PATENT DOCUMENT

Publication Number: US-9568772-B2
Application Number: US-201514684758-A
Country: US
Kind Code: B2

Title: Displays with elevated backlight efficiency

Abstract:
An electronic device may be provided with a display mounted in a housing. The display may include a liquid crystal display module and a reflective polarizer having an in-plane optical axis. The display may also include a backlight unit that includes a light source, a light guide element, and a reflector film coupled to a backside of the light guide element. The display may also include a light retardation layer such as a quarter wave film. The quarter wave film may be arranged between the reflective polarizer and the reflector film of the backlight unit. Partially polarized light that is output from a front side of the light guide element may have a first component parallel to the in-plane optical axis and a second component perpendicular to the in-plane optical axis of the reflective polarizer. The second component may be reflected from the reflective polarizer.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a liquid crystal display module; 
 a backlight unit having a reflector; 
 a prism film; 
 a reflective polarizer interposed between the reflector and the liquid crystal display module; 
 a half wave plate interposed between the reflective polarizer and the prism film; and 
 a uniaxial birefringent compensation film, wherein the half wave plate is interposed between the reflective polarizer and the uniaxial birefringent compensation film. 
 
     
     
       2. The display defined in  claim 1  wherein the prism film has prism elements and a prism axis that runs parallel to the prism elements, wherein the backlight unit emits light that passes through the prism film, wherein the prism film passes more of the emitted light as p-polarized light that has a polarization axis perpendicular to the prism axis than as s-polarized light that has a polarization axis parallel to the prism axis. 
     
     
       3. The display defined in  claim 2  wherein the reflective polarizer has a pass axis, wherein the polarization axis of the p-polarized light and the pass axis of the reflective polarizer are separated by an angle, and wherein the half wave plate has a fast axis that bisects the angle between the polarization axis of the p-polarized light and the pass axis of the reflective polarizer. 
     
     
       4. The display defined in  claim 3  further comprising a diffuser layer. 
     
     
       5. The display defined in  claim 4  wherein the backlight unit has a light guide plate. 
     
     
       6. The display defined in  claim 5  wherein the diffuser layer is interposed between the light guide plate and the prism film. 
     
     
       7. A liquid crystal display, comprising:
 display layers including a liquid crystal layer and a reflective polarizer; 
 a backlight unit having a light guide plate and a reflector, wherein the reflective polarizer is interposed between the liquid crystal layer and the reflector; 
 a prism film; 
 a half wave plate between the reflective polarizer and the prism film; and 
 a uniaxial birefringent compensation film, wherein the half wave plate is interposed between the reflective polarizer and the uniaxial birefringent compensation film. 
 
     
     
       8. The liquid crystal display defined in  claim 7  further comprising a prism film between the light guide plate and the half wave plate. 
     
     
       9. The liquid crystal display defined in  claim 7  wherein the reflective polarizer has a pass axis and wherein the prism film has a prism axis that is oriented at a non-zero angle with respect to the pass axis. 
     
     
       10. The liquid crystal display defined in  claim 9  wherein the prism film passes light with a polarization axis that is perpendicular to the prism axis. 
     
     
       11. The liquid crystal display defined in  claim 10  wherein the half wave plate rotates the polarization axis of the light with the polarization axis that is perpendicular to the prism axis as this light passes through the half wave plate so that the polarization axis of this light is aligned with the pass axis of the reflective polarizer. 
     
     
       12. The liquid crystal display defined in  claim 11  wherein the half wave plate has a fast axis that lies halfway between the pass axis of the reflective polarizer and the polarization axis that is perpendicular to the prism axis. 
     
     
       13. The liquid crystal display defined in  claim 12 , wherein the light guide plate is interposed between the reflector and the prism film. 
     
     
       14. The liquid crystal display defined in  claim 13  further comprising a diffuser between the light guide plate and the prism film. 
     
     
       15. A display, comprising:
 an upper polarizer; 
 a lower polarizer; 
 display layers between the upper polarizer and the lower polarizer that include a layer of liquid crystal material and an array of pixels with rows of pixels that run parallel to a first axis and columns of pixels that run parallel to a second axis that is perpendicular to the first axis; 
 a light guide plate that emits backlight; 
 a half wave plate between the light guide plate and the lower polarizer; 
 a reflective polarizer interposed between the lower polarizer and the light guide plate; 
 a uniaxial birefringent compensation film, wherein the half wave plate is interposed between the reflective polarizer and the uniaxial birefringent compensation film; and 
 a prism film having prism elements and a prism axis that run along a third axis that is between the first and second axes. 
 
     
     
       16. The display defined in  claim 15  wherein the backlight emitted from the light guide plate passes through the prism film, wherein a portion of the backlight that has passed through the prism film has a dominant polarization direction. 
     
     
       17. The display defined in  claim 16  wherein the lower polarizer has a pass axis and wherein the half wave plate rotates the dominant polarization direction of the portion of the backlight into alignment with the pass axis.

