PATENT ABSTRACT
A projection apparatus has a first light beam having a first state of polarization and containing a first set of primary colors, a first light modulator arrangement for spatially modulating the polarization state of the first light beam to encode a first image thereon in the first set of primary colors, a second light beam having a second state of polarization and containing a second set of primary colors, and a second light modulator arrangement for spatially modulating the polarization state of the second light beam to encode a second image thereon. A polarizing beam splitter having first and second input ports to admit the first and second encoded light beams. Light of one polarization state incident on the first port is transmitted to the output port and light of another polarization state incident on the second port is reflected to said output port so that said transmitted and reflected light is combined into a common output beam at said output port. The first and second images having different polarizations contained in the output beam projected onto a display screen can be viewed with the aid of glasses with selective color filters responsive to the first and second sets of primary colors. By using different sets of primary colors considerable efficiencies and economies can be realized relative to a pure polarization-based system.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 USC 119(e) of prior U.S. application No. 60/894,228, filed Mar. 11, 2007, the contents of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to projection display systems, and more particularly, to projection display systems having microdisplay panels, polarizing beam-splitter(s) and a single projection lens that are suitable for displaying high performance two-dimensional (2D) images or videos, and/or three-dimensional (3D) stereoscopic images or videos. 
     BACKGROUND OF THE INVENTION 
     Today, most projection displays are only capable of projecting 2-D images. Stereo 3D displays are useful for many applications. They provide additional depth information and allow viewers to extract information from complex data faster and more accurately. In addition, they create immersive environments that are very useful for visual simulations, 3D gaming and 3D movies. 
     In order to display stereo 3D images, a stereoscopic 3D projection display must be able to show at least two slightly different left-eye and right-eye images to be seen by the viewer&#39;s left and right eyes. There are several approaches to display three-dimensional stereoscopic images or videos by projection means. These include auto-stereoscopic displays which are limited to a single or a few viewers who do not need to wear 3D glasses; and projection displays where viewers wear active or passive 3D glasses. For multi-viewer and large screen applications, 3D stereoscopic projection displays that require viewers to wear 3D glasses are more suitable because they do not limit viewing positions or the number of viewers. Detailed description of each approach can be found in open literature. 
     There are two types of stereoscopic 3D projection displays using 3D glasses: passive and active. In passive stereoscopic 3D displays with glasses, the left- and right-eye images are displayed with light in two different polarizations or in two different sets of colors. They normally require two projectors: one to project the left-eye images in one polarization or set of colors, the other to project the right-eye images in the orthogonal polarization or a different set of colors. Polarizing glasses or color filter glasses are relatively inexpensive and suitable for large audiences such as in a meeting room or a 3D cinema. However, dual projectors are bulky, expensive, and difficult to align. In addition, they are not light efficient, only 12-30% of the light is used in 3D when compared to 100% for displaying 2D images. Single projector versions of passive stereoscopic display systems also exist, such as projection displays using Z-Screens. In this case, the left- and right-eye images are displayed time-sequentially, further reducing light efficiency because the left- and right-eye images are displayed at most only half of the time. Typical light efficiency is only about 12% and thus much higher power lamps must be used. In addition, 3D projectors using Z-screens must operate at faster frame rates. Only expensive 3-chip DLP projectors or cathode ray tubes (CRT) have such capability. 
     In 3D displays with active glasses, the left- and right-eye images are displayed time-sequentially and viewers wear LCD shutter glasses that are synchronized with the appearance of the correct eye image. Only about 16% of light from the projector is used for 3D. Active glasses require power and a wired/wireless link to the projector. In addition, they can generally only work with fast refresh rate CRTs or expensive 3-chip DLP projectors. 
     In stereoscopic 3D projection displays using passive filter glasses, the left- and right-eye images are projected in spectrally separated sets of primary colors, for example, R 1 , G 1 , B 1 , and R 2 , G 2 , B 2 , respectively. Each set of primary colors can form full color images although the color gamut seen by each eye can be slightly different. This difference can be corrected for by the projection system. Using two different sets of primary colors for stereoscopic 3D projection displays has been disclosed. U.S. Pat. No. 7,001,021 discloses an arrangement which combines the left and right-eye color images having different sets of primary colors with a dichroic filter or prism. For projection displays, the projection optical system usually requires reasonably large aperture in order to use as much illumination light as possible, for example, f/2.4 optics with divergent angle of ±12° in air. It is well known that dichroic filters are very sensitive to angles of incidence and polarization states of the incident light and their performance changes significantly with angles and polarization. Since colors R 1  and R 2  are usually very close to each other spectrally, so are colors G 1 , G 2 , and B 1 ,B 2 . It is very difficult or impractical to combine the colors effectively with a dichroic filter or prism in order to project both sets of images simultaneously through a single lens projector. In order to make this type of projection system work, the divergence angle would have to be reduced significantly and this makes it very light inefficient and impractical for use in stereoscopic displays. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide highly light efficient projection display systems that can project left-eye and right-eye images simultaneously on screen through a single projection lens; 
     It is another object of the invention to provide highly light efficient projection display systems that can be switched between 2D and 3D display modes electronically without comprising display performance. 
     According to a first aspect of the invention there is provided a projection apparatus comprising a source of a first light beam having a first state of polarization and containing a first set of primary colors; a first light modulator arrangement for spatially modulating the polarization state of the first light beam to encode a first image thereon in said first set of primary colors; a source of a second light beam having a second state of polarization and containing a second set of primary colors; a second light modulator arrangement for spatially modulating the polarization state of the second light beam to encode a second image thereon in said second set of primary colors; a polarizing beam splitter having first and second input ports for admitting said first and second encoded light beams, and an output port, and wherein light of one polarization state incident on the first port is transmitted to the output port and light of another polarization state incident on the second port is reflected to said output port so that said transmitted and reflected light is combined into a common output beam at said output port; and projection optics for projecting said first and second images having different polarizations contained in said output beam onto a display screen. 
     While it is known to use different polarization states to separate stereoscopic images, such systems require special non-depolarizing screens and expensive optics. By using different primary color sets to display stereoscopic images separated by using light of different polarization states, the equipment can use ordinary screens and achieve lower cross-talk between and left- and right-eye images compared with pure polarization based 3D stereoscopic displays that require the use of polarization preserving screens. 
     It is known that owing to the properties of the human eye, full color images can be formed by combining separate color images in each of a set of primary colors, typically red, green and blue. What is not so commonly known is that the red, green and blue components of the image can each cover a broad range of wavelengths, and it is thus possible to select specific wavelength ranges within this broad range, each of which will act as a primary color. For example, the red image could be formed either by a color R 1  covering the wavelength range from 600 nm to 620 nm or a color R 2  covering the wavelength range from 625 to 645 nm. Similar reasoning applies to the green G 1 , G 2 , and blue images B 1 , B 2 . It is thus possible to have two sets of primary colors, R 1 , G 1 , B 1 , and R 2 , G 2 , and B 1 , B 2 , each of which set is capable of forming a full color image. 
     Thus, although the color pairs R 1 , R 2 , and G 1 , G 2 , and B 1 , B 2  can be similar in color appearance to the human eyes, they are different spectrally and cover different spectral ranges. The colors of pair R 1  and R 2  each appear to represent a red color. In some cases, the pairs can represent different colors also, for example, R 1  could be orange and R 2  red. The important point to realize is that two sets of primary colors, each of which have color components covering different wavelength ranges, are both capable of forming full color images. In the present invention the left-eye images may be formed by a first set of primary colors R 1 , G 1  and B 1  and the right-eye images may be formed by a second set R 2 , G 2  and B 2 . Thus, by wearing color filter glasses that only allow the wavelength ranges of the components of the respective primary color image to enter the respective eye, a viewer will perceive a stereoscopic image without the need to wear polarizing glasses and have a non-depolarizing screen. 
     Unlike the prior art, the present invention uses both different polarizations as well as different primary color sets to display left- and right-eye images. The left-eye and right-eye images are combined by a polarizing beam-splitter rather than a dichroic filter to form single-lens 3D stereoscopic projection displays. Because the polarizing beam-splitter used in the present invention can work over a large angular field the present invention achieves much higher efficiency which is very important in particular for stereoscopic displays because the available illumination has to be shared by left- and right-eye images. In addition, the present invention can work in polarization and/or in color to delivery stereoscopic images and this makes it very versatile for 3D stereoscopic applications. 
     According to a second aspect of the invention there is provided a projection apparatus comprising a plurality of subsystems, each subsystem comprising a source of a first light beam having a first state of polarization; a first light modulator arrangement for spatially modulating the polarization state of the first light beam to encode a first image thereon; a source of a second light beam having a second state of polarization; a second light modulator arrangement for spatially modulating the polarization state of the second light beam to encode a second image thereon; and a polarizing beam splitter having first and second input ports for admitting said first and second encoded light beams, and an output port, and wherein light of one polarization state incident on the first input port is transmitted to the output port and light of another polarization state incident on the second port is reflected to said output port so that said transmitted and reflected light is combined into a common output beam at said output port; and wherein the source of light for each said subsystem is of a different color, and said projection apparatus further comprises a color combiner for combining the output beam of each said subsystem into a common output beam; and projection optics for projecting said common output beam onto a display screen. 
     In a still further aspect the invention provides a projection apparatus comprising a source of an unpolarized light beam containing a plurality of colors; a polarizing beam splitter for separating said unpolarized light beam into a pair of polarized light beams; a pair of subsystems, each subsystem admitting one of said polarized light beams and comprising: a plurality of light modulator arrangements for spatially modulating the polarization state of the said separate polarized light beams to encode respective color images thereon; a color combiner for splitting said polarized light into separate polarized color beams and for combining said encoded separate light beams into a composite beam; and wherein said polarizing beam splitter combines said composite beams of different polarization into an output beam; and projection optics for projecting said common output beam onto a display screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention and exemplary embodiments of the invention will be described in accordance to the following drawings in which: 
         FIG. 1A  is a schematic view of a Type A thin film polarizing beam-splitter which reflects s-polarized light and transmits p-polarized light; 
         FIG. 1B  is a schematic view of a Type B thin film polarizing beam-splitter with frustrated total internal reflection that reflects p-polarized light and transmits s-polarized light; 
         FIG. 1C  is a schematic view of a Type C birefringent multilayer film polarizing beam-splitter that reflects s-polarized light and transmits p-polarized light, or vice versa; 
         FIG. 1D  is a schematic view of a Type D metal-wire grid polarizing beam-splitter that reflects s-polarized light and transmits p-polarized light, or vice versa; 
         FIG. 2A  is a schematic view showing a typical transmissive LCD micro-display panel used in the present invention; 
         FIG. 2B  is a schematic view showing another typical transmissive LCD micro-display panel used in the present invention; 
         FIG. 3A  is a schematic view showing a typical reflective LCOS micro-display panel used in the present invention; 
         FIG. 3B  is a schematic view showing another typical reflective LCOS micro-display panel used in the present invention; 
         FIG. 4A  is a schematic view showing a first typical reflective MEM micro-display panel used in the present invention (the incident light on, and reflected light from, the panel  30  are in the same plane, but they do not follow a co-linear path); 
         FIG. 4B  is a schematic view showing a second typical reflective MEM micro-display panel used in the present invention (the incident light on, and reflected light from, the panel  30  are in the same plane, but they do not follow a co-linear path); 
         FIG. 4C  is a schematic view showing a third typical reflective MEM micro-display panel used in the present invention; 
         FIG. 4D  is a schematic view showing a fourth typical reflective MEM micro-display panel used in the present invention; 
         FIG. 5A  is a schematic view showing a typical cut-off type wavelength selective polarization rotator used in the present invention; 
         FIG. 5B  is a schematic view showing another typical cut-off type wavelength selective polarization rotator used in the present invention; 
         FIG. 5C  is a schematic view showing a typical bandpass type wavelength selective polarization rotator used in the present invention; 
         FIG. 5D  is a schematic view showing another typical bandpass type wavelength selective polarization rotator used in the present invention; 
         FIG. 5E  is a schematic view showing a typical multiple bandpass type wavelength selective polarization rotator used in the present invention; 
         FIG. 5F  is a schematic view showing another typical multiple bandpass type wavelength selective polarization rotator used in the present invention; 
         FIG. 6A  is a schematic view of one of the first type embodiments of the projection display systems in accordance with the present invention having two illumination systems, two transmissive LCD panels, a polarizing beam-splitter with the central angle of incidence on the beam-splitter surface about 45° and a single projection lens; 
         FIG. 6B  is a schematic view of one of the first type embodiments of the projection display systems in accordance with the present invention having two illumination systems, two transmissive LCD panels, a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 6C  is a schematic view of one of the first type embodiments of the projection display systems in accordance with the present invention having two illumination systems, two transmissive LCD panels, a polarizing beam-splitter with central angle of incidence substantially larger than 45° and a single projection lens; 
         FIG. 7A  is a schematic view of one of the second type embodiments of the projection display systems in accordance with the present invention having an illumination system, two transmissive LCD panels, a polarizing beam-splitter with the central angle of incidence on the beam-splitter surface about 45° and a single projection lens; 
         FIG. 7B  is a schematic view of one of the second type embodiments of the projection display systems in accordance with the present invention having an illumination system, two transmissive LCD panels, a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 7C  is a schematic view of one of the second type embodiments of the projection display systems in accordance with the present invention having an illumination system, two transmissive LCD panels, a polarizing beam-splitter with central angle of incidence substantially larger than 45° and a single projection lens; 
         FIG. 8A  is a schematic view of one of the third type embodiments of the projection display systems in accordance with the present invention having an illumination system, two reflective LCOS or MEM panels, a polarizing beam-splitter with the central angle of incidence on the beam-splitter surface about 45° and a single projection lens; 
         FIG. 8B  is a schematic view of one of the third type embodiments of the projection display systems in accordance with the present invention having an illumination system, two reflective LCOS or MEM panels, a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 8C  is a schematic view of one of the third type embodiments of the projection display systems in accordance with the present invention having an illumination system, two reflective LCOS or MEM panels, a polarizing beam-splitter with central angle of incidence substantially larger than 45° and a single projection lens; 
         FIG. 8D  is a schematic view of one of the third type embodiments of the projection display systems in accordance with the present invention having an illumination system, two reflective LCOS or MEM panels, an additional mirror, a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 9A  is a schematic view of one of the fourth type embodiments of the projection display systems in accordance with the present invention having two illumination systems, six transmissive LCD panels, two X-cubes, and a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 9B  is a schematic view of one of the fourth type embodiments of the projection display systems in accordance with the present invention having two illumination systems, six transmissive LCD panels, two X-cubes, a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 10  is a schematic view of one of the fifth type embodiments of the projection display systems in accordance with the present invention having an illumination system, six transmissive LCD panels, two X-cubes, a polarizing beam-splitter with central angle of incidence larger than 45° and a single projection lens; 
         FIG. 11A  is a schematic view of one of the sixth type embodiments of the projection display systems in accordance with the present invention having three illumination systems, six reflective LCOS or MEM panels, an X-cube, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 11B  is a schematic view of one of the sixth type embodiments of the projection display systems in accordance with the present invention having an illumination system, six reflective LCOS or MEM panels, an X-cube, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 11C  is a schematic view of one of the sixth type embodiments of the projection display systems in accordance with the present invention having three illumination systems, six reflective LCOS or MEM panels, a Philips prism, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 11D  is a schematic view of one of the sixth type embodiments of the projection display systems in accordance with the present invention having an illumination system, six reflective LCOS or MEM panels, a Philips prism, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 12A  is a schematic view of one of the seventh type embodiments of the projection display systems in accordance with the present invention having three illumination systems, six reflective LCOS or MEM panels, an X-cube, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 12B  is a schematic view of one of the seventh type embodiments of the projection display systems in accordance with the present invention having an illumination system, six reflective LCOS or MEM panels, an X-cube, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 12C  is a schematic view of one of the seventh type embodiments of the projection display systems in accordance with the present invention having three illumination systems, six reflective LCOS or MEM panels, a Philips prism, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 12D  is a schematic view of one of the seventh type embodiments of the projection display systems in accordance with the present invention having an illumination system, six reflective LCOS or MEM panels, a Philips prism, three polarizing beam-splitters with the central angle of incidence on the beam-splitter surface about 45°, and a single projection lens; 
         FIG. 13A  is a schematic view of one of the eighth type embodiments of the projection display systems in accordance with the present invention having an illumination system, six reflective LCOS or MEM panels, two X-cubes, a polarizing beam-splitter with central angle of incidence greater than 45° and a single projection lens; 
         FIG. 13B  is a schematic view of one of the eighth type embodiments of the projection display systems in accordance with the present invention having an illumination system, six reflective LCOS or MEM panels, two Philips prisms, a polarizing beam-splitter with central angle of incidence greater than 45° and a single projection lens; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     PBS Types 
     In the present invention of projection displays, polarizing beam-splitters are used to separate un-polarized light, and/or to combine polarized light. A polarizing beam-splitter (PBS) reflects light in a first polarization and transmits light in a second polarization. The first polarization and the second polarization are orthogonal to each other. Different types of PBSs can be used in the present invention. By way of examples, several PBS types are incorporated in the present invention and are described in the text below. 
