Thermal camera adapted for use with a reflected infrared band designator

Abstract

A thermal camera comprising an optical chain, a microbolometric sensor receiving light focused by the optical chain, and a signal processing chain for forming an image fed by said microbolometric sensor, the optical chain being adapted to focus onto the sensor light from the scene towards which the sensor is directed, contained within a thermal infrared band (1) delimited on the side of the shorter wavelength values. The optical chain is also adapted to focus onto the sensor light from the scene and contained within a reflected infrared transmission band (2) separated from said thermal infrared band (1). Abbreviated figure: 1

Description

Title of the invention: Thermal camera adapted for the use of a designator in the reflected infrared band Field of invention

[0001] The invention falls within the field of viewing devices based on infrared emitted spontaneously by bodies, these devices being commonly called thermal cameras. Such cameras can be incorporated into various devices having a function requiring viewing, such as for example a shooting scope or multi-function cameras, and also thermal binoculars capable of viewing thermal infrared.

[0002] The long wave infrared (LWIR) range from 8 to 12 or 14 pm (a part of the so-called thermal infrared) allows imaging based on the waves in this range which are emitted by bodies depending on their temperature. This imaging associates the intensity of the radiation, which is broadband, with a temperature and this is visualized on the image by variable colors or in gray levels depending on the measured temperature, which allows the shapes of the bodies to be seen, which generally have temperatures at least slightly distinct from each other.

[0003] Such imaging is used in various applications, such as night vision, surveillance, or combat equipment, and in particular the shooting glasses mentioned above. The images are digital images. The sensor can be of the thermodetector type and react to a change in temperature by varying in particular its electrical resistance, in which case it is called a bolometer. Microbolometers in particular are uncooled thermal sensors, usable at room temperature and immediately, without latency, with low energy consumption. A microbolometer consists of an array of pixels.

[0004] Night vision equipment is also known which may be binoculars and which are also called residual light amplifiers or intensifiers. They use a reflected light amplifier or light intensifier tube (IL tube), in the near infrared - from 800 to 1200 pm, or residual radiation is present in the middle of the night in the open air (this is the so-called reflected near infrared present in a dimly lit scene without the darkness being total, the reflected infrared being opposed to the thermal infrared, of longer wavelength and emitted by any body depending on its temperature, and which was mentioned above). Light intensifiers convert low levels of light photons into electrons, via a photocathode. Such night vision binoculars can be mounted on a rifle scope.

[0005] Finally, common digital photo and video cameras like those found in consumer smartphones are designed to process the visible wavelength range, and for this purpose have a silicon sensor (CCD or CMOS), but this sensor has significant sensitivity up to around 1000 or 1050 nm, i.e. also in the near infrared. Commercially available devices are equipped with a filter that is interposed between the exterior of the device and the sensor, but this filter is fairly easily removable.

[0006] Furthermore, various situations require or benefit from a designation functionality by a beam of light well directed at a particular point. The designation light is emitted by equipment responsible for carrying out the designation, called a pointer or a designator, and which can be carried by the person observing or be installed near them, or on the contrary be very far from them and independent in its movement, the link between the observer and the designator being limited to the fact that they are interested in the same scene, of which a body, point or object has an interest which justifies its highlighting, and therefore its designation, in relation to the other bodies, points or objects of the scene.

[0007] The pointer typically emits laser light, which may be of visible wavelength. This is observed, via its reflection by the designated surface, by the user with the naked eye, or possibly with optics transmitting visible wavelengths.

[0008] In the field of shooting glasses, there are devices called red dot sights which are very popular, including reflex sights. In addition, these devices can be equipped with a see-through channel and this channel allows you to see a designation made with visible light.

[0009] Obviously, a designation made with visible light attracts the attention of people present on the scene, or of observers, and reveals or risks revealing the presence of the designator and of the person for whose benefit the designation is made, who is, in a combat situation, a shooter. It is therefore desired to avoid this, by using non-visible wavelengths for the designation.

