LIGHT DIFFUSERS, BACKLIGHTS, LIQUID CRYSTAL DISPLAYS, AND COMMUNICATION EQUIPMENT

Abstract

The invention relates to a light-diffusing plate 43 having numerous recesses 22 shaped to resemble an inverted polygonal pyramid on the first surface 43a. The first surface 43a has a light-reflecting structure 101. The light-reflecting structure 101 may be, for example, a depositional layer covering the surface of the numerous recesses 22 or a moss eye structure.

Description

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

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

[0002] Liquid crystal display devices are widely used as display devices for various information devices such as notebook computers, smartphones, tablet terminals, etc. The mainstream backlight for liquid crystal display devices is the direct type, in which the light source is placed behind the liquid crystal panel.

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

[0004] JP 2011-129277 A

[0005] However, in conventional direct-type backlights, there is a problem that when the light diffusion sheet is made thinner in order to make the backlight thinner, the luminance uniformity decreases.

[0006] An object of the present disclosure is to provide a light diffusion sheet that can suppress a decrease in brightness uniformity even when the sheet is thin, and a backlight unit, a liquid crystal display device, and an information device that include the light diffusion sheet.

[0007] In order to achieve the above-mentioned object, the light diffusion sheet according to the present disclosure is a light diffusion sheet having a plurality of recesses formed in a roughly inverted polygonal pyramid shape on a first surface, and an anti-reflection structure is provided on the first surface.

[0008] According to the light diffusion sheet of the present disclosure, by providing a light anti-reflection structure on the first surface having a plurality of recesses formed in a generally inverted polygonal pyramid shape, the light transmittance on the first surface is increased while the light reflectance is decreased. This prevents the light reflection on the first surface from reducing the light diffusion effect of the recesses formed in a generally inverted polygonal pyramid shape. Therefore, even when the light diffusion sheet is made thinner, it is possible to prevent a decrease in brightness uniformity, thereby enabling further thinning of backlights.

[0009] In the light diffusion sheet according to the present disclosure, the antireflection structure may be a vapor deposition layer covering the surface of the plurality of recesses. This makes it easy to provide the antireflection structure on the first surface having the plurality of recesses. In this case, if the vapor deposition layer includes a silicon oxide layer and a metal oxide layer, the antireflection structure can easily adjust the light transmittance and light reflectance on the first surface.

[0010] In the light diffusion sheet according to the present disclosure, when the antireflection structure is a moth-eye structure, an antireflection structure with excellent light reflection suppression effect can be obtained.

[0011] In the light diffusion sheet according to the present disclosure, when the plurality of recesses are formed in a substantially inverted quadrangular pyramid shape, a light diffusion sheet with excellent light diffusibility can be obtained.

[0012] In the light diffusion sheet according to the present disclosure, the second surface opposite to the first surface may be a matte or mirror surface, which can prevent a reduction in the light diffusion effect due to the shape of the second surface, which is caused by the recesses formed in a substantially inverted polygonal pyramid shape.

[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 to a display screen, and includes the light diffusion sheet according to the present disclosure described above between the display screen and 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, and therefore can suppress a decrease in brightness uniformity even when the light diffusion sheet is made thinner, thereby enabling further thinning.

[0015] In the backlight unit according to the present disclosure, the light diffusion sheet may be disposed with the first surface facing the display screen. This prevents the light diffusion effect of the recesses on the first surface from being reduced by the second surface. Note that if the first surface is provided with an anti-reflection structure that selectively prevents reflection of light incident in the normal direction to the sheet surface of the light diffusion sheet (normally incident light), i.e., an anti-reflection structure that has a higher transmittance for normally incident light than for other incident light, the light diffusion sheet may be disposed with the second surface facing the display screen.

[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. As the number of light diffusion sheets increases, the luminance uniformity increases.

[0017] The backlight unit according to the present disclosure may further include a color conversion sheet between the plurality of light sources and the light diffusion sheet, which converts the wavelength of the light. This eliminates the need for an expensive white light source, thereby reducing costs. Furthermore, it is possible to avoid a situation in which the color conversion sheet reduces the light diffusion effect of the light diffusion sheet.

[0018] In the backlight unit according to the present disclosure, the plurality of light sources may be two-dimensionally arranged at a pitch of 10 mm or less, whereby the light diffusion sheet having the anti-reflection structure can sufficiently suppress the deterioration of brightness uniformity.

[0019] 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.

[0020] The liquid crystal display device according to the present disclosure includes the backlight unit according to the present disclosure described above, and therefore can accommodate further thinning.

[0021] An information device according to the present disclosure includes the liquid crystal display device according to the present disclosure.

[0022] The information device according to the present disclosure includes the liquid crystal display device according to the present disclosure described above, and therefore can accommodate further thinning.

[0023] According to the present disclosure, it is possible to provide a light diffusion sheet that can suppress a decrease in brightness uniformity even when the sheet is thin, as well as a backlight unit, a liquid crystal display device, and an information device that include the light diffusion sheet.

