Abstract:
An optical sheet capable of enhancing contrast is provided. The optical sheet includes a layer configured to control light incident on the layer and then allow the light to exit towards the observer side. The optical sheet includes: an optical functional sheet layer having multiple prisms capable of transmitting light and multiple light-absorbing parts capable of absorbing light, the multiple prisms and multiple light-absorbing parts being arranged alternately along a sheet plane of the optical sheet; and an electromagnetic-wave shield layer. The electromagnetic-wave shield layer is positioned on a side opposite to the observer side relative to the optical functional sheet layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on the Japanese Patent Application No. 2006-333589 filed on Dec. 11, 2006. The whole contents of the Japanese Patent Application No. 2006-333589 are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an optical sheet that is used in a display, such as a plasma television, in order to control incoming light properly and then allow the light to exit towards the observer side. The present invention also relates to a display comprising the optical sheet. 
     BACKGROUND OF THE INVENTION 
     In a display comprising a plasma display panel (hereinafter also referred to as “PDP”), such as a television (plasma television), an optical sheet (optical member, front filter) are situated on the observer side relative to a light source such as a PDP. The optical sheet has various functions and acts to control light from a source (image light source) so as to output clear and proper image light towards the observer side. 
     Such an optical sheet is made of a laminate of layers having different functions (features). For example, the optical sheet disclosed in Japanese Laid-Open Patent Publication No. 2006-189867 makes it possible to improve image light in transmittance (luminance) and in contrast (light-dark ratio). 
     Because of the recent strong demand for displays improved in fineness and performance, however, there is a need to enhance contrast more greatly than the conventional optical sheet described in Japanese Laid-Open Patent Publication No. 2006-189867 can achieve. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an optical sheet capable of enhancing contrast. 
     An optical sheet (i.e., an optical laminate) according to the present invention that has a plurality of layers configured to control light incident on the plurality of layers and then allow the light to exit towards the an observer side and comprises: an optical functional sheet layer having multiple prisms capable of transmitting light and multiple light-absorbing parts capable of absorbing light, the multiple prisms and the multiple light-absorbing parts being arranged alternately along a sheet plane of the optical sheet; and an electromagnetic-wave shield layer having a function of shielding electromagnetic waves, the electromagnetic-wave shield layer being positioned on a side opposite to the observer side relative to the optical functional sheet layer. 
     The optical sheet according to the present invention may further comprise at least one layer selected from the group consisting of a film layer capable of shielding neon rays, a film layer capable of shielding infrared rays, a film layer capable of correcting color tone, and a film layer capable of preventing reflection of light. 
     In such an optical sheet according to the present invention, the at least one layer selected from the group consisting of a film layer capable of shielding neon rays, a film layer capable of shielding infrared rays, a film layer capable of correcting color tone, and a film layer capable of preventing reflection of light may be positioned in at least one of the following positions: on the observer side relative to both the optical functional sheet layer and the electromagnetic-wave shield layer; on the side opposite to the observer side relative to both the optical functional sheet layer and the electromagnetic-wave shield layer; and between the optical functional sheet layer and the electromagnetic-wave shield layer. 
     Alternatively, in the optical sheet according to the present invention, the film layer capable of preventing reflection of light may be positioned outermost on the observer side, and the optical functional sheet layer may be positioned next to the film layer capable of preventing reflection of light. 
     Alternatively, the optical sheet according to the present invention may further comprise 
     a substrate layer positioned next to the optical functional sheet layer, wherein the film layer capable of preventing reflection of light may be positioned outermost on the observer side, and wherein the substrate layer may be positioned next to the film layer capable of preventing reflection of light. 
     Alternatively, in the optical sheet according to the present invention, the optical functional sheet layer may be positioned outermost on the observer side. 
     The optical sheet according to the present invention may further comprise a substrate layer positioned next to the optical functional sheet layer, wherein the substrate layer may be positioned outermost on the observer side. 
     The optical sheet according to the present invention may further comprise a base plate layer adhered directly or indirectly with the optical functional sheet layer, wherein only the electromagnetic-wave shield layer may be formed as a separate member from the base plate layer. The term “separate member (separate layer)” herein means that a member (layer) is not fixed either directly or indirectly to another object member (layer) with an adhesive. The “substrate layer” can be formed from a material having high light transmittance and required rigidity. Any material can be used for the substrate layer as long as it has high light transmittance and required rigidity, and, glass can be used, for example. 
     Furthermore, in the optical sheet according to the present invention, in a cross section taken along a normal to a light-exiting face of the optical sheet, each of the multiple prisms may be in a nearly trapezoidal shape with a lower base having a greater width and a upper base having a smaller width, the lower base being positioned on the observer side and the upper base being positioned on the side opposite to the observer side, and each of the light-absorbing parts may be in a nearly triangular shape with its base being positioned on the same side as the upper base of the nearly trapezoidal shape section of the each of the multiple prisms. In such an optical sheet according to the present invention, in the cross section taken along the normal to the light-exiting face of the optical sheet, an oblique line extending from one end of the base of the nearly triangular shape section of each of the light-absorbing parts may include such a curved line and/or a broken line that an angle between the oblique line and the normal to the light-exiting face of the optical sheet, determined at one side in a thickness direction of the optical sheet, is different from