Abstract:
The present invention is a method of forming an optical film including the following steps: providing a first film of a first material, extruding a second material to form a second film in a molten state; maintaining the second film in a molten state; bringing the first film proximate the molten second film; patterning the molten second film to form a plurality of structures, the structures defining a plurality of cavities therebetween; and solidifying the molten second film.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application incorporates by reference co-pending applications Ser. No. ______, filed ______, entitled “Composition for Microstructured Screens” by Peter M. Olofson et al. and Ser. No. ______, filed ______, entitled “Microstructured Screen With Light Absorbing Material and Method of Manufacturing” by Patrick A. Thomas et al. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention is directed generally to methods for manufacturing a rear projection screen and the resulting screen. More particularly, the invention relates to a rear projection screen that incorporates totally internally reflecting structures to disperse the light passing through the screen.  
         [0003]     Rear projection screens are generally designed to transmit an image projected onto the rear of the screen into a viewing space. The viewing space of the projection system may be relatively large (e.g., rear projection televisions), or relatively small (e.g., rear projection data monitors). The performance of a rear projection screen can be described in terms of various characteristics of the screen. Typical screen characteristics used to describe a screen&#39;s performance include gain, viewing angle, resolution, contrast, the presence of undesirable artifacts such as color and speckle, and the like.  
         [0004]     It is generally desirable to have a rear projection screen that has high resolution, high contrast and a large gain. It is also desirable that the screen spread the light over a large viewing space. Unfortunately, as one screen characteristic is improved, one or more other screen characteristics often degrade. For example, the horizontal viewing angle may be changed in order to accommodate viewers positioned at a wide range of positions relative to the screen. However, increasing the horizontal viewing angle may also result in increasing the vertical viewing angle beyond what is necessary for the particular application, and so the overall screen gain is reduced. As a result, certain tradeoffs are made in screen characteristics and performance in order to produce a screen that has acceptable overall performance for the particular rear projection display application.  
         [0005]     In U.S. Pat. No. 6,417,966, incorporated herein by reference, Moshrefzadeh et al. disclose a screen having reflecting surfaces disposed so as to reflect light passing therethrough into at least one dispersion plane. The screen thereby permits asymmetric dispersion of image light in a rear projection system and allows the light to be selectively directed towards the viewer. Moshrefzadeh et al. also teach methods for manufacturing the screen, including combinations of steps using casting and curing processes, coating techniques, planarization methods, and removing overcoating materials.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention is a method of forming an optical film including the following steps: providing a first film of a first material, extruding a second material to form a second film in a molten state; maintaining the second film in a molten state; bringing the first film proximate the molten second film; patterning the molten second film to form a plurality of structures, the structures defining a plurality of cavities therebetween; and solidifying the molten second film. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will be further explained with references to the drawing figures below, wherein like structure is referred to by like numerals throughout the several views.  
         [0008]      FIG. 1  is a side elevation view of a microrib screen structure.  
         [0009]      FIG. 2  illustrates a method by which the screen structure of claim  1  may be formed.  
         [0010]      FIG. 3  is a side elevation view of the structure of  FIG. 1  filled with light-absorbing material.  
         [0011]      FIG. 4  is a diagram of one embodiment of a method for filling the structure of  FIG. 1  to produce the structure of  FIG. 3 .  
         [0012]      FIG. 5  is a side elevation diagram of a step of a second method for filling the structure of  FIG. 1  to produce the structure of  FIG. 3 .  
         [0013]      FIG. 6A  is a side elevation view of one embodiment of a screen produced by the method of  FIG. 5 .  
         [0014]      FIG. 6B  is a side elevation view of a second embodiment of a screen produced by the method of  FIG. 5 .  
         [0015]      FIG. 7  illustrates a second embodiment of a screen of the present invention.  
         [0016]      FIG. 8  is a side elevation view of a third embodiment of a screen of the present invention.  
         [0017]      FIG. 9  illustrates the structure of  FIG. 3  with additional layers.  
         [0018]      FIG. 10  is a diagram illustrating one embodiment of a method of the present invention for producing the structure of  FIG. 9 . 
     
    
       [0019]     While the above-identified drawing figures set forth several embodiments of the invention, other embodiments are also contemplated. This disclosure presents illustrative embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope of spirit of the principles of this invention. The drawing figures are not drawn to scale.  
