Abstract:
AVER 2363 This invention relates to a light transmitting filter comprising (a) light absorbing layer having a first and second surface, (b) microspheres embedded in the light absorbing layer and (c) an optically clear, substantially uniform, conformable layer covering the microspheres, wherein the microspheres provide light tunnels through the light absorbing layer. The invention also relates to methods of making the same. The invention also relates to a method of preparing a light transmitting filter comprising the step of laminating a clear substantially uniform conformable layer over transparent microspheres embedded in a light absorbing layer wherein the transparent bead form light tunnels through the light absorbing layer. The light transmitting filters of the present invention have improved light throughput and improved angularity.

Description:
CROSS REFERENCE TO PROVISIONAL APPLICATION  
       [0001]    This application claims priority from provisional application Ser. No. 60/088,591, filed Jun. 9, 1998, the entire disclosure of which is hereby incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to rear projection screens and light filters, specifically those with a conformable coating over transparent microspheres.  
         BACKGROUND OF THE INVENTION  
         [0003]    Rear projection screens transmit an image from the back of the screen to the viewer on the opposite side of the screen. The image is affected by the amount of light transmitted by the screen or filter or throughput. Generally, screens and filters have been limited by their construction to the amount of light transmitted through the screen or filter. It is desirable to have construction which would provide increased light throughput.  
           [0004]    Generally, rear projection screens have suffered from poor angularity. Angularity is the term used to describe the ability of a viewer to see a good image from the screen or filter at some angles other than those which are ordinary to the screen surface. As the viewer moves to the side of the screen or filter, the image quality is decreased. It is desirable to have screens and filters which have improved angularity.  
           [0005]    U.S. Pat. Nos. 5,563,738 and 5,781,344, issued in the name of Vance, relate to light transmitting and dispersing filters having low reflectance. The multi-layer light filters use the addition of optical layers to a basic refractive light filter to allow adjustment of gain, contrast and ambient light rejection of light filters.  
         SUMMARY OF THE INVENTION  
         [0006]    This invention relates to a light transmitting filter comprising (a) light absorbing layer having a first and second surface, (b) transparent microspheres embedded in the light absorbing layer and (c) an optically clear, substantially uniform, conformable layer covering the transparent microspheres, wherein the transparent microspheres provide light tunnels through the light absorbing layer. The invention also relates to methods of making the same. The invention also relates to a method of preparing a light transmitting filter comprising the step of laminating a clear substantially uniform conformable layer over transparent microspheres embedded in a light absorbing layer wherein the transparent microspheres form light tunnels through the light absorbing layer. The light transmitting filters of the present invention have improved light throughput and improved angularity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIGS. 1 a ,  1   b , and  1   c  are cross sectional areas of the light transmitting filter, where  
         [0008]    [0008]FIGS. 1 b  and  1   c  are expanded views of the cross section of a portion of FIG. 1 a.    
         [0009]    [0009]FIG. 2 is a photomicrograph of the top surface of the light transmitting filter of the present invention.  
         [0010]    [0010]FIGS. 3 a - 3   c  are cross sectional views of one embodiment of the method of making the light transmitting filter.  
         [0011]    [0011]FIGS. 4 a - 4   c  are cross sectional views of another embodiment of a method of making the light transmitting filter.  
         [0012]    [0012]FIGS. 5 a - 5   c  are cross sectional views of another embodiment of a method of making the light transmitting filter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    As used in the specification and claims the term substantially uniform refers to a layer of the construction which has a thickness with little variance, such as a variation in thickness of less than about 5 microns, preferably a variation of less than about 2.5 microns, or preferably less than about 1.25 microns. Here and elsewhere in the specification and claims, the range and ratio limits may be combined.  
         [0014]    The term conformable refers to a layer which matches the surface of the transparent microspheres. In other words the layer conforms or cups the transparent microspheres.  
         [0015]    As described above the present invention is directed to a light transmitting filter comprising (a) light absorbing layer having a first and second surface, (b) transparent microspheres embedded in the light absorbing layer and (c) an optically clear, substantially uniform, conformable layer covering, wherein the microspheres provide light tunnels through the light absorbing layer.  
