Abstract:
An optical display system includes a light source, for generating light in first, second and third color bands, and a projection core. The projection core includes a crossed color combiner and first, second and third display panels disposed to direct first, second and third image light into the color combiner. The display panels are arranged for forming images in the light in the first, second and third color bands respectively. An optical relay system relays illumination to the first, second and third imager panels. A first dichroic separator is disposed in the light beam between the light source and the projection core, and separates light in the first color band from light in the second and third color bands. The lengths of optical paths from the first dichroic separator to each of the three display panels are all substantially equal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to illumination systems that may be used in projection systems and projection systems using the illumination systems. More specifically, the invention relates to illumination systems that use a reduced number of optical elements for transferring the illumination light from the light source to the imager panels. 
       BACKGROUND 
       [0002]    Illumination systems have a variety of applications, including projection displays, backlights for liquid crystal displays (LCDs) and others. Projection systems, for example as found in projection televisions and monitors, usually include a source of light, illumination optics, an image-forming device, projection optics and a projection screen. The illumination optics collect the light generated by the light source and direct the collected light to one or more image-forming devices. The image-forming device(s), controlled by an electronically conditioned and processed digital video signal, produces an image light beam corresponding to the video signal. Projection optics magnify the image light beam and project it to the projection screen. 
         [0003]    There is a need that the illumination optical system, i.e. the optics used for transferring the illumination light to the image-forming devices, is simple to assemble and align. Furthermore, it is preferable that the illumination optical system be simple and inexpensive. At the same time, it is preferred that the footprint of the projection system be reduced so that the system can fit into smaller volumes. 
       SUMMARY OF THE INVENTION 
       [0004]    One embodiment of the present invention is directed to an optical display system that includes a light source unit capable of generating a light beam containing light in at least first, second and third color bands and a projection core. The projection core includes a crossed color combiner having first, second and third inputs and an output, and also includes first, second and third display panels disposed to direct first, second and third image light to the first, second and third inputs respectively. The first, second and third display panels are arranged for forming images in the light in the first, second and third color bands respectively. An optical relay system relays light of the light beam to the first, second and third imager panels. A first dichroic separator is disposed in the light beam between the light source and the projection core. The first dichroic separator separates light in the first color band from light in the second and third color bands. The light beam is incident on the first dichroic separator with an incident angle of incidence less than 40°. The light beam is non-telecentric where it is incident on the first dichroic separator. 
         [0005]    Another embodiment of the invention is directed to an optical display system that includes a light source unit capable of generating a light beam containing light in at least first, second and third color bands and a projection core. The projection core includes a crossed color combiner having first, second and third inputs and an output, and first, second and third display panels disposed to direct first, second and third image light to the first, second and third inputs respectively. The first, second and third display panels arranged for forming images in the light in the first, second and third color bands respectively. An optical relay system relays light of the light beam to the first, second and third imager panels. A first dichroic separator is disposed in the light beam between the light source and the projection core. The first dichroic separator separates light in the first color band from light in the second and third color bands. A first optical path length from the first dichroic separator to the first display panel for light in the first color band is substantially equal to a second optical path length from the first dichroic separator to the second display panel for light of the second color band and is substantially equal to a third optical path length from the first dichroic separator to the third display panel for light of the third color band. 
         [0006]    The above summary of the present disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure invention. The figures and the following detailed description more particularly exemplify these embodiments. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The disclosure may be more completely understood in consideration of the following detailed description of various exemplary embodiments in connection with the accompanying drawings, in which: 
           [0008]      FIG. 1  schematically illustrates an embodiment of a projection system according to principles of the present invention; 
           [0009]      FIG. 2  schematically illustrates an embodiment of a projection system showing selected distances between certain system components; 
           [0010]      FIG. 3  schematically illustrated another embodiment of a projection system according to principles of the present invention; and 
           [0011]      FIG. 4  schematically illustrates a polarization converter unit used with the projection system of  FIG. 2 . 
       