an angle between the oblique line and the normal to the light-exiting face of the optical sheet, determined at the other side in the thickness direction of the optical sheet. Alternatively, in the optical sheet according to the present invention, in the cross section taken along the normal to the light-exiting face of the optical sheet, an oblique line extending from one end of the base of the nearly triangular shape section of each of the light-absorbing parts may include such a broken line that an angle between the oblique line and the normal to the light-exiting face of the optical sheet, determined at one side in a thickness direction of the optical sheet, is different from an angle between the oblique line and the normal to the light-exiting face of the optical sheet, determined at the other side in the thickness direction of the optical sheet, and an angle between the oblique line and the normal to the light-exiting face of the optical sheet, determined at any point in the thickness direction of the optical sheet, may be more than zero and equal to or less than 10 degrees. 
     Furthermore, in the optical sheet according to the present invention, the prisms may be made from a resin with a refractive index of Np, and the light-absorbing parts may be made from a resin with a refractive index of Nb, and the refractive index Np may be equal to or greater than the refractive index Nb. 
     Furthermore, in the optical sheet according to the present invention, the light-absorbing parts may include light-absorbing particles with a mean particle diameter of 1 μm or more. 
     Furthermore, in the optical sheet according to the present invention, the electromagnetic-wave shield layer may have a sheet-shaped base and an electrically conductive pattern part formed in a given pattern on one surface of the base. In such an optical sheet of the invention, the electrically conductive pattern part may be situated on a surface, on the observer side, of the base, and irregularities for diffusing light may be formed on the other surface, on the side opposite to the observer side, of the base. Alternatively, in such an optical sheet of the invention, the electrically conductive pattern part may be situated on a surface, on the observer side, of the base, an adhesive layer for bonding the electromagnetic-wave shield layer to other layer may be situated on the observer side of the electromagnetic-wave shield layer, and the adhesive layer may include light-diffusing particles. 
     Furthermore, the optical sheet according to the present invention can further comprise a light-diffusing layer having a function of diffusing light, wherein the light-diffusing layer may be positioned on the side opposite to the observer side relative to the optical functional sheet layer. 
     A display according to the present invention comprises any one of the above-described optical sheets. 
     A first plasma television according to the present invention comprises a plasma display panel and any one of the above-described optical sheets which is situated on an image-displaying side of the plasma display panel. 
     A second plasma television according to the present invention comprises the above-described optical sheet and a plasma display panel, wherein the optical sheet further includes a base plate layer adhered directly or indirectly with the optical functional sheet layer, only the electromagnetic-wave shield layer being formed as a separate member from the base plate layer, and wherein the plasma display panel is adhered directly or indirectly with the electromagnetic-wave shield layer formed as a separate member. 
     According to the present invention, there can be obtained an optical sheet capable of enhancing the contrast of image light to be provided to an observer. 
     Moreover, according to the present invention, a moiré pattern due to the arrangement of the prisms can be made less noticeable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an optical sheet in the first embodiment of the present invention and diagrammatically shows the laminated structure of the optical sheet. 
         FIG. 2  is a view showing an enlarged section of a portion of the optical sheet shown in  FIG. 1 . 
         FIG. 3A  is a view showing another example of the light-absorbing part. 
         FIG. 3B  is a view showing still another example of the light-absorbing part. 
         FIG. 4A  is a view showing a further example of the light-absorbing part. 
         FIG. 4B  is a view showing a still further example of the light-absorbing part. 
         FIG. 5  is a cross-sectional view of an optical sheet in the second embodiment of the present invention and diagrammatically shows the laminated structure of the optical sheet. 
         FIG. 6  is a view for illustrating a modification of the optical sheet in the second embodiment of the invention. 
         FIG. 7  is a cross-sectional view of an optical sheet in the third embodiment of the invention and diagrammatically shows the laminated structure of the optical sheet. 
         FIG. 8  is a view diagrammatically showing the laminated structure of a portion of an optical sheet and a PDP that are incorporated in a plasma television. 
         FIG. 9  is a view showing an example of the path of external light in the plasma television shown in  FIG. 8 . 
         FIG. 10  is a view showing an example of the path of external light in a plasma television provided with a conventional optical sheet. 
         FIG. 11  is a view diagrammatically showing the laminated structure of a portion of a PDP and the optical sheet in the third embodiment that are incorporated in a plasma television. 
         FIG. 12  is a view showing an example of the path of external light in a plasma television. 
         FIG. 13A  is a view showing a modified optical sheet. 
         FIG. 13B  is a view showing another modified optical sheet. 
         FIG. 14  depicts one example of the first embodiment of the resent invention; 
         FIG. 15  depicts another example of the first embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. 
       FIG. 1  is a cross-sectional view of an optical sheet  10  in the first embodiment of the invention and diagrammatically shows the laminated construction of the optical sheet. In  FIG. 1  and also in the other figures, some of the identical parts of the optical sheet are drawn without repeatedly putting reference numerals in order to make the figure neat. The optical sheet  10  is a sheet-shaped member capable of transmitting incoming light towards the observer side, and has the optical and filtering functions. The optical sheet  10  includes an electromagnetic-wave shield layer  11 , an optical functional sheet layer  12 , a PET film layer  17  serving as a substrate layer, a neon-ray-cutting layer  18 , an infrared-cutting layer  19 , a color-tone-correcting layer  20 , a glass layer  21 , and an antireflection layer  22 . In this embodiment, the above-described layers extend along the normal to the sheet plane of the paper on which the figure is drawn, with the cross section shown in  FIG. 1  maintained. These layers will be described below. 
