Patent Publication Number: US-9410677-B2

Title: Light source and display system incorporating same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/502,060, filed on Apr. 13, 2012, which is a national stage filing under 35 U.S.C. 371 of PCT/US2010/053719, filed on Oct. 22, 2010, which claims priority to U.S. Provisional Application No. 61/254,672, filed on Oct. 24, 2009, the disclosure of which is incorporated by reference in its/their entirety herein. 
     RELATED APPLICATIONS 
     This application is related to the following U.S. patent applications which are incorporated herein in their entireties by reference: “Optical Film” filed on Apr. 15, 2009 and having Ser. No. 61/169,466; “Optical Construction and Display System Incorporating Same” filed on Apr. 15, 2009 and having Ser. No. 61/169,521; “Retroreflecting Optical Construction” filed on Apr. 15, 2009 and having Ser. No. 61/169,532; “Optical Film for Preventing Optical Coupling” filed on Apr. 15, 2009 and having Ser. No. 61/169,549; “Backlight and Display System Incorporating Same” filed on Apr. 15, 2009 and having Ser. No. 61/169,555; “Process and Apparatus for Coating with Reduced Defects” filed on Apr. 15, 2009 and having Ser. No. 61/169,427; “Process and Apparatus for A Nanovoided Article” filed on Apr. 15, 2009 and having Ser. No. 61/169,429; and “Optical Construction and Method of Making the Same” filed on Oct. 22, 2009 and having Ser. No. 61/254,243. 
     This application is further related to the following U.S. patent applications, filed on even date herewith and which are incorporated herein in their entireties by reference: “Gradient Low Index Article and Method” having Ser. No. 61/254,673; “Process for Gradient Nanovoided Article” having Ser. No. 61/254,674; “Immersed Reflective Polarizer with High Off-Axis Reflectivity” having Ser. No. 61/254,691; “Immersed Reflective Polarizer With Angular Confinement in Selected Planes of Incidence” having Ser. No. 61/254,692; and “Voided Diffuser” having Ser. No. 61/254,676. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to light sources that include a hollow optically reflective cavity and an optical film that exhibits some low-refractive index-like properties. The invention also relates to illumination devices, backlights and display systems that incorporate such light sources. 
     BACKGROUND 
     Backlights are used as extended area illumination sources in displays such as liquid crystal displays (LCDs). Backlights typically incorporate a light source that includes one or more lamps, a lightguide for producing an extended area light source by extending light from the lamps over the output surface of the backlight, and one or more light management layers such as prismatic light redirecting layers, brightness enhancement layers, reflective polarizer layers, diffuser layers, mirror layers and retarder layers. Lightguides are typically solid and include means for extracting light from the lightguide. 
     SUMMARY OF THE INVENTION 
     Generally, the present inventions relates to light sources. In one embodiment, a light source includes a reflective cavity that includes an input port for receiving light and an output port for transmitting light. The light source also includes a lamp that is disposed at the input port. The light source also includes an optical stack that is disposed at the output port and includes a substantially forward scattering optical diffuser that is disposed at the output port and has an optical haze that is not less than about 20%, an optical film that is disposed on the optical diffuser for enhancing total internal reflection at the interface between the optical film and the optical diffuser. The optical film has an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%. The optical stack also includes a reflective polarizer layer that is disposed on the optical film. Substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other. In some cases, the ratio of the maximum lateral dimension of the optically reflective cavity to the maximum thickness of the optically reflective cavity is not less than about 20, or not less than about 40, or not less than about 60. In some cases, the lamp includes an LED. In some cases, the cavity includes input ports on opposite sides of the cavity. In some cases, the output port of the cavity is located on a top side of the cavity. In some cases, the optical diffuser has a transport ratio that is not less than about 0.2, or not less than about 0.3, or not less than about 0.4, or not less than about 0.5. In some cases, the optical diffuser is a semi-specular partial reflector. In some cases, the optical haze of the optical diffuser is not less than about 30%, or not less than about 40%. In some cases, the optical diffuser includes a surface diffuser, or a volume diffuser, or a combination of a volume diffuser and a surface diffuser. In some cases, the effective index of refraction of the optical film is not greater than about 1.25, or not greater than about 1.2, or not greater than about 1.15, or not greater than about 1.1. In some cases, the optical haze of the optical film is not greater than about 4%, or not greater than about 3%, or not greater than about 2%. In some cases, the optical film includes a plurality of interconnected voids, and in some cases, the optical film also includes particles that can, for example, be or include fumed silica. In some cases, the optical film is laminated to the optical diffuser via an optical adhesive layer. In some cases, the optical film is coated on the reflective polarizer layer. In some cases, the optical stack includes an optical adhesive layer that is disposed on the reflective polarizer layer. In some cases, the reflective polarizer layer includes a multilayer optical film wherein at least some of the layers are birefringent. In some cases, the reflective polarizer layer includes a wire grid reflective polarizer, or a reflective fiber polarizer, or a cholesteric reflective polarizer, or a diffusely reflective polarizing film (DRPF). In some cases, the optically reflective cavity includes one or more specularly reflective side reflectors for at least partially collimating light that is emitted by the lamps. In some cases, the cavity includes a specularly reflective back reflector that faces the output port. In some cases, at least 50%, or at least 70%, or at least 90%, of each two neighboring major surfaces in the optical stack are in physical contact with each other. In some cases, the optical film is disposed between the reflective polarizer layer and the optical diffuser. In some cases, the light source is included in a backlight in a display system. 
     In another embodiment, a light source includes a reflective cavity that includes an input port for receiving light and an output port for transmitting light, a lamp disposed at the input port, and an optical stack that is disposed at the output port and includes an optical film that is disposed at the output port and has an optical haze that is not less than about 30%, and a reflective polarizer layer that is disposed on the optical film, where substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other. In some cases, the ratio of the maximum lateral dimension of the cavity to the maximum thickness of the cavity is not less than about 20, or not less than about 40, or not less than about 60. In some cases, the lamp includes an LED. In some cases, the cavity includes input ports located on opposite sides of the cavity. In some cases, the output port of the cavity is located on the top side of the cavity. In some cases, the optical film has a transport ratio that is not less than about 0.2, or not less than about 0.3, or not less than about 0.4, or not less than about 0.5. In some cases, the optical haze of the optical film is not less than about 40%, or not less than about 50%. In some cases, the optical film includes a binder, a plurality of interconnected voids, and a plurality of particles, where the particles can include fumed silica. In some cases, the optical film is laminated to the reflective polarizer layer via an optical adhesive layer. In some cases, the optical film is directly coated on the reflective polarizer layer. In some cases, the optical stack includes an optically adhesive layer that is disposed on the reflective polarizer layer. In some cases, the reflective polarizer layer includes a multilayer optical film, or a wire grid reflective polarizer, or reflective fiber polarizer, or a cholesteric reflective polarizer, or a diffusely reflective polarizing film (DRPF). In some cases, the cavity comprises a specularly reflective side reflector at least partially collimating light that is emitted by the lamp. In some cases, the cavity includes a specularly reflective back reflector that faces the output port. In some cases, at least 50%, or at least 70%, or at least 90%, of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     In another embodiment, a light source includes an optically reflective cavity that includes an input port for receiving light and an output port for transmitting light, a lamp that is disposed at the input ports, and an optical stack that is disposed at the output port and includes an optical diffuser that is disposed at the output port and has an optical haze that is not less than about 20% and an optical film that is disposed on the optical diffuser for enhancing total internal reflection at the interface between the optical film and the optical diffuser. The optical film has an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%. The optical stack also includes a partially reflective partially transmissive layer that is disposed on the optical film. Substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     In another embodiment, a light source includes an optically reflective hollow cavity that includes an input port for receiving light, a first output port for transmitting light, a second output port for transmitting light, and a lamp that is disposed at the input ports, a first optical stack that is disposed at the first output port, and a different second optical stack that is disposed at the second output port. At least one of the optical stacks includes an optical film that has an optical haze that is not less than about 30%, and a reflective polarizer layer that is disposed on the optical film, where substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other. In some cases, a display system includes a first liquid crystal panel that is disposed on the first optical stack, and a second liquid crystal panel that is disposed on the second optical stack. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention may be more completely understood and appreciated in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic side-view of a display system; 
         FIG. 2  is a schematic illustration of forward and backward scattering; 
         FIG. 3  is a schematic side-view of another display system; 
         FIG. 4  is a schematic side-view of an optical stack; 
         FIG. 5  is a schematic side-view of a display system; 
         FIG. 6  is a schematic side-view of a light source; 
         FIG. 7  is a schematic side-view of an optical stack; 
         FIG. 8  is a schematic side-view of another display system; 
         FIG. 9  is a grayscale conoscopic image of the measured luminance of a display system as a function of viewing angle; 
         FIG. 10  is a schematic side-view of an optical stack; 
         FIG. 11  is a grayscale conoscopic image of the measured luminance of another display system as a function of viewing angle; 
         FIG. 12  is a schematic side-view of a display system; 
         FIG. 13  is a schematic side-view of another display system; 
         FIG. 14  is a schematic side-view of another display system; 
         FIG. 15  is a schematic side-view of an optical stack; 
         FIG. 16  is a schematic side-view of another optical stack; 
         FIG. 17  is a schematic side-view of a display system; 
         FIG. 18  is a schematic side-view of another display system; 
         FIG. 19  is a schematic side-view of an optical construction; 
         FIG. 20  is a schematic side-view of another optical construction; and 
         FIG. 21  is a schematic side-view of an optical stack. 
     
    
    
     In the specification, a same reference numeral used in multiple figures refers to the same or similar elements having the same or similar properties and functionalities. 
     DETAILED DESCRIPTION 
     This invention generally relates to light sources that include a hollow reflective cavity and an optical film that has a low effective index of refraction or exhibits some low-refractive index-like properties. In some cases, the disclosed light sources are extended light sources and can advantageously be incorporated into displays, such as liquid crystal displays (LCDs), to provide extended illumination to an image forming panel. Displays incorporating the disclosed light sources can have reduced thickness and weight. The disclosed light sources can utilize fewer lamps and provide uniform illumination over an extend area by efficient light mixing. 
     The disclosed light sources include an optical film that, in some cases, has a low optical haze and a low effective index of refraction, such as an optical haze of less than about 5% and an effective index of refraction that is less than about 1.3. In some cases, the optical film has a high optical haze and/or high diffuse optical reflectance while exhibiting some low-refractive-index-like optical properties, such as, for example, the ability to support total internal reflection or enhance internal reflection. 
     The optical films disclosed herein include a plurality of voids, such as a plurality of interconnected voids or a network of voids, dispersed in a binder. The voids in the plurality of interconnected voids are connected to one another via hollow tunnels or hollow tunnel-like passages. The voids are not necessarily free of all matter and/or particulates. For example, in some cases, a void may include one or more small fiber- or string-like objects that include, for example, a binder and/or nano-particles. Some disclosed optical films include multiple pluralities of interconnected voids or multiple networks of voids where the voids in each plurality or network are interconnected. In some cases, in addition to multiple pluralities of interconnected voids, the disclosed optical films include a plurality of closed or unconnected voids meaning that the voids are not connected to other voids via tunnels. 
     Some disclosed optical films support total internal reflection (TIR) or enhanced internal reflection (EIR) by virtue of including a plurality of voids. When light that travels in an optically clear non-porous medium is incident on a stratum possessing high porosity, the reflectivity of the incident light is much higher at oblique angles than at normal incidence. In the case of no or low haze voided films, the reflectivity at oblique angles greater than the critical angle is close to about 100%. In such cases, the incident light undergoes total internal reflection (TIR). In the case of high haze voided films, the oblique angle reflectivity can be close to 100% over a similar range of incident angles even though the light may not undergo TIR. This enhanced reflectivity for high haze films is similar to TIR and is designated as Enhanced Internal Reflectivity (EIR). As used herein, by a porous or voided optical film enhancing internal reflection (EIR), it is meant that the reflectance at the boundary of the voided and non-voided strata of the film or film laminate is greater with the voids than without the voids. 
     The voids in the disclosed optical films have an index of refraction n v  and a permittivity ε v , where n v   2 =ε v , and the binder has an index of refraction n b  and a permittivity ε b , where n b   2 =ε b . In general, the interaction of an optical film with light, such as light that is incident on, or propagates in, the optical film, depends on a number of film characteristics such as, for example, the film thickness, the binder index, the void or pore index, the pore shape and size, the spatial distribution of the pores, and the wavelength of light. In some cases, light that is incident on or propagates within the optical film, “sees” or “experiences” an effective permittivity ε eff  and an effective index n eff , where n eff  can be expressed in terms of the void index n v , the binder index n b , and the film porosity or void volume fraction “f”. In such cases, the optical film is sufficiently thick and the voids are sufficiently small so that light cannot resolve the shape and features of a single or isolated void. In such cases, the size of at least a majority of the voids, such as at least 60% or 70% or 80% or 90% of the voids, is not greater than about λ/5, or not greater than about λ/6, or not greater than about λ/8, or not greater than about λ/10, or not greater than about λ/20, where λ is the wavelength of light. 
     In some cases, light that is incident on a disclosed optical film is a visible light meaning that the wavelength of the light is in the visible range of the electromagnetic spectrum. In such cases, the visible light has a wavelength that is in a range from about 380 nm to about 750 nm, or from about 400 nm to about 700 nm, or from about 420 nm to about 680 nm. In such cases, the optical film can reasonably be assigned an effective index of refraction if the size of at least a majority of the voids, such as at least 60% or 70% or 80% or 90% of the voids, is not greater than about 70 nm, or not greater than about 60 nm, or not greater than about 50 nm, or not greater than about 40 nm, or not greater than about 30 nm, or not greater than about 20 nm, or not greater than about 10 nm. 
     In some cases, the disclosed optical films are sufficiently thick so that the optical film can reasonably have an effective index that can be expressed in terms of the indices of refraction of the voids and the binder, and the void or pore volume fraction or porosity. In such cases, the thickness of the optical film is not less than about 100 nm, or not less than about 200 nm, or not less than about 500 nm, or not less than about 700 nm, or not less than about 1000 nm. 
     When the voids in a disclosed optical film are sufficiently small and the optical film is sufficiently thick, the optical film has an effective permittivity ε eff  that can be expressed as:
 
