Imaging optical system including three mirrors

Abstract

The imaging optics (1) with three mirrors (M1, M2, M3) has a configuration adapted to block stray light that could otherwise reach the image sensor (2) of the system, while at the same time allowing a large illumination field, a high aperture, and good system compactness. The system may also incorporate two entrance baffles (11, 12), one located on either side of the optical entrance of said system. The function of said two entrance baffles may be limited to blocking light rays originating from illumination fields angularly distant from the entrance field useful for each captured image. Thus, the two entrance baffles may have a short length upstream, so that the system has a small size.

Description

[Technical Field]

[0001] This specification relates to an imaging optical system including three mirrors, and an optoelectronic imaging apparatus including such a system. [Background technology]

[0002] Imaging optical systems containing three mirrors are used in many applications. In particular, these systems can be telephoto-type systems, and A. Bauer et al., "Concurrent engineering of a next-generation freeform telescope: optical design," SPIE, Vol. 10998, May 14, 2019, pp. 109980W-1~109980W-8, proposes several new imaging optical system configurations, each consisting of three freeform mirrors. Common challenges with imaging optical systems include their size and the possibility of reducing the amount of stray light superimposed on the image formed by the mirrors. To reduce the amount of stray light reaching the image sensor of such imaging optical systems, it is common practice to use one or more baffles arranged in a suitable manner. However, some of these baffles, especially the most efficient baffles, significantly increase the size of the system. In addition to increasing its size, such baffles also increase the rotational inertia of the rotating imaging optical system, which is adapted for scanning large scenes to be captured in several consecutive images. One application of imaging optics requiring high-speed rotation of these systems with high angular acceleration is the supply of optoelectronic pods for surveillance and detection. Optoelectronic pods are intended to be mounted on aircraft, such as helicopters or drones mounted on aircraft. Therefore, it is important to obtain low levels of stray light in the captured image while keeping the baffles incorporated into the imaging optics as small as possible.

[0003] Figure 1 is a diagram of one configuration described in the paper by A. Bauer et al. cited earlier. Such an imaging optical system, collectively referred to as reference number 1, is of the type that includes three mirrors: a first mirror indicated by M1, a second mirror indicated by M2, and a third mirror indicated by M3. These mirrors are adapted and positioned so that rays originating from a scene located in the incident field of the system are first reflected by mirror M1, then by mirror M2, and then by mirror M3 to form an image of the scene within the focal plane of the system indicated by PF. Thus, any rays originating from the scene and contributing to image formation are divided into an initial portion upstream of mirror M1, a first intermediate ray portion between mirrors M1 and M2, a second intermediate ray portion between mirrors M2 and M3, and a terminal ray portion between mirror M3 and the focal plane PF. The second mirror M2 may be convex, and the third mirror M3 may be concave. The curvature direction of the main mirror M1 may vary depending on its position on the mirror. The three mirrors M1, M2, and M3 have free-form reflective surfaces. In known forms, the free-form curved surfaces do not include any surfaces that have rotational symmetry.

[0004] In this specification, the terms upstream and downstream are defined with respect to the propagation direction of rays originating from the scene and forming an image at the focal plane PF. Furthermore, the term “parabasal ray” or “principal ray” is used to refer to a ray originating from the scene, contributing to the image at the focal plane PF by passing through the center of the entrance pupil of system 1, and having zero angular deviation with respect to the optical axis of the system. In Figure 1, the parabasal ray is indicated by reference RP, reference RP0 in the initial portion, reference RP1 and RP2 in the first and second intermediate portions, and reference RP3 in the terminal portion. Rays originating from elements in the scene located at the boundary of the incident field of system 1 and passing through the edge of the entrance pupil of the system are called edge rays of the field.

