Abstract:
An image projection system achieves improved image brightness and optical efficiency by redirecting some of the unused polychromatic light emitted by a primary light source and reflected by a spatially nonuniform light filter back into the lamp assembly housing the light source. The unused portions of the polychromatic light are re-reflected for transmission through a different spatial region of the light filter, resulting in an approximately 30% increase in probability of transmission. Because recirculation of unused light occurs within the lamp assembly, there is no significant reduction in etendue. In a first preferred embodiment, an interference light filter reflects certain colors of light while transmitting other colors of light. In a second preferred embodiment, a polarizing light filter passes light in certain polarization states while reflecting light in other polarization states.

Description:
TECHNICAL FIELD 
     This invention relates to image projection systems and more particularly to a method for improving the brightness of an image produced by and increasing the optical efficiency of an image projection system. 
     BACKGROUND OF THE INVENTION 
     Image projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. 
     The following description is presented with reference to a color image projection system implemented with a color wheel but is applicable to other field sequential image projection systems. Color image projection systems operate on the principle that color images are produced from the three primary light colors: red (“R”), green (“G”), and blue (“B”). With reference to  FIG. 1 , a prior art image projection system  100  includes a primary light source  102  positioned at the focus of a light reflector  104  and emitting light having multiple wavelength bands that propagate in a direction away from light source  102  along a beam propagation path  106  through an optical integrating device  108 , of either a solid or hollow type, to create at its exit end a uniform illumination pattern. The uniform illumination pattern is incident on a rotating color wheel  110 . An exemplary color wheel  110  includes three regions, each tinted in a different one of primary colors R, G, and B. Light exiting color wheel  110  is imaged by a lens element system  112 , reflected off a light reflecting (or transmitting) imaging device  114 , and transmitted through a projection lens  116  to form an image. Popular commercially available image projection systems of a type described above include the LP300 series manufactured by InFocus Corporation, of Wilsonville, Oreg., the assignee of this application. 
     There has been significant effort devoted to developing image projection systems that produce bright, high-quality color images. However, the optical performance of conventional image projection systems is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projection systems in a well-lighted room. 
     Loss of image brightness can, in part, be attributed to the fact that typical image projection systems can utilize only portions of the light beam that are of a specified polarization state or of the color that corresponds to the region of the color wheel aligned with the primary light path at the time of incidence of the light beam on the color wheel. Portions of the light beam that do not correspond to the region of the color wheel aligned with the primary light path at the time of incidence are discarded from the image projection system. As a result, about 60% of the polychromatic light emitted by the primary light source is wasted because it does not pass through the color wheel. This 60% loss of light translates to a significant decrease in image brightness. 
     One attempt to increase image brightness involved recirculating polychromatic light in the optical integrating device, which was typically a light tunnel  108   a , while implementing a spiral color wheel having three color regions simultaneously aligned with the primary light path. With reference to  FIG. 2 , a spiral type color wheel  110   a  includes R, G, and B dichroic coatings arranged in a “spiral of Archimedes” pattern defined by the equation R=aθ. Spiral color wheel  110   a  is located adjacent to an exit end  132  of light tunnel  108   a , and the three color regions move at a nearly constant speed in the radial direction. The spiral color wheel  110   a  may also include a white region that can be used to increase luminous efficiency in non-saturated images. With reference to  FIG. 3 , spiral color wheel  110   a  is positioned such that light exiting the exit end of light tunnel  108   a  is simultaneously incident upon all of the color-selective regions of spiral color wheel  110   a . Further, light tunnel  108   a  includes an entrance end  130  having an entrance aperture through which light emitted by light source  102  propagates. An inner wall  118  of entrance end  130  includes a highly reflective mirror that reflects light that is incident on and reflected by spiral color wheel  110   a . Thus light is recirculated in light tunnel  108   a . While highly reflective inner wall  118  facilitates light recirculation, the image projection system suffers a 60% reduction of input etendue due to the requirement that approximately 60% of the area of inner wall  118  of entrance end  130  is covered such that approximately 60% of the light emitted by light source  102  does not enter light tunnel  108   a . In image projection systems implemented with all but the shortest arc lamps, the efficiency loss due to the etendue reduction is greater than the efficiency increase due to light recirculation within light tunnel  108   a . High brightness projectors require high-power arc lamps which have arc gaps too large for this prior art method of light recirculation to be of significant value. Further, this attempt did not work with more distributed light sources such as electrodeless microwave discharge lamps. 
