Hyperspectral imaging optical system

Abstract

Disclosed is a hyperspectral imaging optical system (1) comprising: - an optical system, called the telescope (T), that is configured to collect light (L) generated by a scene to be observed scrolling in a direction A, so as to form an image, called the intermediate image, in an image focal plane (PF im,t ) of said telescope, - at least one hyperspectral detection assembly (ED) for detecting at least one portion of the light (L) collected by said telescope and comprising: - a filter assembly (EF) comprising n > 60 upstream spectral filters (R i ) placed in proximity to the image focal plane (PF im,t ) of said telescope, each upstream spectral filter being a band-pass filter in an upstream spectral band (Δλ i ) distinct from the others and being arranged so as to spectrally filter a respective sub-beam (SF i ) associated with a field of view (FOVi) of the observed scene, - an optical system called the imaging objective (OI), an object focal plane (PF o,Ol ) of said imaging objective being coincident with the image focal plane of said telescope, - a matrix-array detector (Det) configured to acquire an image of the intermediate image through the imaging objective, the imaging objective being configured to form n spectrally distinct sub-images (l i ) corresponding to said n fields of view (FOV i ) of the scene that are associated with the n sub-beams (SF i ), each of the n sub-images being formed on a sub-region (P i ) of the detector distinct from the others.

Description

DESCRIPTION Title of the invention: Hyperspectral imaging optical system Technical field:

[0001] The present invention relates to the field of hyperspectral imagers and in particular that of pushbroom type hyperspectral imagers (or with a moving linear field). Prior art:

[0002] A hyperspectral imaging system allows the acquisition of a three-dimensional image of a scene to be observed: two classic spatial dimensions and a spectral dimension which corresponds to the decomposition in the spectral domain of each point of the imaged object. For clarification, the term "hyperspectral imaging system" here means an imaging system allowing the simultaneous imaging of at least 60 distinct spectral bands. In contrast, a "multispectral imaging system" typically images ten spectral bands simultaneously or less.

[0003] There are several types of hyperspectral imaging systems that differ in the method of acquiring the different points of the scene to be observed (point by point, line by line or full field) and in the method used to decompose the light collected by the imaging system into several wavelengths (diffraction, refraction, interferometry, etc.).

[0004] Pushbroom imaging systems are most commonly used in remote imaging and airborne systems. They are typically used in situations where there is relative movement between a scene to be observed and the imaging device. This movement may be due to the device being mounted on a vehicle, aircraft, or satellite moving over an area of ​​interest, with the device oriented so that the area of ​​interest intercepts the device's field of view. Alternatively, the scrolling is performed with a scanning mirror. Alternatively, the device may be stationary and the area of ​​interest may move across the field of view, e.g. on a conveyor belt. The hyperspectral image acquired by a pushbroom imager is generally a striped image obtained by imaging a single strip (linear field) of the scene to be observed in the field of view, the relative displacement making it possible to successively obtain a plurality of continuous strips.

[0005] Figure 1A schematically illustrates a prior art pushbroom type hyperspectral imager P comprising a telescope optical system T and a spectro-imager SP. Figure 1B illustrates in more detail the various elements of a spectro-imager SP known from the prior art. Typically, the spectro-imager SP comprises a slit F placed in the image focal plane ^^ ^^ ^^, ^^of the telescope T which collects the light L from the scene to be observed Obj. The telescope T is an optical system known per se and adapted to produce the image of an object located at infinity relative to the focal length of the telescope. In a known manner, it can be made up of a plurality of off-axis parabolas and / or mirrors or even lenses.

[0006] In the example illustrated in Figure 1A, the relative displacement of the scene with respect to the imager P is done in the direction A, perpendicular to the direction of the slit F, called direction B. Thus, only a restricted portion FOV of the field of view of the scene to be observed, called the linear field, crosses the slit F to be imaged on the detector Det of the spectro-imager SP. The image of the portion of the scene observed through the slit is called the linear image.

[0007] The spectro-imager SP comprises a collimation optical system Col adapted to collimate the LF portion of the light passing through the slit F. The collimation optical system is arranged so that its object focal plane ^^ ^^^^, ^^ ^^ ^^is merged with the plane containing the slit F and the image focal plane ^^ ^^ ^^, ^^ of the T telescope. Alternatively, spectro-imagers can use concentric optical assemblies of the Dyson or Offner types in which the beam is not collimated at the level of the dispersive element.

[0008] A dispersive optical element Disp is suitable for spreading spectral information by deflecting rays according to their wavelength. This dispersive optical element can be a prism, a diffraction grating or even a 3 combination of disjointed or grouped dispersive optical elements in transmission. The dispersive element has a dispersion axis along the direction A. Thus, after passing through the dispersive element, the angle of deviation of the rays of the LF portion translates their wavelength. An imaging optical system O located downstream of the telescope on the optical path is adapted to spatially separate the different wavelengths and focus the LF portion which has passed through the dispersive optical element on a matrix detector Det placed in the image focal plane of the imaging optical system O. The image focal plane of the telescope containing the slit F is therefore the conjugate of the image focal plane of the imaging optical system O containing the matrix detector. Thus, the imaging optical system O makes it possible to produce a plurality of imagettes ^^ ^^ ^^ of the slit F corresponding to the restricted portion of the field of view FOV of the scene to be observed, each associated with a distinct spectral band.

[0009] Without slit F in the object focal plane of the collimator, the beam being angularly extended in the dispersion direction, after crossing the dispersive optical element, there would be a spatial overlap on the detector Det of the different wavelengths associated with different points of the scene. It would then be impossible to spatially translate, on the matrix detector, the hyperspectral information of the imaged scene. The slit F imaged on the detector Det therefore allows a spatial separation of the wavelengths on the detector by imaging an entire line of the scene to be observed Obj.

[0010] The matrix detector is typically arranged so that each image ^^ ^^ ^^ is oriented along an axis ^^ (“column”) of the pixel matrix of the detector, the optical imaging system being adapted so that the imagettes ^^ ^^ ^^are aligned along an axis ^^ ("line"), perpendicular to the axis ^^. Thus, in the absence of distortion, each column of the detector is exposed to radiation corresponding to the same linear field of view but at different wavelengths. Each line of the detector records the spectrum of each point in the imaged field of view FOV.

[0011] Hyperspectral instruments known to those skilled in the art and based on dispersive systems have constraints in terms of signal-to-noise ratio, flexibility of the chosen bands, and stray light. It is not For example, it is not possible to optimize the signal-to-noise ratio of a particular spectral band compared to others because all spectral bands are separated and detected in the same way.

[0012] The invention aims to overcome certain problems of the prior art by developing a hyperspectral imaging optical system comprising more than 60 spectral filters arranged in the image focal plane of the telescope observing a moving scene. Thus, the system of the invention, via the modularity of the filters, allows a very good signal-to-noise ratio in all spectral bands, total flexibility of the chosen bands and controlled stray light. Summary of the invention:

[0013] For this purpose, an object of the invention is a hyperspectral imaging optical system comprising: - an optical system called a telescope adapted to collect light from a scene to be observed moving along a direction A so as to form a so-called intermediate image in an image focal plane of said telescope, - at least one hyperspectral detection assembly for at least a portion of the light collected by said telescope and comprising: - a filter assembly comprising ^^ > 60 upstream spectral filters arranged near the image focal plane of said telescope, each upstream spectral filter being band-pass to an upstream spectral band distinct from the others and being arranged so as to spectrally filter a respective sub-beam associated with a field of view of the observed scene, - an optical system called an imaging objective,an object focal plane of said imaging lens being merged with the image focal plane of said telescope - a matrix detector adapted to acquire an image of the intermediate image by the imaging lens, - the imaging lens being configured to form ^^ spectrally distinct sub-images corresponding to said ^^ fields of view of the, scene associated with the ^^ sub-beams, each of the ^^ sub-images being formed on a sub-region of the detector distinct from the others.

[0014] According to a particular embodiment, the upstream spectral filters and said sub-regions extend in a main direction ^^ substantially perpendicular to an optical axis of the imaging objective and substantially perpendicular to said direction A.

[0015] According to a particular embodiment, a so-called lateral dimension of each upstream spectral filter along the direction ^^ is adapted so as to obtain a signal-to-noise ratio greater than a predetermined value in the sub-image associated with said upstream spectral filter.

[0016] According to a particular embodiment, each of the sub-regions comprises a number of pixels along the direction A determined as a function, on the one hand, of a magnification of said imaging objective and, on the other hand, of a so-called lateral dimension along the direction ^^ of the upstream spectral filter filtering said spectral band.

[0017] According to a particular embodiment, the telescope has an aperture number greater than 5, preferably greater than 7, even more preferably greater than 10. Preferably, a so-called lateral dimension of each upstream spectral filter in the direction ^^ is greater than 50 ^^ ^^, preferably greater than 80 ^^ ^^.

