Patent Application
20080055724
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
The present invention is applicable to optical systems and is more particularly applicable to polarized optical systems.
As used herein, the terms “specular reflection” and “specular reflectance” refer to the reflectance of light rays from a body where the angle of reflection is substantially equal to the angle of incidence, where the angles are measured relative to a normal to the body's surface. In other words, when the light is incident on the body with a particular angular distribution, the reflected light has substantially the same angular distribution. The terms “diffuse reflection” or “diffuse reflectance” refer to the reflection of rays where the angle of some of the reflected light is not equal to the angle of incidence. Consequently, when light is incident on the body with a particular angular distribution, the angular distribution of the reflected light is different from that of the incident light. The terms “total reflectance” or “total reflection” refer to the combined reflectance of all light, specular and diffuse.
Similarly, the terms “specular transmission” and “specular transmittance” are used herein in reference to the transmission of light through a body where the angular distribution of the transmitted light is substantially the same as that of the incident light. The terms “diffuse transmission” and “diffuse transmittance” are used to describe the transmission of light through a body, where the transmitted light has an angular distribution that is different from the angular distribution of the incident light. The terms “total transmission” or “total transmittance” refer to the combined transmission of all light, specular and diffuse.
A reflective polarizer film 100 is schematically illustrated in
In the embodiment illustrated in
A cut-away view through a reflective polarizer body according to an exemplary embodiment of the present invention is schematically presented in
Polarizing fibers 204 are disposed within the matrix 202. The polarizing fibers 204 comprise at least two polymer materials, at least one of which is birefringent. In some exemplary embodiments, one of the materials is birefringent while the other material, or materials, is/are isotropic. In other embodiments, two or more of the materials forming the fiber are birefringent. In some embodiments, fibers formed of isotropic materials may also be present within the matrix 202.
The polarizing fibers 204 may be organized within the matrix 202 as single fibers, as illustrated, or in many other arrangements. Some exemplary arrangements include yarns, a tow (of fibers or yarns) arranged in one direction within the polymer matrix, a weave, a non-woven, chopped fiber, a chopped fiber mat (with random or ordered formats), or combinations of these formats. The chopped fiber mat or nonwoven may be stretched, stressed, or oriented to provide some alignment of the fibers within the nonwoven or chopped fiber mat, rather than having a random arrangement of fibers. The formation of a polarizer having an arrangement of polarizing fibers with a matrix is described more fully in U.S. patent application Ser. No. 11/068,157, filed on <date> and incorporated by reference herein.
The refractive indices in the x-, y-, and z-directions for the first fiber material may be referred to as n1x, n1y and n1z, and the refractive indices in the x-, y-, and z- directions for the second fiber material may be referred to as n2x, n2y and n2z. Where the material is isotropic, the x-, y-, and z-refractive indices are all substantially matched. Where the first fiber material is birefringent, at least one of the x-, y- and z- refractive indices is different from the others.
Within each fiber 204 there are multiple interfaces formed between the first fiber material and the second fiber material. When at least one of the first and second fiber materials is birefringent, the interface may be referred to as a birefringent interface. For example, if the two materials present their x-and y-refractive indices at the interface, and n1x≠n1y, i.e. the first material is birefringent, then the interface may be birefringent. Different exemplary embodiments of the polymer fibers containing birefringent interfaces are discussed below.
The fibers 204 are disposed generally parallel to an axis, illustrated as the x-axis in the figure. The refractive index difference at the birefringent interfaces within the fibers 204 for light polarized parallel to the x-axis, n1x−n2x, may be different from the refractive index difference for light polarized parallel to the y-axis, n1y−n2y. The interface is said to be birefringent when the difference in refractive index at the interface is different for different directions. Thus, for a birefringent interface, Δnx≠Δny, where Δnx=|n1x−n2x| and Δny=|n1y−n2y|.
For one polarization state, the refractive index difference at the birefringent interfaces in the fibers 204 may be relatively small. In some exemplary cases, the refractive index difference may be less than 0.05. This condition is considered to be substantially index-matched. This refractive index difference may be less than 0.03, less than 0.02, or less than 0.01. If this polarization direction is parallel to the x-axis, then x-polarized light passes through the body 200 with little or no reflection. In other words, x-polarized light is highly transmitted through the body 200.
