Infrared imaging device

Abstract

Infrared imaging device (1) comprising: - an imaging system (3), - a measurement system (5) comprising: -- a lens array (7) located in an image plane (9) of the imaging system, and -- a photodetector array (11), -- a cryostat (13) configured to cool the photodetector array and the lens array, 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 over an area centered on the main photodetector, the device being configured 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, 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.Figure to be published for the summary: Figure 1.

Description

Title of the invention: Infrared imaging device DOMAIN

[0001] The invention relates to the field of infrared imaging, and in particular to cryostat-cooled infrared imaging devices. STATE OF THE ART

[0002] In the field of cooled infrared imaging, imaging devices are known that comprise a detection unit including a detector and a cryostat configured to cool the detector. A cold screen is integrated into the cryostat, 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 diaphragm defines a numerical aperture of the detection system, limiting the viewing angle of each photodetector in the detector. The cold screen must be cooled to cryogenic temperatures and be sufficiently light to withstand the vibrations that the device may experience and to cool down rapidly. During operation, it is necessary to cool the cold screen before acquiring images. A cooling time is essential.This descent time is not always compatible with the mission of interest.

[0003] There is therefore a need for a cooled infrared imaging device with a shorter cooling time than in the art

[0004] prior. EXPOSED

[0005] One aim of the present presentation is to propose a faster cooled infrared imaging device than in the prior art.

[0006] The goal is achieved by means of an infrared imaging device comprising:

[0007] - an imaging system configured to form an image of an object, - a measurement 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

[0008] — a photodetector array located downstream of the lens array, — a cryostat configured to cool the photodetector array and the lens array,

[0009] 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 on an area centered on the main photodetector, the device being configured

[0010] either in a first configuration in which the distance separating two adjacent principal photodetector centers is equal to twice the dimension of a principal photodetector, the area being less than or equal to the surface area of ​​four principal photodetectors,

[0011] 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 an area of ​​nine main photodetectors.

[0012] Such a device is advantageously and optionally complemented by the following various 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 a surface area less than or equal to a surface area of ​​the main photodetector associated with the lens; - each lens is aligned with the associated main photodetector in a direction parallel to the optical axis; - for each lens, a lens center, an exit pupil center and a center of the main photodetector associated with the lens are aligned on the same straight line; - photodetectors are pixels, and preferably the pixels corresponding to the main photodetectors use different charge capacities than the other pixels; - each photodetector is surrounded by peripheral photodetectors, the device including a mask configured to block the light arriving on the peripheral photodetectors; - the main photodetectors are photodiodes, each photodiode being surrounded in the plane of the photodetector matrix by a peripheral zone configured to block, evacuate or absorb an incident light flux, the peripheral zone corresponding to an area of ​​three main photodetectors or an area of ​​eight main 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; and - a volume separating the lens matrix and the photodetector matrix 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.

[0013] The presentation also relates to a method for infrared imaging of an object comprising:

[0014] - a step in constructing an image of the object by an imaging system on a lens array using light from the object,

[0015] - an optical conjugation step of a plane of an exit pupil of the system imaging 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 the primary photodetector, the conjugation step being performed

[0016] either in a first configuration in which the distance separating two adjacent principal photodetector centers is equal to twice the dimension of a principal photodetector, the area being less than or equal to the surface area of ​​four principal photodetectors,

[0017] 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,

[0018] - a step of cooling the photodetector array and the matrix of lenses,

[0019] - a light measurement step of the main photodetectors. DESCRIPTION OF THE FIGURES

[0020] 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:

[0021] [Fig.1] [Fig.1] is a schematic representation of an example of an infrared imaging device;

[0022] [Fig.2] [Fig.2] is a schematic representation of a detail of an example of an infrared imaging device;

[0023] [Fig.3]

[0024] [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; and

[0025] [Fig.5] [Fig.5] is a schematic representation of an example of an infrared imaging method. DETAILED DESCRIPTION OF THE INVENTION

[0026] With reference to [Fig. 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 radiation, and in particular the infrared radiation emitted by the object 2. The imaging system forms an image of the object 2 in an image plane 9. The imaging system is centered on the optical axis X, an optical axis that can be oriented in the direction of light propagation, 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., 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 materialized by a diaphragm.

