Light diffusion sheet laminate, backlight unit, liquid crystal display device, and information device

Abstract

To improve in-plane luminance uniformity.SOLUTION: A backlight unit 40 is built in a liquid crystal display 50 and guides rays of light emitted from a plurality of light sources 42 toward a display screen 50a. The backlight unit 40 comprises, between the display screen 50a and the plurality of light sources 42, a light diffusion sheet 43 having a first surface 21a to be a light emission surface and a second surface 21b to be a light incident surface. The first surface 21a is provided with a plurality of recesses 22 having a substantially inverted polygonal pyramid shape. The arithmetic average roughness of the second surface 21b is 1.0 μm or more and 3.0 μm or less. The internal haze of the light diffusion sheet 43 is 1.5% or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a light diffusing sheet, a backlight unit, a liquid crystal display device, and an information device. [Background technology]

[0002] In recent years, liquid crystal display devices (hereinafter sometimes referred to as liquid crystal displays) have been widely used as display devices for various information devices such as smartphones, tablet terminals, etc. The mainstream backlights for liquid crystal displays are either a direct type in which a light source is placed on the back of the liquid crystal panel, or an edge light type in which a light source is placed near the side of the liquid crystal panel.

[0003] When a direct-type backlight is adopted, a light diffusion sheet is used to diffuse light from a light source such as an LED (Light Emitting Diode) and increase the uniformity of brightness and chromaticity across the entire screen (see, for example, Patent Document 1).

[0004] The light diffusion sheet diffuses light incident from the light entrance surface by utilizing the diffusion caused by imparting an uneven shape to the light exit surface or by dispersing fine particles having a refractive index different from that of the sheet substrate within the substrate.

[0005] In thin displays of notebook computers, tablet terminals, and the like, a light diffusing sheet is used that has, for example, an inverted pyramid-shaped recess formed on the light exit surface and an embossed light entrance surface. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2011-129277 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, because the light source in a direct backlight is placed directly under the display screen, if the distance from the light source to the light diffusion sheet or the thickness of the light diffusion sheet is reduced as displays become thinner, it becomes difficult for the light diffusion sheet to sufficiently diffuse the light, resulting in a problem of deterioration in the uniformity of brightness within the screen (in-plane brightness uniformity).

[0008] An object of the present disclosure is to provide a light diffusion sheet capable of improving in-plane luminance uniformity. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the light diffusion sheet of the present disclosure is a light diffusion sheet having a first surface serving as a light exit surface and a second surface serving as a light entrance surface, wherein the first surface is provided with a plurality of recesses each having a substantially inverted polygonal pyramid shape, and the second surface has an arithmetic mean roughness of 1.0 μm or more and 3.0 μm or less and an internal haze of 1.5% or less.

[0010] According to the light diffusion sheet of the present disclosure, the arithmetic mean roughness of the second surface, which is the light incidence surface, is 3.0 μm or less, and the internal haze is 1.5% or less, so that the light incident from the second surface reaches the first surface, which is the uneven surface, without being substantially diffused inside the sheet. Therefore, the high-brightness light traveling straight from the light source toward the light diffusion sheet can be uniformly diffused by the concaves of the first surface, so that the light source image can be eliminated and the in-plane brightness uniformity can be improved. In addition, the arithmetic mean roughness of the second surface, which is the light incidence surface, is 1.0 μm or more, so that the decrease in brightness can be suppressed. Therefore, it is possible to respond to further thinning and reduction in the number of light sources.

[0011] In the light diffusion sheet according to the present disclosure, the recesses may be formed in a substantially inverted quadrangular pyramid shape, so that the light traveling straight from the light source can be uniformly diffused on the first surface.

[0012] In the light diffusion sheet according to the present disclosure, the apex angle of the plurality of recesses may be equal to or greater than 80° and equal to or less than 100°. In this way, the light traveling straight from the light source can be uniformly diffused on the first surface.

[0013] The backlight unit according to the present disclosure is incorporated into a liquid crystal display device and directs light emitted from multiple light sources toward a display screen, and includes a light diffusion sheet according to the present disclosure described above between the display screen and the multiple light sources, with the second surface of the light diffusion sheet being positioned toward the multiple light sources.

[0014] According to the backlight unit of the present disclosure, since it is equipped with the light diffusion sheet of the present disclosure described above, it is possible to improve the in-plane brightness uniformity, and therefore it is also possible to accommodate further slimming and a reduction in the number of light sources.

