Abstract:
The invention relates to a transmissive diffuser screen comprising a transparent support ( 50 ), a first face of the support being covered with a first diffusive micro-structure ( 52 ) and a second face of the support being covered with an optical focusing structure ( 54 ) of which the primary focal point is disposed at a lighting source ( 36 ) of the screen and of which the surface is covered with a second diffusive micro-structure ( 56 ).

Description:
[0001]    The present patent application claims the priority benefit of French patent application FR13/56950 which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    The present application relates to a transmissive diffuser screen, for example, for a rear projection display device. 
       DISCUSSION OF THE RELATED ART 
       [0003]    Rear projection display devices are advantageously compatible with different rear projection supports, also called screens. They are in particular compatible with rear projection supports having curved or complex shapes. Such devices are thus particularly capable of providing information in passenger compartments of vehicles, for example cars. Indeed, rear projection display devices may for example be integrated in the central console of the passenger compartment of a car, or also above this central console. 
         [0004]    However, devices integrated in vehicle passenger compartments are subject to significant constraints: such devices should in particular be relatively compact and sufficiently directional to avoid projections towards reflective elements such as the windshield or lateral windows. Further, such devices should ensure the generation of an output light flow sufficient to avoid problems of readability when the vehicle is placed under an illumination of high luminosity, for example, from the sun. 
         [0005]    To limit readability problems, a cap is generally provided above the display screens in vehicle passenger compartments. However, this solution is not adapted to the integration of a screen on a significant surface area, for example, in the central console of the vehicle. 
       SUMMARY 
       [0006]    An object of an embodiment is to provide a transmissive diffuser screen capable of being integrated in a rear projection display device overcoming all or part of the disadvantages of known devices. 
         [0007]    Thus, an embodiment provides a transmissive diffuser screen comprising a transparent support, a first side of the support being covered with a first diffusive microstructure, and a second side of the support being covered with an optical focusing structure having its surface covered with a second diffusive microstructure. 
         [0008]    According to an embodiment, the optical focusing structure is a Fresnel lens. 
         [0009]    According to an embodiment, the second diffusive microstructure is defined on the surface of each of the convex portions of the Fresnel lens. 
         [0010]    According to an embodiment, the focusing structure is placed on the support by means of glue. 
         [0011]    According to an embodiment, the support, the optical focusing structure, the first diffusive microstructure, and the second diffusive microstructure are defined in a single block. 
         [0012]    According to an embodiment, the first diffusive microstructure and the second diffusive microstructure are formed of films placed at the surface, respectively, of the first side of the support and of the optical focusing structure. 
         [0013]    According to an embodiment, the optical focusing structure has a focal distance in the range from 200 to 400 mm. 
         [0014]    Another embodiment provides a rear projection device, comprising a screen of the above-mentioned type. 
         [0015]    Another embodiment provides a central console of a vehicle, comprising a rear projection device of the above-mentioned type. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0017]      FIG. 1  illustrates a portion of an instrument panel of a vehicle, for example, a car; 
           [0018]      FIG. 2  illustrates a rear projection display device; 
           [0019]      FIG. 3  illustrates a portion of a transmissive diffuser screen and a disadvantage of such a device; 
           [0020]      FIG. 4  illustrates a portion of a transmissive diffuser screen according to an embodiment; and 
           [0021]      FIG. 5  illustrates the operation of the device of  FIG. 4 . 
       
    
    
       [0022]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of optical systems, the various drawings are not to scale. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 1  illustrates a portion of the front of the passenger compartment of a vehicle, for example, a car. This drawing shows a central console  10  comprising a plurality of sections  12 , each section comprising either information display screens, or buttons. Central console  10  further comprises an upper portion comprising a cavity having a display screen  14  provided at the bottom thereof. The cavity is topped with a cap  16 . 
         [0024]    The transmissive diffuser screen which is provided herein is particularly capable of being used in a rear projection display device to replace the selection of screen  14  or of the screens placed at the level of sections  12  of central console  10 , or to replace the assembly of sections of the central console, without using a cap, the device being provided to be sufficiently directional to avoid projecting information towards the windshield or the lateral windows and being further sufficiently bright to be readable when the screen is reached by parasitic ambient light rays, for example, from the sun. 
         [0025]    Further, the transmissive diffuser screen provided herein may have a curved shape and thus be integrated in the passenger compartment, and particularly in the central console, “seamlessly”, that is, in a single block at the front of the central console. 
         [0026]      FIG. 2  illustrates a rear projection display device comprising a transmissive diffuser screen capable of being used in the passenger compartment of a car, for example, in the central console thereof. 
