POLARIMETRIC CAMERA

Abstract

The invention relates to a polarimetric camera (100) comprising an optical system (200) and an image sensor (300). The optical system (i) forms, on the image sensor, N images of a scene to be imaged, with N greater than or equal to 2, N being a number of polarization states, (ii) comprises a polarization splitter (220) deflecting the light beams from the scene according to the N polarization states, and (iii) has N output pupils (231). The image sensor comprises a plurality of pixels (P) distributed into N subsets of pixels. Each subset is associated with an output pupil and receives the incident light beams according to the polarization state of the associated output pupil. The image sensor comprises N angular filters (320), each angular filter transmitting the light beams from the associated output pupil to a subset of pixels and filtering out the other light beams. Figure for the abbreviation: Figure 1

Description

Title of the invention: POLARIMETRIC CAMERA technical field

[0001] This description relates generally to imaging systems, and more specifically to so-called polarimetric cameras, adapted to record, for a given scene, information relating to the polarization of the captured light. PRIOR TECHNOLOGY

[0002] Measuring the polarization information of light during image acquisition can be of interest for numerous applications. In particular, it enables the implementation of image enhancement processes, tailored to the specific application. For example, it makes it possible to attenuate or, conversely, enhance reflections on an image from any surface that causes specular reflection, such as glass, water, or the surface of an eye. Furthermore, it makes it possible to detect manufactured objects in a natural environment, as these objects generally exhibit a polarization signature.Among the applications that can benefit from polarization information measurement, we can also mention industrial control applications, biomedical applications, for example applications for the detection of cancer cells (the latter polarize light due to their fibrous nature), contrast enhancement applications for image capture in scattering media (fog, underwater imaging, etc.), or even distance mapping or depth image acquisition applications, in which polarization can provide information on the orientation of the surface of manufactured objects, and thus help 3D reconstruction in addition to another modality such as active illumination by structured light or by time-of-flight measurement.

[0003] To extract relevant information from a given scene using a polarimetric camera, a combination of different polarization states is required. For this, the polarimetric camera (which typically comprises an optical system and an image sensor) must be able to acquire these different polarization states simultaneously and from the same viewpoint so that the images can be perfectly superimposed. The polarizations are therefore separated at the level of the polarimetric camera itself. Two configurations are possible.

[0004] In a first configuration, the polarization separation occurs at the level of the image sensor of the polarimetric camera. The image sensor, having a detection pixel matrix, is then configured so that each of its pixels is sensitive to only one polarization state, for example by means of polarizing filters. Pixels sensitive to different polarization states are interlaced according to an elementary pattern. For this configuration, the optical system used is a standard optical system, and a single image is formed by it on the image sensor.

[0005] One of the limitations of this first configuration is related to the interlacing of these pixels. This implies the implementation of polarizing filters at the pixel scale, and therefore of very small dimensions, which leads to a reduction in performance. Another limitation lies in the fact that, since the pixels sensitive to different polarization states are adjacent in the pixel matrix, polarization crosstalk can appear in the images.

[0006] In a second configuration, the polarization separation occurs at the level of the polarimetric camera's optical system. In this configuration, the optical system is configured to form, on the image sensor, one image per polarization state, each image being spatially offset from the others. The optical system includes, for example, a polarization splitter (also called a polarization router) for this purpose. For this configuration, the image sensor used is a standard image sensor.

[0007] The articles “Imaging polarimetry through metasurface polarization gratings” by Rubin et al., Optics Express, March 2022, vol. 30, no. 6, p. 9389-9412, and “Matrix Fourier optics enables a compact full-Stokes polarization camera” by Rubin et al., Science 365, 2019, eaaxl839 DOI: 10.1126 / science.aaxl839, describe the realization of metasurface-type polarization separators to be placed in the optical system, these metasurfaces making it possible to form, on the image sensor, four images corresponding to four different polarization states.

[0008] One of the limitations of this second configuration is related to image overlap. An image corresponding to one polarization state will overlap, at least partially, the image corresponding to another polarization state. It is possible to use a vignetting screen to limit this overlap; however, this solution leads to a decrease in brightness at the edges of the image.