ε eff   =fε   v +(1 −f )ε b   (1)
 
     In such cases, the effective index n eff  of the optical film can be expressed as:
 
 n   eff   2   =fn   v   2 +(1 −f ) n   b   2   (2)
 
     In some cases, such as when the difference between the indices of refraction of the pores and the binder is sufficiently small, the effective index of the optical film can be approximated by the following expression:
 
 n   eff   =fn   v +(1 −f ) n   b   (3)
 
     In such cases, the effective index of the optical film is the volume weighted average of the indices of refraction of the voids and the binder. For example, an optical film that has a void volume fraction of about 50% and a binder that has an index of refraction of about 1.5, has an effective index of about 1.25. 
       FIG. 1  is a schematic side-view of a display system  1200  that includes a liquid crystal panel  1280  disposed on an extended light source  100 . Light source  100  includes an optical stack  1290  that is disposed on and receives light from an optically reflective cavity  1215 . 
     Optically reflective cavity  1215  includes at least one specularly reflective reflector, an input port for receiving light from a lamp, an output port for transmitting light, and means for improving collimation of light emitted by the lamp, where the improved collimation is in the xz-plane or along a lateral direction, such as along the length and/or width direction, of the optically reflective cavity. In particular, optically reflective cavity  1215  includes specularly reflective side reflectors  1210 A and  1210 B and an input port  1204 A on one (right) side of the optical cavity, specularly reflective side reflectors  1210 C and  1210 D and an input port  1204 B on the opposite (left) side of the optical cavity, lamps  1201  at input port  1204 A, and lamps  1202  at input port  1204 B. Light that is emitted by lamps  1201  is collimated, or partially collimated, by specular side-reflectors  1210 A and  1210 B generally along the length (x−) direction of the optically reflective cavity. Similarly, light that is emitted by lamps  1202  is collimated, or partially collimated, by specular side-reflectors  1210 C and  1210 D generally along the length (x−) direction of the optically reflective cavity. Optically reflective cavity  1215  also includes an output port  1204 C for transmitting light that is emitted by the lamps, and a specularly back or bottom reflector  1212  on the back or bottom side of the cavity facing output port  1204 C. 
     Optical stack  1290  includes a substantially forward scattering optical diffuser  1220  disposed at output port  1204 C, a first optical adhesive layer  1230  disposed on the optical diffuser, an optical film  1240  disposed on the first optical adhesive layer, a reflective polarizer layer  1250  disposed on the optical film, a second optical adhesive layer  1235  disposed on the reflective polarizer layer, and a substrate  1260  disposed on the second optical adhesive layer. Optical film  1240  is disposed between reflective polarizer layer  1250  and the substantially forward scattering optical diffuser layer  1220 . 
     Optical film  1240  has a sufficiently low refractive index and low optical haze and is sufficiently thick so as to promote or enhance total internal reflection at interface  1242  between optical film  1240  and first optical adhesive layer  1230 . The index of refraction of the optical film is not greater than about 1.3, or not greater than about 1.25, or not greater than about 1.2, or not greater than about 1.15, or not greater than about 1.1, or not greater than about 1.05. The thickness of the optical film is not less than about 0.7 microns, or not less than about 0.8 microns, or not less than about 0.9 microns, or not less than about 1 micron, or not less than about 1.1 microns, or not less than about 1.2 microns, or not less than about 1.3 microns, or not less than about 1.4 microns, or not less than about 1.5 microns, or not less than about 1.7 microns, or not less than about 2 microns. The optical haze of the optical film is not greater than about 5%, or not greater than about 4%, or not greater than about 3%, or not greater than about 2%, or not greater than about 1%, or not greater than about 0.5%. 
     For light normally incident on optical film  1240 , optical haze, as used herein, is defined as the ratio of the transmitted light that deviates from the normal (y−) direction by more than 4 degrees to the total transmitted light. Haze values disclosed herein were measured using a Haze-Gard Plus haze meter (BYK-Gardiner, Silver Springs, Md.) according to the procedure described in ASTM D1003. 
     Optical film  1240  has a high optical clarity. For light normally incident on optical film  120 , optical clarity, as used herein, refers to the ratio (T 2 −T 1 )/(T 2 +T 1 ), where T 1  is the transmitted light that deviates from the normal direction between 1.6 and 2 degrees, and T 2  is the transmitted light that lies between zero and 0.7 degrees from the normal direction. Clarity values disclosed herein were measured using a Haze-Gard Plus haze meter from BYK-Gardiner. In the cases where optical film  1240  has a high optical clarity, the clarity is not less than about 50%, or not less than about 60%, or not less than about 70%, or not less than about 80%, or not less than about 90%, or not less than about 95%. 
     Optical film  1240  includes a plurality of voids, such as interconnected voids, dispersed in a binder. The binder can be or include any material that may be desirable in an application. For example, the binder can be a UV curable material that forms a polymer, such as a cross-linked polymer. In general, the binder can be any polymerizable material, such as a polymerizable material that is radiation-curable. 
     In some cases, optical film  1240  also includes a plurality of particles dispersed in the binder and/or the optical film. The particles can be any type particles that may be desirable in an application. For example, the particles in optical film  1240  can be organic or inorganic particles. For example, the particles can be silica, zirconium oxide or alumina particles. The particles in optical film  1240  can have any shape that may be desirable or available in an application. For example, the particles can have a regular or irregular shape. For example, the particles can be approximately spherical. As another example, the particles can be elongated. In such cases, optical film  1240  includes a plurality of elongated particles. In some cases, the elongated particles have an average aspect ratio that is not less than about 1.5, or not less than about 2, or not less than about 2.5, or not less than about 3, or not less than about 3.5, or not less than about 4, or not less than about 4.5, or not less than about 5. In some cases, the particles in optical film  1240  can be in the form or shape of a string-of-pearls (such as Snowtex-PS particles available from Nissan Chemical, Houston, Tex.) or aggregated chains of spherical or amorphous particles, such as fumed silica. 
     The particles in optical film  1240  may or may not be functionalized. In some cases, the particles are not functionalized. In some cases, the particles are functionalized so that they can be dispersed in a desired solvent or binder with no, or very little, clumping. In some cases, the particles can be further functionalized to chemically bond to the host binder. For example, the particles can be surface modified and have reactive functionalities or groups to chemically bond to the binder. In some cases, some of the particles in optical film  1240  have reactive groups and others do not have reactive groups. For example in some cases, about 10% of the particles have reactive groups and about 90% of the particles do not have reactive groups, or about 15% of the particles have reactive groups and about 85% of the particles do not have reactive groups, or about 20% of the particles have reactive groups and about 80% of the particles do not have reactive groups, or about 25% of the particles have reactive groups and about 75% of the particles do not have reactive groups, or about 30% of the particles have reactive groups and about 60% of the particles do not have reactive groups, or about 35% of the particles have reactive groups and about 65% of the particles do not have reactive groups, or about 40% of the particles have reactive groups and about 60% of the particles do not have reactive groups, or about 45% of the particles have reactive groups and about 55% of the particles do not have reactive groups, or about 50% of the particles have reactive groups and about 50% of the particles do not have reactive groups. In some cases, some of the particles can be functionalized with both reactive and non-reactive groups. For example, in some cases, a substantial fraction of the particles, such as at least about 30%, or at least about 40%, or at least about 50%, or at least about 60%, or at least about 70%, or at least about 80%, of the particles can be functionalized with both reactive and non-reactive groups. 
     In some cases, the particles can have an average diameter of greater than about 0.5 microns, or greater than about 1 micron, or greater than about 1.5 microns, or greater than about 2 microns. In some cases, the particles can have an average diameter that is less than about 1 micron, or less than about 0.7 microns, or less than about 0.5 microns, or less than about 0.3 microns, or less than about 0.2 microns, or less than about 0.1 microns, or less than about 0.07 microns, or less than about 0.05 microns. In some cases, the optical film can have a first plurality of larger particles that have an average diameter that is not less than about 1 micron and a second plurality of smaller particles that have an average diameter that is not greater than about 0.5 microns. In such cases, the particle size distribution can have a first peak located at less than about 0.5 microns and a second peak located at greater than about 1 micron. 
     Optical film  1240  can be any optical film that includes a plurality of voids. For example, optical film  1240  can be an optical film described in U.S. Patent Application Ser. No. 61/169,466 titled “Optical Film”, filed on Apr. 15, 2009, and U.S. Patent Application Ser. No. 61/169,521 “Optical Construction and Display System Incorporating Same” filed on Apr. 15, 2009. As another example, optical film  1240  can be an optical film described in U.S. Patent Application Ser. No. 61/254,676 titled “Voided Diffuser”, and U.S. Patent Application Ser. No. 61/254,243 “Optical Construction and Method of Making the Same”, the disclosures of which are incorporated herein in their entireties by reference. 
     Optical diffuser  1220  is a substantially forward scattering diffuser meaning that a substantial portion, such as at least about 20%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60%, or at least about 70%, or at least about 80%, of light incident on the optical diffuser is scattered in a forward direction as forward scattered reflected and/or transmitted lights. 
       FIG. 2  is a schematic illustration of a light ray  205  that is incident at an incident point  225  on an interface  210  between a first medium  215  and a different second medium  220 . Incident point  225  defines a normal plane  230  that is normal to interface  210  at incident point  225 . Plane  230  divides the space into a forward section  235  and a backward section  240 . Portions of incident light  205  that are scattered in a forward direction lie and propagate in forward section  235  and portions of incident light  205  that are scattered in backward direction lie and propagate in backward section  240 . For example, light ray  205  is scattered at interface  210  resulting in a forward scattered transmitted light  245  having a flux F1, a forward scattered reflected light  250  having a flux F2, a backward scattered transmitted light  260  having a flux B1, and a backward scattered reflected light  255  having a flux B2. The total light scattered in the forward direction has a flux F=F1+F2 and the total light scattered in the backward direction has a flux B=B1+B2. The degree of forward scattering of incident light ray  205  by interface  210  can be characterized by a “transport ratio” TR, defined as:
 
TR=( F−B )/( F+B )  (4)
 
where TR can, in general, have a value in a range from zero to one. For example, in the case of a specular reflector, F1, B1 and B2 are zero resulting in a transport ratio of one. As another example, in the case of a Lambertian reflector, F1 and B1 are zero, and F2=B2 resulting in a transport ratio of zero.
 
