Abstract:
A polarized light illumination system includes a light emitting diode (LED) ( 601, 602, 603 ) for providing a source of light that is directed to a non-polarizing dichroic combiner ( 607 ) for combining light from the LEDs into a single light source. A power beam splitter (PBS) ( 608 ) is then used for splitting the single light source into polarized light components and an output waveguide ( 611 ) operates to provide a source of uniformly illuminated light. A condenser lens ( 612 ) then projects the uniformly illuminated light to a microdisplay panel ( 613 ) for use with a television receiver or other type of display monitor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority for this non-provisional application is based on provisional patent application entitled LED Polarizing Optics for Color Illumination System, Ser. No. 60/646,775, filed Jan. 25, 2005; LED Color Illumination Apparatus for Polarized Light Projection System, Ser. No. 60/646,777, filed Jan. 25, 2005; and RGB LED Illumination Apparatus for DLP Projection Applications, Ser. No. 60/646,778, filed Jan. 25, 2005, all owned by Jabil Circuit, Inc. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention generally relates to an LED color illumination apparatus for a polarized light projection system and more specifically to a light projection system for providing uniform light distribution to a display panel. 
     2. Background of the Invention 
     In recent years, digital projection systems using spatial light valve modulators, such as a digital micromirror device (hereafter “DMD”), transmissive liquid crystal display (hereafter “LCD”) and reflective liquid crystal on silicon (hereafter “LCoS”) have been receiving much attention as they provide a high standard of display performance. These displays offer such advantages as high resolution, a wide color gamut, high brightness and a high contrast ratio. 
     Color projection systems of the type based on either LCD technology or LCoS technology require linearly polarized light as the illumination light source. LCD and LCoS devices depend on either the polarization rotation effect or the birefringent effect of the liquid crystal to generate light. The light emitted from the light source must be converted into polarized light for illuminating an LCD or LCoS spatial light modulator. Those skilled in the art will recognize that the optical system contained within a commercial LCD or LCoS projector typically combines a fly&#39;s-eye lens array with a polarizing beam splitter array. Examples of such an arrangement can be found in U.S. Pat. Nos. 6,411,438, 6,776,489, 6,739,726 and 6,092,901 which are all incorporated by reference herein. Two drawbacks to using the fly&#39;s-eye type of optical system are that it is bulky and expensive to manufacture. 
     Most projection systems use short arc gaseous white lamps such as ultra-high pressure mercury, xenon or the like that can achieve a relatively high etendue efficiency required for panel illumination. Etendue refers specifically to the geometric capability of an optical system to transmit radiation such as its throughput. Currently, only a limited number of manufacturers are capable of producing high-quality short arc lamps. The typical operational lifetime of these types of lamps is about 2000 to 6000 hours. Moreover, there are significant amounts of ultraviolet (UV) and infrared (IR) light emitted from this type of lamp. The unfiltered UV light reduces the lifetime of both the optical components and microdisplay panel within the system, while IR light requires additional cooling devices to maintain a desired operating temperature. 
     Significant efforts have been dedicated towards moving away from short arc lamps through the utilization of light emitting diodes (LED) in projection illumination systems. One apparent advantage is that LEDs using three primary colors can produce a wider color gamut than conventional white lamps. In addition, LEDs have a high light efficiency, i.e., the ratio of luminous output to the electrical power required, since all spectra of the red, green and blue light from LEDs can be utilized in a visual system. U.S. Pat. No. 6,224,216, which is incorporated by reference herein, describes a triple-path projector employing three single color LED arrays. The LEDs emit light propagating along separate paths through fiber bundles to respective waveguide integrators and thereafter to respective display devices. A problem exists in this type of system because of the coupling between LEDs and fibers. In practice, due to coupling and transmission loss, it is difficult to efficiently couple light emitting from the LED arrays to the corresponding fiber bundles and waveguides. 
     Similarly, U.S. Pat. No. 6,220,714 discloses a projection system using LEDs for illumination, where light beams emitting from red, green and blue LED arrays are collimated by condenser lenses which pass through fly&#39;s-eye type integrators for illuminating a single panel. Based on the geometry of the fly&#39;s-eye type integrator, only the surface area of light emitting region within a certain field of view can be effectively collected for illuminating a panel. A similar system can be found in U.S. Pat. No. 6,644,814, which describes an LED-illumination-type DMD projector with one panel. Generally, a common problem with these prior art systems is that some light from LEDs cannot enter the corresponding lens of the first and second fly&#39;s-eye lenses due to aberration and aperture limitation of the lens array. Therefore, a portion of the illumination light will fall outside the panel area, resulting in low light efficiency and low contrast. 
