Brightness enhancement film, and methods of making and using the same

Abstract

A brightness enhancement film comprises a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, wherein a first surface of the base film is textured, and wherein a light-redirecting structure is disposed on a first surface of the base film.

Description

BACKGROUND

[0001] In flat panel displays (e.g., backlight computer displays), optical film (which can also be referred to as a sheet, layer, foil, and the like) materials are commonly used, for example, to direct, diffuse, or polarize light. For example, in backlight displays, brightness enhancement films use prismatic structures on the surfaces thereof to direct light along a viewing axis (i.e., an axis normal (perpendicular) to the display). This enhances the brightness of the light viewed by the user of the display and allows the system to consume less power in creating a desired level of on-axis illumination. Such films can also be used in a wide range of other optical designs, such as in projection displays, traffic signals, and illuminated signs.

[0002] Currently, backlight displays, for example, employ a plurality of films arranged in a manner to obtain the desired brightness and diffusion of the light directed to the viewer. It is noted that as the number of films employed increases, the over thickness of the backlight display increases. It is noted, however, that consumers are increasingly demanding thinner backlight display devices. Moreover, it is also desirable to eliminate color bands that may be observable in the back light display device to further meet consumer demands.

[0003] Since a demand exists for increasingly thinner backlight display devices, what is needed in the art is a multifunctional brightness enhancement with no visible color bands.

SUMMARY

[0004] Disclosed herein are brightness enhancement films, and methods of making and using the same.

[0005] One embodiment of a brightness enhancement film comprises a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, wherein a first surface of the base film is textured, and wherein a light-redirecting structure is disposed on a first surface of the base film.

[0006] Another embodiment of a brightness enhancement film comprises a thermoplastic base film comprising greater than or equal to about 80 wt. % polycarbonate, wherein weight percents are based on a total weight of the thermoplastic base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 15 nanometers per inch, and wherein a light-redirecting structure is disposed on a first surface of the base film.

[0007] A third embodiment of a brightness enhancement film comprises a thermoplastic base film comprising about 93 wt. % to about 99.6 wt. % polycarbonate; and about 0.4 wt. % to about 7 wt. % fluorinated phosphonium sulfonate, wherein weight percents are based on a total weight of the thermoplastic base film, and wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, and wherein a light-redirecting structure is disposed on a first surface of the base film.

[0008] One embodiment of a method of making a brightness enhancement film comprises disposing a light-redirecting structure onto a first surface of a base film, wherein a stress retardation gradient of the thermoplastic base film is calculated to be less than or equal to 50 nanometers per inch, wherein the first surface of the thermoplastic base film is textured.

[0009] Another embodiment of a method of making a brightness enhancement film comprises disposing a light-redirecting structure onto a surface of a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 15 nanometers per inch.

[0010] A third embodiment of a method of making a brightness enhancement film comprises disposing a light-redirecting structure onto a surface of a thermoplastic base film comprising about 93 wt. % to about 99.6 wt. % polycarbonate; and about 0.4 wt. % to about 7 wt. % fluorinated phosphonium sulfonate, wherein weight percents are based on a total weight of the thermoplastic base film, and wherein a stress retardation gradient of the thermoplastic base film is calculated to be less than or equal to 50 nanometers per inch.

[0011] An embodiment of a display device comprises an optical source; a light guide in optical and physical communication with the light source; and a brightness enhancement film comprising a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, wherein a first surface of the base film is textured, and wherein a light-redirecting structure is disposed on the first surface of the base film.

[0012] The above-described and other features will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Refer now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike.

[0014] FIG. 1 is a perspective view of an exemplary embodiment of a backlight display device including a brightness enhancement film.

[0015] FIG. 2 is a perspective view of an exemplary embodiment of a brightness enhancement film with prismatic surfaces.

[0016] FIG. 3 is a cross-sectional view of the brightness enhancement film of FIG. 2.

[0017] FIG. 4 is a cross-sectional view and schematic illustration of an exemplary embodiment of a light-diffusing film receptive of light and diffusing the light emanating therefrom.

[0018] FIG. 5 is a perspective view of an exemplary embodiment of two brightness enhancement films.

[0019] FIG. 6 is a perspective view of an exemplary embodiment of a backlight display device including a plurality of brightness enhancement films and a plurality of light-diffusing films.

[0020] FIG. 7 is a schematic view of an extrusion system for producing a base film for a brightness enhancement film.

DETAILED DESCRIPTION

[0021] Disclosed herein are optical films, more particularly brightness enhancement films capable of being employed in a flat panel display (e.g., a backlight display device). It is noted that the brightness enhancement films can be a single layer (e.g., a unitary or monolithic film characterized by the absence of coatings) or a multi-layered structure. The term "total" that is used in relation to reflection is used herein to refer to the combined reflectance of all light from a surface.

[0022] It should further be noted that the terms "first," "second," and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms "a" and "an" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. Furthermore, all ranges disclosed herein are inclusive and combinable (e.g., ranges of "up to about 25 weight percent (wt. %), with about 5 wt. % to about 20 wt. % desired, and about 10 wt. % to about 15 wt. % more desired," is inclusive of the endpoints and all intermediate values of the ranges, e.g., "about 5 wt. % to about 25 wt. %, about 5 wt. % to about 15 wt. %," etc.).

[0023] Several embodiments of backlight display devices are discussed hereunder with reference to individual drawing figures. One of skill in the art will easily recognize that many of the components of each of the embodiments are similar or identical to the others. Each of these elements is introduced in the discussion of FIG. 1, but is not repeated for each embodiment. Rather, distinct structure is discussed relative to each figure/embodiment.

[0024] Referring now to FIG. 1, a perspective view of a backlight display device generally designated 100 is illustrated. The backlight display device 100 comprises an optical source 102 for generating light 104. A light guide 106 in optical communication with optical source 102 guides the light 104 by total internal reflection (TIR) of the light 104 within the light guide 106. A reflective film 108 in physical and/or optical communication with a first surface 110 of light guide 106 reflects the light 104 out of the light guide 106. A brightness enhancement film 112 located in physical and/or optical communication with a second surface 114 of light guide 106 receives the light 104 from the light guide 106.

