Abstract:
The invention relates to a 3-panel transmissive projection system. In particular, the invention relates to a 3-panel transmissive projection system applying reflective type polarizers for both polarizing and analyzing operations in the projection system.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a 3-panel transmissive projection system. In particular, the invention relates to a 3-panel transmissive projection system applying reflective type polarizers for both polarizing and analyzing operations in the projection system.  
       BACKGROUND OF THE INVENTION  
       [0002]     Projection systems, such as described in, for example, US patent application no. 2002/0015135, generally use a reflective LCD array with a single polarizing beam splitter. However, by combining the light path from the light source and the display panels with the light path between the display panels and the projection lens, the light paths cannot be optimized individually.  
         [0003]     The high-temperature (HT) polyfilm technology provides high brightness using small miniaturized LCD panels. However, the combination of miniaturisation and high light output causes extreme high light densities in the light path, thus limiting the lifetime expectancy of the LCD panels and polarizing films. The manufacturers of HT polyfilm projection systems continuously improve the lifetime expectancy of the LCD panels. However, improvements in the lifetime expectancy of the polarizing films has almost come to a halt. Hence, the lifetime expectancy of the HT polyfilm projection system is limited by the lifetime of the polarizing films.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0004]     It is an object of the present invention to provide a projection system having high brightness capabilities in combination with an improved lifetime expectancy.  
         [0005]     It is a further object of the present invention to provide a transmissive type projection system, such as transmissive HT Poly silicon LCDSs, so as to ensure that the light path between the light source and the display panel(s) and the light path between the display panel(s) and the projection lens are completely separated, and therefore may be optimized individually, leading to a higher system efficiency and to a higher brightness.  
         [0006]     A particular advantage of the present invention is the provision of a low-cost projection system having a high brightness and a long lifetime expectancy by using miniaturized transmissive display panels.  
         [0007]     A particular feature of the present invention relates to the provision of polarizers for the analyzing operation and for the polarization operation of the projection system.  
         [0008]     According to a first aspect of the present invention, this object is achieved by a projection system for projecting an image onto a projection surface, the projection system comprising: 
        (a) a light source for supplying light;     (b) an optical element for gathering and focusing said light, thereby providing a light beam;     (c) a first reflective polarizer for polarizing said light beam, thereby generating a polarized light beam;     characterized in that the projection system further comprises:     (d) a transmissive display panel for receiving said polarized light beam and for manipulating said polarized light beam, thereby encoding image information on said polarized light beam and generating an encoded light beam;     (e) means for controlling each pixel of said transmissive display panel so as to control manipulation of said polarized light beam; and     (f) a second reflective polarizer for rejecting unwanted polarizations of said encoded light beam and for transmitting desired polarization of said encoded light beam to said projection surface.        
 
         [0016]     In this context, the term image is to be construed as a frame of a video sequence, a still photograph or a still digital representation or any combination thereof  
         [0017]     The second reflective polarizer according to the first aspect of the present invention may be oriented with respect to the encoded light beam at incident angles in the range between approximately 30° and 60°, such as incident angles of 35°, 45° or 55°. By orienting the second reflective polarizer acting as an analyzer at an angle of approximately 45°, ghost images generated by light bounced back from the second reflective polarizer to the display panel are avoided.  
         [0018]     The projection system according to the present invention may be realised by folding the light path from the light source to the projection surface in a two-layer structure. By folding the light path, the projection system advantageously provides a very compact projection system.  
         [0019]     The transmissive display panel may comprise an electro-optical medium such as liquid crystal or plasma, or electrochromic or electrophoretic elements, light-emitting elements, organic or inorganic light-emitting elements, polymer light-emitting elements, or any combination thereof. Any type of display element may be used for the transmissive display panel as long as the display substrates are transparent or opaque. The flexibility of a transmissive display panel type provides a projection system which may be designed in accordance with a wide variety of customer requirements or specifications.  
         [0020]     The means for controlling each pixel of the transmissive display panel according to the first aspect of the present invention may be implemented by using any processor techniques known to persons skilled in the art. The means for controlling each pixel may be incorporated on the transmissive display panel substrate, thereby reducing the required space and optimizing the production costs.  
         [0021]     The second reflective polarizer may comprise a Moxtek™ beam splitter. By utilising a Moxtek™ beam splitter for the analyzing operation of the projection system, an excellent brightness, low cost, and long lifetime expectancy are obtained. The Moxtek™ beam splitter removes the disadvantages of the polarizer films. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The above, as well as additional objects, features and advantages of the present invention, will be better understood from the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, wherein:  
         [0023]      FIG. 1  is a schematic diagram of the elements and light path for one colour in the preferred embodiment of the present invention; and  
         [0024]      FIG. 2  is a detailed diagram of the elements and light paths for red, green and blue colours, shown unfolded for the sake of simplicity, of the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]     In the following description of various embodiments, reference is made to the accompanying Figures which form a part thereof, and in which various embodiments in which the invention may be practised are shown by way of illustration. It is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present invention.  
         [0026]      FIG. 1  shows a projection system, designated in its entirety by reference numeral  10 , for projecting images onto a projection surface  12 . The projection system  10  comprises a light source  14  supplying the light to be transmitted through the projection system  10 . The projection surface  12  may be formed on any type of surface such as a white wall or a projector screen.  
         [0027]     The light source  14  supplies light to an optical element  16  for gathering and focusing the light, thereby providing a light beam. The optical element  16  may be implemented by a rod integrator. The optical element  16  comprises a first end  18  for receiving the light and a second end  20  for providing the gathered and focused light. A small colour separation prism  22  is placed adjacent to the second end  20 . An entrance surface  24  of the colour separation prism  22  is substantially equal to the surface of the second end  20 . The colour separation prism  22  separates the light into red, blue and green coloured light, respectively, which is subsequently reflected onto separate exit planes of the colour separation prism  22 . For reasons of simplicity,  FIG. 1  shows only one light path for one colour.  
