Infrared imaging device with angularly selective filter

Abstract

The invention relates to a device (3) for detecting infrared electromagnetic radiation, the device comprising a housing (5) that defines a vacuum chamber (7) and comprises a window (9) transparent to the infrared electromagnetic radiation, wherein the device (3) comprises, positioned in the chamber (7), one end of a cold finger (15), a cold shield (11), and a sensor (13), wherein the cold shield (11) and the sensor (13) are configured to be cooled by the cold finger (15), and wherein the cold shield (11) comprises an opening (17) configured to allow the infrared electromagnetic radiation to pass from the window (9) to the sensor (13), and wherein the device (3) comprises a filter (29) positioned along the path of the infrared electromagnetic radiation between the window (9) and the sensor (13), wherein the filter (29) is configured to transmit or reflect rays of the infrared electromagnetic radiation depending on their angle of incidence on the filter (29).

Description

[0001] DESCRIPTION

[0002] TITLE: Angle-selective filter infrared imaging device

[0003] Technical field

[0004] The present invention relates to detection apparatus for thermal imaging.

[0005] A particularly interesting application of the invention relates to a cooled imaging system for the infrared domain, in particular the mid-infrared or even the far-infrared.

[0006] Generally, the invention can also be applied to any part of the infrared spectrum.

[0007] Previous techniques

[0008] A thermal imaging detection device generally operates with an infrared electromagnetic radiation sensor, also called a FPA (Focal Plane Array), positioned in a vacuum chamber and cooled in a cryostat by a cold machine or a Joule-Thomson expander. The chamber is formed by a housing including a window configured to transmit infrared electromagnetic radiation.

[0009] Such a detection device comprises, for example, a cold screen placed upstream of the sensor and which forms the opening diaphragm of said detection device.

[0010] Light rays can thus pass through the window, then through the diaphragm to strike the pixels of the sensor.

[0011] However, new generations of sensors now include more pixels and are larger in size, requiring the design of detection systems with a larger diaphragm opening.

[0012] The diaphragm delimits the solid angles seen by all the pixels but, due to its large aperture, it induces the presence of a parasitic flux, such as Poisson noise, which is quite significant, for example at least 30% greater than the useful flux. In addition, the closer the diaphragm of the cold screen is to the sensor, the greater the parasitic flux will be, even though it would be interesting in terms of optical design to be able to position a field lens close to the sensor, for example for the development of very open imaging systems, or simply to gain in compactness.

[0013] Statement of the invention

[0014] The present invention therefore aims to overcome the aforementioned drawbacks and to provide a detection device whose parasitic flux arriving at the pixels is reduced compared to a device of the prior art.

[0015] The present invention relates to an apparatus for detecting infrared electromagnetic radiation, comprising a housing defining a vacuum chamber and comprising a window transparent to said infrared electromagnetic radiation, the apparatus further comprising, positioned in the chamber, one end of a cold finger, a cold screen and a sensor of said infrared electromagnetic radiation, the cold screen and the sensor being configured to be cooled by the cold finger, the cold screen comprising an opening forming a diaphragm configured to allow the passage of infrared electromagnetic radiation coming from the window to the sensor, the apparatus comprising a filter positioned on the passage of infrared electromagnetic radiation between the window and the sensor,the filter being configured to transmit up to a chosen angle of incidence and reflect beyond this chosen angle of incidence rays of infrared electromagnetic radiation according to their angle of incidence on said filter.,

[0016] In other words, the filter is configured to transmit in one angular range and reflect in a complementary angular range.

[0017] Thus, this angular filtering of the solid angle seen by each pixel makes it possible to divide the parasitic flux by 10. In fact, angular filtering makes it possible to form a virtual pupil moving according to the angle of incidence of the rays of the electromagnetic radiation. In other words, each pixel of the sensor has its own aperture diaphragm.

[0018] Depending on the angle of incidence chosen to perform the angular selection, it is possible to form a variant of telecentric system.

[0019] In addition, this device allows the diaphragm to be brought closer to the cold screen of the sensor, and therefore to reduce the thermal mass of the cold screen to be cooled, and thus improve implementation and cooling times.

[0020] Advantageously, the filter is configured to transmit infrared electromagnetic radiation when the rays of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter between 0° and 25°, the filter being configured to reflect infrared electromagnetic radiation when the rays of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter greater than 30°.

