Abstract:
Generally, the present invention relates to an apparatus for reducing astigmatism in a projection system that is particularly well suited to reducing astigmatism in LCD projection systems. A projection system includes a light source to generate light, conditioning optics to condition the light from the light source and an imaging core to impose on image on conditioned light from the conditioning optics to form image light. The imaging core includes a polarizing beamsplitter and at least one imager, and at least one element in the imaging core is adapted to reduce astigmatism in the image light. The astigmatism may arise in the polarizing beamsplitter. A projection lens system projects the astigmatism-reduced image light from the imaging core.

Description:
RELATED CASES  
       [0001]    This is a continuation application of U.S. Ser. No. 09/878,559, filed on Jun. 11, 2001 and incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention is directed generally to systems for displaying information, and more particularly to reflective projection systems.  
         BACKGROUND  
         [0003]    Optical imaging systems typically include a transmissive or a reflective imager, also referred to as a light valve or light valve array, which imposes an image on a light beam. Transmissive light valves are typically translucent and allow light to pass through. Reflective light valves, on the other hand, reflect only selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid crystal display (LCD) projector configurations may become possible by the use of reflective liquid crystal microdisplays as the imager.  
           [0004]    Many reflective LCD imagers rotate the polarization of incident light. In other words, polarized light is either reflected by the imager with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired grey scale. A 90° rotation provides the brightest state in these systems. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers. A desirable compact arrangement includes a folded light path between a polarizing beamsplitter (PBS) and the imager, wherein the illuminating beam and the projected image reflected from the imager share the same physical space between the PBS and the imager. The PBS separates the incoming light from the polarization-rotated image light. A single imager may be used for forming a monochromatic image or a color image. Multiple imagers are typically used for forming a color image, where the illuminating light is split into multiple beams of different color. An image is imposed on each of the beams individually, which are then recombined to form a full color image.  
           [0005]    It is desirable to use as much light generated by the light source as possible. Where the light source generates light over a wide angle, such as an arc lamp, more light can be passed through the imager system using high f-number optics. A problem, termed “polarization cascade” and associated with a conventional PBS, places a lower limit on the f-number of the illumination optics of traditional optical imaging systems. A conventional PBS used in a projector system, sometimes referred to as a MacNeille polarizer, uses a stack of inorganic dielectric films placed at Brewster&#39;s angle. Light having s-polarization is reflected, while light in the p-polarization state is transmitted through the polarizer. However, wide angle performance is difficult to achieve using these polarizers, since the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from Brewster&#39;s angle, a spectrally non-uniform leak develops. This leak becomes especially severe as the angle of incidence on the film stack becomes more normal than Brewster&#39;s angle. Furthermore, there are contrast disadvantages for a folded light path projector associated with the use of p- and s-polarization.  
           [0006]    Since light in a projection system is generally projected as a cone, most of the rays of light are not perfectly incident on the polarizer at Brewster&#39;s angle, resulting in depolarization of the light beam. The amount of depolarization increases as the system f-number decreases, and is magnified in subsequent reflections from color selective films, for example as might be found in a color-separating prism. It is recognized that the problem of depolarization cascade effectively limits the f-number of the projection system, thereby limiting the light throughput efficiency.  
           [0007]    There remains the need for an optical imaging system that includes truly wide-angle, fast optical components that may allow viewing or display of high-contrast images with low optical aberration.  
         SUMMARY OF THE INVENTION  
         [0008]    Generally, the present invention relates to an apparatus for reducing astigmatism in a projection system that is particularly well suited to reducing astigmatism in LCD projection systems. In particular, the invention is based around an imaging core that includes astigmatism reduction in at least one of its elements, for example in the polarization beamsplitter or, where the imaging core includes imagers for two or more color bands, in the color combiner such as a color prism, an x-cube combiner or a two-color dichroic combiner.  
           [0009]    One particular embodiment of the invention is directed to an optical device that includes a polarizing beamsplitter, a first path being defined through the polarizing beamsplitter for light in a first polarization state, and at least one imager disposed to reflect light back to the polarizing beamsplitter, portions of light received by the at least one imager being polarization rotated, polarization rotated light propagating along a second path from the imager and through the polarizing beamsplitter. An astigmatism compensating element is disposed on the second path to reduce astigmatism in the polarization rotated light caused by the polarizing beamsplitter.  
           [0010]    Another embodiment of the invention is directed to an optical device that includes polarizing beamsplitter means for directing light in a first polarization state along a first path and for directing light, in a second polarization state orthogonal to the first polarization state, along a second path different from the first path, and light imaging means for imposing an image on light by rotating polarization of portions of the light and reflecting the light to the polarizing beamsplitter, image light propagating along the second path through the polarizing beamsplitter means. The device also includes astigmatism correcting means disposed on the second path to reduce astigmatism in the image light caused by the polarizing beamsplitter means.  
           [0011]    Another embodiment of the invention is directed to a projection system that includes a light source to generate light, conditioning optics to condition the light from the light source and an imaging core to impose on image on conditioned light from the conditioning optics to form image light. The imaging core includes a polarizing beamsplitter and at least one imager, and at least one element in the imaging core is adapted to reduce astigmatism in the image light. A projection lens system projects the astigmatism-reduced image light from the imaging core.  
