Laminated light diffusion sheet, backlight unit, liquid crystal display device, and information appliance

Abstract

A laminated light diffusion sheet 100 includes: a first light diffusion sheet 101, on a first side 101a of which are arranged a plurality of recesses 22 formed substantially in the shape of an inverted polygonal pyramid or an inverted polygonal frustum; and a second light diffusion sheet 102 bonded to the first side 101a of the first light diffusion sheet 101. The bonding points between the first light diffusion sheet 101 and the second light diffusion sheet 102 have a peel strength of 5.0 gf / 25 mm or more. Provided that V0 is the volume of the recesses 22 and Va is the volume of the vacant space remaining inside the recesses 22, Va / V0 is between 40% and 100% inclusive in the area excluding the periphery of the first side 101a of the first light diffusion sheet 101.

Description

Laminated light diffusion sheet, backlight unit, liquid crystal display device, and information device

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

[0002] Liquid crystal display devices are widely used as display devices for various information devices such as laptop PCs, televisions, monitors, smartphones, tablet terminals, etc. Two types of backlights have been adopted for liquid crystal display devices: a direct type, in which a light source is placed behind the liquid crystal panel, and a light guide plate type.

[0003] As LCD devices are actively being made thinner, many LCD devices that use a light guide plate have been proposed as devices suitable for thinning. In these cases, various light diffusion sheets have been used to improve the uniformity of in-plane brightness and to prevent the image of the light source arrangement on the light guide surface from remaining due to the moire (interference fringe) phenomenon.

[0004] In recent years, the direct type has become mainstream as a backlight for liquid crystal display devices due to the convenience of being able to individually control the ON / OFF of each LED light source within the same screen.

[0005] When a direct-type backlight is used, a light diffusion sheet is used to eliminate the image of a light source such as an LED (Light Emitting Diode) on the light-emitting surface and increase the uniformity of in-plane brightness. Light diffusion sheets are typically used in a stack of multiple sheets, and it is known that the greater the number of stacked sheets, the more uniform the brightness on the display screen. On the other hand, as the number of optical sheets, including the light diffusion sheet, increases, the handling of the optical sheets becomes more complicated when assembling the backlight unit. In other words, from the perspective of optical performance, the desired performance can be achieved by optimizing the stacking configuration of the optical sheets used in the backlight unit. However, from the perspective of manufacturing, problems may arise in the individual handling and assembly of the multiple optical sheets.

[0006] Problems associated with increasing the number of optical sheets include, for example, an increased risk of damaging the optical sheets when removing the protective films from the optical sheets, and an increased time required for the process of removing the protective films from each optical sheet. Furthermore, the process of inserting each optical sheet into the display frame of the liquid crystal display device to form a laminate of optical sheets also increases the time required, and the possibility of damaging the optical sheets during this process also increases. These problems lead to a decrease in the non-defective product rate, i.e., a decrease in yield, and increase the manufacturing cost of the backlight unit.

[0007] Patent Document 1 discloses a packaging method in which optical films are bundled before being inserted into a display frame, which facilitates handling of the films, reduces the number of steps required to assemble a display device, reduces the possibility of damaging the films, and increases yields.

[0008] Patent No. 5346066

[0009] However, Patent Document 1 discloses that by laminating a brightness-enhancing film, handling is improved, and by managing the thickness of the adhesive layer, peel strength can be controlled while suppressing a decrease in brightness, or brightness can be controlled while suppressing a decrease in peel strength, but it does not disclose any measures to suppress a decrease in performance when laminating a diffusion film.

[0010] Although improving the brightness and brightness uniformity of light diffusion sheets is always a challenge, increasing the number of light diffusion sheets used in a backlight unit improves brightness uniformity, but brightness generally tends to decrease. Furthermore, as mentioned above, increasing the number of light diffusion sheets handled tends to decrease yield.

[0011] An object of the present disclosure is to improve the ease of handling of light diffusion sheets when assembling a backlight unit using a plurality of light diffusion sheets, while suppressing a decrease in brightness and brightness uniformity.

