Infrared imaging device

Abstract

The invention relates to an infrared imaging device (1) comprising: - an imaging system (3), - a measuring system (5) comprising: -- an array of lenses (7) situated in an image plane (9) of the imaging system, and -- an array of photodetectors (11), -- a cryostat (13) configured to cool the array of photodetectors and the array of lenses, each lens (7a, 7b) being associated with a main photodetector (11a, 11b), and each lens being configured to image an exit pupil (14) of the imaging system on an area centred on the main photodetector, the device being configured either in a first configuration, in which a distance separating two centres of adjacent main photodetectors is equal to twice a dimension of a main photodetector, or in a second configuration, in which the distance separating two centres of adjacent main photodetectors is equal to three times the dimension of a main photodetector.

Description

[0001]Description Title: Infrared Imaging Device FIELD The invention relates to the field of infrared imaging, and in particular to cryostat-cooled infrared imaging devices. PRIOR TECHNOLOGY US2015 / 146075 describes a multiband camera and multiband image capture method. US2020 / 241262 describes a spectral imager and spectral imaging method. US2024 / 089569 describes a directional response imaging array. WO2022 / 018484 describes a multispectral light-field device. In the field of cooled infrared imaging, imaging devices are known that comprise a detection block including a detector and a cryostat configured to cool the detector. A cold screen is integrated into the cryostat, the cold screen extending around the photosensitive part of the detector to limit the instrumental background of the device. The cold screen optically defines a diaphragm, called the cold diaphragm, upstream of the photosensitive part. This cold diaphragmdefines a numerical aperture for the detection system, allowing the viewing angle of each photodetector to be limited. The cold screen must be cooled to cryogenic temperatures and be light enough to withstand the vibrations the device may experience and to cool down rapidly. During operation, the cold screen must be cooled before images can be acquired. A cooling time is essential. This cooling time is not always compatible with the mission of interest. Therefore, there is a need for a cooled infrared imaging device with a shorter cooling time than those used in the prior art. PRESENTATION One aim of this presentation is to propose a faster cooled infrared imaging device than those used in the prior art. This aim is achieved through an infrared imaging device comprising: - an imaging system configured to form an image of an object, - ameasurement system located downstream of the imaging system with respect to a direction of light propagation, the measurement system comprising: - a lens array located in an image plane of the imaging system, and - a photodetector array located downstream of the lens array, - a cryostat configured to cool the photodetector array and the lens array, each lens being associated with a primary photodetector and each lens being configured to image an output pupil of the imaging system in a plane of the photodetector array over an area centered on the primary photodetector, the device being configured either in a first configuration in which the distance separating two adjacent primary photodetector centers is equal to twice the dimension of a primary photodetector, the area being less than or equal to the area of ​​four primary photodetectors, or in a second configuration in whichThe distance separating two adjacent principal photodetector centers is equal to three times the dimension of a principal photodetector, the area being less than or equal to the surface area of ​​nine principal photodetectors. Such a device is advantageously and optionally complemented by the following features, taken alone or in combination: - each lens is configured to image the exit pupil of the imaging system in a plane of the photodetector array over an area less than or equal to the surface area of ​​the principal photodetector associated with the lens; - each lens is aligned with the associated principal photodetector along a direction parallel to the optical axis; - for each lens, a lens center, an exit pupil center, and a center of the principal photodetector associated with the lens are aligned on the same straight line; - the photodetectors are pixels, and preferably the pixels corresponding to theThe primary photodetectors use different discharge capacities than the other pixels; each photodetector is surrounded by peripheral photodetectors, the device including a mask configured to block light arriving at the peripheral photodetectors; the primary photodetectors are photodiodes, each photodiode being surrounded in the plane of the photodetector array by a peripheral zone configured to block, evacuate, or absorb incident light flux, the peripheral zone corresponding to an area of ​​three primary photodetectors or an area of ​​eight primary photodetectors; a hood surrounding the measurement system and a screen located between the photodetector array and the hood, the screen being cooled by the cryostat, the screen being configured to block a ray passing through a primary photodetector associated with a first lens and a second lens adjacent to the first lens (preferably the screen).(not materializing the exit pupil of the imaging system); and - a volume separating the lens array and the photodetector array is occupied by a substrate having an optical index greater than 1, preferably greater than or equal to 1.5, and even more preferably greater than or equal to 2. The presentation also relates to a method for infrared imaging of an object comprising: - a step of constructing an image of the object by an imaging system on a lens array using light from the object, - a step of optically conjugating a plane of an exit pupil of the imaging system and a plane of a photodetector array by the lens array, the photodetector array being located downstream of the lens array with respect to a direction of light propagation, each lens being associated with a primary photodetector, and each lens being configured to image the exit pupil over an area centered on themain photodetector, the conjugation step being carried out either in a first configuration in which the distance separating two adjacent main photodetector centers is equal to twice the dimension of a main photodetector, the area being less than or equal to the surface area of ​​four main photodetectors, or in a second configuration in which the distance separating two adjacent main photodetector centers is equal to three times the dimension