    
    
       [0012]    Like numerals in different figures refer to similar elements. 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 
       [0013]    The present invention is applicable to illumination systems for projection displays, and is believed to be particularly useful for rear projection displays such as televisions and monitors, and also for front projection systems. 
         [0014]      FIG. 1  schematically illustrates a projection system  100 . A light source  102  generates illumination light  104  that is directed into a tunnel integrator  106 . The light source  102  may be any suitable type of light source, for example a high pressure mercury lamp, one or more light emitting diodes (LEDs), or some other type of light source. The light source  102  may also include a combination of different types of light sources, for example a combination of a high pressure mercury lamp and one or more LEDs. An optional reflector  108  may be used to increase the amount of illumination light  104  incident at the entrance end of the integrator  106 . Other elements, for example one or more lenses positioned between the light source  102  and the integrator  106 , may also be used to increase the intensity of the light  104  entering the integrator  106 . The integrator  106  may be a tunnel integrator, but this is not a requirement. A tunnel integrator may be trapezoidal, in other words, the integrator  106  it is tapered to expand towards the output. Thus, an integrator  106  that has a square input may have a rectangular output. 
         [0015]    Upon exiting the integrator  106 , the light  104  is directed towards the projection core  110 , which includes an x-cube color combiner  112 , imaging devices  114   a - 114   c  and respective polarizing beamsplitters (PBSs)  116   a - 116   c . One example of a suitable projection core is the Vikuiti™ Optical Core available from 3M Company, St. Paul, Minn. The projection core may include various polarization control elements, such as quarter-wave retarders and the like, not shown in  FIG. 1 . In some embodiments, the light  104  may be directed towards the projection core  110  by an optional first folding mirror  121 . In other embodiments, the light  104  may propagate from the exit of the integrator  106  in the negative Y-direction, without the need for the first folding mirror  121 . 
         [0016]    Each imaging device  114   a - 114   c  and its associated PBS  116   a - 116   c  is used to form an image in a respective color band, which is combined in the color combiner  112  with the colored images produced by the other imaging devices  114   a - 114   c  to form a full color image. The full color image is projected by a projection lens unit  119  to a projection screen  123 . 
         [0017]    The light  104  is split into different color components, associated with different color channels, for separately illuminating the different imaging devices  114   a . For example, the light  104  is split into first and second separated beams  118   a ,  118   b  at a first dichroic separator  120 . The first separated beam  118   a  is directed via a second folding mirror  122  to imaging device  114   c , while the second separated beam  118   b  is split by a second dichroic separator  124  into third and fourth separated beams  126   a  and  126   b  which are directed respectively to imaging devices  114   a  and  114   b . A third folding mirror  128  may be used to direct the second separated light beam  118   b  towards the projection core  110 . 
         [0018]    In other embodiments, the light beam  106  may be incident on the first dichroic separator  120  from a different direction, with the first separated beam  118   a  being reflected by the first dichroic separator  120  and the second separated beam  118   b  being transmitted through the first dichroic separator  120 . 
         [0019]    An image of the output of the integrator  106  is relayed to the imaging devices  114   a - 114   c  using an image relay system that includes a number of lenses. In this exemplary embodiment, the image relay system includes a negative first lens unit  130  positioned close to the output of the integrator  106 , a positive second lens unit  132 , and two third lens units (TLUs)  134   a  and  134   b . The first lens unit  130  and second lens unit  132  are common to all colors, since they are positioned before the first dichroic separator  120 . One of the two TLUs  134   b  is used by one color of light, and the other TLU  134   a  is used by light of two colors. The two TLUs  134   a ,  134   b  are spaced apart from the second lens unit  132  by the same optical path length, and the two TLUs  134   a ,  134   b  may have the same focal length. In some conventional types of illumination systems, different color channels require lenses of different optical lengths, which complicates the assembly. In contrast to these conventional systems, when the TLUs  134   a ,  134   b  have the same focal length, the number of different types of lenses required to assemble the illumination system is reduced. Also, the optical path length for illumination light may be substantially the same from the first dichroic separator  120  to each of the imager devices  114   a - 114   c . This ensures that all image devices  114   a - 114   c  are illuminated using substantially identical illumination beams, so that the color and intensity properties of the resulting full color image are uniform. 
         [0020]    The different lens units  130 ,  132 ,  134   a ,  134   b  may be formed using one or more lens elements. In the exemplary embodiment illustrated in  FIG. 1 , the first lens unit  130  and the third lens units  134   a ,  134   b , each include only a single lens element, while the second lens unit  132  includes two lens elements  132   a ,  132   b . It will be appreciated that the number of lens elements used in each lens unit may be different depending on the type of optical system employed. For example, the second lens unit may be formed of a single lens, such as an aspherical lens, instead of the doublet illustrated in  FIG. 1 . 
         [0021]    Typically, an illumination system that illuminates three imager devices with light of three different colors requires a telecentric light arrangement at the imager devices to provide uniform contrast across the field. This means that identical cones of light illuminate different zones of the imager devices. Although the illumination light  118   a ,  126   a ,  126   b  is incident on the imaging devices as telecentric light, the light is not telecentric at all points within the image relay system. A telecentric system is one where the aperture stop is located at the front focus, resulting in the chief rays being parallel to the optical axis in image space, i.e. the exit pupil is at the infinity. Consequently, in a telecentric light beam, the angular distribution of light at one point of an imager device is the same as the angular distribution of light at another location of the imager device. Where a light beam is non-telecentric, the angular distributions of the light associated with the two points of the imager are different. Thus, in the illumination system illustrated in  FIG. 1 , light in the space of the imager devices  114   a - 114   c  is telecentric, whereas light in the space of the first dichroic separator  120  is non-telecentric. 
