Abstract:
A pseudo light pipe comprises an input end, an output end and a light transmission medium. The input end collects rays of light from a light source. The output end outputs and collimates the rays of the light collected at the input end. The output end has a convex curvature. The light transmission medium interconnects the input and output end, and transmits the rays of the light from the input end to the output end. The convex curvature of the output end is selected to output parallel rays of light. A projection system incorporating the pseudo light pipe and a dual paraboloid reflector (DPR) system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/191,034 filed Sep. 5, 2008, and U.S. Provisional Application Ser. No. 61/233,165 filed Aug. 12, 2009, each of which is incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD OF INVENTION 
       [0002]    This invention relates to a light pipe, more particularly to a pseudo light pipe that operates and functions as a tapered light pipe but is easier to mount and manufacture than a tapered light pipe. The convex curvature of the output end of the pseudo light pipe is selected to provide output with certain divergence and in particular, to provide output parallel rays of light. 
       BACKGROUND OF THE INVENTION 
       [0003]    Tapered light pipe (TLP) is used in many applications to transform a source of light from one area/angle combination to another with substantially the same brightness. The taper angle and length is designed such that there will be minimum loss of brightness. In practical applications, the length is shorter than desired. In such case, the input and output surface are made concave and convex respectively, such that the tapered light pipe appears to be straight to the input and output light. The manufacturing and mounting of the TLP are generally tedious and expensive. Accordingly, the claimed invention proceeds upon the desirability of providing a TLP with a lower cost of fabrication and mounting. 
       SUMMARY OF THE INVENTION 
       [0004]    Therefore, it is an object of the claimed invention to provide a pseudo light pipe that solves the aforesaid problems with the TLP. 
         [0005]    In accordance with an exemplary embodiment of the claimed invention, a pseudo light pipe comprises an input end, an output end and a light transmission medium. The input end collects rays of light from a light source. The input end generally comprises a flat surface. Alternatively, a portion of the input end can have a concave curvature. The output end outputs and collimates the rays of the light collected at the input end. The output end has a convex curvature. Preferably, the curvature of the output end is selected to minimize the etendue mismatch between the input end and the output end. The light transmission medium interconnects the input and output end, and transmits the rays of the light from the input end to the output end. The convex curvature of the output end is selected to output parallel rays of light. Preferably, the surface of the input and output ends of the pseudo light pipe is coated with anti-reflective coating. In accordance with an aspect of the claimed invention, the pseudo light pipe further comprises a mounting surface for mounting the pseudo light pipe. 
         [0006]    In accordance with an exemplary embodiment of the claimed invention, a projection system comprises a projection engine, a light source and a pseudo light pipe. The light source comprises a lamp, a dual paraboloid reflector (DPR) and a retro-reflector, which collects and re-directs the stray rays of light to the DPR. The pseudo light pipe comprises an input end, an output end and a light transmission medium. The input end collects rays of light from a light source. The output end outputs and collimates the rays of the light collected at the input end. The output end has a convex curvature. The light transmission medium interconnects the input and output end, and transmits the rays of the light from the input end to the output end. The convex curvature of the output end is selected to output parallel rays of light. The projection system optionally comprises a fly eye lens and a polarization conversion system between the output end of the pseudo light pipe and the projection engine. Preferably, the projection engine is a liquid crystal display (LCD) or liquid crystal on silicon (LCOS) projection engine. 
         [0007]    In accordance with an exemplary embodiment of the claimed invention, the pseudo light pipe can be used any one of the following light source: a LED, a microwave lamp, an ultra-high pressure mercury lamp, a microwave driven electrodeless lamp, metal halide lamp, fluorescent lamp, and halogen lamp. The light source can combine the lamp with one of the following: a dual paraboloid reflector (DPR), a DPR with a retro-reflector, an elliptical reflector, a parabolic reflector with focusing lens or a dual ellipsoidal reflector (DER) system. The retro-reflector collects and redirects the stray rays of light to the DPR. 
         [0008]    In accordance with an exemplary embodiment of the claimed invention, the light source is positioned near the input end and at a focal point of the output end. 
         [0009]    In accordance with an exemplary embodiment of the claimed invention, the light transmission medium has a round, rectangular or polygonal cross-sectional area. The light transmission medium is made from at least one of the following material: glass, fused silica, plastic, and quartz. 
