Abstract:
An optical non-contact method of reading the three-dimensional shape of a profile in accordance with the luminous section principle consists of scanning the profile with a plane light beam intersecting the profile transversely, simultaneously reading the trace of the plane light beam on the profile by means of an optical receiver having an optical pointing axis at a constant non-zero pointing angle to the light beam at a series of positions along the profile, and deducing the three-dimensional shape of the profile from the readings effected at these various positions. On each reading, the light beam whose trace on the profile is read by the optical receiver is chosen from a plurality of predefined light beams which can be activated alternately. The non-contact optical reading method is particularly suitable for reading the three-dimensional shape of the inside edge of a spectacle frame rim, known as the bezel. A specific device is used to implement the method in this particular application.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an optical non-contact method employing the luminous section principle to read the three-dimensional shape of a profile. It finds one particularly pertinent application in reading the shape of the inside edge of a spectacle frame rim, known as the bezel. The invention also relates to a device for implementing the method when applied to reading a spectacle frame bezel. 
     2. Description of the Prior Art 
     During the fabrication of a pair of spectacles, in order to be able to mount the lenses in the frame, it is necessary to adapt the outside edge of each lens to fit the inside edge of the corresponding rim of the frame, usually called the bezel. A numerically controlled grinding machine is usually employed for this purpose and adapts the outside edge of each lens to the shape of the bezel into which it must be crimped. 
     To perform numerically controlled grinding in this way, it is necessary to have a numerical model of the three-dimensional shape of the bezel concerned. 
     At present, the three-dimensional shape of a bezel is acquired by means of a contact-type measuring device in which a feeler associated with a measuring head rotating about the central axis of the frame rim comes into physical contact with the bezel and slides along the whole of its periphery. However, this contact-type measurement is not entirely satisfactory, for two main reasons. First of all, the feeler, winch is urged at all times against the bezel, can in some cases cause deformation of the frame rim, falsifying the measurements. Secondly, the mechanical system guiding the feeler cannot achieve complete acquisition of the profile of the bezel over the whole of its perimeter in an acceptable time, i.e. in less than one minute. This precision mechanical system is also relatively costly, although it can only provide an acceptable level of accuracy with sufficient radial movement of the feeler. The resulting fabrication and maintenance problems are detrimental to cost. 
     To remedy these drawbacks, a number of non-contact optical methods and devices have previously been proposed for reading the three-dimensional shape of the bezel. 
     Thus the documents FR 2 679 997 and FR 2 713 758 propose a non-contact optical method of reading the shape of a spectacle frame rim bezel using a narrow light beam (rectilinear coherent laser beam) to illuminate certain characteristic points of the bezel and an optical sensor with a matrix of CCD photosensors to register the impact of the beam at each of those points. The spatial coordinates of each of the points illuminated in this way are then computed by triangulation from the position, as acquired by the sensor, of the image of the point of impact of the beam on the frame and the respective spatial configurations of the laser beam and the sensor. To enable illumination and reading of points over the full height of the bezel, the laser beam, to be more precise its source, can move along the axis of the rim concerned of the frame, i.e. in practice vertically. 
     The documents WO 98/45664 and WO 00/03839 propose a similar type of reading method in which the light beam, instead of taking the form of a rectilinear narrow beam, diverges in a plane so that it intersects the bezel transversely. In this method, known as the luminous section method, the sensor receives the image of the trace (or luminous section) of the plane light beam on the bezel. Thus each optical reading does not relate to only one point on the bezel, but to a complete cross section of the bezel. 
     The above luminous section method is described in detail in the following document: 
     “A perspective, on range finding techniques for computer vision”, R. A. Jarvis, IEEE transactions on pattern analysis and machine intelligence, Vol. PAMI-5, N o  2 March 1983. 
     It essentially entails scanning the profile with a plane light beam intersecting the profile transversely and simultaneously reading the trace of the plane light beam at a series of positions along the profile using optical receiver means having an optical pointing axis at a constant non-zero pointing angle to the light beam. Finally, a programmed computer deduces the three-dimensional shape of the profile from the readings effected at the various positions. 
     The documents WO 98/45664 and WO 00/03839 propose non-contact optical devices for reading the three-dimensional shape of the inside edge, referred to as the bezel, of a spectacle frame rim, which devices implement the above method and include a support for the frame rim and a read head which rotates relative to the support about a rotation axis and with which is associated a sensor responsive to its angular position relative to the support, the read head including emitter means adapted to project a plane light beam intersecting the bezel transversely and optical receiver means adapted, regardless of the angular position of the read head relative to the frame support, to read the trace of the plane light beam on the bezel along an optical pointing axis at a constant non-zero pointing angle to the light beam. 
