Abstract:
The invention relates to a backlighting system for a liquid-crystal display screen, comprising:
       an illumination source producing an illumination beam;   an objective illuminated by said illumination beam;   at least one folding mirror illuminated by the illumination beam coming from the objective; and   a Fresnel lens capable of collimating and redirecting the illumination beam reflected by said at least one folding mirror, the beam transmitted by the Fresnel lens being intended to back-light said display screen.

Description:
This application claims the benefit, under 35 U.S.C. § 119 of French Patent Application 0554050, filed Dec. 22, 2005. 
   FIELD OF THE INVENTION 
   The invention relates to the field of liquid-crystal displays or LCDs. 
   More precisely, the invention relates to the backlighting of such displays. 
   PRIOR ART 
   According to the prior art, the backlighting of LCD displays (especially large displays of the television type) are based on a diffuser system illuminated by cold cathode lamps. Such a display  1  is illustrated in  FIG. 1  (in front view) and  FIG. 2  (in side view). The display  1  comprises an LCD screen  10  on its front face, a diffuser  11  placed behind the LCD screen  10  and several lamps  120  to  127  distributed in a regular fashion on the rear of the display. The lamps  120  to  127  emit a light beam towards the diffuser  11 . The diffused incident light beam thus back-lights the LCD screen  10 . 
   This technique has the drawbacks of requiring an acceptable luminance for these large displays, that imply a large number of lamps, and expensive management of the thermal problems. Moreover, the LCD technology of these displays offers a low average contrast (about 300:1) which limits the rendition of the signal and in particular the video signal, and especially for professional applications in which image quality is an aspect of primary importance. 
   Patent document WO 02/069030 entitled “high dynamic range display device” filed by the University of British Columbia discloses a system with backlighting using a first modulator that modulates an illumination beam illuminating a second modulator, which may be of the LCD screen type. Thus, the contrast is enhanced. However, such a system has a large depth and therefore the drawback of being bulky. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to alleviate these drawbacks of the prior art. 
   More particularly, the objective of the invention is to reduce the size of an LCD display with a backlighting system using a projected backlighting beam. 
   For this purpose, the invention proposes a backlighting system for a liquid-crystal display screen, comprising:
         an illumination source producing an illumination beam;   an objective illuminated by the illumination beam;   at least one folding mirror illuminated by the illumination beam coming from the objective; and   a Fresnel lens capable of collimating and redirecting the illumination beam reflected by the folding mirror or mirrors, the beam transmitted by the Fresnel lens being intended to back-light said display screen.       

   Advantageously, at least one folding mirror is an aspherical mirror. 
   According to one particular feature, at least one of said folding mirrors is a concave mirror. 
   Advantageously, according to this feature, the objective is designed to produce an imaging beam and to construct a first image positioned after the objective, the concave mirror being positioned after said first image in the path of the illumination beam and constructing, from the first image, a second image on a projection plane. 
   Preferably, the first image is off-axis with respect to the optical axis of the objective. 
   According to one advantageous feature, the concave mirror has an optical axis positioned on the optical axis of the objective. 
   According to one particular feature, the system includes a mask comprising a black zone that absorbs the parasitic rays and a transparent zone placed in the path of the illumination beam after the concave mirror. 
   Preferably, the source includes a beam modulator. 
   According to one particular feature, the system includes means for polarizing the illumination beam, said means being placed before the beam modulator. For example, this is a polarizer or a polarization recovery device. 
   Advantageously, the source includes means for producing a sequentially coloured beam. 
   According to one particular feature, the system includes a diffuser placed behind the Fresnel lens. 
   The invention also relates to a display device comprising a liquid-crystal display screen and a backlighting system as illustrated above according to the invention, and comprising:
         an illumination source producing an illumination beam;   an objective illuminated by the illumination beam;   at least one folding mirror illuminated by the illumination beam coming from the objective; and   a Fresnel lens capable of collimating and redirecting the illumination beam reflected by the folding mirror or mirrors, the beam transmitted by the Fresnel lens being intended to back-light the display screen.       