[0024] 1 is a cross-sectional view of a liquid crystal display device according to an embodiment. FIG. 2 is a cross-sectional view of a backlight unit according to an embodiment. FIG. 3 is a plan view showing an example of the arrangement of light sources in the backlight unit shown in FIG. 2. FIG. 4 is a perspective view of a light diffusion sheet according to an embodiment. FIG. 5 is a cross-sectional view of a light diffusion sheet according to an example. FIG. 6 is a diagram showing the incidence angle dependency of the transmittance for light with a wavelength of 450 nm when a vapor deposition layer is provided on a 100 μm thick polycarbonate sheet with mirrored surfaces on both sides. FIG. 7 is a diagram showing the incidence angle dependency of the reflectance for light with a wavelength of 450 nm when a vapor deposition layer is provided on a 100 μm thick polycarbonate sheet with mirrored surfaces on both sides. FIG. 8 is a cross-sectional view of a backlight unit used in the evaluation of additional examples.

[0025] Hereinafter, embodiments of the present disclosure 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 modified as desired within the scope of the technical concept of the present disclosure.

[0026] 1 , a liquid crystal display device 50 of this embodiment 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, and a liquid crystal layer 3 provided 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) may be rectangular or square, but is not limited to these, and may be any shape such as a rectangle or square 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, and the light 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 to be 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 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 respectively 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] <Backlight Unit> As shown in Fig. 2, the backlight unit 40 of this embodiment mainly includes a reflective sheet 41, a plurality of small light sources 42 arranged two-dimensionally on the reflective sheet 41, and a light diffusion sheet 43 provided above the plurality of small light sources 42. A plurality of light diffusion sheets 43 (three in this example) may be stacked and arranged. A brightness enhancement sheet 47 may be provided above the light diffusion sheet 43. A color conversion sheet 44 may be provided between the plurality of small light sources 42 and the brightness enhancement sheet 47 (between the plurality of small light sources 42 and the light diffusion sheet 43 in this example). Details of the light diffusion sheet 43 will be described later.

[0032] [Reflective Sheet] The reflective sheet 41 is made of, for example, a white polyethylene terephthalate resin film, a silver vapor deposition film, or the like.

[0033] [Light Source] The type of small light source 42 is not particularly limited, but may be, for example, an LED element or a laser element. LED elements may also be used from the viewpoint of cost, productivity, etc. A lens may be attached to the LED element to adjust the light output angle characteristics of the LED element serving as the small light source 42. For example, as shown in FIG. 3 , multiple small light sources 42 made of LED elements may be arranged on the reflective sheet 41 in a two-dimensional array at regular intervals. The small light source 42 may have a rectangular shape in a planar view, 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). The number of small light sources 42 arranged is also not particularly limited, but when multiple small light sources 42 are distributed, it is preferable to arrange them regularly on the reflective sheet 41. Regular arrangement means arranging them according to a certain rule, such as arranging the small light sources 42 at equal intervals. When the small light sources 42 are arranged at equal intervals, the center-to-center distance (arrangement pitch) between two adjacent small light sources 42 may be 0.5 mm or more (preferably 2 mm or more) and 20 mm or less (preferably 10 mm or less).

[0034] In this example, a blue light source is used as the compact light source 42. The blue light source may emit light such that, for example, x<0.24 and y<0.18 in the CIE 1931 chromaticity coordinates. Alternatively, a white light source may be used as the compact light source 42. The white light source is composed of an LED element having a peak wavelength in the blue region, an LED element having a peak wavelength in the green region, and an LED element having a peak wavelength in the red region, and may emit light such that, for example, 0.24<x<0.42 and 0.18<y<0.48 in the CIE 1931 chromaticity coordinates.

[0035] [Color Conversion Sheet] The color conversion sheet 44 is a wavelength conversion sheet that converts light from a compact light source 42, such as a blue light source, into light with a peak wavelength of a desired color (e.g., green or red). For example, the color conversion sheet 44 converts blue light with a wavelength of 450 nm into green light with a wavelength of 540 nm and red light with a wavelength of 650 nm. In this case, if a compact light source 42 emitting blue light with a wavelength of 450 nm is used, the color conversion sheet 44 partially converts the blue light into green light and red light, so that the light transmitted through the color conversion sheet 44 becomes white light. The color conversion sheet 44 may be, for example, a quantum dot (QD) sheet or a fluorescent sheet. Note that if a white light source is used as the compact light source 42, the color conversion sheet 44 may not be provided. Furthermore, the color conversion sheet 44 may be positioned above or below the light diffusion sheet 43, or between two light diffusion sheets 43, as long as it is between the compact light source 42 and the brightness enhancement sheet 47.

[0036] [Brightness Enhancement Sheet] The brightness enhancement sheet 47 has a structure in which a first prism sheet 45 and a second prism sheet 46 are sequentially stacked from the side closest to the compact light source 42. The first prism sheet 45 and the second prism sheet 46 are each formed with a plurality of adjacent grooves, each having an isosceles triangular cross section, and the apex angle of the prism between each pair of adjacent grooves is approximately 90°. The grooves formed on the first prism sheet 45 and the grooves formed on the second prism sheet 46 are arranged perpendicular to each other. The first prism sheet 45 and the second prism sheet 46 may be integrally formed. The first prism sheet 45 and the second prism sheet 46 may be formed, for example, from a polyethylene terephthalate (PET) film with a prism shape formed using a UV-curable acrylic resin.