         [0010]    In accordance with an exemplary embodiment of the claimed invention, the convex curvature of the output end is one of the following conical shape: parabolic, hyperbolic, or spherical. In general, the convex curvature can be numerically generated surface. Preferably, the convex curvature of the output end is an ellipse. 
         [0011]    In accordance with an exemplary embodiment of the claimed invention, the light transmission medium comprises a plurality of sections. Each section of said light transmission medium is made from one of the following material: glass, fused silica, plastic and quartz. Preferably, a section comprising the input end is made from high temperature material and a section comprising said output end is molded with low temperature glass or plastic. In accordance with an aspect of the claimed invention, the light transmission medium comprises an air gap between each section of said light transmission medium. In accordance with an aspect of the claimed invention, each section of said light transmission medium is made from a different material. The light transmission medium can comprise an input section of air and output section made from one of the following material: glass, fused silica, plastic and quartz. 
         [0012]    In accordance with an exemplary embodiment of the claimed invention, the curvature of the output end is astigmatic such that the output curvature is different in the two perpendicular directions. 
         [0013]    Various other objects, advantages and features of the claimed invention will become readily apparent from the ensuing detailed description, and the novel features will be particularly pointed out in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The following detailed description, given by way of example, and not intended to limit the claimed invention solely thereto, will best be understood in conjunction with the accompanying drawings in which like components or features in the various figures are represented by like reference numbers: 
           [0015]      FIG. 1  shows a cross-sectional view of a dual paraboloid reflector system with a tapered light pipe; 
           [0016]      FIG. 2  shows a cross-sectional view of a tapered light pipe; 
           [0017]      FIG. 3  shows a cross-sectional view of a pseudo light pipe (PLP) in accordance with an exemplary embodiment of the claimed invention; 
           [0018]      FIG. 4  shows a perspective view of the PLP in accordance with an exemplary embodiment of the claimed invention; 
           [0019]      FIG. 5  shows a perspective view of the PLP with a rectangular cross-section in accordance with an exemplary embodiment of the claimed invention; 
           [0020]      FIG. 6  shows a cross-sectional view of the PLP with a concaved input end in accordance with an exemplary embodiment of the claimed invention; 
           [0021]      FIG. 7  shows a cross-sectional view of the PLP fabricated with a combination of material in accordance with an exemplary embodiment of the claimed invention; 
           [0022]      FIG. 8  shows cross-sectional view of the PLP with a light source of dimension d in accordance with an exemplary embodiment of the claimed invention; 
           [0023]      FIG. 9  shows a cross-sectional view of a projection system incorporating the PLP based DPR in accordance with an exemplary embodiment of the claimed invention; 
           [0024]      FIG. 10  shows a cross-sectional view of the PLP with a portion of the output end coated with a reflective coating in accordance with an exemplary embodiment of the claimed invention; 
           [0025]    FIGS.  11 (A)-(C) show cross-sectional views of the output end of the PLP comprising a retro-reflective portion in accordance with an exemplary embodiment of the claimed invention; and 
           [0026]      FIG. 12  is a perspective view of an astigmatic PLP in accordance with an exemplary embodiment of the claimed invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0027]    With reference to the figures, exemplary embodiments of the claimed invention are now described. These embodiments illustrate principles of the invention and should not be construed as limiting the scope of the invention. 
         [0028]      FIG. 1  shows a dual Paraboloid reflector (DPR) system  1000  used in conjunction with a tapered light pipe (TLP)  1100  showing that the small area, large angle Θ i  light incidence at the input of the TLP  1100  is transformed to a larger area, smaller angle at the output. The DPR system  1000  comprises a DPR  1200 , a lamp  1300 , and a retro-reflector  1400 . The arc images onto the input end or surface  1110  of the TLP  1100  using the DPR  1200 , which preserve the brightness of the arc. The size of the TLP  1100  is designed based on the etendue of the DPR system  1000 , which determines the input and output dimensions of the TLP  1100 . The length of the TLP  1100  is generally determined by mechanical limitations and shorter TLPs  1100  are generally preferred. 