of a main photodetector, the area being less than or equal to the surface area of ​​nine main photodetectors, - a cooling step for the photodetector array and the lens array, - a light measurement step for the main photodetectors. DESCRIPTION OF FIGURES Other features and advantages will become apparent from the following description, which is purely illustrative and not limiting, and should be read in conjunction with the accompanying drawings on which:[Fig. 1] Figure 1 is a schematic representation of an example of an infrared imaging device; [Fig. 2] Figure 2 is a schematic representation of a detail of an example of an infrared imaging device; [Fig. 3][Fig. 4] Figures 3 and 4 are schematic representations of an optical image obtained in the plane of a photodetector array of an example of an infrared imaging device; [Fig. 5] Figure 5 is a schematic representation of an example of an infrared imaging method; and [Fig. 6] Figure 6 is a schematic representation of certain elements of the device of Figure 1, and is supplemented by the theoretical position of one or more large screens 12 which are absent from this device according to the invention. DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1, an infrared imaging device 1 comprises an imaging system 3 configured to form an image of an object 2. The imaging system 3 collects a portion of the radiationand in particular of the infrared radiation emitted by object 2. The imaging system forms an image of object 2 in an image plane 9. The imaging system is centered on the optical axis X, an optical axis which can be oriented in the direction of propagation of light, i.e. from the object towards the imaging system 3 or from the imaging system 3 towards the image plane 9. The imaging system 3 has an exit pupil 14 which is located upstream of the image plane 9, i.e. that the exit pupil 14 is located between the imaging system 3 and the image plane 9. Advantageously, the exit pupil 14 is not materialized by a diaphragm. Alternatively, the exit pupil 14 can be represented by a diaphragm. The exit pupil of the imaging system 3 is understood here as the image of the aperture diaphragm of the imaging system 3 by all the optics of the imaging system 3 that are located downstream of the aperture diaphragm, or, where applicable, as the diaphragm itself.The aperture itself, when it is a single element and located downstream of all the optics, is the mechanical component of the imaging system 3 that imposes the maximum angle of the cone of rays reaching a point on the image plane 9 and originating from a point in the field passing through the imaging system 3. The maximum angle allows the calculation of a working aperture number, designated by WF#. The imaging system can be, in particular, a photographic lens. The infrared imaging device 1 also includes a measuring system 5 located downstream of the imaging system 3 with respect to the direction of light propagation. In other words, the imaging system 3 is placed between the object 2 and the measurement system 5. The measurement system 5 is centered on the optical axis X. The measurement system 5 comprises a lens array 7 located in the image plane 9. The lens array comprises a plurality of lenses 7a, 7b, 7c arranged in rows and columns, the rows and theThe columns are orthogonal to each other and to the optical axis X. The lens array 7 is centered on the optical axis. The lenses can advantageously be placed side by side so as to be in contact. The lenses can be microlenses, that is, lenses with a diameter greater than or equal to 10 µm and less than or equal to 90 µm. Since the lens array 7 is located in the image plane 9, each lens receives light from a specific portion of the object 2. Each lens has an aperture number, denoted by F#µL. All lenses have the same aperture number. The measurement system 5 includes a photodetector array 11 located downstream of the lens array 7, i.e., the lens array 7 is located between the imaging system 3 and the photodetector array 11. The photodetector array 11 includes a plurality of photodetectors 11a,11b, 11c are arranged in rows and columns, the rows and columns being orthogonal to each other and to the optical axis X. The photodetector array 11 is centered on the optical axis X. The infrared imaging device 1 also includes a cryostat 13 configured to cool the photodetector array 11. Each lens is associated with a primary photodetector configured to receive light rays passing through a center of the lens and a center of the exit pupil 14. Figure 2 illustrates the general case of such a light ray arriving at a lens at a non-zero angle of incidence. In relation to Figure 2, a light ray 43 is shown passing through the center of the exit pupil 14 and through the center of a lens 7c included in the lens array 7. Lens 7c is not centered on the optical axis X; it is separated from the optical axis X by a length referenced H1 in Figure 2. The light ray 43 defines,Between the exit pupil 14 and the lens array 7, there is an angle of incidence i with the direction of the optical axis X. The length L1 separates the exit pupil 14 and the lens array 7. Downstream of lens 7c, the light ray 43 defines an exit angle r (or angle of refraction r) with the direction of the optical axis X. The exit angle r is different from the angle of incidence i because the refractive index n1 of the medium upstream of lens 7c is different from the refractive index n of the medium 15 downstream of lens 7c. In this case, the angles of incidence i and refraction r are related by Snell's law: n1*sin(i)=n*sin(r). The light ray impacts the main photodetector 11c associated with lens 7c. The length L2, measured along the optical axis X between the apex of a lens and the detector, separates the lens array 7 and the photodetector array 11. The main photodetector 11c is not centered on the optical axis X; it is separated from the axisX-ray optical axis of a length referenced H2 in Figure 2. Lens 7c is associated with the main photodetector 11c which is configured to receive the light ray 43 passing through the center of lens 7c and the center of exit pupil 14. There is a difference ∆ between the length H2 which separates the main photodetector 11c from the X-ray optical axis and the length H1 which separates lens 7c from the X-ray optical axis. H2 is greater than H1. This offset ∆ can be determined based on the parameters of measurement system 5: ∆ = (H2-H1) =(H1*L2) / (n*L1) = H2 / (1+nL1 / L2). In other words, each lens is offset from its primary photodetector by an offset ∆ measured orthogonally to the optical axis X. The offset ∆ is equal to the ratio between the product of the distance L2 between the lens vertex and the detector and the distance H1 from the center of