Abstract:
A low f-number, single-element, catadioptric condenser lens ( 30 ) with four optical surfaces ( 21 - 24 ), which is capable of collecting light at large angles and large pupil diameters. Two of the four surfaces ( 22, 23 ) are reflective surfaces immersed in the lens refractive material ( 25 ), while the other two surfaces ( 21, 24 ) are refractive surfaces and fabricated on the surface of the lens. Two embodiments of the lens are disclosed; a low-obscuration type and a wide-field type. This condenser lens provides improved light collection efficiency overall and has lower obscuration in the center portion of the reformed light source ( 26 ), as well as improved optical aberration and stray light properties. The integration of the optical surfaces into a single package results in a more reliable and lower cost condenser lens.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of optics and particularly to the area of condenser lenses.  
           [0003]    2. Description of the Related Art  
           [0004]    It is difficult to collect collimated light emitted at wide beam angles from a large aperture source without using several powerful lens elements. Often the primary collimating optics (e.g., parabolic reflector) cannot efficiently collect the source light and project it uniformly to the collimator output without introducing significant optical aberrations. Several lens elements typically are required to focus the light from the collimating optics in such a way as to maximize the system luminance. This can increase both the size and cost of the system optics.  
           [0005]    [0005]FIG. 1 is a block diagram for a typical light collection system used in a digital micromirror device (micromirror) video projector. The system consists of: a light source  11 , in this case a xenon arc lamp; an elliptical mirror  12 ; primary condenser optics  10 , consisting of a truncated conical mirror  13  and a conical folding mirror  14 ; a secondary condenser pair  16 ; a micromirror  18 ; and a projection lens  19 . A reformed light source  15  (spot of light) at the output of the light collection optics contains as much of the source emitted light as possible. The secondary condenser pair  16  tailors the light beam into an illumination cone  17 , which is as close as possible to the size of the micromirror  18 , in order to gain the maximum utilization of the available light. Typically, the illumination cone  17  is circular as compared to the micromirror&#39;s rectangular shape, which results in an inefficient use of the light. Rectangular light integration rods can also be used to more closely match the light beam to the surface of the micromirror.  
           [0006]    Of special interest to this invention is the primary condenser optics  10  used to collect as much of the available light from the source as possible. As mentioned above, this consists of a truncated conical mirror  13  and a conical folding mirror  14 . As shown, light from the source  11  is reflected by the elliptical mirror  12  and then double folded by means of the primary condenser optics  10 , resulting in the reformed light source  15 . The light is first folded by the truncated conical mirror  13  back on to the conical folding mirror  14 , which then focuses the light into the reformed spot  15 . This approach also helps to fill in the center portion of the light spot where the lamp lead does not emit light. However, this type of light collection system is often inefficient, introduces significant optical aberrations, tends to be bulky, and is costly.  
           [0007]    There is a need for a single condenser element to replace the rather bulky and costly primary condenser optics  10 , discussed above. The invention disclosed herein addresses this need.  
         SUMMARY OF THE INVENTION  
         [0008]    This invention discloses a low f-number (high numerical aperture), single-element, catadioptric condenser lens that addresses the shortcomings of conventional condenser optics. The disclosed catadioptric condenser is a single lens element with four optical surfaces that is capable of collecting light at large angles and large pupil diameters. Two of the four surfaces are reflective surfaces, which are immersed in the lens refractive material and the other two are refractive surfaces fabricated on the surface of the refractive material.  
           [0009]    The invention relates to the combining of the four optical surfaces in a single catadioptric lens element. Two embodiments of the invention are disclosed: (1) a low-obscuration condenser and (2) a wide-field condenser. The low-obscuration embodiment consists of convex refractive—concave reflective—convex reflective—convex/concave refractive sequential elements from input to output of the lens. In this embodiment, the positive power of the large convex refractive surface bends the light in such a way as to reduce the ray height at the concave secondary reflector, allowing reduced central obscuration and more light to be transmitted. The wide-field embodiment consists of concave refractive—concave reflective—convex refractive—convex/concave refractive sequential elements from input to output of the lens. In this embodiment, the negative power of the large concave refractive surface bends the rays of light in such a way as to reduce the ray incidence angles at the concave primary reflector, allowing the primary surface to be spherical and thereby providing compensation for the collimator optical aberrations.  
           [0010]    This single element catadioptric condenser lens potentially provides some of the following advantages:  
           [0011]    1. efficient source light collection,  
           [0012]    2. fewer lens elements,  
           [0013]    3. common shapes (radii) for the reflective and refractive surfaces,  
           [0014]    4. lower fabrication and assembly cost,  
           [0015]    5. efficient stray light rejection, and  
           [0016]    6. reduced central obscuration.  
