WO2015082514A1

WO2015082514A1 - Cooled ir imaging system with virtually cold filter

Info

Publication number: WO2015082514A1
Application number: PCT/EP2014/076345
Authority: WO
Inventors: Ludovic Perruchot
Original assignee: Thales
Priority date: 2013-12-04
Filing date: 2014-12-03
Publication date: 2015-06-11
Also published as: FR3014194A1

Abstract

The invention concerns a system for imaging a scene, which comprises a chamber (100) at a temperature T, with a cryostat (1) provided with an IR detector (10). According to the invention, the system comprises — in the chamber but outside the cryostat — a cold element with thermal emissivity Te1 <T, and a separator element (3) with a wavelength λs1 comprised within said IR domain, said cold element and said separator element being positioned such that the flow detected by the detector (1) is distributed by the separator element (30) between a flow from the scene and a flow from the cold element, in order to ensure a wavelength band cut-off in said IR domain for the flow from the scene, said band being determined in accordance with λs1, the wavelengths of the flow from the first cold element being comprised within said cut-off band.

Description

COOLED IR IMAGING SYSTEM WITH VIRTUALLY COLD FILTER

The field of the invention is that of cooled IR imaging (3.0 μηι-12μηΊ).

The equipment used today to provide cooled IR imaging functions, an example of which is shown in FIG. 1, comprises, in a known manner, in an enclosure 100:

a cryostat 1 equipped with an IR detector 10 and generally a cold spectral filter 1 1 capable of selecting a band of wavelengths between A _inf and A _sup ,

one or more other spectral filters 12 each capable of transmitting the wavelengths lower than As (As different from one filter to the other) and transmitting for wavelengths greater than As; these filters are located on a filter wheel 60 for example, outside the cryostat not to increase its volume and the number of elements to keep cold;

an optical combination 20 capable of forming the image of a scene on the IR detector 10, the wavelengths of the scene being comprised between A _inf and A _sup ; the wavelengths coming from the scene and arriving on the cryostat are therefore between

For example, a A _inf = 3.4 μητι, A _sup = 4.8 μιτι and As = 4.2 μιτι. The wavelengths arriving on the cryostat are therefore between 3.4 μηπ and 4.2 μηπ.

Today a spectral filter 12 located in the chamber 100 but outside the cryostat 1 is not cooled or said otherwise it is itself emissive IR radiation which produces additional noise related to its own temperature; this reduces the performance of the imager in terms of signal-to-noise ratio.

Consequently, there remains to date a need for a cooled IR imaging system, equipped with at least one spectral filter, simultaneously satisfying all the aforementioned requirements, in terms of in particular cryostat volume, and signal-to-noise ratio at the detector.

The principle of this invention consists in using a separating element such as a splitter plate, in reflection for the wavelength band considered and in transmission for the wavelengths to be rejected. This separator element is associated with a "cold" element such as a narcissus effect mirror or a low temperature black body so as to limit the emissivity of the separating element for the wavelengths to be rejected.

The inverse configuration with a transmission separator element for the wavelength band considered and in reflection for the wavelengths to be rejected, is also envisaged, as will be seen below.

More specifically, the subject of the invention is an imaging system of a scene, which comprises an enclosure whose temperature T is the internal temperature of the system, with in this chamber a cryostat equipped with an IR detector capable of detecting a IR flow. It is mainly characterized in that it comprises, in the chamber but outside the cryostat, a first element with a thermal emissivity Te1 <T, referred to as the first cold element, and a first separating element at a wavelength As1 included in FIG. said IR domain, this first cold element and said first separator element being positioned so that the flux detected by the detector is distributed by the first separator element between a flux coming from the scene and a flow coming from the first cold element, from in order to ensure a cut-off of wavelength band in said IR domain for the flux coming from the scene, this band being determined as a function of As1, the wavelengths of the flux coming from the first cold element being included in said band cut.

This amounts to producing a virtually cold filter, ensuring a cut in a band determined as a function of As for the flux coming from the scene, and allowing the detector to receive a stream coming from the cold element for wavelengths located in this cut strip; while the filter of the equipment described in the preamble absorbs the wavelengths to be filtered and re-emits according to its own temperature.