     In order to facilitate understanding, the optical functional sheet layer  12  will be first described. The optical functional sheet layer  12  has prisms  13 ,  13 , . . . , each prism being in a nearly trapezoidal shape in a cross section taken perpendicularly to the sheet plane (the light-exiting face) of the optical sheet  10 , and light-absorbing parts  14 ,  14 , . . . , one light-absorbing part being situated between two adjacent prisms  13 ,  13 , as shown in  FIG. 1 .  FIG. 2  is a view showing an enlarged section of one light-absorbing part  14  and two prisms  13 ,  13  adjacent to the light-absorbing part  14 , contained in the optical sheet  10  shown in  FIG. 1 . The optical functional sheet layer  12  will be described with reference to  FIGS. 1 and 2  and some other figures. 
     The prisms  13 ,  13 , . . . are so disposed that the upper base, having a smaller width than that of the lower base, and the lower base, having a greater width than that of the upper base, of the nearly trapezoidal shape section of each of prisms are on the sheet plane of the optical sheet  10 . In addition, the prisms  13 ,  13  are so situated that the lower bases of the nearly trapezoidal shape sections of the prisms face PET film layer  17  side. Further, the prisms  13 ,  13 , . . . are made from a light-transmitting resin having a refractive index of Np. The light-transmitting resin is usually a resin having the property of curing in ionizing radiation (e.g., ultraviolet light), etc., such as urethane acrylate. 
     The light-absorbing parts  14 ,  14 , . . . are elements placed between the prisms  13 ,  13 , . . . and, as shown in  FIGS. 1 and 2 , the light-absorbing parts  14 ,  14 , . . . are nearly triangular in a cross section taken along the direction in which the light-absorbing parts  14 ,  14 , . . . are arranged and along the normal to the light-exiting face of the optical sheet  10 . The light-absorbing parts  14 ,  14 , . . . are so disposed that their faces corresponding to the bases of their nearly triangular shape sections are on the side on which the upper bases of the nearly trapezoidal prisms  13 ,  13 , . . . are positioned. The faces of the light-absorbing parts  14 ,  14 , . . . , corresponding to the bases of their nearly triangular shape sections, and the faces of the prisms  13 ,  13 , . . . , corresponding to the upper bases of their nearly trapezoidal shape sections, form a surface, of the optical functional sheet layer  12 , on the side opposite to the observer side (i.e., the surface on the light-entering side). In the cross section taken along the direction in which the light-absorbing parts  14 ,  14 , . . . are arranged (the direction in which the prisms  13 ,  13 , . . . are arranged) and along the normal to the light-exiting face of the optical sheet  10 , the angle between the oblique side of each nearly triangular shape section and the normal to the sheet plane of the optical sheet  10  is preferably more than zero, and equal to or less than 10 degrees. 
     Further, it is not necessary that the inclination of each oblique side be constant. The oblique side may be in the shape of an broken line or even in the shape of a curved line in the cross section taken along the direction in which the light-absorbing parts  14 ,  14 , . . . are arranged and along the normal to the light-exiting face of the optical sheet  10 .  FIG. 3A  shows a case where the oblique sides of the light-absorbing part  14 ′ are formed as a broken line.  FIG. 3B  shows a case where the oblique sides of the light-absorbing part  14 ″ are formed as a curved. In the case shown in  FIG. 3A , each oblique face of the light-absorbing part  14 ′ (each oblique face of the prism  13 ′) is formed not from one plane but from two planes forming an obtusely V-shaped plane. Namely, in the cross section, the light-absorbing part  14 ′ has oblique sides in the shape of obtusely V-shaped lines. Specifically, in the cross section taken along the direction in which the light-absorbing parts  14 ,  14 , . . . are arranged and along the normal to the light-exiting face of the optical sheet  10 , the oblique side of the light-absorbing part  14  forms, on the base side of the light-absorbing part (the left-hand side in the figure, the light-entering side), an angle of θ 1  with the normal to the light-exiting face of the optical sheet. On the other hand, in the cross section taken along the direction in which the light-absorbing parts  14 ,  14 , . . . are arranged and along the normal to the light-exiting face of the optical sheet  10 , the oblique side of the light-absorbing part  14  forms, on the PET film layer  17  side (the right-hand side in the figure, the light-exiting side), an angle of θ 2  with the normal to the light-exiting face of the optical sheet. Preferably, these angles are in the relationship θ 1 &gt;θ 2 , and the two angles θ 1 , θ 2  are more than zero, and equal to or less than 6 degrees. Further, the two planes forming the obtusely V-shaped plane extend in the direction of thickness of the optical functional sheet layer  12  to the distances of widths T 1  and T 2 . It is preferred that the width T 1  and the width T 2  be nearly equal. Although each oblique line side of the nearly triangular shape section is formed from two lines in the cross section shown in  FIG. 3A , the oblique face of the light-absorbing part may be formed from a greater number of planes, i.e., three or more planes. 
     In the case shown in  FIG. 3B , the oblique sides of the light-absorbing part  14 ″ (the oblique sides of the prism  13 ″) are curved lines. Thus, the oblique faces of the light-absorbing parts  14 ″ may also be curved planes. Also in this case, in the cross section taken along the direction in which the light-absorbing parts  14 ,  14 , . . . are arranged and along the normal to the light-exiting face of the optical sheet  10 , the angle between the oblique side having the shape of a curved line and the normal to the light-exiting face of the optical sheet is preferably smaller on the PET film layer  17  side than on the base side. Further, it is preferred that the above angle be more than zero, and equal to or less than 10 degrees, determined at any point on the curved line. Furthermore, it is preferred that the above angle be more than zero, and equal to or less than 6 degrees, determined at any point on the curved line. The angle between the oblique side having the shape of a curved line and the normal to the light-exiting face of the optical sheet is defined as the angle obtained by dividing the oblique side in the shape of a curved line into ten parts having the same distance and measuring the angle between ends of the equally-divided parts and the normal to the light-exiting face of the optical sheet. 
