Patent Publication Number: US-2016238863-A1

Title: Optical sheet, image source unit and image display device

Description:
TECHNICAL FIELD 
     The present invention relates to optical sheets that control light emitted by light sources, to emit the light in observer sides, and image source units and image display devices that include the optical sheets. 
     BACKGROUND ART 
     Light sources and optical sheets are included in image display devices that emit images to observers, like liquid crystal displays, rear projection display devices, display devices using organic electroluminescence, and field emission displays (FEDs). Such an optical sheet is composed of a plurality of layers that have various functions for improving the quality of light emitted by the light source to offer the light to observers. 
     For example, such an optical sheet is disclosed in each of Patent Literatures 1 and 2. 
     The optical sheet (light control film) of Patent Literature 1 has transmissive regions that are arranged in a row along a sheet surface so as to be light-transmissive, and absorptive regions that are arranged in a row, each of which is between transmissive regions, so as to be light-absorptive. It is disclosed that a width at the narrowest region of a transmissive region and pitch of transmissive regions have a certain relationship. 
     The optical sheet of Patent Literature 2 includes an optical functional sheet layer that has prisms arranged in a row along a sheet surface so as to be light-transmissive, and light absorption parts arranged in a row, each of which is between prisms, so as to be light-absorptive. Thereby, an image light and outside light are reflected and absorbed, to improve the quality of the image light. 
     Here, interference fringes and scintillation (so-called glare) are sometimes generated in such an optical sheet, originating from a mode of arrangement of light absorption parts and light transmission parts alternately in a stripe pattern, light concentration due to control of the light, a mode of contact interfaces with other layers, etc. For this generation, Patent Literature 2 discloses that a surface of a base film is prepared to be a rough surface of no less than 0.1 μm in mean roughness (Ra), so that interference fringes can disappear. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] JP 2012-530938 A 
     [Patent Literature 2] JP 2010-217871 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     Having such a light absorption part (absorptive region) makes it possible to absorb light that causes something bad, at the light absorption part to have a proper light blocking effect. “Light blocking effect” means performance in light control in a desired direction. A high light blocking effect makes it possible to control light, not to be emitted in a predetermined direction. On the other hand however, there is a problem that efficiency of light utilization deteriorates because light is absorbed. While the art as Patent Literature 1 is disclosed, conventional arts are not necessarily satisfying in view of having a light blocking effect beyond a certain level and obtaining high efficiency of light utilization. 
     Although the generation of interference fringes can be prevented by providing a rough surface of no less than 0.1 μm in Ra as Patent Literature 2, there sometimes arises a problem that a light blocking effect that is a basic effect of optical sheets deteriorates. 
     In view of the above problems, an object of the present invention is to provide an optical sheet that makes it possible to keep efficiency of light utilization high or to prevent something bad from occurring in a screen like interference fringes, while having a light blocking effect. In addition, the present invention provides an image source unit and an image display device each including the optical sheet. 
     Solution to Problem 
     The present invention will be described hereinafter. 
     A first aspect of the present invention is an optical sheet having a plurality of layers, comprising: a substrate layer; and an optical function layer that is layered on one surface of the substrate layer, and has a plurality of light transmission parts which are arranged in a row along a surface of the substrate layer so as to be light-transmissive, and light absorption parts in a row, each of which is arranged between adjacent ones of the light transmission parts so as to be light-absorptive, wherein in a cross-section of the optical function layer in a layer thickness direction, a cross-sectional area of one of the light transmission parts to a total cross-sectional area of one of the light transmission parts and one of the light absorption parts which are adjacent to each other is 78.2% to 88.5%. 
     A second aspect of the present invention is an optical sheet having a plurality of layers, comprising: a substrate layer; and an optical function layer that is layered on one surface of the substrate layer, and has a plurality of light transmission parts which are arranged in a row along a surface of the substrate layer so as to be light-transmissive, and light absorption parts in a row, each of which is arranged between adjacent ones of the light transmission parts so as to be light-absorptive, wherein optical diffuse reflectance of the optical sheet in a light output surface side is 1.9% to 3.5%. 
     Preferably, optical diffuse reflectance of the optical sheet in a light input surface side is 2.5% to 5.0%. 
     In each of the above aspects, a cross-section of each of the light transmission parts in the layer thickness direction can be a trapezoid. 
     Then, there can be provided an image source unit comprising: a surface light source device; the above optical sheet, which is arranged in a light emission side of the surface light source device; and a liquid crystal panel that is arranged across the optical sheet in an opposite side to the surface light source device. 
     In this image source unit, such a structure can be taken that each of the light transmission parts has a cross-section of a trapezoid, an shorter upper base of the trapezoid thereof facing the surface light source device, a longer lower base of the trapezoid thereof facing the liquid crystal panel, each of the light absorption parts has a cross-section of a trapezoid, a longer lower base of the trapezoid thereof facing the surface light source device, a shorter upper base of the trapezoid thereof facing the liquid crystal panel. 
     There can be further provided an image display device comprising: a housing; and the above image source unit, which is arranged inside the housing. 
     Advantageous Effects of Invention 
     The optical sheet makes it possible to keep efficiency of light utilization high or to prevent something bad from occurring in a screen like interference fringes, while having a light blocking effect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an external perspective view of an image display device  1 . 
         FIG. 2  is an exploded perspective view of an image source unit  5 . 
         FIG. 3  is a cross-sectional view to explain the layer structure of the image source unit  5 . 
