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

Abstract

To provide a light diffusion sheet which can suppress a decrease in brightness uniformity even if it is made thinner.SOLUTION: A light diffusion sheet 43 has a plurality of recesses 22 each formed in substantially inverted polygonal pyramid-shape, on a first surface 43a. The first surface 43a is provided with an antireflection structure 101. The antireflection structure 101 may be, for example, a vapor deposition layer covering surfaces of the plurality of recesses 22, or a moth-eye structure.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] 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 a light source is placed behind the liquid crystal panel.

[0003] When a direct type backlight is adopted, 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 over the entire display screen (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2011-129277 A Summary of the Invention [Problem to be solved by the invention]

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

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

[0007] In order to achieve the above object, the light diffusion sheet according to the present disclosure is a light diffusion sheet having a plurality of recesses formed in a substantially 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 reflection prevention structure on the first surface having a plurality of recesses formed in a substantially inverted polygonal pyramid shape, the light transmittance is increased on the first surface while the light reflectance is decreased. Therefore, it is possible to suppress the reduction in the light diffusion effect due to the recesses formed in a substantially inverted polygonal pyramid shape caused by the light reflection on the first surface. Therefore, even if the light diffusion sheet is made thin, it is possible to suppress the reduction in brightness uniformity, and it is also possible to accommodate further thinning of the backlight.

[0009] In the light diffusion sheet according to the present disclosure, the antireflection structure may be a deposition layer covering the surface of the recesses. In this way, it is easy to provide the antireflection structure on the first surface having the recesses. In this case, if the 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 has a moth-eye structure, it is possible to obtain an antireflection structure having an excellent light reflection suppressing effect.

[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 having 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 surface or a mirror surface, which can suppress a reduction in the light diffusion effect due to the recesses formed in a substantially inverted polygonal pyramid shape caused by the shape of the second 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 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] According to the backlight unit of the present disclosure, since it is equipped with the light diffusion sheet of the present disclosure described above, even if the light diffusion sheet is made thin, it is possible to suppress a decrease in brightness uniformity, and therefore it is possible to accommodate even 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. In this way, it is possible to avoid a situation in which the light diffusion effect due to the recesses of the first surface is reduced by the second surface. In addition, when a light reflection prevention structure that selectively prevents reflection of light (vertically incident light) incident in the normal direction of the sheet surface of the light diffusion sheet is provided on the first surface, that is, a light reflection prevention structure in which the transmittance of vertically incident light is higher than the transmittance of 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, the light diffusion sheet may be a laminate of a plurality of sheets and disposed between the display screen and the plurality of light sources. As the number of light diffusion sheets disposed increases, the luminance uniformity increases.

[0017] In the backlight unit according to the present disclosure, a color conversion sheet for converting the wavelength of the light may be further provided between the plurality of light sources and the light diffusion sheet. In this way, it is not necessary to use an expensive white light source, so that the cost can be reduced. In addition, it is possible to avoid a situation in which the light diffusion effect of the light diffusion sheet is reduced by the color conversion 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 the luminance 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 slimming.

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

[0022] According to the information device according to the present disclosure, since it is equipped with the liquid crystal display device according to the present disclosure described above, it can also accommodate further slimming. Effect of the Invention

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

[0024] [Figure 1] 1 is a cross-sectional view of a liquid crystal display device according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a backlight unit according to the embodiment. [Diagram 3] 3 is a plan view showing an example of the arrangement of light sources in the backlight unit shown in FIG. 2. [Figure 4] 1 is a perspective view of a light diffusion sheet according to an embodiment of the present invention; [Diagram 5] FIG. 2 is a cross-sectional view of a light diffusion sheet according to an embodiment of the present invention. [Figure 6] FIG. 13 is a graph showing the incidence angle dependence of the transmittance for light with a wavelength of 450 nm when a vapor deposition layer is provided on a polycarbonate sheet with a thickness of 100 μm and mirror-finished on both sides. [Figure 7]FIG. 13 is a graph showing the incidence angle dependency of reflectance for light with a wavelength of 450 nm when a vapor deposition layer is provided on a 100 μm-thick polycarbonate sheet with mirror finish on both sides. [Figure 8] FIG. 11 is a cross-sectional view of a backlight unit used in evaluation of additional examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] (Embodiment) 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 may be modified as desired within the scope of the technical concept of the present disclosure.

