EP0099769B1 - Device for analyzing a spatial field for the angular localization of a radiating object - Google Patents

Description

The present invention relates to a device for analyzing a spatial field for the angular localization of a radiating object in the field and in which the video image is formed by circular field scanning. The invention applies more precisely to systems equipped with a detector strip as a photodetector or image sensor, and in which a circular analysis of the image of the instantaneous field observed is carried out.

A more particularly envisaged application is in the field of seeker for missiles, mainly infrared seeker with imaging or pseudo-imaging.

It is known from the book by JM Lloyd entitled "Thermal imaging systems" published by Plenum Press 2nd edition 1979, in particular on pages 319, 322 and 323, to produce a circular scanning of image by rotation of the latter in front of a detector strip arranged radially , using a right reflective dihedral driven in rotation around an axis perpendicular to its edge.

According to a first solution, the axis of rotation is parallel to the axis of the receiving optics and an additional mirror is used to reflect the radiation towards the bar which is offset relative to the axis. According to another solution, the axis of rotation is perpendicular to that of the optics and an internal mirror inclined at 45 ° on the optical axis returns the radiation towards the dihedral which reflects it towards the detector placed at the rear of an opening in the mirror.

None of these solutions is easily adaptable with a gyro-stabilized link of the optics by default of symmetry of execution with respect to the optical axis; these assemblies do not allow the router to be used in a simple manner to rotate the dihedral, there is a photometric loss because of the additional mirror which intercepts part of the radiation, or because of the radiation which is not reflected at the location of the hollowed out part.

The object of the invention is to provide a video image forming device in circular field scanning in which these drawbacks are avoided and which fits well in a gyro-stabilized version.

It is known from patent publication GB-A-2,075,789 an imaging device which uses the above-mentioned rotating scanning with dihedral optics and which is intended for a missile seeker. It includes a dioptric receiving optic with folding of the rays by a set of mirrors, two of which form a dihedral pierced for the passage of rays focused by the receiving lens.

It is also known, according to patent publication US-A-4,266,173, a self-directing missile device with four quadrant detector and equipped with anti-roll stabilization. According to one embodiment, a receiving optical mirror is mounted on the gyroscopic router and is therefore driven in rotation; the connection with the center of suspen'sion is done by a ball mounting allowing the relative axial depointing.

The invention proposes to produce a video image formation device by circular field scanning, comprising a receiving optic which produces the image of the field observed in a plane perpendicular to the axis of the receiving optics, a photodetector device constituted a photodetector strip located in this plane and arranged along a radius of the circular field scan, and intermediate optical means for producing a circular image scan in this plane, these optical means using a straight reflective dihedral driven in rotation around an axis perpendicular to its edge and corresponding to the axis of the receiving optics, the video image forming device being characterized in that the receiving optics is constituted by means of a Cassegrain assembly comprising a main mirror which focuses the incident radiation on said photodetector strip by means of a secondary mirror mounted on the same optical axis, the secondary mirror re being constituted by said rotating dihedral.

The two aforementioned patent publications do not use, as in the proposed solution, a Cassegrain receiving optic, the secondary mirror of which is constituted by a dihedron rotating around the optical axis; such an optic only comprises mirrors which gives it several advantages in particular compared to a receiving lens solution from the point of view of lightness and of the optical transmission coefficient.

• - Fig. 1, un schéma simplifié d'un dispositif de formation d'image vidéo conforme à l'invention;
• - Fig. 2, un schéma rappelant le principe d'élaboration d'un balayage circulaire d'image à l'aide d'un dispositif optique tournant;
• - Fig. 3, le cheminement optique du rayonnement dans un montage dérivé de celui de la Fig. 1 et dans un plan passant par l'axe optique et orthogonal à l'arête du dièdre réfléchissant;
• - Fig. 4, le cheminement du rayonnement reçu dans la même version que la Fig. 3 mais cette fois dans un plan orthogonal au précédent et passant par l'arête du dièdre réfléchissant;
• - Fig. 5, un exemple de réalisation d'un dispositif de formation d'image vidéo conforme à l'invention dans une version gyroscopée à détecteur fixe;
• - Fig. 6, un autre mode de réalisation d'un dispositif de formation d'image vidéo conforme à l'invention dans une version gyroscopée à détecteur porté par la tête stabilisée.

The features and advantages of the present invention will appear in the description which follows, given by way of nonlimiting example, with the aid of the appended figures which represent:

• - Fig. 1, a simplified diagram of a video image forming device according to the invention;
• - Fig. 2, a diagram recalling the principle of developing a circular image scan using a rotating optical device;
• - Fig. 3, the optical path of the radiation in an arrangement derived from that of FIG. 1 and in a plane passing through the optical and orthogonal axis to the edge of the reflecting dihedral;
• - Fig. 4, the path of the radiation received in the same version as FIG. 3 but this time in a plane orthogonal to the previous one and passing through the edge of the reflecting dihedral;
• - Fig. 5, an exemplary embodiment of a video image forming device according to the invention in a gyro version with fixed detector;
• - Fig. 6, another embodiment of a video image forming device according to the invention in a gyroscopic version with detector carried by the stabilized head.