The refractive index difference at the birefringent interfaces in the fibers may be relatively high for light in the orthogonal polarization state. In some exemplary examples, the refractive index difference may be at least 0.05, and may be greater, for example 0.1, or 0.15 or may be 0.2. If this polarization direction is parallel to the y-axis, then y-polarized light is reflected at the birefringent interfaces. Thus, y-polarized light is reflected by the body 200. If the birefringent interfaces within the fibers 204 are substantially parallel to each other, then the reflection may be essentially specular. If, on the other hand, the birefringent interfaces within the fibers 204 are not substantially parallel to each other, then the reflection may be substantially diffuse. Some of the birefringent interfaces may be parallel, and other interfaces may be non-parallel, which may lead to the reflected light containing both specular and diffuse components. Also, a birefringent interface may be curved, or relatively small, in other words within an order of magnitude of the wavelength of the incident light, which may lead to diffuse scattering.
While the exemplary embodiment just described is directed to index matching in the x-direction, with a relatively large index difference in the y-direction, other exemplary embodiments include index matching in the y-direction, with a relatively large index difference in the x-direction.
The polymer matrix 202 may be substantially optically isotropic, for example having a birefringence, n3x−n3y, of less than about 0.05, and preferably less than 0.01, where the refractive indices in the matrix for the x- and y-directions are n3x and n3y respectively. In other embodiments, the polymer matrix 202 may be birefringent. Consequently, in some embodiments, the refractive index difference between the polymer matrix and the fiber materials may be different in different directions. For example, the x-refractive index difference, n1x−n3x, may be different from the y-refractive index difference, n1y−n3y. In some embodiments, one of these refractive index differences may be at least twice as large as the other refractive index difference.
The magnitude of refractive index difference, the extent and shape of the birefringent interfaces, the relative positions of the birefringent interfaces and the density of birefringent interfaces all affect the scattering, determining whether the scattering is predominantly forwards, backwards, or a combination of the two. Where the refractive index difference for a first polarization state is small compared to the second polarization state, light in the first polarization state may be primarily transmitted specularly or diffusely (forward scattered), while light in the second polarization state is primarily diffusely reflected (back scattered).
Suitable materials for use in the polymer matrix and/or in the fibers include thermoplastic and thermosetting polymers that are transparent over the desired range of light wavelengths. In some embodiments, it may be particularly useful that the polymers be non-soluble in water. Further, suitable polymer materials may be amorphous or semi-crystalline, and may include homopolymer, copolymer or blends thereof. Example polymer materials include, but are not limited to, poly(carbonate) (PC); syndiotactic and isotactic poly(styrene) (PS); C1-C8 alkyl styrenes; alkyl, aromatic, and aliphatic ring-containing (meth)acrylates, including poly(methylmethacrylate) (PMMA) and PMMA copolymers; ethoxylated and propoxylated (meth)acrylates; multifunctional (meth)acrylates; acrylated epoxies; epoxies; and other ethylenically unsaturated materials; cyclic olefins and cyclic olefinic copolymers; acrylonitrile butadiene styrene (ABS); styrene acrylonitrile copolymers (SAN); epoxies; poly(vinylcyclohexane); PMMA/poly(vinylfluoride) blends; poly(phenylene oxide) alloys; styrenic block copolymers; polyimide; polysulfone; poly(vinyl chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; unsaturated polyesters; poly(ethylene), including low birefringence polyethylene; poly(propylene) (PP); poly(alkane terephthalates), such as poly(ethylene terephthalate) (PET); poly(alkane napthalates), such as poly(ethylene naphthalate)(PEN); polyamide; ionomers; vinyl acetate/polyethylene copolymers; cellulose acetate; cellulose acetate butyrate; fluoropolymers; poly(styrene)-poly(ethylene) copolymers; PET and PEN copolymers, including polyolefinic PET and PEN; and poly(carbonate)/aliphatic PET blends. The term (meth)acrylate is defined as being either the corresponding methacrylate or acrylate compounds. With the exception of syndiotactic PS, these polymers may be used in an optically isotropic form.
Several of these polymers may become birefringent when oriented. In particular, PET, PEN, and copolymers thereof, and liquid crystal polymers, manifest relatively large values of birefringence when oriented. Polymers may be oriented using different methods, including extrusion and stretching. Stretching is a particularly useful method for orienting a polymer, because it permits a high degree of orientation and may be controlled by a number of easily controllable external parameters, such as temperature and stretch ratio. The refractive indices for a number of exemplary polymers, oriented and unoriented, are provided in Table I below.
PCTG and PETG (a glycol-modified polyethylene terephthalate) are types of copolyesters available from, for example, Eastman Chemical Co., Kingsport, Tenn., under the Eastar™ brand name. THV is a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride, available from 3M Company, St. Paul, Minn., under the brand name Dyneon™. The PS/PMMA copolymer is an example of a copolymer whose refractive index may be “tuned” by changing the ratio of the constituent monomers in the copolymer to achieve a desired value of refractive index. The column labeled “S.R.” contains the stretch ratio. A stretch ratio of 1 means that the material is unstretched and unoriented. A stretch ratio of 6 means that sample was stretched to six times it original length. If stretched under the correct temperature conditions, the polymeric molecules are oriented and the material becomes birefringent. It is possible, however, to stretch the material without orienting the molecules. The column labeled “T” indicates the temperature at which the sample was stretched. The stretched samples were stretched as sheets. The columns labeled nx, ny and nz refer to the refractive indices of the material. Where no value is listed in the table for ny and nz, the values of ny and nz are the same as for nx.