     Referring back to  FIG. 1 , in some cases, such as when optical diffuser layer  1220  is a substantially forward scattering optical diffuser, the optical diffuser layer has a transport ratio that is not less than about 0.2, or not less than about 0.3, or not less than about 0.4, or not less than about 0.5, or not less than about 0.6, or not less than about 0.8. 
     Optical diffuser layer  1220  transmits a portion of an incident light and reflects another portion of the incident light. In some cases, the optical reflectance of optical diffuser layer  1220  is at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%. In some cases, the optical transmittance of optical diffuser layer  1220  is not greater than about 30%, or not greater than about 25%, or not greater than about 20%, or not greater than about 15%, or not greater than about 10%. In some cases, such as when light source  100  provides uniform illumination, the optical haze of substantially forward scattering optical diffuser layer  1220  is not less than about 20%, or not less than about 30%, or not less than about 40%, or not less than about 50%, or not less than about 60%, or not less than about 70%. 
     Optical diffuser layer  1220  can be any optical diffuser that is a substantially forward scattering optical diffuser. For example, in some cases, optical diffuser layer  1220  can be a semi-specular partial reflector that reflects a portion of an incident light and transmits another portion of the incident light, where each of the transmitted and reflected portions includes a specular portion and a diffuse portion. In such cases, a portion of light reflected by layer  1220  is specularly reflected and another portion of light reflected by layer  1220  is diffusely reflected. Similarly, in such cases, a portion of light transmitted by layer  1220  is specularly transmitted and another portion of light transmitted by layer  1220  is diffusely transmitted. In some cases, substantially forward scattering optical diffuser layer  1220  can be or include a substantially forward scattering surface diffuser or a substantially forward scattering volume diffuser or a substantially forward scattering diffuser that is a combination of a surface diffuser and a volume diffuser. In the exemplary display system  1200 , optical diffuser layer  1220  includes a scattering layer  1224  disposed on an optically clear substrate  1222 . 
     Light incident on optical diffuser  1220  is scattered substantially in forward directions. For example, a light ray  1270  incident on the optical diffuser is substantially scattered in the forward direction as a first transmitted light ray  1274  and a first reflected light ray  1272 . Light ray  1274  is subsequently totally internally reflected at interface  1242  as second reflected light ray  1276  which is substantially scattered by scattering layer  1224  in a forward direction as a second transmitted light ray  1278  propagating inside the optical cavity. A substantially forward scattering optical diffuser layer  1220  can provide efficient mixing of light emitted by lamps  1201  and  1202  resulting in light source  100  uniformly illuminating liquid crystal panel  1280 . In some cases, lamps  1201  and  1202  are more optically absorptive than other components, such as, for example, the various specular reflectors, in the optical cavity. In such cases, a substantially forward light scattering optical diffuser  1220  scatters light that is emitted by the lamps substantially in the forward directions and away from the lamps, which can result in light source  100  emitting brighter light. 
     In some cases and specially for light rays propagating at large angles relative to the y-direction, some of the layers disposed on top of optical film  1240 , such as reflective polarizer layer  1250 , substrate  1260 , and/or a liquid crystal panel  1280  that includes one or more light absorbing polarizers, can be more optically absorptive than optical film  1240 . In such cases, optical film  1240  is advantageously positioned in between the more light absorbing layers and optical cavity  1215  to prevent or reduce optical loss by totally internally reflecting light that would otherwise be absorbed in the layers above the optical film. 
     In some cases, optical diffuser  1220  is sufficiently optically diffusive so as to substantially hide at least some detailed features and/or components, such as lamps  1201  and  1202 , in optically reflective cavity  1215  from a viewer  1295  that views display system  1200  from above liquid crystal panel  1280 , specially from larger viewing angles. In some cases, optical diffuser  1220  is sufficiently optically diffusive so as to eliminate or substantially reduce the hall of mirrors effect that can occur and be visible to viewer  1295  when, for example, multiple specular reflections between specular reflectors  1210 A- 1210 D and  1212  create a repeating image pattern. In some cases, optical diffuser  1220  is sufficiently optically diffusive to assist in homogenizing light within optical cavity  1215  so that light with a substantially uniform intensity can be delivered to liquid crystal panel  1280 . 
     In the exemplary display system  1200 , optical diffuser  1220  is a surface diffuser meaning that a thin scattering layer  1224  is disposed on an optically clear non-diffusive substrate  1222 . The scattering layer can, for example, be a plurality of beads disposed on substrate  1222 , where the beads can, for example, be dispersed in a host binder. As another example, scattering layer  1224  can be a surface structure formed in the bottom surface of substrate  1222 . In some cases, optical diffuser  1220  can be a substantially forward scattering volume diffuser. In general, optical diffuser  1220  can be any type optical diffuser or scatterer that is substantially forward scattering. 
     In some cases, side light reflectors  1210 A- 1210 D and back reflector  1212  are substantially specular reflectors. For example, in such cases, the ratio of the specular reflectance to diffuse reflectance of a substantially specular reflector is at least about 100, or at least about 200, or at least about 300, or at least about 400, or at least about 500. In such cases, the diffuse reflectance of the substantially specular reflector is not more than about 2%, or not more than about 1.5%, or not more than about 1%, or not more than about 0.5%. 
     In some cases, at least one of side light reflectors  1210 A- 1210 D and back reflector back reflector  1212  can be a semi-specular reflector meaning that a portion of an incident light is specularly reflected and another portion of the incident light is diffusely reflected. In such cases, the diffusely reflected portion is scattered substantially in the forward direction. For example, in some cases, back reflector  1212  can be a semi-specular reflector. As another example, in some cases, one or more of side reflectors  1210 A- 1210 D can be a semi-specular light reflectors. 
     Specular reflectors  1210 A- 1210 D and  1212  can be any type specular reflectors that may be desirable and/or practical in an application. For example, the reflectors can be aluminized films, silver coated films, or multilayer polymeric reflective films, such as enhanced specular reflector (ESR) films available from 3M Company, St. Paul, Minn. The ESR films have a reflectance of at least about 99% in the wavelength range from about 400 nm to about 1000 nm at normal incidence. 
     Reflective polarizer layer  1250  substantially reflects light that has a first polarization state and substantially transmits light that has a second polarization state, where the two polarization states are mutually orthogonal. In some cases, reflective polarizer  1250  substantially reflects light having a first linear polarization state (for example, along the x-direction) and substantially transmits light having a second linear polarization state (for example, along the z-direction). 
     Any suitable type of reflective polarizer may be used for reflective polarizer layer  1250  such as, for example, a multilayer optical film (MOF) reflective polarizer, a diffusely reflective polarizing film (DRPF), a wire grid reflective polarizer, or a cholesteric reflective polarizer. In some cases, reflective polarizer layer  1250  can be or include a fiber polarizer. In such cases, the reflective polarizer includes a plurality of substantially parallel fibers that form one or more layers of fibers embedded within a binder with at least one of the binder and the fibers including a birefringent material. The substantially parallel fibers define a transmission axis and a reflection axis. The fiber polarizer substantially transmits incident light that is polarized parallel to the transmission axis and substantially reflects incident light that is polarized parallel to the reflection axis. Examples of fiber polarizers are described in, for example, U.S. Pat. Nos. 7,599,592 and 7,526,164, the entireties of which are incorporated herein by reference. 
     In some cases, reflective polarizer layer  1250  can be a partially reflecting layer that has an intermediate on-axis average reflectance in the pass state. For example, the partially reflecting layer can have an on-axis average reflectance of at least about 90% for visible light polarized in a first plane, such as the xy-plane, and an on-axis average reflectance in a range from about 25% to about 90% for visible light polarized in a second plane, such as the xz-plane, perpendicular to the first plane. 
     In some cases, reflective polarizer layer  1250  can be an extended band reflective polarizer that is capable of polarizing light at smaller incident angles and substantially reflecting one polarization state, or two mutually perpendicular polarization states, at larger incident angles as described in U.S. Patent Application Ser. No. 61/254,691 titled “Immersed Reflective Polarizer with High Off-Axis Reflectivity”, and U.S. Patent Application Ser. No. 61/254,692 “Immersed Reflective Polarizer With Angular Confinement in Selected Planes of Incidence”, both filed on even date herewith and the disclosures of which are incorporated herein in their entireties by reference. 
     In some cases, reflective polarizer layer  1250  can be a diffuse reflective polarizer substantially transmitting one polarization state and substantially diffusely reflecting an orthogonal polarization state. Diffuse reflective polarizer films typically include a disperse phase of polymeric particles disposed within a continuous birefringent matrix. The film is oriented, typically by stretching, in one or more directions to develop the birefrengence. Examples of diffuse reflective polarizers are described in, for example, U.S. Pat. Nos. 6,999,233 and 6,987,612 the disclosures of which are incorporated herein in their entireties by reference. 
     Substrate  1260  is optically transparent and is primarily designed to provide support to and strengthen optical stack  1290 . Substrate  1260  can be rigid or flexible. Exemplary materials for the substrate include glass and polymers such as polyethylene terapthalate (PET), polycarbonates, and acrylics. 
     In some cases, first and second optical adhesive layers  1230  and  1235  are primarily designed to bond the layer on one side of the adhesives to the layer on the other side of the adhesives. In such cases, the primary purpose of first optical adhesive layer  1230  is to laminate optical film  1240  to substantially forward scattering optical diffuser layer  1220  and the primary purpose of second optical adhesive layer  1235  is to laminate reflective polarizer layer  1250  to support substrate  1260 . In such cases, the optical adhesive layers can have a high specular optical transmittance. For example, in such cases, the specular optical transmittance of each of the adhesive layers is not less than about 60%, or not less than about 70%, or not less than about 80%, or not less than about 90%. 
     In some cases, one or both of adhesive layers  1230  and  1235  may be absent in display system  1200 . For example, in some cases, display system  1200  may not include first optical adhesive layer  1230 . In such cases, optically diffuser layer  1220  can be coated directly on optical film  1240 . In some cases, optical film  1240  is coated on reflective polarizer layer  1250 . In some cases, optical film  1240  is laminated to reflective polarizer layer  1250  via an adhesive layer not shown expressly in  FIG. 1 . 
     In some cases, optical adhesive layers  1230  and/or  1235  can be optically diffusive. For example, in such cases, the optical haze of an optically diffusive adhesive layer can be at least about 5%, or at least about 10%, or at least about 15%, or at least about 20%. In some cases, the diffuse reflectance of an optically diffusive adhesive layer can be at least about 5%, or at least about 10%, or at least about 15%, or at least about 20%. In such cases, the adhesive layer can be optically diffusive by, for example, including a plurality of particles dispersed in an optical adhesive binder where the particles and the optical adhesive binder have different indices of refraction. The mismatch between the two indices of refraction can result in light scattering. 
     Optical adhesive layers  1230  and  1235  can be or include any optical adhesive that may be desirable and/or available in an application. Exemplary optical adhesives include pressure sensitive adhesives (PSAs), heat-sensitive adhesives, solvent-volatile adhesives, and UV-curable adhesives such as UV-curable optical adhesives available from Norland Products, Inc. Exemplary PSAs include those based on natural rubbers, synthetic rubbers, styrene block copolymers, (meth)acrylic block copolymers, polyvinyl ethers, polyolefins, and poly(meth)acrylates. As used herein, (meth)acrylic (or acrylate) refers to both acrylic and methacrylic species. Other exemplary PSAs include (meth)acrylates, rubbers, thermoplastic elastomers, silicones, urethanes, and combinations thereof. In some cases, the PSA is based on a (meth)acrylic PSA or at least one poly(meth)acrylate. Exemplary silicone PSAs include a polymer or gum and an optional tackifying resin. Other exemplary silicone PSAs include a polydiorganosiloxane polyoxamide and an optional tackifier. 
     In some cases, one or both of optical adhesive layers  1230  and  1235  can be a removable adhesive such as those described in, for example, U.S. Pat. Nos. 3,691,140; 4,166,152; 4,968,562; 4,994,322; 5,296,277; 5,362,516, the disclosures of which are incorporated herein in their entireties by reference. The phrase “removable adhesive” for adhering a film to a substrate means an adhesive that affords convenient, manual removal of the film from the substrate without damaging the substrate or exhibiting excessive adhesive transfer from the film to the substrate. 
