LIGHTING DEVICE COMPATIBLE WITH NIGHT VISION DEVICES

Description

TECHNICAL FIELD

[0001] The invention relates to a light-emitting diode lighting device for flat display screens of the liquid crystal matrix type ("LC matrix" in the following), which offers two backlighting modes for the LC matrix, one suitable for use (observation) of the screen during the day, and the other suitable for use at night and compatible with so-called "NVIS" equipment ( Night Vision Imaging System ) such as light-amplifying night vision binoculars known as "NVG" binoculars (acronym for Night Vision Google ) . The invention applies in particular to display devices, including head-up displays (HUD), used in aircraft cockpits. STATE OF THE ART

[0002] Both day and night modes of LC matrix backlighting each have their own issues. While in both cases, good uniformity of the backlighting (i.e. illumination of the entire rear surface of the LC matrix) is sought, as this determines the optical quality of the image, for the day mode, white light with very high brightness is needed to have sufficient contrast compared to the very high brightness that we have, during the day, inside the cockpit of an aircraft; whereas for the night mode, little light power is needed, but the emission spectrum must be outside the infrared or near infrared band, to be compatible with the observation of the screen with NVIS equipment such as NVG binoculars.

[0003] In the most recent technologies, the lighting sources are based on light-emitting diodes. The daytime source can thus comprise a set of white light-emitting diodes; or arrangements of diodes that emit in different colors, for example arrangements of RGB diodes "red", "green", "blue", which generally allows for better colorimetry. The number of diodes used depends on the space available and the power of the diodes to meet the light output requirement and best ensure uniformity of lighting across the entire rear face of the LC matrix. The nighttime source can use a lighting source designed to emit in the admissible spectrum.Another known embodiment uses one or more white diodes associated individually or collectively with a specific filter called an "NVIS" filter which makes it possible to filter infrared and near infrared radiation to provide an output radiation which has a spectrum compatible with the use of NVIS equipment. Arrangements of RGB diodes can also be used, with an NVIS filter associated with the red diodes.

[0004] The invention is more particularly described below in the context of light-emitting diode lighting sources, white or colored, with adequate NVIS filtering for the night-time source, but the solution of the invention could be applied to other commonly used sources, such as fluorescent or incandescent lamps.

[0005] The invention relates more particularly to the architectures of lighting devices which use the same prismatic light guide to diffuse the light provided by the two backlighting sources, i.e. the "daytime" lighting source and the "nighttime" lighting source.

[0006] A prismatic light guide is generally in the form of a transparent (optical transparency) straight block of low thickness (thin blade) arranged under (at the rear of) the LC matrix of the screen, and the aim is to transmit the light provided by each of the two backlight sources from the lower face of the guide, to diffuse it by the upper face of the guide arranged under the plane of the LC matrix. A corresponding architecture is for example described in applications US 8,628,206 or US 7,924,371. There is then a physical interlacing of the day and night diodes at the rear of the guide, which is not optimal in terms of day diode density, because the night diodes impose larger spacings, due to their individual NVIS filtering cap.This configuration is not optimal either in terms of space requirement, because to ensure a very homogeneous diffusion of the light at the prism exit, it is necessary to place the electronic card supporting the diodes in a plane quite far back from the guide, all the more far back as the diode density is lower.

[0007] Application EP 0 560 614 proposes another architecture which uses edge lighting (" edge lit ") for night vision: the diodes of the night lighting source are placed on one edge of the block. This therefore saves space compared to the previous architecture: the spacing of the daytime diodes being less constrained (there is no longer any need to take into account the space requirement of the nighttime diodes), the daytime diode carpet can be made denser. Nevertheless, the setback distance remains around 10 times the thickness of the light guide. For example, with a prismatic light guide a few millimeters thick, the setback distance of the daytime carpet from the rear face of the guide will be around 4 to 5 centimeters.