[0009] Another limitation, related to the use of metasurfaces, is the presence, around the optical axis of the optical system, of an unpolarized image corresponding to the zero-order diffraction of these metasurfaces. Although these metasurfaces are configured to limit the intensity of this zero-order image, it is nevertheless not possible to eliminate it completely. Description of the invention

[0010] The present invention aims to remedy at least in part the drawbacks of the solutions proposed by the prior art, in particular those set out above.

[0011] To this end, the object of the invention is a polarimetric camera comprising an optical system and an image sensor: • the optical system, having a principal optical axis, being adapted to form on the image sensor at least N spatially distinct images of a scene to be imaged, at a rate of one image per polarization state, with N greater than or equal to 2, N being a predefined number of polarization states, and comprising: • a polarization splitter, adapted to deflect the incident light beams from the scene to be imaged according to the N polarization states, the optical system then presenting at least N exit pupils at least partially offset two by two orthogonally to the main optical axis; • the image sensor, comprising a plurality of detection pixels, each including a photodetector, • the detection pixels being distributed into at least N subsets of pixels, each associated with an exit pupil, and each intended to receive the incident light beams according to the polarization state of the associated exit pupil.

[0012] According to the invention, the image sensor comprises at least N angular filters located between the optical system and the photodetectors, each angular filter being adapted to transmit to a subset of pixels the incident light beams from the associated exit pupil, and to filter at least partially the incident light beams from the other exit pupil(s).

[0013] Such a polarimetric camera advantageously improves the quality of the N images associated with the N polarization states, in particular through the filtering, by angular filters, of at least part of the parasitic light beams which are not associated with the desired polarization state.

[0014] Moreover, such a polarimetric camera advantageously allows, thanks to the association of angular filters with the polarization separator, the use of a standard image sensor.

[0015] According to particular embodiments, the polarimetric camera may include the following characteristics, implemented separately or in each of their technically operative combinations.

[0016] According to particular embodiments, the N angular filters are arranged coplanarly.

[0017] According to particular embodiments, the N angular filters are formed in the same grid having apertures sized to transmit to a subset of pixels the incident light beams coming from the exit pupil associated, and to filter at least partially the incident light beams coming from the other exit pupils.

[0018] According to particular embodiments, the image sensor comprises a plurality of microlenses, adapted to focus the incident light beams on the photodetectors of the detection pixels, located between the optical system and the angular filters.

[0019] According to particular embodiments, the polarization separator is a two-dimensional metasurface.

[0020] According to particular embodiments, the image sensor comprises N polarizing filters located between the optical system and the angular filters, adapted to transmit to a subset of pixels the incident light beams from the associated exit pupil and having the associated polarization state, and to filter at least in part the incident light beams from the other exit pupils and therefore having other polarization states.

[0021] According to particular embodiments, the N polarizing filters are arranged coplanarly.

[0022] According to particular embodiments, the N polarizing filters (340) are made in the same metallic structure. Brief description of the drawings

[0023] Other aspects, objects, advantages and features of the invention will become more apparent upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0024] [Fig-1] is a schematic and partial view, in perspective and exploded view, of an example of a polarimetric camera according to one embodiment;

[0025] [Fig.2A] is an example of an optical diagram of a polarimetric camera which does not include an angular filter at the detection pixel level;

[0026] [Fig.2B] is an optical diagram of a polarimetric camera according to one embodiment;

[0027] [Fig.3A] illustrates an example of angular response of a detection pixel that is not associated with an angular filter, and that of a detection pixel associated with an angular filter according to an embodiment;

[0028] [Fig.3B] is a schematic and partial cross-sectional view of a detection pixel of an image sensor of the polarimetric camera according to one embodiment.

[0029] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0030] In the figures and in the rest of the description, the same reference numerals represent identical or similar elements. Furthermore, the different elements are not The figures are shown to scale to ensure clarity. Furthermore, the various embodiments and variants are not mutually exclusive and may be combined. Unless otherwise specified, the terms "approximately," "around," and "in the order of" mean within 10%, and preferably within 5%. Moreover, the terms "between ... and ..." and equivalents mean that the limits are inclusive, unless otherwise stated.

[0031] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the photodetectors and the electronic control circuits for the image sensors of the polarimetric cameras described have not been detailed, as the described embodiments are compatible with conventional embodiments of these elements.

