Patent Publication Number: US-2005135761-A1

Title: Optical element for uniform illumination and optical system incorporating same

Description:
FIELD OF THE INVENTION  
      This invention generally relates to illumination devices. The invention is particularly applicable to illumination devices producing uniform illumination.  
     BACKGROUND  
      Projection systems typically include a light source, an active light valve for producing an image, an illumination system for illuminating the light valve, and optics for projecting and displaying the image.  
      It is often desirable to illuminate the light valve uniformly and with good optical efficiency so as to increase brightness, resolution and contrast of a displayed image.  
     SUMMARY OF THE INVENTION  
      Generally, the present invention relates to illumination systems.  
      In one embodiment of the invention, an optical element for homogenizing light includes an optical rod having an input face and an output face, where the input face is not parallel to the output face.  
      In another embodiment of the invention, an optical element includes an input face for receiving light from a light source. The optical element further includes a body for homogenizing and transmitting the light received by the input face. The optical element further includes an output face for delivering the homogenized light. The input face cannot be made to overlay the output face by one or more translational shifts of the input face.  
      In another embodiment of the invention, an optical system includes a light source. The optical system further includes an optical element that receives light from the light source from an input face of the element. The optical element homogenizes the received light and transmits the homogenized light from an output face of the optical element. The input face of the optical element is not parallel to the output face of the optical element. The optical system further includes a light valve having an active area. The active area is disposed to receive the light transmitted from the output face of the optical element. The optical system further includes relay optics for delivering the transmitted light to the active area of the light valve. The relay optics images the output face of the optical element onto the active area of the light valve.  
      In another embodiment of the invention, an optical system includes an optical element that is centered on an optical axis. The optical element homogenizes light. The optical element has an input face and an output face. The optical system further includes an active area of a light valve. The active area makes a non-zero angle with a normal to the optical axis. The optical system further includes relay optics that images the output face of the optical element onto the active area of the light valve. The active area lies in an image plane of the output face imaged by a relay optics. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      The invention may be more completely understood and appreciated in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
       FIG. 1  illustrates a schematic side-view of an optical system in accordance with one embodiment of the invention;  
       FIG. 2  illustrates a schematic three-dimensional view of an optical element in accordance with another embodiment of the invention;  
       FIG. 3  illustrates a schematic three-dimensional view of an optical element in accordance with another embodiment of the invention;  
       FIG. 4  illustrates a schematic three-dimensional view of an optical element in accordance with another embodiment of the invention; and  
       FIG. 5  illustrates selected points of illumination intensity of different illumination systems.  
    
    
     DETAILED DESCRIPTION  
      The present invention generally relates to projection displays. The invention is particularly applicable to projection displays with reflective imagers, and even more particularly to projection displays having a digital micromirror imager (DMD).  
       FIG. 1  illustrates a schematic side-view of an optical system  300  in accordance with one embodiment of the invention. Optical system  300  is an optical illumination system providing uniform and efficient illumination to an image source. Optical system  300  includes a light source  340 , a light focusing element  350 , a light homogenizing optical element  303 , relay optics  360 , and a light valve  370  having an active area  380 .  
      Optical element  303  is a light homogenizing element, homogenizing light received from source  340 , where by homogenizing it is meant that light exiting optical element  303  has a more uniform intensity distribution than light entering optical element  303 . Optical element  303  is centered on an optical axis  301  and includes an input face  320 , an output face  330 , and an optical rod  310 . Examples of known light homogenizers may be found in U.S. Pat. Nos. 5,625,738 and 6,332,688; and U.S. Patent Application Publication Nos. 2002/0114167, 2002/0114573, and 2002/0118946. According to one embodiment of the invention, input face  320  is not parallel to output face  330 , meaning that the plane of input face  320  intersects the plane of output face  330 . In other words, the magnitude of the angle between the plane of input face  320  and the plane of output face  330  is greater than zero. Input face  320  is normal to optical axis  301 , that is, the plane of input face  320  makes a 90° angle with optical axis  301 . Output face  330  makes an angle α with a plane normal to optical axis  301 , that is, the plane of output face  330  makes an angle α with a plane normal to optical axis  301 . Active area  380  of light valve  370  makes an angle δ with a plane normal to the optical axis, where the magnitude of angle δ is greater than zero. Angle δ may or may not be equal to angle α.  
      Light focusing element  350  collects light from light source  340  and transmits the collected light to input face  320  of optical element  303 . Optical element  303  receives light from the light source from its input face  320 . Optical rod  310  homogenizes the received light and transmits the homogenized light from output face  330 . Relay optics  360  delivers the light homogenized and transmitted by optical element  303  to an active area  380  of light valve  370 . According to one embodiment of the invention, relay optics  360  images output face  330  of optical element  303  onto active area  380  of light valve  370 . Furthermore, output face  330  defines an object plane  335 , and active area  380  defines an image plane  385 . According to one embodiment of the invention, image plane  385  is an image of object plane  335  imaged by relay optics  360 .  
      The results of tracing rays from output face  330  to selected points on active area  380  are shown in  FIG. 5A , where the following exemplary assumptions and values were used: Optical element  303  was a solid rod having a rectangular cross-section. The output face  320  was a rectangle 8 mm wide and 4.5 mm high. The active area  380  was a rectangle 17.51 mm wide and 9.85 mm high. Relay optics  360  was a lens system with an effective focal length of 51.37 mm and a magnification factor of 2.31. Furthermore, α was 6.8 degrees and δ was 26 degrees. Area  701  in  FIG. 5  corresponds to active area  380  of light valve  370 . The smallness of individual points in  FIG. 5A  indicate that active area  380  lies in the image plane of output face  330  imaged by relay optics  360 . The smallness of the points in  FIG. 5A  further indicates uniform and efficient illumination of active area  380 .  
      For comparison,  FIG. 5B  shows the results of a similar ray tracing for an illumination system where the optical element  303  was replaced with a known rectangular prism light homogenizer, such as one discussed in U.S. Pat. No. 5,625,738. The relatively large points in  FIG. 5B  indicate that the active area of the light valve is not in the image plane of the output face of the known rectangular prism light homogenizer imaged by relay optics  360 . In contrast, a comparison of corresponding spots in  FIGS. 5A and 5B  (such as spots  730  and  720 ) indicates that, for the light homogenizing element of the present invention, output face  330  and active area  380  form an object-image relationship, meaning that, active area  380  lies in an image plane of output face  330 , as imaged by relay optics  360 . Small spot-size image points in  FIG. 5A  indicate uniform illumination. As such, an advantage of the present invention is uniform illumination. Uniform illumination is achieved by using a light homogenizing optical element having an output face which is imaged by relay optics onto an active area of a light valve, where the active area makes a non-zero angle with a normal to the optical axis, that is, the active area is not normal to the optical axis.  
      In general, output face  330  may have a shape that is different than the shape of active area  380 . For example, output face  330  may be a trapezoid and active area  380  may be a square. In some applications, output face  330  and active area  380  may have the same shape, such as a rectangle or a square.  
      Light valve  370  may be a liquid crystal display (LCD), a switchable mirror display, such as a digital micromirror device (DMD) from Texas Instruments, a micro-electromechanical system (MEMS), such as a grating light valve (GLV) discussed, for example, in U.S. Pat. No. 5,841,579. In general, light valve  370  can be any switchable device capable of forming an image.  
      Optical system  300  may be telecentric, meaning that one or both of an entrance pupil and an exit pupil of optical system  300  can be located at or near infinity.  
      Optical system  300  may further include other elements not shown in  FIG. 3 . For example, optical system  300  may include apertures, prisms, mirrors, or any other elements or components that may be suitable for use in an illumination system.  
      The layout in  FIG. 3  shows an unfolded optical system, meaning that optical axis  301  is a straight line, not folded at any point along the optical axis. To economize space, optical system  300  may be folded at one or more points along optical axis  301 .  
