FR2787892A1 - Light box for back-lit screen for example of aircraft panel instrument has light deviation and distribution plate behind diffuser screen - Google Patents

Abstract

The light box (11) has a series of luminous tubes (16) and a light deviation and distribution plate (21) behind the diffuser screen (18), illuminated through its edge (22) by an auxiliary light source (19) and a guide (20) for viewing the screen in a very low light level. The light box (11) has a series of luminous tubes (16) behind a diffuser screen (18) and providing a light intensity that is variable on demand for day or night viewing. It has a light deviation and distribution plate (21) behind the diffuser screen, illuminated through its edge (22) by an auxiliary light source (19) and a guide (20) for viewing the screen in a very low light level. The light guide has a coating of a reflective material or is totally reflective.

Description

LIGHT BOX FOR TRANSMISSION OPTICAL VALVE

  Observable with a light intensifier

  The present invention relates to lighting devices for a transmission display screen, in particular a screen with optical valve.

or a liquid crystal display.

  Generally, a transmission display screen is o equipped with a backlighting device, called a light box, which illuminates the rear face of the screen and the light transmitted by the screen allows the display

  an image on the front of the screen.

  The view of the observer adapting to the average brightness of his environment, reading the information displayed on the screen is comfortable for the observer when the screen brightness differs little from that of the environment observed. More particularly in aeronautics, an on-board instrument with a flat liquid crystal screen comprises a light box placed behind the screen and capable of providing different levels of luminance according to the brightness of the environment. The light box has several modes of operation. We distinguish in particular a day mode corresponding to a high luminance for the observation of the screen in a daytime environment, a night mode with a low luminance for the night use of the screen and an intensification mode corresponding to a very low luminance allowing in particular the operation of the screen by a user wearing vision glasses

  at night with a light intensifier.

  In the known light box shown in FIG. 1 in section along a plane perpendicular to plane 4 of the screen to be lit (the screen is not shown), the day and night modes are produced by a day / night module. 1, and the intensification mode is carried out by a separate intensification module 2. The day / night module 1 is a conventional light box comprising fluorescent tubes 3, generally parallel to the plane 4 of the screen, ensuring through a diffusing screen 5 an illumination of the rear surface of the screen by a flux luminous 6 perpendicular to plane 4 of the screen. For maximum brightness of the light box, all the fluorescent tubes 3 operate and provide high brightness; and when the tubes 3 receive a lower electrical energy, the brightness of the light box is reduced but it remains greater than a minimum value or brightness floor. The minimum or floor brightness provided by the day / night module 1 is approximately one thousand to five thousand times lower than its maximum brightness, which has a value of approximately one thousand candela per square meter. The diffuser screen 5 is located on one side of the day / night module 1, and the intensification module 2 is located on the opposite side. The intensification module 2 comprises a few light tubes 7 parallel to the plane 4 of the screen to be lit and distributed over the surface of the module. The tubes 7 of the intensification module 2 are separated from the tubes 3 of the day / night module 1 by an infrared filter 8 parallel to the plane 4 of the screen. In intensification mode, the tubes 3 of the day / night module 1 are switched off and the tubes 7 of the intensification module 2 provide, through the infrared filter 8 and the diffusing screen 5, a very low energy luminous flux 9 which illuminates the screen . The minimum brightness of the light box in intensification mode is approximately

  one hundred thousand times lower than the maximum brightness in day mode.

  The intensification module 2 illustrated in FIG. 1 leads to a light box, the significant depth of which is annoying in reduced space, in particular in an aircraft cockpit where the space available for

  various on-board instruments is limited.

  The day / night module 1 is located between the intensification module 2 and the screen to be lit. The intensification module 2 provides thermal insulation which hinders the extraction of the heat given off by all the fluorescent tubes.

operating in day mode.

  On the other hand the infrared filter has a large dimension

  substantially equal to that of the screen surface, it is expensive.

  The problem is to make a light box for an optical valve allowing the use of the valve in an environment with very little light or in intensification mode and not having all the

disadvantages of the known box.