[0032] Furthermore, when reference is made to absolute positional qualifiers, such as the terms "top", "bottom", "left", "right", etc., or relative positional qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientational qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0033] Figure 1 illustrates a schematic and partial perspective view of a polarimetric camera 100 according to an embodiment of the invention. The polarimetric camera 100 comprises at a minimum: - an optical system 200, adapted to form on an image sensor 300 at least N spatially distinct images of a scene to be imaged, at a rate of one image per polarization state, with N greater than or equal to 2, N being a predefined number of polarization states, and having at least N exit pupils 231, one per polarization state, at least partly offset two by two orthogonally to a principal optical axis A; - an image sensor 300, comprising a plurality of detection pixels P, which are distributed into at least N subsets of pixels, each associated with an output pupil 231, and each intended to receive the incident light beams according to the polarization state of the associated output pupil.

[0034] The optical system 200 includes a polarization splitter 220, adapted to separate the incident light beams from the scene to be imaged according to the N predefined polarization states. N is at least equal to 2 (at least 2 separate polarization states), but can be equal to 3, 4, or even more. The optical system 200 preferably includes a collimation optic 210 located upstream of the polarization splitter 220, and an imaging optic 230 located downstream of the polarization splitter 220.

[0035] By spatially distinct image, it is understood that the N images formed in the image plane of the optical system 200 are not entirely superimposed two by two in the image plane, but that they are at least partially offset in pairs within the image plane, and preferably completely offset (without overlap) within the image plane. Also, the exit pupils 231 are at least partially offset in pairs orthogonally to the principal optical axis A, and preferably completely offset (without overlap) orthogonally to the principal optical axis A.

[0036] According to the invention, the image sensor 300 comprises at least N angular filters 320 located between the optical system 200 and the photodetectors 303 of the image sensor 300, each angular filter 320 being adapted to transmit to a subset of pixels the incident light beams from the associated output pupil 231, and to filter at least in part the incident light beams from the other output pupil(s) 231.

[0037] Preferably, the optical system 200 comprises a collimation optic 210. Said collimation optic is disposed, along the principal optical axis A, upstream of the polarization separator 220, in the direction of propagation of the incident light beams, preferably perpendicular to the principal optical axis A.

[0038] The collimating optics 210, for example a lens, is adapted to collect the incident light beams from the scene to be imaged and to collimate them towards the polarization splitter 220. Thus, the incident light beams arriving at the polarization splitter 220 are parallel to each other.

[0039] The use of such collimation optics 210, upstream of the polarization splitter 220, optimizes the operation of the polarization splitter 220, particularly when dealing with a two-dimensional metasurface. In such a case, the polarization splitter 220 (metasurface) is located in the plane of the entrance pupil 211. Thus, all light rays from the scene pass through the entrance pupil and cover the entire metasurface. This results in a metasurface with a minimal size, the design of which is simplified. If the metasurface is located upstream or downstream of the entrance pupil 211, a larger metasurface is required, and its design may be more complex.

[0040] The collimating optics 210 here define an entrance pupil 211 of the optical system 200. By entrance pupil, we mean an image of the aperture diaphragm of the optical system 200, seen from the side of the scene to be imaged. This can be the opaque edge of the lens 210.

[0041] Note that, in the case of a more complex optical system 200, this system may include diaphragms arranged to create an entrance pupil 211 in a plane accessible for placing the polarization splitter 220, and to create an exit pupil plane that allows the exit pupils 231 to be spatially separated from each other. Note that the polarization splitter 220 (metasurface) may be one of these diaphragms.

[0042] The polarization splitter 220 is arranged on the main optical axis A of the optical system 200, preferably perpendicular to the main optical axis A. It implements an optical polarization routing (sorting) function, i.e. a function of deflection of the incident light beams according to their polarization state, the light beams being then transmitted to the imaging optics 230.

[0043] In other words, the polarization splitter 220 is configured to separate the incident light beams according to their polarization state and direct them onto spatially distinct areas of the imaging optics 230, as illustrated in [Fig. 1]. These spatially distinct areas of the imaging optics 230 define the exit pupils 231 of the optical system 200. As previously stated, the exit pupils 231 are therefore at least partially offset in pairs orthogonally to the principal optical axis A.

