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 use with a reflected infrared band designator. Field of the invention

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

[0002] The long-wave infrared (LWIR) range of 8 to 12 or 14 pm (a portion of the so-called thermal infrared) allows for imaging based on the waves in this range emitted by bodies according to their temperature. This imaging combines the intensity of the radiation, which is broadband, with a temperature, and this temperature is visualized on the image by varying colors or shades of gray depending on the measured temperature. This allows the shapes of bodies, which generally have temperatures that are at least somewhat distinct from one another, to be seen.

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

[0004] Night vision devices, which can be binoculars, are also known and are also called residual light amplifiers or intensifiers. They use a reflected light amplifier or light intensifier tube (LIT tube) in the near-infrared range—from 800 to 1200 pm—where residual radiation is present in the middle of the night in open air (this is the so-called reflected near-infrared present in a dimly lit scene without total darkness; reflected infrared is the opposite of thermal infrared, which has a longer wavelength and is emitted by all bodies depending on their temperature, and which was mentioned above). Light intensifiers convert low levels of photons Light is transmitted via electrons through a photocathode. Such night vision binoculars can be mounted on a rifle scope.

[0005] Finally, common digital cameras and video recorders, such as those found in consumer smartphones, are designed to process the visible wavelength range and therefore use a silicon sensor (CCD or CMOS). However, this sensor has significant sensitivity up to approximately 1000 or 1050 nm, i.e., also in the near-infrared. Commercially available devices are equipped with a filter that is placed between the exterior of the device and the sensor, but this filter is quite easily removable.

[0006] Furthermore, various situations require or benefit from a designation function using a beam of light directed precisely at a particular point. The designation light is emitted by a device responsible for performing the designation, called a pointer or designator, which can be worn by the observer or installed near them, or conversely, be very far away and independent in its movement. The link between the observer and the designator is limited to the fact that they are interested in the same scene, in which a body, point, or object has an interest that justifies highlighting it, and therefore designating it, in relation to the other bodies, points, or objects in 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 rifle scopes, there are highly regarded red dot sights, including reflex sights. In addition, these sights can be equipped with a see-through aperture, allowing the user to see a designation made with visible light.

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

[0010] Designations are then made using a pointer or rangefinder emitting near-infrared light. The wave from the designator, whose wavelength is, for example, 930 nm or 1064 nm, is reflected by the designated object and observed by a silicon sensor (CMOS or CCD), which is also responsible for filming the scene in visible light and whose sensitivity is extended to the near-infrared (thus notably without the filter found on commercial digital cameras), and which establishes a channel additional in the vision system, in addition for example to a thermal observation channel.

[0011] But these near-infrared designations are also visible by devices These devices are very common, such as smartphones, sometimes requiring only a minor modification such as removing a filter. Therefore, these near-infrared designations are not inconspicuous in a context where detection methods are readily available. Moreover, in addition to the immensely widespread use of mobile phones, night vision binoculars sensitive to the 930 nm wavelength, based on light intensifiers (IL tubes), are widely accessible products.

[0012] InGaAs sensors sensitive to certain waves in the short-wave infrared (SWIR) range (wavelengths between 1.2 and 3 pm, SWIR meaning short-wave infrared) are also available. These sensors allow visualization of a designation made with light specifically emitted in this range, such as 1.5 pm, a wavelength for which numerous devices are available for laser wave emission and transmission. The use of a specific sensor naturally adds complexity to the devices incorporating it, particularly because the fields perceived by the different cameras must be harmonized, and it also entails a significant cost.

[0013] Currently, thermal shooting scopes do not include an optical channel for viewing 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 cleverly going against the principle that, to avoid chromatic aberrations, all wavelengths below 8 pm are blocked in the optical chain focusing the light onto the microbolometer sensor. According to the invention, some of these wavelengths are always blocked, while others are transmitted and used.

[0015] It is indeed proposed to detect, with the microbolometric sensor, the trace of a laser pointer, designator, or rangefinder with a wavelength in the SWIR range, and therefore typically with a wavelength of 1.54 pm, components adapted to this wavelength being readily available. At this wavelength, light is reflected by the bodies 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 fed by said sensor In microbolometric systems, the optical chain is adapted to focus onto the sensor a first light source from the scene towards which the sensor is directed, within a thermal infrared band delimited on the side with shorter wavelengths. Such a delimitation is commonly used, as mentioned above, to avoid chromatic aberrations, and implies that there is a band of mid-infrared wavelengths in the system for which the optical chain is opaque.

