Abstract:
The invention relates to a system for projecting or displaying images comprising a valve exhibiting a plurality of image-forming elements having a light transmission coefficient which can be controlled so as to present the image, and a means of illuminating the valve comprising a light source and an integrator having two lens arrays associated in such a way that each lens of the second array distributes over the valve the light received from a corresponding lens of the first array. This system comprises a means for focusing the illuminating beam onto the integrator. The dimensions of the integrator are thus minimized. The focusing means comprises, for example, a reflector which reflects the light produced by the source, the integrator being arranged substantially in the focal plane of the reflector.

Description:
RELATED APPLICATION 
     This application is a continuation application of U.S. patent application Ser. No. 09/209,204 entitled “SYSTEM FOR PROJECTING OR DISPLAYING IMAGES”, filed Dec. 10, 1998, on behalf of the inventors named herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an illuminating device having an optical integrator, especially for a system for projecting or displaying images. It also relates to such a system. 
     BACKGROUND OF THE INVENTION 
     In the projection (or display) of images, especially of television type, it is known practice to use a liquid-crystal optical valve, the transparency of each image-forming element of which is controlled as a function of the luminance of each point of the image to be formed. 
     The luminous energy for the projection is delivered by an illuminating device behind the valve and an optical system in front of the valve projects the image onto a screen. It is also possible to display the valve either directly or by way of an optical magnifying system. 
     In order for the projected or displayed image to correctly restore the image formed in the valve, it is necessary for the illuminating of the valve to be uniform. This problem is not easily solved, in particular, because the illumination originates from a source of limited extent. 
     One solution consists in using an optical integrator. An example of a known projector having an integrator is depicted in FIG.  1 . 
     In this example, the source  10  is associated with a reflector  12  and the pencils of light reflected by this reflector are collected by a first array  14  of small lenses  14   1 ,  14   2  . . . . 
     The shape (contour) and the proportions of each lens  14   i  correspond, preferably, to the shape and to the proportions of the optical valve  16 . For example, if the valve is rectangular with proportions of 4:3 or 16:9, the lenses will exhibit the same proportions. With each of these lenses  14   i  there is associated another small lens  18   i  of a second array  18 . This lens  18   i  is arranged in such a way that it forms the image of the lens  14   i  over the whole extent of the valve  16 . Thus, even if the various pencils of light originating from the reflector  12  have different energies, this heterogeneity does not result in a heterogeneity of illumination on the valve  16 , since the energy of each pencil is distributed over the whole surface of the valve. Furthermore, if the shape and the proportions of each lens  14   i  correspond to the shape and to the proportions of the valve  16 , losses of light are minimized. 
     The reflector  12  is, for example, elliptical and the source  10  is arranged at the first focus and the pencils received by the lenses  14   i  are slightly convergent. 
     A projection objective  20  is provided downstream of the valve  16 . 
     This projection system is of fairly high cost, especially since its various optical components are of large dimensions. 
     SUMMARY OF THE INVENTION 
     The invention makes it possible to reduce the dimensions of the optical components. 
     It starts from the finding that the aperture of the projection objective depends on the angle a at which the centre 0 of the valve sees the second array  18  of lenses and that it is therefore beneficial to decrease this angle and, as a consequence, to decrease the extent of the integrator. 
     Starting from this finding, the inventors have observed that the surface of the array  18  is not used in an optimal manner. This observation appears in FIG. 1 a  where it may be seen that the array  18 , of circular general shape, is illuminated at localized sites  22   1 ,  22   2 , etc., that is to say sites which are separated from one another, the sum of the areas of the illuminated zones being substantially less than the area of the disk formed by the array  18 . 
     The illuminating device according to the invention is characterized in that it comprises a means for focusing the light pencils from the reflector onto the integrator. 
     Thus, each lens of the first array is illuminated at a considerable angle; as a result of this the associated lens of the second array, which lens is intended to image the first lens on the valve, also receives a light beam of considerable angle, this making it possible to increase, preferably maximize, the area of the illuminated part of the second array of lenses. Furthermore, it is possible to minimize the dimensions of the optical integrator, thereby minimizing its cost. Decreasing dimensions of the arrays of the integrator leads to the desired decrease in the angle at which the valve sees the integrator and hence a decrease in the aperture of the projection objective. 