     Furthermore, the shape of the light-absorbing parts is not limited to examples discussed above in this embodiment, and any modified light-absorbing parts are useful as long as they can properly absorb external light. Examples of modified light-absorbing parts include those ones shown in  FIGS. 4A  and  4 B. The sectional shape of the light-absorbing part  14 ′″ shown in  FIG. 4A  is rectangular, and the sectional shape of the prism  13 ″ is therefore different from that of the prism  13  (see  FIG. 2 ). The sectional shape of the light-absorbing part  14 ″ shown in  FIG. 4B  is pentagonal. Thus, the light-absorbing parts can take various shapes so that their sectional shapes are rectangles, trapezoids, polygons, etc. 
     The light-absorbing parts  14 ,  14 , . . . are made from a given material whose refractive index Nb is equal to or smaller than the refractive index of the material for the prisms  13 ,  13 , . . . . When the refractive index Np of the prisms  13 ,  13 , . . . is equal to or greater than the refractive index Nb of the light-absorbing parts  14 ,  14 , . . . , the image light entering the prisms  13 ,  13 , . . . from a light source under specified conditions can be properly reflected from the interface between the light-absorbing parts  14 ,  14 , . . . and the prisms  13 ,  13 , . . . , thereby providing a bright image to an observer. Although no specific limitations are imposed on the difference between the refractive indexes Np and Nb, the difference is preferably 0 or more, and 0.06 or less. 
     Further, each light-absorbing part  14  in this embodiment is made up of light-absorbing particles  16 ,  16 , . . . and a binder that bridges the gap between the light-absorbing particles so as to form a binder part  15 . Namely, each light-absorbing part  14  includes multiple light-absorbing particles  16 ,  16 , . . . that can absorb light, and a binder part  15  in which the light-absorbing particles  16 ,  16 , . . . are dispersed. Therefore, the light-absorbing parts  14 ,  14 , . . . can absorb the image light entering them without being reflected from the interface between the prisms  13 ,  13 , . . . and the light-absorbing parts  14 ,  14 , . . . . Moreover, the light-absorbing parts  14 ,  14 , . . . can properly absorb external light entering them from the observer side at specified angles, thereby enhancing the contrast of image light. The binder forming the binder part  15  may be a material having a refractive index nearly equal to the refractive index Nb of the material forming the prisms  13 ,  13 , . . . . Any material can be used as the binder. Examples of materials herein useful as the binder include epoxyacrylates having the property of curing in ionizing radiation (e.g., ultraviolet light), etc. It is preferred that the mean particle diameter of the light-absorbing particles  16 ,  16 , . . . be 1 μm or more. Any particles can be used as the light-absorbing particles as long as they can absorb light. For example, black-colored particles can be used as the light-absorbing particles  16 ,  16 , . . . . Black-colored particles are on the market and are easily available. 
     The way in which the parts  14 ,  14 , . . . are provided with the property of absorbing light is not limited to the aforementioned use of the light-absorbing particles  16 ,  16 , . . . . For example, the parts  14 ,  14 , . . . may be colored entirely with a pigment or a dye so that they can act to absorb light. 
     Next, the electromagnetic-wave shield layer  11  will be described. The electromagnetic-wave shield layer  11  is laminated to a surface, of the optical functional sheet layer  12 , on the side on which the bases of the nearly triangular shape sections of the light-absorbing parts  14 ,  14 , . . . are positioned. More specifically, the electromagnetic-wave shield layer  11  is situated next to the optical functional sheet layer  12 , on the side opposite to the observer side (the light-entering side of the optical functional sheet layer  12 ). The electromagnetic-wave shield layer  11  is a layer (filter) having the property of shielding electromagnetic waves, as its name signifies. Any layer having this property can be used as the electromagnetic-wave shield layer  11  regardless of the manner in which it shields electromagnetic waves. A layer includes a sheet-shaped base and an electrically conductive pattern part formed in a given pattern on one surface of the base can be used as the electromagnetic-wave shield layer  11 . Examples of the electrically conductive pattern part include a copper layer patterned into meshes. In order to obtain such an electrically conductive pattern part made of a copper layer patterned into meshes, there can be employed such a technique as etching or vacuum vapor deposition. Etching or vacuum vapor deposition makes it possible to form a fine mesh-pattern in a copper layer. The pitch, etc. in the copper meshes can be suitably determined depending on the electromagnetic waves that should be shielded. The pitch and line width in the meshes can be made about 300 μm and 12 μm, respectively. 
     Thus, in the optical sheet  10  of this embodiment, incorporated in a display  1  (see  FIG. 8 ), the electromagnetic-wave shield layer  11  is situated on the image light source side (opposite to the observer side, the light-entering side) relative to the optical functional sheet layer  12 . The electromagnetic-wave shield layer  11  has a strong tendency to diffusely reflect external light as compared with the other films used in the optical sheet, having other properties. Therefore, by placing the electromagnetic-wave shield layer  11  in the above position, it becomes possible to provide a high-contrast image to an observer. Further, interference fringes due to the prisms  13 ,  13 , . . . disposed with a specified pitch in the optical functional sheet layer  12  are sometimes observed. However, according to this embodiment, the electromagnetic-wave shield layer  11  situated next to the optical functional sheet layer  12  and positioned on the side opposite to the observer side relative to the optical functional sheet layer  12 , can effectively make the interference fringes less noticeable without lowering contrast. These actions and effects will be described later in detail. 
     Next, the PET film layer  17  will be described. The PET film layer  17  is a substrate layer serving as a base on which the optical functional sheet layer  12  is formed. Namely, the optical functional sheet layer  12  is formed on the PET film layer  17 . In addition, the PET film layer  17  is a film layer whose main component is PET. Although the PET film layer  17  is formed mainly from PET, other resins, etc. may also be contained in it. Moreover, a variety of additives may be incorporated in the PET film layer  17 . Examples of additives that can be herein used include antioxidants such as phenolic anti-oxidizing agents, and stabilizers such as lactone stabilizing agents. The term “main component” herein means that an object material (PET, in this case) is contained in the substrate layer in an amount of 50% by weight or more of the whole material forming the substrate layer (the same shall apply hereinafter).  FIG. 15  illustrates one example of the first embodiment of the present invention including the substrate layer. 