The diagram in FIG. 1 represents a video image forming device according to the invention, comprising a receiving optic 1 which produces the image of the field observed in a plane, a photodetector array 3 located in this plane and intermediate optical means using a right dihedron reflective 2 driven into rotation around an axis perpendicular to its edge to produce a circular scan by rotation of the image of the field in front of the bar 3 arranged radially. According to the invention, the receiving optics is constituted by a Cassegrain assembly with a main mirror 1 which focuses the incident radiation via the secondary mirror 2 mounted on the same optical axis Z. The secondary mirror is constituted by the rotating dihedron 2 , the axis of rotation of the reflecting dihedron corresponding to the optical axis Z of the Cassegrain assembly. The radiation received after reflections on the mirrors 1 and 2 is focused in the plane of the bar 3. The block 4 represents means for driving the reflecting dihedron in rotation 2. The block 5 represents the processing and operating circuits of the SV video signal detected by the photodetector element strip 3.

To produce a circular scan using a rotating optical device, it is necessary and sufficient that this device has certain characteristics which are recalled with the aid of FIG. 2. In a longitudinal section plane, called PNI non-inverting plane, the device forms a direct image of an object; thus the object OB has for image O'B 'of the same direction. In a second longitudinal section plane, called the inverting plane PI and perpendicular to the previous one, the optical device forms an inverted image of an object; thus, the object OA will have the image O'A 'of reverse direction. The growth on this diagram has not been taken into account. It appears that the device is equivalent to a symmetry with respect to a straight line in the image plane. If the optical device D is rotated by an angle 8, the image rotates in the same direction by a double angle of 26, this property being general for all these types of devices.

A right-angled dihedral consisting of two plane mirrors placed in an optical path thus provides a symmetrical image with respect to the line of intersection of the two faces of the dihedron, called the edge of the dihedron. This property of geometric optics is well known and if we consider an axis of rotation perpendicular to the edge, we have an inverting plane containing the axis of rotation and perpendicular to the edge of the dihedral and a non-inverting plane containing the axis of rotation and the dihedral edge. The mounting with right dihedral turning thus solves well the problem of the circular sweeping of the field.

The solution chosen which consists in replacing the secondary of the Cassegrain type optics by the dihedral whose edge takes the place of the secondary mirror is particularly advantageous in an application to a seeker with video imaging, given that the axis of rotation is finds it confused with the optical axis of the device. In terms of principle, the condition to check is that the edge of the dihedral passes through the axis of rotation and is perpendicular to this axis. Thus, it is not obligatory that the dihedral is positioned symmetrically with respect to the axis of rotation but, for practical and production considerations, the symmetrical position will be the one chosen because it avoids having disadvantages on the optical plane on the one hand taking into account that the pupil would be asymmetrical, on the mechanical plane on the other hand in particular in a gyro-stabilized version as a result of the imbalance and balancing questions that this is likely to pose .

Figures 3 and 4 show the optical path of the light rays. It was considered that the angular field observed is between the extreme directions given by the rays R2 and R3, the radius R1 corresponding to the middle direction parallel to the optical axis Z of the system. The dihedral edge is perpendicular to the figure plane in Fig. 3 and passes through this plane in the case of FIG. 4. With respect to the structure of FIG. 1, the assembly includes additional optical elements, in particular an image recovery optic 7 which makes it possible to place the detector 3 further downstream on the Z axis. In a version according to FIG. 1 the detector would be placed at the focal plane indicated PF. Each of these versions corresponds to an embodiment described later using FIGS. 5 and 6 where the sensor is fixed or carried by the gyrostabilized part. The Cassegrain assembly grouping the main mirror 1 and the secondary mirror formed by the rotating dihedral 2 focuses the field radiation on the focal plane PF where the image of the field will be located. The optics 7 constituted by a converging lens makes it possible to take up the image of the field in the plane PF to reform it downstream in a plane where the photodetector bar 3 will be positioned. This version corresponds to that of a fixed sensor carried by the missile body. As can be seen, the lens 7 is not useful in its central part and this can possibly be hollowed out. In addition, a spherical mirror 8 is provided, the center of curvature of which is located on the axis Z in the plane where the bar is positioned, so as to return the radiation reflected by the latter towards the detector. The reflecting deposit is distributed on a spherical support, providing the transparent circular area necessary for the passage of useful radiation. The mirror is located where a real pupil PR is formed. The support can be produced using an element made of transparent material, glass for example, or using two opaque elements, metallic for example. The mounting with the mirror 8 is useful in the case of a cooled infrared detector because it makes it possible to limit the noise resulting from the detection of the continuous background by the detector. This technique is described in French patent publication number 2,487,512 (filing no. 80.16126 of July 22, 1980). It is advantageous in the case of an infrared sensor, fixed at the center of suspension of the gyroscope, to place in the real pupil plane PR the spherical mirror 8 whose center is the detector itself which makes it possible to reduce the noise by detector output. The advantage is all the more important since in a concept with a fixed detector, because of the deflections of the line of sight, the field of vision of the detector is necessarily greater than what would be necessary to cover the useful pupil.

