Backlight unit, liquid crystal display device, and information device

Abstract

To provide a light diffusion sheet that can improve luminance uniformity while avoiding a reduction in luminance.SOLUTION: A light diffusion sheet 43 has a plurality of concave parts 22 formed in a substantially inverted polygonal pyramid shape or a substantially inverted polygonal frustum shape on at least a first surface 43a. The arrangement pitch of the plurality of concave parts 22 is 300 μm or more and 1500 μm or less. The width of a boundary part between the adjacent concave parts 22 of the plurality of convex parts 22 is 25% or less of the arrangement pitch.SELECTED DRAWING: Figure 4

Description

Technical Field

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

Background Art

[0002] As a display device for various information devices such as smartphones and tablet terminals, liquid crystal display devices are widely used. As a backlight of a liquid crystal display device, a direct-lit type in which a light source is disposed behind a liquid crystal panel is mainstream.

[0003] When adopting a direct-lit backlight, a light diffusing sheet is used in order to erase the image of a light source such as an LED (Light Emitting Diode) on the light emitting surface and improve the in-plane luminance uniformity (see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in a conventional direct-lit backlight, with thinning due to reduction in the thickness of the light diffusing sheet and the distance between the light source and the light diffusing sheet, and a decrease in the number of light sources for cost reduction, luminance may decrease in the area between light sources (an area where no light source is disposed).

[0006] On the other hand, attempts have been made to print white ink that reflects light in the area directly above the light source of the light diffusing sheet to reduce the luminance in the area directly above the light source and eliminate luminance unevenness that occurs between the area directly above the light source and the area between light sources. However, in this case, a decrease in luminance across the entire screen is inevitable.

[0007] An object of the present disclosure is to provide a light diffusion sheet capable of improving luminance uniformity while avoiding a decrease in luminance.

Means for Solving the Problems

[0008] In order to achieve the above object, the light diffusion sheet according to the present disclosure has at least a first surface with a plurality of recesses formed in a substantially inverted pyramid or a substantially inverted frustum of a pyramid shape, the arrangement pitch of the plurality of recesses being 300 μm or more and 1500 μm or less, and the width of the boundary portion between adjacent recesses in the plurality of recesses being 25% or less of the arrangement pitch.

[0009] According to the light diffusion sheet of the present disclosure, since at least the first surface has a plurality of recesses formed in a substantially inverted pyramid or a substantially inverted frustum of a pyramid shape, luminance uniformity can be improved. Also, since the boundary portion between the recesses is flatter than the recess wall surface and light from the light source is less likely to be diffused (reflected or refracted) there, by setting the arrangement pitch of the recesses to be as large as 300 μm or more, it is possible to reduce the area ratio occupied by this boundary portion on the first surface. Thereby, luminance uniformity can be improved without performing a process that causes a decrease in luminance with respect to the light diffusion sheet.

[0010] In the light diffusion sheet according to the present disclosure, in order to reduce the area ratio of the boundary portion, the width of the boundary portion is set to 25% or less, preferably 20% or less, more preferably 15% or less of the arrangement pitch of the recesses. However, in order to avoid a decrease in wear resistance, the width of the boundary portion is set to 0.5% or more, preferably 1.0% or more of the arrangement pitch of the recesses.

[0011] In the light diffusion sheet according to the present disclosure, when the arrangement pitch of the recesses exceeds 1500 μm, for example, in the case of an inverted quadrangular pyramid with an apex angle of 80° and the thickness of the light diffusion sheet being 1 mm or more, it becomes difficult to make the backlight unit thinner particularly when a plurality of light diffusion sheets are laminated, so the arrangement pitch of the recesses is set to 1500 μm or less.

[0012] In the present disclosure, the "boundary portion between recesses" means the "width of the flat portion intentionally arranged between the recesses" when the recesses are arranged at intervals, and means the "width of the curved portion at the top of the ridge line partitioning the recesses" when the recesses are arranged without gaps.

[0013] In addition, in the present disclosure, considering that it is difficult to form geometrically precise inverted pyramids or frustums of inverted pyramids by ordinary shape transfer techniques, the notations "substantially inverted pyramid" or "substantially frustum of an inverted pyramid" are used, but it goes without saying that these notations include shapes that can be regarded as true or substantially inverted pyramids or frustums of inverted pyramids.

[0014] In the present disclosure, the "light diffusing sheet" shall include a plate-shaped "light diffusing plate" and a film-shaped "light diffusing film".

[0015] In the present disclosure, the "optical sheet" means a sheet having various optical functions such as diffusion, light collection, refraction, and reflection, and the "light diffusing sheet" is one of the "optical sheets".

[0016] In the light diffusing sheet according to the present disclosure, when the arrangement pitch is 1000 μm or less, an increase in the thickness of the light diffusing sheet can be suppressed, and the backlight unit can be made thinner.

[0017] In the light diffusing sheet according to the present disclosure, when the width of the boundary portion is the width of the curved portion at the top of the ridge line partitioning the plurality of recesses, in other words, when the recesses are arranged without gaps, the luminance uniformity can be improved as compared with the case where the recesses are arranged at intervals. In this case, the plurality of recesses are formed in a substantially inverted quadrangular pyramid or a substantially frustum of an inverted quadrangular pyramid, the ridge line extends in a first direction and a second direction, the arrangement pitch is an average value of a first arrangement pitch of the plurality of recesses in the first direction and a second arrangement pitch of the plurality of recesses in the second direction, and the width of the boundary portion may be an average value of the width occupied by the curved portion at the top of the ridge line in the first direction and the width occupied by the curved portion at the top of the ridge line in the second direction. Thereby, the formation of the recesses can be easily performed.

[0018] In the light diffusion sheet according to the present disclosure, when the angle formed by the wall surface of the plurality of concave portions and the sheet surface of the light diffusion sheet is 40 degrees or more and 65 degrees or less, a sufficient effect of improving luminance uniformity can be obtained by the concave portions.

[0019] In the light diffusion sheet according to the present disclosure, when the plurality of concave portions are provided only on the first surface and the second surface of the light diffusion sheet is a flat surface or a matte surface, it is possible to obtain an effect of improving luminance uniformity while suppressing wear and damage on the second surface.

[0020] The backlight unit according to the present disclosure is a backlight unit incorporated in a liquid crystal display device for guiding light emitted from a light source toward a display screen, and includes at least one light diffusion sheet according to the present disclosure described above between the display screen and the light source.

[0021] According to the backlight unit according to the present disclosure, since the light diffusion sheet according to the present disclosure described above is provided, it is possible to improve luminance uniformity while avoiding luminance reduction over the entire screen. In particular, when a plurality of the light diffusion sheets according to the present disclosure described above are used, it is possible to obtain an excellent effect of improving luminance uniformity while suppressing luminance reduction.

[0022] In the backlight unit according to the present disclosure, it is preferable that a plurality of the light diffusion sheets are provided, and the first surface of the light diffusion sheet farthest from the light source among the plurality of light diffusion sheets is an incident surface. By doing so, in a backlight unit including a plurality of light diffusion sheets, the luminance uniformity can be further improved as compared with the case where the first surface (concave portion forming surface) of the light diffusion sheet farthest from the light source is an emission surface.

[0023] In the backlight unit according to the present disclosure, at least one other light diffusion sheet having no concave portions formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape may be further provided between the display screen and the light source. By doing so, both luminance and luminance uniformity can be improved by a combination of different types of light diffusion sheets.

[0024] When the backlight unit according to the present disclosure includes the other light diffusion sheet, a plurality of the light diffusion sheets are provided, the other light diffusion sheet is disposed between the display screen and the plurality of light diffusion sheets, and both surfaces of the other light diffusion sheet may be matte surfaces with a surface roughness Ra of 0.1 μm or more and 10 μm or less. By doing so, both the luminance and the luminance uniformity can be further improved. In this case, the difference in the surface roughness Ra between both surfaces of the other light diffusion sheet is 0.5 μm or more, and it is more preferable that the surface with the smaller surface roughness Ra in the other light diffusion sheet is the light incident surface. Further, it is more preferable that the other light diffusion sheet contains 0.5 parts by mass or more and 1.5 parts by mass or less of a light diffusing agent with respect to 100 parts by mass of the matrix resin.

[0025] When the backlight unit according to the present disclosure includes the other light diffusion sheet, the other light diffusion sheet contains a light diffusing agent and is disposed so as to face the light emitting surface of the light diffusion sheet, and may further include a luminance improvement sheet disposed so as to face the light emitting surface of the other light diffusion sheet. By doing so, both the luminance and the luminance uniformity can be further improved.

[0026] When the backlight unit according to the present disclosure includes the other light diffusion sheet, the other light diffusion sheet may be disposed closer to the display screen than the light diffusion sheet. By doing so, both the luminance and the luminance uniformity can be further improved. In this case, the light diffusion sheet may be disposed such that the second surface faces the light source. By doing so, both the luminance and the luminance uniformity can be further improved.

[0027] The liquid crystal display device according to the present disclosure includes the backlight unit according to the foregoing present disclosure and a liquid crystal display panel.

[0028] According to the liquid crystal display device according to the present disclosure, since the backlight unit according to the present disclosure described above is provided, it is possible to improve the luminance uniformity while avoiding a decrease in luminance over the entire screen.

[0029] The information device according to the present disclosure includes the liquid crystal display device according to the present disclosure described above.

[0030] According to the information device according to the present disclosure, since the liquid crystal display device according to the present disclosure described above is provided, it is possible to improve the luminance uniformity while avoiding a decrease in luminance over the entire screen.

Effect of the Invention

[0031] According to the present disclosure, it is possible to provide a light diffusion sheet capable of improving the luminance uniformity while avoiding a decrease in luminance.