         FIG. 4  is an enlarged view focusing on an optical function layer  21 . 
         FIG. 5  is a view to explain measurement of optical diffuse reflectance. 
         FIG. 6  is a cross-sectional view to explain the layer structure of an image source unit  105 . 
         FIG. 7A  is a view to explain an optical function layer  21 ′ and  FIG. 7B  is a view to explain an optical function layer  21 ″. 
         FIG. 8  is a view to explain the mode of the optical function layer of Example 1. 
         FIG. 9  is a view to explain the mode of the optical function layer of Example 2. 
         FIG. 10  is a view to explain the mode of the optical function layer of Example 3. 
         FIG. 11  is a view to explain the mode of the optical function layer of Example 4. 
         FIG. 12  is a view to explain the mode of the optical function layer of Example 5. 
         FIG. 13  is a view to explain the mode of the optical function layer of Example 6. 
         FIG. 14  is a view to explain the mode of the optical function layer of Comparative Example 1. 
         FIG. 15  is a view to explain the mode of the optical function layer of Comparative Example 2. 
         FIG. 16  is a view to explain the mode of the optical function layer of Comparative Example 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter the present invention will be described with reference to the embodiments shown in the drawings. The present invention is not limited to these embodiments. Because most of elements included in the present invention are actually very minute or very thin layers, parts of the elements are varied, enlarged, etc. to be shown in the drawings for understandability. While signs are appended to the elements, parts of signs which are repeated are sometimes omitted for visibility. 
       FIG. 1  is a view to explain one embodiment, and is a perspective view showing an image display device  1  including an image source unit  5 . The right of the page of  FIG. 1  is an observer side. Here, the image display device  1  of this embodiment is an image display device for the use in cars, and examples thereof include a car navigation device. The image display device  1  includes a housing  2 . The image source unit  5  is built in inside the housing  2 . 
     The housing  2  forms an outer shell of the image display device  1 . The housing  2  is a member that houses thereinside almost all the members constituting the image display device. The housing  2  has an opening. A so-called screen portion of the image source unit  5  is exposed through the opening, which makes it possible to be visually recognized. The image display device  1  includes other various known components for functioning as an image display device. 
       FIG. 2  is an exploded perspective view of the image source unit  5 .  FIG. 2  separately shows part of the layers that constitute the image source unit for understandability. Actually, the layers directly overlie each other or the like, to be layered (see  FIG. 3 ).  FIG. 3  is a cross-sectional view in a thickness direction, including the line shown by in  FIG. 2  (the line in a vertical direction). When the image source unit  5  is disposed in the image display device  1 , the right of the sheet of each of  FIGS. 2 and 3  is an observer side, and the left thereof is a light source side. 
     The image source unit  5  is composed of an image source  10  and a function layer  30  that is arranged in an image emission side of the image source  10  (that is, in the observer side). 
     This embodiment is structured to include a liquid crystal panel  12  in the image source  10 . Specifically, the image source  10  includes a surface light source device  11 , an optical sheet  20  and the liquid crystal panel  12 . That is, in this embodiment, the optical sheet  20  is arranged between the surface light source device  11  and the liquid crystal panel  12 . 
     Here, the surface light source device  11  and the liquid crystal panel  12  of known structures can be used. 
     Examples of the surface light source device  11  include a surface light source device composed of a reflection sheet, a light guide plate (a light emission source is arranged in its side surface), a diffusion sheet, a lens (prism) sheet and a reflective polarizing sheet, which are layered in this order from the light source side (the left of the page of  FIG. 3 ) to the observer side (the right thereof). 
     Examples of the liquid crystal panel  12  include a liquid crystal panel composed of a polarizing film, a glass substrate, a liquid crystal layer, a glass substrate and a polarizing film, which are layered in this order from the light source side (the left of the page of  FIG. 3 ) to the observer side (the right thereof). 
     In this embodiment, the optical sheet  20  consists of a plurality of layers, and is arranged between the surface light source device  11  and the liquid crystal panel  12  in a light emission side of the surface light source device  11 . In this embodiment, the optical sheet  20  includes an optical function layer  21  and a substrate layer  25 , which are arranged in this order from the side of the surface light source device  11 . Each of the layers will be described below. For convenience, the substrate layer  25  will be described first, and after that, the optical function layer  21  will be described. 
     The substrate layer  25  is a layer that is a substrate for forming the optical function layer  21  on one surface thereof. The substrate layer  25 , having translucency, supports the optical function layer  21  so as to prevent the optical function layer  21  from deforming. In view of this, concrete examples of materials that constitute the substrate layer  25  include transparent resin mainly constituted by an acrylic resin, styrene, polycarbonate, polyethylene terephthalate (PET), acrylonitrile and/or triacetyl cellulose (TAC), and epoxy acrylate and urethane acrylate based reactive resin (such as ionizing radiation curable type resin). 
     Among them, it is preferable to use TAC, a metacrylate resin and/or polycarbonate, which is/are with low birefringence, in view of the combination with the liquid crystal panel. Further, it is desirable to use polycarbonate, which has a high glass transition temperature, for the use in cars and so on where a high heat resisting property is required. Specifically, the glass transition temperature of polycarbonate is 143° C. Polycarbonate is suitable for the use in cars, where durability at 105° C. is generally required. 
     Non-limiting thickness of the substrate layer  25  is preferably 25 μm to 300 μm. There is a risk that some problem occurs in processability in a case where thickness of the substrate layer  25  is out of the above range. For example, creases tend to be generated on the substrate layer  25  thinner than 25 If the substrate layer  25  is thicker than 300 it gets difficult to roll up the optical sheet  20 . 