   
     LIST OF THE FIGURES 
     The invention will be more clearly understood, and other features and advantages will become apparent, on reading the following description, which is given with reference to the appended drawings in which: 
       FIGS. 1 and 2  illustrate an LCD display with backlighting known per se; 
       FIG. 3  shows a display device with an LCD display according to one particular embodiment of the invention; 
       FIG. 4  illustrates a projection source for the display of  FIG. 3 ; 
       FIG. 5  shows schematically the backlighting control means for the display of  FIG. 3 ; 
       FIG. 6  shows the diffusion and collimation of the backlighting beam in the display of  FIG. 3 ; and 
       FIGS. 7 to 12  illustrate a display device with an LCD display according to an alternative embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a display device  3  with an LCD display  31  according to one particular embodiment of the invention. 
   The device  3  comprises:
         a projection source  36  that generates a projection beam  37 ;   a first plane folding mirror  35  illuminated by the beam  37  output by the source  36 ;   a second folding mirror  34  illuminated by the beam  37  reflected by the mirror  35 ;   a third folding mirror  33  illuminated by the beam  37  reflected by the mirror  34 ;   means  32  for collimating and diffusing the backlighting beam;   a colour LCD screen  31  back-lit by the beam collimated by and diffused through the means  32 ; and   a case  30  enclosing the elements  31  to  36 .       

   The LCD screen  31  is a screen for displaying an image. For example, this is a screen of the MVA (Multidomain Vertically Aligned), IPS (In-Plane Switching) or TN (Twisted Nematic) type. 
   The system  3  has a configuration allowing it to be thin. The folding mirrors  33  to  35  fold the beam  37  several times and therefore allow the case  30  to have a small depth, preferably less than 25 cm. It therefore makes it possible to maintain a small thickness, such as the conventional LCD displays as illustrated in  FIGS. 1 and 2 . Moreover, the system offers the advantage of increasing the contrast of the LCD screen  31  by a factor of more than 100. 
   The source  36  itself comprises an objective with a front lens group and a rear lens group that are placed on either side of a diaphragm. According to one particular embodiment, the mirror  34  is a convex mirror, preferably a hyperbolic mirror. This allows the depth of the device  3  to be reduced even further. According to one embodiment in which the mirror  34  is a hyperbolic mirror, a first focus of the mirror lies substantially in the plane of the pupil of the front lens group/hyperbola assembly that is located on the opposite side from the hyperbolic mirror with respect to the front lens group, whereas the second focus lies substantially in the plane of the exit pupil of the front lens group. Thus, the hyperbolic mirror  34  is used to conjugate these two pupils. 
   Advantageously, the rear lens group and/or the front lens group comprises at least one optic for correcting geometrical distortions that has a surface in the form of a conic. Preferably, this optic for correcting geometrical distortions is located in the rear lens group and has a surface of hyperbolic shape. In addition, this optic for correcting geometrical distortions is preferably located in a region far from the diaphragm of the objective. The conics of the hyperbolic mirror and of the optic for correcting geometrical distortions may be in a ratio that is substantially proportional to the ratio of the positions of the foci of the hyperbola, that is to say the ratio of the distance between the exit pupil of the front group and the focus of the hyperbola, to the distance between the pupil of the front lens group/hyperbola assembly and the focus of the hyperbola. 
     FIG. 4  illustrates an embodiment of the projection source  36 . 
   The source  36  comprises:
         an elliptical reflector  361 ;   a white light source  360  that produces an illumination beam  362  and is placed at the first focus of the reflector  361  (the reflector  361  thus reflects the beam  362  onto the second focus  367  of the reflector  362 );   a light guide  363 , the entry of which is placed at the second focus  367  which guide allows several images of the source  360  to be created;   a relay lens  364  that collimates the illumination beam output by the light guide  363 ;   an optical modulator  365  placed behind the lens  364 , creating a modulated beam from the illumination beam; and   a projection objective  366  positioned in the path of the modulated beam.       

