COLOR IMAGE INTENSIFIER TUBE AND ASSOCIATED COLOR NIGHT VISION DEVICE

Abstract

The invention relates to a color image intensifier tube (13) comprising the following elements:– an input window (15);– a first filter (30) fixed on an external face of the input window;– a microlens array (32) fixed on the first filter;– a photocathode (16) fixed on an internal face of the input window;– an electron multiplier (18);– a phosphorescent screen (20);– an optical output interface (21); and– a second filter (31), comprising the same set of pixels as the first selective filter, fixed on the output interface. Figure for abstract: Fig 5

Description

Title of the invention: COLOR IMAGE INTENSIFIER TUBE AND ASSOCIATED COLOR NIGHT VISION DEVICE Field of invention

[0001] The invention relates to the field of vision in a very dimly lit environment. Thus, the invention relates to a device conventionally called a “color night vision device”.

[0002] The invention also relates to a specific element of a color night vision device: the image intensifier tube. This is intended to allow, in a known manner, the luminosity of a scene to be amplified. More particularly, the invention aims to observe a night scene in color.

[0003] The invention finds application in many fields where it is sought to improve vision in dark environments.

[0004] In the military and defense field, the invention can be implemented for surveillance applications, detection, recognition and identification of targets, navigation, piloting in low light conditions. The invention can be integrated into optical devices such as night vision binoculars and night weapon sights. In the aeronautics sector, the invention is applicable to assist pilots during landing or takeoff phases in low light and for night surveillance operations. In astronomy, the invention can improve the observation capabilities of optical instruments during observations in low light conditions.

[0005] In search and rescue operations, the invention is beneficial for searching for lost people or animals in night conditions and for navigation in poorly lit areas. In the field of biology and ecology, the invention allows wildlife to be observed in night conditions without disturbance due to artificial light. In industrial and medical fields, the invention finds application in certain endoscopy procedures where lighting is limited. For hunting and outdoor activities, the invention aids in the detection of game in low light and in navigation during night expeditions. In scuba diving, the invention can be used to explore deep and dark ocean areas. In the fields of photography and cinematography, the invention can allow images to be captured without the need for intense artificial lighting.

[0006] This invention therefore presents a multitude of potential applications. State of the art

[0007] As schematically illustrated in Figures 1 and 2a of the state of the art, a night vision device 110 is conventionally in the form of a pair of glasses intended to be placed in front of a sensor, the eye of a user, or both eyes when two devices are juxtaposed to form monocular or binocular night vision binoculars.

[0008] This night vision device 110 integrates several elements placed on the optical axis al of the sensor or the user's eye in order to transform the image of the observed scene. More precisely, the scope comprises, from the scene outside the sensor or the user's eye, an objective 12, an image intensifier tube 130 and an eyepiece 14.

[0009] The objective 12 conventionally comprises one or more lenses for capturing the photons of the electromagnetic radiation of the observed scene. The eyepiece 14, similarly to the objective 12, comprises one or more lenses for capturing and incidentally viewing the photons of the light signal emitted by the image intensifier tube 130.

[0010] The image intensifier tube 130 comprises at least three distinct elements: a photocathode 16, an electron multiplier 18, and a phosphorescent screen 20.

[0011] The photocathode 16 is in the form of a semi-transparent photosensitive layer receiving the photons of the incident electromagnetic radiation, i.e. the photons originating from the observed scene and transmitted by the objective 12. To do this, an entry window 15 transmits the photons from the objective 12 onto the photocathode 16 while guaranteeing the hermeticity of the external wall 23 of the image intensifier tube 130.

[0012] The photocathode 16 is generally produced in the form of a thin layer of metal or semiconductor applied to a layer of glass or material transparent to light. The material of the photocathode 16 is chosen according to its sensitivity to the image of the observed scene. The interaction of the photons of the incident electromagnetic radiation of the observed scene with the photocathode 16 produces, by photoelectric effect, an emission of electrons.

[0013] The emitted electrons, called primary electrons or photoelectrons 28, are then subjected to a first electric field within a first acceleration zone 17 making it possible to direct and accelerate the photoelectrons 28 towards the electron multiplier 18. This first electric field is produced by applying a voltage between the photocathode 16 and the electron multiplier 18, typically a voltage of the order of 50 to 500 volts to guarantee the most rectilinear path possible for the photoelectrons 28.

[0014] The electron multiplier 18, also called an electron amplifier, conventionally comprises a microchannel plate 25 covered by electrodes. This microchannel plate is also known by the acronym GMC or MCP for "microchannel plate" in English literature. It is made from a plate of resistive or dielectric material with a thickness typically between 0.2 and 1 millimeter.

