EP2092282A1

EP2092282A1 - Polarimetric imaging system having a matrix of programmable waveplates based on a material with an isotropic electrooptic tensor

Info

Publication number: EP2092282A1
Application number: EP07821380A
Authority: EP
Inventors: Jérôme BOURDERIONNET; Daniel Dolfi; Sébastien Breugnot
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2006-10-20
Filing date: 2007-10-16
Publication date: 2009-08-26
Also published as: US20100039646A1; FR2907547A1; WO2008046820A1; FR2907547B1

Abstract

The subject of the invention is a polarimetric imaging system having an optical axis and comprising means (35) for detecting and analysing the light backscattered by an object illuminated by a light source and at least one programmable waveplate (33), characterized in that the programmable waveplate comprises a material with an isotropic electrooptic tensor and a set of at least three electrodes placed along directions parallel to the optical axis of the imaging system.

Description

Polarimetric imaging system with programmable material-based waveguide matrix with an isotropic electro-optical tensor

The invention relates to a polarimetric imaging system whose function is to measure the polarization state spatial distribution of light from a scene illuminated by natural light (passive imaging) or by a laser beam (active imaging). The main component of the polarimetric imaging system of the invention is a programmable analysis means of an incident polarization distribution. Such a system effects the projection of a spatial distribution of stokes vectors incident on an arbitrary Stokes vector. By successive measurements (for example from 2 to 4), the system thus gives access to an image coded in polarization (Stokes parameters) of the observed scene. This feature is of great importance for a wide range of applications including decoupling, scene analysis, eg for autonomous driving of unmanned land vehicles, or surface condition analysis.

In general, the measurement of the state of polarization of the light retro-diffused by an object informs on the one hand on the nature of this object, and on the other hand makes it possible to improve the contrast between the object and its environment when a single intensity image is insufficient. The state of polarization of the light is conventionally represented by a vector with 4 components, called Stokes vector:

where I is the total intensity, Q is the component linearly polarized horizontally or vertically, U is the component linearly polarized at 45 ° or 135 ° with respect to the horizontal, and V is the circularly polarized component (right or left). The different effects of the object on the polarization state of the backscattered light are described by the Mueller matrix M associated with the object: S _or t = M x Si _n where S _1n is the state of polarization from the source illumination and S _out the polarization state of the backscattered light. The 16 coefficients m _y of this 4x4 matrix depend on the intrinsic parameters of the object (roughness, nature ...) and the experimental context (angles of incidence and observation, wavelength ...). Different architectures of active or passive type polarimeters then exist, depending on the effect or effects that one seeks to highlight and the number of coefficients of the Mueller matrix that one wishes to measure.

FIG. 1 illustrates an example of an active polarimeter comprising: an illumination source 14, a control means 15 for the state of polarization of this source S, _n , a means of analysis 12 of the state of polarization S _out retro-diffused light 17 from an object 11 illuminated by said illumination source 14 and a detection means 13. Such a system allows the complete characterization of the Mueller matrix by emitting 4 chosen polarization states, and measuring the associated Stokes parameters (Configuration 1: 16 measurements for 16 coefficients). Moreover, it has been shown (S. Breugnot, P. Clemenceau, "Modeling and performance of a polarization active imager at λ = 806 nm", Optical Engineering, Vol.39, No. 10, 2681 -2688, Oct 2000) when the directions of incidence and observation are substantially identical ("single-static" architecture), and for common objects, the Mueller matrix can be considered as diagonal, with coefficients moo, m-π, 1TI ₂₂ and 1TI ₃₃ , mu being the degree of horizontal linear polarization, m ₂₂ the degree of linear polarization at 45 °, and m ₃₃ the degree of circular polarization. The total degree of polarization is in this case defined by (mπ + m ₂ 2 + m3 ₃ ) / 3moo- Depending on the number of these parameters that one wishes to measure, 2, 3 or 4 measurements may be required.