         [0027]    The device comprises a package  20  having projection elements of the rear projection display device integrated therein. In the shown example, package  20  is defined by two substantially parallel first walls  22  and  24 , two second walls  26  and  28  substantially parallel to each other, and a third wall  29 . Walls  26  and  28  form a non-right angle with walls  22  and  24  and, more specifically, the angle between wall  26  and wall  22  is an obtuse angle and the angle between wall  22  and wall  28  is an acute angle. Wall  29  is perpendicular to walls  22  and  24  and is positioned between walls  24  and  26 . Wall  24  is slightly shorter than wall  22  and wall  26  is slightly shorter than wall  28  in the plane of  FIG. 2 . 
         [0028]    Package  20  comprises an output opening in wall  22  having a transmissive diffuser screen  30 , for example, a holographic diffuser screen, positioned therein. Screen  30  allows the transmission of the rays reaching it on the inner side of package  20  to the outside of package  20  with a slight diffusion of these rays. 
         [0029]    Two planar reflective mirrors  32  and  34  respectively positioned along wall  28  and wall  26  are provided inside of the package. A laser projector  36 , for example, a pico projector forming an image by scanning of a laser beam, is substantially positioned at the angle between walls  24  and  29 . In projector  36 , the scanning is for example obtained via a rotating mirror, for example, according to a technology called DLP, for “Digital Light Processing”, in the art. Laser source  36  is positioned to illuminate mirror  34 , so that the beam reflected by mirror  34  reaches mirror  32 , and that the beam reflected by mirror  32  reaches transmissive holographic diffuser  30 . As shown in the example of  FIG. 2 , source  36  may be rotatably assembled along at least two axes to be able to scan the entire surface of holographic diffuser  30 , via the successive reflections on mirrors  34  and  32 . 
         [0030]    The positioning of mirror  32  relative to transmissive holographic diffuser  30 , according to an acute angle, enables to provide a projection with no deformation (a square gives a square). This constraint imposes for mirror  32  to be placed relatively opposite the transmissive holographic diffuser. 
         [0031]      FIG. 3  illustrates a portion of a conventional transmissive diffuser screen and a disadvantage of such a screen. 
         [0032]    The diffuser screen comprises a plate  40  having diffusive microstructures  42 , ensuring a diffusion of the light beams reaching them, provided on one side thereof. Generally, microstructures  42  are placed on plate  40  of the output side thereof, that is, on its non-illuminated side. A laser beam  44  thus reaches plate  40  on the side opposite to that containing microstructures  42 . The laser beam is diffused by microstructures  42 , which forms a large number of diffused beams  46  at the plate output. 
         [0033]    The right-hand side of  FIG. 3  illustrates an observation screen  48  placed behind the screen. A curve  51  in full line illustrates the light intensity received at the level of observation screen  48 . 
         [0034]    As can be seen in  FIG. 3 , this curve is quite irregular. The irregularity of the beam originating from the diffusive screen results from the fact that laser beams have a strong coherence. When the coherent light wave hits rough surface  42  of the diffuser, it is diffracted and the produced diffracted beams, which may be optically associated with secondary light sources, interfere. This phenomenon is called speckle. 
         [0035]    To limit speckle phenomena, it is known to use mobile diffusers, for example, rotating or mobile in translation. By selecting a motion frequency greater than the persistence of vision, the eye thus averages the speckle patterns. However, the use of mobile parts to drive the screen, in particular in the case of a large screen such as that provided in the above application, implies a significant increase of the cost of the device, while increasing the bulk and decreasing the reliability thereof. 
         [0036]    It is thus desired to form transmissive diffuser screens where the coherence, and thus speckle phenomena, are attenuated to obtain an intensity pattern such as that illustrated in dotted lines in curve  53  in  FIG. 3 , that is, a curve having a general Gaussian shape, giving a “smooth” aspect to the projected image. 
         [0037]    Further, particularly for applications such as those provided in relation with  FIG. 1 , it is necessary to provide diffuser screens having a spatially-controlled diffusion, to avoid projections in unwanted direction. Particularly, in the case of the application in a vehicle console, it is necessary to avoid projections towards reflective surfaces such as the windshield or the lateral windows. 
         [0038]    Thus, to overcome all or part of the disadvantages of conventional diffusion plates, a transmissive diffuser comprising different elements enabling to suppress a great part of speckle phenomena and providing a diffusion in a controlled direction, is here provided. 
         [0039]      FIG. 4  illustrates a portion of a transmissive diffuser screen according to an embodiment. 
         [0040]    The screen comprises a transparent plate  50  having diffusive microstructures  52  ensuring a diffusion of rays provided on one side thereof. The side having the microstructures provided thereon is the side intended to be placed on the observer&#39;s side, that is, the output side of the transmissive diffuser screen. On the input side of the screen, that is, opposite microstructures  52 , is provided an optical focusing structure  54 , in the shown example, a Fresnel lens  54 . This lens having its primary focal point located at the level of laser source  36  (see  FIG. 2 ) enables to rectify incident light beams reaching the screen in directions non-normal thereto towards a normal direction. At the surface of the different portions of the Fresnel lens are also provided microstructures  56  ensuring a diffusion. Microstructures  56  may be present on all the areas of the Fresnel lens in contact with the light beam. In the shown example, microstructures  56  are not formed on the sides of the Fresnel lens normal to plate  50 . As a variation, microstructures  56  may be formed on the sides of the Fresnel lens normal to plate  56 . 
