IL195989A

IL195989A - Passive three-field optronic system

Info

Publication number: IL195989A
Application number: IL195989A
Authority: IL
Original assignee: Thales Sa
Priority date: 2006-06-16
Filing date: 2008-12-16
Publication date: 2012-05-31
Also published as: FR2902531B1; EP2030066B1; IL195989A0; WO2007144290A1; KR101387863B1; FR2902531A1; KR20090030309A; EP2030066A1

Description

PASSIVE THREE-FIELD OPTRONIC SYSTEM THALES C: 67533 THREE-FIELD PASSIVE OPTRONIC SYSTEM The field of the invention is that of two-field passive imaging systems, notably infrared (IR) systems in the 3-5 pm band or in the 8-12 pm band used for GC (wide field) observation for the purpose of detecting a target and PC (narrow field) for the purpose of recognition and identification of the target.

A wide field is typically between 8° and 10° and a narrow field between 2° and 4°.

A two-field, PC and GC, passive IR imaging system 100, described with reference to figures 1 a to 1 c typically comprises on the same optical axis: - detection means comprising a two-dimensional IR array detector 10 placed in a cryostat in order to be cooled, - an aperture diaphragm 40 situated outside the cryostat (notably comprising a cold diaphragm 11 and closed by a window 12), but close to the latter, also called an imaging aperture, surrounded a priori by a mirror 41 for reducing the stray thermal radiation of the structure, in order to optimize the photometric behavior of the assembly, as shown in figure 1c, - image forming means, called a "re-imager" 20, comprising a set of lenses able to carry out a scene-detector conjugation whilst guaranteeing a real entrance pupil and a real exit pupil; this aperture transport function imposes the presence of an intermediate focal plane in the optical combination, - a detachable afocal element 30, typically of magnification 3, used in PC mode; the fact that the entrance pupil of the re- imager is real makes it possible to limit the diameters of the components of the afocal element 30.

The term "detachable element" refers to an element that can be removed and then replaced as needed.

In GC mode, an example configuration of which is shown in figure 1a, the system does not comprise the afocal element 30 whilst, in PC mode, a magnifying afocal element 30 is placed on the optical axis upstream of the re-imager 20 as shown in the example of figure 1 b. The upstream- downstream direction is considered to be the direction of propagation of the light arriving from the outside in order to be focused onto the detector, that is to say from right to left in all of the figures.

It is also desired to provide a passive panoramic IR surveillance function, that is to say a very wide field (TGC), typically with 90° of horizontal field (in azimuth) and 30° of vertical field (in elevation). Passive optronic surveillance systems are systems not necessitating emission of radiation and are therefore discreet.

It is desired to provide this surveillance function using the same detector, notably in order to limit the cost.

The optical device of the two-field system has an instantaneous effective field limited to 10° horizontally and vertically whereas it is desired to be able to make an observation in a panoramic field.

A first solution shown in figure 2 consists in associating an existing two-field system with scanning 70 and counter-scanning 80 mirrors driven with rotational movements: the mirrors scan the scene and reflect it to the fixed two-field system 20 in order to be able to observe the scene in the observation TGC, the scanning figures being optimized notably as a function of the observation field and of the scanning speed. Such a scanning device is for example described in the patent FR 2 830 339. The rotation is carried out at a specified angular speed; the latter is limited in particular by the integration time of the detector, typically 3 msec (2 msec integration time and 1 msec reading time). This angular speed is for example between 250° and 3507second in one direction for 14ms in order to acquire the image and then in the other direction for 6ms in order to return to the initial position. It is not therefore possible to provide a scan rate of at least 0.5 Hz necessary to be able to use automatic detection algorithms on operational targets (tanks, missiles, etc).

Another solution consists in increasing the field of the system in order to cover the very wide field, by reducing the focal length F of the system. Conventionally, this reduction of F is carried out for a constant f- number F/D of the system. It therefore results in a reduction of the diameter D of the entrance pupil 60 (figure 3). This results in a great deterioration in the sensitivity of the system.

The purpose of the invention is to obtain an optronic system making it possible to provide the functions of recognition and identification, detection and passive surveillance with the same IR detector without the abovementioned disadvantages.

According to the invention, a two-field optical device able to cover a narrow and a wide field is coupled with a change of field device inside the re-imager making it possible to obtain a larger wide field, of the order of 30° X 30° and the assembly is installed on a scanning platform making it possible to finally cover a very wide horizontal field of 90°. The optical device furthermore comprises a counter-scanning device making it possible to stabilize the image during the image capture.