         [0020]     Moreover, while the embodiments are referred to by the designations “first,” “second,” “third,” etc., it is to be understood that these descriptions are bestowed for convenience of reference and do not imply an order of preference. The designations are presented merely to distinguish between different embodiments for purposes of clarity.  
       DETAILED DESCRIPTION  
       [0021]      FIG. 1  is a side elevation view of a microrib screen structure. Variations of the illustrated embodiments can be utilized for front projection and other screen applications, but they will be described primarily with reference to rear projection screen applications for the purposes of this disclosure. Microrib structure  20  includes a light transmitting base substrate  22  and microstructured diffusive ribs  24 . The term “microstructured” includes features having characteristic dimensions measured in micrometers (μm) or smaller units. In general, microstructured features may have characteristic dimensions ranging from less than 0.01 μm to more than 100 μm. What constitutes a characteristic dimension of a feature depends on the type of feature. Examples include the width of trough-like features in a surface, the height of post-like protrusions on a surface, and the radius of curvature at the point of sharp protrusions or indentations on a surface. Thus, even a macroscopic feature can be said to be microstructured if a characteristic dimension of the feature has dimensions with sub-micrometer tolerances.  
         [0022]     In one exemplary embodiment, linear ribs or microribs  24  are formed of an optical-grade host material such as a resin such as polycarbonate; in particular, the host resin incorporates light scattering particles such as beads so that ribs  24  act as a bulk diffuser. A sufficiently high aspect ratio is chosen for the rib geometry in order to induce total internal reflection (TIR) in the microrib structure  20 . The loading of the light scattering particles within the resin is chosen to control optical properties such as gain and view angle of the screen. A material such as a resin with a high refractive index (RI) is generally chosen for diffusive ribs  24 . In this application, the RI of a rib  24  refers to the RI of the host material. Examples of suitable materials for light diffusive ribs  24  include polymers such as modified acrylics, polycarbonate, polystyrene, polyester, polyolefin, polypropylene, and other optical polymers preferably having a refractive index equal to or greater than about 1.50. Polycarbonate, with a refractive index of 1.59, is particularly useful due to its high glass transition temperature Tg, clarity and mechanical properties. In the embodiment shown in  FIG. 1 , the light diffusive ribs  24  are separated by V-shaped cavities or grooves  26 . While light diffusive structures  24  are described in an exemplary embodiment as ribs that extend across substantially the entire width of base substrate  22 , it is also contemplated that the structures  24 , in an alternative embodiment, form discrete peaks that can be arranged upon base substrate  22  in a staggered, or “checkerboard” pattern, for example. In an exemplary embodiment, each structure  24  has a base  23  and a plurality of walls  25  which narrow the structure  24  as walls  25  extend from base  23 .  
         [0023]      FIG. 2  illustrates a method by which the screen structure of claim  1  may be formed.  FIG. 2  shows one example of a microreplication co-extrusion process that can be used to produce microrib structure  20 , consisting of diffusive ribs  24  on base substrate  22 . The term “microreplication” includes a process whereby microstructured features are imparted from a master or a mold onto an article. The master is provided with a microstructure, for example by micro-machining techniques such as diamond turning, laser ablation or photolithography. The surface or surfaces of the master having the microstructure may be covered with a hardenable material so that when the material is hardened, an article is formed that has a negative replica of the desired microstructured features. The microreplication may be accomplished using rolls, belts, and other apparatuses known in the art.  
         [0024]     Microreplication can be accomplished by techniques including but not limited to extruding, embossing, radiation curing and injection molding.  
         [0025]     In one exemplary embodiment shown in  FIG. 2 , co-extrusion die  28  is a high-temperature, high-pressure die for the simultaneous extrusion of a two-layer film. In one embodiment, die  28  has an extruder orifice diameter  30  of about 44.4 mm (1.75 inch) to about 50.8 mm (2 inches). The two-layer film is composed of material  32  to form base substrate  22  and material  34  to form light diffusive ribs  24 .  
         [0026]     In one embodiment, materials  32  and  34  are heated to about 66° C. (150° F.) and extruded simultaneously from die  28 , which has a temperature of about 293° C. (560° F.). Each material  32  and  34  is isolated from the other until after they are extruded from die  28 . After extrusion, the materials  32  and  34  are brought into contact with each other, wherein at least material  34  is still in a molten state.  
