Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/021—Interferometers using holographic techniques
  • G01B9/023—Interferometers using holographic techniques for contour producing
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
  • G02B27/46—Systems using spatial filters
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V10/00—Arrangements for image or video recognition or understanding
  • G06V10/88—Image or video recognition using optical means, e.g. reference filters, holographic masks, frequency domain filters or spatial domain filters

Definitions

• the subject of the present invention is a method of optical treatment of a family of objects, this treatment being able to consist of a count and / or a location and / or a classification according to a size criterion, etc.
• the treated objects can be grains, bubbles, drops, nodules, coins, letters, signs, etc.
• the method of the invention implements a known technique using FOURIER holography.
• This technique is described in particular in the article by A. VANDER LUGT entitled “Signal Detection by Complex Spatial Filtering” published in the journal IEEE Transactions on Information Theory vol . IT-10 (1964) p. 139 to 145.
• the shape of the object to be treated is compared with a reference of the same shape, by carrying out the optical intercorrelation between the object and the reference.
• a coherent monochromatic source generally a laser
• this hologram is recorded; then the recorded hologram is illuminated at a suitable angle with a field reproducing the spatial FOURIER transform of the element to be tested.
• a second FOURIER transform an image which represents the intercorrelation function between the element to be treated and the reference object.
• the intercorrelation by holography of FOURIER, between an object to be tested and a reference object is a means to determine if, in a given family, there are objects identical to the reference, and if so, how many there are, and where they are located. This technique is difficult to use effectively, mainly due to its lack of selectivity. To show this, we will take the case of circular objects for which the reference objects are disks. The study of the intercorrelation of a reference disc of radius R unit with discs of variable radius R 'was carried out by the applicant and the results obtained made it possible to establish the curves of FIG.
• ring is meant a closed narrow band, which may have various forms circular in most cases, but also elliptical or other.
• Such a ring can be limited by two concentric circles, one inscribed, the other circumscribed.
• the "dimensions” of the ring then refer to the radii of these circles and the “center” of the ring refers to the common center of said circles.
• “narrow” means that the difference in the radii of the two circles delimiting the ring is small compared to the radius of the circumscribed circle. If the ring is circular, its outer and inner edges naturally merge with the circumscribed and inscribed circles.
• the subject of the invention is a method of optical processing of objects in which, after having recorded a hologram of a reference object appropriate, we illuminate this hologram by a plane wave having a field reproducing the spatial Fourier transform of the objects to be treated and we form the spatial Fourier transform of the hologram thus lit, which provides an image representing the function of intercorrelation between the. treated objects and the reference object, this method being characterized in that said objects having contours close to a known contour, we take as reference object at least one ring having this known contour and a width making it possible to include said neighboring contours.
• the reference object is constituted by at least one differential ring, this differential ring being constituted by two concentric rings and of different sizes, these rings being treated, during the recording of the hologram, to give optical contributions of the same energy but of opposite phases.
• the two rings forming the differential ring can be joined or not joined.
• the reference object consists of several offset, homogeneous or differential rings.
• the rings are also circular. They are defined by their radius R which is that of the outer circle and by their width E, the difference between the radii of the inner and outer circles.
• the width E of the ring is chosen according to the fineness of resolution sought. Very generally, the E / R ratio is between 0.01 and 0.3.
• the selectivity of the method is chosen according to the precision of circularity of the family of objects; it can in certain particular cases of spherical or circular objects reach for example 0.001.
• the present invention also relates to a device which implements this method.
• This device comprises means for forming a plane wave having a field which reproduces the Fourier transform space of the objects to be processed, and to illuminate by this wave a hologram of a reference object, means for forming in an image plane the spatial Fourier transform of the hologram thus lit, an optoelectronic receiver arranged behind the image plane , this device being characterized in that the hologram used is that of an object consisting of at least one ring having the outline of the objects to be treated.