     Type A PBS, shown in  FIG. 1A , is based on thin film interference coatings where the thin film beam-splitting (BS) coating is between two transparent prisms. Type A is a MacNeille PBS that reflects s-polarized light and transmits p-polarized light and usually operates at 45° angle of incidence. Type B PBS, shown in  FIG. 1B , is based on thin film interference as well as frustrated total internal reflection in thin film coatings, the thin film beam-splitting coating is also between two transparent prisms. Unlike Type A PBS, Type B PBS reflects p-polarized light and transmits s-polarized light, as disclosed in U.S. Pat. No. 5,912,762. Type C PBS, shown  FIG. 1C , is based on birefringent multilayer thin plastic films, and the beam-splitting films are usually between two transparent prisms, such as the one disclosed in U.S. Pat. No. 6,690,795. Depending on the alignment of the birefringent films, Type C PBS can reflect s-polarized light and transmit p-polarized light, or vice versa. Type D PBS, shown in  FIG. 1D , is based on metal-wire grids on a transparent plate, such as the one disclosed in U.S. Pat. No. 6,122,103, the beam-splitting metal-wire grids are on one side of the plate. Based on the alignment of the metal wires, Type D PBS can reflect s-polarized light and transmit p-polarized light, or vice versa. Type E PBS also reflects s-polarized light and transmits p-polarized light and operates at a central angle of incidence greater than 45° as described in the paper by Li Li and Dobrowolski, Applied Optics, Vol. 39, Issue 16, pp. 2754-2771. Type E is similar to type A that reflects s-polarized light and transmits p-polarized light except that the central angle of incidence is greater than 45°. Type E is also based on thin film interference coatings. Without departing from the spirit of the invention, other beam-splitting angles rather than the ones shown in  FIGS. 1A-1D  and other types of PBSs can be used in the present invention as well. 
     Micro-Display Devices 
     In the present invention of the projection display systems, several different micro-display panels can be used. Typical sizes of micro-display panels are from 0.55″ to 2.0″ in diagonal. The first type of micro-displays is based on transmissive liquid crystal displays (LCDs) that can form images by controlling the polarization states of each individual pixel.  FIG. 2A  shows a typical LCD panel  32  used in accordance with the present invention. The LCD panel  32  consists of a pixelated active liquid crystal structure  130  with addressing matrices and two polarizers  110  and  120 . The polarizer  110  transmits light in a first polarization and blocks light in a second polarization, while the polarizer  120  transmits the second polarization and blocks the first polarization. The first and second polarizations are orthogonal to each other. For “on” pixels, the liquid crystal layer  130  rotates the polarization state of the incident light from the first polarization to the second polarization thus the light transmits through the panel. For “off” pixels, the liquid crystal structure does not rotate the polarization state of the incident light, thus the light is blocked by the second polarizer  120 .  FIG. 2B  shows another typical LCD panel  32  used in accordance with the present invention. In this case, the polarizer  110  blocks light in the first polarization and transmits light in the second polarization, while the polarizer  120  blocks the second polarization and transmits the first polarization. For “on” pixels, the liquid crystal structure rotates the polarization state of the incident light from the second polarization to the first polarization thus the light transmits through the panel. For “off” pixels, the liquid crystal structure does not rotate the polarization state of the incident light, thus the light is blocked by the second polarizer  120 . The micro-display  32  can be optimized to operate for a broadband of wavelengths such as from 400-700 nm in the visible, or it can be optimized to operate for a narrowband of wavelengths such as for red, blue or green colors only. 
     The second type of micro-displays is based on reflective liquid crystal displays that also can form images by controlling the polarization states of each individual pixel. These liquid crystal displays are also called liquid crystal on silicon (LCOS), or direct-drive image light amplifier (D_ILA).  FIG. 3A  shows a typical LCOS  30  panel used in accordance with the present invention with a polarizing beam-splitter  10  (although any type of PBS described above can be used, only a plate PBS is shown here for illustration purposes). The LCOS panel  30  consists of a pixelated reflective liquid crystal structure  300  and an optional waveplate  310 . The PBS  10  reflects light in a first polarization and transmits light in a second polarization. The first and second polarizations are orthogonal to each other. The optional waveplate  310  is usually a quarter-wave plate or combinations of waveplates that can compensate for geometrical depolarization of the incident polarized light (resulting from reflection and transmission of skew rays at optical interfaces) and thus improve display contrast. In  FIG. 3A , the incident light is polarized in the first polarization state and thus is reflected by the polarizing beam-splitter  10  towards the panel  30 . For “on” pixels, the liquid crystal structure rotates the polarization state of the incident light from the first polarization to the second polarization thus the light transmits through the PBS  10  towards a projection lens which is not shown here. For “off” pixels, the liquid crystal structure does not rotate the polarization state of the incident light, thus the light is reflected back by the PBS  10 .  FIG. 3B  shows another typical LCOS panel  30  used in accordance with the present invention with a polarizing beam-splitter  10 . The LCOS panel  30  consists of a pixelated reflective liquid crystal structure  300  and an optional waveplate  310 . The PBS  10  reflects the first polarization and transmits the second polarization. The optional waveplate  310  is usually a quarter-wave plate or combinations of waveplates that can compensate the geometrical depolarization of the incident polarized light and thus improve display contrast. The incident light is polarized in the second polarization state, thus is transmitted through the polarizing beam-splitter  10 . For “on” pixels, the liquid crystal structure rotates the polarization state of the incident light from the second polarization to the first polarization thus the light is reflected by the PBS  10  towards a projection lens which is not shown here. For “off” pixels, the liquid crystal structure does not rotate the polarization state of the incident light, thus the light transmits through the PBS  10  back to the direction of the incident light. The micro-display  30  can be optimized to operate for a broadband of wavelengths such as from 400-700 nm in the visible, or it can be optimized to operate for a narrowband of wavelengths such as for red, blue or green colors only. 
     The third type of micro-displays is based on micro-electrical-mechanical (MEM) devices with plural digital micro-mirrors (digital micro-mirror devices—DMD) or pixels arranged in rows and columns used to form images. A typical MEM device for display applications is Texas Instruments&#39; DLP™ or DMD panels.  FIGS. 4A-4D  shows a MEM panel  30  used in accordance with the present invention with a polarizing beam-splitter  10  (a plate PBS is shown here for illustration purposes). The MEM panel  30  consists of a pixelated reflective MEM device  200  and a waveplate  210 . The PBS  10  reflects light in a first polarization and transmits light in a second polarization. The first and second polarizations are orthogonal to each other. The waveplate  210  is usually a quarter-wave plate or a combination of waveplates that rotate the polarization state of the incident light from the first polarization to the second polarization or vice versa. Preferably, it can also compensate the geometrical depolarization of the incident polarized light and thus improve display contrast or cross-talk.  FIG. 4A  shows a first typical MEM panel  30  used in accordance with the present invention. The incident light is polarized in the first polarization state. For “on” pixels, the light is reflected by the MEM device at a predefined angle and the polarization state of the incident light is rotated by the waveplate  210  from the first polarization to the second polarization, thus the light transmits through the PBS  10  towards a projection lens which is not shown here. For “off” pixels, the light is reflected by the MEM device at a predefined angle, but different than the angle of “on” pixels. Depending on the properties of the waveplate  210  and the PBS  10 , the reflected “off” pixel light may be partially converted to the second polarization, and thus some of the light is reflected and some of the light is transmitted by the PBS  10 . For the “off” pixel light transmitted by the PBS  10 , it travels in a different direction than the “on” pixel light and thus does not reach the projection lens (not showing here) and is absorbed by a light absorber. For the “off” light reflected by the PBS  10 , the light travels in a different direction than the incident light and is absorbed by a light absorber not showing here.  FIG. 4B  shows a second typical MEM panel  30  used in accordance with the present invention with a polarizing beam-splitter  10 . The incident light is polarized in the second polarization state. For “on” pixels, the light is reflected by the MEM device at a predefined angle and the polarization state of the incident light is rotated by the waveplate  210  from the second polarization to the first polarization, thus the light is reflected by the PBS  10  towards a projection lens which is not shown here. For “off” pixels, the light is reflected by the MEM device at a predefined angle, but different than the angle of “on” pixels. Depending on the properties of the waveplate  210  and the PBS  10 , the reflected “off” pixel light may be partially converted to the first polarization, and thus some of the light is reflected and some of the light is transmitted by the PBS  10 . For the “off” pixel light reflected by the PBS  10 , it travels in a different direction than the “on” pixel light and thus does not reach the projection lens (not shown here) and is absorbed by a light absorber. For the “off” light transmitted by the PBS  10 , the light travels in different direction than the incident light and is absorbed by a light absorber not shown here. Depending on the layout of the MEM device, different orientations of the MEM device surface with regard to the PBS surface can be used. The micro-display  30  can be optimized to operate for a broadband of wavelengths such as from 400-700 nm in the visible, or it can be optimized to operate for a narrowband of wavelengths such as for red, blue or green colors only. 
       FIGS. 4C-4D  show a third and fourth typical MEM devices used in the present invention having different alignments between the polarizing beam-splitter and the MEM device. Detailed description of these alignments and other suitable alignments can be found in the U.S. patent application Ser. No. 11/770,247 which is incorporated in the present invention.  FIGS. 4A and 4B  use Type II alignment while  FIGS. 4C and 4D  use Type III or IV alignment as described in the U.S. patent application Ser. No. 11/770,247. 
     Without departing from the spirit of the invention, other arrangements of pixels or types of MEM devices can also be used in the present invention. 
     Wavelength Selective Polarization Rotators 
     In the present invention of the projection display systems, sometimes wavelength selective polarization rotators are used. These rotators are typically made of stretched multilayer birefringent plastic films or crystal plates. They can selectively rotate the polarization state of incident light from a first polarization to a second polarization for some wavelengths but keep the polarization state unchanged for the other wavelengths. The first and the second polarizations are orthogonal to each other.  FIG. 5A  shows a typical cut-off type wavelength selective polarization rotator  90  used in the present invention. The incident light is in the first polarization and consists of colors c 1  and c 2  covering different wavelength regions. The rotator  90  rotates the polarization state of light in color c 2  to the second polarization but keeps the polarization state of light in color c 1  unchanged.  FIG. 5B  shows another typical cut-off type wavelength selective polarization rotator  90  used in the present invention. The rotator  90  rotates the polarization state of light in color c 1  to the second polarization but keeps the polarization state of light in color c 2  unchanged. 