[0018] According to a particular embodiment, the magnification of the imaging objective ^^ is determined as a function of the ground resolution ^^ ^^ ^^ predetermined according to the following formula: ^^ ^^ ^^ = , with ^^ the distance between the scene and the system, ^^ ^^ the image focal length of the telescope T, ^^ the pixel pitch of the detector in direction A.

[0019] According to a particular embodiment, the imaging lens has a magnification of less than 1 in order to obtain a ground resolution of approximately 20 ^^ for said system.

[0020] According to a particular embodiment, the upstream spectral filters form a single-piece structure, the upstream spectral filters being bonded to each other.

[0021] According to a particular embodiment, the upstream spectral filters are separated by a distance of less than 25 ^^ ^^, preferably less than 10 ^^ ^^.

[0022] According to a particular embodiment, the upstream spectral filters are attached or placed on a plate transparent to said light.

[0023] According to a particular embodiment, the system comprises ^^ > 100 upstream spectral filters, preferably ^^ > 200 upstream spectral filters so that said filter assembly transmits a spectral band called total spectral band ranging from 400 ^^ ^^ to 2500 ^^ ^^.

[0024] According to a particular embodiment, the hyperspectral detection assembly further comprises ^^ downstream spectral filters located near the detector and bandpass to a downstream spectral band distinct from the others, a downstream spectral filter being arranged so as to filter a sub-beam associated with an upstream spectral filter, a downstream spectral band of a downstream spectral filter having a non-zero overlap with the upstream spectral band of the associated upstream spectral filter.

[0025] According to a particular embodiment, the system comprises a plurality ^^ > 1 of hyperspectral detection assemblies each adapted to transmit and detect a spectral band called total spectral band different from the others, said system comprising a dichroic splitter arranged on the optical path of the light to spatially separate said light collected by the telescope into ^^ beams having different spectral ranges and to direct each of said ^^ beams towards a hyperspectral detection assembly adapted to detect said spectral range of said beam. Brief description of the figures:

[0026] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0027] [Fig.1A] and [Fig.1B] a schematic view of a prior art pushbroom type hysperspectral imaging optical system;

[0028] [Fig.2A], [Fig.2B] and [Fig.2C] a schematic view of a hyperspectral imaging optical system of the invention;

[0029] [Fig.3] a schematic view of the upstream spectral filters of a hyperspectral imaging optical system of the invention;

[0030] [Fig.4] and [Fig.5] a schematic view of respectively an embodiment of upstream spectral filters of the hyperspectral imaging optical system of the invention.

[0031] [Fig.6] and [Fig.7] a schematic view of a hyperspectral imaging optical system according to embodiments of the invention.

[0032] In the figures, unless otherwise indicated, the elements are not to scale. Detailed description:

[0033] Figures 2A and 2B show a schematic view of a pushbroom type hyperspectral imaging optical system 1 according to the invention. Figure 2A illustrates the arrangement of the system 1 with respect to the scene to be observed Obj. Figure 2B is a more detailed schematic representation of the different elements of the system 1 according to a plane ^^ ^^.

[0034] As in the pushbroom devices of the prior art, the system 1 comprises an optical system called a telescope T adapted to collect light L from the scene to be observed Obj. The telescope T is a catadioptric system known per se adapted to produce the image of an object located at infinity relative to the focal length of the system in the image focal plane ^^ ^^^^ ^^ , ^^of the telescope. In a known manner, it may consist of a plurality of off-axis parabolas and / or mirrors or even lenses. As an illustrative example, in the embodiment of FIG. 2B, the telescope comprises two off-axis parabolas and a mirror.

[0035] To acquire a hyperspectral image of the scene Obj, the system 1 comprises a hyperspectral detection assembly ED of at least a portion of the light L collected by the telescope of the system 1. Figure 2C is a schematic representation according to the plane ^^ ^^ which details more precisely the ED hyperspectral detection set which includes in particular an OI imaging objective and a Det matrix detector.

[0036] The image of the scene formed by the telescope is called the intermediate image ^^ ^^ because the hyperspectral detection ED assembly of system 1 includes an OI imaging objective with an object focal plane ^^ ^^ ^^, ^^ ^^ confused with the image focal plane ^^ ^^^^ ^^ , ^^of the telescope. The imaging objective OI produces the image of this intermediate image ^^ ^^ on the Det detector. The Det matrix detector is a CCD, CMOS sensor or any matrix detector known from the prior art.

[0037] As an illustrative example, in the embodiment illustrated in Figures 2B and 2C, the imaging lens comprises two distinct optical groups, a first group O1 producing the image of the intermediate image, called the first image, at infinity and a second group O2 producing the image of the first image in the image focal plane of the second group ^^ ^^^^ ^^, ^^ ^^. Alternatively, according to another embodiment, the imaging lens comprises a different number of lens groups.

