Abstract:
RD 29133-17-A method for facilitating a reduction of speckle in a screen receiving light from a light source includes positioning at least one optical path distributing screen element such that the light originating from the light source passes through the screen element and emerges decorrelated from the screen element toward an audience space. The method further includes positioning an angular distribution element between the screen element and the audience space such that the angular distribution element distributes the decorrelated light from the screen element toward the audience space.

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to projection television screens, and, more particularly, to projection television screens for use with a coherent projection beam. 
     Rear projection screens transmit an image projected onto the rear of the screen to an audience space and accordingly are sometimes referred to as transmission screens. Rear projection screens typically include a system to diffuse light transmitted into the audience space. One such system includes a plurality of minute colloidal particles. However, when screens with minute colloidal particles are used with high magnification systems in which the projection beam is coherent, a disturbing artifact in the form of a speckle pattern is often observed. Typically, this speckle pattern is more pronounced in screens with high resolution. 
     Speckle reduction techniques include reducing a coherence of an illumination beam to facilitate a reduction in the visibility of speckle. One way to reduce the coherence is to move one diffusing screen with respect to another diffusing screen to separate the diffusing surfaces as described in S. Lowenthal et al., J. Opt. Soc. Am., pp. 847-851 (1971); N. George et al., Opt. Commun., pp. 71-71 (1975); E. G. Rawson et al., J. Opt. Soc. Am., pp. 1290-1294 (1976) and L. G. Shirley et al., J. Opt. Soc. Am. A, pp 765-781 (1989). Another known way to facilitate a reduction in speckle utilizes a rear phase grating surface as described in U.S. Pat. Nos. 5,760,955 and 6,147,801. 
     SUMMARY OF INVENTION 
     In one embodiment, a method for facilitating a reduction of speckle in a screen receiving light from a light source includes positioning at least one optical path distributing screen element such that the light originating from the light source passes through the screen element and emerges decorrelated from the screen element toward an audience space. The method further includes positioning an angular distribution element between the screen element and the audience space such that the angular distribution element distributes the decorrelated light from the screen element toward the audience space. 
     In yet another embodiment, a rear projection television includes a housing, a light engine positioned in the housing, and a screen positioned between the light engine and an audience space. The screen is mounted to the housing and includes at least one optical path distributing screen element and at least one angular distribution element operationally coupled to the optical path distributing screen element. The angular distribution element distributes decorrelated light emerging from the screen element toward the audience space. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an illustration of one embodiment of a speckle reducing projection system. 
         FIG. 2  is a cut away side view of one embodiment of a reduced speckle screen. 
         FIG. 3  is a cross-sectional view of a side  94  of an N-sided polygonal non-imaging concentrator element. 
         FIG. 4  is a cut away side view of an alternative embodiment of a reduced speckle screen. 
         FIG. 5  is a cut away side view of an alternative embodiment of a reduced speckle screen. 
         FIG. 6  is a cut away side view of an alternative embodiment of a reduced speckle screen. 
         FIG. 7  is a perspective view of a plurality of non-imaging elements. 
         FIG. 8  illustrates a geometry for some of the skew elements shown in FIG.  7 . 
         FIG. 9  is a gain profile for the ten by five degree trapezoid non-imaging element shown in FIG.  7 . 
         FIG. 10  is a gain profile for the trapezoidal lenticular non-imaging element shown in FIG.  7 . 
         FIG. 11  is a gain profile for a nine by nine degree trapezoid non-imaging element. 
         FIG. 12  is a gain profile for an eight degree hex non-imaging element. 
         FIG. 13  is a side view with parts cut away of a rear projection speckle-reduced television set. 
         FIG. 14  is an embodiment of a front projection television system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of one embodiment of a speckle reducing projection system  50  including a first pixel  52  and a second pixel  54 . Light enters pixels  52  and  54  from a light engine (not shown in  FIG. 1 ) and exits pixels  52  and  54  entering an optical path distributing element (OPDE)  56 . The light is locally inverted by OPDE  56  and exits OPDE  56  to an angular distribution element  58  that forms a screen surface  60 . In one embodiment, angular distribution element  58  is a conventional diffuser. The light leaves angular distribution element  58  to an observer  62  in an audience space. 