[0008] Furthermore, if we use a standard light guide, the diffusion of night light by the diffusing upper face of the guide is not very homogeneous: almost all the light is extracted at the entrance of the guide. To improve this, the light extraction must be done progressively over the entire length of the guide. This is called an extraction gradient guide, which is achieved, for example, by microstructures whose spacing varies along the length of the guide or by a variation in the thickness of the guide, in the longitudinal direction (corresponding to the lateral injection direction). But this gradient must be compensated on the daytime path, to maintain homogeneous diffusion on the daytime path. This can be done using a technique called " local dimming ", by which the current intensity delivered to each diode is adjusted, depending on its position on the longitudinal axis of light injection. This is a fairly expensive solution, because it is necessary to integrate the adjustment electronics of each diode of the daytime lighting source at least, and to provide a corresponding calibration step, for a given application or product.

[0009] To improve compactness, application WO2016 / 030217 proposes arranging the daytime lighting diodes in row(s) along a lateral face of the guide (input face) and provides for the integration of a light guide specific to the night lane, in the form of a thin blade, between this input face and the row of daytime lighting diodes. The function of this specific guide is to extract the light from a nighttime lighting diode (with filter) which is injected by a lateral face of this specific guide, and which emerges from it through a plurality of diffusing faces of this guide which are arranged at and between the daytime lighting diodes. That is to say, this specific light guide has the technical effect of creating point sources of night lighting diffusion, which are dispersed among or between the daytime diodes, allowing the light from these "nighttime" point sources to be injected through the same input face of the main light guide as the daytime diodes.In doing so, we find at least in part the defects of a physical interlacing between the positions of the daytime diodes and the point sources of nighttime diffusion created by the specific guide. In particular, due to the interlacing of the diffusing faces of the specific guide and the daytime diodes, the design and placement of this specific guide in relation to the daytime lighting diodes are delicate if we want to obtain good homogeneity of the light diffusion for the two modes and sufficient power in daytime mode.

[0010] Document US2005 / 248734 describes a projector architecture compatible with day and night vision and using two independent sources.

[0011] Document US2012 / 230050 describes a lighting module for a flat screen using a particular form of light guide. SUMMARY OF THE INVENTION

[0012] A lighting device according to the invention does not have the various drawbacks of the prior art. More specifically, the invention proposes an architecture which is based on a light guide whose shape is broken down into two straight prisms end to end in a longitudinal direction: a main straight prism which is a straight block and which corresponds to the usual shape of a prismatic light guide for an LC matrix, with a face corresponding to a base of the block which forms the light output surface (by which the light is extracted and diffused towards the rear face of the LC matrix) and a lateral face in a plane orthogonal to the base, which forms a main light input, for the two lighting sources;and an auxiliary right prism in the extension of this lateral entrance face, which is a right prism whose base has the general shape of a right isosceles triangle, and whose three right lateral faces are used, one as the light exit face of the entrance stage and which is of the same rectangular dimensions and placed against, or merging with, the main entrance face; and the other two lateral faces as the light entry face, each for a respective lighting source and at least one of which is semi-reflecting.;

[0013] The expression "general shape of an isosceles right triangle" (or that used later in the description "of the right prism type with an isosceles right triangle base") means that in the invention, variations of angle(s) of the order of a few degrees are possible in practice, so that the triangle is no longer strictly isosceles right to allow optimization of the reflection characteristics by the prism, with the aim of obtaining the most homogeneous possible distribution of light, on the main entry face on the paving stone, whatever the active viewing mode.

[0014] Finally, it seems appropriate to recall the definition of a right prism and the associated terms used in this description: solid having two identical and parallel polygonal faces, superimposed, and which are generally called "bases"; and as many rectangular lateral faces as there are vertices of the base polygon. Thus a right prism with a rectangular base corresponds to a right block and has four rectangular lateral faces. A right prism with a triangular base has three rectangular lateral faces. To say that a lateral face or a base of the prism is a light entry face or an exit face means that it transmits or diffuses light from the outside to the inside of the prism for the first and from the inside to the outside for the second, after possible reflections on the other faces or the other base of the prism.

[0015] That being said, the invention relates to a lighting device according to independent claim 1. Other embodiments are defined by the dependent claims.