           [0012]    The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description, which follow, more particularly exemplify these embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0014]    [0014]FIG. 1 schematically illustrates an embodiment of a projection unit based on a single reflective imager;  
         [0015]    [0015]FIG. 2 schematically illustrates another embodiment of a projection unit based on multiple reflective imagers;  
         [0016]    [0016]FIGS. 3A and 3B illustrates different orientations of a color prism relative to a polarizing beamsplitter;  
         [0017]    [0017]FIG. 4 schematically illustrates a first approach to reducing astigmatism in a projector system, based on a gap in a color prism, according to an embodiment of the present invention;  
         [0018]    [0018]FIG. 5 schematically illustrates another approach to reducing astigmatism in a projector system, based on gaps in a color prism, according to another embodiment of the present invention;  
         [0019]    [0019]FIG. 6 schematically illustrates another approach to reducing astigmatism in a projector system, based on a gap between a wedge prism and a color prism, according to another embodiment of the present invention;  
         [0020]    [0020]FIG. 7 schematically illustrates another approach to reducing astigmatism in a projector system, based on a plate positioned between elements of a color prism, according to another embodiment of the present invention;  
         [0021]    [0021]FIG. 8 schematically illustrates another approach to reducing astigmatism in a projector system, based on plates positioned between and within elements of a color prism, according to another embodiment of the present invention;  
         [0022]    [0022]FIG. 9 schematically illustrates another approach to reducing astigmatism in a projector system having an x-cube color combiner, according to another embodiment of the present invention  
         [0023]    [0023]FIGS. 10A and 10B illustrate different orientations of x-cube color combiner relative to polarization beamsplitter, according to embodiments of the present invention;  
         [0024]    [0024]FIG. 11 schematically illustrates another approach to reducing astigmatism in a projector system, based on a plate positioned within a polarization beamsplitter, according to another embodiment of the present invention;  
         [0025]    [0025]FIG. 12 schematically illustrates another approach to reducing astigmatism in a projector system, based on a second, low index film, according to an embodiment of the present invention;  
         [0026]    [0026]FIG. 13 schematically illustrates an approach to reducing astigmatism in a two imager projection engine, according to an embodiment of the present invention;  
         [0027]    [0027]FIG. 14 schematically illustrates another approach to reducing astigmatism in a two imager projection engine, according to another embodiment of the present invention; and  
         [0028]    [0028]FIG. 15 schematically illustrates another approach to reducing astigmatism in a projector system, based on a wedged component within a polarizing beamsplitter, according to another embodiment of the present invention. 
     
    
       [0029]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0030]    The present invention is applicable to optical imagers and is particularly applicable to low f-number optical imager systems that produce high quality, low aberration, projected images.  
         [0031]    The term optical imager system as used herein is meant to include a wide variety of optical systems that produce an image for a viewer to view, that may be used in, for example, front and rear projection systems, projection displays, head-mounted displays, virtual viewers, heads-up displays, optical computing systems, optical correlation systems and other optical viewing and display systems.  
         [0032]    One approach to overcoming the problem of depolarization cascade is to use a wide-angle Cartesian polarization beamsplitter (PBS), as discussed in U.S. patent application Ser. No. 09/312,917, filed on 17 May, 1999, and incorporated herein by reference. A Cartesian PBS is a PBS in which the polarization of separate beams is referenced to invariant, generally orthogonal, principal axes of the PBS film. In contrast, with a non-Cartesian PBS, the polarization of the separate beams is substantially dependent on the angle of incidence of the beams on the PBS.  
         [0033]    An example of a Cartesian PBS is a multilayer, reflective polarizing beamsplitter (MRPB) film, which is formed from alternating layers of isotropic and birefringent material. If the plane of the film is considered to be the x-y plane, and the thickness of the film is measured in the z-direction, then the z-refractive index is the refractive index in the birefringent material for light having an electric vector parallel to the z-direction. Likewise, the x-refractive index is the refractive index in the birefringent material for light having its electric vector parallel to the x-direction and the y-refractive index is the refractive index in the birefringent material for light having its electric vector parallel to the y-direction. The x-refractive index of the birefringent material is substantially the same as the refractive index of the isotropic material, whereas the y-refractive index of the birefringent material is different from that of the isotropic material. If the layer thicknesses are chosen correctly, the film reflects visible light polarized in the y-direction and transmits light polarized in the x-direction.  
         [0034]    One example of an MRPB film is a matched z-index polarizer (MZIP) film, in which the z-refractive index of the birefringent material is substantially the same as either the x-refractive index or the y-refractive index of the birefringent material. The MZIP film has been described in U.S. Pat. Nos. 5,882,774 and 5,962,114, both of which are incorporated by reference. An improved type of MZIP film, having increased lifetime, uses PET/COPET-PCTG as the alternating layers, as is described in U.S. Patent Application titled “Polarizing Beam Splitter”, filed on even date herewith, with 3M Attorney Docket No. 56718USA7A.002, which is incorporated by reference.  
         [0035]    One embodiment of system  110  that uses an imager is illustrated in FIG. 1, and includes a light source  112 , for example an arc lamp  114  with a reflector  116  to direct light  118  in a forward direction. The light source  112  may also be a solid state light source, such as light emitting diodes or a laser light source. The system  110  also includes a Cartesian PBS  120 , for example a wire grid polarizer or an MRPB film. Light with y-polarization, polarized in a direction parallel to the y-axis, is indicated by the circled x. Light with x-polarization polarized in a direction parallel to the x-axis, is indicated by a solid arrow depicting the polarization vector. Solid lines indicate incident light, while dashed lines show light that has been returned from the imager  126  with a changed polarization state. Light, provided by the source  112 , is conditioned by conditioning optics  122  before illuminating the PBS  120 . The conditioning optics  122  change the characteristics of the light emitted by the source  112  to characteristic that are desired by projection system. For example, the conditioning optics  122  may alter the divergence of the light, the polarization state of the light, and the spectrum of the light. The conditioning optics  122  may include for example, one or more lenses, a polarization converter, a pre-polarizer, and/or a filter to remove unwanted ultraviolet or infrared light. In some embodiments, the conditioning optics  122  may have a low f-number, for example equal to or less than 2.5, in order to use a large fraction of the light from the light source  112 .  
         [0036]    The y-polarized components of the light are reflected by the PBS  120  to the reflective imager  126 . The liquid crystal mode of imager  126  may be smectic, nematic or some other suitable type of reflective imager. If the imager is smectic, the imager  126  may be a ferroelectric liquid crystal display (FLCD). The imager  126  reflects and modulates an image beam having x-polarization. The reflected x-polarized light is transmitted through the PBS  120  and is projected by the projection lens system  128 , the design of which is typically optimized for each particular optical system taking into account all the components between the lens system  128  and the imager(s). A controller  152  is coupled to the imager  126  to control the operation of the imager  126 . Typically, the controller  152  activates the different pixels of the imager  126  to create an image in the reflected light.  