         [0016]    The first layer of the light transmitting filter is a light absorbing layer. This layer generally has a thickness sufficient for embedding the transparent microspheres. The embedding of the transparent microspheres may be at any level provided that the transparent microspheres form a light tunnel through the light absorbing layer. Preferably, the light absorbing layer has a thickness from about 10% to 90% of the transparent microsphere diameter. The light absorbing layer may be any material which is substantially opaque and can be embedded with the transparent microspheres and form light tunnels. The light absorbing layer may be any material which is malleable enough to give way when the transparent microspheres are pushed against it, such as a partially or incompletely crosslinked urethane, a pressure sensitive adhesive, or with the addition of heat, thermoplastic polymers. The light absorbing material may also be a material which forms around the transparent microsphere, such as an asphalt or adhesive (a pigmented pressure sensitive adhesive).  
         [0017]    Typically, the light absorbing layer is a combination of a pigment, usually carbon black, and a polymer, such as a polyolefin, polyacrylatepolyvinylbutyral, such as Butvar resin available from Solutia, polyurethanes, polyesters or polyvinylcarboxylate. The polyolefins may be homopolymers and copolymers of C 2-12  olefins, such as ethylene, propylene, and butylene. The polyacrylates and polymethacrylate may be homopolymers or copolymer of C 1-12  acrylate or methacrylate monomers, such as methyl, ethyl, propyl, butyl, hexyl, or octyl acrylates or methacrylates. Here and elsewhere in the specification and claims, the term for pendant groups is meant to include all isomeric forms of the group. For instance, the use of the term octyl is intended to cover n-octyl, isooctyl, and 2-ethylhexyl groups. The polyvinylcarboxylates include homo or copolymers of C 1-12  vinyl carboxylates, such as vinyl acetate, vinyl propionate and vinyl butyrate.  
         [0018]    In one embodiment, the light absorbing layer is supported by a an optically clear support layer. The optically clear support layer may be a glass or a polymer. The support layer must resist the pressure exerted by the transparent microspheres during the embedding and conforming processes. The support layer is adhered to the light absorbing layer by adhesive, lamination, with or without the layer, or as a result of coextrusion. The support layer may be any material having sufficient strength to provide support to the light absorbing layer and have optically clear characteristics. Examples of support layers include glass, polyacrylics, polycarbonates, polyurethanes, such as two part polyurethanes, and polyesters, such as polyethylene terephthalates.  
         [0019]    Transparent microspheres are embedded into the light absorbing layer. Typically the transparent microspheres are embedded to a level sufficient to provide light tunnels through the light absorbing layer. Generally, the transparent microspheres are embedded at a level of about 10% to about 80%, preferable less than about 50% (preferably from about 30% to about 40%) of their diameter. The transparent microspheres generally have a refractive index from about 1.5 to 2.3, or from about 1.5 to about 2.2, or from about 1.65 to about 1.95. 3They are typically composed of glass, ceramic, plastic or other suitably transparent materials. Transparent microspheres from about 25 to about 300 microns are suitable for construction of light filters as described herein. In one embodiment, the transparent microspheres have a diameter of about 30 to about 120, or from about 40 to about 80, or from about 50 to 65 microns. In one embodiment, the diameter is an average diameter. In one embodiment, the transparent microspheres are substantially uniform in size. The transparent microspheres are present in substantially a monolayer generally covering from 60% to about 91% of the surface area of the assembly, or from about 75% to about 90%, or from about 85% to about 90% of the surface area.  
         [0020]    The third component of the light transmitting filter is the optically clear, substantially uniform conformable layer. This layer typically has a thickness of about 10% to about 90% of the diameter of the average microsphere, or from about 20% to about 80%, or from about 30% to about 70% of the diameter of the average microsphere. The layer comprises any polymer which has the optical clarity needed for light transmitting filters. Typically these polymers are polyolefins, such as optically clear polyolefins from metallocene catalysts, polyacrylates, polymethacrylates, polycarbonates, polyurethanes, polyesters, such as polyethylene terephthates, polyvinylidene dichloride, cellophane, cellulose acetate, polyvinylidene difluoride, polyvinyl chloride, polyvinyl butyral, and polyvinylcarboxylates. These polymers have been described above.  