Brief Description of the Drawings

[0032]

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[0033] (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 can be arbitrarily changed within the scope of the technical idea of the present disclosure.

[0034] <Liquid Crystal Display Device> As shown in FIG. 1, the liquid crystal display device 50 of the present 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 provided to face each other, a liquid crystal layer 3 provided between the TFT substrate 1 and the CF substrate 2, and a sealing material (not shown) provided in a frame shape for enclosing the liquid crystal layer 3 between the TFT substrate 1 and the CF substrate 2.

[0035] In principle, the shape of the display screen 50a of the liquid crystal display device 50 as viewed from the front (above in FIG. 1) is rectangular or square, but it is not limited thereto, and it may be a shape in which the corners of the rectangle are rounded, an elliptical shape, a circular shape, a trapezoidal shape, or any shape such as an instrument panel of an automobile.

[0036] In the liquid crystal display device 50, in each sub-pixel corresponding to each pixel electrode, a voltage of a predetermined magnitude is applied to the liquid crystal layer 3 to change the alignment state of the liquid crystal layer 3. As a result, the transmittance of the light incident from the backlight unit 40 through the first polarizing plate 6 is adjusted. The light with the adjusted transmittance is emitted through the second polarizing plate 7 and an image is displayed.

[0037] The liquid crystal display device 50 of the present embodiment is used as a display device incorporated in various information devices (for example, in-vehicle devices such as car navigation, personal computers, mobile phones, portable information terminals such as notebook computers and tablets, portable game machines, copy machines, ticket vending machines, automated teller machines, etc.).

[0038] The TFT substrate 1 includes, for example, a plurality of TFTs provided in a matrix on a glass substrate, an interlayer insulating film provided to cover each TFT, a plurality of pixel electrodes provided in a matrix on the interlayer insulating film and connected to the plurality of TFTs respectively, and an alignment film provided to cover each pixel electrode. The CF substrate 2 includes, for example, a black matrix provided in a lattice pattern on a glass substrate, a color filter including a red layer, a green layer, and a blue layer provided between the grids of the black matrix respectively, a common electrode provided to cover the black matrix and the color filter, and an alignment film provided to cover the common electrode. The liquid crystal layer 3 is composed 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 provided to sandwich the polarizer layer.

[0039] <Backlight unit> In the example shown in FIG. 2, the backlight unit 40 of the present embodiment includes a reflection sheet 41, a plurality of light sources 42 two-dimensionally arranged on the reflection sheet 41, a first light diffusion sheet 43 provided above the plurality of light sources 42, a second light diffusion sheet 44 provided above the first light diffusion sheet 43, and a brightness enhancement sheet 47 provided above the second light diffusion sheet 44. The first light diffusion sheet 43 is used in at least one sheet, but in this example, two first light diffusion sheets 43 are laminated and used. The brightness enhancement sheet 47 is not particularly limited as long as it can increase the brightness of the light emitted from the light source 42. In this example, as the brightness enhancement sheet 47, a lower first prism sheet 45 and an upper second prism sheet 46 are laminated and used. Although not shown, a polarizing sheet may be provided above the brightness enhancement sheet 47.

[0040] In the present disclosure, the "light diffusion sheet" shall include a plate-shaped "light diffusion plate" and a film-shaped "light diffusion film". The "optical sheet" means a sheet having various optical functions such as diffusion, light collection, refraction, and reflection, and the "light diffusion sheet", "reflection sheet", "brightness enhancement sheet", etc. are included in the "optical sheet".

[0041] [Reflective sheet] The reflective sheet 41 is composed of, for example, a film made of white polyethylene terephthalate resin, a silver vapor-deposited film, or the like.

[0042] [Light source] The type of the light source 42 is not particularly limited, and it may be, for example, an LED element, a laser element, or the like, 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 light source 42, a lens may be attached to the LED element. The 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 10 mm or less (preferably 5 mm or less). The number of arrangements of the light sources 42 is not particularly limited either, but when a plurality of light sources 42 are arranged dispersedly, it is preferable to arrange them regularly on the reflective sheet 41. Arranging regularly means arranging with a certain regularity, and for example, the case where the light sources 42 are arranged at equal intervals corresponds to this. When the light sources 42 are arranged at equal intervals, the center-to-center distance between two adjacent light sources 42 may be 0.5 mm or more (preferably 2 mm or more) and 20 mm or less.

[0043] In this embodiment, for example, as shown in FIG. 3, a plurality of light sources 42 each composed of an LED element are arranged in a two-dimensional array at regular intervals. In other words, the plurality of light sources 42 are arranged along two directions orthogonal to each other. As the light source 42, a white light source may be used. 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, for example, with 0.24 < x < 0.42 and 0.18 < y < 0.48 in the chromaticity coordinates of CIE1931. Alternatively, as the light source 42, a blue light source may be used. The blue light source may emit light, for example, with x < 0.24 and y < 0.18 in the chromaticity coordinates of CIE1931. When using a blue light source, a color conversion sheet is arranged between the light source 42 and the luminance improvement sheet 47. The color conversion sheet is, for example, a wavelength conversion sheet that converts light from the light source 42, which is a blue light source, into light having an arbitrary color (for example, green or red) as the peak wavelength. The color conversion sheet 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, when using the light source 42 that emits blue light with a wavelength of 450 nm, the blue light is partially converted into green light and red light by the color conversion sheet, so the light transmitted through the color conversion sheet becomes white light. As the color conversion sheet, a QD (quantum dot) sheet, a fluorescent sheet, or the like may be used.

[0044] [First Light Diffusion Sheet] The first light diffusion sheet 43 has a base material layer 21. A plurality of recesses 22 are provided on the first surface 43a (the light incident surface in this example) of the first light diffusion sheet 43. The plurality of recesses 22 are formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape. In this example, the plurality of recesses 22 are formed in a substantially inverted regular quadrangular pyramid shape. Adjacent recesses 22 are partitioned by ridge lines 23.

[0045] The array pitch of the recesses 22 is set to, for example, about 50 μm or more. As a feature of the present embodiment, in at least one of the first light diffusion sheets 43, the array pitch of the recesses 22 is set to 300 μm or more and 1500 μm or less, preferably 300 μm or more and 1000 μm or less, more preferably 300 μm or more and 700 μm, and even more preferably 400 μm or more and 550 μm or less.

[0046] The angle formed by the wall surface of the recess 22 (the inclined surface of a substantially inverted pyramid or a substantially inverted truncated pyramid) and the sheet surface of the first light diffusion sheet 43 (the virtual mirror surface without the recess 22) is set to, for example, 40 degrees or more and 65 degrees or less, preferably 45 degrees or more and 60 degrees or less, more preferably 47 degrees or more and 55 degrees or less. In other words, the apex angle of the recess 22 is set to, for example, 50 degrees or more and 100 degrees or less, preferably 60 degrees or more and 90 degrees or less, more preferably 70 degrees or more and 86 degrees or less.

[0047] The second surface 43b of the first light diffusion sheet 43 may be a flat surface (mirror surface), but may also be a matte surface in order to improve diffusibility.

[0048] In this example, the first light diffusion sheet 43 is arranged such that the first surface 43a is the incident surface, but alternatively, the first light diffusion sheet 43 may be arranged such that the first surface 43a is the exit surface. Further, when a plurality of first light diffusion sheets 43 are used, the first light diffusion sheet 43 with the first surface 43a as the incident surface and the first light diffusion sheet 43 with the first surface 43a as the exit surface may be mixed. Further, when a plurality of first light diffusion sheets 43 are used, a plurality of types of first light diffusion sheets 43 having different dimensions, shapes, array pitches, and surface shapes of the second surface 43b of the recess 22 may be used.

[0049] As shown in FIG. 4, a plurality of concave portions 22 having a substantially inverted quadrangular pyramid shape (inverted pyramid shape) are arranged in a two-dimensional matrix on the first surface 21a of the first light diffusion sheet 43. In other words, the plurality of concave portions 22 are arranged along two directions orthogonal to each other. Adjacent concave portions 22 are partitioned by ridge lines 23. The ridge lines 23 extend along the two directions in which the concave portions 22 are arranged. The center of the concave portion 22 (the apex of the inverted pyramid) is the deepest part of the concave portion 22. In FIG. 4, for simplicity, an example in which the concave portions 22 are arranged in a 5×5 matrix is illustrated, but the actual number of arranged concave portions 22 is much larger. In the two-dimensional arrangement of the plurality of concave portions 22, each concave portion 22 may be provided on the first surface 21a without a gap, or may be provided with a predetermined interval. Also, as long as the light diffusion effect is not impaired, some of the concave portions 22 may be randomly arranged.

[0050] The base material layer 21 is configured, for example, with polycarbonate as a base material (matrix resin) and preferably does not contain a diffusing agent, but may contain, for example, about 10% by mass or less of a diffusing agent with respect to 100% by mass of the base material. Known materials can be appropriately used as the diffusing agent. In this example, the first light diffusion sheet 43 has a single-layer structure of the base material layer 21, but instead, it may have a structure of two or more layers including a layer in which the concave portions 22 are formed.

[0051] Details of the first light diffusion sheet 43 will be described later.