       FIG. 5C  shows a typical bandpass type wavelength selective polarization rotator  90  used in the present invention. The incident light is in the first polarization and consists of colors c 1 , c 2  and c 3  with different wavelengths. The rotator  90  rotates the polarization state of light in color c 2  to the second polarization but keeps the polarization state of light in colors c 1  and c 2  unchanged.  FIG. 5D  shows another typical bandpass type wavelength selective polarization rotator  90  used in the present invention. The rotator  90  rotates the polarization state of light in colors c 1  and c 3  to the second polarization but keeps the polarization state of light in color c 2  unchanged. 
       FIG. 5E  shows a typical multi-bandpass type wavelength selective polarization rotator  90  used in the present invention. The incident light is in the first polarization and consists of colors c 1 , c 2 , c 3 , c 4 , c 5  and c 6  with different wavelengths. The rotator  90  rotates the polarization state of light in colors c 2 , c 4  and c 6  to the second polarization but keeps the polarization state of light in colors c 1 , c 3  and c 5  unchanged.  FIG. 5F  shows another typical multi-bandpass type wavelength selective polarization rotator  90  used in the present invention. The rotator  90  rotates the polarization state of light in colors c 1 , c 3  and c 5  to the second polarization but keeps the polarization state of light in colors c 2 , c 4  and c 6  unchanged. 
     The multi-bandpass wavelength selective polarization rotators can be made of combinations of the cut-off or bandpass types wavelength selective polarization rotators. Without departing from the spirit of the present invention, other wavelength selective polarization rotators and rotation combinations can be used as well. The wavelength selective polarization rotator  90  can include additional waveplates to correct depolarization effects due to geometry or non-normal incidence of the light at reflecting or transmitting surfaces. 
     First Type of Embodiments 
     The first type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 6A-6C , comprises of two illumination systems  8 L and  8 R, a polarizing beam-splitter  10  for combining image light, two transmissive LCD micro-display panels  32 L and  32 R, an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . The polarizing beam-splitter  10  reflects light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The illumination systems  8 L and  8 R generate polarized light beams  4 L and  4 R in the first and second polarization, respectively. In the embodiment shown in  FIG. 6A , the central angle of incidence of light at the beam-splitting surface of the polarizing beam-splitter  10  is about 45°; in the embodiment shown in  FIG. 6B , the central angle of incidence is slightly larger than 45°; and in the embodiment shown in  FIG. 6C , the central angle of incidence is substantially larger than 45°. The micro-display panels  32 L and  32 R are of the transmissive micro-display panel  32  described in the preamble. 
     The polarizing beam-splitter  10  is selected from polarizing devices described in the preamble. When the polarizing beam-splitter  10  reflects light in the second polarization and transmits light in the first polarization, the positions of the illumination system  8 L and the micro-display panel  32 L, and the illumination system  8 R and the micro-display panel  32 R are switched. The first and second polarizations can be s- and p-polarized, respectively, or vice versa. 
     The operation of the first type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 6A-6C . 
     The polarized light beam  4 L in the first polarization from the illumination system  8 L is incident upon the micro-display panel  32 L. Images are then encoded by the micro-display panel onto the output light by modulating the polarization state of the incident light. For “on” pixels, the polarization state of the image light is rotated to the second polarization. Thus the “on” pixel image light passes through the polarizing beam-splitter  10  and the optional waveplate  70  and is then projected onto the screen  60  by the projection lens  50 . For “off” pixels, the polarization state of the output image light is unchanged, is still in the first polarization, thus it is absorbed by the internal polarizer within the microdisplay panel  32 L and any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by reflecting the light out of the imaging path of the projection lens. Therefore, very high contrast images can be obtained. 
     Similarly, the polarized light beam  4 R in the second polarization from the illumination system  8 R is incident upon the micro-display panel  32 R. Images are then encoded by the micro-display onto the output light by modulating the polarization states of the incident light. For “on” pixels, the polarization state of the image light is rotated to the first polarization. Thus the “on” pixel image light is reflected by the polarizing beam-splitter  10  and passes through the optional waveplate  70  and is then projected onto the screen  60  by the projection lens  50 . For “off” pixels, the polarization state of the output image light is unchanged, is still in the second polarization, thus it is absorbed by the internal polarizer within the microdisplay panel  32 R and any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by transmitting the light out of the imaging path of the projection lens. Therefore, very high contrast images can also be obtained. 
     If the optional projection screen  60  is reflective, then the projection display system operates as a front projector. If the optional projection screen  60  is transmissive, the projection display system is then a rear projector which may also consist of some additional mirrors to reduce the overall system size that are not shown here. This paragraph regarding the front or rear projection arrangements applies to all types of the embodiments in accordance with the present invention of the projection display systems described hereafter. It is, therefore, not to be repeated and its incorporation in the description of each embodiment type is assumed. 
     The illumination systems  8 L and  8 R can each comprise of a lamp emitting white light such as a UHP lamp and a polarization recovery means for converting un-polarized light into polarized light; in addition, a color filter wheel can be added to generate time-sequential colors, red, green and blue, or any other suitable color combinations. Alternatively, the illumination systems  8 L and  8 R can each comprise of red, green and blue color LED light sources or lasers, or any other suitable color combinations, and a polarization recovery means only if the light sources do not emit polarized light; time-sequential color light is generated by pulsing the LED or laser light sources. 
     If the light from the two illumination systems  8 L and  8 R is white light, then black and white images are shown on screen. If the light from the two illumination systems  8 L and  8 R are time-sequential colors red, green and blue or any other suitable color combinations, and the color images on the micro-display panels are synchronized with the color illumination light, then the full color images are shown on screen. 
     The first type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes: 
     In the first 2D and 3D switchable mode of the first type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this mode, a non-depolarizing projection screen is used. Since the image light from the micro-display panel  32 L is in the second polarization and the image light from the micro-display panel  32 R is in the first polarization, if left-eye and right-eye image signals are fed to the two micro-display panels  32 L and  32 R, respectively, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two microdisplay panels  32 L and  32 R, normal 2D images are then displayed onscreen. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the first type of the embodiments described above, the 3D stereo display mode is realized using two different sets of primary colors for the left- and right-eye images. In this case, a non-depolarizing projection screen or an ordinary screen can be used. The light from the illumination system  8 L consists of a first set of primary colors c 1 L, c 2 L and c 3 L, the light from the illumination system  8 R consists of a second set of primary colors c 1 R, c 2 R and c 3 R. The first and second sets of primary colors are different and occupy different wavelength regions in the wavelength spectrum. More than three colors in each primary color set can be used as well. Thus, the image light from the micro-display panel  32 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L, and the image light from the micro-display panel  32 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R. If left-eye and right-eye image signals are fed to the two micro-display panels  32 L and  32 R, respectively, or vice versa, by wearing matching 3D color filter glasses, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye color filter glasses match the spectrum of the corresponding image sets. The left- and right-eye glasses only allow the corresponding eye color images to pass and block the other eye color images. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L and  32 R, normal 2D images are then displayed onscreen with both the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of ordinary screens and relatively cheap plastic projection lens which may depolarize light, in addition, low cross-talk between the left-eye and right-eye images can be achieved due to the high contrast ratios of color filter glasses. 
     In the third 2D and 3D switchable mode of the first type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations and two different sets of colors for the left- and right eye-images. In this case, a non-depolarizing projection screen or an ordinary screen is used. The light from the illumination system  8 L consists of a first set of primary colors c 1 L, c 2 L and c 3 L, the light from the illumination system  8 R consists of a second set of primary colors c 1 R, c 2 R and c 3 R. The first and second sets of primary colors are different and occupy different wavelength regions in the wavelength spectrum. More than three colors in each primary color set can be used as well. The image light from the micro-display panel  32 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L in the second polarization, and the image light from the micro-display panel  32 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye and right-eye image signals are fed to the microdisplay panels  32 L and  32 R, respectively, or vice versa, and by wearing matching polarizing 3D glasses in the case where a non-depolarizing screen is used, or color filter glasses in the case where a non-depolarizing screen or an ordinary screen is used, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye polarizing or color filter glasses match the polarization or spectrum of the corresponding image sets, they only allow the corresponding polarization or color images of the corresponding eye to pass and block the images for the other eye. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L and  32 R, normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This type of operation mode is desirable in some applications where users can have the flexibility to use either 3D polarizing glasses with a non-depolarizing screen or 3D color filter glasses with a non-depolarizing or an ordinary screen. It is especially suitable for portable projector applications. 
     In the first type of the embodiments of the present invention of the projection display systems, the two images onscreen from the two micro-display panels  32 L and  32 R are well aligned. This is one of the unique advantages of the present invention compared to any existing dual 3D projectors. It allows both high quality 2D and 3D images to be displayed. Existing dual 3D projectors can not be aligned well enough to be used as 2D projectors and they are mostly used as dedicated 3D projectors for displaying 3D images only. In some applications, it may be desirable to slightly off-set the images from the two micro-display panels by a half pixel vertically and horizontally. The off-set approach can be beneficial, for example, it can be used to increase the spatial resolution of displays for 2D images, similar to that of an image wobbling technique, yet in 3D mode, the half-pixel misalignment does not have much impact on the 3D images. However, in this case, the 2D image signals fed to the micro-display panels are not identical but each represents an interlaced half image of the complete 2D images. 
     The above paragraph regarding image alignment applies to all types of embodiments in accordance with the present invention. In the embodiments using two micro-display panels, the two images are from two micro-display panels. In the embodiments using six micro-display panels, the two images are replaced by two sets of the images from two sets of micro-display panels. For simplicity, the above paragraph will not be repeated and its incorporation in the description of each embodiment type is assumed with minor modifications accordingly. 
     The optional waveplate  70  is sometimes used to convert image light from linear polarized light into circular polarized light in 3D operation mode with polarized light. In this case, viewers need to wear matching 3D circular polarizing glasses. The use of circularly polarized light is sometimes beneficial because it allows viewers to tilt their head over a wider range than with linear polarized light while maintaining a good quality 3D viewing experience. This paragraph applies to all embodiment types in accordance with the present invention and thus its incorporation is assumed in the description of the embodiment types hereafter. 
     A person skilled in the art will appreciate that some additional common optical components, for example lens, may be needed in some of the above embodiments and these additional components are not shown in the figures. Without departing from the spirit of the present invention, other embodiments with different arrangements and layouts of the components can be used as well. This paragraph also applies to the second to eighth type of embodiments described below. Furthermore, a person skilled in the art will appreciate that the illumination systems  8  shown for the second to the fifth type of embodiments and the seventh type of embodiments can generate light in the first polarization. 
     Second Type of Embodiments 
     The second type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 7A-7C , comprises of an illumination system  8 , a beam-splitter  3 , two mirrors  5 , a polarizing beam-splitter  10  for combining image light, two transmissive LCD micro-display panels  32 L and  32 R, a optional waveplate  100 , an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . The polarizing beam-splitter  10  reflects light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The waveplate  100  converts light from the second polarization to the first polarization or vice versa. (An optional optical path length compensating plate, that does not change polarization, can be placed before micro-display  32 R, not shown in  FIGS. 7A-7C ). The illumination system  8  generates either polarized or non-polarized light.  FIGS. 7A-7C  show the embodiments in which the illumination system  8  generates the light in the second polarization. If the illumination system  8  generates light in the first polarization, the waveplate  100  is then placed before the micro-display panel  32 R (not shown in  FIGS. 7A-7C ). If the illumination system generates un-polarized light, then the beam-splitter  3  is a polarizing beam-splitter of the type described in the preamble which reflects light in the first polarization and transmits light in the second polarization, or vice versa; thus, the waveplate  100  is not required because the beam-splitter already provides two polarized beams with the second and first polarization states for the two micro-display panels  32 L and  32 R. In the embodiment shown in  FIG. 7A , the central angle of incidence of light at the beam-splitting surface of the polarizing beam-splitter  10  is about 45°; in the embodiment shown in  FIG. 7B , the central angle of incidence is slightly larger than 45°; and in the embodiment shown in  FIG. 7C , the central angle of incidence is substantially larger than 45°. The micro-display panels  32 L and  32 R are transmissive micro-display panels  32  as described in the preamble. 
     The polarizing beam-splitter  10  is selected from polarizing devices described in the preamble. The first and second polarizations can be s- and p-polarized, respectively, or vice versa. 
     The operation of the second type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 7A-7C , assuming the light from the illumination system is in the second polarization and the polarizing beam-splitter  10  reflects light in the first polarization and transmits light in the second polarization. 
     The light in the second polarization from the illumination system  8  is split into two beams by the beam-splitter  3 . The reflected and transmitted light beams in the second polarization from the beam-splitter  3  are reflected by the two mirrors  5  and become beams  4 L and  4 R. The polarized light beam  4 L passes through the waveplate  100  and its polarization state is converted from the second polarization state to the first polarization state. The light is then incident upon the micro-display panel  32 L. Images are then encoded by the micro-display onto the output light by modulating the polarization states of the incident light. For “on” pixels, the polarization state of the image light is rotated to the second polarization. Thus the “on” pixel image light passes through the polarizing beam-splitter  10  and the optional waveplate  70  and is then projected onto the screen  60  by the projection lens  50 . For “off” pixels, the polarization state of the output image light is unchanged, is still in the first polarization, thus it is absorbed by the internal polarizer within the microdisplay panel  32 L and any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by reflecting the light out of the imaging path of the projection lens. Therefore, very high contrast images can be obtained. 
     Similarly, the polarized light beam  4 R in the second polarization is incident upon the micro-display panel  32 R. Images are then encoded by the micro-display onto the output light by modulating the polarization states of the incident light. For “on” pixels, the polarization state of the image light is rotated to the first polarization. Thus the “on” pixel image light is reflected by the polarizing beam-splitter  10  and passes through the optional waveplate  70  and is then projected onto the screen  60  by the projection lens  50 . For “off” pixels, the polarization state of the output image light is unchanged, is still in the second polarization, thus it is absorbed by the internal polarizer within the microdisplay panel  32 R and any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by transmitting the light out of the imaging path of the projection lens. Therefore, very high contrast images can also be obtained. 
     The illumination system  8  can comprise of a lamp emitting white light such as a UHP lamp with or without a polarization recovery means for converting un-polarized light into polarized light; in addition, a color filter wheel can be added to generate time-sequential colors, red, green and blue, or any other suitable color combinations. Alternatively, the illumination system  8  can comprise of red, green and blue, color LED light sources or lasers, or any other suitable color combinations, with or without a polarization recovery means only if the light sources do not emit polarized light; time-sequential color lights are generated by pulsing the LED or laser light sources. 
     If the light from the illumination system  8  is white light then black and white images are shown on screen. If the light from the illumination system  8  consists of time-sequential colors, red, green and blue or any other suitable color combinations. The color images on the micro-display panels are synchronized with the color illumination light, then the full color images are shown on screen. 
     The second type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes: 
     In the first 2D and 3D switchable mode of the second type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this case, a non-depolarizing projection screen is used. In addition, the beam-splitter  3  is a 50/50 beam-splitter which transmits and reflects equally about 50% of the incident light. Since the image light from the micro-display panel  32 L is in the second polarization and the image light from the micro-display panel  32 R is in the first polarization, if left-eye and right-eye image signals are fed to the two micro-display panels  32 L and  32 R, respectively, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two microdisplay panels  32 L and  32 R, normal 2D images are then displayed onscreen. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the second type of the embodiments described above, the 3D stereo display mode is realized using two different sets of primary colors for the left- and right-eye images. In this case, a non-depolarizing projection screen or an ordinary screen can be used. In addition, the beam-splitter  3  reflects light of a first set of primary colors c 1 L, c 2 L and c 3 L and transmits light of a second set of primary colors c 1 R, c 2 R and c 3 R. The first and second sets of primary colors are different and occupy different wavelength regions in the wavelength spectrum. The image light from the micro-display panel  32 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L, and the image light from the micro-display panel  32 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R. If left-eye and right-eye image signals are fed to the two micro-display panels  32 L and  32 R, respectively, or vice versa, by wearing matching 3D color filter glasses, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye color filter glasses match the spectrum of the corresponding image sets. The left- and right-eye glasses only allow the corresponding eye color images to pass and block the other eye color images. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L and  32 R, normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of ordinary screens and relatively cheap plastic projection lens which may depolarize light, in addition, low cross-talk between the left-eye and right-eye images can be obtained due to the high contrast ratios of color filter glasses. 
     In the third 2D and 3D switchable mode of the second type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations and two different sets of colors for the left- and right eye-images. In this case, a non-depolarizing projection screen or an ordinary screen is used. In addition, the beam-splitter  3  reflects light of a first set of primary colors c 1 L, c 2 L and c 3 L and transmits light of a second set of primary colors c 1 R, c 2 R and c 3 R. The first and second sets of primary colors are different and occupy different wavelength regions in the wavelength spectrum. More than three colors in each primary color set can be used as well. The image light from the micro-display panel  32 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L in the second polarization, and the image light from the micro-display panel  32 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye and right-eye image signals are fed to the microdisplay panels  32 L and  32 R, respectively, or vice versa, and by wearing matching polarizing 3D glasses in the case where a non-depolarizing screen is used, or color filter glasses in the case where a non-depolarizing screen or an ordinary screen is used, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye polarizing or color filter glasses match the polarization or spectrum of the corresponding image sets, they only allow the corresponding polarization or color images of the corresponding eye to pass and block the images for the other eye. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L and  32 R, normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This type of operation mode is desirable in some applications where users can have the flexibility to use either 3D polarizing glasses or 3D color filter glasses. 
     Third Type of Embodiments 
     The third type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 8A-8D , comprises of an illumination system  8 , a wavelength selective polarization rotator  90 , a polarizing beam-splitter  10  for separating unpolarized incident light and for combining polarized image light, two reflective micro-display panels  30 L and  30 R, an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . The polarizing beam-splitter  10  reflects light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The illumination system  8  generates polarized light in the first polarization or second polarization state.  FIGS. 8A-8D  show the embodiments in which the illumination system  8  generates the light in the second polarization. In the embodiments shown in  FIGS. 8A and 8D , the central angle of incidence of light at the beam-splitting surface of the polarizing beam-splitter  10  is about 45°; in the embodiment shown in  FIG. 8B  the central angle of incidence is slightly larger than 45°; and in the embodiment shown in  FIG. 8C  the central angle of incidence is substantially larger than 45°. 
     The polarizing beam-splitter  10  is selected from polarizing devices described in the preamble. The first and second polarizations can be s- and p-polarized, respectively, or vice versa. The micro-display panels  30 L and  30 R are of the type of the reflective micro-display panels  30  as described in the preamble. They can be LCOS panels with additional waveplates for compensating geometry depolarization. They can also be MEM panels with additional waveplates for rotating the polarization state of incident light by 90°; in addition, optional waveplates can be used as parts of the panels to compensate geometry depolarization. The wavelength selective polarization rotator  90  is described in the preamble. 
     In the embodiment shown in  FIG. 8D , an additional mirror  5  is added before one of the micro-display panels  30 L (as shown in the figures) or  30 R (not shown in the figures). The mirror  5  can be a plate as shown as solid lines in  FIG. 8D , or it can be a prism  5 L shown as dashed lines with an optical path matching prism  5 R in front of the other micro-display panel. The additional plate mirror  5  or the prism mirror  5 L and matching prism  5 R are necessary for use with some types of MEM devices as disclosed in the U.S. patent application Ser. No. 11/770,247 by the same inventor. Additionally the polarizing beam-splitter  10  as shown in  FIG. 8D  operating with a central angle of incidence of slightly larger than 45° for light incident on the beam-splitting surface can also be configured for operation with a central angle of incidence of about 45°, (as shown in  FIG. 8A ) or substantially larger than 45° (as shown in  FIG. 8C .). 
     The operation of the third type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 8A-8D . 
     The polarized light from the illumination system  8  consists of a first set and a second set of primary colors: c 1 L, c 2 L and c 3 L, and c 1 R, c 2 R and c 3 R. The first and the second color sets are different and occupy different wavelength regions in the wavelength spectrum and they form two different color triangles in the CIE color diagram. More than three colors in each primary color set can be used as well. The wavelength selective polarization rotator selectively rotates the polarization state of some of the incident color light and keeps the rest unchanged. After the light from the illumination system  8  passes through the wavelength selective polarization rotator  90 , the light having the first set of the primary colors c 1 L, c 2 L and c 3 L is in the first polarization state while the light having the second set of primary colors c 1 R, c 2 R and c 3 R is in the second polarization state. 
     The light having the first primary colors c 1 L, c 2 L, c 3 L in the first polarization state is thus reflected by the beam-splitter  10  towards the micro-display panel  30 L. Images are then encoded by the micro-display onto the output reflected light by modulating the polarization states of the incident light. For “on” pixels, the polarization state of the image light is rotated to the second polarization. Thus the “on” pixel image light passes through the polarizing beam-splitter  10  and the optional waveplate  70  and is then projected onto the screen  60  by the projection lens  50 . In the case where the micro-display panel  30 L is a LCOS panel, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from the micro-display panel  30 L is sent back by the polarizing beam-splitter  10  along the direction of the light source. In the case where the micro-display panel  30 L is a MEM panel, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the projection lens. 
     Similarly, the light having the second primary colors c 1 R, c 2 R, c 3 R in the second polarization state is transmitted by the beam-splitter  10  towards the micro-display panel  30 R. Images are then encoded by the micro-display onto the reflected output light by modulating the polarization states of the incident light. For “on” pixels, the polarization state of the image light is rotated to the first polarization. Thus the “on” pixel image light is reflected by the polarizing beam-splitter  10  and passes through the optional waveplate  70  and is then projected onto the screen  60  by the projection lens  50 . In the case where the micro-display panel  30 R is a LCOS panel, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from the micro-display panel  30 L is sent back by the polarizing beam-splitter  10  along the direction of the light source. In the case where the micro-display panel  30 R is a MEM panel, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the projection lens. 
     The illumination system  8  can comprise of a lamp emitting white light such as a UHP lamp and a polarization recovery means for converting un-polarized light into polarized light; in addition, a color filter wheel can be added to generate time-sequential colors, red, green and blue, or any other suitable color combinations. Alternatively, the illumination system  8  can comprise of red, green and blue, color LED light sources or lasers, or any other suitable color combinations, and a polarization recovery means, only if the light sources do not emit polarized light; time-sequential color lights are generated by pulsing the LED or laser light sources. 
     If the light from the illumination system  8  is white light then black and white images are shown on screen. If the light from the illumination system  8  consists of time-sequential colors, red, green and blue or any other suitable color combinations, and the color images on the micro-display panels are synchronized with the color illumination light, then full color images are shown on screen. 
     The third type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes: 
     In the first 2D and 3D switchable mode of the third type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this case, a non-depolarizing projection screen is used. Since the image light from the micro-display panel  30 L is in the second polarization and the image light from the micro-display panel  30 R is in the first polarization, if left-eye and right-eye image signals are fed to the two micro-display panels  30 L and  30 R, respectively, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two microdisplay panels  30 L and  30 R, normal 2D images are then displayed onscreen. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the third type of the embodiments described above, the 3D stereo display mode is realized using two different sets of primary colors for the left- and right-eye images. In this case, a non-depolarizing projection screen or an ordinary screen can be used. The image light from the micro-display panel  30 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L, and the image light from the micro-display panel  30 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R. If left-eye and right-eye image signals are fed to the two micro-display panels  30 L and  30 R, respectively, or vice versa, by wearing matching 3D color filter glasses, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye color filter glasses match the spectrum of the corresponding image sets. The left- and right-eye glasses only allow the corresponding eye color images to pass and block the other eye color images. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L and  30 R, normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of ordinary screens and relatively cheap plastic projection lens which may depolarize light, in addition, low cross-talk between the left-eye and right-eye images can be obtained due to the high contrast ratios of color filter glasses. 
     In the third 2D and 3D switchable mode of the third type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations and two different sets of colors for the left- and right eye-images. In this case, a non-depolarizing projection screen or an ordinary screen is used. The image light from the micro-display panel  30 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L in the second polarization, and the image light from the micro-display panel  30 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye and right-eye image signals are fed to the microdisplay panels  30 L and  30 R, respectively, or vice versa, and by wearing matching polarizing 3D glasses in the case where a non-depolarizing screen is used, or color filter glasses in the case where a non-depolarizing screen or an ordinary screen is used, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye polarizing or color filter glasses match the polarization or spectrum of the corresponding image sets, they only allow the corresponding polarization or color images of the corresponding eye to pass and block the images for the other eye. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L and  30 R, normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This type of operation mode is desirable in some applications where users can have the flexibility to use either 3D polarizing glasses or 3D color filter glasses. 
     In the fourth 2D and 3D switchable mode of the third type of the embodiments described above, the 3D stereo display mode is realized by displaying the left- and right-eye images time sequentially at a fast speed. In this mode, a non-depolarizing projection screen or an ordinary screen can be used. The image light from the micro-display panel  30 L consists of only the first set of primary colors c 1 L, c 2 L and c 3 L in the second polarization, and the image light from the micro-display panel  30 R only consists of the second set of primary colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye signals are fed both to the micro-display panels  30 L and  30 R, and then followed by the right-eye signals being fed both to the micro-display panels  30 L and  30 R (there may be a blank period for reducing ghosting between the switching), the switching between the left- and right-eye images are fast enough, for example, 60 frames per second, by wearing synchronized LCD shutter glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the microdisplay panels  30 L and  30 R, normal 2D images are then displayed onscreen. The 2D and 3D images consist of both the first and second set of the primary colors. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of any type of screen. 
     The reflective micro-display panels  30 L and  30 R can be LCOS panels or MEM device panels as described in the preamble. Different alignments of the MEM devices with regarding to the polarizing beam-splitter can be used, depending on the specifications of the MEM devices. 
     Fourth Type of Embodiments 
     The fourth type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 9A-9B , comprises of two illumination systems  8 L and  8 R for providing polarized light beams  40 L and  40 R having a first set of primary colors c 1 L, c 2 L, c 3 L and a second set of primary colors c 1 R, c 2 R and c 3 R, respectively, two imaging arms  25 L and  25 R, a polarizing beam-splitter  10  for combining image light, an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . The polarizing beam-splitter  10  reflects light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The imaging arm  25 L comprises of two dichroic filters  6 L and  7 L for separating color light beams; plural mirrors  5  for directing the color beams to the corresponding micro-display panels; a waveplate  100 L; a first set of three transmissive micro-display panels  32 L 1 ,  32 L 2  and  32 L 3 ; a X-cube color combiner  16 L for combining color images; and a wavelength selective polarization rotator  90 L. Similarly, the imaging arm  25 R also comprises of two dichroic filters  6 R and  7 R for separating color light beams; plural mirrors  5  for directing the color beams to the corresponding micro-display panels; a waveplate  100 R; a second set of three transmissive micro-display panels  32 R 1 ,  32 R 2  and  32 R 3 ; a X-cube color combiner  16 R for combining color images; and a wavelength selective polarization rotator  90 R. In the embodiments shown in  FIGS. 9A and 9B , the central angle of incidence of light at the beam-splitting surface of the polarizing beam-splitter  10  is slightly larger than 45°. Similar embodiments are possible where the polarizing beam-splitter  10  operates with a central angle of incidence of about 45° (as for example the PBS  10  shown in  FIG. 8A ) and also substantially larger than 45° (as for example the PBS  10  shown in  FIG. 8C ). Two different arrangements for dichroic filters and the mirrors are used in the embodiments shown in  FIGS. 9A and 9B . 