[0015] As illustrated more precisely in [Fig.2b] of the prior art, the microchannels 25 pass right through the wafer of the electron multiplier 18 and are parallel to each other. They have a diameter of between 3 and 12 micrometers. Their internal wall is conventionally coated with a thin semiconductor layer produced by chemical treatment and allowing the transmission of an electric current by jumps. These microchannels 25 have an axis of revolution a2 inclined by an angle a3 of a few degrees, typically between 4 and 12 degrees, relative to the normal of the surface of the electron multiplier 18, so as to induce multiple collisions of the photoelectrons 28 in the microchannels 25.

[0016] In addition to the first electric field created between the photocathode 16 and the electron multiplier 18, a second electric field is created between the two faces of the electron multiplier 18 by means of the electrodes placed on either side of the microchannel plate 25. This electric field makes it possible to charge the internal semiconductor layer of the microchannels 25 so that the multiple collisions of the photoelectrons 28 in the microchannels 25 generate a large number of secondary electrons 29.This electric field also makes it possible, on the one hand, to accelerate the first secondary electrons 29 inside the microchannels 25, so that these first secondary electrons 29 collide again with the surface of the microchannels 25, in turn producing other secondary electrons 29, and so on, generating by this physical process a large number of secondary electrons 29; and on the other hand to accelerate the secondary electrons 29 by supplying energy, in order to orient them from the entrance of the microchannels 25 to the exit of the microchannels 25. Typically, the photoelectrons 28 are multiplied by a factor of between 102 and 104 in the electron multiplier 18.

[0017] At the exit of the microchannels 25, these secondary electrons 29 are then moved linearly towards the phosphorescent screen 20, within a second acceleration zone 19, under the effect of a third electric field generated between the exit of the electron multiplier 18 and the phosphorescent screen 20, typically an electric field generated by a voltage of between 4 and 10 kV.

[0018] The phosphorescent screen 20 makes it possible to transform the secondary electrons 29 into photons producing a luminous intensity.

[0019] It is in the form of a phosphorescent layer or a layer of a luminophore material deposited on a substrate, conventionally made of glass in the form of a network of optical fibers or a glass block.

[0020] At the output of the phosphorescent screen 20, the image formed is transmitted to the eyepiece 14 by optical means, conventionally a network of optical fibers 21 making it possible to return the image formed on the phosphorescent screen 20 to obtain a correct visualization of the observed scene.

[0021] To generate the three electric fields, an electrical power supply unit 22 is conventionally arranged around an internal vacuum enclosure 24 constituting the core of the light intensifier tube 130. The night vision system 110 thus formed therefore has optical elements, an electrical power supply unit 22, and possibly a system for viewing the scene observed on the eyepiece 14. When the night vision system 110 is intended for the human eye, it is conventionally in the form of a telescope and the image intensifier tube 130 is cylindrical.

[0022] In this embodiment, the photocathode 16, the electron multiplier 18, and the phosphorescent screen 20 typically have a diameter of between 15 and 30 millimeters. In order to limit collisions between the electrons and gas molecules, and in particular air, the two acceleration zones 17 and 19 are typically placed under vacuum, at a pressure in the ultra-high vacuum range, of the order of 107 to 10 10 mbar. To do this, the photocathode 16, the electron multiplier 18, and the phosphorescent screen 20 are encapsulated in the internal vacuum enclosure 24.

[0023] There are also, in current night vision systems, solutions proposing to visualize the observed scene in color, in particular by means of color filters arranged at the input and output of the image intensifier tube. To do this, a correspondence is made between the pixels of the input filter and the output filter which are perfectly aligned, in order to preserve the color information generated at the input of the color image intensifier tube.

[0024] In these systems, certain physical phenomena, which can be neglected for a conventional night vision system, must nevertheless be taken into account when it is desired to view the scene in color.

[0025] The first phenomenon to be taken into account is that of optical aberrations inherent in any optical system. These aberrations can take several forms: spherical aberration, coma, astigmatism, field curvature, distortion or chromatic aberrations. These optical aberrations generate a certain deviation of the light ray when passing through the entrance window 15.

[0026] At the output of photocathode 16, the photoelectrons 28 are emitted with an angular distribution and an energy distribution. The electric field applied between the photocathode 16 and the electron multiplier 18 makes it possible to extract and accelerate the photoelectrons 28 perpendicular to the surface of the photocathode 16. However, there is still a spatial dispersion between the normal to the surface of the photo tocathode 16 and the direction of emission of the photoelectrons 28. In the same way, at the output of the electron multiplier 18, the secondary electrons 29 are emitted with an angular distribution also forming a spatial dispersion.

