Abstract:
A display including a light source for generating light, an optical waveguide for receiving and evenly distributing light in a light propagation direction by total internal reflections and a matrix of picture elements constructed on the upper surface of the waveguide, the picture elements including electrically activated micro-mechanical actuators having optical properties for modulating light to produce an image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to displays. More particularly the invention concerns displays comprising an optical waveguide, a light source and a plurality of electrically activated micro-mechanical actuators with optical properties. 
     2. Discussion of the Prior Art 
     Currently liquid crystal displays dominate the flat panel display market. Prior art liquid crystal displays typically comprise a backlight assembly for illumination, light polarizers, color and neutral density filters, and an active matrix liquid crystal layer with thin-film-transistor backplanes. The overall light efficiency of a typical prior art liquid crystal display (LCD) is below 10% mainly due to the fact that light from the backlight assembly has to pass several layers of polarizers, color and neutral density filters. A further problem with LCDs is the slow response time of the liquid crystal resulting in objectionable visible motion artifacts when displaying motion images. 
     Recently, micro-mechanical flat panel displays based on an optical waveguide were proposed as a viable alternate to LCDs. These displays typically consist of a planar waveguide with parallel surfaces on which a matrix of electrically driven micro-mechanical picture elements is constructed. Light from a light source is introduced to the waveguide from one or more sides of the wave guide and is confined within the waveguide by total internal reflections. Light is extracted from the planar surface of the waveguide by coupling to evanescent waves or by deforming the surface of the planar waveguide to produce an image. There is an inherent optical crosstalk problem when picture elements are simultaneously activated to display an image. The state of one picture element changes the brightness of other picture elements. One solution to the optical crosstalk problem is to activate the picture elements sequentially. This requires very fast micro-mechanical actuators and results in very low light efficiency. Displaying color or grey scale images is generally not practical. 
     Another common problem concerns the use of mirror surfaces to redirect light to the viewer. The same mirror surface reflects the ambient light back to the viewer thereby significantly reducing the contrast at high levels of ambient light. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a display that effectively overcomes the optical crosstalk problem typically found in prior art optical waveguide-based displays while displaying the entire image simultaneously. In one form of the invention this object is achieved by providing a display that comprises a light source and an optical waveguide. The optical waveguide distributes light to a plurality of light exits. At each light exit a picture element modulates light by selectively directing the light to the viewer or to a light absorber. 
     Another object of the invention is to provide a high contrast display of the character that operates at high levels of ambient light. Two embodiments of the invention achieve this object by providing a display wherein the majority of the viewing surface is coated with a light-absorbing coating. In the third embodiment of the invention this object is achieved by providing a display which includes a prism film at the upper surface of the display. In this latter embodiment, the prism film redirects light emitting from the display at oblique angles towards the normal so as to improve the viewing angles. The same prism film absorbs most of the ambient light when a light-absorbing coating is applied on the same facets that redirect the display light. 
     Another object of the invention is to provide a display that can compete with LCD&#39;s in light efficiency, picture quality and cost. Increased light efficiency is achieved by providing a display in which light travels most of the light path by total internal reflections and light is modulated by requiring fewer reflections from highly reflective micro-mirrors. Improved picture quality is achieved by providing fast and efficient light modulators. 
     The foregoing as well as other objects of the invention will be achieved by the novel display illustrated in the accompanying drawings and described in the specification that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a generally perspective view of one form of the display of the present invention. 
         FIG. 2  is an enlarged cross-sectional view taken along lines  2 - 2  of  FIG. 1 . 
         FIG. 3  is a greatly enlarged view of the area designated as  3 - 3  in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of an alternate form of display of the invention. 
         FIG. 5  is a greatly enlarged view of the area designated as  5 - 5  in  FIG. 4 . 
         FIG. 6  is a cross-sectional view of still another form of display of the invention. 
         FIG. 7  is a greatly enlarged view of the area designated as  7 - 7  in  FIG. 6 . 
         FIG. 8  is a cross-sectional view of still another form of display of the invention. 
         FIG. 9  is a greatly enlarged view of the area designated as  9 - 9  in  FIG. 8 . 
         FIG. 10  is a cross-sectional view of yet another form of display of the invention. 
