Abstract:
The present invention provides a light redirecting article for redirecting light toward a target angle, the light redirecting article comprising: an input surface comprising a plurality of light redirecting structures. Each light redirecting structure has (i) a near surface having two slopes, sloping away from normal in one direction as defined by a first inclination base angle β 1 , a second inclination angle β 2 , and a first half apex angle α 2 , for accepting incident illumination over a range of incident angles and (ii) a far surface sloping away from normal, in the opposite direction relative to the input surface, as defined by a second base angle γ 1  and a second half apex angle α 1 . In addition, light redirecting structures has (b) an output surface opposing to the input surface, wherein the near and far surfaces are opposed to each other at an angle (α 1 +α 2 ), and the base angle β 1  is greater than or equal to  90  degrees.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to display illumination articles for enhancing luminance from a surface and more particularly relates to a turning film having multiple slopes that redirects light from a light guiding plate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Liquid crystal displays (LCDs) continue to improve in cost and performance, becoming a preferred display type for many computer, instrumentation, and entertainment applications. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light providing surface positioned behind the LCD for directing light outwards, towards the LCD. The challenge of providing a suitable backlight apparatus having brightness that is sufficiently uniform while remaining compact and low cost has been addressed following one of two basic approaches. In the first approach, a light-providing surface is used to provide a highly scattered, essentially Lambertian light distribution, having an essentially constant luminance over a broad range of angles. Following this first approach, with the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution in order to provide a more collimated illumination. 
         [0003]    A second approach to providing backlight illumination employs a light guiding plate (LGP) that accepts incident light from a lamp or other light source disposed at the side and guides this light internally using Total Internal Reflection (TIR) so that light is emitted from the LGP over a narrow range of angles. The output light from the LGP is typically at a fairly steep angle with respect to normal, such as 70 degrees or more. With this second approach, a turning film, one type of light redirecting article, is then used to redirect the emitted light output from the LGP toward normal. Directional turning films, broadly termed light-redirecting articles or light-redirecting films, such as that provided with the HSOT (Highly Scattering Optical Transmission) light guide panel available from Clarex, Inc., Baldwin, N.Y., provide an improved solution for providing a uniform backlight of this type, without the need for diffusion films or for dot printing in manufacture. HSOT light guide panels and other types of directional turning films use arrays of prism structures, in various combinations, to redirect light from a light guiding plate toward normal, or toward some other suitable target angle that is typically near normal relative to the two-dimensional surface. As one example, U.S. Pat. No. 6,746,130 (Ohkawa) describes a light control sheet that acts as a turning film for LGP illumination. 
         [0004]    Referring to  FIG. 1 , the overall function of a light guiding plate  10  in a display apparatus  100  is shown. Light from a light source  12  is incident at an input surface  18  and passes into light guiding plate  10 , which is typically wedge-shaped as shown. The light propagates within light guiding plate  10  until Total Internal Reflection (TIR) conditions are frustrated and then, possibly reflected from a reflective surface  142 , exits light guiding plate at an output surface  16 . This light then goes to a turning film  20  and is directed to illuminate a light-gating device  120  such as an LCD or other type of spatial light modulator or other two-dimensional backlit component that modulates the light. For optimized viewing under most conditions, the emitted light should be provided over a range of relatively narrow angles about a normal V. A polarizer  124  is typically disposed in the illumination path in order to provide light-gating device  120  such as a liquid crystal cell with suitably polarized light for modulation. A reflective polarizer  125  may be provided between absorptive polarizer  124  and turning film  20 . 