[0005] In the system shown in Figure 1, mirrors M1 and M2 are oriented such that the second intermediate portion RP2 of the paraxial ray RP intersects with the initial portion RP0. This configuration of mirrors M1 and M2 is called the α configuration. Furthermore, mirrors M2 and M3 are oriented such that the terminal portion RP3 of the paraxial ray RP passes through the side of mirror M2 on the side opposite to the lateral displacement of mirror M1 relative to mirror M2. In this configuration, the terminal portion RP3 of the paraxial ray RP does not intersect with the first intermediate portion RP1 of the ray. This configuration of mirrors M2 and M3 is called the z configuration. Therefore, system 1 has an overall optical configuration called the α-z configuration.

[0006] System 1 further includes an image sensor 2, and the photosensitive surface S of this image sensor is positioned so as to overlap with the focal plane PF. The photosensitive surface S is located at the upstream boundary L AM From the downstream boundary L AV It extends up to the upstream L of the photosensitive surface S of the image sensor 2. AM and downstream L AV The boundaries are defined in relation to the projection of each of these boundaries onto the initial portion of the paraxial ray RP0, and in relation to the direction of propagation of the paraxial ray RP in this initial portion. [Overview of the project] [Problems that the invention aims to solve]

[0007] Therefore, the present invention aims to propose a new imaging optical system that reduces the amount of stray light reaching the image sensor.

[0008] Another objective of the present invention is to make the imaging optical system compact.

[0009] Another additional object of the present invention is that the imaging optical system can have a large incident field and / or a large entrance pupil.

[0010] Another objective of the present invention is to enable the manufacturing of imaging optical systems at low cost. [Means for solving the problem]

[0011] To achieve at least one of these or other objectives, a first aspect of the present invention proposes an imaging optical system comprising three mirrors of the type described above. The second and third mirrors are oriented such that the upstream boundary of the photosensitive surface of the image sensor is shifted downstream relative to a straight line connecting the upstream edge of the first mirror and the upstream edge of the second mirror, or to the upstream edge of the screen surrounding the second mirror. In this way, the second mirror or the screen surrounding it blocks light rays that would otherwise propagate linearly from the first mirror directly to the photosensitive surface of the image sensor.

[0012] Following the convention described above, the upstream and downstream edges of the first and second mirrors, respectively, are defined in relation to their respective projections onto the initial portion of the paraxial ray. Furthermore, the upstream and downstream edges define the direction of propagation of the paraxial ray in this initial portion. Similarly, the downstream displacement of the upstream boundary of the image sensor's photosensitive surface is parallel to the initial portion of the paraxial ray. This downstream displacement is the displacement in the direction of propagation of the paraxial ray in this initial portion.

[0013] Therefore, such a system configuration, in which the second mirror is positioned between the image sensor and the first mirror, allows for the blocking of stray light. Otherwise, in particular, light rays propagating directly from the first mirror, entering through the optical entrance of the system and reflected toward the image sensor by the first mirror, would reach the image sensor.

[0014] According to an improvement of the present invention, the system may further comprise a first inlet baffle. The first inlet baffle is superimposed on the initial portion of the first field-edge rays on the first side of the same incident field as the image sensor, opposite to the third mirror. In this case, the first inlet baffle may have a downstream edge that connects to the terminal portion of the second field-edge rays. These second field-edge rays may be opposite to the first field-edge rays that enter the system and form the image, especially when the system has a common plane of symmetry for the three mirrors. Such a first inlet baffle blocks a portion of the light that otherwise enters the system through its optical inlet, which is oriented directly toward the image sensor. Furthermore, the first inlet baffle may have a length starting from its downstream edge, with its upstream edge blocking the rays. Otherwise, the rays enter the system through its optical inlet toward the third mirror and are reflected toward the image sensor by the third mirror.

[0015] According to the features of the present invention, since the upstream boundary of the photosensitive surface of the image sensor is shifted downstream with respect to the straight line connecting the respective upstream edges of the first and second mirrors, the first inlet baffle can have a shortened length parallel to the initial portion of the first field edge ray. Therefore, the size of the system including the first inlet baffle is reduced.