the lens to the optical axis, and the product of the refractive index of the medium separating the lens and its primary photodetector and a distance L1 separating thethe exit pupil lens matrix, with the lens being closer to the optical axis than its primary photodetector. This is equivalent to choosing a spatial period in the lens matrix 7 that is not a multiple of the spatial period in the photodetector matrix 11. These two periods are not in an integer ratio. The technical effect associated with this particular choice of periods is to ensure, for each lens, that a cone of light emanating from the exit pupil of the imaging system and passing through the lens is directed precisely and centered towards the primary photodetector associated with that lens. It should be noted that in more specific cases, the average angle of the cones of light rays emanating from the exit pupil of the imaging system and arriving at the lenses may have an incidence of zero or sufficiently small to be neglected. In these cases, each lens is aligned with its primary photodetector in a direction parallel to the axis.optical. This telecentric embodiment corresponds to the case where the light cones from the imaging system 3 arrive at the measurement system 5 at very small angles of incidence. It is as if the exit pupil 14 were very far from the lens array. This mode is particularly advantageous for limiting the spectral deviation of a filter placed between the imaging system 3 and the lens array 7. Each lens is also configured to image the exit pupil 14 in a plane of the photodetector array 11, over an area centered on its main photodetector. Each lens optically conjugates the plane of the exit pupil with the plane of the photodetector array. Each lens optically conjugates the center of the exit pupil with the center of its main photodetector. The device can be configured in two ways. In the first configuration, called the "quad pixel" configuration,The distance separating two adjacent principal photodetector centers is equal to twice the dimension of a principal photodetector. Two principal photodetectors are adjacent if they are directly neighbors; that is, there cannot be a third principal photodetector located between two principal photodetectors. This is equivalent to surrounding each principal photodetector with a buffer zone centered on the principal photodetector, the buffer zone extending over a dimension equal to twice the dimension of a principal photodetector. The surface of the principal photodetector lies at the center of the buffer zone. We can define, in the plane of the photodetector array, an area allocated to each lens as the combination of the surface of its principal photodetector and the buffer zone surrounding it. Such a distance between two principal photodetectors implies geometric conditions in the lens array. In particular, when the lenses areWhen placed in contact with each other, the diameter of a lens is close to twice the size of a primary photodetector. The diameter may, in particular, be less than or equal to twice the size of a primary photodetector. If two adjacent primary photodetector centers are separated by twice the size of a primary photodetector, a peripheral photodetector that is not of the primary type may be located between the two primary photodetectors. In this case, each primary photodetector may be surrounded by eight peripheral photodetectors. Such peripheral photodetectors are the same size as a primary photodetector. The eight peripheral photodetectors include four primary photodetectors that share a common side with the primary photodetector. Each center of one of the first four peripheral photodetectors is obtained by translating the center of the primary photodetector by a distancecorresponding to a dimension of the primary photodetector, the translation occurs in one of two directions along the two axes of the photoreceptor array. There are thus four basic translations: two translations along the rows of the array, respectively to the right and to the left, and two translations along the columns of the array above and below the primary photodetector. Each basic translation applied to the primary photodetector yields one of the first four photodetectors. The eight peripheral photodetectors include four secondary peripheral photodetectors that share a common corner with the primary photodetector. Each center of one of the four secondary peripheral photodetectors is obtained by two translations from the center of the primary photodetector: one basic row translation and one basic column translation. There is a secondary peripheral photodetector respectively above and to the right, above and to the left,below and to the right, and finally below and to the left of the primary photodetector. In this case, the buffer zone associated with a primary photodetector corresponds to a fraction of the area of ​​these eight peripheral photodetectors. More precisely, the buffer zone corresponds to half the area of ​​the first four peripheral photodetectors and a quarter of the area of ​​the second four peripheral photodetectors. Total, the buffer zone corresponds to an area of ​​three photodetectors. In the first configuration, each lens is configured to image the exit pupil of the imaging system in the plane of the photodetector array over an area less than or equal to the area of ​​four primary photodetectors. In other words, the area occupied by the image of the exit pupil by a lens is less than or equal to four times the area of ​​a primary photodetector. This ratio of four times is related to the term "quad-pixel". This area isinscribed within the surface defined by the primary photodetector and the buffer zone described previously. This condition is notably met when the diameter of the exit pupil image is less than or equal to twice the dimension of a pixel. In a second configuration, called the "nona pixel" configuration, the distance separating two adjacent primary photodetector centers is equal to three times the dimension of a primary photodetector. This amounts to surrounding each primary photodetector with a buffer zone centered on the primary photodetector, the buffer zone extending over a dimension equal to three times the dimension of a primary photodetector. The surface of the primary photodetector lies at the center of the buffer zone. We can define, in the plane of the photodetector array, an area allocated to each lens as the combination of the surface of its primary photodetector and the buffer zone surrounding it. Such a gap between twoThe use of primary photodetectors implies specific geometric conditions in the lens array. Specifically, when lenses are in contact with each other, the diameter of a lens is approximately