[0037] Although not shown, a polarizing sheet may be provided above the second prism sheet 46. The polarizing sheet 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.

[0038] In this example, a prism sheet is used as the brightness enhancing sheet 47, but instead of this, other optical sheets capable of increasing the brightness of the light emitted from the small light source 42 may be used.

[0039] <Light Diffusion Sheet> In the backlight unit 40, one or more light diffusion sheets 43 are arranged. When multiple light diffusion sheets 43 are provided, the light diffusion sheets 43 may have the same structure or may have different structures. The light diffusion sheet 43 may be a "light diffusion sheet," or may be a plate-shaped "light diffusion plate" or a film-shaped "light diffusion film."

[0040] The light diffusion sheet 43 has a plurality of recesses 22 formed in a substantially inverted polygonal pyramid shape on its first surface 43a, and the first surface 43a is provided with a light anti-reflection structure 101. When a plurality of light diffusion sheets 43 are provided, at least one of the light diffusion sheets 43 has a plurality of recesses 22 formed in a substantially inverted polygonal pyramid shape on its first surface 43a, and the first surface 43a is provided with a light anti-reflection structure 101. Details of the light anti-reflection structure 101 will be described later.

[0041] The light diffusion sheet 43 has a base layer 21. A plurality of recesses 22 are provided on a first surface (in this example, the light output surface) 43a of the light diffusion sheet 43. In the light diffusion sheet 43 shown in FIG. 2, the first surface 43a on which the plurality of recesses 22 are provided is the light output surface. However, instead, the first surface 43a may be the light input surface. In this example, the plurality of recesses 22 are formed in a substantially inverted square pyramid shape, as shown in FIG. 4. FIG. 4 illustrates an example in which the recesses 22 formed in a substantially inverted square pyramid shape are arranged in a 5×5 matrix on the first surface 43a of the light diffusion sheet 43, but the actual number of recesses 22 arranged is much greater. Adjacent recesses 22 are separated by ridge lines 23. The arrangement pitch of the recesses 22 is, for example, approximately 50 μm or more and approximately 500 μm or less. The angle formed by the wall surface of the recess 22 (the slope of the approximately inverted polygonal pyramid or the approximately inverted polygonal truncated pyramid) and the sheet surface of the light diffusion sheet 43 (a virtual mirror surface without the recess 22) is, for example, 40 degrees or more and 65 degrees or less. In other words, the apex angle of the recess 22 is, for example, 50 degrees or more and 100 degrees or less. The second surface 43b of the light diffusion sheet 43 may be a mirror surface, but may also be a matte surface or may have multiple recesses like the recess 22 to improve light diffusion.

[0042] The multiple recesses 22 are not limited to being substantially inverted square pyramids, but may also be formed in the shape of substantially inverted polygonal pyramids or substantially inverted polygonal truncated pyramids. In the present disclosure, the term "substantially inverted polygonal pyramid" also includes substantially inverted polygonal truncated pyramids. The multiple recesses 22 may also be regularly arranged two-dimensionally. As the "inverted polygonal pyramid (trapezoid)," a triangular pyramid (trapezoid), a square pyramid (trapezoid), or a hexagonal pyramid (trapezoid) is preferred, as they can be arranged two-dimensionally without gaps. A mold (metal roll) is used in the manufacturing process, such as extrusion molding or injection molding, to form the recesses 22. Taking into account the precision of the cutting work on the surface of this mold (metal roll), an inverted square pyramid (trapezoid) may be selected as the "inverted polygonal pyramid (trapezoid)."

[0043] In consideration of the difficulty of forming a recess having a geometrically strict inverted polygonal pyramid or inverted polygonal truncated pyramid shape using ordinary shape transfer technology, the terms "approximately inverted polygonal pyramid" or "approximately inverted polygonal truncated pyramid" are used, but it goes without saying that these terms also include shapes that can be regarded as true or substantially inverted polygonal pyramids or inverted polygonal truncated pyramids. Furthermore, "approximately" means that it can be approximated; for example, "approximately square pyramid" refers to a shape that can be approximated to a square pyramid. Furthermore, shapes that are deformed from an "inverted polygonal pyramid" or "inverted polygonal truncated pyramid" within the range of unavoidable shape variations due to processing accuracy in industrial production are also included in the "approximately inverted polygonal pyramid" or "approximately inverted polygonal pyramid" shapes.

[0044] When the plurality of recesses 22 are regularly arranged two-dimensionally, the recesses 22 may be provided without gaps across the entire surface of the light diffusion sheet 43, or may be provided at a fixed interval (pitch). Furthermore, some of the recesses 22 may be arranged randomly to the extent that the light diffusion effect is not impaired.

[0045] The light diffusion sheet 43 may be configured with a substrate layer 21 that does not contain a diffusing agent, for example, a substrate layer 21 made of clear polycarbonate. When a diffusing agent is contained in the substrate layer 21, the material of the diffusing agent is not particularly limited, and inorganic particles such as silica, titanium oxide, aluminum hydroxide, barium sulfate, etc., and organic particles such as acrylic, acrylonitrile, silicone, polystyrene, polyamide, etc., may be used. From the viewpoint of light diffusion effect, the particle size of the diffusing agent may be, for example, 0.1 μm or more (preferably 1 μm or more) and 10 μm or less (preferably 8 μm or less).