[0016] According to a supplementary improvement of the present invention, the system may further comprise a second inlet baffle. The second inlet baffle is superimposed on the initial portion of the second field edge ray on the second side opposite the image sensor, in the same incident field as the third mirror. The second inlet baffle may then have a downstream edge connected to the upstream edge of the third mirror, or to a screen surrounding the third mirror, or to an opaque base for the third mirror. Alternatively, the downstream edge of the second inlet baffle may be located downstream of a straight line connecting the upstream boundary of the photosensitive surface of the image sensor to the downstream edge of the first inlet baffle. Such a second inlet baffle further reduces the light that would otherwise enter the system through the optical inlet and be directed directly to the image sensor.

[0017] Preferably, the second entrance baffle can have an upstream edge located upstream of a straight line connecting the downstream edge of the first entrance baffle to the downstream boundary of the photosensitive surface of the image sensor. Thus, the first and second entrance baffles cooperate to block all light rays that would otherwise enter the system through its optical entrance and travel directly towards the image sensor or towards the third mirror.

[0018] Also, due to a feature of the present invention, the upstream boundary of the photosensitive surface of the image sensor is offset downstream with respect to a straight line connecting the respective upstream edges of the first and second mirrors, so that the second entrance baffle can reduce the length parallel to the initial portion of the second irradiation field edge ray. Thus, the size of the system including the second entrance baffle is also reduced.

[0019] In a preferred embodiment of the present invention, at least one of the following additional features may be optionally reproduced alone or in combination with some of them.

[0020] - At least one of the first mirror, the second mirror, and the third mirror may have a free-form reflective surface.

[0021] - The vertical dimension of the image sensor determines the first angular field of the system, and the system may be adapted such that this first angular field is 9° (degrees) or more, preferably 18° or more. Thereby, the incident field of the system can be enlarged, but preferably less than 45°.

[0022] - The image sensor may have a matrix array, in which case the horizontal dimension perpendicular to its vertical dimension of the image sensor determines the second angular field of the system. In this case, the system may be adapted such that the second angular field is 12° or more, preferably 24° or more, preferably less than 60°.

[0023] - The system can have an aperture numerical value N less than 5, preferably less than 2, and the aperture numerical value N is equal to f / D, where f is the focal length of the system and D is the dimension of the entrance pupil of the system. Thus, the value of the aperture number N can enlarge the entrance pupil of the system.

[0024] - The focal length f value is 100mm or less, preferably 20mm or less, but may be greater than 2mm.

[0025] - The image sensor may be of a type that is sensitive to at least a portion of the spectral interval ranging from 360 nm (nanometers) to 14 μm (micrometers) in wavelength values ​​of light rays generated from the scene. In particular, the image sensor may be a bolometer type or a microbolometer type thermal sensor.

[0026] -The system may further include a pupil diaphragm positioned on the first or third mirror, preferably the third mirror. For these two positions of the pupil diaphragm, it can have a simple opening shape, in particular, that is circular, square, or rectangular. Furthermore, if it is located on the third mirror, its size is smaller. Advantageously, the pupil diaphragm may be formed by the periphery of the first or third mirror.

[0027] -The system may further include a spectral separator positioned between the third mirror and the image sensor, and an additional image sensor positioned in the image of the focal plane of the system, the image of which may be formed by the spectral separator.

[0028] - The first, second, and third mirrors may be contained within spheres having a diameter of 2 to 6 times the focal length f of the system.

[0029] - At least one of the first, second, and third mirrors may include a rigid portion made of an injected polymer-based material and an optional reflective metal layer.

[0030] Finally, a second aspect of the present invention proposes an optoelectronic imaging device comprising the system according to the first aspect described above. This device may be, but is not limited to, an aircraft guidance system, a thermal camera, a visual assistance device, or an optoelectronic pod for surveillance and detection. [Brief explanation of the drawing]

[0031] The features and advantages of the present invention will become more clearly apparent in the following detailed description of some non-limiting embodiments with reference to the accompanying drawings. [Figure 1] This is an optical diagram of an imaging optical system known prior to the present invention. [Figure 2] This is an optical diagram of the imaging optical system according to the present invention. [Figure 3a] The features of the present invention are shown, corresponding to Figure 2. [Figure 3b] Figure 3a illustrates other features of the present invention. [Figure 4] This corresponds to Figure 2 for an improvement of the present invention. [Figure 5] Figure 2 shows an optoelectronic imaging system incorporating the system shown. [Modes for carrying out the invention]

[0032] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or actual dimensional ratios. Furthermore, identical reference numerals shown in different figures indicate elements that are identical or have the same function. The planes in the drawings constitute the planes of symmetry of the imaging optical system shown, but such symmetry may not be considered essential to the present invention. In fact, the three mirrors in each system can be angled so that the paraxial rays do not lie on the same plane.