         [0037]    Another embodiment of a projection system  200  is illustrated in FIG. 2. The system uses a light source  210 , such as an arc lamp  211  having a curved reflector  213 , which directs light towards the illumination optics  215 . In the illustrated embodiment, the conditioning optics  215  include a collimating lens  217 , a first lenslet array  219 , a second lenslet array  221  and a condensing lens  227 . Between the second lenslet array  221  and the condensing lens  227 , the conditioning optics  215  may include an optional polarization converter  223 , for example of the Geffkcken-type design. Depending on the conversion efficiency of the polarization converter  223 , it may be advantageous to include an optional pre-polarizer  225  following the polarization converter  223 . The pair of lenslet arrays  219  and  221  receives nominally collimated light from the collimating lens  217 . The polarization converter  223  and the prepolarizer  225  polarize the light incident on the PBS  250  in the desired polarization state. It will be appreciated that the illumination optics may include more or fewer optical components than those described for this particular embodiment.  
         [0038]    The lenslet arrays  219  and  221 , and the condensing lens  227 , shape and homogenize the light in order to illuminate the reflective imagers  226 ,  228  and  230  evenly. The PBS  250  redirects the y-polarized light towards the three reflective imagers  226 ,  228  and  230 . The PBS  250  typically includes an MRPB film  252 , such as an MZIP film, that may be free standing, disposed between plates, or encased between prisms  254 , as illustrated. The plates or prisms  254  may be formed from glass and may collectively be referred to as covers for the MRPB film  252 .  
         [0039]    In a multiple-imager system, a color prism  236  separates the light into separate color bands associated with each imager. For the three-imager configuration illustrated, the color prism  236  typically separates the light into primary color bands: red green and blue. Intervening lenses, such as field lenses  238 ,  240  and  242 , may be inserted between each imager and the color prism  236  to further optimize the optical response of the system. The imagers  226 ,  228  and  230  modulate the polarization state of the light upon reflection to varying degrees, depending on particular image information. The color prism  236  then recombines the red, green and blue images and passes the combined image light to the Cartesian PBS  250 , which analyzes the polarization state of the image by passing substantially only x-polarized light. The y-polarized light is redirected back to the light source  212 . The light that passes through the PBS  250  is collected by the projection lens system  234  and may be subsequently focused to a screen (not shown) for viewing. An optional post-polarizer  244  may be inserted between the PBS  250  and the projection lens system  234 . It will be appreciated that other optical configurations may be used with multiple imagers.  
         [0040]    In the illustrated embodiment, the color prism  236  is a Phillips prism, such as is available from Optical Coatings Laboratory, Inc. from Santa Rosa, Calif. For purposes of clarity, the color prism  236  is shown in the conventional orientation with the rotation axes  258  of the first and second color selective surfaces parallel to the rotation axis  256  of the Cartesian PBS  250 , as is illustrated in perspective view in FIG. 3A. A rotation axis is an axis about which a surface would be rotated to move from its real position to a position perpendicular to the light propagation direction. While this relative orientation between the rotation axes  258  of the color selective surfaces and the rotation axis  256  of the PBS is often necessary for conventional types of polarizer, a Cartesian PBS  250  also permits the rotation of the color prism  236  about the principle axis  262  of the beam, so that the first and second imagers  226  and  230  are oriented vertically with respect to one another, and the nominally s-polarized light from the PBS is p-polarized with respect to the color selective surfaces of the color prism  236 . The rotated arrangement is illustrated in perspective view in FIG. 3B, in which the rotation axes  258  of the color selective surfaces are perpendicular to the rotation axis  256  of the PBS  250 . The rotated arrangement is described in U.S. patent application Ser. No. 09/746,933, entitled “Reflective LCD Projection System Using Wide-Angle Cartesian Polarizing Beamsplitter and Color Separation and Recombination Prisms”, by David J. W. Aastuen and Charles L. Bruzzone, filed on Dec. 22, 2000, and incorporated herein by reference.  
         [0041]    The use of a Cartesian PBS  120  or  250  permits the projection system to demonstrate a dynamic range of at least 100:1 in the visible light range where the conditioning optics  215  have an f-number of 2.5 or less. Furthermore, the components between the conditioning optics  215  and the projection lens system  234 , may be referred to as an imaging core. The imaging core typically includes at least a polarizing beamsplitter and one or more imagers. If more than one imager is used, the imaging core may also include color separating and combining optics, such as a color prism, dichroic separator, x-cube or the like. The imaging core does not include lenses, other than optional field lenses disposed between a color separation element and imagers. The imaging core may be telecentric, in which the cone of light incident on the imager is constant over the surface of the imager. Telecentric imager cores typically do not include field lenses.  
         [0042]    One embodiment of Cartesian PBS  250  is an MRPB film  252 , such as an MZIP film, encased between prisms  254 . In order to minimize the birefringence resulting from thermally induced stresses caused by high intensity light beams, the prisms  254  are preferably formed from a material having a low stress-optic coefficient. One of the most suitable materials for this purpose is a glass marketed under the names SF57 (Schott Glass) or PBH55 (Ohara Glass). Both SF57 and PBH55 glass have a refractive index of about 1.85.  
         [0043]    The refractive index of the MRPB film  252  is typically less than that of the surrounding prisms  254 . For example, the refractive index of an MZIP film is approximately 1.56, and its thickness is typically around 125 μm. In assembling the PBS  250 , the MRPB film is attached to the prism faces using approximately 50 μm thick glue with a matching refractive index of about 1.56. One particularly suitable type of glue for use with an MRPB film has been found to be Norland 61, manufactured by the Norland Corporation. Together, the PBS film  252  and the glue form an inclined plate of refractive index of about 1.56 and thickness 225 μm, lying an angle of about 45° to the propagation direction of the light. This relatively low index plate, within relatively higher index prisms  254  introduces astigmatism to the image light. Astigmatism is a problem for light that has been reflected by an imager.  
         [0044]    The astigmatism of an inclined plate of refractive index n in a medium of refractive index n′ is given by the expression:  
             A   =       t         n   2     -       n   ′2          sin   2        θ                [         n   2          cos   2        θ         n   2     -       n   ′2          sin   2        θ         ]               (   1   )                               
 
         [0045]    where t is the thickness of the slab and θ is the angle between the central ray of the optical beam and the slab. The astigmatism is a result of the differential displacement of the sagittal and tangential beams due to passage through an inclined slab of material having a refractive index different from that of its surroundings.  
         [0046]    The values of n and n′ are wavelength dependent due to chromatic dispersion, and so the value of the astigmatism is also wavelength dependent. The wavelength dependence of the refractive indices of an MZIP film, typically comprising polyester-like films and co-polymers, and SF57 glass are provided in Tables I and II respectively.  
