Abstract:
A projector arrangement is provided wherein a spectral splitting and recombination unit (SSR) includes a first and a second polarization selective or polarizing beam splitting device and a wavelength selective optical element (WSOE) which are sufficient to realize the main and/or the entire functionality of said spectral splitting and recombination unit (SSR).

Description:
A PROJECTOR ARRANGEMENT 
   More particular, the present invention relates to a projector arrangement having a compact illumination and beam splitting part which uses three reflective display panels and only two beam splitting devices or cubes. 
   Today, most projector arrangements or projection systems comprise the so-called three panel arrangement which uses three image generating or generation means, units, or devices. In general, primary illumination light, for instance white light is generated and split up in order to generate three spectrally different components which are used to illuminate the image generation units or devices for generating partial images to be superposed in order to finally obtain an image to be displayed. 
   A major difficulty in the progress of building compact projector arrangements is the necessity of a certain number of optical elements in order to split up the provided primary illumination light, to generate the partial images and to recombine the generated partial images in order to finally obtain the image to be displayed as a superposition of said partial images. 
   BACKGROUND AND PROBLEM 
   Today&#39;s three panel projection systems have the drawback of bulky beam splitter units. The white light is first split by many dichroic mirrors, folding mirrors or polarizing beam-splitters into the primary colors red, green and blue and after being reflected or transmitted by the panels, the light again is recombined by several prism cubes before it is projected by a common projection lens. In the case of reflective panels, this results in a long back-focal length BFL (=distance between the panels and the first surface of the projection lens). A long BFL complicates the design of the projection lens.  FIGS. 1   a  and  1   b  show two commonly used architectures of optical engines with three reflective panels. In  FIG. 1   a , the so called “3PBS architecture”, light is split by dichroic mirrors and folding mirrors and is recombined by an X-cube. In  FIG. 1   b , the so-called “ColorQuad” architecture, the light is split and recombined by four polarizing beam-splitters together with wavelength-selective retarders. 
   A  FIG. 2A  shows a basic architecture according to the present invention. It consists of two polarizing beam-splitters PBS 1  and PBS 2 , one wavelength selective optical element WSOE, three display panels P 1 , P 2 , P 3  and two projection lenses PL 1  and PL 2  building the optical projection unit OP. The WSOE is placed between the polarizing beam splitters PBS 1  and PBS 2 . One panel is attached to PBS 1  and two panels are attached to PBS 2 . Projection lens PL 1  is attached to PBS 1  and PL 2  is attached to PBS 2 . 
   White and s-polarized primary illumination light L 1 , w is entering the polarizing beam-splitter PBS 1  and is reflected by the polarizing beam-splitter coating in direction of the wavelength selective optical element WSOE. One spectral part SP 1  of the white light beam L 1 , w is reflected back by the WSOE thereby changing its polarization state from s- to p-polarization. The p-polarized spectral part SP 1  is now transmitting the PBS 1  and is entering the display panel P 1 . The spectral parts SP 2  and SP 3 , which are completely or partly distinct to each other and to SP 1 , are transmitting the WSOE, thereby changing the polarization state of the spectral part SP 2  from s- to p-polarization. The transmitted and still s-polarized spectral part SP 3  is reflected by the beam splitter coating of PBS 2  and is entering the display panel P 3 . The p-polarized spectral part SP 2  is passing the beam splitter coating of PBS 2  and is entering the display panel P 2 . 
   In the ON state the polarization states of the spectral parts SP 1  and SP 2  are changed from p- to s-polarization and the polarization state of the spectral part SP 3  is changed from s- to p-polarization after being reflected by the display panels P 1 , P 2  and P 3  respectively: see  FIG. 2B . Now the s-polarized spectral part SP 1  is reflected by the beam-splitter coating of PBS 1  and is entering projection lens PL 1 . The s-polarized spectral part SP 2  is reflected by the beam-splitter coating of PBS 2  and is entering projection lens PL 2 . The p-polarized spectral part SP 3  is passing PBS 2  and is entering projection lens PL 2 . 
   In the OFF state the polarization state of the spectral parts SP 1  and SP 2  remains p-polarized and the polarization state of the spectral part SP 3  remains s-polarized after being reflected by the display panels P 1 , P 2  and P 3  respectively. All spectral parts SP 1 , SP 2  and SP 3  are now redirected in direction of the illumination unit: see  FIG. 2C . 
   B A common illumination optical unit may be attached to PBS 1 , as shown in  FIG. 3 . White light L 1 , w is emerging from this illumination unit and is entering PBS 1 . The illumination optical unit is adapted to focus the illuminating light beam to the panels P 1 , P 2 , P 3 . The wavelength selective optical element WSOE is placed between PBS 1  and PBS 2  in such a way that the optical distance of spectral part SP 1  to panel P 1  is same like the distance of spectral parts SP 2  and SP 3  to the panels P 2  and P 3  respective. 