[0016] Other characteristics, details and advantages of the invention will emerge from reading the following description, given with reference to the drawings appended by way of example and which represent respectively: there figure 1 , is a perspective view of a lighting device according to the invention according to a first arrangement of a light guide according to an embodiment of the invention and daytime and nighttime lighting sources, for backlighting a liquid crystal matrix of a flat display screen; figure 2 , a schematic sectional view of the general shape of the prismatic light guide of the figure 1 ; and the figure 3 , a schematic perspective view of the light guide alone; the figures 4 et 5 , diagrams showing the optical path in the input stage of the guide, of the radiation from the daytime lighting source ( figure 4 ) and the night lighting source ( figure 5 ) ; THE figures 6 et 7 , a schematic representation of the thin film stacks on the light input side faces of the light guide input stage, which provide the semi-reflective and / or NVIS filtering characteristics of the light input faces of the guide input stage; figure 8 , a schematic top view of the general shape of a prismatic light guide with an input stage according to a second embodiment of the invention and of the arrangement of the two backlighting sources relative to this input stage; figure 9 , a diagram showing the optical path in the input stage of the guide of the figure 8 , respective daytime and nighttime radiation; and the figure 10 , a general perspective view of this guide according to an alternative implementation of the second embodiment. DETAILED DESCRIPTION

[0017] The different figures are represented in a reference frame oriented (x, y, z). The (x, y) plane corresponds to the plane of the light diffusion surface of the guide (or output face) located in a plane parallel and below that of the liquid crystal matrix to be illuminated. The z axis is perpendicular to the (x, y) plane.

[0018] A lighting device according to the invention, suitable for backlighting a liquid crystal matrix of a flat display screen in daytime mode or in nighttime mode and with NVIS compatibility, uses a light guide which is arranged with daytime and nighttime lighting sources, to illuminate the entire rear face of the matrix with the light corresponding to the active viewing mode (daytime or nighttime) in the most homogeneous manner possible and with the least possible loss of luminance in the light guide. Unless otherwise specified, the lighting sources used are conventional lighting sources, formed by at least one row of light-emitting diodes which are regularly arranged on a support card and which emit in a direction orthogonal to the plane of the support card (upward emission).These are, for example, white or colored diodes for the daytime source; white diodes with NVIS filtering for the nighttime source, or colored diodes, RGB, with NVIS filtering associated with the red diodes.

[0019] The figures presented to facilitate understanding of the invention are only indicative of the general arrangement and shapes of the various elements of a lighting device according to the invention. They do not reflect the actual respective dimensions of the various elements. Also, from one figure to another, there may also be changes in scale for one or all of the elements.

[0020] A light guide according to the invention is illustrated in the figure 1 . An LC matrix is ​​symbolically represented in the figure by a rectangular surface M-LC, to illustrate the function of the light guide which is to diffuse the light by an output face fsp located under the rear face of this matrix. This output face has rectangular dimensions at least equal, and generally larger, than those of the matrix to ensure illumination of the entire rear face of the LC matrix.

[0021] This output face fs P corresponds to the rectangular base of a straight block shape PP (rectangular base straight prism). This straight block PP constitutes the main part of a light guide according to the invention, and its shape and dimensions correspond to those of the backlighting light guides of the state of the art. The bases of the straight block PP, one of which constitutes the output face fs P of the guide, are in parallel planes (x,y) superimposed under the matrix plane LC. The straight block also has four lateral faces.

[0022] In the invention, only one lateral face of the right block, denoted fep, is used as the light entry face for extraction and diffusion by the exit face fs P of the right block. The other three lateral faces and the lower base of the right block are reflective (polished glass for example), and the upper base which is the exit face is a diffusing face (frosted glass for example). The right block is advantageously designed to allow optimal extraction of light, towards the diffusing exit face fs P , which is very homogeneous over the entire length L of the block (x axis). This can be obtained by a light guide technology which allows the integration into the mass of diffusing particles. But the diffusion obtained with a lateral injection is generally not optimal.Preferably, a block is used that is designed to have a light extraction gradient in the longitudinal direction x of the block, corresponding to the direction of lateral light injection, from the entry face fe P to the opposite face. For example, the block comprises microstructures formed by surface treatment (lower face and / or upper face) or in the mass, and the size of the microstructures and / or the spacing between them varies to obtain the desired extraction gradient in the injection direction. The function of these microstructures is to break the geometry or air / transparent material interface of the block.The microstructures can be, for example (and not exhaustively) micro-striations oriented orthogonally to the light injection direction, and closer together the further one moves away from the lateral entry face; micro-dots deposited by screen printing of diffusing paint on the rear face; micro-holes (or "bumps") on the surface; or even micro-prisms on the front face. In this case, the gradient function is synthesized by the variations in size and / or position of the microstructures. An extraction gradient can also be achieved by varying the thickness of the block along its length. For example, the rear face of the block can be beveled (option shown on the . figure 1 by the dotted line referenced bv). In this case the main lateral entry face, used for injection, has the height h of the initial block, and this height gradually decreases when approaching the opposite lateral face. It is also possible to combine a variation in thickness along the length of the block with microstructures.