         [0029]    As the TLPs  1100  get shorter, the transformation become non-ideal and the output has a slightly larger etendue than at the input. To overcome this etendue mismatch between the output and input, a TLP  1100  with the output end or surface  1120  having a convex surface (as shown in  FIG. 2 ) instead of a flat surface (as shown in  FIG. 1 ) can be utilized. That is, the convexity or the curvature of the output surface  1120  is selected such that the output etendue of the TLP  1100  matches or near the input etendue of the TLP  1100 . When light enters into the tapered light pipe (TLP)  1100 , the light bounces multiple times off the sidewalls  1130  of the TLP  1100  depending on the angle of incidence Θ i  of the light, the taper angle of the TLP  1100 , and the length of the TLP  1100 . As the length of the TLP  1100  decreases (i.e., a short TLP  1100 ), the curvature of the output surface  1120  of the TLP  1100  needs to increase to make the necessary correction to the input/output etendue mismatch. It is appreciated that if the curvature of the convex output surface  1120  increases too much, it will become non-spherical, e.g., elliptical, and additional calculation will be required to achieve optimum performance. 
         [0030]    Also,  FIG. 2  shows the extreme input angle such that the ray inside the TLP  1100  has a critical angle Θ c , and hits the sidewall of the TLP  1100 . The TLP  1100  comprises an input end  1110  and an output end  1120 . Preferably, the output end  1120  of the TLP  1100  is a convex surface. The multiple bounces or reflections off the sidewalls  1130  of the TLP  1100  operate to mix the light to provide a light output intensity that is uniform in profile. That is, the TLP  1100  functions as a light mixing device. 
         [0031]    Another effect of having a short TLP  1100  is that when the angle Θ is larger than the critical angle Θ c , the incident light will not be reflected by the sidewalls  1130  of the TLP  1100 . In such a case, the TLP  1100  acts as a thick lens more than a tapered light pipe as the incident light exits without any reflection off the sidewall  1130 . In accordance with an exemplary embodiment of the claimed invention, the curvature of the output end  1120  of the TLP  1100  is calculated and determined such that the nominal ray from the center of the input end  1110  will be parallel at the output end  1120 . Since the sidewall  1130  of the TLP  1100  is not used, the TLP  1   100  can be made simply as a straight rectangular or cylindrical rod. 
         [0032]    In accordance with an exemplary embodiment of the present invention, a pseudo light pipe (PLP) or virtual tapered light pipe  2000  is shown in  FIGS. 3 and 4  where the sidewalls exist conceptually, but are not functional and not needed. The PLP  2000  comprises an input end or surface  2200  and output end or surface  2300 . Preferably, the output end  2300  of the PLP  2000  is a convex surface where the curvature is calculated and determined to optimize performance. The virtual sidewall  2100  is at an angle theta (Θ) with the direction of the PLP  2000  such that the virtual sidewall angle θ is adjusted to match with the maximum incident angle Θ i . For example, if the maximum input or incident angle Θ i  is 90 degrees, which is the glazing angle, then the virtual sidewall angle Θ will become the critical angle Θ c . If the virtual sidewall angle Θ is at the critical angle Θ c , the extreme rays of light (input rays of light with an incident angle Θ i  near or at the critical angle Θ c ) from the light source  1300  will propagate along the virtual sidewall  2100 , but will not be incident on the virtual sidewall  2100 . As a result, the sidewall of the PLP  2000  becomes a virtual sidewall without any actual functions. 
         [0033]    To facilitate the fabrication of the PLP  2000 , in accordance with an exemplary embodiment of the claimed invention, actual boundary or extra surfaces  2400  are added to the PLP  2000 . The actual boundary or extra surfaces  2400  also serve no functional purpose, but facilitate mechanical mounting of the PLP  2000  into systems, such as projection and illumination systems. In accordance with an exemplary embodiment of the claimed invention, an outer boundary or shape of the PLP  2000  is shown in  FIG. 4 . The outer boundary of the PLP  2000  comprises one or more mounting surfaces  2500 , an output end or surface  2300 , and an input end or surface  2200 . It is appreciated that the cross-section of the PLP  2000  can be round, rectangular; polygonal and the like depending on the application of the PLP  2000 . Accordingly, the mounting surface  2500  of the PLP  2000  is essentially equivalent to the sidewalls  1130  of the TLP  1100 . 