   C The wavelength selective optical element WSOE comprises two quarter wave layers Q 1  and Q 2 , a dichroic layer D on a transparent substrate S, e.g. glass and a wavelength dependent retarder R as shown in  FIG. 4 . 
   Linear s polarized white light entering the WSOE is passing the first quarter-wave layer, thereby turning the polarization state from linear to circular. Next the light beam hits the dichroic layer D, thereby reflecting spectral part SP 1  and changing the chirality of the circular polarized light, e.g. from left circular to right circular. The reflected back spectral part SP 1  next passes again the first quarter-wave layer, thereby turning the polarization state to linear p. 
   The transmitted spectral parts SP 2  and SP 3  are next passing the second quarter wave layer Q 2 , thereby turning the polarization state from circular to linear. By passing the wavelength selective retarder R the polarization state of spectral part SP 2  is turned from s-polarized to p-polarized or—alternatively the polarization state of spectral part SP 3  is turned from p-polarized to s-polarized. 
   D Light of spectral part SP 2  is reflected in p-polarized mode from the display panel P 2  when the panel is in the OFF state as shown in  FIG. 5 . Ideally, all of the p-polarized light is passing the PBS 2  in direction to PBS 1 . But, as a general attribute of polarizing beam-splitters, about 10% of the p-polarized light is reflected at the polarizing beam-splitter coating. Therefore—even in the OFF (=black) state—about 10% of the light would enter the projection lens, resulting in a worse contrast. 
   To overcome this loss in contrast, an additional clean-up polarizer in front of the projection lens PL 2  is required to block the leaking p-polarized light. This clean-up has to be a wavelength selective polarizer WSP, because only p-polarized light of spectral part SP 2  must be blocked. The p-polarized light of spectral part SP 3 , which comes from display panel P 3  in the ON state, must be transmitted by the WSP. 
   E The wavelength selective polarizer WSP mentioned in section D can be of following types:
         Cholesteric polarizer (CF) with adapted quarter-wave layers. This CF reflects the leaking p-polarized light of spectral part SP 2 , but lets through spectral part SP 3  and all s-polarized light of spectral part SP 2 .   Colour selective retarder, which turns the polarization state of spectral part SP 2  from p polarized to s polarized and from s polarized to p polarized and leaves the polarization state of spectral part SP 3  as it is. An additional absorbing polarizer then absorbs all s-polarized light.   An absorptive wavelength selective polarizer which is adapted in such way that p-polarized light of spectral part SP 2  is absorbed, but p-polarized light of spectral part SP 3  and s-polarized light from SP 2  is transmitted.       

   F Light of spectral part SP 1  is reflected in p-polarized mode from display panel P 1  when the panel is in the OFF state as shown in  FIG. 5 . Ideally, all of the p-polarized light is passing the PBS 1  in direction to PBS 2 . But, as a general attribute of polarizing beam splitters, about 10% of the p-polarized light is reflected at the polarizing beam splitter coating. Therefore—even in the OFF (=black) state—about 10% of the light would enter the projection lens, resulting in a worse contrast. 
   To overcome this loss in contrast, an additional clean-up polarizer or analyser A in front of the projection lens PL 1  is required to block the leaking p-polarized light. In contrast to section D this clean-up polarizer A can be of standard type, as only light in the spectral part SP 1  is influenced. 
   G In reality the diameter of the projection lenses PL 1  and PL 2  exceed the diameter of the polarizing beam-splitter cubes PBS 1  and PBS 2 . As a result, the distance between PBS 1  and PBS 2  must be large enough to fit to both projection lenses PL 1  and PL 2 . Especially rear projection lenses have front lenses with a large diameter. 
   To overcome this space requirement the projection lenses could be split in two separate first lens blocks LB 1  and LB 2  attached to PBS 1  and PBS 2 , respectively, and a common second or front lens block FLB as shown in  FIG. 6A . Folding mirrors FM 1 , FM 2  combine the two separate light paths coming from the first lens blocks LB 1  and LB 2  into the common front lens block FLB. An X prism X combines the light coming from lens block LB 1  with light coming from lens block LB 2 . The X prism X has two different dichroic coatings. One coating is to reflect the light of spectral part SP 1  and the other to reflect the light of spectral parts SP 2  and SP 3  into the common front lens block FLB. 
   H Alternatively, the X prism X can be rotated by 90° in order to fold the common light path together with the front lens block FLB out of the plane as shown in  FIG. 6B . This has the advantage of small footprint and adapts the engine to be used in rear projection cubes. 
   I Instead of using an X prism X to recombine the two light paths into one an arrangement according to  FIG. 7  can be used. It comprises three folding mirrors FM 1 , FM 2 , FM 3  and one dichroic mirror prism DM to recombine the two light paths. 