                                           TABLE I                           Wavelength Dependence of MZIP Refractive Index                Wavelength (nm)   Refractive index                            435.8   1.5745           480   1.5691           546.1   1.5634           589.6   1.5594           643.8   1.5562                      
 
         [0047]    [0047]                                           TABLE II                           Wavelength Dependence of SF57 Refractive Index                Wavelength (nm)   Refractive index                            435.8   1.8939           486.1   1.872           546   1.855           587.5   1.8466           656   1.8365                        
         [0048]    Using expression (1), astigmatism caused by a 225 μm thick film in SF57 glass prisms is calculated for different colors to be: 169 μm for red light (645 nm), 181 μm for green light (546 nm) and 196 μm for blue light (480 nm). In many cases, it may be sufficient to correct for the astigmatism of the green light, concomitantly reducing the astigmatism of the blue and red portions of the light. The viewer will see a substantially astigmatism-free image where the astigmatism for each color band is less than the depth of field of the projection lens system. Thus perfect cancellation of the astigmatism at all wavelengths is not required. When a single value of astigmatism is provided below, it is assumed to be the value of astigmatism for green light at about 546 nm. In other approaches, the astigmatism for different color bands may be corrected separately.  
         [0049]    A first approach to eliminating astigmatism introduced by an inclined plate of relatively low refractive index surrounded by a material of relatively high refractive index is to propagate the light through a second inclined plate that has a refractive index lower than its surrounding material and that is inclined about a rotation axis perpendicular to that of the first plate. The second inclined plate may be formed from any suitable solid, liquid or gaseous material. If the second plate is identical to the first one, in terms of refractive index and thickness, then it should be inclined at the same angle as the first plate in order to minimize the astigmatism. If the second plate is not identical to the first plate, then the magnitude of the astigmatism introduced by the second plate is preferably the same as that introduced by the first plate in order to cancel the astigmatism completely. This requires selection of angle and thickness of the plate and the refractive index difference between the second plate and its surroundings. In the designs discussed below, spherical aberration and coma are sufficiently small that they can be ignored for practical purposes. However, compensation for spherical aberration and coma may be required in an optical system, in addition to astigmatism compensation. Since the introduction of astigmatism compensation may increase other aberrations, it may be preferred partially compensate the astigmatism in order to achieve a balance among aberrations.  
         [0050]    A second approach to eliminating astigmatism introduced by a first inclined plate having a relatively low refractive index compared to its surrounding material is to introduce a second inclined plate having a refractive index higher than the surrounding material. The second inclined plate may be formed of a solid, liquid or gaseous material. The second inclined plate is typically inclined about a rotation axis that is parallel to the rotation axis of the first inclined plate. This requires selection of the material thickness, refractive index and angle of inclination in order to provide compensation for the astigmatism. Specific embodiments using this approach to eliminate astigmatism are discussed later.  
         [0051]    The approaches to reducing astigmatism discussed herein are applicable to projection systems having a wide range of f-numbers, and are believed to be particularly advantageous for projection systems having low f-numbers. The approaches discussed herein may be used to reduce astigmatism or to substantially correct the astigmatism. In many cases, the astigmatism need not be completely cancelled, but need only be reduced to a value less than the depth of field of the projection lens system. The depth of field typically increases with f-number, and so astigmatism correction becomes increasingly more important for low f-number projection systems. The term “substantially correct” means that the astigmatism is reduced to a value less than the depth of field of the projection lens system that is being used.  
         [0052]    Although the discussion herein is directed to reducing astigmatism that arises in a MRPB PBS, it will be appreciated that the approaches to reducing astigmatism discussed below are also useful for reducing astigmatism that arises in other components of a projection system.  
         [0053]    Astigmatism reduction may be introduced based on adaptation of the color prism. Referring again to FIGS. 3A and 3B, in general, when the rotation axes  258  are perpendicular to the rotation axis  256 , astigmatism correction is introduced into the color prism  236  using a plate of relatively low refractive index compared to its surroundings. In contrast, when the rotation axes  258  of the color selecting surfaces are parallel to the rotation axis  256  of the PBS, astigmatism correction is introduced to the color prism using a plate of relatively high refractive index compared to its surroundings.  
         [0054]    First we discuss a specific embodiment of the invention that uses a second inclined plate having a relatively low refractive index. Different designs of color prisms  236  are available, several of which include three or four prisms used for separating the light into two or more color bands. Often a color prism  236  separates the light into its red, green and blue components. In the Philips Prism construction, illustrated in FIG. 4, the color prism  400  is formed from three prisms  402 ,  404  and  406 . Light  410  entering the first prism  402  is incident on the first filter  412 , which reflects light in the first color band and transmits light in the second and third color bands. The light in the first color band  414  is totally internally reflected at the input surface  416  to the first prism, since there is an air gap  417  between the input surface  416  and the PBS  450 , and is directed to the first imager  426 .  
         [0055]    The light transmitted into the second prism  404  is incident on the second filter  418 , which reflects light  420  in the second color band and transmits light  424  in the third color band. The light  420  reflected by the second filter  418  is totally internally reflected at the gap  422 , typically an air gap, between the first and second prisms  402  and  404 , and is directed to the second imager  428 . The light  424  transmitted through the second filter  418  is directed through the third prism  406  to the third imager  430 .  
         [0056]    Typically, the first color band is blue, the second color band is red and the third color band is green. This need not be the case, however, and the different color bands may have different colors.  
         [0057]    The gap  422  between the first and second prisms  402  and  404  is conventionally kept small, typically in the range 10 μm to 25 μm, which is sufficient to permit total internal reflection to take place for the second color band. However, the gap  422  may be increased in size in order to provide astigmatism compensation, as is discussed further in the following example.  
       EXAMPLE 1  
       [0058]    The color prism  400  was formed from low birefringence glass, PBH55, having a refractive index of 1.85. The angle of incidence of the central ray onto the air gap  422  was 21°. The first color band was blue, the second color band was red and the third color band was green. The color prism  400  was in the rotated position relative to the PBS  450 , so that the nominally s-polarized light from the PBS  450  was p-polarized in the color prism  400 .  