         [0034]    In accordance with an exemplary embodiment of claimed invention, the PLP  2000  can be used with various light sources  1300  including but not limited to LED, microwave lamp, ultra-high pressure mercury lamp, microwave driven electrode-less lamp, metal halide lamp, fluorescent lamp, halogen lamp, or other comparable lamps. The light source  1300  can be placed at the focus of light source with reflectors, e.g., a dual paraboloid reflector (DPR), elliptical, parabolic with focusing lens, or a dual elliptical reflector (DER). In accordance with an aspect of the claimed invention, the PLP  2000  can be rotationally symmetric as a round device, non-symmetric in the two directions giving astigmatic output convex surface, or can be linear with a circular or elliptical cross-section for linear lamp applications. 
         [0035]    In accordance with an exemplary embodiment of the claimed invention, the cross-section of the PLP  2000  is rectangular and the output end  2300  is a convex surface. That is, as shown in  FIG. 5 , the input end  2200  of the PLP is rectangular in shape. Additionally, the PLP  2000  can comprise an optional mask  2600  at the input end  2200  for filtering input rays of light such that extreme rays of light (input rays of light with an incident angle Θ i  near or at the critical angle Θ c ) is at a desired angle for hitting the output end or surface  2300  of the PLP  2000 . The optical mask has the effect of limiting the etendue of the system such that not the whole light source is used. This is similar to the input aperture of the standard tapered light pipe, in which the light pipe is designed for a specific etendue. When the mask is not used and the output consist the full etendue of the light source and is made available for the application. As a result, the etendue of the system is limited by the subsequent components, e.g., relay lens, imaging panel, projections lens, or the aperture. 
         [0036]    In accordance with an exemplary embodiment of the claimed invention, the curvature of the output end  2300  of the PLP  2000  is an ellipse for collimating the rays of light. Alternatively, the curvature of the output end  2300  of the PLP  2000  can be different shape to provide different level of collimation, such as a conic shape including but not limited to parabolic, hyperbolic, and spherical. 
         [0037]    Turning now to  FIG. 6 , in accordance with an exemplary embodiment of the claimed invention, there is illustrated a PLP  2000  with an input end  2200 , which is concaved. It is appreciated that the concaved input end  2200  can provide a better coupling or a better match with the system incorporating the PLP  2000 . However, in certain applications, the additional performance improvement may not justify the additional cost of fabricating the PLP  2000  with the concaved input end  2200 . 
         [0038]    The PLP  2000  can be made from plastic, glass, fused silica, quartz and the like depending on the power density requirements of the system incorporating the PLP  2000 . In accordance with the exemplary embodiment of the claimed invention, as shown in  FIG. 7 , the PLP  2000  can also fabricated from multiple sections such that the section comprising the input end  2200  of the PLP  2000  can be made with high temperature material and attached to the curved section of the output end  2300 , which can be molded with low temperature glass or plastic. In accordance with an aspect of the claimed invention, each section of the PLP  2000  can be separated by an air gap. That is, the PLP  2000  can be fabricated from a combination of these materials (e.g., plastic, glass, fused silica, quartz and the like) such that higher melting temperature materials can be placed on the higher intensity side. For example, the PLP  2000  can be fabricated from a glass/plastic combination where section A comprising the input end  2200  is made of glass and section B comprising the output end  2300  is made of plastic. Section A is close to the focus of the light source and receives high power density. The light beam spreads along its path within the PLP  2000  and towards section B made of plastic. In accordance with an aspect of the claimed invention, Section A can be fabricated from quartz for very high power density applications and Section B can be fabricated from glass or plastic. Various other combinations of materials can be also used in fabricating the PLP  2000 , such as a lens for Section B and a transparent material for Section A which can be air, same or different from lens in Section B. In general, there can be more than 2 layers of different materials. 
         [0039]    In accordance with an exemplary embodiment of the claimed invention, various surfaces of the PLP  2000  can be coated with a single or multiple layers of anti-reflective material. 
         [0040]    Since the actual boundary surfaces  2400  of the PLP  2000  is not used optically, as exemplary shown in  FIG. 3 , the boundary surfaces  2400  does not have to be polished. In accordance with an exemplary embodiment of the claimed invention, the boundary surfaces  2400  of the PLP  2000  can be textual for ease of mounting. 