With a passive polarimeter as illustrated in FIG. 2, the illumination source is the fully depolarized ambient light (S _1n = (I, 0.0, 0)). The system then comprises: a means 22 for analyzing the backscattered light 24 coming from an object 21, and a detection means 23. By measuring the polarization state S _or t of the backscattered light 24, the capacity of the object to polarize a depolarized light (1 ^st column of the Mueller matrix, m _O o, m ₁₀ , m ₂ o and IT i ₃₀ ) is measured. For example, such a measure will perfectly discriminate water (polarizing) in a scene. IT) ₁₀ denotes the ability to polarize in a linear polarization horizontal or vertical, m ₂ o the ability to polarize in a linear polarization at 45 ° or 135 °, and ITi ₃₀ the ability to polarize in a circular polarization. Here again, depending on the number of these parameters that one wishes to measure, 2, 3 or 4 measurements will be necessary. The different systems proposed in the state of the art differ in the means of analysis of the retro-diffused polarization, which can be fixed or programmable, in the number of sensors used and in the nature of the information collected, more or less directly related to the coefficients to be calculated. With fixed analysis means, for n measurements to be made, n images of the scene to be characterized on n sensors, through n combinations (analyzer + delay plate) or (analyzer only) fixed. The n images are obtained in various ways (combinations of prisms, holographic gratings, lenses ...) - JD Barter, PHY Lee, "Visible Stokes polarimetric imager", US Patent 6,122,404 (2000), MSShahriar, et al, "Ultrafast Stokesmeter holography for polarization imaging in real time ", Opt.Letters, vol 29, No. 3, 298-300 (2004), GR Gerhart, RM Matchko," Simultaneous 4-Stokes parameterization using a single digital image ", US Patent 2005/0225761 (2005)). The disadvantages of the system (complexity, size, cost, etc.) come from the multiplication of imaging devices and sensors. There is however a system that divides the image into 4 images on a single sensor, but this operation is done to the detriment of the resolution and accuracy of the measurement (error due to the imperfection of the optical divider system, the 4 images must be strictly identical). An alternative is proposed in the literature (K. Oka, T. Kaneko, "Comprehensive Compact Imaging Polarimeter Using Birefringent Wedge Prisms", Opt.Express, Vol.11, No.13, 1510-1519 (2003)) which uses the association of birefringent prisms followed by an analyzer to form on a single sensor an interferogram which allows after a Fourier transform calculation to go back to the components of the incident Stokes vector. In this case, the disadvantage is the relative complexity of this calculation. Finally, a disadvantage common to all systems with fixed analysis means is that one can not choose to measure only one or another component of the Stokes vector according to their relevance. With a programmable analysis means, the n measurements necessary for the n parameters to be measured are performed on a single sensor, but successively. A standard device (programmable waveguide and fixed analyzer) then sequentially performs the n combinations of analyzes required. Conventionally, the programmable waveguide function is performed by mechanical rotation of a wave plate or by means of a polarization controller based on liquid crystals (P.CIémenceau, S. Breugnot, L. Collot, "Apparatus for Polarimetric Imaging, Patent 2,769,980 (1997)). These, however, pose problems of response time (10 to 100ms for the nematic liquid crystals) which can disturb the measurement in the case of variations between successive acquisitions. The response time of the component is also significantly important in the case where the system must be mounted on a mobile platform (drone type, with travel speeds of the order of 50km / h). Indeed, the acquisition of the images must be fast to avoid the so-called "registration" errors, ie the movement of the scene between each image. Ferroelectric liquid crystals are faster, but do not perform all the required functions. Finally, the lithium niobate-based polarization controllers have a very good response time, but the low electro-optical coefficient of this material implies an architecture in integrated optics, which is hardly compatible with the imaging application.

The aim of the invention is to overcome the problems mentioned above by proposing a polarimetric imaging system having an optical axis, and comprising means for detecting and analyzing light that is backscattered by an object illuminated by a light source and at the same time. least one programmable waveguide characterized in that the programmable waveguide comprises a material with an isotropic electro-optical tensor and a set of at least three electrodes arranged in directions parallel to the optical axis of the optical system. imaging.

According to a variant of the invention, the detection and analysis means are located in the same image plane.

According to a variant of the invention, the polarimetric imaging system comprises at least one set of at least two lenses for defining a first intermediate focal plane, the programmable wave plate being located in said first focal plane.

