Optical sheet laminate, backlight unit, liquid crystal display device, information apparatus, and method for manufacturing backlight unit

Abstract

To suppress the reduction in brightness uniformity in a backlight unit.SOLUTION: A laminate of multiple diffusion sheets 43 is incorporated into a backlight unit 40 which includes a pair of prism sheets 44, 45 whose prism extension directions are orthogonal to each other. Multiple recesses 22 in the substantially-inverted square pyramidal shape are arranged in a two-dimensional matrix pattern on a first surface 21a of each diffusion sheet 43. The multiple diffusion sheets 43 include at least two diffusion sheets 43 with different arrangement directions of the recesses 22.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present disclosure relates to an optical sheet laminate, a backlight unit, a liquid crystal display device, an information device, and a method for manufacturing a backlight unit. [Background technology]

[0002] In recent years, liquid crystal display devices (hereinafter sometimes referred to as liquid crystal displays) have been widely used as display devices for various information devices such as smartphones, tablet terminals, etc. As high brightness and high contrast are required for the backlight of liquid crystal displays, the direct type, in which the light source is placed on the back of the liquid crystal panel, has become mainstream.

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

[0004] In direct-type backlight units for liquid crystal displays of notebook computers, tablets, etc., if the distance between the light source and the diffusion sheet is shortened to meet the demand for thinner devices, the diffusion of light from the light source becomes insufficient, resulting in non-uniformity in brightness and chromaticity. Therefore, by using multiple diffusion sheets stacked on top of each other, the uniformity of brightness, etc. is improved. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2011-129277 A Summary of the Invention [Problem to be solved by the invention]

[0006] Portable information terminals such as laptops and tablets are required to consume less power, and as a result, the optical sheets incorporated into backlight units are also required to produce high brightness screens with low power consumption.

[0007] However, in a conventional backlight unit, when an attempt is made to improve the overall brightness, the brightness uniformity tends to decrease.

[0008] The present disclosure aims to suppress a decrease in brightness uniformity in a backlight unit. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the optical sheet laminate of the present disclosure is an optical sheet laminate to be incorporated into a backlight unit, and comprises a plurality of diffusion sheets having a plurality of approximately inverted pyramidal recesses arranged in a two-dimensional matrix on at least one surface, and a pair of prism sheets whose prism extension directions are perpendicular to each other, and the plurality of diffusion sheets includes at least two diffusion sheets in which the arrangement directions of the plurality of recesses are different from each other.

[0010] According to the optical sheet laminate of the present disclosure, a plurality of diffusion sheets (hereinafter sometimes referred to as pyramid sheets) each having a plurality of recesses shaped like an approximately inverted square pyramid on one surface are stacked together, so that the light traveling in a straight line from the light source 42 is repeatedly diffused, thereby improving the luminance uniformity. Furthermore, by using at least two diffusion sheets each having recesses arranged in different directions, the luminance uniformity can be further improved.

[0011] In the optical sheet laminate according to the present disclosure, in order to further improve luminance uniformity, it is preferable that the plurality of diffusion sheets include a first diffusion sheet and a second diffusion sheet, and the arrangement direction of the plurality of recesses in the second diffusion sheet differs from the arrangement direction of the plurality of recesses in the first diffusion sheet by 10° or more. In this case, it is more preferable that the plurality of diffusion sheets further include a third diffusion sheet, and the arrangement direction of the plurality of recesses in the third diffusion sheet differs from each of the arrangement directions of the plurality of recesses in the first diffusion sheet and the second diffusion sheet by 10° or more.

[0012] In the optical sheet laminate according to the present disclosure, the arrangement direction of the recesses in at least one of the diffusion sheets may differ from the prism extension direction by 20° to 70°. In this way, the brightness is increased compared to the case where the angle difference between the arrangement direction of the recesses (inverted pyramids) of the diffusion sheet and the prism extension direction of the prism sheet is close to 0° or close to 90°, when compared with the same light source, the same power, and the same optical sheet laminate configuration. Therefore, a high brightness screen can be realized even with low power in the backlight unit. In this case, if the intersection angle between the arrangement direction of the recesses and the prism extension direction is 25° to 35°, or 55° to 65°, the brightness can be further increased.

[0013] The backlight unit according to the present disclosure is a backlight unit that is incorporated into a liquid crystal display device and directs light emitted from a light source toward a display screen, and includes an optical sheet laminate according to the present disclosure described above between the display screen and the light source, and the multiple diffusion sheets are disposed between the light source and the pair of prism sheets.

[0014] The backlight unit according to the present disclosure includes the optical sheet laminate according to the present disclosure described above, and therefore can suppress deterioration in luminance uniformity.

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

[0016] In the backlight unit according to the present disclosure, the distance between the light source and the plurality of diffusion sheets may be 5 mm or less, preferably 2.5 mm or less, and more preferably 1 mm or less, which allows the backlight unit to be made thinner.

[0017] A liquid crystal display device according to the present disclosure includes the backlight unit according to the present disclosure described above and a liquid crystal display panel.

[0018] According to the liquid crystal display device according to the present disclosure, since the liquid crystal display device includes the backlight unit according to the present disclosure described above, it is possible to suppress a decrease in brightness uniformity.