       
    
    
     DESCRIPTION OF THE VIEWS OF THE DRAWINGS  
       [0017]    The included drawings are as follows:  
         [0018]    [0018]FIG. 1 is a block diagram of a typical light collection system used in a micromirror video projector. (prior art)  
         [0019]    [0019]FIG. 2 is a drawing of the single element, low obscuration, catadioptric condenser lens of the first embodiment of this invention.  
         [0020]    [0020]FIG. 3 a  is a sketch identifying the four surfaces of the catadioptric condenser lens of FIG. 2.  
         [0021]    [0021]FIG. 3 b  defines the radius and vertex for each of the four surfaces for the catadioptric condenser lens of FIG. 2.  
         [0022]    [0022]FIG. 4 illustrates how the catadioptric condenser lens of FIG. 2 is used in a light collection system.  
         [0023]    [0023]FIG. 5 is a drawing of the single-element, wide-field, catadioptric condenser lens of the second embodiment of this invention.  
         [0024]    [0024]FIG. 6 a  is a sketch identifying the four surfaces of the catadioptric condenser lens of FIG. 5.  
         [0025]    [0025]FIG. 6 b  defines the radius and vertex for each of the four surfaces for the catadioptric condenser lens of FIG. 5.  
         [0026]    [0026]FIG. 7 illustrates how the catadioptric condenser lens of FIG. 5 is used in a light collection system.  
         [0027]    [0027]FIG. 8 a  is a system level diagram of a single micromirror projector that uses the single element, catadioptric condenser lens of this invention.  
         [0028]    [0028]FIG. 8 b  is a system level diagram of a three-micromirror, high brightness projector that uses the single element, catadioptric condenser lens of this invention.  
     
    
     DETAILED DESCRIPTION  
       [0029]    This invention discloses two embodiments for a four-surface, single element, catadioptric condenser lens. Both embodiments are comprised of a low f-number (high numerical aperture) condenser lens with four optical surfaces. In both cases, two of these optical surfaces are reflective and are immersed in the lens refractive material. The details of the two embodiments are given below.  
         [0030]    [0030]FIG. 2 is a drawing of the single element catadioptric condenser lens of embodiment one of this invention. In addition to the advantages of the condenser lens of this invention listed in the summary, this embodiment further improves the low obscuration properties of the lens. The condenser lens consists of two immersed zone reflector surfaces and two external zone reflector surfaces, as follows: (i) a surface zone, convex refractive surface  21 , (ii) an immersed zone, concave primary reflector surface  22 , (iii) an immersed zone, convex secondary reflector  23 , and (iv) a surface zone, convex or concave refractive relay lens  24 . Both the concave primary reflector  22  and convex secondary reflector  23  are immersed in the lens&#39; refractive material  25 . In operation, the incoming light rays  20  first strike the convex refractive surface  21  where the light is refracted (bent). The light beam is then doubled folded by reflecting first off the concave primary reflector surface  22  and then off the convex secondary reflector surface  23 . Then the light is again refracted at the relay surface  24  and focused as a reformed image  26  of the source arc. With the reflective surfaces  22 ,  23  being fabricated into the same optical element as the refractive lens surfaces  21 ,  24 , the positive power of the large convex refractive surface  21  bends the rays in such a way as to reduce the ray height at the convex secondary reflector  23 , thereby allowing more light to be transmitted from the center portion of the collimator.  
         [0031]    [0031]FIG. 3 a  includes three views (front, back, and side) of the single-element catadioptric condenser lens  30  that identifies the four optical surfaces; e.g., convex refractive surface (S 1 )  21 , primary concave reflective surface (S 2 )  22 , convex secondary reflective surface (S 3 )  23 , and refractive relay surface (S 4 )  24 . Again, both the concave primary reflector  22  and the convex secondary reflector  23  are immersed in the refractive material  25 , while both the convex refractive surface  21  and the refractive relay  24  are zones located on the surface of the refractive material  25 . Also, both the convex refractive surface  21  and the concave primary reflector  22  have holes in their center portion, as indicated, where the convex secondary reflector  23  and the refractive relay surface  24  are located, respectively.  
         [0032]    [0032]FIG. 3 b  shows the center of radius and vertex locations for each of the surfaces in the catadioptric condenser lens  30  in the first embodiment of this invention. As an example, modeling data showing the radius dimensions, vertex locations, and conic constant for the four surfaces of the first embodiment of the condenser lens  30  are summarized in Table 1 below. The conic constant is a design parameter and is included for illustration purposes only in this example.  