three times the size of a primary photodetector. The diameter may also be less than or equal to three times the size of a primary photodetector. If two adjacent primary photodetector centers are separated by three times the size of a primary photodetector, two non-primary photodetectors may be located between them. In this case, each primary photodetector may be surrounded by eight peripheral photodetectors. Such peripheral photodetectors are the same size as a primary photodetector. As before, the eight peripheral photodetectors comprise four primary and four secondary photodetectors. EachThe center of one of the first four peripheral photodetectors is obtained by a basic translation. Each center of one of the second four peripheral photodetectors is obtained by two translations of the center of the primary photodetector: a basic row translation and a basic column translation. In this case, the buffer area associated with a primary photodetector corresponds to the entire surface area of ​​these eight peripheral photodetectors. In the second configuration, each lens is configured to image the exit pupil of the imaging system in the plane of the photodetector array over an area less than or equal to the surface area of ​​nine primary photodetectors. In other words, the area occupied by the image of the exit pupil by a lens is less than or equal to nine times the surface area of ​​a primary photodetector. This ratio of nine times is related to the term "nona-pixel." This area is inscribed within the surface defined by the photodetector.main and the buffer zone described previously. This condition is met, in particular, when the diameter of the exit pupil image is less than or equal to three times the size of a pixel. Figure 1 illustrates the second configuration, known as "nona-pixels," in which the exit pupil 14 is imaged over an area corresponding to the maximum surface area of ​​nine main photodetectors. The image is centered on the main photodetector 11a and covers eight peripheral photodetectors, including the first peripheral photodetectors 16 and 18, which surround the main photodetector 11a. The peripheral photodetectors 16 and 18 are part of eight main photodetectors surrounding the main photodetector, with the image of the exit pupil 14 extending over each of the eight peripheral photodetectors. Advantageously, when each main photodetector is surrounded by peripheral photodetectors, the device includes a mask configured to block lightarriving at the peripheral photodetectors. The mask can be placed at the level of the photodetector array 11. The mask can be configured to block a light ray whose angle of incidence is not included in the light cone defined by the surface of the main photodetector and the center of the associated lens. It allows the peripheral photodetectors, i.e., the area surrounding each main photodetector, to be blinded. This improves the quality of the images produced of object 2 by preventing blooming or limiting the instrumental background obtained by scattering light from a peripheral photodetector to the main photodetector. By configuring the lens array 7 so that each lens forms an image of the exit pupil of the imaging system 3 centered on its main photodetector, and by requiring that the area allocated to each lens 7 extends over an area corresponding to the surface of four main photodetectors.or to a surface of nine main photodetectors, it is possible to retain, for image production, only the light received by the main photodetector of each lens. For each lens, this selection of the light received only by the main photodetector corresponds to not retaining, for image formation, the light collected by the lens and which corresponds to rays whose angle of incidence on the lens 7 is outside a certain central cone. This ultimately amounts to defining a numerical aperture. It is then possible to use this condition to define the numerical aperture of the detection system 5. This numerical aperture corresponds to a cone of light defined by the surface area allocated to the lens and the center of the lens. Unlike the prior art, and as illustrated in Figure 6, it is no longer necessary to use one or more large cold screen(s) 12 (illustrated in Figure 6 only with dashed lines to indicate its(theoretical position, each screen 12 being absent), slow to cool, to define at its end 120 a numerical aperture of the detection system and to block the rays 121, 122, 123, 124 that would pass outside the cone of incidence defined by the exit pupil. This cold screen 12 is commonly used in the prior art to limit the viewing angle of each photodetector of the detector, in particular so that the photodetectors do not see the hood of the detection system. According to the invention, by retaining, to form an image, only the light received by the main photodetectors, the aperture can be limited, in particular so that the main photodetectors do not see the hood of the detection system. The detection system produces images without needing a large cold screen 12. The cooling of the screen 12 before producing images is no longer necessary, and the cooling of the cooled infrared imaging device is faster thanin the prior art. Furthermore, such a device offers the following advantages: - it is not necessary to deposit thin absorbing materials between the image plane 9 and the photodetector array 11; - there is no shadowing effect between the image plane 9 and the photodetector array 11 because the lenses 7 collect the light flux arriving on their surface and distribute it among the different photodetectors 11; and - when the lenses are microlenses, their optical sag is limited and the number of manufacturing steps for the microlenses is limited. The optical sag of a lens corresponds, for a curved surface of the lens, to a distance measured along the optical axis of the lens between a central point and a peripheral point on the surface. It can be advantageous to choose the lens aperture number F#µL less than or equal to the working aperture number of the imaging system WF#. In this way, there is no light from theimaging system and transmitted by a lens that arrives at the photodetector array outside the area mentioned previously, this area being centered on the primary photodetector associated with the lens. Advantageously, the area occupied by the image of the exit pupil by a lens is, in the first "quad pixel" configuration, strictly less than four times the area of ​​a primary photodetector, or in the second "nona pixel" configuration, strictly less than nine times the area of ​​a primary photodetector. For example, in the first "quad pixel" configuration, the area occupied by the image of the exit pupil by a lens is less than or equal to three times the