Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
  • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/08—Measuring arrangements characterised by the use of optical techniques for measuring diameters

Definitions

• the present invention relates to a method, or characterization, without contact, of three-dimensional objects, more particularly three-dimensional objects which are transparent to visible light or at least translucent with respect to this light.
• an image is formed by means of a calculation code based on an object model chosen a priori.
• the image thus obtained is compared with a simulated radiographic image and the model is then deformed iteratively until the simulated image coincides with the experimental image.
• the reconstruction is based on a hypothesis of symmetry of revolution of the object.
• Interferometry is a precise method, usable in a complex infrastructure, but it is quite delicate to implement.
• the object of the present invention is to remedy the above drawbacks.
• the invention uses a measurement technique by optical ombroscopy ("backlit shadowgraphy"), which applies to the characterization of objects observable from a single angle of view, especially in the case where it is difficult to access. to these objects.
• the invention preferably uses an image acquisition system which is developed on a plane of the object studied.
• the subject of the present invention is a method of non-contact measurement of at least one geometric parameter of a three-dimensional object, this three-dimensional object being translucent or transparent with respect to visible light, this method being characterized in what we determine the optical characteristics of the object, using these optical characteristics, we establish at least one optical model of the propagation of visible light through the object, this model comprising an equation which relates the parameter geometric of the object as a result of an observation carried out directly on an image of the object, this image being acquired by observing this object with visible light, by optical ombroscopy at a single view,
• the geometric parameter of the object is determined using the equation and the result of the observation.
• the optical model is further established from experiments and the image is acquired by means of an image acquisition system in visible light, by performing the development of this image acquisition system. 'images on a sectional plane of the object studied.
• ray tracing software is used, intended to obtain images of objects, to determine the model, this software making it possible to know the influence of the object on the propagation of visible light.
• simulations of optical ombroscopy images of auxiliary objects are also carried out to establish the model, these auxiliary objects having different respective geometric characteristics, and these image simulations are combined by multilinear regression.
• This multilinear regression preferably implements a criterion for minimizing the error in the sense of least squares for example.
• Ray tracing software can be used to perform the simulations.
• the object is a hollow sphere, thus having a wall
• the geometric parameter of the object is the thickness of this wall
• the image of the hollow sphere with a white ring we determine the external radius of the sphere
• we measure the radius of the white ring on the image of the object we determine the thickness of the wall according to the outer radius of the sphere and the radius of the white ring.
• the object is a hollow cylinder, thus having a wall
• the geometric parameter of the object is the thickness of this wall
• the image of the cylinder hollow with a white ring we determine the external radius of the cylinder, we measure the radius of the white ring on the image of the object and we determine the thickness of the wall according to the external radius of the cylinder and the radius of the white ring.
• the external radius can be determined using the directional derivative method.
• the object is hollow and contains a layer or a deposit of a material which is transparent or translucent, and the thickness of this deposit or of this layer is determined.
• the object is hollow and has an internal wall, and the deformation or roughness of this internal wall is determined.
• an optical ombroscopy device comprising a visible light source, means for collimating this source and means for acquiring images, comprising an optic, a sensor for images and means for adjusting the digital aperture of the optic, this optic being placed between the object and the image sensor and making it possible to form the image of the section plane of the object studied on the sensor of images, and the collimation of the source and the digital aperture of the optics are adjusted.
• the image sensor may include a charge transfer device.
• the process which is the subject of the invention has advantages: its cost of implementation is low and the equipment necessary for this implementation is relatively easy to install in a complex infrastructure because this equipment is limited to a light source and a camera.
• FIGS. 1A and 1B schematically illustrate the formation of white bands, for hollow spheres whose respective walls have different thicknesses
• FIGS. 2A and 2B respectively show a real image and a simulated image of a hollow sphere
• FIG. 2C shows the profile of a half line of the simulated image of FIG. 2B
• FIG. 3 shows a radial profile of an image to be processed
• FIG. 4 is a schematic view of a device making it possible to implement a method according to the invention.