Description:
This application is a continuation-in-part of patent application Ser. No. 13/935,110, filed Jul. 3, 2013, which claims the benefit of provisional patent application No. 61/701,544, filed Sep. 14, 2012. Patent application Ser. No. 13/935,110, filed Jul. 3, 2013, is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices with displays. 
     Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user. 
     It can be challenging to form displays for electronic devices. Displays such as liquid crystal displays typically include a backlight unit that generates light for the display. 
     Light is commonly emitted into a light guide plate in the backlight unit and exits the light guide plate into a display module such as a liquid crystal display module. However, if care is not taken, light that exits the light guide plate can sometimes be reflected back toward the light guide plate. This type of reflected light can cause unwanted light leakage from the display, thereby reducing the efficiency of the display. 
     It would therefore be desirable to be able to provide improved displays for electronic devices. 
     SUMMARY 
     An electronic device may be provided with a display mounted within a housing. The display may include a display module such as a liquid crystal display (LCD) panel, a backlight unit, a polarizer such as a reflective polarizer, and a light retardation film such as a quarter wave film, half wave film, or other retarder. The retarder may be arranged between a reflector film within the backlight unit (BLU) and the polarizer. The retarder helps improve backlight efficiency by improving light transmittance through the polarizer. The retarder may also improve light recycling efficiency from the reflective polarizer. 
     The reflective polarizer may be configured to transmit a first polarized light and reflect a second polarized light where the polarization of the second polarized light is orthogonal to the polarization of the first polarized light. The backlight unit may include a light source, a light guide element such as a light guide plate, and a reflector film coupled to a backside of the light guide element. The quarter wave film may be arranged within the backlight unit, among optical films associated with the backlight unit, or between the optical films and the display module such that the quarter wave film is located between the reflector film of the backlight unit and the reflective polarizer. 
     In configurations in which a half wave film is interposed between the backlight unit and the polarizer, the half wave film may help rotate light of a predominant polarization at the output of a prism film into alignment with the pass axis of the polarizer. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device such as a laptop computer with a display in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment of the present invention. 
         FIG. 4  is a perspective view of an illustrative electronic device such as a computer display with display structures in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of an illustrative display including a backlight unit, a reflective polarizer, and a quarter-wave film in accordance with an embodiment of the present invention. 
         FIG. 6  is a cross-sectional side view of an illustrative display in accordance with an embodiment of the present invention. 
         FIG. 7  is a cross-sectional view of an illustrative quarter wave film stack having a uniaxial quarter wave film in accordance with an embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of an illustrative quarter wave film stack having a uniaxial quarter wave film and a uniaxial c-film in accordance with an embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of an illustrative quarter wave film stack having a uniaxial quarter wave film and a uniaxial c-film interposed between the uniaxial wave film and the reflective polarizer in accordance with an embodiment of the present invention. 
         FIG. 10  is a cross-sectional view of an illustrative quarter wave film stack having a uniaxial quarter wave film and multiple uniaxial c-films in accordance with an embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of an illustrative quarter wave film stack having a biaxial quarter wave film in accordance with an embodiment of the present invention. 
         FIG. 12  is a perspective view of an illustrative backlight unit and optical films including two face-up prism films in accordance with an embodiment of the present invention. 
         FIG. 13  is an enlarged view one of the two face-up prism films of  FIG. 12  in accordance with an embodiment of the present invention. 
         FIG. 14  is a perspective view of an illustrative backlight unit and optical films including a face-down prism film in accordance with an embodiment of the present invention. 
         FIG. 15  is an enlarged view of the face-down prism film of  FIG. 14  in accordance with an embodiment of the present invention. 
         FIG. 16  is a cross-sectional view of an illustrative display including a backlight unit, a face-down prism film, a reflective polarizer, and a quarter wave film between a reflective polarizer and a diffuser film in accordance with an embodiment of the present invention. 
         FIG. 17  is a cross-sectional view of an illustrative display including a backlight unit, a face-down prism film, a reflective polarizer, and a quarter wave film between a diffuser film and the face-down prism film in accordance with an embodiment of the present invention. 
         FIG. 18  is a cross-sectional view of an illustrative display including a backlight unit, a face-down prism film, a reflective polarizer, and a quarter wave film between the face-down prism film and a light guide element in accordance with an embodiment of the present invention. 
         FIG. 19  is a cross-sectional view of an illustrative display including a backlight unit, a face-down prism film, a reflective polarizer, and a quarter wave film between a light guide element and a reflector film in accordance with an embodiment of the present invention. 
         FIG. 20  is a cross-sectional of an illustrative display including a backlight unit, two face-up prism films, a reflective polarizer, and a quarter wave film between the reflective polarizer and a top diffuser film in accordance with an embodiment of the present invention. 
         FIG. 21  is a cross-sectional view of an illustrative display including a backlight unit, two face-up prism films, a reflective polarizer, and a quarter wave film between a top diffuser and a top prism film in accordance with an embodiment of the present invention. 
         FIG. 22  is a cross-sectional of an illustrative display including a backlight unit, two face-up prism films, a reflective polarizer, and a quarter wave film between a bottom prism film and a bottom diffuser film in accordance with an embodiment of the present invention. 
         FIG. 23  is illustrates a cross-sectional view an illustrative display including a backlight unit, two face-up prism films, a reflective polarizer, and a quarter wave film between a bottom diffuser film and a light guide element in accordance with an embodiment of the present invention. 