[0027] 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 located downstream of the aperture diaphragm, or, where applicable, as the aperture diaphragm itself when it is a single diaphragm located downstream of all the optics. The diaphragm is the mechanical element 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, which is designated by WF#. The imaging system may, in particular, be a photographic lens.

[0028] The infrared imaging device 1 also includes a measurement 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.

[0029] The measuring system 5 is centered on the optical axis X.

[0030] The measuring system 5 comprises a lens matrix 7 located in the plane Figure 9. The lens array comprises a plurality of lenses 7a, 7b, 7c arranged in rows and columns, the rows and columns being 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.

[0031] The lenses may in particular be microlenses, that is to say lenses having a diameter greater than or equal to 1 Opm and less than or equal to 90 pm.

[0032] Since the lens matrix 7 is located in the image plane 9, each lens receives light from a specific portion of the object 2.

[0033] Each lens has an aperture number which is designated by F#pL. All lenses have the same aperture number.

[0034] The measurement system 5 includes a photodetector matrix 11 located downstream of the lens matrix 7, i.e. the lens matrix 7 is located between the imaging system 3 and the photodetector matrix 11.

[0035] The photodetector matrix 11 comprises a plurality of photodetectors lia, 11b, 1 le distributed in rows and columns, the rows and columns being orthogonal to each other and orthogonal to the optical axis X. The photodetector matrix 11 is centered on the optical axis X.

[0036] The infrared imaging device 1 also includes a cryostat 13 configured to cool the photodetector array 11.

[0037] Each lens is associated with a main photodetector configured to receive light rays passing through a center of the lens and a center of the exit pupil 14.

[0038] Fig. 2 illustrates the general case of such a light ray arriving on a lens at a non-zero incidence.

[0039] In relation to [Fig.2], a light ray 43 is shown which passes through the center of the exit pupil 14 and through the center of a lens 7c included in the lens matrix 7. The lens 7c is not centered on the optical axis X, the lens 7c is separated from the optical axis X by a length referenced H1 in [Fig.2].

[0040] The light ray 43 defines, between the exit pupil 14 and the lens matrix 7, an angle of incidence i with the direction of the optical axis X. The length L1 separates the exit pupil 14 and the lens matrix 7.

[0041] 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 ni 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 nl*sin(i)=n*sin(r).

[0042] The light beam impacts the main photodetector 1 associated with the 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.

[0043] The main photodetector 1 is not centered on the optical axis X, it is separated from the optical axis X by a length referenced H2 in [Fig.2].

[0044] The lens 7c is associated with the main photodetector 1, which is configured to receive the light beam 43 passing through the center of the lens 7c and the center of the exit pupil 14. There is a gap A between the length H2 which separates the main photodetector 1 from the optical axis X and the length H1 which separates the lens 7c from the optical axis X. H2 is greater than Hl.

[0045] This deviation A can be determined according to the parameters of the measuring system 5: A = (H2-H1) =(Hl*L2) / (n*Ll) = H2 / (l+nLl / L2)

[0046] In other words, each lens is offset with its main photodetector by a gap A measured orthogonally to the optical axis X, the gap A being equal to the ratio between a product of the distance L2 between the top of the lens and the detector by the distance Hl from the center of the lens to the optical axis, and a product of an optical index of the medium separating the lens and its main photodetector by a distance L1 separating the lens matrix from the exit pupil, the lens being closer to the optical axis than its main photodetector.

[0047] This amounts to choosing a spatial period in the lens matrix 7 which is not a multiple of the spatial period in the photodetector matrix 11. These two periods are not in an integer ratio.

[0048] The technical effect associated with this particular choice of periods is to ensure for each lens that a cone of light coming from the exit pupil of the imaging system and passing through the lens is directed in a centered manner towards the main photodetector associated with the lens.