[0012] In order to achieve the above object, the inventors of the present application conducted extensive research and found that when a light diffusion sheet having an array of recesses in the shape of an approximately inverted polygonal pyramid is bonded to another sheet, the adhesive material or the substrate of another light diffusion sheet gets into the recesses, resulting in a decrease in brightness and brightness uniformity. Furthermore, the inventors of the present application found that the decrease in brightness and brightness uniformity can be suppressed to a practical level by ensuring that the proportion of voids remaining in the recesses after bonding is 40% or more of the volume of the recesses.

[0013] Specifically, the laminated light diffusion sheet according to the present disclosure is a laminated light diffusion sheet including a first light diffusion sheet having a first surface on which a plurality of recesses formed in the shape of an approximately inverted polygonal pyramid or an approximately inverted polygonal truncated pyramid are arranged, and a second light diffusion sheet bonded to the first surface of the first light diffusion sheet, wherein the peel strength at the bonding point between the first light diffusion sheet and the second light diffusion sheet is 5.0 g force / 25 mm or more, and where the volume of the recesses is V0 and the volume of the voids remaining in the recesses is Va, Va / V0 is 40% or more and 100% or less in the region excluding the peripheral portion of the first surface of the first light diffusion sheet.

[0014] In the laminated light diffusion sheet according to the present disclosure, the first light diffusion sheet and the second light diffusion sheet are bonded together with a peel strength of 5.0 g weight / 25 mm or more, which improves the handleability of the light diffusion sheet when assembling a backlight unit. Furthermore, on the first surface (the active area directly below the display screen excluding the peripheral portion) of the first light diffusion sheet on which recesses formed in the shape of a substantially inverted polygonal pyramid or a substantially inverted polygonal truncated pyramid are arranged, the ratio (volume Va of voids remaining in the recesses) / (volume V0 of the recesses) is 40% or more, which suppresses deterioration in brightness and brightness uniformity due to bonding.

[0015] In the present disclosure, the term "light diffusion sheet" encompasses a plate-shaped "light diffusion plate" and a film-shaped "light diffusion film."

[0016] In addition, in the present disclosure, an "optical sheet" refers to a sheet having various optical functions such as diffusion, light collection, refraction, and reflection, and a "light diffusion sheet" is one type of "optical sheet".

[0017] Furthermore, in this disclosure, the terms "approximately inverted polygonal pyramid" or "approximately inverted polygonal pyramid truncated shape" are used in consideration of the difficulty of forming a recess having a geometrically strict inverted polygonal pyramid or inverted polygonal pyramid truncated shape using conventional shape transfer techniques, but it goes without saying that these terms also include shapes that can be considered to be true or substantially inverted polygonal pyramids or inverted polygonal pyramid truncated shapes.

[0018] In the laminated light-diffusing sheet according to the present disclosure, when the peel strength measured at 85° C. is 10 g weight / 25 mm or more, a laminated light-diffusing sheet having sufficient heat resistance can be obtained.

[0019] In the laminated light-diffusing sheet according to the present disclosure, when the peel strength measured at 85° C. is 100 g / 25 mm or more, a laminated light-diffusing sheet with excellent heat resistance can be obtained.

[0020] In the laminated light diffusion sheet according to the present disclosure, the bonding location may be the entire surface, the peripheral edge, or a plurality of dotted or linear regions on the first surface of the first light diffusion sheet. That is, in the laminated light diffusion sheet according to the present disclosure, the bonding location is not particularly limited as long as the first light diffusion sheet and the second light diffusion sheet are bonded together such that the peel strength between them is 5.0 g weight / 25 mm or more and the ratio of (volume of voids remaining in the recesses Va) / (volume of the recesses V0) is 40% or more for the recesses arranged on the first surface of the first light diffusion sheet.

[0021] In the laminated light diffusion sheet according to the present disclosure, the recesses may be formed in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid, the thickness of the first light diffusion sheet being 30 μm or more and 1000 μm or less, and the portion of the first light diffusion sheet where the recesses are provided and other portions may be integrally molded from the same material. Forming the recesses in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid improves the cutting accuracy of the mold (metal roll) used in the manufacturing process. A thickness of the first light diffusion sheet of 30 μm or more facilitates the improvement of brightness uniformity, while a thickness of the first light diffusion sheet below 30 μm makes it easier for the recesses to penetrate the sheet, making stable processing difficult. A thickness of the first light diffusion sheet of 1000 μm or less allows the backlight unit, i.e., the liquid crystal display device, to be made thinner. However, generally, optical sheets with a thickness exceeding 500 μm have sufficient rigidity even when used alone, making them easier to handle during assembly of the backlight unit without having to be bonded to other optical sheets. When the portion of the first light diffusion sheet where the recesses are provided (recessed portion) and the other portion (base portion) are integrally molded from the same material, it is possible to prevent the recessed portion from peeling off from the base portion during reliability tests, etc.

