Abstract:
The present invention provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising a light input surface for receiving light from the point light source, a light redirecting surface for redirecting light received from the light input surface and a light output surface for outputting at least the light redirected from the light redirecting surface. The light input surface further comprises a composite lens structure having a circular tip segment with a first contact angle, and a first and second elliptical base segments with a second contact angle, the second contact angle being less than the first contact angle and the second contact angle being equal to each other and 
     wherein the circular tip segment satisfies the following equation: 
         y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}
 
     and the elliptical base segments satisfies the following equations: 
         y   4   =d   4   +b   4 ×√{square root over ((1−(( x+c   4 )/ a   4 ) 2 )}
 
         y   5   =d   5   +b   5 ×√{square root over ((1−(( x−c   5 )/ a   5 ) 2 )}

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a light guide film of a light emitting diode (LED) backlight unit, and, more particularly, to a light guide film of an LED backlight unit, which has a plurality of grooves carved into an incident plane of the light guide film to increase an incidence angle of which light can be transmitted through the light guide film. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typically, a liquid crystal display (LCD) for handheld and notebook devices generally employs at least one lateral light emitting diode (LED) as a light source of a backlight unit. Such a lateral LED is generally provided to the backlight unit as shown in FIG. 1 of Yang U.S. Pat. No. 7,350,958. 
         [0003]    Referring to  FIG. 1 , the backlight unit  10  comprises a planar light guide film  20  disposed on a substrate  12 , and a plurality of lateral LEDs  30  (only one lateral LED is shown in  FIG. 1 ) disposed in an array on a lateral side of the light guide film  20 . Light L entering the light guide film  20  from the LED  30  is reflected upwardly by a minute reflection pattern  22  and a reflection sheet (not shown) positioned on the bottom of the light guide film  20 , and exits from the light guide film  20 , providing back light to an LCD panel  40  above the light guide film  20 . Such a backlight unit  20  suffers from a problem as shown in  FIG. 2  when light is incident on the light guide film  20  from the LED  30 . 
         [0004]    As shown in  FIG. 2 , light L emitted from each LED  30  is refracted toward the light guide film  20  by a predetermined angle θ due to difference in refractive index between media according to Snell&#39;s Law when the light L enters the light guide film  20 . In other words, even though the light L is emitted at a beam angle of α 1  from the LED  30 , it is incident on the light guide film  20  at an incidence angle of α 2  less than α 1 . In  FIG. 3 , such an incidence profile of light L is shown. Therefore, there is a problem of increasing the length (l) of a combined region where beams of light L entered the light guide film  20  from the respective LEDs  30  are combined. In addition, light spots H also called “hot spots” and dark spots D are alternately formed in the region corresponding to the length (l) on the incident plane of the light guide film  20 . Each of the light spots H is formed at a location facing the LED  30 , and each of the dark spots D is formed between the light spots H. 
         [0005]    Since the alternately formed light and dark spots are not desirable for the light guide film, they should be minimized and the length (l) should be shortened as much as possible. For this purpose, it is necessary to increase an angle of light entering the light guide film, that is, an incidence angle of light. 
         [0006]    For this purpose, it is suggested to form protrusions on the input surface of the light guide film as shown in  FIG. 4 . Specifically, a plurality of fine prism-shaped structures  24  or arc-shaped structures (not shown) are formed on a light input surface of a light guide film  20 A and light L enters the light guide film at an incidence angle α 3  substantially equal to an orientation angle α 1  of light emitted from a focal point F of a light source. Thus, if orientation angles al of light beams emitted from the focal point F of the light source are identical, the light L enters the light guide film at an incidence angle α 3  wider than the case of  FIGS. 2 and 3 . However, with this solution, there is some secondary light collimation where the light rays are refracted by the wall of the adjacent prism or arc-shaped structure as shown in  FIG. 4 . Secondary light collimation from the walls of the adjacent prism structure turns the light ray back on-axis providing less diffusion of the light from the light source as shown in  FIG. 4 . Thus the continuous prism- or arc-shaped structures on the input surface have limited light diffusing capability. 
         [0007]    Therefore an improved input edge design is needed to provide a more uniform surface illumination of the light guide film without sacrficing the efficiency of the backlight system. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising: a light input surface for receiving light from the point light source; a light redirecting surface for redirecting light received from the light input surface; a light output surface for outputting at least the light redirected from the light redirecting surface; wherein the light input surface further comprises a composite lens structure having a circular tip segment with a first contact angle, and a first and second elliptical base segments with a second contact angle, the second contact angle being less than the first contact angle and the second contact angle being equal to each other; and 
         [0000]    wherein the circular tip segment satisfies the following equation: 
         [0000]        y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}
 
         [0000]    and the elliptical base segments satisfies the following equations: 
         [0000]        y   4   =d   4   +b   4 ×√{square root over ((1−(( x+c   4 )/ a   4 ) 2 )}
 
         [0000]        y   5   =d   5   +b   5 ×√{square root over ((1−(( x−c   5 )/ a   5 ) 2 )}
 
         [0009]    In addition, the invention further provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising: a light input surface for receiving light from the point light source; a light redirecting surface for redirecting light received from the light input surface; a light output surface for outputting at least the light redirected from the light redirecting surface; wherein the light input surface further comprises a composite lens structure having gaps there between, the lens structure having a circular tip segment with a first contact angle, and a first and second elliptical base segments with a second contact angle, the second contact angle being less than the first contact angle and the second contact angle being equal to each other; and 
         [0000]    wherein the circular tip segment satisfies the following equation: 
         [0000]        y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}
 
         [0000]    and the elliptical base segments satisfies the following equations: 
         [0000]        y   4   =d   4   +b   4 ×√{square root over ((1−(( x+c   4 )/ a   4 ) 2 )}
 
         [0000]        y   5   =d   5   +b   5 ×√{square root over ((1−(( x−c   5 )/ a   5 ) 2 )}
 
         [0010]    Further, the invention provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising: a light input surface for receiving light from the point light source; a light redirecting surface for redirecting light received from the light input surface; a light output surface for outputting at least the light redirected from the light redirecting surface; wherein the light input surface further comprises a serrated lens structure that is provided only where the point light source is incident on the light input surface, the lens structure having a circular tip segment with a first contact angle, and a first and second elliptical base segments with a second contact angle, the second contact angle being less than the first contact angle and the second contact angle being equal to each other; and 
         [0000]    wherein the circular tip segment satisfies the following equation: 
         [0000]        y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}
 
         [0000]    and the elliptical base segments satisfies the following equations: 
         [0000]        y   4   =d   4   +b   4 ×√{square root over ((1−(( x+c   4 )/ a   4 ) 2 )}
 
         [0000]        y   5   =d   5   +b   5 ×√{square root over ((1−(( x−c   5 )/ a   5 ) 2 )}
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a schematic diagram illustrating a conventional backlight module; 
           [0012]      FIG. 2  shows a schematic diagram illustrating the distribution of bright/dark bands of a conventional light guide plate; 
           [0013]      FIG. 3  shows a schematic diagram illustrating an embodiment of conventional light-diffusing structures; 
           [0014]      FIG. 4  shows a schematic diagram illustrating another embodiment of conventional light-diffusing structures; 
           [0015]      FIGS. 5   a  and  5   b  shows a schematic diagram illustrating a light guide film according to an embodiment of the invention; 
           [0016]      FIG. 6   a - 6   c  show schematic diagrams illustrating the various segments of the composite lens feature according to an embodiment of the invention; 
           [0017]      FIGS. 7   a  and  7   b  show schematic diagrams illustrating the light diffusing capability of the composite lens feature with a gap between each adjacent feature; 
           [0018]      FIG. 8  shows another embodiment of this invention; 
           [0019]      FIGS. 9   a  and  9   b  show the luminance intensity at various distances from the light input surface for a circular or arc shaped input feature; 
           [0020]      FIGS. 10   a  and  10   b  show the luminance intensity at various distances from the light input surface for a trapezoidal feature or feature with slanted sides; and 
           [0021]      FIGS. 11   a  and  11   b  show the luminance intensity at various distances from the light input surface according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    A light guide film in accordance with the present invention comprises a light output surface, a light redirecting surface and at least one light input surface that joins the light output surface and the light redirecting surface. The light input surface comprises a plurality of concave features consisting of a composite lens array. Each of the composite lenses is separated by a gap that is a flat surface perpendicular to the light output surface. The composite lenses and gaps are disposed along the light input surface, and extend from the output surface to the light redirecting surface. Each of the composite lenses has a symmetric cross-section consisting of a tip portion comprising a circular tip segment of a first contact angle and a base portion comprising two tilted elliptical base segments with a second contact angle, the second contact angle being less than the first contact angle and where the contact angle for each of the two tilted elliptical base segments are equal. 
         [0023]    According to the above embodiment, the geometrical profile of the composite lens allows for comparatively large light deflecting distances; that is, the composite lens structure has better light-diffusing capability. Thus, the distance between the point light source and the active area of the display can be shortened, and the dark spots between the point light sources can be minimized, with the brightness uniformity still being acceptable. The circular tip segment uniformly distributes the light in front of the discrete light source, typically a light emitting diode (LED). The two tilted elliptical base segments uniformly distribute the light between the LEDs. A smooth curvature of the circular tip segment and tilted elliptical base segments maximizes the uniformity of the light spatial distribution so that the light output is uniform. Further, it is also necessary that each two adjacent composite lens structures have a gap or flat therebetween so a greater degree of deflection on the propagation path of the incident light can be achieved to thereby increase the light-diffusing effect. 
         [0024]    Referring to  FIGS. 5   a  and  5   b , a light guide film according to an embodiment of the invention is shown, wherein a planar light guide film  12  is used to receive and guide the light from at least one point light source (such as LEDs  14  shown in  FIG. 5   a ). The side surface of the light guide film  12  next to the LED  14  forms a light input surface  12   a.  The top surface of the light guide film  12  that makes an angle with the light input surface  12   a  forms a light-emitting surface  12   b,  and the bottom surface opposite the light-emitting surface  12   b  forms a light-reflecting surface  12   c.  The light-reflecting surface  12   c  is comprised of a plurality of light reflecting structures. The light emitted from the LED  14  enters the light guide film  12  via the light input surface  12   a  and propagates inside the light guide film  12 . Then, it is guided toward the light-emitting surface  12   b  by the light-reflecting surface  12   c  and finally exits the light guide film  12  through the light-emitting surface  12   b.    
         [0025]    Further, a plurality of concave composite lens structures  16  are serrated on the edge of the light input surface  12   a,  with their longitudinal directions being parallel to each other and having a gap (G) between each adjacent composite lens structure  16 . Referring now to  FIGS. 6   a ,  6   b  and  6   c , the light input surface  12   a,  facing the LED  14 , of the composite lens structure  16  has a circular tip segment  16   a,  and two tilted elliptical base segments  16   b  and  16   c,  respectively. The circular tip segment  16   a  of the concave composite lens structure  16  is the segment furthest from the light input surface  12   a.  Although the composite lens features for the preferred embodiment of this invention are disposed in a concave direction on the light input surface, the composite lens may also be in a convex direction on the light input surface. 
         [0026]    The length T 1  is the distance between the intersections of the extensions of the tangent at the top of the elliptical base segments  16   b  and  16   c,  and the tangent of the circular tip segment  16   a,  where the tangent of the circular tip segment  16   a  is parallel to the light input surface  12   a.  The length T 2  is the total width of the circular tip segment  16   a  taken where the circular tip segment  16   a  intersects the two elliptical base segments  16   b  and  16   c.  Note, T 2  is parallel to T 1 . The contact angle A 1  is the contact angle of the circular tip segment  16   a.  Contact angle A 1  is preferably greater than 0.1 degrees and less than or equal to 85 degrees. Referring now to  FIG. 6   b , the gap G is the distance between each adjacent composite lens. Preferably, the gap G is less than or equal to 0.9 times the pitch P. The pitch P of the linear composite lens array  16  is the distance along the light input edge which includes the gap G distance and the total width B of the composite lens. Preferably the pitch P is greater than or equal to 5 micrometers and less than or equal to 1 millimeter (mm). The total height H of the feature is measured from the light input edge to the tangent of the circular tip segment  16   a.  The total height H of the composite lens is greater than or equal to 3 micrometers and less than or equal to 1 millimeter. The light input surface  12   a  will have a surface finish of 10 nanometers to 2 micrometers. The surface finish of the concave composite lens structures  16  can be the same or different than the gap G portion between the features. 
         [0027]    Advantageously, the shape of an XY section of the circular tip segment  16   a  satisfies the following expression (1): 
         [0000]        y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}  (1)
 
         [0000]    where the circular tip segment  16   a  has a first radius r 1 . The first radius r 1  is defined as the quotient of half the distance T 1  divided by the tangent of half the contact angle A 1 . The value a 1  is defined as the total height H minus the radius r 1  of the circular tip segment  16   a.  The value x is a value in the direction of the light input surface and is preferably set within the range of −r 1 ×sin(A 1 )≦x≦r 1 ×sin(A 1 ). The value y 1  is a value in the light propagation direction. Referring now to  FIG. 6   c , the composite lens structure also comprises two tilted elliptical segments, namely a first elliptical base segment  16   b  and a second elliptical base segment  16   c.  Each elliptical base segment comprises two contact angles, a top contact angle and a bottom contact angle. The first elliptical base segment  16   b  has a top contact angle A 41  and a bottom contact angle A 42 . The second elliptical base segment  16   c  has a top contact angle A 51  and a bottom contact angle A 52 . The top contact angle A 41  is created by a tangent to the first elliptical base segment  16   b  at the point where the circular tip segment  16   a  and the first elliptical base segment  16   b  intersect. The bottom contact angle A 42  is created by a tangent to the first elliptical base segment  16   b  at the point where the first elliptical base segment  16   b  intersects the light input surface  12   a.  The top contact angle A 41  of the first elliptical base segment  16   b  and the top contact angle A 51  of the second elliptical base segment  16   c  are equal. The bottom contact angle A 42  of the first elliptical base segment  16   b  and the bottom contact angle A 52  of the second elliptical base segment  16   c  are equal. The contact angles for each of the two elliptical base segments  16   b  and  16   c  are smaller than the contact angle A 1  of the circular tip segment  16   a.    
         [0028]    Advantageously, the shape of an XY section of the elliptical base segments  16   b  and  16   c  as shown in  FIG. 6   c  satisfy the following expressions (2 and 3) respectively: 
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                                       41 
                                     
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                                       41 
                                     
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         [0029]    Thus, the first elliptical base segment  16   b  has a top contact angle A 41  and a bottom contact angle A 42  and the second elliptical base segment  16   c  has a top contact angle A 51  and a bottom contact angle A 52 . Referencing  FIGS. 6   a  and  6   c , and equation 2, the height H 3  of the first elliptical base segment  16   b  is equal to the total height H of the composite lens feature  16  minus the radius r 1  of the circular tip segment  16   a  times the quantity 1 minus the cosine of contact angle A 1  of the circular tip segment  16   a.  The total width B of the composite lens feature  16  is equal to the pitch P of the composite lens array minus the gap G distance. Preferably gap G is greater than 0 and less than or equal to 0.9 times the pitch P. The pitch P is preferably greater than or equal to 5 micrometers and less than or equal to 1 millimeter. The height of the second elliptical base segment  16   c  is equal to the height H 3  of the first elliptical base segment  16   b.    
         [0030]    The parameter a 4  is equal to the parameter ab 4  times the square root of the quotient of the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  times the quantity 4 times the height H 3  of the elliptical base segment  16   b  plus the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  divided by the quantity the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b.    
         [0031]    The parameter b 4  is equal to 2 times the parameter ab 4  times the square root of the quotient of the quantity of the height H 3  of the first elliptical base segment  16   b  times the quantity the height H 3  of the first elliptical base segment  16   b  times the tangent contact angle A 41  of the top of the first elliptical base segment  16   b  plus the tangent contact angle A 42  at the bottom of elliptical base segment  16   b  plus the tangent contact angle A 41  times the tangent contact angle A 42  times the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  divided by the quantity of tangent contact angle A 41  plus tangent contact angle A 42 . 
         [0032]    The parameter ab 4  is equal to the quotient of the quantity the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  times the quantity twice the height H 3  of the first elliptical base segment  16   b  plus the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  times the quantity twice the height H 3  of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  divided by four times the following quantities the quantity the height H 3  of the first elliptical base segment  16   b  times 4 plus the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a,  plus the quantity the height H 3  of the first elliptical base segment  16   b  times the quantity the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  plus the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  times the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  times the quantity times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a.    
         [0033]    The parameter c 4  is equal to the quotient of the quantity the tangent contact angle A 41  at the top of the first elliptical base segment  16   b  times the quantity twice the height H 3  of the first elliptical base segment  16   b  times the total width B of the composite lens feature  16  plus the tangent contact angle A 42  times the quantity the total width B of the composite lens feature  16  squared minus the total width T 2  of the circular tip segment  16   a  squared, that quantity plus twice the height H 3  of the first elliptical base segment  16   b  times the total width T 2  of the circular tip segment  16   a  times the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  divided by the tangent contact angle A 41  at the top of the first elliptical base segment  16   b  times the quantity the height H 3  of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  times the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  plus quantity the height H 3  of the first elliptical base segment  16   b  times the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  the quantities of the divisor times 4. 
         [0034]    The parameter d 4  is equal to the quotient of the quantity twice the height H 3  of the first elliptical base segment  16   b  plus the tangent contact angle A 41  at the top of the first elliptical base segment  16   b  times the quantity the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a,  the previous quantities times the height H 3  of the first elliptical base segment  16   b  divided by the height H 3  of the first elliptical base segment  16   b  times 4 plus the quantity plus the tangent of contact angle A 41  at the top of the first elliptical base segment  16   b  plus the tangent of contact angle A 42  at the bottom of the first elliptical base segment  16   b  times the quantity the total width B of the composite lens feature  16  squared minus the total width T 2  of the circular tip segment  16   a.    
         [0035]    The coordinate x is a value in the direction of the input edge or more specifically in the direction of the total width of the composite lens feature  16  and is preferably set within the range of −B/2≦x≦−T 2 /2 for the first elliptical base segment  16   b.  The coordinate y 4  is a value in the light propagation direction. 
         [0036]    Referencing  FIGS. 6   a  and  6   c , and equation 3, the height H 3  of the second elliptical base segment  16   c  is equal the height of the first elliptical base segment  16   b.  The total width B of the composite lens feature  16  is equal to the pitch P of the composite lens array minus the gap G distance. Preferably gap G is greater than 0 and less than or equal to 0.9 times the pitch P. 
         [0000]    
       
         
           
             
                 
             
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         [0037]    The parameter a 5  is equal to the parameter ab 5  times the square root of the quotient of the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  times the quantity 4 times the height H 3  of the second elliptical base segment  16   c  plus the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  divided by the quantity the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c.    
         [0038]    The parameter b 5  is equal to 2 times the parameter ab 5  times the square root of the quotient of the quantity of the height H 3  of the second elliptical base segment  16   c  times the quantity the height H 3  of the second elliptical base segment  16   c  times the tangent contact angle A 51  of the top of the second elliptical base segment  16   c  plus the tangent contact angle A 52  at the bottom of elliptical base segment  16   c  plus the tangent contact angle A 51  times the tangent contact angle A 42  times the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  divided by the quantity of tangent contact angle A 51  plus tangent contact angle A 52 . 
         [0039]    The parameter ab 5  is equal to the quotient of the quantity the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  times the quantity twice the height H 3  of the second elliptical base segment  16   c  plus the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  times the quantity twice the height H 3  of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  divided by four times the following quantities the quantity the height H 3  of the second elliptical base segment  16   c  times 4 plus the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a,  plus the quantity the height H 3  of the second elliptical base segment  16   c  times the quantity the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  plus the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  times the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  times the quantity times the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a.    
         [0040]    The parameter c 5  is equal to the quotient of the quantity the tangent contact angle A 51  at the top of the second elliptical base segment  16   c  times the quantity twice the height H 3  of the second elliptical base segment  16   c  times the total width B of the composite lens feature  16  plus the tangent contact angle A 52  times the quantity the total width B of the composite lens feature  16  squared minus the total width T 2  of the circular tip segment  16   a  squared, that quantity plus twice the height H 3  of the second elliptical base segment  16   c  times the total width T 2  of the circular tip segment  16   a  times the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  divided by the tangent contact angle A 51  at the top of the second elliptical base segment  16   c  times the quantity the height H 3  of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  times the quantity of the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a  plus quantity the height H 3  of the second elliptical base segment  16   c  times the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  the quantities of the divisor times 4. 
         [0041]    The parameter d 5  is equal to the quotient of the quantity twice the height H 3  of the second elliptical base segment  16   c  plus the of tangent contact angle A 51  at the top of the second elliptical base segment  16   c  times the quantity the total width B of the composite lens feature  16  minus the total width T 2  of the circular tip segment  16   a,  the previous quantities times the height H 3  of the second elliptical base segment  16   c  divided by the height H 3  of the second elliptical base segment  16   c  times 4 plus the quantity plus the tangent of contact angle A 51  at the top of the second elliptical base segment  16   c  plus the tangent of contact angle A 52  at the bottom of the second elliptical base segment  16   c  times the quantity the total width B of the composite lens feature  16  squared minus the total width T 2  of the circular tip segment  16   a.    
         [0042]    The coordinate x is a value in the direction of the input edge or more specifically in the direction of the total width of the composite lens feature  16  and is preferably set within the range of T 2 /2≦×≦B/2 for the second elliptical base segment  16   c.  The coordinate y 5  is a value in the light propagation direction. 
         [0043]    The contact angles for the composite lens feature can be described where A 41 =A 51 , A 42 =A 52  and A 1 ≧A 42 , A 41 . Preferably, A 1 ≧A 42 , A 41 ≦85 degrees. 