  This is why the invention provides a light box for illuminating by transmission an optical valve observable by day, by night and in a very dim light environment, said light box comprising a network of light tubes located behind a diffusing screen and providing on request an intensity of light whose value is less than a maximum value and greater than a minimum value, for day or night observation of the optical valve, characterized in that it comprises a deflection and distribution plate light located behind the diffusing screen and an auxiliary light source illuminating said plate by the edge, said light box then providing a light intensity lower than said minimum value, for observation of the optical valve

  in an environment with very little light.

  The light source is offset on the back of the light box using a light guide. The guide is preferably a guide by total reflection. It can also have a surface coated with a reflective material. Preferably a conformator selects the light rays of the auxiliary source which have a low incidence on entry into

the light guide.

  Preferably the light guide and the light deflection and distribution plate are separated by an air knife. The thickness of the

  blade is about a few tenths of a millimeter.

  The deflection and distribution plate includes deflecting microprisms. The light received parallel to the plane of the

  plate is returned perpendicularly.

  The light box, the auxiliary source of which operates and the network of light tubes of which is switched off, provides illumination of the optical valve allowing observation of the valve in a very dim light environment. The brightness of the light box can be reduced up to

  worth about one hundredth of a candela per square meter.

  The light box can be equipped with an infrared filter between the auxiliary light source and the deflection and distribution plate. The optical valve lit by this box can be observed without discomfort by a user wearing a night vision device with a light intensifier. Other characteristics and advantages of the invention will appear

  on reading the following detailed description of a particular embodiment

  which is made with reference to the following appended drawings in which the same reference corresponds to the same element: - Figure 1, already described, shows a section of a known light box with an intensification mode

  - Figure 2 is an embodiment of the invention.

  An embodiment of a light box 11 according to the invention is shown in Figure 2 using a section along a plane perpendicular to the plane 4 of the optical valve 12 to be lit. The observer 13 observes the front face 14 of the optical valve 12 and the light box 11 is located opposite the rear face 15 of the valve 12. The light box 11 comprises a light source ensuring a first high level of brightness allowing for example the observation of the valve 12 in a very bright daytime environment. In this embodiment, the light source O is for example constituted by a network of fluorescent tubes 16 parallel to the plane 4 of the valve 12 to be lit and located in a box 17 closed on the side of the optical valve 12 by a diffusing screen 18 parallel in plane 4 of the valve. The fluorescent tubes 16 partially directly illuminate the surface of the diffuser screen 18, they also illuminate the inner wall of the bottom of the box 17 which is preferably reflective so as to return the light rays which it receives towards the diffuser screen 18 at the front of the light box 1 1. The bottom of the box 17 is located at the rear of

the light box 11.

  The diffusing screen 18 ensures homogenization of the light flux directed towards the rear face 15 of the optical valve 12, in particular it prevents the observer 13 from seeing a luminous halo marking the silhouette of each of the fluorescent tubes 16. The diffusing screen 18 can also widen the light beam illuminating the valve 12. It then makes it possible to increase the viewing angle under which the observer 13 can receive light information by transmission of the valve 12. The diffusing screen 18 is produced in a light diffusing material such as a surface diffuser for example with microbeads or a volume diffuser for example a piece of polymethylmethacrylate (PMMA) which is loaded with diffusing particles such as for example alumina particles,

barium or talc sulfate.