According to a variant of the invention, the electrodes are formed by through holes, substantially cylindrical and metallized, pierced in the thickness of the material with an isotropic electro-optical tensor.

According to a variant of the invention, the focal length f of the first lens corresponds to the following equation: where O _h m is the angular acceptance of the programmable waveplate and at the surface defined by the intersection of the half-angle cone at the vertex θ | _im with the plane of the first lens.

According to a variant of the invention, the polarimetric imaging system comprises at least:

A set of two micro-lens arrays for defining a second intermediate focal plane,

A matrix of programmable wave blades located in said second intermediate focal plane.

According to a variant of the invention, the programmable waveguide matrix (83) comprises an array of electrodes, four contact tracks so as to contact all the electrodes, a first (94) and a second one (92). ) tracks being located on a first face of the material, a third (91) and a fourth (93) track being located on a face opposite to said first face, said electrodes having coordinates denoted by a line number j and a number of column k which are integers in a coordinate system corresponding to the plane of the matrix, said first track (94) connecting the electrodes whose coordinates (j, k) correspond to the following equation: k = 4pi-j, where pi is a relative integer, said second track (92) connecting the electrodes whose coordinates

(j, k) correspond to the following equation: k = (4p2 + 2) - j, with P2 relative integer, said third track (91) connecting the electrodes whose coordinates (j, k) correspond to the following equation: k = (4p ₃ +1) + j, with p ₃ relative integer, said fourth track (93) connecting the electrodes whose coordinates (j, k) correspond to the following equation: k = (4p ₄ + 3) + j, with p ₄ relative integer.

According to a variant of the invention, the detection and analysis means comprise the measurement of the components _S0 ,, _n , Si _iin , S ₂ , m t t S3 ,, _n of a Stokes vector of the light backscattered by the object (31), said measurement comprising:

A series of N sets of three steps, allowing N intensity measurements, said steps being, with 1 ≤ j <N:

o The choice of a birefringence ε _j of the wave plate

(33,53) and an orientation θ _j of this birefringence with respect to a predefined axis.

o The determination of the potentials V ₁ to be applied to the electrodes E ₁ (41, 42, 43, 44, 61, 62, 63, 64) so as to obtain the birefringence ε, of orientation θ _j determined in the previous step, said potentials V, corresponding to the following equations: with i integer between 0 and 3, λ the wavelength, no the index of the zero-field material, R the quadratic electro-optical coefficient, e the thickness of the material and the distance between two electrodes facing each other.

o The measurement of the intensity I _j on the sensor.

- The calculation of the values s _o , ιn, s _{1 iin} , s ₂ , _ιn and s _3iin from the N intensity measurements I ₁ , ..., I _j , ..., I _N. The advantages of the proposed structure over the other programmable analysis means (opto-mechanical devices, liquid crystal modulators) result from the properties of the materials with isotropic electro-optical tensor and in particular the electro-optical ceramics involved. These materials have indeed a very fast intrinsic response time (less than a microsecond), the dielectric constant of the material then giving the effective response time of the device (RC filter) of the order of a few microseconds. Moreover, these materials have a broad transmission band (transparent from 500 nm to 7000 nm (G. Haertiing, CE Land, "Hot-pressed (Pb, La) (Zr, Ti) O 3 ferroelectric ceramics for electrooptic applications", J. Am.Ceram.Soc., 51, 1 (1971), KK Li, et al., "Electro-optic ceramic material and devices, PMN-PT", US Patent Application 10/139857, 5/6/02; KK Li, and al, "Electro-optic ceramic material and devices, PZN-PT", US Patent Application 6746618 (2004)) with a substantially constant electro-optic coefficient, it follows that, unlike liquid crystal type solutions, the proposed solution covers a spectral band that extends from 500 to 7000nm, which also allows a better compactness, low control voltages and a great flexibility in the choice of Stokes parameters to be measured.

The invention will be better understood and other advantages will become apparent on reading the detailed description and with the help of the figures among which: FIG. 1 illustrates the operation of an active polarimeter according to the known art. FIG. 2 illustrates the operation of a passive polarimeter according to the known art.

FIG. 3 illustrates a first variant of a polarimetric imaging system according to the invention.