[0026] <Liquid crystal display device> 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 rear side of the liquid crystal display panel 5 via the first polarizing plate 6. The liquid crystal display panel 5 includes a TFT substrate 1 and a CF substrate 2 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 a rectangle or a square, but is not limited to this, and may be any shape, such as a rectangle or square with rounded corners, an ellipse, a circle, a trapezoid, an automobile instrument panel, etc.

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

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

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

[0031] <Backlight unit> As shown in FIG. 2, the backlight unit 40 of this embodiment mainly includes a reflection sheet 41, a plurality of small light sources 42 arranged two-dimensionally on the reflection sheet 41, and a light diffusion sheet 43 provided above the plurality of small light sources 42. The light diffusion sheet 43 may be arranged by stacking a plurality of sheets (three sheets in this example). 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 the small light source 42 is not particularly limited, and for example, it may be an LED element, a laser element, etc., and an LED element may be used from the viewpoints of cost, productivity, etc. In order to adjust the light emission angle characteristics of the LED element serving as the small light source 42, a lens may be attached to the LED element. For example, as shown in FIG. 3, a plurality of small light sources 42 each composed of an LED element may be arranged on the reflection sheet 41 in a two-dimensional array with a certain interval. The small light source 42 may have a rectangular shape in plan view, and in that 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 arrangements of the small light sources 42 is not particularly limited either, but when a plurality of small light sources 42 are arranged dispersedly, it is preferable to arrange them regularly on the reflection sheet 41. Arranging regularly means arranging with a certain regularity, and for example, the case where the small light sources 42 are arranged at equal intervals corresponds to this. When the small light sources 42 are arranged at equal intervals, the center-to-center distance (array 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 small light source 42. The blue light source may emit light with x < 0.24 and y < 0.18 in the chromaticity coordinates of CIE1931, for example. Incidentally, a white light source may be used as the small light source 42. The white light source is composed of an LED element with a peak wavelength in the blue region, an LED element with a peak wavelength in the green region, and an LED element with a peak wavelength in the red region, and may emit light with 0.24 < x < 0.42 and 0.18 < y < 0.48 in the chromaticity coordinates of CIE1931, for example.

[0035] [Color conversion sheet] The color conversion sheet 44 is a wavelength conversion sheet that converts light from the small light source 42, which is, for example, a blue light source, into light with a peak wavelength of any color (for example, green or red). The color conversion sheet 44 converts, for example, 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 small light source 42 that emits blue light with a wavelength of 450 nm is used, the blue light is partially converted into green light and red light by the color conversion sheet 44, so that the light transmitted through the color conversion sheet 44 becomes white light. For example, a QD (quantum dot) sheet, a fluorescent sheet, or the like may be used as the color conversion sheet 44. Note that, when a white light source is used as the small light source 42, the color conversion sheet 44 may not be provided. In addition, the color conversion sheet 44 may be disposed above or below the light diffusion sheet 43, or between the light diffusion sheets 43, as long as it is between the small light source 42 and the brightness improvement sheet 47.

[0036] [Brightness enhancement sheet] The brightness enhancing sheet 47 has a structure in which a first prism sheet 45 and a second prism sheet 46 are laminated in order from the side closer to the small light source 42. For example, a plurality of grooves having an isosceles triangular cross section are formed adjacent to each other in the first prism sheet 45 and the second prism sheet 46, and the apex angle of the prism sandwiched between a pair of adjacent grooves is formed at about 90°. Each groove formed in the first prism sheet 45 and each groove formed in the second prism sheet 46 are arranged so as to be perpendicular to each other. The first prism sheet 45 and the second prism sheet 46 may be formed integrally. For example, the first prism sheet 45 and the second prism sheet 46 may be formed by forming a prism shape on a PET (polyethylene terephthalate) film using a UV-curable acrylic resin.