  The first high brightness level is obtained for example with the commissioning of the fluorescent tubes 16. Its value is then equal to a maximum value. And the light box 11 includes means for controlling the brightness of the light source so as to have one, several or an infinity of other levels of brightness which is, or are, lower than the first , for example by controlling the electrical supply of the tubes 16 by a voltage slot to modulate the light emitted by the tubes 16. The other, or each of the others, brightness levels makes it possible to present on the front face 14 of the valve optics 12 a less bright image which does not shine too much in a darker environment than the daytime environment to which the first level of brightness is adapted. The other, or others, brightness levels are not less than a minimum brightness value. The observer 13 can operate at night in a dimly lit and unobtrusive environment the displays of 1o the optical valve 12 lit with the other, or one of the others, brightness levels. The light box 11 according to the invention comprises an auxiliary light source for lighting the optical valve 12. The auxiliary light source comprises for example at least one auxiliary fluorescent tube 19 preferably placed at the rear of the light box 11 In this embodiment, the light from the auxiliary source is guided via a light guide 20 to a plate 21 for deflecting and distributing the light which is located between the diffusing screen 18 and the network. fluorescent tubes 16 and which returns a luminous flux 23 towards the rear face 15 of the optical valve 12. The auxiliary fluorescent tube 19 is preferably parallel to the plane 4 of the valve 12 to be lit. In FIG. 2, the auxiliary tube 19 is shown parallel to one of the tubes 16, it can be - placed differently, for example perpendicularly. It is held, for example by fixing, in a support 24 having an elongated opening 25 along the axis of the auxiliary tube 19 which allows light rays emitted by the tube 19 to exit from the support 24 and enter the light guide 20. The support 24 is for example made of a plastic material, such as polysulfone or polyphenylene oxide, with its internal face coated with a white paint. The walls of the support 24 are opaque to light radiation. They prevent the propagation in the light box 11 of parasitic radiation not passing through

opening 25.

  In the embodiment of FIG. 2, the auxiliary light source is supplemented by a filter 26, placed between the auxiliary fluorescent tube 19 and the opening 25, which achieves a partial closure of the opening 25 in the frequency domain. Preferably, the filter 26 is fixed to the support 24 by a seal impervious to the radiation cut by the filter 26. The flow of energy emitted over a wide spectrum by the fluorescent tube 19 is filtered to eliminate the propagation to the optical valve 12 of a part of the spectrum produced by the auxiliary light source. The filter 26 cuts for example the frequencies

  to which a type of light intensifier is particularly sensitive.

  The filter 26 can be an infrared filter.

  In an alternative embodiment, the light box 11 does not include the filter 26. In another alternative embodiment, the light box 11 does not include the filter 26 and the auxiliary fluorescent tube 19 is replaced by a light source emitting in a more limited spectrum,

  for example less than a maximum wavelength.

  The light guide 20 is placed on the opening 25 of the support 24, by an inlet end elongated along the axis of the auxiliary tube 19, it extends in the depth of the light box 11 up to an outlet end facing a slice 22 of the plate 21 for deflecting and distributing the light. It is for example made of glass or preferably

  in an acrylic material such as polymethyl methacrylate.

  The guide 20 guides the light between the opening 25 and the edge 22 of the plate 21. The inlet and outlet ends of the guide have very regular surfaces, they are for example obtained by polishing. The polymethylmetacrylate light guide has the advantage of greatly limiting the losses by diffusion, in particular in the field of blue wavelengths. Guidance is

  preferably obtained by total reflection inside the light guide 20.

  In an alternative embodiment, the guidance is carried out using a reflective coating, for example a metallic layer of aluminum, which is located on the surface of the guide 20 and which does not cover the inlet and outlet ends ; the light rays whose incidence on the wall of the guide does not allow total reflection are maintained inside the guide 20 by the reflective coating. This variant has the disadvantage of being more expensive to manufacture because during the metallization of the

  guide it should be spared the inlet and outlet ends.

  The opening 25 ensures a selection of the rays emitted from the auxiliary tube 19, it lets through the rays whose incidence at the entrance of the light guide 20 is limited. The selected incidence values are for example included in the angular range [- 42, + 42] expressed in degrees for an optical index guide equal to 1.5. The opening 25 is a beam former limiting the incidence of the rays of the light beam coming from the auxiliary light source as they enter the light guide 20. The former makes it possible to avoid losses by reflection in

  the light guide operating by total reflection.

  The outlet end of the light guide 20 is preferably perpendicular to the plane 4 of the valve 12 to be lit. It cooperates with the edge 22 of the plate 21 for deflecting and distributing the light to illuminate the plate 21 by the guided light coming from the auxiliary source. The edge 22 is also preferably perpendicular to the plane 4 of the valve 12. But the outlet end of the guide 20 and the edge 22 can be inclined relative to normal to the plane 4 of the valve to be lit. They may also not be planar, for example one has a convex surface and the other a concave surface which fits into the convex surface. Preferably an air gap separates the outlet end of the guide 20 from the wafer 22. Its thickness is approximately a few tenths of a millimeter. In the embodiment illustrated in FIG. 2, the difference between the outlet end of the guide 20 and the edge 22 of the plate 21 is for example 0.3 millimeters. The assembly with an air knife has an advantage: the manufacturing tolerances on the dimensions of the guide 20 and of

  the plate 21 are less severe than for contact mounting.