[0049] It should be noted that in more specific cases, the average angle of the cones of the light rays emanating from the exit pupil of the imaging system and arriving at the lenses may be zero or sufficiently small to be negligible. In these cases, each lens is aligned with its primary photodetector along a direction parallel to the optical axis. This so-called telecentric embodiment corresponds to the case where the light cones from the imaging system 3 arrive at the measurement system 5 with 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 that would be placed between the imaging system 3 and the lens array 7.

[0050] 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 primary 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 primary photodetector.

[0051] The device can be configured according to two configurations.

[0052] In a 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 They are considered adjacent if they are directly neighbors; that is, there cannot be a third primary photodetector located between two primary photodetectors. This is equivalent to surrounding each primary photodetector with a buffer zone centered on the primary photodetector, the buffer zone extending to a dimension equal to twice the dimension of a primary photodetector. The surface of the primary photodetector is located at the center of the buffer zone.

[0053] In the plane of the photodetector matrix, an area allocated to each lens can be defined as the combination of the surface of its main photodetector and the buffer area surrounding it.

[0054] Such a gap between two primary photodetectors implies geometric conditions in the lens array. In particular, when the lenses are placed in contact with each other, the diameter of a lens is close to twice the dimension of a primary photodetector. The diameter may, in particular, be less than or equal to twice the dimension of a primary photodetector.

[0055] If two adjacent primary photodetector centers are separated by twice the dimension 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 have the same size as a primary photodetector.

[0056] The eight peripheral photodetectors comprise four primary peripheral photodetectors that share a common side with the primary photodetector. Each center of one of the four primary peripheral photodetectors is obtained by translating the center of the primary photodetector by a distance corresponding to one dimension of the primary photodetector, the translation being carried out in one of the two directions of the two photoreceptor arrays. 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 four primary photodetectors.

[0057] 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 of the center of the primary photodetector: a basic row translation and a basic column translation. There is a secondary peripheral photodetector located 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.

[0058] In this case, the buffer zone associated with a primary photodetector corresponds to a fraction of the surface area of ​​these eight peripheral photodetectors. More precisely, the buffer zone corresponds to half the surface area of ​​the first four peripheral photodetectors and a quarter of the surface area of ​​the second four peripheral photodetectors. In total, the buffer zone corresponds to the surface area of ​​three photodetectors.

[0059] 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 similar to the term "quad-pixel." This area is contained within the surface defined by the primary photodetector and the buffer zone described previously. This condition is met, in particular, when the diameter of the image of the exit pupil is less than or equal to twice the size of a pixel.

[0060] In a second configuration, called the "nona pixel" configuration, the distance separating two adjacent principal photodetector centers is equal to three times the dimension of a principal photodetector. 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 three times the dimension of a principal photodetector. The surface of the principal photodetector is located at the center of the buffer zone.

[0061] In the plane of the photodetector matrix, an area allocated to each lens can be defined as the combination of the surface of its main photodetector and the buffer area surrounding it.

[0062] Such a difference between two primary photodetectors implies geometric conditions in the lens array. In particular, when the lenses are placed in contact with each other, the diameter of a lens is close to three times the dimension of a primary photodetector. The diameter may, in particular, be less than or equal to three times the dimension of a primary photodetector.

[0063] If two adjacent primary photodetector centers are separated by three times the dimension of a primary photodetector, two non-primary photodetectors 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.

[0064] As before, the eight peripheral photodetectors comprise four first peripheral photodetectors and four second peripheral photodetectors peripherals. Each 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 main photodetector, a basic row translation and a basic column translation.

[0065] In this case, the buffer zone associated with a main photodetector corresponds to the entire surface area of ​​these eight peripheral photodetectors.

[0066] 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 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 area of ​​a primary photodetector. This ratio of nine times is similar to the term "nona-pixel." This area is contained within the surface defined by the primary photodetector and the buffer zone described previously. This condition is met, in particular, when the diameter of the image of the exit pupil is less than or equal to three times the size of a pixel.