[0037] Although not shown in the figure, 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 embodiment, 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 structures different from each other. 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 the first surface 43a, and the first surface 43a is provided with a light antireflection 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 the first surface 43a, and the first surface 43a is provided with a light antireflection structure 101. Details of the light antireflection 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, a 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 set as a light output surface, but instead, the first surface 43a may be set as a light input surface. In this example, the plurality of recesses 22 are formed into a substantially inverted square pyramid as shown in FIG. 4. FIG. 4 illustrates an example in which the recesses 22 formed into a substantially inverted square pyramid are arranged in a 5×5 matrix on the first surface 43a of the light diffusion sheet 43, but the actual number of the recesses 22 arranged is much greater. Adjacent recesses 22 are partitioned by ridges 23. The arrangement pitch of the recesses 22 is, for example, about 50 μm or more and about 500 μm or less. The angle that the wall surface of the recess 22 (the slope of the approximately inverted polygonal pyramid or the approximately inverted polygonal truncated pyramid) makes with 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 to improve light diffusion, or may have a plurality of recesses like the recess 22.

[0042] The recesses 22 are not limited to a substantially inverted square pyramid, and may be formed into a substantially inverted polygonal pyramid or a substantially inverted polygonal truncated pyramid. In the present disclosure, the term "substantially inverted polygonal pyramid" may include a substantially inverted polygonal truncated pyramid. The recesses 22 may be regularly arranged in two dimensions. As the "inverted polygonal pyramid (trapezoid)", a triangular pyramid (trapezoid), a square pyramid (trapezoid), or a hexagonal pyramid (trapezoid) that can be arranged two-dimensionally without gaps are preferable. A mold (metal roll) is used in the manufacturing process such as extrusion molding or injection molding when the recesses 22 are provided, and an inverted square pyramid (trapezoid) may be selected as the "inverted polygonal pyramid (trapezoid)" in consideration of the accuracy of the cutting work of the surface of this mold (metal roll).

[0043] In addition, taking into consideration the difficulty of forming a concave portion having a geometrically strict inverted polygonal pyramid or inverted polygonal truncated pyramid shape by a normal shape transfer technique, the terms "approximately inverted polygonal pyramid" or "approximately inverted polygonal truncated pyramid" are used, but it goes without saying that these terms include shapes that can be regarded as genuine 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 "approximately inverted polygonal pyramid" or "approximately inverted polygonal pyramid truncated pyramid".

[0044] When the recesses 22 are regularly arranged in two dimensions, the recesses 22 may be provided without gaps over the entire surface of the light diffusion sheet 43, or may be provided at a constant interval (pitch). In addition, 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 composed of a base material layer 21 that does not contain a diffusing agent, for example, a base material layer 21 made of clear polycarbonate. When a diffusing agent is contained in the base material layer 21, the material of the diffusing agent is not particularly limited, but 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 the 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 due to the approximately inverted polygonal pyramid shape and the light diffusion effect due to the diffusing agent, it is preferable that the light diffusion sheet 43 does not contain a diffusing agent, but the content of the diffusing agent may be, for example, 0.1 mass% or more (preferably 0.3 mass% or more) and 10 mass% or less (preferably 8 mass% or less) with the material (matrix) constituting the base layer 21 being 100 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 due to the diffusing agent becomes insufficient.

[0047] The resin that forms the matrix of the base 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 multi-layer structure, for example, a two-layer structure of a first base material layer and a second recess-forming layer. In this case, the base material layer and the recess-forming layer may be formed as independent sheets and laminated together to form the light diffusion sheet 43, or the base material layer and the recess-forming layer may 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 recess 22. For example, when a recess having a depth of 20 μm is provided, the thickness of the recess-forming layer is made greater than 20 μm. The light diffusion sheet 43 may have a three-layer structure including the base material layer and the recess-forming layer.

[0050] Hereinafter, a method for manufacturing the light diffusion sheet 43 will be described taking as an example a case where the light diffusion sheet 43 is formed into a sheet shape. 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 the light diffusion sheet 43 is extrusion molded, for example, the line speed may be set to 2 m / min or more and 30 m / min or less, and the compression line pressure may be set to 100 kgf / cm or more and 500 kgf / cm or less.

[0051] The procedure for manufacturing a single-layer light diffusion sheet having a concave-convex shape on its surface using the extrusion molding method is as follows. First, pellet-shaped plastic particles (to which a diffusion agent may be added) are put into a single-screw extruder, and melted and kneaded while being heated. After that, the molten resin extruded by a T-die is sandwiched between two metal rolls and cooled, and then conveyed using a guide roll, and cut into a single flat plate by a sheet cutter, or the continuous sheet is wound into a roll by a winder and then cut into a single flat plate by a sheet cutter, thereby producing a light diffusion sheet. Here, by sandwiching the molten resin using metal rolls having a shape on their surface that is an inversion of the desired concave-convex shape, the inverted shape of the roll surface is transferred to the resin, so that the desired concave-convex shape can be formed on the diffusion sheet surface. In addition, the shape transferred to the resin is not necessarily 100% of the shape of the roll surface, so 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, 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.