[0025] More particularly, in this embodiment, the brightness enhancement film 112 comprises a planar surface 116 in physical and/or optical communication with the second surface 114 of light guide 106, and a prismatic surface 118 in physical and/or optical communication with light-diffusing film 120. Still further, it will be appreciated that the prismatic surfaces 118 can comprise a peak angle, .alpha., a height, h, a pitch, p, and a length, l (see FIGS. 2 and 3) such that the structure of the brightness enhancement film 112 can be deterministic, periodic, random, and the like. For example, films with prismatic surfaces with randomized or pseudo-randomized parameters are described for example in U.S. patent application No. 2003/0214728 to Olcazk. Moreover, it is noted that for each prism the sidewalls (facets) can be straight-side, concave, convex, and the like. The peak of the prism can be pointed, rounded, blunted, and the like. More particularly, in an embodiment the prisms comprise straight-sided facets having a pointed peak (e.g., a peak comprising a radius of curvature of about 0.1% to about 30% of the pitch (p)), particularly about 1% to about 5%).

[0026] The brightness enhancement film 112 receives the light 104 and acts to direct the light 104 in a direction that is substantially normal to the brightness enhancement film 112 as indicated schematically by an arrow representing the light 104 being directed in a z-direction shown in FIG. 1. The light-diffusing film 120 is receptive of the light 104 from the brightness enhancement film 112 and diffuses (e.g., scatters) the light as illustrated schematically in FIG. 4. The light 104 proceeds from the light-diffusing film 120 to a liquid crystal display (LCD) 122.

[0027] Further, it is noted that in various embodiments a backlight display device can comprise a plurality of brightness enhancement films and a plurality of light-diffusing films in optical communication with each other. The plurality of brightness enhancement films and light-diffusing films can be arranged in any configuration to obtain the desired results in the LCD. For example, the plurality of brightness enhancement films can be arranged in physical and/or optical communication with each other as illustrated in FIG. 5. More particularly, referring to FIG. 5, a first brightness enhancement film 212 comprises a first brightness enhancement film planar surface 216 and a first brightness enhancement film prismatic surface 218. A second brightness enhancement film 224 comprises a second brightness enhancement film planar surface 226 and a second brightness enhancement film prismatic surface 228. The first brightness enhancement film 212 and the second brightness enhancement film 224 can be arranged such that the prismatic surfaces (218 and 228, respectively) are positioned at an angle with respect to one another, e.g., 90 degrees. Generally, the arrangement and type of brightness enhancement films and light-diffusing films depends on the backlight display device in which they are employed. It is noted, however, that embodiments are envisioned where one or more brightness enhancement films and/or light-diffusing films can be replaced by a single multifunctional brightness enhancement film, as will be discussed in greater detail below.

[0028] Additionally, as briefly mentioned above, the arrangement, type, and amount of brightness enhancement film (s) and light-diffusing film(s) depends on the backlight display device in which they are employed. An increasingly common use of a backlight display device is in a laptop computer. While reference is made to a laptop computer throughout this disclosure, it is to be understood that one of skill in the art can readily use brightness enhancement films disclosed herein in other applications without undue experimentation.

[0029] An exemplary backlight display device 300 for use in a laptop computer is illustrated in FIG. 6. The backlight display device 300 comprises an optical source 302 for generating light 304. A light guide 306 in optical communication with optical source 302 guides the light 304 by total internal reflection of the light 304, as discussed above in relation to FIG. 1. A reflective film 308 in physical and/or optical communication with a first surface 310 of light guide 306 reflects the light 304 out of the light guide 306. A bottom light-diffusing film 320 and a top light-diffusing film 330 are in optical communication with a first brightness enhancement film 312 and a second brightness enhancement film 324 disposed between the bottom light-diffusing film 320 and the top-diffusing film 330. In an embodiment, the light 304 proceeds from the top light-diffusing film 330 to a liquid crystal display (LCD) 322.

[0030] With regards to the embodiment illustrated in FIG. 6, it is noted that the bottom light-diffusing film 320 can primarily function to enhance the uniformity of the light 304. The top light-diffusing film 330 can primarily function to minimize glare and optical coupling (Newton Rings) between the brightness enhancement films (e.g., 312 and 324). In addition, the top light-diffusing film 330 can also function as a protective film for the brightness enhancement films (312, 324), thereby reducing the likelihood of fracturing or damaging the prismatic surfaces of the brightness enhancement films. Furthermore, it is noted that top light-diffusing films (e.g., 330), i.e., the light-diffusing film nearest to the liquid crystal display (e.g., 322), can comprise a haze value of less than or equal to about 85%, more particularly a haze value of less than or equal to about 50%. Whereas, bottom light-diffusing films (e.g., 320), i.e., the light-diffusing film nearest the light guide (e.g., 306), generally comprise a haze value of greater than or equal to about 90%, more particularly a haze value of greater than or equal to about 95%.

[0031] It is noted that the percent haze can be predicted and calculated from the following equation: % .times. Haze = 100 .times. Total .times. .times. Diffuse .times. .times. Trasmission Total .times. .times. Transmission ( 1 ) ##EQU1##

[0032] wherein total transmission is the integrated transmission; and the total diffuse transmission is the light transmission that is scattered by the film as defined by

[0033] Optical source (e.g., 102, 302) can include any light source suitable to backlight a liquid crystal display (LCD) device, which includes both high-brightness and low brightness light sources. The high-brightness light source can include a cold cathode fluorescent lamp (CCFL), a fluorescent lamp, and the like. The low-brightness light source can include a light emitting diode (LED), and the like.

[0034] Light guide (e.g., 106, 306) preferably comprises a material that assumes a low internal absorption of the light, including an acrylic film and desirably transparent materials including acryl, PMMA (polymethylmethacrylate), polycarbonate, polyethylene, selenium (Se), silver chloride (AgCl), and the like. The shape of the light guide can be in a shape suitable for the desired transmission of the light, such as a bar, a curved surface, a plate, a sheet, and the like. The light guide can be a single sheet or a plurality of sheets.

