Abstract:
The present invention relates to an optical device for black level enhancement of a viewing display. Also disclosed are a system including the optical device and methods of improving black level of a viewing display, such as a plasma display panel, a liquid crystal display panel, an inorganic light emitting diode display panel, or an organic light emitting diode display panel.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an optical device for black level enhancement of a viewing display, such as a plasma display panel, a liquid crystal display (“LCD”) panel, an inorganic light emitting diode (“iLED”) display panel, or organic light emitting diode (“OLED”) display panel and methods thereof. 
       BACKGROUND 
       [0002]    Flat panel screens, in particular plasma display panels (PDPs), enable color pictures with high definition, large screen diagonals and have a compact structure. A plasma screen comprises a gas-filled sealed glass cell with grid-like arranged electrodes. By applying an electric voltage, a gas discharge is caused which mainly generates light in the vacuum ultraviolet range (“VUV”). Fluorescence transforms this VUV light into visible light and the front plate of the glass cell emits this visible light to the viewer. 
         [0003]    When compared to LCD-type large area displays or televisions, PDPs suffer from poor black levels, and therefore comparably poor contrast. The poor black level performance is most evident when ambient room light shines on the plasma TV, and the high reflectance of the whitish-gray light emitters causes the blackest black that is displayed on the PDP to appear whitish-gray. Since LCD and plasma TV&#39;s are now comparable in selling price, contrast performance is becoming a deciding factor in the purchase of a flat panel TV. Plasma TV manufacturers are searching for a simple and low-cost method of improving the black level of their displays, that does not degrade other PDP performance characteristics, such as resolution and on-axis luminance or brightness. 
         [0004]    A prior-art method for improving the black-level of a PDP is presented in  FIG. 1 . In this setup, PDP pixels  10 A and  10 B are situated behind a glass layer  12  of the display panel onto which is installed a film  15  for ambient light absorption. The ambient light absorption film  15  has a substrate  18  onto which is installed a series of black light-absorbing strips  14  between which are transparent apertures  16 . The front face  20  of the ambient light absorption film  15  is transparent, but may be textured to reduce ambient light glare. 
         [0005]    In operation, ambient light ray  30  that originates from a light source in the vicinity of the PDP, typically from an overhead room light, is incident on the front face  20  and refracts into the substrate  18  before striking a black stripe  14  at location  40  where it is absorbed. In this way ambient light is absorbed and prevented from reaching the highly reflective pixels  10 A and  10 B. However, light rays such as ray  32  refract through the front surface  20  into the substrate  18 , but then miss the black stripes  14  and pass through an aperture  16  unattenuated. This ray then passes through the glass layer  12  and is then incident on a PDP pixel  10 A, at location  44  whereupon it is backscattered into a full hemisphere. Some of the backscattered light, such as ray  36 , will be incident on a black stripe and be absorbed, such as at location  48 . However other rays, such as ray  34 , will pass through an aperture  46  between the black stripes and will exit the PDP system. These rays can be easily seen by the TV viewer, and degrade the viewing performance of the PDP by making the black colors appear gray, and by making the saturated colors appear dingy and pale. 
         [0006]    The ambient light absorption film  15  also impacts the brightness of the PDP because a large portion of the light rays emitted by the pixels are absorbed by the black stripes. For example, light ray  62  emitted from pixel  10 B at location  52  passes through the glass  12  and immediately strikes the backside of a black stripe at location  54  and is absorbed. On the other hand, light ray  64  emitted from pixel  10 B at location  50  is able to pass through an aperture of the ambient light absorption film  15  at location  56  unattenuated. 
         [0007]    To obtain maximum brightness then, the ratio of the width of the apertures  16  to the pitch of the black stripes needs to be maximized. But this is at odds with how black-level performance is maximized, and typically a trade-off between transmittance and ambient light absorption must be made at the light absorption film  15 . Because of this compromise generally both the light transmission of the film and the ambient light absorption characteristics are deemed to be inferior to the performance of the LCD-type displays. Consequently there is a genuine need for an ambient light absorption film that has high display light transmission and also high ambient light absorption. The present invention is directed to overcoming these and other deficiencies in the art. 