     Research has also been conducted on using light pipes as means of collecting and homogenizing light for polarize illumination applications. For example, U.S. Pat. No. 6,587,269 discloses a waveguide polarization recovery system comprising an input waveguide that inputs non-polarized light energy into the system. A polarizing beam splitter receives light energy from an input waveguide and transmits light energy of a first polarization type and reflects light energy of a second polarization type. A wave plate modifies the polarization of the transmitted or reflected light energy and an output waveguide removes polarized light energy from the previous system. This type of waveguide polarization recovery system was designed for use with white light sources so that multi-color light sources and beam combiner are not required. 
     Thus, there is a need to provide a light illumination device for LCD or LCoS projection systems or the like which utilizes polarized light with high efficiency and adequate brightness without utilizing complicated and/or expensive components. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings. 
         FIG. 1  is a block diagram illustrating the basic configuration of an RGB LED illumination system in accordance with an embodiment of the invention. 
         FIGS. 2-3  are schematic diagrams of an RGB LED with homogenizer/waveguide assembly used in  FIG. 1 . 
         FIGS. 4-5  are schematic diagrams for embodiments of a non-polarizing dichroic combiner used in  FIG. 1 . 
         FIGS. 6-8  are schematic diagrams for embodiments of a polarization conversion and recovery system used in  FIG. 1 . 
         FIGS. 9-10  are schematic diagrams for embodiments of a light integrator and condenser lens system used in  FIG. 1 . 
         FIG. 11  is a schematic diagram illustrating an embodiment of an LED polarized light illumination system comprising an LCoS or LCD panel and components shown in  FIGS. 2 ,  4 ,  6  and  9 . 
         FIG. 12  is a schematic diagram illustrating another embodiment of an LED polarized light illumination system comprising an LCoS or LCD panel and components shown in  FIGS. 3 ,  4 ,  6 , and  9 . 
         FIG. 13  is a schematic diagram illustrating yet another embodiment of an LED polarized light illumination system comprising an LCoS or LCD panel and components shown in  FIGS. 3 ,  4 ,  7 , and  9 . 
         FIG. 14  is a diagram illustrating yet another embodiment of an LED polarized light illumination system comprising an LCoS or LCD panel and components shown in  FIGS. 3 ,  4 ,  8  and  9 . 
         FIG. 15  is a schematic diagram illustrating yet another embodiment of an LED polarized light illumination system comprising an LCoS or LCD panel and components shown in  FIGS. 2 ,  5 ,  6 , and  9 . 
         FIG. 16  is a diagram illustrating an alternative arrangement of an LED polarized light illumination system comprising an LCoS or LCD panel and components shown in  FIGS. 2 ,  4 ,  6 , and  9 , wherein three LED assemblies are located on the same plate. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the present description is directed in particular to elements forming part of, or cooperating more directly with, the apparatus in accordance with the invention. As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the scope of the invention as a whole. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims. 
       FIG. 1  is a block diagram illustrating the basic configuration of an RGB LED illumination system in accordance with the present invention, which comprises a LED illumination assembly, a polarization conversion and recovery system and a light integrator to provide a linearly polarized light for illuminating a LCD or LCOS image display panel in a projection optical system. The LED illumination assembly shown in  FIG. 1  further includes a color LED with homogenizer/waveguide and a dichroic combiner. The details of the LED illumination system as outlined in  FIG. 1  are disclosed in copending application entitled LED Polarizing Optics for Color Illumination System, Ser. No. 60/646,775, filed Jan. 25, 2005, and assigned to Jabil Circuit, Inc., which is herein incorporated by reference. 