[0035] Reflective film (e.g., 108, 308) can be in any usable shape for reflecting light, e.g., a planar shape, such as a plate, sheet, coating and the like, wherein the reflective film comprises a reflective material. For example, suitable reflective materials include an aluminum, a silver, titanium oxide, and the like, as well as combinations comprising at least one of the foregoing. In other embodiments, the reflective film can comprise a thermoplastic material, e.g., Spectralon.RTM. (available from Labsphere, Inc.), titanium-oxide pigmented Lexan.RTM. (available from General Electric Co.), and the like.

[0036] The brightness enhancement film(s) (e.g., 112, 212, 224, 312, and 324) comprise light-redirecting structure(s) (e.g., prismatic (pyramid-like) cube corners, spheres, edges, and the like) to direct light along the viewing axis (i.e., normal to the display), which enhances the luminance (brightness) of the light viewed by the user of the display and allows the system to use less power to create a desired level of on-axis illumination. Generally, the brightness enhancement film comprises a base film that can comprise an optional curable coating disposed thereon. The light-redirecting structure can be created, for example, by applying the curable coating to the base film and casting the desired light-redirecting structure in the curable coating, by hot-embossing the structure directly onto the base film, or the like. While the base film material can vary depending on the application, suitable materials include those base film materials discussed in published U.S. patent application No. 2003/0108710 to Coyle et al. More specifically, the base film material of the brightness enhancement film can comprise metal, paper, acrylics, polycarbonates, phenolics, cellulose acetate butyrate, cellulose acetate propionate, poly(ether sulfone), poly(methyl methacrylate), polyurethane, polyester, poly(vinylchloride), polyethylene terephthalate, and the like, as well as blends copolymers, reaction productions, and combinations comprising at least one of the foregoing.

[0037] In one embodiment, the base film of the brightness enhancement film is formed from a thermoplastic polycarbonate resin, such as Lexan.RTM. resin, commercially available from General Electric Company, Schenectady, N.Y. Thermoplastic polycarbonate resin that can be employed in producing the base film, include without limitation, aromatic polycarbonates, copolymers of an aromatic polycarbonate such as polyester carbonate copolymer, blends thereof, and blends thereof with other polymers depending on the end use application. In another embodiment, the thermoplastic polycarbonate resin is an aromatic homo-polycarbonate resin such as the polycarbonate resins described in U.S. Pat. No. 4,351,920 to Ariga et al. These polycarbonate resins can be obtained by the reaction of an aromatic dihydroxy compound with a carbonyl chloride. Other polycarbonate resins can be obtained by the reaction of an aromatic dihydroxy compound with a carbonate precursor such as a diaryl carbonate. An exemplary aromatic dihydroxy compound is 2,2-bis(4-hydroxy phenyl) propane (i.e., Bisphenol-A). A polyester carbonate copolymer is obtained by the reaction of a dihydroxy phenol, a carbonate precursor and dicarboxylic acid such as terephthalic acid or isophthalic acid or a mixture of terephthalic and isophthalic acid. Optionally, an amount of a glycol can also be used as a reactant.

[0038] In other embodiments, an anti-static material can optionally be added to the base film of the brightness enhancement film in an amount sufficient to impart anti-static properties to the film. For example, an anti-static material comprising phosphonium sulfonate can be added to a base film comprising polycarbonate. In an embodiment, the anti-static material is that described in U.S. Pat. No. 6,194,497 to Henricus et al. More specifically, the phosphonium sulfonate can be a fluorinated phosphonium sulfonate comprising a fluorocarbon containing an organic sulfonate anion and an organic phosphonium cation. Examples of such organic sulfonate anions include, but are not limited to, perfluoro methane sulfonate, perfluoro butane sulfonate, perfluoro hexane sulfonate, perfluoro heptane sulfonate, and perfluoro octane sulfonate. Examples of the phosphonium cation include, but are not limited to, aliphatic phosphonium such as tetramethyl phosphonium, tetraethyl phosphonium, tetrabutyl phosphonium, triethylmethyl phosphonium, tributylmethyl phosphonium, tributylethyl phosphonium, trioctylmethyl phosphonium, trimethylbutyl phosphonium, trimethyloctyl phosphonium, trimethyllauryl phosphonium, trimethylstearyl phosphonium, triethyloctyl phosphonium and aromatic phosphoniums such as tetraphenyl phosphonium, triphenylmethyl phosphonium, triphenylbenzyl phosphonium, tributylbenzyl phosphonium. More specifically, the fluorinated phosphonium sulfonate can be obtained by any combination comprising at least one of any of these organic sulfonate anions with phosphonium cations.

[0039] Furthermore, even more specifically, the phosphonium sulfonate employed herein can be a fluorinated phosphonium sulfonate having the general formula: {CF.sub.3(CF.sub.2)n(SO.sub.3)}.theta.{P(R.sub.1)(R.sub.2)(R.sub.3)(R.sub- .4)}.PHI.

[0040] wherein F is fluorine; n is an integer of from 1-12, S is sulfur; R.sub.1, R.sub.2, and R.sub.3 can each comprise an aliphatic hydrocarbon radical of 1-8 carbon atoms or an aromatic hydrocarbon radical of 6-12 carbon atoms and R4 is a hydrocarbon radical of 1-18 carbon atoms. Anti-static compositions comprising fluorinated phosphonium sulfonate shown by formula as having the principle component thereof can be used in many different ways to make use of their anti-static and compatibility characteristics and heat resistance in providing such anti-static characteristics to polycarbonate. The phosphonium fluorocarbon sulfonate salts are low melting semi-solid materials, and as such, they can be handled as a molten liquid. Some embodiments are solid crystalline materials at room temperature (i.e., a temperature of about 15.degree. C. to about 25.degree. C.) and are easy to weigh, handle, and add to the polycarbonate.

[0041] While the anti-static material can be added to the polycarbonate at any time in the process, it is desirable to add it to the polycarbonate at the time of polymer production. For example, the polycarbonate and anti-static material can be processed by, for example, extrusion, and the like.

[0042] As briefly mentioned above, the base film of the brightness enhancement film can comprise polycarbonate and an anti-static material. For example, the base film comprises greater than or equal to about 80 wt. % polycarbonate, and more particularly greater than or equal to about 90 wt. % polycarbonate, wherein weight percents are based on a total weight of the base film. For example, in an embodiment, the base film comprises about 93 wt. % to about 99.6 wt. % polycarbonate; and about 0.4 wt. % to about 7 wt. % anti-static material, more specifically, about 0.4 wt. % to about 2 wt. % anti-static material.