The behavior of the refractive index under stretching a fiber is expected to give results similar to, but not necessarily the same as, those for stretching a sheet. Polymer fibers may be stretched to any desired value that produces desired values of refractive index. For example, some polymer fibers may be stretched to produce a stretch ratio of at least 3, and maybe at least 6. In some embodiments, polymer fibers may be stretched even more, for example to a stretch ratio of up to 20, or even more.
A suitable temperature for stretching to achieve birefringence is approximately 80% of the polymer melting point, expressed in Kelvins. Birefringence may also be induced by stresses induced by flow of the polymer melt experienced during extrusion and film formation processes. Birefringence may also be developed by alignment with adjacent surfaces such as fibers in the film article. Birefringence may either be positive or negative. Positive birefringence is defined as when the direction of the electric field axis for linearly polarized light experiences the highest refractive index when it is parallel to the polymer's orientation or aligning surface. Negative birefringence is defined as when the direction of the electric field axis for linearly polarized light experiences the lowest refractive index when it is parallel to the polymer's orientation or aligning surface. Examples of positively birefringent polymers include PEN and PET. An example of a negatively birefringent polymer includes syndiotactic polystyrene.
The matrix 202 and/or the polymer fibers 204 may be provided with various additives to provide desired properties to the body 200. For example, the additives may include one or more of the following: an anti-weathering agent, UV absorbers, a hindered amine light stabilizer, an antioxidant, a dispersant, a lubricant, an anti-static agent, a pigment or dye, a nucleating agent, a flame retardant and a blowing agent. Other additives may be provided for altering the refractive index of the polymer or increasing the strength of the material. Such additives may include, for example, organic additives such as polymeric beads or particles and polymeric nanoparticles, or inorganic additives, such as glass, ceramic or metal-oxide nanoparticles, or milled, powered, bead, flake or particulate glass, ceramic or glass-ceramic. The surface of these additives may be provided with a binding agent for binding to the polymer. For example, a silane coupling agent may be used with a glass additive to bind the glass additive to the polymer.
In some embodiments, it may be preferable that the matrix 202 or a component of the fibers 204 be non-soluble, or at least resistant to solvents. Examples of suitable materials that are solvent resistant include polypropylene, PET and PEN. In other embodiments it may be preferable that the matrix 202 or component of the polymer fibers 204 is soluble in an organic solvent. For example, a matrix 202 or fiber component formed of polystyrene is soluble in an organic solvent such as acetone. In other embodiments, it may be preferable that the matrix is water soluble. For example, a matrix 202 or fiber component formed of polyvinyl acetate is soluble in water.
The refractive index of the materials in some embodiments of optical element may vary along the length of the fiber, in the x-direction. For example, the element may not be subject to uniform stretching, but may be stretched to a greater degree in some regions than in others. Consequently, the degree of orientation of the orientable materials is not uniform along the element, and so the birefringence may vary spatially along the element.
Furthermore, the incorporation of fibers within the matrix may improve the mechanical properties of the optical element. In particular, some polymeric materials, such as polyester, are stronger in the form of a fiber than in the form of a film, and so an optical element containing fibers may be stronger than one of similar dimensions that contains no fibers.
The fibers 204 may be straight, but need not be straight, for example the fibers 204 may be kinked, spiraled or crimped.
A polarizer layer that transmits light of one polarization, either specularly, diffusely or both, and that reflects light of the orthogonal polarization state, may be used in various types of display system. One type of display system 300 that may use such a polarizer is a direct-lit display system schematically illustrated in
An upper absorbing polarizer 308 is positioned above the LC layer 304 and a lower absorbing polarizer 310 is positioned below the LC layer 304. Selective activation of different pixels of the LC layer 304, for example by an attached controller 314, results in the light passing out of the display system 300 at certain desired locations, thus forming an image seen by the viewer. The controller 314 may include, for example, a computer or a television controller that receives and displays television images. One or more optional layers 309 may be provided over the upper absorbing polarizer 308, for example to provide mechanical and/or environmental protection to the display surface. In one exemplary embodiment, the layer 309 may include a hardcoat over the absorbing polarizer 308.
