Backlight unit, liquid crystal display device and information device

Abstract

To provide a light diffusion sheet that can improve in-plane luminance uniformity.SOLUTION: A light diffusion sheet 43 has 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 with a substantially reverse polygonal pyramid shape. The second surface 21b has an arithmetic average roughness of 1.0 μm or more and 3.0 μm or less. The light diffusion sheet 43 has an internal haze of 1.5% or less.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

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

[0003] When a direct backlight is used, a light diffusion sheet is used to diffuse light from a light source such as an LED (Light Emitting Diode) and improve 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 that occurs when an uneven shape is given to the light exit surface or by dispersing fine particles with a refractive index different from that of the sheet substrate within the sheet substrate.

[0005] In thin displays such as those for notebook computers and tablet terminals, a light diffusion 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] Japanese Patent Application Laid-Open No. 2011-129277 Summary of the Invention [Problem to be solved by the invention]

[0007] However, because the light source in a direct backlight is placed directly below the display screen, as displays become thinner, the distance from the light source to the light diffusion sheet and the thickness of the light diffusion sheet are reduced, making it 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 that can improve in-plane luminance uniformity. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the light diffusion sheet according to 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] In the light diffusion sheet according to the present disclosure, the second surface, which serves as the light incidence surface, has an arithmetic mean roughness of 3.0 μm or less and an internal haze of 1.5% or less. Therefore, light incident from the second surface reaches the uneven first surface without substantially diffusing within the sheet. Therefore, high-brightness light traveling directly from the light source toward the light diffusion sheet can be uniformly diffused by the concave portions of the first surface, eliminating the light source image and improving in-plane brightness uniformity. Furthermore, the second surface, which serves as the light incidence surface, has an arithmetic mean roughness of 1.0 μm or more, suppressing brightness degradation. Therefore, the sheet can be made thinner and the number of light sources can be reduced.

[0011] In the light diffusion sheet according to the present disclosure, the plurality of recesses may be formed in a substantially inverted quadrangular pyramid shape, which allows the light traveling straight from the light source to 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, 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] The backlight unit according to the present disclosure is equipped with the light diffusion sheet according to the present disclosure described above, which can improve the in-plane brightness uniformity, thereby enabling further slimming and reducing 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 diffusing sheet, whereby light is further diffused by multiple reflections between the light diffusing sheet and the reflecting sheet, thereby further improving in-plane luminance uniformity.

[0016] In the backlight unit according to the present disclosure, a plurality of the light diffusion sheets may be stacked and disposed between the display screen and the plurality of light sources. In this case, 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, thereby making it possible to suppress deterioration of in-plane luminance uniformity due to the diffusion performance of the light diffusion sheet according to the present disclosure.

[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 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.

[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 possible to accommodate further slimming and a reduction in the number of light sources. [Effects of the Invention]

[0022] According to the present disclosure, it is possible to provide a light diffusion sheet that can improve in-plane luminance uniformity. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a cross-sectional view of a liquid crystal display device according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a backlight unit according to the embodiment. [Figure 3] 1 is a cross-sectional view of a light diffusion sheet according to an embodiment. [Figure 4] FIG. 10 is a cross-sectional view of a light diffusion sheet according to a comparative example. [Figure 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 the evaluation results of the brightness of the light diffusion sheets of Examples 1 to 8 and Comparative Examples 1 and 2. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0024] (Embodiment) Hereinafter, a light diffusion sheet, a backlight unit, a liquid crystal display device, and an information device according to 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 concept 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 back 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 arranged to face each other, a liquid crystal layer 3 provided between the TFT substrate 1 and the CF substrate 2, and a frame-shaped sealant (not shown) for enclosing 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, rectangular or square, but is not limited to this and may be any shape such as a rectangle with rounded corners, an oval, 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 polarizer 6. The light with the adjusted transmittance is then emitted through the second polarizer 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 (for example, 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 covering 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 covering each pixel electrode. The CF substrate 2 includes, for example, a black matrix arranged in a grid pattern on the glass substrate, color filters including red, green, and blue layers arranged between each grid of the black matrix, a common electrode covering the black matrix and the color filters, and an alignment film covering the common electrode. The liquid crystal layer 3 is made of a nematic liquid crystal material containing liquid crystal molecules with electro-optical properties. The first polarizer 6 and the second polarizer 7 include, for example, a polarizer layer having a unidirectional polarization axis and a pair of protective layers sandwiching the polarizer layer.

[0031] As shown in Figure 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 this order above the light diffusion sheet 43, and a polarizing sheet 46 provided above the second prism sheet 45.

[0032] Although Figure 2 illustrates 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 sheets 43 may be used in a single layer, or may be stacked in 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. From the viewpoint of cost, productivity, etc., an LED element may be used. The light source 42 may have a rectangular shape when viewed from above, in which 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. Furthermore, 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 made of, 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 square 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 on the first surface 21a).