[0015] In the backlight unit according to the present disclosure, the plurality of light sources may be disposed on a reflecting sheet provided on the opposite side of the display screen from the light diffusion sheet, whereby the light is further diffused by multiple reflections between the light diffusion sheet and the reflecting sheet, thereby further improving the in-plane luminance uniformity.

[0016] In the backlight unit according to the present disclosure, the light diffusion sheet may be laminated and disposed between the display screen and the light sources. In this way, the first surface of each light diffusion sheet repeatedly diffuses the light traveling straight from the light source, thereby further improving the in-plane luminance uniformity.

[0017] In the backlight unit according to the present disclosure, the distance between the plurality of light sources and the light diffusion sheet may be 10 mm or less. In this way, the diffusion performance of the light diffusion sheet according to the present disclosure can suppress deterioration of in-plane luminance uniformity.

[0018] A liquid crystal display device according to the present disclosure includes the backlight unit according to the present disclosure described above and a liquid crystal display panel.

[0019] According to the liquid crystal display device of the present disclosure, since it is equipped with the backlight unit of the present disclosure described above, it is possible to improve the in-plane brightness uniformity, and therefore it is also possible to accommodate further slimming and a reduction in the number of light sources.

[0020] An information device according to the present disclosure includes the liquid crystal display device according to the present disclosure described above.

[0021] According to the information device of the present disclosure, since it is equipped with the liquid crystal display device of the present disclosure described above, it is possible to improve the in-plane brightness uniformity, and therefore it is also possible to accommodate further slimming and a reduction in the number of light sources. Effect of the Invention

[0022] According to the present disclosure, it is possible to provide a light diffusion sheet capable of improving in-plane luminance uniformity. [Brief description of the drawings]

[0023] [Figure 1] 1 is a cross-sectional view of a liquid crystal display device according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a backlight unit according to the embodiment. [Diagram 3] 1 is a cross-sectional view of a light diffusion sheet according to an embodiment of the present invention. [Figure 4] FIG. 11 is a cross-sectional view of a light diffusion sheet according to a comparative example. [Diagram 5] 1 is a diagram showing the evaluation results of in-plane luminance uniformity of the light diffusion sheets of Examples 1 to 8 and Comparative Examples 1 and 2. FIG. [Figure 6] 1 is a diagram showing evaluation results of the luminance of the light diffusion sheets of Examples 1 to 8 and Comparative Examples 1 and 2. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] (Embodiment) Hereinafter, a light diffusion sheet, a backlight unit, a liquid crystal display device, and an information device according to the embodiments will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments, and can be arbitrarily modified within the scope of the technical idea of ​​the present disclosure.

[0025] FIG. 1 is an example of a cross-sectional view of a liquid crystal display device according to this embodiment, FIG. 2 is an example of a cross-sectional view of a backlight unit according to this embodiment, and FIG. 3 is an example of a cross-sectional view of a light diffusion sheet according to this embodiment.

[0026] 1, a liquid crystal display device 50 includes a liquid crystal display panel 5, a first polarizing plate 6 attached to the lower surface of the liquid crystal display panel 5, a second polarizing plate 7 attached to the upper surface of the liquid crystal display panel 5, and a backlight unit 40 provided on the rear side of the liquid crystal display panel 5 via the first polarizing plate 6. The liquid crystal display panel 5 includes a TFT substrate 1 and a CF substrate 2 provided to face each other, a liquid crystal layer 3 provided between the TFT substrate 1 and the CF substrate 2, and a sealant (not shown) provided in a frame shape to enclose the liquid crystal layer 3 between the TFT substrate 1 and the CF substrate 2.

[0027] The shape of the display screen 50a of the liquid crystal display device 50 when viewed from the front (top of Figure 1) is, in principle, a rectangle or a square, but is not limited to this and may be any shape, such as a rectangle with rounded corners, an ellipse, a circle, a trapezoid, or an automobile instrument panel.

[0028] In the liquid crystal display device 50, a voltage of a predetermined magnitude is applied to the liquid crystal layer 3 in each sub-pixel corresponding to each pixel electrode to change the alignment state of the liquid crystal layer 3. This adjusts the transmittance of light incident from the backlight unit 40 through the first polarizing plate 6. The light with the adjusted transmittance is emitted through the second polarizing plate 7 to display an image.