         FIG. 24  is a cross-sectional view of an illustrative display including a backlight unit, two face-up prism films, a reflective polarizer, and a quarter wave film between a light guide element and a reflector film in accordance with an embodiment of the present invention. 
         FIG. 25  is a cross-sectional view of an illustrative display including a backlight unit, a reflective polarizer, and a quarter wave film between a reflective polarizer and a diffuser film in accordance with an embodiment of the present invention. 
         FIG. 26  is a cross-sectional of an illustrative display including a backlight unit, a reflective polarizer, and a quarter wave film between a diffuser and a light guide element in accordance with an embodiment of the present invention. 
         FIG. 27  is a cross-sectional view of an illustrative display including a backlight unit, a reflective polarizer, and a quarter wave film between a light guide element and a reflector film in accordance with an embodiment of the present invention. 
         FIG. 28  is a diagram showing how light may be output from a light guide element and a face-down prism film in accordance with an embodiment of the present invention. 
         FIG. 29  is a graph showing illustrative luminance intensity output profiles from a light guide element and a face-down prism film in accordance with an embodiment of the present invention. 
         FIG. 30  is a graph showing illustrative light polarization from a light guide element in accordance with an embodiment of the present invention. 
         FIG. 31  is a graph showing illustrative light polarization from a face-down prism film in accordance with an embodiment of the present invention. 
         FIG. 32  is a diagram showing how light may propagate within in a light guide element, a diffuser film and a face-up prism film in accordance with an embodiment of the present invention. 
         FIG. 33  is a diagram showing illustrative light recycling between a reflective polarizer, a quarter-wave film, optical films, and a backlight unit in accordance with an embodiment of the present invention. 
         FIG. 34  is a flow diagram showing how light may be recycled through the reflective polarizer, the quarter wave film, the optical film and the backlight unit in accordance with an embodiment of the present invention. 
         FIG. 35  is a graph showing an illustrative gain for a display having a reflective polarizer in comparison with a conventional display that does not include a reflective polarizer in accordance with an embodiment of the present invention. 
         FIG. 36  is a graph showing an illustrative gain for a display having a reflective polarizer and a quarter wave film in comparison with a conventional display that does not include a reflective polarizer in accordance with an embodiment of the present invention. 
         FIG. 37  is a diagram of an illustrative test system for measuring light luminance from a backlight unit at a normal direction and other angles from the normal direction in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include displays. The displays may be used to display images to a user. Illustrative electronic devices that may be provided with displays are shown in  FIGS. 1, 2, 3, and 4 . 
       FIG. 1  shows how electronic device  10  may have the shape of a laptop computer having upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  may have hinge structures  20  that allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  may be mounted in upper housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, may be placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows how electronic device  10  may be a handheld device such as a cellular telephone, music player, gaming device, navigation unit, or other compact device. In this type of configuration for device  10 , housing  12  may have opposing front and rear surfaces. Display  14  may be mounted on a front face of housing  12 . Display  14  may, if desired, have openings for components such as button  26 . Openings may also be formed in display  14  to accommodate a speaker port (see, e.g., speaker port  28  of  FIG. 2 ). 
       FIG. 3  shows how electronic device  10  may be a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  may have opposing planar front and rear surfaces. Display  14  may be mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  may have an opening to accommodate button  26  (as an example). 
       FIG. 4  shows how electronic device  10  may be a computer display or a computer that has been integrated into a computer display. With this type of arrangement, housing  12  for device  10  may be mounted on a support structure such as stand  27 . Display  14  may be mounted on a front face of housing  12 . 
     The illustrative configurations for device  10  that are shown in  FIGS. 1, 2, 3, and 4  are merely illustrative. In general, electronic device  10  may be a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     Housing  12  of device  10 , which is sometimes referred to as a case, may be formed of materials such as plastic, glass, ceramics, carbon-fiber composites and other fiber-based composites, metal (e.g., machined aluminum, stainless steel, or other metals), other materials, or a combination of these materials. Device  10  may be formed using a unibody construction in which most or all of housing  12  is formed from a single structural element (e.g., a piece of machined metal or a piece of molded plastic) or may be formed from multiple housing structures (e.g., outer housing structures that have been mounted to internal frame elements or other internal housing structures). 
     Display  14  may be a touch sensitive display that includes a touch sensor or may be insensitive to touch. Touch sensors for display  14  may be formed from an array of capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensor components. 
     Display  14  for device  10  includes display pixels formed from liquid crystal display (LCD) components or other suitable image pixel structures. 
     A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . The outermost display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     A cross-sectional side view display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures such as backlight unit  42 , a display module such as display layers  46  (e.g., a liquid crystal display (LCD) panel), and additional light processing layers such as optical films  70 , a light retardation layer such as quarter wave film (QWF)  102 , and reflective polarizer  100 . The QWF helps increase the efficiency of the reflective polarizer. The mechanisms for improving efficiency are explained in further details later. 
     If desired, optical films  70  may be formed as a portion of backlight unit  42  or may be formed separately from backlight unit  42 . Quarter wave film  102  may be located between optical films  70  and reflective polarizer  100 . However, this is merely illustrative. If desired, quarter wave film  102  may be located between one or more of optical films  70 , between optical films  70  and backlight unit  42 , or within backlight unit  42 . 