[0046] From the viewpoint of the reflection and refraction effects of the substantially inverted polygonal pyramid shape and the light diffusion effect of the diffusing agent, it is preferable that the light diffusion sheet 43 does not contain a diffusing agent. However, the content of the diffusing agent may be, for example, 0.1% by mass or more (preferably 0.3% by mass or more) and 10% by mass or less (preferably 8% by mass or less), where the material (matrix) constituting the base layer 21 is 100% by mass. The difference between the refractive index of the diffusing agent and the refractive index of the matrix of the base layer 21 may be 0.01 or more, preferably 0.03 or more, more preferably 0.05 or more, even more preferably 0.1 or more, and most preferably 0.15 or more. If the difference between the refractive index of the diffusing agent and the refractive index of the matrix of the base layer 21 is less than 0.01, the diffusion effect of the diffusing agent will be insufficient.

[0047] The resin that forms the matrix of the base material layer 21 is not particularly limited as long as it is a material that transmits light, but for example, acrylic, polystyrene, polycarbonate, MS (methyl methacrylate-styrene copolymer) resin, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyimide, etc. may be used.

[0048] The thickness of the light diffusion sheet 43 is not particularly limited, but may be, for example, 1200 μm or less and 50 μm or more. If the thickness of the light diffusion sheet 43 exceeds 1200 μm, 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 50 μm, it becomes difficult to achieve the effect of improving brightness uniformity.

[0049] The light diffusion sheet 43 may have a multilayer structure, for example, a two-layer structure consisting of a first base layer and a second recess-forming layer. In this case, the base layer and the recess-forming layer may each be configured as independent sheets and laminated together to form the light diffusion sheet 43, or the base layer and the recess-forming layer may each be disposed separately to form the light diffusion sheet 43. The thickness of the recess-forming layer is greater than the maximum depth of the recesses 22. For example, if a recess with a depth of 20 μm is to be formed, the thickness of the recess-forming layer should be greater than 20 μm. The light diffusion sheet 43 may have a three-layer or greater structure including the base layer and the recess-forming layer.

[0050] Hereinafter, a method for manufacturing the light diffusion sheet 43 will be described taking the case where the light diffusion sheet 43 is formed into a sheet as an example. The method for manufacturing the light diffusion sheet 43 is not particularly limited, and may be, for example, an extrusion molding method, a compression molding method, an injection molding method, or the like. When extrusion molding the light diffusion sheet 43, for example, the line speed may be set to 2 m / min or more and 30 m / min or less, and the compression linear pressure may be set to 100 kgf / cm or more and 500 kgf / cm or less.

[0051] The procedure for producing a single-layer light diffusion sheet having a textured surface using an extrusion molding method is as follows. First, pellet-shaped plastic particles (to which a diffusing agent may be added) are fed into a single-screw extruder, melted, and kneaded while being heated. The molten resin extruded through a T-die is then sandwiched between two metal rolls and cooled, then conveyed using a guide roll, and cut into individual flat plates using a sheet cutter. Alternatively, the continuous sheet is wound into a roll using a winder, and then cut into individual flat plates using a sheet cutter, thereby producing a light diffusion sheet. Here, by sandwiching the molten resin using metal rolls having a surface with a shape that is the inverse of the desired textured shape, the inverse shape of the roll surface is transferred to the resin, allowing the desired textured shape to be formed on the diffusion sheet surface. Furthermore, since the shape transferred to the resin is not necessarily 100% of the shape of the roll surface, the shape of the roll surface may be designed by calculating backwards from the degree of transfer.

[0052] When using an extrusion molding method to manufacture a two-layer light diffusion sheet having an uneven surface, for example, 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.

[0053] Alternatively, a two-layer light diffusion sheet having an uneven 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 that will form 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 light diffusion sheet having an uneven surface.

[0054] Alternatively, a light diffusion sheet 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 to which the concave-convex shape has been transferred by the resin is peeled off from the roll. Finally, the sheet is again irradiated with UV light to completely cure the resin, producing a light diffusion sheet having a concave-convex shape on its surface.

[0055] 5 , the first surface 43 a of the light diffusion sheet 43, which has a plurality of recesses 22 formed in a generally inverted polygonal pyramid shape, is provided with an anti-reflection structure 101 to increase the light transmittance and reduce the light reflectance at the first surface 43 a. The anti-reflection structure 101 increases the light transmittance at the first surface 43 a by 1% or more, preferably 3% or more, and more preferably 5% or more, and decreases the light reflectance at the first surface 43 a by 1% or more, preferably 3% or more, and more preferably 5% or more, over a wide range of angles of incidence of light incident on the light diffusion sheet 43 (the inclination angle with respect to the normal to the sheet surface of the light diffusion sheet 43). Alternatively, the first surface 43 a may be provided with an anti-reflection structure 101 that selectively prevents reflection of light incident in the normal direction to the sheet surface of the light diffusion sheet 43 (normally incident light), i.e., an anti-reflection structure 101 having a higher transmittance for normally incident light than for other incident light.