     An optical path distribution (OPD) of the light across a diameter of OPDE  56  increases as a focal length of OPDE  56  decreases and, by selecting an appropriate focal length, speckle is reduced when the OPD inside a spatial coherence zone of the light is greater than a coherence length of the light. In one embodiment, OPDE  56  is a non-imaging optical screen element, such as, for example, a non-imaging microlens, and angular distribution element  58  is positioned between non-imaging optical screen element  56  and the audience space such that angular distribution element  58  in combination with screen element  56  decorrelates the light and distributes the decorrelated light toward the audience space, facilitating a reduction in speckle. In another embodiment, OPDE  56  is an imaging element, such as, for example, an imaging microlens, and angular distribution element  58  has a diffusion angle approximately equal to an exit angle of screen element  56 , which facilitates a reduction in speckle. 
       FIG. 2  is a cut away side view of one embodiment of a reduced speckle screen  70  including a plurality of OPDEs  72  extending from a substrate  74  to a top section  76 . In another embodiment, screen  70  does not include top section  76 . In one embodiment, OPDEs  72 , substrate  74 , and top section  76  are fabricated from an optical conductor with an index of refraction between about 1.1 and about 3. OPDEs  72  can be total internal reflection (TIR) concentrators wherein a TIR index delta n is between about 0.02 and 1. In an exemplary embodiment, non-imaging optical concentrators  72 , substrate  74 , and top section  76  are fabricated from a polycarbonate having an index of refraction of about 1.59. In another embodiment, OPDEs  72 , substrate  74 , and top section  76  are fabricated from a polycarbonate having an index of refraction greater than or less than 1.59. Additionally, OPDEs  72 , substrate  74 , and top section  76  can all have different indices of refraction. 
     In one embodiment, OPDEs  72 , substrate  74 , and top section  76  are fabricated from a transparent solid material having an index of refraction equal to or greater than 1.1. In an exemplary embodiment, the transparent solid material has a refractive index between about 1.40 and about 1.65 and is selected from the group of polymethylmethacrylate (PMMA), polycarbonate, polyester, poly(4-methyl pentene), polystyrene and polymers formed by photopolymerization of acrylate monomer mixtures. In another embodiment, OPDEs  72 , substrate  74  and top section  76  are fabricated from various transparent solid materials selected from the group of PMMA, polycarbonate, polyester, poly(4-methyl pentene), polystyrene and polymers formed by photopolymerization of acrylate monomer mixtures. In yet another embodiment, OPDEs  72 , substrate  74 , and top section  76  are fabricated entirely from a PMMA. In an alternate embodiment, at least one of OPDEs  72 , substrate  74 , and top section  76  are fabricated from a non-solid material such as a gas, a liquid, and a gel. 
     In an exemplary embodiment, OPDEs  72  are non-imaging optical concentrators and hereinafter are referred to as concentrators  72 . A polymer fill material having an index of refraction less than or equal to the index of refraction of concentrators  72  is utilized to fill a plurality of valleys  78  interposed between concentrators  72 . The polymer material can include an absorbing dye and is selected from the group of polymethylmethacrylate (PMMA), polycarbonate, polyester, poly(4-methyl pentene), polystyrene and polymers formed by photopolymerization of acrylate monomer mixtures. Additionally, the polymer material can be a mixture including at least two of the group of PMMA, polycarbonate, polyester, poly(4-methyl pentene), polystyrene and polymers formed by photopolymerization of acrylate monomer mixtures. Each valley  78  includes a reflective coating  80  and a light absorbing coating  82 . In one embodiment, reflective coating  80  is a mirror fabricated from a metal such as aluminum. Ambient light  84  is absorbed by absorbing layer  82  facilitating a high contrast picture emanating from an audience side  86  of screen  70 . In another embodiment, reflective coating  82  is a TIR coating utilizing a low index dielectric with a high index dielectric and a dye such that reflective coating  80  forms a bulk absorber. In an alternative embodiment, valleys  78  do not include reflective coating  80  or light absorbing coating  82 . Screen  70  further includes a light engine side  88 , and an angular distributing element  90  on audience side  86 . Both light engine side  88  and audience side  86  are planar surfaces. 