  In the embodiment illustrated in Figure 2, the light guide 20 has a rectangular inlet face and parallel to the plane 4 of the valve 12 to be lit, and the guide has the shape of a rectangular parallelepiped cut by an inclined plane d 'an angle of 45 degrees relative to the plane 4 of the valve 12. The section through the inclined plane defines in the guide a surface 27 for bending the light rays. The exit surface of the guide is on the uncut side of the parallelepiped which is perpendicular to the entry face, it is a rectangle adjacent to the folding surface 27. The length of the light guide 20 in the direction perpendicular to the plane of Figure 2 is equal to that of the valve 12 to be lit in this same direction. The folding surface 27 is preferably polished, it allows a specular reflection. The light rays enter the light guide 20 through the entry face under a low incidence (they are substantially perpendicular to the entry surface), then they travel in the guide being channeled by reflection on the walls of the guide and folded by the folding surface 27 for

  finally exit through the guide exit surface with low incidence.

  In alternative embodiments, the folding surface 27 is coated with a reflective material, and it has an angle of inclination ca relative to the plane 4 of the valve 12 to be lit, the value of which is between 30 and 50 in degrees. The walls of the guide adjacent to the entry surface may not be perpendicular to the entry surface, the guide can have any shape insofar as it directs the light from the auxiliary source between the opening25 of the support and the

  edge 22 of the light deflection and distribution plate 21.

  The light deflection and distribution plate 21 is parallel to the plane of the valve 12 to be lit, its surface is substantially equal

  on the surface to be lit. The plate 21 is located behind the diffuser screen 18.

  In the embodiment illustrated in Figure 2, the plate 21 has for example a thickness of 3 millimeters. For example, it is about fifteen

  millimeters from the light tube array 16.

  The plate 21 has for example two flat faces separated by a constant thickness of polymethylmethacrylate and an overmolding of microprisms on the side opposite to the fluorescent tubes 16. In the plate 21, the density of the deflecting microprisms increases as it moves away from the illuminated edge 22. The propagation in the plate 21 of the light entering through the section 22 is disturbed by the deflecting microprisms which return the light towards the bottom of the box 17 surrounding the network of fluorescent tubes 16. By moving away from the section 22, the flow light remaining in the direction of entry parallel to the plate 21 decreases, it encounters an increasing number of microprisms allowing to deflect an almost constant quantity of light out of the plate. The plate 21 ensures both a deviation of about 90 degrees from the mean direction of the light entering through the edge and a distribution of the deflected light over the surface of the plate. The light thus deflected is reflected towards the rear face 15 of the valve 12 by the reflective bottom of the box 17, then crosses the plate 21 without undergoing deflection therein because the flow is then substantially perpendicular to the plane of the plate 21 and crosses the diffuser screen 18 before illuminating the surface of the valve 12 with a flow

auxiliary light 23.

  When the fluorescent tubes 16 are in operation, the light flux returned to the rear face 15 of the valve 12 passes through the deflection and light distribution plate 21 without deflection because it is

  substantially perpendicular to the plate 21.

  The light box 11 according to the invention comprises at least one auxiliary source 19 of light. It may include two auxiliary sources illuminating two opposite sections of the plate 21. When the valve 12 is observed, the illuminated sections are for example at the top and bottom of the valve or to the right and to the left thereof. Each source is located at the rear of the light box 11 using a light guide. Lighting by two light sources allows uniform illumination of an optical valve of

  dimension greater than a square of 12 cm side.