     Decreasing the aperture of the projection objective makes it possible to increase the contrast since it is known that the use of light rays which are steeply inclined with respect to the normal to the plane of a liquid-crystal valve leads to a decrease in the contrast. 
     To focus the beam from the reflector onto the integrator, there is provided either a focusing lens or an appropriately shaped reflector, for example an elliptically shaped reflector in which the source is arranged at the first focus and the integrator at the second focus. 
     The position of the integrator at a point of convergence of the beam makes it possible to use this integrator in a device for projecting or displaying colour images employing a single valve in which each image point is formed from several (in general three) image elements, each of these elements being assigned to a specified colour (red, green and blue, for example), the optical illuminating system being such that a light beam of a specified colour reaches only the image element assigned to this colour. The coloured beams are created from a source of white light and from means of angular separation according to colour. The position of the integrator is such that it does not modify the relative orientation of the rays and thus makes it possible to retain the angular separation of the colours. 
     In one embodiment, each beam of a specified colour illuminates a part of the surface of the integrator which is assigned thereto. Stated otherwise, the surface of the integrator is divided into several distinct parts and each part is used for a single colour. As a variant, provision is made for juxtaposed integrators, the number of integrators being equal to the number of colours. 
     In embodiments in which maximum illumination on the valve is desired, a collecting lens is employed between the integrator and the valve so as to superimpose the various beams reaching the screen. 
     If no collecting lens is used, it is possible to maximize the uniformity of illumination and hence the contrast of the projected or displayed image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will emerge with the description of certain of its embodiments, this being done with reference to the hereinappended drawings in which: 
     FIGS. 1 and 1 a , both already described, depict a known system for projecting images, 
     FIG. 2 is a diagram of a projection system in accordance with the invention, 
     FIG. 3 is a partial view, on a larger scale, of the optical integrator of the system of FIG. 2, 
     FIG. 4 is a diagram of a system according to the invention allowing the projection of colour images with the aid of a single valve, 
     FIGS. 4 a  and  4   b  show details of the system of FIG. 4 and, 
     FIG. 5 is a diagram of a system according to the invention allowing the projection of colour images with the aid of three valves. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The projection system depicted in FIG. 2 comprises a reflector  22  having the shape of a part of an ellipsoid of revolution with axis  24 . A light source  26  is arranged at the first focus F 1  of the ellipsoid. 
     The optical integrator  28  is arranged in the vicinity of the second focus F 2  of the ellipsoid. Thus, according to the invention, the integrator  28  is arranged at the point of convergence of the illuminating beam, or in the vicinity of such a point. 
     The integrator  28  comprises, on the one hand, a first array  30  of lenses  30   1 ,  30   2 , etc. which receives directly the beam  32  originating from the reflecting of the radiation from the source  26  off the internal surface of the reflector  22  and, on the other hand, a second array  34  of lenses  34   1 ,  34   2 , etc. arranged a short distance from the first array  30  on the opposite side from the source  26 . 
     The arrays  30  and  34  are arranged in a plane perpendicular to the axis  24 . 
     Following the integrator  28  is arranged a collecting lens  36 , the role of which will be explained later. 
     The source  26 , the reflector  22 , the integrator  28  and the collecting lens  36  make it possible to illuminate the valve  16  in a uniform manner. Following this valve  16  is arranged an objective  38  for projecting onto a screen (not depicted). 
     The arrays  30  and  34  are associated in such a way that to each lens  30   i  of the array  30  there corresponds a lens  34   i  of the array  34 . 
     The lenses of the arrays  30  and  34  are, for example, made of glass. 
     In a manner known per se, the arrangement of the arrays  30  and  34  and of the valve  16  is such that each lens  30   i  is in a position which is conjugate with the position of the valve  16  with respect to the associated lens  34   i . In other words, each lens  34   i  forms, on the valve  16 , the image of the associated lens  30   i . Of course, in the example, the collecting lens  36  participates in the formation of the image. 