[0038] In the example illustrated in Figure 2A, the relative movement of the scene to be observed with respect to system 1 is in direction A. For example, system 1 may be on board a vehicle or aircraft moving in such a way that the scene to be observed Obj is fixed and intercepts the field of view ^^ ^^ ^^ of the telescope T. Alternatively, system 1 may be fixed and the scene to be observed Obj may move across the field of view, for example on a conveyor belt. Subsequently, the terms "direction A" and "scrolling direction" are considered to be equivalent.

[0039] The system 1 of the invention differs from the hyperspectral pushbroom devices of the prior art at least in that the ED assembly comprises an EF filter assembly enabling hyperspectral operation of the system 1. More precisely, the EF filter assembly comprises ^^ > 60 upstream spectral filters ^^1… ^^ ^^ arranged near the image focal plane ^^ ^^ ^^ ^^, ^^of the telescope. Preferably, as illustrated in Figures 2B and 2C, the upstream spectral filters are arranged in the image focal point ^^ ^^ ^^ ^^, ^^ of the telescope in order to reduce the vignetting produced by each filter. Unlike multispectral devices of the prior art comprising dispersive elements (see figures 1A-1B), the system 1 of the invention does not comprise a slit F in the image focal plane of the telescope.

[0040] Each upstream spectral filter ^^ ^^ is bandpass to an upstream spectral band Δ ^^ ^^ distinct from the others and is arranged to spectrally filter a sub-beam ^^ ^^ ^^ respective associated with a field of view FOV i of the observed scene. Thus, the sub-beam ^^ ^^ ^^ presents spectral information only on the spectral band Δ ^^ ^^. Considering the optical design of system 1, the OI imaging lens is suitable for forming ^^ sub-images ^^ ^^ spectrally distinct corresponding respectively to the ^^ fields of view ^^ ^^ ^^ ^^ of the scene associated with the ^^ under beams ^^ ^^ ^^ . Each of the ^^ sub-images ^^ ^^ is formed on a sub-region ^^ ^^ of the detector distinct from the others. In each sub-image ^^ ^^ , only a restricted portion of the scene is observed (corresponding to the field of view FOV i of each sub-image). The entire FOV field of view i associated with each sub-beam ^^ ^^ ^^ corresponds to the field of view of the telescope FOV. The dimension of each sub-image ^^ ^^ depends on the filter size ^^ ^^ associated and the magnification of the imaging lens. The dimension along the A direction of each sub-region ^^ ^^ is adapted according to the dimension of the sub-image ^^^^ associated. As is known, this dimension can also be adapted according to the time averaging necessary in order to adjust the signal-to-noise ratio of the spectral band associated with the sub-region ^^ ^^ (see further). Furthermore, according to the invention, as will be explained further, the dimension of the filters ^^ ^^ according to direction A (and the dimension of the sub-regions ^^ ^^ associated along direction A) is not necessarily identical for each filter. Indeed, these dimensions can be different to optimize the signal-to-noise ratio (SNR) in the different bands Δ ^^ ^^ .

[0041] Given the small field of view imaged in the sub-images, it is necessary to acquire several of them in order to reconstruct a relevant hyperspectral image of the scene Obj. Also, it is necessary to acquire a plurality ^^ > 1 of sets of sub-images ^^( ^^) = ^^1( ^^) … ,each of the sub-image sets ^^( ^^) = ^^1( ^^) … ^^ ^^ ( ^^) corresponding to a portion of the scene different from the others and imaged by the system at indexed times ^^ ∈ [1, ^^] different from the others when scrolling the scene in direction A. The different sub-images ^^ ^^ ( ^^) are then juxtaposed to create so-called final images

[0042] In Figure 2C, to facilitate understanding of the diagram, three upstream spectral filters ^^1, ^^ are shown ^^ , ^^ ^^ (and the fields of view ^^ ^^ ^^1, ^^ ^^ ^^ ^^ , ^^ ^^ ^^ ^^ and the sub-beams ^^ ^^1, ^^ ^^ ^^ , ^^ ^^ ^^ respectively associated) although the device of the invention comprises more than 60 upstream spectral filters.

[0043] The use of ^^ > 60 filters in the invention rather than a dispersive element as in the devices of the prior art has many advantages thanks to the modularity that the filters allow. In particular, the filters allow the signal-to-noise ratio (SNR) to be optimized in all the upstream spectral bands separately and allows total flexibility of the upstream spectral bands chosen (see below). Conversely, the use of a dispersive element requires the use of the same matrix detector over the entire spectrum because each line of the detector records the spectrum of each point in the imaged field of view.