     It is not necessary that the main component of the substrate layer of the optical sheet be PET. For example, the substrate layer can contain, as a main component, a polyester resin such as polybutylene terephthalate (PBT) resin or polytrimethylene terephthalate (PTT) resin. In this embodiment, however, a resin containing PET as a main component is used as a material preferred from the viewpoint of property, mass-productivity, cost, availability, and so forth. 
     The neon-ray cutting layer  18 , the infrared cutting layer  19 , the color-tone correcting layer  20 , and the antireflection layer  22  have the properties their names signify, respectively. In this embodiment, these layers are laminated on the optical functional sheet layer  12 , on the side opposite to the electromagnetic-wave shield layer  11  side, i.e., on the observer side (light-exiting side) relative to the optical functional sheet layer  12 , as shown in  FIG. 1 . The neon-ray cutting layer  18  cuts mainly neon rays exiting from a display towards the observer side. The infrared cutting layer  19  cuts infrared rays containing near infrared rays, passing through the optical sheet  10 . The color-tone correcting layer  20  is for correcting more properly the color tone of the image light from the source, travelling towards the observer side. The antireflection layer  22  is for preventing external light from being reflected from the optical sheet  10 . Thus the antireflection layer  22  can restrain the external light from returning to the observer side, and thus maintain clearness of the displayed image unclear. 
     Films, etc. that have the above-described properties and are commonly used, can be used as the above layers. For example, it is possible to use commercially available films (sheets) provided with the above properties. 
     The glass layer  21  is made of sheet glass and serves as a base plate layer for supporting the neon-ray cutting layer  18 , the infrared-cutting layer  19 , the color-tone correcting layer  20 , the antireflection layer  22 , etc. that are laminated on its front or back surface either directly or indirectly. 
     The above is the detailed description of the optical sheet  10  according to the first embodiment of the invention. The structure of the optical sheet  10  is not limited to the above-described one. For example, a pressure-sensitive adhesive layer may be placed between any two of the above-described layers to fix these two layers.  FIG. 14  illustrates one example of this embodiment of the present invention. Further, following the recent trend towards thinner flat displays, the component layers of the optical sheet may be laminated on a light source panel (display panel) such as a plasma display panel without using the glass layer  21 . The optical sheet of the invention can also be applied to such a case, and even in this case, the effect of the optical sheet of the invention can be obtained. 
     Next, an optical sheet  30  according to the second embodiment of the invention will be described.  FIG. 5  diagrammatically shows the laminated construction of an optical sheet  30  according to the second embodiment. Like the optical sheet  10  according to the first embodiment, the optical sheet  30  according to the second embodiment includes an electromagnetic-wave shield layer  31 , a optical functional sheet layer  32 , a PET film layer  37  serving as a substrate layer, a neon-ray cutting layer  38 , an infrared cutting layer  39 , a color-tone correcting layer  40 , a glass layer  41 , and an antireflection layer  42 . The details of each layer in the optical sheet  30  according to the second embodiment may be the same as those of the corresponding layer in the optical sheet  10  according to the first embodiment. Therefore, structure of each layer in the optical sheet  30  according to the second embodiment will not be described in detail any more. 
     In the optical sheet  30  according to the second embodiment, all the layers other than the PET film layer  37  and the antireflection layer  42  are situated on the side opposite to the observer side relative to the optical functional sheet layer  32 . Therefore, the optical functional sheet layer  32  can more effectively absorb light entering the optical sheet  30  from the observer side. Namely, it is possible to enhance more greatly the contrast of image light by preventing diffuse reflection of external light. 
       FIG. 6  diagrammatically shows the laminated construction of an optical sheet  30 ′, a modification of the optical sheet  30  according to the second embodiment. In the optical sheet  30 ′ shown in  FIG. 6 , the PET film layer  37 , situated next to the optical functional sheet layer  32  and positioned on the observer side relative to the optical functional sheet layer  32 , is the outermost layer on the observer side (the right-hand side in the figure, the light-exiting side). Namely, the optical functional sheet layer  32  that is formed on the PET film layer  37  so as to form a sheet-shaped member, as mentioned above, may be situated, together with the PET film layer  37 , outermost on the observer side (the right-hand side in the figure, the light-exiting side). 
     Next, an optical sheet  50  according to the third embodiment of the invention will be described.  FIG. 7  diagrammatically shows the laminated construction of the optical sheet  50  according to the third embodiment. In the optical sheet  50  according to the third embodiment, an electromagnetic-wave shield layer  51  exists as a separate layer from an optical functional sheet layer  52 . Namely, in the case shown in  FIG. 7 , the electromagnetic-wave shield layer  51  is not adhered either directly or indirectly to the optical functional sheet layer  52  and forms a sheet-shaped member separate from a sheet-shaped member including the optical functional sheet layer  52 . In this embodiment, the word “directly” implies that two object layers are present adjacently to each other, and the word “indirectly” implies that two object layers are present with another layer between them. In this embodiment, it is possible to fix, with an adhesive, only the electromagnetic-wave shield layer  51  to a light source such as a PDP, as will be described later. Like the optical sheet  10  according to the first embodiment, the optical sheet  50  according to the third embodiment includes an electromagnetic-wave shield layer  51 , an optical functional sheet layer  52 , a PET film layer  57  serving as a substrate layer, a neon-ray cutting layer  58 , an infrared cutting layer  59 , a color-tone correcting layer  60 , a glass layer  61 , and an antireflection layer  62 . The details of each layer in the optical sheet  50  according to the third embodiment may be the same as those of the corresponding layer in the optical sheet  10  according to the first embodiment. Therefore, the structure of each layer in the optical sheet  50  according to the third embodiment will not be described in detail any more. 
     According to the lamination of the optical sheet  50  of the third embodiment, there is no need to laminate the electromagnetic-wave shield layer  51  to the optical functional sheet layer  52  whose structure is complicated, so that the optical sheet (optical member)  50  can be produced with increased productivity. 