[0027] There therefore exists in any image intensification device 130, between the electron multiplier 18 and the phosphorescent screen 20 as well as between the photocathode 16 and the electron multiplier 18, a spatial dispersion of the path of the photoelectrons 28 and the secondary electrons 29 relative to the optical axis a1. The deviation of the photons due to the optical aberrations inherent in the input window 15 is characterized by the measurement of the LSF, for “Line Spread Function” in the Anglo-Saxon literature. On the other hand, the electronic dispersion is characterized by the measurement of the FWHM, for “Full Width at Half Maximum” in the Anglo-Saxon literature; the FWHM representing the width at half-height of the image of a very fine slit, proportional to its spatial widening.

[0028] More precisely, the photoelectrons 28 are distributed in the plane of the input face of the electron multiplier 18 as a function of the spatial dispersion experienced between the photocathode 16 and the electron multiplier 18. In the same way, the secondary electrons 29 are distributed in the plane of the input face of the phosphorescent screen 20 as a function of the spatial dispersion experienced between the electron multiplier 18 and the phosphorescent screen 20.The combination of these two distributions, namely the distribution of the photoelectrons 28 on the plane of the input face of the electron multiplier 18, and the distribution of the secondary electrons 29 on the plane of the input face of the phosphorescent screen 20, are added to obtain a total spatial distribution of the secondary electrons 29 in the plane of the phosphorescent screen 20 which superimposes the color information generated by the different pixels of the input color filter, which are no longer in correspondence with the pixels of the output color filter, resulting in the formation of a blurred and / or monochrome image.

[0029] In order to define the capacity of an image intensifier tube to transmit color information despite the color overlap phenomenon, reference is commonly made to the measurement of the color ratio, defined by the ratio between the percentage of color actually perceived by the user at the output of the color image intensifier tube and the totality of the color information generated at the input of the tube.

[0030] The state of the art discloses several devices for color display of images generated by an image intensifier tube.

[0031] As illustrated in [Fig. 3], document US 5,233,183 proposes to arrange, between the input window 15 and the photocathode 16, a first RGB filter 300, as well as a second RGB filter 310 at the optical output interface 210. The device described in this document does not make it possible to achieve satisfactory color vision, because the phenomenon of spatial dispersion of the electrons between the electron multiplier 18 and the phosphorescent screen 20 as well as the optical aberrations of the input window 15, are not taken into account. Thus, to obtain a color image, it is necessary to degrade the resolution so that the size of each point of the image, that is to say the size of the distinct zones of the filters 300 and 310 is greater than the size of the spatial dispersions of the electrons and the deviations of the photons due to the optical aberrations.

[0032] As illustrated in [Fig.4], the document WO 2004 / 019367 proposes to position two color filters at the input and output of the image intensifier tube, respectively a first RGB filter 301 in front of the photocathode 160 and a second RGB filter 311 after the phosphorescent screen 210. This document also teaches to arrange a microlens array 321 within the objective 120, as well as a second microlens array 322 within the eyepiece lens 140. The device described in this document partially and incorrectly takes into account the phenomena of optical aberration and / or spatial dispersion of the electrons, such that such a vision system also has an unsatisfactory resolution.

[0033] It therefore appears necessary to remedy the physical phenomena causing the overlap of colors in another way. More particularly, it is necessary to provide a solution by producing an image intensifier tube that can guarantee an acceptable color ratio, while ensuring the best possible image resolution, without deteriorating the sensitivity of the color image intensifier tube. Presentation of the invention

[0034] The invention proposes to address this technical problem by placing a microlens array upstream of a first wavelength selective filter, said first selective filter being aligned and identical to a second wavelength selective filter placed after the phosphorescent screen.

[0035] Indeed, within the framework of the invention, it has been demonstrated that arranging a network of microlenses within the objective and / or the eyepiece, without taking into account its positioning with respect to one of the wavelength selective filters, does not make it possible to overcome the drawbacks generated by the physical phenomena responsible for the overlap of the colors.

[0036] Based on this observation, the inventors sought another arrangement of the color filters and the microlens array within the color night vision system, in order to generate color night vision having satisfactory resolution and color ratio.

[0037] Thus, instead of neglecting at least partially the deviation of the photons at the exit of the input window, or the spatial dispersion of the electrons between the photocathode and the electron multiplier and between the electron multiplier and the phosphorescent screen, the inventors sought to take into account these deviations of the photons and these electronic spatial dispersions in the most complete way possible, in order to drastically limit any phenomenon of color overlap without degrading the resolution of the observed scene.

[0038] With this new approach, the inventors determined specific parameters for positioning the microlens array and the wavelength selective filters to meet all of these constraints.