         [0027]     The three-roll extrude-emboss technique shown in  FIG. 2  uses a first roll  36 , a patterned second roll  40 , and a third roll  44 . In one embodiment, each roll  36 ,  40  and  44  is about 0.43 m (17 inches) in diameter. First roll  36  and third roll  44  may be heated or chilled as required by the nature of the materials used to facilitate release of the materials from the roll surfaces. Materials  32  and  34  are simultaneously extruded from die  28  onto patterned roll  40 . In the illustrated embodiment, material  32  is extruded proximate nip roll  36  and material  34  is extruded proximate patterned cast roll  40 . In one embodiment, first or nip roll  36  is heated to greater than or about 52° C. (125° F.) by running heated oil through interior  38  of roll  36 , the oil being heated by an external heat source. In one exemplary embodiment, nip roll  36  is formed of a material such as silicone rubber.  
         [0028]     Cast roll  40  is patterned on outer surface  48  to impart the desired structures upon material  34  to result in light diffusive ribs  24 . In one exemplary embodiment, cast roll  40  is formed of a metal such as chromium, nickel, titanium, or an alloy thereof. In one embodiment, cast roll  40  is heated to greater than or about 204° C. (400° F.), more particularly between about 252° C. (485° F.) and about 282° C. (540° F.), by running heated oil through interior  42  of roll  40 , the oil being heated by an external heat source. Third or carrier roll  44  is generally heated or chilled by running oil or water through interior  46  of roll  44  to assist in the release of microrib structure  20  from cast roll  40 . In one embodiment, carrier roll  44  is heated to greater than or about 66° C. (150° F.) by running heated oil through interior  46  of roll  44 , the oil being heated by an external heat source. In one exemplary embodiment, carrier roll  44  has a smooth outer surface  50  and is formed of a metal such as chromium, nickel, titanium, or an alloy thereof.  
         [0029]     In one embodiment, material  32  for forming base structure  22  is a light transmitting material such as a clear polymer such as polycarbonate, polyester, polyolefin, polypropylene, acrylic or vinyl, for example. In one embodiment, material  34  for diffuser ribs  24  is a high refractive index polymer such as a modified acrylic, polycarbonate, polystyrene, polyester, polyolefin, polypropylene, or other optical polymer. It is particularly suitable for material  34  to have a refractive index greater than or equal to about 1.50. Polycarbonate, with a RI of 1.59 is particularly useful due to its high Tg, clarity and mechanical properties. In one embodiment, material  32  and material  34  are compatible so that they physically bond at the interface therebetween to integrate into a monolithic structure. This is achieved in one exemplary embodiment by using the same polymer material for material  32  and  34 , the difference being that material  34  incorporates light diffusing particles into the polymer. In an alternate embodiment, material  32  and material  34  can have different compositions, but they possess similar processing characteristics and bond to one another at their interface.  
         [0030]     In one embodiment, nip roll  36  and cast roll  40  are in intimate contact to provide high pressure compression of materials  32  and  34 , and particularly material  34 , against cast roll  40 . This is especially important for materials with a high Tg such as polycarbonate, which set up almost immediately upon exiting die  28 . Carrier roll  44  need not be in intimate contact with cast roll  40 ; the purpose of carrier or pull roll  44  is merely to take formed microrib structure  20  off cast roll  40 . In one embodiment, each roll  36 ,  40  and  44  rotates at about 3.6 m (12 feet) per minute, with adjacent rolls rotating in opposite directions.  
         [0031]     In one embodiment, air bar  52  facilitates the release of structure  20  off cast roll  40 . Air bar  52  is a perforated cylinder which emits cooling air onto structure  20  just before the point of separation of structure  20  from cast roll  40 . In one embodiment, air is supplied at about 620 kPa (90 psi) and ambient temperature. Materials  32  and  34  solidify into structure  20 . In one embodiment, tensioning roll assembly  54  is used to provide the proper amount of tension on structure  20  as it travels. Slitter  56  is provided to cut structure  20  to desired widths. Windup roll  58  winds up structure  20  for storage or later retrieval.  
         [0032]     Other cast-emboss and extrude-emboss methods, for example, can also be used. The resulting microrib structure  20  can then be used in the method described with reference to  FIGS. 5, 6A  and  6 B. In another embodiment, single layer extrusion can be used to extrude material  34  for forming light diffusive ribs  24  onto previously formed substrate  22 . In this embodiment, an input feeds substrate  22  so that material  34  in a molten state is extruded thereon. Both materials are pressed together by nip roll  36  so that material  34  is patterned by cast roll  40 . Substrate  22  and material  34  remain in intimate contact during the cooling phase.  