[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 each of two single-screw extruders, and melted and kneaded while being heated. Then, the molten resins for each layer are fed into one T-die, laminated in the T-die, and the laminated molten resin extruded by the T-die is sandwiched between two metal rolls and cooled. Then, the laminated molten resin is conveyed using a guide roll, and cut into a single flat plate by 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 inverted shape of the concave-convex shape to be transferred is filled with uncured UV-curable resin, and a substrate is pressed against the resin. Next, while the roll filled with the UV-curable resin and the substrate are integrated, the resin is cured by irradiating with UV rays. Next, the sheet to which the concave-convex shape has been transferred by the resin is peeled off from the roll. Finally, the sheet is irradiated with UV rays again to completely cure the resin, and a light diffusion sheet having a concave-convex shape on its surface is manufactured.

[0055] <Light reflection prevention structure> As shown in Fig. 5, the first surface 43a of the light diffusion sheet 43, which has a plurality of recesses 22 formed in a substantially inverted polygonal pyramid shape, is provided with a light antireflection structure 101 in order to increase the light transmittance and decrease the light reflectance on the first surface 43a. The light antireflection structure 101 increases the light transmittance by 1% or more, preferably 3% or more, more preferably 5% or more, and decreases the light reflectance by 1% or more, preferably 3% or more, more preferably 5% or more, on the first surface 43a, over a wide range of the incidence angle of light incident on the light diffusion sheet 43 (the inclination angle with respect to the normal line of the sheet surface of the light diffusion sheet 43), compared with the case where the light antireflection structure 101 is not provided. Alternatively, the light antireflection structure 101 that selectively prevents reflection of light incident in the normal line direction of the sheet surface of the light diffusion sheet 43 (vertical incident light), that is, the light antireflection structure 101 having a higher transmittance of vertical incident light than the transmittance of other incident light, may be provided on the first surface 43a.

[0056] The antireflection structure 101 may be, for example, a deposition layer or a sputtering layer that thinly covers the surfaces of the recesses 22 to a thickness of 1 μm or less. The antireflection structure 101 may be a laminate including multiple layers having different refractive indices, for example, a low refractive index layer having a refractive index less than 1.55 and a high refractive index layer having a refractive index of 1.55 or more (preferably 1.80 or more). As the low refractive index layer, for example, a silicon oxide layer, a magnesium fluoride layer, a silicon oxyfluoride layer, or the like having a thickness of about 10 to 500 nm may be used. As the high refractive index layer, for example, a zirconium oxide layer, a hafnium oxide layer, a titanium oxide layer, or the like having a thickness of about 10 to 500 nm may be used. 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 having different refractive indices. When the antireflection structure 101 is a laminate including a low refractive index layer and a high refractive index layer, the uppermost layer of the laminate may be a low refractive index layer such as a silicon oxide layer. Alternatively, the antireflection structure 101 may be a coating layer made of a material having a lower refractive index than the material constituting the base layer 21 of the light diffusion sheet 43 .

[0057] Furthermore, the light reflection preventing 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, four slopes of a substantially inverted quadrangular pyramid). The moth-eye structure has a structure in which protrusions having a structure equal to or 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 when the light diffusion sheet 43 is molded, or may be formed by surface treatment after the light diffusion sheet 43 is molded.

[0058] <Effects of the embodiment> The light diffusion sheet 43 of this embodiment has a plurality of recesses 22 formed in a substantially inverted polygonal pyramid shape on the first surface 43a, and the anti-reflection structure 101 is provided on the first surface 43a. As a result, the light transmittance increases on the first surface 43a, while the light reflectance decreases. This makes it possible to suppress a reduction in the light diffusion effect of the recesses 22 formed in a substantially inverted polygonal pyramid shape due to light reflection on the first surface 43a. Therefore, even if the light diffusion sheet 43 is made thin, it is possible to suppress a reduction in luminance uniformity, and therefore it is possible to accommodate further thinning of the backlight.