Fig. 5 relates to an achievement for a dihedral circular scanning seeker in which the detector or sensor is fixed, carried by the missile body. The detector bar 3 is positioned on the cold table of a cooling device 10, for example a cryostat. The bar is positioned at the center of suspension of the gyroscope which is indicated by the intersection of the Y axis with the Z axis, the Y axis being one of the axes of mechanical rotation of the gimbal. In this arrangement, the optical assembly 1 and 2 is driven in rotation by the gyroscopic router 11 which gives an angular speed of image analysis double the speed of rotation of the router. There is a slight difference in terms of the recovery optics vis-à-vis the representation. 3 and Fig. 4 in the sense that this optic is constituted by two lenses, a lens 7A which transforms the beam coming from the focal plane into a parallel beam and a lens 7B which reconcentrates the beam at the level of the detector 3. The spherical mirror 8 is constituted by a set of two mechanical parts 8A and 8B, the part 8B being integral with the transparent optics 7B which is hollowed out in its central part. The bearings 12 make it possible to separate the terminal optics 7B-8A-8B from the rotation of the router. The router 11 comprises an annular magnet 13 which undergoes the magnetic effects generated by the coils 14 said to be of precession and which are fixedly mounted on a mechanical support 15 which is integral with the body 16 of the missile. The router 11 is decoupled from the frame 17 linked to the central gimbal by the ball bearings 12. The second mechanical axis of the gimbal is perpendicular to the plane of the figure and not visible in the median plane of cut shown. The rotation along the Y axis is decoupled from the body of the missile 16 by the ball bearings 18. The secondary mirror 2 consisting of the right dihedron is supported by arms 19, for example two thin arms to mechanically couple it to the spinning top. Note also the presence of a diaphragm 20 which limits the entry of stray radiation.

Fig. 6 shows a second application to a seeker with circular dihedral scanning and in which this time the sensor is carried by the gimbal and no longer by the missile body, the detector 3 is thus secured to the angular deflection of the aiming axis Z and no longer from the longitudinal axis of the missile. In this version, the detector will preferably consist of a detector strip with multiplexing integrated in the focal plane in order to minimize the number of connecting wires going to downstream processing circuits. In this configuration the optics are particularly simple since it comprises the main mirror of the aspheric Cassegrain, and the secondary mirror constituted by the dihedral 2. There is no dioptric element, the spectral band is therefore very wide. The angular deflections of the aiming axis Z are not limited as in the case of FIG. 5 since the sensor is integral with the gyroscopic structure.

Claims (5)

1. Device for forming a video image by circular field scanning, comprising an optical receiving unit (1) producing the image of the observed field in a plane perpendicular to the axis (Z) of the optical receiving unit, a photodetector device made up by a photodetector bar (3) located in this plane and arranged along a radius of the circular field scanning, and intermediate optical means to produce a circular image scanning in this plane, these optical means using a reflecting upright dihedron (2) rotated about an axis perpendicular to its edge and corresponding to the axis of the optical receiving unit, the video image forming device being characterized in that the optical receiving unit is constituted by means of a Cassegrain arrangement comprising a main mirror (1) focusing the incident radiation on said photodetector bar (3) by a secondary mirror (2) mounted on the same optical axis (Z), the secondary mirror being made up by said rotating dihedron.

2. Device according to claim 1, and wherein an additional optical element is provided to recover the field image formed in the focal plane of the optical receiving unit to restore this image in another plane wherein the bar is located, the image recovering optical element being a convergent lens, characterized in that this lens (7) determines, with the Cassegrain arrangement, an intermediate plane between the lens and the photodetector bar wherein a real diaphragm (PR) of circular shape is formed, and in that a spherical mirror (8) is arranged in this plane having a transparent circular zone corresponding to a diaphragm to pass only the useful radiation, the spherical mirror being reflecting (8A, 8B) outside of this zone to reflect onto the detector the radiation emitted by the latter.

3. Device according to any of the preceding claims, characterized in that it comprises gyroscopic stabilizing means for the optical axis (Z) and that the dihedron is driven by the gyro (11) of the gyroscopic arrangement.

4. Device according to claim 3, used to constitute a missile homing device having a movable detector carried by the gyroscope.

5. Device according to any of the combined claims 2 and 3, used to constitute a missile homing device with a stationary detector, characterized in that the detector bar (3) is positioned in the suspension center of the gyroscope and is joined with the body (16) of the missile.

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