Abstract:
A light collection for an arc lamp includes a parabolic reflector having primary and secondary parabolic reflector sections, the secondary parabolic reflector section divided into subsections, resulting in multiple arc images. The light collection system combines high efficiency and etendue preserving aperture shaping, and is particularly useful in projection display systems.

Description:
TECHNICAL FIELD 
     This invention relates to light collection systems, and more particularly relates to a light collection system for the collection of light from an intense light source such as an arc lamp, which light is to be employed in a projection display system. 
     BACKGROUND AND SUMMARY 
     Projection display systems employ intense light sources in order to provide sufficient light for a bright display at the viewing screen after modulation of the light by one or more light valves. Light sources widely used for this purpose include arc lamps such as UHP and xenon lamps. The elongated shape of the arc of such lamps presents a challenge to the designer to provide efficient optical systems for the collection and shaping of the emitted light for subsequent modulation and display. 
     The most common light collection method in projection is based on a parabolic or elliptical reflector and a lenticular array to correct for non-uniformity of the light source. Due to the large aberrations typical of these reflector types and the mismatch between light source and lenticular geometry, etendue (angular extent of the beam) is not preserved. This accounts for the poor light utilization of present projectors, especially in the case of small light valves. 
     U.S. Pat. No. 6,231,199, issued to Li on May 15, 2001, teaches an optical system for collecting and condensing light from one or more arc lamps down to a small spot size for coupling to a target such as the input face of a single optical fiber. The system includes a retro-reflector for effectively doubling the light output of the arc lamp, and a plurality of concave paraboloid reflectors. 
     An exemplary arrangement using two parabolic reflectors back-to-back to create an arc image at near unit magnification is shown in Applicant&#39;s FIGS. 1A through 1C. 
     FIG. 1A is a longitudinal section view of a light collection system  10  for an arc lamp light source  12  having an elongated arc  14 . This view corresponds to the axial image plane of arc  14 . The light collection system includes retro-reflector  16  and a compound parabolic reflector  18 , composed of primary and secondary parabolic reflector sections  18   a  and  18   b . Light from the lamp represented by rays R 1  and R 2  is retro-reflected back onto the arc  14  by retro-reflector  16 . Parabolic reflector sections  18   a  and  18   b  are coaxial, and arc  14  is located at the focus of the primary parabolic reflector section  18   a . Light from arc  14 , represented by rays R 3  and R 4 , thus forms arc image  20  at the focus of the secondary parabolic reflector  18   b , where an entrance face  22   a  of optic fiber  22  is positioned. 
     Because of the symmetry of this arrangement the huge aberrations of a parabola are largely cancelled. The cone angles of the light emitted from the arc are determined by the lamp&#39;s radiation characteristic. The cone angles of the present UHP lamp are very large. As shown in FIGS. 1B and 1C, the cone angle ψ in the axial image plane, and the cone angle π in the radial plane (view AA′), are about 90 degrees and 180 degrees, respectively, producing an anamorphic cone. Cone angles this large can not be handled downstream by conventional imaging optics. Non-imaging shaping means, e.g., a parabolic reflector, would re-introduce the parabolic aberrations, defeating the purpose of the concept. Thus, although the arrangement of FIG. 1 makes the arc image  20  accessible, it does not solve the problem of how to carry the light from there efficiently and in a manner which preserves etendue. 
     In accordance with the invention, the secondary parabolic section is split into segments and each segment is shifted either longitudinally (along the arc axis) or radially (about the arc axis), causing the formation of multiple arc images (one for each segment) which are correspondingly shifted with to the arc axis. 
     The light cone associated with each segment can be made arbitrarily small such that each arc image can be accommodated by conventional optics. Preferably, however, each arc image from the light collection system is directly coupled into a loss-less, etendue-preserving light guide of the type described in co-pending U.S. patent application Ser. No. 10/161,798, filed Jun. 4, 2002 assigned to the present Assignee 
     By providing a separate light guide input section for each arc image having an input face sized to fit the arc image and then inputting the images separately) into a light guide body section having a common input a sized to fit the array of arc images, aperture shaping can be achieved, for instance, concatenating the separate images by aligning them end-to-end to match the stripe geometry required for a single panel scrolling color projector. 
     A “fitting” light guide is one which has an input face sized to fit a single arc image or an array of arc images which are adjoining or partially overlapping, but not completely superimposed. 
     According to one aspect of the invention, a light collection system comprises a parabolic reflector, the parabolic reflector comprising a primary parabolic reflector section and a secondary parabolic reflector section, the sections being paraboloid sections and being positioned coaxially with respect to one another, so that an object placed at the focal point of the primary parabolic reflector section is imaged at the focal point of the secondary parabolic reflector section, 
     characterized in that the secondary parabolic reflector section is divided into a plurality of segments, whereby for each segment, a separate image is produced. 
     According to another aspect of the invention, a light engine for a projection display system is provided, the light engine comprising the light collection system of the invention, and a loss-less, etendue-preserving light pipe having a plurality of input faces and an output face, each input face positioned to input one of the plurality of images produced by the light collection system. 