       SUMMARY OF THE INVENTION 
       [0008]    An optical device in accordance with embodiments of the present invention includes a microstructure layer having first and second opposing surfaces, wherein the microstructure layer comprises a plurality of transparent microstructures which form a plurality of spaced grooves, said grooves being at least partially filled with an opaque material and positioned to create alternating opaque and transparent sections having refractive index values within 0.03, and a transparent substrate adjacent at least a portion of the first surface of the microstructure layer. 
         [0009]    A system for improving black level of a viewing display in accordance with embodiments of the present invention includes the optical device and a viewing display, wherein the second surface of the microstructure layer is adjacent at least a portion of the viewing display. 
         [0010]    A method for improving contrast of a viewing display in accordance with embodiments of the present invention includes providing an optical device including a microstructure layer having first and second opposing surfaces, wherein the microstructure layer comprises a plurality of transparent microstructures which form a plurality of spaced grooves, said grooves being at least partially filled with an opaque material and positioned to create alternating opaque and transparent sections having refractive index values within 0.03, and a transparent substrate adjacent at least a portion of the first surface of the microstructure layer. At least a portion of the second surface of the microstructure layer is positioned adjacent at least a portion of an output surface of a viewing display, wherein a portion of ambient light is absorbed by the optical device before reaching the viewing display and a portion of ambient light reflected from the viewing display is absorbed by the optical device. 
         [0011]    Accordingly, the present invention provides devices, systems, and methods for improving the black level and/or contrast of viewing displays, such as plasma display panels, LCD display panels, iLED display panels, and OLED display panels. The devices, systems, and methods of the present invention do not degrade other performance characteristics, such as resolution. In particular, light from the display panel passes through the transparent prisms, whereas ambient light will generally strike the blackened areas between the transparent prisms, and be absorbed. In this way ambient light absorption is maximized without unduly impacting display light transmittance through the film. Additionally, the present invention provides a microstructured optical device that is easy and inexpensive to manufacture and which has a compact design. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a partial, cross-sectional view of a prior art device for improving the black level of a plasma display panel; 
           [0013]      FIG. 2  is a partial, cross-sectional view of an optical device and system in accordance with exemplary embodiments of the present invention; 
           [0014]      FIG. 3  is a partial cross-sectional view of the optical device and system illustrated in  FIG. 2  showing the various rays and symbols used to analyze and eliminate the pixel-ghosting problem that arises in the present invention; 
           [0015]      FIG. 4  is a partial, cross-sectional view of an alternate embodiment of an optical device and system in accordance with exemplary embodiments of the present invention; 
           [0016]      FIG. 5  is a partial, front view of an optical device and system in accordance with exemplary embodiments of the present invention which includes a microstructured optical device installed atop the pixels of a display panel in which the microstructure runs horizontally; 
           [0017]      FIG. 6  is a partial, front view of an optical device and system in accordance with exemplary embodiments of the present invention which includes a microstructured optical device installed atop the pixels of a display panel in which the microstructure runs vertically; 
           [0018]      FIGS. 7A-G  are tables illustrating the dependency of the reflectance from an opaque region on the refractive index of the transparent microstructure, the index of the opaque microstructure, and the angle of incidence of the incident light, in accordance with exemplary embodiments of the present invention for improving the black level of a display panel; 
           [0019]      FIG. 8  is a partial, cross-sectional view of an optical device and system in accordance with exemplary embodiments of the present invention in which the sides of the opaque sections are curved; and 