     What is needed, therefore, is an image projection system that exhibits increased optical efficiency and that is implemented with an improved technique for achieving increased image brightness without a significant reduction in etendue. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is, therefore, to provide an apparatus and a method for improving the brightness of an image projected by, and the optical efficiency of, an image projection system. 
     The present invention achieves improved image brightness and optical efficiency by introducing into the image projection system a spatially nonuniform light filter that has multiple spatial regions which transmit light characterized by different sets of optical properties. Each spatial region reflects as unused light components of light characterized by a different set of optical properties and thereby redirects portions of the unused light emitted by the primary light source back into a lamp assembly. The unused light portions may again propagate from the lamp assembly and be transmitted through regions of the light filter characterized by the same set of optical properties, thereby increasing the optical efficiency of the image projection system. Specifically, the light filter reflects the unused portions of light back into the lamp assembly, where the unused portions of light are re-reflected onto optically selective spatial regions of the light filter, resulting in an approximately 30% increase in probability of light transmission. Because recirculation of unused light occurs within the lamp assembly, there is no significant reduction in etendue. 
     In a first preferred embodiment, the spatially nonuniform light filter is of an interference filter type that reflects certain colors of light while transmitting other colors of light. A preferred interference filter is a spiral color wheel having more than two color selective regions. 
     In a second preferred embodiment, the spatially nonuniform light filter is of a polarizing filter type having optically selective regions that pass light in certain polarization states while reflecting light in other polarization states. An exemplary polarizing filter contains a pattern of grids that are orthogonally arranged to create perpendicularly related polarization directions. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric pictorial view of a prior art color image projection system. 
         FIG. 2  is a schematic view of the surface of a prior art spiral color wheel. 
         FIG. 3  is a schematic view of the spiral color wheel of  FIG. 2  (with the center section cut away) positioned adjacent to a light tunnel. 
         FIGS. 4   a  and  4   b  are schematic side elevation views of alternative implementations of a first embodiment of the image projection system of the present invention. 
         FIGS. 5   a  and  5   b  are schematic side elevation views of alternative implementations of the image projection systems of the present invention. 
         FIG. 6  is a schematic isometric view of a second embodiment of the image projection system of the present invention. 
         FIG. 7  is a schematic perspective view of an exemplary light filter that may be implemented in the image projection system of FIG.  6 . 
         FIG. 8  is a schematic fragmentary side elevation view of an alternative implementation of the image projection system of FIG.  6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 4   a , a lamp assembly  120  includes primary light source  102 , which emits polychromatic light that reflects off an inner surface  122  of light reflector  104  and propagates in a direction away from light source  102  along beam propagation path  106 . Primary light source  102  is preferably a high-brightness, high-efficiency lamp system having a long-life burner. An electrodeless microwave discharge lamp is preferred because the absence of electrodes eliminates the possibility of collision of redirected light emissions with lamp electrodes. Such collisions would decrease the optical efficiency of the image projection system. Additionally, an electrodeless microwave discharge lamp does not require the use of a separate optical integrating device  108 , such as a light tunnel  108   a , to generate an illumination patch. Other exemplary primary light sources include high-pressure mercury arc lamps and standard arc lamps. A light tunnel  108   a  is preferably included in an image projection system that is implemented with an arc lamp. 