     Next, the structure of a plasma television  1 , an example of a display in which the above-described optical sheet  10  according to one embodiment of the present invention is incorporated, and how the optical sheet  10  acts when the plasma television  1  displays an image, will be described.  FIG. 8  is a cross-sectional view of the plasma television  1  comprising a PDP  2  and the optical sheet  10  placed on the image-light-exiting side of the PDP  2 . In  FIG. 8 , the PDP  2  and the optical sheet  10  are shown in such a way that the positions of the PDP  2  and the optical sheet  10  can be known clearly. In  FIG. 8 , the right-hand side is the observer side.  FIG. 9  is a view of an enlarged section of a portion of the view of  FIG. 8  and illustrates light path. 
     As  FIG. 8  shows, the optical sheet  10  according to the first embodiment is placed on the image-light-exiting side relative to the PDP  2 , an image light source. Therefore, the electromagnetic-wave shield layer  11 , among the other layers in the optical sheet  10 , is closest to the PDP  2 , and the optical functional sheet layer  12  is situated on the observer side of the electromagnetic-wave shield layer  11 . 
     The light path, especially the path of external light, will be described with reference mainly to  FIGS. 9 and 10 .  FIG. 10  illustrates the path of external light in a conventional optical sheet. In  FIG. 9 , external light L 1  is light entering the optical sheet  10  from the observer side. Such external light L 1  includes sunlight and indoor lamplight. The external light L 1 , a part of external light, is absorbed by the light-absorbing part  14 , as shown in  FIG. 9 . Since the external light is absorbed by the light-absorbing part  14 , the external light does not affect image light, and an image can thus be displayed with high contrast. 
     On the other hand, the electromagnetic-wave shield layer  11  having the electrically conductive pattern part made of a patterned metal film tends to diffusely reflect external light as compared with the other films in the optical sheet, having other properties. Therefore, when the electromagnetic-wave shield layer  111  is situated on the observer side relative to the optical functional sheet layer  112 , as shown in  FIG. 10 , external light L 101  tends to be partly reflected from the observer-side-surface of the electromagnetic-wave shield layer  111  and returned to the observer side (see external light L 101 ′ in  FIG. 10 ). Namely, the optical functional sheet layer  12  in the optical sheet  10  in which the electromagnetic-wave shield layer  11  is situated next to the optical functional sheet layer  12  and situated on the side opposite to the observer side (on the PDP  2  side) relative to the optical functional sheet layer  12 , effectively shows the external-light-absorbing action and can thus effectively improve contrast, as compared with the optical functional sheet layer  112  in the optical sheet  110  in which the electromagnetic-wave shield layer  111  is situated on the observer side relative to the optical functional sheet layer  112 . 
     In the meantime, a portion L 2  of the external light that has entered the optical sheet  10  passes through the prism  13  and reaches the electromagnetic-wave shield layer  11 . However, a portion L 2   a  of the light  12  diffusely reflected from the electromagnetic-wave shield layer  11  is absorbed by the light-absorbing part  14  while the portion L 2   a  of the light  12  is returned to the observer side. Namely, the optical functional sheet layer  12  absorbs not only the external light L 1  travelling towards the PDP side (the side opposite to the observer side) but also the external light L 2   a  travelling towards the observer side. Therefore, the optical sheet  10 , in which the optical functional sheet layer  12  is situated on the observer side relative to the electromagnetic-wave shield layer  11 , can prevent lowering of contrast that usually occurs because of the property of diffusely reflecting light the electromagnetic-wave shield layer  11  has. 
     Also in the optical sheet  30 ,  30 ′ according to the second embodiment, the electromagnetic-wave shield layer  31  is situated on the side opposite to the observer side (on the PDP  2  side), relative to the optical functional sheet layer  32 , as shown in  FIGS. 5 and 6 . Therefore, also when the optical sheet  30 ,  30 ′ according to the second embodiment is used in a display  1 , the optical functional sheet layer  32  can effectively absorb external light and can thus effectively enhance contrast. 
       FIG. 11  is a view similar to that of  FIG. 8  and diagrammatically shows the laminated construction of a plasma television  1  in which the optical sheet  50  according to the third embodiment is incorporated. As  FIG. 11  shows, in the plasma television  1  in which the optical sheet  50  according to the third embodiment is incorporated, the electromagnetic-wave shield layer  51  is attached to a PDP  2  and thus exists as a separate member (separate layer) from the other films in the optical sheet  50 . Also in the plasma television  1  shown in  FIG. 11 , the optical functional sheet layer  52  situated on the observer side relative to the electromagnetic-wave shield layer  51  effectively absorbs external light, so that contrast can be effectively enhanced. 
     In the meantime, in the optical sheet  10 ,  30 ,  30 ′,  50 , the prisms  13 ,  33 ,  53  that transmit light are disposed along the sheet plane of the optical sheet  10 ,  30 ,  30 ′,  50  with a specified pitch. When such an optical sheet  10 ,  30 ,  30 ′,  50  is used together with a PDP  2 , interference fringes (fringe pattern) are sometimes observed clearly. 
     Generally, since the PDP  2  has specified pixels, it is expected that there will occur moiré fringes due to both of the pitch with which the pixels are arranged and the pitch with which the prisms  13 ,  33 ,  53  are arranged. A known measure taken to make the moiré fringes less noticeable is that the pitch with which the pixels are arranged and the pitch with which the prisms  13 ,  33 ,  53  are arranged are adjusted so that the ratio between the two pitches falls in a specified range. It is also known that a layer having the property of greatly diffusing light can make the moiré pattern less noticeable. 
     However, the present inventors have earnestly studied and found that it is impossible to make interference fringes (moiré fringes) that occurred when the optical sheet  10 ,  30 ,  30 ′,  50  is used together with the PDP  2  less noticeable only by controlling the pitch with which the pixels are disposed and the pitch with which the prisms  13 ,  33 ,  53  are disposed. Further, although a layer having the property of greatly diffusing light can make the moiré fringes less noticeable, such a greatly diffusing layer diffusely reflects not only external light but also image light. Namely, although the use of a light-diffusing layer is effective in making the moiré fringes less noticeable, it causes another problem, lowering of contrast. 
