Angle-selective filter infrared imaging device

Abstract

This apparatus (3) for detecting infrared electromagnetic radiation comprises a housing (5) defining a vacuum enclosure (7) and comprising a window (9) transparent to said infrared electromagnetic radiation, the apparatus (3) comprising, positioned in the enclosure (7), one end of a cold finger (15), a cold screen (11) and a sensor (13), 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) configured to allow the passage of infrared electromagnetic radiation coming from the window (9) to the sensor (13), the apparatus (3) comprising 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 or reflect rays of the infrared electromagnetic radiation according to their angle of incidence on said filter (29). Figure for the abstract: Fig. 1

Description

Title of the invention: Infrared imaging apparatus with angularly selective filter Technical field

[0001] The present invention relates to detection devices for thermal imaging.

[0002] 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.

[0003] Generally speaking, the invention can also be applied to any part of the infrared spectrum. Previous techniques

[0004] A detection device for thermal imaging generally operates with an infrared electromagnetic radiation sensor, also called FPA ("Focal Plane Array" in English terms), positioned in a vacuum enclosure and cooled in a cryostat by a cold machine or a Joule-Thomson expander. The enclosure is formed by a housing comprising a window configured to transmit infrared electromagnetic radiation.

[0005] 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.

[0006] Light rays can thus pass through the window, then through the diaphragm to strike the pixels of the sensor.

[0007] However, new generations of sensors now include more pixels and are of larger dimensions, requiring the design of detection systems with a larger diaphragm opening.

[0008] 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.

[0009] Furthermore, 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 quite simply to gain in compactness. Statement of the invention

[0010] 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.

[0011] The present invention relates to an apparatus for detecting infrared electromagnetic radiation, comprising a housing defining a vacuum enclosure and comprising a window transparent to said infrared electromagnetic radiation, the apparatus further comprising, positioned in the enclosure, 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 the infrared electromagnetic radiation between the window and the sensor, the filter being configured to transmit or reflect rays of the infrared electromagnetic radiation according to their angle of incidence on said filter.

[0012] Thus, this angular filtering of the solid angle seen by each pixel makes it possible to divide the parasitic flux by 10. In fact, the 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.

[0013] Depending on the angle of incidence chosen to carry out the angular selection, it is possible to form a variant of the telecentric system.

[0014] In addition, this device makes it possible to bring the diaphragm 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 the implementation and cooling times.

[0015] Advantageously, the filter is configured to transmit the 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 the 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°.

[0016] 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.

[0017] In a first embodiment, the filter is positioned in contact with the window.

[0018] In a second embodiment, the filter is positioned across the opening of the cold screen.

[0019] In a third embodiment, the filter is positioned directly on the sensor.

[0020] 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.

[0021] Advantageously, the filter comprises an alternation of thin layers comprising only germanium on the one hand and silicon dioxide on the other hand.

[0022] Advantageously, the cold screen comprises a matte black internal coating, and an external surface reflecting in the infrared.

[0023] 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. Brief description of the drawings

[0024] 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:

[0025] [Fig.l] is a schematic sectional view of an imaging system comprising an apparatus for detecting infrared electromagnetic radiation according to a first embodiment;

[0026] [Fig.2] is a schematic sectional view of an imaging system comprising an apparatus for detecting infrared electromagnetic radiation according to a second embodiment; and

[0027] [Fig.3] is a schematic sectional view of an imaging system comprising an apparatus for detecting infrared electromagnetic radiation according to a third embodiment.

[0028] Detailed description of at least one embodiment

[0029] [Fig.l] schematically shows an imaging system 1 comprising an apparatus 3 for detecting infrared electromagnetic radiation according to a first embodiment.

[0030] 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 mid-infrared, or between 8 and 9 micrometers for far-infrared.

[0031] The apparatus 3 comprises a housing 5 defining a vacuum enclosure 7.

[0032] The housing 5 is opaque but also includes a transparent window 9 for the infrared electromagnetic radiation. Window 9 is, for example, circular or rectangular in shape.

[0033] The apparatus 3 also comprises, positioned in the vacuum enclosure 7, a cold screen 11 and a sensor 13 of said infrared electromagnetic radiation.

[0034] The cold screen 11 is also called a cold baffle or cooled optical deflector.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] The cold screen 11, 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.

[0040] 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.

[0041] The housing 5 also comprises an internal surface 25 reflecting in the infrared.

[0042] 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.

[0043] 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 infrared electromagnetic radiation according to their angle of incidence on said filter 29.

[0044] 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.

[0045] In this first embodiment, the filter 29 is positioned on the window 9, for example on the surface facing the vacuum enclosure 7.

[0046] 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.

[0047] Preferably, the filter 29 comprises an alternation of layers respectively with a refractive index of less than 1.5 and greater than 3.

[0048] For example, the filter 29 comprises an alternation of thin layers comprising only germanium on the one hand and silicon dioxide on the other hand.

[0049] 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°.

[0050] Furthermore, 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°.

[0051] This configuration corresponds to a detection device 3 having an aperture number of 1.2.

[0052] 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.

[0053] Since transmission through a medium or reflection against the latter 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.

[0054] 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 of 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.

[0055] Similarly, the filter 29 is configured to reflect 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 greater than 30° 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.

[0056] [Fig.2] schematically shows an imaging system 1 comprising an apparatus 3 for detecting infrared electromagnetic radiation according to a second embodiment.

[0057] In this embodiment, the imaging system 1 comprises the same elements and characteristics as the first embodiment illustrated in [Fig.l], with the exception of a change in the position of the filter 29.

[0058] In this second embodiment, the filter 29 is positioned across the opening 17 of the cold screen 11 and not on the window 9.

[0059] For example, a blade 33 of material transparent in the infrared 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.

[0060] The same transmission and reflection characteristics as in the first embodiment apply to the filter 29 in this second embodiment.

[0061] [Fig. 3] schematically shows an imaging system 1 comprising an apparatus 3 for detecting infrared electromagnetic radiation according to a third embodiment.

[0062] In this embodiment, the imaging system 1 comprises the same elements and characteristics as the first embodiment illustrated in [Fig.l], with the exception of a change in the position of the filter 29.

[0063] In this third embodiment, the filter 29 is positioned directly on the sensor 13 and not on the window 9.

[0064] 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.

[0065] The same transmission and reflection characteristics as in the first embodiment apply to the filter 29 in this third embodiment.

[0066] This third embodiment is the most effective embodiment with the objective of reducing the parasitic flux, the filter 29 being positioned as close as possible to the sensor 13.

[0067] The first embodiment is however robust, the filter 29 not being subjected to temperatures as low as in the second and third embodiments.

[0068] The second embodiment represents a good compromise between the first and third embodiments.

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 infrared electromagnetic radiation between the window (9) and the sensor (13),the filter (29) being configured to transmit or reflect 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, wherein 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 shield (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, wherein the cold screen (11) comprises a matte black internal coating (21), and an external surface (23) reflective in the infrared.

10. An 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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