It is recalled that high band cut means that we want to select the small wavelengths, the large ones being rejected; conversely low band cut means that we want to select the long wavelengths, the small ones being rejected.

According to a first embodiment, the first separator element is in reflection for wavelengths greater than As1 of the flux coming from the scene and in transmission for wavelengths lower than As1 of the flux coming from the first cold element, from in such a way as to ensure a low band cut in said IR range for the flux detected by the detector and coming from the scene.

According to an alternative to this first embodiment, the first separator element is in transmission for wavelengths greater than As1 of the flux coming from the scene and in reflection for wavelengths lower than As1 of the flux coming from the cold element, so as to ensure a low band cut in said IR range for the flux detected by the detector and from the scene.

According to a second embodiment, the first separator element is in reflection for wavelengths greater than As1 of the flux coming from the first cold element and in transmission for wavelengths lower than As1 of the flux coming from the scene, of in order to ensure a high band cut in said IR range for the flux detected by the detector and coming from the scene.

According to an alternative to this second embodiment, the first separator element is in transmission for wavelengths greater than As1 of the flux coming from the first cold element and in reflection for wavelengths lower than As1 of the flux coming from the scene, so as to ensure a high band cut in said IR range for the flux detected by the detector and coming from the scene.

According to a third embodiment, the imaging system of a scene combines a low band cut optical configuration with a high band cut optical configuration. It comprises in the enclosure but outside the cryostat and upstream of the first separating element, a second element with thermal emissivity Te2 <T, designated second cold element, and a second separating element at a wavelength As2 included in said domain IR; this second cold element and this second separator element are positioned in such a way that the flux incident on the first separator element and coming from the scene is itself distributed by the second separator element between a flux coming from the scene and a flux originating from the second cold element, so as to ensure for the flow detected by the detector and from the scene, a high band and low band cut determined according to As1 and As2 and thus allow to select an intermediate band in said IR range.

For example, a high band cut configuration upstream of a low band cut configuration can be combined. Several combinations are possible. For example, the low band cut configuration described for the alternative to the first embodiment can be combined with a high band cut configuration with As2> As1 and with the second cold element and the second separating element positioned in such a way as to provide High band cut for the stream coming from the scene that is incident in reflection on the second separator element. It is also possible to combine a low band cut configuration upstream of a high band cut configuration. Again, several combinations are possible. For example, the low-band cut configuration described for the alternative to the second embodiment can be combined with the low-band cut configuration with As 2 <As 1 and with the second cold element and the second separator element positioned in such a way as to ensure low band cut for the stream coming from the incident scene in reflection on the second separator element.

According to one characteristic of the invention, the system comprises:

a plurality of first separator elements each having a different separation wavelength from a first separator element to another, and mounted on switching means of these first separator elements, and / or - A plurality of second separator elements each having a different separation wavelength from a second separator element to the other, and mounted on switching means of these second separator elements.

The switching means are for example a wheel.

Other features and advantages of the invention will appear on reading the detailed description which follows, given by way of non-limiting example and with reference to the appended drawings in which:

FIG. 1 already described schematically represents an example of an optical configuration of a cooled IR imaging system according to the state of the art,

FIGS. 2 schematically represent an example of an optical configuration of a cooled IR imaging system according to the invention, providing a low band cutoff for the flux coming from the scene and detected by the detector, with a reflection splitter plate for wavelengths greater than As and in transmission for wavelengths less than As, associated with a mirror with Narcissus effect (FIG. 2a.3 which is decomposed in FIGS. 2a.1 and 2a.2 respectively corresponding to domains of complementary wavelengths) or to a blackbody (FIG. 2b.3 which is decomposed in FIGS. 2b.1 and 2b.2 respectively corresponding to complementary wavelength domains), and with a transmission splitting plate for wavelengths greater than As and in reflection for wavelengths less than As, associated with a Narcissus effect mirror (FIG. 2c), or with a black body (FIG. 2d),