[0023] According to the invention, this main right prism denoted PP is extended by one of its lateral faces and in a direction orthogonal to this lateral face, by a form of auxiliary right prism PA with a right isosceles triangular base. We thus have in a longitudinal direction of the guide, two right prisms placed end to end: the main block, which offers the output surface fs P of the light guide, for light diffusion, and the auxiliary right prism with a triangular base PA which serves as an input stage of the light guide for each of the two backlighting sources, night and day. This input stage PA is thus located on an edge of the right block (and therefore of the matrix).

[0024] The auxiliary right prism noted PA is based on the general shape of a right-angled isosceles triangle. It therefore has three lateral faces of rectangular shape and two of its lateral faces are in principle at right angles: these are those whose small side of their rectangular shape corresponds to one of the two equal sides of the basic shape are at right angles; and the third face is the one whose small side of its rectangular shape corresponds to the hypotenuse.

[0025] In the invention, these three lateral faces of the auxiliary prism PA are such that a lateral face is an output face fs A of the auxiliary prism and it has the same rectangle dimensions and is attached or merges with a lateral face of the main prism PP , which forms the main input face fe P of the main prism PP . The output face fs A of the input stage PA and the input face fe P of the main part PP merge when the light guide is formed from a single piece: by molding if the guide is made of optical plastic (PMMA, polycarbonate, silicone, etc.); or by machining (engraving, polishing) if the guide is made of glass. Or the two right prisms PA and PP are made separately and then joined by these two faces fe P and fs A , by optical bonding. In this case, the two parts are preferably made of the same optical material (glass or optical plastic). the other two lateral faces form the light input faces of the auxiliary prism PA , each being associated with a respective lighting source.

[0026] In the figures, by way of non-limiting example, the input face denoted fe1 A is that associated with the daytime lighting source, and the input face denoted fe1 B is that associated with the nighttime lighting source.

[0027] In the invention at least one of the input faces of the auxiliary prism is semi-reflecting, to allow injection of the radiation of each lighting source through a respective auxiliary input face (of the input stage) but through the same main input face of the light guide. In this way, the light can be extracted towards the output face fs P of the guide with the same efficiency regardless of the active lighting source, which ensures optimal backlighting, with great homogeneity in night vision mode as well as in day vision mode.

[0028] The specific features of the input stage of the guide according to the invention will now be detailed in two embodiments of the light guide according to the invention with respective arrangements of the lighting sources.

[0029] Throughout the following, we consider lighting sources which are each constituted by at least one row of light-emitting diodes which emit in a direction orthogonal to the source support plane.

[0030] THE figures 1 à 3 show a first embodiment of the light guide according to the invention. We have seen that the auxiliary right prism PA has a base b A in the shape of a right isosceles triangle: the angle at the vertex A of the right isosceles triangle is a right angle and the two sides of equal length make an angle of 45 degrees with the hypotenuse. In practice, the measured angle values ​​can give values ​​equal to or close to 45 degrees, without departing from the scope of the invention. In particular, adjustments of a few degrees can make it possible to optimize the transmission / reflection characteristics, with the aim of obtaining the most homogeneous distribution of light on the main entrance face possible, in each mode.

[0031] In this first embodiment, the two input faces fe1 A and fe2 A are formed by the two right-angled side faces, and these two faces are semi-reflective. The output side face fs A is thus formed by the side face of the prism which corresponds to the hypotenuse, and this face is attached to, or merges with, the input side face fe P of the main block PP. The two faces fep and fs A have in principle the same rectangular surface dimensions.