     In some cases, one or both of optical adhesive layers  1230  and  1235  can be a reusable and/or repositionable adhesive such as those described in, for example, U.S. Pat. No. 6,197,397; U.S. Patent Publication No. 2007/0000606; and PCT Publication No. WO 00/56556, the disclosures of which are incorporated herein in their entireties by reference. The phrases “reusable adhesive” or “repositionable adhesive” for adhering a film to a substrate mean an adhesive that (a) affords a temporary, secure attachment of the film to the substrate while affording convenient, manual removal of the film from the substrate without damaging the substrate or exhibiting excessive adhesive transfer from the film to the substrate, and (b) then affords subsequent reuse of the film on, for example, another substrate. 
     Substantial portions of each two neighboring major surfaces in optical stack  1290  are in physical contact with each other. For example, substantial portions of neighboring major surfaces  1241  and  1251  of respective neighboring layers  1240  and  1250  in optical stack  1290  are in physical contact with each other. For example, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the two neighboring major surfaces are in physical contact with each other. For example, in some cases, optical film  1240  is coated directly on reflective polarizer layer  1250 . 
     In general, substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in optical stack  1290  are in physical contact with each other. For example, in some cases, there may be one or more additional layers, such as a support layer or an adhesive layer, disposed between reflective polarizer layer  1250  and optical film  1240 . In such cases, substantial portions of neighboring major surfaces of each two neighboring layers in optical stack  1290  are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the neighboring major surfaces of each two neighboring layers in the optical stack are in physical contact with each other. 
     In the exemplary optical stack  1290 , optical film  1240  physically contacts reflective polarizer layer  1250 . For example, optical film  1240  can be coated directly on bottom surface  1251  of reflective polarizer layer  1250 . In some cases, one or more layers can be disposed between the two layers. For example,  FIG. 4  is a schematic side-view of an optical stack  1290  that includes an optical adhesive layer  410  and a substrate layer  420  disposed between optical film  1240  and reflective polarizer layer  1250  where substrate layer  420  can be a support layer for optical film  1240  and optical adhesive layer can be a bonding layer for bonding or laminating the optical film to the reflective polarizer layer. 
     Referring back to  FIG. 1 , liquid crystal panel  1280  includes, not expressly shown in  FIG. 1 , a layer of liquid crystal disposed between two panel plates, an upper light absorbing polarizer layer disposed above the liquid crystal layer and a lower absorbing polarizer disposed below the liquid crystal layer. The upper and lower light absorbing polarizers and the liquid crystal layer, in combination, control the transmission of light from reflective polarizer layer  1250  through liquid crystal panel  1280  to viewer  1295 . 
     Lamps  1201  and  1202  can be any type of lamp that may be desirable and/or practical in an application. For example, the lamps can be extended diffuse lamps such as cold cathode fluorescent lamps (CCFLs), smaller area solid state lamps such as light emitting diodes (LEDs), or lasers. In some cases, one or more of lamps  1201  and  1202  can include different type lamps. For example, lamps  1201  can include a combination of LEDs and CCFLs. In some cases, the lamps can emit light in different wavelength regions. For example, lamps  1201  can include a first lamp emitting red light, a second lamp emitting green light, and a third lamp emitting blue light. 
     In the exemplary display system  1200 , substantially forward scattering optical diffuser layer  1220  is placed at output port  1204 C of optically reflective cavity  1215 . In some cases, such as when the primary function of the diffuser layer is to assist in mixing and homogenizing light inside optical cavity  1215 , diffuser layer  1220  can be placed, with proper orientation, at other locations within the optical cavity. For example,  FIG. 3  is a schematic side-view of a display system  1300  that is similar to display system  1200  except that optical diffuser layer  1220  in display system  1300  is placed within the interior of optical cavity  1215  in the xz-plane so that the forward scattering properties of the optical diffuser can assist in efficient mixing of light emitted by the lamps. As another example, optical diffuser  1220  and/or scattering layer  1224  can be disposed on one or more reflectors in the optical cavity. For example,  FIG. 3  illustrates a scattering layer  1324 , similar to scattering layer  1224 , disposed on back reflector  1212 . In the exemplary display system  1300 , optical film  1240  is placed at output port  1204 C of optical cavity  1215 . 
     Low index properties of optical film  1240  and the substantially forward scattering properties of optical diffuser layer  1220  can advantageously provide for a compact and thin optically reflective cavity  1215  and improved light mixing. For example, in some cases, the maximum lateral dimension, such as the size of the diagonal or length of optically reflective cavity  1215  is substantially greater than the maximum thickness of the reflective cavity. For example, in such cases, the ratio of the maximum lateral (in the xz-plane) dimension of optically reflective cavity  1215  to the maximum thickness (along the y-direction) of the optically reflective cavity is not less than about 20, or not less than about 40, or not less than about 60, or not less than about 80, or not less than about 100. In some cases, the maximum thickness hi of optically reflective cavity  1215  in  FIG. 1  is in a range from about 2 mm to about 50 mm, or from about 5 mm to about 40 mm, or from about 7 mm to about 30 mm, or from about 10 mm to about 20 mm. 
     In some cases, a display system, such as an LCD system, can incorporate a backlight for uniform illumination of a liquid crystal panel where the backlight includes light source  100  with no additional layers. In some cases, the backlight can include light source  100  and one or more additional layers, such as one or more additional light management layers or films. Examples of light management films include reflective polarizers, light redirecting films such as a brightness enhancement films (for example, BEF available from 3M Company, Saint Paul Minn.), turning films (for example, an inverted BEF), optical diffusers, or any other light management layer that may be desirable in an application. 
     In the exemplary display system  1200 , layer  1250  is a reflective polarizer. In some cases, layer  1250  can be a non-polarizing partially reflective partially transmissive layer transmitting a portion of an incident light as an unpolarized transmitted light and reflecting a portion, such as least 30% or at least 40% or at least 50%, of the incident light as an unpolarized reflected light. In some cases, a non-polarizing partially reflective partially transmissive layer can also absorb a portion of the incident light. The partially reflective partially transmissive layer can be a multilayer optical film, or a metal, such as Al or Ag or Ni, coated film. In some cases, a non-polarizing partially reflective partially transmissive layer can be or include foams or microreplictated structures. 
     In some cases, optically reflective hollow cavity  1215  can include extraction features to assist in extracting light from the cavity. For example,  FIG. 12  is a schematic side-view of a display system  1232  that is similar to display system  1200  except that optically reflective cavity  1215  in  FIG. 12  includes a plurality of extraction features  1231  disposed on back reflector  1212 . Extraction features  1231  assist in extracting light from the reflective cavity from output port  1204 C. Extraction features  1231  can be any extraction features capable of extracting or assisting in extracting light from the cavity. For example, the extraction features can be features that are for example, printed, cast or stamped, on the back reflector. In some cases, extraction features  1231  can be arranged to enhance or increase the brightness along a desired, such as the on-axis, direction. 
     Optically reflective cavity  1215  can include one or more optical elements not expressly shown in  FIG. 1 . For example,  FIG. 18  is a schematic side-view of a display system  1900 , where optically reflective hollow cavity  1215  includes an optical element  1910  receiving light  1920  emitted by lamps  1202 . Optical element  1910  can be or include an optical filter reflecting and/or absorbing a portion, such as a UV portion, of incident light  1920 . As another example, optical element  1910  can be or include an asymmetric, such as a one-dimensional, optical diffuser for spreading emitted light  1920  more along a particular direction, such as the z-direction, and less along other directions. As another example, optical element  1910  can be or include a wavelength converter for converting, such as down converting, light  1920  to a different, such as longer, wavelength light  1930 . As yet another example, optical element  1910  can be or include a light collimator receiving a less collimated light  1920  and transmitting a more collimated light  1930 . 
     In some cases, light emitted by a lamp in optically reflective cavity  1215  can be delivered to the optical cavity via one or more hollow or solid light guides, such as for example, one or more optical fibers. For example, in  FIG. 18 , light from a lamp  1940  is delivered to optically reflective cavity  1215  at input port  1204 A via optical fibers  1950 . 
     In the exemplary display system  1200  in  FIG. 1 , reflective polarizer  1250  and back reflector  1212  are planar and non-parallel relative to each other. In general, the orientation of the reflective polarizer layer and the back reflector relative to each other can be any orientation that may be desirable in an application. For example, in some cases, the reflective polarizer layer can be parallel to the back reflector. In some cases, the reflective polarizer layer can be non-parallel to the back reflector. In some cases, one or both of the two layers can be planar or non-planar, such as curved. 
     In some cases, optical stack  1290  can be bonded to liquid crystal panel and can have fewer layers. For example,  FIG. 19  is a schematic side-view of an optical construction  1900  that includes liquid crystal panel  1280  disposed on an optical stack  1990 . Optical stack  1990  can, in some cases, replace optical stack  1290  in  FIG. 1  and is laminated or bonded to liquid crystal panel  1280  via optical adhesive layer  1235 . Optical stack  1990  includes optical diffuser layer  1220  disposed at output port  1204 C, optical film  1240  disposed on the optical diffuser layer, reflective polarizer layer  1250  disposed on the optical film, and optical adhesive layer  1235  disposed on the reflective polarizer layer. In some cases, there can be one or more layers between any two neighboring layers in optical stack  1990  or optical construction  1900 . 
     In some cases, a disclosed optical stack can include an optical film disposed between a liquid crystal panel and a reflective polarizer layer. For example,  FIG. 20  is a schematic side-view of an optical construction  2000  that includes liquid crystal panel  1280  disposed on an optical stack  2090 . Optical stack  2090  can, in some cases, replace optical stack  1290  in  FIG. 1  and is laminated or bonded to liquid crystal panel  1280  via optical adhesive layer  1235 . Optical stack  2090  includes optical diffuser layer  1220  disposed at output port  1204 C, reflective polarizer layer  1250  disposed on the optical diffuser layer, optical film  1240  disposed, for example coated, on the reflective polarizer layer, and optical adhesive layer  1235  disposed on the optical film. In some cases, there can be one or more layers between any two neighboring layers in optical stack  2090  or optical construction  2000 . 
       FIG. 5  is a schematic side-view of a display system  1201  that includes liquid crystal panel  1280  disposed on a light source  500 . The light source includes an optical stack  1291  that receives light from optically reflective cavity  1215 . Optical stack  1291  includes a substantially forward scattering optical film  1285  that is disposed at output port  1204 C of optically reflective cavity  1215 , reflective polarizer layer  1250  disposed on the optical film, optical adhesive layer  1330  disposed on the reflective polarizer layer, and substrate  1260  disposed on the optical adhesive layer. 
     A first major surface  1251  of reflective polarizer layer  1250  faces optical film  1285 . Optical film  1285  includes a first major surface  1286  that faces optically reflective cavity  1215  and a second major surface  1287  that faces the reflective polarizer layer and neighbors major surface  1251 . Substantial portions of neighboring major surfaces  1251  and  1287  of the two neighboring layers  1250  and  1285  in optical stack  1291  are in physical contact with each other. For example, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the two neighboring major surfaces are in physical contact with each other. 
     In general, substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in optical stack  1291  are in physical contact with each other. For example, in some cases, there may be one or more additional layers, such as an adhesive layer and/or a substrate layer not expressly shown in  FIG. 5 , disposed in between reflective polarizer layer  1250  and optical film  1285 . In such cases, substantial portions of neighboring major surfaces of each two neighboring layers in optical stack  1291  are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the neighboring major surfaces of each two neighboring layers in the optical construction are in physical contact with each other. 
     Substantially forward scattering optical film  1285  includes a plurality of voids, such as interconnected voids, dispersed in a binder. In some cases, optical film  1285  also includes a plurality of particles dispersed in the binder and/or the optical film. Optical film  1285  can be any optical film disclosed herein that includes voids and is substantially forward scattering. In some cases, optical film  1285  has a low optical haze and diffuse reflectance. 