[0029] The liquid crystal display device 50 of this embodiment is used as a display device incorporated into various information devices (e.g., in-vehicle devices such as car navigation systems, personal computers, mobile phones, personal digital assistants, portable game machines, copy machines, ticket vending machines, automated teller machines, etc.).

[0030] The TFT substrate 1 includes, for example, a plurality of TFTs arranged in a matrix on a glass substrate, an interlayer insulating film arranged to cover each TFT, a plurality of pixel electrodes arranged in a matrix on the interlayer insulating film and connected to each of the plurality of TFTs, and an alignment film arranged to cover each pixel electrode. The CF substrate 2 includes, for example, a black matrix arranged in a lattice on a glass substrate, color filters including a red layer, a green layer, and a blue layer arranged between each lattice of the black matrix, a common electrode arranged to cover the black matrix and the color filter, and an alignment film arranged to cover the common electrode. The liquid crystal layer 3 is made of a nematic liquid crystal material containing liquid crystal molecules having electro-optical properties. The first polarizing plate 6 and the second polarizing plate 7 include, for example, a polarizer layer having a polarization axis in one direction and a pair of protective layers arranged to sandwich the polarizer layer.

[0031] As shown in FIG. 2, the backlight unit 40 includes a reflective sheet 41, a plurality of light sources 42 arranged two-dimensionally on the reflective sheet 41, a light diffusion sheet 43 provided above the plurality of light sources 42, a first prism sheet 44 and a second prism sheet 45 provided in that order above the light diffusion sheet 43, and a polarizing sheet 46 provided above the second prism sheet 45.

[0032] Although Figure 2 shows an example in which two layers of light diffusion sheets 43 having the same structure are stacked and provided in the backlight unit 40, the light diffusion sheet 43 may be used in a single layer, or may be used in a stack of three or more layers.

[0033] The reflective sheet 41 is made of, for example, a white polyethylene terephthalate resin film, a silver vapor deposition film, or the like.

[0034] The type of light source 42 is not particularly limited, and may be, for example, an LED element or a laser element, and may be an LED element from the viewpoint of cost, productivity, etc. The light source 42 may have a rectangular shape when viewed from above, and in this case, the length of one side may be 10 μm or more (preferably 50 μm or more) and 20 mm or less (preferably 10 mm or less, more preferably 5 mm or less). When an LED is used as the light source 42, multiple LED chips may be arranged on the reflective sheet 41 at regular intervals. Also, a lens may be attached to the LED serving as the light source 42 in order to adjust the light emission angle characteristics of the LED.

[0035] 2 and 3, the light diffusion sheet 43 has a base layer 21. The base layer 21 is configured with, for example, clear polycarbonate as a base material (matrix resin). The base layer 21 does not substantially contain a diffusing agent. The light diffusion sheet 43 (base layer 21) has a first surface 21a that serves as a light exit surface and a second surface 21b that serves as a light entrance surface. That is, the light diffusion sheet 43 is disposed with the second surface 21b facing the light source 42.

[0036] A plurality of recesses 22 each having a substantially inverted polygonal pyramid shape, for example a substantially inverted quadrangular pyramid shape (inverted pyramid shape), are two-dimensionally arranged on the first surface 21a of the light diffusion sheet 43. On the other hand, the arithmetic mean roughness of the second surface 21b of the light diffusion sheet 43 is 3.0 μm or less.

[0037] The internal haze of the light diffusion sheet 43 (base layer 21) is 1.5% or less. Note that the "internal haze" refers to the total haze excluding the surface haze caused by the surface shape (specifically, the recesses 22 of the first surface 21a).

[0038] The apex angle θ of the recesses 22 is 80° or more and 100° or less, for example, 90°, and the arrangement pitch p of the recesses 22 is, for example, about 100 μm. Here, the apex angle θ of the recesses 22 refers to the angle formed by the cross-sectional lines of the inclined surfaces in a cross section that appears when a plane (longitudinal cross section) perpendicular to the second surface 21b (horizontal plane) of the light diffusion sheet 43 is cut so as to pass through the apex of the inverted polygonal pyramid and to perpendicularly cross a pair of inclined surfaces facing each other across the apex. The arrangement pitch p of the recesses 22 refers to the horizontal distance (distance along the direction parallel to the second surface 21b) between the apexes of the inverted polygonal pyramids in each of the adjacent recesses 22.