[0021] In one embodiment, the filter is an interference filter configured to transmit and / or reflect infrared electromagnetic radiation of wavelength between 3.8 and 4.1 micrometers, or between 4.5 and 4.8 micrometers, or between 8 and 9 micrometers, preferably only between 3.8 and 4.1 micrometers.

[0022] In a first embodiment, the filter is positioned in contact with the window.

[0023] In a second embodiment, the filter is positioned across the opening of the cold shield.

[0024] In a third embodiment, the filter is positioned directly on the sensor.

[0025] Advantageously, the filter is a stack of thin layers, preferably comprising an alternation of layers respectively with a refractive index of less than 1.5 and greater than 3. Advantageously, the filter comprises an alternation of thin layers comprising only germanium on the one hand and silicon dioxide on the other hand.

[0026] Advantageously, the cold screen includes a matte black internal coating, and an external surface reflective in the infrared.

[0027] The invention also relates to an imaging system comprising the apparatus as defined above, the imaging system further comprising a telecentric and / or Cassegrain type optical system.

[0028] Brief description of the drawings

[0029] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0030] [Fig 1] is a schematic sectional view of an imaging system comprising an apparatus for detecting infrared electromagnetic radiation according to a first embodiment;

[0031] [Fig 2] is a schematic sectional view of an imaging system comprising an apparatus for detecting infrared electromagnetic radiation according to a second embodiment;

[0032] [Fig 3] is a schematic sectional view of an imaging system comprising an apparatus for detecting infrared electromagnetic radiation according to a third embodiment; and

[0033] [Fig 4] is a schematic representation of a pupil with variable position induced by the filter of the detection device according to the invention.

[0034] Detailed description of at least one embodiment Figure 1 schematically shows an imaging system 1 comprising an apparatus 3 for detecting infrared electromagnetic radiation according to a first embodiment.

[0035] In a particular application, the infrared electromagnetic radiation is radiation in the mid-infrared, called MWIR ("Middle Wavelength Infrared" in English terms), and / or in the far-infrared, called LWIR ("Long-Wave Infrared" in English terms). In particular, the wavelength of the electromagnetic radiation is between 3.8 and 4.1 micrometers, or between 4.5 and 4.8 micrometers for the mid-infrared, or between 8 and 9 micrometers for the far-infrared.

[0036] The device 3 comprises a housing 5 defining a vacuum enclosure 7.

[0037] The housing 5 is opaque but also includes a transparent window 9 for infrared electromagnetic radiation. The window 9 is, for example, circular or rectangular in shape.

[0038] The device 3 also comprises, positioned in the vacuum enclosure 7, a cold screen 11 and a sensor 13 of said infrared electromagnetic radiation.

[0039] The cold screen 1 1 is also called cold baffle or cooled optical deflector.

[0040] The cold screen 11 and the sensor 13 are mounted, for example on one end of a cold finger 15, so that the cold screen 11 and the sensor 13 are configured to be cooled via said cold finger 15.

[0041] The cold finger 15 comprises for example one end in the vacuum enclosure 7, its other end exiting the vacuum enclosure. The cold finger 15 comprises for example a heat exchanger taken in its mass in order to cool the sensor 13 and the cold screen 11, preferably to temperatures below 150 Kelvin.

[0042] The cold screen 11 is opaque but includes an opening 17 forming a diaphragm 17 configured to allow the passage of infrared electromagnetic radiation coming from the window 9 towards the sensor 13. The opening 17 is for example circular or rectangular in shape.

[0043] Thus, the window 9, the opening 17 and the sensor 13 are aligned on the same optical axis 19 and the directions of their longitudinal dimensions are orthogonal to said optical axis 19.

[0044] The cold screen 1 1 , when cooled to less than 150 Kelvin, makes it possible to block and therefore reduce the presence of parasitic flows, in particular coming from itself.

[0045] Advantageously, the cold screen 11 comprises a matte black internal coating 21 maximizing its absorption of electromagnetic radiation. Similarly, the cold screen 11 comprises an external surface 23 reflecting in the infrared, in order to avoid untimely heating of said cold screen 11.