     It is therefore clear that, compared to the method of reading points previously cited, the above luminous section method and the device for implementing it have the advantage of reading the complete section of the bezel over its full height each time that the optical receiver means capture an image and without it being necessary to provide for each angular position of the read head any vertical displacement of the laser beam to scan of the section concerned of the bezel transversely. 
     However, whichever type of light beam is used (rectilinear narrow beam or plane divergent beam), several as yet unsolved problems impede practical use of the above non-contact optical reading methods. 
     The first problem results from the diverse sizes of spectacle frames, which impose a minimum depth of field of the order of 4 cm. The constraints for satisfactory crimping of the lens into the bezel, in particular in metal frames, impose a relatively high level of accuracy for grinding the outside edge of the lens in corresponding relationship to the shape of the bezel. The resulting accuracy required in reading the shape of the bezel is of the order of one hundredth of a millimeter. 
     Because these two requirements, relating on the one hand to the depth of field and on the other hand to the accuracy of the measurement, are mutually contradictory, it is not possible at present to satisfy them with components available off the shelf. A compromise consisting of a depth of field of 4 cm for an accuracy of 0.01 mm imposes the use of a sensor providing 4000 measuring points, i.e. a CCD camera with 2000×2000 pixels and a resolution of 0.5 pixel. This resolution is difficult to achieve because a large area camera implies a wide field, which constitutes a source of optical aberrations, especially at the edges, and this impedes obtaining a resolution of less than one pixel unless relatively costly and bulky high-performance optics are used. Furthermore, it is not easy to generate a divergent plane light beam that is sufficiently thin to be contained within a depth of 4 cm, which requires the use of relatively costly and bulky precision optics. 
     Given the above constraints, the only practical solution to the problem of implementing the above reading method with sufficient accuracy is to control the combination of the laser beam and the sensor mechanically to maintain it at a very small distance from the bezel, in order to restrict the depth of field required of the sensor. However, this requires a tracking mechanism whose complexity and cost are added to those of the optical reader device. 
     Optical methods of reading a bezel of a spectacle frame run into a second difficulty. Spectacle frames can be made with very different shapes and from diverse materials having their own optical properties, in particular with regard to reflection, absorption, diffusion and back-scattering. Thus an optical reading method can be validated only on condition that it proves to be effective for all types of frame, in particular those having rims of circular, oval or rectangular shape and made from metal, opaque plastics material, multicolored plastics material or transparent or translucent plastics material, with or without metal inserts. Because of this diversity of shapes and materials, reception by the optical sensor of the image of the trace of the laser beam on the bezel can be rendered impossible by certain optical phenomena depending on the illumination and image capture configuration. In particular, it is necessary to envisage the following difficulties: 
     a specific point or area of excessively intense reflection for a given reading illumination angle, 
     the presence of metal inserts in a transparent or translucent plastics material frame rim causing unwanted reflections masking the trace of the plane light beam on the bezel or making it unusable, 
     an angle of incidence of the light beam producing too much or too little reflection to be usable by the optical sensor, depending on the material from which the frame concerned is made, and 
     a pronounced oblong shape of the rim of the frame preventing illumination of certain sectors of the bezel at a given angle, the light beam impinging on the outside of the rim of the frame. 
     In the above context, one aim of the present invention is to provide an optical non-contact method of reading the three-dimensional shape of a profile, such as the bezel of a spectacle frame rim, avoiding the compromise between resolution and depth of field as much as possible by “increasing” the depth of field, whilst retaining a satisfactory resolution, without it being necessary to use complex and costly optical reader means. 
     Another object of the invention is to minimize luminous signal losses, i.e. to render the image of the trace of the laser beam on the bezel usable by the optical receiver means for the greatest possible variety of situations and types of frame, in particular those previously referred to. 
     SUMMARY OF THE INVENTION 
     With a view to achieving at least one of the above two objects, the invention proposes an optical non-contact method of reading the three-dimensional shape of any profile in accordance with the luminous section principle, the method consisting of: 
     scanning the profile with a plane light beam intersecting the profile transversely, 
     simultaneously reading the trace of the plane light beam on the profile by means of optical receiver means having an optical pointing axis at a constant non-zero pointing angle to the light beam at a series of positions along the profile, and 
     deducing the three-dimensional shape of the profile from the readings effected at these various positions, 
     in which method, on each reading, the light beam whose trace on the profile is read by the optical receiver means is chosen from a plurality of predefined light beams which can be activated alternately. 