[0032] There figure 2 is a sectional view zx in the plane of the base b A of the auxiliary prism PA, of the light guide thus formed of the two main prisms PP and auxiliary PA end to end in a longitudinal direction (x). The figure 3 , which is a perspective view of the guide, shows a corresponding monolithic shape: the exit face fs P is in an x,y plane and the two light entry side faces fe1 A and fe2 A are by construction, each in planes inclined at 45 degrees with the exit face fs P or the opposite face.

[0033] In this embodiment, the lighting sources are arranged in superimposed parallel planes (represented by the plane of their respective support board Pcb N and Pcb D ), on either side of the auxiliary prism, parallel to the plane of the main output face fs P of the guide. They are therefore each arranged at right angles to the lateral output face of the auxiliary prism; and each make an angle β with the respective auxiliary input face, which by construction is equal to the angle α between this input face and the auxiliary output face fs A , i.e. 45 degrees.

[0034] In the example, the night lighting source is associated with the input face fe2 A and they are placed on the side of the output face fs P (upper base) of the main tile PP; and the day lighting source is associated with the input face fe1 A and they are on the other side (lower base) of the main tile PP. We could just as well have the opposite arrangement.

[0035] Furthermore, it has been seen that in this embodiment, the two input faces fe1 A and fe2 A of the auxiliary prism are each semi-reflective. The semi-reflective treatment of these input side faces is defined to allow a transmission of light power at the input fe P of the main block PP which is maximum for the daytime mode. In the configuration of the figures, the input face fe1 A associated with the nighttime source SN has a high transmission coefficient T1 and a low reflection coefficient R1 (R1+T1=100%). And the input side face fe2 A associated with the daytime source has conversely a high transmission coefficient T2 and a low reflection coefficient R2 (R2+T2=100%). For daytime radiation, the transmission coefficient T1 by the face fe1 A must be high, at least 90%, and the reflection coefficient R2 by the other face fe2 A must also be high, at least 90%.As a result of the high values ​​of T1 and R2, the radiation provided by the night source SN will on the contrary be very attenuated: through the associated lateral face fe2 A which therefore has a low transmission coefficient T2, at most 10%, (complement to 100 of R2), then by the low reflection rate R1 on the other lateral face fe1 A , of at most 10%, (complement to 100 of T1). This strong attenuation of the power of the night radiation is balanced / adjusted with the number of diodes of the night source, to obtain the necessary power level, which is naturally low. Here we take advantage of the fact that we can align "night" diodes along the entire length (along y) of the main input face (= auxiliary output face).

[0036] As an indication, the thickness (or height) h of the PP block (thin blade) is generally a few millimeters (3 or 4 millimeters for example), and the length of the lateral entry face, which depends on the dimension of the M-LC matrix to be backlit, can be a few centimeters for small matrices, up to several tens of centimeters for the largest.

[0037] The optical paths of the day and night radiation in an auxiliary prism according to the invention which serves as an input stage for the light guide, are schematically illustrated in the figures 4 et 5 . There figure 4 corresponds to the daytime viewing mode: the daytime source is active (SD On) and the nighttime source is inactive (SN Off). The daytime source is in a plane Pcb D inclined at 45 degrees with the respective lateral input face fe1 A: a major part T1 of the daytime radiation is transmitted by this face fe1 A, towards the lateral input face fe2 A which reflects a major part R2 towards the lateral auxiliary output face fs A. The figure 5 corresponds to the night vision mode: the night source is active (SN On) and the day source inactive (SD Off). The night source is in a Pcb N plane inclined at 45 degrees with the respective input face fe2 A: a small part T2 of the night radiation is transmitted by this face fe2 A, towards the lateral input face fe1 A which reflects a small part R1 towards the lateral auxiliary output face fs A.

[0038] The semi-reflective coating of the input faces of the auxiliary prism is carried out by any technique known in the state of the art. It may in particular consist of a deposition of thin metallic or dielectric layers, adapted to give it the desired reflection / transmission rates [R,T], as illustrated schematically in the figures 6 et 7 It can also be obtained by bonding on the respective face, an optical film having the desired reflection / transmission properties [R,T].

