Abstract:
An optical filter includes an array of light-transmissive beads in a matrix of opaque binder that surrounds one portion of the beads to provide light entrances and exit apertures for incident light to pass substantially only through the beads. A polarizing layer is disposed between the apertures and the viewing surface of the filter for improved rejection of ambient light. The polarizing layer transmits linearly polarized image light aligned with the polarization axis while attenuating randomly or orthogonally polarized ambient light, resulting in improved image contrast.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. § 120 as a continuation-in-part of co-pending U.S. patent application Ser. No. 10/039,622, filed on Dec. 31, 2001, entitled “Light Transmissive Filter Having Anisotropic Properties and Method of Fabrication,” by Charles Robert Wolfe and Dennis W. Vance, the subject matter of this application also is related to commonly-owned U.S. patent application Ser. No. 10/360,470, filed on Feb. 7, 2003, entitled “Method And Apparatus For Correcting Visual Aberrations In Image Projection Systems,” by Charles Robert Wolfe, and U.S. patent application Ser. No. 10/211,785, filed on Aug. 1, 2002, entitled “Lens Optimization For Image Projection Systems,” by Charles Robert Wolfe, now issued as U.S. Pat. No. 6,700,713, the subject matter of which applications are incorporated herein in the entireties by this reference hereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to the field of light filters, and more particularly to light filters with improved rejection of ambient light.  
       BACKGROUND OF THE INVENTION  
       [0003]     Rear projection screens and light diffusers include light filters which provide an optically dispersing medium for transmitting light from an image source on one side of the screen to a viewer on the opposite side of the screen. A basic refractive light filter has been described in U.S. Pat. No. 2,378,252, which includes a refracting lens system as its principal component. The refracting lens system comprises an array of spherical transparent beads embedded in an opaque binder layer and mounted on a transparent support material. Certain known light filters orient the bead layer toward the image source and the transparent support material toward the viewers. (See, for example, U.S. Pat. No. 5,563,738).  
         [0004]     The opaque binder layer affixes the beads to the support material, reduces the reflectivity of the filter, and reduces the amount of light transmitted through the interstices between the beads of the lens system. Light from an image is refracted by the beads and dispersed to the viewer through a transmission area of the beads. This transmission area includes an aperture about the point of contact between the bead and support material and the area surrounding this point where the opaque binder layer is too thin to absorb the refracted light.  
         [0005]     Rear projection screens and light diffusers are characterized by their ambient light rejection, resolution, gain, and contrast as properties that are determined by the structure and composition of the component materials. For example, in traditional light filters the ambient light rejection and contrast of the light filter are determined largely by the opacity of the binder layer.  
         [0006]     However, such traditional light filters still allow a significant amount of ambient light to be reflected from the viewing surface of the filter, decreasing the contrast of the filter. The brightness of traditional beaded screen light filters may be increased by reducing the opacity of the binder layer. However, increasing brightness in this manner would result in reduced contrast for the light filter.  
       SUMMARY OF THE INVENTION  
       [0007]     A multi-layer light filter in accordance with the present invention includes a single layer of glass or resin beads supported in an opaque layer, and includes an additional polarizing layer that transmits linearly polarized image light that is aligned with the polarization axis of the polarizer and blocks or absorbs a fraction of randomly polarized ambient light and ambient light not polarized in alignment with the polarizer. This structure of optical components enhances the contrast relative to non-polarizing light filters. Contrast is improved by linearly polarizing image light while allowing orthogonally or randomly polarizing ambient light, thereby attenuating ambient light. The improved contrast is maintained at various viewing angles, as viewed from the viewer side of the light filter. The improvement in contrast is symmetric, i.e., equivalent for horizontal and vertical angles alike. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a sectional view of a segment of a non-polarizing prior art light filter according to one embodiment of the present invention.  
         [0009]      FIG. 2A  is a sectional view of a segment of a polarizing beaded light filter according to one embodiment of the present invention.  
         [0010]      FIG. 2B  is a sectional view of a segment of a polarizing beaded light filter according to another embodiment of the present invention.  