         [0044]      FIG. 7   a  is a ray tracing for an array of a single composite lens feature  16  of this invention illustrating what happens to the light rays when the individual composite lens features are disposed on the light input surface  12   a  in a contiguous manner such that there is no gap G between adjacent composite lenses.  FIG. 7   b  is a similar ray tracing, but where the individual composite lens feature is separated by a gap G between adjacent features. The gap G is preferably less than or equal to 0.9P where P (as shown in  FIG. 6   b ) is the pitch of the composite lens feature on the input surface  12   a.  In  FIG. 7   a , where the composite lens features are adjacent each other along the input surface, some of the light rays will experience a secondary light collimation as they are refracted when they reach the side of the adjacent feature. This secondary light collimation detracts from the diffusion capability of the composite lens feature  16 . In  FIG. 7   b , the composite lens features are separated by a gap G. The gap allows the light ray to continue in a diffuse manner and thus widens the angle at which the light propagates in the light guide film. There is minimal secondary light collimation when the gap between features is incorporated into the composite lens feature design. In this way, the wider angle of light helps to mitigate the hot spots along the input surface of the light guide film. 
         [0045]    Referring now to  FIG. 8 , the light guide film  12  in  FIG. 8  shows the composite lens features  16  not disposed along the entire input surface  12   a.  Instead, the composite lens features  16  are disposed along the light input surface  12   a  in the region where the LED  14  light is incident. The luminance uniformity of the system is minimally affected as the unpatterned region on the light input surface has minimal light rays in this region. 
       EXAMPLES 
       [0046]      FIG. 9   a  shows a portion of the light input surface  32  of a light guide film  30  with an arc- or circular-type structure  36 . The graph in  FIG. 9   b  illustrates the light intensity for the light guide film  30  at distances 3.5 mm, 4.5 mm and 5.5 mm from the light input surface  32 .  FIG. 9   b  shows that the localized light intensity decreases as the distance increases from the light input surface, but there are still some hot spots evident at 5.5 mm The arc- or circular-type structure solution provides some improvement for hot spots but is more effective at collimating light in line with the LED than widening the incidence angle. This is evident in the graph in  FIG. 9   b . In  FIG. 9   b , the LEDs are located at each of the vertical dotted lines and the light distribution is still not leveled out at 5.5 mm into the light guide film. It is apparent from the graph in  FIG. 9   b  that the arc- or circular-type solution has insufficient diffusion capability. 
         [0047]      FIG. 10   a  shows a portion of the light input surface  42  of a light guide film  40  with a composite lens structure that has flat slanted sides  46 . This result would also be applicable to a trapezoidal shaped light input structure. The graph in  FIG. 10   b  illustrates the light intensity for the light guide film  40  at distances 3.5 mm, 4.5 mm and 5.5 mm from the light input surface  42 .  FIG. 10   b  shows that the localized light intensity actually inverts in the area immediately in front of the LEDs, resulting in a dark spot immediately in front of the LEDs. This overall loss of light intensity immediately in front of the LED is due to the fact that the straight slanted walls diffuse the light more readily through the sides than through the tip. It is also noted that the shape of the light intensity profile across the light guide film does not change significantly as the distance increases from the input surface  42 . 
         [0048]      FIG. 11   a  shows a portion of the light input surface  52  of a light guide film  50  with the composite lens feature  56  of this invention. The composite lens feature utilizes a circular tip segment and two tilted elliptical base segments. The top and bottom contact angles for each of the two tilted elliptical base segments are equal. The top and bottom contact angles for each of the two tilted elliptical base segments are less than the contact angle of the circular tip segment. The circular tip segment uniformly distributes the light in the area immediately in front of the LED. The two tilted elliptical base segments uniformly distribute the light between the LEDs. The smooth curvatures of the circular tip segment and the two tilted elliptical base segments maximize the uniformity of the light spatial distribution so the output light is uniform. The graph in  FIG. 11   b  illustrates that the composite lens  56  of the present invention generates uniform light output across the light guide film at distances of 3.5 mm, 4.5 mm and 5.5 mm from the input surface  52 . 
         [0049]    Hence, an improved light guide film is provided with symmetric light redirecting features to improve light output uniformity without sacrificing light input efficiency. Namely, the improved light guide film  12  having composite lens structure  16  provides enhanced light diffusion in the plane parallel to the light extraction plane and light reflection plane (top and bottom surfaces), allowing greater light redistribution between discrete light sources (light traveling outside the critical angle of planar un-serrated input edge), so that the light output uniformity is improved. Moreover, the light distribution in the plane perpendicular to the light extraction plane and light reflection plane (top and bottom surfaces) is minimized, so that the condition of the total internal reflection is minimized for the inputted traveling light.