         FIG. 11  is a cross-sectional view of a waveguide with embedded dichroic mirrors of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to the drawings and particularly to  FIGS. 1 and 2 , one form of the display of the invention is shown there and generally designated by the numeral  20 . As best seen in  FIG. 1 , display  20  here includes a generally rectangular shaped optical waveguide  21  that is a substantially wedge-shaped cross section. Waveguide  21  is preferably constructed from an optically transparent material, such as acrylic or glass and comprises generally parallel first and second end surfaces  26  and  27  that are joined by parallel side surfaces  28  and  29  (see  FIG. 1 ). Waveguide  21  also includes a specially configured major upper surface  30  and an upwardly inclined lower surface  31  (see also  FIG. 2 ). A plurality of substantially equally spaced-apart grooves  32  are formed on upper surface  30  and, as shown in  FIG. 1 , extend between side surfaces  28  and  29 . An elongated light source  24  is installed proximate the wide edge  26  of the waveguide  21  and a novel matrix of tilting micro-mirrors  33  is constructed on upper surface  30  of the waveguide. In  FIG. 2 , one column of the tilting micro-mirrors is designated as  33   a ,  33   b ,  33   c ,  33   d ,  33   e  and  33   f.    
     Now referring to  FIG. 3  of the drawings, groove  33   e , which is generally representative of all of the grooves formed on the upper surface  30  of the waveguide  21  can be seen to comprise three optically flat facets  34 ,  35  and  36 . As illustrated in  FIGS. 2 and 3 , optically flat facets  34  are inclined downwardly at a steep angle of between about 80 and 90 degrees with respect to the upper surface  30 . Second facets  35  are recessed from and generally parallel to the upper surface  30  and facets  36  are upwardly inclined at angles between about 45 and about 60 degrees with respect to upper surface  30 . 
     As further illustrated in  FIG. 3  multi-layer film coatings are applied on facets  35  and  36 . More particularly, a first layer  37  that comprises a light-absorbing black polymer film is deposited only on facets  36 . A second layer  38  that comprises a conductive specularly reflective mirror film formed from a material such as an aluminum alloy is deposited on facets  35  and on light-absorbing layer  37 . A third layer  39 , that here comprises a transparent electrical insulator, is deposited on conductive mirror film layer  38  only on the flat horizontal sections thereof. 
       FIG. 3  also illustrates one of tilting micro-mirrors  33   e  of the present form of the invention. Micro-mirror  33   e , which typifies the construction of each of the micro-mirrors of the invention, comprises a thin aluminum alloy elastic film that is affixed to the upper surface  30  of the waveguide  21 . In order to bend the micro-mirror at the tilt axis  42  (see  FIG. 3 ), the thickness of each of the micro-mirrors is reduced at the junction of the downwardly inclined facets  34  with the upper surface  30  of the waveguide  21 . For absorbing external light, a thin black polymer film  41  is deposited on upper surface of each micro-mirror. Alternately, the upper surface of the micro-mirror can be blackened by means of a black oxidation process of a character well understood by those skilled in the art. 
     In the present form of the invention, the tilting micro-mirrors  33  operate by electrostatic attraction force and by the counter spring forces generated by the elastic film. The conductive mirror films  38  act as fixed electrodes for the tilting micro-mirrors  33 . When a suitable voltage “V” is applied between the fixed conductive mirror films  38  and a micro-mirror  33 , the micro-mirror tilts by electrostatic attraction force and, when no voltage is applied, the micro-mirror is returned to the flat position by the counter spring force of the elastic film. It is to be understood that instead of grooves, appropriately configured cavities can be formed on the upper surface  30  of the wave guide and the tilting micro-mirrors can be received within the cavities rather than within the grooves. 
     As best seen in  FIG. 2  of the drawings, light rays  43  entering from the wide edge  26  of the waveguide  21  are uniformly distributed in the light propagation direction of the X-axis by total internal reflections and exit the waveguide  21  from downwardly inclined facets  34 . Depending on the positions of the tilting micro-mirrors, light rays are absorbed, or directed, to the viewer. 
