Optical imaging system having multiple parallel optical paths and a common entrance aperture - Patents.com

Abstract

The present invention relates to an athermalized optical imaging system, which includes multiple optical paths (21, 22) arranged in parallel. Moreover, it is suitable for preventing image overlap occurring within the active area of ​​a matrix-like image element (1) common to the optical paths. Also, a thermal compensation device (3) for athermalization is used to support a baffle (4) which eliminates image overlap within the active area. As a result, the system can be very compact and can be used to search for specific gases in a field.

Description

[Technical field]

[0001] The present invention relates to an optical imaging system having multiple parallel arranged optical paths and a common input aperture for all the optical paths. The present invention also relates to a method for acquiring multiple images, where the input optical field is the same for all images. [Background technology]

[0002] In many fields, it is necessary to detect the presence of certain invisible gases in the field. For example, such gas detection is useful in industry when the gas is toxic or polluting and / or to detect the presence of possible gas leaks in facilities that transmit or use this gas. In particular, it is often useful to detect the presence of hydrocarbon gases in industrial facilities. Gas detection in the field is also useful in military applications, especially to detect the presence of possible combat gases in intervention zones.

[0003] In general, the gases of interest have characteristic absorption bands in the wavelength range of 3 μm (micrometers) to 5 μm, usually called the mid-wave infrared (MWIR) range, or in the wavelength range of 8 μm to 14 μm, usually called the long-wave infrared (LWIR) range. To optically detect the gases, optical imaging systems incorporating germanium (Ge)-based lenses are often used, although other materials transparent in the desired wavelength range can alternatively be used in such systems. However, these materials, including germanium, undergo significant changes in their optical properties depending on the thermal history of the environment. Therefore, it is known to use thermal compensation devices, also called athermalization devices, to maintain the clarity of the acquired images regardless of the operating temperature of the system within a given temperature range.

[0004] It is further known to equip optical imaging systems with baffles, the usual function of such baffles being to filter out stray light that would otherwise enter the optical imaging system at highly oblique angles of incidence relative to the optical axis of the system without contributing to image formation.

[0005] Furthermore, selective detection of gases requires that the optical imaging system used implements multiple optical paths in parallel, each optical path working in a different spectroscopic window from one optical path to another, so that the gas or gas family present in the scene can be identified based on the image representation of the gases in the scene for all spectroscopic windows. However, a challenge is to have a multiple optical path optical imaging system that is small in overall size, lightweight, easy to manufacture, and capable of operating over a sufficiently wide operating temperature range, especially with regard to the alignment of the optical components.

[0006] An architecture that offers many advantages for such optical imaging systems is described in [1]. Such a TOMBO system includes: - a matrix image sensor; and - an imaging matrix comprising a plurality of imaging optics arranged in parallel to the effective areas of a matrix-like imager dedicated one-to-one to said imaging optics and adapted to simultaneously form respective images of a scene included in an input optical field common to all imaging optics, the effective areas of the matrix-like imager being separated from one another, and each imaging optic and its corresponding effective area belonging to one of the optical paths of the system separately from the other optical paths.

[0007] It is known to provide partitions between the optical paths in order to prevent an image to be acquired in one of the active areas from spilling over beyond the outer limit of this active area into an adjacent active area. In US Pat. No. 5,399,433 or US Pat. No. 5,399,433, such a TOMBO system is described in which partitions are provided between adjacent optical paths for gas detection applications. Such partitions, which are opaque and generally called low walls, are arranged vertically between adjacent optical paths in the imaging matrix and extend up to the matrix-like imager. However, they have the following disadvantages: If these partition walls extend over the entire imaging matrix, the lenses of the imaging matrix cannot be manufactured as an integral lens matrix, which would require an alignment for each optical path, which is tedious and complicated and would significantly increase the costs of the system. - such partition walls are most effective in preventing image overlap between adjacent optical paths if they extend close or very close to the surface of the matrix-like imager. However, in the MWIR and LWIR wavelength ranges, the matrix-like imager is housed in a vacuum sealed container, which necessarily has a window between the imaging matrix and the matrix-like imager. For this reason, the partition walls end at the window and are therefore located far away from the matrix-like imager, which makes them less effective in preventing image overlap between adjacent optical paths.

[0008] It is also known to provide clearance bands between adjacent active areas in the matrix imager to avoid such overlaps of images within each active area, for example due to each image spilling over the outer limits of its dedicated active area. These clearance bands are dimensioned such that at any value of the system temperature within the specified temperature range, the spillover of each of any image in one of the clearance bands into the adjacent active area is shorter than the width of this clearance band. In general, the active area is determined by limiting the readout selection of the matrix imager during each image acquisition sequence to the light receiving elements belonging to one of the active areas. However, such clearance bands lead to a low utilization of the imager.

