Diffraction-based optronic detection device and vehicle provided with such an optronic device

Abstract

The invention relates to an optronic detection device (D) comprising: an optronic sensor (3) comprising an array of photosensitive elements (E); a filter array (4) comprising at least first filters (B1) transparent to a first wavelength range and second filters (B2) transparent to a second wavelength range, wherein the first filters (B1) and the second filters (B2) are distributed in a regular pattern, each facing one of the photosensitive elements (E); an optical system (5) extending in front of the filter array (4). The optical system (5) comprises at least one diffraction grating (6) arranged such that any incident radiation from an external point source strikes one of the first filters (Bl) and one of the second filters (B2). The invention also relates to a vehicle comprising such a device.

Description

[0001] Optronic Diffraction Detection Device and Vehicle Equipped with Such an Optronic Device

[0002] The present invention relates to the field of optical phenomenon detection and more particularly to an optronic detection device usable for example for the self-protection of military vehicles and the guidance of missiles.

[0003] BACKGROUND OF THE INVENTION

[0004] An optronic detection device generally comprises an optronic sensor and an optical system positioned in front of the optronic sensor. The optronic sensor includes an array of photosensitive elements commonly called pixels (from the English "picture elements") that deliver a signal proportional to the energy transmitted by the light radiation striking them.

[0005] Threat detection for the self-protection of military vehicles relies on detecting threats, such as missiles, based on optical phenomena related, for example, to the ejection of gases from the missile's nozzle. It is understood that to increase the vehicle's chances of survival and to be able to trigger countermeasures, the missile must be detected as early as possible, even when the missile is very far away. This therefore requires the ability to detect a highly localized optical phenomenon within the landscape and to distinguish it from natural optical phenomena such as the sun or a sunspot. The principle is roughly the same for guiding missiles to a target. The early detection of threats / targets depends directly on the signal-to-noise ratio (SNR) provided by the optronic sensor (the higher the signal intensity compared to the noise, the easier it is to detect).

[0006] The classification of detected phenomena (natural phenomena vs. threats / targets) is performed based on a bispectral ratio, or BSR. This means the optronic detection device is configured to provide a signal in two spectral bands, and the classification depends on the ratio of signal intensity in each band. For this purpose, it is known to place a bispectral filtering matrix upstream of the optronic sensor. This matrix comprises first filters that allow light to pass through in one wavelength band and second filters that allow light to pass through in a second wavelength band.The first filters and the second filters are arranged in a Bayer pattern so that a first part of the photosensitive elements is struck by light radiation in the first band of wavelengths and a second part of the photosensitive elements is struck by light radiation in the second band of wavelengths.

[0007] Furthermore, for optronic detection devices to be effective, they must have a relatively small instantaneous field of view (FOV) to achieve sensitivity and resolution suitable for detection. However, the narrower the field of view, the smaller the area covered by the electronic device.

[0008] Optronic detection devices are therefore designed to have an adjustable line of sight. This is referred to as the total field of view or FOR, from the English "field of gaze," to designate the maximum area covered by the optronic device when its line of sight is oriented.

[0009] This architecture has two weaknesses:

[0010] - the interpolation necessary for the calculation of the bi-spectral ratio because this interpolation tends to locally reduce the intensity of the signal in the interpolated parts of the image, interpolated parts which risk being drowned out by noise when the signal / noise ratio is low, especially at the limit of detection (distant threat / target or small dimensions).

[0011] - the refresh rate of the total field of view should be as high as possible but depends directly on the speed of movement of the line of sight.

[0012] The only way to address the first weakness is to optimize the interpolation algorithm.

[0013] One way to address the second weakness is to use a larger optronic sensor, permanently mounted on the vehicle (a so-called strapdown mounting). However, this comes with a drawback that did not exist with steerable line-of-sight optronic devices. Indeed, a threat on a collision course with the threatened vehicle always approaches from the same direction relative to the threatened vehicle. When it is distant, the light emitted by the threat only strikes one filter in the bispectral filtering array and can only be detected by the photosensitive element behind that filter if the light radiation corresponds to the wavelength band of that filter.This problem does not occur with an optronic device with a steerable line of sight because the line of sight sweeps the environment of the vehicle (the light radiation from the threat will successively hit several filters of the filtering matrix, and therefore the photosensitive elements that are behind it).

[0014] A known infrared imager, comprising a matrix sensor in front of which a filter and a diffractive lens are successively placed, is described in US-A-2005189492. This diffractive lens doubles the diameter of the spot formed by the light radiation on the sensor, effectively halving the optical cutoff frequency. A known optical system incorporating a single- or multi-layered diffractive lens, arranged to reduce chromatic aberrations, is also described in JP-A-2010068136.