[0059] In the light diffusion sheet 43 of this embodiment, the light antireflection structure 101 may be a deposition layer that covers the surfaces of the multiple recesses 22. In this way, it becomes easy to provide the light antireflection structure 101 on the first surface 43a having the multiple recesses 22. In this case, if the deposition layer includes a silicon oxide layer and a metal oxide layer, the light 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 the present embodiment, when the antireflection structure 101 has a moth-eye structure, the antireflection structure 101 has 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 has excellent light diffusing properties.

[0062] In the light diffusion sheet 43 of the present embodiment, the second surface 43b opposite to the first surface 43a may be a matte surface or a mirror surface. In this way, it is possible to suppress a reduction in the light diffusion effect caused by 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 the present embodiment may be 50 μm or more and 1200 μm or less. In this way, the backlight unit 40 and the liquid crystal display 50 can be made thinner while obtaining the light diffusion effect of the light diffusion sheet 43.

[0064] The backlight unit 40 of this embodiment is incorporated in 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 the present embodiment, the light diffusion sheet 43 may be disposed with the first surface 43a facing the display screen 50a. In this way, it is possible to avoid a situation in which the light diffusion effect due to the recesses 22 of the first surface 43a is reduced when the light passes through the second surface 43b. In addition, when the first surface 43a is provided with a light reflection prevention structure 101 that selectively prevents reflection of light (vertically incident light) incident in the normal direction of the sheet surface of the light diffusion sheet 43, that is, a light reflection prevention structure 101 having a higher transmittance for vertically incident light than the transmittance 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 the present 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 luminance uniformity increases.

[0067] In the backlight unit 40 of the present embodiment, a color conversion sheet 44 for converting the wavelength of light may be further provided between the multiple small light sources 42 and the light diffusion sheet 43. In this way, it is not necessary to use an expensive white light source, and therefore costs can be reduced. In addition, it is possible to avoid a situation in which the light diffusion effect of the light diffusion sheet 43 is reduced when light passes through the color conversion sheet 44.

[0068] In the backlight unit 40 of the present embodiment, the multiple small light sources 42 may be two-dimensionally arranged 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 the present embodiment, a plurality of small light sources 42 may be disposed on the reflecting sheet 41 provided on the opposite side of the display screen 50a as viewed from the light diffusion sheet 43. In this way, multiple reflections of light between the reflecting sheet 41 and the light diffusion sheet 43 further improve the luminance uniformity.

[0070] The liquid crystal display device 50 of the present embodiment includes the backlight unit 40 of the present embodiment and a liquid crystal display panel 5. According to the liquid crystal display device 50 or an information device including the liquid crystal display device 50, since the backlight unit 40 includes the light diffusion sheet 43 of the present embodiment, 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.

[0071] Examples and Comparative Examples <Evaluation sample> As the light diffusion sheet 43 of the examples and comparative examples, evaluation samples made of polycarbonate with a thickness of 110 μm were prepared. No diffusion agent was added to any of the evaluation samples, and on one side, inverted square pyramid-shaped recesses with a depth of 50 μm and a apex angle of 90° were arranged in a two-dimensional matrix at a pitch of 100 μm, and the other side was a matte surface. In the light diffusion sheet 43 of the first example, a deposition layer was formed as the light reflection prevention structure 101 only on the recess formation surface, and in the light diffusion sheet 43 of the second example, a deposition layer was formed as the light reflection prevention structure 101 on both the recess formation surface and the matte surface. Moreover, in the light diffusion sheet 43 of the first comparative example, a deposition layer was not formed on either side.