       FIGS. 2 and 3  are schematic diagrams for embodiments of an RGB LED assembled with a homogenizer/waveguide referred to in  FIG. 1 . The assembly shown in  FIG. 2  includes either a red LED, or a green LED, or a blue LED with a tapered waveguide. The light beam emitting from the LED is collimated, homogenized and guided by said tapered waveguide. The waveguide can be either a hollow pipe with reflective inner surfaces or an integrator rod. The tapered waveguide can reduce the angle of the input cone at the tapered ratio due to etendue preservation where the product of the illuminated area and the illumination solid angle at the output face of the waveguide is equal to the etendue at the input face of the waveguide. The exit beams from the waveguide can be further converged by a lens attached to the exit surface of the waveguide, as illustrated in another embodiment shown in  FIG. 3 . 
       FIGS. 4-5  are schematic diagrams for embodiments of a non-polarizing dichroic combiner referred to in  FIG. 1 .  FIG. 4  shows a non-polarizing cross-dichroic combiner (X-cube).  FIG. 5  displays a V-type type dichroic prism as a variation to implement a dichroic combiner in accordance with the present invention. The main advantages of a V-type dichroic combiner over a non-polarizing cross-dichroic combiner are low cost and ease of manufacturing. Typically, the angular tolerance of a non-polarizing V-type dichroic prism is a range of a few arc minutes, while the angular tolerance of a cross-dichroic prism is a few arc seconds. 
       FIGS. 6-8  are schematic diagrams for embodiments of a polarization conversion and recovery system referred to in  FIG. 1 .  FIG. 6  shows a polarization conversion and recovery system comprising a polarizing beam split (PBS), a 45-degree prism and a half wave plate.  FIG. 7  shows another embodiment of a polarization conversion and recovery system comprising a polarization conversion and recovery system comprising a PBS cube, a 45-degree prism, and a retro-reflective polarization rotator.  FIG. 8  displays yet another alternative embodiment of a polarization conversion and recovery system comprising a PBS cube, a 45-degree prism, a quarter wave plate and a mirror. 
       FIGS. 9-10  are schematic diagrams for embodiments of a light integrator referred to in  FIG. 1 , wherein  FIG. 9  shows an output integrator waveguide and  FIG. 10  shows a fly&#39;s-eye integrator, comprising a first lens array, a second lens array and a focusing lens. The output integrator waveguide can be a tapered hollow light pipe or a tapered solid rod integrator, increasingly tapered, decreasingly tapered, or straight shaped. 
     As will be evident to those skilled in the art, each component shown in  FIGS. 2-10  can be combined with one another to constitute an LED color illumination system for polarized light projection applications in accordance with the present invention, as illustrated in six exemplary embodiments shown in  FIGS. 11-16 . 
       FIG. 11  illustrates a polarized light illumination system in accordance with an embodiment of the invention, comprising an LCoS or LCD panel and the optical elements illustrated in  FIGS. 2 ,  4 ,  6  and  9 . The system includes a red LED  601 , a green LED  602 , a blue LED  603 , three tapered waveguides  604 ,  605 ,  606 , a non-polarizing cross-dichroic combiner  607 , a PBS  608 , a 45-degree prism  609 , a half wave plate  610 , an output waveguide  611 , a condenser lens  612  and a panel  613 . The light from the red LED  601 , passing through the waveguide  604 , is reflected from the non-polarizing cross-dichroic combiner  607 . Similarly, the light from the blue LED  603 , passing through the waveguide  606 , is reflected from non-polarizing cross-dichroic combiner  607 . The green light of LED  602 , passing through the waveguide  605 , is converged by lens  608  and then transmits through non-polarizing cross-dichroic combiner  607 . The red, green and blue axes are coincident on the exit face of the dichroic combiner  607 . The light output from non-polarizing cross-dichroic combiner  607  is split by PBS  608  into s-polarized light  622  and p-polarized light  621 . The p-component  621  transmits through the PBS  608  while the s-component  622  is reflected upwardly and is further reflected by the 45-degree prism  609  to the half wave plate  610 . The half wave plate  610  rotates the polarization state of the s-component  622  to p-polarized component  623  that propagates in a direction parallel to the direction of the p-component  621 . The beams  621  and  623  are multi-reflected on the inner walls of the output waveguide  611  so that the exit surface of the waveguide  611  is uniformly illuminated with polarized light, and thereby light exiting from waveguide  611  is projected by the condenser lens  612  onto the DLP panel  613 . The shape of the waveguide  611  can be increasingly tapered, decreasingly tapered, or straight, as needed. The exit surface aspect ratio of the waveguide  611  is proportional to that of the panel utilized in a projection system. 