     In some cases, optical film  1285  has a high optical haze. In such cases, the optical haze of the optical film is not less than about 30%, or not less than about 40%, or not less than about 50%, or not less than about 60%, or not less than about 70%, or not less than about 80%, or not less than about 90%. In some cases, optical film  1285  has a high diffuse optical reflectance. In such cases, the diffuse optical reflectance of the optical film is not less than about 30%, or not less than about 40%, or not less than about 50%, or not less than about 60%. In some cases, optical film  1285  has a low optical clarity. In such cases, the optical clarity of the optical film is not greater than about 70%, or not greater than about 60%, or not greater than about 50%, or not greater than about 40%, or not greater than about 30%, or not greater than about 20%, or not greater than about 10%. 
     In some cases, optical film  1285  has a high optical haze and manifests some-low-index like properties. For example, in such cases, optical film  1285  can support TIR or enhance internal reflection. For example, a light ray  1289  that is incident on the interface between the optical film and reflective polarizer layer  1250  with an incident angle θ, can under go TIR because the optical film has a low effective index. In some cases, it may not be possible to assign an effective index to the optical film because of, for example, high optical haze, but the optical film can still enhance internal reflection meaning that the reflection is greater than what the binder of the optical film would produce. 
     An advantage of optical stack  1291  is that optical film  1285  has high optical haze and can substantially scatter light while, at the same time, it can manifest some low-index properties. For example, optical stack  1291  can have an appreciable optical gain. For example, optical gain of optical stack  1291  can be at least about 1.1, or at least about 1.2, or at least about 1.2, or at least about 1.25, or at least about 1.3, or at least about 1.35, or at least about 1.4, or at least about 1.45, or at least about 1.5. As used herein, “gain” or “optical gain” of an optical stack is defined as the ratio of the axial output luminance of an optical or display system with the optical stack to the axial output luminance of the same optical or display system without the optical stack. 
     In the exemplary display system  1201 , major surface  1286  of optical film  1285  is structured and can scatter light. In general, major surface  1286  can have any properties that may be desirable in an application. For example, in some cases, major surface  1285  can be smooth. 
     Optical adhesive layer  1330  bonds reflective polarizer layer  1250  to support substrate  1260 . Optical adhesive layer can be similar to any optical adhesive layer disclosed herein, such as optical adhesive layers  1230  and  1235 . 
     The display systems, light sources, and optically reflective cavities disclosed herein can have any shape and configuration that may be desirable in an application. For example, in some cases, a disclosed display system such as display system  1200 , a disclosed light source such as light source  100 , and/or a disclosed optically reflective cavity such as cavity  1215 , can be planar or curved. 
     In the exemplary display system  1201 , output port  1204 C is substantially the same size as back reflector  1212 . In general, output port  1204 C can have any size and/or shape that may be desirable in an application. For example,  FIG. 14  is a schematic side-view of a display system  1500  that is similar to display system  1201  and includes optical stack  1291  disposed at an output port  1504 C of an optically reflective cavity  1515 , where output port  1504 C is smaller than specular back reflector  1212 . Optical cavity  1515  includes top specular reflectors  1520 A and  1520 B. In some cases, at least one of top reflectors  1520 A and  1520 B can be a semi-specular reflector. 
     Referring back to  FIG. 5 , in some cases, optical stack  1291  can have fewer layers or one or more additional layers. For example,  FIG. 15  is a schematic side-view of an optical stack  1690  that can, for example, replace optical stack  1291 . Optical stack  1690  is bonded to liquid crystal panel  1280  via an optical adhesive layer  1605 . Optical stack  1690  includes optical film  1285 , reflective polarizer layer  1250  disposed on optical film  1285 , a structured light redirecting film  1620  disposed on the reflective polarizer layer, an optical film  1610  disposed on and planarizing the light redirecting film, and optical adhesive layer  1605  disposed on optical film  1610 . Light redirecting film  1620  includes a structured top surface  1621  that includes a plurality of linear prisms  1622  extending along the z-direction. Optical film  1610  planarizes structured surface  1621  and optically couples to liquid crystal panel  1280  via optical adhesive layer  1605 . Optical film  1610  can be any optical film disclosed herein. For example, in some cases, optical film  1610  includes voids dispersed in a binder and has an effective index of refraction that is not greater than about 1.3, or not greater than about 1.25, or not greater than about 1.2, or not greater than about 1.15, or not greater than about 1.1, or not greater than about 1.05. In some cases, the optical haze of optical film  1610  is not greater than about 5%, or not greater than about 4%, or not greater than about 3%, or not greater than about 2%, or not greater than about 1%, or not greater than about 0.5%. 
     Substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in optical stack  1690  are in physical contact with each other. In some cases, there may be one or more additional layers, such as an adhesive layer and/or a substrate layer not expressly shown in  FIG. 15 , disposed in between, for example, reflective polarizer layer  1250  and optical film  1285 . In such cases, substantial portions of neighboring major surfaces of each two neighboring layers in optical stack  1690  are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the neighboring major surfaces of each two neighboring layers in the optical stack are in physical contact with each other. 
     Light redirecting film  1620  can be any film that includes structures capable of redirecting light. Examples of light redirecting films include brightness enhancement films (for example, BEF available from 3M Company, Saint Paul Minn.) and turning films (for example, an inverted BEF). 
     As yet another example,  FIG. 21  is a schematic side-view of an optical stack  2100  that can replace optical stack  1291  or any other optical stack disclosed herein. Optical stack  2100  includes a first light redirecting film  2110 , a first optical film  2120  disposed on and planarizing the first light redirecting film, a first optical adhesive layer  2130  disposed on the first optical film, an optical diffuser layer  2140  disposed on the first optical adhesive layer, a second optical film  2150  disposed on and planarizing the optical diffuser layer, a second optical adhesive layer  2160  disposed on the second optical film, a second light redirecting layer  2170  disposed on the second optical adhesive layer, and a third optical film  2180  disposed on and planarizing the second light redirecting layer. 
     First and second light redirecting films  2110  and  2170  can be any light redirecting films that may be desirable in an application. For example, in some cases, light redirecting films  2110  and  2170  can be similar to light redirecting film  1620 . In some cases, light redirecting films  2110  and  2170  include linear prismatic films with the linear prisms in one light redirecting film being oriented along a first direction and the linear prisms in the other light redirecting film being oriented along a second direction orthogonal to the first direction. For example, in some cases, the linear prisms in light redirecting film  2110  can extend, or be oriented, along the x-direction and the linear prisms in light redirecting film  2170  can extend, or be oriented, along the z-direction. In such cases and with optical stack  2100  disposed at, for example, output port  1204 C, the prisms in first or lower light redirecting film  2110  can efficiently totally internally reflect substantial portions of light that is emitted by light sources  1201  and  1202  and travel along the general x-direction. 
     Optical films  2120 ,  2150  and  2180  can be nay optical films disclosed herein, such as optical films  1240  and  1285 . For example, in some cases, the optical films include voids, such as interconnected voids, dispersed in a binder and have effective indices of refraction that are not greater than about 1.3, or not greater than about 1.25, or not greater than about 1.2, or not greater than about 1.15, or not greater than about 1.1, or not greater than about 1.05. In some cases, the optical haze of the optical films is not greater than about 5%, or not greater than about 4%, or not greater than about 3%, or not greater than about 2%, or not greater than about 1%, or not greater than about 0.5%. 
     Optical adhesive layers  2130  and  2160  can be any optical adhesive layer disclosed herein, such as optical adhesive layers  1230 ,  1235  and  1330 . Optical diffuser layer  2140  can be similar to any optical diffuser layer disclosed herein, such as optical diffuser layer  1220 . In some cases, optical diffuser layer includes a plurality of beads dispersed in a binder, where the beads form a top structured surface. In some cases, the index of the binder and the beads are substantially the same. In such cases, optical diffuser layer  2140  is substantially a surface diffuser and scatters no, or very little, light at a volume diffuser. In such cases, optical diffuser layer  2140  can enhance the optical gain of optical stack  2100 . 
     Substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in optical stack  2100  are in physical contact with each other. In some cases, there may be one or more additional layers, such as an adhesive layer and/or a substrate layer not expressly shown in  FIG. 21 , disposed in between, for example, first optical film  2120  and first light redirecting film  2110 . In such cases, substantial portions of neighboring major surfaces of each two neighboring layers in optical stack  2100  are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the neighboring major surfaces of each two neighboring layers in the optical stack are in physical contact with each other. 
     As another example,  FIG. 16  is a schematic side-view of an optical stack  1790  that can replace optical stack  1291 . Optical stack  1790  is bonded to liquid crystal panel  1280  via an optical adhesive layer  1710  and includes optical film  1285 , an optical film  1730  disposed on optical film  1285 , light redirecting film  1620  disposed on optical film  1730 , optical film  1610  disposed on and planarizing light redirecting film  1620 , an optical adhesive layer  1720  disposed on optical film  1610 , reflective polarizer layer  1250  disposed on optical adhesive layer  1720 , and optical adhesive layer  1710  disposed on reflective polarizer layer  1250 . 
     Optical film  1730  can be any optical film disclosed herein. For example, in some cases, optical film  1730  includes voids dispersed in a binder and has an effective index of refraction that is not greater than about 1.3, or not greater than about 1.25, or not greater than about 1.2, or not greater than about 1.15, or not greater than about 1.1, or not greater than about 1.05. In some cases, the optical haze of optical film  1730  is not greater than about 5%, or not greater than about 4%, or not greater than about 3%, or not greater than about 2%, or not greater than about 1%, or not greater than about 0.5%. 
     Substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in optical stack  1790  are in physical contact with each other. In some cases, there may be one or more additional layers, such as an adhesive layer and/or a substrate layer not expressly shown in  FIG. 16 , disposed in between, for example, light redirecting film  1620  and optical film  1730 . In such cases, substantial portions of neighboring major surfaces of each two neighboring layers in optical stack  1790  are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the neighboring major surfaces of each two neighboring layers in the optical stack are in physical contact with each other. 
       FIG. 13  is a schematic side-view of a display system  1400  that includes a light source  1401  providing illumination to a first liquid crystal panel  1440 A viewable by a viewer  1450 A and a second liquid crystal panel  1440 B viewable by a second viewer  1450 B. Light source  1401  includes an optically reflective cavity  1405  that includes an input port  1403  receiving light from a lamp  1402 , a first output port  1404 A for transmitting light towards and illuminating first liquid crystal panel  1440 A, and a second output port  1404 B for transmitting light towards and illuminating second liquid crystal panel  1440 B. Light source  1401  also includes a first optical stack  1490 A disposed at first output port  1404 A and a second optical stack  1490 B disposed at second output port  1404 B. 
     Optical reflective cavity includes respective first and second specular side reflectors  1410 A and  1410 B and specular end reflector  1410 C. Each of optical stacks  1490 A and  1490 B includes a reflective polarizer layer disposed on an optical film. In particular, first optical stack  1490 A includes a first optical film  1420 A disposed at first output port  1404 A and a first reflective polarizer layer  1430 A disposed on first optical film  1420 A, and second optical stack  1490 B includes a second optical film  1420 B disposed at second output port  1404 B and a second reflective polarizer layer  1430 B disposed on second optical film  1420 A. 
     In some cases, at least one of first and second optical films  1420 A and  1420 B is a substantially forward scattering optical film. In such cases, the optical film has a transport ratio that is not less than about 0.2, or not less than about 0.3, or not less than about 0.4, or not less than about 0.5, or not less than about 0.6, or not less than about 0.8. Each of optical films  1420 A and  1420 B transmits a portion of an incident light and reflects another portion of the incident light. In some cases, the optical reflectance of at least one of optical films  1420 A and  1420 B is at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%. In some cases, the optical transmittance of at least one of optical films  1420 A and  1420 B is not greater than about 30%, or not greater than about 25%, or not greater than about 20%, or not greater than about 15%, or not greater than about 10%. In some cases, such as when light source  1401  provides uniform illumination, the optical haze of at least one of substantially forward scattering optical films  1420 A and  1420 B is not less than about 20%, or not less than about 30%, or not less than about 40%, or not less than about 50%, or not less than about 60%, or not less than about 70%. 
     Substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in each of optical stacks  1490 A and  1490 B are in physical contact with each other. For example, in optical stack  1490 A, substantial portions of the major bottom surface of first reflective polarizer layer  1430 A and the major top surface of first optical film  1420 A are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of each two neighboring major surfaces are in physical contact with each other. 
     In general, substantial portions of neighboring major surfaces (major surfaces that face each other or are adjacent to each other) of each two neighboring layers in each of optical stacks  1490 A and  1490 B are in physical contact with each other. For example, in some cases, there may be one or more additional layers, such as an adhesive layer and/or a substrate layer not expressly shown in  FIG. 13 , disposed in between, for example, reflective polarizer layer  1430 A and optical film  1420 A. In such cases, substantial portions of neighboring major surfaces of each two neighboring layers in optical stack  1490 A are in physical contact with each other. In such cases, at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the neighboring major surfaces of each two neighboring layers in the optical stack are in physical contact with each other. 
     Each of optical films  1420 A and  1420 B includes a plurality of voids, such as interconnected voids, dispersed in a binder. In some cases, at least one of optical films  1420 A and  1420 B also includes a plurality of particles dispersed in the binder and/or the optical film. Optical films  1420 A and  1420 B can be any optical film disclosed herein that includes voids and is substantially forward scattering. In some cases, at least one of optical films  1420 A and  1420 B has a low optical haze and diffuse reflectance. 
     In some cases, at least one of optical films  1420 A and  1420 B has a high optical haze. In such cases, the optical haze of the optical film is not less than about 30%, or not less than about 40%, or not less than about 50%, or not less than about 60%, or not less than about 70%, or not less than about 80%, or not less than about 90%. In some cases, at least one of optical films  1420 A and  1420 B has a high diffuse optical reflectance. In such cases, the diffuse optical reflectance of the optical film is not less than about 30%, or not less than about 40%, or not less than about 50%, or not less than about 60%. In some cases, at least one of optical films  1420 A and  1420 B has a low optical clarity. In such cases, the optical clarity of the optical film is not greater than about 70%, or not greater than about 60%, or not greater than about 50%, or not greater than about 40%, or not greater than about 30%, or not greater than about 20%, or not greater than about 10%. 
     In some cases, at least one of optical films  1420 A and  1420 B has a high optical haze and manifests some-low-index like properties. For example, in such cases, each of first and second optical films  1420 A and  1420 B can support TIR or enhance internal reflection. In some cases, it may not be possible to assign an effective index to the optical film because of, for example, high optical haze, but the film can still enhance internal reflection meaning that the reflection is greater than what the binder of the optical film would produce. 
     An advantage of optical stacks  1490 A and  1490 B is that the optical films can have high optical haze and can substantially scatter light while, at the same time, they can manifest some low-index properties. For example, optical stack  1490 A can have an appreciable optical gain. For example, optical gain of optical stack  1490 A can be at least about 1.1, or at least about 1.2, or at least about 1.2, or at least about 1.25, or at least about 1.3, or at least about 1.35, or at least about 1.4, or at least about 1.45, or at least about 1.5. 
     In the exemplary display system  1400 , major surface  1421 A of optical film  1420 A is structured and can scatter light, and major surface  1421 B of optical film  1420 B is structured and can scatter light. In general, each of major surfaces  1421 A and  1421 B can have any properties that may be desirable in an application. For example, in some cases, at least one of major surfaces  1421 A and  1421 B can be smooth. 
     In some cases, first reflective polarizer layer  1430 A can be bonded to first liquid crystal panel  1440 A via, for example, an optical adhesive layer, and second reflective polarizer layer  1430 B can be bonded to second liquid crystal panel  1440 B also via, for example, an optical adhesive layer not expressly shown in  FIG. 13 . 
     In the exemplary display system  1400 , light source  1401  provides illumination to liquid crystal panels  1440 A and  1440 B for displaying images and/or information to, for example, viewers  1450 A and  1450 B. In some cases, light source  1401  can provide illumination in general lighting applications. 
       FIG. 17  is a schematic three-dimensional view of a display system  1800  for displaying an image and/or data to a viewer  1850 . The display system includes liquid crystal panel  1280  receiving light from a backlight  1810 . Backlight  1810  includes a plurality of light sources  1820  that form an array, such as a regular array. Light sources  1820  can be any light source disclosed herein, such as light source  100  or  500 . In some cases, each light source  1820  can be independently controlled. For example, in such cases, the brightness of light emitted by each light source can be independently controlled. In some cases, rows or columns of light source can be independently controlled. 
     In some cases, backlight  1810  can be actively and locally controlled by, for example, reducing the brightness of a zone of light sources  1820  that corresponds to a dark portion of a displayed image. Such active zonal control of light sources  1820  can reduce power consumption and enhance display contrast. 
     In some cases, backlight  1810  can be a tiled backlight or a tiled light source that includes a plurality of light source tiles  1820 , where at least one of the light source tiles includes a light source disclosed herein. In some cases, the light source tiles can be interleaved meaning that portions of neighboring tiles overlap. In some cases, liquid crystal panel  1280  can be a monolithic image forming panel or a tiled image forming panel that includes a plurality of image forming tiles. 
     The exemplary display system  1800  has a rectangular shape display. In general, the display size and shape can be any size and shape that may be desirable in an application. For example, in some cases, the display can have a regular shape, such as a round shape or an elliptical shape. As another example, in some cases, the display can have an irregular shape. 
     Some of the advantages of the disclosed optical films, layers, stacks, and systems are further illustrated by the following examples. The particular materials, amounts and dimensions recited in these examples, as well as other conditions and details, should not be construed to unduly limit the present invention. 
     Example A 
     A coating solution “A” was made. First, 360 g of Nalco 2327 colloidal silica (40% wt solid) (available from Nalco Chemical Company, Naperville Ill.) and 300 g of solvent 1-methoxy-2-propanol were mixed together under rapid stirring in a 2-liter three-neck flask that was equipped with a condenser and a thermometer. Next, 22.15 g of Silquest A-174 silane (available from GE Advanced Materials, Wilton Conn.) was added. The mixture was stirred for 10 min. Next, an additional 400 g of 1-methoxy-2-propanol was added. The mixture was heated at 85° C. for 6 hours using a heating mantle. The resulting solution was allowed to cool down to room temperature. Next, most of the water and 1-methoxy-2-propanol solvents (about 700 g) were removed using a rotary evaporator under a 60° C. water-bath. The resulting solution was 44% wt A-174 modified 20 nm silica clear dispersed in 1-methoxy-2-propanol. Next, 70.1 g of this solution, 20.5 g of SR 444 (available from Sartomer Company, Exton Pa.), 1.375 g of photoinitiator Irgacure 184 (available from Ciba Specialty Chemicals Company, High Point N.C.), and 80.4 g of isopropyl alcohol were mixed together by stirring to form a homogenous coating solution A. 
     Example B 
     A coating procedure “B” was developed. First, a coating solution was syringe-pumped at a rate of 2.7 cc/min into a 20.3 cm wide slot-type coating die. The slot coating die uniformly distributed a 20.3 cm wide coating onto a substrate moving at 152 cm/min. 
     Next, the coating was polymerized by passing the coated substrate through a UV-LED cure chamber that included a quartz window to allow passage of UV radiation. The UV-LED bank included a rectangular array of 352 UV-LEDs, 16 down-web by 22 cross-web (approximately covering a 20.3 cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heat sinks. The LEDs (available from Cree, Inc., Durham N.C.) operated at a nominal wavelength of 395 nm and were run at 45 Volts at 10 Amps, resulting in a UV-A dose of 0.108 joules per square cm. The UV-LED array was powered and fan-cooled by a Lambda GENH 60-12.5-U power supply (available from TDK-Lambda, Neptune N.J.). The UV-LEDs were positioned above the cure chamber quartz window at a distance of approximately 2.54 cm from the substrate. The UV-LED cure chamber was supplied with a flow of nitrogen at a flow rate of 46.7 liters/min (100 cubic feet per hour) resulting in an oxygen concentration of approximately 150 ppm in the cure chamber. 
     After being polymerized by the UV-LEDs, the solvent in the cured coating was removed by transporting the coating to a drying oven operating at 150° F. Next, the dried coating was post-cured using a Fusion System Model 1300P configured with an H-bulb (available from Fusion UV Systems, Gaithersburg Md.). The UV Fusion chamber was supplied with a flow of nitrogen that resulted in an oxygen concentration of approximately 50 ppm in the chamber. 
     Example 1 
     A light source  2800 , a side-view of which is schematically shown in  FIG. 6 , was made. Light source  2800  included an optically reflective hollow cavity  2810  and an optical stack  2890  that was placed at an open front port  2822  of the reflective cavity. 
     The optical cavity was wedge shaped and included a proximal side reflector  2824 , a distal side reflector  2820 , a bottom reflector  2822 , and a lamp  2830  housed within a parabolic reflector  2840 . The optical cavity had the following dimensions: l 1 =1 mm, l 2 =16 mm, l 3 =400 mm, l 4 =17 mm, and l 5 =21 mm. Lamp  2830  included 12 white LEDs (available under the name Luxeon Rebel from Philips Lumiled Lighting Company, San Jose, Calif.). The LEDs were placed on heat sinks not expressly shown in  FIG. 6 . 
     The interiors of the parabolic reflector, the side reflectors and the bottom reflector were lined with ESR mirror films (available as from 3M Company, St. Paul Minn.) having a 99.5% reflectance in the visible. Optically reflective cavity  2810  had an open output port  2826  for transmitting light that was emitted by lamp  2830 . 
     Optical stack  2890  included an optical diffuser  2850  coated on a reflective polarizer layer  2860 . The reflective polarizer was laminated to a substrate  2880  via an optical adhesive layer  2870 . Substrate  2880  was a 1.5 mm thick polycarbonate (PC) sheet. Optically clear adhesive  2870  was adhesive OCA 8173 (available from 3M Company, St. Paul Minn.). 
     Reflective polarizer layer  2860  had a pass axis along the x-axis and a block axis along the y-axis. The average on-axis (along the z-direction) reflectivity of the reflecting polarizer layer for incident light polarized along the x-axis (the pass axis) was about 68%, and the average on-axis (along the z-direction) reflectivity of the reflecting polarizer layer for incident light polarized along the y-axis (the block axis) was about 99.2%. The reflective polarizer layer was made as described in International Publication No. WO 2008/144656 filed on May 19, 2008, the disclosure of which is incorporated in its entirety herein by reference. 
     The reflective polarizer layer included 274 alternating microlayers of birefringent 90/10 coPEN material and Eastman Neostar Elastomer FN007 (available from Eastman Chemical, Kingsport Tenn.). The 274 alternating microlayers were arranged in a sequence of ¼ wave layer pairs, where the thickness gradient of the layers was designed to provide a strong reflection resonance broadly and uniformly across a bandwidth from approximately 400 nm to 1050 nm for one polarization axis, and a weaker reflection resonance broadly and uniformly across a bandwidth from approximately 400 nm to 900 nm for the orthogonal axis. Two 5 micron thick skin layers of PET-G were disposed on the outside surfaces of the coherent altering microlayer stack. The overall thickness of the reflective polarizer layer, including the alternating microlayers, the protective boundary layers and the skin layers, was approximately 40 microns. The refractive indices, measured at 633 nm, for the alternating 138 microlayers of 90/10 coPEN were n x1 =1.805, n y1 =1.620, and n z1 =1.515; and the indices for the 138 microlayers of FN007 were n x2 =n y2 =n z2 =1.506. 
     Optical diffuser  2850  was prepared using the method described in International Publication No. WO 2008/144656. The diffuser included a plurality of small particles dispersed in a binder. In particular, PMMA beads (MBX-20, available from Sekisui) having an average diameter of about 18 micrometers were dispersed in a solution of Iragacure 142437-73-01, IPA, and Cognis Photomer 6010 (available from Cognis North America, Cincinnati, Ohio). The solution was coated on reflective polarizer layer  2860  and UV cured, resulting in a dried coating thickness of approximately 40 microns. The dispersion of PMMA beads created a partial of hemispheric surface structure, randomly distributed spatially. The average radius of protrusion of the PMMA beads above the mean surface was estimated to be approximately 60% of the average bead radius. The dried matrix was formulated to have approximately the same refractive index as the PMMA beads, minimizing the bulk scattering within the coating. 
     Optical performance of light source  2800  was measured using an Autronic Conoscope Conostage 3 (available from Autronic-Melchers GmbH, Karlsruhe, Germany). LEDs  2830  were driven at 50 mA during the measurements. The axial luminance, maximum luminance, angles of maximum luminance (in degrees) along the x-axis (the down-lightguide direction) and the z-axis (the cross-lightguide direction) relative to the y-direction, and integrated intensity were measured and summarized in Table I. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Measured optical properties for Examples 1 and 2 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Angle of 
                 Angle of 
                   