[0072] The anti-reflection structure 101 was a deposition layer with a thickness of about 200 nm, which was made by alternately laminating ZrO2 thin films and SiO2 thin films. The incidence angle dependence of the transmittance and reflectance for light with a wavelength of 450 nm when this deposition layer was provided on a 100 μm thick polycarbonate sheet with both sides processed to be mirror-finished is shown in FIG. 6 and FIG. 7, respectively. For the measurement of the transmittance and reflectance, an automatic absolute reflectance measuring system ARMN-920 was attached to a spectrophotometer V-770 manufactured by JASCO Corporation was used. As shown in FIG. 6 and FIG. 7, by providing a deposition layer on one side of the sheet, the transmittance increased by about 5% or more and the reflectance decreased by about 5% or more over a wide range (0° to 60°) of the incidence angle of the light incident on the sheet (the inclination angle with respect to the normal line of the sheet surface). Moreover, by providing a deposition layer on both sides of the sheet, the transmittance increased by about 10% or more and the reflectance decreased by about 10% or more over a wide range (0° to 60°) of the incidence angle of the light incident on the sheet. Although the transmittance and reflectance when the incidence angle exceeds 60° are not shown in Figures 6 and 7, even when the incidence angle exceeds 60°, by providing a deposition layer, it is possible to obtain the same effect of increasing the transmittance and decreasing the reflectance as when the incidence angle is 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 Fig. 6 and 7, for the case where a vapor-deposited layer is provided on only one side of the sheet, no significant difference was observed in the transmittance and reflectance whether light is incident from the vapor-deposited side or the non-vapor-deposited side.

[0074] <Brightness uniformity evaluation> 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 a luminance uniformity evaluation was performed.

[0075] Specifically, three light diffusion sheets 43 having the same structure were laminated with the recessed surface as the light output surface on the upper side of the small light source 42 arranged in a two-dimensional manner on the reflection sheet 41 via the color conversion sheet 44, and a first prism sheet 45 and a second prism sheet 46 were arranged on the upper side. A QD sheet was used for the color conversion sheet 44. The prism sheets 45 and 46 were formed by providing a protruding prism portion using a UV-curable acrylic resin made of acrylate on a base layer made of a PET film. The first prism sheet 45 had a total thickness of 90 μm, and the first prism sheet 45 had protruding prism portions with a height of 12 μm and a apex angle of 90° arranged at a pitch of 24 μm. The second prism sheet 46 had a total thickness of 155 μm, and the second prism sheet 46 had protruding prism portions with a height of 25 μm and a apex angle of 90° 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 Fig. 2, a restraining glass for restraining each sheet was arranged on the second prism sheet 46 via an upper light diffusion sheet (top diffuser). For the upper light diffusion sheet, a PET film with a bead coating for diffusion on one side and a bead coating for adhesion prevention on the other side was used.

[0076] The luminance uniformity evaluation was performed as follows, with blue LEDs as the small light sources 42 of the backlight unit 40 arranged in three ways: (1) a square array with a pitch of 2.8 mm (hereinafter referred to as light source array A), (2) a two-dimensional array with a length 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 pitch of 10 mm (hereinafter referred to as light source array C). First, the luminance in the vertical upward direction (the direction from the light source to the holding glass) was measured using a two-dimensional spectroradiometer SR-5000 manufactured by Topcon Technohouse. Next, the two-dimensional luminance distribution in a 40 mm square range was obtained using a luminance unevenness meter, and the overall luminance balance was corrected, and the average luminance value and standard deviation were calculated, and the luminance uniformity was calculated as "luminance uniformity" = "(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 embodiment was 117.7, and the luminance uniformity of the light diffusion sheet 43 of the second embodiment 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 any of the light source arrays A to C, the luminance uniformity was increased by providing the antireflection structure 101 on the surface (first surface 43a) of the light diffusion sheet 43 on which the recesses 22 were formed. In particular, in the first embodiment in which the antireflection structure 101 was provided only on the first surface 43a, the luminance uniformity increased by about 3% to 10% compared to the first comparative example. On the other hand, in the second embodiment in which the antireflection structure 101 was also provided on the second surface 43b having no recesses 22, the luminance uniformity was slightly decreased compared to the first embodiment. In both the first and second embodiments, the decrease in luminance caused by the antireflection structure 101 was about 1 to 2% compared to the first comparative example, which was not a problem.