[0009] It is still possible to space the optical paths laterally from one another to limit or eliminate overlap between images formed by adjacent optical paths, however this also results in reduced utilization of the image sensor.

[0010] Finally, increasing the distance between the imaging matrix and the matrix-like imager in order to reduce the overlap between the images formed by adjacent optical paths would prevent the system from being compact, but there is a strong demand for such optical imaging systems to be compact, for example so that they can be easily carried by an operator or an autonomous vehicle, in particular a drone, or mounted on a steerable turret. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Pat. No. 10,375,327 [Patent Document 2] US Patent Application Publication No. 2018 / 0191967 [Non-patent literature]

[0012] [Non-Patent Document 1] "Thin observation module by bound optics (TOMBO): concept and experimental verification" by J. Tanida et al. (Appl. Opt. vol. 40, pp. 1806-1813 (2001)) Summary of the Invention [Problem to be solved by the invention]

[0013] In view of the above circumstances, one object of the present invention is to provide an athermalize TOMBO system in which image overlap is eliminated in the entire effective area of ​​a matrix-type image sensor.

[0014] A related object of the invention is to provide a system that is small in overall size, lightweight, easy to manufacture, particularly with regard to the alignment of the optical components, and has a wide field of view with a large aperture, which in fact allows the detection of leaks in the scene that extend laterally, and the large aperture allows the signal-to-noise ratio of the acquired image to be increased. [Means for solving the problem]

[0015] In order to achieve at least one of these objects or another, a first aspect of the invention provides a novel optical imaging system having multiple parallel-arranged optical paths of the aforementioned TOMBO system type, which therefore comprises a matrix-like imager and an imaging matrix according to the aforementioned arrangement. The optical imaging system of the invention further comprises: - a thermal compensation device arranged to generate a variable spacing between an imaging matrix and a matrix-like imaging element, the thermal compensation device comprising, in at least one longitudinal section perpendicular to the imaging matrix and to the matrix-like imaging element, a serpentine section including two segments at least partially made of a material with a thermal expansion coefficient value different from that of the other segment, the two segments being connected to each other at their respective distal ends, a proximal end of one of the segments being connected to the imaging matrix and a proximal end of the other segment being connected to the matrix-like imaging element, the difference in the thermal expansion coefficient values ​​and the respective lengths of the two segments being adapted to generate a variable spacing between the imaging matrix and the matrix-like imaging element such that an image formed in the active area of ​​the matrix-like imaging element is sharp when the temperature of the system has any value within a defined range; and a baffle arranged upstream of the imaging matrix with respect to the direction of propagation of radiation entering the system to form an image, the baffle having an opening for radiation common to the optical path of the system.

[0016] According to a first additional feature of the invention, the baffle openings are dimensioned to laterally limit the illumination in each image formed by any optical path, referred to as the optical path under consideration, and to prevent this image from spilling over into the active area of ​​another optical path adjacent to the optical path under consideration, thereby preventing overlap of images in all active areas for any value of the system temperature within the specified temperature range.

[0017] According to a second additional feature of the invention, a distal end of the segment of the thermal compensator is located upstream of the imaging matrix with respect to a propagation direction of radiation entering the system for image formation. Furthermore, a baffle is supported by the thermal compensator, again upstream of the imaging matrix with respect to a propagation direction of radiation entering the system for image formation. Preferably, the baffle may be supported by the thermal compensator at a distal end of the segment of the thermal compensator.

[0018] The baffle is therefore supported by the thermal compensation device. In other words, the invention provides a combination of two functions of the thermal compensation device. The first function consists in adapting the distance between the imaging matrix and the matrix-like imager according to the change in the optical properties of the imaging matrix, essentially the focal length, as a function of temperature. This ensures that the image acquired by the matrix-like imager maintains its sharpness over the entire defined range, which may be large. The second function of the thermal compensation device is to support the baffle. This combination of functions is particularly useful for realizing a compact system. In particular, the additional function of supporting the baffle introduced by the invention does not necessarily require the length of the serpentine part of the thermal compensation device compared to the length required for the athermalization function. According to the invention, the material of each of the two segments of the thermal compensation device is selected to support the baffle in a position that produces the athermalization function and eliminates the overlap of images in the active area of ​​the matrix-like imager.

[0019] According to the invention, the use of partitions between the optical paths is no longer essential: the imaging matrix can consist of an integrated lens matrix, which simplifies the optical alignment of the whole system.