     To be more specific, the above method can advantageously be applied to the situation more particularly referred to hereinabove of reading the shape of the inside edge of a spectacle frame rim, referred to as the bezel. 
     There is also proposed, for implementing the above method, an optical non-contact device for reading the three-dimensional shape of the inside edge, referred to as the bezel, of a rim of a spectacle frame, the device including a support for the frame rim and a read head rotatable relative to the support about a rotation axis and associated with a sensor responsive to its angular position relative to the support, the read head including emitter means adapted to project a plane light beam intersecting the bezel transversely and optical receiver means adapted, regardless of the angular position of the read head relative to the frame support, to read the trace of the plane light beam on the bezel along an optical pointing axis at a constant non-zero pointing angle to the light beam, in which device the emitter means are adapted to project at least two separate light beams. 
     Accordingly, to capture each image, it is possible to choose, from among the plurality of light beams available, the one whose trace on the profile is the most propitious to accurate reading by the optical receiver means along their optical pointing axis. This choice of the most suitable light beam for the configuration encountered can be directed to achieving either or both of the two objects previously cited. 
     In a first aspect, the choice offered by the emitter means between a plurality of light beams can generate an artificial “increase” in the depth of field of the optical receiver means, with no loss of accuracy. As previously mentioned, to preserve good reading accuracy without using complex, bulky and costly optical means, it is necessary to accept a restricted depth of field of the optical receiver means. It follows that with a single light beam, the trace of the beam could not fail to leave the field of the optical receiver means in various areas of the profile, unless the profile were tracked at a constant distance by the optical receiver means. According to the invention, this problem relating to the necessary narrowness of the field of the optical receiver means is compensated by the possibility of illuminating the profile with another light beam, offset relative to its inoperative counterpart, and thereby defining another range of reception in which its trace on the profile (bezel) is totally within the field of the optical receiver means. In other words, the various light beams define, in relation with the depth of field of the optical receiver means, different reading ranges which, on being juxtaposed, are additive and thereby define an overall reading range that is much more extensive than only one of those ranges taken in isolation. When scanning the profile, the light beams are activated alternately so that the trace of the active light beam is in the field of the optical receiver means. This choice between a plurality of beams offered by the emitter means thereby artificially demultiplies the depth of field, and it is therefore possible to use high-precision optical receiver means with a small depth of field, with no costly optical system that is difficult to implement. 
     In a second aspect, the choice between a plurality of light beams offered by the emitter means overcomes the reception difficulties resulting from the diverse shapes and materials of the frames (or other profiles). To capture each image it is possible to change the active light beam as soon as reading becomes impossible or inaccurate with the light beam that is initially active. Clearly, the problems previously cited, relating to the difficulty or impossibility of receiving images or even of illuminating particular areas of the profile, arise only in a relatively precise geometrical configuration of the light beam and the optical receiver means relative to the area concerned of the profile. Consequently, the reception or illumination problems that arise with a particular light beam will generally not arise with another light beam having a different geometrical configuration. This is why alternating between a plurality of light beams overcomes the reception or illumination difficulty in most cases. 
     To improve reading accuracy, it is preferable to have a pointing angle greater than 45°. In practice, a pointing angle of approximately 70° is a good compromise for highly accurate and effective reading in most configurations. 
     The invention provides a number of additional advantageous features contributing to achieving either or both of the two technical effects developed hereinabove. 
     In particular, to increase the depth of field of the optical receiver means artificially, the plurality of light beams can include at least one series of at least two juxtaposed and substantially parallel light beams. In practice, a number of juxtaposed light beams from three to eight in the or each series is satisfactory for reading most frame bezels. 
     Accordingly, when scanning the profile, if the depth of field of the optical receiver means is reduced to the benefit of reading accuracy, the trace of the active light beam may escape from the receive field. In this case, it is sufficient to deactivate that light beam and to activate the adjacent light beam, which, because it is offset relative to initial beam, covers a receive range offset relative to that covered by the initially active beam and therefore illuminates the profile (bezel) along a trace which is within the receive field of the optical receiver means. Accordingly, depending on the distance between the area concerned of the profile (bezel) and optical receiver means (to be more precise the rotation axis of the read head), the choice can be made to activate the parallel juxtaposed light beam covering the receive range corresponding to that distance and whose trace is in the receive field of the optical receiver means. As previously explained, this artificially increases the receive depth of field by creating between the parallel juxtaposed light beams reading ranges corresponding to a particular range of the distance of the profile from the optical receiver means. In other words, the depth of field needed to read a profile in an overall range is divided by the number of parallel juxtaposed light beams, the necessary overall receive range being divided into the same number of small reading ranges, respectively associated with various parallel juxtaposed beams. 