           [0020]      FIG. 9  is a partial, cross-sectional view of an optical device and system in accordance with exemplary embodiments of the present invention in which the substrate of the optical device is located at the display panel side. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    A system  99  including a microstructured optical device  100  in accordance with embodiments of the present invention is illustrated in  FIGS. 2-4 . Referring to  FIGS. 2 and 3 , the optical device  100  includes a substrate  118  comprising a transparent material. Suitable transparent materials include, but are not limited to, polymer sheets or films, such as acrylics, polycarbonates, vinyls, polyethylene terephthalate (“PET”), and polyethylene naphthalate (“PEN”). In one embodiment, as shown in  FIG. 2 , the substrate  118  has a thickness A of from about 0.02 mm to about 10 mm. In another embodiment, the refractive index of the substrate  118  is between 1.4 and 1.6, although lower values of index are preferable to minimize fresnel reflections from the front and rear surfaces. Referring to  FIG. 2 , the substrate  118  has a first surface  120  and a second surface  121 . In one embodiment, the first surface  120  of the substrate, i.e. that which faces the viewer, is treated with an anti-reflective coating or a subwavelength antireflective microstructure to minimize reflections from surface  120 . Furthermore, in another embodiment, first surface  120  has a diffusive surface relief texture to minimize specular glare. 
         [0022]    Referring to  FIG. 2 , adjacent at least a portion of the second surface  121  of substrate  118  is a microstructure layer  101  having a first surface  106  and an opposing second surface  108 . In one embodiment, the microstructure layer  101  has a thickness of from about 0.01 mm to about 1 mm. Normally first surface  106  is a planar, optically smooth surface. First surface  106  is adjacent and in contact with substrate  118 . 
         [0023]    The microstructure layer  101  includes a plurality of transparent microstructures  102 . As used herein, a plurality includes more than one. The microstructures  102  are linear prisms or lenticulars, and can have a trapezoidal cross-sectional shape as shown in  FIG. 2 , although other cross-sectional shapes such as triangular, rectangular, or square, are possible. If the cross-sectional shape of the transparent microstructures  102  is triangular, the triangle can be isosceles, or it can be tilted, asymmetric, or otherwise non-isosceles so that the ambient light absorption, the display light emission, or both, can be asymmetric. Furthermore, although the sides of the microstructures  102  are shown as straight in  FIGS. 2-4 , other embodiments are possible, including curved sides (see description below). The bases of the microstructures  102  at the first surface  106 , which are adjacent the substrate  118 , may be touching, or may be spaced apart. 
         [0024]    In one embodiment, the transparent microstructures  102  are fabricated from UV curable resin in a casting process, or they can be made with a molding process such as injection molding or embossing, using any suitable material, such as acrylic, polycarbonate, or vinyl. In another embodiment, the refractive index of the transparent microstructures  102  is between 1.4 and 1.6, although lower indices perform better as described below. In yet another embodiment, the transparent microstructures  102  have an aspect ratio of from about 0.5 to about 3.0. Normally the transparent microstructures  102  have minimal amounts of haze, although some haze may be beneficial to overcome the louvering effects imparted by the opaque material  114  on the light emitted by the display panel. Furthermore, the normally transparent microstructures  102  can have bulk diffusive properties obtained by dispersing particles of a different refractive index throughout the transparent microstructures  102 . 
         [0025]    The transmittance of the transparent microstructures  102  should not be spectrally dependent, but instead should transmit all wavelengths approximately the same between 400 nm and 700 nm so that it does not impart a strong tint to the viewed image. However, if a mild tint is imparted, the spectral emissive properties of the display panel can be changed to reduce or eliminate the effect. Alternately, tinting can be intentionally added to the transparent microstructures  102  or to the substrate  118  to compensate for spectral irregularities of the light emitted by the display panel. Furthermore, IR absorbing additives can be provided that reduce the amount of infra-red light that is emitted by the display. Such IR emissions have been known to disrupt IR-based handheld remote controls, and blocking these emissions would be beneficial. 
         [0026]    Referring to  FIGS. 2 and 3 , the microstructures  102  form a plurality of triangular-shaped grooves in the microstructure layer  101 , which extend from the second surface  108 . The angle of the sidewalls of the triangular-shaped grooves in  FIGS. 2 and 3  is from about 2° to about 20°, most preferably from about 4° to about 8°, from a line parallel to the optical axis O. As shown in  FIGS. 2 and 3 , in this embodiment, the triangular-shaped grooves extend from the first surface  106  at the narrow end of the triangle to the second surface  108  at the wide end of the triangle (i.e., they taper toward the substrate  118 ). However, in alternative embodiments, the grooves may extend only partially within the microstructure layer (i.e., in this embodiment, the grooves do not extend from first surface  106  to second surface  108 ). 