     When a tilting micro-mirror is in the flat position, such as micro mirrors  33   c  and  33   d  ( FIG. 2 ), light rays reflect from the lower light reflecting surfaces of the micro-mirrors and mirror coatings  38  and are directed to the viewer. When a micro-mirror is tilted down, such as micro-mirrors  33   a  and  33   b , light rays reflect from the lower light reflecting surface of the micro-mirror and mirror coatings  38  and change the angles towards the normal. After multiple reflections, light rays lose their energy and the light is absorbed. Some light rays may change their angles of reflection by reflecting from the micro-mirrors and mirror coatings  38  and such light rays re-enter the light guide from downwardly inclined facets  34  and travel backwards to the direction of the light source. (See micro-mirrors  33   e  and  33   f  as shown in  FIGS. 2 and 3 .) Light rays traveling backwards are absorbed by light-absorbing layers  37 . 
     Depending on the display size and resolution, each picture element may include several tilting micro-mirrors. 
     It is to be understood that all micro-mirrors for each picture element tilt simultaneously when suitable voltage is applied between the fixed electrodes  38  and selected group of micro-mirrors. 
     Referring now to  FIG. 4  of the drawings, a cross-sectional view of the second embodiment of display of the present invention is there shown and generally designated by the numeral  44 . This second embodiment is similar in some respect to the embodiment shown in  FIGS. 1 and 2  of the drawings and like numbers are used in  FIG. 4  to identify like components. 
     The display  44  includes a generally rectangular shaped optical waveguide  45  that is substantially wedge-shaped cross section. Waveguide  45  is constructed from an optically transparent material such as acrylic or glass and comprises of generally parallel first and second end surfaces  46  and  57  that are joined by generally parallel side surfaces. Like display  20 , display  44  also includes a specially configured major upper surface  49  and an upwardly inclined lower surface  47 . As before, a plurality of equally spaced-apart grooves  50  is formed on upper surface  49  and extends between the generally parallel sides. An elongated light source  24  is installed proximate the wide edge  46  of the waveguide  45  and a novel matrix of tilting micro-mirrors  51  is constructed on upper surface  49  of the waveguide  45 . In  FIG. 4  of the drawings one column of the tilting micro-mirrors of the invention can be seen to comprise micro-mirrors  51   a ,  51   b ,  51   c ,  51   d ,  51   e  and  51   f.    
     Turning to  FIG. 5 , the groove  50 , that is there shown is representative of all of the grooves  50  formed on the upper surface  49  of the waveguide  45 , and can be seen to comprise two optically flat facets  52  and  53 . Optically flat facet  52  is inclined downwardly at a steep angle between about 80 and about 90 degrees with respect to the upper surface  49 . Second facet  53  is inclined upwardly at an angle between about 5 degrees and about 15 degrees with respect to the upper surface  49 . Two layer film coatings are applied on facets  53 . The first layer  54  comprises a conductive specularly reflecting mirror, while the second layer  55  comprises a light-absorbing electrical insulator. 
       FIG. 5  also illustrates the construction of tilting micro-mirror  51   e . Micro-mirror  51   e , which is representative of all of the tilting micro-mirrors of this form of the invention, here comprises a thin aluminum alloy elastic film that is attached to the upper surface  49  of the waveguide  45 . In order for the micro-mirror to appropriately bend at the tilt axis  56 , the thickness of each of the micro-mirrors is reduced in the manner previously described. As before, the tilting micro-mirrors  51  are operated by electrostatic attraction force and by the counter spring force of the elastic film. The conductive mirror films  54  act as fixed electrodes for the tilting micro-mirrors  51 . When suitable voltage “V” is applied between the fixed conductive mirror films  54  and a micro-mirror  51 , the micro-mirror tilts by electrostatic attraction force and, when no voltage is applied, the micro-mirror returns to the flat position due to the counter spring forces of the elastic film. 
     As best seen in  FIG. 4  of the drawings, light rays  43  entering from the wide edge  46  of the waveguide  45  are uniformly distributed in the light propagation direction of the X-axis by total internal reflections and exit the waveguide  45  from downwardly inclined facets  52 . Depending on the positions of the tilting micro-mirrors, light rays are either absorbed, or selectively directed to the viewer. When a tilting micro-mirror is in the flat position (see Micro-mirrors  51   a ,  51   b ,  51   e  and  51   f  in  FIG. 4 ), light rays reflect from the lower light reflecting surface of micro-mirrors and are absorbed by the light-absorbing layers  55 . When a micro-mirror is in the tilted down position, such as the micro-mirrors  51   c  and  51   d  shown in  FIG. 4 , light rays reflect from the upper light reflecting surface of the micro-mirror and are directed to the viewer. 