[0052] [Second Light Diffusion Sheet] The second light diffusion sheet 44 does not have recesses formed in a substantially inverted pyramid or substantially inverted truncated pyramid shape. In this example, one second light diffusion sheet 44 is used, but two or more second light diffusion sheets 44 may be used. The second light diffusion sheet 44 may have, for example, a matte surface on the first surface (the surface facing the first prism sheet 45) 44a and a flat surface (mirror surface) on the second surface 44b. The second light diffusion sheet 44 is preferably configured with, for example, polycarbonate as the base material (matrix resin) and contains a diffusing agent. It may contain, for example, about 0.5 to 4 parts by mass (preferably about 0.5 to 1.5 parts by mass) of the diffusing agent with respect to 100 parts by mass of the base material. The second light diffusion sheet 44 is configured by mixing, for example, 1 part by mass of silicone composite powder (average particle diameter 2.0 μm) as a diffusing agent with respect to 99 parts by mass of an aromatic polycarbonate resin.

[0053] [Brightness Enhancement Sheet] In this example, the first prism sheet 45 and the second prism sheet 46 that constitute the brightness enhancement sheet 47 are, for example, films in which a plurality of groove stripes with a cross-section of an isosceles triangle are formed adjacent to each other, and the apex angle of the prism sandwiched between a pair of adjacent groove stripes is formed to be about 90°. Here, each groove stripe formed on the first prism sheet 45 and each groove stripe 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. As the first prism sheet 45 and the second prism sheet 46, for example, a PET (polyethylene terephthalate) film with a prism shape formed using a UV-curable acrylic resin may be used.

[0054] [Other Optical Sheets] Although illustration is omitted, a polarizing sheet may be provided above the second prism sheet 46. The polarizing sheet improves the brightness of the display screen 50a by preventing the light emitted from the backlight unit 40 from being absorbed by the first polarizing plate 6 of the liquid crystal display device 50.

[0055] [Modification Example of Backlight Unit]< In the configuration example of the backlight unit 40 shown in FIG. 2, a combination of two laminated first light diffusion sheets 43 and a second light diffusion sheet 44 is used. Instead of this, as in the modified example shown in FIG. 5, three laminated first light diffusion sheets 43 may be used without using the second light diffusion sheet 44. Alternatively, although not shown, four or more first light diffusion sheets 43 may be laminated. Also in this modified example, in at least one of the first light diffusion sheets 43, the arrangement pitch of the recesses 22 is set to 300 μm or more and 1500 μm or less, preferably 300 μm or more and 1000 μm or less, more preferably 300 μm or more and 700 μm, and even more preferably 400 μm or more and 550 μm or less.

[0056] In addition, in this modified example, the first light diffusion sheet 43 is arranged such that the first surface 43a is the light incident surface, but instead, the first light diffusion sheet 43 may be arranged such that the first surface 43a is the light emitting surface. Also, the first light diffusion sheet 43 with the first surface 43a as the light incident surface and the first light diffusion sheet 43 with the first surface 43a as the light emitting surface may be mixed.

[0057] <Details of the first light diffusion sheet> In the example shown in FIG. 2 or FIG. 5, a plurality of recesses 22 are formed on the first surface 43a of the first light diffusion sheet 43. In addition to this, a plurality of other recesses similar to the recesses 22 may also be formed on the second surface 43b of the first light diffusion sheet 43.

[0058] The plurality of recesses 22 may be formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape. The plurality of recesses 22 may be regularly two-dimensionally arranged. As the "inverted pyramid (truncated pyramid)", a triangular pyramid (truncated pyramid), a quadrangular pyramid (truncated pyramid), or a hexagonal pyramid (truncated pyramid) that can be two-dimensionally arranged without gaps is preferable. In the manufacturing process such as extrusion molding or injection molding when providing the recesses 22, a mold (metal roll) is used. Considering the accuracy of the cutting operation on the surface of this mold (metal roll), an inverted quadrangular pyramid (truncated pyramid) may be selected as the "inverted pyramid (truncated pyramid)".

[0059] In the present disclosure, in consideration of the fact that it is difficult to form geometrically precise inverted pyramids or frustums of inverted pyramids by ordinary shape transfer techniques, the notations "substantially inverted pyramid" or "substantially frustum of an inverted pyramid" are used, but it goes without saying that these notations include shapes that can be regarded as true or substantially inverted pyramids or frustums of inverted pyramids. Further, "substantially" means that approximation is possible. For example, "substantially quadrangular pyramid" refers to a shape that can approximate a quadrangular pyramid. Also, shapes deformed from "inverted pyramid" or "frustum of an inverted pyramid" within the range of inevitable shape variations due to processing accuracy in industrial production are also included in "substantially inverted pyramid" or "substantially frustum of an inverted pyramid".

[0060] When a plurality of recesses 22 are regularly two-dimensionally arranged, the plurality of recesses 22 may be provided without gaps over the entire surface of the first light diffusion sheet 43, or a flat portion having a predetermined width may be provided between the recesses 22.

[0061] The first light diffusing 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 the base material layer 21 contains a diffusing agent, the material of the diffusing agent is not particularly limited. As inorganic particles, for example, silica, titanium oxide, aluminum hydroxide, barium sulfate, etc. may be used, and as organic particles, for example, 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). The first light diffusing sheet 43 is preferably free of a diffusing agent from the viewpoints of the reflection and refraction effects due to a substantially inverted pyramid shape and the light diffusion effect due to the diffusing agent. However, with the material (matrix) constituting the base material layer 21 being 100% by mass, 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). The difference between the refractive index of the diffusing agent and the refractive index of the matrix of the base material layer 21 may be 0.01 or more, preferably 0.03 or more, more preferably 0.05 or more, still 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 material layer 21 is less than 0.01, the diffusion effect by the diffusing agent becomes insufficient.

[0062] The resin serving as the matrix of the base material layer 21 is not particularly limited as long as it is a material that transmits light. For example, acrylic, polystyrene, styrene acrylic, polycarbonate, MS (methyl methacrylate - styrene copolymer) resin, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyimide, etc. may be used.

[0063] The thickness of the first light diffusing sheet 43 is not particularly limited, but for example, it may be 3 mm or less (preferably 2 mm or less, more preferably 1.5 mm or less, still more preferably 1 mm or less) and 0.1 mm or more (preferably 0.2 mm or more, more preferably 0.3 mm or more). If the thickness of the first light diffusing sheet 43 exceeds 3 mm, it becomes difficult to achieve the thinning of the liquid crystal display. On the other hand, if the thickness of the first light diffusing sheet 43 is less than 0.1 mm, it becomes difficult to exert the effect of improving the luminance uniformity.

[0064] When the first light diffusing sheet 43 has a multilayer structure (for example, a base material layer of the first layer and a recess forming layer of the second layer), the thickness of the recess forming layer has a thickness larger than the maximum depth of the recess 22. For example, in the case of a layer provided with a recess having a depth of 20 μm, the thickness is made larger than 20 μm. The first light diffusing sheet 43 may be configured with a structure of three or more layers including a base material layer and a recess forming layer. Alternatively, the base material layer and the recess forming layer may be configured as independent sheets, and the two may be laminated or separately arranged.

[0065] <Manufacturing method of the first light diffusing sheet> Hereinafter, the manufacturing method of the first light diffusing sheet 43 will be described. The manufacturing method of the first light diffusing sheet 43 is not particularly limited, but for example, an extrusion molding method, a compression molding method, a transfer method using a UV curable resin or a thermosetting resin, an injection molding method, etc. may be used. When extruding the first light diffusing sheet 43, for example, the line speed is preferably 2 m / min or more and 20 m / min or less (more preferably 3 m / min or more and 10 m / min or less), and the compression line pressure is preferably 100 kgf / cm or more and 800 kgf / cm or less (more preferably 200 kgf / cm or more and 500 kgf / cm or less). Note that if the line speed exceeds 50 m / min, the shape transfer rate tends to decrease, while if the line speed is less than 1 m / min, the productivity tends to decrease. Also, if the compression line pressure exceeds 1000 kgf / cm, it may exceed the mechanical strength of the manufacturing equipment, while if the compression line pressure is less than 50 kgf / cm, the shape transfer rate tends to decrease.

[0066] The procedure for manufacturing a single-layer light diffusion sheet having an uneven shape on its surface using an extrusion molding method is as follows. First, pellet-shaped plastic particles (diffusing agents may be added) are fed into a single-screw extruder and melted and kneaded while being heated. Then, the molten resin extruded by a T-die is sandwiched between two metal rolls and cooled, and then conveyed using guide rolls and cut into single-sheet flat plates by a sheet cutter machine to produce a light diffusion sheet. Here, by sandwiching the molten resin using a metal roll having a shape with the desired uneven shape inverted on its surface, the inverted shape on the roll surface is transferred to the resin, so that the desired uneven shape can be imparted to the surface of the light diffusion sheet. Also, since the shape transferred to the resin does not necessarily become a shape in which the shape of the roll surface is 100% transferred, the shape of the roll surface may be designed by calculating backward from the transfer degree.

[0067] When manufacturing a two-layer light diffusion sheet having an uneven shape on its surface using an extrusion molding method, for example, pellet-shaped plastic particles necessary for forming 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 produced sheets may be laminated.

[0068] Alternatively, a two-layer light diffusion sheet having an uneven shape on its surface may be produced as follows. First, pellet-shaped plastic particles necessary for forming each layer are fed into each of two single-screw extruders and melted and kneaded while being heated. Then, the molten resin for each layer is fed into a single 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 guide rolls and cut into single-sheet flat plates by a sheet cutter machine to produce a two-layer light diffusion sheet having an uneven shape on its surface.

[0069] Alternatively, a light diffusion sheet may be manufactured as follows by shape transfer using UV (ultraviolet rays). First, an uncured ultraviolet curable resin is filled into a roll having an inverted shape of the uneven shape to be transferred, and a base material is pressed against the resin. Next, while the roll filled with the ultraviolet curable resin and the base material are integrated, ultraviolet rays are irradiated to cure the resin. Next, the sheet having the uneven shape shaped by the resin is peeled off from the roll. Finally, the resin is completely cured by irradiating the sheet with ultraviolet rays again to produce a light diffusion sheet having the uneven shape on the surface.