     During operation of screen  70 , light  92  from a light engine (not shown in  FIG. 2 ) enters screen  70  at light engine side  88  and passes through substrate  74  before entering concentrator  72 . When light  92  leaves concentrator  72  there is an accumulated optical path difference between an on-axis path and an off-axis path of light  92 . Thus the light traveling along these paths is decorrelated. However since the light paths are projecting toward a plurality of different viewer locations in an audience space (shown in  FIG. 1 ) there is no perception of speckle reduction to a viewer in the audience space when angular distributing element  90  is not included in a screen. However screen  70  includes angular distributing element  90  which distributes the decorrelated light such that at each viewer location light is received from multiple paths through concentrator  72 . Accordingly, the combination of angular distributing element  90  and concentrator  72  reduces speckle. More specifically, angular distributing element  90  causes an OPD inside a spatial coherence zone of light  92  that is greater than a coherence length of light  92  facilitating a reduction of speckle. In an alternative embodiment, screen  70  includes a plurality of vertically extending lenticulars arranged horizontally such that light  92  traverses the lenticulars before passing through angular distributing element  90 . The lenticulars and angular distributing element  90  are selected such that an OPD inside a spatial coherence zone of light  92  is greater than a coherence length of light  92 , which facilitates a reduction of speckle. In one embodiment, angular distributing element  90  is a conventional diffuser. 
       FIG. 3  is a cross-sectional view of a facet  94  of an N-sided polygonal non-imaging concentrator element  96  wherein N is between 2 and 12. In an exemplary embodiment, non-imaging concentrator  96  is concentrator  72  (shown in FIG.  2 ). Facet  94  includes a light incidence width  98 , a light emergence width  100 , and a height  102 . Width  98  is between 20 um and 5 mm. Width  100  is between 20 um and 5 mm. Height  102  is also between 20 um and 5 mm. A point “b” defines one side of width  98  and a point “a” is co-linear with a top end  104  of concentrator  76  and distanced from top end  104 . Points a and b are on the same side of element  96 . Let r be the distance along a line  106  from point b to point a such that, at point b, r is equal to zero. Let z be the distance (perpendicular) from r along line  106  that characterizes the shape of an edge  108  of non imaging element  96 . Then edge  108  is such that z=cr^2/(1+sqrt(1−(1+k)c^2r^2))+dr^2+er^+4+fr^6, where −20&lt;c&lt;20, −10&lt;d&lt;10, −10&lt;e&lt;10, −10&lt;f&lt;10, and −10&lt;k&lt;10, where r and z are given in units of microns. In another embodiment, element  96  is a microlens and z is an optical axis of the microlens according to the above equation. In a further embodiment element  96  is a lenticular element and z represents the optical axis of the lenticular element according the above equation. 
     In an alternative embodiment, element  96  is a gradient index (GRIN) lens and z represents an optical axis wherein the index of refraction (n) is according to n^2(z)=n+cr^2+dr^4+er^6, where c is between 0.001 and 10, d is between 0 and 10, and e is between 0 and 10. In further alternative embodiments, element  96  includes elliptical cross sections and/or tapers between polygonal cross-sections and elliptical cross-sections as shown in  FIG. 7  regarding skew elements. When element  96  includes elliptical cross-sections, then a major axis of the elliptical cross-section is between 20 um and 5 mm, and a minor axis of the elliptical cross-section is between 20 um and 5 mm. 
       FIG. 4  is a cut away side view of an alternative embodiment of a reduced speckle screen  110 . Reduced speckle screen  110  is substantially similar to reduced speckle screen  70  shown in  FIG. 2 , and components in reduced speckle screen  110  that are identical to components in reduced speckle screen  70  are identified in  FIG. 3  using the same reference numerals used in FIG.  2 . Accordingly, reduced speckle screen  110  includes a substrate  74  and a plurality of non-imaging optical concentrators  72  extending from substrate  74 . A plurality of valleys  78  are interposed between concentrators  72 . In an alternative embodiment, each valley  78  includes an aluminum layer and a light absorbing layer. Screen  110  further includes a plurality of diffusers  112  positioned at an audience side  114  of concentrators  72  such that each non-imaging concentrator  72  has a corresponding diffuser  112 . 