  The light box according to the invention with one or more auxiliary sources has a reduced overall size compared to the known box shown in FIG. 1. The space taken up by lighting for very poorly lit environments is limited to part of the rear surface of the box 11, and it does not hinder the evacuation of the calories generated by the operation of the tubes 16. In the invention, the lateral space requirement with respect to the surface to be lit is small. It is limited in the vicinity of the plate 21 to the width of the light guide 20, ie a few millimeters. The surface of the filter 26 is greatly reduced compared to that of the filter

corresponding to figure 1.

  The auxiliary luminous flux 23 emitted by an embodiment of the invention is weakly energetic. It preferably has a spectrum whose wavelengths are limited, the maximum wavelength corresponding for example to the MIL L 85762A standard and being worth 615 or 625 nanometers. After transmission by the optical valve 12, the auxiliary flow 23 can be observed using a light intensifier without causing the glare, destruction or saturation thereof. The light box 11 whose network of tubes 16 is switched off and the auxiliary light source 19 is switched on provides lighting of the optical valve 12 compatible with observation of the valve by an observer equipped with an electronic light intensifier mounted for example in glasses

night vision.

Claims (19)

  1. Light box (11) for illuminating by transmission an optical valve (12) observable by day, by night and in a very dim light environment, said light box (11) comprising a network of light tubes (16) located behind a diffusing screen (18) and providing on demand a light intensity whose value is less than a maximum value and greater than a minimum value, for day or night observation of the optical valve (12), characterized in that that it comprises a plate (21) for deflecting and distributing the light situated behind the diffuser screen (18) and an auxiliary light source (19) illuminating said plate (21) by the edge (22), said box with light then providing a light intensity lower than said minimum value, for an observation of the   optical valve (12) in a dimly lit environment.   2. Light box according to claim 1, characterized in that the auxiliary light source (19) is offset on the rear of the light box (11) and is connected by a light guide (20) to the edge (22) of said plate (21).   3. Light box according to claim 2, characterized in that the light guide (20) has a surface coated with a reflective material. 4. Light box according to claim 2, characterized in that   that the light guide (20) is a guide by total reflection.   5. Light box according to one of claims 2 to 4,   characterized in that it comprises a beam former (25) limiting the incidence of the rays of the light beam coming from the auxiliary light source (19) when they enter the light guide (20) to avoid   losses by reflection in the guide by total reflection.   6. Light box according to one of claims 2 to 5,   characterized in that the auxiliary light source is in a box   (24) whose walls are opaque to light radiation.   7. Light box according to claim 6, characterized in that it comprises a filter (26) between the auxiliary light source (19) and the   light deflection and distribution plate (21).   8. Light box according to claim 7, characterized in that the filter (26) transmits only wavelengths less than one maximum wavelength.   9. Light box according to claim 8, characterized in that the filter (26) is fixed to the box (24) by a seal impervious to radiation of wavelengths greater than said maximum wavelength.   10. Light box according to one of claims 8 and 9,   characterized in that the auxiliary light source is in a box (24) whose walls are opaque to light radiation and to radiation of wavelengths greater than said maximum wavelength.   11. Light box according to one of claims 2 to 10,   characterized in that the edge (22) of said light deflection and distribution plate (21) is separated from the light guide (20) by a air blade.   12. Light box according to claim 11, characterized in that   that said air gap has a thickness of a few tenths of a millimeter.   13. Light box according to one of claims 2 to 12,   characterized in that the light guide (20) has a folding surface (27) with an angle between 30 and 50 degrees relative to the plane of the   light deflection and distribution plate (21).   14. Light box according to claim 13, characterized in that   that the folding surface (27) is coated with a reflective material.   15. Light box according to claim 14, characterized in that the light guide (20) has a folding surface (27) with an angle of 45 degrees relative to the plane of the plate (21) for deflection and light distribution.   16. Light box according to claim 15, characterized in that   that the folding surface (27) is polished.   17. Light box according to one of claims 2 to 16,   characterized in that the light guide (20) is made of 1 5 polymethylmetacrylate.   18. Light box according to one of claims 1 to 17,   characterized in that the deflection and distribution plate (21)   light has deflecting microprisms.   19. Light box according to claim 18, characterized in that the plate (21) for deflecting and distributing the light is made of   polymethylmetacrylate with an overmolding of microprisms.