[0067] Figure 1 illustrates the second configuration, known as the "nona-pixel" configuration, in which the exit pupil 14 is imaged over an area corresponding to the maximum surface area of ​​nine primary photodetectors. The image is centered on the primary photodetector 1a and covers eight peripheral photodetectors, including the first peripheral photodetectors 16 and 18, which surround the primary photodetector 1a. The peripheral photodetectors 16 and 18 are part of eight primary photodetectors surrounding the primary photodetector, with the image of the exit pupil 14 extending over each of the eight peripheral photodetectors.

[0068] Advantageously, when each main photodetector is surrounded by peripheral photodetectors, the device includes a mask configured to block the light arriving on the peripheral photodetectors.

[0069] The mask can be placed at the level of the photodetector matrix 11.

[0070] The mask can be configured to block a light ray whose angle the incidence would not be included in the cone of light defined by the surface of the main photodetector and the center of the associated lens.

[0071] 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 a blooming effect or by limiting the instrumental background obtained by scattering of carriers from a peripheral photodetector to the main photodetector.

[0072] By configuring the lens array 7 so that each lens forms an image of the output pupil of the imaging system 3 centered on its photodetector principal and by imposing that the area allocated to each lens 7 extends over an area corresponding to a surface of four principal photodetectors or a surface of nine principal photodetectors, it is possible to retain, to produce an image, only the light received by the principal photodetector of each lens.

[0073] 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.

[0074] 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 allocated to the lens and the center of the lens.

[0075] Unlike the prior art, it is no longer necessary to use a cold screen to define a numerical aperture of the detection system and to block rays that would pass outside the cone of incidence defined by the exit pupil. This cold screen is commonly used in the prior art to limit the viewing angle of each photodetector of the detector, in particular to prevent the photodetectors from seeing the hood of the detection system.

[0076] By retaining only the light received by the main photodetectors to form an image, the aperture can be limited, in particular so that the main photodetectors do not see the cover of the detection system. The detection system produces images without the need for a cold screen. Cooling the screen before image production is no longer necessary, and the cooling of the cooled infrared imaging device is faster than in the prior art.

[0077] In addition, such a device offers the following advantages:

[0078] - it is not necessary to deposit thin absorbent materials between the image plane 9 and the photodetector matrix 11;

[0079] - there is no shadowing effect between the image plane 9 and the photodetector matrix 11 because the lenses 7 collect the light flux arriving on their surface and distribute it between the different photodetectors 11; and

[0080] - when the lenses are microlenses, their optical arrow is limited and the The number of manufacturing steps for microlenses is limited.

[0081] An optical arrow 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 of the surface.

[0082] It may be advantageous to choose the lens aperture number F#pL less than or equal to the working aperture number of the imaging system WF#. In this way, no light from the imaging system is transmitted through a lens which arrives on the photodetector matrix outside the area mentioned previously, this area being centered on the main photodetector associated with the lens.

[0083] 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 main photodetector or in the second "nona pixel" configuration strictly less than nine times the area of ​​a main photodetector.

[0084] 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 a primary photodetector.

[0085] For example, in the first "nona pixel" configuration, the area occupied by the image of the exit pupil 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.

[0086] Even more advantageously, the area occupied by the image of the exit pupil by a lens is less than or equal to the surface area of ​​a main photodetector.

[0087] Each lens produces an image of the exit pupil in the plane of the photodetector array 11, this image being centered on the main photodetector associated with the lens and entirely contained within this main photodetector.

[0088] According to a first alternative concerning the photodetectors, these are pixels of a 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.

[0089] Preferably the pixels corresponding to the main photodetectors use different charging capacities from the other pixels.

[0090] 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 described as the depth of the potential well defined by the pixel.

[0091] In this first alternative, the number of pixels in the photodetector matrix 11 is greater than or equal to the number of lenses in the lens matrix 7. More precisely, the number of pixels is greater than or equal to four times or nine times the number of lenses.