                               TABLE 1                                       Conic       ID   Description   Vertex   Radius   Constant                   S1   Convex refractive   V = −0.98425   R1 = +2.5000   NA           surface       S2   Concave primary   V =   0   R2 = −3.2407   k = −1.00           reflector surface       S3   Convex secondary   V = −0.86614   R3 = −1.2857   k =   8.5388           reflector surface       S4   Refractive relay   V =  0   R4 = −1.2756   k =   0           surface                          
 
         [0033]    Note that the vertices for surfaces S 2  and S 4  are the same and are located at the reference point (0 inches) in this example. The radii for surfaces S 2 , S 3 , and S 4  are negative, relative to the reference point (0 inches), while the radius for S 1  is positive, as shown. The condenser lens  30  Is optimized for operation over the visible spectrum from 450-650 nanometers.  
         [0034]    [0034]FIG. 4 is a block diagram illustrating how the catadioptric condenser lens  30  of the first embodiment of this invention is used in a light collection system. The single-element, low-obscuration catadioptric condenser lens  30 , described above, is shown on the right side of the diagram. This condenser lens  30  is optically coupled to illumination source collection optics  40 , shown on the left side of the diagram. The conventional collection optics  40  consist of a light source (lamp)  41 , a parabolic reflector  42 , and a light source window  43  (optional). The collection optics  40  are shown in conjunction with the catadioptric condenser lens  30  in this figure for illustrative purposes only. In operation, the parabolic reflector  42  reflects light  20  from the lamp  41 . The function of the single-element, low-obscuration catadioptric condenser lens  30  is to capture as much of the incoming light  20  as possible and efficiently produce a reformed light source image  26 . The condenser lens  30  is capable of collecting light at both large input angles and large pupil diameters. The positive power of the large aperture refractive surface of the condenser lens  30  bends the light rays in such a way as to reduce the ray height at the convex secondary reflector  23 , thereby reducing the central obscuration caused by the lamp&#39;s leads, allowing more light to be transmitted and at the same time providing a uniform reformed light source image  26 . The immersed configuration of the single-element condenser lens  30  also significantly improves the stray light rejection properties of the lens.  
         [0035]    [0035]FIG. 5 is a drawing of the single-element catadioptric condenser lens of the second embodiment of this invention. In addition to the advantages of the condenser lens listed in the summary, this embodiment further improves the wide-angle properties of the lens. The condenser lens consists of two immersed zone reflector surfaces and two external zone reflector surfaces, as follows: (i) a surface zone, concave refractive surface  51 , (ii) an immersed zone, concave primary reflector surface  52 , (iii) an immersed zone, convex secondary reflector  53 , and (iv) a surface zone, convex or concave refractive relay lens  54 . Both the concave primary reflector  52  and the convex secondary reflector  53  are immersed in the lens&#39; refractive material  55 . In operation, the incoming light rays  50  first strike the concave refractive surface  51 , where the light is refracted. The light beam is then double folded by reflecting first off the concave primary reflector surface  52  and then off the convex secondary reflector surface  53 . Then the light is again refracted at the relay surface  54  and focused as a reformed image  56  of the source arc. With the reflective surfaces  52 ,  53  being fabricated into the same optical element as the refractive surfaces  51 ,  54 , the negative power of the large concave refractive surface  51  bends the rays in such a way as to reduce the ray incidence angles at the concave primary reflector  52 , thereby allowing more light to be transmitted. The convex secondary reflector  53  is made larger to improve the wide-field properties of the condenser lens.  
         [0036]    [0036]FIG. 6 a  is a sketch (front, back, and side views) of the single-element catadioptric condenser lens  60  that identifies the four optical surfaces; e.g., concave refractive surface (S 1 )  51 , concave primary reflective surface (S 2 )  52 , convex secondary reflective surface (S 3 )  53 , and refractive relay surface (S 4 )  54 . Again, both the concave primary reflector  52  and the convex secondary reflector  53  are immersed in the refractive material  55 , while both the concave refractive surface  51  and the refractive relay  54  are zones located on the refractive material  55 . Also, both the concave refractive surface  51  and the concave primary reflector  52  have holes in their center portion, as indicated, where the convex secondary reflector  53  and the refractive relay surface  54  are located, respectively.  
         [0037]    [0037]FIG. 6 b  shows the radius and vertex locations for each of the surfaces in the catadioptric condenser lens  60  in the second embodiment of this invention. As an example, modeling data showing the radius dimensions and vertex locations for the four surfaces of the second embodiment of the condenser lens  60  are summarized in Table 2 below.  