[0056] The antireflection structure 101 may be, for example, a vapor-deposited or sputtered layer that thinly coats the surfaces of the multiple recesses 22 to a thickness of 1 μm or less. The antireflection structure 101 may also be a laminate including multiple layers with different refractive indices, such as a low-refractive index layer with a refractive index of less than 1.55 and a high-refractive index layer with a refractive index of 1.55 or higher (preferably 1.80 or higher). Examples of low-refractive index layers include silicon oxide layers, magnesium fluoride layers, and silicon oxyfluoride layers, each having a thickness of approximately 10 to 500 nm. Examples of high-refractive index layers include zirconium oxide layers, hafnium oxide layers, and titanium oxide layers, each having a thickness of approximately 10 to 500 nm. The antireflection structure 101 may include two or more low-refractive index layers and two or more high-refractive index layers, or may include three or more layers with different refractive indices. When the antireflection structure 101 is a laminate including low-refractive index layers and high-refractive index layers, the uppermost layer of the laminate may be a low-refractive index layer, such as a silicon oxide layer. Alternatively, the anti-reflection structure 101 may be a coating layer made of a material having a lower refractive index than the material making up the base layer 21 of the light diffusion sheet 43 .

[0057] Furthermore, the anti-reflection structure 101 may be, for example, a moth-eye structure, which is fine irregularities with a height difference of 1 μm or less provided on the surface of the plurality of recesses 22 (in this example, the four slopes of a substantially inverted square pyramid). The moth-eye structure has a structure in which protrusions having a structure smaller than the wavelength of incident light are densely packed, specifically, a structure in which nanometer-sized fine irregularities are uniformly arranged, and this structure reduces the light reflectance of the first surface 43 having the plurality of recesses 22. The moth-eye structure may be formed during molding of the light diffusion sheet 43, or may be formed by surface treatment after molding of the light diffusion sheet 43.

[0058] Effect of the embodiment The light diffusion sheet 43 of this embodiment has a plurality of recesses 22 formed in a generally inverted polygonal pyramid shape on the first surface 43a, and the anti-reflection structure 101 is provided on the first surface 43a. This increases the light transmittance on the first surface 43a while decreasing the light reflectance. This prevents a reduction in the light diffusion effect of the recesses 22 formed in a generally inverted polygonal pyramid shape due to light reflection on the first surface 43a. Therefore, even when the light diffusion sheet 43 is made thinner, it is possible to prevent a decrease in brightness uniformity, thereby enabling further reduction in the thickness of backlights.

[0059] In the light diffusion sheet 43 of this embodiment, the antireflection structure 101 may be a vapor deposition layer that covers the surface of the plurality of recesses 22. This makes it easy to provide the antireflection structure 101 on the first surface 43a having the plurality of recesses 22. In this case, if the vapor deposition layer includes a silicon oxide layer and a metal oxide layer, the antireflection structure 101 can easily adjust the light transmittance and light reflectance on the first surface 43a.

[0060] In the light diffusion sheet 43 of this embodiment, when the antireflection structure 101 has a moth-eye structure, the antireflection structure 101 can have an excellent light reflection suppression effect.

[0061] In the light diffusion sheet 43 of this embodiment, when the recesses 22 are formed in a substantially inverted quadrangular pyramid shape, the light diffusion sheet 43 can have excellent light diffusion properties.

[0062] In the light diffusion sheet 43 of this embodiment, the second surface 43b opposite to the first surface 43a may be a matte or mirror surface, which can prevent a reduction in the light diffusion effect of the recesses 22 formed in a substantially inverted polygonal pyramid shape due to the shape of the second surface 43b.

[0063] The thickness of the light diffusion sheet 43 of this embodiment may be 50 μm or more and 1200 μm or less, which allows the backlight unit 40 and the liquid crystal display 50 to be made thinner while still obtaining the light diffusion effect of the light diffusion sheet 43.

[0064] The backlight unit 40 of this embodiment is incorporated into a liquid crystal display device 50 so as to guide light emitted from a plurality of small light sources 42 to a display screen 50, and includes a light diffusion sheet 43 of this embodiment between the display screen 50 and the plurality of small light sources 42. Therefore, even if the light diffusion sheet 43 is made thin, it is possible to suppress a decrease in brightness uniformity, and therefore it is possible to accommodate further thinning.

[0065] In the backlight unit 40 of this embodiment, the light diffusion sheet 43 may be disposed with the first surface 43a facing the display screen 50a. This prevents the light diffusion effect of the recesses 22 on the first surface 43a from being reduced when light passes through the second surface 43b. Note that, when the first surface 43a is provided with a light antireflection structure 101 that selectively prevents reflection of light incident in the normal direction to the sheet surface of the light diffusion sheet 43 (normally incident light), i.e., a light antireflection structure 101 having a higher transmittance for normally incident light than for other incident light, the light diffusion sheet 43 may be disposed with the second surface 43 facing the display screen 50a.

[0066] 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 small light source 42. As the number of light diffusion sheets 43 disposed increases, the brightness uniformity increases.

[0067] The backlight unit 40 of this embodiment may further include a color conversion sheet 44 that converts the wavelength of light between the multiple small light sources 42 and the light diffusion sheet 43. This eliminates the need for an expensive white light source, thereby reducing costs. It also prevents the light diffusion effect of the light diffusion sheet 43 from being reduced when light passes through the color conversion sheet 44.