         [0021]    In one embodiment, the light filter has a tie layer to improve the adhesion of the conformable coating to the transparent microspheres and the light absorbing layer. This is any material which improves this adhesion. In another embodiment, the transparent microspheres and the light absorbing layer may be treated to improve adhesion. Examples of suitable tie layer resins include “Platamid”, available from Elf Atochem, “CXA”, available from DuPont, and “Plexar” available from Chemplex.  
         [0022]    The methods of making the light transmitting filters may be through heat lamination. It is essential that the substantially uniform conformable layer is formed on the transparent microspheres. The Inventor has discovered that the vicat softening point of the polymer of the conformable layer must be higher than the vicat softening point of the molding layer. The molding layer is in contact with the conformable polymeric layer during preparation of the light filter. The molding layer may be any thermoplastic polymer with the appropriate vicat softening point. If the molding layer is composed of a polymer of similar nature to the conformable layer then a layer of silicone release layer, such as those used for pressure sensistive adhesive liners, may be used to enhance ease of separation of the layers. In one embodiment, the molding layer is composed of polyolefins, such as low, medium and high density polyethylene, propylene or mixtures thereof. The lower vicat softening point of the molding layers helps form the conformable layer by softening and/or melting to conform to the surface of the transparent microspheres. Under the pressure and temperature of preparation the molding layer presses the conformable layer against the transparent microspheres.  
         [0023]    The invention may be further understood by reference to the attached figures. FIG. 1 a  contains a cross section of light filter  10 . Box  10   a  is expanded in FIG. 1 b  to show a conformable coating  11 , light absorbing layer  12 , transparent microspheres  13  and clear support layer  14 . FIG. 1 c  represents another embodiment where conformable coating  11  is attached to tie layer or corona treatment layer  15 . The light filter still contains light absorbing layer  12 , transparent microspheres  13  and clear support layer  14 .  
         [0024]    [0024]FIG. 3 a  is a method of preparing the light filters. Construction  30  contains layer  33 , which comprises the composition of the conformable coating, layer  32  which is a molding layer, and layer  31  which is the substrate layer (typically paper or PET film such as Mylar®). Construction  30   a  contains transparent microspheres  36  embedded in light absorbing layer  35  on support layer  34  (in one embodiment, a polyacrylate) and removable support layer  39  (typically paper or PET film). Constructions  30  and  30   a  are laminated together as shown in FIG. 3 b . Lamination temperatures and pressures depend on the materials (polymers) used but preferably the lamination occurs at a temperature from about 175 to about 400° F., or from about 250 to about 350° F. The laminating pressure is preferably between about 50 to about 150, or from about 75 to about 125 pounds per square inch. After lamination and shown in FIG. 3 b , a construction having support layer  34  in contact with transparent microspheres  36  and light absorbing layer  35 . The transparent microspheres  36  and light absorbing layer  35  are also in contact with the layer  33  which is the composition of the conformable coating. Layer  33  is in contact with molding layer  32  and support layer  31 . As shown in FIG. 3 c , the layers  31  and  32  are removed leaving a light filter with conformable coating  37  over transparent microspheres  36 , which are embedded in light absorbing layer  35 , which is supported by layers  34  and  39 . Removable support layer  39  is present to provide support during manufacture and processing. It is typically removed before the light filter is used.  
         [0025]    An alternative means for preparing the light filter is shown in FIGS. 4 a - 4   c . Construction  40  comprises support layer  41  and molding layer  42  laminated to layer  43 , which is composed of the materials for the conformable coating, and having transparent microspheres  44  embedded therein. Construction  40   a  comprises a light absorbing layer  45 , clear support layer  46  (e.g. a polyacrylate) and removable support layer  47  (e.g. paper or PET film). Construction  40  and  40   a  are brought together with heat and pressure as described herein. After constructions  40  and  40   a  have been combined, FIG. 4 b  represents a multilayer construction which comprises support layer  41  on molding layer  42  which in turn is in contact with layer  43  which is the composition of the conforming coating. Transparent microspheres  44  are embedded in light absorbing layer  45  and in contact with clear support layer  46  to form light tunnels. A portion of the multilayer construction is removed and as illustrated in FIG. 4 c . In FIG. 4 c , support layer  41  and molding layer  42  are removed from the construction. The remaining portion of the construction comprises removable support layer  47  and clear support layer  46  which is in contact with transparent microspheres  44  to form light tunnels. Support layer  47  is removed before using the light filter. The transparent microspheres are embedded in the light absorbing layer  45 . A conformable coating  48  covers the transparent microspheres  44  and light absorbing layer  45 .  