     A cross-sectional side view of one illustrative configuration for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4  or other suitable electronic devices) incorporating the elements of  FIG. 5  is shown in  FIG. 6 . 
     As shown in  FIG. 6 , backlight structures  42  may be used for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 5 ) and passes through display pixel structures in display layers  46 . This illuminates any images that are being produced by the display pixels for viewing by a user. For example, backlight  44  may illuminate images on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing  12  or display layers  46  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     In a configuration in which display layers  46  are used in forming a liquid crystal display, display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  56  and  58  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of thin-film transistors and associated electrodes (display pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits such as components  68  on printed circuit  66  of  FIG. 5 ) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed from circuitry  68  to display driver integrated circuit  62  using a signal path such as a signal path formed from conductive metal traces in flexible printed circuit  64  (as an example). 
     Display driver integrated circuit  62  may be mounted on thin-film-transistor layer driver ledge  82  or elsewhere in device  10 . A flexible printed circuit cable such as flexible printed circuit  64  may be used in routing signals between printed circuit  66  and thin-film-transistor layer  58 . If desired, display driver integrated circuit  62  may be mounted on printed circuit  66  or flexible printed circuit  64 . Printed circuit  66  may be formed from a rigid printed circuit board (e.g., a layer of fiberglass-filled epoxy) or a flexible printed circuit (e.g., a flexible sheet of polyimide or other flexible polymer layer). 
     Backlight structures  42  may include a light guide element such as light guide plate  78 . Light guide plate  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes. 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed in dimensions X and Y throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-scattering features such as pits or bumps. The light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide plate  78 . 
     Light  74  that scatters upwards in direction Z from light guide plate  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upwards direction by reflector  80 . Reflector  80  may be formed from a reflective material such as a layer of white plastic or other shiny materials. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70  and additional optical layers such as quarter wave film (QWF) stack  104 . 
     Optical films  70  may include diffuser layers (diffuser films) for helping to homogenize backlight  44  and thereby reduce hotspots, compensation films for enhancing off-axis viewing, and brightness enhancement films (also sometimes referred to as turning films) or other prism films for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide plate  78  and reflector  80 . For example, if light guide plate  78  has a rectangular footprint in the X-Y plane of  FIG. 5 , optical films  70  and reflector  80  may have a matching rectangular footprint. 
     QWF stack  104  may include a light retardation film such as quarter wave film  102  and a polarizer such as reflective polarizer  100 . QWF  102  may be located between reflective polarizer  100  and optical films  70  as shown in  FIG. 6 . However, this is merely illustrative. If desired, QWF  102  may be located at any suitable location between reflective polarizer  100  and reflector  80  of backlight structures  42 . 
     QWF stack  104  may vary in various embodiments as shown in  FIGS. 7, 8, 9, 10, and 11 . As shown in  FIG. 7 , QWF stack  104  may include a QWF  102  formed from a uniaxial QWF having an in-plane slow axis and an in-plane fast axis, which are both parallel to the surface of QWF  102 . The uniaxial QFW may be formed by stretching a polymer film along a single direction. This stretching direction is either along the in-plane slow axis or the in-plane fast axis. 
     As shown in  FIG. 8 , QWF stack  104  may include a QWF  102  formed from a uniaxial QWF and a uniaxial birefringent compensation film (c-film)  312  on a bottom surface of the uniaxial QWF. Uniaxial c-film  312  has an optical axis normal to plane of c-film  312 . Uniaxial c-film  312  may be included to compensate for the viewing angle dependence of QWF  102 . 
     As shown in  FIG. 9 , QWF stack  104  may include a QWF  102  formed from a uniaxial QWF and uniaxial c-film  312  on the top surface of the uniaxial QFW  102  (e.g., between QWF  102  and reflective polarizer  100 ). Uniaxial c-film  312  has an optical axis normal to the plane of c-film  312 . Uniaxial c-film  312  may be included to compensate for the viewing angle dependence of QFW  102 . 
     As shown in  FIG. 10 , QWF stack  104  may include a QWF  102  formed from a uniaxial QWF, uniaxial c-film  312   a  on the top surface of the uniaxial QWF  102 , and uniaxial c-film  312   b  on the bottom surface of the uniaxial QWF  102 . Uniaxial c-film  312   a  and uniaxial c-film  312   b  both have an optical axis normal to their own film surface. Uniaxial c-film  312   a  and uniaxial c-film  312   b  may be included to compensate for the viewing angle dependence of QFW  102 . 
     As shown in  FIG. 11 , QWF stack  104  may include a QWF  102  formed from a biaxial QWF having an in-plane phase retardation between an in-plane slow axis and an in-plane fast axis, and also an out-of-plane phase retardation. A biaxial QWF of this type may be formed by stretching a semi-crystalline polymer film along two perpendicular directions. Such a semi-crystalline polymer film includes but not limited to polypropylene film, polyethylene terephthalate (PET) film, or other polymer films. 
     QWF  102  may be a wave retarder that is formed of birefringent materials. Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. A wave retarder alters the polarization state or phase of light traveling therethrough. The wave retarder has a slow axis that is also referred to an extraordinary axis, and a fast axis that is also referred to an ordinary axis. As polarized light passes through the wave retarder, the light along the fast axis travels more quickly than along the slow axis. 
     A quarter wave film retards the velocity of one of the polarization components (x or y), such that the one of the polarization components is one quarter of a wavelength out of phase from the other polarization component (y or x). Polarized light passing through a quarter wave retarder thus becomes circularly polarized, which is also referred to as twisting or rotating the polarized light. The retardation of a QWF is expressed in Equation (1)
 