[0038] The apex angle θ of the recesses 22 is equal to or greater than 80° and equal to or less than 100°, 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 perpendicularly intersect a pair of slopes that face each other across the apex. Furthermore, the arrangement pitch p of the recesses 22 refers to the horizontal distance (distance along a direction parallel to the second surface 21b) between the apexes of the inverted polygonal pyramids in 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 its 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 an inverted square pyramid shape) are arranged two-dimensionally to create an uneven shape on the first surface 21a, but the recesses 22 may have another approximately inverted polygonal pyramid shape, or 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 using conventional shape transfer technology. However, "approximately" means that it can be approximated; 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 shapes" with flat apexes are also included in the "approximately inverted polygonal pyramid" category if the apex area is small enough that the effects of the present invention are not lost. Furthermore, shapes that deform from an "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" category.

[0042] Furthermore, 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 the mold (metal roll) used in the 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 arranged without gaps on the first surface 21a, or may be arranged 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 approximately 90°. The grooves formed in the first prism sheet 44 and the grooves formed in the second prism sheet 45 are arranged 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 thereon using a UV-curable acrylic resin.

[0044] For example, a 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 each having a substantially inverted polygonal pyramid shape. 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, light incident from the second surface 21b reaches the uneven first surface 21a without being substantially diffused inside the light diffusion sheet 43 (base layer 21). Therefore, high-brightness light traveling straight from the light source 42 toward the light diffusion sheet 43 can be uniformly diffused by the recesses 22 on the first surface 21a, thereby eliminating the image of the light source 42 on the display screen 50a and improving in-plane brightness uniformity. This allows for further slimming and a reduction in 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 denoted by the same reference numerals. In the light diffusion sheet 43A of the comparative example, light traveling straight from the light source 42 is diffused randomly 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 varies depending on the position of the light source 42 on the first surface 21a. 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 incident surface, of the light diffusion sheet 43 is set to 3.0 μm or less. From the viewpoint of improving 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 deterioration in in-plane luminance uniformity 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 diffused uniformly 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 into a liquid crystal display device 50, and guides light emitted from a plurality of light sources 42 toward a 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 in-plane luminance uniformity, which can accommodate further slimming and a reduction in the number of light sources.

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

[0053] In the backlight unit 40 of this embodiment, a plurality of light diffusion sheets 43 may be stacked and disposed between the display screen 50a and the light source 42. In this case, 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 this 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 more effectively prevent deterioration of in-plane brightness uniformity than in the past.

[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. Similar effects can also 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 to be arranged is not particularly limited, but when multiple light sources 42 are arranged in a dispersed manner, it is preferable to arrange them regularly on the reflective sheet 41. Arranging them regularly means arranging them according to a certain rule, and corresponds to, for example, arranging the light sources 42 at equal intervals. 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 (substrate layer 21) may contain a diffusing agent or other additives to the extent that 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 addition, in this embodiment, the resin that forms the matrix of the base material 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] Furthermore, in this embodiment, the thickness of the light diffusion sheet 43 is not particularly limited, but 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 aforementioned effect of improving brightness uniformity. The light diffusion sheet 43 may be in the form of a film or a plate.

[0060] In this embodiment, the method for manufacturing 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 textured surface using extrusion molding is as follows: First, pellet-shaped plastic particles (which may contain a diffusing agent) are fed into a single-screw extruder and melted and kneaded while being heated. The molten resin is then extruded through a T-die, sandwiched between two metal rolls, cooled, transported using a guide roll, and cut into flat sheets using a sheet cutter to produce a light diffusion sheet. Here, by sandwiching the molten resin using metal rolls whose surfaces have an inverted shape of the desired textured shape, the inverted shape of the roll surface is transferred to the resin, allowing the desired textured shape to be formed on the surface of the light diffusion sheet. Furthermore, the shape transferred to the resin is not necessarily a 100% copy of the shape of the roll surface, so the shape of the roll surface can be designed by working 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-layer diffusion sheet having a textured surface may be produced as follows. First, pellet-shaped plastic particles required for forming each layer are fed into two single-screw extruders, and melted and kneaded while heated. The molten resins for each layer are then fed into a T-die and laminated within the T-die. The laminated molten resin extruded through the T-die is sandwiched between two metal rolls and cooled. The laminated molten resin is then transported using guide rolls and cut into individual flat plates using a sheet cutter, thereby producing a two-layer diffusion sheet having a textured 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 inverse 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, UV light is irradiated to cure the resin. Next, the sheet onto which the concave-convex shape has been transferred by the resin is peeled off from the roll. Finally, the sheet is irradiated with UV light again to completely cure the resin, producing a light diffusion sheet having a concave-convex shape on its surface.

[0065] Furthermore, in this embodiment, a direct-type backlight unit in which a plurality of light sources 42 are distributed on the back side of the display screen 50a of the liquid crystal display device 50 is used as the backlight unit 40. Therefore, 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 brightness of the portions of the display screen 50a located in the areas between the distributed light sources 42 is lower than that of other portions (brightness unevenness).

