EP0028548A1 - Real-time optical correlation system - Google Patents

Abstract

The invention relates to a correlation system providing the correlation function of two objects (A and B) illuminated in coherent light, using the principle of double diffraction. The correlation system according to the invention is essentially characterized in that it uses as recording medium a slide (10) of recyclable material, that is to say writable and erasable at will, such as bismuth oxide- silicon. The recording made in the slide by the superposition of the beams of illumination of the objects is read by a beam (FL) undergoing an angular scanning making it possible to optimize the diffraction efficiency for all the correlation peaks in an observation plane. Application to pattern recognition, the pursuit of targets.

Description

The invention relates to optical correlation systems making it possible to obtain the function of correlating one image with another. Such systems allow for example to recognize a predetermined graphic in a composite pattern.

A known correlation method consists in recording on a photosensitive support a system of interference fringes representing the diffraction pattern provided by a lens which corresponds to two light beams on the path of which are interposed respectively two objects with non-uniform transparency, generally l object to analyze and a reference object. This photosensitive support is read by a reading beam and a characteristic intensity distribution is obtained in the focal plane of a second lens in certain zones of the correlation product between the two objects. In the case where the reference object carries a pattern that one seeks to find in the object to be analyzed, the image obtained is formed of peaks indicating the presence and the position of the reference pattern in the object to be analyzed. . This correlation method has been tested with interference fringe supports of photographic and thermoplastic types.

Such supports require a treatment, chemical or thermal, between the recording and reading phases, which implies a delay between the two operations. In addition, they are generally not erasable. They therefore do not allow operation in real time.

The object of the invention is to use the correlation method described above in real-time applications such as automatic reading, target tracking, machine guidance, etc. To this end, the correlation system according to the invention comprises a photosensitive support that is continuously recyclable, that is to say writable without development and erasable at will. Particularly suitable materials are electro-optical materials such as bismuth-silicon oxide, in which spatial variations in light intensity can be converted in real time into spatial variations in refractive index. Registration is done by volume, and not on the surface, the optimal reading conditions are defined by Bragg's law which imposes a value distinct from the reading angle for each spatial frequency recorded. Knowing that the correlation peaks are linked to the recorded spatial frequencies, the invention provides an angular scanning of the read beam making it possible to explore the whole spectrum of the recorded spatial frequencies.

• D'autres caractéristiques et avantages de l'invention apparaîtront dans la description ci-après, en référence aux figures annexées où :

• - Les figures 1 et 3 représentent un système de corrélation de type connu ;
• - Les figures 2 et 4 sont des figures explicatives du fonctionnement du système représenté sur les figures 1 et 3 ;
• - La figure 5 représente un mode de réalisation de l'invention ;
• - Les figures 6 et 7 représentent des variantes de réalisation de l'invention.

The object of the present invention is an optical correlation system making it possible to obtain the correlation function of a first object by a second, comprising means for illuminating the objects by respectively two coherent beams, first focusing means projecting in a focal plane (PF) an illumination representative of the algebraic sum of the Fourier transforms of the light amplitudes transmitted by the two objects respectively, a photosensitive support recording this illumination, other means of illuminating the photosensitive support, second means of focusing projecting into a focal plane (P) an illumination representative of the Fourier transform of the recorded illumination, and means for detecting correlation peaks located in an area of the focal plane (P) characterizing the correlation function, characterized in that the photosensitive support consists of a continuously recyclable material in which the recording const itue a three-dimensional network of fringes and in that the other means for illuminating the photosensitive support include angular scanning means ensuring optimum diffraction efficiency successively for the different points of the observed area of the plane (P):

• Other characteristics and advantages of the invention will appear in the description below, with reference to the appended figures where:

• - Figures 1 and 3 show a known type of correlation system;
• - Figures 2 and 4 are explanatory figures of the operation of the system shown in Figures 1 and 3;
• - Figure 5 shows an embodiment of the invention;
• - Figures 6 and 7 show alternative embodiments of the invention.