FIG. 4 illustrates an example of a programmable waveguide that can be used in the first variant of a polarimetric imaging system according to the invention.

FIG. 5 illustrates a second variant of a polarimetric imaging system according to the invention. FIG. 6 illustrates an example of a programmable waveguide that can be used in the second variant of a polarimetric imaging system according to the invention.

FIG. 7 illustrates the example of wave plate in the second variant of a polarimetric imaging system according to the invention, seen in section.

FIG. 8 illustrates a third variant of a polarimetric imaging system according to the invention.

FIG. 9 illustrates an example of a programmable waveguide matrix that can be used in the third variant of a polarimetric imaging system according to the invention.

FIG. 10 illustrates a detailed view of the programmable waveguide array in the third variant of a polarimetric imaging system according to the invention.

A first variant of a polarimetric imaging system according to the invention is illustrated in FIG.

The first variant of a polarimetric imaging system according to the invention comprises an objective 32, a programmable waveguide 33, a detection means 35 and a polarizer 34 located between said programmable waveguide and said detection means 35. The polarizer projects the polarization along its (fixed) axis. The combination of the waveguide and polarizer allows projection of the polarization along any axis. The system measures the state of polarization of the light backscattered by an object 31, illuminated by a light source. The programmable wave plate 33 comprises in particular a block of a material with an isotropic electro-optical tensor, and comprising a set of at least three electrodes. An exemplary embodiment of such a programmable waveguide is illustrated in FIG. 4. The programmable waveguide 33 is in the form of a block of parallelepipedic electro-optical material 45 on whose edges there are four electrodes E, referenced 44, 41, 42 and 43 in FIG.

The programmable waveguide 33 consists of a single pixel whose useful area (space between the electrodes in which the applied electric field is substantially uniform) corresponds to the size of the sensor. We then consider an incident light of stokes vector distribution S _in (x, y) of components s _o , in (x, y), S ₁ J _n (X ₁ Y), s ₂ , in (x, y) and s _3ii n (x, y). The programmable analysis function of Sj _{n is performed} by applying to the electrodes E ₁ the potentials Vj. The potentials V, satisfy the following equation with i between 0 and 3: with λ the wavelength, n ₀ the index of the zero-field material, R the quadratic electro-optical coefficient, e the thickness of the material, and the distance between two facing electrodes. The wave plate then has the birefringence ε along the neutral axes oriented at an angle θ relative to the axis defined by the electrodes E ₀ (44) and E ₂ (42) illustrated in FIG. 4.

His Mueller matrix is:

with C = cos 2Θ and S = sin 2Θ. The Mueller matrix of the polarizer oriented according to θ = 0 is:

The intensity measured on the sensor is l (x, y) = s _{0, O} ut (x, y), with and s _0iO ut (x, y), if, _O ut (χ, y), s ₂ , _O ut (χ, y) and s ₃ , out (χ, y) the components of S _or t (x, y ). We thus have: l (x, y) = - (Λ- ₀ ", (x, y) + (c ² + S ² cos ε) s _{] <ιn} {x, y) + SC {\ - cos ε) s _2jn (x, y) + S sin ε S _{3 m} (x, y

For example, four pairs of angle and birefringence values (0, ε) are chosen so as to obtain four similar equations forming an invertible system, and S _n (X, y) is then completely determined. It is also possible to do only two or three measurements, depending on the number of components of S, _n that we are interested in. This architecture completely fulfills the function of programmable analysis, however, a pixel of the size of the sensor, corresponding to a distance of 2 centimeters between electrodes, leads to very high control voltages of the order of a few kVolts even for thicknesses of 1 centimeter substrate.

In order to reduce the control voltages, a second variant of a polarimetric imaging system according to the invention is illustrated in FIG. 5. The second variant of a polarimetric imaging system according to the invention comprises: a lens 32 , a programmable waveguide 53, a detection means 35 and a polarizer 34. This system further comprises two lenses 51 and 52 for defining a first intermediate focal plane, the wave plate 53 being located in this focal plane intermediate. This variant makes it possible to reduce the control voltages by reducing the size of the pixel, and by using a focusing lens 51 in the pixel to preserve the illumination of the sensor 35.