[0068] In the backlight unit 40 of this embodiment, the multiple small light sources 42 may be arranged two-dimensionally at a pitch of 10 mm or less. In this way, the light diffusion sheet 43 provided with the anti-reflection structure 101 can sufficiently suppress the deterioration of brightness uniformity.

[0069] In the backlight unit 40 of this embodiment, a plurality of small light sources 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 way, multiple reflections of light between the reflective sheet 41 and the light diffusion sheet 43 further improve brightness uniformity.

[0070] The liquid crystal display device 50 of this embodiment includes the backlight unit 40 of this embodiment and a liquid crystal display panel 5. In the liquid crystal display device 50 or an information device including the liquid crystal display device 50, the backlight unit 40 includes the light diffusion sheet 43 of this embodiment, so that even if the light diffusion sheet 43 is made thin, a decrease in brightness uniformity can be suppressed, and therefore further thinning can be achieved.

[0071] (Examples and Comparative Examples) <Evaluation Samples> Evaluation samples made of polycarbonate and 110 μm thick were prepared as light diffusion sheets 43 for the examples and comparative examples. No diffusion agent was added to any of the evaluation samples. One side of the sheet had inverted pyramidal recesses, each 50 μm deep and with a 90° apex angle, arranged in a two-dimensional matrix at a 100 μm pitch, and the other side was matte. In the light diffusion sheet 43 of the first example, a vapor deposition layer was formed as the anti-reflection structure 101 only on the recessed surface. In the light diffusion sheet 43 of the second example, a vapor deposition layer was formed as the anti-reflection structure 101 on both the recessed surface and the matte surface. Furthermore, in the light diffusion sheet 43 of the first comparative example, no vapor deposition layer was formed on either side.

[0072] The anti-reflection structure 101 is made of ZrO 2 Thin film and SiO 2 A vapor-deposited layer approximately 200 nm thick, consisting of alternately laminated thin films and a thin film, was used. Figures 6 and 7 show the incidence angle dependence of the transmittance and reflectance for light with a wavelength of 450 nm when this vapor-deposited layer was provided on a 100 μm-thick polycarbonate sheet with mirror-finished surfaces on both sides. The transmittance and reflectance were measured using a JASCO Corporation V-770 spectrophotometer equipped with an ARMN-920 automatic absolute reflectance measurement system. As shown in Figures 6 and 7 , providing a vapor-deposited layer on one side of the sheet increased the transmittance by approximately 5% or more and decreased the reflectance by approximately 5% or more over a wide range (0° to 60°) of the incidence angle of light incident on the sheet (the angle of inclination relative to the normal to the sheet surface). Furthermore, providing a vapor-deposited layer on both sides of the sheet increased the transmittance by approximately 10% or more and decreased the reflectance by approximately 10% or more over a wide range (0° to 60°) of the incidence angle of light incident on the sheet. Although the transmittance and reflectance for incident angles exceeding 60° are not shown in Figures 6 and 7, even for incident angles exceeding 60°, by providing a vapor deposition layer, it is possible to obtain the same transmittance-increasing effect and reflectance-reducing effect as for incident angles of 60° or less.

[0073] 6 and 7, for the case where a vapor-deposited layer is provided on only one side of the sheet, the solid lines show the transmittance and reflectance when light is incident from the non-vapor-deposited side, and the dashed lines show the transmittance and reflectance when light is incident from the vapor-deposited side. As shown in Figures 6 and 7, when a vapor-deposited layer is provided on only one side of the sheet, there was no significant difference in transmittance and reflectance whether light was incident from the vapor-deposited side or the non-vapor-deposited side.

[0074] <Evaluation of Brightness Uniformity> For the light diffusion sheets 43 of the first and second examples and the first comparative example, the backlight unit 40 shown in FIG. 2 was constructed and brightness uniformity was evaluated.

[0075] Specifically, three light diffusion sheets 43 with the same structure were laminated on the upper side of the compact light sources 42 arranged two-dimensionally on the reflective sheet 41, with the recessed surfaces serving as light output surfaces, via a color conversion sheet 44. A first prism sheet 45 and a second prism sheet 46 were placed on top of the three light diffusion sheets 43. A QD sheet was used for the color conversion sheet 44. The prism sheets 45 and 46 were formed by providing a base layer made of PET film with protruding prism portions using a UV-curable acrylic resin made of acrylate. The first prism sheet 45 had a total thickness of 90 μm, and protruding prism portions with a height of 12 μm and a 90° apex angle were arranged at a pitch of 24 μm. The second prism sheet 46 had a total thickness of 155 μm, and protruding prism portions with a height of 25 μm and a 90° apex angle were arranged at a pitch of 50 μm. The first prism sheet 45 and the second prism sheet 46 were arranged so that the arrangement direction of the small light sources 42 and the prism extension direction of the second prism sheet 46 intersected at an angle of 40°, and the prism extension direction of the second prism sheet 46 and the prism extension direction of the first prism sheet 45 intersected at an angle of 90°. Furthermore, although not shown in Figure 2, a restraining glass for holding down each sheet was placed on the second prism sheet 46 via an upper light diffusion sheet (top diffuser). The upper light diffusion sheet was made of PET film with a bead coating for diffusion on one side and a bead coating for adhesion prevention on the other side.