         [0022]    Since the light  104  in the space of the first dichroic separator  120  is non-telecentric, various portions of the light beam  104  coming onto different points of the imager devices  114   a - 114   c  are incident at the first dichroic separator  120  with different angular distributions. The dichroic separator  120  is typically formed of a multilayer dielectric coating whose optical properties are dependent on the angle of incidence. Consequently, the spectrum of light passed by the first dichroic separator  120  is not uniform across the imagers  114   a - 114   c . This phenomenon is often referred to as color shift. A first trim filter  136  may be disposed in the first separated beam  118   a  to uniformize the spectrum of light incident at the third imager device  114   c . A second trim filter  138  may also be used to trim the spectrum of light whose wavelength band is adjacent the wavelength band of the light in the first separated beam  118   a . For example, where the light in the fourth separated beam  126   b  is green and the light in the first separated beam  118   a  is red, then a second trim filter  138  may be disposed in the fourth separated beam  126   b . In some exemplary embodiments, the trim filters  136  and  138  are tilted with respect to the incident light beams. This eliminates the reflection of some light back to the imager devices,  114   a - 114   c , which can otherwise result in ghosting effects. In some exemplary embodiments the trim filters  136  and  138  may be tilted at an angle of about 12° relative to an axis of the incident light. 
         [0023]    The first dichroic separator  120  is oriented so that the axis of light  104  incident on the first dichroic separator  120  is less than 45°. Instead, the light is incident on the first dichroic separator at an angle of less than 40°, and may be less than 35° or even 30°. This permits the illumination path lengths to the imager devices  114   a - 114   c  to be the same when using an x-cube color combiner for combining the image light beams. The actual angle of incidence on the first dichroic separator  120  is a design choice that is affected, at least in part, by the size of the system components, particularly the diameter of the third lens unit  134   a  and the width of the light beam  118   a . In some embodiments the angle of incidence of the light  118   b  on the third folding mirror  128  is substantially the same as the angle of incidence of the light  104  on the first dichroic separator  120 , and so the light reflected by the third folding mirror  128  is substantially parallel to the light  104  incident on the first dichroic separator. 
         [0024]    The angle of incidence on the first dichroic separator  120  being less than 45° and the ability to maintain the same optical path lengths for all three color channels can be understood further with reference to  FIG. 2 . The figure shows substantially the same system  100  as  FIG. 1 , but with distances between certain components labeled. The distance labeled “a” is the distance in the y-direction from the center of the x-cube combiner unit  112  to the point where the axial ray of light beam  118   b  is incident on the third folding mirror  128 . The distance “b” is the distance along the z-axis between the center of the x-cube combiner unit  112  and the point where the axial ray of light beam  118   a  is incident on the second folding mirror  122 . The distance “c” is the distance in the y-direction between the point where the axial ray of light beam  118   a  is incident on the second folding mirror  122  and the point where the axial ray of light beam  104  is incident on the first dichroic separator  120 . The distance “d” represents the distance between the center of the x-cube combiner unit  112  and the optical center of the PBS  116   a - 116   c . The distance “d” is identical for all three color channels. In most embodiments, there is no significant gap between the PBSs  116   a - 116   c  and the color combiner unit  112 , and so the distance “d” is set by the dimensions of the optical components themselves and is not a variable. The angle “γ” is the angle between the direction of beam  118   b  before and after reflection at the third folding mirror  128 . Thus, if the beam  118   b  is incident at the third folding mirror at 30°, then the value of “γ” is 60°. 
         [0025]    From a consideration of the geometry shown in  FIG. 2 , and under the assumption that the optical path lengths are the same, it can be shown that: 
         [0000]        a +( b−d )/(sin γ)= b+c    (1) 
         [0000]      and 
         [0000]        a −( b−d )/(tan γ)= c−d    (2) 
         [0000]    From (1) and (2) it can be shown that: 
         [0000]        b=d ×(1+cos(γ)+sin(γ))/(1+cos(γ)−sin(γ))   (3) 
         [0000]      and 
         [0000]        c=a−b +( b−d )/sin(γ)   (4) 
         [0000]    Thus, the designer may select a desired value of “γ” and then calculate “b” using (3). The value of “a” can be selected within a range of distances in which the beam  118   b  is not vignetted by the lens units  132  and  134   a . For the smallest system footprint, the value of “a” is selected as the smallest value within the non-vignetting range, from which the value of “c” can then be calculated. 
         [0026]    In addition, aperture stops  140  and  142  may be positioned on the first and second separated beams  118   a  and  118   b . The actual position of the aperture stops is dependent on the optical design of the image relay system. Furthermore, in some embodiments, a pre-polarizer  144  may be used to pre-polarize the light incident at the PBSs  116   a - 116   c . In the illustrated embodiment, a pre-polarizer  144  is positioned closely following the exit of the integrator  106 , where the light beam  104  has a small cross-section and thus the pre-polarizer  144  can also be small. Thus, the costs of the pre-polarizer may also be reduced. It will be appreciated, however, that the pre-polarizer may be positioned elsewhere. The pre-polarizer may be any suitable type of polarizer, for example a wire grid polarizer, a multilayer film polarizer or a PBS. 
         [0027]    In some embodiments of projector system, the illumination light  104  may be unpolarized, but the PBSs  116   a - 116   c  direct light in only one polarization state to the imager devices  114   a - 114   c , and so 50% of the light  104  would otherwise be unused. In the projection system  200  schematically illustrated in  FIG. 3 , a polarization converter unit  310  is used to convert light from the unused polarization state to the useful polarization state. The polarization converter  310  may replace the pre-polarizer  144 . An embodiment of the polarization converter unit  310  is schematically illustrated in  FIG. 4 . Light  104  enters the polarization converter unit  310  from the second lens unit  132 . The light  104  passes through a polarization beamsplitter  312 , which reflects s-polarized light  314 , and transmits p-polarized light  316 . The p-polarized light  316  is reflected by a prism  318  to propagate substantially parallel to the s-polarized light  314  and then passes through a polarization rotator  320 , for example a half-wave retardation plate, to become s-polarized. 
         [0028]    There are two important points related to the relative orientation of the integrator and the polarization converter unit:
       1. The relative orientation of the polarization converter unit  310 , first folding mirror  121  and the integrator  106  can affect the uniformity of the illumination light reaching the imager devices  114   a - 114   c . In many embodiments, the exit end of the integrator  106  is rectangular, and it is desirable that the orientation of the imager devices match the image of the exit end of the integrator  106 , after all reflections are taken into account.   2. If the light integrator  106  is trapezoidal in shape, then the axial symmetrical angular distribution of light after the light source  102 , and entering into integrator  106 , is converted into an elliptical angular distribution after the integrator  106 . It is, therefore, advantageous to orient the long side of integrator exit window along the short side of polarization converter unit  210  to provide high collection efficiency.       
 