[0070] <Features of the First Light Diffusion Sheet> Hereinafter, the features of the first light diffusion sheet 43 of the present embodiment will be described in detail with reference to FIGS. 6 to 11.

[0071] As shown in FIG. 6, on the first surface 43a of the first light diffusion sheet 43, a plurality of recesses 22 formed in a substantially inverted regular square pyramid shape, for example, are provided. The plurality of recesses 22 may be formed in a substantially inverted regular square pyramid frustum shape. The center 22a of the recess 22 is the deepest part of the recess 22. The plurality of recesses 22 are arranged along the X direction (first direction) and the Y direction (second direction) orthogonal to each other. Adjacent recesses 22 are partitioned by a ridge line 23. The ridge line 23 extends along the X direction and the Y direction.

[0072] In the first light diffusion sheet 43, the ridge line 23 may have a shape recessed between the intersections 23a of the ridge line 23 with respect to the straight lines Lx and Ly connecting the intersections 23a of the ridge line 23. The maximum height difference d between the straight lines Lx and Ly connecting the intersections 23a and the ridge line 23 may be 1 μm or more and 10 μm or less, preferably 1.5 μm or more and 7 μm or less, more preferably 2.5 μm or more and 5 μm or less.

[0073] In the first light diffusion sheet 43, the ridge line may be recessed between all the intersections 23a of the ridge line, or the ridge line 23 may not have a recessed shape between some of the intersections 23a.

[0074] FIG. 7 shows an example of the shape of the ridge line 23 extending in the X direction along the Ax - Bx line in FIG. 6 when viewed from a direction parallel to the sheet surface and perpendicular to the X direction, and FIG. 8 shows an example of the shape of the ridge line 23 extending in the Y direction along the Ay - By line in FIG. 6 when viewed from a direction parallel to the sheet surface and perpendicular to the Y direction. As shown in FIG. 7, with respect to the straight line Lx connecting the intersections 23a of the ridge line 23 in the X direction, the ridge line 23 may have a shape that is concave between the intersections 23a. In this case, taking the arrangement pitch Px in the X direction of the recess 22, the ridge line 23 extending in the X direction has, for example, the lowest point 23b at a position of Px / 2 (half pitch) from the intersection 23a, and the distance (maximum height difference) from the straight line Lx to the lowest point 23b is dx. Also, as shown in FIG. 8, with respect to the straight line Ly connecting the intersections 23a of the ridge line 23 in the Y direction, the ridge line 23 may have a shape that is concave between the intersections 23a. In this case, taking the arrangement pitch Py in the Y direction of the recess 22, the ridge line 23 extending in the Y direction has, for example, the lowest point 23b at a position of Py / 2 (half pitch) from the intersection 23a, and the distance (maximum height difference) from the straight line Ly to the lowest point 23b is dy.

[0075] When the recess 22 is formed in an inverted regular square pyramid shape, the arrangement pitch Px in the X direction of the recess 22 is equal to the interval (horizontal distance) between the intersections 23a in the X direction, and the arrangement pitch Py in the Y direction of the recess 22 is equal to the interval (horizontal distance) between the intersections 23a in the Y direction.

[0076] Taking the average value of the maximum height difference dx in the X direction and the maximum height difference dy in the Y direction as the maximum height difference d, the maximum height difference d may be set to 1 μm or more and 10 μm or less, preferably 1.5 μm or more and 7 μm or less, more preferably 2.5 μm or more and 5 μm or less.

[0077] The concave shape of the ridge line 23 between the intersections 23a is not particularly limited. For example, as shown in FIG. 9, with respect to the straight line L connecting the intersections 23a, the ridge line 23 may be concave in a substantially arc shape (FIG. 9(A)), a substantially parabolic shape (FIG. 9(B)), a substantially triangular shape (FIG. 9(C)), or a substantially trapezoidal shape (FIG. 9(D)) between the intersections 23a.

[0078] As a characteristic of the first light diffusion sheet 43, when the arrangement pitch of the recesses 22 is P and the dimension occupied by the curved portion at the top of the ridge line 23 in the arrangement direction of the recesses 22 is Wr, the ratio Wr / P needs to be 0.25 (25%) or less, preferably 0.2 (20%) or less, and more preferably 0.15 (15%) or less. However, in order to avoid a decrease in wear resistance, the ratio Wr / P should be 0.005 (0.5%) or more, preferably 0.01 (1.0%) or more.

[0079] In addition, in the present disclosure, when the recesses 22 are arranged without gaps as shown in FIG. 6, the "curved portion at the top of the ridge line 23" is regarded as the "boundary portion between the recesses 22". When the recesses 22 are arranged with an interval, the "flat portion intentionally arranged between the recesses 22" is regarded as the "boundary portion between the recesses 22".

[0080] FIG. 10 shows an example of the cross-sectional configuration of the first light diffusion sheet 43 along the Cx-Dx line in FIG. 6, and FIG. 11 shows an example of the cross-sectional configuration of the first light diffusion sheet 43 along the Cy-Dy line in FIG. 6. Specifically, FIG. 10 shows the cross-sectional configuration when the first light diffusion sheet 43 is cut by a plane passing through the centers 22a of the recesses 22 adjacent to each other in the X direction and the midpoint between the intersection points 23a on the ridge line 23 located between the recesses 22 and perpendicular to the sheet surface. FIG. 11 shows the cross-sectional configuration when the first light diffusion sheet 43 is cut by a plane passing through the centers 22a of the recesses 22 adjacent to each other in the Y direction and the midpoint between the intersection points 23a on the ridge line 23 located between the recesses 22 and perpendicular to the sheet surface.

[0081] In the cross-sectional configuration shown in FIG. 10, the distance (horizontal distance) between the centers 22a of the recesses 22 adjacent to each other in the X direction is equal to the arrangement pitch Px of the recesses 22 in the X direction. The dimension occupied by the curved portion (boundary portion) at the top of the ridge line 23 in the X direction is Wrx. The dimensions occupied by the straight portions of the respective wall surfaces (inclined surfaces of the inverted square pyramid) of the recesses 22 adjacent to each other with the ridge line 23 interposed therebetween in the X direction are Wsx1 and Wsx2. The angle formed between the wall surface (inclined surface of the inverted square pyramid) of the recess 22 and the sheet surface in the X direction is θx. The height from the center 22a of the recess 22 to the apex (midpoint between the intersection points 23a) of the ridge line 23 (ridge line 23 extending in the Y direction) is Hx.

[0082] In the cross-sectional configuration shown in FIG. 11, the distance (horizontal distance) between the centers 22a of the recesses 22 adjacent to each other in the Y direction is equal to the arrangement pitch Py of the recesses 22 in the Y direction. The dimension occupied by the curved portion (boundary portion) at the top of the ridge line 23 in the Y direction is Wry. The dimensions occupied by the straight portions of the respective wall surfaces (inclined surfaces of the inverted square pyramid) of the recesses 22 adjacent to each other with the ridge line 23 interposed therebetween in the Y direction are Wsy1 and Wsy2. The angle formed between the wall surface (inclined surface of the inverted square pyramid) of the recess 22 and the sheet surface in the Y direction is θy. The height from the center 22a of the recess 22 to the apex (midpoint between the intersection points 23a) of the ridge line 23 (ridge line 23 extending in the X direction) is Hy.

[0083] When the recess 22 is formed in an inverted square pyramid, it is necessary to set the ratio Wr / P, where P is the average value of the arrangement pitch Px and the arrangement pitch Py and Wr is the average value of the dimension Wrx and the dimension Wry, to 0.25 (25%) or less, preferably 0.2 (20%) or less, and more preferably 0.15 (15%) or less.

[0084] FIG. 12 shows an example of the results of measuring the shape and dimensions of the X-direction ridge lines shown in FIG. 7 using a laser microscope. FIG. 13 shows an example of the results of measuring the shape and dimensions of the Y-direction ridge lines shown in FIG. 8 using a laser microscope. FIG. 14 shows an example of the results of measuring the shape, dimensions, and angles of the cross-sectional configuration shown in FIG. 10 using a laser microscope. FIG. 15 shows an example of the results of measuring the shape, dimensions, and angles of the cross-sectional configuration shown in FIG. 11 using a laser microscope. In the measurement of the maximum values (maximum height differences) dx and dy of the distances between the straight lines Lx and Ly connecting the intersections 23a of the ridge lines 23 and the ridge lines 23, the maximum values of the lengths of the perpendiculars drawn perpendicularly from the points on the ridge lines 23 to the straight lines Lx and Ly were taken as dx and dy. Also, in the measurement of the array pitches Px and Py, the "horizontal distances between the intersections 23a" in the X direction and the Y direction were determined as Px and Py, respectively. Even with the method of measuring the "horizontal distance between the intersections 23a" in this way, the array pitches Px and Py can be obtained easily and accurately.

[0085] <Effects of the Embodiment (Including Modification Examples)> As described above, the first light diffusion sheet 43 of the present embodiment has at least on the first surface 43a a plurality of recesses 22 formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape, the array pitch of the recesses 22 is 300 μm or more and 1500 μm or less, and the width of the boundary portion between adjacent recesses 22 is 25% or less of the array pitch.