         [0041]    In accordance with an exemplary embodiment of the claimed invention, the curvature of the input and output surface  2200 ,  2300  are optimized by analytical formulas or by ray tracing. Typically, a light source  1300  is not a point source, but has a dimension d, as shown in  FIG. 8 . That is, the light source  1300  generates an input beam with a dimension d. Rays or beam of light from such light source  1300  will subtend an angle φ 1  inside the PLP  2000  and will exit the output end  2300  of the PLP  2000  at an output angle φ 2 . As the size of the PLP  2000  increases, the angle φ 1  will decrease resulting in a smaller output angle φ 2  for the same light source with dimension d. That is, the area of the input surface/end  2200  and the output surface/end  2300  of the PLP  2000  will increase with the size of the PLP  2000 . This results in conservation of etendue or minimizes the input/output etendue mismatch. As a result, a smaller PLP  2000  will have a larger output angle φ 2 , but a smaller output surface area  2300 . A larger PLP  2000  will have a smaller output angle φ 2 , but a larger output surface area  2300 . 
         [0042]    Turning now to  FIG. 9 , there is illustrated an exemplary application of the PLP  2000  in accordance with an exemplary embodiment of the claimed invention. The DPR system  3000  of  FIG. 9  is similar to the DPR system  1000  of  FIG. 1 . Instead of incorporating the TLP  1100 , the DPR system  3000  incorporates the PLP  2000  in accordance with an exemplary embodiment of the claimed invention. The DPR system  3000  can be used with a liquid crystal display (LCD) or liquid crystal on silicon (LCOS) projection engine  4100  to provide an illumination/projection system  4000 . The collimated light output  3100  from the PLP  2000  is inputted into the LCD/LCOS projection engine  4100 . Alternatively, the projection system  4000  comprises an optional fly eye lens  3100  and/or an optional polarization conversion system between the output end  2300  of the PLP  2000  and the input end  4110  of the LCD/LCOS projection engine  4100 . That is, the collimated light output  3100  is incident on an optional fly eye lens  3100  and/or an optional polarization conversion system  3200  before entering the LCD/LCOS projection engine  4100 . It is appreciated that the light source or lamp  1300  can be LED, ultra-high pressure mercury lamp, microwave driven electrode-less lamp, metal halide lamp, or other lamps suitable for use with the DPR system  3000 . 
         [0043]    In accordance with an exemplary embodiment of the claimed invention, the curvature of the output end  2300  of the PLP  2000  can be astigmatic with different curvature in the two perpendicular directions, as exemplary shown in  FIG. 121  The curvature of the output end  2300  in X and Y direction can be same or different to provide an astigmatic PLP  2000 . 
         [0044]    In accordance with an exemplary embodiment of the claimed invention, as shown in  FIG. 10 , the output end  2300  of the PLP  2000  comprises a retro-reflective portion  2310 , preferably a spherical in shape. The retro-reflective portion  2310  of the output end  2300  is coated with a reflective coating or coupled to a reflector to provide retro-reflection. That is, the retro-reflective portion  2310  reflects a portion or part of the light emitted by the light source  1300  back into the light source  1300  to provide recycling of the light via retro-reflection. 
         [0045]    Turning now to  FIG. 11(A) , there is illustrated a perspective view of the output end  2300  of the PLP  2000  with recycling. The output end or surface  2300  of the PLP comprises a collimating surface  2320  for outputting a collimated light and a retro-reflective portion  2310  for reflecting a portion of the emitted light back to the input end  2200  and to the light source  1300 . In accordance with an exemplary embodiment of the claimed invention, as shown in  FIGS. 11(B) and 11(C) , the retro-reflective portion  2310  comprises a plurality of retro-reflective sections  2330 . Each retro-reflective section  2330  comprises a parabolic surface pairs  2340 , such that light incident on a first parabolic surface  2340  collimates onto the second parabolic surface  2340  (as shown in FIG.  11 (C)), and focused back into the light source  1300 . The number and size of the retro-reflective sections  2330  is determined such that all reflections off the parabolic surface pairs  2340  is by total internal reflection, thereby eliminating the need to coat the retro-reflective portion  2310  with a reflective coating. Additionally, this advantageously lowers the cost of manufacturing the claimed PLP  2000 , particularly when the PLP  2000  is fabricated by a molding process. 
         [0046]    The invention, having been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the following claims.