         [0005]    Referring to  FIG. 2 , there is shown a schematic cross-sectional view of a conventional turning film  20   a  used with light guiding plate  10 , showing key angles and geometric relationships. Turning film  20   a  has a number of prismatic structures facing downward toward light guiding plate  10 , each structure having a near surface  24  (being near relative to light source  12 , as shown in the embodiment of  FIG. 1 ) and a far surface  26 , both sides slanted from a film normal direction V as determined by an apex angle α, and base angles β 1  and β 2 , relative to a horizontal S. Light from light guiding plate  10  is incident over a small range of angles about a central input angle θ in . The output angle θ out  of light delivered to the LC display element is determined by a number of factors including the central input angle θ in , the refractive index n of turning film  20   a , and the base angle β 1  at which far surface  26  is slanted. Output angle θ out  for emitted light is preferably normal with respect to turning film  20   a , however output angle θ out  can be considered a target angle, which may be at some inclination with respect to normal for some applications. For most conventional turning films, the target angle is near normal. In a typical arrangement, base angles β 1  and β 2  are about 56 degrees, and apex angle α, 68 degrees. The primary (or principal) ray  50   a  having an input angle around θ in ≈70° is redirected to near normal direction. However, some secondary rays  50   c ,  50   c   1  having an input angle around θ in &lt;70° may take paths as shown in  FIG. 2 . Secondary ray  50   c   1  is redirected toward a relative large angle from the normal direction. Further, secondary ray  50   c  is totally reflected back by the light exiting surface  92 . Consequently, the light utilization of this existing turning film is not satisfactory. 
         [0006]    Referring to  FIG. 3 , there is shown a schematic cross-sectional view of an improved turning film  90   a  having multiple slopes. The turning film  90   a , while improving the light utilization, has base angle β 1  generally less than 90°. Moreover, during the manufacture of the turning film  90   a , it may be difficult to precisely control the base angle β 1  due to the asymmetry of the turning film  90   a . Thus, while there have been solutions proposed for turning films suitable for some types of display apparatus and applications, there remains a need for improved turning films. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a light redirecting article for redirecting light toward a target angle, the light redirecting article comprising: (a) an input surface comprising a plurality of light redirecting structures each light redirecting structure having: (i) a near surface having two slopes, sloping away from normal in one direction as defined by a first inclination base angle β 1 , a second inclination angle β 2 , and a first half apex angle α 2 , for accepting incident illumination over a range of incident angles; (ii) a far surface sloping away from normal, in the opposite direction relative to the input surface, as defined by a second base angle γ 1  and a second half apex angle α 1 ; and (b) an output surface opposing to the input surface, wherein the near and far surfaces are opposed to each other at an angle (α 1 +α 2 ), and the base angle β 1  is greater than or equal to 90 degrees. 
         [0008]    The present invention also provides a light redirecting article for redirecting light toward a target angle, the light redirecting article comprising: (a) an input surface comprising a plurality of light redirecting structures, each light redirecting structure having: (i) a near surface having two slopes, sloping away from normal in one direction as defined by a first inclination base angle β 1 , a second inclination angle β 2 , and a first half apex angle α 2 , for accepting incident illumination over a range of incident angles; (ii) a far surface sloping away from normal, in the opposite direction relative to the input surface, as defined by a second base angle γ 1  and a second half apex angle α 1 ; and (b) an output surface opposing to the input surface, wherein the near and far surfaces are opposed to each other at an angle (α 1 +α 2 ), two neighboring light redirecting structures have a gap G and a pitch P, and G/P is in the range of between about 0.08 and 0.12. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a cross sectional view showing components of a conventional display apparatus; 
           [0010]      FIG. 2  is a schematic cross-sectional view showing a turning film with prismatic structure facing downward, toward the light guiding plate; 
           [0011]      FIG. 3  is a schematic cross-sectional view showing a single unit of a prior turning film having two slopes on the near surface of the prismatic structures with a base angle less than 90°; 
           [0012]      FIG. 4A  is a schematic cross-sectional view showing two units of a turning film having a base angle equal to or greater than 90° according to the present invention; 
           [0013]      FIG. 4B  is a schematic cross-sectional view showing two unit of a turning film having a gap G according to the present invention; 
           [0014]      FIG. 5  is a schematic cross-sectional view showing a turning film of the present invention in an LCD display system; 
           [0015]      FIG. 6A  is a schematic top view showing an LCD with a pair of polarizers oriented at 45 degrees relative to the grooves of the light redirecting structure of the turning film; 
           [0016]      FIG. 6B  is a schematic top view showing an LCD with a pair of polarizers oriented at parallel or perpendicular to the grooves of the light redirecting structure of the turning film; and 
