Abstract:
A film for use with a backlit display has prismatic structure on one surface. The structures have a serpentine shape. The average periods and amplitudes of the serpentine shapes may be optimized to minimize moiré interference with the pixels of the display.

Description:
BACKGROUND OF THE INVENTION 
   Electronic displays often use a light gating device such as a liquid crystal display (LCD) in front of a backlight. Backlights may be as simple as a lighting element such as a fluorescent tube behind the light gating device or may be more complicated arrangements. A common arrangement is to use a lighting element such as a cold cathode fluorescent tube (CCFT) or one or more light emitting diodes (LED&#39;s) placed adjacent an edge light guide lying behind the light gating device. The light guide conducts and distributes the light over the entire light gating device. 
   Various types of optical films maybe positioned between the light source or light guide and the light gating device. Examples include prismatic films with the prisms directed away from the light source, prismatic films with the prisms directed toward the light source, diffusing films, and reflective polarizers. The prismatic films with the prisms on the side of the film directed toward the light gating device improve the on axis brightness of the display by causing light from the backlight to be restricted to a narrower range of angles than it would be absent the film. The prism films with the prisms facing toward the backlight are used with a special class of light guides that emit light at a large angle, typically over 70°, to the normal. These prism films are used to turn the light towards the normal. The diffusers and reflective polarizers do just what their names imply. 
   One known problem when using prismatic films with a light gate having a rectangular array of pixels is moiré interference that arises because of the interaction of the regular pattern of linear prisms with the rows and columns of pixels in the light gating device. Various attempts have been made to avoid this problem by arranging the prismatic films such that the prisms run at an angle other than zero or 90° with the rows and columns of pixels or by choosing a prism size such that the moiré interference has a spatial frequency sufficiently different than that of the display. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a film for enhancing the apparent brightness of a display; 
       FIG. 2  is a display utilizing the film of  FIG. 1 ; 
       FIG. 3  is a schematic demonstration of the moiré performance of the prior art; 
       FIG. 4  is a drawing of a film according to the invention; 
       FIG. 5A through 5F  are schematic demonstrations of the moiré performance of various films according to the invention; 
       FIG. 6  is a drawing of a single prism of a film according to the invention; and 
       FIG. 7  is a schematic drawing an alternative display according to the invention. 
   

   DETAILED DESCRIPTION 
   The present invention will be described in the context of a liquid crystal display although those of skill in the art will recognize that it would work with any display capable of selectively blocking or transmitting light. Such displays are known as light gating devices.  FIG. 1  shows a film  18  intended to increase the on axis brightness of a display. Film  18  has a first surface  20  and a second surface  22 . Second surface  22  includes a plurality of structures. Generally these structures are triangular prisms such as prism  24  although other shapes may be used. Prism  24  has a peak  26 . Experience has shown that the gain (i.e. the ratio of the on axis brightness of a backlight with the film to the on axis brightness of the same backlight without the film) will be greatest if peak  26  is a sharp corner and forms an angle of 90°, although other considerations may cause a user to choose a rounded corner or other angles. Between prism  24  and adjacent prism  28  is a valley  30 . As with peak  26 , film  18  will have the highest gain if valley  30  is sharp and forms an angle of 90° although valley  30  could be rounded or form other angles. 
   The prisms have a peak to peak separation known as the pitch, P, of film  18 . Generally the pitch should be small enough that the prisms will not be visible to a user viewing a display including film  18 , but not so small that diffractive effects reduce performance. Commercial products have generally had pitches between about 24 μm and 50 μm. 
   Typically first surface  20  will be a smooth surface. In this context, a smooth surface will be understood to mean that any structures on surface  20  are small in comparison to the size of the prisms such as prism  24  on side  22  of film  18 . Commonly surface  20  will be a matte surface in order to help hide any structure of the backlight behind film  18 . Alternatively, side  20  might have other structures such as a moth eye structure for reducing fresnel reflections from surface  20 . 