[0039] Thus, according to a first aspect, the invention relates to a color image intensifier tube comprising the following elements: - an input window configured to receive and transmit photons; - a first wavelength selective filter, comprising a set of pixels, fixed on an external face of the input window; - a photocathode capable of converting photons transmitted by the input window into photoelectrons, said photocathode being fixed on an internal face of the input window; - an electron multiplier comprising two parallel faces, capable of multiplying photoelectrons into secondary electrons; - a phosphorescent screen transforming secondary electrons into photons; - an optical output interface configured to capture the photons emitted by the phosphorescent screen in order to transmit them to the output of the color image intensifier tube; and - a second wavelength selective filter, comprising the same set of pixels as the first selective filter, fixed on the output interface.

[0040] The invention is characterized in that the color image intensifier tube also comprises a microlens array fixed to the first selective filter so as to focus, at the output of the input window, the photons in the plane of the photocathode.

[0041] Thus, the characteristics of the microlenses are determined so as to anticipate the optical aberrations of the input window and / or the dispersion of the electrons between the electron multiplier and the phosphorescent screen so as to limit the overlap of the colors at the optical output interface.

[0042] With this precise positioning of the microlens array and wavelength selective filters, the main physical phenomena responsible for color overlap are limited as completely as possible.

[0043] According to one embodiment, in order to limit the loss of light power available at the input of the color image intensifier tube, the first and second filters are filters incorporating transparent pixels. Preferably, but not excluded sively, the first and second filters are SPARSE matrix type color filters, making it possible to maximize the conservation of light power while providing a color image of the observed scene. A SPARSE matrix is, for example, a matrix which has at least as many transparent pixels as wavelength selective pixels.

[0044] According to a preferred variant, the first and second filters are color filters of the Bayer matrix type, RGBW matrix, or any other matrix of colored filters.

[0045] Preferably, for the purpose of preserving the wavelength selective filters, the second wavelength selective filter is secured to a protective plate made of transparent material. The protective plate may be made of glass or resin. This resin may typically be an epoxy, polyurethane, acrylic or silicone resin.

[0046] Preferably, in order to avoid interreflection phenomena between the different components at the output of the image intensifier tube, the second wavelength selective filter is integral with an anti-reflective layer. This anti-reflective layer is positioned either on the face of the second selective filter facing the objective, or on the face of the second selective filter facing the eyepiece or the sensor.

[0047] According to one embodiment, in order to limit the color overlap and to achieve sufficient image resolution, the microlens array has at least two, three or four microlenses per pixel of the first selective filter.

[0048] According to one embodiment, in order to obtain a color image from several spectral bands, the phosphorescent screen is based on P45 type phosphor.

[0049] According to a second aspect, the invention also relates to a color night vision system comprising: an objective; a color image intensifier tube according to the first aspect of the invention; and an eyepiece and / or a sensor. Brief description of the figures

[0050] The invention will be better understood on reading the following description, given solely by way of example, and carried out in relation to the appended drawings, in which identical references designate identical or similar elements, and in which:

[0051] [Fig.l] illustrates a schematic sectional view of a state-of-the-art night vision device;

[0052] [Fig.2a] illustrates a perspective view of the image intensifier tube of the night vision device of [Fig.l];

[0053] [Fig.2b] illustrates a schematic sectional view of the electron multiplier of the night vision device of [Fig.l];

[0054] [Fig.3] illustrates a schematic sectional view of an image intensifier tube in an enclosure of a color night vision device according to a prior art embodiment;

[0055] [Fig.4] illustrates a schematic sectional view of an image intensifier tube in an enclosure of a color night vision device according to another embodiment of the prior art;

[0056] [Fig.5] illustrates a schematic sectional view of a color image intensifier tube in an enclosure of a color night vision device according to a first embodiment of the invention;

[0057] [Fig.6] illustrates a schematic sectional view of a color image intensifier tube in an enclosure of a color night vision device according to a second embodiment of the invention;

[0058] [Fig.7a] illustrates a schematic view of a set of sixteen square pixels of a selective filter according to an embodiment of the invention;

[0059] [Fig.7b] illustrates a schematic view of a microlens array according to one embodiment of the invention;

[0060] [Fig.7c] illustrates a schematic view in detail of the superposition of the set of sixteen square pixels illustrated in [Fig.7a] with the microlens array illustrated in [Fig.7b];

[0061] [Fig.8a] illustrates a schematic view of a set of sixteen square pixels of a selective filter according to an embodiment of the invention;

[0062] [Fig.8b] illustrates a schematic view of a microlens array according to another embodiment of the invention;