     Light reflector  104  focuses polychromatic light (indicated by light rays  124 ) emitted by primary light source  102  onto either a spatially nonuniform light filter  126 , as shown in  FIG. 5   b , or onto an entrance end  130  of light tunnel  108   a , which directs the polychromatic light onto spatially nonuniform light filter  126 . Spatially nonuniform light filter  126  is preferably one of two types of light reflecting filters, which are described below with reference to  FIGS. 5-7 . Light reflector  104  is preferably an annular reflector of hollow shape positioned about and spaced from primary light source  102 . Depending on the design goals and the details of the downstream optics, light reflector  104  may be of any suitable shape including ellipsoidal, paraboloidal, spherical, generally aspheric, or faceted form. Inner surface  122  of light reflector  104  reflects and redirects light (indicated by light rays  128 ) reflected by light filter  126 . Inner surface  122  is preferably of uniform smoothness. Other characteristics such as size, length, focal length, and thermal properties are determined by the design goals of the image projection system. 
     With reference to  FIG. 5   a , an alternative implementation lamp assembly  120   b  includes a sulfur bulb  102   b  that is surrounded by a bulb fill  134 . Bulb fill  134  may be any of a variety of bulb fills including a minimally reflective single element fill or a conventional mercury or metal halide fill. The fill preferably operates at a low pressure. Bulb  102   b  is also surrounded by a reflective jacket  150 , preferably made of ceramic, having an entrance aperture through which light emitted by sulfur bulb  102   b  propagates into light reflector  104   b . Light reflected by light filter  126  undergoes multiple reflections off inner walls  122   b  such that the reflected light is again incident on light filter  126 . An exemplary commercially available lamp assembly is the Bytelight™ manufactured by Fusion Lighting. Lamp assembly  120   b  has various advantages over other lamp assemblies. One advantage is increased lamp life, which can be greater than 20,000 hours. Another advantage is highly consistent, high brightness output, as great as 1500-7000 lumens over the course of the bulb&#39;s life. A final advantage is increased light uniformity as compared to prior art discharge lamps, which typically have a localized bright spot. 
     As shown in  FIG. 5   b , an alternative implementation lamp assembly  120   c  includes an electrodeless light source  102  positioned in light reflector  104 . 
     As shown in  FIG. 4   a , an image projection system of the present invention may include an optical integrating device  108   a  positioned between lamp assembly  120  and light filter  126 . A preferred optical integrating device  108  is a light tunnel  108   a , preferably a solid or hollow glass rod whose interior surfaces have been coated with a highly reflective dielectric coating. Also, the glass rod preferably includes an entrance aperture that can be adjusted to maximize the efficiency of the image projection system. Polychromatic light emitted by primary light source  102  reflects off of light reflector  104  and converges to a focus at an entrance end  130  of light tunnel  108   a . The polychromatic light propagating through light tunnel  108   a  undergoes multiple reflections off of its walls so that the light emitted at an exit end  132  of light tunnel  108   a  is of uniform intensity. 
     An alternative implementation of optical integrating device  108  is the trapezoidal-shaped light tunnel  108   b  shown in  FIG. 4   b . The entrance end  130  of light tunnel  108   b  preferably corresponds to the size of the light spot emitted by light source  102 , which is dictated by the type of light source implemented in the image projection system. Light tunnel  108 b maximizes the amount of light that is recirculated while reducing the likelihood that the recirculated light will be incident on the electrodes contained in the light source. 
     With reference to  FIG. 5   b , the image projection system of the present invention also includes a spatially nonuniform light filter  126  that has multiple regions that transmit light characterized by different sets of optical properties. Each of the two preferred embodiments of light filter  126  has optically selective spatial regions  142  and  144  that transmit light beam portions characterized by different ones of two sets of optical properties and reflect light beam portions characterized by the set of optical properties of the light transmitted by the other spatial region. Light filter  126  is positioned to direct the light beam portions reflected by spatial regions  142  and  144  in directions generally opposite to the direction of propagation along beam propagation path  106 . Light filter  126  and light reflector  104  are positioned in optical association with each other such that at least some of the light beam portions reflected by spatial regions  142  and  144  reflect off of inner surface  122  of light reflector  104  and propagate through the one of spatial regions  142  and  144  other than that which reflected the light beam portions. 