   According to an alternative embodiment, the light guide  363  and the relay lens  364  are replaced with any other means allowing the illumination of the light modulator  365  to be made uniform (for example a matrix of microlenses). 
   The objective  366  comprises lenses shaped so as to enlarge the illumination rectangle created by the beam  367 . To reduce the cost of the objective  366 , a field lens is placed at the exit of this integrator, making it possible in particular for the objective to work with telecentric illumination. Telecentric illumination at the modulator makes it possible in particular to increase the flux if the optical modulator is of the DMD type and the contrast if the optical modulator is of the LCD or LCOS type. According to an alternative embodiment of the invention, the illumination beam is convergent and not telecentric. 
   The optical modulator  365  is, for example, of the LCD, LCOS or DMD type. If the optical modulator  365  is of the LCD type, the illumination of the LCD is prepolarized, a polarizer or polarization recovery device being placed before the optical modulator. Since a polarized beam illuminates the LCD screen  31 , an analyser is placed after the modulator, the polarization at the exit of the analyser being chosen so as to have the same direction as the entry polarizer of LCD screen  31  (the illumination beam output by the analyser is polarized in a direction such that the polarization is not lost at the entry of the polarizer located in front of the LCD matrix of the screen  31 ). This makes it possible to reduce the loss of flux by a factor of at least 50%. If the optical modulator is of the LCOS type, it is positioned between a polarizer and an analyser placed at the entry and the exit of a grating polarizer (of the Moxtek type) or glass-type polarization splitter. According to one embodiment of the invention, there is no analyser after the optical modulator, a polarizer being positioned before the LCD display, allowing the imaging beam transmitted by the modulator to be analysed. In the case in which the modulator is of the DMD type, the signal does not need to be polarized. 
   The contrast of the modulator  365  does not need to be high. A value of 100 to 200:1 is sufficient to enhance the contrast of the LCD-TV. This also makes it possible to reduce the constraints on the contrast of the LCD screen  31  itself. The latter may also have a contrast of around 300:1—the final contrast of the image may reach a value of more than 30000:1 to 60000:1. Preferably, the luminance is between 10 −2  Cd/m 2  and 3×10 4  Cd/m 2 . 
     FIG. 5  describes how the display  31  and the modulator  365  are controlled. 
   The system comprises a controller  50  for controlling the display  31 . It receives data representative of an image to be displayed and drives the display  31  so that it displays the corresponding image. The controller  50  transmits all or some of this data to an electronic controller  51  that controls the modulator  365 . 
   The resolution of the modulator  365  is low and, for example, of the VGA type comprising a matrix of 640×480 pixels. The modulator  365  modulates the signal in the white. Advantageously, the modulator comprises specific regions  520 ,  529 , for example comprising a matrix of size of 20 pixels by 20 pixels. Within a region  520 ,  529 , the pixels are controlled at the same time. Thus, the modulator is simplified and its cost is reduced. In the case of a modulator  365  of the LCD type, it is positioned between a polarizer and an analyser. The electronic controller calculates the modulation signal and controls the modulator  365  via this signal. The brighter the region of a displayed image, the brighter the intensity of the beam passing through the corresponding region of the modulator  365 , this region letting through a greater light flux of the illumination beam. Thus, the intensity of the regions of the modulator  365  is modulated according to the video content of the image that addresses the LCD screen  31 . This allows the contrast of the image displayed by the screen  31  to be enhanced. 
   According to a variant, the modulator  365  has a high resolution and each region corresponds to one pixel and is associated with a pixel displayed by the LCD screen  31 . 
   According to one embodiment of the invention, the illumination beam illuminating the modulator  365  is a colour beam. Such a colour beam sequentially takes on the primary colours of the image displayed (for example, red, green and blue) and is, for example, obtained using colour light-emitting diodes that illuminate the modulator  365  or by inserting a colour wheel in the path of the illumination beam (for example at the second focus  367 ). The colour sequence of the illumination beam is synchronized with the corresponding pixels displayed in the colour LCD screen. Such synchronization is provided by the control elements  50  and  51 . According to an alternative embodiment that allows the cost to be reduced and the actual resolution of the LCD screen to be increased (typically by a factor of 3), the LCD screen is in black and white (it does not include pixels with a colour filter) and it is the colour backlighting beam that determines the colour of the pixels displayed. Advantageously, the modulator  365  is a high-speed modulator, for example of the DMD or LCOS type. 
   According to a variant of the invention, a projection source is used that does not include a modulator. Moreover, this source is similar to the source  36  and comprises:
         the elliptical reflector  361 ;   the white light source  360  that produces an illumination beam  362  and is placed at the first focus of the reflector  361 ;   the light guide  363 , the entry of which is placed at the second focus  367 , which light guide allows several images of the source  360  to be created;   a relay lens  364  that collimates the illumination beam output by the light guide  363 ; and   a projection objective  366  positioned behind the relay lens in the path of the illumination beam.       