         [0011]      FIG. 3A  is a graph comparing on-axis contrast of a non-polarizing and a polarizing light filter for various ambient light levels according to one embodiment of the present invention.  
         [0012]      FIGS. 3B and 3C  are graphs comparing on-axis contrast of a non-polarizing and a polarizing light filter for various ambient light levels according to two embodiments of the present invention.  
         [0013]      FIG. 4  is a graph comparing contrast of a non-polarizing and two polarizing light filters at various viewing angles according to one embodiment of the present invention.  
         [0014]      FIG. 5  is a flowchart illustrating a method of manufacture of a light filter according to one embodiment of the present invention.  
         [0015]      FIG. 6  is a process schematic of a roll-to-roll lamination process according to one embodiment of the present invention.  
         [0016]      FIG. 7  is a process schematic of a roll-to-sheet lamination process according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Referring now to  FIG. 1 , there is shown a sectional view of a segment of a conventional non-polarizing beaded light filter  10 . This conventional filter  10  includes a transparent support layer  12  in contact with a plurality of spherical glass or resin transparent beads  14  that are oriented substantially in contiguous array. This allows transmission of light  18  through a plurality of transmission apertures  20  and through the support layer  12  for viewing at different horizontal or vertical viewing angles. The beads  14  each have a radius about equal to a selected value R. The interstices ( 30 ) between beads  14  on the support layer  12  are filled with an opaque layer  16  that inhibits the passage of incident light  18  through the filter  10  otherwise than through the front center of the beads  14 . Transparent support layer  12  is laminated to the beads  14  and opaque layer  16  by a transparent layer of pressure-sensitive adhesive (PSA)  22 .  
         [0018]     The light filter  10  may include an additional conformal layer of light transmissive material  24  disposed over the protruding beads  14  to a substantially uniform thickness between about 0.1 R and 1.0 R, the thickness measured normal to the protruding spherical surfaces  26  of the beads  14 .  
         [0019]     The conformal layer  24  defines a plurality of lenses  28  for reducing dispersion of incident light and increasing the transmittance of the light filter  10 . Each such lens  28  is disposed on the protruding spherical surface  26  of a bead  14  and has a substantially spherical or curved incident surface with a radius of curvature about 1.1 to 2 times the radius of the bead  14  or an average thickness around the beads of about 0.1 to 1 times the radius of the beads  14 .  
         [0020]     Light  18  that is approximately collimated from an effectively distant image source  18  is incident on filter  10  at back surfaces  26  of beads  14  and back surface  30  of the opaque binder layer  16  between the beads  14 . These surfaces define an incident or image side of light filter  10 . Outer surface  32  of the support layer  12  may define a front or viewing side of light filter  10  through which viewers observer the transmitted image light. Thus, light  18  incident on beads  14  is refracted and transmitted through the beads  14  and the associated transmission apertures  20 , and passes through the support layer  12  to emerge as signal light  38 . Light  18  incident on back surface  30  of binder layer  16  between beads  14  is absorbed to reduce transmission of this light through the filter  10 .  
         [0021]     For the light filter  10  illustrated in  FIG. 1 , ambient light incident  34  on the viewing surface  32  may be partially reflected  36  at the viewing surface  32 , at the interface between the opaque layer  16  and transparent support layer  12 , and at the interface between the beads  14  and transparent support layer  12 . Such ambient light reflecting off the light filter  10  can reduce contrast of the viewing image. To reduce ambient light reflecting  36  off the viewing surface  32 , in one embodiment, the outer surface  32  of the transparent support layer  12  may include an anti-reflective (AR) or anti-glare (AG) coating. However, the light filter  10  of  FIG. 1  still allows for significant reflection of ambient light  36 .  
         [0022]     Referring now to  FIG. 2A , there is shown a sectional view of a segment of a polarizing beaded light filter  11  according to one embodiment of the present invention. The polarizing light filter  11  includes beads  14  and an opaque layer  16 , a transparent support layer  12 , and a conformal layer  24  on the incident light  18  side of the polarizing filter  11 . A transparent support layer  12  contacts a plurality of spherical glass or resin transparent beads  14  that are oriented substantially in contiguous array. The interstices between beads  14  and the support layer  12  are filled with an opaque layer  16  that inhibits the passage of incident light  18  through the filter  11  otherwise than through the front center of the beads  14 . Transparent support layer  12  is laminated to the beads  14  and opaque layer  16  by a layer of pressure-sensitive adhesive (PSA)  22 .  