           [0017]      FIG. 6C  is a schematic top view showing a turning film with arcuate grooves. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The apparatus of the present invention uses light-redirecting structures that are generally shaped as prisms. True prisms have at least two planar faces. Because, however, one or more surfaces of the light-redirecting structures need not be planar in all embodiments, but may be curved or have multiple sections, the more general term “light redirecting structure” is used in this specification. 
         [0019]      FIG. 3  shows one unit of a turning film  90   a , which comprises a substrate  96  having a light inputting surface  94  and a light exiting surface  92 . On the side of the light inputting surface  94  of the film  90   a  is a prismatic structure which is described by points P 1 , P 2 , P 3 , and P 4  and characterized by a near surface  24  and a far surface  26 , and the near surface is composed of at least first flat segment  24   a  and second flat segment  24   b , the angle β 2  between the first segment  24   a  and the horizontal direction S is smaller than the angle β 1  between the second segment  24   b  and the horizontal direction S. The prismatic structure can be further described by two half apex angles α 1  and α 2 , the pitch P and height H, and three projection dimensions L 1 , L 2 , and L 3 . The prismatic structure is made of a material of refractive index n, and the substrate may have its index of refraction greater than, equal to, or less than n. The shape and the refractive index n of the prismatic structure are chosen so that the primary ray  50   a  from the light guide plate  10 , secondary ray  50   b  having larger incident angle than the primary ray  50   a , and secondary ray  50   c  having smaller incident angle than the primary ray  50   a  are characterized as following: the primary ray  50   a  is refracted by the first segment  24   a  of the near surface  24 , subsequently reflected due to the total internal reflection at the far surface  26 , and finally emerges out toward the target angle (normally within 5 degrees from the normal of the film); the secondary ray  50   b  is also refracted by the first segment  24   a  of the near surface  24 , subsequently reflected due to the total internal reflection at the far surface  26 , and finally emerges out in a direction that is bent more from its original direction than the primary ray  50   a ; and the secondary ray  50   c  is refracted by the second segment  24   b  of the near surface  24 , subsequently reflected due to the total internal reflection at the far surface  26 , and finally emerges out in a direction that is closer to the target direction than it would if the second segment  24   b  has the same slope as the first segment  24   a.    
         [0020]      FIG. 4A  shows two neighboring units of a turning film  90   b  according to the present invention, which comprises a substrate  96  having a light inputting surface  94  and a light exiting surface  92 . On the side of the light inputting surface  94  of the film  90   b  is a prismatic structure which is described by points P 1 , P 2 , P 3 , and P 4  for one unit and points P 1 ′, P 2 ′, P 3 ′, and P 4 ′ for another unit, and characterized by a near surface  24  and a far surface  26 , and the near surface is composed of at least first flat segment  24   a  and second flat segment  24   b , the angle β 2  between the first segment  24   a  and the horizontal direction S is smaller than the angle β 1  between the second segment  24   b  and the horizontal direction S. The prismatic structure can be further described by two half apex angles α 1  and α 2 , the pitch P and height H, and three projection dimensions L 1 , L 2 , and L 3 . The prismatic structure is made of a material of refractive index n, and the substrate may have its index of refraction greater than, equal to, or less than n. The shape and the refractive index n of the prismatic structure are chosen so that the primary ray  50   a  from the light guide plate  10 , secondary ray  50   b  having larger incident angle than the primary ray, and secondary ray  50   c  having smaller incident angle than the primary ray have similar characteristics as the turning film  90   a  shown in  FIG. 3 . 
         [0021]    According to one aspect of the present invention, the improved turning film  90   b  has a near based angle β 1 ≧90°. Note that in  FIG. 4A , the based angle β 1  is sharp, but it can also be rounded, meaning that there may be a curvature near Points P 1 ′ and P 1 . 
         [0022]    According to another aspect of the present invention, the improved turning film  90   b  has a gap G between the base points P 4 ′, P 1  of two neighboring prisms. As a result, projection dimension L 1  is negative as it is the difference between the projected coordinates onto the horizontal direction S of two neighboring points of one prism, while keeping L 1 /P+L 2 /P+L 3 /P=1. 
         [0023]    Inventive (denoted as “I”) and comparative examples (denoted as “C”) of turning film  90   b  are shown in Table 1-Table 2. In all of these examples, refractive index n is held constant at 1.5, and pitch P of the prisms is about 50 μm, though it can be in the range of 15 to 150 μm, preferably in the range of 20 to 75 μm, more preferably in the range of 25 to 50 μm. When n and P are held constant, there are 4 independent parameters to specify the shape of turning film  90   b , which are chosen to be L 1 /P, L 2 /P, β 1 , and β 2 . The height H and angles can be calculated as 
         [0000]    
       