où λ₁ est la longueur d'onde optique des faisceaux FA et F_B et α₀ le demi- angle entre les deux faisceaux qui interfèrent. La répartition d'intensité résultante selon des axes x,y du plan PF est proportionnelle au carré du module de la transformée de Fourier de la répartition d'amplitude dans le plan objet PO. Les positions des objets A et B dans ce plan sont représentés sur la figure 2. x , y étant les axes parallèles à x,y dans le plan PO, on appelera centres des deux objets les points de coordonnées respectives (a, o) et (- b, o), si bien que les transmittances des deux objets peuvent s'exprimer sous la forme : A(x - a, y ) et B(x + b, y ). Leurs transformées de Fourier peuvent s'écrire respectivement : TA e^2πjax, TBe^-2πjbx, si bien que la répartition d'intensité dans le plan PF s'écrit :

FIG. 1 represents a known optical system carrying out the recording trement of the algebraic sum of the Fourier transforms of two two-dimensional functions. The two functions represent the transmittances of the two objects A and B lit by parallel beams FA and FB contiguous or not coming from the same coherent source. Objects A and B are placed on either side of the optical axis z of a lens L ₁ of focal distance fl, in the same plane PO perpendicular to this axis. In the focal plane PF of the lens L ₁ , an amplitude distribution proportional to the Fourier transform of the amplitude distribution in the object plane is obtained. A photosensitive support 1: photographic or thermoplastic being placed in the plane PF records the superposition of fringe systems of intensity of different steps, the average step p being equal to

where λ ₁ is the optical wavelength of the beams FA and F _B and α ₀ the half angle between the two beams which interfere. The resulting intensity distribution along x, y axes of the PF plane is proportional to the square of the module of the Fourier transform of the amplitude distribution in the object plane PO. The positions of the objects A and B in this plane are represented in figure 2. x, y being the axes parallel to x, y in the plane PO, we will call centers of the two objects the points of respective coordinates (a, o) and (- b, o), so that the transmittances of the two objects can be expressed in the form: A (x - a, y) and B (x + b, y). Their Fourier transforms can be written respectively: TA e ^2πjax , TBe ^-2πjbx , so that the intensity distribution in the plane PF is written:

incidence normale. Les différents réseaux enregistrées diffractent le faisceau FL selon des angles θ qui dépendent des pas p : où λ₂ est la longueur d'onde du faisceau F_L.Once recording on the photosensitive directed support, it undergoes chemical treatment or appropriate heat then is read by the optical system shown in Figure 3. The reading is performed by a beam F _L parallel coherent illuminating the photosensitive medium 1 penny

normal incidence. The different networks recorded diffract the beam FL at angles θ which depend on the steps p: where λ ₂ is the wavelength of the beam F _L.

A new Fourier transformation is carried out by a second lens L ₂ with focal distance f ₂ . We therefore obtain in its focal plane P with respect to axes X, Y parallel to the axes x, y an intensity distribution I (X, Y) equal to the sum of three terms:

. On obtient les produits de corrélation des deux fonctions A et B, centrés autour des points M : (k(a+b),o) et N : (-k(a+b),o).The sign Ⓧ expresses the correlation product. k is the magnification ratio:

. We obtain the correlation products of the two functions A and B, centered around the points M: (k (a + b), o) and N: (-k (a + b), o).

The limits of the images in the plane P of the three terms of the expression above have been represented in FIG. 4: I, II, III in the case where, in the plane PO, the two objects are squares of side 1 Depending on the correlation between the two objects, appear in the plane P, which is that of the figure, peaks of light intensity whose position is included in the frames shown, on side 2K1 and is characteristic of the presence of 'same signal in both objects. By way of example, the same cross-shaped pattern, represented in the plane PO in FIG. 2, occupies in the two objects the respective positions (x _A , y _A ) and (-x _B , -y _B ). The presence of this signal corresponds in the image plane P to two peaks of intensity P _II and P _III symmetrical with respect to the axis Y of coordinates in the axes X, Y: ± k (x _A + x _B ), k (y _A + y _B ). Because of the presence of the term T _I , whose image I is centered on the intersection O of the axes X, Y, it is preferable, to avoid any superposition of the three terms, that the distance from the centers of the objects: a + b is greater than their width 1. Of course, all that has been said for the direction x would also be valid for the direction y, in the general case where the centers of objects A and B are not located on the x axis . In the description of the invention which follows, it will be considered for simplicity that a = b and that the objects A and B are centered on the axis x.