         [0023]     Light  18  that is approximately collimated from an effectively distant image source  18  is incident on filter  11  at back surfaces  26  of beads  14  and back surface  30  of the opaque binder layer  16  between the beads  14 . Thus, light  18  incident on beads  14  is refracted, transmitted through the beads  14  and the associated transmission apertures  20 , and passes through the support layer  12 , emerging as signal light  42 . Light  18  incident on back surface  30  of binder layer  16  between beads  14  is absorbed to reduce transmission of this light through the filter  11 .  
         [0024]     In addition, the polarizing filter  11  has a second layer of pressure sensitive adhesive  22 , and a linear polarizing layer  40  sandwiched in between the PSA layers  22 . The additional linear polarizing layer  40  has a two-fold effect on rejection of ambient light  34 . Image light  18  passes through the filter  1  and emerges polarized  42  from the viewing surface  32 , in contrast to emerging light  38  of the non-polarized filter  10  of  FIG. 1 . This polarization of image light  18  is linear, in a direction determined by the polarization axis of the polarizer layer  40 . In addition, ambient light  34  incident upon the filter  11  passes through the polarizing layer  40  twice. The ambient light  34  passes through the polarizing layer  40  once before the ambient light  34  reaches the opaque layer  16  and again after the light partially reflects from the opaque layer  16 . As a result, a fraction of the ambient light (that which is polarized orthogonal to the polarizing axis of the polarizer)  34  incident upon the filter  11  is blocked or extinguished by the polarizing layer. This effect significantly attenuates the amount of the reflected ambient light  44  as seen by the viewer. The combination of linear polarization  42  of the incident light  18  and the random or orthogonal polarization of the ambient light  34  results in improved contrast over non-polarizing filters, e.g., light filter  10  of  FIG. 1 . In addition, the improved brightness is achieved without attenuation of the image light  18 .  
         [0025]     In one embodiment, the polarizing layer  40  is positioned between the transparent support layer  12  and the beads  14  and opaque layer  16 . The light filter  11  of  FIG. 2A  shows this embodiment, with the polarizing layer adjacent to the beads  14  and opaque layer. However, the polarizing layer can be placed anywhere between the bead apertures  20  and the source of ambient light  34 .  
         [0026]     For example, referring now to  FIG. 2B , there is shown a sectional view of a segment of a polarized beaded light filter  13  according to another embodiment of the present invention. In this embodiment, the polarizing layer  40  is on the viewing surface  32  side of the support layer  12 . Ambient light  34  still passes twice through the polarizing layer  40 , with similar attenuation effects. Specifically, ambient light incident upon the filter  13  passes through the polarizing layer  40  a first time. Then, after passing through the support layer  12 , the ambient light  34  is partially reflected off the opaque layer  16 , and passes through the polarizing layer  40  a second time, emerging significantly attenuated  46 .  
         [0027]     In addition, ambient light rejection in the light filter of the present invention, e.g. light filters  11  and  13 , is affected by the opacity of binder layer  16 . Thus, if the opacity of binder layer  16  is increased to improve ambient light rejection, the amount of image light  18  transmitted through the transmission apertures  20  around the point of contact between beads  14  and transparent support layer  12  decreases.  
         [0028]     The light filters  11 ,  13  may include an additional conformal layer of light transmissive material  24  disposed over the protruding beads  14  to a substantially uniform thickness between about 0.1 R and 1.0 R, the thickness measured normal to the protruding spherical surfaces  26  of the beads  14 .  
         [0029]     The conformal layer  24  defines a plurality of lenses  28  for controlling dispersion of incident light and increasing the transmittance of the light filter  10 . Each such lens  28  is disposed on the protruding spherical surface  26  of a bead  14  and has a substantially spherical or curved incident surface with a radius of curvature about 1.1 to 2 times the radius of the bead  14  or an average thickness around the beads of about 0.1 to 1 times the radius of the beads  14 .  