[0086] According to the first light diffusion sheet 43 of the present embodiment, since at least the first surface 43a has a plurality of recesses 22 formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape, the luminance uniformity can be improved. Also, since the boundary portion between the recesses 22 is flatter than the wall surface of the recesses 22 and light from the light source 42 is less likely to be diffused (reflected or refracted) at this portion, by setting the array pitch of the recesses 22 to be as large as 300 μm or more, it is possible to reduce the area ratio occupied by this boundary portion on the first surface 43a. As a result, the luminance uniformity can be improved without performing a process that causes a decrease in luminance with respect to the first light diffusion sheet 43.

[0087] In the first light diffusion sheet 43 of the present embodiment, in order to reduce the area ratio of the boundary portion, the width of the boundary portion is set to 25% or less, preferably 20% or less, more preferably 15% or less of the arrangement pitch of the concave portions 22. However, in order to avoid a decrease in wear resistance, the width of the boundary portion is set to 0.5% or more, preferably 1.0% or more of the arrangement pitch of the concave portions 22.

[0088] In the first light diffusion sheet 43 of the present embodiment, when the arrangement pitch of the concave portions 22 exceeds 1500 μm, for example, in the case of an inverted quadrangular pyramid with an apex angle of 80° and a sheet thickness of 1 mm or more, it becomes difficult to make the backlight unit 40 thinner, especially when a plurality of first light diffusion sheets 43 are stacked. Therefore, the arrangement pitch of the concave portions 22 is set to 1500 μm or less.

[0089] In the first light diffusion sheet 43 of the present embodiment, when the arrangement pitch of the concave portions 22 is 1000 μm or less, it is possible to suppress an increase in sheet thickness and make the backlight unit 40 thinner.

[0090] In the first light diffusion sheet 43 of the present embodiment, the width of the boundary portion between the concave portions 22 may be the width of the curved portion at the top of the ridge line 23 that partitions the concave portions 22. In this case, since the concave portions 22 are arranged without gaps, the luminance uniformity can be improved as compared with the case where the concave portions 22 are arranged with intervals. Further, when the concave portions 22 are formed in a substantially inverted quadrangular pyramid or a substantially inverted quadrangular frustum shape, the concave portions 22 can be easily formed.

[0091] In the first light diffusion sheet 43 of the present embodiment, when the angle formed by the wall surface of the concave portion 22 and the sheet surface is 40 degrees or more and 65 degrees or less, a sufficient effect of improving luminance uniformity can be obtained by the concave portion 22.

[0092] In the first light diffusion sheet 43 of the present embodiment, when the concave portions 22 are provided only on the first surface 43a and the second surface 43b is a flat surface or a matte surface, an effect of improving luminance uniformity can be obtained while suppressing wear and damage on the second surface 43b.

[0093] In the first light diffusion sheet 43 of the present embodiment, when the ridge line 23 (the opening edge of the recess 22) that partitions the recess 22 causes wear or damage, if the ridge line 23 has a shape that is recessed between the intersection points 23a of the ridge line 23, wear and damage are less likely to occur even when used in overlap with other optical sheets or other light diffusion sheets. Further, the dimension Wr occupied by the curved portion of the top of the ridge line 23 in the arrangement direction of the recesses 22 is suppressed to 25% or less of the arrangement pitch P of the recesses. For this reason, since the top of the ridge line 23 can maintain a steep shape, the luminance uniformity is less likely to decrease even if the ridge line 23 is recessed between the intersection points 23a. Further, when the maximum height difference d between the straight line connecting the intersection points 23a and the ridge line 23 is 1 μm or more, the scratch resistance is improved, and when the maximum height difference d is 10 μm or less, a decrease in luminance uniformity can be suppressed. In particular, when the maximum height difference d is 1.5 μm or more and 7 μm or less, both the scratch resistance and the luminance uniformity can be further improved, and when the maximum height difference d is 2.5 μm or more and 5 μm or less, both the scratch resistance and the luminance uniformity can be further improved.

[0094] The backlight unit 40 of the present embodiment is a backlight unit 40 that is incorporated into the liquid crystal display device 50 and guides the light emitted from the light source 42 toward the display screen 50a, and at least one sheet of the first light diffusion sheet 43 of the present embodiment is provided between the display screen 50a and the light source 42.

[0095] According to the backlight unit 40 of the present embodiment, since the first light diffusion sheet 43 is provided, it is possible to improve the luminance uniformity while avoiding a decrease in luminance over the entire screen. In particular, when a plurality of the first light diffusion sheets 43 are used, it is possible to obtain an excellent effect of improving the luminance uniformity while suppressing a decrease in luminance.

[0096] In the backlight unit 40 of the present embodiment, a plurality of the first light diffusion sheets 43 are provided, and the first surface 43a of the first light diffusion sheet 43 that is farthest from the light source 42 among the plurality of the first light diffusion sheets 43 may be the light incident surface. By doing so, the luminance uniformity can be further improved as compared with the case where the first surface 43a (the surface on which the recesses 22 are formed) of the first light diffusion sheet 43 that is farthest from the light source 42 is the light emitting surface.

[0097] In the backlight unit 40 of this embodiment, at least one additional second light diffusion sheet 44 (other light diffusion sheet) that does not have a concave portion formed in a substantially inverted pyramid or substantially inverted truncated pyramid shape may be provided between the display screen 50a and the light source 42. By doing so, both the luminance and the luminance uniformity can be improved by combining different types of light diffusion sheets.

[0098] When providing the second light diffusion sheet 44, a plurality of first light diffusion sheets 43 may be provided, and both surfaces of the second light diffusion sheet 44 may be matte surfaces with a surface roughness Ra of 0.1 μm or more and 10 μm or less. By doing so, both the luminance and the luminance uniformity can be further improved. In this case, the first surface 43a of the first light diffusion sheet 43 that is farthest from the light source 42 among the plurality of first light diffusion sheets 43 is the light-emitting surface, the second light diffusion sheet 44 is arranged to face the first surface 43a, the difference in the surface roughness Ra between both surfaces of the second light diffusion sheet 44 is 0.5 μm or more, and it is preferable that the surface with the smaller surface roughness Ra in the second light diffusion sheet 44 is the light-incident surface. Further, the second light diffusion sheet 44 preferably contains 0.5 parts by mass or more and 1.5 parts by mass or less of a light diffusing agent with respect to 100 parts by mass of the matrix resin.

[0099] When providing the second light diffusion sheet 44, the second light diffusion sheet 44 contains a light diffusing agent and is arranged to face the light-emitting surface of the first light diffusion sheet 43 (when a plurality of first light diffusion sheets 43 are provided, the first light diffusion sheet 43 that is farthest from the light source 42), and a luminance improvement sheet 47 arranged to face the light-emitting surface of the second light diffusion sheet 44 may be further provided. By doing so, both the luminance and the luminance uniformity can be further improved.

[0100] When providing the second light diffusion sheet 44, the second light diffusion sheet 44 may be disposed closer to the first prism sheet 45 (when a plurality of first light diffusion sheets 43 are provided, the first light diffusion sheet 43 farthest from the light source 42) than the first light diffusion sheet 43 (i.e., closer to the display screen 50a). By doing so, both the luminance and the luminance uniformity can be further improved. In this case, the first light diffusion sheet 43 (when a plurality of first light diffusion sheets 43 are provided, the first light diffusion sheet 43 farthest from the light source 42) may be disposed such that the second surface 43b faces the light source 42. By doing so, both the luminance and the luminance uniformity can be further improved.

[0101] In addition, in the backlight unit 40 of the present embodiment, when the light source 42 is disposed on the reflection sheet 41 provided on the opposite side of the display screen 50a as viewed from the first light diffusion sheet 43, the luminance uniformity is further improved.

[0102] 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.

[0103] According to the liquid crystal display device 50 of the present embodiment and the information device including the liquid crystal display device 50, since the backlight unit 40 of the present embodiment is provided, it is possible to improve the luminance uniformity while avoiding a decrease in luminance over the entire screen.

[0104] (Example) Hereinafter, the examples will be described together with comparative examples and reference examples.

[0105] <Light diffusion sheets used>[ Table 1 shows the configurations and optical characteristics of various light diffusion sheets (Sheets #1 to #16) used in the examples, comparative examples, and reference examples. In Table 1, the "width Wr of the ridge line" means the "width of the curved portion at the top of the ridge line", that is, the "width of the boundary portion between the recesses".

[0106]

Table 1

[0107] Sheet #1 was produced as follows. First, an aromatic polycarbonate resin with a melt mass flow rate of 15 g / 10 min measured in accordance with ISO 1133 was charged into an extruder, melt-kneaded, and then the resin was extruded from a T-die. Thereafter, as one of the two metal rolls, a roll having the shape shown in FIGS. 16(A) and (B) ((B) is a shape view seen from the X-Y cross-sectional direction of (A)) (a pyramid shape of a regular square pyramid with a height of 107 μm, a pitch of 180 μm, and an apex angle of 80 degrees) on its surface was used as a casting roll, and as the other roll, a roll having a random mat shape (surface roughness Ra = 2.6 μm) on its surface was used as a pressing roll. The molten resin extruded from the T-die was sandwiched between the two rolls and cooled while transferring the shape. Thereby, a single-layer sheet #1 with a thickness of 650 μm was produced by an extrusion molding method. As shown in Table 1, sheet #1 does not contain a diffusing agent, has a recess (inverted square pyramid) with a height (depth) H of 87 μm depending on the height of the regular square pyramid on the roll on one surface (first surface 43a), and the other surface (second surface 43b) is a mat surface with a surface roughness Ra = 0.47 μm. Also, the arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 180 μm, 80 degrees, 36 μm, and 20%, respectively.