[0022] For example, in a light diffusion sheet in which a UV-curable resin is shaped and laminated onto a substrate film to form a concave portion, the concave portion may peel off from the substrate film during reliability tests, etc. On the other hand, a light diffusion sheet in which the substrate portion and the concave portion are integrally molded, such as an extrusion molded product or a compression molded product, can prevent the concave portion from peeling off from the substrate portion during reliability tests, etc. Furthermore, in general, in an optical sheet bonded to another optical sheet, stress tends to concentrate at the bonding points due to the adhesive effect associated with the bonding. However, if the substrate portion and the concave portion are not integrally molded, there is also the problem that the substrate portion and the concave portion are more likely to peel off due to the difference in the linear expansion coefficients of each portion.

[0023] In the laminated light diffusing sheet according to the present disclosure, when the recesses are formed in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid, a plurality of other recesses each having an approximately inverted square pyramid or an approximately inverted square truncated pyramid shape may be arranged on one surface of the second light diffusing sheet, the other surface of the second light diffusing sheet may be flat or matte, and the first surface of the first light diffusing sheet and the other surface of the second light diffusing sheet may be bonded together. In this way, a laminated light diffusing sheet that can exhibit an excellent effect of improving luminance uniformity can be obtained relatively easily.

[0024] In the laminated light diffusion sheet according to the present disclosure, one surface of the second light diffusion sheet may have a plurality of other recesses formed in the shape of a substantially inverted polygonal pyramid or a substantially inverted polygonal truncated pyramid, and the arrangement direction of the recesses may be different from the arrangement direction of the other recesses. In this manner, even if there is variation in Va / V0 (hereinafter also referred to as the "spatial volume ratio") among the recesses, the occurrence of mottled patterns on the display screen and a decrease in brightness and brightness uniformity can be suppressed. In this case, the recesses and the other recesses may be formed in the shape of a substantially inverted square pyramid or a substantially inverted square truncated pyramid, a substantially inverted triangular pyramid or a substantially inverted triangular truncated pyramid, or a substantially inverted hexagonal pyramid or a substantially inverted hexagonal truncated pyramid. This facilitates the regular two-dimensional arrangement of the recesses and the other recesses. Furthermore, if the arrangement direction of the recesses differs from the arrangement direction of the other recesses by 8° or more, the occurrence of mottled patterns on the display screen can be further suppressed. The upper limit of the angle difference between the arrangement direction of the recesses and the arrangement direction of the other recesses varies depending on the shape of the inverted polygonal pyramid or inverted polygonal truncated pyramid. When the shape of the recesses and the other recesses is an inverted square pyramid or an inverted square truncated pyramid, the angle difference is preferably 82° or less, and more preferably 75° or less. In other words, when the shape of the recesses and the other recesses is an inverted square pyramid or an inverted square truncated pyramid, the arrangement relationship between the recesses and the other recesses is the same whether the angle difference is 8° or 82°, and therefore a similar effect of suppressing mottling can be obtained. Furthermore, when the shape of the recesses and the other recesses is an inverted triangular pyramid or an inverted triangular truncated pyramid, the angle difference is preferably 52° or less, and more preferably 45° or less. In other words, when the shape of the recess and the other recess is an N-sided pyramid or an N-sided truncated pyramid (N is an integer of 3 or more), the angle difference is preferably [(180° × (N-2) / N) - 8°] or less, and more preferably [(180° × (N-2) / N) - 15°] or less.

[0025] In the laminated light-diffusing sheet according to the present disclosure, when the Va / V0 is 50% or more, the deterioration in luminance and luminance uniformity caused by lamination can be further suppressed.