[0039] In this embodiment, the light diffusion sheet 43 has a single-layer structure of a base layer 21 having an uneven shape (recesses 22) on the first surface 21a. However, instead of this, the light diffusion sheet 43 may have a two-layer structure of a base layer having flat surfaces and a layer having an uneven shape on one surface, or may have a three-layer or more layer structure including a layer having an uneven shape on one surface.

[0040] In addition, in this embodiment, the recesses 22 having an inverted pyramid shape (approximately inverted square pyramid shape) are arranged two-dimensionally to provide an uneven shape on the first surface 21a, but the recesses 22 may have another approximately inverted polygonal pyramid shape, and the recesses 22 may be arranged randomly to the extent that the effect of the present invention is not lost.

[0041] In this disclosure, the term "approximately inverted polygonal pyramid" is used in consideration of the difficulty of forming a geometrically strict inverted polygonal pyramid recess by a normal shape transfer technique, but the term "approximately inverted polygonal pyramid" includes shapes that can be regarded as genuine or substantially inverted polygonal pyramids. Furthermore, "approximately" means that it can be approximated, and for example, "approximately inverted square pyramid" refers to a shape that can be approximated to an inverted square pyramid. For example, "inverted polygonal pyramid truncated" with a flat apex is also included in the "approximately inverted polygonal pyramid" if the apex area is small enough that the effect of the present invention is not lost. Furthermore, shapes that are deformed from the "inverted polygonal pyramid" within the range of unavoidable shape variations due to processing accuracy in industrial production are also included in the "approximately inverted polygonal pyramid".

[0042] Moreover, the "inverted polygonal pyramid" shape of the recesses 22 is preferably a triangular pyramid, a square pyramid, or a hexagonal pyramid, which can be arranged two-dimensionally without gaps. An inverted square pyramid may be selected as the "inverted polygonal pyramid" in consideration of the accuracy of the surface cutting work of a mold (metal roll) used in a manufacturing process such as extrusion molding or injection molding when forming the recesses 22. When the recesses 22 are regularly arranged two-dimensionally, the recesses 22 may be provided on the first surface 21a without gaps, or may be provided at predetermined intervals.

[0043] The first prism sheet 44 and the second prism sheet 45 are, for example, films in which a plurality of grooves having an isosceles triangular cross section are formed adjacent to each other, and the apex angle of the prisms sandwiched between a pair of adjacent grooves is formed at about 90°. The grooves formed in the first prism sheet 44 and the grooves formed in the second prism sheet 45 are disposed so as to be perpendicular to each other. The first prism sheet 44 and the second prism sheet 45 may be formed integrally. The first prism sheet 44 and the second prism sheet 45 may be, for example, a PET (polyethylene terephthalate) film having a prism shape formed by using a UV-curable acrylic resin.

[0044] For example, DBEF series manufactured by 3M may be used as the polarizing sheet 46. The polarizing sheet 46 prevents the light emitted from the backlight unit 40 from being absorbed by the first polarizing plate 6 of the liquid crystal display device 50, thereby improving the brightness of the display screen 50a.

[0045] According to the light diffusion sheet 43 of the present embodiment described above, the first surface 21a, which is the light exit surface, is provided with a plurality of recesses 22 having a substantially inverted polygonal pyramid shape, and the second surface 21b, which is the light entrance surface, has an arithmetic mean roughness of 3.0 μm or less and an internal haze of 1.5% or less. Therefore, the light incident from the second surface 21b reaches the first surface 21a, which is an uneven surface, without being substantially diffused inside the light diffusion sheet 43 (base material layer 21). Therefore, the high-luminance light traveling straight from the light source 42 toward the light diffusion sheet 43 can be uniformly diffused by the recesses 22 of the first surface 21a, so that the image of the light source 42 on the display screen 50a can be eliminated and the in-plane luminance uniformity can be improved. Therefore, it is possible to further reduce the thickness and reduce the number of light sources.

[0046] FIG. 4 shows a cross-sectional configuration of a light diffusion sheet 43A of a comparative example in which a concave-convex shape is provided on the second surface 21b by embossing. In FIG. 4, the same components as those of the light diffusion sheet 43 of the present embodiment shown in FIG. 3 are given the same reference numerals. In the light diffusion sheet 43A of the comparative example, the light traveling straight from the light source 42 is randomly diffused on the second surface 21b, and the light cannot be diffused uniformly by the recesses 22 on the first surface 21a. In other words, the degree to which the image of the light source 42 is eliminated on the first surface 21a varies depending on the position of the light source 42. As a result, a problem occurs in that the in-plane luminance uniformity deteriorates.