[0020] Furthermore, the absence of a partition allows a window to be inserted between the imaging matrix and the matrix-shaped imaging element, and in particular a window of a vacuum-sealed container can be inserted if such a container is required for the operation of the imaging element.

[0021] Generally speaking, when the active areas aligned between two opposite edges of the matrix-like imager are counted sequentially from one of these edges, the opening of the baffle and the distance between this baffle and the imaging matrix can be dimensioned such that the spillover of the image corresponding to the second active area in a clearance band midway between the first and second active areas is limited by the baffle and stops at the boundary between the clearance band and the first active area. Alternatively, the opening of the baffle and the distance between this baffle and the imaging matrix can be dimensioned such that the spillover of the image corresponding to the first active area in a clearance band midway between the first and second active areas is limited by the baffle and stops at the boundary between the clearance band and the second active area. This is the condition of non-overlapping between the images of adjacent optical paths expressed with respect to the laterally outermost clearance band.

[0022] In preferred embodiments of the invention, at least one of the following additional features may optionally be present, either alone or in multiple combinations: The separation distance between the baffle and the imaging matrix, measured parallel to the common direction of each optical axis of the imaging optics, may be between 1.0 and 6.0 times the average focal length of the imaging optics. The optical imaging system may further comprise a container arranged to accommodate the matrix imaging element, the container comprising a window transparent to the imaging radiation, the window being located between the imaging matrix and the matrix imaging element. The imaging optics may have a row and column arrangement in an imaging matrix, with each row perpendicular to each column, and the imaging matrix may be oriented such that each row of the imaging optics is parallel to the row or column direction of the light receiving elements of the matrix image sensor. The row and column arrangement of the imaging optics in the imaging matrix can be in one of the following dimensions: 1×2, 2×1, 2×2, 2×3, 3×2, 3×3, 2×4, 4×2, 3×4, 4×3 and 4×4. However, it may be preferable to limit the number of optical paths, so as not to require too wide clearance bands when they are largely offset relative to the central optical axis of the imaging matrix. If the row and column arrangement of the imaging matrix includes at least three imaging optics aligned along a row or column direction, then any two clearance bands that are parallel to each other may have the same width. Each imaging optical system may include at least one germanium-based lens. The imaging matrix may include an integral lens matrix. Each imaging optical system may include a spectral filter, the spectral filters of two imaging optical systems that differ from one another in the imaging matrix may determine different spectral transmission windows. - the spectral filter of each imaging optical system may be arranged between the two lenses of this imaging optical system, in particular to avoid false images which may result from multiple reflections between this spectral filter and the matrix-like imaging element and to avoid an increase in the distance between the imaging matrix and the matrix-like imaging element. - The numerical aperture (commonly expressed as F#) of each optical path of the optical imaging system may be between 0.9 and 1.5, corresponding to a large value of the pupil diameter. Such numerical aperture values, coupled with the fact that the diameter of the optical system must be equal to or smaller than the individual size of the image area allocated on the detector according to the TOMBO architecture, can be advantageously obtained by using materials with high refractive index values, such as germanium. However, these materials generally have a higher thermal sensitivity than materials with lower refractive index values, such as chalcogenide glasses. The thermal compensation device provided by the invention is therefore even more advantageous. - the effective field of view (commonly known as FOV) of each optical path can be between 20°(°C) and 60°, corresponding to a wide or ultra-wide field of view; and The specified temperature range may have a range of 60° C. (Celsius) or more, preferably 80° C. or more. Typically, the temperature specified range may be described in the optical imaging system or in a hardware or electronic manual accompanying this system.

[0023] A second aspect of the invention provides a method of acquiring a plurality of images associated with the same input field of view for all such images, the method being carried out using an optical imaging system according to the first aspect of the invention.

[0024] Such a method may be intended to reveal the presence of a gas in the input field of view, in which case each optical path is provided with a spectral filter as described above, which may be advantageously selected such that the spectral transmission window of at least one spectral filter is in the transmission band of the gas and the spectral transmission window of at least one other spectral filter is in the absorption band of the gas.

[0025] In particular, the gases sought by such methods may be natural gas, hydrocarbon gases, toxic gases (such as hydrogen sulfide (H2S) or carbon monoxide (CO)), or greenhouse gases (particularly carbon dioxide (CO2)). [Brief description of the drawings]

[0026] The features and advantages of the present invention will become more apparent in the following detailed description of non-limiting exemplary embodiments, with reference to the accompanying drawings.