     A surface of the substrate layer  25  which is not in touch with the optical function layer  21  may be a rough surface. In a case where a rough surface is formed, its surface roughness is preferably 0.1 μm to 0.2 μm in Ra (μm) (JIS B 0601 (2001) arithmetical mean roughness). The surface roughness within the above range makes it possible to hold down: generation of interference fringes due to so-called optical contact when the optical sheet  20  touches other layers; and generation of something bad in the exterior view such as scintillation (so-called glare on the screen), which originates from people&#39;s easy recognition of unevenness based on the roughness. 
     In this embodiment, the optical function layer  21  has a function of changing a direction of an image light to the front direction, and absorbing part of an image light that is emitted by the image source  10 , so that the image light is not reflected in a windshield of a car. That is, the optical function layer  21  is a layer having a light blocking effect of controlling a direction where light travels.  FIG. 4  shows a partially enlarged view of  FIG. 3 , focusing on the optical function layer  21 . 
     In this embodiment, the optical function layer  21  has a cross-section shown in  FIGS. 3 and 4 , and has a shape extending in a direction into and out of the page. In this embodiment, this extending direction is a horizontal direction in a state where the image display device  1  is installed in a car. This prevents an image light from being reflected in a windshield as described below. 
     The optical function layer  21  includes light transmission parts  22 , which are trapezoids in the cross-section appearing in  FIGS. 3 and 4 , and light absorption parts  23  having cross-sections of trapezoids, each of which is formed between two adjacent light transmission parts  22 . Thus, in this embodiment, the light transmission parts  22  and the light absorption parts  23  are arranged in a row alternately in a vertical direction in a state where the image display device  1  is installed in a car. 
     A light transmission part  22  is a part whose main function is to transmit light. In this embodiment, the light transmission part  22  is an isosceles trapezoid having a longer lower base in the substrate layer  25  side (observer side) and a shorter upper base in the opposite side (surface light source device side), in the cross-section appearing in  FIGS. 3 and 4 . The light transmission part  22  extends along a surface of the substrate layer  25  while keeping the cross-section, and some light transmission parts  22  are arranged in a row at intervals in a direction different from this extending direction. A space of a trapezoidal cross-section is formed between adjacent light transmission parts  22 . Thus, this space has the trapezoidal cross-section of a longer lower base in the upper base side of the light transmission part  22  and a shorter upper base in the lower base side of the light transmission part  22 . The space is filled with necessary materials described later, which forms a light absorption part  23 . This embodiment has a linking part  24  that links the adjacent light transmission parts  22  together in their longer lower base sides. 
     A refractive index of the light transmission part  22  is represented by Nt. Such a light transmission part  22  can be formed by curing transmission part constituting components. A non-limiting value of the refractive index Nt is preferably no less than 1.55 in view of proper total reflection of light on the interface between inclines of the trapezoidal cross-sections of the light transmission part  22  and the light absorption part  23  as described below. More preferably the refractive index is no less than 1.56. It is noted that the refractive index Nt is preferably no more than 1.61 because most of materials of excessively high refractive indexes easy to crack. 
     Here, examples of components constituting the light transmission part include epoxy acrylate, urethane acrylate, polyether acrylate, polyester acrylate and polythiol based ionizing radiation (like ultraviolet) curable type resin. 
     The light absorption part  23  is arranged in the above described space that is formed between the adjacent light transmission parts  22 , and its shape of a cross-section is same as that of the space. Therefore, the cross-section of the light absorption part  23  is an isosceles trapezoid whose shorter upper base faces the substrate layer  25  side (observer side) and whose longer lower base faces the opposite side (surface light source device  11  side). A refractive index of the light absorption part  23  is represented by Nr. The light absorption part  23  is configured so as to be able to absorb light. Specifically, light absorbing particles disperse into binder having the refractive index of Nr. The refractive index Nr is lower than the refractive index Nt of the light transmission part  22 . A non-limiting value of the refractive index Nr is preferably no more than 1.50, and more preferably no more than 1.49. The reference index Nr is preferably no less than 1.47 in view of availability. 
     Non-limiting difference between the refractive index Nt of the light transmission part  22  and the refractive index Nr of the light absorption part  23  is preferably no less than 0.05. As well, a non-limiting upper limit of the difference in refractive index is preferably no more than 0.14 in view of availability of materials. 
     Non-limiting examples of materials used as the binder here include photocurable resin such as urethane (meth)acrylate, polyester (meth)acrylate, epoxy (meth)acrylate and butadiene (meth)acrylate. 
     While light absorbing colored particles such as carbon black are preferably used as the light absorbing particles, the light absorbing particles are not limited thereto. Colored particles that selectively absorb a specific wavelength may be used for the light absorbing particles depending on characteristics of an image light. Specific examples of colored particles include carbon black, graphite, metallic salts such as black iron oxide, organic fine particles colored with dyes, pigments and the like, and colored glass beads. Especially, colored organic fine particles are preferably used in view of the cost, quality, availability and so on. An average particle size of colored particles is preferably 0.01 μm to 20 μm. 
     In this embodiment, 78.2% to 88.5% is a percentage of the cross-sectional area of the light transmission part  22  in the total cross-sectional area of one adjacent light transmission part  22  and light absorption part  23  (this is the cross-sectional area that is in the cross-section appearing in  FIG. 4 , and that is in the direction orthogonal to the direction where the light transmission part  22  and the light absorption part  23  extend) (this percentage is referred to as “a proportion of the cross-sectional area of the light transmission part”). Here, the light transmission part  22  is that existing between adjacent light absorption parts  23 , and in this embodiment, the cross-sectional area of the light transmission part  22  is that of an isosceles trapezoid, as shown by S 1  in  FIG. 4 . The cross-sectional area of the light absorption part  23  is that of an isosceles trapezoid in this embodiment as shown by S 2  in  FIG. 4 . 