[0044] Note that the exit pupils 231 can obviously be located elsewhere than in the imaging optics 230, particularly when the optical system 200 is more complex than two lenses. In this case, the exit pupils 231 can then be located upstream of the imaging optics 230, and the entrance pupil 211 may not be confused with the lens 210.

[0045] By exit pupil, we mean an image of the aperture diaphragm by the optical system 200 seen from the side of the image sensor 300. It is a spatially delimited area through which light passes to reach the image sensor 300.

[0046] In this embodiment, N is equal to 4. Also, the polarization separator 220 is adapted to route the incident light beams according to four different polarization states, denoted: PSI, PS2, PS3, PS4. There are thus at least 4 exit pupils 231, each exit pupil 231 being arranged to receive incident light beams according to one of the four polarization states PSI, PS2, PS3, PS4.

[0047] For example, the polarization splitter 220 is adapted to deflect incident light beams along four different polarization orientations, for example linear polarizations along four directions respectively forming angles of 0°, 90°, +45°, -45° with respect to a reference direction.

[0048] In the case where the polarization splitter 220 is a two-dimensional metasurface, the optical system 200 may include a fifth exit pupil 231, which is associated with the zero-order diffraction of the metasurface. This exit pupil is not shown in [Fig. 1], but is represented in [Fig. 2A] and [Fig. 2B]. The image of the object point A (located on the principal optical axis A) is denoted A'DOo (DO0, for Diffraction Order 0) and is also located on the principal optical axis A.

[0049] As described above, in a preferred embodiment, the polarization splitter 220 is a two-dimensional metasurface, as illustrated in Figures 1, 2A, and 2B. The two-dimensional metasurface comprises a two-dimensional array of portions 221 of a first material, laterally surrounded by a filling material with a different refractive index. Preferably, the first material has a higher refractive index than the filling material. The portions 221 of the two-dimensional metasurface have sub-wavelength lateral dimensions, i.e., the largest lateral dimension of each portion 221 is smaller than the principal wavelength intended to be measured by the underlying detection pixel P (e.g., the wavelength for which the quantum efficiency of pixel P is maximal).For example, for P pixels intended to measure visible or near-infrared radiation, for example radiation with a wavelength less than 1 pm, the largest dimension of each portion is between 1 pm and 500 nm (here, half the maximum wavelength of the detection spectral band, here 1 pm), for example between 80 nm and 300 nm.

[0050] The portions 221 of the two-dimensional metasurface 220 preferably have varied lateral dimensions. The dimensions and arrangement of the portions 221 are defined according to the desired optical function. Thus, to achieve the polarization routing function, portions 221 may have asymmetrical shapes in top view, for example, rectangular or elliptical, it being understood that the portions 221 may have any shape in top view. The portions 221 may have vertical edges, oblique edges, or stepped edges, comprising at least one step. Furthermore, each portion 221 may be made of a single material or a stack of layers of different materials.The pattern of the two-dimensional metasurface can be defined using an electromagnetic simulation tool, for example by using inverse design methods, for example of the type described in the article entitled "Phase-to-pattem inverse design paradigm for fast realization of functional metasurfaces via transfer leaming" by Zhu, R., Qiu, T., Wang, J. et al. Nat. Commun. 12, 2974 (2021), or in the article entitled "Matrix Fourier optics enables a compact full-Stokes polarization camera" by Rubin et al., SCIENCE, Vol. 365, Issue 6448 - 5 July 2019.

[0051] The portions 221 of the two-dimensional metasurface preferably all have the same height, for example of the same order of magnitude as the principal wavelength intended to be measured by each detection pixel P, for example between 20 nm and 2 pm, preferably between 100 nm and 1000 nm, for radiation with a wavelength less than 1 pm. The use of portions 221 of constant height makes it possible to simplify the fabrication of the two-dimensional metasurface.

[0052] In the example illustrated in [Fig. 1], the polarization splitter 220 comprises a single two-dimensional metasurface. Alternatively, the polarization splitter of the optical system could comprise several two-dimensional metasurfaces, arranged coplanarly or not.