                                   TABLE 2                                   ID   Description   Vertex   Radius                           S1   Concave refractive   V = −0.765   R1 = −2.899               surface           S2   Concave primary   V =   0   R2 = −3.277               reflector surface           S3   Convex secondary   V = −0.765   R3 = −1.825               reflector surface           S4   Refractive relay   V = +0.039   R4 = −3.277               surface                                  
 
         [0038]    In this case, the vertex locations for the concave refractive surface S 1  and the convex secondary reflector surface S 3  are the same. As in the case for first embodiment of this invention, the condenser lens  60  is optimized for operation over the visible spectrum from 450-650 nanometers.  
         [0039]    [0039]FIG. 7 is a block diagram illustrating how the catadioptric condenser lens  60  of the second embodiment of this invention is used in a light collection system. The single-element, wide-field catadioptric condenser lens  60 , described above, is shown on the right side of the diagram. This condenser lens is optically coupled to illumination source optics  70 , shown on the left side of the diagram. This conventional collection optics  70  consists of a light source (lamp)  71 , a parabolic reflector  72 , and a light source window  73  (optional). The collection optics  70  is shown in conjunction with the catadioptric condenser lens  60  in this figure for illustrative purposes only. In operation, the parabolic reflector  72  reflects light  50  from the lamp  71 . The function of the single-element, wide-field catadioptric condenser lens  60  is to capture as much as possible of the incoming light  50  and efficiently produce a reformed light source image  56 . The condenser lens  60  is capable of collecting light at large input angles and large pupil diameters. The negative power of the large concave refractive surface  51  of the condenser lens  60  bends the light rays in such a way as to reduce the ray incidence angles at the at the concave primary reflector  52 , thereby allowing more light with less optical aberrations to be transmitted. The immersed configuration of the single-element condenser lens  60  also significantly improves the stray light rejection properties of the lens.  
         [0040]    [0040]FIG. 8 shows a single and a three-chip spatial light module (SLM) projection display system, respectively, using one or the other catadioptric condenser lens of this invention. As a first example, FIG. 8 a  is a block diagram for a single micromirror device projection display, which consists of light source  80  (lamp and collector), the catadioptric condenser lens  81  of this invention, a color filter wheel and motor assembly  82 , a secondary condenser lens  83 , a micromirror chip  84 , a zoom projection lens  85 , and a projection screen  86 . In operation, the catadioptric condenser lens  81  collects and reforms the light from the light source  80 . The color filter wheel  82 , which has red, green, and blue filter segments, filters the reformed light source and sequentially applies first red, then green, and then blue light (color sequence, example only) to the micromirror. The secondary condenser lens  83  consists of illumination optics used to reform/reconstruct as much as possible of the available light produced at the output of the catadioptric lens  81  onto the micromirror  84  surface. Light reflected from the micromirror pixels is then projected by means of a zoom lens  85  onto a viewing screen  86 . The catadioptric condenser lens  81  of this invention improves the light collection efficiency and therefore the brightness of the projection system and significantly reduces optical aberrations in the overall projection system.  
         [0041]    As another example, FIG. 8 b  is a block diagram for a three-micromirror projection display, which consists of a light source  90  (lamp and collector), the catadioptric condenser lens  91  of this invention, a secondary condenser lens  92 , a turning mirror  93 , a total-internal-reflective (TIR) prism  94 , three micromirrors  95  (one each for red, green, and blue light frequencies), color splitting and combining prisms  96 , a zoom projection lens  97 , and a projection screen  98 . Here three micromirrors  95 , one dedicated to each of the there primary colors (red, green, and blue) are used. The total-internal-reflective (TIR) prism  94  and color-splitting and color combining prisms  96  are added to direct the appropriate red, green, and blue light to the appropriate micromirror. In this case, the secondary condenser lens  92  consists of illumination optics used to reform/reconstruct as much as possible of the available light at the output of the catadioptric condenser lens  91  into the TIR prism  94  and ultimately onto the micromirror surfaces  95 . Light reflected from the three micromirror pixels is recombined by the combining prisms  96  and then projected by means of a zoom lens  97  onto a viewing screen  98 . The three-micromirror projector is used in higher brightness applications, such as for cinema and/or large conference centers. As in the case of the single micromirror projector, the catadioptric condenser lens  91  of this invention improves the light collection efficiency and therefore-the brightness of the projection system. In addition, there is a significant reduction in the optical aberrations in the overall brightness of the 3-micromirror projection system.  
         [0042]    While this invention has been described in the context of two preferred embodiments, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.