         
           
             
               H 
               = 
               
                 P 
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                 α 
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             α 
             ≡ 
             
               
                 α 
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               + 
               
                 α 
                 2 
               
             
           
         
       
       
         
           
             
               
                 γ 
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                 where 
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                 2 
               
               ≡ 
               
                 
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                   2 
                 
                 P 
               
             
             , 
             
               h 
               ≡ 
               
                 
                   H 
                   P 
                 
                 . 
               
             
           
         
       
     
         [0000]    When α 1 =α 2 , it follows 
         [0000]    
       
         
           
             
               
                 l 
                 2 
               
               = 
               
                 
                   
                     1 
                     - 
                     
                       l 
                       1 
                     
                   
                   2 
                 
                 - 
                 
                   
                     
                       l 
                       1 
                     
                     2 
                   
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                         ( 
                         
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                           1 
                         
                         ) 
                       
                     
                     
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                         ( 
                         
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                         ) 
                       
                     
                   
                 
               
             
             , 
             
               
                 or 
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                   l 
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         [0000]    Consistent with the above discussion, when β 1 =90°, 
         [0000]    
       
         
           
             
               
                 
                   l 
                   1 
                 
                 ≡ 
                 
                   
                     L 
                     1 
                   
                   P 
                 
               
               = 
               0 
             
             ; 
           
         
       
     
         [0000]    and when β 1 ≧90°, 
         [0000]    
       
         
           
             
               l 
               1 
             
             ≡ 
             
               
                 L 
                 1 
               
               P 
             
             &lt; 
             0. 
           
         
       
     
         [0024]    In Table 1-2, Columns L 1 /P, β 1 , and β 2  are independent parameters. L 2 /P is chosen to be 
         [0000]    
       
         
           
             
               
                 L 
                 2 
               
               / 
               P 
             
             = 
             
               
                 l 
                 2 
               
               = 
               
                 
                   
                     1 
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         [0000]    to ensure α 1 =α 2 =90°−β 2 , and α≡2α 1 . The four right most columns represent the output of turning film in terms of total power, maximum intensity ratio, maximum intensity angle, and on-axis intensity ratio. The turning film of the present invention has: Power ≧85%, Maximum intensity ratio ≧1.1 and Maximum intensity angle is within −5° and −5°. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 summarizes impact of β 1  and G/P when β 1  ≧ 90°. 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                 Maximum 
                 Maximum 
                 On-axis 
               
               
                   
                   
                   
                 β 1   
                   
                   
                 Intensity 
                 Intensity 
                 Intensity 
               
               
                 Ex 
                 L 1 /P 
                 L 2 /P 
                 (°) 
                 G/P 
                 Power 
                 Ratio 
                 angle (°) 
                 ratio 
               
               
                   
               
             
          
           
               
                 C1.1 
                 −0.36029 
                 0.25498 
                 105.9 
                 0.32 
                 0.870 
                 0.810 
                 3.5 
                 0.699 
               
               
                 C1.2 
                 −0.32143 
                 0.24770 
                 104.7 
                 0.3 
                 0.881 
                 0.859 
                 1.5 
                 0.766 
               
               
                 C1.3 
                 −0.28472 
                 0.24082 
                 103.4 
                 0.28 
                 0.888 
                 0.937 
                 2.5 
                 0.846 
               
               
                 C1.4 
                 −0.25000 
                 0.23431 
                 102.2 
                 0.26 
                 0.893 
                 0.990 
                 1.5 
                 0.950 
               
               
                 C1.5 
                 −0.21711 
                 0.22814 
                 100.9 
                 0.24 
                 0.895 
                 1.013 
                 0.5 
                 1.012 
               
               
                 C1.6 
                 −0.18590 
                 0.22229 
                 99.6 
                 0.22 
                 0.896 
                 1.066 
                 −0.5 
                 1.046 
               
               
                 C1.7 
                 −0.15625 
                 0.21673 
                 98.3 
                 0.2 
                 0.895 
                 1.081 
                 −0.5 
                 1.080 
               
               
                 I1.1 
                 −0.12805 
                 0.21145 
                 97.0 
                 0.18 
                 0.895 
                 1.114 
                 0.5 
                 1.105 
               
               
                 I1.2 
                 −0.10119 
                 0.20641 
                 95.7 
                 0.16 
                 0.895 
                 1.124 
                 1.5 
                 1.110 
               
               
                 I1.3 
                 −0.07558 
                 0.20161 
                 94.3 
                 0.14 
                 0.894 
                 1.129 
                 1.5 
                 1.117 
               
               
                 I1.4 
                 −0.05114 
                 0.19703 
                 93.0 
                 0.12 
                 0.894 
                 1.157 
                 1.5 
                 1.132 
               
               
                 I1.5 
                 −0.02778 
                 0.19265 
                 91.7 
                 0.1 
                 0.894 
                 1.157 
                 1.5 
                 1.137 
               
               
                 I1.6 
                 −0.00543 
                 0.18846 
                 90.3 
                 0.08 
                 0.893 
                 1.167 
                 1.5 
                 1.146 
               
               
                 C1.8 
                 0.01596 
                 0.18445 
                 89.0 
                 0.06 
                 0.893 
                 1.189 
                 1.5 
                 1.157 
               
               
                 C1.9 
                 0.03646 
                 0.18061 
                 87.7 
                 0.04 
                 0.892 
                 1.173 
                 1.5 
                 1.134 
               