       FIG. 2  illustrates a three-dimensional schematic of an optical element  200  as one particular embodiment of optical element  303  of  FIG. 1 . Optical element  200  homogenizes light that enters the optical element through an input face  220  of optical element  200 . Optical element  200  further includes an optical rod  210  and an output face  230 . Light entering optical element  200  is homogenized, for example, as it travels along the optical rod by, for example, reflection or total internal reflection off of the sides  211  of the optical rod. Sides  211  can form an interface between the optical rod and what surrounds the optical rod, such as air. Sides  211  can be planar. Sides  211  can be curved, for example, to focus the light as it propagates along the optical rod. As an example, and without loss of generality, optical element  200  is centered on an optical axis  201  that extends along the y-axis. The index of refraction of a solid optical rod  210  may vary along optical axis  201 . For example, optical rod  210  may have a gradient index along optical axis  201  to, for example, bend or focus the light as it propagates along the optical rod. Input face  220  is a rectangle and is normal to the optical axis  201 . Output face  230  is also a rectangle and makes an angle β with the xz-plane where the magnitude of angle β is greater than zero. Optical rod  210  has a rectangular cross-section  
      According to one embodiment of the invention, input face  220  is not parallel to the output face  230 . In this particular embodiment of the invention, an input plane defined by input face  220  intersects and makes the angle β with an output plane defined by output face  230 . It will be appreciated that no one or multiple translational shifts of input face  220  can result in input face  220  overlay or coincide output face  230 .  
      It will further be appreciated that input face  220 , output face  230 , and a cross-section of optical rod  210  can have a shape other than rectangle, such as a trapezoid, a square, or any other shape that may be desirable in an application. Furthermore, input face  220 , output face  230  and a cross-section of optical rod  210  can have different shapes. For example, input face  220  can be a rectangle, while output face  230  and a cross-section of optical rod  210  can be squares. A cross-section of optical rod  210  can be different at different locations along the optical rod. For example, optical rod  210  may be tapered along its major length along optical axis  201 . The sides of a cross-section of optical rod  210  may be straight or curved. An example of a tapered optical rod is described in U.S. Pat. No. 6,332,688.  
      A portion of or the entire optical element  200  can be solid or hollow. An example of a hollow light homogenizer is illustrated in reference to  FIG. 3 .  
       FIG. 3  illustrates a three-dimensional schematic of a light homogenizing optical element  600  in accordance with one embodiment of the invention. Optical element  600  can function as optical element  303  in  FIG. 1 . Optical element  600  includes an input face  620 , an output face  630 , and an optical rod  610 . Output face  630  is not parallel to input face  620 . In particular, output face  630  makes an angle ω with input face  620 , where the magnitude of angle ω is greater than zero. Optical rod  610  includes a core  617 , and a cladding  650 . Cladding  650  includes an interior surface  615  and an exterior surface  616 .  
      Core  617  may be air in which case optical element  600  is hollow. In such a case, interior surface  615  may be highly reflective, for example, by including a reflective layer, such as a metal coating, a metal coating with a multilayer enhancement coating, or a multilayer dielectric coating. The multilayer dielectric coating can include organic layers, such as a multilayer optical film (MOF) discussed in, for example, U.S. Pat. No. 5,882,774. The MOF can function as a filter and a reflector by, for example, reflecting light in the visible region of the spectrum and transmitting light in the infrared region of the spectrum. Examples of metals that can be used in a metal coating include silver, aluminum, and gold. Light can be homogenized by multiple reflections from a highly reflective interior surface  616 . The entire cladding  650  may be made of a metal.  
