Abstract:
An arrangement for projecting a multi-color image, including at least two image-generating elements which enable generation of one partial color image each of the multi-color image, and a color combiner which superimposes the light partial beam paths and emits it as a common beam. The arrangement further includes projection optics. The image scales being different for the partial color images, one optical device each is arranged in one or more of the partial beam paths of the partial color images whose image scales differ and each optical device is configured such that, due to the change in the image scale of the corresponding partial color image effected by it, the image scale of the partial color image corresponds to a predetermined, common image scale.

Description:
FIELD OF INVENTION 
   The invention relates to an arrangement for projecting a multi-color image onto a projection surface. 
   BACKGROUND 
   Said arrangement comprises at least two image-generating elements which are controllable using a control unit and which enable generation of a partial color image of the multi-color image, and a color combiner arranged following the image-generating elements, wherein the light for the corresponding partial color image coming from the image-generating elements is guided, in one partial beam path each, to the color combiner, which superimposes the light guided in the partial beam paths and emits it as a common beam, said arrangement further comprising projection optics arranged following the color combiner, on which projection optics the common beam impinges and which project the muilticolor image onto the projection surface, and the image scales given by said arrangement being different for two or more partial color images. 
   In such arrangements for projecting a multi-color image, there is a need to correct the chromatic aberration of magnification of the projection optics (the different image scales), which causes the differently colored components of a pixel, which is to be represented on the projection surface, not to overlie each other. In conventional objective lenses, the chromatic aberration of magnification is corrected by combining optical glasses differing in dispersion (e.g. optical crown/optical flint), and for higher requirements, use may be made of expensive special media having a specific partial dispersion. However, in an arrangement for projecting a multi-color image of the above-mentioned type, correction of the chromatic aberration of magnification is made more difficult, on the one hand, because the color combiner requires a telecentric beam path for the projection light, as the color combiner would otherwise not function sufficiently well, and, on the other hand, because the intercept distance of the projection optics is substantially increased due to the color combiner arranged between the image-generating elements and the projection optics. This leads to more complex and, thus, more expensive projection optics, with the required narrow tolerancing of the dispersion of the employed optical glasses, in particular, leading to a further increased cost. 
   In view of the above, it is the object of the present invention to improve an arrangement for projection of the aforementioned type in a manner allowing adjustment of the different image scales of the partial color images to each other with minimum effort. 
   SUMMARY OF THE INVENTION 
   According to the invention, this object is achieved by an arrangement for projecting a multi-color image onto a projection surface of the aforementioned type in that one optical device each is arranged in one or more of the partial beam paths of the partial color images, whose image scales differ, and each of said optical devices is generated such that, by the change in image scale of the corresponding partial color image effected by it, the image scale of said partial color image corresponds to a predetermined, common image scale. 
   Thus, according to the invention, by arranging the optical device in a partial beam path or by arranging several optical devices in several partial beam paths, use is made of the fact that the arrangement for projecting a multi-color image comprises several color channels in which one partial color image each is generated. Thus, the corresponding optical device in the partial beam path or color channel (and, thus, before the location where the light of the other color channels is superimposed on the light of this partial beam) allows a desired prior magnification or prior reduction to be effected only for this color channel, independently of the other color channels. Said prior magnification or reduction is chosen such that the desired achromatization is achieved. In cases where one optical device each is arranged in several color channels, the prior magnification or prior reduction, and thus, finally, the magnification or reduction (image scale) of the arrangement according to the invention for each color channel or each partial color image can be defined separately and independently of the other color channels (partial color images). This is always effected in such a way that the image scale of the partial color images corresponds to a predetermined, common image scale, so that the chromatic aberration of magnification or of reduction is considerably reduced or, if possible, completely eliminated. 
   In a preferred further embodiment of the arrangement according to the invention, the optical device generates a virtual intermediate image of the partial color image generated by the image-generating element of the first partial beam path. The advantage of generating a virtual intermediate image is that it may be generated very near the object (the image-generating element) (for example, between the image-generating element and the optical device), so that there is practically no change in the dimensions of the entire arrangement. 
   In particular, the optical device in the arrangement according to the invention may comprise at least one lens, which is preferably plano-concave. Using this lens, the optical device can be realized in a simple manner, so that the desired achromatization may be realized by only one lens arranged in the corresponding color channel. This also has the result that the construction of the projection optics is considerably simplified as compared to projection optics wherein achromatization is effected in the projection optics themselves. Of course, the optical device may also comprise several lenses. 