[0108] Regarding sheet #2, 0.8 parts by mass of silicone composite powder (average particle diameter 2.0 μm) as a diffusing agent was premixed with 100 parts by mass of the same aromatic polycarbonate resin as sheet #1, charged into an extruder, and through the same process as sheet #1, a sheet #2 with a thickness of 650 μm was produced. As shown in Table 1, sheet #2 contains 0.8 parts by mass of a diffusing agent with respect to 100 parts by mass of the matrix resin, has a recess (inverted square pyramid) with a height (depth) H of 85 μm depending on the height of the regular square pyramid on the roll on one surface (first surface 43a), and the other surface (second surface 43b) is a mat surface with a surface roughness Ra = 0.39 μm. Also, the arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 180 μm, 80 degrees, 41 μm, and 23%, respectively.

[0109] For Sheets #3 to #5, the same resin as that of Sheet #1 was used. As one of the rolls, a roll having a shape shown in FIGS. 16(A) and (B) (a pyramid shape of a regular square pyramid with a height of about 300 μm, a pitch of 500 μm, and an apex angle of 80 degrees) on its surface was used as a casting roll, and as the other roll, a roll having a random mat shape similar to that of Sheet #1 on its surface was used as a pressing roll. Sheets #3 to #5 were produced by adjusting the thickness to 650 μm for Sheet #3, 750 μm for Sheet #4, and 850 μm for Sheet #5. Incidentally, the surface roughness and manufacturing conditions of the pressing roll were selected so as to obtain a sheet with the target surface roughness. As shown in Table 1, Sheets #3 to #5 do not contain a diffusing agent. Sheet #3 has a recess (inverted square pyramid) with a height (depth) H of 259 μm depending on the height of the regular square pyramid on the roll on one surface (the first surface 43a), and the other surface (the second surface 43b) is a mat surface with a surface roughness Ra = 0.45 μm. The arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 500 μm, 80 degrees, 40 μm, and 8%, respectively. Sheet #4 has a recess (inverted square pyramid) with a height (depth) H of 249 μm depending on the height of the regular square pyramid on the roll on one surface (the first surface 43a), and the other surface (the second surface 43b) is a mat surface with a surface roughness Ra = 0.41 μm. The arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 500 μm, 80 degrees, 52 μm, and 10%, respectively. Sheet #5 has a recess (inverted square pyramid) with a height (depth) H of 239 μm depending on the height of the regular square pyramid on the roll on one surface (the first surface 43a), and the other surface (the second surface 43b) is a mat surface with a surface roughness Ra = 0.38 μm. The arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 500 μm, 80 degrees, 64 μm, and 13%, respectively.

[0110] Regarding the light diffusion sheets of the double-sided mats of sheets #6, #8 to #10, and #15, resins and diffusing agents with the compositions shown in Table 1 were used respectively. For the roll for the first side, a matte roll with a surface roughness Ra of 4.5 μm was used as the casting roll, and for the roll for the second side, the roll used for sheet #1 (a roll with a random matte shape (surface roughness Ra = 2.6 μm) on the surface) was used as the pressing roll. Thus, sheets #6, #8 to #10, and #15 having a matte surface 1 (M1 surface) with a relatively large surface roughness Ra and a matte surface 2 (M2 surface) with a relatively small surface roughness Ra were produced. As shown in Table 1, sheet #6 has a thickness of 1000 μm, does not contain a diffusing agent, one surface (the first surface 43a) is a matte surface with a surface roughness Ra = 2.3 μm, and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.91 μm. As shown in Table 1, sheet #8 has a thickness of 1000 μm, contains 0.8 parts by mass of a diffusing agent with respect to 100 parts by mass of the matrix resin, one surface (the first surface 43a) is a matte surface with a surface roughness Ra = 2.2 μm, and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.92 μm. As shown in Table 1, sheet #9 has a thickness of 1000 μm, contains 2.0 parts by mass of a diffusing agent with respect to 100 parts by mass of the matrix resin, one surface (the first surface 43a) is a matte surface with a surface roughness Ra = 4.0 μm, and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.53 μm. As shown in Table 1, sheet #10 has a thickness of 1200 μm, contains 0.8 parts by mass of a diffusing agent with respect to 100 parts by mass of the matrix resin, one surface (the first surface 43a) is a matte surface with a surface roughness Ra = 1.9 μm, and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.90 μm. As shown in Table 1, sheet #15 has a thickness of 1200 μm, contains 2.0 parts by mass of a diffusing agent with respect to 100 parts by mass of the matrix resin, one surface (the first surface 43a) is a matte surface with a surface roughness Ra = 2.1 μm, and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.68 μm.

[0111] Regarding Sheet #7, a resin and a diffusing agent having the composition shown in Table 1 were used. For the roll for the first surface, a matte roll with a surface roughness Ra of 4.5 μm, the same as that of Sheet #6, was used as the casting roll, and for the roll for the second surface, a mirror roll was used as the pressing roll to produce Sheet #7 having a rough matte surface and a mirror surface. As shown in Table 1, Sheet #7 has a thickness of 1000 μm, contains 0.8 parts by mass of a diffusing agent with respect to 100 parts by mass of the matrix resin, one surface (the first surface 43a) is a matte surface with a surface roughness Ra = 3.2 μm, and the other surface (the second surface 43b) is a mirror surface (flat surface) with a surface roughness Ra = 0.04 μm.

[0112] Regarding Sheet #11, while using the same resin composition and the same rolls as Sheet #3, the forming was carried out with a reduced linear pressure of the rolls. As a result, a sheet #11 with a thickness of 650 μm and a low shape transfer rate was obtained. As shown in Table 1, Sheet #11 does not contain a diffusing agent, has a recess (inverted square pyramid) with a height (depth) H of 180 μm depending on the height of the regular square pyramid on the roll on one surface (the first surface 43a), and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.47 μm. Also, the arrangement pitch P, apex angle, width Wr of the ridge line, and Wr / P of the inverted square pyramid are 500 μm, 80 degrees, 175 μm, and 35%, respectively.

[0113] Regarding Sheet #12, using the same resin as Sheet #1, as one roll, a roll having a shape similar to (A) and (B) in Fig. 16 (a pyramid shape of a regular square pyramid with a height of about 149 μm, a pitch of 250 μm, and an apex angle of 80 degrees) on the surface was used as the casting roll, and as the other roll, a roll having the same random matte shape as Sheet #1 on the surface was used as the pressing roll, and Sheet #12 was produced by adjusting to a thickness of 650 μm. As shown in Table 1, Sheet #12 does not contain a diffusing agent, has a recess (inverted square pyramid) with a height (depth) H of 130 μm depending on the height of the regular square pyramid on the roll on one surface (the first surface 43a), and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.47 μm. Also, the arrangement pitch P, apex angle, width Wr of the ridge line, and Wr / P of the inverted square pyramid are 250 μm, 80 degrees, 38 μm, and 15%, respectively.

[0114] For Sheet #13, using the same resin as Sheet #1, as one roll, a roll with a shape similar to (A) and (B) in Fig. 16 (a pyramid shape of a regular square pyramid with a height of about 191 μm, a pitch of 320 μm, and an apex angle of 80 degrees) on the surface was used as a casting roll, and as the other roll, a roll with the same random matte shape as Sheet #1 on the surface was used as a pressing roll, and Sheet #13 was created by adjusting to a thickness of 650 μm. As shown in Table 1, Sheet #13 does not contain a diffusing agent, has a recess (inverted square pyramid) with a height (depth) H of 166 μm depending on the height of the regular square pyramid on the roll on one surface (the first surface 43a), and the other surface (the second surface 43b) is a matte surface with a surface roughness Ra = 0.47 μm. Also, the arrangement pitch P, apex angle, width Wr of the ridge line, and Wr / P of the inverted square pyramid are 320 μm, 80 degrees, 40 μm, and 13%, respectively.

[0115] For Sheets #14 and #16, the same resin as that of Sheet #1 was used. As one of the rolls, a roll having the shape shown in FIGS. 16(A) and (B) (a pyramid shape of a regular square pyramid with a height of about 300 μm, a pitch of 500 μm, and an apex angle of 80 degrees) on its surface was used as the casting roll, and as the other roll, a roll having a random mat shape similar to that of Sheet #1 on its surface was used as the pressing roll. Sheets #14 and #16 were prepared by adjusting the thicknesses to 450 μm for Sheet #14 and 650 μm for Sheet #16. Incidentally, the surface roughness and manufacturing conditions of the pressing roll were selected so as to obtain a sheet with the target surface roughness. As shown in Table 1, Sheets #14 and #16 do not contain a diffusing agent. Sheet #14 has a recess (inverted square pyramid) with a height (depth) H of 284 μm, which depends on the height of the regular square pyramid on the roll, on one surface (the first surface 43a), and the other surface (the second surface 43b) is a mat surface with a surface roughness Ra = 2.0 μm. The arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 500 μm, 80 degrees, 24 μm, and 5%, respectively. Sheet #16 has a recess (inverted square pyramid) with a height (depth) H of 263 μm, which depends on the height of the regular square pyramid on the roll, on one surface (the first surface 43a), and the other surface (the second surface 43b) is a mat surface with a surface roughness Ra = 8.0 μm. The arrangement pitch P, apex angle, ridge width Wr, and Wr / P of the inverted square pyramids are 500 μm, 80 degrees, 40 μm, and 8%, respectively.