         [0027]    Although in this embodiment of the present invention, the microstructure layer  101  includes symmetric triangular-shaped grooves (i.e., isosceles triangle-shaped grooves in cross-section), other shapes of grooves may be used including, but not limited to, non-isosceles triangles, rectangular, square, and trapezoidal, and their side and base surfaces can be flat as shown in  FIGS. 2 and 3 , or one of more of them can be curved or non-linear. An example of trapezoidal-shaped grooves is shown in  FIG. 4 . 
         [0028]    The grooves are typically either linear and parallel or arcuate and concentric, although other configurations are possible. In one embodiment of the present invention, the grooves extend horizontally across the optical device  100 . In another embodiment, as illustrated in  FIG. 6 , the grooves extend vertically across the optical device  100 . In yet another embodiment, the microstructure layer  101  includes multiple sets of grooves. For example, the multiple sets of grooves can be positioned such that they are cross-hatched (bi-directional) wherein two sets of grooves are orthogonal to each other or three sets of grooves can be positioned so that they are rotationally 60 degrees apart. Furthermore, two or more sets of optical devices  100  can be used, either crossed or running parallel (either vertically, horizontally, or some other arbitrary angle to minimize moiré). 
         [0029]    Referring to  FIGS. 2 and 3 , the triangular-shaped grooves are filled with an opaque material  114  to create alternating transparent and opaque sections on surface  108  of the microstructure layer  101 . Alternately the triangular-shaped grooves can be partially filled with an opaque material  114  as long as the sides of the grooves are coated with the opaque material. In this case the void behind the partially filled triangular-shaped groove could be filled with a second material, or it can be left vacant. The opaque material  114  has a light absorbing characteristic. Also referring to  FIG. 2 , the distance D between adjacent opaque sections  114  is from about 0.01 mm to about 1 mm and the width B of the opaque sections  114  is from about 0.005 mm to about 0.5 mm. Suitable opaque materials  114  include, but are not limited to, a UV curable resin, a solvent-cured material, a paint, a heat-curing material, or any other material that polymerizes without the use of UV radiation. In one embodiment, light absorbing particles are mixed into, for example, a UV curable resin to form the opaque material  114 . Suitable light absorbing particles include, but are not limited to, carbon, dyes, inks, or stains. 
         [0030]    In one embodiment, the opaque material  114  has a refractive index of from about 1.4 to about 1.6. In one particular embodiment of the present invention, the refractive index of the microstructures  102  and opaque material  114  are substantially equal. This reduces fresnel reflection of light (both ambient light and light emitted from the display). In one preferred embodiment, the difference in refractive indices between the transparent microstructures  102  and the opaque material  114  is 0.03 or less. In another preferred embodiment, the refractive index of the opaque material  114  is greater than the refractive index of the microstructure  102  so that Total Internal Reflection of ambient light or light emitted from a pixel  10  does not occur at the interface between the two materials. 
         [0031]    In addition, the opaque material  114  preferably has an optical density greater than 1.0, most preferably greater than 3.0, and superior ambient light absorbance is achieved when the optical density is 5.0 or more. 
         [0032]    In yet another embodiment, the opaque material  114  is composed of a dielectric material. However, in alternate embodiments, the opaque material may contain metallic components, particularly light-absorbing ferrous materials that can be magnetically mixed, dispersed, or deposited throughout a dielectric matrix of a supporting medium. The opaque material  114  may also contain particles of metallic oxides. 
         [0033]    In a further embodiment, a non-symmetric microstructure  102 , such as a parallelogram cross-section, or slanted trapezoid, can be tailored to produce opaque regions  114  that preferentially absorb ambient light from a predetermined direction, such as from overhead. 