[0043] While it is noted that the thickness of the base film of the brightness enhancement film can vary depending on the desired application, the base film can comprise a thickness sufficient for use in a flat panel display, e.g., for use in a laptop computer. For example, the base film can comprise a thickness of about 25 micrometers to about 1,000 micrometers, specifically about 175 micrometers to about 750 micrometers.

[0044] In embodiments comprising a curable coating on the base film of the brightness enhancement film, the curable coating comprises a curable composition, which generally comprises a polymerizable compound. Polymerizable compounds, as used herein, are monomers or oligomers comprising one or more functional groups capable of undergoing radical, cationic, anionic, thermal, and/or photochemical polymerization. Suitable functional groups include, for example, acrylate, methacrylate, vinyl, epoxide, and the like.

[0045] For example, the curable composition can include monomeric and dimeric acrylates, for example, cyclopentyl methacrylate, cyclohexyl methacrylate, methylcyclohexylmethacrylate, trimethylcyclohexyl methacrylate, norbomylmethacrylate, norbomylmethyl methacrylate, isobornyl methacrylate, lauryl methacrylate 2-ethylhexyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hexanediol acrylate, 2-phenoxyethyl acrylate, 2-hydroxyethyl acrylate, 2-hydoxypropyl acrylate, diethyleneglycol acrylate, hexanediol methacrylate, 2-phenoxyethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydoxypropyl methacrylate, diethyleneglycol methacrylate, ethylene glycol dimethacrylate, ethylene glycol diacrylate, propylene glycol dimethacrylate, propylene glycol diacrylate, allyl methacrylate, allyl acrylate, butanediol diacrylate, butanediol dimethacrylate, 1,6hexanediol diacrylate, 1,6-hexanediol dimethacrylate, diethyleneglycol diacrylate, trimethylpropane triacrylate, pentaeryritol tetraacrylate, hexanediol dimethacrylate, diethyleneglycol dimethacrylate, trimethylolpropane triacrylate, trimethylpropane trimethacrylate, pentaeryritol tetramethacrylate, and combinations comprising at least one of the foregoing acrylates.

[0046] Additionally, the curable composition can comprise a polymerization initiator to promote polymerization of the curable components. Suitable polymerization initiators include photoinitiators that promote polymerization of the components upon exposure to ultraviolet radiation. Suitable photoinitiators include, but are not limited to benzophenone and other acetophenones, benzil, benzaldehyde and O-chlorobenzaldehyde, xanthone, thioxanthone, 2-chlorothioxanthone, 9,10-phenanthrenenquinone, 9,10-anthraquinone, methylbenzoin ether, ethylbenzoin ether, isopropyl benzoin ether, 1-hydroxycyclohexyphenyl ketone, .alpha.,.alpha.-diethoxyacetophenone, .alpha.,.alpha.-dimethoxyacetoophenone, 1-phenyl-,1,2-propanediol-2-o-benzoyl oxime, 2,4,6-trimethylbenzoyldiphenyl phosphine oxide, and, .alpha.,.alpha.-dimethoxy-.alpha.-phenylacetopheone, as well as combinations comprising at least one of the foregoing. In one embodiment, the polymerization initiator is present in an amount of about 0.1 wt. % to about 10 wt. %, more specifically about 1 wt. % to about 8 wt. %, wherein weight percents are based upon a total weight of the curable composition.

[0047] In an embodiment, the curable composition comprises multifunctional (meth)acrylates, substituted or unsubstituted arylether (meth)acrylate monomer, brominated aromatic (meth)acrylate monomer, and polymerization initiator. The curable coating has a refractive index of greater than or equal to about 1.50, more specifically greater than or equal to about 1.61. The refraction index of the curable coating can be increased by including metal oxide nanoparticles in the curable composition. Examples of suitable metal oxides include, but are not limited to, titanium oxide, zinc oxide, indium tin oxide, indium oxide, tin oxide, cadmium tin oxide, and combinations comprising at least one of the forgoing oxides. Further, suitable metal oxide nanoparticles and methods for their preparation are also described, for example, in U.S. Pat. No. 6,261,700 to Olson et al. and U.S. Pat. No. 6,291,070 to Arpac et al. For example, metal oxide nanoparticles can be prepared by a method comprising hydrolyzing a metal alkoxide with an acidic alcohol solution, wherein the acidic alcohol solution comprises an alkyl alcohol, water, and an acid to form a first sol comprising metal oxide nanoparticles; treating the first sol (i.e., a colloidal solution) with an organosilane to form a second sol comprising treated metal oxide nanoparticles; and treating the second sol with an organic base in an amount of about 0.1:1 to about 0.9:1 molar ratio of organic base to acid to form a third sol comprising treated metal oxide nanoparticles. The metal of the metal alkoxide can be, for example, titanium, zinc, indium, tin, cadmium, and combinations comprising at least one of the foregoing. The alkoxide of the metal alkoxide can be, for example, a linear or branched C.sub.1-C.sub.12 alkoxide.

[0048] The curable coating can comprise a thickness of about 10 micrometers to about 100 micrometers, specifically about 35 micrometers to about 100 micrometers, and more specifically about 60 micrometers to about 80 micrometers.

[0049] As will be discussed in greater detail, it has been discovered that a flat panel display comprising no color bands and/or shadows (when viewed from all viewing angles in a display device with the backlight on) can be obtained when a brightness enhancement film is employed in the flat panel display, wherein the base film of the brightness enhancement film comprises a base film with a low stress retardation variation. It is noted that term "retardation" is a term readily understood in the art. A low stress retardation variation base film can be defined mathematically as a film comprising a low stress retardation gradient from a stress retardation profile (i.e., stress retardation as a function of location within the film), wherein a stress retardation gradient is less than or equal to 50 nanometers per inch (nm/in), more particularly less than or equal to 30 nm/in, still more particularly less than or equal to 15 nm/in. Stress retardation can be measured using, for example, a SCA1500 System from Strainoptic Technologies (now Strainoptic, Inc.) according to ASTM D4093. Stated another way, the stress retardation gradient is the first derivative of the stress retardation profile.