               
               
                   
                   
                   
                 maximum 
                 maximum 
               
               
                   
                 Axial 
                 Maximum 
                 luminance 
                 luminance 
                 Integrated 
               
               
                   
                 luminance 
                 luminance 
                 (z-axis) 
                 (x-axis) 
                 intensity 
               
               
                 Example 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
                 (degrees) 
                 (degrees) 
                 (lm/m 2 ) 
               
               
                   
               
               
                 1 
                 1201.1 
                 1214.8 
                 63 
                 270 
                 2589.9 
               
               
                 2 
                 1274.4 
                 1352.7 
                 63 
                 273 
                 2890.8 
               
               
                   
               
            
           
         
       
     
     Example 2 
     A light source similar to light source  2800  was made except that optical stack  2890  was replaced with an optical stack  2900 , a side-view of which is shown schematically in  FIG. 7 . 
     Optical stack  2900  included 1.5 mm thick PC substrate  2880 , OCA 8173 optically clear adhesive layer  2870 , reflective polarizer layer  2860  having the same optical properties as the reflective polarizer layer in optical stack  2890 , an optical film  2940  coated on polarizer layer  2860 , an optical diffuser  2910  similar to optical diffuser  2850  and coated on a substrate  2920 , and an optical adhesive layer  2930  laminating substrate  2920  to optical film  2940 . Optical stack  2900  was placed at output port  2826  of optically reflective cavity  2810  in  FIG. 6 . 
     Optical film  2940  was made by coating solution A from Example A on reflective polarizer layer  2860  using the coating method described in Example B, except that the syringe pump rate was 6 cc/min and the UV-LEDs were ran at 13 Amps (resulting in a UVA dose of 0.135 joules per square cm). The optical film had an index of refraction of about 1.22 and a thickness of about 5 microns. 
     Optical diffuser  2910  was prepared using the method described in International Publication No. WO 2008/144656. The diffuser included a plurality of small particles dispersed in a binder. In particular, PMMA beads (MB30X-8, available from Soken Chemical Company, Ltd, Tokyo Japan) having an average diameter of about 8 micrometers (22.5% by weight) were mixed with Cognis 6010 resin (15% by weight) (available as Photomer 6010 from Cognis North America, Cincinnati Ohio), photoinitiator Esacure (0.1% by weight) (available from Lamberti S.p.A., Gallarate, Italy), radiation curing silicone additive Tego Rad 2250 (0.1% by weight) (available from Evonik Goldschmidt Corporation, Hopewell Va.), and solvent Dowanol PM (61.9% by weight) (available from Dow Chemical Company, Midland Mich.). The components were mixed in a high shear mixer with the beads added last. The solution was coated on a 0.051 mm thick PET substrate  2920 , dried and UV cured, resulting in a dried coating thickness of approximately 8 microns. 
     Optical performance of light source  2800  was measured using the procedure described in Example 1. The results are summarized in Table I. 
     Example 3 
     A display system  800 , a side-view of which is schematically shown in  FIG. 8 , was made. Display system  800  included a rectangular liquid crystal panel  820  disposed on an extended light source  801 . Liquid crystal panel  820  had a length (x-direction) of about 895 mm and width (z-direction) of about 515 mm. The extended light source  801  had a rectangular emissive or light emitting area that was 705 mm long (x-direction) and 400 mm wide (z-direction). Extended light source  801  illuminated a similar size area of liquid crystal panel  820  that was smaller than the size of the panel. 
     Light source  801  included an optical stack  890  that was disposed on and received light from an optically reflective hollow cavity  810 . The optical cavity had a proximal side reflector  824 , a distal side reflector  820 , a bottom reflector  822 , and a lamp source assembly  870  that included six light engines. Each light engine included a lamp  830  that was housed within a parabolic reflector  840 . Each lamp  830  included 12 cool white LEDs (available under the name Luxeon Rebel from Philips Lumiled Lighting Company, San Jose, Calif.) arranged in a linear array with a pitch of about 9.8 mm. The light engines were attached to aluminum heat sinks for thermal management. The optical cavity had the following dimensions: l 1 =1 mm, l 2 =17 mm, l 3 =400 mm, l 4 =17 mm, and l 5 =21 mm. The interiors of the parabolic reflector, the side reflectors, and the bottom reflector were lined with ESR mirror films (available as from 3M Company, St. Paul Minn.) having a 99.5% reflectance in the visible. Optically reflective hollow cavity  810  had an open output port  826  for transmitting light that was emitted by lamps  830 . 
     Optical stack  890  included an optical diffuser  850  coated on a reflective polarizer layer  860 . The reflective polarizer was bonded to liquid crystal panel  820  via an optical adhesive layer  870 . Reflective polarizer layer  860  was similar to reflective polarizer layer  2860  and was made as described in Example 1. 
     Optical diffuser  850  was prepared as follows: 15 kg of Photomer 6010 (available from Cognis USA, Cincinnati, Ohio) and 62.1 kg of 1-methoxy-2-propanol were combined under rapidly stirring until the Photomer 6010 was completely dissolved. Then, 0.1 Kg of Tego Rad 2250 (available from Evonik Goldschmidt Corp. Hopewell, Va.), 0.53 kg of Esacure ONE (available from Lamberti, Conshohocken, Pa.), and 22.5 kg of MB30X-8 (available from Sekisui Plastics Co, Ltd. Tokyo, Japan) were added and rapidly stirred until a homogenous coating solution was obtained. The resulting solution was then coated on reflective polarizer layer  860  using a coating pump at a pump rate of about 800 g/min with the reflective polarizer layer moving at about 30.5 m/min. Next, the coating was dried by passing it through a first oven at 160° F. and a second oven at 200° F. The dried coating was then UV cured using a Light Hammer 6 UV light source that included H bulbs (available from Fusion UV Systems, INC. Gaithersburg, Md.) and operated at 100% UV under nitrogen. The resulting coated reflective polarizer layer was laminated to liquid crystal panel  820  via optical adhesive layer  870  (adhesive OCA 8173 available from 3M Company, St. Paul Minn.). 
     Optical performance of display system  800  was measured using EZ Contrast XL 88W Conoscope (Model XL88W-R-111124, available from Eldim-Optics, Hérouville Saint-Clair France). The display system had an axial luminance of about 93 nits, a maximum luminance of about 100 nits, a contrast ratio of about 146, a viewing angle of about 63 degrees along the x-axis and a viewing angle of about 30 degrees along the z-axis.  FIG. 9  is a grayscale conoscopic image of the measured luminance of display system  800  as a function of viewing angle. The grid overlaying the image is provided for reference purposes to show the azimuthal angle φ ranging from 0 to 360 degrees, and the polar angle θ ranging from 0 at the center to about 88 degrees at the periphery, with concentric circles provided for each 20 degree increment of θ. 
     Example 4 
     A display system similar to display system  800  was made except that optical stack  890  was replaced with an optical stack  1090 , a side-view of which is shown schematically in  FIG. 10 . Optical stack  1090  included optical diffuser  850  coated on the bottom major surface of reflective polarizer layer  860 , optical film  1010  coated on the top major surface of reflective polarizer layer  860 , and optical adhesive layer  870 . 
     Optical film  1010  was prepared and coated on reflective polarizer layer  860  as follows. In a 2 liter three-neck flask, equipped with a condenser and a thermometer, 960 grams of IPA-ST-UP organosilica elongated particles (available from Nissan Chemical Inc., Houston, Tex.), 19.2 grams of deionized water, and 350 grams of 1-methoxy-2-propanol were mixed under rapid stirring. The elongated particles had a diameter in a range from about 9 nm to about 15 nm and a length in a range from about 40 nm to about 100 nm. The particles were dispersed in a 15.2% wt IPA. Next, 22.8 grams of Silquest A-174 silane (available from GE Advanced Materials, Wilton, Conn.) was added to the flask. The resulting mixture was stirred for 30 minutes. The mixture was kept at 81° C. for 16 hours. Next, the solution was allowed to cool down to room temperature. Next, about 950 grams of the solvent in the solution were removed using a rotary evaporator under a 40° C. water-bath, resulting in a 42.1% wt A-174-modified elongated silica clear dispersion in 1-methoxy-2-propanol. 
     Next, 95 grams of this clear dispersion, 26.8 grams of SR 444 (available from Sartomer Company, Exton, Pa.), 102 grams of isopropyl alcohol, 0.972 grams of photoinitiator Irgacure 184 and 0.167 grams of photoinitiator Irgacure 819 (both available from Ciba Specialty Chemicals Company, High Point N.C.) were mixed together and stirred resulting in a homogenous coating solution with 30.4% wt solids. Next, the coating solution was coated on the top major surface of reflective polarizer layer  860  using the coating method described below: 
     The coating solution was syringe-pumped at a rate of 2.5 cc/min into a 20.3 cm wide slot-type coating die. The slot coating die uniformly distributed a 20.3 cm wide coating onto a substrate moving at 152 cm/min. Next, the coating was polymerized by passing the coated reflective polarizer layer through a UV-LED cure chamber that included a quartz window to allow passage of UV radiation. The UV-LED bank included a rectangular array of 352 UV-LEDs (available from Cree, Inc., Durham, N.C.), 16 down-web (coating direction) by 22 cross-web (approximately covering a 20.3 cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heat sinks. The UV-LEDs operated at a nominal wavelength of 395 nm and were run at 45 Volts at 13 Amps, resulting in a UV-A dose of about 0.1352 joules per square cm. The UV-LED array was powered and fan-cooled by a Lambda GENH 60-12.5-U power supply (available from TDK-Lambda, Neptune N.J.). The UV-LEDs were positioned above the cure chamber quartz window at a distance of approximately 2.54 cm from the reflective polarizer layer. The UV-LED cure chamber was supplied with a flow of nitrogen at a flow rate of 46.7 liters/min resulting in an oxygen concentration of approximately 150 ppm in the cure chamber. 
     After being polymerized by the UV-LEDs, the solvent in the cured coating was removed by transporting the coating on a web to a drying oven operating at 150° F. for 2 minutes at a web speed of about 152 cm/mim. Next, the dried coating was post-cured using a Fusion System Model 1300P configured with an H-bulb (available from Fusion UV Systems, Gaithersburg Md.). The UV Fusion chamber was supplied with a flow of nitrogen that resulted in an oxygen concentration of approximately 50 ppm in the chamber. 
     The resulting optical film  1010  had a thickness of about 5 microns and an effective refractive index of 1.16 measured using a Metricon Model 2010 Prism Coupler (available from Metricon Corp., Pennington, N.J.). A similar optical film  1010  coated on a 0.051 mm thick PET substrate had a total optical transmittance of about 94.9% and an optical haze of about 1.1% as measured with a Haze-Gard Plus haze meter (available from BYK-Gardiner, Silver Springs Md.). 
     The display system had an axial luminance of about 302 nits, a maximum luminance of about 312 nits, a contrast ratio of about 555, all of which were more than three times greater than the corresponding measurements reported in Example 3. The display system had a viewing angle of about 63 degrees along the x-axis and a viewing angle of about 35 degrees along the z-axis.  FIG. 11  is a grayscale conoscopic image of the measured luminance of the display system as a function of viewing angle. 
     Item 1 is a light source comprising:
         an optically reflective hollow cavity comprising:
           one or more input ports for receiving light and an output port for transmitting light; and   one or more lamps disposed at the one or more input ports; and   
           an optical stack disposed at the output port and comprising:
           a substantially forward scattering optical diffuser disposed at the output port and having an optical haze that is not less than about 20%;   an optical film disposed on the substantially forward scattering optical diffuser for enhancing total internal reflection at an interface between the optical film and the substantially forward scattering optical diffuser, the optical film having an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%; and   a reflective polarizer layer disposed on the optical film, wherein substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other.   
               