         [0033]     Referring to  FIG. 5 , co-extrusion can also be used to extrude the dual layer of shield  86  and light absorbing adhesive  85 . Suitable optical materials for light absorbing adhesive  85  include those discussed with reference to  FIGS. 5, 6A  and  6 B, for example.  
         [0034]      FIG. 3  is a side elevation view of the structure of  FIG. 1  filled with light-absorbing material  62 . Embedded microstructured film  60  includes filling material  62 . Material  62  typically incorporates a black pigment or dye to absorb ambient light and improve contrast in the final screen construction. Material  62  has a low refractive index so that a relatively high difference in refractive index exists between light absorbing material  62  and the material composing light diffusive ribs  24 . A refractive index difference of at least about 0.06 is desired. Such a difference induces efficient internal reflection and high screen performance. In one exemplary embodiment, microrib structure  20  is filled with a black pigmented high melt flow PMMA light absorbing material  62 . This construction yields a desirably high refractive index difference of about 0.08 to 0.09 between light absorbing material  62  and ribs  24 . Internally reflecting surfaces  64  are formed by the interfaces between light diffusive ribs  24  and light absorbing material  64 . In one exemplary embodiment, front surface  66  of embedded microstructured film  60  is a smooth or slightly matte surface with minimal land on the rib top surfaces  68 . Totally internally reflecting surfaces  64  disperse light through optically transmitting areas  68  of front surface  66 . Front surface  66  preferably has a matte surface finish that assists in scattering the light propagating therethrough.  
         [0035]      FIG. 4  is a diagram of one embodiment of a method for filling structure  20  of  FIG. 1  to produce structure  60  of  FIG. 3 . Planarization process  70  coats light absorbing material  62  onto the microrib structure  20  to form embedded microstructured film  60 . Planarization process  70  uses resin coating station  74 , precision nip roll  76 , a smooth, matte or microstructured cylinder  78 , ultraviolet lamp  80 , precision nip roll  82  and embedded microstructured film rewind  84 .  
         [0036]     Microrib structure  20  is first unwound from substrate unwind station  72 . Microrib structure  20  continues on to resin coating station  74 , where it is overcoated with light absorbing material  62 . The composite structure is pressed by precision nip roll  76  against cylinder  78 . Cylinder  78  may be smooth, matte or microstructured to impart a desired texture upon front surface  66  of the resulting embedded planar microstructured film  60  shown in  FIG. 3 . After light absorbing material  62  is cast onto microrib structure  20 , the film proceeds to be cured by ultraviolet lamp  80 . A completed embedded microstructured film  60  emerges from precision nip roll  82  to be wound upon embedded microstructured film rewind  84 .  
         [0037]      FIG. 5  is a side elevation diagram of a step of a second method for filling structure  20  of  FIG. 1  to produce structure  60  of  FIG. 3 . In one embodiment of the method of the present invention, microrib structure  20  is formed by the co-extrusion process discussed above with respect to  FIG. 2  to impart light diffusive ribs  24  having V-shaped grooves  26  onto base structure  22 . An alternate filling process illustrated in  FIG. 5  eliminates the planarization process  70  shown in  FIG. 4  and additionally laminates a protective shield to microrib structure  20 . This is achieved by introducing a light absorbing adhesive  85  which serves both light absorbing and adhesive functions. The term “adhesive” used with reference to light absorbing adhesive  85  need not be an adhesive in the normal sense, but needs only to have bonding capabilities with light diffusive ribs  24 , and also to shield  86  if used. By combining the light absorption and adhesive functions in one material, savings in materials and manufacturing steps are obtained. Light absorbing adhesive  85  is disposed on rear surface  88  of shield  86 . Shield  86 , with light absorbing adhesive  85  disposed thereon, is brought together with microrib structure  20 . As shown by arrow  90 , for example, shield  86  and microrib structure  20  are laminated together.  
         [0038]     The thickness of light transmitting base film  22  can be chosen to meet the requirements of each particular application. For example, a thin base film with a thickness of about 0.127 mm (5 mils) to about 0.254 mm (10 mils) can be chosen to provide for ease of manufacturing; alternatively, a thick film with a thickness of about 0.508 mm (20 mils) to about 1.016 mm (40 mils) can be chosen to provide additional product stiffness. Suitable materials include polycarbonate, polyester, acrylic and vinyl films, for example. In one exemplary embodiment, back surface  91  of base substrate  22  has a matte finish to reduce specular reflection back into the imaging system.  