[0053] In another embodiment of the polarization splitter, said polarization splitter may be a Wollaston prism. The Wollaston prism is adapted to separate incident light beams into two orthogonal polarization states. The Wollaston prism generally consists of two prisms made of birefringent materials placed side by side, for example calcite or quartz, whose optical axes are orthogonal to each other. The advantage of using a Wollaston prism is the absence of a zero-order diffraction and therefore the absence, around the optical axis of the optical system, of an unpolarized image corresponding to this zero-order diffraction.

[0054] The polarization splitter 220 may comprise a single Wollaston prism. Alternatively, the polarization splitter 220 of the optical system 200 may comprise several Wollaston prisms, each prism separating the incident light beams into two different orthogonal polarization states.

[0055] In a preferred embodiment, particularly illustrated in the figures, the imaging optics 230 is a focusing lens adapted to focus the incident light beams onto the plane of the image sensor 300. The imaging optics 230 is configured, on the one hand, to receive the incident light beams spatially distributed according to their polarization state and, on the other hand, to focus these incident light beams according to the N polarization states onto the corresponding N zones of the image sensor 300, thus forming N images of the scene on said image sensor, the N images being spatially offset from one another, as illustrated in [Fig. 1]. It should be noted that an additional image, associated with the zero-order diffraction of the two-dimensional metasurface 220, can also be formed on the image sensor 300. Here, the exit pupils 231 are located at the level of the imaging optics 230.

[0056] Figure 2A illustrates an example of an optical diagram of a polarimetric camera 100, similar to that of the invention, but which would not include angular filters 320 described below. In this example, the polarization splitter 220 is a two-dimensional metasurface.

[0057] A point A is placed in the object plane of the optical system 200, on the principal optical axis A. Light beams from point A are collected by the optics collimation 210, which transmits them, collimated (parallel to each other and to the main optical axis A), towards the polarization separator 220.

[0058] The polarization splitter 220 deflects the light beams according to, in this case, two different polarization states P1 and PS2. Note that the light beams associated with the zero-order diffraction of the polarization splitter 220 are not deflected. The light beams associated with the polarization states P1 and PS2 are thus directed towards the imaging optics 230 in pairs, offset from the principal optical axis A (above and below the principal optical axis A, according to the orientation of [Fig. 2A]), at a distance from the principal optical axis A. This results in an exit pupil 23lPS1 associated with the P1 state, an exit pupil 231PS2 associated with the PS2 state, and an exit pupil associated with the zero-order diffraction. These exit pupils are offset in pairs orthogonally to the principal optical axis A, and therefore do not completely overlap.

[0059] The imaging optics 230 then focuses the incident light beams onto the image plane of the optical system 200, into distinct areas of the image sensor 300, thus forming several distinct images. The image of point A associated with the PSI state is denoted A'PSI, and the image of point A associated with the PS2 state is denoted A'PS2. The image of point A associated with the zero-order diffraction of the polarization splitter 220 is also denoted A'DOo. The optical axes associated with these three output pupils are distinct from one another, and the three image points are also spatially distinct from one another. This yields two distinct images associated with the two polarization states PSI and PS2, and one image associated with the zero-order diffraction that is also distinct from the other two.

[0060] However, it can be seen that the image B' of a point B in the scene, located off the optical axis and not shown in Figures 2A and 2B, associated with zero diffraction order, can be detected by the detection pixel that receives the image point A'PS1. Similarly, the image B” of another point in the scene, associated with zero diffraction order, can be detected by the detection pixel that receives the image point A'PS2. Furthermore, it is also possible that an image point associated with the PS2 polarization state is detected by the subset of pixels associated with the PSI polarization state, and vice versa.

[0061] Fig. 2B illustrates an example of an optical diagram of a polarimetric camera 100 according to one embodiment, which includes angular filters 320. In this example also, the polarization splitter 220 is a two-dimensional metasurface.

[0062] As stated above, at least N angular filters 320 are located between the optical system 200 and the photodetectors 303 of the image sensor 300. Each angular filter 320 is adapted to transmit the beams to a subset of pixels incident light beams from the associated exit pupil 231, and to filter at least partially incident light beams from the other exit pupil(s) 231.