FIGS. 3 schematically represent an example of an optical configuration of a cooled IR imaging system according to the invention, providing a high band cutoff for the flux coming from the scene and detected by the detector, with a reflection splitter plate for wavelengths greater than As and in transmission for wavelengths less than As, associated with a Narcissus mirror (FIG. 3a) or a black body (FIG. 3b), and with a transmission splitter blade for lengths waves greater than As and in reflection for wavelengths lower than As, associated with a mirror with Narcissus effect (FIG. 3c), or with a black body (FIG. 3d), FIGS. 4 schematically represent an example of an optical configuration of a cooled IR imaging system according to the invention, combining the low band cut configurations and the high band cut configurations, to obtain an intermediate band cut, with for the flow from the scene a high band cut upstream of a low band cut (fig 4a), or a low band cut upstream of a high band cut:

with a first As1 splitter plate in reflection for the wavelengths less than As1 of the flux coming from the scene and, upstream, a second As2 splitter plate in reflection for the wavelengths greater than As2 of the flux coming from the scene (fig 4b),

with a first As1 splitter plate in reflection for the wavelengths greater than As1 of the flux coming from the scene and, upstream, a second As2 splitter plate in reflection for the wavelengths greater than As2 of the flux coming from the scene (fig 4c).

FIGS. 5 illustrate broadband detection means that can complete the band-cut configurations.

From one figure to another, the same elements are identified by the same references. The figures show the wavelength domains that pass through the system; in solid lines, those coming from the scene, in dotted lines, those coming from the cold element.

In general, an IR imaging system of a scene, which is a cooled system comprises an enclosure 100 whose temperature T is the internal temperature of the system, with in this chamber, a cryostat 1 equipped with an IR detector 10 able to detect a flow. It also includes in the enclosure but outside the cryostat:

an element with thermal emissivity Te <T, called cold element, and

a separator element 30 at a wavelength λ contained in said IR domain, which transmits or reflects the wavelengths to be selected.

This cold element and this separator element 30 are positioned in such a way that the flux detected by the detector 10 is distributed by the separator element between a stream coming from the scene, for a band of wavelengths determined according to As, and a stream coming from the cold element, for wavelengths outside of said wavelength band. This cold element is not emissive (= not emitting photons), unlike the filter used in the equipment described in the preamble.

This amounts to producing a virtually cold filter, ensuring a cut in a band determined as a function of As for the flux coming from the scene and allowing the detector to receive a flux from the cold element for wavelengths located in this region. cut tape.

This separator element is for example a separating plate or a separating prism with a dichroic treatment or a separator cube with also a dichroic treatment. In the following we will take a separator blade as an example of separator element.

This cold element is for example a black body 42 or a mirror with Narcissus effect 41 able to send back to the detector its own image.

Such a scene imaging system makes it possible to vary the spectral band observed as a function of As while maintaining an optimum signal-to-noise ratio.

This invention is exploitable whatever the field of use (terrestrial, airborne, maritime) of such a scene imaging system, IR cooled.

It solves the problem of performance of an imaging system of a scene, cooled IR, filter "classic" thus emissive which generates noise related to its own temperature. By making this spectral filter virtually virtual, this invention makes it possible to obtain the optimum performance of the sensor whatever the spectral band observed.

Such a system makes it possible to ensure a high band or low band or intermediate band cut for the stream coming from the scene, according to the chosen embodiment.

With reference to FIGS. 2a and 2b, a first embodiment of a cooled IR imaging system according to the invention is described. a low band cut for the stream coming from the scene and detected by the detector.

It comprises in the chamber 100 but outside the cryostat 1, a separator plate 30 to As of the IR domain considered, able to send the rays from the IR scene in reflection to the detector 10 for wavelengths greater than As , and in transmission in the enclosure for wavelengths less than As; she is not emissive. In other words, the plate 30 behaves like a mirror for wavelengths greater than As as shown in FIGS. 2a.1 and 2b.1, but is transparent for wavelengths less than As as shown in FIG. 2a.2 and FIG. 2b.2; the combination of these two functions of mirror and transparent blade is shown in Figures 2a.3 and 2b.3.

But the blade is at room temperature Te of the enclosure and the detector therefore also receives a flux emitted by the separating plate itself, independently of the scene; this "hot" flow which contributes only as additional noise, therefore deteriorates the signal-to-noise ratio of the detector.