     It is to be understood that instead of grooves, appropriately configured cavities can be formed on the upper surface of the waveguide and the tilting micro-mirrors can be received within the cavities rather than within the grooves. 
     Referring now to  FIG. 6  of the drawings, still another alternate embodiment of the display of the latest embodiment is there shown and generally designated by the numeral  62 . This latest embodiment is similar in some respect to the embodiments shown in  FIGS. 1 and 2  of the drawings and like numbers are used in  FIG. 6  to identify like components. 
     The display  62  here includes a generally rectangular shaped optical waveguide  63  that is substantially wedge-shaped cross section. As before, waveguide  63  is constructed of an optically transparent material such as acrylic or glass, and comprises generally parallel first and second end surfaces  66  and  68  that are joined by generally parallel side surfaces. Waveguide  63  also comprises a specially configured major upper surface  65  and an upwardly inclined lower surface  64 . An elongated light source  24  is installed proximate wide edge  66  of the waveguide  63 . Formed on the upper surface  65  of the wave guide is a plurality of equally spaced-apart grooves  69  that extend between the sides. A plurality of micro-prisms  70  are dispensed within the grooves  69  and a matrix of picture elements  67  is assembled proximate the upper surface of the waveguide  63 . In  FIG. 6 , one column of the picture elements can be seen to comprise picture element  67   a ,  67   b  and  67   c.    
     Referring to  FIG. 7 , one of the grooves  69 , which is representative of all of the grooves formed on the upper surface  65  of the waveguide  63 , can be seen to comprise optically flat facets  71 ,  72  and  73 . Optically flat facet  71  is inclined downwardly at a relatively steep angle between about 80 and about 90 degrees with respect to the upper surface  65 . Second facet  72  is recessed from and generally parallel to the upper surface  65 , while facet  73  is upwardly inclined at an angle between about 45 to about 60 degrees with respect to the upper surface  65 . 
     As further illustrated in  FIG. 7 , multi-layer film coatings are applied to facets  72 ,  73  and to the upper surface  65  of the waveguide  63 . First layer  74 , which is formed from a material such as aluminum here, comprises a conductive specularly reflecting mirror film. Second layer  76  here comprises a light-absorbing black polymer insulator film that is deposited on the mirror film  74 . Third layer  75  comprises a cladding that is applied only to the flat lower sections of light-absorbing layer  76 . Cladding, which is well known to those skilled in fiber optics technology, comprises a transparent dielectric material that is coated on the surface of a fiber or on the waveguide and preferably has lower refractive index to facilitate total internal reflections. 
     Also illustrated in  FIG. 7  is one of micro-prisms  70  that resides within one of the grooves  69 . To construct each of the micro-prisms  70 , a UV-hardening transparent liquid polymer is deposited into the grooves  69 . The polymer preferably has the same refractive index as does the waveguide  63  following the application of the coatings  74 ,  76  and  75 . Each micro-prism comprises a light input facet  80  that is optically coupled to a selected one of the downwardly inclined facets  71  of the grooves  69 . In a similar fashion, a second facet  81  is optically coupled to a selected one of the light-absorbing films  76  and a third facet  83  is optically coupled to cladding layer  75 . The fourth facet  82  here comprises a light exit facet. 
     Also depicted in  FIG. 7  is picture element  67   c  that is assembled proximate the upper surface of the waveguide  63  using spacers  84  that are manufactured with alternating heights. The construction of picture element  67   c  is representative of the construction of all of the picture elements shown in  FIG. 6 . Picture elements  67  here comprise electrically activated micro-mechanical actuators that are constructed from transparent materials having the same or greater refractive index as do the micro-prisms  70  and each has a light diffusing upper surface. In operation, the conductive mirror films  74  act as a fixed electrode for picture elements  67 . Each picture element  67  comprises a transparent elastic film  85 , a light diffusing section  77  formed from a rigid transparent material, a light coupling section  78  formed from a relatively soft transparent material and an electrode  79  formed from an opaque conductive material. 