[0026] In the laminated light diffusion sheet according to the present disclosure, the first light diffusion sheet and the second light diffusion sheet may be bonded together using an optically clear adhesive (OCA), which simplifies the bonding process.

[0027] In the laminated light diffusion sheet according to the present disclosure, the first light diffusion sheet and the second light diffusion sheet may be bonded together using an ink containing an ultraviolet-curable resin. In this way, the thickness of the adhesive layer bonding the first light diffusion sheet and the second light diffusion sheet together can be changed depending on the type of ink. However, compared to bonding using an OCA, this method requires an additional step of curing the ink by irradiating it with ultraviolet (UV) light or the like.

[0028] In the laminated light diffusion sheet according to the present disclosure, the first light diffusion sheet and the second light diffusion sheet may be bonded together by laser welding using an infrared absorbing agent. In this way, sufficient peel strength can be obtained even when only the four corners of the sheets are bonded together. However, compared to bonding by OCA, this requires an additional step of welding by irradiating an infrared laser.

[0029] Another laminated light diffusion sheet according to the present disclosure is a laminated light diffusion sheet including a first light diffusion sheet having a first surface on which a plurality of recesses formed in the shape of approximately inverted polygonal pyramids or approximately inverted polygonal truncated pyramids are arranged, and a second light diffusion sheet bonded to the first surface of the first light diffusion sheet, wherein the peel strength at the bonding point between the first light diffusion sheet and the second light diffusion sheet is 5.0 g weight / 25 mm or more, and one surface of the second light diffusion sheet has a plurality of other recesses formed in the shape of approximately inverted polygonal pyramids or approximately inverted polygonal truncated pyramids arranged in a direction different from the direction of arrangement of the other recesses.

[0030] According to another laminated light diffusion sheet of the present disclosure, the first light diffusion sheet and the second light diffusion sheet are bonded together with a peel strength of 5.0 g / 25 mm or more, thereby improving the handleability of the light diffusion sheet during assembly of a backlight unit. Furthermore, because the arrangement direction of the recesses on the first light diffusion sheet differs from the arrangement direction of the other recesses on the second light diffusion sheet, even if the spatial volume ratio Va / V0 varies among the recesses on the first light diffusion sheet, the occurrence of mottled patterns on the display screen and reduced brightness and brightness uniformity can be suppressed. In this case, the recesses and the other recesses may be formed in the shape of an approximately inverted square pyramid or approximately inverted square truncated pyramid, an approximately inverted triangular pyramid or approximately inverted triangular truncated pyramid, or an approximately inverted hexagonal pyramid or approximately inverted hexagonal truncated pyramid. This facilitates the regular two-dimensional arrangement of the recesses and the other recesses. Furthermore, if the arrangement direction of the recesses differs from the arrangement direction of the other recesses by 8° or more, the occurrence of mottled patterns on the display screen can be further suppressed. The upper limit of the angle difference between the arrangement direction of the recesses and the arrangement direction of the other recesses varies depending on the shape of the inverted polygonal pyramid or inverted polygonal truncated pyramid. When the shape of the recesses and the other recesses is an inverted square pyramid or an inverted square truncated pyramid, the angle difference is preferably 82° or less, and more preferably 75° or less. In other words, when the shape of the recesses and the other recesses is an inverted square pyramid or an inverted square truncated pyramid, the arrangement relationship between the recesses and the other recesses is the same whether the angle difference is 8° or 82°, and therefore a similar effect of suppressing mottling can be obtained. Furthermore, when the shape of the recesses and the other recesses is an inverted triangular pyramid or an inverted triangular truncated pyramid, the angle difference is preferably 52° or less, and more preferably 45° or less. In other words, when the shape of the recess and the other recess is an N-sided pyramid or an N-sided truncated pyramid (N is an integer of 3 or more), the angle difference is preferably [(180° × (N-2) / N) - 8°] or less, and more preferably [(180° × (N-2) / N) - 15°] or less.

[0031] The backlight unit according to the present disclosure is incorporated into a liquid crystal display device and directs light emitted from a light source toward a display screen, and includes the laminated light diffusion sheet according to the present disclosure or another laminated light diffusion sheet between the display screen and the light source. This makes it possible to improve the handleability of the light diffusion sheets when assembling a backlight unit using multiple light diffusion sheets, while suppressing deterioration in brightness and brightness uniformity.