[0076] The luminance uniformity evaluation was performed as follows using blue LEDs as the compact light sources 42 of the backlight unit 40, arranged in three ways: (1) a square array with a 2.8 mm pitch (hereinafter referred to as light source array A), (2) a two-dimensional array with a height of 3.5 mm and a width of 4.5 mm (hereinafter referred to as light source array B), and (3) a square array with a 10 mm pitch (hereinafter referred to as light source array C). First, the luminance in the vertical upward direction (the direction from the light source toward the holding glass) was measured using a two-dimensional spectroradiometer SR-5000 manufactured by Topcon Technohouse Corporation. Next, the two-dimensional luminance distribution in a 40 mm square area was obtained using a luminance unevenness meter, and after correcting for the overall luminance balance, the average luminance value and standard deviation were calculated. The luminance uniformity was then calculated as "(average luminance value) / (standard deviation of luminance)."

[0077] <Evaluation Results> In light source array A, the luminance uniformity of the light diffusion sheet 43 of the first comparative example was 106.9, while the luminance uniformity of the light diffusion sheet 43 of the first example was 117.7, and the luminance uniformity of the light diffusion sheet 43 of the second example was 112.1.

[0078] In light source array B, the luminance uniformity of the light diffusion sheet 43 of the first comparative example was 37.8, while the luminance uniformity of the light diffusion sheet 43 of the first embodiment was 40.8, and the luminance uniformity of the light diffusion sheet 43 of the second embodiment was 39.2.

[0079] In light source array C, the luminance uniformity of the light diffusion sheet 43 of the first comparative example was 14.6, while the luminance uniformity of the light diffusion sheet 43 of the first embodiment was 15.0 and the luminance uniformity of the light diffusion sheet 43 of the second embodiment was 14.7.

[0080] From the above results, it was found that, in all cases of light source arrays A to C, providing the anti-reflection structure 101 on the surface (first surface 43a) of the light diffusion sheet 43 where the recesses 22 are formed increases brightness uniformity. In particular, in the first example in which the anti-reflection structure 101 is provided only on the first surface 43a, brightness uniformity increased by approximately 3% to 10% compared to the first comparative example. On the other hand, in the second example in which the anti-reflection structure 101 is also provided on the second surface 43b, which does not have the recesses 22, brightness uniformity decreased slightly compared to the first example. Note that in both the first and second examples, the decrease in brightness caused by the anti-reflection structure 101 was approximately 1% to 2% compared to the first comparative example, which was not a problem.

[0081] (Additional Examples and Additional Comparative Examples) <Evaluation Samples> Evaluation samples made of polycarbonate and 110 μm thick were prepared as light diffusion sheets 43 for the additional examples and comparative examples. No diffusing agent was added to any of the evaluation samples. One side had inverted pyramidal recesses, each 50 μm deep and with a 90° apex angle, arranged in a two-dimensional matrix at a 100 μm pitch, and the other side was matte. In the light diffusion sheet 43 of the third example, a vapor deposition layer was formed as the anti-reflection structure 101 only on the recessed surface. In the light diffusion sheet 43 of the fourth example, a vapor deposition layer was formed as the anti-reflection structure 101 on both the recessed surface and the matte surface. Furthermore, in the light diffusion sheet 43 of the second comparative example, no vapor deposition layer was formed on either side.

[0082] The antireflection structure 101 used in the first and second examples had an antireflection effect over a wide range of angles of incidence of light incident on the light diffusion sheet 43 (the inclination angle with respect to the normal to the sheet surface of the light diffusion sheet 43) (see FIGS. 6 and 7). In contrast, in the third and fourth examples, an antireflection structure 101 that selectively prevents reflection of light incident in the normal direction to the sheet surface of the light diffusion sheet 43 (normally incident light), that is, an antireflection structure 101 with a higher transmittance for normally incident light than for other incident light, was provided on the first surface 43a. Specifically, the antireflection structure 101 in the third and fourth examples was made of TiO 2 Thin film and SiO 2 The deposited layer was about 600 nm thick and consisted of alternately stacked thin films.

[0083] <Evaluation of Brightness Uniformity> For the light diffusion sheets 43 of the third and fourth examples and the second comparative example, a backlight unit 40 shown in Fig. 8 was constructed and a brightness uniformity evaluation was performed. Note that the backlight unit 40 shown in Fig. 8 differs from the backlight unit 40 shown in Fig. 2 in that the three light diffusion sheets 43 are arranged with their first surfaces 43a (recess-forming surfaces) serving as light entrance surfaces.