[0047] To solve this problem, the arithmetic mean roughness of the second surface 21b, which is the light incidence surface, of the light diffusion sheet 43 is set to 3.0 μm or less. From the viewpoint of improving the in-plane luminance uniformity, the arithmetic mean roughness of the second surface 21b of the light diffusion sheet 43 is preferably 0.5 μm or less, more preferably 0.3 μm or less, even more preferably 0.1 μm or less, and even more preferably 0.05 μm or less. On the other hand, from the viewpoint of suppressing a decrease in luminance, the arithmetic mean roughness of the second surface 21b of the light diffusion sheet 43 is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0048] The problem of in-plane luminance uniformity being deteriorated also occurs when light diffusion is performed by dispersing fine particles (diffusing agent) having a refractive index different from that of the base layer 21 in the base layer 21. That is, in the light diffusion sheet 43 of this embodiment, the smaller the diffusing agent content, i.e., the smaller the internal haze, the better. Specifically, the internal haze of the light diffusion sheet 43 is preferably 5% or less, more preferably 3% or less, even more preferably 1.5% or less, and even more preferably 1.0% or less.

[0049] In the light diffusion sheet 43 of this embodiment, when the recesses 22 are formed in a substantially inverted quadrangular pyramid shape, the light traveling straight from the light source 42 can be uniformly diffused on the first surface 21a.

[0050] In the light diffusion sheet 43 of this embodiment, when the apex angle of the recesses 22 is equal to or greater than 80° and equal to or less than 100°, the light traveling straight from the light source 42 can be uniformly diffused on the first surface 21a.

[0051] The backlight unit 40 of this embodiment is incorporated in a liquid crystal display device 50, and guides light emitted from a plurality of light sources 42 to the display screen 50a. In the backlight unit 40, a light diffusion sheet 43 of this embodiment is disposed between the display screen 50a and the light sources 42, with the second surface 21b facing the light sources 42. Therefore, the light diffusion sheet 43 can improve the in-plane luminance uniformity, and can also accommodate further slimming and a reduction in the number of light sources.

[0052] In the backlight unit 40 of the present embodiment, the light source 42 may be disposed on the reflecting sheet 41 provided on the opposite side of the display screen 50a as viewed from the light diffusion sheet 43. In this way, the light is further diffused by multiple reflections between the light diffusion sheet 43 and the reflecting sheet 41, so that the in-plane luminance uniformity is further improved.

[0053] In the backlight unit 40 of the present embodiment, a plurality of light diffusion sheets 43 may be laminated and disposed between the display screen 50a and the light source 42. In this way, the first surface 21a of each light diffusion sheet 43 repeatedly diffuses the light traveling straight from the light source 42, thereby further improving the in-plane luminance uniformity.

[0054] In the backlight unit 40 of the present embodiment, when the distance between the light source 42 and the light diffusion sheet 43 is 10 mm or less, the diffusion performance of the light diffusion sheet 43 can prevent deterioration of the in-plane luminance uniformity more effectively than before.

[0055] The liquid crystal display device 50 of this embodiment includes the backlight unit 40 of this embodiment and a liquid crystal display panel 5. Therefore, the backlight unit 40 can improve the in-plane luminance uniformity, which can accommodate further slimming and a reduction in the number of light sources. The same effect can be obtained in information devices (personal computers, mobile phones, etc.) incorporating the liquid crystal display device 50 of this embodiment.

[0056] In this embodiment, the number of light sources 42 is not particularly limited, but when multiple light sources 42 are distributed, it is preferable to arrange them regularly on the reflection sheet 41. Arranging regularly means arranging according to a certain rule, and corresponds to arranging the light sources 42 at equal intervals, for example. When arranging the light sources 42 at equal intervals, the center-to-center distance between two adjacent light sources 42 may be 0.5 mm or more (preferably 2 mm or more) and 20 mm or less.

[0057] In this embodiment, the light diffusion sheet 43 (base layer 21) may contain a diffusing agent or other additives within a range in which the effects of the present invention are not lost. The additives that can be contained are not particularly limited, and may be, for example, inorganic particles such as silica, titanium oxide, aluminum hydroxide, barium sulfate, etc., or organic particles such as acrylic, acrylonitrile, silicone, polystyrene, polyamide, etc.

[0058] In this embodiment, the resin that forms the matrix of the base layer 21 is not particularly limited as long as it is made of a material that transmits light, and may be, for example, acrylic, polystyrene, polycarbonate, MS (methyl methacrylate-styrene copolymer) resin, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyimide, etc.