       FIGS. 4   a ,  4   b ,  5   a , and  5   b  are schematic views of the alignment of lamp assembly  120  and a color wheel type light filter implemented in four exemplary image projection systems of the present invention. The image projection system shown in  FIG. 4   a  includes light tunnel  108   a . The image projection system shown in  FIG. 4   b  includes light tunnel  108   b.    FIGS. 5   a  and  5   b  show image projection systems without a light tunnel. In  FIGS. 4   a ,  4   b ,  5   a , and  5   b , light filter  126  is positioned transversely of beam propagation path  106  to receive light reflected by inner surface  122  of light reflector  104 . 
     In a first preferred embodiment, light filter  126  is a spatially nonuniform color wheel of an interference filter type that is wavelength selective such that the color wheel transmits light of certain wavelengths and reflects light of other wavelengths back into lamp assembly  120 . Thus, the color wheel reflects certain colors of light and transmits other colors of light. The color wheel is preferably positioned very close to exit end  132  of optical integrating device  108  or light reflector  104 . The gap between the two components is preferably sufficiently small to prevent undesirable light “leakage” that can occur around the perimeter of the interface between the color wheel and exit end  132  or between the color wheel and light reflector  104 . When polychromatic light reaches the color wheel, light of a given color propagates through the one of spatial regions  142  and  144  that is covered by a transmissive coating of the corresponding color and reflects off the other one of spatial regions  142  and  144 . For example, in an image projection system having a spiral color wheel light filter, red light is transmitted through the spatial region of the spiral color wheel covered by the red dichroic coating while all other colors of light are reflected back into lamp assembly  120 . The reflected light reflects off of inner surface  122  of light reflector  104  and is thereby directed in the direction of beam propagation path  106  onto one of spatial regions  142  and  144  of the color wheel. A portion of the reflected light may be incident on a corresponding spatial region of the color wheel resulting in transmission of that portion of the reflected light through the image projection system. For example, reflected blue light will be transmitted by the spatial region of the color wheel covered by a blue dichroic coating. This effect occurs continuously with light of all three colors. This process is repeated several times until all the light emitted by primary light source  102  is transmitted, absorbed, or scattered by or through the color wheel. In an alternative implementation of an image projection system of the present invention as shown in  FIG. 5   b , light filter  126  has three optically selective spatial regions  142 ,  144 , and  146  but operates in a manner analogous to that described above. 
     A preferred interference type light filter is a spiral (or scrolling) color wheel having R, G, and B color regions. The spiral color wheel may also include a white (“W”) region, whose presence increases the luminous efficiency of non-saturated images. Use of the spiral color wheel has three advantages: (1) all colors are simultaneously present in the illumination area so less light is wasted as compared to a conventional field sequential image projection system; (2) there is a reduction in the occurrence of “color separation artifacts” caused by quick eye movements or a fast changing screen; and (3) small spiral color wheels are commercially available and thereby enable the design of a more compact image projection system. An exemplary commercially available spiral color wheel is manufactured by Unaxis. Other exemplary interference type light filters include rotating color drums, dichroic filters, and color filters with two or more color bands. 
     In a second preferred embodiment of the present invention, shown in  FIG. 6 , light filter  126  is of a light polarizing filter type having regions that transmit light in certain polarization states and reflect light in other polarization states. Portions of light in a polarization state that differs from that transmitted by the one of spatial regions  142  and  144  on which the light is incident are reflected into lamp assembly  120 , where they reflect off light reflector  104  and are redirected to light filter  126 . Light filter  126  is of a reflective wire-grid polarizer type having a pattern of grids orthogonally arranged to create orthogonally aligned polarization directions. An exemplary commercially available linear polarizing filter is the High Transmission Proflux polarizer (Part No. PPLD2C manufactured by Moxtek), a diagram of which is shown in FIG.  7 . Spatial regions  142  and  144  shown in  FIG. 6  indicate, respectively, horizontal and vertical polarization directions. Light filter  126  can, in cooperation with other optical components, operate with light in other polarization states, including circular or elliptical. 
     Light filter  126  is preferably positioned very close to the exit end of optical integrating device  108  (if present) or light reflector  104 . Alternatively, light filter  126  may be positioned within lamp assembly  120 , as shown in FIG.  8 . 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. For example, multiple light filters may be implemented as necessary to maximize the optical goals of the image projection system. The scope of the present invention should, therefore, be determined only by the following claims.