[0010] Designations made with a pointer or a rangefinder emitting light in the near infrared are then used. The wave of the designator, whose length is for example 930 nm or 1064 nm, is reflected by the designated body, and it is observed by a silicon sensor (CMOS or CCD), also responsible for filming the scene in visible light and whose sensitivity is extended to the near infrared (therefore notably without the filter that we know on commercial digital devices), and which establishes an additional channel in the vision device, in addition for example to a thermal observation channel.

[0011] But these near infrared designations are also visible by very common devices such as smart mobile phones (smartphones), sometimes with a small adaptation, namely the removal of a filter. Thus, these near infrared designations are not discreet in a context where means of discovering them are readily available. Moreover, in addition to the immensely widespread mobile phones, night vision binoculars sensitive to the 930 nm wavelength, based on light intensifiers (IL tubes) are widely accessible products.

[0012] InGaAs sensors are also available that are sensitive to certain waves in the short infrared or SWIR range (wavelengths between 1.2 and 3 pm, SWIR meaning short-wave infrared). They allow visualization of a designation made with light specifically emitted in this range, such as 1.5 pm, which is a wavelength for which many devices are available, with regard to the emission and transmission of the laser wave. The use of a specific sensor of course imposes complexity on the devices on board, in particular because it is necessary to harmonize the fields perceived by the different cameras, and also has a significant cost.

[0013] Currently, thermal rifle scopes do not include an optical path to view a laser designation made by a pointer or rangefinder.

[0014] To solve this problem, in particular, it is proposed to take advantage, in an original way, of the non-optimal but still significant sensitivity properties of microbolometer type sensors in the reflected infrared wavelength range, to extend the sensitivity of an uncooled infrared camera. This involves going, in a clever way, against the principle according to which, to avoid chromatic aberrations, all wavelengths below 8 pm are blocked in the optical chain focusing the light on the microbolometric sensor. Some of these wavelengths are, according to the invention, always blocked, while others are transmitted and used.

[0015] It is in fact proposed to detect, with the microbolometric sensor, the trace of a pointer, or designator or of a laser rangefinder of wavelength in the SWIR range, and therefore typically of wavelength 1.54 pm, components adapted to this wavelength being particularly available. At this wavelength, the light is reflected by the bodies that it encounters.

[0016] Thus, a thermal camera is proposed comprising an optical chain, a microbolometric sensor receiving light focused by the optical chain, and a signal processing chain for forming an image supplied by said microbolometric sensor, the optical chain being adapted to focus on the sensor a first light coming from the scene towards which the sensor is directed included in a band thermal infrared delimited on the side of low wavelength values. Such delimitation is commonly practiced, as mentioned above, to avoid chromatic aberrations, and implies that there exists in the system a band of mid-infrared wavelengths for which the optical chain is opaque.

[0017] According to original characteristics, the optical chain is also adapted to focus in a superimposed manner on the sensor a second light coming from the scene and included in a reflected infrared transmission band separated from said thermal infrared transmission band. When the width of the bands is expressed in wavelength as is done conventionally, the reflected infrared transmission band can be for example 40 to 600 times narrower than the thermal infrared band. The reflected infrared transmission band can be described as a window.

[0018] And in an original manner, the signal processing chain generates a digital image based on the signals provided by the microbolometric sensor simultaneously receiving the first and second lights.

[0019] The imaging is therefore still carried out by the thermal camera method, but we benefit from an observation window on the reflected infrared which can be placed by construction in the SWIR range (and therefore be limited in the direction of the visible, and not overflow towards the near infrared), to benefit from the sensitivity of the micro-bolometer in this range, and to distinguish in superposition on the thermal image, designations made by pointers operated by friends or third parties in this reflected infrared wavelength range.