[0039] Regarding the NVIS filter, noted F NVIS in the figures, which is a filter that allows wavelengths in the red and infrared to be cut, it can be individual to each diode, in which case it is presented in a known manner, in the form of a cap, as schematically illustrated in the figures 4 et 5 ; or collective, in which case it is in the form of a deposit in thin dielectric layers ensuring the desired function, or the deposit of an optical filter having the NVIS filtering function, over the semi-reflective treatment [R,T] of the input face fe2A associated with the night source, as illustrated in the figure 7 This collective filter configuration is less expensive and also reduces the constraints in terms of spacing between the diodes on the night source electronic support board.

[0040] Finally, it should be noted that just as the angles between the faces of the auxiliary prism could have values ​​not strictly equal to those indicated, the measurements of the different angles between the planes of the sources and the different parts of the guide could also not be strictly equal to the values ​​indicated. In particular, tolerances in the mechanical assembly of the guide and the lighting sources must be taken into account. Such variations are covered by the invention. Also, in the figures, the nighttime lighting source (SN ) is located on the same side as the main output face (fs P ) of the guide. But the daytime source could also be placed on this side, without departing from the scope of the invention.

[0041] A second embodiment of the invention is illustrated in the figures 8 et 9 . In this embodiment, the two lateral faces of the auxiliary prism which are at right angles, corresponding to the two equal sides of the basic right-angled isosceles triangle shape of this prism, form one, a first auxiliary input face, fe2 A in the example and the other, the auxiliary output face fs A . The second auxiliary input face, fe1 A in the example, is produced by the lateral face of the prism which corresponds to the hypotenuse of the basic triangle: in this embodiment the two auxiliary input faces fe1 A and fe2 A make an angle γ of 45° and only the input face fe1 A receives a semi-reflective treatment [R,T ].

[0042] Taking the illustrated example where the daytime source is associated with the face fe1 A , and the nighttime source is associated with the face fe2, the arrangement of the sources in relation to the auxiliary prism which serves as the input stage of the light guide is then as follows: the nighttime source and the respective input face fe2 A are placed in superimposed parallel planes, parallel to the output face plane fs P of the main block. And the daytime source and the auxiliary output face fs A are in superimposed parallel planes on either side of the lateral input face fe1 A .

[0043] In this case, the input side face fe2 A associated with the night source does not receive a semi-reflective treatment, but it can include a multi-layer treatment (stack of thin layers) to achieve a collective function (for all the diodes of the night source) of optical filter F NVIS as explained in relation to the figure 7 Such a collective filter allows the night source diodes to be placed closer together, since they no longer have an individual NVIS filtering cap.

[0044] There figure 9 schematically illustrates for this second embodiment, the optical paths and the application of the reflection rates R1' and transmission T1' of the input face fe1 A which make it possible to inject the light with the power adapted according to the activated source: maximum for daytime radiation, with a coefficient T2' of at least 90%; and consequently very attenuated, by a factor R1' (complement to 100 of T1' - therefore at most 10%) for nighttime radiation. When it is the nighttime source SN, the radiation emitted by the diodes is filtered (F NVIS) individually ( figure 9 ) or collective ( figure 7 ) is transmitted without losses by the lateral face fe2 A (which is in a plane orthogonal to the direction of the radiation) inside the auxiliary prism, towards the other input face fe1 A which reflects only the small proportion R1' towards the output face fs A . When it is the daytime source SD , ​​the radiation emitted by the diodes is mainly transmitted by the semi-reflecting lateral face fe1 A (transmission coefficient T1 ') towards the output face fs A . In practice, T1 ' is preferably chosen between 90 and 95%.

[0045] We could reverse the position of the lighting sources, and associate the daytime source with the entrance face fe2 A, and the nighttime source with the face fe1 A: then it would be T1' which would be weak, at most 10% and R1' which would be strong, at least 90%.

[0046] Everything that has been said about the production of the light guide shape according to the invention in two parts assembled by gluing or in a single piece by machining or molding; or about the angle values ​​in the prism or between the different elements of the assembly, which may deviate from the theoretical values, for reasons of optimization of the reflection / transmission characteristics, or for reasons of tolerances of the manufacturing or mechanical assembly processes; or about the adjustment of the number of diodes according to the reflection / transmission losses to obtain the desired power applies to this second embodiment.

