Abstract:
The present invention pertains to an optical device useful in a projection system. The optical device can be designed for easy replacement of selected parts, if necessary, without the need for extensive realignment procedures for the projection system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/298,007 filed Jun. 13, 2001 and U.S. Provisional Application Ser. No. 60/385,050, filed May 30,2002, which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention provides an optical device that is useful in a projection system. In particular, the device provides for proper alignment of the components constituting the optical core and provides for a system that allows for access and removal of some of the components. 
     BACKGROUND 
     A projection system typically requires proper alignment of the components, particularly the optical components, used therein. It is also desirable to be able to change certain parts of the projector components that may need replacement due to normal usage. For example, it is a common practice to replace a burnt light bulb in a projection system with a new one. It is desirable that such replacement procedures are user friendly so that the majority of users can follow them. 
     There is a need for similar versatility and ease in replacing other parts in a projection system. 
     SUMMARY 
     The present invention provides for an optical device designed so as to allow for precise placement of the components making up the optical core. The optical core comprises of a polarizing beam splitter and a imaging unit, which further comprises at least one color prism, at least one imager, and optionally, at least one heat dissipating unit. 
     Advantageously, the design of optical device and the various optical core components allow for easy removal of a polarizing beam splitter (PBS). The design desirably yields a system whereby the PBS can be removed and replaced by a typical user such that additional alignment of the optical components is usually not necessary. Thus, the design is robust and is user friendly. 
     In brief summary, the present invention provides an optical device in a projection system, the device comprising: (a) a polarizing beam splitter ( 130 ) further comprising: (i) first, second, third, and fourth sides, and top and bottom surfaces, wherein the first and third sides, the second and fourth sides, and the top and bottom surfaces are substantially parallel to one another, and wherein the first, third, and fourth sides define a first aperture ( 134 ), a second aperture ( 132 ), and a third aperture ( 131 ) respectively; (ii) a first means for spacing the polarizing beam splitter and a projection lens unit ( 120 ), the first means for spacing ( 136   a ) disposed on the first side of the polarizing beam splitter; (iii) a second means for spacing ( 136   b ) the polarizing beam splitter and an optical core frame, the second means disposed on the fourth side of the polarizing beam splitter; (iv) a third means for spacing ( 136   c ) the polarizing splitter and the optical core frame, the third means disposed on the top surface of the polarizing beam splitter; and (v) a first axis located at the geometric center of the polarizing beam splitter; (b) a carrier assembly ( 700 ) for guiding the insertion and removal of the polarizing beam splitter, the carrier attached to at least a portion of the bottom surface of the polarizing beam splitter and located proximate to the second side of the polarizing beam splitter, the carrier further comprising: (i) means for grasping the carrier ( 703 ); and (ii) at least one guide member ( 707 ); and (c) an imaging unit ( 140 ) having a second geometric center axis, the imaging unit located proximate to the third side of the polarizing beam splitter and in optical communication with the polarizing beam splitter. 
     In this document, the term “about” is presumed to modify all numerical recitation of a physical property such as, but not limited to, dimensions (length, width, height) and thickness of a material. For example, a film having a thickness of 1 mm is presumed to be a film having a thickness of “about” 1 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of the various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram illustrating projection unit  100  in accordance with one aspect of the invention; 
     FIG. 2 is a front view of a polarizing beam splitter  130  in accordance with one embodiment of the invention; 
     FIG. 3 is a front view of an imaging unit  400  in accordance with one embodiment of the invention; 
     FIG. 4 is a cross sectional view of imaging unit  400  showing a simplified tracing of ray  152 ; 
     FIG. 5 a  and  5   b  are front views of carrier assembly  700  disengaged from optical core frame  600  in accordance with the present invention while FIG. 5 c  shows the carrier assembly  700  engaged in the frame  600 ; 
     FIG. 6 is a front view of carrier assembly  700  partially engaged in optical core frame  600 ; and 
     FIG. 7 is a perspective view of an imaging unit in accordance with another embodiment of the invention. 