     The waveguide can be either a hollow pipe with reflective inner surfaces or an integrator rod with total or partial internal reflection. The tapered waveguide can be used to collimate the light, homogenize it and shape the beam. As shown in  FIG. 11 , the area of the output face of the input waveguide is larger than that of the input face; therefore, the cone angle of the output beam is smaller than that of the input beam and hence collimation is achieved. Another function of the waveguide is that the waveguides can always be used as light homogenizers to change a spatially non-uniform distributed light on the input face to an output light with essentially uniform intensity. Furthermore, the aspect ratio of the output surface of the waveguide may be different from that of the input surface. This is especially useful in those applications when one requires the shape of the light source to be proportional to that of the panel it is illuminating. Even though the waveguide may introduce some optical loss when compared with a lens or a lens-array-based illumination system, the waveguide-based system is relatively compact in size, simple in structure, and inexpensive to manufacture. 
     In the configuration shown in  FIG. 11 , a gap is formed between the red waveguide  604  and the upper side surface of the non-polarizing cross-dichroic combiner  610 , and a gap is formed between the blue waveguide  606  and lower side surface of the dichroic combiner  610 . Thus, the light exiting from the green waveguide  605  reflects internally on both side surfaces of the dichroic combiner  610  and leakage of green light into the red and blue waveguides can be prevented. 
     The illumination system in  FIG. 11  can be further modified in many different configurations. For instance, lenses can be attached to the exit surface of the waveguides to make the exit beams from three waveguides become further converged. Furthermore, a tapered waveguide, cross-dichroic prism, PBS prism and 45 degree prism may be regarded as a combining light integrator to achieve uniform light at the polarization conversion output. The uniform light can be directly relayed to LCOS or LCD panel, as shown in  FIG. 12 , wherein the homogenizer/waveguide assembly shown in  FIG. 2  is replaced with one shown in  FIG. 3 . The light emitting from red LED  701 , green LED  702  and blue LED  703  are collimated, homogenized, and guided by tapered waveguides  704 ,  705  and  706 , respectively. The light beams exiting from three waveguides  704 ,  705  and  706  are further converged by three lenses  707 ,  708  and  709  and thereafter enter into three entrance surfaces of the non-polarizing cross-dichroic combiner  710 . If the cross-dichroic combiner, PBS and 45-degree prism are all made of solid glass, they can form a combined light integrator to achieve more uniform light at the polarization conversion output. An imaging lens can directly relay the uniform output to LCOS or LCD panel. 
       FIG. 13  shows a variation to implement an illumination system in accordance with an embodiment of the present invention, comprising an LCoS or LCD panel and components demonstrated in  FIGS. 3 ,  4 ,  7  and  9 . The light emitting from red LED  801 , green LED  802  and blue LED  803  are homogenized and guided by tapered waveguides  804 ,  805  and  806 , respectively. The light beams exiting from three waveguides  804 ,  805  and  806  are further converged by three lenses  807 ,  808  and  809  and thereafter enter into three entrance surfaces of the non-polarizing cross-dichroic combiner  810 . If the waveguide is made of solid glass rod, the lens and corresponding rod can be integrated to a one-piece optical component. The embodiment employs the polarization conversion system shown in  FIG. 7 , which comprises a PBS cube  812 , a 45-degree prism  813 , and a retro-reflective polarization rotator  811 . 
     A detailed description of a retro-reflective polarization rotator can be found in U.S. Patent Publication No. 2004/0090763, which is herein incorporated by reference. The main advantages of this apparatus are that it is not sensitive to wavelength variations of the incoming light, temperature changes and polarization alignment errors. The incident light entering the PBS  812  is split into the s-polarized light  822  and the p-polarized light  821 . The p-component  821  transmits through PBS  812  while the s-component  822  reflects downwardly. Unlike the embodiment shown in  FIG. 12 , a polarization rotator  811  is used to replace the half waveplate to rotate the polarization direction of the s-component  822  coming from PBS  812  by 90 degrees. After being reflected from the polarization rotator  811 , the otherwise wasted s-component  822  becomes p-polarized beam  823  and passes through the PBS cube  812  to the prism  813 . The 45-degree prism  813  then redirects p-polarized beam  823  in a propagation direction parallel to the direction of the p-component  821 . 