[0033] In FIGS. 1-4, in the orthogonal coordinate system x, y, z, the x-axis is perpendicular to the plane of the figure, the z-axis is parallel to the initial portion RP0 of the paraxial ray RP, and is oriented in the propagation direction of the ray on this portion, and the y-axis is oriented so as to coincide with the propagation direction of the ray contributing to the image formed on the focal plane PF, and the end portions of these rays are oriented so as to have a projection on the positively oriented y-axis. The y-z plane, which is the plane of the figure, may be a symmetry plane of system 1 including the symmetry plane of each of the reflecting surfaces of mirrors M1, M2, and M3. The terms upstream and downstream are defined with respect to this z-axis by comparing the respective positions of the projections of the boundaries or edges of the optical components on the z-axis. In particular, the downstream edge B AV1 , B AV2 of each is on the opposite side of each of the upstream edges B AM1 , B AM2 with respect to each of mirrors M1, M2.

[0034] The straight line D0 shown in FIG. 1 connects the upstream edges of mirrors M1, M2, and is indicated by B AM1 , and B AM2 respectively. It shows that the image sensor 2 is still at least partially shifted upstream with respect to this straight line D0 with respect to the z-axis. Due to the relative position between the straight line D0 and the image sensor 2, stray light can propagate directly from mirror M1 to the image sensor 2. This stray light is generated from the scene towards which the optical entrance of system 1 is directed and can be reflected by mirror M1 towards the image sensor 2. Then, the stray light passes upstream of mirror M2 and reaches the image sensor 2 directly. The reference R1 in FIG. 1 indicates the ray of this stray light. The ray R1 of this stray light has a slight inclination with respect to the optical axis of system 1 at its light incidence and has a slight inclination with respect to the z-axis before being reflected by mirror M1. Removal of it by a mask of the irradiation field edge disposed at the optical entrance of system 1 requires that this mask has a large length in the upstream direction of the optical entrance.

[0035] FIG. 2 is of the same type as FIG. 1 but shows system 1 modified according to the present invention. Also, in the present invention, mirrors M2, M3 are such that the photosensitive surface S of the image sensor 2 is with respect to the straight line D0 downstream It is positioned and tilted so as to be completely offset. In other words, it is the upstream boundary L of the photosensitive surface S. AM It is located downstream of the line D0. In this way, according to the present invention, stray light cannot be directly propagated from mirror M1 to image sensor 2, and all rays similar to ray R1 are blocked. Clearly, the upstream edge of mirror M2 can be replaced so as to define the line D0 by the upstream edge of the peripheral screen of mirror M2 that extends upstream of the mirror.

[0036] In system 1 of Figure 2, mirror M3 constitutes the entrance pupil.

[0037] In the embodiment of the present invention shown in Figure 2, the dimensions of the photosensitive surface S of the image sensor 2 appearing in the yz plane of the figure are such that the associated field of view is equal to 18°. In the overall context of this description, this dimension of the photosensitive surface S is referred to as the vertical dimension, and the associated field of view is referred to as the first field of view. This first field of view is denoted below by α1.