The backlight 312 provides light for the display system 300 behind the LC panel 302. In this embodiment, the backlight 312 includes a number of light sources 316 disposed behind the LC panel 302, in the so-called “direct-lit” configuration. The light sources 316 often used in a LCD-TV or LCD monitor are linear, cold cathode, fluorescent tubes that extend along the height of the display system 300. Other types of light sources may be used, however, such as filament or arc lamps, light emitting diodes (LEDs), flat fluorescent panels or external fluorescent lamps. This list of light sources is not intended to be limiting or exhaustive, but only exemplary.
The backlight 312 may include a reflector 318 for reflecting light propagating downwards from the light sources 316, in a direction away from the LC panel 302. The reflector 318 may also be useful for recycling light within the display system 300, as is explained below. The reflector 318 may be a specular reflector or may be a diffuse reflector. One example of a specular reflector is Vikuiti™ Enhanced Specular Reflection (ESR) film available from 3M Company, St. Paul, Minn. Examples of suitable diffuse reflectors include polymers, such as PET, PC, PP, PS loaded with diffusely reflective particles, such as titanium dioxide, barium sulphate, calcium carbonate or the like.
An arrangement 320 of light management films, which may also be referred to as a light management unit, is positioned between the backlight 312 and the LC panel 302. The light management films affect the light propagating from backlight 312 so as to improve the operation of the display system 300. For example, the arrangement 320 of light management films includes a diffuser plate 322. The diffuser plate 322 is used to diffuse the light received from the light sources, which results in an increase in the uniformity of the illumination light incident on the LC panel 302.
The light management unit 320 may also include a reflective polarizer layer 324. The light sources 316 typically produce unpolarized light but the lower absorbing polarizer 310 only transmits a single polarization state, and so about half of the light generated by the light sources 316 is not transmitted through to the LC layer 304. The reflecting polarizer 324, however, may be used to reflect the light that would otherwise be absorbed in the lower absorbing polarizer, and so this light may be recycled by reflection between the reflecting polarizer 324 and the reflector 318. At least some of the light reflected by the reflecting polarizer 324 may be depolarized, and subsequently returned to the reflecting polarizer 324 in a polarization state that is transmitted through the reflecting polarizer 324 and the lower absorbing polarizer 310 to the LC layer 304. In this manner, the reflecting polarizer 324 may be used to increase the fraction of light emitted by the light sources 316 that reaches the LC layer 304, and so the image produced by the display system 300 is brighter. The reflective polarizer layer may be, for example, a layer like that shown in
A polarization control layer 326 may be provided in some exemplary embodiments, for example between the diffuser layer 322 and the reflective polarizer 324. Examples of polarization control layer 326 include a quarter wave retarding layer and a polarization rotating layer, such as a liquid crystal polarization rotating layer. A polarization control layer 326 may be used to change the polarization of light that is reflected from the reflective polarizer 324 so that an increased fraction of the recycled light is transmitted through the reflective polarizer 324.
The arrangement 320 of light management layers may also include one or more brightness enhancing layers. A brightness enhancing layer is one that includes a surface structure that redirects off-axis light in a direction closer to the axis of the display. This increases the amount of light propagating on-axis through the LC layer 304, thus increasing the brightness of the image seen by the viewer. One example is a prismatic brightness enhancing layer, which has a number of prismatic ridges that redirect the illumination light, through refraction and reflection. Examples of prismatic brightness enhancing layers that may be used in the display device include the Vikuiti™ BEFII and BEFIII family of prismatic films available from 3M Company, St. Paul, Minn., including BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, and BEFIIIT.
The exemplary embodiment shows a first brightness enhancing layer 328a disposed between the reflective polarizer 324 and the LC panel 302. A prismatic brightness enhancing layer typically provides optical gain in one dimension. A second brightness enhancing layer 328b may also be included in the arrangement 320 of light management layers, having its prismatic structure oriented orthogonally to the prismatic structure of the first brightness enhancing layer 328a. Such a configuration provides an increase in the optical gain of the display unit in two dimensions. In other exemplary embodiments, the brightness enhancing layers 328a, 328b may be positioned between the backlight 312 and the reflective polarizer 324.
Another display system 350 is schematically illustrated in
The arrangement 354 of light management films may include layers like those used in a direct-lit configuration, although some layers may be omitted. For example, only a single brightness enhancing layer 328 may be used. Also, the diffuser layer 322 may be omitted. Additionally, the edge-lit display 350 may include a turning film 360 for directing light emitted by the light guide 358 into a direction towards the LC panel 302.