[0020] A wavelength is thus chosen in the SWIR range, the micro-bolometer sensor retaining sufficient sensitivity at this wavelength, for example more than 10% of its sensitivity in the thermal infrared, which has not been exploited until now. Beyond the value of 1.54 pm, we are interested in the 1.2 pm to 3 pm band, or even slightly beyond, without going too low, so that the designation is not visible by night vision binoculars with an IL tube or a smartphone camera from which its filter has been removed. These wavelengths 1.2 pm to 3 pm are not detectable by the CMOS (silicon) sensors fitted to smartphones, even after the filter that protects them from the near infrared has been removed.And the effect is achieved with a single sensor - the microbolometric sensor - and not two sensors as would have been natural, in this case a specific sensor for SWIR in addition to the LWIR sensor, which would have been heavier, more fragile and more expensive. There is no need for a second camera. The recognition range of the designation point is of the same order of magnitude as the range of the thermal camera.

[0021] According to optional and original characteristics,

[0022] - the optical chain can be designed by its materials and surface treatments to selectively transmit, to the exclusion of other infrared wavelengths, said thermal infrared band and said reflected infrared transmission band;

[0023] - the optical chain may comprise a first and a second sub-chain pa alleles, the first and second sub-chains being respectively adapted to selectively transmit, to the exclusion of other infrared wavelengths, said thermal infrared band and said reflected infrared transmission band;

[0024] - the two sub-chains can be concentric, or non-concentric;

[0025] - furthermore, in its signal processing chain, the camera may include a means for detecting a time signature in said light included in the reflected infrared transmission band to provide suitable signage to the user.

[0026] - the transmission band can be between 10 and 100 nm wide.

[0027] - the optical chain can, for example, comprise optical components in silicon or chalcogenide.

[0028] The invention may take the following forms: a shooting scope comprising a camera according to the invention, the shooting optics of the scope comprising said camera and making it possible to align the sight on a superposition of said image and said light included in a reflected infrared transmission band; monocular, binocular or bi-ocular night vision binoculars, which can be worn hands-free, and comprising a camera according to the invention, or even a multifunction camera comprising a thermal camera mode according to the invention. Brief description of the drawings

[0029] The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation and thanks to the appended figures among which:

[0030] [Fig.l] is a representation of the light transmissions in the wavelength ranges of the invention, for different materials, and the assembly according to the invention.

[0031] [Fig. 2] is an illustration, according to the principles of the invention, of the visibility ranges of a pointer as a function of its power.

[0032] [Fig. 3] is a schematic view of one embodiment of the invention.

[0033] [Fig.4] is a schematic view of a second embodiment of the invention.

[0034] [Fig.5] shows an operating mode of the invention.

[0035] [Fig.6] shows a detail of an embodiment of the invention, in a variant.

[0036] [Fig.7] shows an alternative embodiment of the invention. Detailed description of the drawings

[0037] We will therefore present a thermal camera adapted for the exploitation of a SWIR band designator.

[0038] [Fig.l] In [Fig.l], the wavelengths of the range 0 to 18 pm are represented on the abscissa and the percentages of transmission of the waves of the indicated wavelength are represented on the ordinate, for the assembly according to the invention.

[0039] Thus the assembly has a transmission band 1 of 7 to 13 pm, with a transmission close to 90% between 8 and 13 pm (this is the IR3 band) and a transmission band 2 around 1.5 pm, with a width of approximately 50 nm.

[0040] Between transmission band 1 and transmission band 2, there is an opacity band 3, several pm wide, in which the light is blocked with a maximum extinction rate (extinction greater than 98% for example).

[0041] As already mentioned, when the width of the bands is expressed in wavelength as is conventionally done, the reflected infrared transmission band can be 40 to 600 times narrower than the thermal infrared band. The reflected infrared transmission band can be referred to as a window.