     Another alternative embodiment for a polarization recovery apparatus introduced in  FIG. 8  is shown in  FIG. 14 , which includes three LED  901 ,  902  and  903 ; three tapered waveguides  904 ,  905  and  906 ; three lenses  907 ,  908  and  909 ; a non-polarizing cross-dichroic combiner  910 ; a PBS cube  912 ; a 45-degree prism  911 ; a quarter wave plate  913  and a mirror  914 ; an output waveguide  915 ; a condenser lens  916 ; and an LCoS or LCD panel  917 . The polarization conversion and recovery system includes the PBS cube  912 , the 45-degree prism  911 , the quarter wave plate  913  and the mirror  914 . The light entering the PBS  912  is split into the s-polarized light  922  and the p-polarized light  921 . The p-component  921  transmits through PBS  912 . The s-component  922  passes the quarter wave plate  913  and becomes circularly polarized. After reflected from the mirror  914  and again passes through the quarter wave plate  913 , it becomes p-polarized. The recovered p-component  923  passes through the PBS cube  912  to the prism  911 . The 45-degree prism  911  then redirects the beam  923  in a propagation direction parallel to the direction of p-component  921 . 
       FIG. 15  shows another variation to implement an illumination assembly in accordance with the present invention, comprising an LCoS or LCD panel and components shown in  FIGS. 2 ,  5 ,  6 , and  9 , wherein a non-polarizing V-type dichroic combiner in  FIG. 5  is employed to replace the non-polarizing cross-dichroic combiner shown in  FIG. 4 . The inner surface of the V-type dichroic combiner  308  is coated with two different dichroic coatings  1030  and  1031 . Coating  1030  transmits green and red color and reflects blue color, while coating  1031  transmits green and blue color and reflects red color. The light emitting from green LED  1002 , guided by waveguide  1005 , passes through the dichroic coatings  1030  and  1031  while the light emitting from the blue LED  1003 , guided by waveguide  1006 , is reflected from the coating  1030  and passes through the coating  1031 . 
     The optical axes of the red path, green path and blue path are coincident on the exit face of the V-type dichroic combiner  1008 . The function of the gap between the blue waveguide  1006  and dichroic combiner  1008  and the gap between the glass volume  1007  and dichroic combiner  1008  is similar to that of  FIG. 11 . The light exiting from the green waveguide reflects internally on both side surfaces of the V-type dichroic combiner and leakage of green light into the blue and red waveguides can be prevented. 
       FIG. 16  illustrates an alternative configuration in accordance with the present invention, comprising components shown in  FIGS. 2   4 ,  6 , and  9 . The system includes a red LED  1101 , a green LED  1102 , a blue LED  1103 , three tapered waveguides  1104 ,  1105  and  1106 , a non-polarizing cross-dichroic combiner  1107 , two 45-degree prisms  1108  and  1109 , a PBS  1110 , a 45-degree prism  1111 , a half wave plate  1112 , an output waveguide  1113 , a condenser lens  1114  and a panel  1115 . Different from the first embodiment, three LEDs are configured to be on the same plate to make the system more compact. Two 45-degree prisms  1108  and  1109  are arranged next to two side surfaces of non-polarizing cross-dichroic combiner  1107  with gaps. The light from the red LED  1101 , passing through the waveguide  1104 , is reflected from the 45-degree prism  1108  and is then reflected from the non-polarizing cross-dichroic combiner  1107 . Similarly, the light from the blue LED  1103 , passing through the waveguide  1106 , is reflected from the 45-degree prism  1109  and is then reflected from non-polarizing cross-dichroic combiner  1107 . The green light of LED  1102 , passing through the waveguide  1105 , transmits through non-polarizing cross-dichroic combiner  1107 . The red, green and blue optical axes are coincident on the exit face of the dichroic combiner  1107 . 
     In summary, the present invention provides a polarized light illumination apparatus using light beams emitted from multicolor LEDs for LCD or LCoS projection applications. The invention greatly improves the color gamut of the imaging, eliminates unwanted UV and IR light as well as low efficiency color wheels. Thus, the luminous efficiency and the operating life of the light source are significantly increased by providing a light engine comprised of a unique combination of optical components. Overall, the light engine collimates, combines and converts RGB LED light with high efficiency and compact size.