[0081] Additional Examples and Comparative Examples <Evaluation sample> As the light diffusion sheet 43 of the additional example and the additional comparative example, an evaluation sample made of polycarbonate with a thickness of 110 μm was prepared. No diffusion agent was added to any of the evaluation samples, and on one side, inverted square pyramid-shaped recesses with a depth of 50 μm and a apex angle of 90° were arranged in a two-dimensional matrix at a pitch of 100 μm, and the other side was a matte surface. In the light diffusion sheet 43 of the third example, a deposition layer was formed as the light reflection prevention structure 101 only on the recess formation surface, and in the light diffusion sheet 43 of the fourth example, a deposition layer was formed as the light reflection prevention structure 101 on both the recess formation surface and the matte surface. In addition, in the light diffusion sheet 43 of the second comparative example, a deposition layer was not 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 (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, the antireflection structure 101 selectively prevents reflection of light incident in the normal direction of the sheet surface of the light diffusion sheet 43 (vertical incident light), that is, the antireflection structure 101 having a higher transmittance for vertically 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 a deposition layer having a thickness of about 600 nm, in which TiO2 thin films and SiO2 thin films were alternately laminated.

[0083] <Brightness uniformity evaluation> 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 luminance 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 three light diffusion sheets 43 are arranged with their first surfaces 43a (recessed portion forming surfaces) serving as light entrance surfaces.

[0084] Specifically, three light diffusion sheets 43 having the same structure were laminated with the recessed surface as the light entrance surface on the upper side of the small light source 42 arranged in a two-dimensional manner on the reflection sheet 41 via the color conversion sheet 44, and a first prism sheet 45 and a second prism sheet 46 were arranged on the upper side. A QD sheet was used for the color conversion sheet 44. The prism sheets 45 and 46 were formed by providing a protruding prism portion using a UV-curable acrylic resin made of acrylate on a base layer made of a PET film. 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 apex angle of 90° 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 apex angle of 90° 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 Fig. 8, a restraining glass for restraining each sheet was arranged on the second prism sheet 46 via an upper light diffusion sheet (top diffuser). For the upper light diffusion sheet, a PET film with a bead coating for diffusion on one side and a bead coating for adhesion prevention on the other side was used.

[0085] The luminance uniformity evaluation was performed as follows, with blue LEDs arranged in the above-mentioned light source array B as the small light sources 42 of the backlight unit 40. First, the luminance in the vertical 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. Next, a luminance unevenness meter was used to obtain a two-dimensional luminance distribution in 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 calculated as "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 embodiment was 17.1, and the luminance uniformity of the light diffusion sheet 43 of the fourth embodiment was 15.8.

[0087] From the above results, it was found that the luminance uniformity was increased by providing the antireflection structure 101 on the surface (first surface 43a) of the light diffusion sheet 43 on which the recesses 22 were formed. In particular, in the third embodiment in which the antireflection structure 101 was provided only on the first surface 43a, the luminance uniformity increased by about 22% compared to the second comparative example. Also, in the fourth embodiment in which the antireflection structure 101 was provided also on the second surface 43b having no recesses 22, the luminance uniformity increased by about 13% compared to the second comparative example.

[0088] (Other embodiments) Although the embodiments of the present disclosure (including modified examples and examples; the same applies below) have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the disclosure. In other words, the description of the above-described embodiments is essentially merely illustrative, and is not intended to limit the present disclosure, its applications, or its uses. For example, it goes without saying that the configuration (layer structure, material, etc.) of the light diffusion sheet is not limited to the configuration of the light diffusion sheet 43 of the above-described embodiment. It 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 of the above-described embodiment. [Explanation of symbols]

[0089] 1 TFT substrate 2 CF board 3 Liquid crystal layer 5 Liquid crystal display panel 6 First polarizing plate 7 Second polarizing plate 21 Base material layer 22 Recess 23 Ridgeline 40 Backlight unit 41 Reflective Sheet 42 Small light source 43 Light Diffusion Layer 43a 1st page 43b 2nd side 44 Color conversion sheet 45 First prism sheet 46 Second prism sheet 47 Brightness enhancing sheet 50 LCD display device 50a display screen 101 Anti-reflection structure

Claims

1. A light diffusion sheet having a first surface on which a plurality of recesses are formed in a substantially inverted polygonal pyramid shape, The first surface is provided with an anti-reflection structure. Light diffusion sheet.

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

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

4. The antireflection structure is a moth-eye structure. The light diffusing sheet according to claim 1 .

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

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

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

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

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

10. 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. The backlight unit according to claim 7.

11. A color conversion sheet for converting the wavelength of the light is further provided between the plurality of light sources and the light diffusion sheet. The backlight unit according to claim 7.

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

13. The backlight unit according to claim 7 ; A liquid crystal display panel is provided. LCD display device.

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

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