Abstract:
An electro-optical display system includes a rear reflective heterogeneous polarizer having first and second regions. The first region transmits a first polarization state and reflects a second polarization state and the second region transmits the second polarization state and reflects the first polarization state. The system further includes a front heterogeneous polarizer having third and fourth regions. The third region is substantially aligned with and approximately the same size and shape as the first region and the fourth region is substantially aligned with and approximately the same size and shape as the second region.

Description:
FIELD OF THE INVENTION 
   One embodiment of the present invention is directed to a liquid crystal display. More particularly, one embodiment of the present invention is directed to a liquid crystal display having polarization light recycling. 
   BACKGROUND INFORMATION 
   A liquid crystal display (“LCD”) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It has many advantages over competing technologies because it uses very small amounts of electric power and is therefore suitable for use in battery-powered electronic devices, and because of its thinness. 
   Each pixel in an LCD consists of a layer of liquid crystal (“LC”) molecules suspended between two transparent electrodes, and sandwiched between two crossed linear polarizers (i.e., polarizers with axes of transmission which are perpendicular to each other). Without the liquid crystals between them, light passing through one polarizer would be blocked by the other. The liquid crystals act as polarization modifying light valves by changing the polarization state of the light coming from the rear polarizer. In order to function in this manner, the liquid crystal molecules must be correctly aligned so that they accept light of the polarization state transmitted by the rear polarizer and can rotate it to the polarization state that is transmitted by the front polarizer. Various techniques are known for achieving the appropriate alignment of the liquid crystal molecules. These include mechanical rubbing, which introduces microscopic grooves, or use of oriented linearly polarized UV illumination of an appropriate alignment layer substrate. Application of an electric field, by applying a voltage to the transparent electrodes, can modify the degree of polarization rotation, thus enabling fine control of the light passing through the pixel. The operation of LCDs depends on the correct relationship between the axis of transmission of the rear polarizer, the alignment of the liquid crystal layer (both for light entering and leaving) and the axis of transmission of the front polarizer. 
   The pixels by themselves do not generate light and therefore an LCD requires external illumination, either from ambient sources for a “reflective” LCD or from a backlight for a “transmissive” LCD (or from a combination in the case of a “transflective” LCD). A large portion of the power consumption of a transmissive LCD is devoted to the backlight. However one problem with known transmissive LCDs is that the vast majority of this power is expended in producing light that is ultimately not used in the display output, since it is filtered out. A typical light yield (i.e., the fraction of generated light that is transmitted by a fully active pixel) of these known LCDs is approximately 5%-7%. 
   Light loss that is intrinsic to LCD designs is typically due to the following elements (assuming illumination from an unpolarized white source):
         color filter set: approximately 28% transmission;   aperture ratio: approximately 70% transmission; and   rear and front polarizers: approximately 40% transmission.
 
Color filters are required since backlight typically generate white light. The aperture ratio arises since some of the area of an LCD does not transmit light.
       

   Polarization losses arise from intrinsic aspects of the design of LCDs. As has been described, LCDs require illumination to be linearly polarized and appropriately oriented, which typically results in the loss of at least half of the light available from the backlight. 
   Various attempts have been made to improve the light yield of LCDs, which could greatly improve the electrical efficiency of LCDs, therefore enabling more power efficient appliances, extending battery life for mobile devices, reducing needed backlight illumination components since fewer or lower power lamp elements would be needed to provide a certain level of brightness, and improving heat management in display units since much of the lost light is absorbed as heat. One method of improving light yield is through polarization light recycling, disclosed in, for example, U.S. Pat. No. 7,038,745.  FIG. 1  is a cross-sectional view of a typical prior art transmissive LCD  10  which includes polarization light recycling in order to improve the light yield.  FIG. 1  shows only those elements directly involved in light recycling and does not show many other elements often found in LCDs (e.g., color filters, prism sheet, diffusers, etc). LCD  10  includes a liquid crystal layer  17 . Liquid crystal layer  17  is sandwiched by a homogenous front polarizer  19  and a homogenous rear reflective polarizer  18 . Rear polarizer  18  transmits light of a first polarization (“P 1 ”) and reflects light of a second polarization (“P 2 ”). Front polarizer  19  is crossed with rear polarizer  18  (i.e., has perpendicular transmission axes) and transmits P 2  light. Liquid crystal layer  17  functions as a plurality of polarization modifying light valves (i.e., pixels) each of which can rotate incident P 1  light to include a P 2  component, depending on the amount of applied voltage. LCD  10  further includes a backlight unit  11  that includes a backlight  12  for generating unpolarized white light, a rear reflector  14  for reflecting light, and a light guide  30  for guiding and homogenizing generated and reflected light. 
   In operation, backlight unit  11  produces a uniform distribution of white unpolarized illumination that includes both P 1  and P 2  light (arrow  20 ). This light is incident on homogenous rear reflecting polarizer  18  that transmits one polarization P 1  (arrow  21 ) and reflects the majority of the other component, P 2  (arrow  23 ), back into backlight unit  11 . LC layer  17  converts some of the polarization of arrow  21  to P 2  light and transmits that as arrow  24 , of which the P 2  light portion is ultimately transmitted by homogeneous front polarizer  19  as arrow  25 . 