               
                 C1.10 
                 0.05612 
                 0.17693 
                 86.3 
                 0.02 
                 0.892 
                 1.168 
                 1.5 
                 1.161 
               
               
                 C1.11 
                 0.07500 
                 0.17339 
                 85.0 
                 0 
                 0.892 
                 1.196 
                 1.5 
                 1.155 
               
               
                   
               
             
          
         
       
     
         [0025]    In Table 1, Ex. C1.1-C1.7, C1.8-C.11 and I1.1-I1.6 show the impact of β 1 , L 1 /P, L 2 /P, and Gap/P, given α 1 =α 2 =α=34°, and β 2 =56°. Turning films of inventive examples I1.1 through I1.6 all have β 1 ≧90° and meet the criteria: high power (&gt;0.88), large maximum peak intensity ratio (≧1.10), and small maximum intensity angle from the normal (≦±5°). When β 1  is out of the preferred range between 90° and 98°, or Gap/P is out of the preferred range of between 0.19 and 0.07, the outputs from comparative examples C1.1-C1.7 do not meet all of the criteria, in terms of power (&gt;0.85), maximum intensity ratio (≧1.10), and maximum intensity angle (≦±5°, indicating inferior performance. 
         [0026]    Compared to comparative examples C1.8-C1.11 which have either G/P=0, or G/P out of the preferred range, the present invention of examples I1.1-I1.6 may be easier for manufacturing due to the existence of the gap between the base points P 4 , P 4 ′ (or P 1 , P 1 ′) of two neighboring prisms, and/or the base angle β 1 ≧90°. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 summarizes impact of G/P when β 1  ≧ 90°. 
               
             
          
           
               
                   
                   
                   
                 Maximum 
                 Maximum 
                 On-axis 
               
               
                   
                   
                   
                 Intensity 
                 Intensity 
                 Intensity 
               
               
                 Ex 
                 G/P 
                 Power 
                 Ratio 
                 angle (°) 
                 ratio 
               
               
                   
               
             
          
           
               
                 I2.1 
                 0.00000 
                 0.899 
                 1.119 
                 2.5 
                 1.062 
               
               
                 I2.2 
                 0.01961 
                 0.899 
                 1.112 
                 1.5 
                 1.073 
               
               
                 I2.3 
                 0.03846 
                 0.898 
                 1.107 
                 3.5 
                 1.061 
               
               
                 I2.4 
                 0.05660 
                 0.897 
                 1.130 
                 1.5 
                 1.061 
               
               
                 I2.5 
                 0.07407 
                 0.897 
                 1.137 
                 1.5 
                 1.090 
               
               
                 I2.6 
                 0.09091 
                 0.897 
                 1.154 
                 2.5 
                 1.108 
               
               
                 I2.7 
                 0.10714 
                 0.895 
                 1.153 
                 2.5 
                 1.110 
               
               
                 I2.8 
                 0.12281 
                 0.897 
                 1.154 
                 0.5 
                 1.133 
               
               
                 I2.9 
                 0.13793 
                 0.894 
                 1.131 
                 1.5 
                 1.095 
               
               
                 I2.10 
                 0.15254 
                 0.896 
                 1.121 
                 1.5 
                 1.108 
               
               
                 I2.11 
                 0.16667 
                 0.894 
                 1.112 
                 0.5 
                 1.101 
               
               
                 I2.12 
                 0.18033 
                 0.895 
                 1.131 
                 1.5 
                 1.118 
               
               
                 I2.13 
                 0.19355 
                 0.895 
                 1.129 
                 −0.5 
                 1.125 
               
               
                 I2.14 
                 0.20635 
                 0.894 
                 1.144 
                 0.5 
                 1.128 
               
               
                 I2.15 
                 0.21875 
                 0.893 
                 1.133 
                 −0.5 
                 1.125 
               
               
                 I2.16 
                 0.23077 
                 0.893 
                 1.115 
                 −0.5 
                 1.093 
               
               
                 I2.17 
                 0.24242 
                 0.893 
                 1.118 
                 −0.5 
                 1.095 
               
               
                 C2.1 
                 0.25373 
                 0.894 
                 1.090 
                 −0.5 
                 1.063 
               
               
                 C2.2 
                 0.26471 
                 0.894 
                 1.093 
                 −1.5 
                 1.045 
               
               
                 C2.3 
                 0.27536 
                 0.893 
                 1.089 
                 −1.5 
                 1.059 
               
               
                 C2.4 
                 0.28571 
                 0.893 
                 1.086 
                 −2.5 
                 1.016 
               
               
                   
               
             
          
         
       
     