[0059] In the present embodiment, the thickness of the light diffusion sheet 43 is not particularly limited, and may be, for example, 3 mm or less (preferably 2 mm or less, more preferably 1.5 mm or less, and even more preferably 1 mm or less) and 0.1 mm or more. If the thickness of the light diffusion sheet 43 exceeds 3 mm, it becomes difficult to achieve a thin liquid crystal display. On the other hand, if the thickness of the light diffusion sheet 43 is less than 0.1 mm, it becomes difficult to achieve the above-mentioned effect of improving brightness uniformity. The light diffusion sheet 43 may be in the form of a film or a plate (board).

[0060] In this embodiment, the method for producing the light diffusion sheet 43 is not particularly limited, but may be, for example, an extrusion molding method, an injection molding method, or the like.

[0061] The procedure for manufacturing a single-layer light diffusion sheet having a concave-convex shape on its surface using the extrusion molding method is as follows. First, pellet-shaped plastic particles (which may contain a diffusion agent) are put into a single-screw extruder, melted and kneaded while being heated. After that, the molten resin extruded by a T-die is sandwiched between two metal rolls and cooled, then conveyed using a guide roll, and cut into a flat sheet by a sheet cutter to produce a light diffusion sheet. Here, by sandwiching the molten resin using metal rolls having a surface with a shape that is the inverse of a desired concave-convex shape, the inverse shape of the roll surface is transferred to the resin, so that the desired concave-convex shape can be formed on the surface of the light diffusion sheet. In addition, the shape transferred to the resin is not necessarily 100% the shape of the roll surface, so the shape of the roll surface may be designed by calculating backwards from the degree of transfer.

[0062] When using an extrusion molding method to manufacture a two-layer light diffusion sheet having an uneven surface, for example, the pellet-shaped plastic particles required to form each layer are fed into each of two single-screw extruders, and then the same procedure as described above is carried out for each layer, and the resulting sheets are laminated.

[0063] Alternatively, a two-layered diffusion sheet having an uneven surface may be produced as follows. First, pellet-shaped plastic particles required for forming each layer are fed into each of two single-screw extruders, and melted and kneaded while being heated. Then, the molten resins for each layer are fed into one T-die and laminated in the T-die, and the laminated molten resin extruded by the T-die is sandwiched between two metal rolls and cooled. Then, the laminated molten resin is transported using a guide roll, and cut into individual flat plates by a sheet cutter, thereby producing a two-layered diffusion sheet having an uneven surface.

[0064] Alternatively, the light diffusion sheet 43 may be manufactured by shape transfer using UV (ultraviolet rays) as follows. First, a roll having an inverted shape of the concave-convex shape to be transferred is filled with uncured UV-curable resin, and a substrate is pressed against the resin. Next, while the roll filled with the UV-curable resin and the substrate are integrated, the resin is cured by irradiating with UV rays. Next, the sheet to which the concave-convex shape has been transferred by the resin is peeled off from the roll. Finally, the sheet is irradiated with UV rays again to completely cure the resin, and a light diffusion sheet having a concave-convex shape on its surface is manufactured.

[0065] In the present embodiment, a direct-type backlight unit in which a plurality of light sources 42 are distributed on the rear side of the display screen 50a of the liquid crystal display device 50 is used as the backlight unit 40. For this reason, in order to reduce the size of the liquid crystal display device 50, it is necessary to reduce the distance between the light sources 42 and the light diffusion sheet 43. However, reducing this distance is likely to cause a phenomenon in which the luminance of the display screen 50a in the portion located on the region between the distributed light sources 42 is lower than that of the other portions (luminance unevenness).

[0066] In contrast, the use of the light diffusion sheet 43 of this embodiment is useful for suppressing uneven brightness. In particular, in view of the trend toward thinner small and medium-sized liquid crystal displays in the future, the usefulness of the present invention is considered to become even more pronounced when the distance between the light source and the light diffusion sheet is set to 15 mm or less, preferably 10 mm or less, more preferably 5 mm or less, further preferably 2 mm or less, and ultimately 0 mm.

[0067] Examples and Comparative Examples Examples and comparative examples will be described below.