[0042] [Fig.2] In [Fig.2], the visibility distance or detection range (for an object frequency of 1 cy) of the pointer, or range of the device, is represented as a function of the power of the pointer. The curve relates to, in accordance with the invention, a 1.556 pm laser and a microbolometer sensor. The visibility is 21 km, the beam divergence is 1.5 mrd and the emission diameter is 6 mm. The detector has an objective with a focal length of F, a numerical aperture close to F / l, a transmission of 75% in the 8-12 pm range (i.e. in the LWIR band, the extreme values ​​of this band can be slightly adapted by convention) and a relative transmission at 1.54 pm (relative transmission compared to the average transmission between 8 and 12 pm, i.e. in the LWIR band) of 85%.

[0043] The detector, taken as a non-limiting example, is a 640x512 pixel detector of 12 pm NETD (Noise Equivalent Thermal Difference or Noise Equivalent Temperature) 50 mK, having a relative quantum efficiency at 1.54 pm (expressed relative to the efficiency in the LWIR range) QEij54|jm / QELW1R equal to approximately 25% with a silicon input window of thickness of approximately 1 mm. The residual sensitivity at 1.54 pm is in fact of the order of 20% compared to the sensitivity peak in the IR3 band.

[0044] The abscissa axis is graduated from 0 to 2 W for the laser power, and two vertical lines have been drawn, for a class 1 laser and a class 3B laser.

[0045] The ordinate axis is graduated from 0 to 800 m, and a horizontal line has been drawn at the distance corresponding to the recognition of a kneeling man with a resolving power of 3.5 pairs of lines - about 360 m.

[0046] The silicon input window can be replaced by a chalcogenide window.

[0047] [Fig.3] [Fig.3] shows an embodiment of the invention. It is a portable shooting system 100, of the handgun or rifle type, comprising a thermal shooting scope.

[0048] It comprises an integrated laser designator 110, of the pointer or rangefinder type, emitting laser radiation of 1.54 pm, in the aiming direction.

[0049] The shooting system 100 is further equipped with an uncooled IRNR infrared camera 120 meeting the principles of the invention, allowing the user wishing to aim at a target to view in the viewing screen 130 which presents the scene observed in the thermal infrared also the designation at 1.54 pm. The viewing system is optionally of the reflex sight type, that is to say that it has an optical viewfinder which allows the user to look through a partially reflective glass element and to see an illuminated projection of another image superimposed on the field of vision by eliminating parallax errors, using optics and / or a curved or non-curved mirror.

[0050] Its use involves two optical transfer chains in parallel

[0051] For thermal infrared, the temperature difference of the different zones of the scene or the different bodies observed, gives an apparent intensity difference for the optical system, after passing through the atmosphere, and taking into account the atmospheric conditions, then attenuation by passing through the objective of the device and taking into account its modulation transfer function MTF, then passing through the thermal imaging display path, with its modulation transfer function MTF, which defines the minimum resolvable or observable temperature difference MRTD (for an object frequency of 3.5 cy which corresponds to the recognition criterion)

[0052] For the designation in the SWIR band, the difference in contrast of the different zones of the scene or the different bodies observed and the luminance of the observed designation spot, give a difference in apparent contrast for the optical system, after passing through the atmosphere, and taking into account the atmospheric conditions, then attenuation by crossing the objective of the device and taking into account its modulation transfer function MTF, then crossing the viewing path of the pointer, with its modulation transfer function MTF, which defines the minimum resolvable or observable contrast MRC (for an object frequency of 1 cy which corresponds to the detection criterion).

[0053] [Fig.4] [Fig.4] shows another embodiment of the invention. It is again a portable shooting system 200, of the handgun or rifle type, comprising a thermal shooting scope. This time it does not include the laser designator, which is placed elsewhere, and is independent in its movements from the firing system. Typically, it can be a laser designator carried by an aerial vehicle such as an aircraft 210, or a land vehicle, such as a car, a combat vehicle, or even a two-wheeler - these vehicles can notably carry a multi-function camera responsible for emitting the laser designation - but the laser designation can be emitted by a single-function device dedicated to this emission.