         [0039]     Shield  86  can also be varied to provide for different functionalities. Shield  86  can range in thickness from thin (less than about 0.508 mm (20 mils)) to semi-rigid (about 0.508 mm (20 mils) to about 1.016 mm (40 mils)) to rigid (greater than about 1.016 mm (40 mils)). The thickness of base substrate  22  and protective shield  86  can be chosen to yield a wide variety of products with these options impacting total material cost, optical functionality, and ease of processing. In one exemplary embodiment, light diffusive ribs  24  are formed of a polycarbonate loaded with light diffusing particles. In one exemplary embodiment, shield  86  is a clear PMMA.  
         [0040]     In one exemplary embodiment, light absorbing adhesive  85  is a photopolymerizable, low refractive index material which adheres to both light diffusive ribs  24  and shield  86 . In an exemplary embodiment, the refractive indices of light diffusive ribs  24  and light absorbing adhesive  85  differ enough to cause total reflection rather than transmittance at the interface therebetween. In an exemplary embodiment, the refractive index of the microrib material of light diffusive ribs  24  varies from 1.49 for simple acrylate materials to 1.58 or higher for materials such as aromatic polycarbonates. The refractive index requirement for the groove filler material  85  is, therefore, dependent on the optical properties (such as refractive index) of the microrib  24  material. For the high refractive index microrib materials, such as polycarbonate, commercially available photolaminating adhesives may be adequate. Exemplary adhesives  85  have a RI of less than about 1.50. Particularly suitable adhesives  85  have a RI of less than about 1.45. In some embodiments, adhesive  85  is a pigmented blend of one or more of the following components: urethane acrylate oligomers; substituted acrylate, diacrylate, and triacrylate monomers; fluorinated acrylates; perfluoroalkylsulfonamidoalkyl acrylates; acrylated silicones, acrylated silicone polyureas and UV or visible light activated photoinitiators.  
         [0041]     If the viscosity of the groove filler  85  is too low, it will flow during the groove filling process. This can waste material, give nonuniform thickness, and contaminate the process equipment. If the viscosity is too high, filling the grooves  24  can be a slow, difficult process and the possibility of introducing bubbles (optical defects) increases significantly. While photolamination can be accomplished with fluids having viscosities as low as about 150 centipoises, many processes can benefit from a viscosity of at least about 400 centipoises before polymerization. While viscosities as high as about 5,000 centipoises before polymerization can be used, viscosities no higher than about 1,500 centipoises before polymerization are especially suitable for reasonable process speed and bubble-free coatings.  
         [0042]     A standard measure of adhesion between substrates and coatings is the amount of force required to separate them, known as the peel force. The peel force of a system containing excellent interfacial adhesion at the interface between layers will be very high. While peel force strength of at least about 35.7 kg/m (2 pounds/inch) is probably adequate between polycarbonate diffusive ribs  24  and light absorbing adhesive  85 , it is more desirable to have peel force of at least about 71.4 kg/m (4 pounds/inch). This high peel force should be maintained under environmental test conditions of high temperature and humidity. Adequate adhesion may be achieved by modification of the substrate surfaces by treatment, such as with corona discharge or plasma, or priming; it is preferred, however, that the adhesive  85  adhere to the light diffusive ribs  24  and shield  86 , if used, without the necessity of surface modification.  
         [0043]     One suitable embodiment of light absorbing adhesive  85  is constructed by warming the following resin components to about 70° C. (158° F.) to lower the viscosity sufficiently to allow for agitation: 16.0 g aliphatic urethane acrylate oligomer; 19.0 g ethoxyethoxyethyl acrylate; 5.5 g hexanediol diacrylate; 5.0 g tetrahydrofurfuryl acrylate; 44.5 g N-methyl-perfluorobutylsulfonamidoethyl acrylate; 10.0 g acryloyloxyethoxyperfluorobutane; and 1.0 g phenyl bis( 2 , 4 , 6  trimethyl benzoyl) phosphine oxide photoinitiator.  