[0050] Without being bound by theory, suitable low stress retardation variation base films used to produce brightness enhancement films that when employed in a display device do not produce color bands, include but is not limited to, base films comprising a first surface (i.e., the surface of the brightness enhancement film that faces toward a LCD when employed in a flat panel display, more particularly the surface of the brightness enhancement film that faces toward the user of the device) comprising a textured surface, e.g., a matte surface. Further, in an embodiment, the base film comprises a second surface (i.e., the surface that faces toward the light guide, more particularly the surface that faces away from the user of the device), which can comprise a polished surface or a textured surface (e.g., a matte surface, velvet surface, and the like). It is noted that when a textured second surface is employed in a laptop computer, the overall thickness of the films employed can advantageously be decreased by at least the thickness of a light diffusing film. More particularly, the overall thickness can be decreased by greater than or equal to about 50 micrometers, more specifically greater than or equal to about 100 micrometers.

[0051] For example, suitable base films include, but are not limited to, base films comprising a first surface comprising a polish surface, matte surface, and the like; and a second surface comprising a polish surface, a matte surface, a velvet surface, and the like, wherein the light-redirecting structures are disposed on the first surface of the base film, wherein the resulting base film is a low stress retardation variation base film comprising a stress retardation gradient less than or equal to 50 nanometers per inch (nm/in), as discussed above.

[0052] The terms "polish", "matte", and "velvet" are all terms readily understood by those skilled in the art. For example, a polish surface can comprise a surface roughness (Ra) of less than 0.3 micrometers; a matte (e.g., fine matte, medium matte, course matte, and the like) surface can comprise a surface roughness (Ra) of 0.3 micrometers to 2.2 micrometers; and a velvet surface can comprise a surface roughness (Ra) greater than 2.2 micrometers. It is noted that the term surface roughness (Ra) is a term readily understood by those skilled in the art. Generally, the Ra is a measure of the average roughness of the film. It can be determined by integrating the absolute value of the difference between the surface height and the average height and dividing by the measurement length for a one dimensional surface profile, or the measurement area for a two dimensional surface profile. More particularly, surface roughness can be measured using a Serfcorder SE4000K, commercially available from Kosaka Laboratory Ltd., wherein the surface roughness is measured according to ASME B46.1-1995.

[0053] Additionally, it is noted that embodiments of the brightness enhancement film disclosed herein comprise a brightness performance equivalent to a brightness enhancement film comprising polish/polish surfaces. For example, all else being equal (e.g., same base film materials, prismatic structures, etc.) the brightness enhancement film comprising matte/polish surfaces can comprise a relative luminance of about 99.5% to about 100.5%compared to a base film comprising polish/polish surfaces. While base films comprising a haze value of about 20% to about 80% and transmission greater than or equal to about 85% can be produced, those base films comprising a haze value of less than or equal to about 50% and transmission greater than or equal to about 89% are particularly useful in obtaining the desired luminance comparable to a polish/polish film.

[0054] In other embodiments, the base film of the brightness enhancement film can comprise a haze value sufficient to eliminate at least one light-diffusing film (e.g., a bottom light diffusing film (e.g., 320)) in a backlight display device. In other words, the brightness enhancement film can be a multifunctional brightness enhancement film acting as both a traditional brightness enhancement film, for example, to direct light along a viewing axis (i.e., an axis normal (perpendicular) to the display), and as a light diffusing film. The terms "top" and "bottom" used herein with regards to light-diffusing films, as well as any other film employed in a display device, e.g., a backlight display device, are readily understood by those skilled in the art. The term "top" generally refers to a side of a film or the film itself that is closest to the LCD (i.e., the side or the film itself that is closest to and/or faces toward the viewer). Conversely, the term "bottom" generally refers to a side of a film or the film itself that is farthest away from the LCD (i.e., the side of the film itself that is farthest away from and/or faces away from the viewer).

[0055] In an embodiment of making a brightness enhancement film, the method comprises forming a base film by feeding a thermoplastic resin(s) (e.g., polycarbonate resin) to an extruder; melting the thermoplastic resin to a temperature greater than or equal to the glass transition temperature (Tg) of the thermoplastic resin while it advances through the extruder; extruding the resulting molten resin through a die into a nip or gap between two calendering rolls; and cooling the resulting film to below its glass transition temperature. The resulting film can be rolled and stored for subsequent processing (e.g., coating and casting, embossing, and the like). Alternatively, the base film can be feed directly to a coating and casting station, embossing station, and the like.

[0056] In an embodiment, the molten thermoplastic resin used to produce the base film of the brightness enhancement film is passed through two calendering rolls such that the resulting base film is a low stress retardation variation base film comprising a stress retardation gradient less than or equal to 50 nanometers per inch (nm/in), as discussed above. Without being bound by theory, a low stress variation base film can be obtained when at least one calendering roll employed comprises a material comprising a hardness suitable for producing the low stress retardation variance base film. For example, the roll(s) can comprise an elastomeric material (e.g., an EPDM (ethylene propylene diamine monomer) based rubber). It is noted that in various embodiments the roll can be made entirely of the elastomeric material. Alternatively, the elastomeric material can be disposed on an outer surface of the roll, i.e., the surface of the roll that is in physical communication with the base film.

[0057] For example, in making the base film, a textured rubber calendering roll can be employed to texture the first surface of the base film, as discussed above (e.g., the surface is a matte surface). In various embodiments, the second surface can have a polished surface or textured surface. As discussed throughout this disclosure, the selection of surface roughness of the second surface is a design choice. Embodiments comprising a polished second surface and a matte first surface can advantageously produce a brightness enhancement film comprising a luminance (brightness) equivalent to the luminance as a polish/polish film without color bands. Moreover, embodiments comprising a textured second surface and a matte first surface can advantageously produce a multifunctional brightness enhancement film capable of acting as both a traditional brightness enhancement film, for example, to direct light along a viewing axis (i.e., an axis normal (perpendicular) to the display), and as a light diffusing film.