     This makes it possible to realize a high level of the efficiency of light utilization while a predetermined light blocking effect is kept. If the proportion of the cross-sectional area of the light transmission part is smaller than 78.2%, the efficiency of utilization of light supplied from the light source gets low. On the other hand, if the proportion of the cross-sectional area of the light transmission part is beyond 88.5%, the light blocking effect deteriorates, which results in transmission of light that is to be removed. In addition, if the proportion of the cross-sectional area of the light transmission part is beyond 88.5%, the light absorption part gets minuter, which makes it difficult to make the light absorption part of a high accuracy. 
     There is nothing special to be limited in addition to the above. The light transmission part  22  and the light absorption part  23  may be formed as follows as an example. That is, a pitch of the light transmission part  22  and the light absorption part  23 , which is shown by Pk in  FIG. 3 , is preferably 30 μm to 100 μm. It is also preferable that: the interface between inclines of the light absorption part  23  and the light transmission part  22 , and a normal line to the surface of the optical function layer  21  be at an angle within the range of 0° to 10°, which is shown by θk in  FIG. 4 . It is also preferable that thickness of the light absorption part  23 , which is shown by Dk in  FIG. 3 , be 60 μm to 150 μm. The pitch, angle and thickness within the above ranges allows balance between light transmission and light absorption to be much better. 
     This embodiment illustrates that the interface between the light transmission part  22  and the light absorption part  23  (leg part) is in a straight line on the cross-section. The interface is not limited thereto, and may be in a zigzag line, a curved convex surface, a curved concave surface, or the like. A plurality of the light transmission parts  22  and the light absorption parts  23  may have the same shape of the cross-sections, and may have different shapes thereof with predetermined regularities. 
     In this embodiment, the optical sheet  20  can be structured so that its optical diffuse reflectance measured in its light output side is 1.9% to 3.5%. This makes it possible to prevent interference fringes and scintillation, and at the same time to highly secure a demanded light blocking effect of the optical sheet  20 . If this optical diffuse reflectance is smaller than 1.9%, interference fringes and scintillation (glare) are easy to be generated. In contrast, if this optical diffuse reflectance is larger than 3.5%, there is a risk that some problem arises in the light blocking effect. 
     This optical diffuse reflectance measured in the light output side can be obtained with, for example, Haze Meter HR100 (Murakami Color Research Laboratory). More specifically, this optical diffuse reflectance is obtained as follows.  FIG. 5  shows a view for explanation. 
     The optical sheet  20  is disposed on an integrating sphere  50  so that the surface of the optical sheet  20  in its light output side faces the inside of the integrating sphere  50 . Then, incident light Ln is irradiated, inclining at an angle of 45° to a normal line n to the light output surface of the optical sheet  20 . As a result, the incident light Ln with which the optical sheet  20  is irradiated divides into specular reflection light Ls, diffuse reflection light Lk and transmission light Lt, to travel in their own directions. In this situation, the specular reflection light Ls and the transmission light Lt are released to the outside of the integrating sphere  50 , so as not to return to the inside of the integrating sphere  50 . Whereby, all the light inside the integrating sphere  50  is the diffuse reflection light Lk. Measuring its total strength makes it possible to obtain strength of the diffuse reflection light Lk. The obtained strength of the diffuse reflection light Lk is divided by strength of the incident light Ln, and is represented in percentage terms, to calculate optical diffuse reflectance (%). 
     Means of making the optical diffuse reflectance measured in the light output side of the optical sheet  20 , 1.9% to 3.5% is not especially limited. Examples of this means include known light diffusing means such as: making a surface of the substrate layer  25  rough; and dispersing light-scattering particles into the substrate layer  25 . 
     Such optical diffuse reflectance is not decided only depending on a mode of the light diffusing means, but is also influenced by modes of the light transmission part and the light absorption part. Thus, the optical diffuse reflectance is related to a whole mode of the optical sheet. 
     Further, optical diffuse reflectance measured in the light input side of the optical sheet  20  is preferably 2.5% to 5.0%. This makes it possible to hold down scintillation (so-called glare). 
     The above described method of obtaining optical diffuse reflectance in the light output side can be followed by a method of obtaining optical diffuse reflectance in the light input side: the light input side of the optical sheet is directed towards the inside of the integrating sphere  50 ; measurement is carried out; strength of the diffuse reflection light Lk is obtained; and its percentage to the incident light Ln is calculated. 
     Nothing particular is limited in order to obtain such optical diffuse reflectance in the light input side, and the light input surface of the optical function layer  21  may be adjusted. In addition, for example, a structure as shown in  FIG. 6  can be taken.  FIG. 6  corresponds to  FIG. 3 , and is a cross-sectional view showing a layer structure of an image source unit  105 . The image source unit  105  includes an optical sheet  120 . This optical sheet  120  is structured so that a transparent resin layer  121  is layered on a surface of the above described optical function layer  21  of the optical sheet  20  in the opposite side to the substrate layer  25 . Optical diffuse reflectance in its light input side can be a desired value by employing light diffusing means such as: making a surface of the transparent resin layer  121  rough; containing light-scattering particles in the transparent resin layer  121 ; and employing both means. 
     In this case as well, the optical diffuse reflectance is not decided only depending on a mode of the light diffusing means, but is also influenced by modes of the light transmission part and the light absorption part. Thus, the optical diffuse reflectance is related to a whole mode of the optical sheet. 
       FIG. 7  shows an example of asymmetric leg parts in the trapezoidal cross-sections.  FIG. 7  corresponds to  FIG. 4 .  FIG. 7A  shows an example of an optical function layer  21 ′ and  FIG. 7B  shows an example of an optical function layer  21 ″. 