         [0044]     The components are then shaken until a clear solution results. The solution is then pigmented for light absorption. One suitable pigment is carbon black; in one exemplary embodiment, the pigment is used in a concentration between about 50 ppm (parts per million) and about 20,000 ppm; in one exemplary embodiment, the pigment is used in a concentration greater than about 1,000 ppm and less than about 9,000 ppm. A concentration of about 3,000 ppm is particularly suitable, based on mass ratios of the carbon black material to the resin material. In one embodiment, the formulation is disposed onto shield  86  by a conventional method such as knife coating. The coated shield is then pressed onto microrib structure  20  as shown in  FIG. 5 , for example, to partially or completely fill grooves  26 . Excess adhesive  85 , if any, is expelled by running a rubber roller over the construction. The construction is passed under a 11.81 W/mm (300 Watt/in) Fusion Systems D lamp several times at about 6.1 m (20 feet) per minute. In an alternate method, the formulation may be coated directly onto the microrib structure  20 , and shield  86  then adhered to the microrib structure  20  with adhesive  85  already disposed thereon. Thereafter, the steps of removing excess adhesive  85  and curing the construction are the same as discussed above.  
         [0045]      FIG. 6A  is a side elevation view of one embodiment of a screen produced by the method of  FIG. 5 . The step of  FIG. 5  can result in a completely filled structure  93  illustrated at  FIG. 6A . In one exemplary embodiment, light absorbing adhesive  85  has a low refractive index to produce efficient TIR within ribs  24 . Light absorbing adhesive  85  is formulated to effectively bond diffuser ribs  24  to shield  86 . Light absorbing adhesive  85  can possess low shrinkage properties to produce a cosmetically acceptable lamination result. Moreover, it is particularly suited that light absorbing adhesive  85  is curable by ultraviolet light in order to allow for convenient processing and a fast cure.  
         [0046]     In one embodiment, light diffusive ribs  24  are replicated from a tooling mold using a high refractive index diffuser resin, as shown in the coextrusion process of  FIG. 2 . In this application, all percentages are by mass unless otherwise indicated. One suitable resin is about 79% aliphatic urethane acrylate oligomer, about 19% 2-phenoxyethyl acrylate, and about 2% 2-hydroxy-2-methyl-1-phenyl-1-propanone photoinitiator. Another suitable resin is about 69% aliphatic urethane acrylate oligomer, about 29% 2-(1-naphthyloxy)-ethyl acrylate and about 2% 2-hydroxy-2-methyl-1-phenyl-1-propanone photoinitiator.  
         [0047]     Then, a pigmented, typically black, light absorbing adhesive  85  is applied to a second substrate such as shield  86 . One suitable light absorbing adhesive  85  is formed from a resin having about 30% “Formulation A,” (the “Formulation A” having about 38.5% aliphatic urethane acrylate oligomer, about 26.9% ethoxyethoxyethyl acrylate, about 28.8% isobornyl acrylate, about 5.8% hexanediol diacrylate and about 1% α,α-diethoxyacetophenone (DEAP) photoinitiator); about 10% aliphatic urethane diacrylate; about 30% trifluoroethyl acrylate; and about 30% N-methyl-perfluorobutylsulfonamidoethyl acrylate. Another suitable light absorbing material  85  is formed from a resin having about 50% “Formulation A,” discussed above, and about 50% N-methyl-perfluorobutylsulfonamidoethyl acrylate. In one exemplary embodiment, light absorbing adhesive  85  contains a pigment such as carbon black. In one exemplary embodiment, the pigment is used in a concentration between about 50 ppm and about 20,000 ppm. In one exemplary embodiment, the pigment is used in a concentration greater than about 1,000 ppm and less than about 9,000 ppm. A concentration of about 3,000 ppm is particularly suitable, based on mass ratios of the carbon black material to the adhesive material.  
         [0048]     Light absorbing adhesive  85  can be applied to a second substrate such as shield  86  in sufficient quantity to completely fill diffuser ribs  24 , allowing a slight excess to ensure complete fill, in the lamination method illustrated in  FIG. 5 . The excess adhesive squeezes out of completely filled structure  93  upon lamination. Completely filled structure  93  is then exposed to radiation under conditions similar to those discussed above for microreplication process  120 . The exposure can, for example, result in a partial or complete polymerization of the material. After at least partial polymerization, light absorbing adhesive  85  is a copolymer of its components.  
         [0049]      FIG. 6B  is a side elevation view of another embodiment of a screen produced by the method of  FIG. 5 . When a small thickness or amount of light absorbing adhesive  85  is used in the step illustrated in  FIG. 5 , partially filled structure  95  results. In partially filled structure  95 , air gaps  97  are left in V-shaped grooves  26 . A benefit of air gap  97  is that the low refractive index air fills the rib grooves  26  and creates a large refractive index difference between the grooves  26  and the light diffusive ribs  24 , further enhancing “TIR efficiency.” Because the refractive index of air is 1.0, the difference in refractive index between air gap  97  and light diffusive ribs  24  is usually greater than about 0.5. Because air gap  97  creates the bulk of the diffuser rib interface, light absorbing adhesive  85  need not possess as low a refractive index as when the ribs are completely filled in structure  93 . This allows for the selection of an adhesive  85  to optimize other important properties, such as low shrinkage and high peel strength adhesion, for example. Since the adhesive contact area between light absorbing adhesive  85  and diffuser ribs  24  is smaller, light absorbing adhesive  85  may possess greater adhesive properties in partially filled structure  95  than completely filled structure  93 .  