[0044] In addition, the filters of the invention make it possible to reduce parasitic noise. Indeed, performing spectral filtering in the image focal plane of the telescope greatly reduces the noise associated with diffuse reflections caused by the passage of the imaging lens and detected by the detector because each sub-beam is spectrally filtered before passing through the imaging lens. This is notably different from the arrangement implemented in certain multispectral devices of the prior art in which a small number (typically less than 10) of spectral filters is located near the matrix detector.

[0045] Preferably, the set of EF filters comprises ^^ > 100 upstream spectral filters, preferably ^^ > 200 upstream spectral filters so that the set of EF filters transmits a spectral band called total spectral band ranging from 400 ^^ ^^ to 2500 ^^ ^^. Thus, the system 1 of the invention makes it possible to carry out hyperspectral imaging. covering the visible range up to the SWIR range (for Short Wavelength Infrared in English).

[0046] In state-of-the-art hyperspectral devices, telescopes have a low aperture number ^^ ( ^^ -number in English, also noted ^^ / # ) – typically less than 3 – in order to maximize the signal-to-noise ratio of the sub-images. Indeed, a low aperture number allows a greater flux to be collected. For the record, the aperture number ^^ ^^ of the telescope T is worth ^^ ^^ = ^^ ^^ / ^^ ^^ , with ^^ ^^the image focal length of the telescope and ^^ ^^ the diameter of the entrance pupil of the telescope T. After many tests and simulations, the inventors realized that, in the invention, the use of a number ^^ > 60 filters near the plane ^^ ^^ ^^ ^^, ^^ with a telescope having an aperture number less than 5 produced a beam in the image focal plane which was of too large a transverse dimension and induced too much vignetting in the sub-images. In order to overcome this problem, the inventors determined a preferred embodiment, denoted MP, in which the telescope T of the invention has an aperture number ^^ ^^ greater than 5, preferably greater than 7 and even more preferably greater than 10.

[0047] Thus, the collected light L is focused by the telescope T into a sufficiently thin beam FF in the image focal plane ^^ ^^ ^^ ^^, ^^to ensure optimal filter performance. This makes it possible, for example, to limit crosstalk between filters. The use of a telescope T with a high aperture number as in the invention is particularly counterintuitive because it reduces the flux collected by the telescope and therefore leads to a reduction in the SNR. However, the invention makes it possible to compensate for this reduction in flux by time averaging or by filters with suitable lateral dimensions (see below), which was not possible in prior art devices using dispersive elements.

[0048] Preferably, in the MP embodiment, the lateral dimension ℎ ^^ of each upstream spectral filter ^^ ^^ is greater than 50 ^^ ^^, preferably greater than 80 ^^ ^^ still preferably greater than 90 ^^ ^^ in order to reduce the vignetting of the FF beam by the filters upstream spectral. Even more preferentially, the lateral dimension ℎ ^^ of each upstream spectral filter ^^ ^^ is between 90 ^^ ^^ and 750 ^^ ^^, this lateral dimension ℎ ^^ being preferably chosen according to the power spectral density of the light L in the upstream spectral band Δλ ^^ filtered by the upstream spectral filter ^^ ^^ . Indeed, it is possible to compensate for a low power spectral density in the upstream spectral band Δλ ^^ by a lateral dimension ℎ ^^ relatively larger (see below).

[0049] Preferably, the imaging objective has a magnification G adapted according to the aperture number of the telescope T in order to obtain a predetermined ground resolution in the system of the invention. Indeed, in the device of the invention, it is demonstrated that the ground resolution (also called GSD for Ground Sampling Distance in English) is given by the following formula: ^^ ^^ ^ ^^× ^^

[0050] ^ = ^^ ^^ × ^^

[0051] with ^^ the distance between the scene and system 1 (see figure 2A), ^^ ^^ the image focal length of the telescope T, ^^ the pixel pitch of the detector in direction A and ^^ the magnification of the imaging lens.

[0052] For example, according to a variant of the MP embodiment, the magnification G of the imaging lens is less than 1 to compensate for the aperture number ^^ ^^relatively high so as to obtain a predetermined ground resolution in the system of the invention. According to a preferred variant of the embodiment MP, the imaging objective has a magnification of less than 1 in order to obtain a ground resolution of approximately 20 ^^ in the system 1. For example, for an aperture number ^^ ^^ > 7 of the telescope T, we have a magnification ^^ < 0.8 of the imaging objective. According to a specific example, ^^ ^^ = 9.6 and ^^ = 0.73.