[0015] SUBJECT OF THE INVENTION

[0016] The invention is intended in particular to improve the detection capabilities of an optronic detection device.

[0017] SUMMARY OF THE INVENTION

[0018] For this purpose, the invention provides an optronic detection device comprising:

[0019] - a building,

[0020] - an optronic sensor comprising an array of photosensitive elements;

[0021] - a filtering matrix extending in front of the optronic sensor and comprising at least first filters transparent to a first range of wavelengths and second filters transparent to a second range of wavelengths, the first filters and the second filters being distributed in a regular pattern on a surface of the filtering matrix each opposite one of the photosensitive elements;

[0022] - an optical system extending in front of the filtering matrix;

[0023] The optical system includes at least one diffraction grating arranged so that any incident radiation from an external point source strikes one of the first filters and one of the second filters.

[0024] It is important to remember that a diffractive grating, which comprises a set of parallel lines, differs from a diffractive lens, which exhibits rotational symmetry. Thus, thanks to the diffraction grating, the light emanating from each point in the observed scene will strike two photosensitive elements, each corresponding to a specific wavelength band. In other words, the individual line of sight of the photosensitive elements is modified by the diffractive grating without any spreading of the spot formed by the light emanating from the sensor.Expressed more technically, the centroid of the point spread function (describing the optronic sensor's response to a point source of radiation—this function is commonly called the PSF, or point spread function), also known as the spatial impulse response, is not located on a single photosensitive element but on two photosensitive elements positioned behind one of the first and second filters, respectively. Therefore, the risk of a missed detection is low, even if the line of sight is fixed. Furthermore, interpolation between the two wavelength bands is no longer necessary because the information acquired in both wavelength bands corresponds to the same element of the observed scene.

[0025] Depending on optional features, used individually or in whole or in combination:

[0026] - the diffraction grating is supported by a blade;

[0027] - the diffraction grating is positioned at one end of the optical system close to the filtering matrix;

[0028] - the optical system includes at least a first group at the input of the optical system and a second group at the output of the optical system and the diffraction grating between the two groups;

[0029] - the diffraction grating is positioned close to the first group;

[0030] - the diffraction grating is positioned close to the second group;

[0031] - the diffraction grating is positioned at one end of the optical system close to the filtering matrix;

[0032] - the optical system includes at least one scanning blade; - the scanning blade carries the diffraction grating.

[0033] The invention also relates to a vehicle having a structure on which such an optronic device is fixed, the optronic device being fixed in position relative to said structure.

[0034] Other features and advantages of the invention will become apparent from the following description of particular and non-limiting embodiments of the invention.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Reference will be made to the attached drawings, including:

[0037] [Fig. 1] is a schematic view of an optronic detection device according to a first embodiment, in cross-section along its optical axis, in a version mounted on a vehicle;

[0038] [Fig. 2] is a view analogous to figure 1 of an optronic device according to a second embodiment;

[0039] [Fig. 3] is a view analogous to figure 1 of an optronic device according to a third embodiment;

[0040] [Fig. 4] is a view analogous to figure 1 of an optronic device according to a fourth embodiment;

[0041] [Fig. 5] is a view analogous to figure 1 of an optronic device according to a fifth embodiment;

[0042] [Fig. 6] is an enlarged view of area VI of figure 5;

[0043] [Fig. 7] is a view analogous to Figure 1, partial, of an optronic device according to a variant of the fifth embodiment;

[0044] [Fig. 8] is a partial schematic top view of a filter matrix according to a first embodiment; [Fig. 9] is a view analogous to Figure 8 showing different possible shifts on this filter matrix; [Fig. 10] is a partial schematic top view of a filter matrix according to a second embodiment; [Fig. 11] is a partial schematic top view of a filter matrix according to a third embodiment;

[0045] [Fig. 12] is a partial schematic top view of a filtering matrix according to a fourth embodiment;

[0046] [Fig. 13] is a schematic cross-sectional view of a diffraction plate used in this optronic detection device, according to a first embodiment;

[0047] [Fig. 14] is a schematic cross-sectional view of a diffraction plate according to a second embodiment;

[0048] [Fig. 15] is a partial schematic top view of an optronic sensor used in the optronic sensing device according to the invention;

[0049] [Fig. 16] is a schematic cross-sectional view showing the diffraction effect obtained by the invention;

[0050] [Fig. 17] is an enlarged view of area XVII of figure 16.

[0051] DETAILED DESCRIPTION OF THE INVENTION

[0052] With reference to figure 1, the optronic detection device according to the invention, generally designated as 1, is mounted on a vehicle V.