         [0034]      FIG. 5  is a front view of the present invention, showing the pixels  10  of the display panel in the background behind the opaque material  114  and  214  (described below). A duty factor of the opaque material  114  can be defined as the ratio of the width of the widest part of an opaque material  114 , designated as “W” in  FIG. 5  divided by the pitch, P. That is, the duty factor D=W/P. Larger duty factors allow for greater light absorption while smaller duty factors allow for greater display light transmittance through the optical device  100 . A typical value for D is 0.15, although it can range from about 0.05 up to about 0.85. 
         [0035]    The absorbance of the opaque material  114  should not be spectrally dependent, but instead should absorb all wavelengths approximately the same between 400 nm and 700 nm so that it does not impart a strong tint to the viewed image. However, if a mild tint is imparted, the spectral emissive properties of the display panel can be changed to reduce or eliminate the effect. Alternately, tinting can be intentionally added to the opaque material  114  to compensate for spectral irregularities of the light emitted by the display panel. Furthermore, IR absorbing additives can be added to the opaque material  114  that reduce the amount of infra-red light that is emitted by the display. Such IR emissions have been known to disrupt IR-based handheld remote controls, and blocking these emissions would be beneficial. 
         [0036]    In another embodiment, the grooves filled with opaque material  114  have an aspect ratio, defined as the ratio of H/B (see  FIG. 2 ), of greater than one for optimal ambient light absorption as described below. The material of the opaque material  114 , the transparent microstructure  102 , or both can have elastomeric properties to facilitate molding of the high aspect ratio microstructure. 
         [0037]    Referring to  FIG. 5 , in one embodiment, the microstructures  102  have a pitch P of from about 10 μm to about 1 mm, which should be much less than the width of a pixel  10  so that moiré interference does not occur. The pitch of the microstructures can be such that there are at least two, and preferably five or more, transparent microstructures  102  per pixel  10  of the viewing display. 
         [0038]    In one exemplary embodiment, the thickness of the optical device  100  including the substrate  118  and microstructure layer  101  is less than about 1 mm, preferably in the range of from about 0.1 mm to about 2.5 mm. In general it is desirable to keep the thickness of the optical device  100  as small as possible, in keeping with the trend to thinner displays, and the total thickness can be kept as low as 0.45 mm (0.02 mm for an adhesive layer, 0.10 mm microstructure layer  101 , and 0.15 mm substrate  118  thickness) although other thicknesses can be provided to best suit the application. 
         [0039]    Referring to  FIG. 2 , second surface  108  of the microstructure layer  101  is adhered to an output surface of a front face panel  12  of a viewing display using an adhesive layer  104 , such as a pressure sensitive adhesive (PSA). Alternatively, the optical device  100  can be installed onto a light-transmissive sheet of material that is then placed in front of the display panel. The transmittance of the adhesive layer  104  should not be spectrally dependent, but instead should transmit all wavelengths approximately the same between 400 nm and 700 nm so that it does not impart a strong tint to the viewed image. However, if a mild tint is imparted to the adhesive layer  104 , the spectral emissive properties of the display panel can be changed to reduce or eliminate the effect. Alternately, tinting can be intentionally added to the adhesive layer  104  to compensate for spectral irregularities of the light emitted by the display panel. Furthermore, IR absorbing additives can be added to the adhesive layer to reduce the amount of infra-red light that is emitted by the display. Such IR emissions have been known to disrupt IR-based handheld remote controls, and blocking these emissions would be beneficial. 
         [0040]    In one exemplary embodiment, the refractive index of the adhesive layer  104  is between that of the microstructures  102  and the output surface of the viewing display  12  to reduce unwanted fresnel reflections at these interfaces. 
         [0041]    In one embodiment, the viewing display is a flat panel display. Suitable viewing displays include, but are not limited to, pixelated displays, such as plasma display panels, LCD display panels, iLED display panels, and OLED display panels.  FIGS. 2-4  show examples of pixelated displays including pixels  10 A,  10 B, and  10 C. In another embodiment, the display panel is curved, and the optical device  100  of the present invention can be formed to fit the curvature of such a non-flat device. 