   A further advantageous embodiment of the arrangement according to the invention consists in that the optical device comprises a material interface of the color combiner located preceding the point of superimposition in the light propagation direction, said material interface being curved (preferably spherical). This has the advantageous effect that no additional optical element is required in order to realize the optical device. It is merely required to provide an existing material interface with a suitable curvature. 
   In particular, the arrangement according to the invention may be further embodied such that three image-generating elements are provided for the colors green, red and blue, the optical device being arranged in the partial beam path of the image-generating element for the color green. The optical device is preferably adapted to effect a prior reduction. In this case, the achromatization for the colors red and blue may be realized either by correspondingly provided projection optics or by a corresponding optical device in the red and/or blue partial beam path. In this manner, an arrangement is provided with which multi-color images having very good color properties can be displayed on the basis of the primary colors, red, green and blue. 
   An advantageous embodiment of the arrangement according to the invention consists in that the image-generating elements are provided in the form of LCD modules. These modules enable a very high resolution. They may be provided as transmitting or reflecting modules. If they are provided as reflecting modules, then the color combiner is advantageously also employed as color splitter, in particular, for white illumination light. Further, it is also possible to realize the image-generating elements by tilting mirror matrices including a plurality of tilting mirrors arranged in lines and columns, which may be brought into at least two different tilting positions. In this case, too, the color combiner is preferably also employed as a color splitter for the illumination light. 
   The arrangement according to the invention can project the multi-color image generated by the image-generating elements in magnified, reduced or unmagnified form (the image scale then being  1 ). 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is explained in more detail below, essentially by way of example, with reference to the drawings, wherein: 
       FIG. 1  shows a view of the arrangement for projecting a multi-color image according to the invention; 
       FIG. 2  shows an enlarged view of the color unit and of the projection optics of the arrangement shown in  FIG. 1 , and 
       FIG. 3  shows an enlarged view of the detail A of FIG.  2 . 
       FIG. 4  shows an enlarged view of the detail A of  FIG. 2  for another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The arrangement for projecting a multi-color image comprises an image-generating portion  1 , on which white light  2  from a light source  3  may be incident, and a color unit  4 , which is employed both as a color splitter and as a color combiner, as will be described in more detail hereinafter. 
   The color unit  4  splits the white light  2 , which is incident thereon, up into the colors red, green and blue, and guides the light of each of these colors to an image-generating element  5 ,  6  and  7  which, in the embodiment example presently shown, is a reflecting LCD module in each case. These LCD modules  5  to  7  comprise individually controllable image pixels, which are arranged in a matrix of lines and columns and are controlled by a control unit  8 . Each image pixel may be switched to at least a first condition, wherein the polarization direction of the reflected light is not rotated, and to a second condition, wherein the polarization direction of the reflected light is rotated, preferably through 90°, so that the light reflected by the LCD modules  5  to  7  is polarization-modulated. The reflected light is superimposed to form a common beam by the color unit  4  and is projected, by projection optics  9  arranged following the color unit  4 , onto a projection surface  10  so as to represent the multi-color image on the latter. 
     FIG. 2  shows the color unit  4  and the projection optics  9  in an enlarged view, and for better comprehension of the function of these elements, projection beam paths are indicated. The white illumination light  2  first impinges on a polarizer  11 , which only allows light to pass which is polarized perpendicular to the drawing plane, which light then impinges on a retarder  12  arranged following the polarizer  11 , which passes through the light incident thereon and is adapted to rotate only the polarization direction of the green light through 90°, but does not change the polarization direction of the remaining light. The green light, which is now polarized in parallel with the drawing plane, and the remaining light (red and blue light), which is polarized perpendicular to the drawing plane, subsequently reaches a polarization splitter cube  13  containing a polarization splitter layer  14 , which is inclined at 45° relative to the propagation direction of the white light  2 . The light polarized in parallel with the drawing plane (i.e. the green light) is reflected by this polarization splitter layer  14 , and the light polarized perpendicular to the drawing plane is allowed to pass, so that the green light is deflected to the left, as viewed in  FIG. 2 , while the red and blue light passes through the polarization splitter layer  14  and is upwardly propagated. 
   By a color-splitting layer  16  (dichroic layer) in a color-splitting cube  15  arranged following the polarization splitter cube  13 , the color-splitting layer  16  being aligned in parallel with the polarization splitter layer  14 , said red and blue light is then split up into the red and blue color components, such that the blue light is reflected and, thus, deflected to the left, as viewed in  FIG. 2 , as well as directed to the LCD module  6 , and that the red light passes through the color-splitting layer  16  and impinges on the LCD module  7 . 