[0038] The image sensor 2 may be of a matrix type, in which case its photosensitive surface S has another dimension parallel to the x-axis. This other dimension is referred to as the transverse dimension of the photosensitive surface S in the general part of this specification. In the embodiment of Figure 2, this transverse dimension of the photosensitive surface S of the image sensor 2 is like the relevant field of view, referred to as the second field of view, and is equal to 24°. Thus, system 1 in Figure 2 has a large full field of view of 18° × 24°. However, such an imaging optical system configuration can yield a larger or smaller field of view. According to the embodiments described herein, if the photosensitive surface S of the image sensor 2 is rectangular, the image sensor is preferably oriented such that the maximum transverse dimension of its photosensitive surface is perpendicular to the plane of symmetry of system 1, i.e., perpendicular to the plane of Figure 2.

[0039] In the embodiment shown in Figure 2 as an example, the image sensor 2 has 240 pixels of 12 μm (micrometers) in its vertical dimension and 320 pixels in its horizontal dimension. The focal length value f of system 1 is equal to 9 mm (millimeters), its numerical aperture N is equal to 1.5, and it corresponds to an entrance pupil size of 6 mm.

[0040] Figures 3a and 3b repeat the same embodiment of the present invention as in Figure 2, showing that the three mirrors M1, M2, and M3 of System 1, like the image sensor 2, are contained within a sphere with a diameter equal to 40 mm, indicated by SPH. Thus, System 1 is particularly compact and suitable for integration into aircraft guidance systems, thermal cameras, visual aids, and optoelectronic imaging devices such as optoelectronic pods for surveillance and detection. Figure 5 shows such an optoelectronic pod for surveillance and detection, indicated by reference numeral 20, mounted on an unmanned aerial vehicle 30 and incorporating System 1.

[0041] In some possible embodiments of the present invention, some or all of the optical components of System 1 may be manufactured by three-dimensional printing, commonly known as 3D printing.

[0042] In other possible embodiments, some or all of the optical components of System 1 may be manufactured from a polymer-based material into which injection is performed. Such other embodiments may have particularly low cost prices. Furthermore, at least one of the mirrors M1, M2, and M3 formed by injection may be produced directly using a self-positioning system for the mirrors.

[0043] Each of the mirrors M1, M2, and M3 may consist of a rigid base that provides the shape of its reflective surface and a reflective metal layer placed on its surface. The rigid base may be manufactured from a solid 3D printed material or based on an injected polymer. For mirror M2, the base and reflective layer of this mirror are indicated in Figure 2 by reference numbers M2b and M2r, respectively.

[0044] Figures 3a and 3b further show two inlet baffles. Inlet baffles are added to system 1 to further reduce the amount of stray light that could otherwise reach the image sensor 2. The inlet baffle indicated by reference numeral 11 is referred to as the first inlet baffle in the whole part of this specification, and the inlet baffle indicated by reference numeral 12 is referred to as the second inlet baffle. The optical inlet of system 1 is indicated by reference numeral O. Inlet baffle 11 is located on the edge of the optical inlet O, close to the image sensor 2. Inlet baffle 12 is located on the edge of the optical inlet O opposite to inlet baffle 11. Thus, inlet baffle 12 is close to mirror M3. In fact, due to the α-z configuration of system 1, the image sensor 2 is located close to or very close to the optical inlet O, while being shifted laterally relative to it in the opposite direction from mirror M3.

[0045] In the yz plane of Figures 3a and 3b, the incident field of system 1 is bounded between two field edge rays indicated by references RM1 and RM2. Thus, the initial portions of these field edge rays RM1 and RM2 form the aforementioned field of view α1 between them. The inlet baffle 11 is superimposed on the initial portion of the field edge ray RM1. The inlet baffle 12 is superimposed on the initial portion of the field edge ray RM2. Furthermore, the inlet baffle 11 may extend downstream of the terminal portion of the field edge ray RM2. The inlet baffle 12 is positioned at the upstream edge B of mirror M3. AM3 It may extend downstream. In other words, the downstream edge B of the inlet baffle 11 AV11 This is the irradiated field edge ray RM 2. It can be located on the terminal portion of the inlet baffle 12. AV12 This is the upstream edge B of mirror M3. AM3 It can be coupled with the following. Outside the yz plane in Figures 3a and 3b, the inlet baffles 11 and 12 are preferably superimposed on the irradiated field edge rays RM1 and RM2 that are close to the irradiated field edge rays.