[0063] Thus, as shown in the figure, the detection pixels associated with the exit pupil 23 lPSi transmit light beams exhibiting the polarization state PSI, but filter at least partially those associated with the polarization state PS2, as well as those associated with the zero-order diffraction. Similarly, the detection pixels associated with the exit pupil 231PS2 transmit light beams exhibiting the polarization state PS2, but filter at least partially those associated with the polarization state PSI, as well as those associated with the zero-order diffraction.

[0064] This results in a polarimetric camera with improved performance, since the quality of the two images associated with the polarization states PS1 and PS2 is improved by filtering, at least partially, the light beams that can be described as parasitic, i.e., those not associated with the desired polarization state. Furthermore, the combination of the angular filters 320 with the polarization splitter 220 allows the use of standard photodetectors.

[0065] Figure 3A illustrates an example of the angular response of a detection pixel P of the image sensor 300 of the polarimetric camera 100 according to one embodiment. This is the amplitude of the measurement signal of the detection pixel, for a given incident luminous flux, as a function of the angle of incidence of the detected light beams.

[0066] We consider here a detection pixel dedicated to the polarization state PSI. Light beams associated with the polarization state PS2, which form parasitic beams, are likely to reach this detection pixel.

[0067] A solid line represents the angular response of a detection pixel of a polarimetric camera similar to that of [Fig.2A], i.e. without angular filter 320, and a dashed line represents that of a detection pixel of a polarimetric camera according to an embodiment of the invention, i.e. with angular filter 320.

[0068] The light beams associated with the PSI polarization state are incident on the detection pixel within an angular cone denoted A0PSi. The integral of the response I over the angular amplitude A0PSi contributes to forming the measurement signal.

[0069] However, it is noted that light beams associated with the PS2 polarization state are also incident on the detection pixel within an angular cone denoted A0PS2. In the case of a polarimetric camera without an angular filter 320, these light beams also contribute to forming the measurement signal, which degrades the performance of the polarimetric camera.

[0070] On the other hand, the angular response of a detection pixel associated with an angular filter (dashed line) shows reduced sensitivity to light beams located outside of The angular amplitude A0PSi. In other words, the angular filter filters out light beams that are not associated with the intended polarization state (and therefore do not originate from the exit pupil associated with the detection pixel). The performance of the polarimetric camera according to the invention is thus improved.

[0071] Returning to [Fig. 1]. The image sensor 300 is located in the image plane of the optical system 200. It comprises a plurality of detection pixels P. Each detection pixel P comprises a photodetector 303. As previously stated, the detection pixels are grouped into subsets, each subset being associated with an exit pupil. Thus, there are at least N subsets of detection pixels, for N exit pupils associated with the N polarization states. A subset of detection pixels, centered on the principal optical axis A, can be associated with the exit pupil linked to the zero-order diffraction of the polarization splitter 220 (in the case of a two-dimensional metasurface).

[0072] Fig. 3B is a schematic and partial cross-sectional view of an image sensor 300 of a polarimetric camera according to one embodiment.

[0073] The photodetectors 303 of the detection pixels are formed in a semiconductor substrate 310. The semiconductor substrate 310 is made for example of a single-crystal semiconductor material, for example silicon.

[0074] Trenches or isolation walls 302, extending vertically within the semiconductor substrate 301, laterally separate, electrically and / or optically, the photodetectors 303 of the detection pixels from each other, so as to limit crosstalk. The isolation walls 302 are, for example, made of a dielectric material, for example silicon oxide.

[0075] In the example of [Fig.3B], the image sensor 100 is a backside illumination sensor, also called a BSI sensor (from the English "Back Side Illumination"), that is to say that the light beams from the scene to be imaged illuminate the semiconductor substrate from one side, called the back side 304. The back side 304 of the semiconductor substrate 310 of the image sensor is arranged in the image plane of the optical system 200.

[0076] The image sensor 300 further includes, on the side of the front face 305, corresponding to the lower face of the semiconductor substrate 101, a stack (not shown in the figures) of insulating and conductive layers (for example metallic), commonly called an interconnect stack, in which interconnecting elements of the detection pixels P of the image sensor 300 are formed.

[0077] It is clear that the embodiments described also apply to front side illumination sensors or FSI sensors (from the English "Front Side Illumination"). "), that is, sensors in which the semiconductor substrate is intended to be illuminated by its face in contact with the interconnect stack.