FIG. 5 represents an embodiment of the invention. Part of the elements of the correlation device are common with those of Figures 1 and 3 and bear the same references. The interference fringes resulting from the superposition of the beams F _A and F _B which illuminate the objects A and B, after the focusing effected by the lens L are recorded in a photosensitive plate 10 centered on the focal image plane PF of the lens L ₁ and made of an electro-optical material polarized by a field electric obtained by means of a voltage source V. Its orientation is such that the electric field produces a transverse electro-optical effect. Spatial variations in light intensity existing in the PF plane are instantly reflected in the plate by spatial variations in refractive index, the interference planes being almost perpendicular to the direction of the applied electric field. The index modulation disappears as soon as its cause, that is to say the presence of objects A and B in the path of the beams disappears. We therefore obtain a real-time registration, erasable at will. To obtain all the information with maximum resolution, the thickness of the crystal must be equal to or greater than the width of the diffraction zone corresponding to the intersection of the diffraction ellipsoids of the two beams FA and FB whose dimensions depend on the numerical aperture of the lens L ₁ . A useful thickness can be defined, which is in any case clearly greater than the wavelength of the two beams so that the recording in the slide can be considered as three-dimensional. It is a superposition of arrays of surfaces. When the width of the blade 10 in the direction perpendicular to the plane of the figure is not too large, (typically of the same order of magnitude as the thickness), these surfaces can be assimilated to planes perpendicular to the plane of the figure and whose pitch p and inclination f relative to the z axis depend on the angle of the interfering rays, the wavelength λ ₁ and the refractive index n of the plate 10.

The materials which can be used to form the strip 10 must be both photosensitive and electro-optical. Bismuth-silicon oxide (Bi ₁₂ Si 0 ₂₀ ) and bismuth-germanium oxide (Bi ₁₂ Ge 020) are particularly suitable for the invention because they are very sensitive, in the range of wavelengths commonly used (visible and infrared spectrum) and it is known to obtain single crystals of sufficient dimensions (several cm) and having good optical qualities. Other materials could also be suitable but generally do not have as good optical qualities: potassium niobate (KNb0 ₃ ), KTN, SBN.

Il traverse ensuite un élargisseur de faisceau 6 et est renvoyé par une lame semi-transparente L en direction de la lame 10. L'élargissement apporté par l'élargisseur 6 permet d'éclairer toute la zone enregistrée de la lame 10. La lame semi-transparente L s'interpose sur le trajet des faisceaux FA et F_B et doit être conçue de façon à laisser passer ces faisceaux. Elle introduit inévitablement un déphasage, qui n'est pas génant car identique pour les deux faisceaux. L'orientation par rapport à la lame 10 du faisceau de lecture, parallèle, est variable en fonction du temps et commandée par le déflecteur 5. Après diffraction par la deuxième lentille L₂ et traversée d'un filtre 2 et d'un polariseur 3, on obtient dans le plan focal P de la lentille L₂ des pics de corrélation analogues à ceux obtenus par exemple avec une plaque photographique. Toutefois, à chaque instant, pour une orientation donnée, seuls sont obtenus avec un rendement maximum les points situés sur une droite perpendiculaire au plan de la figure et auxquels on peut associer une inclinaison ϕ et un pas p de réseau de plans dans la lame 10 pour lequel l'incidence e du faisceau par rapport aux plans est l'incidence de Bragg : défini par