     The polarizing beam-splitter  10  is selected from polarizing devices described in the preamble. The first and second polarizations can be s- and p-polarized, respectively, or vice versa. The two sets of the micro-display panels  32 L 1 ,  32 L 2  and  32 L 3 , and  32 R 1 ,  32 R 2  and  32 R 3  are of the type of the transmissive micro-display panel  32  as described in the preamble. The wavelength selective polarization rotators  90 L and  90 R are the type of the wavelength selective polarization rotator  90  described in the preamble. 
     The operation of the fourth type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 9A and 9B . 
     In the image arm  25 L, light beam  40 L in the second polarization from the illumination system  8 L consists of the first set of three primary colors c 1 L, c 2 L and c 3 L, such as red, green and blue primary colors. The beam  40 L is separated into three color beams  41 L,  42 L and  43 L having colors c 1 L, c 2 L and c 3 L, respectively by the dichroic filters  6 L and  7 L. The three color beams are then directed to the associated three micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  by the plural mirrors  5 . In the cases of the micro-display panels  32 L 1  and  32 L 3 , the panels encode images onto the output light by converting the polarization states of the incident light from the second polarization to the first polarization state for the “on” pixels; and for “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizers within the micro-display panels. In the case of the micro-display panel  32 L 2 , the polarization state of the incident beam  42 L is rotated from the second polarization state to the first polarization state by the waveplate  100 L, the panel then encodes images on the output light by converting the polarization state of the incident light from the first polarization to the second polarization state for “on” pixels; for the “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizer within the micro-display panel. Therefore, the image light from the micro-display panel  32 L 1  is in the first polarization state with the c 1 L color; the image light from the micro-display panel  32 L 2  is in the second polarization state with the c 2 L color; and the image light from the micro-display panel  32 L 3  is in the first polarization state with the c 3 L color. The color X-cube  16 L then combines all three image light beams into a single image light beam. In this case, preferably, the first polarization is s-polarized and the second polarization is p-polarized. The wavelength selective polarization rotator  90 L selectively rotates the polarization state of the image light in colors c 1 L and c 3 L from the micro-display panel  32 L 1  and  32 L 3  from the first polarization state to the second polarization state, but keeps the polarization state of the image from micro-display panel  32 L 2  unchanged. Thus, the final emerging image light from the imaging arm  25 L consists of three color c 1 L, c 2 L and c 3 L images in the second polarization state, these three color images are called the first set of color images. The final emerging image light passes through the polarizing beam-splitter  10 , the optional waveplate  70 , and the first set of color images are projected by the projection lens  50  onto the optional projection screen  60 . Any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by reflecting the light out of the imaging path of the projection lens. Therefore, very high contrast images can be obtained. 
     Similarly, in the image arm  25 R, light beam  40 R in the second polarization from the illumination system  8 R consists of the second set of three primary colors c 1 R, c 2 R and c 3 R, such as red, green and blue primary colors. The beam  40 R is separated into three color beams  41 R,  42 R and  43 R having color c 1 R, c 2 R and c 3 R, respectively, by the dichroic filters  6 R and  7 R. The three color beams are then directed to the associated three micro-display panels  32 R 1 ,  32 R 2  and  32 R 3  by the plural mirrors  5 . In the cases of the micro-display panels  32 R 1  and  32 R 3 , the panels encode images onto the output light by converting the polarization states of the incident light from the second polarization to the first polarization state for the “on” pixels; and for “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizers within the micro-display panels. In the case of the micro-display panel  32 R 2 , the polarization state of the incident beam  42 R is rotated from the second polarization state to the first polarization state by the waveplate  100 R, the panel then encodes images on the output light by converting the polarization state of the incident light from the first polarization to the second polarization state for “on” pixels; for the “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizer within the micro-display panel. Thus, the image light from the micro-display panel  32 R 1  is in the first polarization state with the c 1 R color; the image light from the micro-display panel  32 R 2  is in the second polarization state with the c 2 R color; and the image light from the micro-display panel  32 R 3  is in the first polarization state with the c 3 R color. The color X-cube  16 R then combines all three image light beams into a single image light beam. In this case, preferably, the first polarization is s-polarized and the second polarization is p-polarized. The wavelength selective polarization rotator  90 R selectively rotates the polarization state of the image light in colors c 2 R from the micro-display panel  32 R 2  from the second polarization state to the first polarization state, but keeps the polarization state of the image light in colors c 1 R and c 3 R from micro-display panel  32 R 1  and  32 R 3  unchanged. Thus, the final emerging image light from the imaging arm  25 R consists of three color c 1 R, c 2 R and c 3 R images in the first polarization state, these three color images are called the second set of color images. The final emerging image light is reflected by the polarizing beam-splitter and passes through the optional waveplate  70 , and the second set of color images are projected by the projection lens  50  onto the optional projection screen  60 . Any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by transmitting the light out of the imaging path of the projection lens. Therefore, very high contrast images can also be obtained. 
     The illumination systems  8 L and  8 R can each comprise of a lamp emitting white light such as a UHP lamp and a polarization recovery means for converting un-polarized light into polarized light. Alternatively, the illumination systems  8 L and  8 R can each comprise of red, green and blue, color LED light sources or lasers, or any other suitable color combinations, and a polarization recovery means only if the light sources do not emit polarized light. 
     The fourth type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes: 
     In the first 2D and 3D switchable mode of the fourth type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this mode, a non-depolarizing projection screen is used. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R, can be identical (or substantially similar), or different, thus, the first and second sets of images can be identical (or substantially similar), or different in colors. It is preferred to have the two sets of colors identical (or substantially similar), so that in 3D mode the left- and right-eye images are identical in colors. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  is in the second polarization and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  is in the first polarization. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the fourth type of the embodiments described above, the 3D stereo display mode is realized using two different sets of colors for the left- and right-eye images. In this mode, a non-depolarizing or an ordinary projection screen can be used. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R, are different and occupy different wavelength regions in the wavelength spectrum, thus, the first and second sets of images are formed with different sets of primary colors. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  consists of only colors c 1 L, c 2 L and c 3 L, and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  consists of only colors c 1 R, c 2 R and c 3 R. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching color filter 3D glasses, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye color filter glasses match the spectrum of the corresponding image sets. The left- and right-eye glasses only allow the corresponding eye color images to pass and block the other eye color images. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of ordinary screens and relatively cheap plastic projection lens which may depolarize light, in addition, low cross-talk between the left-eye and right-eye images can be obtained due to the high contrast ratios of color filter glasses. 
     In the third 2D and 3D switchable mode of the fourth type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations and two different sets of colors for the left- and right eye-images. In this case, a non-depolarizing projection screen or an ordinary screen is used. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R, are different and occupy different wavelength regions in the wavelength spectrum, thus, the first and second sets of images are formed with different sets of primary colors. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  consist of colors c 1 L, c 2 L and c 3 L in the second polarization and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  consists of only colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, and by wearing matching polarizing 3D glasses in the case where a non-depolarizing screen is used or color filter glasses in the case where a non-depolarizing screen or an ordinary screen is used, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because of its flexibility in the use of screens or 3D glasses. 
     In the fourth 2D and 3D switchable mode of the fourth type of the embodiments described above, the 3D display mode is realized by displaying the left- and right-eye images time sequentially at a fast speed. In this mode, a non-depolarizing projection screen or an ordinary screen can be used. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R, can be identical (or substantially similar), or different, thus, the first and second sets of images are formed with identical (or substantially similar), or different sets of primary colors. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  consist of colors c 1 L and c 2 L and c 3 L in the second polarization and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  consists of only colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye signals are fed both to the first and second sets of micro-display panels, and then the right-eye signals are fed both to the first and second sets of micro-display panels, the switching between the left- and right-eye images are fast enough (for example, 60 frames per second), by wearing synchronized LCD shutter glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen. The 2D and 3D images consist of both the first and second set of the primary colors. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of any type of screen. 
     Fifth Type of Embodiments 
     The fifth type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIG. 10 , comprises of an illumination system  8  for providing a polarized or un-polarized light beam  40 , a beam-splitter  3  for separating the polarized or un-polarized light  40  into two light beams  40 L and  40 R, two imaging arms  25 L and  25 R, a polarizing beam-splitter  10  for combining image light, an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . The polarizing beam-splitter  10  reflects light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The imaging arm  25 L comprises of two dichroic filters  6 L and  7 L for separating color light beams; plural mirrors  5  for directing the color beams to the corresponding micro-display panels; a waveplate  100 L; a first set of three transmissive micro-display panels  32 L 1 ,  32 L 2  and  32 L 3 ; a X-cube color combiner  16 L for combining color images; and a wavelength selective polarization rotator  90 L. Similarly, the imaging arm  25 R also comprises of two dichroic filters  6 R and  7 R for separating color light beams; plural mirrors  5  for directing the color beams to the corresponding micro-display panels; a waveplate  100 R; a second set of three transmissive micro-display panels  32 R 1 ,  32 R 2  and  32 R 3 ; a X-cube color combiner  16 R for combining color images; and a wavelength selective polarization rotator  90 R. In the embodiment shown in  FIG. 10 , the central angle of incidence of light at the beam-splitting surface of the polarizing beam-splitter  10  is slightly larger than 45°. Similar embodiments are possible where the polarizing beam-splitter  10  has a central angle of incidence of about 45° and also substantially larger than 45°. 
     The illumination system  8  generates either polarized or non-polarized light.  FIG. 10  shows the embodiment in which the illumination system  8  generates the light in the second polarization. If the illumination system  8  generates light in the first polarization, a waveplate is then used to convert light from the first polarization state to the second polarization state. If the illumination system generates un-polarized light, then the beam-splitter  3  is a polarizing beam-splitter of the type described in the preamble which reflects light in the first polarization state and transmits light in the second polarization state, or vice versa; Thus, a waveplate is required in one of the beams, either  40 L or  40 R, to convert the light to the desired polarization (not shown in  FIG. 10 ). 
     The polarizing beam-splitter  10  is selected from polarizing devices described in the preamble. The first and second polarizations can be s- and p-polarized, respectively, or vice versa. The two sets of the micro-display panels  32 L 1 ,  32 L 2  and  32 L 3 , and  32 R 1 ,  32 R 2  and  32 R 3  are the type of the transmissive micro-display panels  32  as described in the preamble. The wavelength selective polarization rotators  90 L and  90 R are the type of the wavelength selective polarization rotator  90  described in the preamble. 
     The operation of the fifth type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIG. 10 , assuming the light from the illumination system is in the second polarization and the polarizing beam-splitter  10  reflects light in the first polarization and transmits light in the second polarization. 
     In the image arm  25 L, light beam  40 L in the second polarization consists of the first set of three primary colors c 1 L, c 2 L and c 3 L, such as red, green and blue primary colors. The beam  40 L is separated into three color beams  41 L,  42 L and  43 L having colors c 1 L, c 2 L and c 3 L, respectively by the dichroic filters  6 L and  7 L. The three color beams are then directed to the associated three micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  by the plural mirrors  5 . In the case of the micro-display panels  32 L 1  and  32 L 3 , the panels encode images onto the output light by converting the polarization states of the incident light from the second polarization to the first polarization state for the “on” pixels; and for “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizers within the micro-display panels. In the case of the micro-display panel  32 L 2 , the polarization state of the incident beam  42 L is rotated from the second polarization state to the first polarization state by the waveplate  100 L, the panel then encodes images on the output light by converting the polarization state of the incident light from the first polarization to the second polarization state for “on” pixels; for the “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizer within the micro-display panel. Therefore, the image light from the micro-display panel  32 L 1  is in the first polarization state with the c 1 L color; the image light from the micro-display panel  32 L 2  is in the second polarization state with the c 2 L color; and the image light from the micro-display panel  32 L 3  is in the first polarization state with the c 3 L color. The color X-cube  16 L then combines all three image light beams into a single image light beam. In this case, preferably, the first polarization is s-polarized and the second polarization is p-polarized. The wavelength selective polarization rotator  90 L selectively rotates the polarization state of the image light in colors c 1 L and c 3 L from the micro-display panel  32 L 1  and  32 L 3  from the first polarization state to the second polarization state, but keeps the polarization state of the image from micro-display panel  32 L 2  unchanged. Thus, the final emerging image light from the imaging arm  25 L consists of three color c 1 L, c 2 L and c 3 L images in the second polarization state, these three color images are called the first set of color images. The final emerging image light passes through the polarizing beam-splitter, the optional waveplate  70 , and the first set of color images are projected by the projection lens  50  onto the optional projection screen  60 . Any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by reflecting the light out of the imaging path of the projection lens. Therefore, very high contrast images can be obtained. 