[0078] The detection pixels P of the image sensor 300 are preferably arranged in a matrix of rows and columns, as illustrated in [Fig. 1]. The detection pixels P are preferably all identical, within manufacturing variations, or similar.

[0079] The detection pixels P of the image sensor are divided into N subsets of pixels. Each subset of detection pixels P comprises a set of adjacent pixels. The N subsets of pixels are distinct from one another. Thus, each detection pixel P belongs to only one subset.

[0080] In the example of [Fig. 1], the detection pixels P are distributed into four subsets of pixels. Each detection pixel subset P is intended to receive incident light beams, coming from its associated output pupil, according to one of the four polarization states P1, PS2, PS3, PS4.

[0081] Furthermore, the image sensor 300 also includes N angular filters 320. They are located between the optical system 200 and the photodetectors 303. As previously stated, each angular filter 320 is adapted to transmit to a subset of pixels the incident light beams from the associated output pupil 231, and to filter at least partially the incident light beams from the other output pupil(s) 231.

[0082] Each angular filter 320 is therefore associated respectively with a subset of detection pixels and with the associated output pupil 231. Thus, in the example of [Fig. 1] where the polarization splitter 220 ensures the deflection of the light beams according to the four polarization states P1, PS2, PS3 and PS4, the polarization state P1 is associated with the output pupil (denoted here 231PS1) then with the angular filter (denoted here 320psi) and with the subset of detection pixels (denoted here EPPS1), the polarization state PS2 is associated with the output pupil 231PS2 then with the angular filter 320PS2 and with the subset EPPs2 of detection pixels, and so on.

[0083] In the example of [Fig.3B], the angular filters 320 are opaque structures comprising through-openings 321, each through-opening 321 being arranged opposite the photodetector 303 of a detection pixel P. The transverse dimensions of the through-openings 321 are defined to allow the transmission of light beams from the associated exit pupil, and to filter at least in part those from the other exit pupils.

[0084] By way of example, opaque structures are made of a metallic material, such as aluminum, copper, or tungsten, among others. Preferably, the N angular filters are arranged coplanarly. Preferably, the N 320° angular filters are made within the same opaque structure.

[0085] Thus, as shown in [Fig.3B], the light beams from the associated exit pupil (solid lines) are transmitted through the angular filter 320, while those from the other exit pupils (dotted lines) are at least partially filtered, and here completely filtered.

[0086] In one embodiment, the image sensor 300 may include a plurality of microlenses 330. The microlenses 330 are preferably arranged between the optical system 200 and the angular filters 320. They ensure focusing of the incident light beams onto the detection pixel associated with the microlens 330. In the case where the image sensor 300 does not include microlenses 330, it may include an angular filter similar to that of the filters 320 to provide an angular selection function similar to that of the microlenses 330.

[0087] In one embodiment, but not a limiting one, and as illustrated in [Fig. 1], each microlens 330 extends, in dimension, opposite a pixel. In other words, there are as many microlenses as there are pixels in the image sensor. Note that the microlenses 330 can be offset transversely to the opposite pixel to take into account the principal angle at which the light beams intended to be captured by that pixel arrive. The microlenses 330, the polarizing filters 340, and the through-holes 321 of the angular filters 320 are then offset according to the value of this angle of incidence.

[0088] In one embodiment, as illustrated in [Fig.3B], the image sensor may include, in addition to the N angular filters, N polarizing filters 340. The N polarizing filters 340 are arranged here between the microlenses 330 and the angular filters 320, but they could be located between the angular filters 320 and the photodetectors 303.

[0089] Each polarizing filter 340 is associated respectively with a subset of pixels, and therefore with an exit pupil 231 of the optical system 200.

[0090] Each polarizing filter 340 is adapted to transmit incident light beams according to a predefined polarization state and to block incident light beams according to other polarization states.

[0091] Thus, the image sensor 300 comprises N polarizing filters 340 each having different polarization orientations and are thus each adapted to transmit only the incident light beams according to a given polarization state, more particularly the incident light beams coming respectively from the associated exit pupil.

[0092] In a preferred embodiment, the polarizing filters 240 are metallic structures comprising through-holes and transmitting predominantly the incident light beams according to a predefined polarization state, that is to say the incident light beams coming only from the associated exit pupil, and absorbing or reflecting the radiation according to the other polarization states.