[0063] [Fig.8c] illustrates a schematic view in detail of the superposition of the set of sixteen square pixels illustrated in [Fig.8a] with the microlens array illustrated in [Fig.8b];

[0064] [Fig.9a] illustrates a schematic view of a set of sixteen square pixels of a selective filter according to an embodiment of the invention;

[0065] [Fig.9b] illustrates a schematic view of a microlens array according to another embodiment of the invention;

[0066] [Fig.9c] illustrates a schematic view in detail of the superposition of the set of sixteen square pixels illustrated in [Fig.9a] with the microlens array illustrated in [Fig.9b];

[0067] [Fig. 10] illustrates a schematic view of a set of twelve rectangular pixels of a SPARSE type selective filter according to an embodiment of the invention; and

[0068] [Fig. 11] illustrates a graph of the emission spectrum of the P45 type phosphor constituting the phosphorescent screen. Detailed description of the invention

[0069] Figures 5 to 6 illustrate a color night vision device 11 incorporating a color image intensifier tube 13 according to the invention. As illustrated in [Fig.l] of the prior art, the color night vision device 11 is in the form of a goggle intended to be placed in front of a sensor, the eye of a user, or both eyes when two devices are juxtaposed to form monocular or binocular night vision binoculars.

[0070] This color night vision device 11 integrates several elements placed on the optical axis al of the sensor or the user's eye in order to transform the image of the observed scene. More precisely, the scope comprises, from the scene outside the sensor or the user's eye, an objective 12, the color image intensifier tube 13 of the invention and an eyepiece 14.

[0071] As described above, the objective 12 comprises one or more lenses for capturing the photons of the electromagnetic radiation of the observed scene. The eyepiece 14, similarly to the objective 12, comprises one or more lenses for capturing and incidentally viewing the photons of the light signal emitted by the color image intensifier tube 13.

[0072] The color image intensifier tube 13 comprises at least three distinct elements integrated in an internal vacuum enclosure 24: a photocathode 16, an electron multiplier 18, and a phosphorescent screen 20.

[0073] As described previously, the photocathode 16 is in the form of a semi-transparent photosensitive layer receiving the photons of the incident electromagnetic radiation, i.e. the photons transmitted by the objective 12. To do this, an input window 15 transmits the photons from the objective 12 onto the photocathode 16 while guaranteeing the hermeticity of an external wall 23 of the color image intensifier tube 13.

[0074] The photocathode 16 is generally produced in the form of a thin layer of metal or semiconductor applied to a glass layer. The interaction of the photons of the incident electromagnetic radiation of the observed scene with the photocathode 16 produces, by photoelectric effect, an emission of electrons.

[0075] The emitted electrons, called primary electrons or photoelectrons 28, are then subjected to a first electric field within a first acceleration zone 17 making it possible to direct the photoelectrons 28 towards the electron multiplier 18. This first electric field is produced by applying a voltage between the photocathode 16 and the electron multiplier 18, typically a voltage of between 50 and 500 volts to best guarantee a rectilinear path for the photoelectrons 28.

[0076] In addition to the first electric field created between the photocathode 16 and the multiplier of electrons 18, a second electric field is created between the two faces of the electron multiplier 18 by means of the electrodes placed on either side of the microchannel plate 25. This electric field makes it possible to charge the internal semiconductor layer of the microchannels 25 so that the multiple collisions of the photoelectrons 28 in the microchannels 25 generate a large number of secondary electrons 29. This electric field also makes it possible, on the one hand, to accelerate the first secondary electrons 29 inside the microchannels 25, so that these first secondary electrons 29 collide again with the surface of the microchannels 25 producing in turn other secondary electrons 29, and so on generating by this physical process a large number of secondary electrons 29; and on the other hand to accelerate the secondary electrons 29 by supplying energy, in order to orient them from the entrance of the microchannels 25 to the exit of the microchannels 25.Typically, the primary electrons 28 are multiplied by a factor between 102 and 104 in the electron multiplier 18. .

[0077] At the exit of the microchannels 25, these secondary electrons 29 are then accelerated in a rectilinear manner towards the phosphorescent screen 20, within a second acceleration zone 19, under the effect of a third electric field generated between the electron multiplier 18 and the phosphorescent screen 20, typically an electric field generated by a voltage of between 4 and 10 kV.

[0078] The phosphorescent screen 20 makes it possible to transform the secondary electrons 29 into photons producing a luminous intensity. It is in the form of a phosphorescent layer or a layer of a luminophore material deposited on a surface of the optical output interface 21, said surface of the optical output interface 21 being opposite the electron multiplier 18.