     Similarly, in the image arm  25 R, light beam  40 R in the second polarization consists of the second set of three primary colors c 1 R, c 2 R and c 3 R, such as red, green and blue primary colors. The beam  40 R is separated into three color beams  41 R,  42 R and  43 R having color c 1 R, c 2 R and c 3 R, respectively, by the dichroic filters  6 R and  7 R. The three color beams are then directed to the associated three micro-display panels  32 R 1 ,  32 R 2  and  32 R 3  by the plural mirrors  5 . In the cases of the micro-display panels  32 R 1  and  32 R 3 , the panels encode images onto the output light by converting the polarization states of the incident light from the second polarization to the first polarization state for the “on” pixels; and for “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizers within the micro-display panels. In the case of the micro-display panel  32 R 2 , the polarization state of the incident beam  42 R is rotated from the second polarization state to the first polarization state by the waveplate  100 R, the panel then encodes images on the output light by converting the polarization state of the incident light from the first polarization to the second polarization state for “on” pixels; for the “off” pixels, the polarization state of the light is unchanged and thus the light is absorbed by the internal polarizer within the micro-display panel. Thus, the image light from the micro-display panel  32 R 1  is in the first polarization state with the c 1 R color; the image light from the micro-display panel  32 R 2  is in the second polarization state with the c 2 R color; and the image light from the micro-display panel  32 R 3  is in the first polarization state with the c 3 R color. The color X-cube  16 R then combines all three image light beams into a single image light beam. In this case, preferably, the first polarization is s-polarized and the second polarization is p-polarized. The wavelength selective polarization rotator  90 R selectively rotates the polarization state of the image light in colors c 2 R from the micro-display panel  32 R 2  from the second polarization state to the first polarization state, but keeps the polarization state of the image light in colors c 1 R and c 3 R from micro-display panel  32 R 1  and  32 R 3  unchanged. Thus, the final emerging image light from the imaging arm  25 R consists of three color c 1 R, c 2 R and c 3 R images in the first polarization state, these three color images are called the second set of color images. The final emerging image light is reflected by the polarizing beam-splitter and passes through the optional waveplate  70 , and the second set of color images are projected by the projection lens  50  onto the optional projection screen  60 . Any residue light from the “off” pixels is further reduced by the polarizing beam-splitter  10  by transmitting the light out of the imaging path of the projection lens. Therefore, very high contrast images can also be obtained. 
     The illumination system  8  can comprise of a lamp emitting white light such as a UHP lamp, alternatively, the illumination system  8  can comprise of red, green and blue color LED light sources or lasers. In the case where the beam-splitter  3  is a polarizing beam-splitter, no polarization recovery means is required within the illumination system. Otherwise, a polarization recovery means is part of the illumination system if the light sources do not emit polarized light. 
     The fifth type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes. 
     In the first 2D and 3D switchable mode of the fifth type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this mode, a non-depolarizing projection screen is used. In addition, the beam-splitter  3  is a 50/50 beam-splitter which transmits and reflects equally about 50% of incident light. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R are substantially similar. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  is in the second polarization and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  is in the first polarization. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the fifth type of the embodiments described above, the 3D stereo display mode is realized using two different sets of colors for the left- and right-eye images. In this mode, a non-depolarizing or an ordinary projection screen can be used. In addition, the beam-splitter  3  reflects light of a first set of primary colors c 1 L, c 2 L and c 3 L and transmits light of a second set of primary colors c 1 R, c 2 R and c 3 R. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R, are different and occupy different wavelength regions in the wavelength spectrum, thus, the first and second sets of images are formed with different sets of primary colors. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  consists of only colors c 1 L, c 2 L and c 3 L, and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  consists of only colors c 1 R, c 2 R and c 3 R. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching color filter 3D glasses, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye color filter glasses match the spectrum of the corresponding image sets. The left- and right-eye glasses only allow the corresponding eye color images to pass and block the other eye color images. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of ordinary screens and relatively cheap plastic projection lens which may depolarize light, in addition, low cross-talk between the left-eye and right-eye images can be obtained due to the high contrast ratios of color filter glasses. 
     In the third 2D and 3D switchable mode of the fifth type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations and two different sets of colors for the left- and right eye-images. In this case, a non-depolarizing projection screen or an ordinary screen is used. The first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R, are different and occupy different wavelength regions in the wavelength spectrum, thus, the first and second sets of images are formed with different sets of primary colors. In addition, the beam-splitter  3  reflects light of a first set of primary colors c 1 L, c 2 L and c 3 L and transmits light of a second set of primary colors c 1 R, c 2 R and c 3 R. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  consist of colors c 1 L and c 2 L and c 3 L in the second polarization and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  consists of only colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, and by wearing matching polarizing 3D glasses in the case where a non-depolarizing screen is used or color filter glasses in the case where a non-depolarizing screen or an ordinary screen is used, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3  and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because of its flexibility in the use of screens or 3D glasses. 
     In the fourth 2D and 3D switchable mode of the fifth type of the embodiments described above, the 3D display mode is realized by displaying the left- and right-eye images time sequentially at a fast speed. In this mode, a non-depolarizing projection screen or an ordinary screen can be used. In addition, the beam-splitter  3  is a 50/50 beam-splitter which transmits and reflects equally about 50% of incident light. Preferably, the first and second sets of primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R are substantially similar. The first set of images from the image arm  25 L having the first set of micro-display panels  32 L 1 ,  32 L 2  and  32 L 3  consist of colors c 1 L and c 2 L and c 3 L in the second polarization and the second set of images from the image arm  25 R having the second set of microdisplay panels  32 R 1 ,  32 R 2  and  32 R 3  consists of only colors c 1 R, c 2 R and c 3 R in the first polarization. If left-eye signals are fed both to the first and second sets of micro-display panels, and then the right-eye signals are fed both to the first and second sets of micro-display panels, the switching between the left- and right-eye images are fast enough (for example, 60 frames per second), by wearing synchronized LCD shutter glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  32 L 1 ,  32 L 2  and  32 L 3 , and  32 R 1 ,  32 R 2  and  32 R 3 , normal 2D images are then displayed onscreen. The 2D and 3D images consist of both the first and second set of the primary colors. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of any type of screen. 
     Sixth Type of Embodiments 
     The sixth type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 11A-11D , comprises of an illumination system  8  or three illumination systems  8   a ,  8   b  and  8   c ; plural mirrors  5 ; three sets of polarizing image combiners  26   a ,  26   b  and  26   c ; a color combining prism  16  for combining image light, an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . In the embodiments shown in  FIGS. 11A , and  11 C, three separate illumination systems  8   a ,  8   b  and  8   c  are used which provides three color beams  40   a ,  40   b  and  40   c  having colors c 1 , c 2  and c 2 , respectively. In the embodiments shown in  FIGS. 11B and 11D , only one illumination system  8  is used which provides the light beam  40  that is subsequently separated into three color beams  40   a ,  40   b  and  40   c  by two dichroic color filters  6  and  7 . The color beams  40   a ,  40   b  and  40   c  also have colors c 1 , c 2  and c 3 , respectively. The polarizing image combiner  26   a  comprises of a polarizing beam-splitter  10   a  and two reflective micro-display panels  30 L 1  and  30 R 1 . Similarly, the polarizing image combiner  26   b  comprises of a polarizing beam-splitter  10   b  and two reflective micro-display panels  30 L 2  and  30 R 2 . Also, the polarizing image combiner  26   c  comprises of a polarizing beam-splitter  10   c  and two reflective micro-display panels  30 L 3  and  30 R 3 . 
     In the case where three illumination systems  8   a ,  8   b  and  8   c  are used as shown in the embodiments in  FIGS. 11A and 11C , preferably, the light sources comprise of color LEDs or lasers. In the case where a single illumination system  8  is used as shown in the embodiments in  FIGS. 11B and 11D , preferably, the illumination system comprises of a white light source, such as an UHP lamp. 
     In the embodiments shown in  FIGS. 11A and 11B , the color combiner  16  is an X-cube which transmits c 2  color and reflects c 1  and c 3  colors; in the embodiments shown in  FIGS. 11C and 11D , the color combiner  16  is a Philips prism which also transmits c 2  color and reflects c 1  and c 3  colors. 
     The reflective micro-displays panels  30 L 1 ,  30 R 1 ,  30 L 2 ,  30 R 2 ,  30 L 3 ,  30 R 3  are described in the preamble as reflective micro-display panel  30 . They can be LCOS panels with additional waveplates for compensating geometry depolarization. They can also be MEM panels with additional waveplates for rotating the polarization state of incident light by 90°; in addition, optional waveplates can be used as parts of the panels to compensate geometry depolarization. 
     The polarizing beam-splitters  10   a ,  10   b  and  10   c  are selected from the types of the polarizing beam-splitters  10  described in the preamble that reflect light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The first and second polarizations can be s- and p-polarized, or vice versa. In the embodiments shown in  FIGS. 11A-11D  the central angle of incidence of the polarizing beam-splitters  10   a ,  10   b  and  10   c  is about 45°. Other similar embodiments to each of the embodiments shown in  FIGS. 11A-11D  are possible where the central angle of incidence of the polarization beam-splitters  10   a ,  10   b  and  10   c  are each slightly larger than 45° and also each substantially larger than 45°. The three polarizing beam-splitters  10   a ,  10   b  and  10   c  can be identical and operate over a large band of wavelengths covering colors c 1 , c 2  and c 3 , or they can be different, and only operate at the specific band of wavelengths such as c 1 , c 2  and c 3 , respectively. 
     The operation of the sixth type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 11A-11D . 
     In the first set of the polarizing image combiner  26   a , the un-polarized light beam  40   a  from the illumination system consists of light both in the first and second polarization states. The polarizing beam-splitter  10   a  reflects the light in the first polarization towards the micro-display panel  30 L 1  and transmits the light in the second polarization towards the micro-display panel  30 R 1 . The reflective micro-display panels  30 L 1  and  30 R 1  encode images onto the output light by rotating the polarization state of the “on” pixel light by 90°, from the first polarization state to the second polarization state in the case of the micro-display panel  30 L 1 ; and from the second polarization state to the first polarization state in the case of the micro-display panel  30 R 1 . Therefore, the image light from the two micro-display panels  30 L 1  and  30 R 1  are in the second and first polarization states, respectively. The polarizing beam-splitter  10   a  then combines the image light and directs the image light to the color combiner  16 . In the case of the micro-display panels  30 L 1  and  30 R 1  are LCOS panels, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from both the micro-display panels  30 L 1  and  30 R 1  are sent back by the polarizing beam-splitter  10   a  along the direction of the light source. In the case of the micro-display panels  30 L 1  and  30 R 1  are MEM panels, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the color combiner  16 . 
     Similarly, in the second set of the polarizing image combiner  26   b , the un-polarized light beam  40   b  from the illumination system consists of light both in the first and second polarization states. The polarizing beam-splitter  10   b  reflects the light in the first polarization towards the micro-display panel  30 L 2  and transmits the light in the second polarization towards the micro-display panel  30 R 2 . The reflective micro-display panels  30 L 2  and  30 R 2  encode images onto the output light by rotating the polarization state of the “on” pixel light by 90°, from the first polarization state to the second polarization state in the case of the micro-display panel  30 L 2 ; and from the second polarization state to the first polarization state in the case of the micro-display panel  30 R 2 . Therefore, the image light from the two micro-display panels  30 L 2  and  30 R 2  are in the second and first polarization states, respectively. The polarizing beam-splitter  10   b  then combines the image light and directs the image light to the color combiner  16 . In the case where the micro-display panels  30 L 2  and  30 R 2  are LCOS panels, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from both the micro-display panels  30 L 2  and  30 R 2  are sent back by the polarizing beam-splitter  10   a  along the direction of the light source. In the case where the micro-display panels  30 L 2  and  30 R 2  are MEM panels, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the color combiner  16 . 
     Also, in the third set of the polarizing image combiner  26   c , the un-polarized light beam  40   c  from the illumination system consists of light both in the first and second polarization states. The polarizing beam-splitter  10   c  reflects the light in the first polarization towards the micro-display panel  30 L 3  and transmits the light in the second polarization towards the micro-display panel  30 R 3 . The reflective micro-display panels  30 L 3  and  30 R 3  encode images onto the output light by rotating the polarization state of the incident light for the “on” pixel light by 90°, from the first polarization state to the second polarization state in the case of the micro-display panel  30 L 3 ; and from the second polarization state to the first polarization state in the case of the micro-display panel  30 R 3 . Therefore, the image light from the two micro-display panels  30 L 3  and  30 R 3  are in the second and first polarization states, respectively. The polarizing beam-splitter  10   c  then combines the image light and directs the image light to the color combiner  16 . In the case where the micro-display panels  30 L 3  and  30 R 3  are LCOS panels, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from both the micro-display panels  30 L 3  and  30 R 3  are sent back by the polarizing beam-splitter  10   a  along the direction of the light source. In the case where the micro-display panels  30 L 3  and  30 R 3  are MEM panels, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the color combiner  16 . 
     The color combiner  16  combines the color image light from the polarizing image combiners  26   a ,  26   b  and  26   c . The combined image light then passes through the optional waveplate  70  which can convert linear polarized light into circular polarized light that can be advantageous. The projection lens  50  then project the images onto the optional screen  60 . The X-cube prism may be preferred when image light c 1 , c 2  and c 3  each consists of a narrow range of wavelengths such as light from LEDs and lasers while the Philips prism is preferred when image light c 1 , c 2  and c 3  each consists of a relatively broadband of wavelengths. The colors c 1 , c 2  and c 3  can be red, green and blue, or any suitable primary color combinations. 
     In the sixth type of the embodiments in accordance with the present invention of the projection display systems, the images from the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in the second polarization state, they form the first set of primary color images. Therefore, the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are called the first set of micro-display panels. Similarly, the image light from the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  is in the first polarization state, they form the second set of primary color images. Therefore, the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are called the second set of micro-display panels. 
     The sixth type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes. 