Retardation=( n   e   −n   o ) t   Equation (1)
 
where n e  is a refractive index along an extraordinary axis, n o  is a refractive index along an ordinary axis, and t is a thickness of the QWF.
 
     Referring to  FIGS. 7-11  again, in order to improve light recycling efficiency, the uniaxial QWF and the biaxial QWF may have an in-plane retardation ranging from 50 nm to 200 nm (for example) between the in-plane slow axis and the in-plane fast axis of the QWF. The in-plane retardation between the in-plane slow axis and the in-plane fast axis may be in the range of 100 nm to 160 nm (for example). In a particular embodiment, the in-plane retardation between the in-plane slow axis and the in-plane fast axis may be about 137.5 nm. 
     Referring to  FIGS. 7-11  again, reflective polarizer  100  may transmit linearly polarized light along a first axis and reflect linearly polarized light along a second axis that is perpendicular to the first axis. The in-plane slow axis of the uniaxial QWF or the in-plane slow axis of the biaxial QWF may form an angle ranging from 30 to 60 degrees or ranging from 120 to 150 degrees (as examples) with respect to the first axis or the second axis of the reflective polarizer. 
     In one particular embodiment, the in-plane slow axis of the uniaxial QWF or the in-plane slow axis of the biaxial QWF may form an angle ranging from about 40-50 degrees or about 130-140 degrees (as examples) with respect to the first axis or the second axis of the reflective polarizer. 
     In another particular embodiment, the in-plane slow axis of the uniaxial QWF or the in-plane slow axis of the biaxial QWF may form an angle about 45 degrees or about 135 degrees (as examples) with respect to the first axis or the second axis of the reflective polarizer. 
     Referring to  FIGS. 7-11  again, reflective polarizer  100  may transmit a first circularly polarized light (right-handed or left-handed) and reflect a second circularly polarized light (left-handed or right-handed) that is orthogonal to the first circularly polarized light. The in-plane slow axis of the uniaxial QWF or the biaxial QWF may have no orientation constraint as long as the in-plane slow axis is substantially parallel to the surface of QWF  102 . 
     Optical films that may be used in various embodiments described herein include diffuser films and prism films. A diffuser film is used to provide more uniform light distribution toward the LCD panel and hide cosmetic defects. A prism film is used to collimate the light such that the light output from the light guiding element can be collimated toward the LCD panel in a direction that is perpendicular to (e.g. normal to) the LCD panel surface. 
     As shown in  FIG. 12 , in one suitable configuration, optical films  70  may include two face-up prism films such as face-up prism film  108  and face-up prism film  110 . Prism films  108  and  110  may be interposed between diffuser film  106  and diffuser film  112 . Face-up prism film  108  may have prism elements  109  facing toward the reflective polarizer and display module  46 . Face-up prism film  110  may have prism elements  111  facing toward the prism film  108 . As shown, prism elements  109  and  111  of face-up prism films  108  and  110  may be arranged to be perpendicular to each other (e.g., prisms  109  and  111  may be offset from one another by a 90 degree angle). 
     Two crossed face-up prism films of this type can rotate the angle of the light output from light guide element  78  to be normal to the LCD panel surface. 
       FIG. 13  illustrates an enlarged view of one of the face-up prism films of  FIG. 12 . The prism pitch or distance between two nearest prisms  109  may vary from, for example, 5 microns to 300 microns. Prism elements  109  (or prism elements  111 ) of each of the two face-up prisms may also have an apex angle of 90 degrees as shown in  FIG. 13 , although prisms with different apex angles may be used. 
     The configuration of  FIGS. 12 and 13  is merely illustrative. If desired, optical films  70  may include a different set of optical films from those described above in connection with  FIG. 12 . 
     As shown in  FIG. 14 , in another suitable configuration, optical films  70  may include a single diffuser film such as diffuser film  106  and a turning film such as face-down prism film  120  having prism elements  121 . Face-down prism film  120  has prisms  121  facing toward light guide element  78  of backlight unit  42 . 
       FIG. 15  illustrates an enlarged view of face-down prism  120  of  FIG. 14 . In the example of  FIG. 15 , face-down prism film elements  122  have an apex angle of 68 degrees. However, this is merely illustrative. Generally, the apex angle may vary from 58 degrees to 78 degrees (for example). The pitch (i.e., the distance between prisms  121 ) may vary from, for example, 5 microns to 300 microns (for example). 
     When partially polarized light passes through a stack of optical films  70 , which may include a bottom diffuser, one or more prism films or turning films, and a top diffuser, the partially polarized light may become further depolarized. For example, if optical films  70  include two face-up prism films (e.g., films  108  and  110  of  FIG. 12 ), the partially polarized light becomes less polarized before entering reflective polarizer  100 . If optical films  70  include a single face-down prism (e.g., film  120  of  FIG. 14 ), the input partially polarized light also becomes less polarized. However, the depolarization effect of a single face-down prism is usually less than that of two face-up prisms. This can be verified by measuring the luminance ratio of two polarized components, such as the ratio of a p-component to an s-component of the light. 
     Light may have a p-component and an s-component that is perpendicular to the p-component. A luminance ratio of the p-component to the s-component may be used to determine the degree of polarization light. Generally, an optical film with a luminance ratio (p-component/s-component) close to 1 generates a relatively larger depolarization effect than another optical film with a luminance ratio (p-component/s-component) that is relatively further away from 1 (e.g., either smaller than or larger than 1). When the luminance ratio is lower than 1, the s-component is more dominant than the p-component. On the contrary, when the luminance ratio is greater than 1, the p-component is more dominant than the s-component. 
     Diffuser films (e.g., top diffuser  106  and bottom diffuser  112  of  FIG. 12  or diffuser  106  of  FIG. 14 ) may also have an impact on the degree of polarization of light that has passed through the top diffuser or the bottom diffuser. 
     Diffuser films may also vary from manufacturer to manufacturer. For example, a diffuser from one manufacturer may depolarize more than a diffuser from an alternative manufacturer. 
     In order to reduce this type of depolarization effect, the base film of a diffuser film may be a film that is either optically isotropic or a film that has relatively low (near zero) in-plane birefringence. In this way, the base film of the diffuser sheet may be arranged to have a small or negligible impact on the polarization of the input light. 
     In order to reduce depolarization effects from prism films, the base film of a prism film may be a film that is either optically isotropic or a film that has relatively low (near zero) in-plane birefringence. Alternatively, to have less depolarization effect, the base film of the prism film may have an in-plane optical axis that is either parallel or perpendicular to the prism orientation direction. 
     Display  14  may have optical films  70  and QWF stack  104  each having one of several arrangements. As examples, QWF stack  104  may include a reflective polarizer and a quarter wave film or may include only a reflective polarizer without any quarter wave film. As examples, optical films  70  may include two crossed face-up prism films, a single face-down prism film, or may be provided without any prism films. 
     It has been found that, for any of the above-mentioned optical film stacks, providing a display with a reflective polarizer increases the efficiency with which light is output from the display and providing a display with a reflective polarizer and a quarter wave film that is interposed between the reflective polarizer and the reflector of the backlight unit further increases the efficiency with which light is output from the display. 