[0032] The backlight unit according to the present disclosure may further include a color conversion sheet between the display screen and the light source, which converts the wavelength of light emitted from the light source. This eliminates the need for an expensive white light source, thereby reducing the cost of the backlight unit. The location of the color conversion sheet is not particularly limited as long as it is between the display screen and the light source. For example, the color conversion sheet may be located between the light source and the laminated light diffusion sheet or another laminated light diffusion sheet, or between the display screen and the laminated light diffusion sheet or another laminated light diffusion sheet.

[0033] In the backlight unit according to the present disclosure, the content of the light diffusing agent in the first light diffusing sheet is not particularly limited as long as the light diffusing effect due to reflection and refraction in the recesses formed in the shape of an approximately inverted polygonal pyramid or an approximately inverted polygonal truncated pyramid is not impaired. For example, the first light diffusing sheet may contain 0 to 10 parts by mass of the light diffusing agent per 100 parts by mass of the matrix resin.

[0034] The liquid crystal display device according to the present disclosure includes the backlight unit according to the present disclosure and a liquid crystal display panel. Therefore, when assembling a backlight unit using multiple light diffusion sheets, it is possible to improve the ease of handling of the light diffusion sheets while suppressing a decrease in brightness and brightness uniformity. The same effects can be obtained in information devices equipped with the liquid crystal display device according to the present disclosure.

[0035] In addition, it is preferable that the concave-shaped portion and the base material portion of the first light diffusion sheet are molded from the same material so that interfacial peeling does not occur between the concave-shaped portion and the base material portion during reliability tests, etc., and it is particularly preferable that the concave-shaped portion and the base material portion are molded integrally by a method such as extrusion molding or compression molding.

[0036] In the first light diffusion sheet, the arrangement pitch of the recesses is preferably 30 μm or more and 1000 μm or less, and the width of the boundary between adjacent recesses is preferably 25% or less of the arrangement pitch. Specifically, in order to reduce the area ratio of the boundary, the width of the boundary is set to 25% or less, preferably 20% or less, and more preferably 15% or less of the arrangement pitch. However, in order to avoid a decrease in abrasion resistance, the width of the boundary is set to 0.5% or more, preferably 1.0% or more of the arrangement pitch. In the present disclosure, "boundary between recesses" means "the width of the flat portion intentionally arranged between the recesses" when the recesses are arranged with a gap between them, and means "the width of the curved portion of the top of the ridge line separating the recesses" when the recesses are arranged without gaps.

[0037] In the first light diffusion sheet, when the arrangement pitch is 1000 μm or less, an increase in the thickness of the first light diffusion sheet can be suppressed, and the backlight unit can be made thinner.

[0038] In the first light diffusing sheet, when the width of the boundary portion is the width of the curved portion of the apex of the ridge line separating the recesses from each other, in other words, when the recesses are arranged without any gaps, brightness uniformity can be improved compared to when the recesses are arranged with intervals. In this case, the recesses may be formed in a substantially inverted square pyramid or a substantially inverted square pyramid truncated shape, the ridge line may extend in a first direction and a second direction, the arrangement pitch may be the average of a first arrangement pitch of the recesses in the first direction and a second arrangement pitch of the recesses in the second direction, and the width of the boundary portion may be the average of a width occupied by the curved portion of the apex of the ridge line in the first direction and a width occupied by the curved portion of the apex of the ridge line in the second direction. This makes it easy to form the recesses.

[0039] In the first light diffusion sheet, when the angle formed by the wall surface of the recess and the sheet surface of the first light diffusion sheet is 40 degrees or more and 65 degrees or less, the recess can achieve a sufficient effect of improving brightness uniformity.

[0040] In the first light diffusion sheet, if the recesses are provided only on the first surface and the second surface of the first light diffusion sheet is a flat or matte surface, it is possible to achieve the effect of improving brightness uniformity while suppressing wear and damage on the second surface.

[0041] According to the present disclosure, it is possible to improve the handleability of the light diffusion sheets when assembling a backlight unit using a plurality of light diffusion sheets, while suppressing a decrease in brightness and brightness uniformity.