     The optical function layer  21 ′ shown in  FIG. 7A  has light transmission parts  22 ′ and light absorption parts  23 ′. Materials constituting this, and ways of grasping its refractive index, proportion of the area of the light transmission part, and optical diffuse reflectance are same as those of the optical function layer  21 . 
     As is seen from  FIG. 7A , the leg parts of the light absorption part  23 ′ in the trapezoidal cross-section have a further feature in this example. Two leg parts of the light absorption part  23 ′ form different angles with a normal line to a layer surface of the optical function layer  21 ′. In more detail, the angle formed by a leg part  23 ′ a , which is a lower leg part in a state where the image display device is installed, is smaller than that formed by a leg part  23 ′ b , which is an upper leg part in this state. More preferably, the angle formed by the leg part  23 ′ a  is 0° (that is, the leg part  23 ′ a  is parallel to the normal line to the layer surface of the optical function layer  23 ′). This makes it possible to efficiently hold down light (image light) travelling upward, and to further hold down reflection in a windshield in a case of installation in a car. 
     On the other hand, the optical function layer  21 ″ shown in  FIG. 7B  has light transmission parts  22 ″ and light absorption parts  23 ″. Ways of grasping its proportion of the area of the light transmission part, and optical diffuse reflectance are same as those of the optical function layer  21 . The optical function layer  21 ″ has a mode that inclines of the leg parts have an inverse relationship to those of the optical function layer  21 ′. Whereby emission of image light diagonally upward increases. In a case where it is no problem to reflect light in a windshield, a clear image can be observed in a view diagonally upward. 
     Such an optical sheet  20  is made as follows for example. 
     First, the light transmission part  22  is formed on one surface of the substrate layer  25 . For this, a substrate sheet that is to be the substrate layer  25  is inserted between a mold roll having such a shape on its surface that the shape of the light transmission part  22  can be copied thereon and a nip roll arranged so as to face the mold roll. Then, the mold roll and the nip roll are rotated while composition for constituting the light transmission part is supplied between the substrate sheet and the mold roll. Whereby, the composition for constituting the light transmission part is filled into grooves (have a shape of an inverted light transmission part) formed on the surface of the mold roll, which correspond to the light transmission part, and thus the shape of the composition corresponds to that of the surface of the mold roll. 
     The composition for constituting the light transmission part, which is sandwiched and filled between the mold roll and the substrate sheet, is irradiated with light for curing in the substrate sheet side by a light irradiation device, which makes it possible to cure the composition and fix its shape. Then, the substrate layer  25  and the molded light transmission part  22  are separated from the mold roll by means of a release roll. 
     Next, the light absorption part  23  is formed. In order to form the light absorption part  23 , first, composition for constituting the light absorption part is filled into intervals between the above molded light transmission parts  22 . Thereafter, an excessive amount of the composition is scraped off by a doctor blade or the like. Then, the remaining composition is irradiated with ultraviolet rays in the light transmission part  22  side to be cured, to form the light absorption part  23 . 
     Examples of the function layer  30  include known layers arranged nearer to the observer side than the liquid crystal panel and having various functions, such as anti-reflection layers, anti-glare layers and hardcoat layers. 
     The image source unit  5  having the structure as described above can be made as follows for example: that is, the made optical sheet  20  is arranged in the light emission side of the surface light source device  11  so that a side thereof opposite to the substrate layer  25  (that is, the optical function layer  21 ) faces the surface light source device  11 . The liquid crystal panel  12  and the function layer  30  are layered on the observer side of the optical sheet  20 . 
     The image source unit  5  structured as above is housed into the housing  2 , and the housing  2  is arranged so that the function layer  30  side is in the observer side, which makes it possible to structure the image display device  1 . At this time, electric circuits, power supply circuits and the like for operating the image source unit  5  are also provided if necessary. 
     Such an image display device is installed in a car, and operates as follows for example, which will be described with examples of optical paths. These examples of optical paths are for the description and conceptual, and do not illustrate reflection, refraction or the like strictly. 
     When the image display device  1  is operated, the surface light source device  11  emits lighting as shown in  FIG. 3 . Light L 10  emitted by the surface light source device  11  transmits the optical sheet  20  without reaching any interface between the light transmission part  22  and the light absorption part  23 , transmits the liquid crystal panel  12  as obtaining image information in the liquid crystal panel  12 , and also transmits the function layer  30 , to reach the observer side. An observer can observe the image light. 
     Light L 11  emitted by the surface light source device  11  reaches an interface between the light transmission part  22  and the light absorption part  23 . Total reflection occurs in the light L 11  in relation to difference between both in refractive index and its incidence angle at the interface. The light L 11  transmits the liquid crystal panel  12  as obtaining image information in the liquid crystal panel  12 , and also transmits the function layer  30 , to be emitted in the observer side. At this time, since the interface between the light transmission part  22  and the light absorption part  23  inclines to the normal line to the light output surface of the optical sheet  20  as described above, the direction of the light L 11  is changed downward, and the light is blocked from travelling upward (light blocking is carried out), which prevents the light L 11  from being reflected in a windshield. Since the light L 11  directs in the front direction, such light L 11  also contributes to improvement of front brightness of image light. 
     Light L 12  emitted by the surface light source device  11  reaches an interface between the light transmission part  22  and the light absorption part  23 , transmits the interface in relation to difference between both in refractive index and its incidence angle at the interface, and is absorbed by the light absorption part  23 . This blocks the light from travelling upward (light blocking is carried out), and prevents the light from being reflected in a windshield. 