         [0042]    In one embodiment, referring to  FIGS. 2-4 , the present invention relates to a method of making an optical device  100 / 200 . This method involves providing a transparent substrate  118 / 218  and applying a microstructure layer  101 / 201  having first and second opposing surfaces  106 / 206  and  108 / 208 . In accordance with one embodiment, the microstructure layer  101 / 201  is cast on substrate  118 / 218  with a casting process in which a UV curable resin is placed into a microstructured mold which is then brought into contact with the substrate  118 / 218 , and then the UV curable resin is exposed to UV light which polymerizes the resin and causes it to harden and attach to the substrate  118 / 218 . The mold is then removed. This process is typically done in a continuous roll-to-roll process in which the mold is in the form of a cylinder in which a negative of the microstructures  102 / 202  is formed into the surface, and then the UV resin and substrate  118 / 218  are continually rolled over the mold&#39;s surface as it rotates about its axis. A tie coat, such as PET or acrylic, can be provided between the UV-curable resin and the substrate to improve the adhesion of the UV cured material to the substrate. 
         [0043]    Alternately, the microstructure layer  101 / 201  can be formed directly into the substrate by the use of an embossing molding process, a compression molding process, or an injection molding process. 
         [0044]    Next the grooves are filled with opaque material  114 / 214 . Filling can be achieved by methods known to one of ordinary skill in the art. In particular, the opaque material,  114  and  214 , can be installed between the transparent microstructures  102  and  202 , respectively, in any of a number of different ways. By way of example only, the grooves and transparent microstructures  102  and  202  can both be sprayed with the opaque material, and the transparent microstructures can be wiped or squeegeed so that they are free of the opaque material, with the result that the opaque material is only present in the grooves. Alternately, the opaque material can simply be squeegeed across the grooves and transparent microstructures  102  and  202  with the result that the transparent microstructures are free of the opaque material but the opaque material will be present in the grooves. Yet another method is to use a pair of nip rollers to force the opaque material into the grooves (and leave the microstructures  102  and  202  substantially free of the opaque material) as the optical device  100 / 200 , with a bead of opaque material at the nip, passes between the rollers. The surface  108 / 208  of the microstructure layer  101 / 201  is then attached to the output surface of the viewing display  12  using an adhesive  104 / 204 , resulting in the final construction shown in  FIGS. 2-4 . 
         [0045]    Referring back to  FIG. 2 , the operation of the device  100  can be illustrated by describing how a few different types of rays interact with the device  100 . Ambient light ray  130  originates at an ambient light source, such as an overhead room lamp, or it could be reflected off of a wall of a room of the ambient environment. Regardless of its source, it is highly desirable to prevent ambient light ray  130  from being reflected back into the viewing environment. Ambient light ray  130  refracts through the first surface  120  of a substrate  118  of the optical device  100 , and thereafter enters into the transparent microstructure  102 . After propagating some distance into the transparent microstructure  102 , the ambient light ray  130  becomes incident upon a groove filled with opaque material  114  at location  140 . If the refractive index of the opaque material  114  is substantially the same as the refractive index of the transparent microstructure  102 , then ambient light ray  130  will be substantially absorbed at location  140 , regardless of the angle of incidence of the ambient light ray  130  at location  140 . In this way, good ambient light absorption is achieved. 
         [0046]    Ambient light ray  130  also illustrates an advantage of the present invention over the prior art. If the grooves of opaque material  114  were instead replaced with thin opaque stripes  14  of the prior art, then ray  130  would not be absorbed at location  140 , but instead would propagate along path  131  and pass through a transparent section at location  141 . This ray would then be backreflected by pixel  10 A, seen by a viewer, and result in an apparent reduction in screen black level. 
         [0047]    Consider another ambient light ray  132 . This ray passes through the first surface  120  of the substrate  118 , and passes through the transparent microstructure  102 , a transparent section  142 , the glass layer  12 , and eventually reaches a substantially reflective pixel  10 A at location  144 . This ray is then diffusely back-reflected at location  144  into several rays including ray  134  and ray  136 . Ray  136  is then absorbed at location  148  at the base of a triangular-shaped groove filled with opaque material  114 , and does not contribute to a reduction in display black level. On the other hand ray  134  is not absorbed by a groove filled with opaque material  114 , and exits the optical device  100  and does contribute to a reduction in screen black level. The present invention can reduce the amount of ambient light that is backreflected by 80%, and in some cases more than 95%. 