. Sont également obtenus avec un rendement réduit les points voisins pour lesquels l'incidence est comprise dans une gamme

où n est l'indice de réfraction de la lame 10 et d l'épaisseur de la zone utile de diffraction dans la lame 10. Pour examiner la totalité de la zone III (ou II) centrée autour du point N (ou M), il faut balayer toute la plage angulaire correspondante. Tous les pics de corrélation apparaissent ainsi séquentiel- lement.When reading, to obtain an optimum efficiency in one of the diffraction orders, it is necessary to respect the Bragg condition which defines, for each interference system, the angle between the paral beam the reading line and the diffraction planes. Since this condition cannot be achieved simultaneously for all of the superimposed systems, the invention provides for angular scanning of the reading beam F _L. This is supplied by a laser 4 of low power and of wavelength λ ₂ chosen outside the range of wavelengths to which the material constituting the lamelO is sensitive. The beam F _L is deflected by a conventional acousto-optical or mechanical deflector 5 carrying out the angular scanning in a manner which will be detailed later. The beam F _L is represented in the figure in its average position, corresponding to a network of planes parallel to z (ϕ = o) of step

It then crosses a beam expander 6 and is returned by a semi-transparent strip L in the direction of the strip 10. The widening provided by the spreader 6 makes it possible to illuminate the entire recorded area of the strip 10. The semi strip -transparent L is interposed on the path of the beams FA and F _B and must be designed so as to allow these beams to pass. It inevitably introduces a phase shift, which is not annoying since it is identical for the two beams. The orientation relative to the blade 10 of the reading beam, parallel, is variable as a function of time and controlled by the deflector 5. After diffraction by the second lens L ₂ and crossing of a filter 2 and a polarizer 3 , one obtains in the focal plane P of the lens L ₂ correlation peaks similar to those obtained for example with a photographic plate. However, at each instant, for a given orientation, only the points located on a line perpendicular to the plane of the figure are obtained with maximum efficiency and to which an inclination ϕ and a pitch p of network of planes in the blade 10 can be associated for which the incidence e of the beam with respect to the planes is the Bragg incidence: defined by

. Neighboring points for which the incidence is within a range are also obtained with reduced efficiency.

where n is the refractive index of the plate 10 and d the thickness of the useful diffraction zone in the plate 10. To examine the whole of the zone III (or II) centered around the point N (or M), it is necessary to scan the entire corresponding angular range. All the correlation peaks thus appear sequentially.

The detection of the correlation peaks is carried out by means 18 such as, for example: mosaic of detectors or vidicon tube connected to a television system. In the latter case, the scanning beam scanning speed is advantageously equal to the television scanning speed.

By way of nonlimiting example, the device was produced with a monocrystalline bismuth-silicon oxide blade of length 2 mm and thickness 1 mm polarized by a voltage V ₀ of the order of 2000 V, which provides an electric field of the order of 10 kV / cm ¹ , the wavelength of the illumination beams λ, was 0.5 μm, which corresponds to good sensitivity of the crystal. The reading beam F _L came from a Helium-Neon laser of a few mW of power and wavelength λ ₂ = 0.6 μm.

The focal length of the lens L ₁ was: 30 cm and that of the lens L ₂ : 10 cm. The magnification k was therefore equal to 0.4.

The objects were 2 cm x 2 cm slides. The extent of each zone II and III was thus 0.8 x 0.8 cm, observable with a vidicon tube whose diameter is typically 1.5 cm. Instead of using a HeNe laser, a semiconductor laser with a wavelength of 0.8 µm can also be used.

The system represented in FIG. 5 admits numerous variants, in particular as to the means supplying the beams F _A , F _B , F _L , to the means for detecting the correlation peaks obtained in the plane P and at the respective location of the different elements optical. FIG. 6 represents an alternative embodiment concerning the means supplying the bundles F _A and F _B. It avoids the use of a lens L ₁ with a large aperture. In fact, according to the previous embodiment, the width of the objects being typically 2 or 3 cm and the distance between their centers at least equal to this value, the necessary diameter of the lens L ₁ reaches close to 10 cm. According to the proposed variant, the lens L ₁ is replaced by two lenses LA and L _B , smaller since their dimensions correspond to those of objects A and B, and whose optical axes are merged respectively with the axes of the beams FA and F _B which are no longer parallel but each form with respect to the z axis an angle ± αo, which remains unchanged after the lenses. The beams FA and F _B come from a single beam F delivered by a laser 7, argon for example, after widening by a widener 13 and separation and return by mirrors 14, 15, 16, 17. Objects A and B are centered with respect to the respective axes of the two beams. The correlation system is represented in the case of its application to the tracking of targets: Object A is the reference object. It is for example constituted by a slide representing a fixed landscape. Object B has a variable pattern. It consists of an electro-optical modulator controlled by a signal S from, for example, a television camera aiming at the object to be pursued. The correlation system allows the detection of the coincidence between the target landscape and the fixed landscape.