         [0050]     In both completely filled structure  93  and partially filled structure  95 , the level of light absorbing material used in light absorbing adhesive  85  is chosen based on the desired amount of contrast enhancement and ambient light absorption. The light absorbing material in an exemplary embodiment is a black pigment such as carbon black. In completely filled structure  93 , the black pigment concentration can be relatively low and yet yield an acceptable total fixed absorbence, or optical density value, because the thickness of the layer of light absorbing adhesive  85  is large. A suitable loading concentration of pigment such as carbon black in completely filled structure  93  in one embodiment is between about 50 ppm and about 20,000 ppm. In an exemplary embodiment, the concentration is greater than about 1,000 ppm and less than about 9,000 ppm. A concentration of about 3,000 ppm is particularly suitable, based on mass ratios of the carbon black material to the adhesive material. However, in partially filled structure  95 , the coating thickness is small; therefore, the black pigment concentration must be larger to yield the same optical density. In the latter case, the ambient light absorption is larger per unit of coating thickness than in the former case. A suitable loading concentration of pigment such as carbon black in partially filled structure  95  in one embodiment is between about 50 ppm and about 20,000 ppm. In an exemplary embodiment, the concentration is greater than about 5,000 ppm and less than about 10,000 ppm, based on mass ratios of the carbon black material to the adhesive material.  
         [0051]     A challenge in both completely filled structure  93  and partially filled structure  95  is the removal of excess adhesive  85  from front surface  66  of diffuser ribs  24  during lamination. If all of the light absorbing adhesive  85  is not removed from front surface  66  of the diffuser ribs  24  during lamination, some image light can be lost due to absorption during TIR transmission. In a partially filled structure  95  with more highly pigmented adhesive  85 , more loss of image light can occur for the same residual black layer thickness.  
         [0052]      FIG. 7  illustrates a second embodiment of a screen of the present invention. In one embodiment, overcoat layer  92  is made of a material which is multifunctional to serve as a low refractive index component as well as a hard coat. In this way, the “TIR efficiency” is maintained, but the potential need to laminate to a protective shield is eliminated since the material of overcoat layer  92  is scratch-resistant due to its inherent hard properties. This combination of functions within one material further reduces material usage and costs. Suitable materials for overcoat layer  92  include hard coat materials incorporating a pigment such as carbon black. In one embodiment, the pigment is used in a concentration between about 50 ppm and about 20,000 ppm. In an exemplary embodiment, the concentration is greater than about 1,000 ppm and less than about 9,000 ppm. A concentration of about 3,000 ppm is particularly suitable, based on mass ratios of the carbon black material to the hard coat material.  
         [0053]     One suitable hard coat material is disclosed in U.S. Pat. No. 5,104,929 to Bilkadi, hereby incorporated by reference. Bilkadi teaches a photocurable abrasion resistant coating including colloidal silicon dioxide particles dispersed in ethylenically unsaturated aliphatic and/or cycloaliphatic monomers that are substituted by a protic group. In particular, the coating composition curable to an abrasion and weather resistant coating includes a non-aqueous dispersion of colloidal silicon dioxide particles of diameters less than about 100 nanometers in a protic group-substituted ester or amide of acrylic or methacrylic acid.  
         [0054]     Another suitable hard coat material is disclosed in U.S. Pat. No. 5,633,049 to Bilkadi, hereby incorporated by reference. Bilkadi teaches an acid- and abrasion-resistant coating prepared from a silica-free protective coating precursor composition including a multifunctional ethylenically unsaturated ester of acrylic acid, a multifunctional ethylenically unsaturated ester of methacrylic acid, or a combination thereof; and an acrylamide.  
         [0055]     Other hard coat materials include room-temperature curing silicone resins derived from functionalized silane monomers; coatings derived from hydrolyzable silanes; polymers derived from a combination of acryloxy functional silanes and polyfunctional acrylate monomers; polymers such as acrylic with colloidal silica; and polymerized acrylate or methacrylate functionalities on a monomer, oligomer or resin; for example.  