[0046] Figure 3a shows the complete paths of the field edge rays RM1 and RM2 in system 1, as well as their contribution to the image formed on the photosensitive surface S of the image sensor 2. The field edge ray RM1 is located at the downstream boundary L of the photosensitive surface S of the image sensor 2. AV It contributes to image formation in the field edge. In addition, the field edge ray RM2 is upstream boundary L AM It contributes to the formation of images in [location].

[0047] Referring to Figure 3b, in the yz plane, reference symbol F0 represents the incident field of system 1, and reference symbols F1 and F2 represent angular irradiation fields that are outside the incident field F0 but are angularly close to it. Reference symbols F3 and F4 represent angular irradiation fields that are angularly located on the opposite side of irradiation fields F1 and F2, respectively, relative to the incident field F0. Therefore, irradiation fields F1 and F2 are called adjacent irradiation fields to the incident field F0. Irradiation fields F3 and F4 are called non-adjacent irradiation fields to the incident field F0.

[0048] Light rays coming from adjacent illumination fields F1 and F2, which can be reflected by mirror M1, then mirror M2, and finally mirror M3, reach the image sensor 2 outside its photosensitive surface S. In principle, light rays originating from non-adjacent illumination fields F3 and F4 do not sequentially follow the planned path within system 1 via the three mirrors. However, the light rays are either directed directly to the image sensor 2 if they originate from the non-adjacent illumination field F4, or they reach the image sensor 2 after being reflected on mirror M3 if they originate from the non-adjacent illumination field F3 or the mirror.

[0049] As provided by the α-z configuration, by positioning the image sensor 2 near the optical inlet O of system 1, it becomes possible to prevent stray light originating from the adjacent illumination field F1 from being reflected towards the image sensor 2 by the mirror M3. Therefore, the function of the inlet baffle 11 includes blocking stray light originating from the non-adjacent illumination field F3 that could be reflected towards the image sensor 2 by the mirror M3, but does not include blocking stray light originating from the adjacent illumination field F1 that is directed toward the mirror M3. This allows the length of the upstream inlet baffle 11 of system 1 to be shortened.

[0050] The inlet baffle 11 also blocks some of the light rays coming from the non-adjacent illumination field F4 while it is oriented toward the image sensor 2, meaning the light rays from the non-adjacent illumination field F4 that are not tilted very much with respect to the z-axis. These are, in fact, blocked by the downstream portion of the inlet baffle 11.

[0051] Furthermore, the non-adjacent field of view F4 rays that are most inclined with respect to the z-axis while oriented toward the image sensor 2 are blocked by the inlet baffle 12. To block these rays, the inlet baffle 12 is positioned at the downstream edge B of the inlet baffle 11. AV11 and the downstream boundary L of the photosensitive surface S of the image sensor 2 AV Upstream edge B, located upstream of the straight line D1 connecting the two points. AM12 It can have this. However, because mirror M2 blocks light rays propagating linearly between mirror M1 and image sensor 2, the inlet baffle 12 does not need to block stray light from adjacent illumination field F2, which would otherwise be reflected toward the image sensor 2 on mirror M1 and would also reflect light rays with a low gradient from non-adjacent illumination field F4. Therefore, the α-z configuration of system 1 makes it possible to position the image sensor 2 near its optical inlet O so that the inlet baffle 12 blocks only the most inclined parasitic rays from non-adjacent illumination field F4, without requiring the inlet baffle 12 to block light rays from adjacent illumination field F2 or light rays with a low gradient from non-adjacent illumination field F4. Therefore, the upstream edge B of the inlet baffle 12 AM12 It can be positioned on a straight line D1 without necessarily extending beyond the upstream. Therefore, the inlet baffle 12 may also have an upstream length that means a length that extends in front of the short light inlet O. Also, the downstream edge B of the inlet baffle 12 AV12 This is the upstream edge B of mirror M3. AM3 Instead of connecting them, the downstream edge B of the inlet baffle 11 AV11 and the upstream boundary L of the photosensitive surface S of the image sensor 2 AM It should be placed on the straight line D2 connecting the two points.