[0047] Generally speaking, the sources will be arranged in a centered manner relative to their respective input face, to capture the maximum radiation (divergence of the diodes). As we have seen, the number of diodes on each row (or even the number of rows) will be adapted for each source in relation to the reflection and transmission coefficients which apply respectively to obtain the necessary power at the main input fe P for each mode of vision.

[0048] It will be added that the well-known techniques of current control of each diode individually (local dimming) can be applied to a lighting device integrating a light guide according to the invention.

[0049] Finally, a light device incorporating a light guide according to the invention is suitable, as we have seen, for light sources with light-emitting diodes, which can be white diodes, but also colored light-emitting diodes, typically red, green and blue (with filters on the red diodes for the night-time source)...

[0050] There figure 10 illustrates a variant applicable to the second embodiment, which has the advantage of giving a final general shape of an elongated straight block to the light guide, which can simplify the handling and assembly of the guide as well as the mechanical assembly with the lighting sources. In this variant, a third straight prism PB is provided, of identical shape to the auxiliary prism PA, and the two prisms PA and PB are assembled by their diagonal face f hyp to form a straight block. The two bases b A and b B thus assembled form a square.

[0051] The advantages of a lighting device according to the invention with two modes of daytime vision and NVIS compatible night vision are to allow in a simple and inexpensive way by a light guide with an input stage according to the invention, both to ensure a great homogeneity of the backlighting in each mode with a power adapted to each mode, in a configuration which favors by construction the transmission on the daytime channel which is the one which requires the most power, while maintaining a reduced size due to the lateral injection by the same input face. In particular, it was possible to verify that at least 81% of the light from the daytime source is transmitted by the input stage PA in the main block PP. The guide is compatible with state-of-the-art gradient structuring techniques applied to the main block making it possible to maximize both the extraction (overall efficiency of the backlighting) and the uniformity of extraction.

[0052] A dual-mode lighting device for day and night vision, compatible with NVIS according to the invention, also offers great compactness. In particular, the thickness of the auxiliary prism(s) and source assembly is not so different from the thickness of the main block; mechanical integration is facilitated, including in terms of thermal management and connectivity (to other cards).

[0053] All these advantages contribute to a reduction in the overall cost for the manufacture of the guide and the integration of the sources to form the lighting device.

Claims

1. A backlighting device (1) for a flat display screen of the liquid-crystal matrix array type, intended to be placed behind said liquid-crystal matrix array, said device being compatible with night-vision equipment, and containing at least: a first lighting source (SD) configured for a day-vision mode, formed of at least one row of light-emitting diodes (DD) aligned parallel to a direction Y and emitting in a direction orthogonal to a source-carrier plane (PcbD); a second lighting source (SN) configured for a night-vision mode, formed of at least one row of light-emitting diodes (DN) aligned parallel to the direction Y and emitting in a direction orthogonal to a source-carrier plane (PcbN), with an optical filter (FNVIS) suitable for making the light emitted by said second source compatible with the use of night-vision equipment; and a light guide (GL) configured to diffuse, via a main output face (fsP) arranged in a plane XY, the light delivered by the first or the second lighting source depending on which, day- or night-, vision mode is active, said light being for lighting a back face of said liquid-crystal matrix array; the light guide (GL) comprising a main right-prism shape with rectangular bottom (b'P) and top (bP) bases forming a rectangular cuboid (PP), the top base of said cuboid forming the main output face (fsP) of the light guide, said rectangular cuboid comprising at least one lateral face (feP) arranged in a plane YZ, in a perpendicular manner to said main output face; and the assembly of the light guide together with the first and the second lighting sources being such that the light emitted by each of the sources is injected into said main rectangular cuboid shape via said lateral face (feP) of said cuboid, named lateral face for injection, wherein - the lateral face for injection of light (feP) into said main rectangular cuboid (PP) of the guide is the same for the first and the second lighting sources; and - the shape of the light guide (GL) is made up of two right-prism shapes end-to-end in a longitudinal direction of the guide, comprising said shape of a main rectangular cuboid (PP) and an auxiliary right-prism shape, the base of which is an isosceles right-angled triangle (PA), with a hypotenuse and two sides at right angles, defining, by construction, three rectangular-shaped lateral faces, two lateral faces of which being at right angles, where: • a first lateral face is an output face (fsA) for light from said auxiliary prism, said output face (fsA) having the same rectangular dimensions as the lateral injection face (feP) of the main cuboid (PP) to which it is bonded or with which it is coincident; and • a second lateral face is a first light input face (fe1A) for one lighting source among the first and the second lighting sources, towards the inside of said auxiliary right prism in the direction of another lateral face of the auxiliary prism; • a third lateral face is a second light input face (fe2A) for the other lighting source, towards the inside of said auxiliary prism in the direction of said first input face (fe1A); and - at least the first input face is semi-reflective, allowing reflection of the light transmitted via said second input face in the direction of said output face.