[0116] <Measurement of the Arrangement Pitch, Apex Angle, Ridge Width, and Height of the Inverted Square Pyramid> The shape measurement of the recesses (inverted square pyramids) formed in Sheets #1 to #5, #11 to #14, and #16 shown in Table 1 was performed using a laser microscope. Specifically, the array pitches Px and Py (the horizontal distances between the intersection points 23a in the X and Y directions) of the recesses 22 shown in FIGS. 7 and 8 were measured, and the array pitch P was obtained as the average value thereof. Also, the angles θx and θy (the angles formed between the wall surfaces (the slopes of the inverted regular square pyramids) of the recesses 22 and the sheet surface in the X and Y directions) shown in FIGS. 10 and 11 were measured, and the apex angle (apex angle = 180 degrees - (θx + θy)) was obtained based on the average value thereof. Further, the dimensions Wrx and Wry (the dimensions occupied by the curved portions at the tops of the ridge lines 23 in the X and Y directions) shown in FIGS. 10 and 11 were measured, and the width Wr of the ridge line was obtained as the average value thereof. Note that the unit of the ratio Wr / P of the width Wr to the array pitch P is %. Also, the heights Hx and Hy (the heights from the centers 22a of the recesses 22 to the vertices of the ridge lines 23) shown in FIGS. 10 and 11 were measured, and the height H of the inverted square pyramid was obtained as the average value thereof.

[0117] <Measurement of the surface roughness (Ra) of the light diffusing sheet> The surface roughness (Ra) of the matte surface or mirror surface of Sheets #1 to #16 shown in Table 1 was measured using an SJ-210 manufactured by Mitutoyo Corporation in accordance with JIS B-601.

[0118] <Measurement of optical properties> The light transmittance and light reflectance at a wavelength of 450 nm of Sheets #1 to #16 shown in Table 1 were measured using a V-670 manufactured by JASCO Corporation, and the haze was measured using an HZ-2 manufactured by Suga Test Instruments Co., Ltd. in accordance with JIS-K7361:2000. Note that the measurement of the optical properties was performed as follows: (1) for Sheets #1 to #5, #11 to #14, and #16 having inverted square pyramids, with the surface having the inverted square pyramid as the incident surface, (2) for Sheet #7 having a matte surface and a mirror surface, with the matte surface as the incident surface, and (3) for Sheets #6, #8 to 10, and #15 having double-sided matte surfaces, with the matte surface (M1 surface) having a larger surface roughness Ra as the incident surface.

[0119] <Measurement of luminance and luminance uniformity> In the Examples, Reference Examples, and Comparative Examples described below, the measurement of luminance and luminance uniformity was carried out with a configuration similar to that of the backlight unit 40 shown in, for example, FIG. 2 or FIG. 5, using three light diffusion sheets selected from Sheets #1 to #16 shown in Table 1 and an LED array in which light sources 42 were arranged as shown in FIG. 3. That is, an optical sheet such as a light diffusion sheet or a luminance improvement sheet was placed on the light sources (LEDs) 42 arranged in an array, and the measurement of luminance and luminance uniformity was carried out. Specifically, as the LED array, an array in which blue LEDs (product number XPGDRY-L1-0000-00501) manufactured by Cree, which serve as the light sources 42, were arranged at a pitch of 12.5 mm was used. Three light diffusion sheets were placed on the LED array, and on top of them, a luminance improvement sheet 47 (prism sheets 45 and 46) was stacked via a color conversion sheet so that the ridge lines of the prism sheets 45 and 46 were orthogonal to each other, and the measurement of luminance and luminance uniformity was performed. Note that the three light diffusion sheets used may include two or three sheets of the same type.

[0120] In the measurement of luminance uniformity, first, using an LED array (6×6) as shown in FIG. 3, the two-dimensional luminance distribution was measured on the surface of the uppermost luminance improvement sheet 47 in the sheet stacking configuration of the aforementioned backlight unit. Then, the average value and standard deviation were calculated for the actually measured luminance values of all 22,500 pixels (150×150 pixels with a pixel pitch of 0.25 mm) within the area of 3×3 LEDs vertically and horizontally. This average value was used for the evaluation of luminance, and the luminance uniformity was calculated using the average value and standard deviation of luminance according to the following formula: Luminance uniformity = (average value of luminance (cd / m 2 ))÷(standard deviation of luminance (cd / m 2 )) The higher the value of the luminance uniformity obtained in this way, the more uniform the luminance is.

[0121] <Evaluation of Luminance Uniformity> The evaluation of the luminance uniformity of the Examples, Comparative Examples, and Reference Examples described below was carried out according to the following criteria. AA: The value of the luminance uniformity is 55 or more, and the luminance uniformity is very excellent. A: The value of the luminance uniformity is 45 or more and less than 55, and the luminance uniformity is excellent. B: The value of the luminance uniformity is 35 or more and less than 45, and the luminance uniformity is somewhat excellent. C: The value of the luminance uniformity is 25 or more and less than 35, and the luminance uniformity meets the minimum requirement. X: The value of the luminance uniformity is less than 25, and the luminance uniformity is insufficient.

[0122] In Tables 2 to 5 described later showing the evaluation results of the luminance uniformity, the orientation of stacking the light diffusion sheets was described as follows. i) For Sheets #1 to #5, #11 to #14, and #16 where the first surface has an inverted square pyramid and the second surface is a matte surface, if the first surface is the incident surface, it is described as "under the inverted square pyramid", and if the first surface is the exit surface, it is described as "above the inverted square pyramid". ii) For Sheet #7 where the first surface is a matte surface and the second surface is a mirror surface, if the matte surface is the incident surface and the mirror surface is the exit surface, it is described as "under the M surface, above the mirror surface", and conversely, if the mirror surface is the incident surface and the matte surface is the exit surface, it is described as "under the mirror surface, above the M surface". iii) For Sheets #6, #8 to #10, and #15 with matte surfaces on both sides, the surface with a larger surface roughness Ra of the matte surface is defined as the M1 surface, and the surface with a smaller surface roughness Ra is defined as the M2 surface. If the M1 surface is the incident surface and the M2 surface is the exit surface, it is described as "under the M1 surface", and if the M2 surface is the incident surface and the M1 surface is the exit surface, it is described as "above the M1 surface".

[0123] <Examples 1 to 8, Comparative Examples 1 to 4, Reference Examples 1 to 2> In Examples 1 to 8, Comparative Examples 1 to 4, and Reference Examples 1 to 2, three light diffusion sheets selected from Sheets #1 to #5 and #11 shown in Table 1 were stacked in the order and orientation shown in Table 2, and the luminance and luminance uniformity were measured. Table 2 shows the total thickness of the three light diffusion sheets, the measured values of the luminance and luminance uniformity, and the evaluation results of the luminance uniformity.

[0124]

Table 2

[0125] As shown in Table 2, in Examples 1 to 8, by using two or more sheets #3 to #5 in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 500 μm, compared with Comparative Examples 1 to 4 using sheets #1 and #2 in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 180 μm, it was possible to improve the luminance uniformity while suppressing the luminance decrease. Further, as can be seen from the comparison between Example 1 and Example 2, the comparison between Example 3 and Example 4, the comparison between Example 5 and Example 6, and the comparison between Example 7 and Example 8, when the same type of light diffusion sheets were stacked in the same order, for the light diffusion sheet farthest from the light source, by using the surface provided with the concave portions having an inverted square pyramid shape as the incident surface, the luminance uniformity could be further improved. Incidentally, as can be seen from Reference Examples 1 to 2, when using sheet #11 in which Wr / P exceeds 25% because the width of the ridge line (the width of the boundary portion between the concave portions) is large, even if the concave portions having an inverted square pyramid shape were arranged at a pitch of 500 μm, the luminance uniformity was not sufficiently improved.

[0126] <Examples 9 to 20, Reference Examples 3 to 4> In Examples 9 to 20 and Reference Examples 3 to 4, three light diffusion sheets selected from sheets #3 to #5, #6, #8 to #10 shown in Table 1 were stacked in the order and orientation shown in Table 3, and the luminance and luminance uniformity were measured. Table 3 shows the total thickness of the three light diffusion sheets, the measured values of the luminance and luminance uniformity, and the evaluation results of the luminance uniformity. Incidentally, in Examples 9 to 20 and Reference Examples 3 to 4, as the light diffusion sheet farthest from the light source, any one of sheets #6, #8 to #10 whose both surfaces are matte surfaces (having no concave portions having an inverted square pyramid shape) was used, and as the other two light diffusion sheets, any one of sheets #3 to #5 in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 500 μm was used.

[0127]

Table 3

[0128] As shown in Table 3, in Examples 9 to 20, by arranging one of the sheets #8 to #10 with a matte surface on both sides on top of two light diffusion sheets selected from the sheets #3 to #5 in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 500 μm, it was possible to improve the luminance uniformity while suppressing the luminance decrease as compared with Comparative Examples 1 to 4 (see Table 2). Further, as can be seen from the comparison between Example 9 and Example 10, the comparison between Example 11 and Example 12, the comparison between Example 13 and Example 14, the comparison between Example 15 and Example 16, the comparison between Example 17 and Example 18, and the comparison between Example 19 and Example 20, when the same type of light diffusion sheets were stacked in the same order, by using the M1 surface having a larger surface roughness Ra of the matte surface as the light emitting surface in the sheets #8 to #10 (the light diffusion sheet farthest from the light source) with a matte surface on both sides, in other words, by using the M2 surface having a smaller surface roughness Ra of the matte surface as the light incident surface, the luminance uniformity could be further improved. Furthermore, as can be seen from the comparison between Examples 17 to 20 and Reference Examples 3 to 4, in order to improve the luminance uniformity, it is preferable to contain a diffusing agent in the light diffusion sheet having a matte surface on both sides, and particularly, it is preferable to contain 0.5 parts by mass or more and 1.5 parts by mass or less of the diffusing agent with respect to 100 parts by mass of the matrix resin.