     Since the proportion of the cross-sectional area of the light transmission part is constituted as described above in this embodiment, a stray light like the light L 12  is absorbed, and at the same time light emitted in the observer side like the light L 10  and the light L 11  can be efficiently obtained, which makes it possible to keep the efficiency of utilization of light high. 
     Optical diffuse reflectance in at least the light output side of the optical sheet is constituted as described above in this embodiment, occurrence of interference fringes can be held down, and light can be blocked from travelling in an unintended direction due to diffusion of the light (in this embodiment, reflection in a windshield due to light travelling upward), which makes it possible to carry out light blocking. Therefore, a high light blocking effect can be kept, and high front brightness can be achieved. 
     EXAMPLES 
     As Examples, optical sheets according to six examples where shapes of the cross-sections of the light transmission parts and the light absorption parts varied (Example 1 to Example 6) were made, to check the efficiency of light utilization. As Comparative Examples, optical sheets according to three examples (Comparative Example 1 to Comparative Example 3) were made, to check the efficiency of light utilization as well. 
     Example 1 
     In Example 1, an optical sheet that included an optical function layer having a shape of the cross-section shown in  FIG. 8  was made. This optical sheet was formed of the optical function layer and a substrate layer, and was made by the above described method. See the following for more details: 
     Polycarbonate resin of 130 μm in thickness was used for the substrate layer; 
     UV-curable urethane acrylate of 1.56 in refractive index was used for the light transmission part. The cross-section of the light transmission part was an isosceles trapezoid of 29 μm in upper base, 35 μm in lower base and 102 μm in height; 
     UV-curable urethane acrylate of 1.49 in refractive index was used for the binder of the light absorption part. In the binder, 25 mass % of acrylic beads containing carbon black was included. The cross-section of the light absorption part was an isosceles trapezoid of 4 μm in upper base, 10 μm in lower base and 102 μm in height; and 
     The linking part was 25 μm in thickness (Tk in  FIG. 3 ). 
     A 6.5-inch liquid crystal display (LQ65T5GG03, manufactured by Sharp Corporation) was equipped with the optical sheet as the above, to be a liquid crystal display. More specifically, the optical sheet was arranged in the light output surface side of a surface light source device that consisted of a light guide plate in a side surface of which a light emission source was arranged, a prism sheet, a light diffusion film and a reflective polarizing plate; and a liquid crystal panel was arranged in the light output surface side of the optical sheet. 
     Example 2 
     Example 2 was the same as Example 1 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 9 . 
     Example 3 
     Example 3 was the same as Example 1 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 10 . 
     Example 4 
     The example shown in  FIG. 5  was followed by Example 4. Example 4 was an example that angles of inclination of two leg parts of the light transmission part and the light absorption part were different. Specifically, shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 11 . Here, an angle of inclination of the leg part  23 ′ a  was 0° (that is, the leg part  23 ′ a  was parallel to the normal line to the layer surface of the optical sheet). Other details were same as Example 1. 
     Example 5 
     Example 5 was the same as Example 4 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 12 . 
     Example 6 
     Example 6 was the same as Example 4 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 13 . 
     Comparative Example 1 
     Comparative Example 1 was the same as Example 1 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 14 . Here, some signs used in  FIGS. 14 to 16  represent an optical function layer  121 , a light transmission part  122  and a light absorption part  123 . 
     Comparative Example 2 
     Comparative Example 2 was the same as Comparative Example 1 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 15 . 
     Comparative Example 3 
     Comparative Example 3 was the same as Comparative Example 1 except that shapes of the cross-sections of the light transmission parts and the light absorption parts were changed as  FIG. 16 . 
     Brightness of each of the above made liquid crystal displays according to Examples 1 to 6 and Comparative Examples 1 to 3 was measured in the normal line direction of the optical sheet, assuming that the normal line direction was 0°, and at every 10° of inclination from the normal line to ±80°, with EZContrast (manufactured by ELDIM). Then, their integrated values of brightness were obtained. Thus, it could be said that a larger integrated value of brightness indicated more light emission, which was accompanied by high efficiency of light utilization. 
     On the other hand, as to a light blocking effect, brightness was measured in the normal line direction of the optical sheet, assuming that the normal line direction was 0°, and at every 10° of inclination from the normal line to ±80°, with EZContrast (manufactured by ELDIM). Then, brightness at 45° (the mean of 40° and 50° in brightness) was measured and represented in percentage terms assuming that the front brightness was 100%. A case where a value represented in percentage terms was no more than 10% was regarded as a good light blocking effect. 
     Table 1 shows characteristic shapes of the light transmission parts and the light absorption parts other than the above description and the measurement results. 
     “Proportion of Cross-sectional Area of Light Transmission Part (%)” represents a percentage of the cross-sectional area of the light transmission part  22  or  122  in the total cross-sectional area of one adjacent light transmission part  22  or  122  and light absorption part  23  or  123 , respectively. That is, this percentage can be calculated by “an area of one light transmission part/(an area of one light transmission part+an area of one light absorption part) in each cross-section shown in  FIGS. 8 to 16 . 
     “Proportion of Opening (%)” represents a proportion of an area of the light transmission part  22  or  122  per pitch in a surface side of the optical function layer where incident light comes (in these examples, the shorter upper base side of the light transmission part  22  or  122 ). That is, this proportion can be calculated by “a length of the upper base of one light transmission part/(a length of the upper base of one light transmission part+a length of the lower base of one light absorption part)” in each cross-section shown in  FIGS. 8 to 16 . 
     “Integrated Brightness” and “Light Blocking Effect” are as described above. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Proportion of 
                   