[0068] In the examples and comparative examples, a light diffusion sheet having a substrate layer with a thickness of 130 μm and made of clear polycarbonate as a base material was used. In both the examples and comparative examples, a plurality of recesses having a substantially inverted pyramid shape with a 90° apex angle (inverted pyramid shape) were two-dimensionally arranged at a pitch of 100 μm on the first surface (light exit surface) of the light diffusion sheet. As examples, four types of light diffusion sheets having second surfaces (light entrance surfaces) processed to have arithmetic mean roughnesses Ra of 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm, respectively, were prepared. As comparative examples, a light diffusion sheet having a second surface (light entrance surface) processed to have arithmetic mean roughness Ra of 3.4 μm was prepared.

[0069] The manufacturing method of the light diffusion sheet of the embodiment is as follows. First, the pellet-shaped base resin (plastic resin) is made into a resin film by an extrusion molding machine. Then, one of the two metal rolls is a roll with a convex pyramid shape on the surface, and the other roll is a mirror roll. The two rolls are pressed against the resin film to produce a single-layer light diffusion sheet with an inverted pyramid shape on one side and a mirror surface on the other side.

[0070] The manufacturing method of the light diffusion sheet of the comparative example is as follows. First, a pellet-shaped base resin (plastic resin) was made into a resin film by an extrusion molding machine. Then, one of two metal rolls was a roll with a convex pyramidal surface, and the other roll was an embossing roll with a random matte shape, and both rolls were pressed against the resin film to produce a single-layer light diffusion sheet with an inverted pyramidal shape on one side and an embossed shape on the other side. The difference in roughness of the surface with the embossed shape was controlled by the roughness of the embossing roll surface.

[0071] The surface roughness (arithmetic mean roughness Ra) of the light diffusion sheets in the examples and comparative examples was measured using a Mitutoyo SJ-210 in accordance with JIS B 0601-1994, with the measurement speed set to 0.5 mm / s, the measurement distance set to 4 mm, and the cutoff value λc set to 0.8 mm.

[0072] The internal haze and total light transmittance of the light diffusion sheet of the embodiment were 0.6% and 90.8%, respectively. The internal haze and total light transmittance were measured by filling the recesses (inverted pyramids) on the first surface of the light diffusion sheet with a UV curable resin (the same resin as the matrix resin of the light diffusion sheet). The UV curable resin used had the same refractive index as the matrix resin of the light diffusion sheet. The internal haze and total light transmittance were measured using a haze meter HZ-2 manufactured by Suga Test Instruments Co., Ltd. in accordance with JIS K 7136.

[0073] The in-plane luminance uniformity of the light diffusion sheets of the Examples and Comparative Examples was evaluated as follows. First, two or three light diffusion sheets of the Examples (four types) and Comparative Examples were stacked and arranged on a blue LED array arranged at a pitch of 2.8 mm, two prism sheets were placed on top of them, and a transparent glass plate was placed on top of them to prevent the sheets from floating. The luminance in the vertical upward direction (the direction from the LED array to the glass plate) was measured using a two-dimensional color luminance meter UA-200 manufactured by Topcon Technohouse. Next, the obtained two-dimensional luminance distribution image was corrected for the variation in the emission intensity of each LED, and a filtering process was performed to suppress bright and dark spot noise caused by foreign matter, etc., and then the average value and standard deviation of the luminance of all pixels were calculated. Finally, the "in-plane luminance uniformity" was defined as "average luminance value / standard deviation of luminance", and the in-plane luminance uniformity of the light diffusion sheets of the Examples and Comparative Examples was calculated.

[0074] The evaluation results of the luminance and in-plane luminance uniformity of the light diffusion sheets of the examples and comparative examples are shown in Table 1. The luminance shown in Table 1 is a relative luminance, with the luminance (average value) of the comparative example with the same number of overlapping sheets taken as 1.

[0075] [Table 1]

[0076] In Table 1, Examples 1 to 4 are the results of evaluating the luminance and in-plane luminance uniformity by stacking two light diffusion sheets of the above-mentioned Example having a second surface (light incident surface) processed to have an arithmetic mean roughness Ra of 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm, respectively. Comparative Example 1 is the result of evaluating the luminance and in-plane luminance uniformity by stacking two light diffusion sheets of the above-mentioned Comparative Example having a second surface processed to have an arithmetic mean roughness Ra of 3.4 μm. Examples 5 to 8 are the results of evaluating the luminance and in-plane luminance uniformity by stacking three light diffusion sheets of the above-mentioned Example having a second surface processed to have an arithmetic mean roughness Ra of 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm, respectively. Comparative Example 2 is the result of evaluating the luminance and in-plane luminance uniformity by stacking three light diffusion sheets of the above-mentioned Comparative Example having a second surface processed to have an arithmetic mean roughness Ra of 3.4 μm.