         [0059]    The size of the air gap was adjusted to compensate for an astigmatism value of 181 μm. Before adjustment, the PBS/color prism assembly was used in a projector system that projected a pattern of horizontal and vertical lines on a screen. It was possible to focus on either the horizontal lines or vertical lines, but not both simultaneously. If, for example, the horizontal lines were focused at 178 cm distance from the projection lens, then the vertical lines were in focus at 105 cm, a focal distance ratio of 1.7:1. If the best simultaneous focus were used, then both sets of lines became significantly blurred.  
         [0060]    To adjust the gap  422 , the first and second prisms  402  and  404  were separated and then re-assembled with an air gap  422  of 100 μm using Monosized Microsphere Size Standard Beads from Duke Scientific Corp., Palo Alto, Calif., as spacers. The beads had a diameter of 100 μm.  
         [0061]    After reassembling with the 100 μm gap  422 , the astigmatism of the system was again measured for red and green light. The vertical lines focused at 135 cm whereas the horizontal lines focused at 178 cm, a focal distance ratio of 1.32:1. Furthermore, the qualitative appearance of the lines when the focus was optimized was dramatically improved from the situation where the gap  422  was 10 μm.  
         [0062]    The gap  422  was readjusted to 140 μm by replacing the 100 μm spacer beads with 140 μm spacer beads, also from Duke Scientific. When tested for astigmatism, it was difficult to quantify the difference between the focal points of the vertical and horizontal lines. It appeared that the saggital rays were focused between 160 and 170 cm from the projector, for a focal ratio of less than 1.1:1. When re-focused to provide the best overall focus, there was no apparent blur to either the vertical or horizontal lines.  
         [0063]    It will be appreciated that adjusting the air gap  422  does not affect the astigmatism for the light  414  in the first color band. A qualitative test was made to determine whether correction of the red and green astigmatism alone would lead to an acceptable image. The blue, red and green images were carefully aligned and images of different contrast were observed. It was determined that any blue blur could only be discerned by careful examination of white lines on a dark background, but was not noticeable for dark lines on a bright background. This suggests that reduction of the blue astigmatism may not be as important as reduction of green and red astigmatism. A possible reason for this is that the density of blue receptors in the human eye is less than that for green and red receptors, and so the normal resolution of blue images is less than for green or red images.  
         [0064]    Astigmatism for the first color band may be corrected, however, using the approach illustrated in FIG. 5, which shows a color prism similar to that illustrated in FIG. 4, except that the first prism  402  is formed from two parts  402   a  and  402   b , with an air gap  502  therebetween. A blunt tip  504  is desired on the acute angle end of prism  402   b  for manufacturing reasons. Preferably, the size and position of the gap  502  are such that the air gap  502  does not obstruct the light  410  entering the color prism  400  from the PBS  450 . Also, the size and position of the gap  502  are such that the gap  502  is not in the path of the light  414  of the first color band until the light  414  has totally internally reflected off the input face  416 . Using expression (1) above, the air gap  502  should be around 0.875 mm in width, at an angle of about 32.25°, to compensate for an astigmatism of 196 μm, whereas the astigmatism corrected for in the other gap  422  may be of a different value. While this rather large separation may induce other aberrations, it is possible to use smaller gaps that introduce smaller aberrations, in order to partially compensate the astigmatism. Those skilled in the art will appreciate that it is possible to optimize the image either through optical simulations on a computer, or through empirical trials.  
         [0065]    It will be appreciated that the air gaps  422  and  502  are examples of sheets of lower refractive index material, air, surrounded by higher index material, for example prism glass. The gaps  422  and  502  need not be filled only with air, although air is useful since it gives a large refractive index difference with the prism material. The gaps  422  and  502  may also be filled with another material of a relatively low refractive index, other than air. It will be understood, however, that the refractive index difference between, for example the second prism  404  and the gap  422 , should be sufficient to maintain total internal reflection of the light  420 , even when the gap  422  is not filled with air. Likewise, other gaps discussed below need not be filled with air, but need only be filled with a material that has a lower refractive index than the material surrounding the gap.  
         [0066]    Another approach to correcting the astigmatism is described with reference to FIG. 6. In this embodiment, a wedge prism  662  is disposed between the color prism  600  and the PBS  650 , with a gap  664  between the wedge prism  662  and the color prism  600 . The color prism  600 , known as a modified Philips prism, is formed from first, second and third prisms  602 ,  604  and  606 , with a totally internally reflecting gap  622  between the first and second prisms  602  and  604 . In the illustrated embodiment, the third prism  606  also includes a totally internally reflecting surface  656 . This need not be the case, and the third prism  606  may be formed using a geometry that does not include a totally reflecting surface.  
         [0067]    In conventional wedge prism systems, the air gap  664  between the wedge prism  662  and the first prism  602  is only sufficiently large as to permit total internal reflection of light  614  of the first color band reflected within the first prism  602 . However, the air gap  664  between the wedge prism  662  and the first prism  602  may be selected to have a larger width so as to substantially reduce and correct the astigmatism arising within the PBS  650 . The width of the gap  664  is selected according to expression (1).  
         [0068]    For example, where the astigmatism of the PBS  650  is 181 μm, and the wedge angle of the wedge prism  662  is 10°, expression (1) suggests that the astigmatism may be corrected by an air gap  664  of around 2.104 mm.  
         [0069]    It will be appreciated that, although the low index plate has been described with reference to FIGS.  4 - 6  as an air gap, other materials having a low refractive index may also be used, for example a low index polymer film. Furthermore, it is possible to use a combination of gaps between prisms of the color prism and a gap between the color prism and the wedge prism to compensate for astigmatism. It will further be appreciated that astigmatism reduction may be implemented in different embodiments of color prisms other than those illustrated here.  
         [0070]    The second approach to correcting for the astigmatism in the PBS introduced above is to introduce a plane of relatively high refractive index that is inclined about an axis parallel to the axis of inclination of the PBS polarizer film. This approach is useful where the color prism is not rotated relative to the PBS and, therefore, the nominally s-polarized light from the PBS is also nominally s-polarized within the color prism.  
         [0071]    One particular embodiment of this approach is illustrated in FIG. 7, which shows a color prism  700  formed from first, second and third prisms  702 ,  704  and  706 . A high index plate  760 , formed from a transparent material having a higher refractive index than the first and second prisms  702  and  704  is disposed on the output surface of the first prism  702 . An air gap  722 , typically about 10 μm wide, is provided between the high index plate  760  and the second prism  704  so that light in the second color band is internally reflected within second prism  704  towards the output face  727 .  