The polarizer layer may include fibers that are arranged within the matrix in many different ways. For example, the fibers may be positioned randomly across the cross-sectional area of the matrix, for example as is shown for fibers 204 in matrix 202, shown in
In another exemplary embodiment, schematically illustrated in cross-section in
In another embodiment of optical element 420, schematically illustrated in
While the fibers may all be substantially parallel to the x-axis, this need not be the case, and some fibers may lie with greater or smaller angles to the x-axis. For example, in the example optical element 430 illustrated in
Such an arrangement may be useful where the fibers in one row are effective for light in a first wavelength band and the fibers in another row are effective for light in a second wavelength band different from the first wavelength band. Consider the illustrative example where the fibers 434 in the first row 436a are effective at reflectively polarizing light in a red bandwidth and the fibers 434 in the second row 436b are effective at reflectively polarizing light in a blue bandwidth. Therefore, where the optical element 430 is illuminated with a mixture of red and blue light, the first row 436a of fibers 434 passes all the blue light while transmitting red light polarized at the angle θ1. The second row 436a of fibers 434 would transmit the red light polarized at the angle θ1 while also transmitting blue light polarized parallel to the angle θ2. Where the angles θ1 and θ2 are separated by 90°, the element 430 transmits red light in one polarization state and blue light in the orthogonal polarization state. Likewise, the reflected blue light is polarized orthogonally to the reflected red light. It will be appreciated that different numbers of rows of fibers 434 may be aligned at each angle, and be used for each color band.
In some embodiments, the density of the fibers may be constant within the optical element or may vary within the optical element. For example, the density of fibers may decrease from one side of the optical element, or may vary in some other manner. In the embodiment schematically illustrated in
Such a variation in the fill factor may be useful, for example, to improve the uniformity of light transmitted through the element 440 from a light source 446. This may be important, for example, where the element 440 is included in a direct view screen lit by discrete light sources: in such devices it is important to present the viewer with an image of uniform illumination. When a light source is placed behind a uniform diffuser, the brightness of the light transmitted through the diffuser is highest above the light source. The variation in fill factor illustrated in
In other embodiments, some fiber optical property may vary across the optical element. Thus, instead of, or in addition to, the fiber density varying across the optical element, some other property of the fiber may be varied. For example, polarizing fibers that diffusely transmit light more may be used in some regions of the optical element while polarizing fibers that diffusely transmit light less may be used in other portions of the optical element. In other examples, the amount of light back-scattered by a fiber, or the spectrum of the light back-scattered by a fiber, at one position of the optical element may be different from one or more of those properties of a fiber at another position of the optical element. Thus, fiber optical properties that may be varied across the optical element include the amount of diffuse transmission, the amount of backscattering and the back-scattering spectrum.
The optical element may have flat surfaces, for example the flat surfaces parallel to the x-y plane as shown in
Structured surfaces may be formed on the matrix using any suitable method. For example, the matrix may be cured while its surface is in contact with the surface of a tool, such as a microreplication tool, whose tool surface produces the desired shape on the surface of the polymer matrix. Furthermore, the polarizing fibers 454 may be located within the prismatic surface structures 457.
Another exemplary embodiment of the invention is schematically illustrated in
In different embodiments of polarizer, different fibers within a polarizer may be designed to preferentially reflect light in one polarization state in different wavelength ranges. For example, one set of polarizing fibers within the polarizer may reflect light with a reflectivity peak at a first wavelength while a second set of fibers within the polarizer reflects light with a reflectivity peak at a second wavelength different from the first wavelength. To illustrate, one set of fibers may have a broad reflectivity peak for blue and/or green wavelengths while another set of fibers has a broad reflectivity peak for green and/or red wavelengths. In such a case, the two sets of fibers together may provide polarized reflection over a broad wavelength range.
In addition, the reflection spectrum of different sets of fibers may be set to reflect light at different intensity peaks of the spectrum of light produced by the light sources used in the display system. For example, where the light source generates light having intensity peaks at two different wavelengths, the reflectance spectrum of one set of fibers may be matched to one intensity peak while the reflectance spectrum of another set of fibers is matched to the second intensity peak.
In the different embodiments of polarizer discussed above, and other embodiments encompassed by the invention, some or all of the fibers present in the polarizer layer may be polymeric polarizing fibers. In other embodiments, some of the fibers may be formed of an isotropic material, such as an isotropic polymer or an inorganic material, such as glass, ceramic or glass-ceramic. The use of inorganic fibers in a film is discussed more detail in U.S. patent application Ser. No. 11/125,580, incorporated herein by reference. Inorganic fibers provide additional stiffness to a polarizer layer, and resistance to curling and shape changes under differential conditions of humidity and/or temperature.