To obtain better linearity in the response of the electro-optical crystal, it may be advantageous to create a constant level of modulation by means of a reference light beam analogous to that present in a conventional holographic system. The illumination due to this reference beam creates a first variation of index which is not spatially modulated, to which are added the variations due to interference systems due to the illumination beams of objects A and B. additional interference is formed but it can be ensured, by suitably choosing the inclination of the reference beam, that the reflected rays which result therefrom are clearly outside the zones examined, centered around I and J. An exemplary embodiment of system in which a constant level of index modulation is created is shown in FIG. 7. The reference beam FR comes from the same source 7 as the beams F _A and F _B. A semi-reflecting plate 8 and a mirror 9 make it possible to separate the beam F _R. The beams F _A F _B on the one hand and F _R on the other hand are widened by means of wideners 11 and 12. The rest of the system is similar to that of FIG. 5 or of one of the variants thereof. .

Claims (8)

1. Optical correlation system making it possible to obtain the correlation function of a first object by a second, comprising means for illuminating the objects by respectively two coherent beams, first focusing means projecting into a first focal plane (PF ) an illumination representative of the algebraic sum of the Fourier transforms of the light amplitudes transmitted by the two objects respectively, a photosensitive support, located in the first focal plane (PF), recording this illumination in real time, and consisting of a material continuously recyclable in which the recording constitutes a three-dimensional network of fringes, other means of illuminating the photosensitive support, second focusing means projecting into a second focal plane (P) an illumination representative of the Fourier transform of the illumination recorded, and means for detecting correlation peaks located in an area of the second focal plane (P) and charac testing the correlation function, characterized in that the other means for illuminating the photosensitive support include angular scanning means ensuring optimum diffraction efficiency successively for the different points of the observed area of the second focal plane (P). 2. Optical correlation system according to claim 1, characterized in that the means for illuminating the objects comprise a laser and optical means providing two beams parallel to the axis of the lenses on either side of this axis, the objects being respectively centered on the axes of the two beams. 3. Optical correlation system according to claim l, characterized in that the means for illuminating the objects comprise a laser and means for separating the laser beam providing two parallel beams whose axes form between them a predetermined angle and intersect at the vicinity of the center of the photosensitive support, the first focusing means being made up of two spherical lenses whose axes coincide respectively with the axes of the two beams and whose focal points coincide with the meeting point of these axes, the objects being respectively centered on the axes of the two beams. 4. Optical correlation system according to any one of claims 1 to 3, characterized in that the other means for illuminating the photosensitive support comprise a laser. 5. Optical correlation system according to any one of claims 1 to 4, characterized in that the photosensitive support is a monocrystalline plate of bismuth-silicon oxide. 6. Optical correlation system according to any one of claims 1 to 4, characterized in that the photosensitive support is a monocrystalline plate of bismuth-germanium oxide. 7. Optical correlation system according to any one of claims 1 to 6, characterized in that the photosensitive support is polarized by an electric field obtained by means of a voltage source. 8. Correlation system according to any one of claims 1 to 7, characterized in that it further comprises third means for illuminating the photosensitive support making it possible to create in this photosensitive support a constant level of modulation, superimposed on the network. three-dimensional fringes corresponding to the recording of the illumination representative of the algebraic sum of the Foùrier transforms of the light amplitudes transmitted by the two objects.

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