         [0028]     The coloured light  26  exiting the colour separation prism  22  is directed through a first lens  28  focusing the coloured light  26  onto a first polarizer  30  which is transmissive to unwanted polarizations of coloured light and reflective to desired polarizations, i.e. reflecting a polarized light beam  32 . In an alternative embodiment of the present invention, the first polarizer  30  may be reflective to unwanted polarizations of coloured light and transmissive to desired polarizations. This, however, obviously requires a change of the design and the light path from the light source to the projection surface.  
         [0029]     In addition, the first polarizer  30  may be reflective to both desired and undesired polarizations of the coloured light. The desired polarizations of the coloured light are directed in one direction and the undesired polarizations are directed in another direction.  
         [0030]     The polarized light beam  32  is focused through a second and third lens  34  communicating the polarized light beam to a transmissive display panel  36  which modulates the polarized light beam so as to encode image information thereon. The transmissive display  36  panel is controlled by a processor controlling each pixel of the transmissive display panel  36 .  
         [0031]     The transmissive display panel  36  may be implemented in a number of ways. By way of example, a transmissive display panel having an opaque substrate may utilise an electro-optical medium such as liquid crystal or plasma, or electrochromic or electrophoretic elements, light-emitting elements, organic or inorganic light-emitting elements, polymer light-emitting elements, or any combination thereof  
         [0032]     In the preferred embodiment of the present invention, the transmissive display panel  36  utilises a liquid crystal display array.  
         [0033]     As described above, the colour separation prism  22  is placed adjacent to the optical element  16  so as to form an extension on the optical element  16 . Hence, the colour separation prism may be made very small. This, however, necessitates the coloured light to be expanded in cross-sectional area so as to match the transmissive display panel  36 . The expansion of the coloured light is performed by the second lens  34 .  
         [0034]      FIG. 1  shows a single transmissive display panel  36  for simplicity only. It is to be understood that each coloured light separated by the colour separation prism  22  is communicated to a specific transmissive display panel.  
         [0035]     The transmissive display panel generates an encoded light beam  38 , which is communicated to a second polarizer  40  operating as an analyzer rejecting unwanted polarizations of the encoded light beam from the light path.  
         [0036]     The second polarizer  40  is transmissive to unwanted polarizations of the encoded light beam and reflective to desired polarizations of the encoded light beam. In an alternative embodiment of the present invention, the second polarizer may be reflective to unwanted polarizations of coloured light and transmissive to desired polarizations. This, however, obviously requires a change of the design and the light path from the light source to the projection surface.  
         [0037]     As described with reference to the first polarizer  30 , the second polarizer  40  may be reflective to both desired and undesired polarizations of the encoded light beam. The desired polarizations of the encoded light beam are directed in one direction and the undesired polarizations are directed in another direction.  
         [0038]     In the preferred embodiment of the present invention, the first and second polarizers  30 ,  40  may be implemented by a Moxtek™ beam splitter. However, the first and second reflective polarizers  30 ,  40  may be implemented by a wide variety of polarizers such as wire-grid polarizers, cholesteric polarizers, interference films, holographic structures, stacks of thin birefringent films, beam splitters, mirrors, or any combination thereof.  
         [0039]     The polarized and encoded light  42  is received in a recombination prism  44  gathering each polarized and encoded light beam from each coloured light path, i.e. the red, green and blue light paths. The recombined light forms a complete image to be projected through a projection lens  46  onto the projection surface  12 .  
         [0040]     The two prisms  22  and  44  may be implemented in a wide variety of ways. However, in the preferred embodiment of the present invention, the prisms  22  and  44  are implemented by a first and a second dichroic cube.  
         [0041]      FIG. 2  shows a projection system designated in its entirety by reference numeral  50 . In contrast to  FIG. 1 ,  FIG. 2  shows three light paths: a red light path  51   a , a green light path  51   b , and a blue light path  51   c.    
         [0042]     Elements of the projection system  10  described with reference to  FIG. 1 , which are identical to elements in  FIG. 2 , are denoted by the same reference numerals.  
         [0043]     The light source  14  supplies the light of the projection system  50 , and the optical element  16  focuses and gathers the light from the light source  14  prior to directing the light to a colour separation prism  22 . The colour separation prism is shown in  FIG. 2  as prisms denoted by reference numerals  22   a ,  22   b  and  22   c . The prism  22   a  provides the red light through the red light path  51   a  to a first transmissive display panel  36   a . The prism  22   b  provides the green light through the green light path  51   b  to a second transmissive display panel  36   b.  The prism  22   c  provides the blue light through the blue light path  51   a  to a third transmissive display panel  36   c.    
         [0044]     Each transmissive display panel  36   a ,  36   b  and  36   c  modulates the light in accordance with the generation of particular images. The transmissive display panels  36   a ,  36   b  and  36   c  are controlled by one or more processors controlling each pixel of the transmissive display panels  36   a ,  36   b  and  36   c.    
         [0045]     The encoded lights: encoded red, encoded green, and encoded blue are enhanced through sets of lenses  52   a ,  52   b ,  52   c  and  54   a,    54   b  and  54   c.  The sets of lenses allow the use of a very small dichroic cube for the colour recombination prism  44 .  
         [0046]     As described with reference to  FIG. 1 , the light now recombined is projected on the projection surface through a projection lens  46 .  
         [0047]     The projection system  50  may be folded into a two-layer configuration using polarizers for the polarizing and analyzing operation, similarly as described with reference to  FIG. 1 .