    
    
     These figures are idealized, are not to scale, and are intended to be illustrative and non-limiting. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1., in one aspect, the present invention is a projection display apparatus  100  comprising projection lens unit  120 , PBS  130 , imaging unit  140 , and illumination unit  150 . In very simplified form, in use, polarized light having polarization state P 1  enters PBS unit  130  in the direction indicated by arrow  151 . Birefringent multi-layer film  138 , which functions as a reflective polarizer, reflects light having polarization P 1  in direction  152  into imaging unit  140 . The imaging unit may further contain electronic imaging devices, not shown, that can rotate the polarization of illumination  152  in a pixel-wise manner, to form at least one image. 
     In a preferred embodiment, imaging unit  140  comprises three imaging devices, each one illuminated by a different color, wherein imaging unit  140  divides incoming light  151  into three colors, each directed at one of the three imaging devices. The color images formed in this manner are then recombined by imaging unit  140  to form imaging beam  153 , which is then incident on birefringent multi-layer film  138 . Because some of the light in beam  153  comes from pixels that have rotated the polarization of incident illumination  152 , this light is transmitted by birefringent multi-layer film  138 , while light having unrotated polarization is reflected back toward illumination unit  150 . As a result, the image produced by imaging unit  140  is transmitted to projection lens unit  120 , which projects image light rays  155  onto screen  160 . 
     The PBS  130  has first, second, third, and fourth sides and top and bottom surfaces. The first and third sides, second and fourth sides, and top and bottom surfaces are substantially parallel to each other. The first side corresponds to a first aperture  134 , the third side corresponds to the second aperture  132 , and the fourth side corresponds to the third aperture  131 . As shown, incident polarized light  151  enters the fourth side of the PBS. The second side of the PBS is a non-aperture surface and lies proximate to a carrier assembly, as further described below. 
     The quality of the image projected by lens unit  120  onto screen  160  is improved if lens unit  120 , PBS  130 , and imaging unit  140  are properly positioned and aligned relative to one another. Moreover, it is preferred that positioning and alignment be robust, so that PBS  130  can be replaced without significant detrimental effects on projected image quality. It has been found that good image quality can be achieved if PBS  130  and imaging unit  140  can be located within ±0.1 millimeters, in all directions, of their design positions, and orientation of PBS  130  and imaging unit  140  can be maintained within ±0.1 degree about three coordinate axes similar to the x-, y-, and z-coordinate axes. In this document, the three coordinate axes include: (1) the optical axis, i.e., the geometric center of the PBS  130 , imagining unit  140  or projection lens unit  120  represented as line  121 , and (2) a first axis perpendicular to  121  running vertically and lies in the plane of the page (not shown), and (3) a second axis perpendicular to  121  and comes out of the plane of the page (not shown). 
     In one aspect of the invention, proper positioning and alignment of lens unit  120 , PBS  130 , and imaging unit  140  can be achieved through the use of compression springs, as further explained below. The forces exerted from such springs are schematically shown in FIG. 1 as arrows  162  and  164 , which push the imaging unit  140  against the PBS  130  which in turn pushes against projection lens unit  120 . Proper alignment of lens unit  120 , PBS  130 , and imaging unit  140  means that the optical axes of the components are substantially coaxially aligned. 
     Projection lens unit  120  comprises input aperture  122 , output aperture  124 , and a system of projection lenses, located internal to enclosure  126 , preferably aligned along optical axis  121 , but not shown. 
     Polarizing Beam Splitter 
     A PBS is an optical component that splits incident light rays into a first (transmitted) polarization component and a second (reflected) polarization component. 
     For projection systems that use reflective liquid crystal display (LCD) imagers, a folded light path where the illuminating light beam and the projected image share the same physical space between a polarizing beam splitter (PBS) and an imager offers a compact design. Most reflective LCD imagers are polarization rotating, i.e., polarized light is either transmitted with its polarization state substantially unmodified for the darkest state or transmitted with its polarization state rotated to provide a desired gray scale. Thus, a polarized light beam is generally used as the input beam. Use of a PBS offers an attractive design because it can function to polarize the input beam and fold the light path. 
     WO 00/70386, in FIG. 1, discloses a Cartesian PBS element  50  that includes a multi-layer birefringent film  52  encased in a glass cube  54 , and oriented so as to reflect light incident with x-polarization (i.e., approximately s-polarization). See page 11, lines 9 to 11. For incident rays of light in a large cone angle, the Cartesian PBS has been demonstrated to provide a higher contrast than a PBS that discriminates only on the basis of s-polarization vs. p-polarization. The Cartesian PBS is one useful PBS that can be used in the present invention. 