2. The lighting device according to claim 1, wherein said two lateral faces at right angles correspond to the first input face (fe1A) and the second input face (fe2A) of the auxiliary prism and they are semi-reflective, the first light input face (fe1A) being a face for input of light from the associated lighting source in the direction of the second input face (fe2A) and the second semi-reflective input face allowing reflection of the light transmitted via said first input face in the direction of said output face.

3. The lighting device according to claim 2, the first and second lighting sources being arranged in superposed parallel planes on either side of the auxiliary prism, parallel to the output face plane (fsP) of the main rectangular cuboid, each facing one respective input face among the first and the second input faces of the auxiliary prism.

4. The lighting device according to one of claims 2 to 3, wherein the input face associated with the daytime lighting source features a semi-reflective treatment configured to favour a transmission coefficient T1 over a reflection coefficient R1 of said face, where R1 + T1 = 100, and the input face associated with the night-time lighting source features a semi-reflective treatment configured to favour a reflection coefficient R2 over a transmission coefficient T2 of said face, where R2 + T2 = 100.

5. The lighting device according to claim 4, wherein T1 and R2 are each at least equal to 90% and / or between 90 and 95%.

6. The lighting device according to claim 1, wherein the auxiliary prism has a single semi-reflective input face, which is the first input face, and said two lateral faces at right angles of the auxiliary prism correspond to the output face (fsA) and the second input face (fe2A) of the auxiliary prism.

7. The lighting device according to claim 6, wherein the first and second lighting sources are arranged in planes at right angles, one lighting source, associated with the second input face, being in a plane superposed over and parallel to the plane of said second input face (fe2A), the other lighting source, associated with the first input face (fe1A), being in a plane parallel to and superposed over the output face plane (fsA) of the auxiliary prism, with the first input face (fe1A) between said other source and said output face.

8. The lighting device according to claim 7, wherein the first input face (fe1A) is associated with the daytime lighting source (SD) and features a semi-reflective treatment configured to favour a transmission coefficient T1 over a reflection coefficient R1 of said face, where R1 + T1 = 100.

9. The lighting device according to any one of claims 6 to 8, the input stage of the light guide containing a complementary right prism (PB), the shape of which is identical to that of the auxiliary prism (PA), the two prisms being adjoined to one another so as to form a rectangular cuboid.

10. The lighting device according to any one of claims 1 to 9, wherein the second, night-time lighting source (SN) lies in a plane that is parallel to and on the same side as the main output face (fsP) of the guide.

11. The lighting device according to any one of claims 1 to 10, wherein the auxiliary prism (PA) is joined to the main rectangular cuboid (PP) by bonding.

12. The lighting device according to any one of claims 1 to 10, wherein the main rectangular cuboid (PP) and the auxiliary prism (PA) form a monolithic optical part obtained by moulding or machining.

13. The lighting device according to any one of the preceding claims, wherein a semi-reflective treatment of an input face comprises an operation of surface-depositing metal or dielectric thin films.

14. The lighting device according to any one of the preceding claims, wherein the optical-filter (FNVIS) function associated with the second lighting source is a collective filter common to all of the diodes of the night-time source and formed by a dielectric thin-film structure surface-deposited on the second lateral input face of the auxiliary input prism.