area of ​​a primary photodetector. For example, the diameter of the image of the exit pupil is less than 1.5 times the size of a primary photodetector or less than 1.2 times the size of aprimary photodetector. For example, in the first "nona pixel" configuration, the area occupied by the image of the exit pupil formed by a lens is less than or equal to five times the area of ​​a primary photodetector, or less than or equal to three times the area of ​​a primary photodetector. For example, the diameter of the image of the exit pupil is less than twice the size of a primary photodetector, or less than 1.5 times the size of a primary photodetector. Even more advantageously, the area occupied by the image of the exit pupil formed by a lens is less than or equal to the area of ​​a primary photodetector. Each lens produces an image of the exit pupil in the plane of the photodetector array 11, this image being centered on the primary photodetector associated with the lens and entirely contained within that primary photodetector. According to a first alternative concerning the photodetectors, these are pixels of aA photographic sensor based on a charge-transfer device. The pixels are sensitive to infrared radiation, the sensor being configured to produce images in this spectral range. Preferably, the pixels corresponding to the primary photodetectors use different charge capacities than the other pixels. The charge capacity of a pixel corresponds to the amount of charge (electrical, such as electrons or holes) that a pixel can store. Charge capacity can also be defined as the depth of the potential well defined by the pixel. In this first alternative, the number of pixels in the photodetector array 11 is greater than or equal to the number of lenses in the lens array 7. More precisely, the number of pixels is greater than or equal to four or nine times the number of lenses. It is then possible to associate with each lens an area of ​​four pixels or an area of ​​nine pixels in the plane ofThe photodetector array 11. Each pixel composing the surface is associated with the lens and can receive a portion of the light collected by the lens. In all cases, the pixel corresponding to the main photodetector receives at least a portion of the light collected by the lens. Each pixel associated with a lens can receive light rays whose angles of incidence are contained within a cone of light centered on an angle of incidence defined by the angle formed between the optical axis X and the line passing through the center of the pixel and the center of the lens. The cone of light is defined by the surface of the pixel and the center of the lens. With reference to Figure 1, and in the case of lens 7a, the main photodetector 11a corresponding to a pixel associated with lens 7a is placed at the center of the other pixels 16 and 18, which are also associated with lens 7a. The other pixels 16 and 18 are, as already mentioned above, peripheral pixels to the photodetector.The principal pixel 11a receives, for example, the light cone 21 from lens 7a. This cone corresponds upstream of the lens to a light cone 19 and covers a central part of the exit pupil of the imaging system 3. Pixels 16 and 18 receive, for example, the light cones 25 and 29 respectively from lens 7a. These cones correspond upstream of the lens to light cones 27 and 23 respectively. In this example, these cones cover peripheral parts of the exit pupil of the imaging system 3. As mentioned previously, only the light received by the principal pixel 11a is retained to produce an image, and more generally, for each lens, only the light received by its principal pixel. This pixel selection corresponds to retaining only the rays whose angle of incidence is within the central cone defined by the cross-section of the principal pixel and the center of the lens, and ultimately toDefine the numerical aperture of the measurement system. 5. In this first alternative, the instrumental background is captured by the peripheral pixels of the main pixel. These peripheral pixels are called blind because the received signal is not taken into account in image formation. In particular, one can choose main pixels with a charge capacity lower than that of the peripheral pixels. In this way, one limits the risk that the peripheral pixels will contain more charge than they can store, which could lead to an alteration of the main pixel's charge, such as in a blooming effect (glare). This improves the quality of the images produced, especially for long exposure times. In this first alternative, it can be advantageous to include a specific readout circuit for the photodetector array, this readout circuit being configured to manage the charge reading of the pixels.exhibiting different charging capacities. According to a second alternative concerning the photodetectors, the array comprises only photodiodes. The number of photodiodes is greater than or equal to the number of lenses. Each lens is associated with a single photodiode that corresponds to its primary photodetector. Each photodiode is surrounded in the plane of the photodetector array by a peripheral zone configured to dissipate or absorb incident light. This peripheral zone corresponds to the buffer zone defined previously. In this way, the risk of incident radiation around the primary photodetector altering the signal of the photodiode corresponding to the primary photodetector is limited. This improves the quality of the images produced, particularly in the case of long exposure times. In this second alternative, the instrumental background is managed by the passivation zones around the central photodiodes. It can be advantageousIn this second alternative, a specific detection circuit, and even a specific readout circuit, is used for the photodiode array. This detection and readout circuits are configured to recover only the signal from the primary photodetectors. Figures 3 and 4 illustrate two implementations of the infrared imaging device. In Figure 3, each lens is configured to image a portion of the plane containing the output pupil in a plane of the photodetector array over an area corresponding to the surface area of ​​nine primary photodetectors. The image of the imaging system's pupil is centered on the primary pixel. As mentioned previously, a preferred mode corresponds to the case where the image of the imaging system's pupil is entirely contained within the primary pixel. Each square shown in Figure 3 corresponds to the surface area of ​​a primary photodetector. When the photodetector array is a pixel array,Each square corresponds to a pixel, and when the photodetector array comprises photodiodes according to the second alternative, only certain squares correspond to a