[0093] By way of non-limiting example, the metallic structures of the polarizing filters are made of aluminum, tungsten, or copper. Preferably, the N polarizing filters are arranged coplanarly. In one embodiment, the N polarizing filters are made within the same metallic structure.

[0094] A filling layer 350 can be arranged between the angular filters 320 and the polarizing filters 340. Similarly, a filling layer 360 can be arranged between the polarizing filters 340 and the microlenses 330.

[0095] Alternatively, the angular filters 320 and the polarizing filters 340 can be made in the same layer: the opaque part of the layer 320 remains unchanged, and the polarizing filters 340 are located in the through openings 321.

[0096] The filling layer(s) 350, 360 can, for example, be made of a dielectric material, transparent to the wavelengths to be detected, such as silicon oxide, silicon nitride, alumina, or tantalum oxide.

[0097] Similarly, the openings in the metallic structures of the angular filters 320 and the polarizing filters 340 can be filled with a dielectric material, transparent to the wavelengths to be detected, such as silicon oxide, silicon nitride, alumina, or tantalum oxide, among others.

[0098] Alternatively, the openings in the metal structures of the angular filters and polarizing filters can be left empty or filled with air.

[0099] The foregoing description clearly illustrates that, through its various features and their advantages, the present invention achieves the stated objectives. In particular, the present invention provides a polarimetric camera that makes it possible to block the stray incident light beams related to image overlap on the image sensor and those from zero order generated by the polarization splitter, when the latter is a two-dimensional metasurface.

Claims

1.

2.

3. Demands Polarimetric camera (100), comprising an optical system (200) and an image sensor (300): the optical system (200), having a principal optical axis (A), being adapted to form on the image sensor (300) at least N spatially distinct images of a scene to be imaged, at a rate of one image per polarization state, with N greater than or equal to 2, N being a predefined number of polarization states, and comprising: a polarization splitter (220), adapted to deflect the incident light beams from the scene to be imaged according to the N polarization states, the optical system then presenting at least N exit pupils (231) at least partially offset two by two orthogonally to the principal optical axis (A); the image sensor (300), comprising a plurality of detection pixels (P) each comprising a photodetector (303), the detection pixels (P) being distributed into at least N subsets of pixels, each associated with an exit pupil (231), and each intended to receive the incident light beams according to the polarization state of the exit pupil associated; characterized in that the image sensor (300) comprises at least N angular filters (320) located between the optical system (200) and the photodetectors (303), each angular filter (320) being adapted to transmit to a subset of pixels the incident light beams from the associated exit pupil (231), and to filter at least in part the incident light beams from the other exit pupil(s) (231). Polarimetric camera (100) according to claim 1, in which the N angular filters (320) are arranged coplanarly. Polarimetric camera (100) according to claim 1 or 2, in in which the N angular filters (320) are formed in the same grid having openings sized to transmit to a subset of pixels the incident light beams from the associated exit pupil, and to filter at least partially the incident light beams from the other exit pupils.

4. Polarimetric camera (100) according to claim 3, wherein the grid is opaque.

5. Polarimetric camera (100) according to any one of the preceding claims, wherein the image sensor (300) comprises a plurality of microlenses (330), adapted to focus the incident light beams onto the photodetectors of the detection pixels (P), located between the optical system (200) and the angular filters (320).

6. Polarimetric camera (100) according to any one of the preceding claims, wherein the polarization separator (220) is a two-dimensional metasurface.

7. Polarimetric camera (100) according to any one of the preceding claims, wherein the image sensor (300) comprises N polarizing filters (340) located between the optical system (200) and the angular filters (320), adapted to transmit to a subset of pixels the incident light beams from the associated exit pupil and having the associated polarization state, and to filter at least in part the incident light beams from the other exit pupils and therefore having other polarization states.

8. Polarimetric camera (100) according to claim 7, wherein the N polarizing filters (340) are arranged coplanarly.

9. Polarimetric camera (100) according to any one of claims 7 or 8, wherein the N polarizing filters (340) are made in the same metallic structure.

10. Polarimetric camera (100) according to any one of the preceding claims, wherein the pixels of a subset of pixels are adjacent.

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