[0058] In various other embodiments, one of the calendering roll can comprise a chrome or chromium plated roll comprising a polished surface or texture surface (e.g., a velvet surface). Furthermore, it is generally noted that the size of the rollers, material of the rollers, number of rollers, the film wrap around the rollers, and the like can vary with the system employed. Further, it is noted that processing conditions (e.g., the temperature of the calendering rollers, the line speed, nip pressure, and the like) are controlled to produce the desired haze value and luminance in the base film for the resulting brightness enhancement film.

[0059] Referring to FIG. 7, a schematic view of an exemplary extrusion system, generally designated 400 is illustrated. Molten thermoplastic resin 402 is extruded from slot die 404. The molten thermoplastic resin is then passed through a nip or gap 406 formed by calendering rolls 408 and 410, is cooled, and is then passed through pull rolls 412. The cooled film can be rolled (stored) to be subsequently processed, or the cooled film can feed directly to a station (device) to form the light-redirecting structure on the cooled film (base film) to form the brightness enhancement film (e.g., a coating and casting station, embossing station, and the like).

[0060] Having formed the base film of the brightness enhancement film, the method of making the brightness enhancement film further comprises creating light-redirecting structure(s) on the first surface of base film, i.e., the surface comprising the matte surface. As briefly noted above, the light-redirecting structure (e.g., prismatic structure) can be created by applying a curable coating onto the first surface of the base film and casting the structure into the curable coating as it is curing, by hot-embossing the structure onto the base film, or the like. For example, prismatic structures can be formed by disposing a curable coating on the base film, and curing the coating (e.g., by exposing the coating to ultra violet (UV) radiation) while the coating is in physical communication with a cast, wherein the cast comprises the negative image of the desired surface structure.

[0061] Methods of coating a curable composition on a surface of a substrate are described, for example, in U.S. Pat. No. 5,175,030 to Lu et al., U.S. Pat. No. 5,183,597 to Lu, U.S. Pat. No. 5,271,968 to Coyle et al., U.S. Pat. No. 5,468,542 to Crouch, 5,626,800 to Williams et al., and U.S. Pat. No. 6,280,063 to Fong et al., as well as U.S. patent application Publication No. 2003/0108710 Al to Coyle et al. For example, suitable methods of disposing the coating in physical communication with the first surface of the brightness enhancement film include, but is not limited to, spraying, brushing, electro-deposition, dipping, flow coating, roll coating, gravure, and screen printing. Moreover, it is noted that coating can be applied as continuous coating or as patches that correspond with a pattern on the cast.

[0062] In other embodiments, the light-redirecting structures can be formed by hot-embossing the base film, wherein the method comprises heating the base film to a temperature sufficient to soften the base film, and embossing the desired structure into the base film. It is noted that roll embossing, stamping, or the like can be employed to emboss the light-redirecting structure (e.g., prism(s)) into the base film. More particularly, the embossing tool comprises a negative image of the desired surface.

[0063] For protection and convenience of handling in between preparation of brightness enhancement film and its incorporation into a device, the brightness enhancement film can additionally/optionally comprise a masking layer(s). For example, the brightness enhancement film can comprise a masking layer disposed over the first surface of the film (e.g., in embossed embodiments), the second surface of the base film, and/or the curable coating (e.g., in coated embodiments). Suitable masking layers include single or co-extruded layers of polyethylene, polypropylene, polyester, and combinations comprising at least one of the foregoing, wherein the adhesion to the brightness enhancement film is controlled by a pressure sensitive adhesive, by static, and/or the like.

[0064] With regards to the light-diffusing film (e.g., 120) of the backlight display device, the light-diffusing film can be a textured light-diffusing film and/or a bulk light-diffusing film (e.g., light-diffusing can be imbedded into the film to impart the light-diffusing properties to the film). Generally, the light-diffusing film comprises a thermoplastic substrate such as polyester, polycarbonate, or combinations comprising the foregoing. As noted above, the haze value for the light-diffusing film can vary depending on the application. For example, it is noted that top light-diffusing films (e.g., 330) can comprise a haze value of less than or equal to about 85%, more particularly a haze value of less than or equal to about 50%. Whereas, bottom light-diffusing films (e.g., 320) generally comprise a haze value of greater than or equal to about 90%, more particularly a haze value of greater than or equal to about 95%.

[0065] As briefly mentioned above, the brightness enhancement films disclosed herein can be employed in various backlight display devices, e.g., a laptop computer. In various embodiments, the brightness enhancement film can be a multifunctional brightness enhancement film as described above. For example, referring again to FIG. 1, the brightness enhancement film 112 can be disposed in physical communication with light guide 106. More particularly, brightness enhancement film 112 can be in physical communication with the second surface 114 of the light guide 106, i.e., the surface opposite the first surface 110 of the light guide 106. In other words, a bottom light-diffusing film is not disposed in physical communication with brightness enhancement film 112 and light guide 106.

EXAMPLES

Comparative Example 1

[0066] Polycarbonate resins were extruded at 270.degree. C. into base films comprising a thickness of about 175 micrometers. The film was extruded between two polished chrome calendering rolls maintained at 127.degree. C. A base film was achieved with stress retardation gradient greater than 50 nanometers per inch retardation across a 50 inch (127 centimeter) wide film. It is noted that in constructing a stress profile from which the stress retardation gradient was obtained, stress retardation was measured at every 0.25 inches (0.64 centimeters) across the length of the film.

[0067] This base film was then coated with a coating comprising about 60 wt. % brominated epoxy acrylate, and about 40 wt. % phenylthiolethylacrylate, with a trace amount of a photoinitiator (i.e., a bis(acyl)phosphine oxide sold as IRGACURE 819, commercially available from Ciba Geigy, Inc., wherein weight percents were based on a total weight of the coating. The coating composition was applied to the bottom-masked base film by gravure roll at a thickness of about 30 micrometers. After the coating was applied to the film, prismatic structures were formed by curing the coating while it was in contact with a cast comprising a surface with the negative image of the desired surface structures as described above. The film was then masked, and subsequently converted into a format suitable for handling and assembly in a backlight module in a liquid crystal display. Color-band was observed when this coated film was assembled in a back light display device and covered by crossed polarizer films.