photodiode, one square out of nine in this case. Figure 3 corresponds to the so-called telecentric embodiment, that is, the case where, for each cone of light from the imaging system 3 arriving at the measurement system 5, the mean angle of the cone corresponds to a very small angle of incidence. The mean angle of the cones of the light rays from the exit pupil of the imaging system arriving at the lenses has an incidence of zero or sufficiently small to be neglected. In these cases, each lens is aligned with its main photodetector in a direction parallel to the optical axis. The main photodetector 11a associated with lens 7a is centered on the optical axis X. A projection along the optical axis of lens 7a onto the plane of the photodetector array is represented byCircle 35. Since the lens is aligned with its primary photodetector along the optical axis, the projection 35 is centered on the primary photodetector 11a. This set of nine squares is designated 'nona-pixel', as mentioned previously. In Figure 3, the set of nine squares, or 'nona-pixel' 31, corresponds to lens 7b and is centered on the primary photodetector 11b associated with lens 7b. In Figure 4, each lens is configured to image a portion of the plane containing the output pupil in a plane of the photodetector array over an area corresponding to the surface area of ​​four primary photodetectors, with the image of the pupil of the imaging system centered on the primary pixel. As mentioned previously, a preferred mode corresponds to the case where the image of the pupil of the imaging system is entirely contained within the primary pixel. Each square shown in Figure 3 corresponds to the surface area of ​​a primary photodetector. When the photodetector arrayis a pixel matrix, each square corresponding to a pixel, and when the photodetector matrix includes photodiodes according to the second alternative, only some squares correspond to a photodiode, one square out of four in this case. Figure 4 corresponds to the so-called telecentric embodiment, that is, the case where the light cones from the imaging system 3 arrive at the measurement system 5 with very small angles of incidence. The average angle of the cones of the light rays from the exit pupil of the imaging system and arriving at the lenses has an incidence of zero or low enough to be neglected. In these cases, each lens is aligned with its main photodetector in a direction parallel to the optical axis. The main photodetector 11a associated with lens 7a is centered on the optical axis X. A projection along the optical axis of lens 7a onto the plane of the photodetector matrix is ​​represented by theCircle 37. As the lens is aligned with its primary photodetector along the optical axis, the projection 37 is centered on the primary photodetector 11a. The nine squares centered on the photodetector 11a include: - the photodetector 11a, which receives light only from lens 7a; - the first four squares, which receive light from two lenses: lens 7a and the one above, below, to the right, and to the left of lens 7a; and - the second four squares, which receive light from four lenses: lens 7a and three lenses that form the corners of the large square formed by the eight squares surrounding square 11a. When the squares correspond to peripheral photodetectors, the first four squares are the first four peripheral photodetectors mentioned previously, and the second four squares are the second four peripheral photodetectors. The area allocated to a lens corresponds to a surface offour main photodetectors. This area has been represented and referenced 33 for lens 7b and main photodetector 11b. As mentioned previously, each lens is associated with a surface of four squares or 'quad-pixels', the central photodetector of which is the main photodetector of the lens. According to a first option, and with reference to Figure 1, the infrared imaging device may include a hood 45 surrounding the measurement system 5. The hood 45 allows the measurement system to be evacuated for cooling. Advantageously, when the infrared imaging device includes a hood 45, the device includes, for each main photodetector, a mask placed at the level of the photodetector array 11 configured to cover the buffer area associated with the main photodetector. The mask is configured to block a light ray whose angle of incidence would not be included in the cone of light defined by the surface of the photodetector.The main lens and the center of the associated lens are protected. The mask allows, among other things, the blocking of radiation emitted by the hood 45 that could reach the photodetector array 11. The hood 45 is not cooled by the cryostat 13. In relation to the first option, the infrared imaging device may include, in addition to the hood 45, a screen 17 (significantly smaller than the state-of-the-art screen 12) located between the photodetector array 11 and the hood 45. The screen 17 is cooled by the cryostat and configured to block a ray passing through a main photodetector associated with a first lens and a second lens adjacent to the first lens. When cooled, the screen 17 allows the blocking of grazing rays, for example, those emitted by the hood 45. The hood is at ambient temperature and therefore emits radiation that could unintentionally reach the detector. Screen 17 is configured forThe screen 17 is a cold screen that blocks ray beams passing through a primary photodetector associated with a first lens and a second lens separated from the first by one or more lenses of the lens array. The hood 45 is not cooled by the cryostat 13. The screen 17 is a cold screen that blocks so-called 'grazing' rays, i.e., rays with a very high angle of incidence with the optical axis. These are, for example, rays passing through a primary photodetector associated with a first lens and a second lens adjacent to the first lens, but also rays passing through a primary photodetector associated with a first lens and a second lens separated from the first by one or more lenses. The screen 17 thus protects the primary photodetector from these high-angle rays and improves the quality of the images produced. Thanks to this screen 17, the primary photodetector receives little to no light.light passes outside the exit pupil and through the lens associated with an adjacent primary photodetector. This ensures that the only light received by the primary photodetector is light passing through the exit pupil and the lens associated with the primary photodetector. It should be noted that the screen 17 does not block all the radiation emitted by the hood 45 towards a primary photodetector, only the grazing portion. In other words, the screen 17 does not represent the exit pupil of the imaging system 3 as can be