     Turning next to  FIGS. 6 and 7 , it can be seen that light rays  43  entering from the wide edge  66  of the waveguide  63  are uniformly distributed in the light propagation direction of the X-axis by total internal reflections and exit the waveguide  63  from downwardly inclined facets  71 . As best seen in  FIG. 7 , light rays exiting the waveguide  63  enter the micro-prisms  70  from light input facets  80 . Inside of the micro-prisms  70  the light rays propagate by total internal reflections from the upper surface  82  and from the lower surface  83  of the micro-prisms  70  until they reach the light-absorbing layer  76  formed on upwardly inclined facets  73 . To modulate light, suitable voltage “V” is applied between fixed conductive mirror films  74  and the respective electrodes  79  of the picture elements. This causes the picture elements such as element  67   b  to move by attractive electrostatic force at close distance to the upper surface of the micro-prisms  70  within a fraction of the wavelength. Light rays penetrate from the upper surface of the micro-prisms  70  into the picture element  67   b  by coupling to evanescent waves and exit the upper light diffusing surface of the picture element  67   b.    
     Referring now to  FIG. 8  and  FIG. 9  of the drawings, still another alternate embodiment of the display of the latest embodiment is there shown and generally designated by the numeral  90 . This latest embodiment is similar in some respect to the embodiments shown in  FIG. 6  and  FIG. 7  of the drawings and like numbers is used in  FIG. 8  and  FIG. 9  to identify like components. 
     The main difference between this latest embodiment and that shown in  FIG. 6  and  FIG. 7  of the drawings concerns the construction of the picture elements and optical coatings that are applied to the facets of the optical waveguide. 
     The display  90  here includes a generally rectangular shaped optical waveguide  63  that is substantially wedge-shaped cross section. As before, waveguide  63  is constructed of an optically transparent material such as acrylic or glass, and comprises generally parallel first and second end surfaces  66  and  68  that are joined by generally parallel side surfaces. Waveguide  63  also comprises a specially configured major upper surface  65  and an upwardly inclined lower surface  64 . An elongated light source  24  is installed proximate wide edge  66  of the waveguide  63 . Formed on the upper surface  65  of the wave guide is a plurality of equally spaced-apart grooves  69  that extend between the sides. A plurality of micro-prisms  70  are dispensed within the grooves  69  and a matrix of picture elements  67  is assembled proximate the upper surface of the waveguide  63 . In  FIG. 8 , one column of the picture elements can be seen to comprise picture element  92   a ,  92   b  and  92   c.    
     Referring to  FIG. 9 , one of the grooves  69 , which is representative of all of the grooves formed on the upper surface  65  of the waveguide  63 , can be seen to comprise optically flat facets  71 ,  72  and  73 . Optically flat facet  71  is inclined downwardly at a relatively steep angle between about 80 and about 90 degrees with respect to the upper surface  65 . Second facet  72  is recessed from and generally parallel to the upper surface  65 , while facet  73  is upwardly inclined at an angle between about 45 to about 60 degrees with respect to the upper surface  65 . 
     As further illustrated in  FIG. 9 , a conductive specularly reflecting mirror film  74  is formed from a material such as aluminum on facets  72 ,  73  and to the upper surface  65  of the waveguide  63 . 
     Also illustrated in  FIG. 7  is one of micro-prisms  70  that resides within one of the grooves  69 . Each micro-prism comprises a light input facet  80  that is optically coupled to a selected one of the downwardly inclined facets  71  of the grooves  69 . In a similar fashion, a second facet  81  and a third facet  83  are optically coupled to a selected one of the light reflecting mirror films  74 . The fourth facet  82  here comprises a light exit facet. 
     Also depicted in  FIG. 9  is picture element  92   c  that is assembled proximate the upper surface of the waveguide  63  using spacers  84  that are manufactured with alternating heights. The construction of picture element  92   c  is representative of the construction of all of the picture elements shown in  FIG. 8 . Picture elements  92  here comprise electrically activated micro-mechanical actuators that are constructed from transparent materials having the same or greater refractive index as do the micro-prisms  70 . In operation, the conductive mirror films  74  act as a fixed electrode for picture elements  92 . Each picture element  92  comprises a transparent elastic film  85 , a light diffusing section  77  formed from a rigid transparent material, a light coupling section  93  formed from a relatively soft light absorbing material and an electrode  79  formed from an opaque conductive material. 
     Turning next to  FIG. 8  and  FIG. 9 , it can be seen that light rays  43  entering from the wide edge  66  of the waveguide  63  are uniformly distributed in the light propagation direction of the X-axis by total internal reflections and exit the waveguide  63  from downwardly inclined facets  71 . Light rays exiting the waveguide  63  enter the micro-prisms  70  from light input facets  80 . Inside of the micro-prisms  70 , the light rays propagate by total internal reflections from the upper surface  82  and from the lower surface  83  of the micro-prisms  70 . Light rays that reach the light reflecting layer  74  formed on upwardly inclined facets  73  are deflected to the viewer (see  FIG. 9 ). 