     According to a further aspect of the invention, a projection display system is provided, the system comprising the light engine of the invention, at least one light valve for modulating light from the light engine to produce a display in accordance with a display signal, and a projection lens for projecting the display onto a display surface. 
     The light collection system of the invention provides low aberration reflective optics that can be replicated at low cost, and enables high-efficiency light collection and aperture shaping without loss of etendue. This opens the way for high-efficiency projection with small size light valves. Particularly, single panel scrolling color systems using narrow stripe illumination will benefit from this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIGS. 1A and 1B are longitudinal section views, and FIG. 1C is a cross-section view of a light collection system of the prior art for use with an arc lamp light source, having a compound parabolic reflector with primary and secondary parabolic sections. 
     FIG. 2A is a longitudinal section view of the compound parabolic reflector of FIGS. 1A and 1B, showing a modification of the secondary parabolic section in accordance with the invention. 
     FIG. 2B is a cross-section view similar to that of FIG. 1C, showing another modification of the secondary parabolic section in accordance with the invention. 
     FIG. 2C is a longitudinal section view of a compound secondary parabolic section of a compound parabolic reflector of the invention. 
     FIG. 3A is a side elevation view of one embodiment of a light guide for the loss-less guiding of light from the collection system of FIGS. 2B and 2C. 
     FIGS. 3B and 3C are end views of an output face and an input face, respectively, of the light guide of FIG.  3 A. 
     FIGS. 4A,  4 B and  4 C are side elevation views, and FIG. 4D is an end view, of another embodiment of a light guide for the loss-less light guiding of light from the collection system of FIGS. 2B and 2C. 
     FIG. 5 is a side elevation view of an embodiment of a light guide for the loss-less bending of light. 
     FIG. 6 is a schematic illustration of a light beam splitting and scrolling engine. 
     FIG. 7 is a schematic illustration of a light valve projection display system including the light collection system of FIGS. 2-5 and the beam splitting and scrolling engine of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 2A through 2C illustrate one way of dividing the secondary parabolic reflector into multiple segments, radially as well as axially. FIG. 2A, a longitudinal section view of compound parabolic reflector  30 , shows how moving the secondary parabolic reflector section  32   b  away from parabolic reflector section  32   a  along longitudinal axis L from position C to position D, results in moving the image of arc  34  along axis L from position C′ to position D′. 
     FIG. 2C shows that by dividing secondary parabolic reflector section  32   b  into segments  32   e  and  32   f , and shifting these segments axially away from the primary parabolic reflector  32   a , an array of images ( 42 ,  44 ) along axis L are produced. Segments  32   e  and  32   f  are connected by an annular band  32   g . As light cones of smaller angular extent (defined by rays; R 6  and R 13 , and R 8  and R 14 , respectively) are associated with each segment, the images produced by these light cones can be individually handled more easily by the downstream optics, e.g., by the light guides as shown in FIGS. 3,  4  and  5 . 
     FIG. 2B, a section view BB′, shows that by dividing secondary parabolic reflector section  32   b  into segments  32   c  and  32   d , and shifting these segments radially away from one another, an array of separate images ( 38 ,  40 ) are produced on either side of axis L by light cones of angular extent φ, defined by rays R 9  and R 10 , and R 11  and R 12 , respectively. 
     Splitting the secondary reflector section into even smaller segments and shifting each segment either axially or radially enables the production of light cones associated with each image of arbitrarily small angular extent, such that each image can be accommodated easily by “conventional” optics. 
     However, a preferred way of handling the images is to directly couple each image into a fitting light guide. This type of light guide is described in my co-pending U.S. patent application Ser. No. 10/161,798, filed Jun. 4, 2002 entitled “Loss-less etendue preserving Light guide including bends”, the disclosure of which is incorporated herein by reference. 
     One embodiment of a fitting light guide suitable for assembling multiple images is shown in FIG.  3 . The light guide  50  has a pair of input sections  52  and  54 , spaced apart to input arc images  42  and  44  from the secondary parabolic reflector section  32   b  in FIG. 2C, at input faces  52   a  and  54   a , respectively. The light rays from the two arc  42  and  44 , represented by rays R 9  and R 8 , respectively, are guided by sidewalls ( 52   b ,  52   c ) and ( 54   b ,  54   c ) of input guide sections  52  and  54  to output faces  52   d  and  54   d , respectively, whence they enter coupling sections  56  and  58  through input faces  56   a  and  58   a , respectively. The rays are guided by internal reflection from faces  56   b ,  56   c ,  58   b ,  58   c  to output faces  56   c  and  58   c , respectively. The angle of incidence of the rays on faces  56   c  and  58   c  determines whether the rays are internally reflected or outputted to main guide section  60 . Thence, the rays are inputted to main guide section via stepped input faces  60   a  and  60   b , and guided by sidewalls  60   c  and  60   d  to common output face  60   e.    
     In addition to loss-less, etendue-preserving guiding of light, the light guide also achieves aperture shaping, by concatenating the images  42  and  44 , and conforming them to the cross-sectional shape of the main guide section  60 , which corresponds to the shape of output face  60   e , shown in the end view of FIG.  3 B. This cross-sectional shape is an elongated rectangle, for instance to match the stripe geometry in scrolling color projection. 