       The arrangement of polarization converter unit  210 , folding mirror  121  and integrator  106  shown in FIG. 3 conforms with both of these points. 
       [0031]    An advantage of the arrangement illustrated in  FIG. 1  is that each illumination beam can be directed towards its respective imager device  114   a - 114   c  substantially independently of the other illumination beams. For example, folding mirror  122  may be directed to align the light beam  118   a  to the imager device  114   c . Also, folding mirror  128  may be directed to align the beam  126   b  to the imager device  114   b , while the dichroic separator  124  may be oriented to direct the light beam  126   a  to the imager device  114   a . While rotation of the folding mirror  122  affects the direction of both separated beams  128   a  and  128   b , the direction of separated beam  128   a  can be independently adjusted using the second dichroic separator  124 . 
       EXAMPLES 
       [0032]    Two exemplary optical systems are presented. In the first optical system, there is no pre-polarizer or polarization converter, and the second lens unit is a single, plastic aspheric lens. In the second optical system, the second lens unit comprises two spherical lenses. The co-ordinates are relative to the y-z axis shown in  FIG. 1 , with the origin at the center of the x-cube color combiner  122 . The x-axis is directed into the plane of the figure. The exemplary systems are arranged in a plane, and so the x-value for all elements is zero. The measurements of radius, thickness and clear aperture are given in mm. The values of y and z are also in mm. 
       Example 1 
       [0033]      
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Clear 
                 Coordinates (x = 0) 
               