         [0026]    Another means of preparing the light filter is shown in FIGS. 5 a - 5   f . In FIG. 5 a , transparent microspheres  51  are placed on a clear support layer  52  and within retainers  53 . In FIG. 5 b , light absorbing material, such as asphalt or black pressure sensitive adhesive is placed between the transparent microspheres  54 . FIG. 5 c  shows the transparent spheres  51  set in the light absorbing material  54  and on support layer  52  without retainers  53 . FIG. 5 d  shows lamination of the construction from FIG. 5 c  to a construction comprising layer  56  which is the composition of the conformable coating, molding layer  55  and support layer  57 . The resulting construction is illustrated in FIG. 5 e  and comprises a clear support layer  52  in contact with glass microspheres  51  to form light tunnels through light absorbing layer  54 . Light absorbing layer  54  and the transparent microspheres  51  are in contact with the layer  55  which is the composition of the conformable coating. The layer  55  is in contact with molding layer  56  and support layer  57 . Support layer  57  and molding layer  56  are removed from the multilayer construction of FIG. 5 e . A portion of layer  55  is removed with the molding layer  56  and support layer  57 . The other portion of the construction comprises a clear support layer  52  and transparent microspheres  51  in contact to form light tunnels through light absorbing layer  54 . Over the transparent microspheres and light absorbing layer  54  is conformable coating  58 .  
         [0027]    Following are examples of light filters. These examples are illustrative and are not be considered limiting to the scope of the invention.  
       EXAMPLE 1  
       [0028]    A 75 micron layer of polyethylene terephthalate is laminated to a 75 micron layer of polymethylmethacrylate. A 10 micron layer of black polyvinylbutyral is placed on top of the polymethylmethacrylate. Glass microspheres having a refractive index of 1.80 and having an average diameter of 52 microns are embedded into the polyvinylbutyral forming a light tunel to make construction  1 . A paper facestock is coated with 35 microns of low density polyethylene. Onto the layer of polyethylene is placed a 20 micron thick layer of clear polyvinylbutyral to make construction  2 . Construction  1  and  2  are pressed together at a temperature of 285 F. and 100 psi using a roll laminator. After cooling the paper and the low density polyethylene are separated from the other materials. The polyethylene terephthalate is removed from the polymethylmethacrylate layer to provide a light filter with a conformable coating.  
       EXAMPLE 2  
       [0029]    A paper is coated with 35 microns of low density polyethylene and layer of clear polyvinylbutyral. Glass microspheres of example 1 are embedded in the polyvinylbutyral to form light tunnels. This construction is then laminated to a construction comprising of a 75 micron layer of polyethylene terephthalate coated with 75 microns of polymethylmethacrylate and 10 microns of black polyvinylbutyral at 100 psi and 285 F. The paper and the polyethylene as well as the polyethylene terephthalate are removed to form the light filter with a conformable coating.  
       EXAMPLE 3  
       [0030]    Glass microspheres of Example 1 are placed on a layer of polymethylmethacrylate supported by a layer of polyethylene terephthalate. Black pressure sensitive adhesive is added to a level of about 40% of the diameter of the glass microspheres. A 20 micron layer of clear polyvinylbutyral on low density polyethylene coated paper is laminated to the transparent microspheres in the pressure sensitive adhesive at a temperature of 230 F. and 80 psi. The paper and polyethylene as well as polyethylene terephthalate are removed to leave the desired light filter with a conformable coating.  
         [0031]    The light filter may be adhered to a rigid structure, such as sheet polycarbonate (Lexan a ) or polymethylmethacrylate (Plexiglas a ).  
         [0032]    While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.