[0053] The possibility of optimizing the GSD from the magnification of the relay optics is an option that was not possible in state-of-the-art devices using dispersive elements (see Figures 1A and 1B) in which the ground GSD resolution is given by the following relationship: ^^ ^^ ^^ = ^^ × ^^ ^^ / ^^ ^^ with ^^ the distance between the scene and the device, ^^ ^^ the image focal length of the telescope T and ^^ ^^the transverse dimension of the slot F (along direction A).

[0054] Figure 3 schematically illustrates only the EF filter assembly and the matrix detector Det and illustrates how the adaptation of the filters and the detector allows an adaptation of the SNR associated with each upstream spectral band Δλ ^^ . As an illustrative example, in Figure 3, the upstream spectral filters are all arranged in the image focal plane ^^ ^^ ^^ ^^, ^^ of the telescope. In addition, the shape of the upstream spectral filters is rectangular and they extend along the main direction ^^ substantially perpendicular to the optical axis AO OI of the imaging objective OI and substantially perpendicular to the direction A. Similarly, to adapt the detection, each sub-region ^^ ^^extends along the main direction ^^. Alternatively, according to an embodiment different from that illustrated in Figure 3, the upstream spectral filters are square, elliptical or even circular in shape.

[0055] According to a first embodiment, this SNR optimization is carried out via the lateral dimension ℎ ^^ of each upstream spectral filter ^^ ^^ according to the direction ^^. Indeed, in order to improve the signal to noise ratio in an upstream spectral band Δλ ^^ given it is possible to increase the lateral dimension ℎ ^^ of the upstream spectral filter ^^ ^^ transmitting this spectral band because it allows the transmission of the sub-beam ^^ ^^ ^^ with a higher photon flux in this spectral band. Also, in the first embodiment, the lateral dimension ℎ ^^is adapted so as to obtain a signal-to-noise ratio greater than a predetermined value in the sub-image ^^ ^^ associated with the filter ^^ ^^ . This embodiment is particularly interesting when the power spectral density of light L in an upstream spectral band Δλ ^^ filtered by the upstream spectral filter ^^ ^^ is low. This may be the case, for example, when the user of the device requires a spectral resolution (i.e. a bandwidth Δλ ^^ ) low, for example 10 ^^ ^^, in a given spectral band for a specific use.

[0056] As a non-limiting example, the filters ^^1… ^^ ^^ transmit spectral bands Δλ1… Δλ ^^ adjacent with a central frequency increasing from filter ^^1 to filter ^^ ^^. Moreover, as illustrated in the example of Figure 3, the lateral dimension of the filters is decreasing from the first filter ^^1 to the filter ^^ ^^ for example because light L has a decreasing power spectral density from the Δλ1 band to the Δλ band ^^ .

[0057] According to a second embodiment, the improvement of the SNR in the upstream spectral band Δλ ^^ is performed by time averaging in the sub-region ^^ ^^ associated with Δλ ^^ . Indeed, in order to improve the signal to noise ratio in a Δλ band ^^ , it is possible to perform an average according to the direction A of sub-images ^^ ^^ ( ^^), each being obtained in the sub-region ^^ ^^ , at a different instant ^^. This temporal averaging of the sub-images ^^ ^^ ( ^^), in the region ^^ ^^ is made possible by the number of pixel lines along the A direction in the region ^^^^ and allows you to obtain a sub-image ^^ ^^ with a better signal-to-noise ratio. We recall that the dimension of a sub-image ^^ ^^ on the detector depends on the lateral dimension ℎ ^^ of the associated filter and the magnification of the imaging objective OI. Also, according to the second embodiment, to optimize the signal-to-noise ratio in the upstream spectral band Δλ ^^ , the number of pixels along the direction A of the sub-region ^^ ^^ associated with this band depends on the magnification of the imaging objective OI and depends on the lateral dimension ℎ ^^ of the upstream spectral filter ^^ ^^ transmitting the Δλ band ^^ . Just like the first embodiment, the second embodiment makes it possible to improve the SNR for example when the power spectral density of the light L in an upstream spectral band Δλ ^^ transmitted by the upstream spectral filter ^^ ^^is low. It is understood that the number of lines depending on the direction may be different between the sub-regions ^^ ^^ according to the specificities of the Δλ bands ^^ associated.