the case in the prior art. It is smaller and lighter in mass than the cold screens 12 of the prior art. The cooling time associated with the screen 17 can be sufficiently short to keep the infrared imaging device 1 sufficiently fast. It should be noted that the first and second options can be combined, as illustrated in Figure 1. The volume separating the matrix fromThe lenses 7 and the photodetector array 11 can be partially occupied by a substrate having a refractive index greater than or equal to 1, preferably greater than or equal to 1.5, and even more preferably greater than or equal to 2. By choosing a high-index material separating the lens array 7 and the photodetector array 11, it is easier to obtain a very low aperture number for the lens. This is an advantage for an infrared configuration compared to the visible range. Furthermore, when the lenses are microlenses, their diffractive effects can be limited by configuring the system to define the largest possible generalized Fresnel number FN' thanks to the material between the microlens and the detector having a high refractive index. Example implementation: In the case of a photodetector array that is a pixel array, in the case of a nona-pixel configuration whereEach lens has an allocated area in the photodetector array plane corresponding to a surface of nine pixels. A 1024x780 pixel array can be chosen, with each pixel having a size of 10µm and operating in the mid-infrared (MWIR, corresponding to a wavelength range between 3µm and 5µm). The lens array comprises microlenses with a focal length of 55µm, a diameter of 30µm, a radius of curvature of 35µm, and a refractive index of 2.75. The aperture number of the microlens is 0.67. The generalized Fresnel number (FN) is 2.8. At the edge of the photodetector array, the gap ∆ between the microlens and its main photodetector is approximately 5 µm (5.11 µm precisely, taking H2 = 5.12 mm). The very low number of apertures in the microlens is noteworthy. This configuration is possible because the distance between the microlens and the detector is embedded in a high-refractive-index material. This is an advantage.For an infrared configuration relative to the visible range: In this example, the cone defined by the cross-section of the main pixel and the center of the lens corresponds, upstream of the lens, to an upstream cone defined by a cold diaphragm of an equivalent cold screen, placed upstream of the lens array and downstream of the imaging system. The aperture number of the cone defined by the cross-section of the main pixel and the center of the lens corresponds to an upstream cone aperture number of 2. The cold diaphragm of the equivalent cold screen is separated, along the direction of the optical axis, from the lens array by a distance of 20 mm. The gap ∆ is calculated based on this distance of 20 mm. More generally, when one wishes to replace a cold screen defining an aperture (or an exit pupil) of diameter Φ and positioned at a distance L1 from the image plane of the imaging system 3, one can dimension the imaging device and in particular thelenses from the following elements. The parameter t represents a length of the main photodetector. L1 denotes the distance between the exit pupil and the lens. L2 is the distance between the lens and the photodetector array. The angle of incidence i of a ray passing through the edge of the exit pupil and the center of the lens is given by the following relation: ^ = atan The angle of refraction r in the lens with refractive index n is given by the relation: ^ = atan The refractive index angle i is related to the angle of refraction r by Snell's law: sin(θ) = θ sin(θ) ≈ θ ( ... ^ ^ ^ ^ = ^ ^ ^ ^ ^ ^In a nona-pixel configuration, φµL = 3t, where φµL is the diameter of the lens. The working aperture number of the 3WF# imaging system is defined by: The aperture number of the lens F#µL is defined by: Furthermore, the radius of curvature of the lens can be calculated as a function of the distance and the refractive index n of the material: The gap Δ is defined so that the distance L1 corresponds to the distance between a cold diaphragm of an equivalent cold screen and the image plane of the imaging system: ∆ = ^2 ^ ^2 ≈ ^ 1 + ^^1 ^^1 ^ ^ In the case of microlenses, the size of the microlenses (such as the diamètre can be sufficiently weak and close to the wavelength λ, so that diffractive effects appear and predominate over the refractive properties from which the preceding equations are derived. To determine the principal mode of operation, we can define the generalized Fresnel number by the following relation: Since the distance between the microlens and the detector is immersed in a medium of refractive index n, the microlens diffracts as if the wavelength were equal to λ / n and the generalized Fresnel number adapts as follows: Depending on the FN value, two regimes can be distinguished: If FN is significantly greater than 1 (typically FN > 10), the laws of refraction apply, and ray tracing allows us to describe the operation of the optical component. If FN is less than 1 or close to 1, diffraction phenomena must be taken into account to describe the component's operation. Optimal system operation is generally obtained for FN >> 1. To achieve this, a small lens aperture and large pixels relative to the wavelength are required. Regarding this last point, the trend in both the infrared and visible ranges is towards a reduction in pixel pitch, which approaches the operating wavelength. Using the infrared imaging device as just described, it is possible to implement an infrared imaging process P. Referring to Figure 5, process P comprises the following steps. In a first step S1, an image of object 2 is constructed (or formed) by the imaging system 3 on the lens array 7 using the light from object 2. The imaging system 3, positioned between object 2 and the lens array 7, allows, for example, this first step S1 to be carried out.In a second step S2, optical conjugation is performed between the plane of the exit pupil of the imaging system and the plane of the photodetector array by the lens array. Each lens is associated with its primary photodetector, and each lens is configured to image the exit pupil over an area centered on the primary photodetector. The conjugation step is performed either in a quad-pixel or nona-pixel configuration. In a third step S3, the photodetector array 11 and the lens array 7 are cooled, with the photodetector array 11 located downstream of the lens array 7 with respect to the direction of light propagation. In a fourth step S4, the light received by the photodetector array 11 is measured, recovering only the signal from the primary photodetectors. This produces an image of object 2.