• FIG. 5A shows the ombroscopic image of a hollow cylinder
• Figure 5B shows the profile of the image of Figure 5A.
• the principle of measurement which is used in the invention for the measurement of an object, is based on the observation of the object by ombroscopy in visible light, in association with an optical model of light propagation.
• This measurement principle takes into account the physical phenomena of light propagation in different translucent or transparent materials that the object comprises, in particular at the various interfaces of the object, and makes it possible to connect the measurement directly carried out on the image of ombroscopy with the internal physical dimensional quantities of the object studied, by means of the equation of the model.
• ombroscopy Admittedly, to study a planar object, ombroscopy is an inexpensive and simple to use measurement method. By direct measurement on the image of the object, it is possible to know for example the size of the object.
• the direct analysis of the image does not provide enough information because the observed image of a section of the object is not only the image of the section through the lens of the ombroscopy device used but also the image of the section through the objective and the object itself.
• Figure 1A illustrates schematically, in cross section, the formation of this ring, or band, at point A for a sphere, or ball, hollow 4, whose external radius is lOOO ⁇ m and the thickness lOO ⁇ m (respectively 200 ⁇ m).
• References 6, 8 and 10 respectively designate the light source of the ombroscopy device used for image formation, the objective of this device and the light rays which come from the source 6 and interact with the sphere 4 and goal 8.
• the radius of this white ring is directly related to the thickness and the external radius of the studied sphere.
• the goal of the modelizations is to relate this thickness and this external radius with the radius of the white ring.
• This method is used for measuring the thickness of a hollow sphere.
• FIG. 2A schematically represents an image 12 of an actual hollow sphere.
• the external radius of this sphere is 578 ⁇ m and its thickness is 66 ⁇ m.
• FIG. 2A the presence of a white ring 14 and a black zone 16 is observed (the corresponding elements of FIG. 2B having the same references). We found that:
• the width of the black zone depends on the numerical aperture (“numerical aperture”) of the image acquisition system that includes the ombroscopy device used.
• FIG. 2C shows the profile of a half line of the simulated image, the pixel numbers (Pix) being on the abscissa and the amplitudes (gray levels) on the ordinate (Ampl).
• the white ring 14 has been identified as well as the black area 16.
• the model is obtained by a multilinear regression which is based on the criterion of the minimization of the error in the sense of least squares (see the document mentioned above).
• E being the vector of error between the modeling and the tests.
• the radius of this ring is measured on each photograph.
• Rbde 0, 0089 + 0, 9871R ext -l, 156e for R ex t belonging to the interval [800 ⁇ m; 1400 ⁇ m] and e belonging to the interval [25 ⁇ m; 250 ⁇ m].
• This result is used to determine, from real shadow images, the thickness of the hollow sphere by measuring the external radius and the radius of the corresponding white ring.
• This method is based on the cancellation of the gradient of the image and on the maximization of the second derivative.
• Figure 3 shows one of these profiles.
• the pixel numbers (Pix) are plotted on the abscissa and the amplitudes (gray levels) are plotted on the ordinate (Ampl).
• Point A is obtained by canceling the second derivative.
• Point B is obtained by reducing the profile study area (to the area delimited by circle C in the example shown) and by looking for the local maximum. To have a sub-pixel coordinate, the profile is locally adjusted to a Gaussian law.
• the thickness of the sphere, for this radius is obtained by using the model equation.
• the internal and external surfaces of the sphere are reconstructed and it is then possible to know the average thickness of the sphere on its equator.
• the comparison is made on the measurement of the average thickness of a sphere.
• the measurements which have been obtained are given at ⁇ 3 ⁇ m at 2 ⁇ for X-ray radiography and at ⁇ 2 ⁇ m at 2 ⁇ for interferometry.
• the ombroscopic method of measuring the thickness of a hollow sphere according to the invention has the advantage of being inexpensive and of being able to be implemented very easily and quickly.