         [0041]      FIG. 5  illustrates the operation of the transmissive diffuser screen of  FIG. 4 . This drawing illustrates a first light beam  60  reaching laser source  36  at the surface of the transmissive diffuser screen along a direction normal to its surface. Light beam  60  reaches microstructured surface  56  of Fresnel lens  54 . Microstructure  56  implies the diffusion of light beam  60 , which forms a set of light beams  62  originating from a plurality of secondary sources within plate  50 . When light beams  62  reach microstructure  52  on the output surface of the diffuser screen, they diffuse again in a set of beams  64 . 
         [0042]    Microstructure  56  thus implies forming a plurality of secondary sources  62  having their beams, after diffusion on microstructure  52 , exhibiting an attenuated coherence. Indeed, beams  62  travel different distances in plate  50 , which at least largely cancels the spatial coherence of the different beams originating from microstructure  52 . Thus, at the device output, the obtained beam substantially has an intensity in the form of that of curve  53  of  FIG. 3 . 
         [0043]    In  FIG. 5 , a second light beam  66  is illustrated, beam  66  reaching the surface of the transmissive diffuser screen with a non-zero angle of incidence. In the same way as for beam  60 , beam  66  is diffused at the level of microstructure  56  to form beams  68  in plate  50 , and beams  68  diffuse again at the level of microstructure  52  to form a set of output beams  70 . The use of Fresnel lens  54  enables to rectify beam  66  towards an observer placed on the output side of the device. The rectification of the incident beam thus still more clearly appears for a still more oblique beam  72  which provides diffused beams  74 - 76  having the same respective directions as beams  62 - 64  and  68 - 70 . 
         [0044]    The cumulated used of the Fresnel lens and of the two microstructures, on the input side and the output side of the diffuser screen, thus provides a good focusing of incident light beams, limits speckle phenomena, while ensuring the main function of the plate, that is, the diffusion of the incident beam. 
         [0045]    When the screen of  FIG. 4  is used to replace screen  30  of  FIG. 2 , the beam originating from mobile laser  36  scans the entire surface of the transmissive diffuser screen according to different angles of incidence. The use of the Fresnel lens thus enables to rectify the rays, while providing a good control of the diffusion. 
         [0046]    It should be noted that microstructure  56  is preferably formed of a holographic-type diffuser which enables to control the angles for which light is diffused, these angles corresponding to the width at mid-height of the indicatrix of diffusion. It should also be noted that the diffusion angle of microstructure  56  is selected so that the beam thus diffused by an angle θ generates an image spot on the output side of plate  50  located at a distance e from the input surface of plate  50 , compatible with the desired resolution r, that is, satisfying equation: r=2.e.tan(θ/2). 
         [0047]    As an example, microstructures  52  and  56  may be formed of holographic films formed at the surface, respectively, of plate  50  and of Fresnel lens  54 , topped with a metallization. Such microstructures are particularly known and commercialized by Luminit. 
         [0048]    The microstructures may be obtained by molding or by printing. Such microstructures are of pseudo-random nature. As an example, the mold or the printing pattern may be obtained by recording a speckle pattern by a holographic method. The characteristic dimensions of such microstructures are for example an average pitch in the range from 1 to 200 μm, and a depth (or outgrowth height) in the range from 0.5 to 5 μm. 
         [0049]    It should be noted that the screen of  FIG. 4  may be assembled in different ways. It may in particular be formed of a single transparent block having, on one side, a Fresnel lens and microstructures  56  and, on the other side, microstructures  52 , formed thereon. It may also be provided to place, for example, by means of transparent glue, a Fresnel lens having microstructures  56  formed thereon on a transparent plate, the second surface of the transparent plate being microstructured. Finally, microstructures  52  and  56  may also be themselves in the form of films respectively placed respectively at the surface of the Fresnel lens and at the surface of the central transparent plate. 
         [0050]    Further, the forming of a Fresnel lens  54  at the surface of the plate easily enables to obtain a lens having a focal distance in the range from 200 to 400 mm, for example, in the order of 300 mm, while having a relatively small bulk (for example, with a pitch in the range from 0.2 to 0.3 mm). 
         [0051]    Specific embodiments have been described. Various alterations and modifications will occur to those skilled in the art. A diffusion screen where the optical focusing structure is a Fresnel lens has in particular been provided in the drawings. It should be noted that this lens may be replaced with any optical focusing device, microstructure  56  being then defined on the surface of this optical focusing device.