[0052] Therefore, by shortening the upstream lengths of the two inlet baffles 11 and 12, the entire system 1 including these inlet baffles 11 and 12 has a small size.

[0053] Figure 4 also corresponds to the embodiment of the invention in Figure 2 and shows the possible integration of an additional image sensor into System 1. Reference numeral 13 indicates a spectral separation device, such as a dichroism separator. The device 13 generates an image PF' of the focal plane PF. The additional image sensor 2' can then be positioned so that its photosensitive surface is superimposed on the image PF' of the focal plane. As an example, the additional image sensor 2' may be silicon-based and sensitive in the visible light range.

[0054] It will be understood that the present invention can reproduce the secondary aspects of the embodiments described in detail above, while still retaining at least some of the cited advantages. In particular, the imaging optical system according to the present invention can be used for applications other than those described above. In addition, any numerical values ​​mentioned are for illustrative purposes only and can be changed depending on the specific application. Those skilled in the art will understand how to adapt values ​​such as focal length, angle of view, and entrance pupil size to each application without hindrance.

Claims

1. An imaging optical system (1) comprising three mirrors, including a first mirror (M1), a second mirror (M2), and a third mirror (M3), wherein the first mirror (M1), the second mirror (M2), and the third mirror (M3) are fitted and arranged such that light rays originating from a scene located in the incident field of the system are reflected first by the first mirror, then by the second mirror, and then by the third mirror to form an image of the scene in the focal plane (PF) of the system. The light rays generated from the aforementioned scene and contributing to the formation of the aforementioned image are divided into an initial portion upstream of the first mirror (M1), a first intermediate ray portion between the first mirror and the second mirror (M2), a second intermediate ray portion between the second mirror and the third mirror (M3), and a terminal ray portion between the third mirror and the focal plane (PF). In the imaging optical system (1), the first (M1) and second (M2) mirrors are oriented such that the second intermediate portion of the paraxial ray (RP) of the system (1) intersects with the initial portion (RP 0) of the paraxial ray (RP), and the second and third mirrors (M3) are oriented such that the terminal portion of the paraxial ray (RP) does not intersect with the first intermediate portion of the paraxial ray (RP), and the terminal portion of the paraxial ray (RP) passes along the side of the second mirror opposite to the lateral displacement of the first mirror relative to the second mirror. The system (1) further comprises an image sensor (2), the photosensitive surface (S) of the image sensor is superimposed on the focal plane (PF), and the photosensitive surface is at the upstream boundary (L ＡＭ ) from the downstream boundary (L ＡＶ The image sensor is positioned to extend to the upper and lower boundaries of the photosensitive surface of the image sensor, and the upper and lower boundaries of the upper and lower boundaries are defined in relation to the projection of the upper and lower boundaries onto the initial portion (RP 0) of the paraxial ray (RP), and are defined in relation to the propagation direction of the paraxial ray (RP) in the initial portion (RP 0) of the paraxial ray (RP). The system (1) is the upstream boundary (L) of the photosensitive surface (S) of the image sensor (2). ＡＭ ) is the upstream edge (B) of the first mirror (M1) ＡＭ１ ) and the upstream edge (B) of the second mirror (M2) ＡＭ２ The second mirror and the third mirror are oriented such that they are misaligned along the direction of downstream misalignment with respect to a straight line (D 0) connecting the first mirror or the upstream edge of the screen surrounding the second mirror, and that the second mirror or the screen surrounding the second mirror blocks light rays that would propagate linearly from the first mirror to the photosensitive surface of the image sensor. Each upstream edge portion (B ＡＭ１ , B ＡＭ２ ), and downstream edge portion (B ＡＶ１ , B ＡＶ２ ) of the first mirror (M1) and the second mirror (M2) is defined in relation to the projection of each of the upstream and downstream edge portions of the first mirror and the second mirror onto the initial portion (RP0) of the paraxial ray (RP), and is defined in relation to the propagation direction of the paraxial ray (RP) in the initial portion (RP0) of the paraxial ray (RP). The upstream boundary (L) of the photosensitive surface (S) of the image sensor (2) with respect to the straight line (D 0) ＡＭ The direction of the downstream displacement of the ) is parallel to the initial portion (RP 0) of the paraxial ray (RP) and is oriented according to the propagation direction of the paraxial ray (RP) in the initial portion (RP 0) of the paraxial ray (RP), wherein the imaging optical system (1)