Example 2

[0068] Polycarbonate resins were extruded at 270.degree. C. into base films comprising a thickness of about 175 micrometers. The film was extruded between a polished chrome calendering roll maintained at 127.degree. C. and a steel calendering roll coated with 0.5 inch (about 1.3 cm) thick, 70 durometer (Shore A) silicone rubber calendering roll cooled with water at temperature of 43.degree. C. A base film was achieved with stress retardation gradient less than 20 nanometers per inch retardation and 40% haze at a line speed of 19 feet per minute (ft/min) (about 5.8 meters per min (m/min)). It is noted that in constructing a stress profile from which the stress retardation gradient was obtained, stress retardation was measured at every 0.25 inches (0.64 centimeters) across the length of the film.

[0069] This base film was then coated with a coating comprising about 60 wt. % brominated epoxy acrylate, and about 40 wt. % phenylthiolethylacrylate, with a trace amount of a photoinitiator (i.e., a bis(acyl)phosphine oxide sold as IRGACURE 819, commercially available from Ciba Geigy, Inc., wherein weight percents were based on a total weight of the coating. The coating composition was applied to the bottom-masked base film by gravure roll at a thickness of about 30 micrometers. After the coating was applied to the film, prismatic structures were formed by curing the coating while it was in contact with a cast comprising a surface with the negative image of the desired surface structures as described above. The film was then masked, and subsequently converted into a format suitable for handling and assembly in a backlight module in a liquid crystal display. Substantially the same luminance was obtained after replacing a brightness enhancement film made from a polish/polish base film in a backlight display device with a brightness enhancement film comprising a matte/polish base film. More particularly, the luminance at a zero degree view angle (i.e., on-axis) was measured (with the liquid crystal panel removed from a backlight display device) using an Eldim EZ Contrast 160D instrument, and found to be about 102% to about 104% of the value obtained when a commercial brightness enhancement film was used, i.e., a BEF II film commercially available from 3M, Inc. No color-band was observed when this coated film was assembled in a back light display device and covered by crossed polarizer films.

Example 3

[0070] Polycarbonate resins and an antistatic agent tetrabutylphosphonium perfluorobutylsulfonate ("FC-1") present in an amount of about 1.1 wt. % based on a total weight of the blend were extruded at 263.degree. C. into base films comprising a thickness of about 125 micrometers. The film was extruded between a polished chrome calendering roll maintained at 135.degree. C. and a steel calendering roll coated with 0.5 inch (about 1.3 cm) thick, 70 durometer (Shore A) silicone rubber calendering roll cooled with water at a temperature of 40.degree. C. A base film was achieved with less than 15 nm/in stress retardation gradient and 40% haze at a line speed of 16 feet per minute (ft/min) (about 4.9 meters per min (m/min)). A coating was applied in the same manner and using the same coating material as discussed in Example 1. In this example however, the surface static decay of the resulting film decreased by 3 to 4 orders of magnitude. The film was masked, and subsequently converted into a format suitable for handling and assembly in a backlight module in a liquid crystal display. Substantially the same luminance was obtained after replacing a brightness enhancement film made from a polish/polish base film in a backlight display device with a brightness enhancement film comprising a matte/polish base film. No color-band was observed when this coated film was assembled in a back light display device and covered by polarizer films.

Example 4

[0071] Polycarbonate resins and an antistatic agent tetrabutylphosphonium perfluorobutylsulfonate ("FC-1") present in an amount of about 1.1 wt. % based on a total weight of the blend were extruded at 263.degree. C. into base films comprising a thickness of about 125 micrometers. The film was extruded between a velvet steel calendering roll maintained at 135.degree. C. and a steel calendering roll coated with 0.5 inch (about 1.3 cm) thick, 70 durometer (Shore A) silicone rubber calendering roll cooled with water at a temperature of 40.degree. C. A base film was achieved with less than 15 nm/in stress retardation gradient and 45% haze at a line speed of 12 feet per minute (ft/min) (about 3.7 meters per min (m/min)). The base film had a matte surface on a first surface and a velvet texture on a second surface. The first surface of the base film was then coated with a coating comprising about 60 wt. % brominated epoxy acrylate, and about 40 wt. % phenylthiolethylacrylate, with a trace amount of a photoinitiator (e.g., same as that discussed above in Example 1), wherein weight percents are based on a total weight of the coating. After the coating was applied to the film, prismatic structures were formed by curing the coating while it was in contact with cast comprising a surface with the negative image of the desired surface structures as described above. The film was then masked, and subsequently converted into a format suitable for handling and assembly in a backlight module in a liquid crystal display.

[0072] It was noted that in a backlight module, most defects (scratches, point defects, ripples, and the like) generated during current processing and handling conditions, which were visible in a polish/polish base film, were not visible in this textured base film. Additionally, the luminance at a zero degree view angle (i.e., on-axis) was measured (with the liquid crystal panel removed from a backlight display device) using an Eldim EZ Contrast 160D instrument, and found to be about 92% of the value obtained when a commercial brightness enhancement film was used, i.e., a BEF II film commercially available from 3M, Inc. Grid lines from light guide were not visible after replacing a bottom diffuser with the brightness enhancement film described above. Additionally, it is noted that similar results were also obtained when the second surface comprised a matte surface.

[0073] Advantageously, embodiments of the brightness enhancement films disclosed herein do not produce color bands when employed in a flat panel display device (e.g., a backlight display devices), and perform equivalent to a polish/polish. For example, it is noted that color bands were observed in comparative example 1, but not color bands were observed in Examples 2-4. Additionally, it is noted that embodiments are disclosed herein wherein the brightness enhancement films comprising a base film comprising matte/polish surfaces can comprise a relative luminance of about 99.5% to about 100.5% compared to a base film comprising polish/polish surfaces. In other words, the brightness enhancement films disclosed herein comprise substantially the same brightness performance as a polish/polish film, but have the advantage of not producing color bands.

[0074] Other embodiments are disclosed herein wherein at least one light-diffusing film, or brightness enhancement film can be eliminated in a backlight display device. In other words, the brightness enhancement film can be a multifunctional brightness enhancement film acting as both a traditional brightness enhancement film, for example, to direct light along a viewing axis (i.e., an axis normal (perpendicular) to the display), and as a light diffusing film. Moreover, embodiments are envisioned where a plurality of brightness enhancement films and a plurality of light-diffusing films can be replaced by one or more multifunctional film, thereby advantageously reducing the overall thickness of the backlight display device.

