]      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 . 
         [0108]    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 . 
         [0109]    To ensure that the light flux is 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. 
         [0110]    After it has passed through the ophthalmic lens  14 , the light of the beam  21  issued from the backscatterer  12  then passes through the collimating lens  30 , is reflected by the redirecting mirror  29 , passes through the half-silvered mirror  28  and reaches the objective  35  of the video camera  31 . 
         [0111]    The objective is focused so that the sensor  36  of the video camera  31  takes images of the backscatterer  12 . 
         [0112]    These images are displayed on the displaying unit  32 , which is connected to the video camera  31 . 
         [0113]    Thus, an observer looking at the displaying unit  32  sees images allowing the micro-engravings  16  present on the face  15  of the ophthalmic lens  14  to be identified and localized. 
         [0114]    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 operator when he wants to edge the ophthalmic lens  14 , i.e. cut the edges of the ophthalmic lens  14  to the shape of the frame in which it must be fitted. 
         [0115]    In practice, the centering pin used to attach 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. 
         [0116]      FIG. 6  shows elements allowing the centering pin to be placed automatically. 
         [0117]    In addition to being connected to the displaying unit  32 , the video camera  31  is connected to an image-analyzing device  37  that is capable of identifying and localizing the micro-engravings  16 . A device  38  for automatically positioning a centering pin is connected 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. 
         [0118]    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. 
         [0119]    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 comprised between 700 nm and 1000 nm, i.e. in the infrared near the spectrum of visible light. 
         [0120]    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. 
         [0121]    Of course, the sensor  36  of the video camera  31  is chosen to be sensitive in this wavelength range. 
         [0122]    Generally, the instrument  10  is here configured for micro-engravings  16  the width of which is comprised between 10 and 80 μm. 
         [0123]    It is important for the image of the micro-engravings  16  projected onto the backscatterer  12  to be contrasted. Specifically, this makes it possible to use a video camera  31  with an aperture  35  of relatively large size. Such an aperture limits the loss of light flux on route to the sensor  36  of the video camera  31 . 
         [0124]    Thus, enough light flux is received by the sensor  36  of the video camera  31  to allow a fluid observation of the micro-engravings  16 , i.e. the user may move the ophthalmic lens  14  over the holder  13  while the displaying unit  32  is refreshed in real time (in practice, at a frequency at least equal to 15 Hz). 
         [0125]    It has been observed that with the aforementioned range of wavelengths, a pinhole  27  with a diameter D smaller than or equal to one fiftieth of the focal length F of the collimating lens  30  (distance between the lens  30  and its focal point) makes it possible to ensure that the image of the micro-engravings  16  projected onto the backscatterer  12  is contrasted. 
         [0126]    It is believed that this is the result of the good match between the transverse spatial coherence width of the beam  20  and the width of the micro-engravings  16 . 
         [0127]    Generally, given the aforementioned lower limit of 10 μm for the width of the micro-engravings, it is advantageous for the transverse spatial coherence width of the beam  20  to be larger than or equal to 5 times the width of the micro-engravings  16 . 
         [0128]    By definition, the transverse spatial coherence width is equal to Fλ/D, where λ is the wavelength of the light flux. 
         [0129]    If the width of the micro-engravings is denoted a, the following relationship is obtained: D≦Fλ/5a 
         [0130]    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,       
 
         [0134]    then the diameter D of the pinhole  27  is smaller than or equal to 680 μm. 
         [0135]    It has been observed that excellent results are obtained for micro-engravings  16  having a width a comprised between 30 μm and 60 μm when the wavelength of the light flux λ is comprised between 800 and 900 μm and the focal length F is comprised between 150 and 300 mm. 
         [0136]    As indicated above, with a light source  25  emitting at a wavelength comprised between 700 nm and 1000 nm, and a light-source diameter D smaller than or equal to one 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 smaller than or equal to one hundredth, one hundred and fiftieth, one two hundredth or one two hundred and fiftieth of the focal length F. 
         [0138]    It has also been observed that parameters favorable to making 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 portion 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 an ophthalmic lens  14  is present or not), such calibrating patterns no longer being perceptible when the backscatterer is rapidly rotating. 
         [0143]    It will be noted that micro-engravings such as  16  are more precise than the marks borne by ophthalmic lenses as supplied by their manufacturers; and that the instrument according to the invention allows the micro-engravings to be used directly, to the benefit of precision. 
         [0144]    Such a precision, for example for the centering, is important since lenses are becoming increasingly personalized. 
         [0145]    It will be noted that the instrument  10  is easily integratable into an already existing instrument, for example a reader-blocker or a grinder. 
         [0146]    It will also be noted that one possible use of the instrument according to the invention is to measure any shift between a reference given by the micro-engravings and other reference marks present on the lens, for example the marks with which the lens is delivered; and/or that another possible use of the instrument according to the invention is to produce marks very precisely with respect to the micro-engravings by virtue of a read-out with the instrument  10 . 