2. The imaging optical system (1) according to claim 1, wherein at least one of the first mirror (M1), the second mirror (M2), and the third mirror (M3) has a free-form reflective surface.

3. On the first side of the incident field, opposite to the third mirror (M3) and the same as the image sensor (2), there is a first inlet baffle (11) superimposed on the initial portion of the first field edge ray, and the first inlet baffle has a downstream edge (B) that connects to the terminal portion of the second field edge ray. ＡＶ１１ The imaging optical system (1) according to claim 1 or 2, having ).

4. The system further comprises a second inlet baffle (12) located on the second side of the incident field, opposite to the image sensor (2) and the same as the third mirror (M3), which is superimposed on the initial portion of the second field edge ray, and the second inlet baffle is located at the upstream edge (B) of the third mirror. ＡＭ３ ), the screen surrounding the third mirror, or the downstream edge (B) connected to an opaque base for the third mirror ＡＶ１２ ) has or the downstream edge of the second inlet baffle has the upstream boundary (L) of the photosensitive surface (S) of the image sensor (2). ＡＭ ) and the downstream edge (B) of the first inlet baffle (11) ＡＶ１１ ) and the straight line (D ２ The imaging optical system (1) according to claim 3, which is arranged downstream of the ).

5. The second inlet baffle (12) is located between the downstream edge (BAV11) of the first inlet baffle (11) and the downstream boundary (L) of the photosensitive surface (S) of the image sensor (2). ＡＶ ) and the straight line (D １ The upstream edge (B) located upstream of ) ＡＭ１２ The imaging optical system (1) according to claim 4, characterized by having ).

6. The vertical dimension of the image sensor (2) is such that the first field of view (α) of the system is １ ) decided, The system (1) has the first field of view (α １ An imaging optical system (1) according to any one of claims 1 to 5, which is adapted such that the angle is 9° or greater.

7. The image sensor (2) has a matrix arrangement, the horizontal dimension of the image sensor is perpendicular to the vertical dimension, and the second field of view of the system is determined. The imaging optical system (1) according to claim 6, wherein the system (1) is further adapted so that the second field of view is 12° or more.

8. The imaging optical system (1) according to any one of claims 1 to 7, having an numerical aperture N of less than 5, wherein the numerical aperture N is equal to f / D, where f is the focal length f of the system and D is the dimension of the entrance pupil of the system.

9. The imaging optical system (1) according to any one of claims 1 to 8, further comprising a pupil diaphragm, wherein the pupil diaphragm is located on the first mirror (M1) or the third mirror (M3).

10. The imaging optical system (1) according to any one of claims 1 to 9, further comprising a spectral separator (13) disposed between the third mirror (M3) and the image sensor (2), and an additional image sensor (2') disposed at the image (PF') of the focal plane (PF) of the system, wherein the image is formed by the spectral separator.

11. The imaging optical system (1) according to any one of claims 1 to 10, wherein the first (M1), second (M2), and third (M3) mirrors are contained within a sphere having a diameter of 2 to 6 times the focal length f of the system.

12. The imaging optical system (1) according to any one of claims 1 to 11, wherein at least one of the first (M1), second (M2), and third (M3) mirrors comprises a rigid portion (M2b) made of an injected polymer-based material and an arbitrary reflective metal layer (M2r).

13. An optoelectronic imaging device (100) comprising a system (1) according to any one of claims 1 to 12, wherein the optoelectronic imaging device is selected from aircraft guidance systems, thermal cameras, vision assistance devices, and optoelectronic pods for monitoring and detection.

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