[0017] According to original features, the optical chain is also adapted to focus, superimposed on the sensor, a second light source from the scene and contained within a reflected infrared transmission band separate from said thermal infrared transmission band. When the bandwidths are expressed in wavelengths, as is conventionally done, 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 way, the signal processing chain generates a digital image on the basis of the signals provided by the microbolometric sensor receiving simultaneously the first and second lights.

[0019] Imaging is therefore always 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 spill over into the near infrared), to benefit from the sensitivity of the microbolometer in this range, and to distinguish in superposition on the thermal image, designations made by pointers operated by friends or third parties in this range of reflected infrared wavelength.

[0020] A wavelength is thus chosen from the SWIR range, the microbolometer 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. The focus is on the 1.2 pm to 3 pm band beyond the value of 1.54 pm, or even slightly beyond, without going too low, so that the designation is not visible to night vision binoculars with an IL tube or a smartphone camera with its filter removed. These wavelengths from 1.2 pm to 3 pm are not detectable by the CMOS (silicon) sensors in smartphones, even after the filter protecting 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 one might have naturally expected, namely a dedicated SWIR sensor 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 range of... Recognition of the designation point is of the same order of magnitude as the range of the thermal camera.

[0021] According to optional and original features,

[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 parallel, 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 subchains can be concentric, or non-concentric;

[0025] - moreover, in its signal processing chain, the camera may include a means of detecting a temporal signature in said light included in the reflected infrared transmission band to provide user-friendly signage.

[0026] - the transmission band can have a width between 10 and 100 nm.

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

[0028] The invention can take the following forms: a shooting scope comprising a camera according to the invention, the shooting optics of the scope comprising said camera and enabling the sight to be aligned 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 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 become apparent upon reading the following description, given by way of non-limiting example, and with reference to the accompanying figures, among which:

[0030] Fig. 1 is a representation of light transmissions in the wavelength ranges of the invention, for different materials, and the assembly according to the invention.

[0031] Figure 2 illustrates, according to the principles of the invention, 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] Figure 5 presents one mode of operation of the invention.

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

[0036] Figure 7 presents an alternative embodiment of the invention. Detailed description of the drawings

[0037] We will therefore present a thermal camera adapted for use with a SWIR band designator

[0038] [Fig.1] In [Fig.1], 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 from 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 light is blocked with a maximum extinction rate (extinction greater than 98% for example).

[0041] As already mentioned, when the bandwidth is expressed in wavelengths 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 described 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 shown as a function of the pointer power. The curve relates, according to the invention, to a laser at 1.556 pm 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 lens with a focal length 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 being slightly adjusted by convention), and a relative transmission at 1.54 pm (relative transmission with respect 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 with a NETD (Noise Equivalent Thermal Difference) of 50 mK and a relative quantum efficiency of 1.54 pm (expressed relative to the efficiency in the LWIR range) QEi,54Mm / QELWIR equal to approximately 25%, with a silicon input window approximately 1 mm thick. The sensitivity residual at 1.54 pm is indeed around 20% compared to the peak sensitivity in the IR3 band.

[0044] The x-axis is graduated from 0 to 2 W for 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 line pairs - approximately 360 m.

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

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

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

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

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

[0051] For thermal infrared, the temperature difference of the different areas of the scene or the different bodies observed gives a difference in apparent intensity for the optical system, after passing through the atmosphere, and taking into account atmospheric conditions, then attenuation by passing through the lens of the device and taking into account its modulation transfer function MTF, then passing through the thermal imaging viewing channel, 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 designation in the SWIR band, the contrast difference of the different areas of the scene or the different bodies observed and the luminance of the observed designation spot, give an apparent contrast difference for the optical system, after passing through the atmosphere, and taking into account atmospheric conditions, then attenuation by passing through the lens of the device and taking into account its MTF modulation transfer function, then passing through the channel of visualization 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] Figure 4 shows another embodiment of the invention. This is again a portable firing system, 200, of the handgun or rifle type, comprising a thermal sight. This time it does not include the laser designator, which is located elsewhere, and is independent of the firing system in its movements. Typically, it can be a laser designator mounted on 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 also be emitted by a single-function device dedicated to this purpose.

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

[0055] [Fig. 5] Figure 5 shows the display on the firing system screen, with an optional feature consisting of digital image processing from the p-bolometer sensor to identify a temporal hash-type (also called frequency-domain) signature of the designation element(s). Thus, a means of detecting the temporal signature is available in the signal processing chain. Depending on the recognized signature, which can, for example, be classified into two types, "friendly" and "enemy," using a database if necessary, a different designation symbol is displayed, for example, using two different colors, or different geometric shapes or crosshairs. Here, an enemy designation 50 is represented by a triangle, which can be red, and a friendly designation 60 is represented by 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 telegraphed by an enemy, and the invention therefore makes it possible to warn him, if he is a friendly soldier, so that he can take cover or escape the telemetry to which he is being subjected.