     Furthermore, likewise conventionally, each lens  30   i  has substantially the rectangular shape and the proportions of the valve  16  and the magnification of each lens  34   i  is such that the image of the lens  30   i  occupies the entire surface of the valve  16 . 
     The first array  30  has a rectangular general shape and the second array  34  exhibits any shape such as that of a disc, a square or a hexagon. 
     In the example depicted in FIG. 3, the array  30  is formed on the first face  52  of a glass plate  50  and the array  34  is formed on the second face  54  of the same glass plate  50 . As a variant (not shown), each array is formed on a distinct glass plate and the two plates are affixed back to back. 
     The focal plane of the lenses  30   i  lies substantially in the plane of the lenses  34   i  so that all of the light originating from the source passes through the lenses of the second array. 
     According to another arrangement of the invention, the lenses  30   i  and  34   i  as well as the reflector  22  are configured in such a way that a converging beam  40  (FIG. 3) on a lens  30   i , that is to say a beam whose vertex passes through the optical centre of the lens  30   i , occupies almost the whole of the pupil of the associated lens  34   i . Thus, the lenses  34   i  being adjoining, the light flux occupies at most the surface of the array  34 . 
     The numerical aperture of the lenses  30  and  34  is, stated otherwise, of the same order of magnitude as the numerical aperture of the illumination in the plane of the focus F 2 . For an angle of incidence β of the order of 25°, the numerical aperture is 1:1.5 approximately. 
     The integrator  28  being arranged at a site where the energy is concentrated, the components of this integrator can be of reduced size and hence relatively inexpensive. 
     Moreover, the angle β at which the valve  16  sees the integrator being small, it follows that the numerical aperture of the objective  38  downstream of the valve  16  is likewise of a small value, thus making it possible to minimize the cost of this projection objective. 
     It should be noted that the minimizing of the angular aperture of the beam illuminating the valve  16  also leads to an improvement in the contrast of the projected image, since the rays which are steeply inclined with respect to the normal to the plane of the valve  16  are eliminated, these inclined rays impairing the contrast. 
     As a variant (not depicted), the reflector has the shape of a paraboloid of revolution and the light source lies at the focus of this paraboloid. In this case, the beam produced by the reflector is focused with the aid of one (or more) lens(es), the integrator then lying in the focal plane, or in the vicinity of the focal plane, of this lens. 
     In the example described in conjunction with FIGS. 2 and 3, the collecting lens  36  makes it possible to convey the whole of the light flux onto the valve  16 . In a variant (likewise not depicted), this collecting lens  36  is not provided. In this case, a loss of light flux may occur, this loss depending on the characteristics of the lamps used. However, it is then possible to maximize the degree of uniformity of the illumination and hence the contrast. 
     The embodiment described in conjunction with FIGS. 2 and 3 corresponds to the displaying or projecting of a monochrome image with a liquid-crystal valve  16 . Two example applications of the invention to the projecting or to the displaying of colour images will now be described in conjunction with FIGS. 4,  4   a  and  5 . 
     Reference is made firstly to FIGS. 4 and 4 a.    
     In this example, the valve  60  is such that the image-forming elements are grouped in triplets  72   R ,  72   V  and  72   B  (FIG. 4 a ) and each of the elements of a triplet is associated with a colour, generally red R, green G and blue B. Furthermore, the illuminating device produces light beams in each of the colours which have different orientations. Thus, it may be seen in FIG. 4 that the central pencil  64  of the green-coloured beam is along the axis  66  of the projection system, whilst the central pencil  68  of the red-coloured beam R is inclined with respect to the axis  66 , and the central pencil  70  for the blue colour is inclined with respect to the axis by the same angle as the pencil  68 , but symmetrically with respect to the axis  66 . 
     The inclinations of these pencils  64 ,  68  and  70  are such that each beam converges, by virtue of an array  62  of lenses  62   1 ,  62   2 , etc. (FIG. 4 a ), only towards the image-forming elements assigned to the corresponding colour. Thus, the green-coloured beam reaches only the image elements  72   G . Likewise, the respectively red- and blue-coloured beams reach only the image elements  72   R  and  72   B  provided for these respective colours. 