     On the other hand, the optical sheet  10 ,  30 ,  30 ′,  50  in which the optical functional sheet layer  12 ,  32 ,  52  is situated on the observer side relative to the electromagnetic-wave shield layer  11 ,  31 ,  51  can effectively make the moiré fringes less noticeable, as supported by the results of the evaluations made in the following Examples. Although the mechanism that makes the moiré fringes less noticeable has not yet been fully understood, one possible mechanism will be explained below with reference mainly to  FIG. 12 . However, this mechanism is not explained as limiting the present invention. 
     As  FIG. 9  shows, external light that has entered the optical sheet  10 ,  30 ,  30 ′,  50  situated on the observer side relative to the PDP  2  is partly absorbed by the light-absorbing parts  14 ,  34 ,  54 . On the other hand, as  FIG. 12  shows, external light L 21  entering the optical sheet  10 ,  30 ,  30 ′,  50  at a small angle relative to the normal to the light-exiting face of the optical sheet  10 ,  30 ,  30 ′,  50  partly passes through the prisms  13 ,  33 ,  53  and travels towards the PDP  2  from the observer side. Since the PDP  2  has phosphors for emitting visible light, the phosphors reflects the external light L 21  entering the phosphors from the observer side, towards the observer side at a high reflectance. The intensity of the reflected light L 22  traveling again towards the optical functional sheet layer  12 ,  32 ,  52  varies periodically along the direction in which the prisms  13 ,  33 ,  53  are arranged. The cycle with which the intensity of the light L 22  varies corresponds to the pitch with which the prisms  13 ,  33 ,  53  are arranged. 
     It is expected that, because of the cycle of this cyclic light L 22  and the pitch with which the prisms  13 ,  33 ,  53  are disposed, interference fringes (hereinafter also referred to as “self-moiré” in order to distinguish them from moiré fringes due to both of the pitch with which pixels are disposed on the PDP  2  and to the pitch with which the prisms  13 ,  33 ,  53  are disposed) will occur. Since the cycle of the cyclic light L 22  corresponds to the pitch with which the prisms  13 ,  33 ,  53  are disposed, it is presumed that controlling the pitch with which the prisms  13 ,  33 ,  53  are disposed is not effective in making the self-moiré less noticeable. On the other hand, it is presumed that the self-moiré can be made less noticeable by placing a layer having the function of diffusing light although such a layer lowers contrast. 
     In the aforementioned optical sheet  10 ,  30 ,  30 ′,  50 , the electromagnetic-wave shield layer  11 ,  31 ,  51  is situated between the optical functional sheet layer  12 ,  32 ,  52  and the PDP  2 . The electromagnetic-wave shield layer  11 ,  31 ,  51  diffuses light more greatly than the other layers in the optical sheet  10 ,  30 ,  30 ′,  50 , though the absolute amount of the light the electromagnetic-wave shield layer diffuses is extremely small. Therefore, the light L 21  that has passed through the optical functional sheet layer  12 ,  32 ,  52  and entered the electromagnetic-wave shield layer  11 ,  31 ,  51  is diffused as shown in  FIG. 12  by the dotted lines. The light L 22  that has been reflected from the PDP  2  and re-entered the electromagnetic-wave shield layer  11 ,  31 ,  51  is further diffused as shown in the figure by the dotted lines. Consequently, the electromagnetic-wave shield layer  11 ,  31 ,  51  diffuses two times the light that is the cause of self-moiré. It is thus presumed that the periodicity of the light L 22  re-entering the optical functional sheet layer  12 ,  32 ,  52  will be reduced. Namely, it is considered that, since the electromagnetic-wave shield layer  11 ,  31 ,  51  diffuses light two times so as to reduce the periodicity of the light, no interference fringes (self-moiré) occur. 
     Further, as mentioned above, the electromagnetic-wave shield layer  11  diffusely reflects light, so that, regardless of the angle at which external light has entered the optical sheet  10 ,  30 ,  30 ′,  50 , the optical functional sheet layer  12 ,  32 ,  52  can partly absorb the light L 22  that has been reflected from the PDP  2  and is traveling towards the observer side. It is therefore considered that the electromagnetic-wave shield layer can prevent the occurrence of interference fringes (self-moiré) regardless of the angle at which external light comes in the optical sheet  10 ,  30 ,  30 ′,  50  and of the angle at which an observer observes the optical sheet  10 ,  30 ,  30 ′,  50 . 
     Even when the electromagnetic-wave shield layer  11 ,  31 ,  51  is situated on the observer side relative to the optical functional sheet layer  12 ,  32 ,  52 , external light that causes self-moiré passes through the electromagnetic-wave shield layer  11 ,  31 ,  51  two times. In this case, however, the light-diffusing property of the electromagnetic-wave shield layer  11 ,  31 ,  51  acts only to scatter the once produced interference fringes (self-moiré) to make them less noticeable, and does not act to prevent interference fringes (self-moiré) for occurring. It can therefore be presumed that the electromagnetic-wave shield layer situated on the observer side of the optical functional sheet layer cannot fully make the interference fringes less noticeable, as supported by the results of the evaluations made in the following Examples. 
     In the meantime, the image light from the PDP  2  passes, only once, through the electromagnetic-wave shield layer  11 ,  31 ,  51  that has the property of slightly diffusing light. Therefore, the image light is never diffused excessively so as to form a poor-quality image. 
     Further, from the above viewpoint, it is effective that a light-diffusing layer  70  having the function of diffusing light is further placed on the side opposite to the observer side relative to the optical functional sheet layer  12 ,  32 ,  52 , when the self-moiré cannot be fully made less noticeable. 
     Specifically, the above embodiment is as follows. As shown in  FIG. 13A , in the electromagnetic-wave shield layer  11 ,  31 ,  51 , an electrically conductive pattern part  72  is formed on a surface, on the observer side of the base  71 , and irregularities  73  that diffuse light are made in the other surface of the base  71 . In this case, the electromagnetic-wave shield layer  11 ,  31 ,  51  also functions as the light-diffusing layer  70 . In  FIG. 13A , the electromagnetic-wave shield layer  11 ,  31 ,  51  is fixed to the optical functional sheet layer  12 ,  32 ,  52  with an adhesive layer  75 . 
     Another possible embodiment is as follows. As shown in  FIG. 13B , in the electromagnetic-wave shield layer  11 ,  31 ,  51 , an electrically conductive pattern part  72  is formed on a surface, on the observer side of the base  71 , and an adhesive layer  75  containing light-diffusing particles  76 , useful for bonding the electromagnetic-wave shield layer  11 ,  31 ,  51  to other layer, is situated on the observer side of the electromagnetic-wave shield layer  11 ,  31 ,  51 . The adhesive layer herein encompasses a pressure-sensitive adhesive layer. 