FIGS. 6 and 7 illustrate an example of a programmable waveguide that can be used in the second variant of a polarimetric imaging system according to the invention. The programmable waveguide 53 is in the form of a block of parallelepipedal electro-optical material 65 in which four holes are drilled whose walls are covered with four electrodes 61, 62, 63 and 64. The electrodes in 61 and 63 are located at a distance from one another. It is the same for the electrodes 62 and 64. The block of parallelepipedic electro-optical material 65 has a thickness e.

Advantageously, the electrodes 61, 62, 63 and 64 are formed by through holes, substantially cylindrical and metallized, pierced in the thickness of the material with an isotropic electro-optical tensor. The electrodes 61, 62, 63 and 64 are made in the volume of the material to optimize on the one hand the overlap between the optical wave and the applied electric field and, on the other hand, the homogeneity of the applied electric field. This has the effect of optimizing the effectiveness of the component.

In the second variant of a polarimetric imaging system according to the invention, the limiting factor is the number of resolved points of the image formed on the sensor. The programmable waveguide acts as a aperture diaphragm illustrated in Figure 7. Indeed, the phase shift induced by the component is directly proportional to the thickness of material traversed. The angular acceptance 0 _{\\ m} of the programmable waveguide must be limited to ensure a homogeneous phase shift (typically, 6> ι _im ~ 15 ° for a phase shift homogeneous to 5%). Thus, to limit the angle of incidence of the light rays passing through the programmable waveguide, the numerical aperture of the focusing lens is limited to the programmable waveguide. The numerical aperture ON is defined by ON = f / D, where f is the focal length of the lens and D the diameter of the lens. The maximum angle of incidence of the light rays passing through the programmable waveguide is then atan (2 / ON).

The number of resolved points in the image is then given by: N ₌ πσt ^ θ _1≡ _ ₍₁₎

At where σ is the component of the useful surface (surface where the applied electric field is uniform, in practice, σ is estimated by πd ^2/16), λ the wavelength and θ _hm the angular acceptance of the blade of programmable wave.

According to an exemplary embodiment of the second variant of a polarimetric imaging system according to the invention, to obtain 1000 x 1000 resolved points in the image, the spacing d between the electrodes opposite must be 2.5 mm, which leads to control voltages of the order of 500Volts for a substrate whose quadratic electro-optical coefficient R is typically of the order of 2.10 ^"16 m ² V ^{~ 2} , and with a thickness e of 5mm.

Advantageously, the focal length f of the first lens 51 corresponds to the following equation:

[Â / π

/ = - Q hm where θiim is the angular acceptance of the programmable waveguide and At the surface defined by the intersection of the cone of half-angle at the vertex θ | _im with the plane of the first lens 51.

An example of use makes it possible to illustrate the influence of an inhomogeneity of birefringence on a measurement. Consider four couples values of (θ, ε): (0, 0), (π / 4, π), (π / 2, π), (π / 4, π / 2). We obtain the following system of equations:

1 ₁ = 1/4 (S ₀ ,, n + Si _{| ln} )

I ₄ = 1/4 (S ₀ ,, n + S3, ιn) that can be written as follows:

S ₀ , ιn = 2 (I ₁ + I ₂ ) If, m = 2 (I ₁ - I ₂ ) S _{2, ιn} = _4I ₃ - 2 (I ₁ + I ₂ ) s _3lin = _4I ₄ - 2 (I ₁ + I ₂ ) with So.in, S ₁ ,, _n , s ₂ , _ιn and s _3i ιn the components of the state of polarization to be measured and h, I ₂ , I ₃ and I ₄ the intensities measured on the sensor. Consider that we are trying to measure a state of polarization

S _1n = (1, 0,0,1). Deviations Δεi, Ae ₂ Δε ₃ and Δε ₄ at the desired birefringence during the four successive measurements lead to the following measurements: h = s _o , ιn / 4 I ₃ = s _o , _ιn / 4 + sin (Δε ₃ ) / 8 ^* s ₃ ,, _n

I ₄ = s _{o> ιn} / 4 + cos (Δε ₄ ) / 4 ^* s _{3 | in} Using the preceding equations, we then deduce:

So.out ⁼ So, m ⁼ 1 S _{1 1OU} t = O s ₂ , _out = sin (Δε ₃ ) / 2 ^* s ₃ , _ιn = sin (Δε ₃ ) / 2

S ₃ , out = COS (Δε ₄ ) * S _3l ιn = COS (Δε ₄ ) with So.out, Si _{> O} ut, s _2iOu t and s _3i0U t the components of the measured polarization state S _or t- If we take Δε ^ = 0.05 ^* π and Δε ₄ = 0.05 ^* π / 2

(corresponding to an inhomogeneity of 5%), we obtain: s _out = (1, 0, 0.078, 0.997). There is an error of 7.8% between the actual value s _2iin and the measured value s _{2 or} t-

A third variant of a polarimetric imaging system according to the invention is illustrated in FIG. 8. The third variant of a polarimetric imaging system according to the invention comprises: a lens 32 forming a first image of the object 31 in a plane 85. The plane 85 corresponds to the object focal plane of a first micro-lens array 81 which focuses the light in the plane of a blade array. programmable wave 83, in the center of each elementary programmable waveguide of said matrix 83. A second micro-lens array 82 reforms the image on the sensor 35. The system further comprises a polarizer 34. This third variant makes it possible to to reduce the size of the system as well as the control voltages.

FIGS. 9 and 10 show an exemplary embodiment of a programmable waveguide array 83 used in the third variant of a polarimetric imaging system according to the invention. The matrix 83 comprises electrodes 96 distributed regularly in a matrix manner. The matrix 83 further comprises four contact tracks. A first track 94 and a second track 92 are located on a first face of the material. A third track 91 and a fourth track 93 are located on a face opposite to said first face. A pixel 106 is formed by four adjacent electrodes connected by four different tracks.

According to the type of embodiment presented as an example, the pixels of the matrix are collectively controlled, a possible variation of pixel pixel characteristics that can be corrected to the calibration processing. Note also in the example of Figure 10 that the generated electric fields are opposite for two adjacent pixels. This does not influence the operation of the device since the electro-optical effect used is quadratic, therefore independent of the polarity of the electric field. It is also possible to envisage an individual addressing of each pixel.

In the third variant of a polarimetric imaging system according to the invention, the number of resolved points in the image reconstructed on the sensor 35 is then n × n × N with n × n the number of pixels of the programmable waveguide matrix. and N the value given by equation (1). Compared with the second variant of a polarimetric imaging system according to the invention, a factor n is thus obtained over the spacing between electrodes necessary to obtain a given number of resolved points in the image. However, the product nxn is bounded higher by the number of pixels of the sensor 35. According to an exemplary embodiment of the third variant of a polarimetric imaging system according to the invention, for 25x25 elements, 1000x1000 resolved points in the image are obtained for an inter-electrode spacing of 100 microns. Control voltages of the order of 6OVoItS are then obtained for a substrate whose electro-optical coefficient is typically of the order of 2.10 ^"16 m ² V ^{" 2} and whose thickness e is 500 microns.

Advantageously, the focal length f _m of the microlenses of the first microlens array 81 satisfies the following equation: where θ | _im is the angular acceptance of a programmable waveguide of the matrix 83 and A the surface defined by the intersection of the cones of half-angle at the vertex θι, _m with the plane of the first micro-lens matrix 81 . The flow collected on the sensor is proportional to ΩA, where Ω is the solid angle defined by the cone of half-angle at the top θι _ιm . The use of a matrix component thus makes it possible to gain a factor n on the bulk of the optical system, for an identical collected flow.

Advantageously, the electro-optical material is of the ceramic type.

Advantageously, the ceramic is (Pb _{_IX} _x) (Zr _Y tiz) ₁ - ₄ _X z O ₃ (PLZT) or the [Pb (Mg ₁ Z ₃ ₂₇₃ Nb) O _3] IX [PbTiO _3] X (PMN-PT).