[0129] <Examples 21 to 26> In Examples 21 to 26, three light diffusion sheets selected from the sheets #1, #3, #5, and #7 shown in Table 1 were stacked in the order and orientation shown in Table 4, and the luminance and luminance uniformity were measured. Table 4 shows the total thickness of the three light diffusion sheets, the measured values of the luminance and luminance uniformity, and the evaluation results of the luminance uniformity. In Examples 21 to 26, as the light diffusion sheet farthest from the light source, the sheet #7 having a rough matte surface and a mirror surface (not having concave portions having an inverted square pyramid shape) was used, and as the other two light diffusion sheets, either the sheet #1 in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 180 μm or the sheets #3 and #5 in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 500 μm were used.

[0130]

Table 4

[0131] As shown in Table 4, in Examples 21 to 26, by arranging Sheet #7 having a matte surface and a mirror surface on two light diffusion sheets selected from Sheets #1, #3, and #5 on which concave portions having an inverted square pyramid shape were arranged, it was possible to improve the luminance uniformity while suppressing the luminance decrease as compared with Comparative Examples 1 to 4 (see Table 2). Further, in Examples 21 to 22, even when only one light diffusion sheet (Sheet #3) in which the concave portions having an inverted square pyramid shape were arranged at a pitch of 500 μm was used, an effect of improving the luminance uniformity was obtained. In addition, as can be seen from the comparison between Example 25 and Example 15 (see Table 3) and the comparison between Example 26 and Example 16 (see Table 3), as the light diffusion sheet farthest from the light source, using Sheet #8 having matte surfaces on both sides (the concentration of the diffusing agent is the same as that of Sheet #7) as compared with Sheet #7 having a matte surface and a mirror surface made it possible to further improve the luminance uniformity.

[0132] <Examples 27 to 28, Comparative Examples 5 to 6> In Examples 27 to 28, three sheets of Sheet #13 shown in Table 1 were stacked in the directions shown in Table 5, and in Comparative Examples 5 to 6, three sheets of Sheet #12 shown in Table 1 were stacked in the directions shown in Table 5, and the luminance and luminance uniformity were measured respectively. Table 5 shows the total thickness of the three light diffusion sheets, the measured values of the luminance and luminance uniformity, and the evaluation results of the luminance uniformity.

[0133]

Table 5

[0134] As shown in Table 5, in Examples 27 to 28 using Sheet #13 in which the arrangement pitch P of the inverted square pyramids was 320 μm, it was possible to improve the luminance uniformity while suppressing the luminance decrease as compared with Comparative Examples 1 to 4 (see Table 2). However, in Comparative Examples 5 to 6 using Sheet #12 in which the arrangement pitch P of the inverted square pyramids was 250 μm, the luminance uniformity could not be improved.

[0135] <Examples 29 to 33> In Examples 29 to 33, three light diffusing sheets selected from Sheet #3, #5, #10, #14, #15, and #16 shown in Table 1 were stacked in the order and orientation shown in Table 6, and the luminance and luminance uniformity were measured. Table 6 shows the total thickness of the three light diffusing sheets, the measured values of the luminance and luminance uniformity, and the evaluation results of the luminance uniformity.

[0136]

Table 6

[0137] As shown in Table 6, in Examples 31 to 33, as the first and second sheets, Sheets #3 and #16 with an array pitch P of the inverted square pyramid of 500 μm were used with the surface provided with the inverted square pyramid-shaped recesses as the light-emitting surface, and as the third sheet, Sheets #10 and #15 with both sides being matte surfaces (without inverted square pyramid-shaped recesses) were used, whereby excellent luminance uniformity was obtained. In particular, in Example 33, as the first and second sheets, Sheet #16 with an inverted square pyramid-shaped recess provided on the first surface and a matte surface with a surface roughness (Ra) of 8.0 μm on the second surface was used with the first surface as the light-emitting surface, and as the third sheet, Sheet #10 with both sides being matte surfaces (without inverted square pyramid-shaped recesses) was used, whereby very excellent luminance and luminance uniformity were obtained.

[0138] (Other Embodiments) As described above, the embodiments (including examples; the same shall apply hereinafter) of the present disclosure have been described. However, the present disclosure is not limited to the foregoing embodiments only, and various modifications are possible within the scope of the disclosure. That is, the description of the foregoing embodiments is merely illustrative in nature and is not intended to limit the present disclosure, its applications, or its uses.

[0139] For example, the configuration (layer structure, material, etc.) of the light diffusion sheet is not limited to the configuration of the first light diffusion sheet 43 of the foregoing embodiment as long as it is a light diffusion sheet having "a plurality of recesses formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape on at least the first surface, the arrangement pitch of the recesses being 300 μm or more and 1500 μm or less, and the width of the boundary portion between the recesses being 25% or less of the arrangement pitch".

[0140] In addition, the configuration of the backlight to which the light diffusion sheet is applied and the liquid crystal display device including the backlight are also not limited to the configuration of the backlight unit 40 and the liquid crystal display device 50 of the foregoing embodiment as long as they include at least one light diffusion sheet having "a plurality of recesses formed in a substantially inverted pyramid or a substantially inverted truncated pyramid shape on at least the first surface, the arrangement pitch of the recesses being 300 μm or more and 1500 μm or less, and the width of the boundary portion between the recesses being 25% or less of the arrangement pitch". That is, as long as at least one light diffusion sheet of the present invention is provided without impairing the effects of the present invention, it is possible to appropriately combine and use other optical sheets having different structures. For example, it may be possible to combine and use one light diffusion sheet having "a plurality of recesses formed in a substantially inverted quadrangular pyramid shape on the first surface, the arrangement pitch of the recesses being 300 μm or more and 1500 μm or less, and the width of the boundary portion between the recesses being 25% or less of the arrangement pitch" and one other light diffusion sheet having "a plurality of recesses similar to the recesses of the light diffusion sheet on the first surface, the arrangement pitch of the recesses being 300 μm or less".

Explanation of Reference Numerals

[0141] 1 TFT substrate 2 CF substrate 3 Liquid crystal layer 5 Liquid crystal display panel 6 First polarizing plate 7 Second polarizing plate 21 Base material layer 22 Recess 22a Center 23 Ridge line 23a Intersection point 23b Lowest point 40 Backlight unit 41 Reflective sheet 42 Light source 43 First light diffusion sheet 43a First surface 43b Second surface 44 Second light diffusion sheet 44a First surface 44b Second surface 45 First prism sheet 46 Second prism sheet 47 Brightness enhancement sheet 50 Liquid crystal display device 50a Display screen

Claims

Claim 1. A backlight unit incorporated in a liquid crystal display device for guiding light emitted from a light source toward a display screen, wherein a plurality of light diffusion sheets are provided between the display screen and the light source; the plurality of light diffusion sheets each have at least a first surface with a plurality of recesses formed in a substantially inverted pyramid or substantially inverted frustum of a pyramid shape; the first surface of the light diffusion sheet furthest from the light source among the plurality of light diffusion sheets is an incident surface; the arrangement pitch of the plurality of recesses is 300 μm or more and 1500 μm or less; the width of the boundary portion between adjacent recesses among the plurality of recesses is 25% or less of the arrangement pitch; the top of the ridge line partitioning the plurality of recesses has a curved portion; the width of the boundary portion is the width of the curved portion. A backlight unit.

2. The arrangement pitch is 1000 μm or less. The backlight unit according to Claim 1.

3. The plurality of recesses are formed in a substantially inverted quadrangular pyramid or substantially inverted frustum of a quadrangular pyramid shape; the ridge line extends in a first direction and a second direction; the arrangement pitch is an average value of a first arrangement pitch of the plurality of recesses in the first direction and a second arrangement pitch of the plurality of recesses in the second direction; the width of the boundary portion is an average value of the width occupied by the curved portion of the top of the ridge line in the first direction and the width occupied by the curved portion of the top of the ridge line in the second direction. The backlight unit according to Claim 1.

4. The angle formed by the wall surface of the plurality of recesses and the sheet surface of the light diffusion sheet is 40 degrees or more and 65 degrees or less. The backlight unit according to Claim 1.

5. The plurality of recesses are provided only on the first surface; the second surface of the light diffusion sheet is a flat surface or a matte surface. The backlight unit according to Claim 1.

6. wherein at least one additional light diffusion sheet having no recesses formed in a substantially inverted pyramid or substantially inverted frustum of a pyramid shape is further provided between the display screen and the light source. The backlight unit according to Claim 1.

7. The additional light diffusion sheet is disposed between the display screen and the plurality of light diffusion sheets, and both surfaces of the additional light diffusion sheet are matte surfaces with a surface roughness Ra of 0.1 μm or more and 10 μm or less. The backlight unit according to Claim 6.

8. The difference in surface roughness Ra between both surfaces of the additional light diffusion sheet is 0.5 μm or more. The surface with the smaller surface roughness Ra in the other light diffusion sheet is the light incident surface. The backlight unit according to claim 7.

9. The other light diffusion sheet contains 0.5 parts by mass or more and 1.5 parts by mass or less of a light diffusing agent with respect to 100 parts by mass of the matrix resin. The backlight unit according to claim 7.

10. The other light diffusion sheet contains a light diffusing agent and is arranged so as to face the light emitting surface of the light diffusion sheet. The backlight unit further includes a brightness enhancement sheet arranged so as to face the light emitting surface of the other light diffusion sheet. The backlight unit according to claim 6.

11. The other light diffusion sheet is arranged closer to the display screen than the light diffusion sheet. The backlight unit according to claim 6.

12. A backlight unit according to any one of claims 1 to 11, and a liquid crystal display panel, a liquid crystal display device.

13. An information device including the liquid crystal display device according to claim 12. An information device.

Images