      Core  617  may include a solid material. The solid material may be made of glass or organic material. Exemplary glass materials include soda lime glass, borosilicate glass, borate glass, silicate glass, oxide glass and silica glass, or any other glass material that may be suitable for use as a core material. Exemplary organic materials include polycarbonate, acrylic, polyethylene terephthalate (PET), polyvinyl chloride (PVC), polysulfone, and the like. Where core  617  includes a solid material, cladding  650  preferably has an index of refraction that is lower than the index of refraction of the core material to facilitate reflection including total internal reflection of light propagating along the optical rod.  
      Core  617  may include a fluid. Exemplary fluids include ethylene glycol, mixtures of ethylene glycol and glycerol, mixtures of ethylene glycol and water, liquids including a siloxane polymer having methyl, phenyl, and hydrophilic side groups, and liquids including mixtures of a siloxane polymer having methyl and phenyl side groups and a siloxane polymer having methyl and hydrophilic side groups. A fluid core material can assist in heat sinking light source  340  or focusing element  350 .  
      Core  617  can scatter light. For example, core  617  may include particles dispersed in a host material where the index of refraction of the particles is different than that of the host material, in which case, particles can scatter light, thereby assist in homogenizing light.  
       FIG. 4  illustrates a schematic side view of an optical element  100  in accordance with one embodiment of the invention. Optical element  100  homogenizes light received from a light source  140 . Optical element  100  is centered on an optical axis  101 . Optical axis  101  can be straight, as shown in  FIG. 4 , curved, or a combination of one or more straight and curved line segments. In general, optical axis  101  can have any shape that may be desirable in an application. Optical element  100  includes an input face  120 , a body  110 , and an output face  130 . Optical element  100  receives light from light source  140  through input face  120 . Light entering body  110  becomes more uniform, for example, as it travels along the body. Body  110  can be a rod, where at least a portion of the rod may be solid or hollow. Homogenized light exits body  110  through output face  130 . By homogenization, it is meant that light exiting optical element  100  is more uniform than light entering the optical element.  
      According to one embodiment of the invention, input face  120  is not parallel to output face  130 . For example, input face  120  cannot be made to overlay or coincide output face  130  by one or more translational shifts of input face  120  along, for example, x, y, or z-axes, or a combination thereof. Translational shifts of input face  120  combined with one or more rotations or tilts of input face  120  may cause the input face to overlay the output face. One or both of input face  120  and output face  130  may be planar or non-planar. One or more of input face  120 , output face  130 , and a cross-section of body  110  may have any shape having a regular or irregular perimeter. For example, the perimeter of one or more of input face  120 , output face  130 , and a cross-section of body  110  may be a circle, an ellipse, a polygon, such as a quadrilateral, a rhombus, a parallelogram, a trapezoid, a rectangle, a square, or a triangle.  
      Input face  120  can define an input plane  121  and output face  130  can define an output plane  131 . According to one embodiment of the invention, input plane  121  and output plane  131  are not parallel, meaning that planes  121  and  131  intersect, for example, when one or both are sufficiently extended. According to one embodiment of the invention, input plane  121  cannot be made to overlay or coincide output plane  131  by one or more translational shifts of the input plane  121 , where by a translational shift of input plane  121 , it is meant a shift of input plane  121  that can be expressed as a combination of shifts along the x, y, and z-axes Input face  120  can be normal to optical axis  101 .  
      Optical element  100  can have any three-dimensional shape, for example, a polyhedron, such as a hexahedron. Optical element  100  can be solid or hollow. Optical element  100  may homogenize an input light by any suitable optical method such as reflection, total internal reflection, refraction, scattering, or diffraction, or any combination thereof.  
      Optical transmittance of optical element  100  is preferably no less than 50%, more preferably no less than 70%, and even more preferably no less than 80%, where optical transmittance is the ratio of total light intensity exiting output surface  130  to total light intensity incident on input face  120 .  
      All patents, patent applications, and other publications cited above are incorporated by reference into this document as if reproduced in full. While specific examples of the invention are described in detail above to facilitate explanation of various aspects of the invention, it should be understood that the intention is not to limit the invention to the specifics of the examples. Rather, the intention is to cover all modifications, embodiments, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.