The separating blade 30 is therefore associated with a low emissivity element called cold element. The plane of the separating blade delimits a first half-space called "front space" that is to say including the cryostat, and a second half-space called "rear space", complementary to the first. This cold element is placed behind the blade 30, that is to say in the "back" space with respect to the detector so as to face the detector through the blade; thus the detector 10 receives a flux emitted by this cold element, which passes through the transmission blade for wavelengths less than As. In fact, if this cold element was not put in place, the detector would see in transmission through the blade. separator, the mechanical structure of the enclosure throughout the IR band, in addition to the "hot" flow of the blade: instead of receiving a noise corresponding to the entire IR band, the detector receives a noise corresponding only to the high band. This combination of the separator blade and the cold element thus makes it possible to cut the high band (greater than As) so that the detector receives only the signal of the desired low band and to limit the noise of the blade to that of the band. high band, and thus improve the S / N ratio. At wavelengths less than As, the detector sees the cold element as if there was no separating blade.

When the cold element is a black body 42 shown in FIGS. 2b, the flux emitted by this black body is omnidirectional but a part of this flux reaches the detector 10 in transmission through the splitter plate 30 which is "transparent" for this flow for the wavelengths lower than As. The flow of the black body towards the detector can optionally be directed with an optical system to limit the size of said black body.

When the cold element is a Narcissus effect mirror 41 shown in FIGS. 2a, the cold flow emitted by the cooled detector and transmitted by the splitter plate 30 in transmission to the Narcissus effect mirror 41 (for wavelengths greater than As therefore), is reflected by this mirror towards the detector in the opposite direction, through the separating plate 30 in transmission also, always for wavelengths lower than As.

According to an alternative to this optical configuration ensuring a low band cutoff, a splitter plate is used in reflection for wavelengths less than As and in transmission for wavelengths greater than As. This type of splitter blade is less common use. The splitter blade at As is positioned to send the rays from the IR scene in transmission to the detector for wavelengths greater than As, and in reflection in the chamber for wavelengths less than As. And the cold element is placed in the space "back of the blade", so as to be reflected on the blade towards the detector for wavelengths greater than As, as shown in the example of Figures 2c and 2d. .

FIGS. 3a and 3b show a second embodiment of a cooled IR imaging system according to the invention, providing a high band cut for the flux coming from the scene.

It comprises in the chamber 100 but outside the cryostat 1, a separator plate 30 to As of the IR domain considered, capable of sending the rays from the IR scene in transmission to the detector 10 for the wavelengths less than As, and reflection in the chamber for wavelengths greater than As.

This splitter plate 30 is associated with a cold element placed in the "before" space of the blade, so as to send the flux from this cold element to the detector in reflection on the splitter plate for wavelengths greater than As indeed, if one did not put this element cold, the detector would see in reflection on the separating plate, the mechanical structure of the enclosure in all the band IR. When the cold element is a black body 42 shown in FIG. 3b, the flux emitted by the black body is omnidirectional but part of this flux reaches the detector 10 by reflection on the splitter plate 30 for wavelengths greater than As.

When the cold element is a Narcissus mirror 41 shown in FIG. 3a, the cold flow emitted by the detector cooled and reflected by the splitter plate 30 to the Narcissus effect mirror (for wavelengths less than As, for example), is reflected by this mirror to the separating plate which also reflects it to the detector 10 in the opposite direction, for wavelengths greater than As.

According to an alternative to this optical configuration ensuring a high band cutoff, a splitter plate is used in reflection for wavelengths less than As and in transmission for wavelengths greater than As. in order to send the rays coming from the IR scene in reflection towards the detector for the wavelengths lower than As, and in transmission in the chamber for the wavelengths higher than As. And the cold element is placed in the space "back of the blade", so as to be sent by transmission through the blade to the detector for wavelengths greater than As, as can be seen in Figure 3c. In FIG. 3d, the Narcissus mirror 41 has been replaced by a black body 42.