     Yet another useful PBS is disclosed in U.S. patent application Ser. No. 09/878,575 entitled “Polarizing Beam Splitter,” filed on Jun. 11, 2001, by the assignee of this invention, which application is hereby incorporated by reference in its entirety. This application disclosed a PBS comprising: (a) a birefringent film having a pass axis, the birefringent film comprising multi-layers of at least a first material layer and a second material layer, each material layers having an absorption edge in the visible spectrum such that in the ultraviolet region, the absorption edge is at least 40 nm less than the shortest wavelength of light that illuminates the polarizing beam splitter and in the infrared region, the absorption edge is at least 40 nm greater than the longest wavelength of light that illuminates the polarizing beam splitter; and (b) at least one prism having a refractive index greater than 1.6 but less than a value that would create total internal reflection along the pass axis of the birefringent film. The PBS is said to have extended durability in the near UV and blue light of the visible spectrum. The term “pass axis” means the optical axis of transmission of the polarizer, i.e., of the birefringent multi-layer film. 
     In a projection system, such as a front or rear projection system, typically two substantially right angle triangular prisms will be used to form substantially a cube-shaped PBS. In this case, the birefringent film is sandwiched between the hypotenuses of the two prisms using an attachment means, as discussed below. A cube-shaped PBS is preferred in most projection systems because it provides for a compact design, i.e., the light source and other components, such as filters, can be positioned so as to provide a small, light-weight, portable projector. For some systems, the cube-shaped PBS may be modified such that one or more faces are not square. If non-square faces are used, a matching, parallel face should be provided by the next adjacent component, such as the color prism or the projection lens. 
     Although a cube is one preferred embodiment, other PBS shapes can be used. For example, a combination of several prisms can be assembled to provide a rectangular PBS. Although the PBS disclosed in WO 00/70386 and U.S. patent application Ser. No. 09/878,575 are exemplary examples of useful PBS, other types of PBS can be used in the present invention. 
     The prism dimension and thus the resulting PBS dimension depend upon the intended application. In an illustrative front projector, the PBS is a cube of 40 mm in length and width, with a 57 mm hypotenuse when using a small arc high pressure Hg type lamp, such as the UHP type sold commercially by Philips Corp., with its beam prepared as an F/2.2 cone of light and presented to the PBS cube for use with 0.78 inch diagonal imagers, such as the SXGA resolution imagers available from Three-Five Systems. The f/# of the beam, optical distance (i.e., sum of actual distances divided by the index of refraction for each unit of distance) separating the imager(s) from the PBS, and the imager size are some factors that determine the PBS size. 
     FIG. 2 shows a front view of a substantially cubic-shaped PBS  130  formed from first prism  137  and second prism  139  and having birefringent multi-layer film  138  embedded along the hypotenuse of the prisms. Typically, film  138  extends beyond the hypotenuse of prisms  137  and  139 . The prisms are substantially right angle prisms. The PBS has a first aperture surface  134  corresponding to the first side where a first means for spacing  136   a  are located. Tabs  136   a  function to create a desired spacing between the PBS and the projection lens unit. On the third aperture surface  131 , second means for spacing  136   b  are located. Tabs  136   b  function to create a desired spacing between the PBS and an optical core frame. On the top surface of PBS  130 , a third means for spacing  136   c  is located. Tab  136   c  also function to create a desired spacing between the PBS and the optical core frame. Although tabs  136   a,    136   b,  and  136   c  are shown as discrete tabs that overlap to the top surface of the PBS, they may take on any configuration so long as the tabs do not interfere with the light path. Furthermore tabs  136   a,    136   b,  and  136   c  function to stabilize the PBS in the carrier frame, i.e., to minimize rotation and movement of the PBS. 
     In one preferred embodiment, it has been found that a low friction material can function as the first, second, and third means for spacing. A suitable low friction material is polytetrafluoroethylene (PTFE) film or tape is particularly useful. A commercially available PTFE film is TEFLON tape. In another embodiment, tabs  136   a  is a polyethylene terephthalate film having a thickness of 0.127 mm (0.005 inches) affixed to the PBS by a pressure sensitive adhesive tape. 