More precisely, the subject of the invention is a passive imaging system comprising: - an IR array detector, - a two-field optical device, able to form an image on the detector and comprising on an optical axis image forming means, a detachable afocal element of specified magnification G placed upstream of the image forming means, an aperture diaphragm disposed between the detector and the image forming means.

It is principally characterized in that it comprises means of scanning the detector and two-field optical device assembly in azimuth, and in that the aperture diaphragm is detachable and in that the optical device furthermore comprises a counter-scanning mirror able to be locked, disposed upstream of the image forming means, and in that the image forming means comprise means for obtaining a third field, the counter-scanning mirror and the means for obtaining a third field being disposed on said optical axis and able to form an image on said detector in order to obtain a three-field imaging system.

In this way a three-field IR system is obtained using a single detector and a single optical axis.

Once the aperture diaphragm is retracted, the aperture of the system is formed by the cold diaphragm of the cryostat; this makes it possible to reduce the f-number F/D in TGC configuration and therefore to maintain a virtually constant size of the entrance pupil of the re-imager whilst increasing the field.

The resultant field is of the order of 30°. It is sufficiently wide for a conventional rate of 1 Hz to be sufficient for a scanning of the 90° x 30" zone.

Moreover, the system obtained is compact. It thus makes it possible to use an entrance window of acceptable size: 130 mm X 75 mm instead of 75 mm X 75 mm for a two-field system. However, the solution of the prior art according to which the detector is fixed with respect to the scanning device necessitates a window of very large size, of the order of 60 cm in order to cover the angular displacement of the very wide field.

The means for obtaining another field comprise detachable optical components able to be inserted on the optical axis between the counter-scanning device and the detector, or optical elements that are movable in translation along the optical axis; these field-changing optical elements are disposed in such a way as to maintain a virtually fixed position of the entrance pupil of the re-imager.

According to one feature of the invention, the counter-scanning mirror is placed between the image forming means and the afocal element.

According to another feature of the invention, the image-forming means have a specified aperture and the means for obtaining another field modify that aperture.

Other features and advantages of the invention will appear on reading the following detailed description, given by way of non-limiting example and with reference to the drawings in which: the already-described figures 1 diagrammatically represent a two-field system in GC configuration (figure 1a), in PC configuration (figure 1 b) and a zoom on the cryostat and the entrance diaphragm (figure 1 c), the already-described figure 2 is a diagrammatic representation of a known three-field system, the already-described figures 3 are a diagrammatic illustration of the effect of changing focal length on the diameter of the entrance pupil of an optical system operating with a constant f-number, the figures 4 are a diagrammatic representation of a three-field system according to the invention in PC configuration (figure 4a), in GC configuration (figure 4b) and in TGC configuration (figure 4c), figure 5 is a diagrammatic. representation of the scanning platform and of the counter-scanning device controlled by the processing unit.

The same elements are given the same references in all of the figures.

With reference to the figures 4, the passive imaging system 200 according to the invention comprises the following elements, only some of which are used depending on the chosen mode: - detection means comprising a two-dimensional IR array detector 10 placed in the conventional manner in a cooling device such as a cryostat, used for all three modes PC, GC and TGC, - a re-imager 20' that is adjustable so that it can change from a configuration common to the PC and GC modes to a configuration for the TGC mode, - a detachable afocal element 30 of magnification G (G=3 for example), used only in PC mode, - a detachable aperture diaphragm 40, used only in PC or GC mode, - a counter-scanning device used in TGC mode, comprising a counter-scanning mirror able to be locked in a fixed position for the PC and GC modes.

These elements, which are on the same optical axis, are mounted on a platform 70' equipped with means of rotation in azimuth in order to obtain the 90° horizontal scan of the TGC mode; this platform is locked in a fixed position for the PC and GC modes.

The scanning platform 70' and the counter-scanning device 80' are controlled by a processing unit 75 and are shown in figure 5.

The array detector 10 comprises for example 240 x 320 pixels. This detector, which is for example sensitive in the 3-5 μιη band, can be composed of a mercury, cadmium and tellurium (HgCdTe) material. Other materials can be used such as multi-quantum well materials including the gallium arsenide/gallium and aluminum arsenide compounds (AsGa/AsGaAI); the indium antimonide (InSB) compound can also be used.