[0027] [Figure 1] FIG. 1 is a cross-sectional view of a portion of an optical imaging system according to the present invention. [Diagram 2] FIG. 2 shows useful rays of the optical path of the optical imaging system of FIG. [Diagram 3] 3 is a perspective view of an optical path assembly support usable in the optical imaging system of FIG. [Figure 4]FIG. 4 is a perspective view of the appearance of the optical imaging system of FIG. [Figure 5a] FIG. 5a shows multiple optical paths of the optical imaging system of FIG. 1 arranged side-by-side in a longitudinal cross-section. [Figure 5b] FIG. 5b shows the thermal and geometric dimensioning parameters of the optical imaging system of FIG. 1 in the longitudinal cross section of FIG. 5a. [Figure 5c] FIG. 5c corresponds to FIG. 5a and shows the optical and geometric dimensioning parameters of the optical imaging system of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] For clarity, the dimensions of the elements shown in these drawings do not correspond to actual dimensions or to actual dimension ratios. Moreover, some of these elements are represented only symbolically, and the same reference signs shown in different drawings refer to the same elements or elements having the same functions.

[0029] The embodiment of the invention described herein with reference to figures 1 to 4 comprises a 2x3 matrix of juxtaposed light paths. Reference number 2 refers to this matrix of light paths as a whole, which has been referred to as the imaging matrix in the general part of this specification. Figure 1 shows the following elements of an optical imaging system: - a matrix image sensor designated by reference numeral 1; - two optical paths arranged side by side and designated by the references 21 and 22 respectively; - a mechanical connection between the matrix-like imager 1 and the imaging matrix 2 (symbolically represented by a dashed line and consisting of a thermal compensation device 3); and - Baffle 4

[0030] The matrix-like imaging element 1 may be of the bolometer or microbolometer type arranged in a matrix of columns and rows, for example 1024 columns by 768 rows. It may advantageously be of the operating type without a cooling system. This matrix-like imaging element 1 is housed in a vacuum-sealed container 1a, the latter comprising a window 1b transparent to the radiation intended to be detected by the matrix-like imaging element 1. For example, in particular for gas detection applications, the spectral sensitivity range of the matrix-like imaging element 1 may include at least one of the following two wavelength ranges: 3 μm to 5 μm and 8 μm to 14 μm.

[0031] In such an embodiment, all the optical paths may have a similar configuration, each of which may include two lenses and a spectral filter. Thus, optical path 21 may include lenses 211 and 221 and a spectral filter 231, optical path 22 may include lenses 212 and 222 and a spectral filter 232, and similarly for the other optical paths of the system. Advantageously, lenses 211 and 212, and their counterparts in the other optical paths of the system, may be formed from a single piece of transparent material, for example germanium, to form a first lens matrix 21. Similarly, lenses 221 and 222, and their counterparts in the other optical paths of the system, may be formed from a single other piece of transparent material, which may also be germanium, to form a second lens matrix 22. Such a design of all the optical paths of the optical imaging system facilitates optical alignment of the entire system due to the fact that the lenses are fabricated directly parallel to each other within each lens matrix. Preferably, the imaging matrix 2 is oriented such that the rows or columns of its optical paths are parallel to the rows of the bolometers or microbolometers in the matrix imager 1 .

[0032] The optical paths are differentiated from one another by their respective spectral filters, which have different transmission windows all within the spectral sensitivity range of the matrix-like imaging element 1. The transmission windows of these spectral filters can be selected as a function of the gas to be searched for, for which the optical imaging system is dedicated. The skilled person can select the spectral filters depending on the absorption and transmission bands of the gas to be detected.

[0033] Each optical path combines a scene contained in the input optical field of the optical imaging system common to all optical paths with a respective part of the matrix of the imager 1, called the active area of ​​the matrix imager for this optical path. Thus, during each readout sequence, the matrix imager 1 simultaneously acquires all images of the scene formed in its active area by all optical paths. Such a multiple optical path optical imaging system architecture is known in the prior art as TOMBO, an acronym for "Thin Observation Module by Bound Optics".

[0034] 2 shows the light rays transmitted by one optical path, for example optical path 21. The effective area of ​​the matrix-like imager 1 for each optical path may be, for example, 254 columns by 319 rows, and two of the optical paths that are adjacent optical paths with respect to the imaging matrix 2 may be separated by 130 rows or 130 columns of the imager matrix 1. ZU1 designates the effective area of ​​the optical path 21. In such an embodiment, all optical paths have a length L of the order of 13 mm (millimeters), measured between the front surface of the lens matrix 21 and the photosensitive surface of the matrix-like imager 1. mod may have:

[0035] In a particularly advantageous embodiment of the optical imaging system, some components of its optical paths may be supported by a common assembly support 20 for them, as shown in FIG. 1. An example of such a support 20 is shown in FIG. 3. It comprises holes T, for example cylindrical, assigned one-to-one to the optical paths, allowing the light rays to reach the matrix-like imager 1. These holes T are separated from one another by portions of the material of the support 20 that are opaque for the spectral range of sensitivity of the matrix-like imager 1. These portions of opaque material constitute a longitudinal separation, i.e. a partition, between adjacent optical paths. This eliminates stray light that may pass from one optical path to an adjacent one. Finally, in order to hold in place the respective spectral filters of the optical paths, the support 20 may be provided on one side with raised ribs NR on these partitions. Thus, FIG. 3 shows the support 20 from its side intended to face the site to be analyzed, i.e. its side opposite the matrix-like imager 1 in the system. Rims R 21 is intended to support the lens matrix 21 and has a similar rim R 22 is provided on the other side of the support 20 and supports a lens matrix 22 in front of the matrix-like imager 1 (see FIG. 1). A wedge and centering system within the knowledge of a person skilled in the art may further be provided to align the lens matrices 21 and 22 with the edges R 21 and R 22 The support 20 may be pressed against the support 20 with a force of 0.5 V.

[0036] The support 20 and the baffle 4 are shown in Fig. 1 and Fig. 4. The baffle 4 has a single entrance aperture O4 common to all the optical paths through which the light rays enter the system for all these optical paths until they reach the active area on the matrix-like imager 1 which corresponds to each of them individually. Advantageously, the optical imaging system can be protected at its top by a thermal screen 9, commonly known as a cap, so as not to be subjected to non-uniform thermal variations caused by temporary exposure to the sun's rays or cold air. Indeed, as will become clear later, the thermal compensation device 3 used in the optical imaging system requires, in order to operate correctly, that the temperature in the system is substantially uniform and stable.

[0037] The athermalization function of the thermal compensation device 3 is to maintain the sharpness of the image acquired by the matrix-like imager 1 when the system temperature changes. For this purpose, the thermal compensation device 3 induces a change in the distance between the optical center of the imaging matrix 2 and the matrix-like imager 1 as a function of the system temperature. This distance is the focal length of the imaging matrix 2, denoted by f, and its change as a function of temperature is approximately given by the formula known to the person skilled in the art:

[0038]

number

[0039] where α is the thermal expansion coefficient of the material of the lens matrices 21 and 22, in this case germanium, n is the optical refractive index of this material, and T is the temperature of the optical imaging system. The factor in brackets is commonly called the thermo-optical coefficient and is denoted β. Figure 5a shows the focal length f of the optical imaging system that is the subject of this detailed description. In this figure, the imaging matrix 2 is replaced by a matrix of single lenses, each equivalent to an optical doublet of the corresponding optical path. The plane of this figure is perpendicular to the plane of Figure 1.

[0040] The configuration of the thermal compensation device 3 used in the present invention is shown in Fig. 5b. In the same longitudinal section of the system as in Fig. 5a, in the plane of this figure perpendicular to both the imaging matrix 2 and the matrix-like imager 1, the thermal compensation device 3 has a serpentine shape that extends upstream of the imaging matrix 2, i.e. opposite the matrix-like imager 1. It comprises two segments 31 and 32, each made of a material different from the other segment, the difference between the respective thermal expansion coefficient values ​​of these two materials being non-zero and denoted ΔCTE. In a simplified approach, the two segments 31 and 32 are parallel to each other and have a common length denoted L'. The respective proximal ends of the two segments 31 and 32 are not directly connected to each other. The proximal end 31p of the segment 31 is connected to the matrix-like imager 1 by an intermediate portion 33, the thermal changes of which are negligible. The proximal end 32p of the segment 32 is connected to the support 20 of the imaging matrix 2. The two segments 31 and 32 extend upstream of the imaging matrix 2 or in front of it opposite the matrix-like imager 1, as shown in Fig. 5b, and their respective distal ends 31d and 32d are connected to each other. When the thermal compensation device 3 has such a configuration, the thermal change it produces with respect to the separation distance between the imaging matrix 2 and the matrix-like imager 1 is:

[0041]

number

[0042] The inset in Fig. 5b shows the length change ΔL of each of the two segments 31 and 32. 31 and ΔL 32 The thermal change over distance L' is expressed as the difference between ΔL' and ΔL': 31 -ΔL 32 The thermal compensation device 3 is effective to maintain the clarity of the image acquired by the sensor 1 when the system temperature changes (i.e. due to the athermalization function) when ΔL′=Δf. To meet this condition, two parameters are available for dimensioning the device 3: L′ and ΔCTE. When so selected, i.e. when the following condition is met:

[0043]

number

[0044] The optical imaging system is said to be athermalized, the previous condition being called the athermalization condition. The thermal interval of use of the system specified by the manufacturer, which in the general part of this specification is called the specified interval of the optical imaging system, is therefore extended compared to a system without a thermal compensation device. The thermal compensation device 3 just described is often called a passive device, since it does not use active components such as user-controlled motors or drives to adjust the distance between the imaging matrix 2 and the matrix-like imaging element 1. The aforementioned athermalization condition leaves one parameter of the thermal compensation device 3 still available to generate an additional function: the difference in thermal expansion coefficient values ​​ΔCTE. This additional function will be to support the baffles 4, preferably at the distal ends 31d and 32d.