[0046] The housing 5 also includes an internal surface 25 which is reflective in the infrared.

[0047] Aligned on the same optical axis 19, the imaging system 1 comprises an optical system 27 configured to direct electromagnetic radiation towards the sensor 13. The optical system 27 is for example a telecentric optical system or an optical system of the Cassegrain telescope type.

[0048] The apparatus 3 further comprises a filter 29 positioned on the passage of the infrared electromagnetic radiation between the window 9 and the sensor 13. This filter 29 is an angularly selective filter 29, the filter 29 being configured to transmit or reflect rays 31 of the infrared electromagnetic radiation according to their angle of incidence on said filter 29.

[0049] The device 3 equipped with the filter 29 makes it possible to bring the optical system 27 as close as possible to the housing 5, the optical system 27 comprising for example a field lens making it possible to obtain a very open and compact imaging system 1.

[0050] In this first embodiment, the filter 29 is positioned on the window 9, for example on the surface facing the vacuum enclosure 7. Advantageously, the filter 29 is a stack of thin layers deposited on the window 9. The filter 29 is therefore a dichroic filter 29, or also called interference, angularly selective.

[0051] Preferably, the filter 29 comprises an alternation of layers respectively with a refractive index of less than 1.5 and greater than 3.

[0052] For example, filter 29 comprises an alternation of thin layers comprising only germanium on the one hand and silicon dioxide on the other hand.

[0053] In a preferred embodiment, the filter 29 is configured to transmit infrared electromagnetic radiation when the rays 31 of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter between 0° and 25°.

[0054] In addition, the filter 29 is configured to reflect infrared electromagnetic radiation when the rays 31 of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter 29 greater than 30°.

[0055] This configuration corresponds to a detection device 3 having an aperture number of 1.2.

[0056] This maximizes the detection of rays whose direction is almost normal to the surface of the filter 29, and minimizes the detection of rays arriving at an angle, generally rays inducing a parasitic flux.

[0057] Since transmission through a medium or reflection against it is never perfect, in other words the luminous flux is never transmitted or reflected at 100%, transmission and reflection respectively mean a transmission of at least 51% of the luminous flux in a particular wavelength range, and a reflection of at least 51% of the luminous flux in a particular wavelength range.

[0058] Preferably, the filter 29 is configured to transmit the infrared electromagnetic radiation when the rays 31 of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter 29 of between 0° and 25° and for wavelengths between 3.8 and 4.1 micrometers, or between 4.5 and 4.8 micrometers, or between 3.8 and 4.1 and 4.5 and 4.8 micrometers, or between 8 and 9 micrometers, preferably only between 3.8 and 4.1 micrometers.

[0059] Likewise, the filter 29 is configured to reflect infrared electromagnetic radiation when the rays 31 of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter 29 greater than 30° and for wavelengths between 3.8 and 4.1 micrometers, or between 4.5 and

[0060] 4.8 micrometers, or between 3.8 and 4.1 and 4.5 and 4.8 micrometers, or between 8 and 9 micrometers, preferably only between

[0061] 3.8 and 4.1 micrometers.

[0062] Figure 2 schematically shows an imaging system 1 comprising an apparatus 3 for detecting infrared electromagnetic radiation according to a second embodiment.

[0063] In this embodiment, the imaging system 1 includes the same elements and features as the first embodiment illustrated in FIG. 1, except for a change in the position of the filter 29.

[0064] In this second embodiment, the filter 29 is positioned across the opening 17 of the cold screen 11 and not on the window 9.

[0065] For example, a blade 33 of infrared-transparent material is positioned across the opening 17, and a multi-layer treatment is applied to this blade 33, on its external face as shown or on its internal face, so as to form the angularly selective filter 29.

[0066] The same transmission and reflection characteristics as in the first embodiment apply to filter 29 in this second embodiment.

[0067] Figure 3 schematically shows an imaging system 1 comprising an apparatus 3 for detecting infrared electromagnetic radiation according to a third embodiment.

[0068] In this embodiment, the imaging system 1 includes the same elements and features as the first embodiment illustrated in FIG. 1, except for a change in the position of the filter 29.

[0069] In this third embodiment, the filter 29 is positioned directly on the sensor 13 and not on the window 9.