   In a variant of the invention, placed behind the relay lens, substantially in a region corresponding to the imaged plane of the source  360 , are a uniformity filter making the beam illuminating the LCD screen uniform, should this be necessary (especially if there is a loss of illumination in the corners), and/or a polarization filter. 
   Preferably, the light guide  363  is of rectangular cross section with proportions equal to or close to the proportions of the LCD screen so that the illumination beam has substantially the same cross section as the LCD screen at the screen (preferably it is the same size as or slightly larger than the display in order to illuminate the entire display). 
   According to a variant of the invention, a reflector is used whose shape allows a rectangular beam to be obtained, or a mask that allows the rays outside a rectangle to be eliminated so that the shape and size of the beam are matched to those of the LCD screen. 
   According to one particular embodiment of this variant, there is no relay lens, the projection function of the illumination beam being provided by an objective that combines the optical functions of the relay lens  364  with those of the projection objective  366 . 
   According to one particular embodiment not employing a modulator, the source includes means for sequentially colouring the beam. These means and their functions are similar to the corresponding means and functions of the abovementioned colour-wheel variant. 
     FIG. 6  shows schematically, in a top view, the means  32  for collimating and diffusing the backlighting beam, which means comprise:
         a Fresnel lens  320  that collimates the incident beam  37 ; and   a diffuser  321  that diffuses the collimated beam onto the screen  31 .       
   In  FIG. 6 , the beam  37  shown in dotted lines is in the unfolded form in order to make the figure easier to examine. In reality, the beam  37  output by the source  36  is folded by the folding mirrors, and especially the mirror  33 . 
   The beam diffused by the diffuser  321  back-lights the LCD screen, comprising in succession a polarizer, an LCD layer and an analyser. 
   The diffuser allows the collimated beam to be diffused over a region that preferably makes an angle of ±85° in a horizontal plane along a normal to the display. Thus, a relatively wide field of view is obtained. 
   According to the invention, the device is thin. The angles of incidence of the illumination beam are therefore high. To limit parasitic reflections of the illumination beam, the Fresnel lens  320  has a plane exit face and, on its prismatic entry face, it comprises:
         a central region  327 , called refractive region, where the rays of the incident beam  37  are refracted by a prism entry face towards the exit face of the lens  320 ; and   a peripheral region  326 , called reflective region, where the rays of the incident beam  37  are refracted by a prism entry face towards a second prism face that reflects the refracted rays towards the exit face of the lens  320 .       

   Of course, according to the invention, other Fresnel lenses may be employed, especially Fresnel lenses with only refractive prisms or only reflective prisms and/or with a prismatic exit face (it then being possible for the entry face to be plane). 
     FIG. 7  shows schematically (and in exploded form) a display device  7  with an LCD display  31  according to one particular embodiment of the invention, with a concave aspherical folding mirror. 
   The device  7  comprises:
         a source  37 ;   an objective  71  illuminated by the beam  37  produced by the source  37 ;   a concave aspherical mirror  72  that enlarges the image of the beam  37  to form a beam  73 , while folding the beam;   a folding mirror  74  that receives the beam  73 , preferably a vertical plane mirror;   means  32  for collimating and diffusing the backlighting beam, which means are illuminated by the beam  73  reflected by the mirror  74 ;   a colour LCD screen  31  back-lit by the beam collimated by and diffused through the means  32 ; and   a case  70  enclosing the elements  37 ,  71  to  74 ,  31  and  32 .       