         [0027]    Table 2 shows the impact of G/P when the other parameters are kept constant; L 1 /(P−G)=−0.10119, L 2 /(P−G)=0.20641, β 1 =95.7°, β 2 =56°, α 1 =α 2 =α=34°, and n=1.5. The shape of the prisms specified by those parameters is identical to example I1.2 in Table 1. 
         [0028]    Turning films of inventive examples I2.1-I2.17 meet the criteria: high power (&gt;0.88), large maximum peak intensity ratio (≧1.10), and small maximum intensity angle from the normal (≦±5°), while comparative examples C2.1-C2.4 do not. Note that the maximum intensity ratio first decreases with G/P, then increases with G/P. The maximum intensity ratio reaches a local maximum value of about 1.15 when G/P is in the range of between about 0.08 and 0.12. As G/P further increases, the maximum intensity ratio decreases and then increases to a second local maximum value of about 1.14 when G/P is about 0.21. When G/P is greater than about 0.25, the maximum intensity ratio becomes below than 1.10, as shown in examples C2.1-C2.4. 
         [0029]    Thus, Table 2 shows a turning film according to the present invention having a selective G/P ratio to maximize the maximum intensity ratio while allowing easy manufacture. 
         [0030]    Though the preferred G/P range of between about 0.08 and 0.12 is found for a turning film  90   b  having a base angle β 1 ≧90°, applicants find that this G/P range also works well for a turning film  90   c  having a base angle β 1 &lt;90°.  FIG. 4B  is a schematic cross-sectional view showing two neighboring units of a turning film  90   c  having two slopes on the near surface of the prismatic structures and the based angle β 1 &lt;90° and a gap G according to the present invention. Like parts in  FIGS. 3 ,  4 A and  4 B are designated by the same parts number. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 summarizes impact of G/P when β 1  &lt; 90°. 
               
             
          
           
               
                   
                   
                   
                 Maximum 
                 Maximum 
                 On-axis 
               
               
                   
                   
                   
                 Intensity 
                 Intensity 
                 Intensity 
               
               
                 Ex 
                 G/P 
                 Power 
                 Ratio 
                 angle (°) 
                 ratio 
               
               
                   
               
             
          
           
               
                 I3.1 
                 0.0099 
                 0.892 
                 1.204 
                 0.5 
                 1.177 
               
               
                 I3.2 
                 0.0291 
                 0.891 
                 1.173 
                 1.5 
                 1.165 
               
               
                 I3.3 
                 0.0476 
                 0.889 
                 1.185 
                 1.5 
                 1.169 
               
               
                 I3.4 
                 0.0654 
                 0.890 
                 1.178 
                 0.5 
                 1.175 
               
               
                 I3.5 
                 0.0826 
                 0.889 
                 1.189 
                 −0.5 
                 1.175 
               
               
                 I3.6 
                 0.0991 
                 0.888 
                 1.172 
                 −0.5 
                 1.162 
               
               
                 I3.7 
                 0.1071 
                 0.888 
                 1.199 
                 0.5 
                 1.193 
               
               
                 I3.8 
                 0.1150 
                 0.888 
                 1.179 
                 −0.5 
                 1.170 
               
               
                 I3.9 
                 0.1228 
                 0.887 
                 1.188 
                 −1.5 
                 1.182 
               
               
                 I3.10 
                 0.1379 
                 0.886 
                 1.153 
                 −1.5 
                 1.134 
               
               
                 I3.11 
                 0.1453 
                 0.887 
                 1.158 
                 −0.5 
                 1.147 
               
               
                 I3.12 
                 0.1525 
                 0.886 
                 1.168 
                 −1.5 
                 1.124 
               
               
                 I3.13 
                 0.1597 
                 0.885 
                 1.165 
                 −1.5 
                 1.120 
               
               
                 I3.14 
                 0.1667 
                 0.886 
                 1.166 
                 −1.5 
                 1.116 
               
               
                 I3.15 
                 0.1736 
                 0.887 
                 1.169 
                 −1.5 
                 1.109 
               
               
                 I3.16 
                 0.1803 
                 0.886 
                 1.162 
                 −1.5 
                 1.123 
               
               
                 I3.17 
                 0.1870 
                 0.885 
                 1.148 
                 −1.5 
                 1.127 
               
               
                 I3.18 
                 0.1935 
                 0.886 
                 1.141 
                 −2.5 
                 1.105 
               
               
                 I3.19 
                 0.2000 
                 0.884 
                 1.130 
                 −0.5 
                 1.120 
               
               
                 I3.20 
                 0.2063 
                 0.885 
                 1.170 
                 −2.5 
                 1.100 
               
               
                 I3.21 
                 0.2126 
                 0.886 
                 1.140 
                 −1.5 
                 1.119 
               
               
                 I3.22 
                 0.2188 
                 0.884 
                 1.140 
                 −2.5 
                 1.084 
               
               
                 I3.23 
                 0.2248 
                 0.885 
                 1.114 
                 −2.5 
                 1.064 
               
               
                 I3.24 
                 0.2308 
                 0.884 
                 1.157 
                 −2.5 
                 1.105 
               
               
                 I3.25 
                 0.2366 
                 0.884 
                 1.128 
                 −2.5 
                 1.066 
               
               
                 I3.26 
                 0.2424 
                 0.885 
                 1.147 
                 −2.5 
                 1.045 
               
               
                 I3.27 
                 0.2481 
                 0.882 
                 1.134 
                 −1.5 
                 1.072 
               
               
                 I3.28 
                 0.2537 
                 0.885 
                 1.160 
                 −2.5 
                 1.073 
               
               
                 I3.29 
                 0.2593 
                 0.882 
                 1.118 
                 −2.5 
                 1.075 
               
               
                 I3.30 
                 0.2647 
                 0.885 
                 1.130 
                 −2.5 
                 1.080 
               
               
                 I3.31 
                 0.2701 
                 0.883 
                 1.143 
                 −2.5 
                 1.096 
               
               
                 I3.32 
                 0.2754 
                 0.884 
                 1.116 
                 −3.5 
                 1.063 
               
               
                 I3.33 
                 0.2857 
                 0.885 
                 1.113 
                 −1.5 
                 1.037 
               
               
                 I3.34 
                 0.2908 
                 0.883 
                 1.115 
                 −2.5 
                 1.035 
               
               
                 I3.35 
                 0.2958 
                 0.885 
                 1.112 
                 −2.5 
                 1.066 
               
               
                 I3.36 
                 0.3007 
                 0.883 
                 1.110 
                 −3.5 
                 1.045 
               
               
                 C3.1 
                 0.3056 
                 0.885 
                 1.098 
                 −1.5 
                 1.016 
               
               
                 C3.2 
                 0.3377 
                 0.882 
                 1.077 
                 −2.5 
                 0.989 
               
               
                 C3.3 
                 0.3421 
                 0.883 
                 1.096 
                 −2.5 
                 0.978 
               
               
                 C3.4 
                 0.4737 
                 0.881 
                 1.002 
                 −6.5 
                 0.788 
               
               
                   
               
             
          
         
       
     
         [0031]    Table 3 shows the impact of G/P when the other parameters are kept constant; L 1 /(P−G)=0.077, L 2 /(P−G)=0.16633, β 1 =85°, β 2 =56°, α 1 =α 2 =α=34°, and n=1.5. The shape of the prism specified by L 1 /(P−G)=0.077, L 2 /(P−G)=0.16633, β 1 =85°, β 2 =56°, α 1 =α 2 =α=34°, and n=1.5. 
         [0032]    The inventive examples I3.1-I3.36 meet the criteria: high power (&gt;0.88), large maximum peak intensity ratio (≧1.10), and small maximum intensity angle from the normal (≦±5°). When the ratio of gap over the pitch G/P is out of the preferred range of between 0 and 0.3, the outputs from comparative examples C3.1-C3.4 do not meet all of the criteria, in terms of power (&gt;0.85), maximum intensity ratio (≧1.10), and maximum intensity angle (≦±5°), indicating inferior performance. 
         [0033]    Examples I3.3-I3.9 in Table 3 also show a more preferred range for G/P is between 0.08 and 0.12. In this range, the turning film of the present invention is easy to fabricate, and also has reasonably good optical performance as shown by the large maximum peak intensity ratio greater than 1.17. 
       Display Apparatus and Orientation of Polarizers 
       [0034]    The apparatus and method of the present invention allow a number of possible configurations for support components to provide light for an LCD.  FIG. 5  is a schematic cross-sectional view showing a display apparatus  60  using turning film  90 , which can be  90   b  or  90   c  according to the present invention. An LC spatial light modulator  70  modulates light received from light guiding plate  10  and turning film  90 . A back polarizer  72  and a front polarizer  73  are provided for LC spatial light modulator  70 . 
         [0035]      FIG. 6A  is a schematic top view showing polarized light transmission axes  172  and  173  for LC spatial light modulator  70 , using a pair of polarizers that are oriented at 45 degrees relative to light redirecting structures  75  and grooves of turning film  90  that extend vertically in the view of  FIG. 6A . In this case, the LC spatial light modulator  70  can be a twisted nematic (TN) LCD, which is the dominant mode used in a notebook and monitor display. 
         [0036]      FIG. 6B  is a schematic top view showing polarized light transmission axes  172  and  173  for LC spatial light modulator  70 , using a pair of polarizers oriented at parallel or perpendicular relative to the grooves and light redirecting structures  75  of turning film  90 . In this case, the LC spatial light modulator  70  can use vertically aligned (VA) LCD or IPS LC elements. Rear polarizer transmission axis  172  is parallel to the plane of the cross section. 
         [0037]    In one embodiment the display apparatus comprises a pair of crossed polarizers, wherein the light redirecting structures are elongated in an elongation direction and wherein each of the crossed polarizers is oriented either substantially parallel or perpendicular to the elongation direction of the light redirecting article. In another embodiment the display apparatus comprises a pair of crossed polarizers, wherein the light redirecting structures are elongated in an elongation direction and wherein the polarizers are substantially oriented at ±45 degrees relative to the elongation direction of the light redirecting article. 
         [0038]      FIG. 6C  is a schematic top view showing turning film  90  with arcuately elongated light redirecting structures  75  in another embodiment. This arrangement is advantageous for employing a point light source such as Light Emitting Diode (LED) at one or more corners of light guiding plate  10  in order to have a more compact design. The rear polarizer transmission axis  172  is more or less parallel to the plane of the cross section. 
       Materials for Forming Turning Film 
       [0039]    Turning film  90   b - 90   c  of the present invention can be fabricated using polymeric materials having indices of refraction ranging typically from about 1.40 to about 1.66. Possible polymer compositions include, but are not limited to: poly(methyl methacrylate)s, poly(cyclo olefin)s, polycarbonates, polysulfones and various co-polymers comprising various combinations of acrylate, alicyclic acrylate, carbonate, styrenic, sulfone and other moieties that are known to impart desirable optical properties, particularly high transmittance in the visible range and low level of haze. Various miscible blends of the aforementioned polymers are also possible material combinations that can be used in the present invention. The polymer compositions may be either thermoplastic or thermosetting. The former are manufacturable by an appropriate melt process that requires good melt processability while the latter can be fabricated by an appropriate UV cast and cure process or a thermal cure process. 
         [0040]    Turning film  90   b - 90   c  of the present invention may be fabricated using materials having an index of refraction in the range of 1.12 and 1.40. Example materials are inorganic materials, for example, MgF. Also, materials having a grating formed between a common polymeric material having refractive index in the range of 1.48 and 1.59 and air (n=1). Further, a mix of low index materials (n&lt;1.4) and materials having indices of refraction from about 1.40 to 1.50 may be used as well. 
       Maximum Intensity Ratio (or Optical Gain), Maximum Intensity Angle (or Peak Angle), and Power of a Turning Film 
       [0041]    In general, light distribution is specified in terms of spatial and angular distributions. The spatial distribution of light can be made quite uniform, achieved by careful placement of micro features on top and/or bottom sides of a light guide plate. The angular distribution of light is specified in terms of luminous intensity I as a function of polar angle θ and azimuthal angle. The angular distribution of light is measured with EZ Contrast 160 (available from Eldim, France). Polar angle θ is the angle between the light direction and the normal of the light guide plate V. The azimuthal angle is the angle between the projection of the light onto a plane that is perpendicular to the normal direction V and a direction that is parallel to the length direction of the light guide plate. The length direction of the light guide plate is perpendicular to the light source  12  and the normal direction V. The angular distribution of light can also be specified in terms of luminance L as a function of polar angle θ and azimuthal angle. The luminance L and the luminous intensity I are related by L=I/cos(θ). 
         [0042]    The maximum intensity angle, also referred as peak angle of a light distribution is defined as the polar angle at which the maximum luminous intensity occurs. Each luminous intensity distribution then defines a maximum (or peak) luminous intensity and a maximum intensity (or peak) angle. 
         [0043]    The maximum intensity ratio, also referred as optical gain, or normalized peak intensity, of a turning film, is defined as a ratio of the maximum luminous intensity of the light that is transmitted through the turning film over the maximum luminous intensity of the light that is emitted from a light guide plate. As a result, the maximum intensity ratio of a turning film is not dependent upon the absolute level of the light source, but is primarily dependent upon the turning film design itself. 
         [0044]    The power of a turning film is the ratio of the total amount of light passing through the turning film over the total amount of light incident upon the turning film. Thus, various turning film designs can be compared in terms of two critical quantities: maximum intensity ratio (or optical gain) and maximum intensity angle of the light that is transmitted through the turning film.