• FIG. 2 shows a network of curves showing the variations in the intercorrelation between a reference ring and discs of different radii
• FIG. 3 shows a network of curves showing the variations in the intercorrelation between a reference ring and rings of different radii
• FIG. 4 shows a network of curves showing the variations in the intercorrelation between a differential reference ring and discs of different radii
• FIG. 7 illustrates a differential reference ring with contiguous (a) or non-contiguous (b) rings usable in the invention
• FIG. 8 shows an experimental arrangement to obtain the hologram of rings
• FIG. 9 illustrates a particular embodiment of a device according to the invention.
• FIG. 10 illustrates a variant of the device in which the hologram operates by reflection at incidence, of Bragg;
• FIG. 11 illustrates means for obtaining a hologram
• FIG. 12 illustrates a variant in which several differential rings are used.
• FIG. 2 shows that the method of the invention has an appreciable advantage over the previous method linked to the fact that the intercorrelation function has, for the disks of radius equal to or greater than that of the reference ring , a marked maximum and the "noise" due to discs of lower spokes can be practically eliminated.
• the probability distribution corresponds to the variation, as a function of the reference radius, of the number of disks in the family whose radius is between this reference radius and a neighboring radius whose deviation relative to the reference radius is predetermined.
• the maximum of the intercorrelation function is very marked for the rings of radius R and low for the rings of neighboring radii less than or greater than R.
• this technique has the disadvantage of being difficult to implement. Indeed, the optical transformatio ⁇ of a ring disk actually corresponds to a derivation of the signal. This derivation can only be validly carried out if the signal is pure or, otherwise expressed, only if the disc is perfect. Indeed, if this is not the case, the derivation will only accentuate the faults without making it possible to achieve a satisfactory result.
• the objects to be studied are rarely in the form of perfect discs and the use of this technique may not be satisfactory.
• the invention in an implementation variant, however, achieves an equivalent result, while avoiding these drawbacks.
• This variant uses a differential reference ring. It is a ring made up of a first ring of radius R 1 and of small width E in front of R 1 , of a second concentric ring of radius R 2 greater than R 1 . and of width E 2 small in front of R 2 , the two rings being contiguous or not; these two rings are treated so that their optical contributions during the formation of the hologram, are of equal energies but in phase opposition. This point will be taken up later.
• the width of the rings will be chosen according to the precision and finesse of resolution sought. In general, the rings have the same surface.
• the characteristics of the differential ring are then simple to define. Indeed, it suffices to set the width and the radius of a rings, E 2 and R 2 for example, to automatically deduce the radius R 1 of the other ring and its width E 1
• E 1 R 1 - R 0 I1 should be noted that E 2 being chosen generally weak before R 2 , E 1 , will have a value very close to E 2 .
• the width of the peak at mid-height (linked to the spatial separating power) is of the order of E 2 , which means that the energy which passes through the object disk is concentrated at its center in the autocorrelation plane. It is essential, for the process to give any satisfaction, that the reference ring and the establishment of its hologram be carried out with maximum care.
• the reference rings preferably used in the method of the invention and its variant are produced by means of a microbelinograph. Using this device, each information point on a photographic plate is illuminated with a luminance calculated by computer, the photographic plate is then developed, then it is placed in the optical assembly intended to establish the hologram.
• a homogeneous reference ring usable in the invention is illustrated in Figure 6 with the notations already defined: width E equal to R 1 - R 0 , R 0 being the radius of the inner circle and R 1 the radius of the outer circle.
• a differential reference ring that can be used in the invention is produced by light diffracted by the objects illustrated in FIG. 7: with contiguous rings on part (a) and non-contiguous rings on part (b). These rings are modulated in density by a sinusoid of pitch "p" symbolically represented by vertical hatching. The modulation of the first ring is in spatial phase opposition with that of the second ring.
• the rings When taking the hologram of such an object, the rings are illuminated by a plane wave inclined obliquely on the plane of the rings so that the diffracted light has a wave plane parallel to the plane of the rings.
• the spatial phase opposition of the sinusoids modulating the rings induces a temporal phase opposition on the light diffracted by said rings, which makes the ring "differential".