         [0048]    Fortunately rays such as ray  134  are in the minority, as most rays are incident on the base of a triangular-shaped groove filled with opaque material  114  as seen with ray  136 , or are incident on the side of a groove filled with opaque material  114 , as seen with ray  137 . Ray  137  is absorbed at location  166  on the side of a groove filled with opaque material  114 , and does not contribute to a reduction in screen black level. Note, however, that if the groove filled with opaque material  114  were replaced with thin opaque stripes  14  of the prior art, then ray  137  would instead exit the optical device  100  and contribute to a reduction in screen black level. 
         [0049]    Now consider light rays emitted by the display panel pixels themselves, such as light rays  162 ,  164 , and  168  emitted by pixel  10 B at locations  150 ,  152 , and  154 . Emitted light ray  162  is absorbed at the base of a triangular-shaped groove filled with opaque material  114  at location  158 , and reduces the apparent brightness of the display panel. Light ray  164  passes through an transparent section  156  and subsequently passes through the optical device  100  and contributes to the brightness of the display panel. The optical device  100  of the present invention will reduce the amount of transmitted light (emitted by the display panel) by less than 50%, although in some cases it may approach 75%, or be as little as 20%, depending on the ambient light absorbing characteristics of the film. 
         [0050]    Light ray  168  exits the pixel  10 B at an oblique angle and is subsequently incident on the side of a groove filled with opaque material  114  at location  170 . Light ray  168  is nominally absorbed, but if the refractive index of the clear microstructure  102  is different than the refractive of the opaque material  114 , then a reflection ray  172  exists. To a viewer, reflection ray  172  appears to originate at pixel  10 C, by way of virtual ray  174  which appears to originate at location  176 . To the viewer, then, pixel  10 B and pixel  10 C appear to overlap to some extent, and results in a phenomenon that will be referred to as “pixel blur”. This pixel blur manifests itself as a reduction in spatial resolution of the display panel. 
         [0051]    However, pixel blur can be easily remedied by substantially matching the refractive index of the opaque material  114  to the refractive index of the transparent microstructure  102 , as this will reduce or eliminate the Fresnel reflection, or Total Internal Reflection (TIR) that can occur at the point of incidence. 
         [0052]    The analysis of the light reflection at the interface between the opaque material  114  and the transparent microstructure  102  can be facilitated by referring to  FIG. 3 . In this figure, θ 1  is the angle of incidence that the emitted ray  168  makes at the interface between the opaque material  114  and the transparent microstructure  102 ; θ T  is the angle of exittance of the light ray transmitted into the opaque material  114 ; θ C =90°−θ 1 ; θ S  is the slope angle of the sidewalls of the grooves filled with the opaque material  114  relative to a normal line  184  and is typically less than 15°; θ PV  is the apparent emission angle of virtual ray  174  as it leaves a pixel  10 C at location  176 ; θ PR  is the emission angle of real ray  168  as it leaves a pixel  10 B at location  154 ; and θ Out  is the final output angle of the light ray  172  as it leaves the display panel relative to a normal line  182 . By inspection: 
         [0000]      θ C =θ PR −θ S   (Equation 1) 
         [0000]      θ PV =2θ C −θ PR   (Equation 2) 
         [0000]      Sin(θ Out )= n   C  Sin(θ PV )  (Equation 3A) 
         [0000]      θ Out   =A  sin [ n   C  Sin(θ PV )]  (Equation 3B) 
         [0000]        n   C  Sin(θ 1 )= n   O  Sin(θ T )  (Equation 4A) 
         [0000]      θ T   =A  sin [ n   C  Sin(θ 1 )/ n   O ]  (Equation 4B) 
         [0000]    where n C  is the refractive index of the transparent microstructure  102  and n O  is the refractive index of the opaque material  114 . 
         [0053]    As discussed above, it is highly desirable to minimize the power in reflected rays  172 , which is accomplished by controlling the relative refractive indices of the opaque material  114  and the transparent microstructure  102 . The amount of power in the reflected rays  172  is known to follow the Fresnel reflection equations. There are two Fresnel equations which are used to compute the amount of reflected power: one for light whose E-field is oriented perpendicular to the plane of incidence (s-polarization), and another for light whose E-field is oriented parallel to the plane of incidence (p-polarization). These two equations are: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     s 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               n 
                               C 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   I 
                                 