         [0030]     In addition, in one embodiment, the outer surface  32  of the transparent support layer  12  of the polarized light filters  11 ,  13  may include an anti-reflective (AR) or anti-glare (AG) coating to further reduce the effects of ambient light. Examples of the effects of the polarizing layer are described herein in greater detail with respect to FIGS.  3 A-C and  FIG. 4 .  
         [0031]      FIG. 3A  is a graph comparing on-axis contrast of a non-polarizing and a polarizing light filter for various ambient light levels according to one embodiment of the present invention. On-axis contrast is the contrast of the light filter screen viewed at a 90-degree angle with the surface of filter, i.e., viewed from directly in front of the screen. The dotted line  315  represents contrast at various ambient light levels for a non-polarized light filter. The solid line  320  represents contrast at various ambient light levels for a polarizing light filter such as described in the various embodiments of the present invention. The ambient light level is measured in lux, the International Standard (SI) unit of measure for luminous flux density at a surface. One lux equals one lumen per square meter. In the embodiment depicted in  FIG. 3A , the polarizing light filter  320  polarizing layer has a single-pass transmission for aligned polarized light (TPP) of 0.70 a single-pass transmission of unpolarized ambient light (TPA) of 0.20. Note that both the TPP and TPA for the non-polarizing filter equal 1 (i.e., there is no polarizing layer). The extinction of the polarizer layer is given by the ration TPP/TPA. In  FIG. 3  this ratio equals 3.5, which is typical of a class of low cost liquid crystal polarizing materials manufactured by Optiva, Inc. of South San Francisco Calif. As indicated in  FIG. 3A , the screen in this example is sized one square meter, and the projector has a contrast of 1000:1 and output of 500 lumens.  
         [0032]     As shown by  FIG. 3A , contrast is greater for the polarizing light filter than for the non-polarizing filter at all light levels. Significantly, at 500-1000 lux, the level of light common in offices, the increased contrast is most significant for the polarizing filter. In addition, at 10,000 lux, which approximates outdoor sun light, a significant level of increase in contrast still can be seen in the contrast of the polarizing filter over the non-polarizing filter.  
         [0033]      FIGS. 3B and 3C  are graphs comparing on-axis contrast of a non-polarizing and a polarizing light filter for various ambient light levels according to two embodiments of the present invention. These figures illustrate how the contrast is further increased at various light levels when the polarizing layer extinction increases. In the embodiment depicted in  FIG. 3B , the polarizing light filter  320  polarizing layer has a single-pass transmission for aligned polarized light (TPP) of 0.90 a single-pass transmission of unpolarized ambient light (TPA) of 0.15. Thus, the polarizing layer extinction is 6.0. In the embodiment depicted in  FIG. 3C , the polarizing light filter  320  polarizing layer has a single-pass transmission for aligned polarized light (TPP) of 0.90 a single-pass transmission of unpolarized ambient light (TPA) of 0.09. Thus, the polarizing layer extinction is 10.0. The TPP and TPA for the non-polarizing filter in  FIGS. 3B and 3C  equals 1 (i.e., there is no polarizing layer).  
         [0034]     In the various embodiments, the thickness of the polarizing layer should be such that it does not add significantly to the total thickness of the film layers applied to the surface of the support layer  12 . The support layer thickness typically ranges 0.100″-0.200″ for rigid materials: 0.010″-0.020″ thickness is typical for flexible, roll-up support materials. Typically, the thickness of the polarizing layer is much less, &lt;10 microns typically are achieved.  
         [0035]     The graph of  FIG. 4  shows f two polarizing light filters (lines  415 ,  420 ) with significant gains in contrast over a non-polarizing light filter (line  425 ) at various viewing angles. Note that the increased contrast has circular symmetry in the viewing space, i.e., the angles listed are measured from either left or right, or up or down relative to on-axis viewing. In this figure, the ambient light level remains constant (at  200  lux), and the plots show contrast  405  for various viewing angles  410 , as measured from normal (on-axis) viewing.  