         [0072]    Where the first filter  712  is disposed on the second surface  762  of the high index plate  760 , the light in the first color band  714  passes through the high index plate twice before exiting the first prism  702 , whereas the light  720  in the second color band and the light  724  in the third color band only pass through the high index plate  760  once before exiting the second and third prisms  704  and  706 . Thus, the light in the first color band experiences a different amount of astigmatism correction from the second and third color bands. Since the astigmatism of blue light is less significant to the viewer&#39;s perception of an image than green or red light, as has been discussed above, this embodiment may provide adequate astigmatism compensation where the first color band is blue light.  
         [0073]    In another embodiment, the first filter  712  may be placed on the output surface  703  of the first prism. In this embodiment, the light  714  in the first color band does not pass through the high index plate  760 , and so the light  714  in the first color band experiences no astigmatism correction. As is discussed above, where the light  714  in the first color band is blue, the astigmatism correction to the green and red light only may provide sufficient correction for viewing.  
         [0074]    In another embodiment, illustrated in FIG. 8, the first prism  702  may be split into two parts  702   a  and  702   b . A second high index plate  862  may be positioned between the prism parts  702   a  and  702   b , having a thickness, angle of orientation and refractive index selected to reduce astigmatism in the first color band. This embodiment is particularly useful where the filter  712  is positioned between the first prism part  702   a  and the high index plate  760 . Thus, the color prism  800  may provide correction for all three color bands.  
         [0075]    Astigmatism correction may also be implemented in an X-cube beamsplitter/combiner. An embodiment of a projection engine  900  that uses an X-cube beamsplitter and combiner is partially illustrated in FIG. 9. Light  902  from a light source (not shown) is incident on an X-cube beamsplitter  904 , that separates the light  902  into three color bands. Light  906  in the first color band is transmitted through the X-cube beamsplitter  904  to the first reflector  908 , while light  910  in the second color band is reflected by the X-cube beamsplitter  904  into the plane of the figure towards the second reflector  912 . Light  914  in the third color band is reflected in a direction out of the plane of the figure towards a third reflector. Optical elements for operating on the third color band are not shown in the figure for the sake of clarity. In the projection engine  900  that uses three PBSs, the back focal length may be reduced, thus permitting the use of a simplified projection lens system. Furthermore, the weight of the projection lens system required for wide field angle may be reduced.  
         [0076]    The first and second reflectors  908  and  912  respectively reflect light in the first and second color bands towards first and second polarizing beamsplitters  916  and  918 . The first and second reflectors may be mirrors, for example multilayer mirrors or metal mirrors, or may be reflecting polarizers oriented to reflect light in the desired polarization state towards the first and second polarizing beamsplitters  916  and  918 .  
         [0077]    Light in the first color band  906  is reflected by the first PBS  916 , having an MPBR film  917 , towards a first reflecting imager  920  that reflects the light  906  in the first color band and rotates polarization of selected portions of the wavefront of the light  906  to create an imaged beam  922  of light in the first color band that is transmitted through the first PBS  916  to the X-cube combiner  924 . Similarly, light  910  in the second color band is reflected by the second PBS  918  towards the second reflecting imager  926 . The second reflecting imager  926  produces an imaged beam  928  of light in the second color band that is transmitted through the second PBS  918  towards the X-cube combiner  924 .  
         [0078]    It will be appreciated that the projection engine  900  also includes a third reflector (not shown), a third PBS (not shown) and a third imager (not shown) to produce an imaged beam  930  of light in the third color band that is directed to the X-cube combiner  924  from a direction out of the plane of the figure. The three imaged beams  922 ,  928  and  930  are combined in the X-cube combiner to produce a three color image beam  932  that is typically projected to a screen by a set of projection optics.  
         [0079]    A more detailed illustration of the X-cube combiner  924  is presented in FIG. 10A, showing a cross-section through the X-cube combiner in the plane of the imaged beams  922 ,  928  and  930 . The X-cube combiner  924  is assembled from four right-angled prisms  1002 ,  1004 ,  1006  and  1008 , having various reflective coatings, for example multilayer dielectric reflective coatings, between certain interfaces of the prisms  1002 - 1008 . Coatings  1010  and  1012  reflect the imaged beam  928  in the second color band and coatings  1014  and  1016  reflect the imaged beam  930  in the third color band.  
         [0080]    Two slabs  1020  and  1022  are inserted into the X-cube combiner  924  in positions so that the light in each imaged beam  922 ,  928  and  930 , except for a small central portion of the first imaged beam  922 , passes through either one of the slabs  1020  or  1022  only once. In the illustrated embodiment, the first slab  1020  is disposed between the fourth prism  1008  and the first prism  1002 , and the second slab  1022  is disposed between the first and second prisms  1002  and  1004 .  
         [0081]    In the embodiment illustrated in FIGS. 9 and 10A, the axis of rotation of the MPBR film  917  and the axes of rotation of the slabs  1020  and  1022  are perpendicular. Therefore, the refractive index of the slabs  1020  and  1022  is selected to be less than the refractive index of the prisms  1002 - 1008 . For example, the prisms may be formed from SF57 glass, whereas the slabs  1020  and  1022  are formed from a lower index glass, such as BK7, having a refractive index of 1.517. The thickness of the slabs  1020  and  1022  is preferably selected to at least partially compensate for the astigmatism arising in the PBSs. For example, where the astigmatism is 181 μm, the prisms  1002 - 1008  are formed from SF57 glass, and the slabs  1020  and  1022  are formed from BK7, the astigmatism is corrected where the slab thickness is 150 μm. It is assumed that the angle of incidence in the X-cube combiner  924  is 45°.  
         [0082]    The central portion of the first imaged beam  922 , having a width d 1 , does not make a single pass through the entire thickness of either of the slabs  1020  and  1022 , and so is may not be corrected for astigmatism. Typically, the area of the central portion is small relative to the clear aperture of the beam  922 , and so the amount of light that is not corrected for astigmatism is small, a few % of the total output light. The central portion may be uncorrected for astigmatism, or may be blocked, for example using black paint, which produces less than 5% power loss. The overall effect of not correcting the central portion of the beam  922  may be reduced if the beam  922  contains light of a color band that produces a smaller astigmatism effect in the viewer&#39;s on the eye, for example blue light.  