             
          
           
               
                 Component 
                 Radius 
                 Thickness 
                 Material 
                 Aperture 
                 y 
                 z 
               
               
                   
               
             
          
           
               
                 Integrator 
                   
                 50.0 
                   
                 5.9 × 5.9 
                 103.00 
                 −166.00 
               
               
                 (106) 
                   
                 7.0 
                   
                 10.5 × 5.9  
                 103.00 
                 −116.00 
               
               
                 First lens 
                 −14 
                 4.0 
                 SK5 
                 15.0 
                 103.00 
                 −108.00 
               
               
                 unit (130) 
                 −15.844 
                 15.08 
                   
                 20.0 
                 103.00 
                 −104.00 
               
             
          
           
               
                 1 st  folding 
                 Front surface mirror 
                 38 × 24 
                 103.00 
                 −88.92 
               
               
                 mirror 
               
               
                 (121) 
               
             
          
           
               
                 2 nd  lens 
                 194.97 
                 12.0 
                 PMMA 
                 44.0 
                 84.00 
                 −88.92 
               
               
                 unit (132) 
                 −28.331 
                 25.72 
                   
                 44.0 
                 72.0 
                 −88.92 
               
               
                 1 st  dichroic 
                 — 
                 1.0 
                 BK7 
                 50 × 34 
                 46.28 
                 −88.92 
               
               
                 (120) 
                   