                   
                   
               
               
                   
                 Cross-Sectional 
               
               
                   
                 Area of Light 
                 Proportion 
                 Integrated 
                 Light 
               
               
                   
                 Transmission Part 
                 of Opening 
                 Brightness 
                 Blocking 
               
               
                   
                 (%) 
                 (%) 
                 (cd/m 2 ) 
                 Effect 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 82.1 
                 74.4 
                 2841 
                 6.7% 
               
               
                 Example 2 
                 78.2 
                 71.8 
                 2610 
                 5.2% 
               
               
                 Example 3 
                 88.5 
                 82.1 
                 3200 
                 9.8% 
               
               
                 Example 4 
                 82.1 
                 74.4 
                 2841 
                 6.7% 
               
               
                 Example 5 
                 78.2 
                 71.8 
                 2610 
                 5.2% 
               
               
                 Example 6 
                 88.5 
                 82.1 
                 3200 
                 9.8% 
               
               
                 Comparative 
                 72.3 
                 53.2 
                 2450 
                 4.7% 
               
               
                 Example 1 
               
               
                 Comparative 
                 78.1 
                 75.0 
                 2587 
                 4.5% 
               
               
                 Example 2 
               
               
                 Comparative 
                 76.7 
                 69.8 
                 2530 
                 7.0% 
               