[0019] An information device according to the present disclosure includes the liquid crystal display device according to the present disclosure described above.

[0020] According to the information device according to the present disclosure, since it includes the liquid crystal display device according to the present disclosure described above, it is possible to suppress deterioration in luminance uniformity.

[0021] The method for manufacturing a backlight unit according to the present disclosure is a method for manufacturing a backlight unit that is incorporated in a liquid crystal display device and guides light emitted from a light source to a display screen. The method for manufacturing a backlight unit according to the present disclosure includes the steps of: arranging a plurality of diffusion sheets, at least one of which has a plurality of recesses having a shape of an approximately inverted square pyramid arranged in a two-dimensional matrix, on the display screen side as viewed from the light source; and arranging a pair of prism sheets, the prism extension directions of which are orthogonal to each other, on the display screen side as viewed from the plurality of diffusion sheets. In the step of arranging the plurality of diffusion sheets, luminance uniformity is evaluated while changing the arrangement direction of the plurality of recesses in each of the plurality of diffusion sheets, and the arrangement direction of the plurality of recesses in each of the plurality of diffusion sheets is determined based on the evaluation result.

[0022] According to the manufacturing method of the backlight unit of the present disclosure, in the step of arranging a plurality of diffusion sheets, the arrangement direction of the recesses in each diffusion sheet is determined based on the result of evaluating the luminance uniformity while changing the arrangement direction of the recesses in each diffusion sheet, so that the arrangement direction of the recesses in each diffusion sheet can be set so as to improve the luminance uniformity. Effect of the Invention

[0023] According to the present disclosure, it is possible to suppress a decrease in brightness uniformity in a backlight unit. [Brief description of the drawings]

[0024] [Figure 1] 1 is a cross-sectional view of a liquid crystal display device including a backlight unit according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a backlight unit incorporating the optical sheet laminate according to the embodiment. [Diagram 3] 4 is a cross-sectional view of a diffusion sheet included in the optical sheet laminate according to the embodiment. FIG. [Figure 4] FIG. 2 is a perspective view of a diffusion sheet included in the optical sheet laminate according to the embodiment. [Diagram 5]5 is a diagram showing an example of a relationship between the arrangement directions of recesses of each diffusion sheet in the optical sheet laminate according to the embodiment; FIG. [Figure 6] 5A and 5B are diagrams illustrating the relationship for explaining the arrangement angles of a diffusion sheet and a prism sheet in the optical sheet laminate according to the embodiment; [Figure 7] 11 is a diagram showing the relationship between the recess arrangement direction of each diffusion sheet in the optical sheet laminate according to the example and luminance uniformity. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] (Embodiment) Hereinafter, an optical sheet laminate, a backlight unit, a liquid crystal display device, an information device, and a method for manufacturing a backlight unit according to the embodiments will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments, and can be arbitrarily modified within the scope of the technical concept of the present disclosure.

[0026] <Configuration of Liquid Crystal Display Device> As shown in FIG. 1, the liquid crystal display device 50 includes a liquid crystal display panel 5, a first polarizing plate 6 attached to the lower surface of the liquid crystal display panel 5, a second polarizing plate 7 attached to the upper surface of the liquid crystal display panel 5, and a backlight unit 40 provided on the rear side of the liquid crystal display panel 5 via the first polarizing plate 6.

[0027] The liquid crystal display panel 5 includes a TFT substrate 1 and a CF substrate 2 arranged facing each other, a liquid crystal layer 3 arranged between the TFT substrate 1 and the CF substrate 2, and a frame-shaped sealant (not shown) for enclosing the liquid crystal layer 3 between the TFT substrate 1 and the CF substrate 2.

[0028] The shape of the display screen 50a of the liquid crystal display device 50 when viewed from the front (top of Figure 1) is, in principle, a rectangle or a square, but is not limited to this and may be any shape, such as a rectangle with rounded corners, an oval, a circle, a trapezoid, or an automobile instrument panel.

[0029] In the liquid crystal display device 50, a voltage of a predetermined magnitude is applied to the liquid crystal layer 3 in each sub-pixel corresponding to each pixel electrode to change the alignment state of the liquid crystal layer 3. This adjusts the transmittance of light incident from the backlight unit 40 through the first polarizing plate 6. The light with the adjusted transmittance is emitted through the second polarizing plate 7, and an image is displayed on the display screen 50a.

[0030] The liquid crystal display device 50 of this embodiment can be used as a display device to be incorporated into various information devices (e.g., in-vehicle devices such as car navigation systems, personal computers, mobile phones, portable information terminals such as notebook computers and tablets, portable game machines, copy machines, ticket vending machines, automated teller machines, etc.).

[0031] The TFT substrate 1 includes, for example, a plurality of TFTs arranged in a matrix on a glass substrate, an interlayer insulating film arranged to cover each TFT, a plurality of pixel electrodes arranged in a matrix on the interlayer insulating film and connected to each of the plurality of TFTs, and an alignment film arranged to cover each pixel electrode. The CF substrate 2 includes, for example, a black matrix arranged in a lattice on a glass substrate, color filters including a red layer, a green layer, and a blue layer arranged between each lattice of the black matrix, a common electrode arranged to cover the black matrix and the color filter, and an alignment film arranged to cover the common electrode. The liquid crystal layer 3 is made of a nematic liquid crystal material containing liquid crystal molecules having electro-optical properties. The first polarizing plate 6 and the second polarizing plate 7 include, for example, a polarizer layer having a polarization axis in one direction and a pair of protective layers arranged to sandwich the polarizer layer.

[0032] <Configuration of Backlight Unit and Optical Sheet Laminate> 2, the backlight unit 40 includes a reflective sheet 41, a plurality of light sources 42 arranged two-dimensionally on the reflective sheet 41, and an optical sheet laminate 100 provided above the plurality of light sources 42. The optical sheet laminate 100 includes a plurality of diffusion sheets 43 arranged on the side of the light sources 42, and a pair of prism sheets 44 and 45 provided above the plurality of diffusion sheets 43 (on the side of the display screen 50a).

[0033] In this embodiment, for example, a white light source may be used as the light source 42. Alternatively, for example, a blue light source may be used as the light source 42. In this case, a color conversion sheet may be provided between the plurality of diffusion sheets 43 and the prism sheets 44 and 45. The plurality of diffusion sheets 43 may be, for example, three diffusion sheets 43 each having a plurality of recesses 22 each having a substantially inverted quadrangular pyramid shape on one surface, stacked together. Alternatively, the plurality of diffusion sheets 43 may be two or four or more diffusion sheets 43 stacked together. The pair of prism sheets 44 and 45 may be a lower prism sheet 44 and an upper prism sheet 45 whose prism extension directions (extending directions of the prism ridges) are perpendicular to each other. The diffusion sheet 43 and the prism sheets 44 and 45 constituting the optical sheet laminate 100 may be in the form of a film or a plate (plate). These optical sheets may be stacked by their own weight in the frame (not shown) of the backlight unit 40 without using adhesive.

[0034] [Reflective sheet] The reflective sheet 41 is made of, for example, a white polyethylene terephthalate resin film, a silver vapor deposition film, or the like.

[0035] [light source] The type of the light source 42 is not particularly limited, and for example, it may be an LED element, a laser element, etc., and an LED element may be used from the viewpoints of cost, productivity, etc. The light source 42 may have a rectangular shape in plan view, and in that case, the length of one side may be 100 μm or more (preferably 50 μm or more) and 1 mm or less. When using an LED element as the light source 42, a plurality of LED chips may be arranged on the reflection sheet 41 at regular intervals. 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 number of arrangements of the light source 42 is not particularly limited, but when a plurality of light sources 42 are arranged dispersedly, it is preferable to arrange them regularly on the reflection sheet 41. Arranging regularly means arranging with a certain regularity, for example, it corresponds to the case where the light sources 42 are arranged at equal intervals. When arranging the light sources 42 at equal intervals, the center-to-center distance between two adjacent light sources 42 may be 0.5 mm or more (preferably 2 mm or more) and 20 mm or less. When using a white light source as the light source 42, the white light source is composed of an LED element with a peak wavelength in the blue region, an LED element with a peak wavelength in the green region, and an LED element with a peak wavelength in the red region, and may emit light, for example, with 0.24 < x < 0.42 and 0.18 < y < 0.48 in the chromaticity coordinates of CIE1931. When using a blue light source as the light source 42, the blue light source may emit light, for example, with x < 0.24 and y < 0.18 in the chromaticity coordinates of CIE1931.

[0036] [Diffusion sheet] The diffusion sheet 43 has a base layer 21 as shown in FIG. 2 and FIG. 3. The diffusion sheet 43 (base layer 21) has a first surface 21a which is a light exit surface and a second surface 21b which is a light entrance surface. That is, the diffusion sheet 43 is disposed with the second surface 21b facing the light source 42. The base layer 21 is not particularly limited as long as it is made of a resin material that transmits light, and may be, for example, acrylic, polystyrene, polycarbonate, MS (methyl methacrylate-styrene copolymer) resin, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyimide, or the like. The base layer 21 may contain a diffusing agent or other additives, or may substantially not contain additives. The additive that can be contained in the base layer 21 is not particularly limited, and may be, for example, inorganic particles such as silica, titanium oxide, aluminum hydroxide, barium sulfate, or organic particles such as acrylic, acrylonitrile, silicone, polystyrene, polyamide, or the like.

[0037] The thickness of the diffusion sheet 43 is not particularly limited, and may be, for example, 3 mm or less (preferably 2 mm or less, more preferably 1.5 mm or less, and even more preferably 1 mm or less) and 0.1 mm or more. When the thickness of the diffusion sheet 43 is 3 mm or less, the liquid crystal display can be made thinner. When the thickness of the diffusion sheet 43 is 0.1 mm or more, the brightness uniformity is improved. The diffusion sheet 43 may be in the form of a film or a plate.

[0038] As shown in FIG. 4, a plurality of recesses 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 diffusion sheet 43. In other words, the plurality of recesses 22 are arranged along two directions perpendicular to each other. Adjacent recesses 22 are partitioned by ridge lines 111. The ridge lines 111 extend along the two directions in which the recesses 22 are arranged. The center 112 (the apex of the inverted pyramid) of the recess 22 is the deepest part of the recess 22. For simplicity, FIG. 4 illustrates an example in which the recesses 22 are arranged in a 5×5 matrix, but the actual number of recesses 22 arranged is much greater. In the two-dimensional arrangement of the plurality of recesses 22, each recess 22 may be provided on the first surface 21a without any gaps, or may be provided at a predetermined interval. In addition, some recesses 22 may be arranged randomly to the extent that the light diffusion effect is not impaired.

[0039] The apex angle θ of the recesses 22 may be, for example, 90°, the arrangement pitch p of the recesses 22 may be, for example, 100 μm, and the depth of the recesses 22 may be, for example, 50 μm. The apex angle θ of the recesses 22 refers to the angle formed by the cross-sectional lines of the inclined surfaces in a cross section that appears when a plane (longitudinal cross section) perpendicular to the arrangement surface of the diffusion sheet 43 is cut through the center (vertex 112 of the inverted pyramid) of the recesses 22 and perpendicularly crosses a pair of inclined surfaces facing each other across the center. The arrangement pitch p of the recesses 22 refers to the distance between the centers (vertices 112 of the inverted pyramids) of adjacent recesses 22 (the distance along the direction parallel to the arrangement surface of the diffusion sheet 43).

[0040] The second surface 21b of the diffusion sheet 43 may be, for example, a flat surface (mirror surface) or an embossed surface. The diffusion sheet 43 may be configured with a single-layer structure of a base material layer 21 having an uneven shape (recesses 22) on the first surface 21a. The diffusion sheet 43 may be configured with a two-layer structure of a base material layer that is flat on both sides and a layer having an uneven shape on one surface. The diffusion sheet 43 may be configured with a three or more layer structure including a layer having an uneven shape on one surface.

[0041] [Manufacturing method of diffusion sheet] The method for manufacturing the diffusion sheet 43 is not particularly limited, but may be, for example, an extrusion molding method, an injection molding method, or the like.

[0042] The procedure for manufacturing a single-layer diffusion sheet having a concave-convex shape on its surface using the extrusion molding method is as follows. First, pellet-shaped plastic particles to which a diffusion agent has been added (pellet-shaped plastic particles to which a diffusion agent has not been added may also be mixed) are put into a single-screw extruder, melted and kneaded while being heated. Thereafter, the molten resin extruded by a T-die is sandwiched between two metal rolls and cooled, then conveyed using a guide roll, and cut into a sheet flat plate by a sheet cutter to produce a diffusion sheet. Here, by sandwiching the molten resin using metal rolls having a surface with a shape that is the inverse of a desired concave-convex shape, the inverse shape of the roll surface is transferred to the resin, so that the desired concave-convex shape can be formed on the diffusion sheet surface. In addition, the shape transferred to the resin is not necessarily 100% the shape of the roll surface, so the shape of the roll surface may be designed by calculating backwards from the degree of transfer.

[0043] When using an extrusion molding method to manufacture a two-layer diffusion sheet having an uneven surface, for example, the pellet-shaped plastic particles required to form each layer are fed into each of two single-screw extruders, and then the same procedure as described above is carried out for each layer, and the resulting sheets are laminated.

[0044] Alternatively, a two-layered diffusion sheet having an uneven surface may be produced as follows. First, pellet-shaped plastic particles required for forming each layer are fed into each of two single-screw extruders, and melted and kneaded while being heated. Then, the molten resins for each layer are fed into one T-die and laminated in the T-die, and the laminated molten resin extruded by the T-die is sandwiched between two metal rolls and cooled. Then, the laminated molten resin is transported using a guide roll, and cut into individual flat plates by a sheet cutter, thereby producing a two-layered diffusion sheet having an uneven surface.

[0045] Alternatively, the diffusion sheet may be manufactured by shape transfer using UV (ultraviolet rays) as follows. First, a roll having an inverted shape of the concave-convex shape to be transferred is filled with uncured UV-curable resin, and a substrate is pressed against the resin. Next, while the roll filled with the UV-curable resin and the substrate are integrated, the resin is cured by irradiating with UV rays. Next, the sheet to which the concave-convex shape has been transferred by the resin is peeled off from the roll. Finally, the sheet is irradiated with UV rays again to completely cure the resin, and a diffusion sheet having a concave-convex shape on its surface is manufactured.

[0046] In this disclosure, the term "approximately inverted square pyramid" is used in consideration of the difficulty of forming a geometrically strict inverted square pyramid recess by a normal shape transfer technique, but the term "approximately inverted square pyramid" includes shapes that can be regarded as genuine or substantially inverted square pyramids. Furthermore, "approximately" means that it can be approximated, and "approximately inverted square pyramid" refers to a shape that can be approximated to an inverted square pyramid. For example, an "inverted square pyramid trapezoid" with a flat apex is also included in the "approximately inverted square pyramid" if the apex area is small enough that the effect of the present invention is not lost. Furthermore, shapes that are deformed from an "inverted square pyramid" within the range of unavoidable shape variations due to processing accuracy in industrial production are also included in the "approximately inverted square pyramid".

[0047] [Prism sheet] The prism sheets 44 and 45 are formed mainly from a transparent (for example, colorless and transparent) synthetic resin since they need to transmit light. The prism sheets 44 and 45 may be formed integrally. As shown in FIG. 2, the lower prism sheet 44 has a base layer 44a and a protrusion row consisting of a plurality of protruding prism portions 44b laminated on the surface of the base layer 44a. Similarly, the upper prism sheet 45 has a base layer 45a and a protrusion row consisting of a plurality of protruding prism portions 45b laminated on the surface of the base layer 45a. The protruding prism portions 44b and 45b are laminated in a stripe shape on the surfaces of the base layers 44a and 45a, respectively. The protruding prism portions 44b and 45b are triangular prisms whose back surfaces are in contact with the surfaces of the base layers 44a and 45a, respectively. The extending direction of the protruding prism portions 44b and the extending direction of the protruding prism portions 45b are mutually perpendicular. This allows the light rays incident from the diffusion sheet 43 to be refracted toward the normal direction by the lower prism sheet 44, and the light rays exiting the lower prism sheet 44 to be refracted by the upper prism sheet 45 so as to travel approximately perpendicular to the display screen 50a.

[0048] The lower limit of the thickness of the prism sheets 44 and 45 (the height from the back surface of the base layer 44a and 45a to the apex of the protruding prism portion 44b and 45b) may be, for example, about 50 μm, more preferably about 100 μm. The upper limit of the thickness of the prism sheets 44 and 45 may be, for example, about 200 μm, more preferably about 180 μm. The lower limit of the pitch of the protruding prism portion 44b and 45b in the prism sheets 44 and 45 may be, for example, about 20 μm, more preferably about 25 μm. The upper limit of the pitch of the protruding prism portion 44b and 45b in the prism sheets 44 and 45 may be, for example, about 100 μm, more preferably about 60 μm. The apex angle of the protruding prism portion 44b and 45b may be, for example, 85° or more and 95° or less. The lower limit of the refractive index of the protruding prism portion 44b and 45b may be, for example, 1.5, more preferably 1.55. The upper limit of the refractive index of the protruding prism portions 44b and 45b may be, for example, 1.7.

[0049] The prism sheets 44 and 45 may be formed by providing base material layers 44a and 45a made of, for example, a PET (polyethylene terephthalate) film with protruding prism portions 44b and 45b whose shape is transferred using a UV-curable acrylic resin, or by integrally molding the protruding prism portions 44b and 45b with the base material layers 44a and 45a.

[0050] [Other optical sheets] Although not shown, a polarizing sheet may be provided above (on the display screen 50a side of) the prism sheets 44 and 45. The polarizing sheet prevents the light emitted from the backlight unit 40 from being absorbed by the first polarizing plate 6 of the liquid crystal display device 50, thereby improving the brightness of the display screen 50a.

[0051] <Features of the embodiment> The optical sheet laminate 100 of this embodiment is incorporated into a backlight unit 40. The optical sheet laminate 100 includes a plurality of diffusion sheets 43 having a plurality of recesses 22 each having a substantially inverted quadrangular pyramid shape arranged in a two-dimensional matrix on a first surface 21a, and a pair of prism sheets 44 and 45 in which the extension directions of protruding prism portions 44b and 45b (hereinafter also referred to as prism extension directions) are perpendicular to each other.

[0052] When multiple pyramid sheets, ie, diffusion sheets 43, are stacked together, it is preferable to set the arrangement direction of the recesses 22 in each diffusion sheet 43 to the same direction in order to reduce the loss caused by cutting from the sheet roll when forming the diffusion sheets 43.

[0053] In previous studies, the inventors of the present application have found that by setting the angle difference between the arrangement direction of recesses 22 in diffusion sheet 43 and the prism extension direction in prism sheets 44 and 45 to be 20° or more and 70° or less, preferably 25° or more and 35° or less, or 55° or more and 65° or less, the brightness is increased compared to when the angle difference is close to 0° or close to 90°. Since the extension direction of protruding prism portion 44b and the extension direction of protruding prism portion 45b are perpendicular to each other, when the extension direction of one of protruding prism portion 44b or 45b satisfies the above-mentioned angle difference, the extension direction of the other protruding prism portion 44b or 45b also satisfies the above-mentioned angle difference.

[0054] However, the inventors of the present application have discovered that when multiple diffusion sheets 43, each with the same arrangement direction of the recesses 22, are stacked together and the angle difference between the arrangement direction of the recesses 22 and the prism extension direction is set within the aforementioned range, while the brightness increases, the brightness uniformity may decrease.

[0055] Therefore, the inventors of the present application evaluated the brightness uniformity while changing the arrangement direction of the recesses 22 in each of the multiple diffusion sheets 43, and found that it may be possible to suppress the decrease in brightness uniformity by using multiple diffusion sheets 43 in which the arrangement directions of the recesses 22 are different from each other.

[0056] That is, the optical sheet laminate 100 of this embodiment is based on the above findings, and the multiple diffusion sheets 43 include at least two diffusion sheets 43 in which the arrangement directions of the recesses 22 are different from each other.

[0057] According to the optical sheet laminate 100 of the present embodiment described above, since a plurality of diffusion sheets 43 having a plurality of substantially inverted pyramidal recesses 22 arranged in a two-dimensional matrix on the first surface 21a are stacked, the light traveling in a straight line from the light source 42 is repeatedly diffused, thereby improving the luminance uniformity. Moreover, by using at least two diffusion sheets 43 having recesses 22 arranged in different directions, the luminance uniformity can be further improved.

[0058] In the optical sheet laminate 100 of this embodiment, in order to further improve the luminance uniformity, the plurality of diffusion sheets 43 preferably include a first diffusion sheet and a second diffusion sheet, and the arrangement direction of the recesses 22 in the second diffusion sheet is different from the arrangement direction of the recesses 22 in the first diffusion sheet by at least 10° or more. In this case, the plurality of diffusion sheets 43 further preferably include a third diffusion sheet, and the arrangement direction of the recesses 22 in the third diffusion sheet is different from each of the arrangement directions of the recesses 22 in the first diffusion sheet and the second diffusion sheet by at least 10° or more. Specifically, the arrangement directions of the recesses 22 in the plurality of diffusion sheets 43 may differ from each other by about 10° to 45°. Since the recesses 22 are arranged in a two-dimensional matrix, the angle difference between the arrangement directions of the recesses 22 between the two diffusion sheets 43 is a maximum of 45°.

[0059] FIG. 5 shows an example of the relationship of the arrangement directions of the recesses 22 of a plurality (three sheets) of diffusion sheets 43 in the optical sheet laminate 100 of this embodiment. In FIG. 5, the arrangement of the recesses 22 in two rows and two columns of each diffusion sheet 43 is shown center-to-center. As shown in FIG. 5, the recesses 22 in the first diffusion sheet 43 (the diffusion sheet 43 closest to the light source 42) are arranged along the X-axis and Y-axis that are perpendicular to each other, and the arrangement direction along the X-axis is defined as the arrangement direction D1. In this case, the arrangement direction D2 of the recesses 22 in the second diffusion sheet 43 is different from the arrangement direction D1, and the arrangement direction D3 of the recesses 22 in the third diffusion sheet 43 is different from the arrangement directions D1 and D2.

[0060] Although Figure 5 shows a case where the arrangement directions of the recesses 22 in the three diffusion sheets 43 are all different, alternatively, the arrangement directions of the recesses 22 in two of the three diffusion sheets 43 may be the same.

[0061] In the optical sheet laminate 100 of this embodiment, the arrangement direction of the recesses 22 in at least one of the multiple diffusion sheets 43 may differ from the prism extension direction of the prism sheets 44 and 45 by 20° to 70°. In this way, the brightness is increased compared to the case where the angle difference between the arrangement direction of the recesses (inverted pyramids) 22 and the prism extension direction is close to 0° or close to 90°, when compared with the same light source, the same power, and the same optical sheet laminate configuration. Therefore, a high-brightness screen can be realized even with low power in the backlight unit 40. In this case, if the intersection angle between the arrangement direction of the recesses 22 and the prism extension direction is 25° to 35°, or 55° to 65°, the brightness can be further increased.

[0062] The backlight unit 40 of this embodiment is incorporated in a liquid crystal display device 50, and guides light emitted from a light source 42 to a display screen 50a. The backlight unit 40 includes the optical sheet laminate 100 of this embodiment between the display screen 50a and the light source 42, and a plurality of diffusion sheets 43 are disposed between the light source 42 and the prism sheets 44 and 45.

[0063] The backlight unit 40 of the present embodiment includes the optical sheet laminate 100 of the present embodiment, and therefore can suppress deterioration in luminance uniformity.

[0064] In the backlight unit 40 of the present embodiment, the light source 42 may be disposed on the reflecting sheet 41 provided on the opposite side of the display screen 50a from the multiple diffusion sheets 43. In this way, the light is further diffused by multiple reflections between the diffusion sheet 43 and the reflecting sheet 41, improving the luminance uniformity.

[0065] In the backlight unit 40 of the present embodiment, if the distance between the light source 42 and the multiple diffusion sheets 43 (more precisely, the distance between the light source 42 and the diffusion sheet 43 closest to the light source 42) is 5 mm or less, the backlight unit 40 can be made compact. In addition, in anticipation of future thinning of small and medium-sized liquid crystal displays, the distance between the light source 42 and the multiple diffusion sheets 43 may be more preferably 2.5 mm or less, further preferably 1 mm or less, and ultimately 0 mm.

[0066] The liquid crystal display device 50 of this embodiment includes the backlight unit 40 of this embodiment and a liquid crystal display panel 5. Therefore, the optical sheet laminate 100 incorporated in the backlight unit 40 can suppress deterioration in luminance uniformity. The same effect can be obtained in an information device (for example, a portable information terminal such as a notebook computer or a tablet) incorporating the liquid crystal display device 50.

[0067] The method for manufacturing the backlight unit 40 of this embodiment is a method for manufacturing the backlight unit 40 that is incorporated in the liquid crystal display device 50 and guides light emitted from the light source 42 to the display screen 50a side. The method for manufacturing the backlight unit 40 of this embodiment includes a step of arranging a plurality of diffusion sheets 43 having a plurality of recesses 22 of a substantially inverted quadrangular pyramid shape arranged in a two-dimensional matrix on the first surface 21a on the display screen 50a side as viewed from the light source 42, and a step of arranging a pair of prism sheets 44 and 45 whose prism extension directions are perpendicular to each other on the display screen 50a side as viewed from the plurality of diffusion sheets 43. In the step of arranging the plurality of diffusion sheets 43, the luminance uniformity is evaluated while changing the arrangement direction of the recesses 22 in each diffusion sheet 43, and the arrangement direction of the recesses 22 in each diffusion sheet 43 is determined based on the evaluation result.

[0068] According to the manufacturing method of the backlight unit 40 of the present embodiment, in the step of arranging a plurality of diffusion sheets 43, the arrangement direction of the recesses 22 in each diffusion sheet 43 is determined based on the result of evaluating the luminance uniformity while changing the arrangement direction of the recesses 22 in each diffusion sheet 43. Therefore, it is possible to set the arrangement direction of the recesses 22 in each diffusion sheet 43 so as to improve the luminance uniformity.

[0069] (Example) Examples will now be described.

[0070] The optical sheet laminate 100 used in the embodiment was made by stacking three diffusion sheets 43 of the same structure and thickness of 110 μm in the same orientation, on top of which a lower prism sheet 44 and an upper prism sheet 45 whose prism extension directions are perpendicular to each other were placed.

[0071] The diffusion sheet 43 was formed by extrusion molding polycarbonate, which would become the base layer 21, and by two-dimensionally arranging inverted pyramid-shaped recesses 22, each having an apex angle of 90° and a depth of 50 μm, at a pitch of 100 μm, into a single layer structure. The diffusion sheet 43 was disposed so that the surface on which the recesses 22 were formed (first surface 21a) served as the light exit surface, and the surface (second surface 21b) opposite to the surface on which the recesses 22 were formed (first surface 21a) was processed into a matte surface.

[0072] Prism sheets 44 and 45 are formed by providing protruding prism portions 44b and 45b using a UV-curable acrylic resin made of acrylate on base material layers 44a and 45a made of PET film. Lower prism sheet 44 has a total thickness of 145 μm, and protruding prism portions 44b with a height of 12 μm and a apex angle of 94° are arranged at a pitch of 25 μm. Upper prism sheet 45 has a total thickness of 128 μm, and protruding prism portions 45b with a height of 24 μm and a apex angle of 93° are arranged at a pitch of 51 μm.

[0073] A light source 42 was placed on the lower side (the side of the diffusion sheet 43) of the optical sheet laminate 100 of the embodiment, and the brightness uniformity of the light passing through the optical sheet laminate 100 was examined while changing the arrangement direction of the recesses 22 in each diffusion sheet 43.

[0074] The light source 42 used was a white LED array light source in which multiple blue LEDs with a peak wavelength of 456 nm (full width at half maximum 19 nm), green LEDs with a peak wavelength of 535 nm (full width at half maximum 53 nm), and red LEDs with a peak wavelength of 631 nm (full width at half maximum 10 nm) were arranged alternately in a two-dimensional pattern with a pitch of 8.4 mm.

[0075] As shown in FIG. 6, in the initial state for evaluating brightness uniformity, diffusion sheet 43 was positioned so that one of the arrangement directions of recesses 22 was aligned with the reference direction (X-axis direction) (at an arrangement angle of 0°), lower prism sheet 44 was positioned so that the extension direction of protruding prism portion 44b was rotated 100° counterclockwise about the X-axis (at an arrangement angle of 100°), and upper prism sheet 45 was positioned so that the extension direction of protruding prism portion 45b was rotated 10° counterclockwise about the X-axis (at an arrangement angle of 10°).

[0076] Next, the arrangement angle (the angle that the arrangement direction of the recesses 22 makes in the counterclockwise direction with respect to the X-axis) of the three diffusion sheets (inverted pyramid sheets) 43 was changed from the initial state in various ways, and the luminance uniformity was evaluated.

[0077] As comparative examples, three different types of diffusion sheets 43 were prepared, and two of each type of diffusion sheet 43 were stacked together with the arrangement direction of the recesses 22 aligned, and a similar luminance uniformity evaluation was performed. For one of the diffusion sheets 43, the arrangement directions of the recesses 22 in the two diffusion sheets 43 were made to differ by 45° as another example.

[0078] Specifically, the luminance uniformity evaluation was performed in the following procedure. First, the optical sheet laminate 100 of the embodiment or comparative example was placed on the light source 42 (LED array), and a transparent glass plate was placed on top of it to prevent the sheets from floating. The luminance in the vertical upward direction (the direction from the LED array to the glass plate) was measured in a 33 mm square area using a two-dimensional color luminance meter UA-200 manufactured by Topcon Technohouse. Next, the obtained two-dimensional luminance distribution image was corrected for the variation in the emission intensity of each LED, and a filtering process was performed to suppress bright and dark spot noise caused by foreign matter, etc., and then the average value and standard deviation were calculated for the luminance of all pixels. Finally, the luminance uniformity was evaluated by defining it as "luminance uniformity = average value / standard deviation".

[0079] The evaluation results of the brightness uniformity described above are shown in Fig. 7. Fig. 7 shows the relationship between the arrangement direction of the recesses 22 of each diffusion sheet 43 in the optical sheet laminate 100 according to the example and the brightness uniformity. In Fig. 7, the angle indicated on the horizontal axis indicates "arrangement angle of the diffusion sheet on the light source side" / "arrangement angle of the diffusion sheet on the prism sheet side" for the two-sheet diffusion sheet 43, and indicates "arrangement angle of the diffusion sheet on the light source side" / "arrangement angle of the intermediate diffusion sheet" / "arrangement angle of the diffusion sheet on the prism sheet side" for the three-sheet diffusion sheet 43.

[0080] As shown in FIG. 7, for the three-sheet diffusion sheet 43, for example, in the cases of 0° / 60° / 30°, 30° / 60° / 0°, and 60° / 30° / 0°, the brightness uniformity was comparable to that of the case of 0° / 0° / 0°. In other cases, the brightness uniformity did not decrease by more than 10% compared to the case of 0° / 0° / 0°. On the other hand, in the embodiment, by including the diffusion sheet 43 with a large arrangement angle, the brightness increase effect due to the angle difference with the prism extension direction described above can be obtained, which is superior to the case of 0° / 0° / 0°. For the two-sheet diffusion sheet 43 (two-stage configuration C in FIG. 7), for example, in the cases of 45° / 0° and 0° / 45°, the brightness uniformity was comparable to that of the case of 0° / 0°, and considering the brightness increase effect, it is superior to the case of 0° / 0°.

[0081] (Other embodiments) In the above-described embodiment (including the examples, the same applies below), the optical sheet laminate 100 is configured from a plurality of diffusion sheets 43 and the prism sheets 44 and 45. However, the optical sheet laminate 100 may further include other optical sheets.

[0082] In the above embodiment, the inverted polygonal pyramid shape of the recesses 22 provided on the first surface 21a of the diffusion sheet 43 included in the optical sheet laminate 100 is an inverted quadrangular pyramid, but instead of this, other shapes that can be arranged two-dimensionally, such as an inverted triangular pyramid or an inverted hexagonal pyramid, may be used. Alternatively, instead of the recesses 22 that can be arranged two-dimensionally, a protrusion row such as a protruding prism portion may be provided. In addition, the second surface 21b of the diffusion sheet 43 is a flat surface (mirror surface) or an embossed surface, but the second surface 21b of the diffusion sheet 43 may be provided with a recess in the shape of an inverted polygonal pyramid that can be arranged two-dimensionally or a protrusion row such as a protruding prism portion.

[0083] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the disclosure. In other words, the description of the above-described embodiments is merely illustrative in nature and is not intended to limit the present disclosure, its applications, or its uses. [Explanation of symbols]

[0084] 1 TFT substrate 2 CF board 3 Liquid crystal layer 5 Liquid crystal display panel 6 First polarizing plate 7 Second polarizing plate 21 Base material layer 21a 1st page 21b 2nd side 22 Recess 40 Backlight unit 41 Reflective Sheet 42 Light source 43 Diffusion Sheet 44 Lower prism sheet 44a Base material layer 44b Protruding prism part 45 Upper prism sheet 45a Base material layer 45b Protruding prism part 50 LCD display device 50a display screen 100 Optical sheet laminate 111 Ridgeline 112 Center of concave portion (vertex of inverted pyramid)

Claims

1. An optical sheet laminate to be incorporated in a backlight unit having a pair of prism sheets whose prism extension directions are perpendicular to each other, a plurality of diffusion sheets on at least one surface of which a plurality of recesses each having an inverted polygonal pyramid shape or a shape that can approximate an inverted polygonal pyramid are arranged in a two-dimensional matrix; The plurality of diffusion sheets include at least two diffusion sheets in which the arrangement directions of the plurality of recesses are different from each other. Optical sheet laminate.

2. The shape of the plurality of recesses is an inverted square pyramid or a shape that can approximate an inverted square pyramid. The optical sheet laminate according to claim 1 .

3. The shape of the plurality of recesses is an inverted triangular pyramid or a shape that can approximate an inverted triangular pyramid. The optical sheet laminate according to claim 1 .

4. The shape of the plurality of recesses is an inverted hexagonal pyramid or a shape that can approximate an inverted hexagonal pyramid. The optical sheet laminate according to claim 1 .

5. the plurality of diffusion sheets include a first diffusion sheet and a second diffusion sheet, The arrangement direction of the plurality of recesses in the second diffusion sheet differs from the arrangement direction of the plurality of recesses in the first diffusion sheet by 10° or more. The optical sheet laminate according to any one of claims 1 to 4.

6. The plurality of diffusion sheets further includes a third diffusion sheet, The arrangement direction of the plurality of recesses in the third diffusion sheet differs from the arrangement direction of the plurality of recesses in the first diffusion sheet and the arrangement direction of the plurality of recesses in the second diffusion sheet by 10° or more. The optical sheet laminate according to claim 5 .

7. The arrangement direction of the plurality of recesses in at least one of the plurality of diffusion sheets differs from the extending direction of the prisms by 20° to 70°. The optical sheet laminate according to any one of claims 1 to 6.

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