                                 ) 
                               
                             
                           
                           - 
                           
                             
                               n 
                               O 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   T 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               n 
                               C 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   I 
                                 
                                 ) 
                               
                             
                           
                           + 
                           
                             
                               n 
                               O 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   T 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     R 
                     p 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               n 
                               C 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   T 
                                 
                                 ) 
                               
                             
                           
                           + 
                           
                             
                               n 
                               O 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   I 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               n 
                               C 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   T 
                                 
                                 ) 
                               
                             
                           
                           - 
                           
                             
                               n 
                               O 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   θ 
                                   I 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
         [0054]    Given that the light emitted by a display panel&#39;s pixel is generally randomly polarized, containing 50% P-polarization and 50% S-polarization, the total reflectance becomes an average of these two: 
         [0000]      % R =( R   S   +R   P )/2×100%  (Equation 7) 
         [0055]    The tables in  FIGS. 7A-G  present the percentage amount of power in the reflected ray (column % R) compared to the amount of power in the incident ray, as a function of various values of incidence angle (Theta Inc. or θ 1 ), refractive index of the transparent (clear) microstructure  102  (n_clear or n C ), and refractive index of the opaque material  114  (n_opaque or n O ). As a general rule of thumb, for the pixel-blur to be minimized, the amount of power in the reflected ray should be less than 10% of the amount of power in a ray emitted by a pixel, but preferably the amount of reflected power should be less than 2%, for any given angle of incidence. This condition occurs when the refractive index difference is less than 0.01, although differences as high as 0.03 may be acceptable for some applications. Furthermore, the refractive index of the opaque material  114  should be greater than the refractive index of the transparent microstructure  102  in order to avoid total internal reflectance (TIR) conditions which can occur at large values of θ 1  and small differences in refractive index. TIR can produce 100% reflectance, which clearly will result in objectionable pixel blur. 
         [0056]    One potential problem with the film construction depicted in  FIG. 2  is that for large aspect ratio grooves, where H/B&gt;1.5, it can be difficult to produce a mold for the microstructure or it can be difficult to separate the microstructure  102  from the mold, or both problems can occur. A remedy is to simply change the design so that the transparent microstructures  102  and the grooves filled with opaque material  114  have trapezoidal shapes, or some other shape having small values of θ S  and H/B&lt;1.5. 
         [0057]    Referring back to  FIG. 4 , one such alternate configuration is shown in which θ S  is small and H/B&lt;1.5. This configuration was obtained by simply removing the peaked portions of the opaque material  114  and adjacent transparent microstructure  102  material. The resulting opaque material  214  is now trapezoidal in cross-section. The operation of this embodiment is similar to that described in connection with  FIGS. 2 and 3  and is briefly described below, although the ambient light absorption will be somewhat reduced because the surface area of the sides of the opaque material  214  is reduced. This compromise is often justified as the small reduction in performance is more than offset by the reduced cost of production. 
         [0058]    Referring to  FIG. 4 , ambient light ray  230  originates at an ambient light source. Ambient light ray  230  refracts through the first surface  220  of a substrate  218  of the optical device, and thereafter enters into the transparent microstructure. After propagating some distance into the transparent microstructure, the ambient light ray  230  becomes incident upon a groove filled with opaque material  214  at location  240 . At this point ambient light ray  230  is absorbed and does not have the opportunity to be scattered or otherwise back-reflected to the viewer and thereby degrade the black-level performance of the display. 
         [0059]    Consider another ambient light ray  232 . This ray passes through the first surface  220  of the substrate  218 , and passes through the transparent microstructure, a transparent section  242 , the glass layer  12 , and eventually reaches a substantially reflective pixel  10 A at location  244 . This ray is then diffusely back-reflected at location  244  into several rays including ray  234  and ray  236 . Ray  236  is then absorbed at location  248  at the base of a trapezoidal-shaped groove filled with opaque material  214 , and does not contribute to a reduction in display black level. On the other hand ray  234  is not absorbed by a groove filled with opaque material  214 , and exits the contrast enhancing film and does contribute to a reduction in screen black level. This embodiment of the present invention can reduce the amount of ambient light that is backreflected by 50%, and in some cases more than 95%. 
         [0060]    As described above, rays such as ray  234  are in the minority, as most rays are incident on the base of a trapezoidal-shaped groove filled with opaque material  214  as seen with ray  236 , or are incident on the side of a groove filled with opaque material  214 , as seen with ray  237 . Ray  237  is absorbed at location  266  on the side of a groove filled with opaque material  214 , and does not contribute to a reduction in screen black level. Note, however, that if the groove filled with opaque material  214  were replaced with thin opaque stripes  14  of the prior art, then ray  237  would instead exit the contrast enhancement film and contribute to a reduction in screen black level. 
         [0061]    Now consider light rays emitted by the display panel pixels themselves, such as light rays  262  and  264  emitted by pixel  10 B at locations  250  and  252 . Emitted light ray  262  is absorbed at the base of a trapezoidal-shaped groove filled with opaque material  214  at location  258 , and reduces the apparent brightness of the display panel. Light ray  264  passes through transparent section  256  and subsequently passes through the optical device  100  and contributes to the brightness of the display panel. The optical device  100  of the present invention will reduce the amount of transmitted light (emitted by the display panel) by less than 50%, although in some cases it may approach 75%, or be as little as 20%, depending on the ambient light absorbing characteristics of the film. 
         [0062]    One alternate microstructure configuration is shown in  FIG. 8  where the transparent microstructure  302  has sides that are non-linear in cross-section or curved. Non-linear sides can have several potential advantages over a linear cross-sectional shape, such as the ability to fabricate molds or tools quickly and at a lower cost, faster and less costly molding processes, and better optical performance of the finished part. 
         [0063]    Another alternate embodiment is as shown in  FIG. 9  where the optical device  100  is positioned in a reverse orientation wherein the substrate  118  is attached with PSA  104  onto the front face panel  12  of the display. The grooves filled with opaque material  114  are now facing the viewer. The operation of this configuration follows that as described in connection with  FIGS. 2-4 , including the relative refractive index values of the opaque material and the transparent microstructures  102 . 
         [0064]    Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.