   Meanwhile, in backlight unit  11 , elements such as rear reflector  14  convert P 2  of arrow  23  so that it now has components of both P 1  and P 2  (arrow  26 ), thus allowing an increase in the amount of “recycled” P 1  (arrow  27 ) available for transmission. A portion of the recycled P 1  (arrow  27 ) is converted to recycled P 2  (arrow  28 ) by LC layer  17 , which is ultimately transmitted (arrow  29 ) by homogenous front polarizer  19 . This process is repeated many times to increase the total amount of P 2  light transmitted to the viewer. Commercial versions of the recycling technology shown in  FIG. 1  are known to increase the light yield by approximately 30%. 
   Based on the foregoing, there is a need for an LCD system that has an improved light yield relative to known systems. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention is an electro-optical display system that includes a rear reflective heterogeneous polarizer having a first and a second region. The first region transmits a first polarization state and reflects a second polarization state and the second region transmits the second polarization state and reflects the first polarization state. The system further includes a front heterogeneous polarizer having a third and a fourth region. The third region is substantially aligned with and approximately the same size and shape as the first region and the fourth region is substantially aligned with and approximately the same size and shape as the second region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a typical prior art transmissive LCD which includes polarization light recycling in order to improve the light yield. 
       FIG. 2  is a cross-sectional view of a portion of an LCD in accordance with one embodiment of the present invention. 
       FIG. 3  is a top view of a heterogeneous front polarizer in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   One embodiment of the present invention is an LCD with a heterogeneous front and a complementary heterogeneous rear polarizer to achieve substantially improved light yield. 
     FIG. 2  is a cross-sectional view of a portion of an LCD  50  in accordance with one embodiment of the present invention.  FIG. 2  shows only those elements directly involved in one embodiment, and does not show other elements that can be used in combination (e.g., color filter elements, prism sheets, alignment layers, diffusers, other polarizing elements, other reflecting elements, etc). LCD  50  includes a backlight unit that includes a backlight (not shown) for generating unpolarized light, a rear reflector  14  for reflecting light, and a light guide  30  for guiding and homogenizing the generated and reflected light. A rear relative polarizer  48  is heterogeneously polarized with alternating orientation regions, two of which are shown in  FIG. 2  ( 46  and  47 ). LCD  50  further includes a front polarizer  49  which is heterogeneously polarized with alternating orientation regions, two of which are shown in  FIG. 2  ( 51  and  52 ) and each regions is complimentary to the corresponding region of rear polarizer  48  (e.g., region  46  transmits only P 1  light and region  51  transmits only P 2  light). Regions  46  and  51  are of approximately the same shape and size. Regions  47  and  52  are approximately the same shape and size. 
   LCD  50  further includes an LCD layer  70  that includes regions  71  and  72  that are each appropriately oriented to the corresponding regions of rear polarizer  48  and front polarizer  49 . In one embodiment, LC layer  70  is formed of Twisted Nematic liquid crystals that have an orientation dependence such that they must be appropriately aligned with respect to the transmission axes of both the front and rear polarizers. Given that one embodiment of the present invention has alternating regions of orientations of both front and rear polarizers, LC regions  71  and  72  should also have alternating orientations. This type of “alignment” can be accomplished by either mechanical rubbing (i.e., creation of micro grooves) or by using oriented linearly polarized UV. 
   In operation, the backlight unit produces a uniform distribution of unpolarized illumination that includes both P 1  and P 2  light. Some of this light (arrow)  60 ) is incident on region  46  of heterogeneous rear reflective polarizer  48  that transmits one polarization P 1  (arrow  61 ) and reflects the majority of the other component, P 2  (arrow  62 ), back towards the backlight unit. LC layer  70  (specifically region  71 ) converts some of the polarization of arrows  61  to P 2  light and transmits as arrow  73 , of which the P 2  light portion is ultimately transmitted by front polarizer  49  (specifically though region  51 ) as arrow  53 . 
   Meanwhile, rear reflector  14  converts P 2  of arrow  62  so that it now has components of both P 1  and P 2  (arrow  63 ), of which the “recycled” P 2  portion (arrow  45 ) is transmitted by region  47  of rear reflective polarizer  48 . A portion of the recycled P 2  is converted to recycled P 1  (arrow  74 ) by region  72  of LC layer, which is ultimately transmitted (arrow  54 ) by region  52  of front polarizer  49 . This process is repeated many times such that light reflected from the rear polarizer  46  (either P 1  or P 2  light depending on which region of polarizer  46  it is reflected by) can be reflected and recycled many times until it is ultimately transmitted by an appropriate region of the present invention, thereby enabling an increase in total light transmitted by the LCD. Embodiments of the present invention do not depend on the ability of rear reflector  14  to convert between polarization states. Rather, embodiments would be effective even if the rear reflector did nothing more than reflect, since by multiple reflections each polarization state would ultimately reach a region through which it could be propagated. 