     Another embodiment of a fitting light guide, which is suitable for assembling multiple off-normal arc images is shown in FIG.  4 . Off-normal arc images are formed by light cones whose axes are neither perpendicular to the L (arc/parabola) axis nor lying in the axial plane (plane of the page) and are formed by radial segments. Examples of off-normal arc images are arc images  38  and  40  in FIG. 2B, whose associated light cones have axes E and E′, respectively. FIGS. 4A,  4 B and  4 C are side elevation views of a compound light guide  70  (FIG. 4A) composed of a pair of light guides  72  (FIG. 4B) and  74  (FIG. 4C) sandwiched together. Light guide  72  includes input section  76  having a fitting input face  76   a  for inputting arc image  38 , an internally reflecting face  76   b  and output face  76   c . Joined to input section  76  is light guide body section  78 , having input face  78   a , sidewalls  78   b  and  78   c , and an output face, not shown. Light guide  74  includes compound input section  80 , having segments  82 ,  84  and  86 . Input segments  82  and  84  each have an input face ( 82   a ,  84   a ) which together form a fitting input face ( 82   a ,  84   a ) for inputting arc image  40 . Each segment ( 82 ,  84 ,  86 ) also has one or more internally reflecting faces ( 82   b ,  86   b  and  84   a ,  84   d , depending on the angle of incidence) and a common output face ( 84   d ,  86   c ). Joined to input section  80  is light guide body section  88 , having input face  88   a , sidewalls  88   b  and  88   c , and an output face, not shown. 
     FIG. 4D is a top view G—G′ of the compound light guide  70  showing the arc images  38  and  40  incident on the fitting input faces  76   a  and ( 82   a ,  84   a ), which are in turn in contact with body sections  78  and  88 , respectively. 
     Off-normal arc images which are not aligned can be aligned to a preferred common orientation by passing them around a bend or fold in a loss-less light guide. This is accomplished inherently in the compound light guide  70  of FIG.  4 . FIG. 5 shows another embodiment of a light guide  90  with a bend or fold, enabling loss-less transport of light “around the corner”. Output surface  92   d  of straight light guide  92  and input surface  96   a  of straight light guide  96 , having longitudinal axes Y and Z, respectively, are coupled with coupling element  94 , having an input surface  94   a  and internally reflecting surfaces  94   b  and  94   c . Output surface  94   c  of coupling element  94  is either internally reflecting or transparent to the guided light, depending on the angle of incidence, as demonstrated by the path of light ray R 15 . 
     Such a light guide with a fold is particularly useful in the present invention, in that off-normal light from a radial segment (represented by rays R 10  and R 11 ) can be re-aligned to a preferred common orientation. 
     The light collection system of the invention is useful in any application where efficient collection and/or beam shaping of an elongated arc light source is needed, particularly projection display systems. The light collection system of the invention is especially useful in single panel color projection display systems. 
     A single panel scrolling color projection display system is characterized by a single light modulator panel such as a liquid crystal display (LCD) panel having a raster of individual picture elements or pixels, which panel is illuminated by horizontally elongated red, green and blue illumination bars or stripes. The stripes are continuously scrolled vertically across the panel while synchronously addressing the rows of pixels with display information corresponding to the color of the then incident stripe. See, for example, U.S. Pat. No. 5,410,370, “Single panel color projection video display improved scanning” issued to P. Janssen on Mar. 25, 1994, and U.S. Pat. No. 5,416,514, “Single panel color projection video display having control circuitry for synchronizing the color illumination system with reading/writing of the light valve” issued to P. Janssen et al. on May 16, 1995, the entire disclosures of which are hereby incorporated herein by reference. 
     FIG. 6 is a schematic illustration of a beam splitting and scrolling engine  600  used in such a single panel scrolling color display system. White light from source S is split into a Blue component and a Green/Red component G/R by dichroic element  2 . The Blue component is directed by lens  603  and mirror  604  to prism scanner  605 . The G/R component is passed by dichroic element  602  through lens  606  to dichroic element  607 , which splits the G/R component into a Green component and a Red component. The Green component is reflected by element  607  to prism scanner  608 , while the Red component is passed through dichroic element  607  to prism scanner  609 . The scanned Red, Green, Blue components are then directed to recombination dichroic elements  610  and  611  by mirror  612  and relay lenses  613  through  617 . 
     FIG. 7 is a block diagram of a single panel color projection display system  700  incorporating a light collection system of the invention. Light engine  710  includes light collection system  720 , which a light collecting parabolic reflector such as that shown in FIG. 2, and a light guide of the type shown in FIGS. 3-5. Light collection system  720  provides an illumination beam of stripe-shaped cross-section to beam splitting and scrolling engine  730 . Engine  730  generates sequentially scrolling red, green and blue stripes, for sequentially scrolling across the surface of light valve panel  740 , which modulates the scrolling light beams synchronously with the input of display information from electrical signal input source  750 . Projection lens  760  projects the modulated light onto a display surface, not shown. 
     The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims.