Abstract:
A backlight includes a source of diffuse light and a light transmissive film having a structured surface facing the light source where the film directs light from the light source into a plurality of primary intensity lobes in different directions.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to backlights for use with electronic displays and more particularly to backlights having major, maximum output lobes in a plurality of directions.  
       BACKGROUND  
       [0002]     Many electronic displays utilize a light valve that is illuminated by a backlight. The most common type of light valve currently in use is the liquid crystal display (“LCD”). Back lit LCD&#39;s are familiar to almost any user of electronic devices ranging from wristwatches to laptop computers.  
         [0003]     In order to optimize the efficiency of backlights, a variety of film materials are used. Among the films used are a variety of brightness enhancing films. The need for brightness enhancing materials is increased by the fact that typical LCD&#39;s absorb 90 to 94% of the light that impinges on them.  
         [0004]     Since LCD&#39;s necessarily operate on polarized light, and an absorbing polarizer inherently absorbs and discards at least half of the light impinging on it, many backlights utilize a reflective polarizer that transmits one polarization of light and reflects the other. The light that is reflected will either have its polarization reversed or randomized. In either case the light is reflected back to the reflective polarizer allowing more of the light to be transmitted. Backlights that use reflective polarizers in this way are characterized as polarization recycling backlights.  
         [0005]     Another type of backlight utilizes directional recycling films. A backlight that utilizes a directional recycling film is known as a directionally recycling backlight. It is possible for a backlight to be both directional and polarization recycling by using both types of materials.  
         [0006]      FIG. 1  shows a display  10  utilizing a directionally recycling backlight. In display  10 , a lamp  12 , such as a cold cathode fluorescent tube, directs light into an input surface  14  of light guide  16 . A reflector  18  behind lamp  12  helps to direct the light into light guide  16 . Light is conducted along light guide  16  by total internal reflection from surfaces  20  and  22  of light guide  16 . Light may be extracted from light guide  16  in a variety of manners. The most common manner is by providing a series of small dots  24  on the back or bottom surface of light guide  16 . Typically dots  24  are screen-printed of a white, highly diffuse material in order to provide color independent and diffuse reflection. Typically also, the density of dots  24  increases along the course of the light guide. Thus, the density of the dots will be greater closer to end  26  of light guide  16  than it will be close to input end  14 . This helps to increase the uniformity of light extraction from light guide  16 .  
         [0007]     Light extracted through side  20  of light guide  16  encounters diffuser  28 . Diffuser  28  serves to further randomize the direction of the light as well as to hide dots  24  which would otherwise appear to a viewer of the display as bright spots. After diffuser  28 , light enters directional recycling film  30 . Directional recycling film  30  could be of a variety of forms but typically has a plurality of prisms thereon. One type of directional recycling film that may be used and the function of such directional recycling films is described in U.S. Pat. No. 6,354,709, the teaching of which is incorporated herein by reference. The effect of directional recycling film  30  is to reflect light which is traveling in directions closer to the axis of the display back toward light guide  16  while refracting light traveling in directions further from the axis of the display towards the axis of display. Thus, the light emerging from direction recycling film  30  does so in a smaller range of angles than the light entering directional recycling film  30 . Thus, directional recycling  30  collapses the light traveling through it into a smaller range of angles in the dimension perpendicular to the prisms.  
         [0008]     Typically a second directional recycling film  34  is also used. The prisms  36  on directional recycling film  34  cannot be directly seen in  FIG. 1  because they run in a direction perpendicular to lamp  12  but are indicated by dashed line  38 . Directional recycling film  34  works like directional recycling film  30  except in a direction perpendicular to the direction in which directional recycling film  30  operates. The light reflected by directional recycling films  30  and  34  pass back through diffuser  28 , light guide  16  and strike reflector  40 . In addition, light emitted through side  22  of light guide  16  also strikes reflector  40 . Reflector  40  could be either a highly reflective diffuse reflector or a specular reflector such as Enhanced Specular Reflector (“ESR”) available from 3M Company. After being reflected by reflector  40 , the light passes back through light guide  16  and diffuser  28  and enters directional recycling film  30 . Since the direction of the light has now been randomized by two passes through diffuser  28 , as well as reflection from reflector  40  if reflector  40  is a diffuse reflector, the light now makes a wide variety of angles with directional recycling film  30  and the operation of directional recycling films  30  and  34  are repeated.  
         [0009]     The result of the operation of the backlight that includes light guide  16 , diffuser  28 , directional recycling films  30  and  34  is that the light entering liquid crystal display panel  42  has a much narrower range of angles than the light initially emitted through surface  20  of light guide  16 . This narrow range of angles is concentrated on the axis of the display, the direction most useful to a viewer. This permits the designer of the display to obtain a desired on-axis brightness while using a smaller output lamp than would otherwise be required.  
         [0010]      FIG. 2  is a graph showing the amount of illumination provided by a display of the type shown in  FIG. 1  as a function of the angle from which the display is viewed. In  FIG. 2  an angle of 0° corresponds to an observer viewing the display from directly in front of it and increasing positive and negative angles correspond to viewing from increasing angles. As may be seen from  FIG. 2 , the on-axis apparent brightness is greatest on the display axis or normal to the display surface. Because of its shape and size, the central portion of the graph is known as the major or primary lobe. Outside of the central primary lobe  50 , the apparent brightness drops off significantly until, at fairly large angles, it will typically rise again. The smaller maxima  52  and  54  that occur at large angles are known as the minor or secondary lobes. Although there is no reason to desire such secondary lobes, they typically result from the operation of the directional recycling films. Alternatively phrased, the backlight of  FIG. 1  has a single major peak in gain which occurs on the display axis.  
         [0011]     Another type of film that is used to help provide high on-axis brightness in a backlit display is known as turning film.  FIG. 3  shows a display  60  that utilizes a turning film. In display  60 , a lamp  62  emits light such as light beam  64  which enters a light guide  66  through input surface  68 . Light is conducted through light guide  66  by total internal reflection from surfaces  70  and  72 .  
         [0012]     As may be seen, light guide  66  takes the form of a wedge. This means that surfaces  70  and  72  are not parallel to one another as they were in the slablike guide  16  of  FIG. 1 . As light is reflected from the surfaces, it continues to make smaller and smaller angles to the normals to the surfaces. Eventually, light ray  64  will strike a surface at an angle that is no longer greater than the critical angle. For light ray  64 , this occurs at point  74  on surface  70 . Since light ray  64  strikes surface  70  at an angle less than the critical angle, it is not totally internally reflected and passes through surface  70 . When light ray  64  passes through surface  70 , it is refracted to an angle highly oblique to the normal to surface  70 .  
         [0013]     Since all of the extraction occurs when the various light rays reach angles that are less than the critical angle, they tend to all be extracted when traveling at approximately the same angle. Thus, the light emitted from light guide  66  tends to be highly collimated, but collimated in a direction itself highly oblique to the normal to surface  70 , which corresponds to the axis of the display. Light ray  64  then encounters turning film  76 . Turning film  76  has a plurality of linear prisms such as prism  78 . Prism  78  has input surface  80  and reflecting surface  82 . Light ray  64  enters a prism such as prism  78  through entry surface  80  and is reflected by reflecting surface  82 . After reflection by reflecting surface  82 , the light is turned to a direction highly collimated along the axis of the display. It then passes through liquid crystal display panel  84 .  
         [0014]     In addition to the features described above, display  60  will typically include a reflector  86  for recovering light that escapes through surface  72  of light guide  66 . Furthermore, although not necessary in theory, light extraction from light guide  66  may be enhanced through the use of screen printed dots such as dots  24  of the display of  FIG. 1 , diffusing structures on surface  70  or  72  or bulk diffusing materials embedded in light guide  66 .  
         [0015]      FIG. 4  is a plot of the light intensity of a backlight according to  FIG. 3  as a function of viewing angle. As may be seen, the light intensity has a strong maximum or major or primary lobe  90  on the display axis. Typically turning film based systems do not exhibit secondary lobes unless they are used in conjunction with a directional recycling film, whose prisms would be typically directed in a direction perpendicular to the prisms of the turning film.  
         [0016]     The displays described above provide strong maximum output on axis. This is typically desirable because a viewer will normally look at the display along the display axis. Sometimes, however, a display is intended to be viewed by a plurality of people at one time. Heretofore, designers of displays have provided a strong primary lobe and accepted that as two or more viewers look at the display none, or at most one, will be viewing the display from the direction of the primary lobe. A preferred design would provide a plurality of primary lobes, one for each intended viewer.  
       SUMMARY  
       [0017]     According to the invention, a backlight includes a source of diffuse light. Light from the light source enters a light transmissive film having a structured surface facing the light source. The film directs light from the light source into a plurality of primary intensity lobes in different directions 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a side view of a display using a backlight of the prior art;  
         [0019]      FIG. 2  is a graph of the light output of a backlight according to  FIG. 1 ;  
         [0020]      FIG. 3  is a side view of a display using another backlight of the prior art;  
         [0021]      FIG. 4  is a graph of the light output of a backlight according to  FIG. 3 ;  
         [0022]      FIG. 5  is a side view of a display using a backlight according to one embodiment of the invention;  
         [0023]      FIG. 6  is a graph of the light output of a backlight according to  FIG. 5 ;  
         [0024]      FIG. 7  is a side view of a display using another embodiment of the invention;  
         [0025]      FIG. 7A  is a top view of the backlight according to  FIG. 7 ;  
         [0026]      FIG. 8  is a side view of a display using another embodiment of the invention;  
         [0027]      FIG. 9  is a side view of a display using another embodiment of the invention;  
         [0028]      FIG. 10  is a side view of a display using another embodiment of the invention; and  
         [0029]      FIG. 11  is a side view of another film that may be used in the invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]     As described above, a backlight according to  FIG. 1  will operate with either a diffuse or a directional source of light. The system of  FIG. 3 , on the other hand, uses a highly directional source of light. In each of the systems of  FIGS. 1 and 3 , the resulting light output has a single principle node on the axis of the display. The present invention utilizes a light directing film with a diffuse light source in order to provide a plurality of major lobes in the output distribution. Preferably the light source is a lambertian source, although, in general, any diffuse light source will suffice.  
         [0031]      FIG. 5  shows a backlight display  100  according to the present invention. Backlit display  100  includes a lamp  102  which works in cooperation with a reflector  104  to provide light to input surface  106  of light guide  108 . An optional additional lamp  110  operates in conjunction with reflector  112  to provide light to second input surface  114  of light guide  108 . In the embodiment shown in  FIG. 5 , input surfaces  106  and  114  are parallel to one another. Both lamps  102  and  110  are preferably cold cathode fluorescent tubes. Light guide  108  conducts light from lamps  102  and  110  of total internal reflection from surfaces  118  and  119 . Light guide  108  also includes a diffuse extraction mechanism. Typically a diffuse extraction mechanism is provided in the form of diffusely reflecting, screen printed dots  120 . Light traveling in light guide  108  will strike one of dots  120  and be diffusely reflected and extracted from light guide  108 . A reflector  122  is provided behind light guide  108 , although it is less important than reflectors  40  and  86  of  FIGS. 1 and 3  respectively because the backlight of  FIG. 5  is not recycling and is unlikely to extract a significant amount of light through back surface  118 .  
         [0032]     Extracted light emerges from light guide  108  through front surface  119  and enters light directing film  124 . Light directing film  124  is light transmissive and has structures  126  on the side adjacent light guide  108 . Structures  126  may have a variety of shapes. In one embodiment, structures  126  are triangular prisms. Structures  126  of light directing film  124  may have the shape of isosceles triangles with 90 degree included angles. The product BEF II 90/50, commercially available from 3M Company, works well as a light directing film according to the present invention. If BEF II 90/50 is used as a light directing film, it should be installed upside down from the way in which it is normally installed.  
         [0033]     In operation structures  126  on side  128  of light directing film  124  operate by way of refraction to separate the light into two principle output lobes. The angular location and strength of those lobes will depend on the output distribution of light guide  108 , geometry of structures  126  and the index of refraction of light directing film  124 . The light emerges through surface  130  of light directing film  124 . Surface  130  of light directing film  124  may be an optically smooth surface or may have a matte finish or other optically functional structure. After emerging from light directing film  124 , the light passes through LCD panel  132 .  