[0053] The optronic device 1 includes a frame 2, an optronic sensor 3, a filtering matrix 4 and an optical system 5.

[0054] The frame 2 is fixed to a structure of the vehicle V so that it is immobile relative to it (strapdown mounting).

[0055] The optronic sensor 3 is fixed to the frame 3 and includes in a known manner within itself an array of photosensitive elements E (see figure 15).

[0056] The filter matrix 4 extends in front of the optronic sensor 3 and contains, in a manner known to itself, first filters B1 transparent to a first wavelength range and second filters B2 transparent to a second wavelength range. The first filters B1 and the second filters B2 are distributed in a regular pattern on a surface of the filter matrix 4, each opposite one of the photosensitive elements E (Figure 15). The pattern is, for example, a Bayer checkerboard. The first filters B1 here allow the blue MWIR wavelength band (3.6 µm; 4.1 µm) to pass through, and the second filters B2 here allow the red MWIR wavelength band (4.5 µm; 4.9 µm) to pass through.

[0057] The optical system 5 extends in front of the filter matrix 4 and comprises, along an optical axis, a first group of lenses 5.1 arranged at the entrance of the optical system 5 and a second group of lenses 5.2 arranged at the exit of the optical system 5 to direct the incident light radiation towards the optronic sensor 3 and form an image on it. This arrangement is known in itself and will not be described in further detail here.

[0058] The optical system 5 further comprises at least one diffraction grating 6 arranged so that any incident radiation from a point source strikes one of the first filters B1 and one of the second filters B2. The resulting shift on the filter matrix 4 is illustrated in Figures 16 and 17.

[0059] The diffraction grating can be arranged to cause a shift of any odd number of filters, in either direction.

[0060] - along the X-axis (rows) of the filtering matrix (arrows D1, D2 in Figure 9); and / or

[0061] - along the Y-axis (columns) of the filtering matrix (arrows D3, D4 in Figure 9); and / or

[0062] - along any axis of the filtering matrix (arrows D5, D6 in figure 9).

[0063] Offsets D1 and D3 are linear offsets of a single filter; offset D2 is a linear offset of three filters in a straight line; offset D4 is a linear offset of seven filters in a straight line; offsets D5 and D6 are L-shaped offsets of three filters. Other offset values ​​are, of course, possible.

[0064] It is understood that the light radiation from each point in the scene will arrive on a filter B1 and a filter B2, and therefore on two photosensitive elements E, each corresponding to a band of wavelengths. In other words, the centroid of the point's spreading function (or spatial impulse response) is not located on a single photosensitive element E, but on two photosensitive elements E positioned behind one of the first filters B1 and one of the second filters B2, respectively.

[0065] In the first embodiment shown in Figure 1, the diffraction grating 6 is carried by a blade 60 positioned near the group of lenses 5.2 at the output of the optical system 5.

[0066] For photosensitive elements E with dimensions of 15 µm, the diffractive equation of the diffractive grating 6 is, for example, 0.0036X for a shift along the X-axis of the filtering matrix 4 (recall that the optical phase is defined modulo 2n and that X is the slope of the line defining the surface of the diffractive grating 6, a line which is folded every 2n increments to obtain a sawtooth surface profile such as that visible in Figure 13). The diffractive equation of the diffractive grating 6 is, for example, 0.0036Y for a shift along the Y-axis of the filtering matrix 4.

[0067] According to a first embodiment of the blade 60, shown in Figure 13, the blade 60 is made of germanium and has a surface with reliefs forming the diffraction grating 6.

[0068] According to a second embodiment of the plate 60, shown in Figure 14, the plate 60 is a phase plate incorporating local modifications of the refractive index distributed and arranged in the thickness of the plate 60 to form the diffraction grating 6.

[0069] In the second embodiment of the optronic device, shown in Figure 2, the diffraction grating 6 is carried by a blade 60 positioned at the output of the optical system 5 near the filtering matrix 6.

[0070] For photosensitive elements E of dimension 15 m, the diffractive equation of the diffractive grating 6 is for example 0.035X for a shift along the X axis of the filtering matrix 4. The diffractive equation of the diffractive grating 6 is for example 0.035Y for a shift along the Y axis of the filtering matrix 4.

[0071] In the third embodiment shown in Figure 3, the diffraction grating 6 is mounted on a plate 60 positioned near the first group of lenses 5.1. For photosensitive elements E of dimension 15 µm, the diffractive equation of the diffractive grating 6 is, for example, 0.0066X for a shift along the X-axis of the filtering matrix 4. The diffractive equation of the diffractive grating 6 is, for example, 0.0066Y for a shift along the Y-axis of the filtering matrix 4.