[0092] It is then possible to associate with each lens a surface of four pixels or a surface of nine pixels in the plane of the photodetector array 11. Each of the pixels 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.

[0093] Each pixel associated with a lens can receive light rays whose angles of incidence lie 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 [Fig. 1], and in the case of lens 7a, the main photodetector lia corresponding to a pixel associated with lens 7a is located at the center of the other pixels 16 and 18 also associated with lens 7a. The other pixels 16 and 18 are, as mentioned above, peripheral pixels to the main photodetector or main pixel lia.

[0094] The main pixel lia receives for example the cone of light 21 from the lens 7a, this cone corresponding upstream of the lens to a cone 19 of light and covering a central part of the exit pupil of the imaging system 3.

[0095] Pixels 16 and 18 receive, for example, respectively the light cones 25 and 29 from lens 7a, these cones corresponding upstream of the lens respectively to the light cones 27 and 23. In this example, these cones cover peripheral parts of the exit pupil of the imaging system 3.

[0096] As mentioned previously, only the light received by the principal pixel 1la is retained to produce an image, and more generally for each lens only the light received by its principal pixel.

[0097] This pixel selection corresponds to retaining only the rays whose angle of incidence is included in the central cone defined by the section of the principal pixel and the center of the lens and ultimately to defining the numerical aperture of the measurement system 5.

[0098] In this first alternative, the instrumental background is recovered by the peripheral pixels of the main pixel, these peripheral pixels are said to be blind because the received signal is not taken into account in the formation of the image.

[0099] In particular, one can choose primary pixels that have a lower charge capacity than the peripheral pixels. In this way, one limits the risk that the peripheral pixels contain more charge than they can store, which could lead to an alteration of the charge of the primary pixel, for example in a blooming (glare) effect. This allows to improve the quality of the images produced, particularly for long exposure times.

[0100] In this first alternative, it may be advantageous to provide a specific reading circuit for the photodetector matrix, this reading circuit being configured to manage the charge reading of pixels with different charge capacities.

[0101] 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, which 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.

[0102] In this second alternative, the instrumental background is managed by the passivation zones around the central photodiodes.

[0103] In this second alternative, it may be advantageous to provide a specific detection circuit and also a specific reading circuit for the photodiode matrix, this detection circuit and this reading circuit being configured to recover only the signal from the main photodetectors.

[0104] Figures 3 and 4 illustrate two implementations of the infrared imaging device.

[0105] In [Fig. 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 of nine 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 [Fig. 3] corresponds to the surface 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 some squares correspond to a photodiode, one square out of nine in this case.

[0106] 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 measuring 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 a zero or sufficiently weak to be neglected. In these cases, each lens is aligned with its main photodetector along a direction parallel to the optical axis.

[0107] The main photodetector 1la 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 by circle 35. Since the lens is aligned with its main photodetector along the optical axis, the projection 35 is centered on the main photodetector 1la. This set of nine squares is designated 'nona-pixel', as mentioned previously. In [Fig. 3], the set of nine squares or 'nona-pixel' 31 corresponds to lens 7b and is centered on the main photodetector 11b associated with lens 7b.

[0108] In [Fig. 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 of four primary photodetectors, with the image of the imaging system's pupil 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 [Fig. 3] corresponds to the surface 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 some squares correspond to a photodiode, one square out of four in this case.

[0109] 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 at 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 is 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.

[0110] The main photodetector 1 la associated with the lens 7a is centered on the optical axis X. A projection along the optical axis of the lens 7a into the plane of the photodetector array is represented by the circle 37. As the lens is aligned with its main photodetector along the optical axis, the projection 37 is centered on the main photodetector lia.

[0111] The nine squares centered on the photodetector include: - the photodetector lia which only receives light 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 - four second squares which receive light from four lenses, lens 7a and three lenses which form the corners of the large square formed by the eight squares which surround square lia.

[0112] When the squares correspond to peripheral photodetectors, the first four squares are the first four peripheral photodetectors mentioned above and the second four squares are the second four peripheral photodetectors.