[0077] Fig. 5 shows the relationship between the surface roughness (arithmetic mean roughness) Ra and the in-plane luminance uniformity of the light incident surface of the light diffusion sheet in each of Examples 1 to 8 and Comparative Examples 1 and 2. Fig. 6 shows the relationship between the surface roughness (arithmetic mean roughness) Ra and the luminance of the light incident surface of the light diffusion sheet in each of Examples 1 to 8 and Comparative Examples 1 and 2.

[0078] As shown in Table 1 and Fig. 5, regardless of the number of overlapping light diffusion sheets, the smaller the surface roughness Ra of the light incident surface, the better the in-plane luminance uniformity. In particular, when the surface roughness Ra of the light incident surface was the smallest at 0.03 µm (mirror surface) (Examples 4 and 8), the in-plane luminance uniformity was the greatest for each number of overlapping sheets. Furthermore, when the surface roughness Ra of the light incident surface was 3.0 µm or less, the decrease in in-plane luminance uniformity was suppressed compared to the mirror surface for each number of overlapping sheets.

[0079] On the other hand, as shown in Table 1 and Fig. 6, when the surface roughness Ra of the light incident surface was the smallest at 0.03 µm (mirror surface) (Examples 4 and 8), a decrease in brightness was observed at each number of layers. Also, when the surface roughness Ra of the light incident surface was 1.0 µm or more, no decrease in brightness was observed at each number of layers.

[0080] From the results shown in Table 1, Figures 5 and 6, it was found that the luminance and in-plane luminance uniformity required for the product can be met by appropriately setting the surface roughness Ra of the light incident surface within the range of 3.0 μm or less and adjusting the number of overlapping light diffusion sheets. For example, in a product that requires improved in-plane luminance uniformity and suppression of luminance degradation, the surface roughness Ra of the light incident surface of the light diffusion sheet can be set to 1.0 μm or more and 3.0 μm or less.

[0081] Although the embodiments of the present disclosure (including examples; the same applies below) have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the disclosure. In other words, the description of the above-described embodiments is essentially merely illustrative, and is not intended to limit the present disclosure, its applications, or its uses. [Explanation of symbols]

[0082] 1 TFT substrate 2 CF board 3 Liquid crystal layer 5 Liquid crystal display panel 6 First polarizing plate 7 Second polarizing plate 21 Base material layer 21a 1st page 21b 2nd side 22 Recess 40 Backlight unit 41 Reflective sheet 42 Light source 43 Light diffusion sheet 44 First prism sheet 45 Second prism sheet 46 Polarizing Sheet 50 LCD display device 50a display screen

Claims

1. A laminate of a plurality of light diffusion sheets to be incorporated into a backlight unit, Each of the plurality of light diffusion sheets has a first surface serving as a light exit surface and a second surface serving as a light incident surface, The first surface is provided with a plurality of recesses each having a substantially inverted polygonal pyramid shape, the arithmetic mean roughness of the second surface is 1.0 μm or more and 3.0 μm or less; The plurality of light diffusion sheets have an internal haze of 1.5% or less. Light diffusion sheet laminate.

2. The plurality of recesses are formed in a substantially inverted quadrangular pyramid shape. The light-diffusing sheet laminate according to claim 1 .

3. The apex angle of the plurality of recesses is equal to or greater than 80° and equal to or less than 100°. The light-diffusing sheet laminate according to claim 1 or 2.

4. A backlight unit incorporated in a liquid crystal display device that guides light emitted from a plurality of light sources toward a display screen, a light diffusing sheet laminate according to any one of claims 1 to 3 between the display screen and the plurality of light sources; The plurality of light diffusion sheets are arranged with the second surfaces facing the plurality of light sources. Backlight unit.

5. The plurality of light sources are disposed on a reflecting sheet provided on the opposite side of the display screen from the light diffusion sheet laminate. The backlight unit according to claim 4 .

6. The distance between the plurality of light sources and the light diffusion sheet laminate is 10 mm or less. The backlight unit according to claim 4 or 5.

7. The backlight unit according to any one of claims 4 to 6, A liquid crystal display panel is provided. LCD display device.

8. An information device comprising the liquid crystal display device according to claim 7.

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