     Instead of, or in combination with, the plurality of juxtaposed light beams, to overcome the reception problems resulting from the diverse shapes and materials of the frames in particular areas of the bezel of the frame rim, or more generally of the profile concerned, the plurality of light beams can include at least two light beams with an angle between them greater than 20°. 
     Accordingly, a given area of the profile (bezel) can be illuminated alternately by one or the other of these two beams at two different angles of incidence. Then, if the illumination of a given area of the profile by one of the light beams cannot produce a satisfactory reading, for any of the reasons previously explained, that beam is deactivated in favor of an angularly offset beam, to illuminate the area concerned of the profile at another angle with a very high probability of producing a satisfactory reading, or at least a better reading than the first beam. Similarly, if, in a particular area of the profile, to be more specific in a particular area of the bezel of a frame rim, the light beam is reflected by a metal insert buried in a translucent plastics material, causing abnormal reflection of the beam, preventing accurate reading of its trace, it is again sufficient to deactivate the beam encountering the metal insert and activate the other, angularly offset beam. The latter, illuminating the profile at a different angle, will not encounter the metal insert and the unwanted reflection from that insert will therefore be eliminated, so that a reading can be obtained under the normal conditions. 
     In one advantageous configuration the plurality of light beams includes at least one pair of light beams symmetrical to each other with respect to a median reading plane containing the optical pointing axis. Accordingly, if the frame rim to be measured is oblong in shape, and the initially active beam no longer illuminates the bezel from the inside of the rim, but to the contrary illuminates the outside of the rim, it is sufficient to activate the symmetrical beam which, being on the inside of the rim, will illuminate the bezel correctly. It is therefore possible in particular to provide that each of the two symmetrical beams is at a pointing angle to the optical pointing axis of approximately 70° to obtain maximum reading accuracy and efficacy. 
     In one preferred embodiment, combining the choice between a plurality of parallel juxtaposed light beams and a plurality of angularly offset light beams, the plurality of light beams could include a first series of at least two parallel juxtaposed light beams and a second series of at least two parallel juxtaposed light beams, the beams of the two series being symmetrical in pairs with respect to the median reading plane. 
     In an advantageous embodiment, each plane light beam is derived from a rectilinear coherent light beam on whose trajectory is placed a spreader lens causing the rectilinear coherent light beam to diverge in the required plane transverse to the bezel. 
     To be more precise, each rectilinear coherent light beam is emitted by its own laser source which is disposed vertically, parallel to the rotation axis of the read head, and an oblique mirror is disposed on the trajectory of the rectilinear coherent light beam, upstream of the spreader lens, to reorient the trajectory of the rectilinear coherent light beam, and consequently that of the divergent plane light beam derived therefrom, toward the bezel of the rim of the frame carried by the support. This vertical disposition of the laser sources considerably reduces the overall size of the read head in the radial direction, i.e. its overall size in the direction perpendicular to its rotation axis. 
     Similarly, to reduce the overall axial size of the read head, the optical receiver means include a matrix optical sensor and an associated objective lens both placed on a common optical axis and at least one mirror is placed obliquely to the optical axis to reorient it along a broken-line path whose final branch forms the required optical pointing axis. 
     For example, the optical axis is oblique to the rotation axis of the read head and at least two mirrors are placed on respective opposite sides of the axis of the read head to reorient the optical axis along a zig-zag path with at least three branches. 
     Finally, for rapid and convenient reading of both rims of the same frame, the read head is rotatably mounted on a carriage that can slide relative to the support for the frame between two reading positions in which the read head alternately faces one or the other of the two rims of the frame. 
     On the other hand, the various materials (metal, opaque or translucent plastics material, etc.) from which frames are made having very different optical properties, in particular with regard to their reflectivity characteristics, the power of the light beam can advantageously be regulated as a function of the luminous flux received by the optical receiver means to maintain that flux at a substantially constant level suited to the capabilities of the receiver means for satisfactory reading accuracy regardless of the type of frame. 
     Finally, for reading the shape of the outside edge of a glass template, the device advantageously includes additional support means for a template adapted to be turned relative to the read head about an axis parallel to the axis of the read head by associated drive means to move the outside edge of the template in front of the emitter means and the optical receiver means while the read head is held fixed in a particular angular position. 
     Other features and advantages of the invention will become apparent on reading the following description of one particular embodiment of the invention, which description is given by way of non-limiting example only and with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall perspective view of an optical reading device according to the invention. 