We have for example for these embodiments in the IR2 domain (3μηΊ-5μηι):

Aint = 3.4 μηη, A _sup = 4.8 μηι and As = 3.6 μηι or 3.8 μηι or 4.2 μηπ.

In the IR3 domain (8μηΊ-12μηπ), there are, for example: A _inf = 8 μΐΎΐ, A _sup = 10 μm and

As = 8.5 μm or 9 μιτι or 9.5 μm.

With reference to FIGS. 4, a third embodiment of a cooled IR imaging system according to the invention is described, making it possible to select an intermediate band, that is to say ensuring a cut at the same time high band and low band for the stream coming from the scene.

For this purpose, a low band cut configuration is combined with a high band cut configuration in the following manner.

According to a first embodiment shown in FIG. 4a, it comprises in the enclosure 100 but outside the cryostat 1, a first separating plate 30a with As1 of the considered IR domain, able to send the rays coming from the IR scene in transmission towards the detector 10 for wavelengths greater than As1, and reflection in the chamber for wavelengths lower than As1. It is thus a configuration ensuring a low band cut for the flow coming from the scene. This first separating blade 30a is associated with a first thermal emissivity element Tel <T designated first cold element 41 placed in the chamber outside the cryostat, in the "before" space of the blade, so as to send the flow from this first cold element towards the detector 10 in reflection on the first separating plate 30a for the wavelengths lower than As1.

The stream coming from the scene, incident in transmission on this first splitter plate 30a, is itself from a high-band cut configuration for the flux coming from the scene.

Indeed, the imaging system further comprises in the chamber 100 but outside the cryostat 1, and upstream of the first separator blade 30a (the upstream direction -> downstream is the scene-> detector direction), a second thermal emissivity element Te2 <T, designated second cold element 41b, and a second separator plate 30b at a wavelength As2 included in said IR domain, with As2> As1. This second cold element 41b and this second separating plate 30b are positioned in such a way that the transmission incident flux on the first separating plate 30a it is itself distributed by the second splitter plate 30b between a flux from the scene and a flow from the second cold element 41b.

More specifically, the second splitter plate 30b to As2 is adapted to send the rays from the IR scene in reflection to the first splitter plate 30a for wavelengths less than As2, and in transmission in the enclosure for the lengths of the wavelength. wave greater than As2. It is therefore a configuration ensuring a high band cut, located upstream of the low band configuration as shown in the figure. This second separating plate 30b is associated with a second cold element 41b which is placed behind the second plate 30b, that is to say in the "rear" space, on the optical axis coming from the detector and passing through this second transmission splitter plate so as to face the detector through the two blades 30a, 30b; thus the first splitter plate receives a flux emitted by this second cold element 41b, which passes through the second transmission blade for wavelengths greater than As2.

This third embodiment thus makes it possible to ensure for the stream coming from the scene a high band and low band cut so as to select an intermediate band, that is to say in a band between As1 and As2 with As1> A _inf the lower wavelength of said IR domain and As2 <A _sup the upper wavelength of said IR domain. This intermediate band is usually a narrow band.

This third embodiment has been described using a first As1 splitter plate in reflection for wavelengths less than As1 of the flux from the scene and a second As2 splitter plate in reflection for wavelengths less than As2 of the stream coming from the scene.

This embodiment may be declined in the following manner using:

a first As1 splitter plate in reflection for wavelengths less than As1 and a second As2 splitter plate in reflection for wavelengths longer than As2;

a first As1 splitter plate in reflection for wavelengths longer than As1 and a second As2 splitter plate in reflection for wavelengths less than As2; a first As1 splitter plate in reflection for wavelengths longer than As1 and a second As2 splitter plate in reflection for wavelengths longer than As2. We have for example for this third embodiment in the IR2 domain:

A _inf = 3.4 μηη, A _sup = 4.8 μηι and

As1 = 3.6 μηπ with As2 = 3.8 μηπ, or As1 = 3.8 μιτι with As2 = 4 μιτι.

In the IR3 domain, for example:

A μητι _inf = 8, A = 10 _sup μηπ and

As = 8.5 μιτι or 9 μιτι or 9.5 μm.