[0084] Specifically, three light diffusion sheets 43 with the same structure were laminated on the upper side of the compact light source 42 arranged two-dimensionally on the reflective sheet 41, with the recessed surface serving as the light entrance surface, via the color conversion sheet 44. A first prism sheet 45 and a second prism sheet 46 were placed on top of the three light diffusion sheets 43. A QD sheet was used for the color conversion sheet 44. The prism sheets 45 and 46 were formed by providing a base layer made of PET film with protruding prism portions using a UV-curable acrylic resin made of acrylate. The first prism sheet 45 had a total thickness of 130 μm, and the first prism sheet 45 had protruding prism portions with a height of 20 μm and a 90° apex angle arranged at a pitch of 40 μm. The second prism sheet 46 had a total thickness of 90 μm, and the second prism sheet 46 had protruding prism portions with a height of 12 μm and a 90° apex angle arranged at a pitch of 24 μm. The first prism sheet 45 and the second prism sheet 46 were arranged so that the arrangement direction of the small light sources 42 and the prism extension direction of the second prism sheet 46 intersected at an angle of 40°, and the prism extension direction of the second prism sheet 46 and the prism extension direction of the first prism sheet 45 intersected at an angle of 90°. Furthermore, although not shown in Figure 8, a restraining glass for holding down each sheet was placed on the second prism sheet 46 via an upper light diffusion sheet (top diffuser). The upper light diffusion sheet was made of PET film with a bead coating for diffusion on one side and a bead coating for adhesion prevention on the other side.

[0085] The luminance uniformity evaluation was performed as follows, using blue LEDs arranged in the aforementioned light source array B as the compact light sources 42 of the backlight unit 40. First, the luminance in the vertically upward direction (the direction from the light source toward the retaining glass) was measured using a two-dimensional spectroradiometer SR-5000 manufactured by Topcon Technohouse Corporation. Next, a luminance unevenness meter was used to obtain the two-dimensional luminance distribution over a 40 mm square area, and after correcting the overall luminance balance, the average luminance value and standard deviation were calculated, and the luminance uniformity was determined by the formula "luminance uniformity" = "(average luminance value) / (standard deviation of luminance)."

[0086] <Evaluation Results> The luminance uniformity of the light diffusion sheet 43 of the second comparative example was 14.0, whereas the luminance uniformity of the light diffusion sheet 43 of the third example was 17.1, and the luminance uniformity of the light diffusion sheet 43 of the fourth example was 15.8.

[0087] From the above results, it was found that brightness uniformity was increased by providing the anti-reflection structure 101 on the surface (first surface 43a) where the recesses 22 of the light diffusion sheet 43 were formed. In particular, in the third example in which the anti-reflection structure 101 was provided only on the first surface 43a, brightness uniformity increased by about 22% compared to the second comparative example. Furthermore, in the fourth example in which the anti-reflection structure 101 was also provided on the second surface 43b without the recesses 22, brightness uniformity increased by about 13% compared to the second comparative example.

[0088] (Other Embodiments) Although the embodiments of the present disclosure (including variations and examples; the same applies hereinafter) 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. For example, it goes without saying that the configuration of the light diffusion sheet (layer structure, materials, etc.) is not limited to the configuration of the light diffusion sheet 43 in the above-described embodiment. It also goes without saying that the configuration of the backlight unit to which the light diffusion sheet is applied and the configuration of the liquid crystal display device including the backlight unit are not limited to the configuration of the backlight unit 40 and the liquid crystal display device 50 in the above-described embodiment.

[0089] REFERENCE SIGNS LIST 1 TFT substrate 2 CF substrate 3 Liquid crystal layer 5 Liquid crystal display panel 6 First polarizer 7 Second polarizer 21 Base layer 22 Recess 23 Ridge line 40 Backlight unit 41 Reflective sheet 42 Small light source 43 Light diffusion layer 43a First surface 43b Second surface 44 Color conversion sheet 45 First prism sheet 46 Second prism sheet 47 Brightness enhancement sheet 50 Liquid crystal display device 50a Display screen 101 Light reflection prevention structure

Claims

1. A light diffusion sheet having a plurality of depressions formed in a generally inverted polygonal pyramid shape on a first surface, the first surface being provided with a light reflection prevention structure.

2. The light diffusion sheet according to claim 1, wherein the anti-reflection structure is a vapor deposition layer that covers the surfaces of the plurality of recesses.

3. The light diffusing sheet according to claim 2, wherein the deposition layer includes a silicon oxide layer and a metal oxide layer.

4. The light diffusion sheet according to claim 1, wherein the anti-reflection structure is a moth-eye structure.

5. The light diffusion sheet according to claim 1, wherein the plurality of recesses are formed in a substantially inverted quadrangular pyramid shape.

6. The light diffusion sheet according to claim 1, wherein a second surface opposite to the first surface is a matte surface or a mirror surface.

7. A backlight unit that is incorporated in a liquid crystal display device and directs light emitted from a plurality of light sources to a display screen, the backlight unit comprising a light diffusion sheet according to any one of claims 1 to 6 between the display screen and the plurality of light sources.

8. The backlight unit according to claim 7, wherein the light diffusion sheet is disposed with the first surface facing the display screen.

9. The backlight unit according to claim 7, wherein the light diffusion sheet is disposed such that a second surface opposite to the first surface faces the display screen.

10. The backlight unit according to claim 7, wherein 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.

11. The backlight unit according to claim 7, further comprising a color conversion sheet that converts the wavelength of the light between the plurality of light sources and the light diffusion sheet.

12. The backlight unit according to claim 7, wherein the plurality of light sources are arranged two-dimensionally at a pitch of 10 mm or less.

13. A liquid crystal display device comprising the backlight unit according to claim 7 and a liquid crystal display panel.

14. An information device comprising the liquid crystal display device according to claim 13.

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