[0054] Again, the firing system 200 is further equipped with an uncooled IRNR infrared camera 220 meeting the principles of the invention, allowing the user wishing to aim at a target to view in the viewing screen 230 which presents the scene observed in the thermal infrared also the designation at 1.54 pm. The viewing system is optionally of the reflex sight type.

[0055] [Fig.5] [Fig.5] shows the visualization in the screen of the firing system, with an optional characteristic, consisting of a digital processing of the image from the p-bolometer sensor making it possible to identify a temporal signature of the temporal hashing type (also called frequency) of the designation element(s). There is therefore, in the signal processing chain, a means of detecting the temporal signature. Depending on the recognized signature, which can for example be classified into two types "friend" and "enemy", using a database if necessary, there is a different display of a designation symbol for example by two different colors, or using different geometric shapes or reticles. Here, an enemy designation 50 has been represented with a triangle which can be red, and a friendly designation 60 has been represented with a cross or a star which can be blue.If an enemy designation is observed, this indicates that the object of this designation, which is often a soldier, is being telemetered by an enemy, and the invention therefore makes it possible to warn him, if it is a friendly soldier, so that he takes cover or escapes the telemetry to which he is subject.

[0056] The invention makes it possible, using this signature analysis, to increase the detection range and improve the visualization ergonomics in full contour mode.

[0057] In the absence of a study of a time signature of the designation points in the display device, the display is carried out using the microbolometer sensor and its sensitivity which depends on the wavelengths, so that the designation point is perceived by the user as a circular element of characteristic dimension presenting a local temperature difference with respect to the environment or a temperature gradient from the center of the circle towards the outside of the circle depending on the display modes proposed by the device.

[0058] Regarding the production of the optical chain, according to a particular embodiment, there is a single optical transmission chain made up of components chalcogenide and / or silicon optics, the optical components having been treated by a combination of optical treatments or a single optical treatment, including for example an anti-reflection treatment judiciously chosen in combination with the exact materials of the optical components, the assembly being sized to transmit the luminous flux according to the desired template, namely

[0059] - A thermal infrared transmission band limited to the 8-12 pm band, the cut-offs at 8 and 12 pm, intended to reduce the chromatic correction constraints on optics for thermal imaging - these cut-offs are made gradually but quickly over an interval which can be 1 pm between the absence of transmission, and the effective transmission of the transmission band (for example 75%). The thermal infrared transmission band is adapted to the optimal response spectral range of a bolometric sensor (between 8 and 13 pm).

[0060] - A reflected infrared transmission band about 50 nm wide around of 1.54 pm in order to visualize the designation point, this reflected infrared transmission band being separated, in terms of wavelength relative to the thermal imaging band, from the thermal infrared transmission band, by a band 4 to 5 pm wide without transmission. In addition, the wavelengths closer to the visible are blocked, to guarantee the selection of designations made voluntarily at SWIR wavelengths, and typically, for practicality of construction, at the wavelength of 1.54 pm.

[0061] - And an opacity band between the reflected infrared transmission band and the thermal infrared transmission band, approximately between the wavelength 2 pm and the wavelength 6.5 pm (taking these values, a width of 4.5 pm). Thus, the thermal infrared transmission band is limited towards the low wavelengths, since it is followed in this direction by an opacity band, and this makes it possible to avoid chromatic aberrations for the thermal imaging function.

[0062]

[0063] [Fig.6] [Fig.6] shows another embodiment of the apparatus including two separate optical chains for the two frequency ranges to be transmitted for the light to be focused on the microbolometer sensor 300. In this particular case a spectral pupil is used, or a spectral diaphragm, and the two separate optical chains are concentric. The outer, peripheral, annular part 310 allows the wavelengths of the LWIR band to be passed for thermal imaging, and the central part 320 allows the wavelengths of the reflected infrared transmission band, or SWIR infrared to be passed for visualizing the designation. An inverted construction is provided as a variant. A focusing lens 330 brings the beam, initially collimated, onto the detector 300. The thermal infrared and reflected infrared lights are superimposed on the scale of the pixel.