                 76.25 
                   
                 50 × 34 
                 45.57 
                 −89.63 
               
             
          
           
               
                 3 rd  folding 
                 Front surface mirror 
                 40 × 36 
                 83.93 
                 −23.73 
               
               
                 mirror 
               
               
                 (128) 
               
             
          
           
               
                 3 rd  lens 
                 55.04 
                 9.0 
                 BK7 
                 44.0 
                 58.68 
                 −23.73 
               
               
                 unit (134 a) 
                 −92.79 
                 25.61 
                   
                 44.0 
                 49.68 
                 −23.73 
               
               
                 2 nd   
                 — 
                 1.0 
                 BK7 
                 30 × 28 
                 24.07 
                 −23.73 
               
               
                 Dichroic 
                   
                 14.44 
                   
                 30 × 28 
                 23.36 
                 −24.44 
               
               
                 (124) 
               
               
                 1 st  PBS 
                   
                 — 
                   
                 17.5 × 30.0 
                 24.07 
                 −8.75 
               
               
                 (116 a) 
               
               
                 2 nd  PBS 
                   
                 — 
                   
                 17.5 × 30.0 
                 8.75 
                 −24.07 
               
               
                 (116 b) 
               
             
          
           
               
                 2 nd  folding 
                 Front surface mirror 
                 55 × 30 
                 −24.07 
                 −89.11 
               
               
                 mirror 
               
               
                 (122) 
               
             
          
           
               
                 3 rd  lens 
                 55.04 
                 9.0 
                 BK7 
                 44.0 
                 −24.07 
                 −58.68 
               
               
                 unit (134 b) 
                 −92.79 
                 40.93 
                   
                 44.0 
                 −24.07 
                 −49.68 
               
               
                 3 rd  PBS 
                   
                 — 
                   
                 17.5 × 30.0 
                 −24.07 
                 −8.75 
               
               
                 (116 c) 
               
               
                   
               
             
          
         
       
     
         [0034]    The conic constant of the aspheric lens used in the second lens unit  132  is −0.6646. The angle of incidence of axial light on the first and second folding mirrors  121  and  122 , and on the second dichroic separator  124 , is 45°. The angle of incidence on the first dichroic separator and the third folding mirror  128  is 30°. In Example 1, the pre-polarizer and field stops are omitted. 
       Example 2 
       [0035]      
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Clear 
                 Coordinates (x = 0) 
               
             
          
           
               
                 Component 
                 Radius 
                 Thickness 
                 Material 
                 Aperture 
                 y 
                 z 
               
               
                   
               
             
          
           
               
                 Integrator 
                   
                 50.0 
                   
                 5.9 × 5.9 
                 123.27 
                 −197.78 
               
               
                 (106) 
                   
                   
                   
                 10.5 × 5.9  
                 123.27 
                 −147.78 
               
               
                 First lens 
                 −12.032 
                 4.0 
                 SK5 
                 15.0 
                 123.27 
                 −138.78 
               
               
                 unit (130) 
                 −12.993 
                   
                   
                 19.0 
                 123.27 
                 −134.78 
               
               
                 Pre- 
                   
                 1.0 
                   
                 25.0 × 25.0 
                 123.27 
                 −133.00 
               
               
                 polarizer 
                   
                   
                   
                   
                 123.27 
                 −132.00 
               
               
                 (144) 
               
             
          
           
               
                 1 st  folding 
                 Front surface mirror 
                 50.0 × 40.0 
                 123.27 
                 −103.30 
               
               
                 mirror (121) 
               
             
          
           
               
                 2 nd  lens unit 
                 −680.28 
                 7.5 
                 BK7 
                 56.0 
                 93.77 
                 −103.30 
               
               
                 (132 a) 
                 −64.342 
                   
                   
                 56.0 
                 86.27 
                 −103.30 
               
               
                 2 nd  lens unit 
                 111.522 
                 7.0 
                 PMMA 
                 56.0 
                 86.17 
                 −103.30 
               