[0058] As a non-limiting example, in the example of figure 3, the sub-regions ^^ ^^ and ^^ ^^ associated with filters ^^ ^^ and ^^ ^^ respectively present more lines in direction A than the sub-region ^^1 associated with the filter ^^1 in order to allow temporal averaging of the sub-images ^^ ^^ ( ^^) and ^^ ^^ ( ^^) and thus improve the SNR in the Δλ spectral bands ^^ and Δλ ^^ . This mode of realization is advantageous when these bands Δλ ^^ and Δλ ^^ are more absorbed by the atmosphere than the Δλ band ^^ For example.

[0059] The first and second embodiments allow, via the use of filters, a great modularity of the device 1 according to the type of illumination and the imaged scene by optimizing the SNR on each of the spectral bands. This constitutes an advantage compared to the hyperspectral devices of the pushbroom type of the prior art using dispersive elements in which the detection of the different spectral bands could not be optimized separately.

[0060] Figure 4 illustrates an embodiment in which the upstream spectral filters form a single-piece structure. That is, the upstream regions ^^ ^^ are stacked on top of each other in direction A. The manufacturing process of these upstream spectral filters is thus simplified. The regions ^^ ^^can for example be reported or stuck together. Preferably, in the embodiment of figure 4, the upstream spectral filters are separated by a distance of less than 25 ^^ ^^, preferably less than 10 ^^ ^^ which is due to the presence of glue between the filters.

[0061] Figure 5 illustrates an embodiment in which the upstream spectral filters are attached or joined to a transparent plate LT to the light L collected by the telescope. By transparent, we mean here a transmission greater than 99%. The advantage of this embodiment is to allow the upstream spectral filters to be deposited directly on the plate LT and therefore to manufacture the EF assembly in a single step.

[0062] Figure 6 schematically illustrates an embodiment of the invention, in which the hyperspectral detection ED assembly further comprises ^^ downstream spectral filters ^^ 2 ^^, located near the detector and bandpass to a downstream spectral band Δ ^^ 2 ^^ distinct from the others. Each downstream spectral filter ^^ 2 ^^ is arranged to filter a sub-beam ^^ ^^ ^^ associated with an upstream spectral filter ^^ ^^ . In addition, a downstream spectral band Δ ^^ 2 ^^ of a downstream spectral filter has a non-zero overlap with the upstream spectral band Δ ^^ ^^ of the associated upstream spectral filter. Thus, the upstream spectral filter ^^ ^^ and downstream ^^ 2 ^^ associated are bandpass to a common spectral portion of the collected light. In this way, the sub-image ^^ ^^ East representative of the spectral information of the observed scene associated with this field of view and this common spectral portion.

[0063] By using upstream spectral filters combined with downstream spectral filters, the noise associated with diffuse reflections caused by the passage of the imaging lens and detected by the detector is significantly reduced.

[0064] In addition, the intensity of the portion of light passing through the imaging lens and reflected by each downstream spectral filter ^^ 2 ^^ towards the imaging lens then presents a spectrum such as: |Δ ^^ ^^ − Δ ^^ 2 ^^|. Depending on how the downstream and upstream spectral filters are chosen, the spectral bandwidth of the portion reflected by each downstream spectral filter can be very small or even zero if the associated upstream and downstream spectral bands are identical. Critically, even if the spectral bandwidth of the reflected portion is not zero, when the portion reflected by each downstream spectral filter reflects off the various components of the imaging lens, it will be spectrally filtered by the other regions ^^ of the EF filter assembly and will therefore not be detected by the Det detector. Indeed, the regions ^^ ≠ ^^ are each bandpass at a distinct downstream bandwidth of the spectral band |Δ ^^ ^^ − Δ ^^ 2 ^^ | of the reflected portion. Thanks to the upstream spectral filters, the contribution of these rays to the stray light with respect to the sub-image associated with the region ^^ ≠ ^^is greatly reduced.

[0065] Figure 7 schematically illustrates an embodiment of the invention in which the system 1 comprises a plurality ^^ > 1 of hyperspectral detection assemblies ED, ED' each adapted to transmit and detect a so-called total spectral band different from the others. In the embodiment of Figure 7, the system comprises a dichroic splitter MD - for example one or more dichroic mirrors - arranged on the optical path of the beam FF to separate it spatially into ^^ beams having different spectral ranges and to direct each of the ^^ beams towards a hyperspectral detection assembly adapted to detect the spectral range of this beam. As an illustrative example, in the embodiment illustrated in Figure 7, the system 1 comprises two paths separated by a plate or a dichroic mirror MD and two hyperspectral detection assemblies ED, ED' The embodiment of Figure 7 makes it possible to maximize the detection efficiency of the detector Det, Det' of each hyperspectral detection set ED, ED'. Indeed, the detector of the hyperspectral detection set is optimized specifically for the spectral range of the beam directed in this set.