[0079] At the output of the phosphorescent screen 20, the image formed is transmitted to the eyepiece 14 by an optical output interface 21, conventionally in the form of a network of optical fibers 35 or a glass output interface 36. The case of using the network of optical fibers 35 with an inversion function making it possible to return the image formed on the phosphorescent screen 20 to obtain a correct visualization of the observed scene.

[0080] To generate the three electric fields, an electrical power supply unit 22 is conventionally arranged around the internal vacuum enclosure 24. The color night vision system 11 thus formed therefore has optical elements, an electrical power supply unit 22, and possibly a system for viewing the scene observed on the eyepiece 14. When the color night vision system 11 is intended for the human eye, it is conventionally in the form of a telescope and the color image intensifier tube 13 is cylindrical.

[0081] In the embodiment of [Fig.5], the photocathode 16, the multiplier electrons 18, and the phosphorescent screen 20 typically have a diameter of between 15 and 30 millimeters. The phosphorescent screen is preferably based on P45 type phosphor, having an emission spectrum suitable for detailed color vision in several spectral bands located between 400 and 750 nm, and more particularly between 400 and 700 nm, as illustrated in [Fig. 11].

[0082] More specifically, still with reference to [Fig.l 1], the emission spectrum of the P45 type phosphor constituting the phosphorescent screen 20 is represented, emitting a brightness in a plurality of spectral bands between 400 nm and 750 nm. More particularly, this graph represents the intensity of the brightness of the P45 type phosphor as a function of the wavelength, expressed in nanometers. The brightness is expressed as a percentage, and normalized as a function of the lowest brightness and the most intense brightness for each wavelength over the entire spectrum ranging from 400 nm to 750 nm.

[0083] In order to limit collisions between electrons and gas molecules, and in particular air, the two acceleration zones 17 and 19 are conventionally placed under vacuum, at a pressure in the ultra-high vacuum range, of the order of 107 to 10 10 mbar. To do this, the photocathode 16, the electron multiplier 18, and the phosphorescent screen 20 are encapsulated in the internal vacuum enclosure 24.

[0084] As illustrated in [Fig.5], a first wavelength selective filter 30, comprising a set of pixels, is fixed on an external face of the input window 15, and a second wavelength selective filter 31, comprising the same set of pixels as the first selective filter, is fixed on the face of the optical output interface 21 opposite the eyepiece 14 in the form of a fiber network 35. According to a variant, a protective plate made of transparent material, such as a glass or resin plate, for example of the epoxy, polyurethane, acrylic or silicone type, can be fixed on the second wavelength selective filter 31 for protection purposes. Optionally, an anti-reflective layer is positioned on the face of the second wavelength-selective filter 31 facing the objective, or on the face of the second wavelength-selective filter 31 facing the eyepiece or the sensor.

[0085] The first wavelength-selective filter 30 and the second wavelength-selective filter 31 advantageously comprise transparent pixels, the first filter 30 and the second filter 31 being, for example, in the form of color filters of the SPARSE matrix type, i.e. a matrix which comprises at least as many transparent pixels as wavelength-selective pixels. For example, as illustrated in [Fig. 10], a SPARSE type matrix SI can integrate RGB elementary cells, reproduced as many times as necessary to form the entire filter, with two red pixels R, two green pixels G and two blue pixels B and six transparent pixels T, interposed between the red pixels R, green pixels G and blue B.

[0086] Alternatively, the first filter 30 and the second filter 31 may also be in the form of color filters of the Bayer matrix type, RGBW matrix, or any other matrix of colored filters. Generally speaking, the first filter 30 and the second filter 31 may be in any form of matrix of colored filters according to various matrix compositions which would present a preservation of the sensitivity of the incident radiation thanks to transparent pixels.

[0087] The second wavelength-selective filter 31 can be positioned on one of the two faces of the optical output interface 21, depending on the nature of the optical output interface 21. In this case, as illustrated in [Fig. 5], the second wavelength-selective filter 31 is positioned on the face of the optical output interface 21 opposite the phosphorescent screen 20.

[0088] More particularly, if the optical output interface 21 is in the form of a network of optical fibers, generally in the form of a block of optical fibers, the second selective filter 31 is positioned on the face of the optical output interface opposite the eyepiece 14 or the sensor. Alternatively, if the optical output interface 21 is in the form of a glass block, the second selective filter 31 is positioned between the phosphorescent screen 20 and the optical output interface 21.

[0089] According to the invention, and as illustrated in Figures 5 to 9c, the color image intensifier tube 13 also comprises a microlens array 32 fixed to the first selective filter 30 so as to focus, at the output of the input window 15, the photons in the plane of the photocathode 16.