     Imaging Unit 
     After the incident polarized light  152  leaves the PBS, it enters the imaging unit. FIG. 3 shows imaging unit  400  having a first color prism  450  and an associated first imager  452 , a second color prism  460  and an associated second imager  462 , and a third color prism  470  and an associated third imager  472 . Optionally, heat dissipating units  454 ,  464 , and  474  can be used to cool the imagers, and are located proximate to the imagers. Polarized beam  152  is directed towards a color splitter/combiner prisms  450 ,  460 , and  470  that splits the polarized beam  152  into three sub-beams. The three sub-beams are reflected and modulated off red, green, and blue reflective imagers  452 ,  462 , and  472 . A controller, not shown, can be coupled to the imagers to control their operation. Typically, the controller activates different pixels of the imagers to create an image in the reflected light. The reflected and modulated sub-beams are recombined by the color splitter/combiner prisms (hereinafter referred to as “color prism” for convenience). The modulated components of the combined beams  153  pass through PBS  130  and are projected as an image by projection lens unit  120 . 
     Imagers  452 ,  462 , and  472  are affixed to the color prisms by attachment means  410  and  420 . Although FIG. 3 shows attachment means  410  and  420  as brackets, other attachment means can be used. For example, the imagers and the color prisms can be adhesively attached. For attaching the imagers to the color prisms, first, positioning and alignment of the imagers is done by holding each imager in an alignment fixture. The fixture also hold the combined prisms  450 ,  460 , and  470 . Position adjustment can be done while the imagers display a color test image. When the colors are in proper registration, bracket  410  is soldered to bracket  420 . The brackets are preferably metallic when a soldering process is used. Bracket  420  can be attached to the color prisms  450 ,  460 , and  470  using any suitable adhesive that is capable of withstanding soldering temperatures. On aperture surface  434 , a fourth means of spacing  446  and  447  can be used space between the joined color prism and the PBS (not shown). 
     FIGS. 3,  4 , and  5   a  illustrate one embodiment of the brackets  410 ,  420  with three independent brackets  420  coupled to each prism  450 ,  460 ,  470 . By independent, it is meant that a bracket are not integrally formed with another independent bracket, but the independent brackets are separate structures. One of the independent brackets  420   a  is coupled on one side of each prism (see FIG. 3) and two independent brackets  420   b ,  420   c  are coupled to the opposing side of each prism (see FIG. 5 a ). 
     FIG. 4 shows a simplified tracing of polarized light ray  152 . As ray  152  enters illumination aperture  434 , it is transmitted to color prism interface  510 , where light of a first color is reflected as ray  551 , to reflective surface  570 , and to imager  452 , where it is reflected, in a pixel wise manner, with the image being formed by rotation or nonrotation of the polarization plane of incident light  552 . Light reflected from imager  452  retraces paths  552 ,  551 , and  152 , to emerge from aperture  434 . Light not reflected by the color filter at interface  510  is transmitted along  553  to prism interface  520 , where a second color is reflected along rays  554  and  555  to imager  462 , where it undergoes pixel wise polarization rotation and is reflected back along rays  555 ,  554 ,  553 , and  152 . The remaining light not reflected by prism interface  520  is transmitted along  556  and  557  to imager  472 , where it is reflected in with polarization rotated in a pixel wise manner, back to aperture  434 . 
     Carrier Assembly 
     The carrier assembly allows for quick and easy removal of the PBS from the optical core frame. In one preferred embodiment, the carrier assembly is a thermoplastic molded part. 
     FIG. 5 b  shows a carrier assembly  700  having a base  702 , a means for grasping the assembly  703  and guide members  707 , first cam surfaces  705 , and support member  704 . As shown, PBS  130  has been inserted into the carrier assembly such that the third side of the PBS with aperture  132  is exposed. Proximate and parallel to base  702  is the second side, i.e., the non-aperture surface, of the PBS (not shown). PBS  130  is affixed to the carrier assembly at support member  704 . Typically, multiple support members are used. Preferably, PBS  130  is adhesively bonded, e.g., with epoxy, to support member  704 . 