     Illustrative configurations for display  14  in which display  14  includes a reflective polarizer and optical films  70  that include a diffuser film and a single face-down prism film are shown in  FIGS. 16, 17, 18, and 19 . Illustrative configurations for display  14  in which display  14  includes a reflective polarizer and optical films  70  that include top and bottom diffuser films and two crossed face-up prism films are shown in  FIGS. 20, 21, 22, 23, and 24 . Illustrative configurations for display  14  in which display  14  includes a reflective polarizer and optical films  70  that include a diffuser film and no prism film are shown in  FIGS. 25, 26, and 27 . 
     As shown in  FIG. 16 , display  14  may include backlight unit  42 , optical films  70  having diffuser film  106  and face-down prism film  120 , reflective polarizer  100 , and quarter wave film  102  formed between reflective polarizer  100  and diffuser film  106 . Reflective polarizer  100  may be formed between optical films  70  and display module  46 . Reflective polarizer  100  may be formed adjacent to lower polarizer  60  of display module  46 . In the example of  FIG. 16 , quarter wave film  102  forms a top layer of optical films  70 . In this example, QWF  102  may be laminated to the bottom surface of reflective polarizer  100  or may be a standalone film. The configuration of display  14  of  FIG. 16  in which optical films  70  include diffuser layer  106  and face-down prism film  120  and in which QWF  102  is formed between diffuser  106  and reflective polarizer  100  is merely illustrative. QWF  102  may be formed in other locations in display  14  having optical films  70  that include diffuser layer  106  and face-down prism  120  as described below in connection with  FIGS. 17, 18, and 19 . 
     If desired, QWF  102  may be formed between diffuser film  106  and face-down prism film  120  as shown in  FIG. 17 . In this type of configuration, QWF  102  may be laminated to the bottom surface of diffuser  106 , or may be a standalone film. 
     If desired, QWF  102  may be formed below prism film  120  (e.g., between prism film  120  and light guide element  78  of backlight unit  42 ) as shown in  FIG. 18 . In this type of configuration, QWF  102  may be laminated to the top surface of light guide element  78  or may be a standalone film. 
     If desired, QWF  102  may be formed as a layer of backlight unit  42  (e.g., between light guide element  78  and reflector  80  of backlight unit  42 ) as shown in  FIG. 19 . In this type of configuration, QWF  102  may be laminated to the top surface of reflector  80  or may be a standalone film. 
     As shown in  FIG. 20 , display  14  may include optical films  70  having diffuser film  106  implemented as a top diffuser, two crossed face-up prism films  108  and  110 , and bottom diffuser  112 . Optical films  70  may be interposed between QWF  102  and backlight unit  42 . In the example of  FIG. 20 , reflective polarizer  100  and quarter wave film  102  are formed between diffuser film  106  and display module  46 . Reflective polarizer  100  may be formed adjacent to lower polarizer  60  of display module  46 . Quarter wave film  102  may be formed adjacent to diffuser film  106 . In this example, QWF  102  may be laminated to the bottom surface of reflective polarizer  100  or may be a standalone film. The configuration of display  14  of  FIG. 20  in which optical films  70  include a top diffuser, two crossed face-up prism films, and a bottom diffuser and in which QWF  102  is formed between diffuser  106  and reflective polarizer  100  is merely illustrative. QWF  102  may be formed in other locations in display  14  having optical films  70  that include a top diffuser, two crossed face-up prism films, and a bottom diffuser as described below in connection with  FIGS. 21, 22, 23, and 24 . 
     If desired, QWF  102  may be formed between diffuser film  106  and face-up prism film  108  as shown in  FIG. 21 . In this type of configuration, QWF  102  may be laminated to the bottom surface of diffuser  106 , or may be a standalone film. 
     If desired, QWF  102  may be formed between diffuser film face-up prism film  110  and diffuser film  112  as shown in  FIG. 22 . In this type of configuration, QWF  102  may be laminated to the top surface of diffuser  112 , or may be a standalone film. 
     If desired, QWF  102  may be formed below diffuser film  112  (e.g., between diffuser  112  and light guide element  78  of backlight unit  42 ) as shown in  FIG. 23 . In this type of configuration, QWF  102  may be laminated to the top surface of light guide element  78  or may be a standalone film. 
     If desired, QWF  102  may be formed as a layer of backlight unit  42  (e.g., between light guide element  78  and reflector  80  of backlight unit  42 ) as shown in  FIG. 24 . In this type of configuration, QWF  102  may be laminated to the top surface of reflector  80  or may be a standalone film. 
     As shown in  FIG. 25 , display  14  may include backlight unit  42 , optical films  70  having diffuser film  106  and no prism films, reflective polarizer  100 , and quarter wave film  102  formed between reflective polarizer  100  and diffuser film  106 . Reflective polarizer  100  may be formed between optical films  70  and display module  46 . Reflective polarizer  100  may be formed adjacent to lower polarizer  60  of display module  46 . In the example of  FIG. 25 , quarter wave film  102  forms a top layer of optical films  70 . In this example, QWF  102  may be laminated to the bottom surface of reflective polarizer  100  or may be a standalone film. The configuration of display  14  of  FIG. 25  in which optical films  70  include diffuser layer  106  and QWF  102  formed between diffuser  106  and reflective polarizer  100  is merely illustrative. QWF  102  may be formed in other locations in display  14  having optical films  70  that include a diffuser layer and no prism films as described below in connection with  FIGS. 26 and 27 . 
     If desired, QWF  102  may be formed below diffuser film  106  (e.g., between diffuser film  106  and light guide element  78  of backlight unit  42 ) as shown in  FIG. 26 . In this type of configuration, QWF  102  may be laminated to the top surface of light guide element  78  or may be a standalone film. 
     If desired, QWF  102  may be formed as a layer of backlight unit  42  (e.g., between light guide element  78  and reflector  80  of backlight unit  42 ) as shown in  FIG. 27 . In this type of configuration, QWF  102  may be laminated to the top surface of reflector  80  or may be a standalone film. 
       FIG. 28  illustrates light output from the light guide element of a display such as display  14  in a configuration that includes a face-down prism. As shown in  FIG. 28 , light  74  from light source  72  enters light guide element  78 . A light ray such as light ray  1006  may exit light guide element  78  at about 70 degrees from the normal direction of light guide element  78  and refract into prism film  120  at a surface  1010 A. The light then reflects from a surface  1010 B and refracts from the prism at a surface  1010 C, such that ray  1006 B exiting from face-down prism  120  is nearly in a normal direction with respect to surface  1010 C. 
     In another example, a light ray such as ray  1008  may exit light guide element  78  at an angle of 30 degrees from the normal direction, enters surface  1010 D of prism film  120  at a very small incident angle and then be reflected at surface  1010 C back into the prism. 
       FIG. 29  is a graph that illustrates luminous intensity output profiles of light from a light guide element and a single face-down prism. As shown, curve  1002  represents the light output from the light guide element, while curve  1004  represents light output from the single face-down prism. The example of  FIG. 29  represents light output for a face-down prism in which the prism pitch is 50 microns and the apex angle is 66 degrees. 
     A Brewster angle is an angle of incidence at which light with a particular polarization is transmitted without reflection. When unpolarized light is incident at the Brewster angle, the light that is reflected from the surface is completely polarized. If a prism or a light guide element is made of polymer (e.g., polymethyl methacrylate), the Brewster angle of the prism is about 34 degrees. 
     At the Brewster angle, no p-polarized light or p-component is reflected from the surface. The reflected light is s-polarized or s-component light. The light transmittance output from the light guide element varies with polarization direction, as shown in Equations (2) and (3) below. 
     In equations 2 and 3 below, Tp is the light transmittance for a polarization component parallel to the incident plane, and Ts is light transmittance for a polarization component perpendicular to the incident plane. Both Tp and Is are dependent upon an incident angle θ i  and a refraction angle θ t  from a normal direction that is perpendicular to the light guide element.
 