According to an alternative to this third embodiment, the high and low band configurations are reversed for the stream coming from the scene: the low band configuration is upstream of the high band configuration as can be seen in FIG. 4b. to select an intermediate band.

It comprises in the chamber 100 but outside the cryostat 1, a first splitter plate 30a to As1 of the IR domain considered, capable of sending the rays from the IR scene in reflection to the detector 10 for wavelengths less than As1, and in transmission in the enclosure for wavelengths greater than As1. It is thus a configuration ensuring a high band cut for the flow coming from the scene. This first separating plate is associated with a first low-emissivity element Tel <T said first cold element such as a cold black body or a mirror Narcissus effect 41a which reflects the detector 10 cooled. This cold element 41a is placed behind the first blade 30a, that is to say in the "rear" space relative to the detector; thus the detector receives a flux emitted by this first cold element 41a, which passes through the first plate 30a in transmission for wavelengths greater than As1.

The flux coming from the scene incident in reflection on this first splitter blade is itself from a low band cut configuration.

Indeed, the imaging system further comprises in the chamber but outside the cryostat, a second element with thermal emissivity Te2 <T, designated second cold element 41b, and a second separator plate 30b at a wavelength As2 included in said IR domain, with As2 <As1. This second cold element 41b and this second separating plate 30b are positioned in such a way that the incident flux in reflection on the first separating plate 30a is itself distributed by the second separating plate 30b between a flux coming from the scene and a flow from the second cold element 41 b.

More specifically, the second splitter plate 30b to As2 is able to send the rays from the IR scene in reflection to the first splitter plate 30a for wavelengths greater than As2, and in transmission in the enclosure for the lengths of wave less than As2. It is therefore a configuration providing a low band cut for the stream from the scene, located upstream of the high band configuration as shown in the figure.

The second cold element 41b is placed in the space

"Before" of the second blade 30b, so as to send the flow from the second cold element to the first separating blade 30a in transmission on the second separator blade, for wavelengths less than As2.

This alternative to the third embodiment has been described using a first As1 splitter plate in reflection for wavelengths lower than As1 of the flux from the scene and a second wavelength reflecting As2 splitter plate. greater than As2 of the stream from the scene.

This alternative can be declined in the following manner by adapting the positioning of the detector and using:

a first As1 splitter plate in reflection for wavelengths longer than As1 and a second As2 splitter plate in reflection for wavelengths less than As2;

a first As1 splitter plate in reflection for wavelengths greater than As1 and a second As2 splitter plate in reflection for wavelengths greater than As2. This embodiment is illustrated in FIG. 4c.

We have for example for this alternative:

A _inf = 3.4 μηη, A _sup = 4.8 μηι and

As1 = 3.8 μιτι with As2 = 3.6 μιτι, or As1 = 4 μιτι with As2 = 3.8 μιτι.

According to one characteristic of the invention, the system comprises, for each high-band cut configuration, a plurality of separating blades, each having a spectral separation wavelength λ different from one blade to the other; these blades are mounted on switching means 60 (or 60a) of these blades. Likewise, for each low-band cut configuration, the system comprises a plurality of separating blades, each having a spectral separation wavelength λ different from one blade to the other; these blades are mounted on switching means 60 (or 60b) of these blades.

These switching means are for example a wheel such as a filter wheel on which the filters have been replaced by the separating blades, or a carousel, or ...

These band-cut configurations can of course be supplemented to also provide broadband detection. A mirror 50 able to reflect the scene towards the detector whatever the wavelength, can be installed in the enclosure 100. It is for example mounted on switching means 60 with the splitter plate 30, as shown in FIG. Figure 5a. Another solution shown in FIG. 5b that does not include a mirror allows the detector to receive the image of the scene regardless of the wavelength. The system is then called broadband.

Each cold element used in these different configurations, may be a cold black body, that is to say Te <T temperature or a mirror with narcissus effect that reflects the cooled detector.

For example, we use:

dichroic blades having a reflection coefficient of 95% and a transmission coefficient of 5%, a black body regulated at a temperature of 2 ° C or even lower than 0 ° C if the environment is controlled (nitrogen-type dry gas),

a mirror with a concave Narcissus effect reflecting the detector itself having a reflection of> 99.8%,

- a detector of 640 x 512 pixels in step of 15μηι technology

InSb or MCT cooled between 77 ° K and 120 ° K.