   To make  FIG. 7  easier to examine, the beam  73  has been shown not folded by the mirror  74 ). The optical part of the device  7  has an optical axis  76 , the optical beam  37 ,  73  produced being off-axis (as is therefore the modulator) with respect to this axis  76 . The mirror  72  is such that, seen from the means  32 , the beam  73  seems to come from a pupiliary region, corresponding to a pupil P F  located between the mirror  72  and the means  32  in the path of the beam  73 . 
   The concave aspherical mirror  72  has an axisymmetric shape, the reflecting surface of which is given by the following aspherical surface equation: 
             Z   ⁡     (   r   )       =           r   2     /   R       1   +       1   -       (     1   +   c     )     ⁢       (     r   /   R     )     2               +       a   1     ⁢   r     +       a   2     ⁢     r   2       +       a   3     ⁢     r   3       +       a   4     ⁢     r   4       +       a   5     ⁢     r   5       +       a   6     ⁢     r   6       +   …           
where:
         r represents the distance of a given point from the optical axis, the axis of the mirror  72  being positioned on the optical axis of the objective;   Z represents the distance of this point from a plane perpendicular to the optical axis;   the coefficient c is the conic;   the parameter R corresponds to the radius of curvature of the surface; and   the parameters a 1 , a 2 , . . . a i  are asphericity coefficients of order 1, 2 and i, respectively.       
     FIG. 10  shows in greater detail the objective  71  adapted to the concave mirror  72 . 
   The objective  71  comprises a rear group of lenses  711  to  713  and a front group of lenses  714  to  716 . 
   The last lens  716  of the objective  71  in the path of the beam  37  is preferably an aspherical meniscus lens  72 . Its shape is therefore preferably given by an aspherical surface equation as shown above. 
   As an illustration, in one particular embodiment the radius R of the concave mirror  72  is  60 , the parameters c and a 1  to a 8  are, respectively, the following: −1.59311 mm; 0; 0; −8.94×10 −6 ; 0; 1.64×10 −9 ; −9.74×10 −13 ; −7.84×10 −14 ; and 2.31×10 −16 . The radius R of the first surface (the modulator side) of the meniscus  206  is 44.94711 mm, and the parameters c and a 1  to a 8  have, respectively, the following values: 0; 0; 0; −3.1×10 −4 ; 2.88×10 −5 ; 1.96×10 −6 ; 7.14×10 −8 ; 4.15×10 −10 ; and −4.30×10 −10 . The radius R of the second surface (the imager side) of the meniscus  206  is 29.49554 mm and the parameters c and a 1  to a 8  have, respectively, the following values: 0; 0; 0; −2.7×10 −4 ; 9.97×10 −6 ; 6.34×10 −7 ; −1.41×10 −7 ; 8.98×10 −9 ; and −1.78×10 −10 . 
   Advantageously, according to the invention, the front lens group comprises three lenses, the two lenses located in the extreme positions having a power opposite that of the lens located in the middle. Thus, the front group contributes to generating a curvature of the intermediate image so that the projected image is plane, while comprising a small number of lenses. This makes its manufacture easier and reduces its cost. 
   Thus, the front lens group illustrated in  FIG. 10  comprises two negative-power lenses  714  and  716  (divergent lenses) surrounding a positive-power lens  715  (convergent lens). 
   According to a variant of the invention, the objective  71  is replaced with an objective of lower quality, in particular with an MTF (Modulation Transfer Function), the distortion moreover remaining similar to that of the objective  71 . Such an objective is less expensive, the material of the lenses itself being less expensive and the number of lenses being reduced. 
   According to a variant of the invention, the front lens group of the objective comprises two positive (convergent) lenses surrounding a negative lens. An objective  110  comprising such a front group and able to replace the objective  71  is illustrated in  FIG. 11 . 
   The objective  110  comprises a rear lens group consisting of the lenses  111  to  118  and a front lens group consisting of the lenses  119 ,  1110  and  1111 , said lens groups being placed on either side of the exit pupil P E  of the objective. 
   As an illustration, the characteristics of the objective  110  are given in the following table (the radii, thicknesses and diameters all being expressed in millimetres and the material of the lenses  111  to  119 ,  1110  and  1111  corresponding to the references of the products provided by the company OHARA®): 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
             
             
                 
               Radius of 
                 
                 
                 
             
             