A fourth variant of a polarimetric imaging system according to the invention uses the elements constituting the third variant of a polarimetric imaging system according to the invention, illustrated in FIGS. 8, 9 and 10. In FIG. it is noted that the electrodes of 4 adjacent pixels 106 used for the architecture 3 form in the center of these pixels 106 another pixel 105, for which the generated electric field is symmetrical to the field of the other 4 pixels with respect to the diagonal axes. The proposed variant exploits the central pixel 105 to reduce the number of images necessary to obtain the system of equations giving the Stokes parameters. Indeed, in this case, an image on the sensor gives access to 2 equations of the type l (x, y) = f (s _o , Si, S2, s ₃ ). However, this variant is accompanied by a loss of resolution, since in this case, the information obtained on each of the pixels is averaged, the number of resolved points becoming half the number of pixels of the component.

Claims

1. Polarimetric imaging system having an optical axis, and comprising means for detecting and analyzing (35) the backscattered light by an object illuminated by a light source and at least one programmable waveguide (33) characterized in that it comprises:

A set of two micro-lens matrices making it possible to define a second intermediate focal plane,

A matrix of programmable wave plates located in said second intermediate focal plane, the programmable wave plates comprising a material with an isotropic electro-optical tensor and a set of at least three electrodes arranged in directions parallel to the optical axis of the imaging system.

2. polarimetric imaging system according to claim 1, characterized in that the electrodes are formed by through holes, substantially cylindrical and metallized in the thickness of the material with an isotropic electro-optical tensor.

Polarimetric imaging system according to one of claims 1 or 2, characterized in that the programmable waveguide array (83) comprises an array of electrodes, four contact tracks so as to contact all the electrodes, a first (94) and a second (92) track being located on a first face of the material, a third (91) and a fourth (93) track being located on a face opposite to said first face, said electrodes having coordinates identified by a line number j and a column number k which are integers in a coordinate system corresponding to the plane of the matrix, said first track (94) connecting the electrodes whose coordinates (j, k) correspond to the equation following: k = 4P ₁ -j, where pi is a relative integer, said second track (92) connecting the electrodes whose coordinates (j, k) correspond to the following equation: k = (4p ₂ + 2) - j, with p ₂ relative integer, said third track (91) connecting the electrodes whose coordinates (j, k) satisfy the following equation: k = (4p ₃ +1) + j, with p ₃ relative integer, said fourth track (93) connecting the electrodes whose coordinates

(j, k) correspond to the following equation: k = (4p ₄ + - 3) + - j, with p ₄ relative integer.

Polarimetric imaging system according to one of the preceding claims, characterized in that the focal length (f _m ) of the microlenses of the first microlens array (81) corresponds to the following equation: where θι, m is the angular acceptance of a programmable waveguide of the matrix (83) and A the surface defined by the intersection of the cones of half-angle at the vertex θ |, _m with the plane of the first microlens matrix (81).

5. polarimetric imaging system according to one of the preceding claims characterized in that the electro-optical material is ceramic type.

Polarimetric imaging system according to claim 5, characterized in that the ceramic is (PLZT) or the [Pb (Mgi / ₃ Nb _2/3) O _3] ix [PbTiθ3] χ (PMN-PT).

7. polarimetric imaging system according to one of the preceding claims characterized in that the detection and analysis means comprise the measurement of the components s _o ,, _n , Si ,, _n , S _2, m and s _{3 jn} a Stokes vector of light backscattered by the object (31), said measurement comprising:

A series of N sets of three steps, allowing N intensity measurements, said steps being, with 1 <j <N: o The choice of a birefringence ε _j of the wave plate (33,53) and an orientation O _j of this birefringence with respect to a predefined axis.

o The determination of the potentials V ₁ to be applied to the electrodes E ₁ (41, 42, 43, 44, 61, 62, 63, 64) so as to obtain the birefringence ε _i of orientation θ _j determined in the previous step, said potentials V ₁ responding to the following equations: with i integer between 0 and 3, λ the wavelength, n ₀ the index of the zero-field material, R the quadratic electro-optical coefficient, e the thickness of the material and the distance between 2 electrodes opposite. .

o The measurement of the intensity I ₁ on the sensor.

The computation of the values s _Oiin , Si ,, _n , s ₂ , m and s _3iin from the N intensity measurements I ₁ , ..., I _j , ..., IN.