[0070] For example, a multi-layer treatment is applied directly to the surface of the sensor 13 so as to form the angularly selective filter 29.

[0071] The same transmission and reflection characteristics as in the first embodiment apply to filter 29 in this third embodiment.

[0072] This third embodiment is the most effective embodiment in the objective of reducing the parasitic flux, the filter 29 being positioned as close as possible to the sensor 13.

[0073] The first embodiment is however robust, the filter 29 not being subjected to temperatures as low as in the second and third embodiments.

[0074] The second embodiment represents a good compromise between the first and third embodiments.

[0075] Figure 4 schematically illustrates the effect provided by the present invention. Figure 4 schematically shows a sensor 13 from which three pixels P1, P2 and P3 are highlighted. The useful flux F1, F2 and F3 respectively of each of these pixels P1, P2 and P3 is shown in order to illustrate the effect of the angular filtering of the filter 29.

[0076] As previously mentioned, the angular filtering induced by the presence of the filter 29 makes it possible to form a mobile virtual pupil moving in front of each pixel of the sensor 13 according to the angle of incidence of the rays of the electromagnetic radiation. In other words, each pixel of the sensor 13 has its own aperture diaphragm.

[0077] Without the installation of the filter 29, the pupil would be fixed, symbolized by the zone Z, and the same for each pixel of the sensor 13. The pupil would be defined only by the window 9 or the optical system 27 and would not allow the suppression of parasitic flows arriving on the sensor 13 with a significant angle of incidence.

Claims

CLAIMS 1. Apparatus (3) for detecting infrared electromagnetic radiation, comprising a housing (5) defining a vacuum enclosure (7) and comprising a window (9) transparent to said infrared electromagnetic radiation, the apparatus (3) further comprising, positioned in the enclosure (7), one end of a cold finger (15), a cold screen (11) and a sensor (13) of said infrared electromagnetic radiation, the cold screen (11) and the sensor (13) being configured to be cooled by the cold finger (15), the cold screen (11) comprising an opening (17) forming a diaphragm configured to allow the passage of infrared electromagnetic radiation coming from the window (9) to the sensor (13), characterized in that it comprises a filter (29) positioned on the passage of the infrared electromagnetic radiation between the window (9) and the sensor (13),the filter (29) being configured to transmit up to a chosen angle of incidence and reflect beyond this chosen angle of incidence rays of infrared electromagnetic radiation according to their angle of incidence on said filter (29)., 2. Apparatus (3) according to claim 1, wherein the filter (29) is configured to transmit infrared electromagnetic radiation when the rays (31) of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter (29) between 0° and 25°, the filter (29) being configured to reflect infrared electromagnetic radiation when the rays (31) of said infrared electromagnetic radiation have an inclination relative to the normal to the surface of the filter (29) greater than 30°.

3. Apparatus (3) according to one of claims 1 and 2, wherein the filter (29) is an interference filter (29) configured to transmit and / or reflect infrared electromagnetic radiation with a wavelength between 3.8 and 4.1 micrometers, or between 4.5 and 4.8 micrometers, or between 3.8 and 4.1 and 4.5 and 4.8 micrometers, or between 8 and 9 micrometers, preferably only between 3.8 and 4.1 micrometers.

4. Apparatus (3) according to any one of claims 1 to 3, in which the filter (29) is positioned in contact with the window (9).

5. Apparatus (3) according to any one of claims 1 to 3, wherein the filter (29) is positioned across the opening (17) of the cold screen (11).

6. Apparatus (3) according to any one of claims 1 to 3, wherein the filter (29) is positioned directly on the sensor (13).

7. Apparatus (3) according to any one of claims 1 to 6, in which the filter (29) is a stack of thin layers, preferably comprising an alternation of layers respectively with a refractive index of less than 1.5 and greater than 3.

8. Apparatus (3) according to claim 7, wherein the filter (29) comprises an alternation of thin layers comprising only germanium on the one hand and silicon dioxide on the other hand.

9. Apparatus (3) according to any one of claims 1 to 8, in which the cold screen (11) comprises a matte black internal coating (21), and an external surface (23) reflecting in the infrared.

10. Imaging system (1) comprising the apparatus (3) according to any one of claims 1 to 9, the imaging system (1) further comprising a telecentric and / or Cassegrain type optical system.

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