     FIG. 2 is a partial perspective view of the read head on its own, with its casing represented as if it were transparent. 
     FIG. 3 is a partial top view of the interior of the read head on its own, with its casing represented as if it were transparent. 
     FIG. 4 is a partial view in axial section of the read head on its own. 
     FIG. 5 is a view analogous to FIG. 1 showing a different embodiment of the device according to the invention including dedicated means for reading the outside edge of a glass template. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The non-contact optical reader device described by way of example with reference to the above figures is specifically designed for reading the three-dimensional shape of the inside edge D, referred to as the bezel, of a rim C of a spectacle frame M. 
     It is nevertheless clear that the essential features of the device could be transposed to other, similar applications for non-contact optical reading of any profile, in particular, although not exclusively, a closed profile. 
     Referring to FIGS. 1 and 2 in particular, the optical non-contact reader device according to the invention includes a chassis  1  having a generally U-shaped profile including a base  1 . 1  and two lateral walls  1 . 2 ,  1 . 3  to the top of which is attached a support  2  for a frame M. The support  2  overlies the base  1 . 1  and essentially comprises two jaws  3 ,  4  adapted to grip by their edge the two rims C of the frame M in a vertical locating plane P perpendicular to the lateral walls  1 . 2 ,  1 . 3 . 
     A mobile read head  9  is mounted on the base  1 . 1  of the chassis  1 . To be more precise, the read head  9  has a body or casing  10  of generally cylindrical shape with an axis  11 , mounted on a sliding carriage  5  so that it can rotate about its axis  11 , which is vertical. Rotation of the casing  10  is driven by a motor-encoder  13  whose body is fixed to the carriage  5  through a circular rack  12  at its periphery and a drive pinion  13 . 1 . The motor-encoder  13  therefore drives rotation of the casing  10  of the read head  9  about its axis  11  and simultaneously senses its angular position about that axis relative to the carriage  5  and therefore the base  1 . 1 . 
     The carriage  5  is mounted on the base  1 . 1  of the chassis  1  so that it can slide in a sliding direction  6  parallel to the vertical locating plane P of the frame support  2 , between two reading positions in which the read head  9  is vertically aligned with one or the other of the two rims C of the frame M. For moving it in translation on the base  1 . 1 , the carriage  5  includes a motor  7  whose body is fixed to the carriage  5  and whose shaft is fitted with a gear meshing with a rectilinear rack  8  associated with the base  1 . 1 . 
     In its two reading positions, the carriage  5  abuts against the two vertical walls  1 . 2 ,  1 . 3  of the chassis  1 , for example. Of course, other types of physical or optical, and possibly adjustable, abutment could be provided for immobilizing the carriage  5  in each of its two positions. An alternative solution would be to use a motor  7  integrating a coding function enabling it to deliver an electrical signal representative of the position of the carriage  5  so that translation of the carriage  5  with an offset corresponding to the distance between the centers of the two rims C of the frame could be controlled automatically. 
     The cylindrical casing  10  of the read head  9  has a cylindrical lateral wall  15  closed at respective ends by a plane bottom wall  16  and a plane top wall  17 . The top wall  17  has a generally trapezoidal reading slot  18  whose function is further explained later. 
     Inside its cylindrical housing  10 , the read head  9  includes emitter means  20 ,  21  adapted to project a plurality of plane light beams F 1  to F 8  oriented to intersect the bezel D of the rim C of the frame M transversely. 
     In this example the emitter means are divided into two parts  20  and  21  each adapted to emit a plurality of parallel juxtaposed light beams. To be more precise, the first part  20  of the emitter means is adapted to emit four parallel juxtaposed light beams F 1  to F 4  and the second part  21  of the emitter means is adapted to emit four parallel juxtaposed light beams F 5  to F 8 . 
     The light beams F 1  to F 4  and F 5  to F 8  of the two series are symmetrical in pairs with respect to a median reading plane  22  containing the axis  11  of the read head  9 . The beam F 5  emitted by the second part  21  of the emitter means is therefore symmetrical with respect to the plane  22  to the beam F 1  emitted by the first part  20  of the emitter means. Similarly, the beam F 6  is symmetrical to the beam F 2 , the beam F 7  is symmetrical to the beam F 3 , and the beam F 8  is symmetrical to the beam F 4 . 
     FIG. 3 shows the angle a, referred to hereinafter as the pointing angle, between each of the light beams F 1  to F 8  and the median plane  22 . Because of the parallel relationship of the beams of each series and the symmetrical relationship of the two series of beams, all the beams F 1  to F 8  are of course at the same angle a to the median plane  22 . 