               
                 (132 b) 
                 −181.441 
                   
                   
                 56.0 
                 79.17 
                 −103.30 
               
               
                 1 st  dichroic 
                   
                 1.0 
                 BK7 
                 51.0 × 46.0 
                 45.11 
                 −103.30 
               
               
                 (120) 
                   
                   
                   
                   
                 44.27 
                 −103.82 
               
               
                 Stop 142 
                   
                   
                   
                 35.5 
                 65.00 
                 −62.55 
               
             
          
           
               
                 3 rd  folding 
                 Front surface mirror 
                 44.0 × 40.0 
                 83.93 
                 −23.73 
               
               
                 mirror (128) 
               
             
          
           
               
                 3 rd  lens unit 
                 58.325 
                 6.5 
                 SK5 
                 40.0 
                 55.40 
                 −23.74 
               
               
                 (134 a) 
                 −241.6 
                   
                   
                 40.0 
                 48.90 
                 −23.74 
               
               
                 2 nd  dichroic 
                   
                 1.0 
                 SK5 
                 32.0 × 34.0 
                 24.07 
                 −23.74 
               
               
                 (124) 
                   
                   
                   
                   
                 23.36 
                 −24.45 
               
               
                 Trim filter 
                   
                 1.0 
                 BK7 
                 25.0 × 20.0 
                 13.48 
                 −23.87 
               
               
                 (138) 
                   
                   
                   
                   
                 12.50 
                 −24.07 
               
               
                 2 nd  PBS 
                   
                 — 
                 SK5 
                 17.5 × 30.0 
                 8.75 
                 −24.07 
               
               
                 (116 b) 
               
               
                 1 st  PBS 
                   
                 — 
                 SK5 
                 17.5 × 30.0 
                 24.07 
                 −8.75 
               
               
                 (116 a) 
               
               
                 Stop (140) 
                   
                   
                   
                 35.5 
                 3.00 
                 −103.51 
               
             
          
           
               
                 2 nd  Folding 
                 Front Surface Mirror 
                 55.0 × 40.0 
                 −24.07 
                 −103.51 
               
               
                 mirror (122) 
               
             
          
           
               
                 3 rd  lens unit 
                 58.325 
                 6.5 
                 SK5 
                 40.0 
                 −24.07 
                 −55.75 
               
               
                 (134 b) 
                 −241.6 
                   
                   
                 40.0 
                 −24.07 
                 −49.25 
               
               
                 Trim filter 
                   
                 1.0 
                 BK7 
                 25.0 × 20.0 
                 −23.87 
                 −13.48 
               
               
                 (136) 
                   
                   
                   
                   
                 −24.07 
                 −12.50 
               
               
                 3 rd  PBS 
                   
                   
                 SK5 
                 17.5 × 30.0 
                 −24.07 
                 −8.75 
               
               
                 116 c 
               
               
                   
               
             
          
         
       
     
         [0036]    The co-ordinates for mirrors show the geometrical center of the mirror surface. Each mirrors is offset −6.5 mm down in the plane of mirror. The trim filters are set so that the light is incident at an angle of 12°. The co-ordinates for the first dichroic separator  120  are for the geometrical center of the surface. The first dichroic separator  120  is offset +2.0 mm up in the plane of the reflecting layer. The co-ordinates for the second dichroic separator  124  are for the geometrical center of the surface. The second dichroic separator  124  is offset +3.0 mm up in the plane of the reflecting layer. The angle of incidence on the first dichroic separator and the third folding mirror  128  is 32°. 
         [0037]    In the above description, the term angle of incidence, when used to describe the incidence of light having an angular distribution on a surface, refers to the angle that the axial ray makes relative to the normal to the surface. 
         [0038]    The present disclosure should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present disclosure may be applicable will be readily apparent to those of skill in the art to which the present disclosure is directed upon review of the present specification. The claims are intended to cover such modifications and devices.