     In use, as carrier assembly  700  slides into optical core frame  600  (shown in FIG. 5 a ), first cam surfaces  705  push against second cam surfaces on brackets that are bonded to either side of color prism  450 . This sliding action pushes the color prisms away from the PBS so that aperture  132  of PBS  130  does not come into contact with the color prism. When the PBS  130  has slid close to its proper vertical height, slot features on the carrier assembly  700  and slot features on the optical core frame allow the PBS to move forward (towards the projection unit  120 ) and to the side of the optical core frame to the PBS final resting position. FIG. 6 schematically shows the cam surfaces  610  of the optical core frame  600  as the carrier assembly  700  slides into the frame, through the use of guide members  707  contacting guide rails  706 . 
     As shown in FIG. 5 c,  optical core frame  600  contains optional extension springs  602 , compression springs  604 , and optional leaf springs  606 , all functioning to position and align the imaging unit  400 , the PBS and its carrier assembly against the projection lens unit. 
     Illumination Source 
     A typical light source includes a lamp and a reflector. Suitable lamps include xenon, incandescent, laser, light emitting diode (LED), metal halide arc light source, and high-pressure mercury light source. Such light sources can emit light in the blue and near ultraviolet wavelength. 
     Another Embodiment 
     FIG. 7 illustrates another embodiment of an imaging unit  800 . The imaging unit  800  includes an x-cube structure  802  for directing and combining light, three PBS structures  804 ,  805 , and  806 , and three imagers  808 ,  810 ,  812 . Examples of such imaging units and the component structures can be found in, for example, U.S. Patent Application Ser. No. 09/878,559 and U.S. Patent Application Ser. No. 10/159,694, entitled “PROJECTION SYSTEM HAVING LOW ASTIGMATISM”, filed May 29, 2002, both of which are incorporated herein by reference. 
     Each imager  808 ,  810 , and  812  is mounted to the corresponding PBS structure  804 ,  805 ,  806 , respectively, using two mounting brackets  820 ,  822  disposed on the PBS structure and two imager brackets  830 ,  832  mounted on the imager. The two mounting brackets  820 ,  822  are independent of each other. The two imager brackets  830 ,  832  provide three or more (preferably three) mounting sites  840 ,  842 ,  844  for coupling to the mounting brackets  820 ,  822 . 
     In the preferred embodiment, one imager bracket  830  has two mounting sites  840 ,  842  that are spaced apart from each other along a width dimension, w, (where the width dimension is defined as the smaller of the width and length dimensions of the imager) of the imager  808 . These two mounting sites  840 ,  842  can then be coupled to a single mounting bracket  820 . The second imager bracket  832  is spaced apart from the first imager bracket along the length dimension,  1 , of the imager  808 . The second imager bracket  832  has a single mounting site  844  that can be coupled to the mounting bracket  822 . This arrangement can provide stability of the positioning of the imager while reducing the effects along the longest dimension of the different thermal expansion coefficients of the mounting brackets  820 ,  822  and the PBS structure  804 . 
     The mounting brackets  820 ,  822  are adhesively or otherwise coupled to the PBS structure. The mounting brackets  820 ,  822  are preferably, but not necessarily, similarly shaped, as illustrated in FIG. 7 with structures  850 ,  852 ,  854  to couple to any of the mounting sites  840 ,  842 ,  844 , even though only one or two of those structures will be used. This preferred arrangement can reduce the complexity of the assembly of the imaging unit  800 . 
     The imager brackets  830 ,  832  can be mounted to the imager, for example, to a heat sink portion of the imager, using any mounting technique, including, for example, adhesive or mechanical (using screws, bolts, etc.) or welding or soldering techniques. The imager brackets  830 ,  832  can be coupled to the mounting brackets  820 ,  822  using any mounting technique including adhesive mounting. In one embodiment, the imager brackets  830 ,  832  and mounting brackets  820 ,  822  are soldered together to facilitate easy of mounting or readjustment to align the imager or both. 
     In one embodiment, the imager bracket  832  is configured and arranged, as illustrated in FIG. 7, to permit flexing along the length dimension of the imager. This can be achieved, for example, by using a relatively thin piece of material that is appropriately shaped so that the imager bracket  832  can flex along the length dimension of the imager. This flexing can be useful to accommodate the differential thermal expansion between the imager and the polarizing beam splitter or other optical element. The other imager bracket  830  can be configured and arranged to resist flexing along the length dimension of the imager.