[0113] The area allocated to a lens corresponds to a surface of four main photodetectors. This area has been shown and referenced 33 for lens 7b and main photodetector 11b.

[0114] As mentioned previously, each lens is associated with a surface of four squares or 'quad-pixel' of which the central photodetector is the main photodetector of the lens.

[0115] According to a first option, and with reference to [Fig.1], the infrared imaging device may include a hood 45 which surrounds the measuring system 5. The hood 45 allows the measuring system to be evacuated for cooling.

[0116] Advantageously, when the infrared imaging device includes a hood 45, the device includes, for each main photodetector, a mask located at the level of the photodetector array 11 configured to cover the buffer zone associated with the main photodetector. The mask is configured to block a light ray whose angle of incidence is not included in the cone of light defined by the surface of the main photodetector and the center of the associated lens. The mask allows, among other things, the blocking of radiation emitted by the hood 45 that may reach the photodetector array 11.

[0117] Hood 45 is not cooled by cryostat 13.

[0118] In relation to the first option, the infrared imaging device may include, in addition to the hood 45, a screen 17 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 primary photodetector associated with a first lens and a second lens adjacent to the first lens. When cooled, the screen 17 blocks grazing rays, for example, those emitted by the hood 45. The hood is at ambient temperature and therefore emits radiation that may unintentionally reach the detector.

[0119] The screen is configured to block a ray passing through a primary photodetector associated with a first lens and a second lens separated from the first lens by one or more lenses of the lens matrix.

[0120] The hood 45 is not cooled by the cryostat 13.

[0121] The screen 17 is a cold screen which allows blocking so-called 'grazing' rays, that is- These are rays with a very high angle of incidence with the optical axis. Examples include rays passing through a primary photodetector combined with a first lens and a second lens adjacent to the first, but also rays passing through a primary photodetector combined 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.

[0122] Thanks to this screen, the main photodetector receives little or no light passing outside the exit pupil and through the lens associated with an adjacent main photodetector. This ensures that the only light received by the main photodetector is light passing through the exit pupil and the lens associated with the main photodetector.

[0123] 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 materialize the exit pupil of the imaging system 3 as may be the case in the prior art. It is smaller and lighter in mass than the cold screens 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.

[0124] It should be noted that the first option and the second option can be combined, as illustrated in [Fig.1].

[0125] The volume separating the lens array 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 highest possible generalized Fresnel number FN' thanks to the material between the microlens and the detector having a high refractive index. Example of a completed project

[0126] In the case of a photodetector array that is a pixel array, in the case of a nona-pixel configuration where each lens has an allocated area in the plane of the photodetector array corresponding to an area of ​​nine pixels, a 1024x780 pixel array can be chosen, each pixel having a size of 1 Opm and operating in the mid-infrared (or MWIR, corresponding to a wavelength range between 3 pm and 5 pm). The lens array comprises microlenses with a focal length of 55 pm, a diameter of 30 pm, a radius of curvature of 35 pm, 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 length of the photodetector array, the gap A between the microlens and its main photodetector is about 5pm (5.1 Ipm precisely taking H2= 5.12mm).

[0127] Note the very low number of apertures in the microlens. This configuration is possible because the distance between the microlens and the detector is embedded in a high-index material. This is an advantage for an infrared configuration compared to the visible range.

[0128] In this embodiment, the cone defined by the cross-section of the principal 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 principal 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 A is calculated based on this distance of 20 mm.

[0129] More generally, when it is desired to replace a cold screen defining a diaphragm (or exit pupil) of diameter <e>and positioned at a distance L1 from the image plane of the imaging system 3, we can dimension the imaging device and in particular the lenses from the following elements.

[0130] The parameter t represents a length of the main photodetector.

[0131] L1 denotes the distance between the exit pupil and the lens.

[0132] L2 is the distance between the lens and the photodetector array.