   The green light, which is reflected to the left, as viewed in  FIG. 2 , by the polarization splitter layer  14 , is guided through a glass block  17 , which is subsequently arranged in this direction and is provided to adapt the path in glass for the green light to the paths in glass for the red and blue light, and is then directed to the LCD module  5  by the field lens  18 . Said field lens  18  is provided to compensate for the chromatic aberration of magnification of the arrangement for projecting a multi-color image, in particular of the projection optics, of the color green relative to the colors red and blue. The exact function of the field lens  18  will be explained in further detail with reference to FIG.  3 . 
   Each of the LCD modules  5  to  7  comprises a cover glass  19 ,  20 ,  21  and, behind it, the image-generating area  22 ,  23 ,  24 , which is now schematically represented in the Figures by a line. 
   In operation, the LCD modules  5  to  7  are controlled by the control unit  8  such that those image pixels which are to be brightened rotate the polarization direction of the light reflected by them through 90°, while the remaining image pixels, which are to be darkened, do not rotate the polarization direction of the light reflected by them, so that a green, a blue and a red partial color image of the multi-color image to be projected are generated by the LCD modules  5 ,  6  and  7  (for adjustment of the brightness of the individual image pixels, the time period during which they are brightened, may be correspondingly selected). Said partial color images are superimposed in a common light beam by the color-splitting layer  16  and the polarization beam-splitting layer  14 , with the reflected green light, whose polarization direction is not rotated (light polarized in parallel with the drawing plane), being downwardly reflected, as viewed in  FIG. 2 , by the polarization beam-splitting layer  14 , while the light polarized perpendicular to the drawing plane passes through the polarization beam-splitting layer  14 . The same applies to the red and blue light, in which case the reflected light which is polarized perpendicular to the drawing plane passes through the polarization beam-splitting layer  14 , while the reflected light polarized in parallel with the drawing plane is reflected to the right, as viewed in  FIG. 2 , by the polarization beam-splitting layer  14 . 
   This common beam formed by the polarization beam-splitting layer  14  then impinges on a retarder  25 , which only rotates the polarization direction of the green light through 90° and does not influence the polarization direction of the remaining light (the red and blue light), so that, following the retarder  25 , the red, green and blue light is, respectively, polarized in parallel with the drawing plane. Said light then impinges on an analyzer  26 , which only allows light to pass which is polarized in parallel with the drawing plane. The retarder  25  and the analyzer  26  serve to increase contrast, since the polarization beam-splitting layer  14  undesirably also allows light to pass, which is polarized in parallel with the drawing plane (i.e. actually darkened pixels), due to the different angles at which the green light reflected by the LCD module  5  impinges on the polarization beam-splitting layer  14 . However, the retarder  25  and the analyzer  26  fade out the majority of said light. The same applies to the reflected red and blue light, which is polarized perpendicular to the drawing plane and should actually not be reflected by the polarization beam-splitting layer  14  in the direction of the projection objective  9 . 
   The retarders  12 ,  25 , the polarizers  11 ,  26 , the polarization beam-splitter cube  13  comprising the polarization beam-splitter layer  14 , and the color-splitting cube  15  comprising the color-splitting layer  16 , are selected by the skilled person on the basis of their technical knowledge. 
   The analyzer is followed by the projection optics  9 , which, on the basis of the color unit  4 , jointly project the partial color images generated by the LCD modulators  5  to  7  onto the projection surface  10  as the desired multi-color image. 
   The projection optics  9  are designed such that they are achromatized for the red and blue partial color images, so that the red and blue color components of a pixel are superimposed on one another on the projection surface  10 . 
   In order to also achromatize the green partial color image relative to the red and green partial color images, the field lens  18  is provided, whose shape is selected such and which is arranged in the color partial beam path (i.e. the beam path of the green light before being superimposed by the color unit  4 ) such that the green partial color image is subjected to a slight prior reduction. Together with the magnification of the projection objective  9 , this will then lead to an achromatization also of the green partial color image. 
     FIG. 3  schematically shows the prior reduction by the field lens  18 , said representation not being to scale and the cover glass  19  of the LCD module  5  not being shown. The field lens  18  is a plano-concave lens which forms a virtual intermediate image  27  of the image-generating area  22  of the LCD module  5 , which area is located between the field lens  18  and the LCD module, said virtual intermediate image  27  being smaller than the image-generating area  22  of the LCD module  5 . In the presently described case, the chromatic aberration of magnification for the green light is 1‰, so that, in order to correct it, the distance s of the field lens  18  from the image-generating area  22  was selected to be 2.3 mm for a focal length f=−3248. Therefore, the intermediate image  27  is located nearly in the plane of the image-generating area  22 . 
   The exact configuration of the optical system shown in  FIG. 2  is evident from the following Tables, wherein the radiuses of curvature are indicated without algebraic signs, since  FIG. 2  shows whether the corresponding surface is convex or concave: 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Radius of 
                 