The re-imager 20' such as used in the PC and GC modes, comprises a group of N divergent and convergent lenses disposed for example as follows: one group of lenses allowing the image transport, such as for example the three lenses 21 , 22, 23 that are the most downstream in the figures 4, and a group of lenses forming an eyepiece, such as the two lenses 24, 25 downstream of the counter-scanning mirror; it also comprises means of varying the field.

According to a first embodiment, the re-imager 20' is of the zoom type. The means of varying the field comprise the convergent lenses 22 and 23 respectively called the compensator 22 and the variator 23 and means of moving these lenses in translation on the optical axis into a variable position depending on the chosen mode. The variator principally makes it possible to obtain a variation of the focal length of the optical system and the compensator is an element principally making it possible to maintain the focal plane virtually fixed. In TGC mode (figure 4c), the variator 23 is in its maximum downstream position and the compensator 22 is in its maximum upstream position, they thus make it possible to obtain a 30° field. For the PC (figure 4a) or GC (figure 4b) modes, the variator 23 is in its maximal upstream position and the compensator 22 is in its maximal downstream position. Moreover, the optical combination of the variator 22 and the compensator 23 is calculated in such a way as to minimize the necessary size of the counter-scanning mirror 81 whilst keeping the image of the aperture in the vicinity of the latter.

According to a second embodiment, the means of varying the field comprise detachable optical elements; these are for example a group of lenses which is inserted on the optical axis between the eyepiece and the transport constituting the re-imager 20' in order to obtain a 30° field and which is removed in order to change to PC or GC modes. It is for example mounted on a structure which can be rotated in order to be able to be placed on the optical axis or withdrawn depending on the chosen mode.

The afocal element 30 typically comprises a divergent lens and a convergent lens. It is recalled that an afocal element is an optical element such that a beam which is collimated on one side is still collimated on the other side after having traversed the afocal element. It is inserted on the optical axis upstream of the re-imager in PC mode and is removed in the GC and TGC modes The aperture diaphragm 40 is used only in PC or GC mode with an aperture of about F/3; it is removed in TGC mode for which the aperture 11 of the cryostat acts as a pupil. The withdrawal of the aperture diaphragm 40 consequently makes it possible to reduce the f-number F/D of the system to about F/1.2 in the TGC position and therefore to retain a virtually constant size of the entrance pupil of the re-imager whilst increasing the field.

The counter-scanning device 80' makes it possible to compensate for the movements of the image due to the scanning of the scene by the platform 70'; it makes it possible to stabilize the image during the acquisition of the images. It comprises, for example, a mirror 81 which, in the rest state, is oriented at 45° on the optical axis, associated with means of rotating said mirror. The rotation has an amplitude of ± 15° in order to cover a 30° field. This mirror 81 , which also has the effect of bending the path of the beam, also makes it possible to reduce the dimensions of the optical device. The processing unit 75 synchronizes the acquisition of the images by the detection means with the counter-scanning movements. More precisely, as the acquisition time of an image is composed on the one hand of a time of integration Tint by the detector 10 and on the other hand of a time Tiect necessary for reading the information provided by the detector, the counter-scanning means 80' provide a rotational movement contrary to that of the scanning platform 70' during the time Tint and make it possible to keep the observed zone fixed on the detector during the time necessary for the integration of the signal and thus to avoid a blurring effect on the image. They return to their initial position during the time T|eC|.

The previously described elements are installed in a casing 90 fixed to the scanning platform 70'. The casing possibly comprises other elements. It is provided with at least one transparent window or port 95, preferably inclined with respect to the optical axis in order to avoid the narcissus effect; it is for example inclined at about 18°. The vertical dimension of the window, conditioned by the PC beams, is of the order of 75 mm. Its horizontal dimension is of the order of 130 mm to allow the TGC beams to pass without vignetting, which represents an acceptable oversizing in comparison with the original size of 75 mm. Finally an oblong-shaped window of 130 mm X 75 mm is obtained. The window is typically at a distance of 100 mm from the counter-scanning mirror.