         [0147]    In the embodiment of the instrument  10  that was just described, the backscatterer  12  is formed by an unapertured rotatable platen, i.e. a platen containing no aperture. 
         [0148]    A second embodiment of the optical instrument according to the invention, in which the backscatterer  12  is replaced by a backscatterer containing an aperture and the assembly for receiving and analyzing light is placed under this backscatterer, i.e. on that side of the backscatterer which is opposite the holder  13  provided to hold the ophthalmic lens  14 , will now be described with reference to  FIGS. 8 to 13 . 
         [0149]    The holder  13 , the aperture in the backscatterer and the assembly for receiving and analyzing light 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 its 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. 
         [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 outline 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 assembly  42  for receiving and analyzing light is placed centered on the center of rotation  40 . 
         [0157]    As may be seen in  FIG. 9 , the holder  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 assembly  42  for receiving and analyzing light. 
         [0159]    As will be understood in light of  FIG. 8 , at any instant a portion of the assembly  42  for receiving and analyzing light is in line with the aperture  41 . 
         [0160]    Thus, at any instant, a section of the assembly  42  for receiving and analyzing light receives light that has passed through the aperture  41 . 
         [0161]    Because of the rotary movement of the backscatterer  112 , each section of the assembly  42  for receiving and analyzing light is, at a certain moment, in line with the aperture  41  when the backscatterer  112  makes a turn. 
         [0162]    Therefore, on each turn of the backscatterer  112 , the entirety of the assembly  42  for receiving and analyzing light receives light that has passed through the ophthalmic lens  14  and through the aperture  41 . 
         [0163]    Thus, on each turn of the backscatterer  112 , the assembly  42  for receiving and analyzing light 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 turn of the backscatterer  112 , the assembly  42  for receiving and analyzing light is able to determine optical data of the lens  14 , and more precisely of the zone through which the light passed before reaching the assembly  42  for receiving and analyzing light. 
         [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 turn, with a section of the backscatterer  112  not forming part of the aperture  41 , i.e. an unapertured portion. 
         [0167]    Here, where the aperture  41  takes the form of an angular sector having its apex 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 turn and therefore in line with a section of the backscatterer  112  not forming part of the aperture  41  during 11/12 of a turn. 
         [0168]    The images of the backscatterer  112  displayed by the displaying 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]    Measurements that make it possible to prevent the aperture  41  from appearing at all on the displaying 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 apex of which is a distance away from the center of rotation  40 ; furthermore, the assembly  42  for receiving and analyzing light is off-center with respect to the center of rotation  40 , as is the holder  13  provided to hold the ophthalmic lens  14 ; the holder  13  and the assembly  42  for receiving and analyzing light are centered one relative to the other. 
         [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 assembly  42  for receiving and analyzing light. 
         [0172]    As will be understood in light of  FIG. 10 , at any instant a portion of the assembly  42  for receiving and analyzing light is in line with the aperture  141 . 
         [0173]    Thus, at any instant, a section of the assembly  42  for receiving and analyzing light receives light that has passed through the aperture  141 . 
         [0174]    Because of the rotary movement of the backscatterer  112 , each section of the assembly  42  for receiving and analyzing light is, at a certain moment, in line with the aperture  141  when the backscatterer  112  makes a turn. 
         [0175]    Therefore, on each turn of the backscatterer  112 , the entirety of the assembly  42  for receiving and analyzing light receives light that has passed through the ophthalmic lens  14  and through the aperture  141 . 
         [0176]    Thus, on each turn of the backscatterer  112 , the assembly  42  for receiving and analyzing light 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 displaying 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 assembly  42  for receiving and analyzing light is off-center with respect to the center of rotation  40 , as is the holder  13  provided to hold the ophthalmic lens  14 ; the holder  13  and the assembly  42  for receiving and analyzing light are centered with relative to the other. 
         [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 assembly  42  for receiving and analyzing light. 
         [0181]    As will be understood in light of  FIG. 12 , at any instant a portion of the assembly  42  for receiving and analyzing light is in line with the aperture  241 . 
         [0182]    Thus, at any instant, a section of the assembly  42  for receiving and analyzing light receives light that has passed through the aperture  241 . 
         [0183]    Because of the rotary movement of the backscatterer  112 , each section of the assembly  42  for receiving and analyzing light is, at a certain moment, in line with the aperture  241  when the backscatterer  112  makes a turn. 
         [0184]    Therefore, on each turn of the backscatterer  112 , the entirety of the assembly  42  for receiving and analyzing light receives light that has passed through the ophthalmic lens  14  and through the aperture  241 . 