[0075] Additionally, it is noted that embodiments are disclosed herein comprising anti-static agent. As noted above, the static decay can be decrease by 3 to 4 orders of magnitude compared to embodiments not comprising the anti-static agent.

[0076] Furthermore, in various embodiments, the base film of the brightness enhancement film comprises polycarbonate, e.g., the base film comprise greater than or equal to about 80 wt. % polycarbonate, and more particularly greater than or equal to about 90 wt. % polycarbonate, wherein weight percents are based on a total weight of the base film. Compared to commercially available brightness enhancement films comprising polyethylene terephthalate (PET), the brightness enhancement films disclosed herein comprising polycarbonate have superior long-term stability. For example, in a thermal cycle test, a polycarbonate base film can out perform the PET base film, i.e., the film flatness of the polycarbonate can remain more flat compared to the PET film. Generally, in the thermal cycle test, a base film is placed in a chamber where the temperature is cycled between 85.degree. C. and -35.degree. C. (minimal moisture content in the air, e.g., less than or equal to 60% relative humidity), with the temperature held at each extreme for 1 hour and then changed to the other extreme at a rate of 2.degree. C. per minute. Generally, 100 such cycles are run and then the base films are compared.

[0077] While the invention has been described with reference to several embodiments thereof, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A brightness enhancement film comprising: a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, wherein a first surface of the base film is textured, and wherein a light-redirecting structure is disposed on the first surface of the base film.

2. The brightness enhancement film of claim 1, wherein the stress retardation gradient is calculated to be less than or equal to 30 nanometers per inch.

3. The brightness enhancement film of claim 2, wherein the stress retardation gradient is calculated to be less than or equal to 15 nanometers per inch.

4. The brightness enhancement film of claim 1, wherein the first surface comprises a surface roughness (Ra) of 0.3 micrometers to 2.2 micrometers.

5. The brightness enhancement film of claim 1, wherein the second surface is a polished surface comprising a surface roughness (Ra) of less 0.3 micrometers.

6. The brightness enhancement film of claim 1, wherein the second surface is a textured surface comprising a surface roughness (Ra) greater than 0.3 micrometers.

7. The brightness enhancement film of claim 1, wherein the base film comprises a haze value of about 20% to about 80% as measured according to ASTM D1003, and a transmission of greater than or equal to about 85%.

8. The brightness enhancement film of claim 1, wherein the base film comprises a haze value of less than or equal to about 50% as measured according to ASTM D1003, and a transmission of greater than or equal to about 89%.

9. The brightness enhancement film of claim 1, further comprising a curable coating disposed on the first surface of the base film, wherein the curable coating comprises an index of refraction of greater than or equal to about 1.5

10. The brightness enhancement film of claim 9, wherein the curable coating comprises a polymerizable compound comprising a functional group selected from the group consisting of acrylate, methacrylate, vinyl, and epoxide.

11. The brightness enhancement film of claim 9, wherein the curable coating further comprises metal oxide nanoparticles.

12. The brightness enhancement film of claim 1, wherein the base film comprises polycarbonate and phosphonium sulfonate.

13. The brightness enhancement film of claim 12, wherein the phosphonium sulfonate is fluorinated phosphonium sulfonate.

14. The brightness enhancement film of claim 1, wherein the light-redirecting structure is a random prismatic structure, and wherein the random prismatic structure comprises a plurality of prisms each comprising a peak comprising a radius of curvature of about 0.1% to about 30% of a pitch of the prismatic structure.

15. A brightness enhancement film comprising: a thermoplastic base film comprising greater than or equal to about 80 wt. % polycarbonate, wherein weight percents are based on a total weight of the thermoplastic base film, wherein a stress retardation gradient of the thermoplastic base film is calculated to be less than or equal to 15 nanometers per inch, and wherein a light-redirecting structure is disposed on a first surface of the thermoplastic base film.

16. A brightness enhancement film comprising: a thermoplastic base film comprising about 93 wt. % to about 99.6 wt. % polycarbonate; and about 0.4 wt. % to about 7 wt. % fluorinated phosphonium sulfonate, wherein weight percents are based on a total weight of the thermoplastic base film, and wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, and wherein a light-redirecting structure is disposed on a first surface of the base film.

17. The brightness enhancement film of claim 16, wherein the first surface of the base film is textured.

18. The brightness enhancement film of claim 17, wherein a second surface is textured.

19. A method of making a brightness enhancement film comprising: disposing a light-redirecting structure onto a first surface of a base film, wherein a stress retardation gradient of the thermoplastic base film is calculated to be less than or equal to 50 nanometers per inch, wherein the first surface of the thermoplastic base film is textured.

20. The method of claim 19, further comprising forming the base by melting a thermoplastic resin at a temperature greater than or equal to a glass transition temperature of the thermoplastic resin; extruding the thermoplastic resin into a gap between a first calendering roll and a second calendering roll, wherein the first calendering roll comprises a textured surface and an elastomeric material; and cooling the thermoplastic resin to a temperature below the glass transition temperature of the thermoplastic resin to produce the thermoplastic base film.

21. The method of claim 19, further comprising coating the first surface of the base film with a curable coating material.

22. A method of making a brightness enhancement film comprising: disposing a light-redirecting structure onto a surface of a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 15 nanometers per inch.

23. A method of making a brightness enhancement film comprising: disposing a light-redirecting structure onto a surface of a thermoplastic base film comprising about 93 wt. % to about 99.6 wt. % polycarbonate; and about 0.4 wt. % to about 7 wt. % fluorinated phosphonium sulfonate, wherein weight percents are based on a total weight of the thermoplastic base film, and wherein a stress retardation gradient of the thermoplastic base film is calculated to be less than or equal to 50 nanometers per inch.

24. A display device comprising: an optical source; a light guide in optical and physical communication with the light source; and a brightness enhancement film comprising a base film, wherein a stress retardation gradient of the base film is calculated to be less than or equal to 50 nanometers per inch, wherein a first surface of the base film is textured, and wherein a light-redirecting structure is disposed on the first surface of the base film.