• the energy of the wave diffracted by such a differential ring must be the same for the two rings which compose it, so that one generally takes rings of the same surface; but one could take rings of different surfaces, provided that this difference is compensated by an attenuation of the sinusoid on the largest ring to restore the equality of the diffracted waves.
• the hologram of the rings used in the invention (homogeneous or differential) is established with an apparatus which, by way of nonlimiting example, is illustrated in FIG. 8.
• a monochromatic light source (laser) 1 provides a coherent light beam 2. A fraction of it passes through a separating plate 3, is reflected by a mirror 4 and passes through a spatial filter 5 then a lens 6 whose focal length is such that the optical assembly thus formed makes it possible to illuminate, at an angle ⁇ , with a plane wave the object plane 7 which contains the reference ring. The diffracted light at 7 is collected by a lens 8 whose focal distance is f; this object forms the FOURIER transform of plane 7 into a plane 9.
• the separating plate 3 operating in reflection sends another fraction of the beam 2 onto a mirror 10, the reflected beam passes through a spatial filter 11, then, an objective 12 whose focal length is such that the optical assembly 3, 10, 11 12 thus formed produces a plane wave surface which illuminates at an angle ⁇ 2 the plane 9.
• the optical paths using the elements 3, 4, 5, 6, 7, 8, 9 and 3, 10, 11, 12, 9 are equal and there is interference in the plane 9. It is enough to place a holographic plate in the plane 9 to record the hologram of the ring placed in the plane 7 , which, developed, will be used in the process of the invention.
• the angle ⁇ 1 (inclination of the direction of illumination with the normal to the plane containing the ring) will be a function of the wavelength ⁇ of the coherent source used and the step p of the phase sinusoid hatching the rings (see Figure 7) according to the relation being small).
• this angle ⁇ 1 is zero, which corresponds, in the preceding relation, to an infinite step p.
• the reference rings can be made using the thin film technique.
• This technique consists in covering a transparent plate with a thin metallic layer obtained for example by vacuum evaporation, taking care, during the operation, to hide the surface corresponding to the desired ring.
• To make the differential ring it suffices to prepare according to the same process a ring whose surface is that of the two rings of the differential ring then to deposit on one of these rings, taking care to hide the rest of the surface, a dielectric material whose thickness is that of a half-wave plate. The two rings will thus be in phase opposition. It is also possible to produce a differential ring hologram from a simple ring with a conventional assembly, operating in two stages.
• the hologram of the first ring is conventionally established then, during the second stage, the hologram of the second ring is established on the same holographic plate, ensuring that they are concentric and contiguous and that they have the same surface and by introducing for the second ring a phase shift of ⁇ by means of a phase plate placed on the reference beam.
• This variant embodiment of the reference ring is however more difficult to implement than the first.
• the hologram of the reference ring being established, it is used in a device making it possible to obtain an intercorrelation between the objects to be tested and the reference ring.
• Several assemblies are possible. They all derive more or less from the double diffraction arrangement illustrated in FIG. 9.
• the assembly includes:
• a lens 14 whose focal length is such that it allows to illuminate by a plane wave an object plane 15 containing the objects to be identified;
• variable focal length objective 16 of the same optical axis as the objective 14, which creates on a plane 17 the spatial FOURIER transform of the objects contained in the plane 15 with a diameter proportional to its focal distance, - a plane 17 containing l 'hologram of the previously established reference ring;
• variable focal length lens 18 which is coupled with the objective 16, so that their focal lengths always have an equal value, so that in the focal plane 19, the dimension of the pupil is always equal to that of plan 15;
• the optical axis of the objective 18 forms with the optical axis of the objectives 14 and 16 the angle ⁇ 1 defined during the establishment of the hologram (cf. FIG. 8);
• the objective 18 produces, by FOURIER transform, in the image plane 19, the desired correlation function between the objects placed in the plane 15 and the reference object whose hologram has been placed in the plane 17;
• a receiver 20 transforming the light flux received by the image plane 19 into an electrical signal; this receiver 20 can for example be a television camera or a photomultiplier.