[0045] Thus, due to the variable distance between the matrix-like imager 1 and the imaging matrix 2 (i.e. due to the athermalization function) and the presence of a window between these two components, a vertical partition cannot be provided between the optical paths extending from the imaging matrix 2 to the photosensitive surface of the matrix-like imager 1. The function of such partitions, known before the present invention, is to eliminate image overflow between adjacent optical paths, which generates image overlaps. These image overlaps, also known as image superpositions, occur at the edges of parallel adjacent images. For example, with reference to FIG. 5a, the image formed by the optical path 21 thus overflows from the effective area ZU1 assigned to this optical path in the matrix-like imager 1 into the effective area ZU3 assigned to the optical path 23 beyond the border between both effective areas. In the image overlap areas, the image information is mixed and therefore can no longer be used, and in particular the presence of the desired gas in the part of the input optical field of the system related to these image overlaps cannot be detected. It is possible to separate the effective areas of two adjacent optical paths by an unused intermediate band in the matrix of the matrix-like imager 1. Such a separation band is denoted BS in Fig. 5a and is called a clearance band in the general part of this specification. Thus, if at least one of the two images formed by the optical paths 21 and 23 overflows into a separation band BS that is intermediate between the active areas ZU1 and ZU3, this overflow does not directly affect the other of these active areas as long as it remains within the separation band BS. Nevertheless, such separation bands are areas in which the photosensitive surface of the matrix-like imager is not used, and the invention described below makes it possible to reduce the width of these separation bands BS and thus the number of photosensitive elements (pixels) of the matrix-like imager 1 that are not used.

[0046] According to the invention, the images formed by all the light paths on the matrix-like imager 1 respectively are limited by an input aperture O4 of a baffle 4 common to all these light paths. This input aperture O4 is preferably located centrally with respect to the imaging matrix 2 and the matrix-like imager 1. The effectiveness of the baffle 4 in eliminating the overlap of the images is determined by the dimensions of its input aperture O4 and the position of the edge of this aperture O4 in front of the imaging matrix 2. When each image is limited in this way, the effective areas can be brought closer to each other so that the width of each separation strip BS can be reduced. The number of unused light-receiving elements of the matrix-like imager 1 is therefore reduced. In other words, this sensor has a higher utilization of its photosensitive surface.

[0047] Further according to the invention, the thermal compensation device 3 is further used to support the baffle 4. If it comprises more than two materials with different thermal expansion coefficient values, the remaining available parameters of the thermal compensation device 3, or one of its remaining available parameters, is used to adjust the position of the baffle 4 in front of the imaging matrix 2. In continuation of the asermalization of the optical imaging system previously described with reference to Fig. 5b, such a selection of the parameter ΔCTE will now be explained below with reference to Fig. 5c.

[0048] Figure 5c is drawn in a longitudinal section of the system parallel to the rows or columns of the imaging matrix 2, for example also in the plane of Figures 5a and 5b. The imaging matrix 2 is assumed to have N optical paths juxtaposed in this longitudinal section, N being an integer greater than or equal to 2, preferably equal to 2 or 3, and generally less than or equal to 4. The following notation is adopted: H det is the size of the matrix-type image sensor 1 in the longitudinal section; l BS are the widths of the separation strips BS, which are all assumed to be equal to one another; and FOV is the effective field of view of the system for all optical paths in the longitudinal section under consideration, and the size of any effective area ZU in this longitudinal section is 2-f-tan(FOV / 2), where tan(.) is the tangent function, and:

[0049]

number

[0050] The field of view FOV is common to all optical paths, the image of whose contents is formed individually by each optical path within the effective area ZU of the matrix-like imager 1 dedicated to that optical path.