[0090] To do this, the microlenses 40 are designed with a thickness and a radius of curvature adapted so that the rays incident on these microlenses 40 pass through the entrance window 15 and have a maximum of convergence on a plane coplanar to that of the photocathode 16. For the purposes of the invention, the focusing in the plane of the photocathode 16 corresponds to a focusing on the plane of an internal surface of the photocathode 16, said surface of the photocathode 16 being opposite the entrance window 15.

[0091] The focusing of the photons in the plane of the photocathode 16 is determined so as to anticipate the optical aberrations of the input window 15 and / or the dispersion of the electrons between the photocathode 16 and the electron multiplier 18 as well as between the electron multiplier 18 and the phosphorescent screen 20, so as to limit the overlap of the colors at the optical output interface 21 without degrading the resolution of the image at the output of the color image intensifier tube 13.

[0092] As illustrated in Figures 7a to 9c and with reference to Figures 5 and 6, the microlens array 32 has a number of microlenses 40 which can be variable. the species, the microlens array 32 has at least two microlenses 40, three microlenses 40 or at least four microlenses 40 per pixel of the first selective filter 30.

[0093] The number of microlenses 40 can thus be adapted according to the desired resolution, while taking into account the desired resolution and the desired color ratio. The microlenses 40 are ideally square or rectangular in shape.

[0094] The first filter 30 and the second filter 31 can alternately have square pixels 39 or rectangular pixels 43.

[0095] The microlens array 32 is ideally formed so as to have several microlenses 40 per square pixel 39 or rectangular pixel 43 and 46. Ideally, the projected area of ​​the microlenses of each square pixel 39 or rectangular pixel 43 and 46 must represent at least 90% of the total area of ​​said pixel, the projected area being understood as the projected area of ​​the microlens according to the normal of the first filter 30 and / or the second filter 31.

[0096] According to a first exemplary embodiment illustrated in FIGS. 7a to 7c, the first filter 30 and the second filter 31 have square pixels 39, and the microlens array 32 has four square microlenses 40 per square pixel 39 of the first filter 30 and second filter 31. Thus, in this exemplary embodiment, the length of the side of each square pixel 39 corresponds to at least a multiple by two of the length of the side of the microlenses 40.

[0097] [Fig.7b] shows a microlens array 32 of the same size as a set of sixteen square pixels 37 from the first filter 30 and / or the second filter 31, illustrated in [Fig.7a]. [Fig.7c] thus represents a subset of four square pixels 38 from the superposition of the set of sixteen rectangular pixels 37 and the microlens array 32 illustrated in [Fig.7b].

[0098] According to a second exemplary embodiment illustrated in FIGS. 8a to 8c, the first filter 30 and the second filter 31 have rectangular pixels 43, and the microlens array 32 has two square microlenses 40 per rectangular pixel 43 of the first filter 30 and second filter 31. Thus, in this exemplary embodiment, the length of each rectangular pixel 43 corresponds to at least a multiple by two of the length of the side of the microlenses 40, while the width of each rectangular pixel 43 corresponds at least to the length of the side of each microlens 40.

[0099] [Fig.8b] shows a microlens array 32 of the same size as a set of sixteen rectangular pixels 41 from the first filter 30 and / or the second filter 31, illustrated in [Fig.8a]. [Fig.8c] thus represents a subset of four rectangular pixels 42 from the superposition of the set of sixteen rectangular pixels 41 and the microlens array 32 illustrated in [Fig.8b].

[0100] According to a third exemplary embodiment illustrated in Figures 9a to 9c, the first filter 30 and the second filter 31 have rectangular pixels 46, and the microlens array 32 alternately has three square microlenses 40 per rectangular pixel 46 of the first filter 30 and second filter 31. Thus, in this exemplary embodiment, the length of each rectangular pixel 46 corresponds to at least a multiple by three of the length of the side of the microlenses 40, while the width of each rectangular pixel 46 corresponds at least to the length of the side of each square microlens 40.

[0101] [Fig.9b] shows a microlens array 32 of the same size as a set of sixteen rectangular pixels 44 from the first filter 30 and / or the second filter 31, illustrated in [Fig.9a]. [Fig.9c] thus represents a subset of four rectangular pixels 45 from the superposition of the set of sixteen rectangular pixels 44 and the microlens array 32 illustrated in [Fig.9b].

[0102] As illustrated in Figures 7a to 9c, the set of square pixels 37 and the set of rectangular pixels 41 and 44 have at least three different color filters, these filters advantageously being in the form of a blue color filter, a red color filter and a green color filter. These sets of square pixels 37 or rectangular pixels 41 and 44 also have transparent pixels, said transparent pixels also being covered by the microlens array during the assembly of a color image intensifier tube 13 according to the invention.