     In the example, the reflector  80  has the shape of a paraboloid of revolution about the axis  66  and the source  82 , at the focus of this paraboloid, produces white light. 
     The parallel beam reflected by the reflector  80  reaches a device  84  for separating the colours. This device  84  is for example of the type described in French Patent No. 9308470 in the name of Thomson CSF, that is to say it comprises a means for dispersing the colours by means of a holographic grating and a mask with apertures. The apertures are arranged in triplets; each aperture corresponds to a specified colour and is associated with a corresponding image-forming element of the valve  60 . 
     When the colours are correctly separated, the apertures of the mask comprise no filters. If the distribution of the colours is not satisfactory, filters are provided on the apertures. 
     The beam originating from the device  84  is focused by a lens  86 . As may be seen in FIG. 4, the light beams issuing from this lens  86  have different orientations and reach different zones of the integrator  90 . Thus, the integrator  90  is separated into three parts, a central part  92  for the green beam, a lateral part  94  for the red beam and another lateral part  96  for the blue beam. 
     The separation of the integrator  90  into three parts is effected with the aid of a mask  89  arranged, in the example, on that side of the integrator facing the valve  60 . This mask is depicted in plan in FIG. 4 b . In this figure, the direction d 1  corresponds to the direction d in FIG.  4 . It comprises apertures  92 ,  94  and  96  of rectangular shapes. 
     These apertures, and hence the zones  92 ,  94  and  96 , exhibit contours which are homothetic with the contours of the corresponding zones of the pixels, each zone being imaged, by a lens  62   i , in the plane of the pixels of the corresponding colour. Stated otherwise, the zones  92 ,  94  and  96  are homothetic with the triplets of pixels (image elements,  72   B ,  72   G  and  72   R . 
     As in the example of FIG. 2, a collecting lens  98  is provided. 
     Moreover, a field lens  100  is arranged in front of the array  62 . 
     With this integrator, uniformity of illumination is obtained for each colour, the colour-based integrators being separate. Under these conditions, on the valve  60 , the quality of the colours is optimal. 
     Moreover, this projection system also allows correct saturation of the colours. 
     As in the embodiment of FIG. 2, good uniformity of illumination and optimal use of the light flux are obtained. 
     The embodiment depicted in FIG. 5 is a colour image projection system in which the images originating from three valves,  110  for the green colour,  112  for the red colour and  114  for the blue colour respectively, are superimposed. In this example, an integrator is provided for each colour, that is to say for each valve. 
     The colours are separated by virtue of dichroic mirrors which are placed upstream of the integrators. This results in a saving of space, that is to say, as compared with conventional systems, a reduction in bulk. 
     The source  116  emits white light which is reflected, by the reflector  118 , towards a first dichroic mirror  120 . Thus, this mirror  120  transmits a red beam onto the integrator  122  and reflects the remaining components, green and blue, towards a second dichroic mirror  124 . The beam originating from the mirror  120  is thus partially reflected towards the integrator  126  assigned to the green colour and is partially transmitted towards the integrator  128  assigned to the blue colour. 
     With each integrator there is associated a collecting lens, as in the embodiment depicted in FIGS. 2 and 3, as well as a projection lens,  130 ,  132  and  134  respectively. 
     Deviating mirrors make it possible to combine all of the beams onto the same axis  140 , which is, in the example, the axis of the blue beam. Thus, the red beam is deviated through 90° by a mirror  142  arranged between the collecting lens assigned to the integrator  122  and the projection objective  132 . The beam reflected by the mirror  142  passes through a semi-reflecting mirror  144  so as to be reflected by another semi-reflecting mirror  146  which conveys the red beam into the direction of the axis  140 . The green beam originating from the valve  110  is reflected by the mirror  144  as well as by the mirror  146 . 
     A mirror  150  conveys the blue beam into the direction of the axis  140 , parallel to the axis of the reflector  118 .