Claims

AMENDED CLAIMS received by the International Bureau on May 24, 2024 (05.24.2024) 1. Hyperspectral imaging optical system (1) comprising: an optical system called a telescope (T) having an aperture number greater than 5 and adapted to collect light (L) from a scene to be observed moving in a direction A so as to form a so-called intermediate image in an image focal plane (PF im t ) of said telescope, at least one set (ED) for hyperspectral detection of at least a portion of the light (L) collected by said telescope and comprising: a set of upstream spectral filters (EF) (R i ) arranged near the image focal plane (PF im t ) of said telescope, each upstream spectral filter being band-pass to an upstream spectral band (Δλ i ) distinct from the others and being arranged so as to spectrally filter a sub-beam (SF i) respective associated with a field of view (FOVi) of the observed scene, an optical system called imaging objective (Ol), an object focal plane (PF o ,OI ) of said imaging lens being merged with the image focal plane of said telescope a matrix detector (Det) adapted to acquire an image of the intermediate image by the imaging lens, the imaging lens being configured to form n sub-images ( / i ) spectrally distinct corresponding to said n fields of view (FOV) i ) of the scene associated with the n sub-beams (SF i ), each of the n sub-images being formed on a sub-region (P i ) of the detector distinct from the others.

2. System according to the preceding claim, in which said upstream spectral filters and said sub-regions extend in a main direction z substantially perpendicular to an optical axis (AO oI ) of the imaging lens (01) and substantially perpendicular to said direction A.

3. System according to the preceding claim, in which a so-called lateral dimension (h i ) of each upstream spectral filter (R i ) along direction A is adapted so as to obtain a signal-to-noise ratio greater than a predetermined value in the sub-image ( / i ) associated with said upstream spectral filter. AMENDED SHEET (ARTICLE 19) 4. System according to claim 2 or 3, in which each of the sub-regions (P i ) comprises a number of pixels in direction A determined as a function, on the one hand, of a magnification of said imaging objective (Ol) and, on the other hand, of a so-called lateral dimension (h i ) according to the direction A of the upstream spectral filter filtering said spectral band.

5. System according to any one of the preceding claims, in which said telescope has an aperture number greater than 7, even more preferably greater than 10.

6. System according to the preceding claim, in which a so-called lateral dimension of each upstream spectral filter in direction A is greater than 50 μm, preferably greater than 80 μm.

7. System according to any one of the preceding claims, in which the magnification of the imaging objective G is determined as a function of the predetermined ground resolution GSD according to the following formula: with H the distance between the scene and the system, f T the image focal length of the telescope T, p the pixel pitch of the detector in direction A.

8. A system according to any preceding claim, wherein said imaging lens has a magnification of less than 1 in order to obtain a ground resolution of approximately 20 m for said system.

9. System according to any one of the preceding claims, in which the upstream spectral filters form a single-piece structure, the upstream spectral filters being bonded to each other. AMENDED SHEET (ARTICLE 19) 10. System according to the preceding claim, in which the upstream spectral filters are separated by a distance of less than 25 μm, preferably less than 10 μm. 1 1. System according to any one of claims 1 to 8, in which the upstream spectral filters are attached or attached to a transparent plate (LT) to said light (L).

12. System according to any one of the preceding claims, comprising n > 100 upstream spectral filters, preferably n > 200 upstream spectral filters so that said filter assembly (EF) transmits a spectral band called total spectral band ranging from 400 nm to 2500 nm.

13. System according to any one of the preceding claims, in which the hyperspectral detection assembly further comprises n downstream spectral filters (R 2i ), located near the detector and bandpass to a downstream spectral band (Δλ 2i ) distinct from the others, a downstream spectral filter being arranged so as to filter a sub-beam (SF i ) associated with an upstream spectral filter, a downstream spectral band (Δλ 2i ) of a downstream spectral filter having a non-zero overlap with the upstream spectral band (Δλ 2i ) of the associated upstream spectral filter.

14. System according to any one of the preceding claims, comprising a plurality m > 1 of hyperspectral detection assemblies (ED, ED') each adapted to transmit and detect a spectral band called total spectral band different from the others, said system comprising a dichroic splitter (MD) arranged on the optical path of the light to spatially separate said light collected by the telescope into m beams having different spectral ranges and to direct each of said m beams towards a hyperspectral detection assembly adapted to detect said spectral range of said beam. AMENDED SHEET (ARTICLE 19)

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