 Tp =sin(2θ i )sin(2θ t )/sin 2 (θ i +θ t )/cos 2 (θ i −θ t )  Equation (2)
 
 Ts =sin(2θ i )sin(2θ t )/sin 2 (θ i +θ t )  Equation (3)
 
       FIG. 30  illustrates light polarization from a light guide element according to embodiments of the present disclosure. As shown, the transmitted polarized light  1104 (Tp) and  1106 (Ts), reflected polarized light  1110 (Rs) and  1108 (Rp), as well as degree of polarization  1102 (P) vary with incident angles. The light output from the light guide element is partially polarized. The light guide element emission may be predominately at angles ranging from about 50 degrees to about 80 degrees from the normal direction (as examples) when the incident angle inside the light guide element ranges from 31 degrees to 40 degrees. 
       FIG. 31  illustrates light polarization from a face-up prism film according to embodiments of the present disclosure. As shown, the transmitted polarized light  1114 (Tp) and  1116 (Ts), reflected polarized light  1118 (Rp) and  1120 (Rs), as well as degree of polarization  1112 (P) vary with incident angles. When the incident angle inside the face-up prism film is about 25 degrees, the light comes out of the prism film nearly in a normal direction with respect to the LCD panel. As shown in  FIG. 31 , the light coming out of face-up prism film may also be partially polarized. 
       FIG. 32  illustrates the light propagation in a light guide element, a diffuser film, and a face-up prism film such as face-up prism film  110  according to embodiments of the present disclosure. Light output from light guide element  78  may be angled at about 70 degrees (for example) from the normal direction to the light guide element, as shown by rays  1202 A. Rays  1202 A from the light guide element are collimated slightly by the diffuser film, as shown by rays  1202 B. Rays  1202 B are further collimated by the face-up prism film toward a normal direction to the reflective polarizer, as shown by rays  1202 C. 
       FIG. 33  illustrates light recycling between reflective polarizer  100 , quarter wave film  102 , and BLU  42  (which may include some or all of optical films  70 ) according to embodiments of the present disclosure. As shown, P 1  and P 2  represent light components transmitting through reflective polarizer  100  for respective first and second times. Elements s 1  and s 2  represent the s-component reflected from reflective polarizer  100  and recycled by QWF  102 , optical films  70  and BLU  42  for respective first time and second times. 
     As shown in  FIG. 33 , the light rays 1ST, 2ND, 3RD and 4TH represent light passing through the QWF for respective first, second, third, and fourth times. The 2ND and 3RD times in the QWF represent a round trip for one recycle. 
       FIG. 34  illustrates a flow process representing light that is recycled through the reflective polarizer, the quarter wave film, the optical films and the BLU according to embodiments of the present disclosure. As shown in  FIG. 34 , the light output from BLU  42  (and optical films  70 ) is partially polarized. This partially polarized light passes through QWF  102  for a first time and is still partially polarized. The partially polarized light has a p-component that passes through reflective polarizer  100  to LCD panel  46  while an s-component is reflected back from reflective polarizer  100 . The reflected s-component enters QWF  102  for a second time and becomes circularly polarized. This circularly polarized light then goes through the round trip as light rays 2ND and 3RD (see  FIG. 33 ) in one recycle in the optical films and the BLU. 
     The polarization of the circularly polarized light is partially depolarized and partially preserved, such that the output includes unpolarized light and circularly polarized light. The circularly polarized light enters into QWF  102  for a third time and becomes linearly polarized light that rotates about 90 degrees which is now a p-component. This p-component passes through reflective polarizer  100  and enters into LCD panel  46  to illuminate it. 
     The unpolarized portion of the partially polarized light from the optical films and the BLU goes through the light recycling over again. As one can see from the flow diagram of  FIG. 34 , the partial polarization preservation property of the optical films and BLU enable the QWF to help improve the reflective polarizer light recycling efficiency. 
     The total transmittance of the reflective polarizer without a QWF can be expressed by Equation (4) as follows:
 
 T= 0.5+0.5* r*p/[ 1− r *(1− p )]  Equation (4)
 
     The total transmittance of the reflective polarizer with a QWF can be expressed by Equation (5) as follows:
 
 T= 0.5+0.5* r *(1− p )/(1− r*p )  Equation (5)
 
where r is the light reflectance from optical films and BLU during light recycling, p is the polarization conversion efficiency for the reflective polarizer, which counts the percentage of linearly polarized light being rotated 90 degrees after a round-trip in the optical films and the BLU. A brightness gain of the reflective polarizer is defined by T/0.5. Typically, a display having a reflective polarizer has a gain ranging from 1.3 to 1.35 (for example) compared to a display that does not include reflective polarizer.
 