     Item 2 is the light source of item 1, wherein a ratio of a maximum lateral dimension of the optically reflective hollow cavity to a maximum thickness of the optically reflective cavity is not less than about 20. 
     Item 3 is the light source of item 1, wherein a ratio of a maximum lateral dimension of the optically reflective hollow cavity to a maximum thickness of the optically reflective hollow cavity is not less than about 40. 
     Item 4 is the light source of item 1, wherein a ratio of a maximum lateral dimension of the optically reflective hollow cavity to a maximum thickness of the optically reflective hollow cavity is not less than about 60. 
     Item 5 is the light source of item 1, wherein the one or more lamps comprises one or more LEDs. 
     Item 6 is the light source of item 1, wherein the one or more input ports are located on opposite sides of the optically reflective hollow cavity and the output port is located on a top side of the optically reflective hollow cavity. 
     Item 7 is the light source of item 1, wherein the substantially forward scattering optical diffuser has a transport ratio that is not less than about 0.2. 
     Item 8 is the light source of item 1, wherein the substantially forward scattering optical diffuser has a transport ratio that is not less than about 0.3. 
     Item 9 is the light source of item 1, wherein the substantially forward scattering optical diffuser has a transport ratio that is not less than about 0.4. 
     Item 10 is the light source of item 1, wherein the substantially forward scattering optical diffuser has a transport ratio that is not less than about 0.5. 
     Item 11 is the light source of item 1, wherein the substantially forward scattering optical diffuser comprises a semi-specular partial reflector. 
     Item 12 is the light source of item 1, wherein the optical haze of the substantially forward scattering optical diffuser is not less than about 30%. 
     Item 13 is the light source of item 1, wherein the optical haze of the substantially forward scattering optical diffuser is not less than about 40%. 
     Item 14 is the light source of item 1, wherein the substantially forward scattering optical diffuser comprises a substantially forward scattering surface diffuser. 
     Item 15 is the light source of item 1, wherein the substantially forward scattering optical diffuser comprises a substantially forward scattering volume diffuser. 
     Item 16 is the light source of item 1, wherein the substantially forward scattering optical diffuser comprises a light scattering layer disposed on an optically transparent substrate. 
     Item 17 is the light source of item 1, wherein the effective index of refraction of the optical film is not greater than about 1.25. 
     Item 18 is the light source of item 1, wherein the effective index of refraction of the optical film is not greater than about 1.2. 
     Item 19 is the light source of item 1, wherein the effective index of refraction of the optical film is not greater than about 1.15. 
     Item 20 is the light source of item 1, wherein the effective index of refraction of the optical film is not greater than about 1.1. 
     Item 21 is the light source of item 1, wherein the optical haze of the optical film is not greater than about 4%. 
     Item 22 is the light source of item 1, wherein the optical haze of the optical film is not greater than about 3%. 
     Item 23 is the light source of item 1, wherein the optical haze of the optical film is not greater than about 2%. 
     Item 24 is the light source of item 1, wherein the optical film comprises a plurality of interconnected voids. 
     Item 25 is the light source of item 1, wherein the optical film comprises a binder and a plurality of interconnected voids. 
     Item 26 is the light source of item 1, wherein the optical film comprises a binder, a plurality of interconnected voids, and a plurality of particles. 
     Item 27 is the light source of item 1, wherein the optical film is laminated to the substantially forward scattering optical diffuser via an optical adhesive layer. 
     Item 28 is the light source of item 1, wherein the optical film is coated on the reflective polarizer layer. 
     Item 29 is the light source of item 1, wherein the optical stack comprises an optically adhesive layer disposed on the reflective polarizer layer. 
     Item 30 is the light source of item 1, wherein the reflective polarizer layer comprises a multilayer optical film comprising alternating layers, wherein at least one of the alternating layers comprises a birefringent material. 
     Item 31 is the light source of item 1, wherein the reflective polarizer layer comprises a wire grid reflective polarizer. 
     Item 32 is the light source of item 1, wherein the reflective polarizer layer comprises a plurality of substantially parallel fibers, the fibers comprising a birefringent material. 
     Item 33 is the light source of item 1, wherein the reflective polarizer layer comprises a cholesteric reflective polarizer. 
     Item 34 is the light source of item 1, wherein the reflective polarizer layer comprises a diffusely reflective polarizing film (DRPF). 
     Item 35 is the light source of item 1, wherein the optically reflective hollow cavity comprises one or more specularly reflective side reflectors, light that is emitted by the one or lamps being collimated by the one or more specularly reflective side reflectors along a lateral direction of the optically reflective hollow cavity. 
     Item 36 is the light source of item 1, wherein the optically reflective hollow cavity comprises a specularly reflective reflector facing the output port. 
     Item 37 is the light source of item 36, wherein the output port is smaller than the specularly reflective reflector. 
     Item 38 is the light source of item 1, wherein the optical reflective hollow cavity comprises one or more specular reflectors. 
     Item 39 is the light source of item 38, wherein the one or more specular reflectors include one or more enhanced specular reflectors (ESRs). 
     Item 40 is the light source of item 1, wherein at least 50% of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     Item 41 is the light source of item 1, wherein at least 70% of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     Item 42 is the light source of item 1, wherein at least 90% of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     Item 43 is the light source of item 1, wherein the optical film is disposed between the reflective polarizer layer and the substantially forward scattering optical diffuser. 
     Item 44 is a backlight for providing illumination in a display system, the backlight comprising the light source of item 1. 
     Item 45 is a display system comprising the light source of item 1 and a liquid crystal panel disposed on the optical stack. 
     Item 46 is the display system of item 45, wherein the optical stack is bonded to the liquid crystal panel via a removable adhesive. 
     Item 47 is the display system of item 45, wherein the optical stack is bonded to the liquid crystal panel via a repositionable adhesive. 
     Item 48 is the light source of item 1, wherein the optically reflective hollow cavity further comprises an optical element disposed near an input port in the one or more input ports, the optical element comprising an optical filter, an asymmetric optical diffuser, a wavelength converter, or a light collimator. 
     Item 49 is a tiled light source comprising a plurality of light source tiles, at least one of the plurality of light source tiles comprising the light source of item 1. 
     Item 50 is a display system comprising the tiled light source of item 49. 
     Item 51 is the display system of item 50 comprising a monolithic image forming panel. 
     Item 52 is the display system of item 50 comprising a tiled image forming panel. 
     Item 53 is a light source comprising:
         an optically reflective hollow cavity comprising:
           one or more input ports for receiving light and an output port for transmitting light; and   one or more lamps disposed at the one or more input ports; and   
           an optical stack disposed at the output port and comprising:
           a substantially forward scattering optical film disposed at the output port and having an optical haze that is not less than about 30%; and   a reflective polarizer layer disposed on the optical film, wherein substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other.   
               

     Item 54 is the light source of item 53, wherein a ratio of a maximum lateral dimension of the optically reflective hollow cavity to a maximum thickness of the optically reflective hollow cavity is not less than about 20. 
     Item 55 is the light source of item 53, wherein a ratio of a maximum lateral dimension of the optically reflective hollow cavity to a maximum thickness of the optically reflective hollow cavity is not less than about 40. 
     Item 56 is the light source of item 53, wherein a ratio of a maximum lateral dimension of the optically reflective hollow cavity to a maximum thickness of the optically reflective hollow cavity is not less than about 60. 
     Item 57 is the light source of item 53, wherein the one or lamps comprise one or more LEDs. 
     Item 58 is the light source of item 53, wherein the one or more input ports are located on opposite sides of the optically reflective hollow cavity and the output port is located on a top side of the optically reflective hollow cavity. 
     Item 59 is the light source of item 53, wherein the substantially forward scattering optical film has a transport ratio that is not less than about 0.2. 
     Item 60 is the light source of item 53, wherein the substantially forward scattering optical film has a transport ratio that is not less than about 0.3. 
     Item 61 is the light source of item 53, wherein the substantially forward scattering optical film has a transport ratio that is not less than about 0.4. 
     Item 62 is the light source of item 53, wherein the substantially forward scattering optical film has a transport ratio that is not less than about 0.5. 
     Item 63 is the light source of item 53, wherein the optical haze of the substantially forward scattering optical film is not less than about 40%. 
     Item 64 is the light source of item 53, wherein the optical haze of the substantially forward scattering optical film is not less than about 50%. 
     Item 65 is the light source of item 53, wherein the substantially forward scattering optical film comprises a plurality of interconnected voids. 
     Item 66 is the light source of item 53, wherein the substantially forward scattering optical film comprises a binder and a plurality of interconnected voids. 
     Item 67 is the light source of item 53, wherein the substantially forward scattering optical film comprises a binder, a plurality of interconnected voids, and a plurality of particles. 
     Item 68 is the light source of item 53, wherein the substantially forward scattering optical film is laminated to the reflective polarizer layer via an optical adhesive layer. 
     Item 69 is the light source of item 53, wherein the substantially forward scattering optical film is coated on the reflective polarizer layer. 
     Item 70 is the light source of item 53, wherein the optical stack comprises an optically adhesive layer disposed on the reflective polarizer layer. 
     Item 71 is the light source of item 53, wherein the reflective polarizer layer comprises a multilayer optical film comprising alternating layers, wherein at least one of the alternating layers comprises a birefringent material. 
     Item 72 is the light source of item 53, wherein the reflective polarizer layer comprises a wire grid reflective polarizer. 
     Item 73 is the light source of item 53, wherein the reflective polarizer layer comprises a plurality of substantially parallel fibers, the fibers comprising a birefringent material. 
     Item 74 is the light source of item 53, wherein the reflective polarizer layer comprises a cholesteric reflective polarizer. 
     Item 75 is the light source of item 53, wherein the reflective polarizer layer comprises a diffusely reflective polarizing film (DRPF). 
     Item 76 is the light source of item 53, wherein the optically reflective hollow cavity comprises one or more specularly reflective side reflectors, light that is emitted by the one or lamps being collimated by the one or more specularly reflective side reflectors along a lateral direction of the optically reflective hollow cavity. 
     Item 77 is the light source of item 53, wherein the optically reflective hollow cavity comprises a specularly reflective reflector facing the output port. 
     Item 78 is the light source of item 53, wherein the optical reflective hollow cavity comprises one or more specular reflectors. 
     Item 79 is the light source of item 53, wherein the one or more specular reflectors include one or more enhanced specular reflectors (ESRs). 
     Item 80 is the light source of item 53, wherein at least 50% of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     Item 81 is the light source of item 53, wherein at least 70% of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     Item 82 is the light source of item 53, wherein at least 90% of each two neighboring major surfaces in the optical stack are in physical contact with each other. 
     Item 83 is a backlight for providing illumination in a display system, the backlight comprising the light source of item 53. 
     Item 84 is a display system comprising the light source of item 53 and a liquid crystal panel disposed on the optical stack. 
     Item 85 is a light source comprising:
         an optically reflective hollow cavity comprising:
           one or more input ports for receiving light and an output port for transmitting light; and   one or more lamps disposed at the one or more input ports; and   
           an optical stack disposed at the output port and comprising:
           a substantially forward scattering optical diffuser disposed at the output port and having an optical haze that is not less than about 20%;   an optical film disposed on the substantially forward scattering optical diffuser for enhancing total internal reflection at an interface between the optical film and the substantially forward scattering optical diffuser, the optical film having an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%; and   
           a partially reflective partially transmissive layer disposed on the optical film, wherein substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other.       

     Item 86 is a light source comprising:
         an optically reflective hollow cavity comprising:
           one or more input ports for receiving light;   first and second output ports for transmitting light; and   one or more lamps disposed at the one or more input ports; and   
           first and second optical stacks disposed at respective first and second output ports, each optical stack comprising:
           an optical film having an optical haze that is not less than about 30%; and   a reflective polarizer layer disposed on the optical film, substantial portions of each two neighboring major surfaces in the optical stack being in physical contact with each other.   
               

     Item 87 is a display system comprising:
         a first liquid crystal panel disposed on the first optical stack of the light source of item 86; and a second liquid crystal panel disposed on the second optical stack of the light source of item 86.       

     As used herein, terms such as “vertical”, “horizontal”, “above”, “below”, “left”, “right”, “upper” and “lower”, “top” and “bottom”, “clockwise” and “counter clockwise” and other similar terms, refer to relative positions as shown in the figures. In general, a physical embodiment can have a different orientation, and in that case, the terms are intended to refer to relative positions modified to the actual orientation of the device. For example, even if display system  1200  in  FIG. 1  is flipped as compared to the orientation in the figure, reflector  1212  is still considered to be a “bottom” reflector. 
     All patents, patent applications, and other publications cited above are incorporated by reference into this document as if reproduced in full. While specific examples of the invention are described in detail above to facilitate explanation of various aspects of the invention, it should be understood that the intention is not to limit the invention to the specifics of the examples. Rather, the intention is to cover all modifications, embodiments, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.