     To obtain good reading accuracy, as explained hereinafter, it is preferable for the pointing angle a to be greater than 45°. To be more precise, optimization calculations and experiments have shown that an optimum pointing angle for reading the bezel D accurately and effectively is around 70°, as adopted in the example shown in the figures; however, it could be from 60 to 80°. 
     Each plane light beam F 1  to F 8  is in practice derived from a rectilinear coherent light beam f 1  to f 8  on whose trajectory is placed a cylindrical spreader lens  25  causing the rectilinear coherent light beam f 1  to f 8  to diverge in the required plane transverse to the bezel D. 
     To be more precise, each rectilinear coherent light beam f 1  to f 8  is emitted by its own laser source  26  disposed parallel to the axis  11  of the read head  9 , i.e. vertically. An oblique mirror  27  is placed on the trajectory of each rectilinear coherent light beam f 1  to f 8 , upstream of the spreader lens  25 , to reorient the trajectory of the rectilinear coherent light beam concerned, and consequently that of the divergent plane light beam F 1  to F 8  derived from it, toward the bezel D of the rim C of the frame M carried by the support  2 . 
     In this example, each of the two parts  20  and  21  of the emitter means has a single mirror  27  in the form of an extremely elongate rectangle placed on the trajectory of the four rectilinear coherent light beams f 1  to f 4  and a single spreader lens in the form of an elongate cylindrical rod  25  from which the four divergent plane light beams F 1  to F 4  emerge. Similarly, the second part  21  of the emitter means includes a single mirror  27  and a single spreader lens  25  identical to those of the first part  20  of the emitter means. 
     The casing  10  of the read head  9  further contains optical receiver means including a matrix optical sensor  31 , an associated objective lens  32  and two mirrors  33 ,  36 . The sensor  31  and the associated objective lens  32  are disposed on an optical axis  34  oblique to the axis  11  of the read head  9 . The mirrors  33 ,  36  are placed against the inside face of the cylindrical lateral wall  15  of the casing  10 , one on each side of the axis  11 , obliquely to the optical axis  34 , to reorient the latter along a path forming a zig-zag broken line with three branches, i.e. a Z-shaped line, whose last branch forms a final optical pointing axis  35  of the receiver means situated, like the optical axis  34 , in the median reading plane  22  and oriented obliquely in that plane. 
     To reduce overall size in the heightwise direction, i.e. along the axis  11  of the read head, in this example the optical axis  34  is at an angle to the axis  11  close to a right angle. To reduce further the overall size in this direction, the angle between the optical axis  34  and the axis  11  could be increased by placing one or more additional mirrors to obtain a zig-zag path of the optical axis with four or more branches. 
     The matrix optical sensor  31  can advantageously be a CMOS sensor which has the two-fold advantage over a conventional CCD sensor of being less costly and of capturing images faster. 
     As indicated by the double-headed arrow in FIG. 4, the optical sensor  31  can be moved along the optical axis  34  by adjustment means (not shown in the figures) for focussing the optical receiver means. The sensor  31  could instead be fixed, the optical receiver means being focussed by moving the objective lens  32  along the optical axis  34 . 
     The optical pointing axis  35  and the general orientation of each of the light beams F 1  to F 8  being oblique, the plane light beams F 1  to F 8  and the optical pointing axis of the optical receiver means pass obliquely through the reading slot  18  in the top wall  17  of the casing  10  of the read head  9  to impinge on the bezel D of the rim C of the frame M. 
     The reading device finally includes a central control and computer unit (not shown in the figures) for storing and processing the reading data for each reading position of the read head  9 , namely the image read by the sensor  31 , the identity of the active beam F 1  to F 8  and the angular position of the read head  9  relative to the base  1  (and therefore relative to the frame support  2 ), and controls the rotation of the read head  9  and the activation of the light beams F 1  to F 8 . 
     In practice, the procedure is as follows: 
     The frame rims C whose bezels D have to be read are placed on and fixed to the support  2  by gripping them in the manner of a vise between the jaws  3 ,  4  so that their central axes are approximately in the locating plane P containing the axis  11  of the read head  9 . The position of the read head  9  is adjusted by moving the sliding carriage  5  so that the read head  9  is vertically aligned with one of the two rims C, with its axis  11  approximately coincident with the central axis of the rim C concerned, this configuration corresponding to the first position of the carriage  5 . Reading the rim C can then begin. 