[0103] According to another embodiment illustrated in [Fig.6], the optical output interface 21 is this time in the form of a glass optical output interface 36. In this embodiment, a transparent protective plate 34 made of resin, for example of the epoxy, polyurethane, acrylic or silicone type, is integral with the second selective filter 31, in order to protect the integrity of the second wavelength selective filter 31.

[0104] Alternatively, the protective plate 34 may be made of glass. Generally, the protective plate 34 may be made of any type of material having a transparency to the operating spectrum of the photocathode 16, defined by the capacity of the material to transmit the entire operating spectrum of the photocathode 16 without attenuation, diffusion or absorption. In other words, the material constituting the protective plate 34 has a transmittance of at least 90%, and preferably 95% or 99% over the entire spectrum between 400 and 1000 nanometers. Optionally, an anti-reflective layer is positioned on the face of the second wavelength-selective filter 31 facing the objective, or on the face of the second wavelength-selective filter 31 facing the eyepiece or the sensor.

[0105] With reference to Figures 5 to 9c, the microlens array 32 can be made by thermal reflow and by photolithography. Other methods of manufacturing the microlens array 32 can be envisaged, such as by hot embossing / stamping, by projection of plastic microdroplets, or by ultra-precision machining technologies. This microlens array 32 can thus be produced directly on the first wavelength-selective filter 30.

[0106] The first filter 30 and the second filter 31 are conventionally produced in the form of a layer of gelatin deposited on the desired surfaces, in this case respectively on the external face of the entrance window 15 facing the objective 12 and on the face of the phosphorescent screen 20 facing the electron multiplier 18. The layer of gelatin is then divided into several sections by photolithography and colored using various dyes in order to form the different colors of the first filter 30 and the second filter 31. Advantageously, the gelatin before coloring has a transmittance of at least 90%, and preferably from 95% to 99% over the entire spectrum between 400 and 1000 nanometers, in order to allow the transparent pixels a maximum of transmittance.

[0107] With this precise positioning of the selective filters and the microlens array 32, the invention makes it possible to drastically limit the phenomenon of color overlap, the color image intensifier tube 13 thus being able to generate colored images without degrading the image resolution and without compromising the color ratio, for optimized color night vision.

Claims

Claims

1. A color image intensifier tube (13) comprising the following elements: - an input window (15) configured to receive and transmit photons; - a first wavelength-selective filter (30), comprising a set of pixels, fixed on an external face of the input window; - a photocathode (16) capable of converting photons transmitted by the input window (15) into photoelectrons (28), said photocathode (16) being fixed on an internal face of the input window (15); - an electron multiplier (18) comprising two parallel faces (38, 39), capable of multiplying the photoelectrons (28) into secondary electrons (29); - a phosphorescent screen (20) transforming the secondary electrons (29) into photons; - an optical output interface (21) configured to capture the photons emitted by the phosphorescent screen (20) in order to transmit them to the output of the color image intensifier tube (13);and - a second wavelength-selective filter (31), comprising the same set of pixels as the first selective filter, fixed to the output interface; characterized in that the color image intensifier tube (13) also comprises a microlens array (32) fixed to the first selective filter (30) so as to focus, at the output of the input window (15), the photons in the plane of the photocathode (16).;

2. A color image intensifier tube (13) according to claim 1, wherein the first and second filters (30, 31) are filters incorporating transparent pixels.

3. A color image intensifier tube (13) according to claim 2, wherein the first and second filters (30, 31) are SPARSE matrix type color filters.

4. Color image intensifier tube (13) according to one of claims 1 to 3, wherein the microlens array (32) has at least two microlenses per pixel of the first selective filter (30).

5. A color image intensifier tube (13) according to one of claims 1 to 4, wherein the microlens array (32) has three lenses per pixel of the first selective filter (30).

6. A color image intensifier tube (13) according to one of claims 1 to 4, wherein the microlens array (32) has at least four lenses per pixel of the first selective filter (30).

7. A color image intensifier tube (13) according to one of claims 1 to 6, wherein the phosphorescent screen (20) is based on P45 type phosphor.

8. Color image intensifier tube (13) according to one of claims 1 to 7, in which the second wavelength-selective filter (31) is integral with a protective plate (33, 34) made of transparent material.

9. A color image intensifier tube (13) according to claim 8, wherein said protective plate (33, 34) is made of glass or resin.

10. A color night vision system (11) comprising: - a lens (12); - a color image intensifier tube (13) according to one of the preceding claims; and - an eyepiece (14) and / or a sensor.

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