     In the first 2D and 3D switchable mode of the sixth type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this mode, a non-depolarizing projection screen is used. The first set of color images from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  is in the second polarization, and the second set of color images from the second set of microdisplay panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2  and  30 L 3 , and  30 R 1 ,  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the seventh type of the embodiments described above, the 3D display mode is realized by displaying the left- and right-eye images time sequentially at a fast speed. In this mode, a non-depolarizing projection screen or an ordinary screen can be used. The first set of color images from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in the second polarization state, and the second set of color images from the second set of microdisplay panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization state. If left-eye signals are fed both to the first and second sets of micro-display panels, and then the right-eye signals are fed both to the first and second sets of micro-display panels, the switching between the left- and right-eye images are fast enough (for example, 60 frames per second), by wearing synchronized LCD shutter glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2  and  30 L 3 , and  30 R 1 ,  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of any type of screen. 
     There are variations of the sixth type of the embodiments, in which the illumination systems  8  or  8   a ,  8   b  and  8   c , and the image combining devices  26   a ,  26   b  and  26   c  can be arranged to allow the three color beams  40   a ,  40   b  and  40   c  perpendicular to the plane of the drawing paper surface, instead of within the plane of the drawing paper surface. These variations are useful when available space is limited. The operation of these variations is similar to the above sixth type of the embodiments with some minor modifications. 
     Seventh Type of Embodiments 
     The seventh type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 12A-12D , comprises of an illumination system  8  or three illumination systems  8   a ,  8   b  and  8   c ; plural mirrors  5 ; three optional clean-up polarizers  35   a ,  35   b  and  35   c ; three sets of polarizing image combiners  26   a ,  26   b  and  26   c ; a color combining prism  16  for combining image light, an optional wavelength selective polarization rotator  90 , an optional waveplate  70 , a projection lens  50  and an optional projection screen  60 . In the embodiments shown in  FIGS. 12A and 12C , three separate illumination systems  8   a ,  8   b  and  8   c  are used which provide three polarized color beams  40   a ,  40   b  and  40   c  having colors c 1 , c 2  and c 2 , respectively. In the embodiments shown in  FIGS. 12B and 12D , only one illumination system  8  is used which provides the polarized light  40  that is subsequently separated into three polarized color beams  40   a ,  40   b  and  40   c  by two dichroic color filters  6  and  7 . The color beams  40   a ,  40   b  and  40   c  also have colors c 1 , c 2  and c 3 , respectively. The polarizing image combiner  26   a  comprises of an optional polarizer  35   a , a wavelength selective polarization rotator  90   a   1 , a polarizing beam-splitter  10   a , two reflective micro-display panels  30 L 1  and  30 R 1  and an optional wavelength selective polarization rotator  90   a   2 . Similarly, the polarizing image combiner  26   b  comprises of an optional polarizer  35   b , a wavelength selective polarization rotator  90   b   1 , a polarizing beam-splitter  10   b , two reflective micro-display panels  30 L 2  and  30 R 2  and an optional wavelength selective polarization rotator  90   b   2 . Also, the polarizing image combiner  26   c  comprises of an optional polarizer  35   c , a wavelength selective polarization rotator  90   c   1 , a polarizing beam-splitter  10   c , two reflective micro-display panels  30 L 3  and  30 R 3  and an optional wavelength selective polarization rotator  90   c   2 . 
     In the case where three illumination systems  8   a ,  8   b  and  8   c  are used as shown in the embodiments in  FIGS. 12A and 12C , preferably, the light sources comprise of color LEDs or lasers. In the case where a single illumination system  8  is used as shown in the embodiments in  FIGS. 12B and 12D , preferably, the illumination system comprises of a white light source, such as an UHP lamp. Also, polarization recovery means in the illumination systems are used to convert un-polarized light from the light sources to polarized light, if the light sources do not emit polarized light. Although the light from illumination systems  8   a ,  8   b  and  8   c  or  8  can be either in the first or second polarization state, for the simplicity of the following descriptions, the second polarization is assumed as shown in the  FIGS. 12A-12D . 
     The colors c 1 , c 2  and c 3  can be red, green and blue, or any suitable primary color combinations. In addition, color c 1  consists of two sub-colors c 1 L and c 1 R. The two sub-colors are similar in color appearance, for example, both in the red spectrum of the visible, but have different wavelength regions in the wavelength spectrum. Similarly, color c 2  consists of two sub-colors c 2 L and c 2 R which are similar but have different wavelength regions in the wavelength spectrum. Also, color c 3  consists of two sub-colors c 3 L and c 3 R which are similar but have different wavelength regions in the wavelength spectrum. Colors c 1 L, c 2 L, c 3 L form a first set of primary colors, and colors c 1 R, c 2 R and c 3 R form a second set of primary colors. The first and second sets of the primary colors c 1 L, c 2 L, c 3 L, and c 1 R, c 2 R, c 3 R are different and occupy different wavelength regions in the wavelength spectrum. 
     In the embodiments shown in  FIGS. 12A and 12B , the color combiner  16  is an X-cube which transmits c 2  color and reflects c 1  and c 3  colors; in the embodiments shown in  FIGS. 12C and 12D , the color combiner  16  is a Philips prism which also transmits c 2  color and reflects c 1  and c 3  colors. The Philips prism is preferred when image light c 1 , c 2  and c 3  each consists of light in both the first and second polarizations. 
     The reflective micro-displays panels  30 L 1 ,  30 R 1 ,  30 L 2 ,  30 R 2 ,  30 L 3 ,  30 R 3  are described in the preamble as reflective micro-display panel  30 . They can be LCOS panels with additional waveplates for compensating geometry depolarization. They can also be MEM panels with additional waveplates for rotating the polarization state of incident light by 90°; in addition, optional waveplates can be used as parts of the panels to compensate geometry depolarization. 
     The polarizing beam-splitters  10   a ,  10   b  and  10   c  are selected from the types of the polarizing beam-splitters  10  described in the preamble that reflect light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The first and second polarizations can be s- and p-polarized, or vice versa. In the embodiments shown in  FIGS. 12A-12D  the central angle of incidence of the polarizing beam-splitters  10   a ,  10   b  and  10   c  is about 45°. Other similar embodiments to the embodiments shown in  FIGS. 12A-12D  are possible where the central angle of incidence of the polarizing beam-splitters  10   a ,  10   b  and  10   c  are each slightly larger than 45° and also each substantially larger than 45°. The three polarizing beam-splitters  10   a ,  10   b  and  10   c  can be identical and operate over a large band of wavelengths covering colors c 1 , c 2  and c 3 , or they can be different, and only operate at the specific band of wavelengths such as c 1 , c 2  and c 3 , respectively. 
     The wavelength selective polarization rotators  90 ,  90   a   1 ,  90   a   2 ,  90   b   1 ,  90   b   2 ,  90   c   1 ,  90   c   2  are the type of the wavelength selective polarization rotator  90  described in the preamble. 
     The operation of the fifth type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 12A-12D . 
     In the first set of the polarizing image combiner  26   a , the polarized light beam  40   a  from the illumination system having color c 1  in the second polarization state is directed to the optional clean-up polarizer  35   a  which removes residue light in the undesired polarization if required. The color light beam having color c 1  consists of the two sub-colors c 1 L and c 1 R. The wavelength selective polarization rotator  90   a   1  rotates the polarization state of one of the sub-colors from the second polarization state to the first polarization state and keeps the polarization state of the other sub-color light unchanged. For simplicity of explanation, it is assumed that the sub-color light c 1 L and c 1 R are in the first and second polarization states, respectively, after leaving the wavelength selective polarization rotator  90   a   1 . Thus, the polarizing beam-splitter  10   a  reflects the sub-color light c 1 L in the first polarization towards the micro-display panel  30 L 1  and transmits the sub-color light c 1 R in the second polarization towards the micro-display panel  30 R 1 . The reflective micro-display panels  30 L 1  and  30 R 1  encode images onto the output light by rotating the polarization state of the incident light for the “on” pixels by 90°, thus, from the first polarization state to the second polarization state in the case of the micro-display panel  30 L 1 ; and from the second polarization state to the first polarization state in the case of the micro-display panel  30 R 1 . Therefore, the image light from the two micro-display panels  30 L 1  and  30 R 1  are in the second and first polarization states, respectively. The polarizing beam-splitter  10   a  then combines the image light and directs the image light to the color combiner  16 . In the case of the micro-display panels  30 L 1  and  30 R 1  are LCOS panels, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from both the micro-display panels  30 L 1  and  30 R 1  are sent back by the polarizing beam-splitter  10   a  along the direction of the light source. In the case where the micro-display panels  30 L 1  and  30 R 1  are MEM panels, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the color combiner  16 . The optional wavelength selective polarization rotator  90   a   2  can rotate the polarization state of the image light in color c 1 L from the micro-display panel  30 L 1  from the second polarization state to the first polarization state, but keeps the polarization state of the image light in color c 1 R from the micro-display panel  30 R 1  unchanged, still in the first polarization state. This polarization state rotation operation is advantageous in some embodiments when X-cube is used for combining color images as shown in  FIGS. 12A and 12B , but is not necessary when the Philips prism is used for combining color images as shown in  FIGS. 12C and 12D  where the polarization rotator  90   a   2  is not used. 
     Similarly, in the second set of the polarizing image combiner  26   b , the polarized light beam  40   b  from the illumination system having color c 2  in the second polarization state is directed to the optional clean-up polarizer  35   b  which removes residue light in the undesired polarization if required. The color light beam having color c 2  consists of the two sub-colors c 2 L and c 2 R. The wavelength selective polarization rotator  90   b   1  rotates the polarization state of one of the sub-colors from the second polarization state to the first polarization state and keeps the polarization state of the other sub-color light unchanged. For simplicity of explanation, it is assumed that the sub-color light c 2 L and c 2 R are in the first and second polarization states, respectively, after leaving the wavelength selective polarization rotator  90   b   1 . Thus, the polarizing beam-splitter  10   b  reflects the sub-color light c 2 L in the first polarization towards the micro-display panel  30 L 2  and transmits the sub-color light c 2 R in the second polarization towards the micro-display panel  30 R 2 . The reflective micro-display panels  30 L 2  and  30 R 2  encode images onto the output light by rotating the polarization state of the “on” pixel light by 90°, from the first polarization state to the second polarization state in the case of the micro-display panel  30 L 2 ; and from the second polarization state to the first polarization state in the case of the micro-display panel  30 R 2 . Therefore, the image light from the two micro-display panels  30 L 2  and  30 R 2  are in the second and first polarization states, respectively. The polarizing beam-splitter  10   b  then combines the image light and directs the image light to the color combiner  16 . In the case where the micro-display panels  30 L 2  and  30 R 2  are LCOS panels, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from both the micro-display panels  30 L 2  and  30 R 2  are sent back by the polarizing beam-splitter  10   b  along the direction of the light source. In the case where the micro-display panels  30 L 2  and  30 R 2  are MEM panels, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the color combiner  16 . The optional wavelength selective polarization rotator  90   b   2  can rotate the polarization state of the image light in color c 2 R from the micro-display panel  30 R 2  from the first polarization state to the second polarization state, but keeps the polarization state of the image light in color c 2 L from the micro-display panel  30 L 2  unchanged, still in the second polarization state. This polarization state rotation operation is advantageous in some embodiments when X-cube is used for combining color images as shown in  FIGS. 12A and 12B , but is not necessary when the Philips prism is used for combining color images as shown in  FIGS. 12C and 12D  where the polarization rotator  90   b   2  is not used. 
     Also, in the third set of the polarizing image combiner  26   c , the polarized light beam  40   c  from the illumination system having color c 3  in the second polarization state is directed to the optional clean-up polarizer  35   c  which removes residue light in the undesired polarization if required. The color light beam having color c 3  consists of the two sub-colors c 3 L and c 3 R. The wavelength selective polarization rotator  90   c   1  rotates the polarization state of one of the sub-colors from the second polarization state to the first polarization state and keeps the polarization state of the other sub-color light unchanged. For simplicity of explanation, it is assumed that the sub-color light c 3 L and c 3 R are in the first and second polarization states, respectively, after leaving the wavelength selective polarization rotator  90   c   1 . Thus, the polarizing beam-splitter  10   c  reflects the sub-color light c 3 L in the first polarization towards the micro-display panel  30 L 3  and transmits the sub-color light c 3 R in the second polarization towards the micro-display panel  30 R 3 . The reflective micro-display panels  30 L 3  and  30 R 3  encode images onto the output light by rotating the polarization state of the “on” pixel light by 90°, thus, from the first polarization state to the second polarization state in the case of the micro-display panel  30 L 3 ; and from the second polarization state to the first polarization state in the case of the micro-display panel  30 R 3 . Therefore, the image light from the two micro-display panels  30 L 3  and  30 R 3  are in the second and first polarization states, respectively. The polarizing beam-splitter  10   c  then combines the image light and directs the image light to the color combiner  16 . In the case where the micro-display panels  30 L 3  and  30 R 3  are LCOS panels, the “off” pixels do not change the polarization state of incident light, thus the reflected “off” light from both the micro-display panels  30 L 3  and  30 R 3  are sent back by the polarizing beam-splitter  10   c  along the direction of the light source. In the case where the micro-display panels  30 L 3  and  30 R 3  are MEM panels, the “off” pixels reflect the incident light at different direction from the “on” pixel light, the reflected “off” light is absorbed by a light absorber and it does not reach to the color combiner  16 . The optional wavelength selective polarization rotator  90   c   2  can rotate the polarization state of the image light in color c 3 L from the micro-display panel  30 L 3  from the second polarization state to the first polarization state, but keeps the polarization state of the image light in color c 3 R from the micro-display panel  30 R 3  unchanged, still in the first polarization state. This polarization state rotation operation is advantageous in some embodiments when X-cube is used for combining color images as shown in  FIGS. 12A and 12B , but is not necessary when the Philips prism is used for combining color images as shown in  FIGS. 12C and 12D  where the polarization rotator  90   c   2  is not used. 