• the use of this method requires a judicious choice of the digital aperture of the image acquisition system, which comprises the ombroscopy device used, and of the emission diagram of the light source which this device comprises, in order to '' obtain the optimal conditions for correctly viewing the white stripe.
• the calculated model is valid only for a certain range of radii and a certain range of thicknesses for a given hollow sphere. This model can be improved by improving the precision on the optical characteristics of the material of which the sphere is made.
• the uncertainty of the measurement depends essentially on the spatial resolution of the image.
• the center of the sphere is observed in order to be able to trace the radial profiles.
• the larger the sphere the greater the micrometer conversion coefficient per pixel is large, and therefore the greater the measurement uncertainty. This measurement uncertainty therefore depends on the radius of the studied sphere.
• the apparatus used for ombroscopy is conventional. It includes a collimated light source, which emits visible light and which is associated with an image acquisition system which is intended to be developed on a plane of the object under study and whose digital aperture is adjustable.
• the possibility of modifying the digital aperture of the image acquisition system facilitates the detection of the radius of the white ring.
• Figure 4 is a schematic view of an ombroscopy device for implementing the method of the invention.
• This device comprises a source 18 of visible light, means 20 for adjustable collimation of this source and image acquisition means, comprising an optic 22 which is provided with means 24 for varying the digital aperture of this optic.
• CCD sensor 26 which is provided with image processing means 28, with which a display device 30 is associated.
• a hollow sphere 32 which we want to study, is placed between the source 18 and the optic 22.
• This optic 22 makes it possible to form the image of a plane of section of the hollow sphere 32 on the CCD sensor 26.
• the invention relates essentially to the method used to determine the thickness of the hollow sphere, namely: determination of the experimental conditions conducive to easy detection of the radius of the white ring (digital opening of the image acquisition system, collimation of the light source), elaboration of the equation of the mathematical model based on the characteristics of the object studied and on the phenomenon observed on the image
• Figure 5A shows the shadow image 34 of a hollow cylinder 36 with an external radius of 1000 ⁇ m and a thickness of 300 ⁇ m.
• the profile of this image is shown in Figure 5B. This profile is drawn along line X of FIG. 5A.
• a white band B is observed in FIG. 5A.
• This white band corresponds to zone C in FIG. 5B.
• the edge of the cylinder is marked with the arrow D.
• the position of the white strip is linked to the external radius and to the thickness of the hollow cylinder.
• Knowing the distance between the center of the white ring and each point of the latter makes it possible to determine the surface condition of the internal wall of the hollow cylinder, in terms of deformation and roughness, according to an equator or two generatrices of the cylinder, in the observation plane (which is perpendicular to the optical observation axis).
• the method object of the invention makes it possible to measure the thickness of the internal layer subject to knowledge of the thickness of the wall of the object, called the external layer, which is then measured beforehand.
• the roughness and deformation of the internal surface of the bilayer object can also be measured.
• the process which is the subject of the invention can be used regardless of the diameter of the sphere or of the cylinder. Indeed, the use of an optical chain with a suitable magnification coefficient makes it possible to observe the whole of an object on a CCD sensor of 6.6 mm by 8.8 mm. It is even possible to observe only one part of the object, provided you have an appropriate optical system.
• a hollow object for example a hollow sphere
• account must be taken of the resolution of the optical system used for this measurement: for a given resolution, the sphere must be thick enough so that the white band can be easily distinguished.

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• General Physics & Mathematics (AREA)
• Length Measuring Devices By Optical Means (AREA)

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• 2003
  • 2003-03-12 FR FR0350045A patent/FR2852389B1/fr not_active Expired - Fee Related
• 2004
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  • 2004-03-10 CN CNB2004800065848A patent/CN100376865C/zh not_active Expired - Fee Related
  • 2004-03-10 US US10/547,783 patent/US7307740B2/en not_active Expired - Fee Related
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  • 2004-03-10 CA CA2518702A patent/CA2518702C/fr not_active Expired - Fee Related
  • 2004-03-10 EP EP04718994A patent/EP1601932A2/de not_active Withdrawn
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