         [0036]     The solid line  425  of  FIG. 4  represents the contrast level of a non-polarizing light filter, and the dotted  415  and dashed  420  lines respectively represent contrast levels for polarizing light filters in accordance with the present invention. Both polarizing light filters  415 ,  420  show significant increases in contrast at all viewing angles over the non-polarizing light filter  425 . This increase is most pronounced at viewing angles commonly encountered, i.e., 0-45 degrees in either direction from on-axis viewing, although contrast improvement is achieved at high incident angles (glancing incidence) also.  
         [0037]     The polarizing light filter represented by the dashed line  420  shows an even greater increase in contrast over the non-polarizing filter  425  than does the polarizing filter shown by the dotted line  415 . Note that the transmission of both polarized image light (TPP) and unpolarized ambient light (TPA) are different between these filters. Both the TPP and the TPA of the polarizing filter  420  have reduced values compared with these parameters of the polarizing filter  415 . The fact that contrast is increased despite the reduction of image light (TPP), indicates that reduction of ambient light (TPA) has a larger effect upon contrast.  
         [0038]     Therefore, improved contrast may be established using filter structures according to the present invention that promote greater rejection of ambient light in comparison with non-polarizing light filters.  
         [0039]     Light filters of the type described in this patent are commonly made by first preparing the layered materials in film form by roll coating. For example, the beaded structure, the pressure sensitive adhesive (PSA), the anti-glare (AG) film, anti-reflection (AR) film and polarizing layer materials are commonly made as films and supplied in roll form. Screen manufacturing typically involves two lamination processes: roll-to-roll lamination and roll-to-sheet lamination.  
         [0040]      FIG. 5  is a flowchart illustrating a method of manufacture of a light filter according to one embodiment of the present invention. The process begins with two film sub-assembly materials  505  in rolls. Using roll-to-roll lamination, the two film sub-assembly materials  505   a ,  505   b  are combined  510  into a multi-layer assembly. In one embodiment, the sub-assembly materials  505  are pressure-sensitive adhesive (PSA)  505   a , which is applied to the front surface of a beaded film  505   b  to produce a multi-layer assembly  615 . This embodiment is depicted in  FIG. 6 .  
         [0041]     In the embodiment of  FIG. 6 , the process begins with a supply roll ( 600 ) of PSA  505   a  sandwiched between two release liners  605 ,  607 . The first sub-assembly (e.g., PSA  505   a ) is separated  620 , releasing it from one of the liners  605 . In addition, a supply roll ( 630 ) of the second sub-assembly (e.g., beaded film  505   b ) on a release liner  610  is separated  640 . Then, the two sub-assemblies are combined (step  510  of  FIG. 5 ), e.g., PSA  505   a  (with liner  607 ) and beaded film  505   b  are combined, or laminated, using soft rollers  650  that can apply both heat and pressure, or only pressure to the layers  505 . The result is a multi-layer assembly  615  that includes the PSA  505   a  with liner  607  and the beaded film  505   b.    
         [0042]     Other examples of sub-assembly materials  505  that may be combined include: applying PSA to the back surface of an anti-glare or anti-reflective film, applying PSA to the polarizing layer, or applying the polarizing layer/PSA sub-assembly to the beaded film/PSA sub-assembly.  
         [0043]     Referring again to  FIG. 5 , the multi-layer assembly is then laminated to a sheet substrate using a roll-to-sheet lamination process. This step produces a light filter end product  530 .  FIG. 7  depicts an example of the roll-to-sheet lamination process. Starting with the multi-layer assembly  615  of  FIG. 6  in a supply roll  700 , the liner  607  is removed  710 . Then, soft rollers  720  may apply both pressure and heat, or only pressure to laminate the PSA  505   a  and beaded film  505   b  to a sheet substrate  730 . In one embodiment, the sheet substrate  730  is rigid plastic (e.g., acrylic). In another embodiment, the sheet substrate  730  is glass. The result is a completed light filter  740 , e.g., the light filters described in conjunction with  FIGS. 2A and 2B , which is ready for inspection and shipping to a purchaser.