     The laser sources  26  are energized successively by the central control unit to activate the plane beams F 1  to F 8  successively until the trace of one of them on the bezel D can be read by the optical receiver means, i.e. until that trace is in the field of the optical receiver means shown by the chain-dotted lines  40  in FIGS. 3 and 4. In the configuration shown in FIG. 3, for example, it can be seen that only the symmetrical beams F 2  and F 6  illuminate the bezel D of the rim C within the field  40  of the optical receiver means. The other light beams either do not illuminate the frame rim C at all or illuminate it in an area that is outside the field  40  of the optical receiver means. 
     Consequently, if the beams F 1  to F 8  are activated in the order of their reference numbers, the central control unit will stop the illumination sequence at the beam F 2  which is the first beam whose trace on the bezel D of the frame rim C is within the field  40  of the optical receiver means. 
     However, if the trace of the beam F 2  on the bezel D cannot be read by the optical receiver means, or cannot be read sufficiently because of a particular optical configuration, such as one of those previously mentioned, for example assuming that the beam F 2  illuminates a metal insert buried in a translucent resin from which the rim C of the frame is made, the central control unit will deactivate the beam F 2  and activate the beam F 6  which is symmetrical to the beam F 2  to illuminate the bezel D of the frame rim C at a different angle, preventing the metal insert from interfering with the reading. 
     The reading of the trace of the beam on the bezel D chosen in this way is stored by the central unit which, on the basis of position parameters for the beam to be activated and the angular position of the read head  9  delivered by the motor-encoder  13 , determines the three-dimensional geometrical definition of the section concerned of the bezel D using standard triangulation calculations, such as those described in the following documents: 
     “Applications des lasers”, R. Farcy—p153-163; Masson 1993, 
     “A perspective, on range finding techniques for computer vision”, R. A. Jarvis, IEEE transactions on pattern analysis and machine intelligence, Vol. PAMI-5, N o  2 March 1983. 
     The central unit then controls the motor-encoder  13  so that the read head  9  rotates about its axis  11  by a particular angular increment. In this example, the trace of the light beam initially activated can remain within the field  40  of the optical receiver means or leave it. 
     If the trace of the beam initially activated remains in the field  40  of the optical receiver means, the beam in question remains active and the central unit performs a new computation to determine the three-dimensional geometrical definition of the new section concerned and of the bezel D as a function of the new angular position of the read head  9 . 
     On the other hand, if the trace of the light beam initially active leaves the field  40  of the optical receiver means, the central unit deactivates the light beam concerned and activates another light beam whose trace on the bezel D is within the field  40  of the optical receiver means. The central unit chooses the new light beam to be activated. This can be done as previously by means of a procedure for sequentially activating the beams F 1  to F 8  in a predetermined order until the trace of one of the beams is within the field  40  of the optical receiver means. 
     However, to speed up the overall reading procedure, it may be beneficial for the central unit to make this choice by detecting the direction in which the trace of the light beam initially activated progresses before leaving the field  40  of the optical receiver means, and deduce therefrom, assuming that the bezel is continuous, the adjacent light beam whose trace will most probably enter the field  40  of the optical receiver means. For example, if, subject to the hypothesis previously envisaged, the beam initially active is the beam F 2  that is part of the series of beams emitted by the first part  20  of the emitter means, and if the trace of that beam F 2 , as seen by the optical receiver means, moves to the right in the configuration shown by way of example in FIG. 3, that signifies that the trace of that beam F 2  on the bezel D is moving away from the mirror  36  of the optical receiver means. It is then necessary to activate the next light beam in the order in which the beams move farther away from the mirror  34 , i.e. the beam F 3  which, in conjunction with the field  40  of the optical receiver means, defines a reception range farther away than that defined by the initial beam F 2 . 
     As previously, assuming that the trace of the beam chosen in this way on the bezel D is impossible or difficult for the optical receiver means to read, for any reason of optical disturbance, such as the presence of a metal insert in the section concerned of the frame rim C, the central unit deactivates the beam chosen in this way and activates the corresponding symmetrical beam of the other series. Accordingly, under the hypothesis envisaged, the beam initially chosen for its reception range being the beam F 3 , on noticing the unwanted reflections the central unit will deactivate the beam F 3  and, to replace it, activate the symmetrical beam F 7  of the second part  21  of the emitter means. 
     The correct light beam having been determined in this way, the central unit stores the reading of the trace of the beam concerned delivered by the sensor  31  and deduces the three-dimensional geometrical definition of the corresponding section of the bezel D of the frame rim C from it, by computation, and as a function of the geometrical data relating to the beam activated in this way and the angular position of the read head  9  delivered by the motor-encoder  13 . 