[0133] 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:

[0134] _ afan^ j (case of small angles)

[0135] The angle of refraction r in the lens of index n is given by the relation:

[0136] r _ atan ^es pe(jts angies}

[0137] The refractive index angle i is related to the angle of refraction r by Snell's law:

[0138] sin(ï) = wsin(r)

[0139] i^nr (case of small angles)

[0140] By combining the different equations, we obtain the following relationships: 101411 101421 ^=4*

[0143] In a nona-pixel = 3t configuration, with the lens diameter. The working aperture number of the 3WF# imaging system is defined by:

[0144]

[0145] The aperture number of lens F#L|I is defined by:

[0146] _d_h_ F if uL ~ n wl

[0147] Furthermore, the radius of curvature of the lens can be calculated as a function of the distance L2 and the refractive index n of the material:

[0148] = ¾

[0149] The gap A is defined so as to have a distance L1 corresponding to the distance between a cold diaphragm of an equivalent cold screen and the image plane of the imaging system:

[0150] *

[0151] In the case of microlenses, the size of the microlenses (such as the diameter ^fiL) can be sufficiently small and close to the wavelength >1, 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, the generalized Fresnel number can be defined by the following relation:

[0152] = 4727

[0153] 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 X / n and the generalized Fresnel number adapts as follows:

[0154] V FN = 4777 = 44¾

[0155] Depending on the value of FN, two regimes can be distinguished:

[0156] If FN is significantly greater than 1 (typically FN > 10), the laws of refraction apply and ray tracing allows the operation of the optical component to be described. If FN is less than 1 or close to 1, diffraction phenomena must be taken into account to describe the operation of the component.

[0157] 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. On this latter point, the trend in both the infrared and visible ranges is towards a reduction in pixel pitch, which approaches the operating wavelength. Infrared imaging method

[0158] Using the infrared imaging device as just described, it is possible to implement an infrared imaging method P. With reference to [Fig. 5], the method P comprises the following steps.

[0159] In a first step SI, an image of the object 2 is constructed (or formed) by the imaging system 3 on the lens matrix 7 using the light coming from the object 2. The imaging system 3 placed between the object 2 and the lens matrix 7 allows, for example, this first step SI to be carried out.

[0160] In a second step S2, an optical conjugation of the exit pupil plane of the imaging system and the photodetector matrix plane is performed by the lens matrix, each lens being associated with its main photodetector and each lens being configured to image the exit pupil on an area centered on the main photodetector, the conjugation step being performed either according to the quad-pixel configuration or according to the nona-pixel configuration.

[0161] In a third step S3, the photodetector matrix 11 and the lens matrix 7 are cooled, the photodetector matrix 11 being located downstream of the lens matrix 7 with respect to the direction of propagation of the light.

[0162] In a fourth step S4, the light received by the photodetector array 11 is measured by recovering only the signal from the main photodetectors. This makes it possible to produce an image of the object 2.< / e>

Claims

Demands

1. 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 (1a, 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, Faire being less than or equal to the surface area of ​​nine principal photodetectors.

2. 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 a surface of the main photodetector associated with the lens.

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

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

5. Device according to any one of claims 1 to 4, wherein the photodetectors are pixels, and preferably the pixels corresponding to the main photodetectors use different charge capacities from the other pixels.

6. 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 on the peripheral photodetectors.

7. 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 an incident luminous flux, 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 primary 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, wherein a volume separating the lens matrix and the photodetector matrix 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. A method for infrared imaging an object comprising: - a step of constructing (S1) an image of the object by an imaging system onto a lens array using light from the object, - a step of optical conjugation (S2) of a plane of an exit pupil of the imaging system and a plane of a photodetector array by the lens array, the array of photodetectors 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 the primary photodetector, the conjugation step being carried out either in a first configuration in which a distance separating two centers of adjacent primary photodetectors is equal to twice the dimension of a primary photodetector, the area being less than or equal to an area of ​​four primary photodetectors, either 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, Faire being less than or equal to an area of ​​nine principal photodetectors, - a cooling stage (S3) of the photodetector array and the lens array, - a light measurement step (S4) of the main photodetectors.

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