• the operating principle of this device is as follows: in plane 15 is placed a family of objects, for example disks of different radii, and in plane 17 the hologram of a homogeneous ring of radius R.
• the focal lengths objectives 16 and 18 are set to the value fo used for recording the hologram of the ring (cf. FIG. 8).
• the dimension of the FOURIER transform of the family of these disks is the same as that of the ring recorded on the hologram. There will then be a maximum intercorrelation in the plane 19 where as many light points will appear as disks of radius at least equal to R, these points being located at the same coordinates as the centers of the disks in question in the plane 15.
• a threshold amplifier can be used to eliminate the parasitic terms of the feet from the cross-correlation functions. It then suffices to clip the remaining peaks and count them digitally or analogically.
• a lens 23 whose focal length is such that the system allows to illuminate by a plane wave an object plane 24 containing the family of objects to be treated;
• variable focal length lens 26 which performs the spatial FOURIER transform of the plane 24 into a plane 27; - Said plane 27 containing the hologram of the reference ring and the normal of which makes with the optical axis, an angle ⁇ equal to that defined during the establishment of the hologram of the ring (cf. FIG. 8 ); a plane-r-image 28 receiving by reflection on the separating wool 25 the function of intercorrelation of the plane 2.4 with the reference object whose hologram has been placed in the plane 27.
• the light rays pass twice through the same objective with variable focal length which performs the transformation of FOURIER on the outward and return journey.
• This assembly comprises:
• a separating plate 32 dividing the beam from 31 into two beams
• a lens 38 of focal length fo which collects the light diffracted by the plane 37 and which is in fact the spatial FOURIER transform in a plane 39;
• a mirror 40 receiving the beam transmitted by the. plate 32 and which reflects it towards a spatial filter 41,
• a lens 42 delivering a plane wave which constitutes a reference wave, the normal to the plane 39 making an angle ⁇ 2 with respect to the optical axis of the lens 42.
• the assembly comprises: an object slot 50 having a certain width, a first objective 51, a plane 52 where the multiple hologram in question obtained is placed with reference rings whose centers are spaced apart by an equal interval at the width of the slot 50, a second objective 53, an image plane 54 in which one obtains as many images as the reference rings used. These images are adjacent and offset along a direction perpendicular to the slit. Each of these images represents the intercorrelation function of the object to be studied, diaphragmed by the slot 50 with one of the reference rings.
• the photodetector (not shown) can then advantageously be of the vidicon type operating by line-by-line scanning of the image plane in a direction parallel to the object slot.
• the method of the invention can be used in particular to study the whole range of objects having a substantially circular or spherical shape.
• the study of the grain size of the grains in suspension in a liquid can be carried out either directly on the sample by placing it in a transparent sheath arranged in the object plane of the device, or by means of photographs of these samples. Mention may also be made, as an application of the invention, of the study of the particle size of underwater nodules. This study is carried out using images obtained via a camera and a photographed television screen or via a camera and a coherent imager.
• the method of the invention can naturally be applied to the study of objects which, although not circular, are in the form of neighboring homothetic surfaces delimited by simple closed curves. Indeed, it will then suffice to complete the optical arrangements illustrated above with an appropriate optical system - prism or mirror - allowing the object plane or the hologram plane to rotate.
• the invention is also applicable to the recognition of shapes and in particular letters, numbers or various signs

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Theoretical Computer Science (AREA)
• Optics & Photonics (AREA)
• Holo Graphy (AREA)
• Instruments For Measurement Of Length By Optical Means (AREA)
• Length Measuring Devices By Optical Means (AREA)
• Character Discrimination (AREA)

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• 1979
  • 1979-06-18 FR FR7915519A patent/FR2459449A1/fr active Granted
• 1980
  • 1980-06-17 JP JP50134680A patent/JPS56500710A/ja active Pending
  • 1980-06-17 EP EP80400884A patent/EP0021982A1/fr not_active Ceased
  • 1980-06-17 WO PCT/FR1980/000096 patent/WO1980002882A1/fr not_active Ceased

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