     On the other hand, the various materials (metal, opaque or translucent plastics material, etc.) of which frames are made having very different optical properties, in particular as to their reflectivity characteristics, the power of the light beams is regulated as a function of the luminous flux received by the optical receiver means to maintain that flux at a substantially constant level matched to the capabilities of the receiver means for satisfactory accuracy reading regardless of the type of frame. 
     The reading operation is repeated in this way, turning the read head  9  through the predefined angular increment each time. After one complete rotation of the read head, the central unit holds in its memory geometrical definitions of the sections of the bezel corresponding to the various angular positions of the read head  9  for which a luminous section reading has been effected. 
     Based on the geometrical definitions of these various sections of the bezel, the three-dimensional shape of the bezel throughout its periphery can be established using appropriate trigonometrical functions known in the art, such as those proposed in the following documents, for example: 
     “A perspective, on range finding techniques for computer vision”, R. A. Jarvis, IEEE transactions on pattern analysis and machine intelligence, Vol. PAMI-5, N o  2 March 1983, and 
     “System of optical non contact microtopography” M. Costa and J. Almeida, Applied Optics Vol. 32, N o  25, Sep. 1, 1993, or “Light sectioning with large depth and high resolution” G. Häusler, W. Heckel, Dec. 15, 1988, Vol. 27 N o  4, Applied Optics. 
     The device is calibrated using a calibrated frame to store corrections for taking manufacturing disparities into account. 
     The angular increment between reading positions of the read head  9  represent a compromise between, on the one hand, the accuracy of the geometrical definition of the bezel throughout its periphery, which may prove insufficient if the gap between measurement positions is too great, and on the other hand the speed of the overall reading operation, which depends directly on the number of reading positions and which is the subject of severe demands by the profession (the read head preferably having less than 10 seconds to scan the bezel). Computation and experiments indicate that a number of reading positions from 600 to 1000 is generally satisfactory. In practice, a number of reading positions approximately equal to 800 can be used, corresponding to an angular increment of approximately 0.45°. 
     When reading the first rim C of the frame M in this way is completed, it suffices for the central unit to control the motor  7  to slide the carriage  5  towards its second position in which the read head  9  is vertically aligned with the second rim C of the frame M, with its axis  11  approximately coincident with the central axis of the second rim C. The bezel D of the second rim C can then be read optically in the manner previously described for the first rim C. 
     FIG. 5 shows a different embodiment of the reading device in accordance with the invention. The device has the same main components, designated by the same reference numbers, as that previously described with reference to FIGS. 1 to  4  (chassis  1 , sliding carriage  5  and read head  9 ), and further includes dedicated support means for reading the outside edge E of a glass template G. 
     The dedicated means include a support column  50  with a vertical axis  51  parallel to the axis  11  of the read head  9 . The column  50  has at the top a top face  52  slightly higher than the top face  17  of the read head  9  and adapted to have the template G to be read rest on it. 
     The top face  52  is provided with means for fixing the template G, such as suction or sucker means for fixing the template G temporarily, in particular with the template G prevented from rotating relative to the column  50  by friction. However, any other means can be used to fix the template G to the top of the column  50 , for example a simple adhesive pad. 
     The column  50  is adjacent the read head  9  so that when the template G is attached to its top face  52  at least a portion of the periphery of the template G is vertically aligned with the top face  17  of the read head  9 . 
     The column  50  is mounted on the carriage  5  to turn about its axis  51 . To drive it in rotation, it is provided with a toothed ring  53  that meshes with a pinion  54  associated with the shaft of a drive motor  55  whose body is fixedly mounted on the carriage  5 . 
     The three-dimensional shape of the outside edge E of the template G carried by the column  50  in this way is read in the following manner. 
     The read head  9  is placed in the angular position shown in FIG. 5, in which the reading slot  18  lies substantially in the plane containing the axes  11  and  51 , which in this instance corresponds to the locating plane P previously defined with reference to FIG.  1 . 
     The read head is held in one angular position throughout the reading operation, and therefore remains fixed, in contrast to the embodiment previously described. 
     The outside edge E of the template G is scanned by the light beams and its luminous section is read at a series of positions along its perimeter by turning the support column  50 , and therefore the template G, about the axis  51 , to cause the outside edge E of the template G to move through successive angular increments in vertical alignment with the reading slot  18  of the read head  9 , facing the emitter and receiver means. To this end, the central unit controls the drive motor  55  of the column  50  so that it turns the template G stepwise relative to the read head  9 .