         [0083]    Another embodiment of X-cube combiner  1050  is illustrated in FIG. 10B. Light  1070 , of one color band, enters the PBS  1054  and is reflected to the imager  1072 , which rotates polarization of certain portions of the light  1070  to form image light  1074 . The image light  1074  is transmitted through the PBS  1054  to the X-cube combiner  1050 . Image light  1076  of one or more color bands is directed into the X-cube combiner  1050  and combined with the image light  1074 .  
         [0084]    In this embodiment, the rotation axis of the MPBR film  1052  in the PBS  1054  is parallel to the axis of rotation of the slabs  1056  and  1058 . Accordingly, the refractive index of the slabs  1056  and  1058  is selected to be greater than the refractive index of the prisms  1060 - 1066  that form the X-cube combiner.  
         [0085]    The glass selection for the X-cube combiner  1050  is not limited to high index glasses, and so the combiner  1050  may be formed from a more common type of glass, such as BK7. If the astigmatism introduced by PBS  1054  is around 181 μm, then the thickness of the slabs  1056  and  1058  needed to achieve astigmatism correction is calculated to be around 1.1 mm where the slabs  1056  and  1058  are formed from PBH71 glass and the prisms  1060 - 1066  are formed from BK7.  
         [0086]    Another particular embodiment of astigmatism correction in a projector system that uses a plate of a relatively high refractive index material, illustrated in FIG. 11, is to include the plate of relatively high refractive index material in the PBS  1100 .  
         [0087]    The PBS  1100  is formed from two prisms  1102  and  1104  with two layers, an MRPB/adhesive layer  1106  and a high index layer  1108 , sandwiched between the prisms  1102  and  1104 . The refractive index, n 2 , of the high index layer  1108  is higher than the refractive index, n 0 , of the prisms  1102  and  1104 . Where the refractive index of the MRPB/adhesive layer  1106  is given by n 1 , the following relationship holds: n 2 &gt;n 0 &gt;n 1 . The thickness, d 2 , of the high index layer  1108 , is selected so that the astigmatism introduced by the high index layer  1108  reduces the astigmatism arising from the MRPB/adhesive layer  1106 . For example, where the prisms  1102  and  1104  are formed from PBH55 glass with a refractive index of 1.85 and the MRPB/adhesive layer  1106  has a thickness of 225 μm with a refractive index of 1.56, the astigmatism is 181 μm. This value of astigmatism may be compensated using a 3.8 mm thick layer of PBH71 glass, having a refractive index of 1.92, as the high index layer  1108 . It will be appreciated that an adhesive layer may be used for attaching the high index layer to the prism: the effect of such an adhesive layer has been ignored here for simplicity. Chromatic dispersion in the PBS  1100  may lead to color shift effects where light at one color is translated across the image relative to light of another wavelength. The effect of color shift may be reduced using, for example, a second PBS following the first PBS, where the second PBS is oriented to transmit the image light and to provide a color shift that compensates for the color shift arising in the first PBS  1100 .  
         [0088]    The PBS  1100  may be used where there is only one imager, and no color prism is present. One of the advantages of using only a single imager is that there is no need to align the image formed by one imager over the image formed by another imager, as is the case in a multiple-imager projection engine. Another advantage is that, since there is no requirement for a color separator/combiner, such as a color prism, x-prism, or the like, the back focal length of the engine can be reduced, and so low f-number projection lens systems may be used, for example as low as f/1.8 or less.  
         [0089]    Usually, single panel imagers operate with some kind of color selection schemes, such as a color wheel or fast tunable color filters. Accordingly, only about one third of the light incident on the imager, contained within one of three color bands, is used at any one time, and so high light efficiency is even more desirable in a single panel engine than in a three panel engine. With an f-number of f/1.8, the system étendue is 2.7 times greater than that of an engine having an f-number of f/3.0, and so the total light throughput of the engine is increased at lower f-numbers. Additionally, the coherence length of the projection engine is reduced at lower f-number, resulting in lower speckle.  
         [0090]    Another approach to compensating astigmatism in a system that uses only a single imager is illustrated in the embodiment shown in FIG. 12. Light  1202  from a light source (not shown) is reflected towards the imager  1204  by a PBS  1206  formed from an MRPB film  1208  sandwiched between glass prisms  1210 . The image light  1212  reflected from the imager  1204  is transmitted through the PBS  1206 . The image light  1212  is astigmatic due to the passage through the PBS  1206 .  
         [0091]    The image light  1212  is passed through an astigmatism-correcting cube  1214 , having a film  1216  of relatively low refractive index sandwiched between two prisms  1218  of relatively high refractive index. The plane of the film  1216  is rotated around a rotation axis  1220  that is perpendicular to the rotation axis  1222  of the MRPB film  1208  in the PBS  1206 . The thickness and angle of the film  1216  may be selected to reduce or substantially correct astigmatism arising in the PBS  1206  or in other components of the projection system.  
         [0092]    In one embodiment, the cube  1214  may be formed from an MRPB film  1216  similar to the MRPB film  1208 , sandwiched between two glass prisms  1218  similar to the glass prisms  1210  of the PBS  1206 . In such a case, the MRPB film  1216  is oriented so as to transmit the image light  1212 . The second MRPB film  1216  may be used as a post-polarizer, thus increasing the contrast by reducing the transmission of the light in the polarization state blocked by the PBS  1206 .  
         [0093]    The optical requirements of the first MRPB film  1208 , namely high transmission of one polarization state and high reflection of the other polarization state, are high so that good contrast is obtained in the image beam  1212 . This means that only the best performing sections of a manufactured length of MRPB film are suitable for use as the first MRPB film  1208 . However, the optical requirements of the second MRPB film  1216  are more relaxed, since it is not the primary means of generating contrast, and is used primarily for astigmatism compensation and for clean up. The extinction ratio for transmitted light may be in the range 100:1-10:1. Therefore, the second MRPB film  1216  may be formed from less than optimally performing sections of a manufactured length of MRPB film, thus increasing the fraction of a manufactured length of MRPB film that is useful.  
         [0094]    The cube  1214  may also be a MacNeille PBS having a thick plate. It is possible to use the MacNeille PBS in this embodiment because it is only operating in transmission, and light reflected by the MacNeille PBS, which contains mixed polarization states, is disregarded. Where a MacNeille PBS is used, the second cube may be formed from BK7 glass.  