                 
               curvature 
               Thickness 
                 
               Diameter 
             
             
               Lens 
               (in mm) 
               (in mm) 
               Material 
               (in mm) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               111 
               36.43399 
               16.245 
               acrylic 
               48 
             
             
                 
               −104.8294 
               0.500 
                 
               48 
             
             
               112 
               206.176 
               9.538 
                 
               46 
             
             
               113 
               −48.868 
               5.389 
                 
               46 
             
             
               114 
               33.95 
               14.933 
                 
               46 
             
             
                 
               −83.67 
               0.500 
                 
               46 
             
             
               115 
               infinite 
               12.658 
                 
               36 
             
             
               116 
               −22.648 
               2.626 
                 
               36 
             
             
                 
               infinite 
               0.500 
                 
               36 
             
             
               117 
               47.785 
               9.381 
                 
               28 
             
             
               118 
               −27.09 
               2.493 
                 
               28 
             
             
                 
               infinite 
               5.974 
                 
               28 
             
             
               PS 
               infinite 
               41.998 
                 
               16.58 
             
             
               119 
               33.768 
               6.849 
                 
               38 
             
             
                 
               120.842 
               14.926 
                 
               38 
             
             
               1110 
               −77.81 
               16.188 
                 
               41 
             
             
                 
               46.246 
               24.299 
                 
               41 
             
             
               1111 
               −86.258 
               9.816 
                 
               60 
             
             
                 
               −41.781 
               — 
                 
               60 
             
             
                 
             
           
        
       
     
   