         [0185]    Thus, on each turn of the backscatterer  112 , the assembly  42  for receiving and analyzing light 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 displaying 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 assembly  42  for receiving and analyzing light 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 pierced 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 depending 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, the reader may for example refer to French patent application 2 825 466 which corresponds to the U.S. Pat. No. 6,888,626, or to international patent application WO 95/34800. 
         [0191]    As may be seen in  FIG. 11 , the image sensor  46  is connected to an image-analyzing device  47  that is capable of determining the optical characteristics of the ophthalmic lens  14 . 
         [0192]    The image-analyzing device  47  is connected to the displaying unit  32  that may thus display the optical characteristics of the ophthalmic lens  14  determined by the image-analyzing device  47 . 
         [0193]    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 connected not only to the image-analyzing device  37 , but also to the image-analyzing device  47 , this meaning that the device  38  for automatically positioning a centering pin may obtain 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 the aperture such as  41 ,  141  or  241  from appearing on the displaying unit  32 . 
         [0196]    The video camera  31  and the device  50  for driving the backscatterer  112  to rotate are connected to a controlling device  51  configured so that each time interval during which the video camera  31  takes an image corresponds to an integer number of turns of the backscatterer  112 . 
         [0197]    In the illustrated example, each image is taken during 1/20 s and the backscatterer  112  makes one turn in 1/20 s, i.e. at a speed of 1200 rotations/mn. 
         [0198]    As explained above, because the center of rotation  40  is not located in the 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 turn, with a section of the backscatterer  112  not forming part of the aperture (unapertured section). 
         [0200]    Likewise, each pixel of the sensor  36  of the video camera  31  receives, during part of each turn, light from a section of the backscatterer  112  not forming part of the aperture. 
         [0201]    As each image is taken during one or more integer turns of the backscatterer  112 , each pixel receives the same proportion of the light coming from the section 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 any void corresponding to the aperture such as  41 ,  141  or  241 . 
         [0203]    An operator looking at the displaying unit  32  sees the same images as with the unapertured backscatterer  12 , except that all or some of 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 of the backscatterer  112  in its entirety is 11/12 the perceived brightness of the backscatterer  12 , all else moreover being equal. 
         [0205]    The backscatterer  112  having the aperture  41  shown in  FIGS. 8 and 9  is thus perceived as an entirely unapertured backscatterer (just like the backscatterer  12 ) while still letting light pass to the unit  42  for receiving and analyzing light. 
         [0206]    For the backscatterer  112  having the aperture  141  shown in  FIGS. 10 and 11 , a central section, the radius of which corresponds to the distance between the center of rotation  40  and the apex 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 these cases, the image of the backscatterer  112  allows the micro-engravings  16  to be seen in their entirety. 
         [0208]    The implementation of the controlling device  51  for example involves a common time-base generator 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 determining 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 controlling device such as  51  to control the video camera  31  and the device  50  for driving the backscatterer  12  to rotate, provision is made for a device  55  connected to the device  50  for driving the backscatterer  112  to rotate 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 controlling device  55  is configured to make the light source  25  emits flashes, each time interval during which the light source  25  emits a flash corresponding to an integer number of turns of the backscatterer  112 . 
         [0212]    For example, if the backscatterer  112  turns at a speed of 1200 rotations/mn, and therefore makes one turn 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 displaying unit  32 . 
         [0214]    As each moment in which an image appears on the displaying unit  32  lasts one or more integer turns of the backscatterer  112 , each displayed image contains no void corresponding to the aperture such as  41 ,  141  or  241 . 
         [0215]    An operator looking at the displaying unit  32  sees the same images as with a controlling device such as  51 , except that the images have a brightness that oscillates with the rhythm of the flashes emitted by the light source  25 . 
         [0216]    The implementation of the controlling device  55  for example involves a common time-base generator 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 rotations/mn. 
         [0218]    In variants (not illustrated) of the second embodiment of the optical instrument according to the invention, the aperture  41 ,  141  or  241  is replaced by an aperture of different shape/form not containing the center of rotation  40 , for example an aperture not extending as far as the periphery of the backscatterer  112  or an aperture the outline of which is larger than the assembly  42  for receiving and analyzing light; and/or is replaced by a plurality of apertures. 
         [0219]    In variants (not illustrated) of the first and second embodiment of the optical instrument according to the invention:
       there is no redirecting mirror such as  29  or the mirror  29  is replaced by a curved mirror conjointly playing the role of the mirror  29  and of the collimating lens  30 ;   the point light source is configured differently, for example it is a laser emitting a pencil beam of predetermined diameter;   the collimating lens is replaced by a different collimating member, for example made up of a plurality of lenses;   the video camera  31  is replaced by another image-capturing unit;   the displaying unit  32  is replaced by another image-exploiting unit, for example an image-analyzing device such as  37 ; and/or   the micro-engravings such as  16  are placed elsewhere than on the front face such as  15 , for example on the back face or in the thickness of the lens.       
 
         [0226]    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 shown.