[0051] The image overlap is most significant between the first two or the last two effective areas of the matrix-like image sensor 1, counting the effective areas sequentially from one edge of the sensor 1 to its opposite edge in the longitudinal section under consideration. In other words, the image overlap occurs first between the first two optical paths or between the (N-1)th and Nth optical paths. The extreme field of view of the second optical path that defines the maximum image overlap with the first optical path without spilling over into the intermediate separation zone within the effective area of ​​this first optical path is called FOV. ext or, by symmetry, the extreme field of view of the (N-1)th path that defines the maximum image overlap with the Nth path is denoted as FOV ext From Figure 5c we get:

[0052]

number

[0053] where L is the distance in front of the imaging matrix 2 where the baffle 4 is located, and Φ is the individual pupil diameter of the optical paths, which is assumed to be the same for all optical paths. In an optimized embodiment of the optical imaging system, two optical paths that are adjacent optical paths are assumed to be continuous in the imaging matrix 2 (i.e., the respective pupils of the optical paths are adjacent to each other). At the same time, the extreme field of view FOV extBy the definition of , the following equation corresponds to the separation zone BS between the first two or last two effective areas ZU in the longitudinal section.

[0054]

number

[0055] Between the previous two equations, tan(FOV ext / 2), the pupil diameter Φ is equal to f / F#, where F# is the numerical aperture in the system for each optical path, and the H det Using this formula, the result is:

[0056]

number

[0057] This formula, which is of a purely optical and geometrical nature, gives the distance L at which the baffle 4 must be placed in front of the imaging matrix 2 in order to avoid overlapping of images within the effective area of ​​the matrix-like imager 1. It was obtained for the overflow of the image formed by the second optical path into the effective area of ​​the first optical path.

[0058] In a preferred embodiment of the invention, which reduces the overall size of the complete system, the baffle 4 can be supported by the distal ends 31d and 32d of the segments 31 and 32 of the thermal compensation device 3, which distal ends 31d and 32d are connected to each other, as shown in Figure 5b. In other words: L = L'. The remaining available parameter ΔCTE is then given by:

[0059]

number

[0060] Under these conditions, the length of the thermal compensation device 3 in front of the imaging matrix 2 corresponds to the position of the baffle 4 in order to eliminate the overlap of images in the useful area. Thus, the optical imaging system with multiple optical paths is as compact as possible.

[0061] As an example, the following values ​​can be used: the matrix imager 1 may have dimensions of 1024 columns by 768 rows with a row-column pitch equal to 17 μm, corresponding to a sensor size of 17.4 mm (millimeters) by 13.1 mm; - three optical paths can be juxtaposed along the length of this matrix imager 1: N=3; Each separation strip BS may correspond to 130 adjacent columns of the matrix-like image sensor 1, BS = 2.21 mm; - The field of view FOV may be equal to 30° (°C) for each optical path; - the numerical aperture F# may also be equal to 1.2 for each optical path; and Both lens matrices 21 and 22 may be made of germanium. For these values, the parameters of the thermal compensation device 3 are L = L' = 40 mm and ΔCTE = 20.5 μm.m -1 .℃ -1 . This value of ΔCTE can be obtained by using aluminum and an alloy commercially designated Invar® for each of the two segments 31 and 32 of the thermal compensation device 3. The quotient L / f of the distance L to the focal length f is then equal to 5.6. Under these conditions, the defined temperature interval can be from -40°C to +60°C. In another embodiment, where one of the segments 31 and the other 32 is made of aluminum and the other of ABS (acrylonitrile-butadiene-styrene) polymer, the quotient L / f can be equal to 1.8.

[0062] It is reiterated that the mathematical model described herein is simplified and is intended only to illustrate the principles of the present invention, and those skilled in the art will be able to complete it to make it more accurate, taking into account the specific details of the optical imaging system.

[0063] Furthermore, it is possible to place the baffle 4 at a level along the length of the thermal compensator 3 different from that of the distal ends 31d and 32d. The use of the present invention then results in values ​​of the parameter ΔCTE different from those obtained from the last equation above, but the estimation of these values ​​is within the knowledge of one skilled in the art after setting the geometry of the thermal compensator 3 with respect to the imaging matrix 2 and the baffle 4. Finally, the thermal compensator 3 can be composed of more than two segments whose changes in length are combined to produce asermalization, while supporting the baffle 4 at the appropriate position to eliminate image overlap in the active area. Additional parameters are then available that may facilitate the use of the present invention for a particular application.

[0064] Finally, it is understood that the invention may be reproduced by modifying further secondary aspects of the simplified embodiment detailed above while retaining at least some of the cited advantages. In particular, the optical imaging system according to the invention may be used for applications other than the search for one or more gases in an environment. Moreover, all numerical values ​​are cited purely by way of example and may be modified depending on the application under consideration.