     During operation of screen  110 , light from a light engine (not shown in  FIG. 3 ) enters screen  110  at light engine side  88  and passes through substrate  74  before entering concentrator  72 . The light leaves concentrator  72  with an accumulated optical path difference between an on-axis path and an off-axis path, and passes through diffuser  112  before exiting audience side  86 . Diffuser  112  redistributes the decorrelated light such that, at a plurality of viewer locations in an audience space, light is received from multiple paths through concentrator  72 , which reduces speckle. More specifically, diffuser  112  causes an OPD inside a spatial coherence zone of the light that is greater than a coherence length of the light as the light leaves audience side  86  of screen  70  facilitating a reduction of speckle. Additionally, in one embodiment, the light exiting at least eighty percent of screen  110  from audience side  86  is at least two times the coherence length of the light. In an alternative embodiment, the light exiting at least ninety percent of screen  110  from audience side  86  is at least two times the coherence length of the light. In an exemplary embodiment, the light exiting approximately all of screen  110  from audience side  86  is at least two times the coherence length of the light. 
       FIG. 5  is a cut away side view of an alternative embodiment of a reduced speckle screen  120 . Reduced speckle screen  120  is substantially similar to reduced speckle screen  110  shown in  FIG. 4 , and components in reduced speckle screen  120  that are identical to components in reduced speckle screen  110  are identified in  FIG. 5  using the same reference numerals used in FIG.  4 . Accordingly, reduced speckle screen  120  includes a substrate  74  and a plurality of non-imaging optical concentrators  72  extending from substrate  74 . A plurality of valleys  78  are interposed between concentrators  72 . Each valley  78  includes a reflective layer  80  covered by an absorbing layer. In one embodiment, a filler material is utilized to fill valleys  78  such that screen  120  includes a substantially planar top surface  122 . Screen  120  further includes a microoptic top structure  124  positioned at an audience side  126  of concentrator  72 . In one embodiment, microoptic top structure  124  includes microlenses. In another embodiment, microoptic top structure  124  forms a lenticular top structure. Screen  120  further includes a light engine diffuser  128  positioned at a light engine side  88 . 
     During operation of screen  120 , light from a light engine (not shown in  FIG. 4 ) enters screen  120  at light engine side  88  and passes through light engine diffuser  128  and substrate  74  before entering concentrator  72 . The light leaves concentrator  72  and passes through microoptic top structure  124  before exiting audience side  86 . Microoptic top structure  124  in combination with concentrator  72  facilitates a reduction in speckle as explained above. Additionally, microoptic top structure  124  in combination with concentrator  72  and light engine diffuser  128  facilitates a reduction in speckle more than if diffuser  128  were not present in screen  120 . 
       FIG. 6  is a cut away side view of an alternative embodiment of a reduced speckle screen  140  including a light engine side  142 , an audience side  144 , and a mask  146  positioned between light engine side  142  and audience side  144 . Light engine side  142  includes a plurality of microoptic structures  148 . In one embodiment, microoptic structures  148  are microlenses. In another embodiment, microoptic structures  148  form a lenticular structure. Mask  146  includes a plurality of apertures  150 . A diffuser  152  is positioned at audience side  144 . 
     During operation of screen  140 , light from a light engine (not shown in  FIG. 6 ) enters microoptic structures  148  from light engine side  142 . The light travels through screen  140  toward mask  146 . While some of the light encounters mask  146  and is absorbed by mask  146 , most of the light passes through apertures  150  and is diffused by diffuser  152  before exiting audience side  144 . Microoptic structures  148  are locally inverting and are optical path distributing elements. Accordingly, microoptic structures  148  in combination with diffuser  152  facilitate a reduction in speckle. 
       FIG. 7  is a perspective view of a plurality of non-imaging concentrator elements  160  including an array  162  of ten by five degree trapezoid non-imaging elements  164 , an array  166  of trapezoidal lenticular non-imaging elements  168 , and a plurality of skew elements  170 . Skew elements mean that the concentrators are less than completely rotationally symmetric and thus the light makes multiple bounces (reflections) around a perimeter of the element before exiting the element. These multiple bounces facilitate an increase in optical path difference. Additionally, positioning a diffuser such as diffuser  112  between the light engine and skew element  170  enhances the increase in OPD. 