     Based upon Equations (4) and (5), the gain versus polarization conversion efficiency can be plotted.  FIG. 35  shows an exemplary gain versus polarization conversion efficiency for a reflective polarizer without a QWF. Linear lines  1402 A,  1402 B,  1402 C, and  1402 D represent gain versus polarization conversion efficiencies for the optical films and BLU with a reflectance of r equal to 0.3, 0.4, 0.5, and 0.6, respectively. At p=0.4 and r=0.55, the empirical gain with the reflective polarizer but without the QWF is 1.33. It should be appreciated that these are sample values and may vary between embodiments. 
       FIG. 36  shows an exemplary gain versus polarization conversion efficiency for a reflective polarizer with a QWF according to embodiments of the present disclosure. Linear lines  1412 A,  1412 B,  1412 C, and  1412 D represent gain versus polarization conversion efficiencies for the optical films and BLU with a reflectance of r equal to 0.3, 0.4, 0.5, and 0.6, respectively. As shown in  FIG. 36 , when p is less than 0.5, the gain of the reflective polarizer with QWF is higher than that of the reflective polarizer without QWF. For example, at p=0.4 and r=0.55, the gain (GAIN 2) is 1.43 (for example) for the reflective polarizer with the QWF. 
     Comparing  FIGS. 35 and 36 , the brightness gain increases 7.5% with the QWF added to the backside of reflective polarizer. When p=0.5, the QWF may not affect the gain of the reflective polarizer, because the optical films and BLU do not preserve the polarization. 
       FIG. 37  illustrates a test system such as system  900  that may be used for measuring light intensity from display  14  (e.g., light that is emitted from BLU  42  and passes through optical films  70  and, if desired, a reflective polarizer and a quarter wave film) during design and/or manufacturing operations for a display such as display  14 . 
     In the example of  FIG. 37 , system  900  is configured to measure light output a normal angle with respect to the surface of display  14  and an additional angle such as 70 degrees from normal. Photodetector  1502 A may be configured to detect backlight generated by backlight unit  42  in the normal direction with respect to optical films  70  and BLU  42 . Photodetector  1502 B may be configured to detect light from light guide element  78  in absence of the optical films  70 . A system of this type may allow measurements of luminance ratios between p-components and s-components in various configurations of a display such as display  14 . 
     If desired, quarter wave film  102  of  FIG. 8  may be formed from a half wave plate (half wave film) rather than a quarter wave film under reflective polarizer  100 . This type of arrangement may be used to maximize the amount of light from light guide plate  78 . 
     Consider, as an example, a scenario in which light from light guide plate  78  of  FIG. 32  passes through diffuser  106  of  FIG. 32  and prism film  110  of  FIG. 32  before passing through the structures of  FIG. 8  (i.e., the structures of  FIG. 8  that include a half wave plate between film  312  and polarizer  100 ). In the example of  FIG. 32 , prism film  110  is in a face-up configuration and is characterized by an orientation axis (sometimes referred to as the prism axis or groove axis) that runs along the peaks of the prisms in film  110 . 
     To minimize Moiré effects, it may be desirable to orient the prism axis at a non-zero angle to the array of pixels in display  14 . The pixels in display  14  may form a rectangular array with rows of pixels running parallel to axis X and columns of pixels running parallel to axis Y. If the prism film were oriented with its prism axis parallel to one of these lateral dimensions (e.g., along the Y axis), Moiré patterns associated with Moiré interference between the prisms of the prism film (which may have a periodicity of 10-30 microns or other suitable periodicity) and the pixels in display layers  46  (e.g., subpixels) might become visible. 
     With one suitable arrangement, prism film  110  is oriented with its prism axis at a non-zero angle A of 61° with respect to the X axis. Other orientations may be used if desired. The pass axis of reflective polarizer  100  may be oriented in alignment with the X axis (as an example). Prism film  110  may tend to polarize light emitted from light guide plate  78 . In particular, prism film  110  will emit more p-polarized light (light polarized along the axis perpendicular to the prism axis) than s-polarized light (light polarized parallel to the prim axis). Because more p-polarized light is emitted by prism film  110  than s-polarized light (i.e., because the dominant polarization direction of the light emitted by the prism film is perpendicular to the prism axis), backlight efficiency can be enhanced by using a half wave plate to rotate this more prevalent p-polarized light into alignment with the pass axis of polarizer  100  (i.e., the half wave plate rotates the dominant polarization direction of the portion of the backlight with the dominant polarization direction into alignment with the pass axis). 
     The p-polarized light (in this example) runs perpendicular to the prism axis, which is oriented at 61° from the X axis. As a result, there is an angular separation of 29° (90°−61°) between the polarization axis of the p-polarized light and the X axis. The half wave plate is interposed between prism film  110  and polarizer  100 . To rotate the p-polarized light by 29° so that the p-polarized light is aligned with the pass axis of polarizer  100 , the fast axis of the half wave plate is oriented at an angle that bisects the angle between the polarization axis of the p-polarized light and the X axis (i.e., the fast axis is located 14.5° from the X axis and 14.5° from the polarization axis of the p-polarized light in this example). When the half wave plate is oriented so that the fast axis of the half wave plate lies halfway between the polarization axis of the most prominent polarization of the backlight passing through the prism film and the pass axis of the lower polarizer in the display, the amount of the backlight passing through the lower polarizer is maximized (neglecting recycled light). 
     S-polarized light from prism film  110  will reflect downwards from reflective polarizer  100 . This reflected S-polarized light will pass downwards through the half wave plate, will reflect upwards from reflector  80 , and will pass upwards through the half wave plate before reaching the reflective polarizer again. In theory, the s-polarized light will remain s-polarized because it has passed through the half wave plate twice and therefore experiences no polarization rotation. In practice, however, the presence of layers such as the diffuser layer will scramble the polarization of the recycled s-polarized light so that a significant portion of this light can pass through the reflective polarizer, further enhancing backlight efficiency. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.

Metadata:
Filing Date: 20150413
Publication Date: 20170214
Grant Date: 20170214
Priority Date: 20120914
Inventors: ZHU XINYU
QI JUN
JIAO MEIZI
ROLAND NICHOLAS G.
YIN VICTOR H.
CHEN WEI
XU MING
CHEN CHENG
GE ZHIBING
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/133536", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13363", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F2001/133638", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133504", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0073", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F2413/01", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0053", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0073", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0053", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133638", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133536", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13363", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133504", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133638", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2413/01", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2413/01", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133504", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133536", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13363", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 54006698