     FIG. 1  shows four exemplary light rays. The first, light ray  36 , approaches surface  20  at a grazing angle, i.e. an angle to the normal approaching 90°. Light ray  36  will be refracted towards the normal to film  18  as it passes through surface  20 . Upon reaching structured surface  22 , it will be refracted again. Because of the structures on structured surface  22  it will be refracted so that again it will make a smaller angle to the normal to film  18 . Light ray  38  approaches smooth surface  20  at an angle much closer to the normal to film  18 . It is also refracted as it passes through surface  20 , but to a lesser extent. Upon exiting through surface  22 , it is refracted again so that it is on the opposite side of the normal to film  18  that it was on upon first striking surface  20 . Light ray  40  approaches at an angle close to or at the normal to film  18  and is totally internally reflected twice by structured surface  22  in return toward the backlight. Light ray  42  approaches surface at an angle similar to that of light ray  38  but in a location such that it is totally internally reflected by one side of a prism on structured surface  22  but not by the second side. As a result it emerges at a large angle to the normal to film  18 . Because such a reflection only occurs to a light ray that is traveling in direction that forms a high incidence angle to the side it strikes, the prisms provide a very small cross section to such rays. In addition, many of those rays will re-enter the next prism and be returned into the display. 
   As may be seen from this discussion, light that, absent film  18 , would have emerged from the display at a high angle to the axis of the display is redirected into a direction closer to that axis. A small amount of light will be directed out at a large angle to the axis. Thus, we may say that light that enters film  18  through surface  20  with an angle of incidence greater than a predetermined angle is directed into a narrower range of angles than the range of angles through which it entered. Light that enters film  18  through smooth surface  20  at an angle of incidence less than that predetermined angle will be reflected back towards the backlight. 
   The light that is reflected back towards the backlight will be diffused and reflected back towards film  18 . In general, it will make a different angle than it made the first time. The process is then repeated so that more of the light is redirected into the narrower range of angles. The key aspect of the operation of film  18  is that film  18  must be capable of reflecting light striking it in a first predetermined range of angles and passing but refracting light striking it in a second predetermined range of angles wherein the angles in the second range of angles are greater than those in the first range of angles and wherein the light in the second range of angles is refracted into a narrower range of angles than those through which they entered. 
     FIG. 2  is a typical display utilizing two films as shown in  FIG. 1 . Although a single film could be used, most LCD displays use two of such films. 
   Display  50  includes a case  52  which encloses the display. Inside case  52  is a light source  54 . Typically light source  54  is a cold cathode fluorescent tube. Partially surrounding light source  54  is a reflector  56 . Reflector  56  could be any highly reflective material. An example of a material which could be used as reflector  56  is a multi-layer optical film as described in U.S. Pat. No. 5,882,774. Optically coupled to light source  54  is a light guide  58 . Light guide  58  could be acrylic or any other highly transparent material. Typically, light guide  58  has a pattern of dots screen printed on the side away from the liquid crystal in order to help extract light to illuminate the LCD. Light extracted from light guide  58  passes through films  18   a  and  18   b . Film  18   b  has structures similar to those on  18   a , but they cannot be seen in the view of  FIG. 2  because they run perpendicular to those of  18   a . The effect of using two films of the type of film  18  of  FIG. 1  is that the gain is increased by moving light that would emerge from the display at high angles in both the horizontal and vertical dimensions to angles closer to the access of the display. 
   After emerging from film  18   b , light passes through a diffuser  60 . Diffuser  60  helps to hide both the prisms and the dots printed on light guide  58  from the user. After diffuser  60  the light passes through liquid crystal display  62  and protective cover sheet  64 . 
   In the prior art, the prisms of film  18  were typically linear prisms. A problem with linear prisms is that they tend to produce moiré interference with the linear rows and columns of pixels in the LCD display.  FIG. 3  illustrates this. In  FIG. 3  an array of horizontal lines such as horizontal line  70  and vertical lines such as vertical line  72  that represent the pixel array in a typical LCD display. Each pixel is square and consists of three horizontally aligned sub-pixels, one each for red, green and blue. Therefore, the pixel array has three times as many vertical lines as horizontal lines. Overlaid on this pixel array is a series of vertical lines such as line  74  representing a film such as film  18  having linear presence. The lines such as line  74  represent the peaks of the prisms. When viewed from a distance of approximately 6 meters,  FIG. 1  will have the visual effect of viewing a typical LCD display with a film such as film  18  which linear prisms from a distance of about 45 centimeters, a typical viewing distance for a person using a computer with an LCD display. When  FIG. 3  is viewed from this distance the light and dark bands of the moire interference are clearly visible. 
