Optics: measuring and testing – Shape or surface configuration
Reexamination Certificate
2001-10-02
2004-11-02
Mathews, Alan (Department: 2851)
Optics: measuring and testing
Shape or surface configuration
C033S200000, C359S201100
Reexamination Certificate
active
06813033
ABSTRACT:
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
Almeras Emmanuel
Farcy René
Guillermin Laurent
Guirriec Florent
Essilor International (Compagnie Generale d'Optique)
Mathews Alan
Young & Thompson
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