Claims

1. Multiple optical paths (2 １ , 2 ２ 1. An optical imaging system comprising: The system a matrix-type image sensor (1), and an imaging matrix (2) comprising a number of imaging optics arranged in parallel to an effective area (ZU) of the matrix-like imager (1) dedicated to said imaging optics in a one-to-one correspondence and adapted to form simultaneously for all said imaging optics respective images of a scene contained in the same input optical field, said effective areas of said imager being spaced apart from one another, and each imaging optic and its corresponding effective area belonging to one of the optical paths of the system apart from each of the other optical paths, Furthermore, the optical imaging system a thermal compensation device (3) arranged to generate a variable spacing between the imaging matrix (2) and the matrix-like imaging element (1), the thermal compensation device (3) comprising, in at least one longitudinal section perpendicular to the imaging matrix and to the matrix-like imaging element, a serpentine section including two segments (31, 32) including a material having a thermal expansion coefficient value different from that of the other segment, the two segments being connected to each other at their respective distal ends (31d, 32d), one proximal end (32p) of the segment being connected to the imaging matrix and the other proximal end (31p) of the segment being connected to the matrix-like imaging element, the difference in thermal expansion coefficient values ​​and the respective lengths of the two segments being adapted to generate the variable spacing between the imaging matrix and the matrix-like imaging element such that an image formed in the effective area (ZU) of the matrix-like imaging element is sharp when the temperature of the system has any value within the defined range; a baffle (4) arranged upstream of the imaging matrix (2) with respect to the direction of propagation of radiation entering the system to form an image, said baffle (4) being arranged upstream of the optical path (2) of the system; １ , 2 ２ The radiation aperture (O ４ ), a baffle (4) having a The opening (O) of the baffle (4) ４ ) are dimensioned to laterally limit the illumination in each image formed by any one of the optical paths, referred to as the optical path under consideration, in order to avoid said images spilling over into the effective area (ZU) of another optical path adjacent to the optical path under consideration, thereby preventing overlap of images in all of the effective areas for any value of the temperature of the system within the specified range, Furthermore, the distal ends (31d, 32d) of the segments (31, 32) of the thermal compensation device (3) are located upstream of the imaging matrix (2) with respect to the propagation direction of the radiation entering the system for image formation, An optical imaging system, wherein the baffle (4) is supported by the thermal compensation device (3) upstream of the imaging matrix (2) also with respect to the propagation direction of the radiation entering the system for image formation.

2. 2. The optical imaging system of claim 1, wherein the baffle (4) is supported by the thermal compensation device (3) at the distal ends (31d, 32d) of the segments (31, 32) of the thermal compensation device.

3. 3. The optical imaging system of claim 1, wherein a separation distance between the baffle (4) and the imaging matrix (2), measured parallel to a common direction of the optical axes of the imaging optics, is between 1.0 and 6.0 times the average focal length of the imaging optics.

4. An optical imaging system according to any one of claims 1 to 3, further comprising a container (1a) arranged to accommodate the matrix-like imaging element (1), the container having a window (1b) transparent to the radiation forming an image, the window being located between the imaging matrix (2) and the matrix-like imaging element.

5. 5. The optical imaging system of claim 1, wherein the imaging optics has a row and column arrangement in the imaging matrix (2), each row being perpendicular to each column, and the imaging matrix is ​​oriented such that each row of the imaging optics is parallel to a row or column direction of the light receiving elements of the matrix-shaped imager (1).

6. 6. The optical imaging system of claim 5, wherein the row and column arrangement of the imaging optics in the imaging matrix (2) has one of the following dimensions: 1x2, 2x1, 2x2, 2x3, 3x2, 3x3, 2x4, 4x2, 3x4, 4x3, and 4x4.

7. Each imaging optical system includes at least one germanium-based lens (21 １ , 21 ２ , 22 １ , 22 ２ The optical imaging system according to any one of claims 1 to 6, comprising:

8. Each imaging optical system is a spectral filter (23 １ , 23 ２ 8. The optical imaging system according to claim 1, wherein the spectral filters of two imaging optics that differ from one another in the imaging matrix (2) determine different spectral transmission windows.

9. A method for acquiring multiple images associated with the same input optical field for all said images, implemented using an optical imaging system according to any one of claims 1 to 8.

10. 9. The optical imaging system of claim 8, wherein the optical imaging system is intended to reveal the presence of gas within the input optical field, and the spectral filter (23) １ , 23 ２ 10. The method of claim 9, wherein at least one of the spectral filters has a spectral transmission window that is within a transmission band of the gas and at least one other of the spectral filters has a spectral transmission window that is within an absorption band of the gas.

11. 11. The method of claim 9 or claim 10, wherein the gas is a natural gas, a hydrocarbon gas, a toxic gas, or a greenhouse gas.

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