     Referring to the embodiments that are considered to be most practical and preferred at the present time, the present invention has been described. However, the present invention is not limited to the above-described embodiments. The aforementioned embodiments are to be considered in all respects as illustrative and not restrictive, and various changes may be made without departing from the scope of the invention. 
     For example, in the aforementioned embodiments, the optical sheet  10 ,  30 ,  30 ′,  50  includes the base plate layer  21 ,  41 ,  61 , and the optical functional sheet layer  12 ,  32 ,  52  is bonded to the base plate layer  21 ,  41 ,  61  either directly or indirectly. The present invention is not limited to this, and the optical functional sheet layer  12 ,  32 ,  52  may be bonded to a plasma display panel  2  together with the electromagnetic-wave shield layer  11 ,  31 ,  51 . Similarly, the layers in the optical sheet  10 ,  30 ,  30 ′,  50  other than the optical functional sheet layer  12 ,  32 ,  52  and the electromagnetic-wave shield layer  11 ,  31 ,  51  also may be bonded to a plasma display panel  2 . Moreover, the base plate layer  21 ,  41 ,  61  may be eliminated from the optical sheet  10 ,  30 ,  30 ′,  50 . 
     EXAMPLES 
     By way of the following Examples, the present invention will now be explained more specifically. However, these examples are not intended to limit or restrict the scope of the invention in any way. 
     Plasma televisions according to Examples and Comparative Examples were produced in the following manner, and contrast and self-moiré on each plasma television were evaluated. 
     Samples 
     Example 1 
     An electromagnetic-wave shield layer (EMI) was placed on the image-light-exiting face of a plasma display panel (PDP), and a optical functional sheet layer (CRF) was bonded to the observer-side-surface of the electromagnetic-wave shield layer with an adhesive layer (PSA). The electromagnetic-wave shield layer was the previously-mentioned layer having a transparent base and an electrically conductive pattern part made of a meshed copper film, formed on the observer-side-surface of the transparent base. The adhesive layer contained no light-diffusing particles. In this manner, there was produced a plasma television according to Example 1, composed of the plasma display panel (PDP), the electromagnetic-wave shield layer (EMI), the adhesive layer (PSA), and the optical functional sheet layer (CRF). 
     Example 2 
     A plasma television according to Example 2 was produced in the same manner as in Example 1, except that light-diffusing particles were disposed in the adhesive layer with which the electromagnetic-wave shield layer and the optical functional sheet layer were bonded to each other. 
     Example 3 
     A plasma television according to Example 3 was produced in the same manner as in Example 1, except that irregularities were made in the PDP-side-surface of the transparent base of the electromagnetic-wave shield layer so as to make the surface matted. 
     Comparative Example 1 
     A plasma television according to Comparative Example 1 was produced in the same manner as in Example 1, except that the electromagnetic-wave shield layer (EMI) and the optical functional sheet layer (CRF) were laminated in the order reverse to that in the plasma television according to Example 1. 
     Comparative Example 2 
     The same optical functional sheet layer (CRF) as in Example 1 was placed on the image-light-exiting face of a plasma display panel (PDP). In this manner, there was produced a plasma television according to Comparative Example 2 composed of the plasma display panel (PDP) and the optical functional sheet layer (CRF). 
     Comparative Example 3 
     The same electromagnetic-wave shield layer (EMI) as in Example 1 was placed on the image-light-exiting face of a plasma display panel (PDP). In this manner, there was produced a plasma television according to Comparative Example composed of the plasma display panel (PDP) and the electromagnetic-wave shield layer (EMI). 
     &lt;Evaluation of Contrast&gt; 
     Lamplight is applied, from the observer side, to the display face of the plasma display at an angle of 45° relative to the normal to the display face such that the illuminance on the display face is 150 lux. The ratio of the brightness of the plasma display panel (PDP) displaying white in the lamplight relative to the brightness of the PDP displaying black in the lamplight is taken as contrast. 
     The results of the evaluations of the plasma televisions of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1. 
     &lt;Evaluations of Interference Fringes&gt; 
     At different angles of projection, external light was projected on each plasma television displaying no image, and the plasma television was observed as to whether interference fringes were noticeable or not. The results are shown in Table 1. In Table 1, the plasma television on which the interference fringes were noticeable when external light was projected at a certain angle is indicated by x, and the plasma television on which the interference fringes were not noticeable regardless of the angle at which external light was projected is indicated by O. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Results of Evaluations 
               
             
          
           
               
                   
                   
                   
                 Interference 
               
               
                 Sample 
                 Laminated Structure 
                 Contrast 
                 Fringes 
               
               
                   
               
               
                 Example 1 
                 PDP/EMI/PSA/CRF 
                 46.7 
                 ◯ 
               
               
                 Example 2 
                 PDP/EMI/PSA(containing 
                 46.0 
                 ◯ 
               
               
                   
                 light-diffusing particles)/CRF 
               
               
                 Example 3 
                 PDP/EMI(with a matted surface 
                 46.0 
                 ◯ 
               
               
                   
                 on the PDP side)/PSA/CRF 
               
               
                 Comparative 
                 PDP/CRF/PSA/EMI 
                 41.9 
                 X 
               
               
                 Example 1 
               
               
                 Comparative 
                 PDP/CRF 
                 46.5 
                 X 
               
               
                 Example 2 
               
               
                 Comparative 
                 PDP/EMI 
                 26.0 
                 ◯ 
               
               
                 Example 3