     FIG. 4  illustrates an alternative version of film  18 . Film  18  of  FIG. 4  includes prisms such as prism  80  that are curved or serpentine. The serpentine pattern could be random or regular and periodic as shown. Regular and periodic curves will generally have less effect in reducing the gain of film  18 . 
     FIGS. 5A to 5F  are illustrations similar to  FIG. 3  except that the lines representing prism peaks are serpentine rather than linear. When viewed from a distance of 6 meters most of these exhibit no discernible moiré pattern. Even when a moiré pattern is visible it is reduced in comparison to that produced by linear prisms. 
     FIG. 6  is a top view of a single prism of film  18  of  FIG. 5 . Regardless of whether the serpentine pattern is random or regular and periodic. The serpentine curves will have an average amplitude A and an average period L. When the pattern is regular and periodic, the average amplitude and average period will simply be equal to the actual amplitude and period. The average amplitude and average period should be appropriately chosen to minimize visible moiré interference. Generally the amplitude should be greater than about one-quarter of the sub-pixel size and the amplitude should be at least about equal to the pixel size and the ratio of the period to the amplitude should not be too great. Experiments have shown that ratio of the period to the amplitude up to 12 to 1 can be effective at limiting visible moiré interference, however, larger ratios may work to eliminate visible moiré. Even if visible moiré is not totally eliminated, serpentine prisms tend to reduce it. As a result prisms having a serpentine pattern but not meeting the amplitude and period limitations above are still useful. μm 
     FIG. 7  is a schematic view of an alternative display designated generally as  90 . Display  90  includes a light guide  92  that receives light from a CCFT  94 . Reflector  96  collects light from CCFT  94  and directs it into light guide  92 . 
   Light guide  92  is tapered or wedge shaped. As light is conducted along light guide  92 , the angle it makes with the surfaces of light guide  92  will change with each reflection due to the tapered shape of light guide  92 . Eventually the light will strike one of the surfaces of light guide  92  at an angle less than the critical angle for total internal reflection and will be emitted. A reflector  97  is provided to reflect light that is emitted from the back side of light guide  92 . The light that is emitted from light guide  92 , will be emitted at a very large angle to the normal to the surface of light guide  92 . Prism film  98  captures that light and redirects it by total internal reflection from the facets of the prism so that it is traveling along the axis of the display. A diffuser  100  and a liquid crystal panel  102  are also provided. 
   As with the display of  FIG. 2 , the prism of film  98  can create moiré interference with the pixel array of liquid crystal panel  102 . By providing a serpentine pattern to those prisms, the moire interference can be reduced or eliminated. The same considerations apply for choosing amplitude and pitch of the serpentine pattern as apply to the display of  FIG. 2 . 
   Films according to the present invention could be made of any transparent polymer material such as polycarbonate or an acrylic. Films used in displays such as the one depicted in  FIG. 2  generally should have a high index of refraction. An index of 1.72 is generally considered optimum although commercial products typically have an index around 1.585. Films for use in displays of the type depicted in  FIG. 7  generally have a lower index. Films according to the invention could be manufactured by any known method such as extrusion or cast and cure of a U-V curable resin. 
   Masters for the tools used for manufacturing films according to the invention, whether by extrusion or by a cast and cure process, may be made by known diamond turning techniques. Typically the tools are made by diamond turning on a cylindrical blank known as a roll. The surface of the roll is typically of hard copper, although other materials may be used. The prism structures are formed in continuous patterns around the circumference of the roll. In a preferred embodiment the grooves are produced by a technique known as thread cutting. In thread cutting, a single, continuous groove is cut on the roll while the diamond tool is moved in a direction transverse to the turning roll. If the structures to be produced have a constant pitch, the tool will move at a constant velocity. A typical diamond turning machine will provide independent control of the depth that the tool penetrates the roll, the horizontal and vertical angles that the tool makes to the roll and the transverse velocity of the tool. 
   In the invention a fast tool servo (FTS) is used to superimpose a back and forth motion on the constant transverse motion of the diamond tool. It is also possible to use the FTS to move the diamond tool to different depths to provide varying heights for the prisms as taught in U.S. Pat. No. 6,354,709, the teaching of which is incorporated herein by reference.