               
                 Example 3 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 1, the integrated brightness of Examples 1 to 6 was larger than that of Comparative Examples 1 to 3, and the efficiency of light utilization in Examples 1 to 6 was found to be high. Here, as can be seen from the comparison between Example 1 and Comparative Example 2 for example, even if the proportions of openings were approximately same, integrated brightness largely differed. All Examples had no more than 10% of good light blocking effects. 
     Liquid crystal displays were made with optical sheets according to six examples that optical diffuse reflectance in the light input sides and optical diffuse reflectance in the light output sides varied (Example 7 to Example 12), to check interference fringes, scintillation (glares) and light blocking effects. As Comparative Examples, liquid crystal displays were made with optical sheets according to two examples (Comparative Example 4 and Comparative Example 5), to check the same. 
     An optical sheet that included an optical function layer having light transmission parts and light absorption parts whose shapes of the cross-sections were as shown in  FIG. 4  was made. More particularly, this optical sheet was formed of the optical function layer, a substrate layer, and layers having micro-roughness. See the following for further details: 
     Matte polycarbonate resin of 130 μm in thickness was used for the substrate layer. That is, a micro-roughness surface formed on a mold was copied on a surface of the substrate layer which was in the opposite side to the optical function layer, to form micro-roughness (matte surface); 
     UV-curable urethane acrylate of 1.56 in refractive index was used for the light transmission part. The cross-section of the light transmission part was an isosceles trapezoid of 29 μm in upper base, 35 μm in lower base and 102 μm in height (Dk in  FIG. 3 ); 
     UV-curable urethane acrylate of 1.49 in refractive index was used for the binder of the light absorption part. In the binder, 25 mass % of acrylic beads containing carbon black was included. The cross-section of the light absorption part was an isosceles trapezoid of 4 μm in upper base, 10 μm in lower base and 102 μm in height (Dk in  FIG. 3 ); 
     The linking part was 25 μm in thickness (Tk in  FIG. 3 ); and 
     In addition, a layer having micro-roughness (matte) was formed of the materials same as the light transmission part, on a surface of the optical function layer which was in the opposite side to the substrate layer, with a mold having a micro-roughness surface. Whereby, the optical sheet whose thickness was 280 μm, which was total thickness of the substrate layer, the optical function layer and the layers having micro-roughness, was obtained. 
     Optical diffuse reflectance in the light input side and optical diffuse reflectance in the light output side were adjusted according to the substrate layers, and change in degree of micro-roughness (surface roughness) provided on the surface of any mold for forming the layers having micro-roughness, to obtain the optical sheets of Examples 7 to 12 and Comparative Examples 4 and 5. Micro-roughness provided on the surface of the molds was formed by copper-plating on the surface of the molds and blasting process on the copper plating with glass beads. Its roughness was adjusted according to diameters of the beads for the blasting process and pressure for the blasting process. 
     The optical diffuse reflectance in the light input side and the optical diffuse reflectance in the light output side of each optical sheet of the above described Examples and Comparative Examples were measured as described above. The measurement results will be shown in Table 1 later along with their evaluation results. 
     Liquid crystal displays of 6.5-inch (LQ65T5GG03, manufactured by Sharp Corporation) were equipped with the optical sheets as the above, to be liquid crystal displays. More specifically, an optical sheet was arranged in the light output surface side of a surface light source device that consisted of a light guide plate in a side surface of which a light emission source was arranged, a prism sheet, a light diffusion film and a reflective polarizing plate; and a liquid crystal panel was arranged in the light output surface side of the optical sheet. 
     A light blocking effect, occurrence of interference fringes and scintillation (glare) of each of the above made liquid crystal displays according to Examples 7 to 12 and Comparative Examples 4 and 5 were evaluated visually. 
     As to “Light Blocking Effect”, brightness was measured in the normal line direction of the optical sheet, assuming that the normal line direction was 0°, and at every 10° of inclination from the normal line to ±80°, with EZContrast (manufactured by ELDIM). Then, a case where a proportion of brightness at 45° (the mean of 40° and 50° in brightness) in the front brightness was no more than 7% was regarded as a good light blocking effect, and is indicated by an open circle. A case where this proportion was no less than 7% is indicated by a cross. 
     It was checked whether “Interference Fringe” and “Scintillation” occurred visually upon turning off the liquid crystal display. A case where no fringe and scintillation occurred is indicated by an open circle, and a case where fringe or scintillation occurred is indicated by a cross. 
     The results are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Optical 
                 Optical 
                   
                   
                   
               
               
                   
                 Diffuse 
                 Diffuse 
               
               
                   
                 Reflectance 
                 Reflectance 
               
               
                   
                 in Light 
                 in Light 
                 Light 
                 Inter- 
               
               
                   
                 Output Side 
                 Input Side 
                 Blocking 
                 ference 
                 Scintil- 
               
               
                   
                 (%) 
                 (%) 
                 Effect 
                 Fringe 
                 lation 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 7 
                 3.2 
                 2.9 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Example 8 
                 3.2 
                 3.8 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Example 9 
                 1.9 
                 2.5 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Example 10 
                 3.5 
                 5.0 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Example 11 
                 3.5 
                 2.5 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Example 12 
                 1.9 
                 5.0 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Comparative 
                 1.5 
                 1.9 
                 ∘ 
                 x 
                 x 
               
               
                 Example 4 
               
               
                 Comparative 
                 3.8 
                 3.2 
                 x 
                 ∘ 
                 ∘ 
               
               
                 Example 5 
               
               
                   
               
            
           
         
       
     
     As is seen from Table 2, in a case where the optical diffuse reflectance in the light output surface side of the optical sheet was 1.9% to 3.5%, all of a light blocking effect, interference fringe and scintillation could be “good”. 
     REFERENCE SIGNS LIST 
     
         
           1  image display device 
           5  image source unit 
           10  image source 
           11  surface light source device 
           12  liquid crystal panel 
           20  optical sheet 
           21  optical function layer 
           22  light transmission part 
           23  light absorption part 
           25  substrate layer