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

[0057] In the absence of a temporal signature study of the designation points in the display device, the display is carried out using the microbolometer sensor and its wavelength-dependent sensitivity, such that the designation point is perceived by the user as a circular element of dimension characteristic showing a local temperature difference relative to the environment or a temperature gradient from the center of the circle to the outside of the circle depending on the visualization modes offered by the device.

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

[0059] - A thermal infrared transmission band limited to the 8-12 pm band, The cutoffs at 8 and 12 pm serve to reduce the chromatic correction requirements on optics for thermal imaging. These cutoffs occur gradually but rapidly over an interval that can be as small as 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 spectral response range of a bolometric sensor (between 8 and 13 pm).

[0060] - A reflected infrared transmission band approximately 50 nm wide around A 1.54 pm wavelength is used to visualize the designation point. This reflected infrared transmission band is separated, in terms of wavelength relative to the thermal imaging band, from the thermal infrared transmission band by a 4-5 pm wide non-transmission band. Furthermore, wavelengths closer to the visible spectrum are blocked to ensure that designations are intentionally selected at SWIR wavelengths, and typically, for practical construction reasons, at the 1.54 pm wavelength.

[0061] - And an opacity band between the reflected infrared transmission band and the The thermal infrared transmission band is approximately between wavelengths 2 pm and 6.5 pm (which, using these values, gives a width of 4.5 pm). Thus, the thermal infrared transmission band is limited towards shorter wavelengths, as it is followed in this direction by an opacity band, thereby avoiding chromatic aberrations for thermal imaging.

[0062]

[0063] [Fig. 6] Figure 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 onto the microbolometer sensor 300. In this particular case, a spectral pupil, or spectral diaphragm, is used, and the two separate optical chains are concentric. The external, peripheral, annular portion 310 allows the transmission of wavelengths from the LWIR band. For thermal imaging, the central section 320 allows the wavelengths of the reflected infrared (SWIR) transmission band to be passed through for visualization. An inverted design is available as an alternative. A focusing lens 330 directs the initially collimated beam onto the detector 300. The thermal and reflected infrared light are superimposed at the pixel level.

[0064] [Fig. 7] Figure 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 blade 350 positioned at 45° to the beams, and the two separate optical chains 360 and 370 are parallel and fused as they approach the microbolometer sensor 300. The upper part in the figure allows the transmission of wavelengths from the reflected infrared or SWIR infrared band for visualizing the designation, and the lower part allows the transmission of wavelengths from the LWIR band for thermal imaging. An inverted construction is provided as an alternative. The thermal infrared and reflected infrared lights are superimposed at the pixel level.

[0065] The variants of figures 6 and 7 can incorporate optical treatments to promote transmission in the desired frequency bands, and exclude wavelengths in the undesired bands.

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

[0067] The invention can also be implemented on a multifunction camera (typically equipped with several of the following functions: rangefinder, magnetic compass, laser pointer, gyroscope, and optical direct channels) incorporating an IRNR channel and a 1.54 pm laser designator. The pointer or rangefinder can be integrated into the multifunction camera. This camera has several modes, including a daytime vision mode and a mode based on a cooled infrared sensor.

Claims

Demands

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 from the scene towards which the sensor is directed included in a thermal infrared band (1) delimited on the side of the short 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 from the scene and included in a reflected infrared transmission band (2) separated from said thermal infrared band (1) by an opacity band (3),The signal processing chain generates a digital image based on the signals provided by the microbolometric sensor, which simultaneously receives 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-chains, the first and second sub-chains being respectively adapted to selectively transmit, to the exclusion of 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, fused by a beam combination blade (350).

5. A thermal camera according to any one of claims 1 to 4, further comprising, in the signal processing chain, a means for detecting a temporal 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 any one of claims 1 to 5, characterized in that the reflected infrared transmission band (2) has a width between 10 and 100 nm.

7. Thermal camera according to any one of claims 1 to 6, characterized in that the optical chain comprises optical components made of silicon or chalcogenide.

8. A shooting scope (100; 200) comprising a camera according to any one of claims 1 to 7, characterized in that the shooting optics of the scope comprise said camera and allow the sight to be aligned on a superposition of said image and said second light contained in a reflected infrared transmission band.

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

10. Multi-function camera comprising a thermal camera according to any one of claims 1 to 7.

Images