                 
             
             
               Surface 
               curvature 
                 
               Distance between vertexes 
             
             
               number 
               (mm) 
               Surfaces 
               (mm) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               101 
               ∞ 
               101-102 
               1.4 
             
             
               102 
               ∞ 
               102-103 
               0.9 
             
             
               103 
               ∞ 
               103-104 
               2.5 
             
             
               104 
               1700.0 
               104-105 
               2.7 
             
             
               105 
               ∞ 
               105-106 
               69.5 
             
             
               106 
               ∞ 
               106-107 
               2.2 
             
             
               107 
               ∞ 
               107-108 
               1.5 
             
             
               108 
               ∞ 
               108-109 
               1.4 
             
             
               109 
               ∞ 
               109-110 
               3.9 
             
             
               110 
               57.3 
               110-111 
               7.5 
             
             
               111 
               57.3 
               111-112 
               2.2 
             
             
               112 
               40.1 
               112-113 
               9.0 
             
             
               113 
               40.1 
               113-114 
               2.5 
             
             
               114 
               144.3 
               114-115 
               0.5 
             
             
               115 
               23.1 
               115-116 
               6.8 
             
             
               116 
               63.9 
               116-117 
               2.5 
             
             
               117 
               63.9 
               117-118 
               13.5 
             
             
               118 
               54.0 
               118-119 
               4.0 
             
             
               119 
               18.3 
               119-120 
               1.5 
             
             
               120 
               11.2 
               120-121 
               5.8 
             
             
               121 
               11.2 
               121-122 
               1.5 
             
             
               122 
               42.3 
               122-123 
               53.1 
             
             
               123 
               21.7 
               123-124 
               4.5 
             
             
               124 
               37.2 
               124-125 
               4.9 
             
             
               125 
               34.6 
               125-126 
               5.5 
             
             
               126 
               76.6 
               126-10  
               358.8 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
                 
                 
               Abbe 
                 
             
             
                 
               Number of the 
               Refractive 
               dispersion 
               Diameter 
             
             
                 
               optical element 
               index 
               number 
               (mm) 
             
             
                 
                 
             
           
           
             
                 
               19 
               1.519 
               63.96 
               22 
             
             
                 
               18 
               1.519 
               63.96 
               22 
             
             
                 
               17 + 13 
               1.855 
               23.64 
             
             
                 
               25 
               1.519 
               63.96 
               31 
             
             
                 
               26 
               1.519 
               63.96 
               31 
             
             
                 
               31 
               1.621 
               49.54 
               33 
             
             
                 
               32 
               1.489 
               70.23 
               29 
             
             
                 
               33 
               1.813 
               25.16 
               29 
             
             
                 
               34 
               1.489 
               70.23 
               23 
             
             
                 
               35 
               1.813 
               25.16 
               23 
             
             
                 
               36 
               1.813 
               25.16 
               10 
             
             
                 
               37 
               1.610 
               56.37 
               11 
             
             
                 
               38 
               1.624 
               36.11 
               11 
             
             
                 
               39 
               1.839 
               42.48 
               12 
             
             
                 
               40 
               1.527 
               56.26 
               48 
             
             
                 
               41 
               1.776 
               49.38 
               62 
             
             
                 
                 
             
           
        
       
     
   
   The lens surface indicated by reference numeral  123  is aspherically ground, with the aspheric parameter being indicated in the ISO nomenclature in Table 3. The aspheric equation according to ISO is as follows. 
       z   =           h   2     rad     ⁢     1     1   +       1   -       (     1   +   k     )     ⁢       h   2       rad   2                   +       ∑     i   =   2     5     ⁢           ⁢       c   ⁡     (   i   )       ⁢     h     2   ⁢   i                 
 
wherein h is the distance to the optical axis OA and z is the distance to the vertex plane (the plane which is perpendicular to the optical axis OA and contains the point of intersection of the vertex of the surface comprising the optical axis OA).
 
   
     
       
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               rad 
               k 
               c(2) 
               c(3) 
               c(4) 
               c(5) 
             
             
                 
             
           
           
             
               21.70000 
               −9.59292 · 10 −1   
               5.3713940 · 10 −7   
               −7.238548 · 10 −9   
               1.4979671 · 10 −11   
               −1.630338 · 10 −14   
             
             
                 
             
           
        
       
     
   
   In an alternative embodiment (not shown), the surface  105  of the glass block  17  may have a curved design, instead of the field lens  18 , and assume the function of the field lens  18 , so that there is, advantageously, no need to provide an additional lens.