   The device  7  has the advantage of a relatively small height h below the LCD screen  31 , typically between 10 and 20 cm for a display with a diagonal of about 1.50 m. This height h is in fact sufficient to house the objective  71  and the mirror  72 , while still forming a correct backlighting beam image  73  on the means  32  without the beam  37  encountering the objective  71 . Preferably, the height h is equal to one fifth (approximately) of the height of the display. More precisely, the height h is less than or equal to the height of the display divided by 5. 
     FIGS. 8 and 9  illustrate a side view and a top view, respectively, of the device  7  as shown schematically in  FIG. 7 . 
   To reduce the depth of the device  7 , a folding mirror  75  is interposed between the objective  72  and the concave mirror  72 . The dotted lines and the full lines represent, respectively, the elements with the beam  37 ,  73  not folded and folded, respectively. The mirror  74  is vertical (that is to say parallel to the screen  31 ), the optical axis of the beam  37  before the mirror  72  being horizontal. The long side of the imager  365  of the device  7  is horizontal (for a vertical LCD screen  32  whose long side is horizontal). 
   The objective  71  is placed along the side and folded preferably in a horizontal plane placed along the side, preferably with an axis parallel to the screen  32 , thereby making it possible to reduce the depth of the device  7 . The angle α that the mirror  75  makes with a normal to the screen  32  depends on the angle that the optical axis of the objective  70  makes with the screen  32 . When the objective  70  is parallel to the screen  32 , the angle α is equal to 45°. The distance between the objective  70  and the mirror  72  is such that the beam  73  does not encounter the objective  70 . 
   In general, all the optical axes of the various elements of the unfolded projection system are perpendicular to the plane of projection, assumed to be vertical. They are therefore horizontal (for a system as shown in unfolded form, with the exception of the folding due to the concave folding mirror). 
   In the device  7 , the real axis of the objective  70  remains horizontal, the screen  32  being vertical. The device  7  has a relatively small “chin”, that is to say a relatively small value of h. Thus, for a device with a 50″ LCD screen, the height h is for example less than 20 cm and typically equal to 12 cm. 
   However, in alternative embodiments that allow the illumination part to be housed more easily (inclination of the optical illumination core, lamp casing, electronic card attached to the modulator  365 ), the real axis of the objective is inclined. This is because the axis of one element of the projection system may become non-horizontal after being folded by a folding mirror. For example, if the large mirror is inclined, all the following elements will also be inclined through twice the angle, in particular the concave mirror. 
   According to a variant, the device  7  comprises other folding mirrors, especially in the objective and the mirror  72 . 
     FIG. 12  illustrates the optical properties of the device  7 . More precisely, the objective  71  creates, from the incident illumination beam, a first image comprising two points A and B indicated by way of illustration. Emanating from these two points A and B are two beams,  1202  and  1201  respectively, which form, after passing through the objective  70  comprising at least one lens  1202  and an exit pupil P E    1203 , two points A′ and B′ belonging to the image I S  created by the objective  71 . 
   The beams  1202  and  1201  are reflected, in non-discrete regions A″ and B″ respectively, off the mirror  72  and converge on a region corresponding to the pupil P F , the image of the pupil P E  via the mirror  72 . 
   It should be noted that the pupil P F  is relatively close to the mirror  72  and that the pupil P E  is further away from the mirror  72 . Typically, the distance of the exit pupillary region P F  from the vertex of the concave mirror  72  is between 25 mm and 60 mm. Preferably, the distance of the exit pupil  1203  from the concave mirror  72  must be as large as possible. 
   According to a variant, the objective comprising a diaphragm S consists of a first assembly formed from at least one lens and of a second assembly formed from at least one lens. The second assembly is positioned after the diaphragm S in the path of the imaging beam and is closer to the image I S  than the diaphragm S. Preferably, the distance d 1  between the second assembly and the exit pupil P′ E  of the first assembly is equal to or greater than three times the distance d 2  between the first assembly and the modulator  365 . The exit pupil P′ E  corresponds to an image of the diaphragm S formed by the first lens assembly. The second lens assembly positions the exit pupil P E  of the objective substantially at infinity. Thus, the second lens assembly rectifies the rays of the imaging beam and enables the size of the concave mirror to be reduced, while keeping, moreover, its shape. 
   Advantageously, according to a variant, the device  7  includes a mask (for example an apertured black plate, or glass or plastic plate) that includes a transparent region for passage of the projection beam  73 . The thickness of the mask is chosen to be as small as possible and preferably less than 2 mm, and even more preferably equal to 1 mm or less. This mask is located near the pupil P F  (typically at a distance of 5 mm or less from the pupillary region P F  corresponding to the exit pupil of the system comprising the objective and concave mirror) and absorbs the parasitic rays via a black region which does not cut off the beam  37 ,  73 . The black peripheral region corresponds either to a bulk-tinted region of the mask or to a treatment on one or both faces of the mask. Preferably, when the transparent region is a full region, it undergoes an antireflection treatment using techniques well known to those skilled in the art. The mask preferably extends as far as the boundary of the case of the device  7  and thus keeps the objective, the concave mirror and the corresponding modulator free of dust and/or eliminates (or reduces) the parasitic rays emanating from these elements or from the outside. The boundary of the transparent region of the mask comprise normal walls (thereby facilitating its manufacture) or inclined walls (so as to bring the path of the backlighting beam close to the surface of the mask). 
   Of course, the embodiments of the invention that include a concave mirror are compatible with the variants presented above and especially with implementation without a modulator, with or without means for providing a sequential colour beam (for example a colour wheel), with or without a uniformity filter in the image plane of the light guide (or any other means of creating a uniform source). 
   The invention is not limited to the embodiments described above and a person skilled in the art will be able to adapt the various elements of the systems and devices described above while still remaining within the scope of the invention. 
   In particular, the invention is compatible with any type of LCD screen (especially any shape and size). 
   According to the invention, there may be any number of folding mirrors even though preferably the backlighting system comprises at least two folding mirrors. Folding mirrors may also have any shape. Advantageously, the final folding mirror is plane. Preferably at least one folding mirror is not plane and, for example, is aspherical, and the construction of the objective and its relative position with respect to the non-plane folding mirror are such that the image of the illumination beam projected onto the LCD screen is substantially plane. 
   The invention is also compatible with any type of projection source with light-emitting diodes or an incandescent lamp associated with a reflector, with or without a colour wheel, with or without a modulator. 
   Likewise, the invention may be used within the context of a wall with several displays. Thus, according to the invention, several LCD displays may be used (for example two or three rows of superposed or juxtaposed LCD screens), all of the displays (replacing a display in the abovementioned embodiments) being back-lit by a single illumination source, and corresponding optic (objective, folding mirror, Fresnel lens) with or without a modulator.