Claims

CLAIMS 1. An infrared imaging device (1) comprising: - an imaging system (3) configured to form an image of an object (2), - a measurement system (5) located downstream of the imaging system with respect to a direction of light propagation, the measurement system comprising: - a lens array (7) located in an image plane (9) of the imaging system, and - a photodetector array (11) located downstream of the lens array, - a cryostat (13) configured to cool the photodetector array and the lens array, each lens (7a, 7b) being associated with a primary photodetector (11a, 11b) and each lens being configured to image an exit pupil (14) of the imaging system in a plane of the photodetector array over an area centered on the primary photodetector,the device being configured either in a first configuration in which the distance separating two adjacent principal photodetector centers is equal to twice the dimension (t) of a principal photodetector, the area being less than or equal to the surface area of ​​four principal photodetectors, or in a second configuration in which the distance separating two adjacent principal photodetector centers is equal to three times the dimension of a principal photodetector, the area being less than or equal to the surface area of ​​nine principal photodetectors.

2. A device according to claim 1, wherein each lens is configured to image the exit pupil of the imaging system in a plane of the photodetector array over a surface area less than or equal to the surface area of ​​the primary photodetector associated with the lens.

3. A device according to any one of claims 1 and 2, wherein each lens is aligned with the associated primary photodetector in a direction parallel to the optical axis.

4. A device according to any one of claims 1 to 3, wherein, for each lens, a lens center, a center of the exit pupil, and a center of the primary photodetector associated with the lens are aligned on the same straight line.

5. A device according to any one of claims 1 to 4, wherein the photodetectors are pixels, and preferably the pixels corresponding to the primary photodetectors use different charge capacities than the other pixels. 6.A device according to any one of claims 1 to 5, wherein each main photodetector is surrounded by peripheral photodetectors (16, 18), the device comprising a mask configured to block light arriving at the peripheral photodetectors. A device according to any one of claims 1 to 4, wherein the main photodetectors are photodiodes, each photodiode being surrounded in the plane of the photodetector array by a peripheral area configured to block, evacuate, or absorb a flux. incident light, the peripheral area corresponding to an area of ​​three main photodetectors or an area of ​​eight main photodetectors.

8. Device according to any one of claims 1 to 7, comprising a hood (45) surrounding the measuring system and a screen (17) located between the photodetector array and the hood, the screen being cooled by the cryostat, the screen being configured to block a ray passing through a main photodetector associated with a first lens and a second lens adjacent to the first lens.

9. Device according to any one of claims 1 to 8, in which a volume separating the lens array and the photodetector array is occupied by a substrate (15) having an optical index greater than 1, preferably greater than or equal to 1.5 and even more preferably greater than or equal to 2.10.Infrared imaging method of an object comprising: - a construction step (S1) of an image of the object by an imaging system on a lens array using light from the object, - an optical conjugation step (S2) of a plane of an output pupil of the imaging system and a plane of a photodetector array by the lens array, the photodetector array being located downstream of the lens array with respect to a direction of propagation of the light, each lens being associated with a main photodetector and each lens being configured to image the output pupil on a. area centered on the main photodetector, the conjugation step being carried out either in a first configuration in which a distance separating two centers of adjacent main photodetectors is equal to twice the dimension of a main photodetector, the area being less than or equal to a surface of four main photodetectors, or in a second configuration in which the distance separating two centers of adjacent main photodetectors is equal to three times the dimension of a main photodetector, the area being less than or equal to a surface of nine main photodetectors,- a cooling step (S3) of the photodetector matrix and the lens matrix,- a light measurement step (S4) of the main photodetectors.

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