[0066] In contrast, the use of the light diffusion sheet 43 of this embodiment is useful for suppressing brightness unevenness. In particular, in view of future thinning of small and medium-sized liquid crystal displays, 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, even more 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 130 μm-thick substrate layer made of clear polycarbonate was used. In both the examples and comparative examples, a plurality of recesses each having a substantially inverted square pyramidal shape with a 90° apex angle (inverted pyramidal shape) were two-dimensionally arranged at a 100 μm pitch on the first surface (light exit surface) of the light diffusion sheet. As examples, four types of light diffusion sheets were prepared, each having a second surface (light entrance surface) processed to have an arithmetic mean roughness Ra of 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm, respectively. As a comparative example, a light diffusion sheet was prepared, each having a second surface (light entrance surface) processed to have an arithmetic mean roughness Ra of 3.4 μm.

[0069] The manufacturing method of the light diffusion sheet of the example is as follows. First, a pellet-shaped base resin (plastic resin) was formed into a resin film using an extrusion molding machine. Then, two metal rolls, one of which had a convex pyramidal surface and the other a mirror-finished roll, were used, and these rolls were pressed against the resin film to produce a single-layer light diffusion sheet with an inverted pyramidal surface on one side and a mirror-finished surface on the other side.

[0070] The manufacturing method of the light diffusion sheet of the comparative example is as follows. First, pellet-shaped base resin (plastic resin) was made into a resin film using an extrusion molding machine. Then, two metal rolls were used, one of which had a convex pyramidal surface and the other an embossing roll with a random matte surface, and both rolls were pressed against the resin film to produce a single-layer light diffusion sheet with an inverted pyramidal surface on one side and an embossed surface on the other side. The difference in roughness of the embossed surface 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 this example 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 from the Examples (four types) and Comparative Examples were stacked on top of a blue LED array arranged at a 2.8 mm pitch. Two prism sheets were then placed on top of that, and a transparent glass plate was placed on top of that to prevent the sheets from floating. The luminance in the vertical upward direction (the direction from the LED array toward the glass plate) was measured using a Topcon Technohouse UA-200 two-dimensional color luminance meter. Next, the obtained two-dimensional luminance distribution image was corrected for variations in the luminance intensity of each LED and filtered to suppress bright and dark spot noise caused by foreign matter, etc., and the average luminance and standard deviation for all pixels were calculated. Finally, the in-plane luminance uniformity of the light diffusion sheets of the Examples and Comparative Examples was calculated, defining "in-plane luminance uniformity" as "average luminance value / standard deviation of luminance."

[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 relative luminance, with the luminance (average value) of the comparative example with the same number of overlapping sheets being taken as 1.

[0075] [Table 1]

[0076] In Table 1, Examples 1 to 4 show the results of evaluating the luminance and in-plane luminance uniformity of two stacked light diffusion sheets of the aforementioned Examples, each 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 shows the results of evaluating the luminance and in-plane luminance uniformity of two stacked light diffusion sheets of the aforementioned Comparative Example, each having a second surface processed to have an arithmetic mean roughness Ra of 3.4 μm. Examples 5 to 8 show the results of evaluating the luminance and in-plane luminance uniformity of three stacked light diffusion sheets of the aforementioned Examples, each 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 shows the results of evaluating the luminance and in-plane luminance uniformity of three stacked light diffusion sheets of the aforementioned Comparative Example, each 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 sheets 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 sheets in each of Examples 1 to 8 and Comparative Examples 1 and 2.

[0078] As shown in Table 1 and Figure 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 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 Figure 6, when the surface roughness Ra of the light incident surface was the smallest at 0.03 μm (mirror finish) (Examples 4 and 8), a decrease in brightness was observed at each number of layers. Furthermore, 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] The results shown in Table 1, Figures 5 and 6 indicate that the brightness and in-plane brightness uniformity required for a product can be met by appropriately setting the surface roughness Ra of the light incident surface to within the range of 3.0 μm or less and adjusting the number of light diffusion sheets stacked. For example, in a product that requires improved in-plane brightness uniformity while suppressing brightness 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 embodiments (including examples; the same applies hereinafter) of the present disclosure 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 merely illustrative in nature 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 LCD 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 backlight unit incorporated in a liquid crystal display device that guides light emitted from multiple light sources toward a display screen, a light diffusion sheet is provided between the display screen and the plurality of light sources; the light diffusion sheet 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 light diffusion sheet does not contain a diffusing agent and has an internal haze of 1.5% or less; the light diffusion sheet is disposed with the second surface facing the plurality of light sources; The light diffusion sheet is a laminate of a plurality of sheets and is disposed between the display screen and the plurality of light sources. Backlight unit.

2. The plurality of recesses are formed in a substantially inverted quadrangular pyramid shape. The backlight unit 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 backlight unit according to claim 1 or 2.

4. The plurality of light sources are disposed on a reflecting sheet provided on the opposite side of the display screen from the light diffusing sheet. The backlight unit according to any one of claims 1 to 3.

5. The distance between the plurality of light sources and the light diffusion sheet is 10 mm or less. The backlight unit according to any one of claims 1 to 4.

6. The backlight unit according to any one of claims 1 to 5, A liquid crystal display panel is provided. LCD display device.

7. An information device comprising the liquid crystal display device according to claim 6.

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