Claims

An imaging system of a scene, which comprises a chamber (100) whose temperature T is the internal temperature of the system, with in this chamber, a cryostat (1) equipped with a detector (10) IR capable of detecting a IR flow, characterized in that it comprises in the chamber but outside the cryostat, a first element with thermal emissivity Te1 <T, said first cold element, and a first separating element (30, 30a) with a length of wave As1 included in said IR domain, this first cold element and this first separator element being positioned in such a way that the flux detected by the detector (10) is distributed by the first separator element (30, 30a) between a flux coming from the scene and a flux coming from the first cold element, so as to ensure a wavelength band cut in said IR domain for the flux coming from the scene, this band being determined according to As1, the wavelengths flu x from the first cold element being included in said cut strip.

A scene imaging system according to claim 1, characterized in that the first splitter element (30, 30a) is in reflection for wavelengths greater than As1 of the flux from the scene and in transmission for lengths lower than As1 wave of the flow from the first cold element, so as to ensure a low band cut in said IR range for the flux detected by the detector and from the scene.

A scene imaging system according to claim 1, characterized in that the first splitter element (30, 30a) is in transmission for wavelengths greater than As1 of the flux from the scene and in reflection for lengths wave less than As1 of the flux from the cold element, so as to ensure a low band cut in said IR range for the flux detected by the detector and from the scene. A scene imaging system according to claim 1, characterized in that the first splitter element (30, 30a) is in reflection for wavelengths greater than As1 of the flux from the first cold element and in transmission for wavelengths less than As1 of the flux from the scene, so as to ensure a high band cut in said IR range for the flux detected by the detector and coming from the scene.

An imaging system of a scene according to the preceding claim, characterized in that the first separating element (30, 30a) is in transmission for wavelengths greater than As1 of the flux coming from the first cold element and in reflection for wavelengths less than As1 of the flux from the scene, so as to ensure a high band cut in said IR range for the flux detected by the detector and coming from the scene.

Imaging system of a scene according to claim 1, characterized in that it comprises in the enclosure (100) but outside the cryostat (1) and upstream of the first separator element (30a), a second element to thermal emissivity Te2 <T, designated second cold element, and a second separator element (30b) at a wavelength As2 included in said IR domain, this second cold element and this second separator element being positioned in such a way that the flux incident on the first separator element (30a) and coming from the scene is itself distributed by the second separator element (30b) between a flux from the scene and a flow from the second cold element, so as to ensure for the flow detected by the detector and coming from the scene, a high band and low band cut determined according to As1 and As2 and thus make it possible to select an intermediate band in said IR domain .

A scene imaging system according to the preceding claim combined with claim 3, characterized in that As2> As1 and in that the second cold element and the second separator element are positioned to provide a high band cutoff for the flux coming from the incident scene in reflection on the second separator element (30a).

8. A scene imaging system according to claim 6 combined with claim 5, characterized in that As2 <As1 and in that the second cold element and the second separator element are positioned to provide a low band cutoff for the flux coming from the incident scene in reflection on the second separator element (30a).

9. Imaging system of a scene according to one of the preceding claims, characterized in that it comprises a plurality of first separator elements (30, 30a) each having a different separation wavelength of a first separator element to the other, and mounted on switching means (60, 60a) of these first separator elements.

10. A scene imaging system according to one of the preceding claims, characterized in that it comprises a plurality of second separating elements (30b) each having a different separation wavelength from a first separating element to a separating element. other, and mounted on switching means (60b) of these second separator elements. 1 1. Scene imaging system according to one of claims 8 or 9, characterized in that the switching means are a wheel.

12. Scene imaging system according to one of the preceding claims, characterized in that at least one cold element is a black body (42, 42a, 42b).

13. Scene imaging system according to one of the preceding claims, characterized in that at least one cold element is a narcissus effect mirror (41, 41 a, 41 b).

14. Scene imaging system according to one of the preceding claims, characterized in that the domain I R is IR2 or IR3.