In some embodiments, the inorganic fiber material has a refractive index that matches the refractive index of the matrix, and in other embodiments the inorganic fiber has a refractive index that is different from the refractive index of the matrix. Any transparent type of glass may be used, including high quality glasses such as E-glass, S-glass, BK7, SK10 and the like. Some ceramics also have crystal sizes that are sufficiently small that they can appear transparent if they are embedded in a matrix polymer with an index of refraction appropriately matched. The Nextel™ Ceramic fibers, available from 3M Company, St. Paul, Minn., are examples of this type of material, and are already available as thread, yarn and woven mats. Glass-ceramics of interest have compositions including, but not limited to, Li2O—Al2O3—SiO2, CaO—Al2O3—SiO2, Li2O—MgO—ZnO—Al2O3—SiO2, Al2O3—SiO2, and ZnO—Al2O3—ZrO2—SiO2, Li2O—Al2O3—SiO2, and MgO—Al2O3—SiO2.
In one exemplary embodiment the birefringent material is of a type that undergoes a change in refractive index upon orientation. Consequently, as the fiber is oriented, refractive index matches or mismatches are produced along the direction of orientation. By careful manipulation of orientation parameters and other processing conditions, the positive or negative birefringence of the birefringent material can be used to induce diffuse reflection or transmission of one or both polarizations of light along a given axis. The relative ratio between transmission and diffuse reflection is dependent on a number of factors such as, but not limited to, the concentration of the birefringent interfaces in the fiber, the dimension of the fiber, the square of the difference in the index of refraction at the birefringent interfaces, the size and geometry of the birefringent interfaces, and the wavelength or wavelength range of the incident radiation.
The magnitude of the index match or mismatch along a particular axis affects the degree of scattering of light polarized along that axis. In general, the scattering power varies as the square of the index mismatch. Thus, the larger the mismatch in refractive index along a particular axis, the stronger the scattering of light polarized along that axis. Conversely, when the mismatch along a particular axis is small, light polarized along that axis is scattered to a lesser extent and the transmission through the volume of the body becomes increasingly specular. Diffusion transmission is related to haze, which can be measured by many commercially available haze-meters and is defined according to ASTM D1003. A common tool for measuring haze is the BYK Gardner Haze-Gard Plus (Cat. No. 4725), which defines haze as the fraction of light transmitted that is scattered outside an 8° cone divided by the total amount of light transmitted. In some of the polarizer films according to the present invention, the haze is at least 10%, and may be at least 30% or at least 50%.
If the index of refraction of the non-birefringent material matches that of the birefringent material along some axis, then incident light polarized with electric fields parallel to this axis will pass through the fiber unscattered regardless of the size, shape, and density of the portions of birefringent material. In addition, if the refractive index along that axis is also substantially matched to that of the polymer matrix of the polarizer body, then the light passes through the body substantially unscattered. For purposes of this disclosure, substantial matching between two refractive indices occurs when the difference between the indices is less than at most 0.05, and preferably less than 0.03, 0.02 or 0.01.
If the indices between the birefringent material and non-birefringent material are not matched along some axis, then the fiber scatters or reflects light polarized along this axis. The strength of the scattering is determined, at least in part, by the magnitude of the index mismatch for scatterers having a given cross-sectional area with dimensions larger than approximately λ/30, where A is the wavelength of the incident light in the polarizer. The exact size, shape and alignment of a mismatched interface play a role in determining how much light will be scattered or reflected into various directions from that interface. If the density and thickness of the scattering layer is sufficient, according to multiple scattering theory, incident light will be either reflected or absorbed, but not transmitted, regardless of the details of the scatterer size and shape.
Prior to use in the polarizer, the fibers are preferably processed by stretching and allowing some dimensional relaxation in the cross stretch in-plane direction, so that the index of refraction difference between the birefringent material and the non-birefringent materials are relatively large along a first axis and small along the other two orthogonal axes. This results in a large optical anisotropy for electromagnetic radiation of different polarizations.
Some of the polarizers within the scope of the present invention are elliptically diffusing polarizers. In general, elliptically diffusing polarizers use fibers having a difference in index of refraction between the birefringent and non-birefringent materials along both the stretch and non-stretch directions, and may diffusely transmit or reflect light of one polarization. The birefringent material in the fiber may also form birefringent interfaces with the polymer matrix material, in which case these interfaces may also include an index mismatch for both the stretch and cross-stretch directions.
The ratio of forward-scattering to back-scattering is dependent on the difference in refractive index between the birefringent and non-birefringent materials, the concentration of the birefringent interfaces, the size and shape of the birefringent interfaces, and the overall thickness of the fiber. In general, elliptical diffusers have a relatively small difference in index of refraction between the birefringent and non-birefringent materials.