       FIG. 8  illustrates a geometry for some of skew elements  170  (shown in FIG.  7 ). A first angle θ 1  refers to a top  172  of skew element  170  and a second angle θ 2  refers to a bottom  174  of skew element  170 . In one embodiment, θ 1  is less than θ 2  and accordingly an edge  176  of skew element  170  rotates right-handedly as edge  176  traverses from bottom  174  to top  172 . An absolute value of θ 1 −θ 2  is less than 180°. In another embodiment, θ 2  is less than θ 1  and accordingly edge  176  rotates left-handedly as edge  176  traverses from bottom  174  to top  172 . 
       FIG. 9  is a gain profile for ten by five degree trapezoid non-imaging element  164  (shown in FIG.  7 ).  FIG. 10  is a gain profile for trapezoidal lenticular non-imaging element (shown in FIG.  7 ).  FIG. 11  is a gain profile for a nine by nine degree trapezoid non-imaging element (not shown), and  FIG. 12  is a gain profile for an eight degree hex non-imaging element (not shown). Each profile includes a horizontal gain profile  180  and a vertical gain profile  182 . As illustrated in the gain profiles, ten by five degree trapezoid non-imaging element  164  produces a favorable gain over the widest range of viewing angles of the four profiles, with a horizontal three decibel angle of 68° and a vertical three decibel angle of 20°. 
     During operation of a screen, such as, for example, screens  70 ,  110 , and  120 , incorporating ten by five degree trapezoid non-imaging element  164  in combination with angular distribution element  58  (shown in  FIG. 1 ) or diffuser  112  (shown in FIG.  2 ), a reduction in speckle is facilitated as explained above. Additionally, gain is favorable over a wide range of viewing angles as shown in FIG.  9 . 
     Accordingly, a screen is provided that is cost-effective and facilitates a reduction in speckle while also delivering a high gain image over a wide range of viewing angles. The screen is economical to manufacture and, in one embodiment, the screen is of integral construction. In another embodiment, the screen is of unitary construction. 
       FIG. 13  is a side view with parts cut away of a rear projection speckle-reduced television set  200  including a housing  202  and a light valve or light engine  204  positioned within housing  202 . Television set  200  further includes a reflection mirror  206  and reduced speckle screen  70  (shown in FIG.  2 ). In an alternative embodiment, television set  200  includes screen  110  (shown in  FIG. 3 ) or screen  120  (shown in  FIG. 5 ) instead of screen  70 . 
     During operation of television set  200 , light leaves light engine  204  and is reflected off reflection mirror  206  to light engine side  88  of screen  70 . As explained above regarding  FIG. 2 , the light passes through substrate  74  before travelling through concentrator  72 . The light passes through top portion  156  before being diffused by diffuser  90  and exiting audience side  86 . As explained above, diffuser  90  in combination with concentrator  72  facilitates a reduction in speckle. More specifically, diffuser  90  causes an OPD inside a spatial coherence zone of light  92  that is greater than a coherence length of light  92  as light  92  leaves audience side  86  of screen  70  facilitating a reduction of speckle. In one embodiment, the distribution of optical paths i.e., OPD, is at least two times the coherence length of the light. In another embodiment, the distribution of optical paths is more than or less than two times the coherence length of the light. 
       FIG. 14  is an embodiment of a front projection television system  220  including a light engine  222  and a reflection screen  224 . Light engine  222  is positioned in an audience space  223  including a plurality of viewer locations (not shown). Reflection screen  224  includes a plurality of divots  226  shaped identical to concentrator element  96  (shown in FIG.  4 ). In one embodiment, divots  226  are voids. In another embodiment, divots  226  are fabricated from an optical conductor with an index of refraction between about 1.1 and about 3. Screen  224  further includes a reflective coating  228  on a light engine side  230 . 
     In one embodiment, reflective coating  228  is a diffusing reflective coating. Light from light engine  222  enters divots  226  from light engine side  230  and reflects off coating  228  returning toward light engine  222 . More particularly, at least some of the light entering divots  226  make multiple reflections off coating  228  which causes an optical path difference between an on-axis path and an off-axis path of the light to accumulate before the light returns toward audience space  223 . Additionally, the multiple reflections of the light off of diffusing reflective surface  228  distributes the decorrelated light to audience space  223  such that at each viewer location light is received from multiple paths through divots  226 , which facilitates a reduction in speckle. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.