[0064] [Fig.7] [Fig.7] shows another embodiment of the apparatus again including two separate optical chains for the two frequency ranges to be transmitted for the light to be focused onto the microbolometer sensor 300. In this particular case a beam combining means is used, such as a plate 350 placed at 45° to the beams, and the two separate optical chains 360 and 370 are parallel and merged as they approach the microbolometer sensor 300. The upper part, in the figure, allows the wavelengths of the reflected infrared or SWIR infrared transmission band to be passed for visualizing the designation, and the lower part allows the wavelengths of the LWIR band to be passed for thermal imaging. An inverted construction is provided as an alternative. The thermal infrared and reflected infrared lights are superimposed at the pixel scale.

[0065] The variants of Figures 6 and 7 may incorporate optical treatments to promote transmission in the desired frequency bands, and exclude wavelengths from the unwanted bands.

[0066] The invention can be implemented on hands-free night vision binoculars incorporating an IRNR channel allowing the viewing of a 1.54 pm laser pointer or rangefinder.

[0067] The invention can also be implemented on a multi-function camera (typically equipped with several of the following functions: rangefinder, magnetic compass, laser pointer, gyrometer and direct optical channels) integrating an IRNR channel and a 1.54 pm laser designation. The pointer or rangefinder can be integrated into the multi-function camera. The latter has several modes, such as in particular a day vision mode, or also a mode based on a cooled infrared sensor.

Claims

Claims

1. Thermal camera comprising an optical chain, a microbolometric sensor (300) receiving light focused by the optical chain, and a signal processing chain for forming an image, the signal processing chain being powered by said microbolometric sensor (300), the optical chain being adapted to focus on the sensor (300) a first light coming from the scene towards which the sensor is directed included in a thermal infrared band (1) delimited on the side of the low wavelength values, said camera being characterized in that the optical chain is also adapted to focus, in a superimposed manner, on the microbolometric sensor (300) a second light coming from the scene and included in a reflected infrared transmission band (2) separate from said thermal infrared band (1),the signal processing chain generating a digital image on the basis of the signals provided by the microbolometric sensor simultaneously receiving the first and second lights.,

2. Thermal camera according to claim 1, characterized in that the optical chain is designed by its materials and the treatments of its surfaces to selectively transmit, to the exclusion of other infrared wavelengths, said thermal infrared band (1) and said reflected infrared transmission band (2).

3. Thermal camera according to claim 1 or claim 2, characterized in that the optical chain comprises a first and a second parallel sub-chain, the first and the second sub-chains being respectively adapted to selectively transmit, to the exclusion of the other infrared wavelengths, said thermal infrared band (1) and said reflected infrared transmission band (2).

4. Thermal camera according to claim 3, characterized in that the two sub-chains are concentric (310, 320), or non-concentric (360, 370) and in this case, merged by a beam combining blade (350).

5. Thermal camera according to one of claims 1 to 4, further comprising, in the signal processing chain, means for detecting a time signature in said second light included in the reflected infrared transmission band to provide the user with suitable signage (50, 60).

6. Camera according to one of claims 1 to 5, characterized in that the reflected infrared transmission band (2) has a width of between 10 and 100 nm.

7. Thermal camera according to one of claims 1 to 7, characterized in that the optical chain comprises optical components made of silicon or chalcogenide.

8. Shooting scope (100; 200) comprising a camera according to one of claims 1 to 7, characterized in that the shooting optics of the scope comprise said camera and make it possible to align the sight on a superposition of said image and said second light included in a reflected infrared transmission band.

9. Monocular, binocular or bi-ocular night vision binoculars, capable of being worn hands-free, and comprising a camera according to one of claims 1 to 7.

10. A multi-function camera comprising a thermal camera according to one of claims 1 to 7.

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