         [0095]    It will be appreciated that the embodiment of astigmatism correction illustrated in FIG. 12 may also be implemented in a multiple-imager imager core, where a color separator/combiner is used between the PBS  1206  and the imagers.  
         [0096]    Another particular embodiment of astigmatism correction, that is advantageous for correcting astigmatism in a projection engine  1300  based on two imagers, is schematically illustrated in FIG. 13. In this embodiment, light  1302   a  and  1302   b , from a light source (not illustrated) is incident on respective Cartesian PBSs  1304   a  and  1304   b . The different light beams  1302   a  and  1302   b  may be generated by separating the light from a light source using a reflective dichroic filter or by any other suitable method for producing two color bands. The PBSs  1304   a  and  1304   b  may use respective MRPB films  1306   a  and  1306   b  to reflect light in a particular polarization state. The light  1308   a  and  1308   b  reflected from the PBSs  1304   a  and  1304   b  is directed to the respective imagers  1314  and  1318 . Image light  1312   a  reflected by the first imager  1314  is transmitted through the PBS  1304   a  to the dichroic combiner  1310 . Image light  1312   b    0 reflected by the second imager  1318  is transmitted through the PBS  1304   b  to the dichroic combiner  1310 . The image light  1312   a , in the first color band, is transmitted through the dichroic combiner  1310  while the image light  1312   b , in the second color band, is reflected by the dichroic combiner  1310  so as to combine with the first image light  1312   a , and produce the combined image light output  1320 .  
         [0097]    The dichroic combiner  1310  is formed from two prisms  1322  and  1324 , typically glass prisms. The prisms  1322  and  1324  are formed from material having a first refractive index. Each prism  1322  and  1324  has a respective plate  1326  and  1328  of high index material, for example high index glass, along its base. A dichroic film  1330  is disposed between the two plates  1326  and  1328  of high index material.  
         [0098]    The plates  1326  and  1328  of high index material are selected to have thicknesses that substantially reduce astigmatism, for example the astigmatism arising in the PBSs  1304   a  and  1304   b . The plates  1326  and  1328  may be selected to have equal thicknesses, as illustrated. The plates  1326  and  1328  may also be selected so that one plate is thicker than the other, as shown in FIG. 14. This latter embodiment may be advantageous, for example, where it is determined that one color band requires more astigmatism correction than the other color band. For example, the color band having the shorter wavelength range may be determined to require less astigmatism correction than the light in the longer wavelength band. Where the first plate  1326   a  has a thickness d 1  and the second plate  1328   a  has a thickness d 2 , the light  1312   a  in the first color band passes through a combined thickness of high index material of d 1 +d 2 . On the other hand, light  1312   b  in the second color band passes through a combined thickness of high index material of 2×d 2 . Thus, where d 1 &gt;d 2 , the image light  1312   a  in the first color band experiences a greater amount of astigmatism correction than the image light  1312   b  in the second color band.  
         [0099]    In addition to adding slabs of high index or low index to the optical system for astigmatism reduction, astigmatism may also be reduced by introducing a wedged component into the optical system. One particular embodiment of a wedged astigmatism correction element is featured in FIG. 15, which shows a PBS  1500  formed of two glass prisms  1502  and  1504 , with an MRPB film  1506  sandwiched therebetween. Light  1508  from a light source (not shown) is reflected by the MRPB film  1506  to at least one imager  1510 . If more than one imager  1510  is used, a color prism  1512  may be placed between the PBS  1500  and the multiple imagers.  
         [0100]    A wedge plate  1514  is disposed between the MRPB film  1506  and one of the prisms  1502  and  1504 . The wedge plate  1514  may be formed of any suitable transparent material. For example, the wedge plate  1514  may be formed of glass or polymer. In one particular embodiment, the wedge plate  1514  is formed from optical adhesive, such as Norland 61 that adheres the MRPB film  1506  to the prism  1504 .  
         [0101]    The embodiment is illustrated further with an example. For glass prisms  1502  and  1504  formed from SF57 glass and an MRPB film/adhesive layer thickness of 225 μm, the wedge angle, α, required for astigmatism correction is between 0.15°-0.25°, calculated using a ray tracing program, ZEMAX. For a prism height of h, the wedge thickness, w, on the wide side of the wedge  1514  is given by the expression:  
             w   =     h   ·     2     ·   α   ·     π   180               (   2   )                               
 
         [0102]    Where h=35 mm, the thickness, w, is calculated to be 129 μm, and so the optical path length change at the center of the PBS is equal to 65 μm. The wedge may be formed of optical adhesive by placing a 129 μm spacer on one side of the prism  1504  and filling the resulting wedged space with optical adhesive. The optical adhesive may then be cured using UV light.  
         [0103]    The spacers may be glass or plastic spheres deposited along only the wide side of the wedge. Alternatively, the spacers may be structures embossed into the MRPB film  1506  or attached to the PBS prism  1504 . If manufacturing tolerances are suitably high, there may be no spacer at all. A machine may automatically create the gap for the wedge to be filled with adhesive during manufacture simply by tilting one of the prisms with respect to the other. The shape of the other prism  1502  may be adjusted to correct for non-parallelism in the PBS  1500  in the imaging path.  
         [0104]    One of the advantages of using a wedged element  1514  to correct for astigmatism is that the total thickness of the PBS is less than, for example, the embodiment illustrated in FIG. 11, where the addition of the high index pate increased the optical path by over 5 mm. Since the wedge angle is small, the wedge  1514  may be formed simply from the adhesive used to attach the MRPB film  1506  to the prisms  1502  and  1504 . No extra optical components, such as slabs, are required in the wedged PBS assembly. It will be appreciated that wedge astigmatism compensation may be introduced in other components, for example in a dichroic separator/combiner or in an X-cube combiner.  
         [0105]    As noted above, the present invention is applicable to display devices, and is believed to be particularly useful in reducing astigmatism in a projection system, for example astigmatism introduced by a polarizing beamsplitter that uses a polymeric multilayer, reflective polarizing beamsplitter film. A common type of polymeric multilayer, reflective polarizing beamsplitter film is a matched index multilayer film. The invention may also be used to reduce astigmatism that arises in other components of the projection system. Furthermore, the invention is applicable to projection systems having a wide range of f-number, but is believed to be particularly useful in projection systems having a low f-number.  
         [0106]    The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.