The materials selected for use in the fibers in accordance with the present invention, and the degree of orientation of these materials, are preferably chosen so that the birefringent and non-birefringent materials in the finished fiber have at least one axis for which the associated indices of refraction are substantially equal. The match of refractive indices associated with that axis, which typically, but not necessarily, is an axis transverse to the direction of orientation, results in substantially no reflection of light at the internal fiber interfaces in that plane of polarization. A degree of intentional mismatching of refractive index for this plane, however, may be used to create some degree of light diffusion, as described elsewhere
One exemplary embodiment of a polarizing fiber that has internal birefringent interfaces, and that may be used in some embodiments of polarizer discussed above, is a multilayer polarizing fiber. A multilayer fiber is a fiber that contains multiple layers of different polymer materials, at least one of which is birefringent. In some exemplary embodiments, the multilayer fiber contains a series of alternating layers of a first material and a second material, where at least one of the materials is birefringent. In some embodiments, the first material has a refractive index along one axis about the same as that of the second material and the refractive index along an orthogonal axis different from that of the second material. Such structures are discussed at greater length in, for example, U.S. Pat. No. 5,882,774, incorporated herein by reference.
A cross-section through one exemplary embodiment of a multilayer polarizing fiber 500 is schematically illustrated in
The fiber 500 may be surrounded by a cladding layer 508. The cladding layer 508 may be made of the first material, the second material, the material of the polymer matrix in which the fibers are embedded, or some other material. The cladding may functionally contribute to the performance of the overall device, or the cladding may perform no function. The cladding may functionally improve the optics of the reflective polarizer, such as by minimizing the depolarization of light at the interface of the fiber and the matrix. Optionally, the cladding may mechanically enhance the polarizer, such as by providing the desired level of adhesion between the fiber and the continuous phase material. In some embodiments, the cladding 508 may be used to provide an antireflection function, for example by providing some refractive index matching between the fiber 400 and the surrounding polymer matrix.
The fiber 500 may be formed with different numbers of layers and with different sizes, depending on the desired optical characteristics of the fiber 500. For example, the fiber 500 may be formed with from about ten layers to hundreds of layers, with an associated range in thickness. There is no limitation on the width of the fiber 500, although preferred values of the width may fall in a range from 5 μm to about 5000 μm, although the fiber width may also fall outside this range.
A multilayer fiber 500 may be fabricated by coextruding multiple layers of material into a multilayer film, followed by a subsequent step of stretching so as to orient the birefringent material and produce birefringent interfaces. Multilayer fibers may be obtained by slicing a multilayer sheet. Some approaches to manufacturing multilayer sheets containing birefringent interfaces are described further, for example, in U.S. Pat. Nos. 5,269,995; 5,389,324; and 5,612,820, incorporated by reference.
Some examples of suitable polymer materials that may be used as the birefringent material include PET, PEN and various copolymers thereof, as discussed above. Some examples of suitable polymer materials that may be used as the non-birefringent material include the optically isotropic materials discussed above.
Other configurations of multilayer fiber may be used. For example, another exemplary embodiment of multilayer fiber 520 may be formed with concentric layers of alternating first material 522 and second material 524, where the first material 522 is birefringent and the second material 524 may be either isotropic or birefringent. In this exemplary embodiment, the fiber 520 includes concentric birefringent interfaces 526, between the alternating layers 522, 524, that extend along the fiber 520.
The outer layer 528 of the fiber 520 may be formed of one of the first and second material, the same polymer material as is used in the polymer matrix of the polarizer, or some other material.
The fiber 520 may be formed with any suitable number of layers and layer thicknesses to provide desired optical characteristics, such as reflectivity and wavelength dependence. For example, the concentric fiber 520 may contain from around ten layers to hundreds of layers. The concentric fiber 520 may be formed by coextruding a multilayer form followed by stretching to orient the birefringent material. Any of the materials listed above for use in the flat multilayer fiber 500 may also be used in the concentric fiber 520.
Multilayer fibers having different types of cross-sections may also be used. For example, concentric fibers need not be circular in shape and may have some other shape, such as elliptical.
Another exemplary embodiment of a multilayer polarizing fiber is a spiral wound fiber, described in greater detail in U.S. patent application Ser. No. 11/278,348, incorporated herein by reference. An exemplary embodiment of a spiral wound fiber is schematically illustrated in
Another exemplary embodiment of a polarizing fiber having internal birefringent interfaces is a composite polarizing fiber, which contains multiple scattering fibers infiltrated with a polymer filler. An example of a cross-section through an exemplary composite polarizing fiber 540 is schematically illustrated in
Composite polarizing fibers are described further in U.S. patent application Ser. No. 11/068,157. A composite polarizing fiber can take on different cross-sectional shapes and may be, for example, circular as shown in
The positions of the scattering fibers 542 within the cross-section of the composite fiber may be random, although other cross-sectional arrangements of the scattering fibers 542 may be used. For example, the scattering fibers 542 may be regularly arranged within the cross-section of the composite polarizing fiber 540, for example as discussed in U.S. patent application Ser. No. 11/068,157. and U.S. patent application Ser. No. 11/068,158, incorporated herein by reference. In some embodiments, the scattering fibers 542 may be arranged to form a photonic crystal for light incident on the polarizer. Additionally, the scattering fibers 542 and/or the composite fibers 540 need not all be of the same size, or may vary in size along their lengths.