     When suitable voltage “V” is applied between fixed conductive mirror films  74  and the respective electrodes  79  of the picture elements, the picture elements such as  92   b  shown in  FIG. 8  move by attractive electrostatic force at close distance to the upper surface of the micro-prisms  70  within a fraction of the wavelength. Light rays penetrate from upper surface  82  of micro-prisms  70  to light absorbing section  93  of picture element  92   b.    
     Yet another alternate form of display of the invention is shown in  FIG. 10  and is generally designated by the numeral  95 . The main difference between this latest embodiment and that shown in  FIG. 4  of the drawings concerns the installation of a prism film  96  at the upper surface of the display  44 . Prism film  96  functions to improve the viewing angles of the display and enhances the contrast at high levels of ambient light. 
     Prism film  96  is preferably constructed from a transparent polymer and has a major planar upper surface  97  and a lower surface that includes alternating downwardly inclined flat facets  98  and upwardly inclined flat facets  99 . Facets  98  are downwardly inclined at angles between about 80 to about 90 degrees, while facets  99  are upwardly inclined at angles between about 40 to about 60 degrees with respect to the upper surface  97 . Two coating layers are applied to upwardly inclined facets  99 . The first layer comprises a cladding  100  and the second layer comprises a light-absorbing layer  101 . 
     As shown in the  FIG. 10 , light rays are emitted from the display  44  at oblique angles of less than 45 degrees with respect to the display surface. The light rays emitted from the display  44  enter the prism film from the downwardly inclined facets  98  and deflect from upwardly inclined facets  99  towards the normal. This improves the viewing angles of the display. 
     To illustrate the ambient light-absorbing feature of the prism film  96 , an external light source  102  is placed near the upper surface  97  of the prism film  96 . Light rays emitted from the external light source  102  reflect from downwardly inclined facets  98 , upwardly inclined  99 , or from the upper surface of the micro-mirrors and are absorbed by the light-absorbing layer  101 . This substantially improves the display contrast for viewing in high levels of ambient light. Instead of cladding and light-absorbing layers, a conductive specularly reflecting mirror film formed from an aluminum alloy can be coated on facets  99 . Similarly, the outside surface of the mirror film can be blackened by means of a black oxidation process. In any event, the optical functionality will be substantially the same and the viewer will see a black surface when no light is emitted from the display and most of ambient light will be absorbed by the black oxide layer. Electrically, this can be used as a second fixed electrode for the tilting micro-mirrors. 
     Two alternative uses for these conductive mirror films are as an electrostatic shield to prevent external electrostatic discharge to the display and as an EMI shield to reduce electromagnetic emission from the display. 
     For color displays wherein each picture element comprises red, green and blue sub-pixels, one form of optical waveguide having embedded dichroic color filters is illustrated in  FIG. 11  and generally designated by the number  104 . 
     Waveguide  104  here comprises first and second sections  105  and  106 . First section  105  is preferably constructed from optically transparent material such as acrylic or glass, and has a planar upper surface  108 . Second section  106  includes stripes of dichroic mirror coatings  107   r ,  107   g  and  107   b  which are deposited on the planar upper surface  108  and slices of a substantially transparent material  109  which are selectively deposited on the dichroic mirrors. Slices  109  preferably have substantially the same refractive index as does section  105 . 
     In operation, the Dichroic filters  107   r  transmit red light and reflect green and blue light, the dichroic filters  107   g  transmit green light and reflect red and blue light, and the dichroic filters  107   b  transmit blue light and reflect red and green light. 
     It is to be understood that, while the previously described embodiments of the invention involve the use of an edge-lit optical waveguide having a wedge shape cross section, many other designs of optical waveguides can be used. Similarly, while the previously described embodiments comprise grooves or cavities formed on a planar surface of the waveguide, the various components can also be built up from a flat planar surface, or alternately can be formed on a separate substrate that can be combined with a backlight assembly. 
     Having now described the invention in detail in accordance with the requirements of the patent statutes, those skilled in this art will have no difficulty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions. Such changes and modification may be made without departing from the scope and spirit of the invention, as set forth in the following claims.