     The color combiner  16  combines the color image light from the polarizing image combiners  26   a ,  26   b  and  26   c . The combined image light then passes through the optional wavelength selective polarization rotator  90 , the optional waveplate  70  which can convert linear polarized light into circular polarized light that can be advantageous in some 3D operation modes. The projection lens  50  then project the images onto the optional screen  60 . 
     In the seventh type of the embodiments in accordance with the present invention of the projection display systems, the images from the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in colors c 1 L, c 2 L and c 3 L, they form the first set of primary color images. Therefore, the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are called the first set of micro-display panels. Similarly, the image light from the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  is in colors c 1 R, c 2 R and c 3 R, respectively, they form the second set of primary color images. Therefore, the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are called the second set of micro-display panels. The seventh type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes. 
     In the first 2D and 3D switchable mode of the seventh type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this mode, a non-depolarizing projection screen is used. There are several approaches to get the two sets of the images from the two sets of the micro-display panels to have two different polarizations representing the left- and right-eye images. In the first approach, the optional wavelength selective polarization rotators  90   a ,  90   b ,  90   c  and  90  are not used, thus the first set of the images from the first set of the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in the second polarization states having colors c 1 L, c 2 L and c 3 L, respectively, and the second set of the images from the second set of the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization state having colors c 1 R, c 2 R and c 3 R, respectively. In the second approach, the optional wavelength selective polarization rotators  90   a ,  90   b ,  90   c  and  90  are used together, for example, the wavelength selective polarization rotator  90  can be used to rotate the polarization states of the image light from the micro-display panels  30 L 1  and  30 L 3  in colors c 1 L and c 3 L, respectively, from the first polarization state to the second polarization state, the image light from the micro-display panel  30 R 2  in color c 2 R from the second polarization state to the first polarization state, but keep the polarization states of the image light from the micro-display panels  30 R 1 ,  30 L 2  and  30 R 3  unchanged, as a result, the first set of the images from the first set of the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in the second polarization states having colors c 1 L, c 2 L and c 3 L, respectively, and the second set of the images from the second set of the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization state having colors c 1 R, c 2 R and c 3 R, respectively. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2 ,  30 L 3  and  30 R 1  and  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the seventh type of the embodiments described above, the 3D stereo display mode is realized using two different sets of colors for the left- and right-eye images. In this mode, a non-depolarizing or an ordinary projection screen can be used. In addition, the optional wavelength selective polarization rotators  90   a   2 ,  90   b   2 ,  90   c   2  and  90  can be used as well. The first set of color images from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  consist of only colors c 1 L, c 2 L and c 3 L, and the second set of color images from the second set of microdisplay panels  30 R 1 ,  30 R 2  and  30 R 3  consists of only colors c 1 R, c 2 R and c 3 R. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching color filter 3D glasses, viewers will be able to see 3D stereoscopic images on screen. The left- and right-eye color filter glasses match the spectrum of the corresponding image sets. The left- and right-eye glasses only allow the corresponding eye color images to pass and block the other eye color images. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2 ,  30 L 3  and  30 R 1  and  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of ordinary screens and relatively cheap plastic projection lens which may depolarize light, in addition, low cross-talk between the left-eye and right-eye images can be obtained due to the high contrast ratios of color filter glasses. 
     In the third 2D and 3D switchable mode of the seventh type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations and two different sets of colors for the left- and right eye-images. In this case, a non-depolarizing projection screen or an ordinary screen is used. There are several approaches to get the two sets of the images from the two sets of the micro-display panels to have two different polarizations representing the left- and right-eye images. In the first approach, the optional wavelength selective panels,  90   a ,  90   b ,  90   c  and  90  are not used, thus the first set of the images from the first set of the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in the second polarization states having colors c 1 L, c 2 L and c 3 L, respectively, and the second set of the images from the second set of the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization state having colors c 1 R, c 2 R and c 3 R, respectively. In the second approach, the optional wavelength selective polarization rotators  90   a ,  90   b ,  90   c  and  90  are used together, for example, the wavelength selective polarization rotator  90  can be used to rotate the polarization states of the image light from the micro-display panels  30 L 1  and  30 L 3  in colors c 1 L and c 3 L, respectively, from the first polarization state to the second polarization state, the image light from the micro-display panel  30 R 2  in color c 2 R from the second polarization state to the first polarization state, but keep the polarization states of the image light from the micro-display panels  30 R 1 ,  30 L 2  and  30 R 3  unchanged, as a result, the first set of the images from the first set of the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are in the second polarization states having colors c 1 L, c 2 L and c 3 L, respectively, and the second set of the images from the second set of the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization state having colors c 1 R, c 2 R and c 3 R, respectively. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, and by wearing matching polarizing 3D glasses in the case where a non-depolarizing screen is used or color filter glasses in the case where a non-depolarizing screen or an ordinary screen is used, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2 ,  30 L 3  and  30 R 1 ,  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen with the first and second sets of the primary colors combined. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because of its flexibility in the use of screens or 3D glasses. 
     In the fourth 2D and 3D switchable mode of the seventh type of the embodiments described above, the 3D display mode is realized by displaying the left- and right-eye images time sequentially at a fast speed. In this mode, a non-depolarizing projection screen or an ordinary screen can be used. In addition, the optional wavelength selective polarization rotators  90   a   2 ,  90   b   2 ,  90   c   2  and  90  can be used. The first set of color images from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  consist of colors c 1 L and c 2 L and c 3 L, and the second set of color images from the second set of microdisplay panels  30 R 1 ,  30 R 2  and  30 R 3  consists of only colors c 1 R, c 2 R and c 3 R. If left-eye signals are fed both to the first and second sets of micro-display panels, and then the right-eye signals are fed both to the first and second sets of micro-display panels, the switching between the left- and right-eye images are fast enough (for example, 60 frames per second), by wearing synchronized LCD shutter glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2 ,  30 L 3 , and  30 R 1 ,  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen. Both 2D and 3D operation modes are highly light efficient. The 2D and 3D images consist of both the first and second set of the primary colors. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of any type of screen. 
     There are variations of the seventh type of the embodiments, in which the illumination systems  8  or  8   a ,  8   b  and  8   c , and the image combining devices  26   a ,  26   b  and  26   c  can be arranged to allow the three polarized color beams  40   a ,  40   b  and  40   c  to be directed perpendicular to the plane of the drawing paper surface, instead of within the plane of the drawing paper surface. These variations are useful when there is space limitation. The operation of the variations is similar to the above seventh type of the embodiments with some minor modifications. 
     Eighth Type of Embodiments 
     The eighth type of the embodiments of the projection display systems in accordance with the present invention as shown in  FIGS. 13A and 13B , comprises of an illumination system  8  for providing un-polarized light; a polarizing beam-splitter  10 ; two imaging arms  25 L and  25 R; an optional waveplate  70 ; a projection lens  50  and an optional projection screen  60 . The polarizing beam-splitter  10  reflects light in a first polarization and transmits light in a second polarization, or vice versa, the first and second polarizations are orthogonal to each other. The imaging arm  25 L comprises of a color separating and combining prism  16 L and a first set of three micro-display panels  30 L 1 ,  30 L 2  and  30 L 3 . Similarly, the imaging arm  25 R also comprises of a color separating and combining prism  16 R and a second set of three micro-display panels  30 R 1 ,  30 R 2  and  30 R 3 . 
     In the embodiment shown in  FIG. 13A , the color separating and combining prisms  16 L and  16 R are X-cubes which transmit c 2  color and reflects c 1  and c 3  colors; in the embodiments shown in  FIG. 13B , the color separating and combining prisms  16 L and  16 R are Philips prisms which also transmit c 2  color and reflects c 1  and c 3  colors. 
     The reflective micro-displays panels  30 L 1 ,  30 R 1 ,  30 L 2 ,  30 R 2 ,  30 L 3 ,  30 R 3  are described in the preamble as reflective micro-display panel  30 . They can be LCOS panels with additional waveplates for compensating geometry depolarization. They can also be MEM panels with additional waveplates for rotating the polarization state of incident light by 90°; in addition, optional waveplates can be used as parts of the panels to compensate geometry depolarization. 
     The polarizing beam-splitter  10  is selected from the type of the polarizing beam-splitter  10  described in the preamble. The first and second polarizations can be s- and p-polarized, or vice versa. In the embodiments shown in  FIGS. 13A and 13B , the central angle of incidence of light at the beam-splitting surface of the polarizing beam-splitter  10  is slightly larger than 45°; similar embodiments are possible where the central angle of incidence is about 45°, and also where the central angle of incidence is substantially larger than 45°. 
     The operation of the fifth type of the embodiments in accordance with the present invention of the projection display systems is described as follows with the help of  FIGS. 13A and 13B . 
     The un-polarized light from the illumination system  8  consists of colors c 1 , c 2  and c 3 , such as red, green or blue or any suitable color combinations. The un-polarized light having the first and second polarizations is first split into two beams having the first and second polarization states, respectively, by the polarizing beam-splitter  10 ; the reflected light in the first polarization state towards the image arm  25 L and the transmitted light in the second polarization state towards the image arm  25 R. 
     In the image arm  25 L, the color separating and combining prism  16 L separates the incident polarized light in the first polarization into three polarized color beams having colors c 1 , c 2  and c 3 , respectively. The three polarized color beams are incident onto the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3 . The first set of the reflective micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  encode images onto the output light by rotating the polarization state of the “on” pixel light by 90° relative to the incident light, thus, from the first polarization state to the second polarization state. Subsequently, the three output image lights are combined by the color separating and combining prism  16 L and form the first set of color images in the second polarization state. The first set of color image light then passes through the polarizing beam-splitter  10 , the optional waveplate  70  and the projection lens  50 , finally, the images are then projected onto the optional screen  60 . For “off” pixel light from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3 , if the micro-display panels are LCOS panels, the polarization state of the “off” pixel light is unchanged, thus it is sent back by the polarizing beam-splitter  10  along the direction of the light source; if the micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  are MEM panels, the “off” pixel light is reflected by the panels at different directions from the “on” pixel light and does not enter the projection lens  50  and is absorbed by a light absorber. 
     Similarly, in the image arm  25 R, the color separating and combining prism  16 R separates the incident polarized light in the second polarization into three polarized color beams having colors c 1 , c 2  and c 3 , respectively. The three polarized color beams are incident onto the second set of micro-display panels  30 R 1 ,  30 R 2  and  30 R 3 . The second set of the reflective micro-display panel  30 R 1 ,  30 R 2  and  30 R 3  encode images onto the output light by rotating the polarization state of the “on” pixel light by 90° relative to the incident light, thus, from the second polarization state to the first polarization state. Subsequently, the three output image lights are combined by the color separating and combining prism  16 R and form the second set of color images in the first polarization state. The second set of color image light is then reflected by the polarizing beam-splitter  10 , and passes through the optional waveplate  70  and the projection lens  50 , and finally, the images are then projected onto the optional screen  60 . For “off” pixel light from the second set of micro-display panels  30 R 1 ,  30 R 2  and  30 R 3 , if the micro-display panels are LCOS panels, the polarization state of the “off” pixel light is unchanged, thus it is sent back by the polarizing beam-splitter  10  along the direction of the light source; if the micro-display panels  30 R 1 ,  30 R 2  and  30 R 3  are MEM panels, the “off” pixel light is reflected by the panels at different directions from the “on” pixel light and does not enter the projection lens  50  and is absorbed by a light absorber. 
     The eighth type of the embodiments in accordance with the present invention can be configured to work in one or combinations of the following 2D and 3D switchable modes: 
     In the first 2D and 3D switchable mode of the eighth type of the embodiments described above, the 3D stereo display mode is realized by using two orthogonal polarizations for the left- and right eye-images. In this mode, a non-depolarizing projection screen is used. The first set of color images from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  is in the second polarization, and the second set of color images from the second set of microdisplay panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization. If left-eye and right-eye image signals are fed to the first and second sets of micro-display panels, respectively, or vice versa, by wearing matching polarizing 3D glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2  and  30 L 3 , and  30 R 1 ,  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen. Both 2D and 3D operation modes are highly light efficient. In switching from 2D to 3D mode, no light is wasted, the available light is equally shared between the left-and right-eye images. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of cheap polarizing 3D glasses. 
     In the second 2D and 3D switchable mode of the eighth type of the embodiments described above, the 3D display mode is realized by displaying the left- and right-eye images time sequentially at a fast speed. In this mode, a non-depolarizing projection screen or an ordinary screen can be used. The first set of color images from the first set of micro-display panels  30 L 1 ,  30 L 2  and  30 L 3  is in the second polarization, and the second set of color images from the second set of microdisplay panels  30 R 1 ,  30 R 2  and  30 R 3  are in the first polarization. If left-eye signals are fed both to the first and second sets of micro-display panels, and then the right-eye signals are fed both to the first and second sets of micro-display panels, the switching between the left- and right-eye images are fast enough (for example, 60 frames per second), by wearing synchronized LCD shutter glasses, viewers will be able to see 3D stereoscopic images on screen. In addition, if identical 2D image signals are fed to the two sets of microdisplay panels  30 L 1 ,  30 L 2  and  30 L 3 , and  30 R 1 ,  30 R 2  and  30 R 3 , normal 2D images are then displayed onscreen. The 2D and 3D modes can be switched electronically and no hardware needs to be added or removed during switching. This operation mode is advantageous in some applications because it allows the use of any type of screen.