[0072] In the fourth embodiment shown in Figure 4, the diffraction grating 6 is carried by a 90° reflecting plate 5.3 positioned between the two groups of lenses 5.1 and 5.2.

[0073] For photosensitive elements E of dimension 15 m, the diffractive equation of the diffractive grating 6 is for example 0.0029X for a shift along the X axis of the filtering matrix 4. The diffractive equation of the diffractive grating 6 is for example 0.0029Y for a shift along the Y axis of the filtering matrix 4.

[0074] In the fifth embodiment shown in Figures 5 and 6, the diffraction grating 6 is mounted on a blade 60 positioned as in the first embodiment, and the optical system 5 includes at least one scanning blade 7 positioned near the first group of lenses 5.1. The scanning blade 7 is a blade with flat, parallel faces, mounted to pivot about its central axis forming an acute angle with the optical axis of the optical system 5, resulting in two extreme positions shown respectively as a solid line and a dashed line in Figure 6. The scanning blade 7 is arranged to offset the line of sight by a few sensitive elements so that the optronic detection device has no blind spots. For photosensitive elements E with dimensions of 15 µm, the angle between the central pivot axis of the scanning blade 7 and the optical axis of the optical system 5 is approximately 2.16°.

[0075] It is possible to produce the diffractive beam directly on the scanning blade 7. In such a case, the scanning blade is inclined at 2.11° and the diffractive equation is 0.0058X (or Y as appropriate).

[0076] In the variant of the fifth embodiment shown in Figure 7, the scanning blade 7 is positioned in the second lens group 5.2. The angle between the central pivot axis of the scanning blade 7 and the optical axis of the optical system 5 is approximately 1.12°. Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0077] In particular, the optronic detection device according to the invention may have a different structure from that described.

[0078] Instead of a checkerboard pattern as shown in Figures 8 and 9, the first and second filters can be arranged in rows as in Figure 10, in columns, or diagonally. The filter matrix can also include, in addition to the first and second filters, third filters transparent to a third wavelength (see Figure 11 with a diagonal filter arrangement, for example), or even fourth filters transparent to a fourth wavelength (see Figure 12 with a row filter arrangement, for example), or more. The first band of wavelengths can be separated from the second band of wavelengths as in the example described, or the two bands of wavelengths can be placed side by side.For example, one could have: the first wavelength band corresponding to the MWIR band and the second wavelength band corresponding to the SWIR band; or the first wavelength band corresponding to the MWIR band and the second wavelength band corresponding to the LWIR band; or the first wavelength band corresponding to the SWIR band and the second wavelength band corresponding to the LWIR band. It should be noted, however, that the optronic device of the invention is not limited to infrared detection applications but can also be configured to operate in other domains, for example, in the visible spectrum.

[0079] The optical system may comprise a single group of lenses or more than two groups of lenses.

[0080] The diffractive grating 6 can be made directly on a lens of the optical system 5.

[0081] The 7 sweeping blade is optional.

Claims

DEMANDS 1. Optronic detection device (1), comprising: - a building (2), - an optronic sensor (3) comprising a matrix of photosensitive elements (E); - a filtering matrix (4) extending in front of the optronic sensor (3) and comprising at least first filters (B1) transparent to a first range of wavelengths and second filters (B2) transparent to a second range of wavelengths, the first filters (B1) and the second filters (B2) being distributed in a regular pattern on a surface of the filtering matrix (4) each opposite one of the photosensitive elements (E); - an optical system (5) extending in front of the filtering matrix (4); characterized in that the optical system (5) comprises at least one diffraction grating (6) arranged so that any incident radiation from an external point source strikes one of the first filters (B1) and one of the second filters (B2).

2. Device according to claim 1, in which the diffraction grating (6) is carried by a blade.

3. Device according to claim 1, in which the diffraction grating (6) is disposed at one end of the optical system (5) close to the filtering matrix (4).

4. Device according to claim 1, wherein the optical system comprises at least a first group at the input of the optical system and a second group at the output of the optical system and the diffraction grating (6) is between the two groups.

5. Device according to claim 4, wherein the diffraction grating (6) is arranged near the first group.

6. Device according to claim 4, wherein the diffraction grating (6) is arranged near the second group.

7. Device according to claim 1, wherein the optical system (5) comprises at least one scanning blade (7).

8. Device according to claim 7, wherein the scanning blade (7) carries the diffraction grating (6).

9. Vehicle (V) having a structure on which is fixed an optronic device (1) according to any one of the preceding claims, the optronic device (1) being fixed in position relative to said structure.

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