         [0056]      FIG. 8  is a side elevation view of a third embodiment of a screen of the present invention. Embedded microstructured film  60  is provided with a hard coat  94  to protect the film against scratching and other damage. Hard coat  94  may be applied by spraying, dipping, or roll coating, for example. This process eliminates the need for a separate protective shield  86 .  
         [0057]      FIG. 9  illustrates the structure of  FIG. 3  with additional layers: Shielded screen  96  incorporates embedded microstructured film  60  with back surface  98  and adhesive  100  on front surface  66  for the attachment of a light transmitting shield  86 . Shield  86  is a protective layer that can be a film or sheet made of transparent material such as acrylic, polycarbonate or glass, for example. Shield  86  functions as a protective element so that embedded microstructured film  60  is not damaged by contact. Shield  86  is an optional component, though most applications benefit greatly from this protection. Shield  86  can be made to be anti-glare (matte), anti-reflective, anti-static, anti-scratch or smudge resistant, for example, through coatings, surface textures, or other means. In one embodiment, shield  86  is a 3 millimeter thick acrylic panel from Cyro Corporation with a non-glare, matte outward-facing surface.  
         [0058]     The thickness of base film  22  can be chosen to meet the requirements of each particular application. For example, a thin base film with a thickness of about 0.127 mm (5 mils) to about 0.254 mm (10 mils) can be chosen to provide for ease of manufacturing; alternatively, a thick film with a thickness of about 0.508 mm (20 mils) to about 1.016 mm (40 mils) can be chosen to provide additional product stiffness. Suitable materials include polycarbonate, polyester, acrylic, polyolefin, polypropylene and vinyl films, for example. In one exemplary embodiment, back surface  98  of embedded microstructured film  60  has a matte finish to reduce specular reflection back into the imaging system.  
         [0059]     Shield  86  can also be varied to provide for different functionalities. Shield  86  can range in thickness from thin (less than about 0.508 mm (20 mils)) to semi-rigid (about 0.508 mm (20 mils) to about 1.016 mm (40 mils)) to rigid (greater than about 1.016 mm (40 mils)). The thickness of base substrate  22  and protective shield  86  can be chosen to yield a wide variety of products with these options impacting total material cost, optical functionality, overall construction stiffness and ease of processing. In one exemplary embodiment, light diffusive ribs  24  are formed of a polycarbonate loaded with light diffusing particles.  
         [0060]      FIG. 10  is a diagram illustrating one embodiment of a method of the present invention for producing the structure of  FIG. 9 . In one embodiment, lamination process  102  directly follows the planarization or filling process in a single assembly line. Lamination process  102  uses adhesive unwind  104 , lamination nip assembly  106  and lamination nip assembly  108 . Either of lamination nip assemblies  106  or  108  may be driven, or separate drive wheels or other drive mechanisms can be used to propel components through process  102 . The adhesive material disposed on adhesive unwind  104  is typically a layer of pressure-sensitive adhesive  100  sandwiched between two liner layers. When the adhesive material is unwound from adhesive unwind  104 , top liner  110  is separated therefrom and wound upon top liner rewind  112 . The remaining adhesive material  114  is contacted with embedded microstructured film  60 , which is unwound from film unwind  84 . Embedded microstructured film  60  and adhesive material  114  pass through lamination nip  106 , where they are pressed together.  
         [0061]     Thereafter, bottom liner  116  of the adhesive composite  114  is removed and wound onto bottom liner rewind  118 . A shield  86  is introduced on a transversely traveling feed web or other suitable mechanism and disposed onto the exposed adhesive  100 . The structure then passes through lamination nip  108 , where shield  86  is pressed onto microstructured film  60  and adhered thereto by adhesive  100 . The embedded microstructured film  60  can be severed between discrete shields  86  to form individual shielded screens  96 .  
         [0062]     Although the present invention has been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, while particular shapes for light diffusive and light absorbing structures are illustrated, it is contemplated that the structures may be formed in different shapes, incorporating additional or different planes or angles, additional edges, and curved surfaces. It is further noted that the light diffusive structures on a particular substrate need not all be of the same height or shape, for example. Similarly, the light absorbing structures on a particular substrate need not all be of the same height or shape, for example. Moreover, components of the materials and processes described therein are combinable in numerous ways; only a few of those possibilities have been specifically described by way of example, although all are regarded to be within the scope of the invention.