   Existing projection systems with three reflective display panels are using three to four beam splitter cubes and need a long back-focal length of the projection lens. The invention describes a projection system with outstanding compact illumination and beam splitter part using only two beam-splitter cubes. The projection lens or lenses has/have a short back focal length. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIGS.  1 A,B are schematical block diagrams of prior art projector arrangements. 
       FIGS. 2A-C  are schematical block diagrams elucidating a first embodiment of the inventive projector arrangement. 
       FIGS. 3-7  demonstrate by means of schematical block diagrams aspects of further embodiments of the inventive projector arrangement. 
   

   DETAILED DESCRIPTION 
   In the following similar elements and structures with respect to their functionality and construction are denoted by the same reference symbols. A detailed description is not repeated in each case of their occurrence. 
   As already indicated above, prior art projector arrangements  100  as shown in  FIGS. 1A and 1B  possess a comparable large number of optical elements and they are therefore more complicated as introduced by the present invention, as can be obtained by the schematical block diagrams of  FIGS. 1A  und  1 B. 
     FIGS. 2A to 2C  demonstrate by means of schematical block diagrams the basic structure of an embodiment of the inventive projector arrangement  10  with respect to three operational states. 
   The embodiment of the inventive projector arrangement  10  as shown in  FIG. 2A to 2C  comprises a spectral splitting and recombination unit SSR and an optical projection unit OP as basic constituents. Additionally, first, second, and third image generating/generation means/devices or units P 1 , P 2 , and P 3  are provided, which are adapted and assigned for generating first to third partial images Ir, Ig, Ib of an image I to be displayed. 
   The spectral splitting and recombination unit SSR comprises a first polarization selective or polarizing beam splitting device PS 1 ,  11  and a second polarization selective or polarizing beam splitting device PS 2 ,  12 . Said first and said second polarization selective or polarizing beam splitting devices PBS 1 ,  11 ; PBS 2 ,  12  may be built by so called beam splitting cubes, or the like. Said first polarization selective or polarizing beam splitting device PBS 1 ,  11  comprises a first, a second, a third, and a fourth surface  11 - 1 ,  11 - 2 ,  11 - 3 , and  11 - 4 , respectively. Additionally, a polarization selecting or selective interface  11   c  is provided. 
   In a similar manner said second polarization selective or polarizing beam splitting device PBS 2 ,  12  comprises a first, a second, a third, and a fourth surface  12 - 1 ,  12 - 2 ,  12 - 3 , and  12 - 4 , respectively, as well as a polarization selective/selecting interface  12   c.    
   The first surface  11 - 1  of said first polarization selective or polarizing beam splitting device PBS  1 ,  11  serves as a light entrance for the spectral splitting and recombination unit SSR and therefore for the inventive projector arrangement  10 . White light w—here having a s-polarized polarization state and being constituted by first to third spectrally separated, non-overlapping and complementary primary illumination light components L 1   r , L 1   g , L 1   b ; SP 1 , SP 2 , SP 3 , respectively—enters as primary illumination light L 1  said first surface  11 - 1  as said light entrance section or portion. Because of its s-polarized polarization state said primary illumination light L 1 , and in particular its first to third primary illumination light components L 1   r , L 1   g , L 1   b ; SP 1 , SP 2 , SP 3  are reflected from said first surfaces  11 - 1  by said polarization selective interface  11   c  to said second surface  11 - 2  of said first polarization selective or polarizing beam splitting device PBS 1 ,  11  and thereby leave said first polarization selective or polarizing beam splitting device PBS 1 ,  11  in order to interact the wavelength selective optical element WSOE, which is provided between said second surface  11 - 2  of said first polarization selective or polarizing beam splitting device PBS 1 ,  11  and the first surface  12 - 1  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12  or in an optical path between these surfaces. 
   Upon interaction the first to third primary illumination light components L 1   r , L 1   g , L 1   b ; SP 1 , SP 2 , SP 3  which may be referred to as secondary illumination light components L 2   r , L 2   g , and L 2   b , respectively, of secondary illumination light L 2  as light for said partial images Ir, Ig, and Ib, respectively, to be generated, primary illumination light L is split up into its distinct spectral components. The first secondary illumination light component L 2   r  stems from the first primary illumination light component L 1   r , SP 1  and is reflected by the interaction of the wavelength selective optical element WSOE and thereby obtains a p-polarized polarization state and re-enters the first polarization selective or polarized beam splitting device PBS 1 ,  11  via its second surface  11 - 2  thereof. Because of its p-polarized polarization state said first secondary illumination light component L 2   r  is transmitted by the polarization selective interface  11   c  of said first polarization selective or polarizing beam splitting device PBS  1 ,  11  in order to hit the third surface  11 - 3  thereof to leave the same in order to interact as light for said first partial image Ir to be generated with a first image generating means/unit P 1  for said first partial image Ir, which is provided in the neighbourhood or vicinity of said first surface  11 - 3 . 