     FIG. 3  is a top view of heterogeneous front polarizer  49  in accordance with one embodiment of the present invention. As shown, polarizer  49  includes alternating orientation regions. As disclosed, rear polarizer  48  has a similar pattern of alternating regions that are the same shape and size as those of the front polarizer  49  but are complimentary with each region being crossed with its corresponding region. LC layer  70  also has a similar pattern of alternating regions with the relevant alignment directions. 
   The patterned heterogeneous regions of polarizers  48  and  49  can be of any shape or size that is convenient to manufacture. In one embodiment, the aggregate area of each orientation covers approximately 50% of the total display area. In other embodiments, the shape and layout of heterogeneous regions can be optimized to take advantage of the properties of elements of the backlight unit and to also optimize evenness of illumination. 
   In one embodiment, rear heterogeneous polarizer  48  is non-absorbing, and front polarizer  49  is any type as long as the regions are the same shape and size and complementary to the equivalent regions of rear polarizer  48 . Other components typically used in LCDs and backlights, such as color filters, diffusers, compensating films, prism sheets, collimating sheets etc., may also be used in conjunction with embodiments of the invention. Any of these additional elements that requires a matched polarization orientation should also have appropriate heterogeneous regions. 
   Heterogeneous polarizers such as polarizers  48  and  49  are known. For example, in the specialized area of 3D stereoscopic displays, heterogeneously polarized output is used to encode and present different images to the right and left eyes of observers, which are then decoded by using suitable complementary polarized eye glasses. In these known systems, a heterogeneously polarized output is used to deliver different perspectives to the left and right eyes of observers by adding a patterned polarizing layer to the front of an otherwise conventional electro-optical display. These systems typically use either a micro-polarized array that is alternating for adjacent pixels or narrow strips of the display or alternating phase retarder regions to give distinctly circularly polarized views to each eye. However, these known systems do not include both rear and front heterogeneous polarizers. 
   The incorporation of more than one polarization orientation in embodiments of the present invention allows more total light to be output by polarization light recycling. Prior art polarization recycling has relied on homogeneously polarized reflective rear polarizers and reflection or other recycling or conversion techniques to convert a portion of the untransmitted light to a state that can be transmitted. In contrast, embodiments of the present invention do not have to rely on polarization conversion of the untransmitted light. Therefore, embodiments of the present invention are not limited by the efficiency of the conversion process. This theoretically enables near zero polarization losses, which can substantially improve power efficiency and panel brightness. 
   Patterned alignment regions in LCD panels such as LCD layer  70  of  FIG. 2  and methods of manufacturing them are known. For example multi-domain pixels have been designed to improve contrast and viewing angles in which the pixels are divided into more than one domain, each of which has different alignment directions, although all are homogeneously polarized. Known manufacturing methods for creating these differently aligned domains are used in one embodiment to manufacture the patterned alignment regions of LC layer  70  (e.g., regions  71  and  72 ) that match the respective heterogeneous regions of the patterned polarizers  48  and  49 . 
   In one embodiment, complementary regions of the front and rear polarizers  48  and  49  and related LC layer  70  patterns should be designed so that interfaces between adjacent regions of the heterogeneous polarization are not visible. To achieve this, in one embodiment boundaries can be arranged to be in areas of the panel that are between pixel elements of the display. 
   Embodiments of the present invention can be implemented for any electro-optical display technology using a light modulation unit that depends on polarization modification to enable its light valves to work. These light modulation units can be pixilated (i.e., have a regular array of pixels of the same shape and size), or they can have other patterns, where the elements are of different shapes and sizes, such as a “seven segment” display as commonly used in watches. The light modulation unit can be of any type, including liquid crystal such as Twisted Nematic, Super Twisted Nematic or non Nematic liquid crystal. 
   The present invention can be utilized with black and white displays, color displays and 3D displays. It can be utilized with any backlight source technology, including Cold Cathode Fluorescent Lamps (“CCFL”), electroluminescent or Light Emitting Diodes (“LED”). For backlights with LEDs, embodiments of the present invention provide particular advantages for manufacturers of displays since the number of lamp units can be reduced, enabling LED backlights to compete for larger display segments. Further, embodiments of the present invention can be used in combination with other known methods of improving efficiency and performance, such as color display schemes that incorporate a white sub pixel (disclosed, for example, in U.S. Pat. No. 6,989,876), field sequence color filterless displays which use multiple monochromatic primary color strobed light sources to avoid losses from color filters (disclosed, for example, in U.S. Pat. No. 6,480,247) and multi-domain vertical alignment LCDs. Such displays still suffer from large polarization losses and hence can benefit from embodiments of the present invention. 
   Several embodiments of the present invention are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. 
   For example some embodiments may include transmission axes of the front and rear polarizer regions that are parallel. Further, some embodiments may use more than two orientations of primary axes of transmission in the heterogeneous polarizers and each polarizer can have more than two regions of different polarization orientations. Further, some embodiments of the invention may incorporate heterogeneous polarization elements into the backlight unit, instead of on the rear of the light modulation unit.