Another method for generating the desired internal structure that contains polymer birefringent interfaces in a fiber is to use two polymers which are not miscible, where at least one of the polymers is birefringent. The polymers may be coextruded, cast, or otherwise formed into a fiber. Upon processing, a continuous phase and a dispersed phase are generated. With subsequent processing or orientation, the dispersed phase can assume rod-like or layered structures, depending on the internal structure of the polymer fiber. Furthermore, the polymer materials may be oriented so that there is substantial refractive index matching between the two materials for one polarization direction and a relatively large index mismatch for the other polarization. The generation of a dispersed phase in a film matrix is described in greater detail in U.S. Pat. No. 6,141,149, included herein by reference.
This type of birefringent polymer fiber may be referred to as a dispersed phase polarizing fiber. An example of a dispersed phase polarizing fiber 550 is schematically illustrated in
The dispersed phase may also be formed of liquid crystal droplets, liquid crystal polymers or polymers. The dispersed phase could, alternatively, be comprised of air (microvoids). In any case, the interfaces between the dispersed and continuous phases within the dispersed phase fiber can induce desired optical properties, including reflective polarization.
In another approach to forming a birefringent polymer fiber, a fiber may be formed in a manner similar to a composite fiber, with a first polymer being used as the filler, but with second and third polymers being used for the scattering fibers. In some embodiments, the second and third polymers are not miscible with each other, and at least one of the second and third polymers is birefringent. The second and third polymers may be mixed as extruded as scattering fibers in a composite fiber. Upon processing, the first polymer forms the filler portion of the composite fiber, and the scattering fibers contain both a continuous phase and a dispersed phase, from the second and third polymers, respectively. This type of fiber is referred to as a dispersed phase composite fiber. An example of a dispersed phase composite fiber 560 is schematically illustrated in
Similarly, the concentric multilayer fiber and non-concentric multilayer fibers may be made of alternating layers with one of the layer types comprised of a first polymer and the second layer type comprised of a mixture of two polymers or materials which are not miscible. Upon processing in those cases, alternating layers are produced with some layers comprising the first polymer and some other layers comprising both a dispersed phase and a continuous phase. Preferably, one or both of the continuous phase and the dispersed phase are birefringent. With subsequent processing or orientation, the dispersed phase in the second type of layers can assume rod-like or layered structures.
The size requirements for the scattering fibers or birefringent regions in a layered fiber are similar among all the various embodiments. The size of the fiber or thickness of a layer in a multilayer device can be scaled up or down appropriately to achieve the desired size scale for the systems comprising layers or fibers containing a continuous and disperse phase, dependent on the desired operating wavelength or wavelength range. In some embodiments that include quarter-wave multilayered fibers, requirements of reflectivity and wavelength may determine the cross-sectional size of a fiber.
Another type of polymer fiber that may be used in a polarizer of the present invention is now described with reference to
The yarn 600 may include lengths of fiber, commonly referred to as staple fiber, that do not extend over the entire length of the yarn 600. The yarn 600 may be encapsulated within the polymer matrix, with the matrix filling the spaces between the fibers 602 that comprise the yarn 600. In other embodiments, the yarn 600 may have a filler between the fibers 602.
In general, the birefringent interfaces of the polymer fibers are elongated, extending in a direction along the fibers. In some exemplary embodiments, the birefringent fibers lie parallel to the x-axis, and so the diffusely reflected light is scattered mostly into the plane perpendicular to the fibers, the y-z plane, and there is little scattering in the x-z-plane.
Another embodiment of yarn 700, schematically illustrated in
The fibers may be included in the polymer matrix in the form of a tow, a parallel-type arrangement of fibers or yarns that are discrete. The fibers in the tow can be composite fibers, multilayer fibers, fiber yarn, any other suitable type of fibers, inorganic fibers, or a combination thereof. In particular, the tow or tows may form a set of fibers or yarns that are substantially parallel to each other. An embodiment of a fiber tow 800 is schematically illustrated in
The fibers may also be included in the matrix in the form of one or more fiber weaves. A weave 900 is schematically illustrated in
In some embodiments, more than one weave may be included within a matrix. For example, a polarizer film may include one or more weaves that contain polarizing fibers and one or more weaves that contain only inorganic fibers. In other embodiments, different weaves may include both polarizing fibers and inorganic fibers.
The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.