   Upon interaction of the second primary illumination light component L 1   g , SP 2  with said wavelength selective optical element WSOE between the second surface  11 - 2  and the first surface  12 - 1  of said first polarization selective or polarizing beam splitting device PBS 1 ,  11  and said second polarization selective or polarizing beam splitting device PBS 2 ,  12 , respectively, the polarization state of said second primary illumination light component L 1   g , SP 2  is changed to a p-polarized polarization state, thereby generating a second secondary illumination light component L 2   g  as light for said second partial image Ig to be generated. Said second secondary illumination light component L 2   g  is because of its p-polarized polarization state directly transmitted from said first interface  12 - 1  to said second surface  12 - 2  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12  via its polarization selecting interface  12   c  in order to leave said second polarization selective or polarizing beam splitting device PBS 2 ,  12  via its second surface  12 - 2  and in order to interact with a second image generating means/unit P 2  for said second partial image Ig, which is provided in the neighbourhood or vicinity of said second surface  12 - 2  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12 . 
   Upon interaction of said third primary illumination light component L 1   b , SP 3  with said wavelength selective optical element WSOE between said second surface  11 - 2  and said first surface  12 - 1  of said first polarization selective or polarizing beam splitting device PBS 1 ,  11  and said second polarization selective or polarizing beam splitting device PBS 2 ,  12 , respectively, said third primary illumination light component L 1   b , SP 3  obtains a s-polarized polarization state to thereby form a third secondary illumination light component L 2   b  as light for said third partial image Ib to be generated. Because of its s-polarized polarization state said third secondary illumination light component L 2   b  is reflected by said polarization selecting interface  12   c  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12  directly from said first surface  12 - 1  to said third surface  12 - 3  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12  in order to leave said second polarization selective or polarizing beam splitting device PBS 2 ,  12  via its third surface  11 - 3  and in order to interact with a third image generating means/device P 3  for said third partial image Ib to be generated. 
   Upon interaction of said first, second and third secondary illumination light components L 2   r , L 2   g , and L 2   b , respectively, as light for said first, second, and third partial images Ir, Ig, Ib, respectively, to be generated, first, second, and third tertiary illumination light components L 3   r , L 3   g , and L 3   b  as light of said first, second, and third partial images Ir, Ig, and Ib are generated having a s-polarized polarization state, a s-polarized polarization state, and a p-polarized polarization state, respectively. These tertiary illumination light components L 3   r , L 3   g , and L 3   b  re-enter the respective first and second polarization selective or polarizing beam splitting devices PBS 1 ,  11 ; PBS 2 ,  12  via its respective third surface  11 - 3 , second surface  12 - 2 , and third surface  12 - 3 , respectively. Upon further interaction with respective polarization selecting interfaces  11   c  and  12   c , respectively, these first tertiary illumination light components L 3   r , L 3   g , and L 3   b , respectively, are reflected, reflected and transmitted from the third surface  11 - 3  to the fourth surface  11 - 4  of said first polarization selective or polarizing beam splitting device PBS 1 ,  11 , from said second surface  12 - 2  to said fourth surface  12 - 3  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12 , and from said third surface  12 - 3  to said fourth surface  12 - 4  of said second polarization selective or polarizing beam splitting device PBS 2 ,  12 , respectively, in order to leave the respective polarization selective beam splitting devices PBS  1 ,  11 ; PBS 2 ,  12  and in order to enter the optical projection unit OP, which is provided in the vicinity or neighbourhood of said fourth surfaces  11 - 4  and  12 - 4  or in an optical path thereof. 
     FIG. 2A  demonstrates the basic architecture of this embodiment, whereas  FIGS. 2B and 2C  demonstrate the ON state and the OFF state, respectively, of the first, second and third image generating means or devices P 1 , P 2  and P 3 , respectively, which are in the case of  FIGS. 2A to 2C  reflective imaging panels.  FIG. 2C  also indicates the off light or lost waste light components SP 1 ′, SP 2 ′, and SP 3 ′ with the respective components L 3   r ′, L 3   g ′, L 3   b′.    
     FIG. 3  shows an embodiment which is similar to that of  FIGS. 2A to 2C  but now additionally illustrates the provided illumination unit which is arranged and adapted in order to provide the primary illumination light L 1 . 
   In  FIG. 4  a possible embodiment for the wavelength selective optical element WSOE is indicated as already described above. 
     FIG. 5  illustrates the provision of several blocking mechanisms to further enhance the functionality and the reliability of the inventive projector arrangement as already mentioned above. 
     FIGS. 6A ,  6 B, and  7  elucidate further embodiments of the inventive arrangement having different architectures for the optical projection unit OP.