The scanning platform 70', an example of which is shown in figure 5, is equipped with means of rotation provided for example by universal shafts each having one degree of freedom. The platform 70' is constituted by a support 71 upon which is mounted the detector-optical device assembly placed in the casing 90, a rotating joint 72 and a motor 73 allowing the rotation of the support about a vertical axis 50. The detector and the lenses of the re-imager, which are situated on an axis perpendicular to the plane of the figure and behind the mirror 81 , are not shown in order not to overload the figure. A second motor 82 makes it possible to drive the counter-scanning mirror 81 about the same axis of rotation 50 as that of the overall support. The two motors are controlled by a processing unit 75 which makes it possible to manage the counter-scanning.

The system 200 according to the invention will now be described for each mode.

In PC mode, the system described with reference to figure 4a comprises: - the detection means comprising the two-dimensional IR array detector 10 placed conventionally in the cooling device, - the aperture diaphragm 40 surrounded by the flux reducing mirror, - the re-imager 20' in common PC/GC configuration if it is a re-imager of the zoom type, without the elements specific to the TGC mode if the re-imager is equipped with a device for changing the field by the insertion of optical elements in the combination, - the afocal element 30 of magnification G (G=3 for example), - the counter-scanning device 80' locked in a fixed common PC/GC position.

These elements are mounted on the platform 70' locked in a common PC/GC position.

In GC mode, the system described with reference to figure 4b comprises: - the detection means comprising the two-dimensional IR array detector 10 placed conventionally in the cooling device, - the aperture diaphragm 40 surrounded by the structural flux reducing mirror, - the re-imager 20" in common PC/GC configuration if it is a re-imager of the zoom type, without the elements specific to the TGC mode if the re-imager is equipped with a device for changing the field by the insertion of optical elements in the combination, - the counter-scanning device 80' locked in a fixed common PC/GC position.

In TGC mode, the system 200 described with reference to figure 4c comprises: - the detection means comprising a two-dimensional IR array detector 10 placed conventionally in a cooling device such as a cryostat; in TGC mode it is the cold diaphragm 11 of the cryostat which acts as a pupil, - the re-imager 20' in TGC configuration if it is an imager of the zoom type, or including the additional optical elements specific to the TGC configuration if the re-imager is equipped with a device for changing the field by the insertion of optical elements in the combination, - the counter-scanning device 80'.

These elements are mounted on the platform 70' equipped with means of rotation in azimuth in order to obtain the 90° scan of the TGC mode.

According to another embodiment the platform has two degrees of freedom, in elevation and in azimuth.

The scanning platform 70' and the counter-scanning device 80' are controlled by the processing unit 75.

Claims

1. CLAIMS 1 , A passive IR imaging system (200) comprising: - an IR array detector (10), - a two-field optical device, able to form an image on the detector and comprising on an optical axis means of forming the image (20'), a detachable afocal element (30) of specified magnification G placed upstream of the image forming means, an aperture diaphragm (40) disposed between the detector and the image forming means, characterized in that it comprises means (70') of scanning the detector and two-field optical device assembly in azimuth, and in that the aperture diaphragm (40) is detachable and in that the optical device furthermore comprises a counter-scanning mirror (81 ) able to be locked, disposed upstream of the image forming means, and in that the image forming means comprise means for obtaining a third field, the counter-scanning mirror (81 ) and the means for obtaining a third field being disposed on said optical axis and able to form an image on said detector (10) in order to obtain a three-field imaging system. The imaging system as claimed in the preceding claim, characterized in that the means for obtaining another field comprise detachable optical components able to be inserted on the optical axis between the counter-scanning device and the detector. The imaging system as claimed in claim 1 , characterized in that the means for obtaining another field are of the zoom type, that is to say comprising optical elements (22, 23) that are movable in translation along the optical axis. The imaging system as claimed in any one of the preceding claims, characterized in that the counter-scanning mirror (81 ) is placed between the image forming means and the afocal element. The imaging system as claimed in any one of the preceding claims, characterized in that the image-forming means have a specified aperture and the means for obtaining another field modify that aperture. The imaging system as claimed in any one of the preceding claims, characterized in that the other field is a very wide field. The imaging system as claimed in any one of the preceding claims, characterized in that the counter-scanning mirror (81 ) is disposed at 45° on the optical axis. The imaging system as claimed in any one of the preceding claims, characterized in that the counter-scanning mirror (81 ) is included in a counter-scanning device (80') and in that the scanning platform (70') and the counter-scanning device (80') are connected to a processing device (75) able to control them. The imaging system as claimed in any one of the preceding claims, characterized in that the detector is placed in a cryostat comprising a cold diaphragm (11 ). For th^Ap licant,