         [0034]      FIG. 6  shows a graph of light output as a function of viewing angle for a display according to  FIG. 5 . The data for  FIG. 6  came from a backlight having highly diffuse output and using BEF II 90/50 as a light directing film. BEF II 90/50 has right isosceles prisms having a peak to peak pitch of 50 μm. It is made of an acrylate resin having an index of refraction of 1.586 cast on a polyester substrate. As may be seen the light has two principle output lobes at approximately plus and minus 45 degrees.  
         [0035]      FIG. 7  shows an embodiment of the display system of  FIG. 5  including lamps  104 ,  110  and an additional lamp  134 . This arrangement is sometimes referred to as a “U” lamp arrangement since lamps are provided on three sides of the light guide.  FIG. 7A  is a top view of light guide  108  and lamps  194 ,  110 , and  134  for clarity. Alternatively a “L” arrangement could be used by providing lamps on 2 adjacent sides of light guide  108 . Thus, for example, an “L” arrangement could include lamps  104  and  134  but not lamp  110 .  
         [0036]     Although the examples given with respect to  FIGS. 5, 6  and  7  include cold cathode fluorescent tubes, other lamps may be used as well. For example, light guide  108  may be eliminated all together and replaced with an electroluminescent panel. Generally electroluminescent panels provide highly diffuse outputs that would work very well with the present invention.  
         [0037]     Another type of lamp that can be used with the present invention is an LED. In fact, one or more LED&#39;s could be used with the present invention. When an LED is used, it is typically desirable to provide some system for helping to evenly distribute the light in the light guide and to extract the light from the light guide. This could be a reflective structure, typically operating by total internal reflection. Such structures are described in U.S. Pat. No. 6,167,182, the disclosure of which is incorporated herein by reference.  FIG. 9  shows a light guide using such reflective structures. According to the system of  FIG. 9  and LED  150 , emits light into light guide  152 . Reflective structures  153  help to distribute the light evenly in light guide  152  and to extract light from light guide  152 . Typically such reflective structures will be positioned on the back of light guide  152 . After exiting from light guide  152 , the light encounters direction control film  154  which divides the light into two primary lobes. Finally, the light is modulated by LCD panel  156 .  
         [0038]     As an alternative to the reflective technology described in conjunction with  FIG. 9 , diffractive technology may be used. In a diffractive light guide, diffractive rather than reflective structures help to provide uniform light distribution in the light guide and extract light from the guide. European published patent application 1,016,817 A1, the disclosure of which is incorporated herein by reference, describes such a diffractive light guide.  FIG. 10  shows a display including a diffractive light guide. Light from a LED  160  goes into light guide  162 . Light guide  162  includes diffractive structures  163 . Typically diffractive structures  163  are positioned on the front surface of light guide  162 , but in some circumstances could be on the back surface or even on both surfaces. Diffractive structures  163  help to provide uniform illumination of the light guide and extract the light from the light guide. After extraction from the light guide, the light encounters direction control film  164 , which separates it into two major lobes. Finally, the light is modulated by LCD panel  166 .  
         [0039]     All of the embodiments shown and discussed utilize light directing films having isosceles prisms. Such prisms are generally desirable when symmetric positioning of the major lobes are required. However, other designs could be utilized. For example, asymmetric prisms could be used if it is desirable to have the major lobes positioned asymmetrically with respect to a normal to the light directing film. Such a configuration might be desirable, for example, in a navigation display in an automobile if that display is to be positioned in a location other than in the center of the dashboard. This would allow the major lobes to be located for easy reading by both the driver and the front seat passenger.  
         [0040]     Shapes other than triangular prisms may also be used. Such shapes would generally be desirable when more than two lobes are desired.  FIG. 11  shows a prism film  170  having prisms  172 . Each of prisms  172  has four active faces  174 ,  176 ,  178 , and  180 . Such a film would provide four distinct principle lobes. Other designs are certainly possible as may be desirable for a particular display.