[0042] 1 is a cross-sectional view of a liquid crystal display device according to an embodiment. FIG. 2 is a cross-sectional view of an example of a backlight unit according to an embodiment. FIG. 3 is a plan view showing an example of the arrangement of light sources in the backlight unit shown in FIG. 2. FIG. 4 is a perspective view of a light diffusion sheet constituting the laminated light diffusion sheet according to an embodiment. FIG. 5 is a cross-sectional view of another example of a backlight unit according to an embodiment. FIG. 6 is an enlarged perspective view of recesses formed in a light diffusion sheet constituting the laminated light diffusion sheet according to an embodiment. FIG. 7 is a schematic view showing an example of the shape of X-direction ridge lines defining recesses in a light diffusion sheet constituting the laminated light diffusion sheet according to an embodiment. FIG. 8 is a schematic view showing an example of the shape of Y-direction ridge lines defining recesses in a light diffusion sheet constituting the laminated light diffusion sheet according to an embodiment. FIG. 9 is a schematic view showing variations in the shape of ridge lines defining recesses in a light diffusion sheet constituting the laminated light diffusion sheet according to an embodiment. FIG. 10 is a schematic view showing an example of a cross-sectional configuration when a light diffusion sheet constituting the laminated light diffusion sheet according to an embodiment is cut along a plane that passes through the centers of recesses adjacent to each other in the X direction and the midpoint of the ridge line located between the recesses and is perpendicular to the sheet surface. 11A and 11B are schematic diagrams showing an example of a cross-sectional configuration when a light diffusion sheet constituting a laminated light diffusion sheet according to an embodiment is cut along a plane perpendicular to the sheet surface and passing through the centers of adjacent recesses in the Y direction and the midpoints of ridge lines located between the recesses.

[0033] FIG. 7 is a diagram showing 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.

[0034] FIG. 8 is a diagram showing an example of the results of measuring the shape, dimensions, etc. of the cross-sectional configuration shown in FIG. 10 using a laser microscope.

[0035] FIG. 11B is a diagram showing an example of the results of measuring the shape, dimensions, etc. of the cross-sectional configuration shown in FIG. 11 using a laser microscope.

[0036] FIG. 11C is a diagram showing the shape of a square pyramid on a roll used in manufacturing a light diffusion sheet constituting a laminated light diffusion sheet according to an example.

[0037] FIG. 11D is a diagram showing an example of a method for measuring the spatial volume ratio Va / V0 in a laminated light diffusion sheet according to an example.

[0038] FIG. 11E is a photograph showing an example of a cross-sectional configuration of a laminated light diffusion sheet according to an example.

[0039] FIG. 11F is a photograph showing another example of a method for measuring the spatial volume ratio Va / V0 in a laminated light diffusion sheet according to an example.

[0039] 1 is a plan view showing the arrangement of light sources in a backlight unit used to evaluate the luminance and luminance uniformity of the laminated light diffusion sheet according to an embodiment of the present invention.

[0043] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments and can be modified as desired within the scope of the technical concept of the present disclosure.

[0044] 1 , a liquid crystal display device 50 of this embodiment includes a liquid crystal display panel 5, a first polarizing plate 6 attached to the lower surface of the liquid crystal display panel 5, a second polarizing plate 7 attached to the upper surface of the liquid crystal display panel 5, and a backlight unit 40 provided on the back side of the liquid crystal display panel 5 via the first polarizing plate 6. The liquid crystal display panel 5 includes a TFT substrate 1 and a CF substrate 2 arranged to face each other, a liquid crystal layer 3 provided between the TFT substrate 1 and the CF substrate 2, and a frame-shaped sealant (not shown) for enclosing the liquid crystal layer 3 between the TFT substrate 1 and the CF substrate 2.

[0045] The shape of the display screen 50a of the liquid crystal display device 50 when viewed from the front (top of Figure 1) is, in principle, rectangular or square, but is not limited to this and may be any shape such as a rectangle with rounded corners, an oval, a circle, a trapezoid, or an automobile instrument panel.

[0046] In the liquid crystal display device 50, a voltage of a predetermined magnitude is applied to the liquid crystal layer 3 in each sub-pixel corresponding to each pixel electrode to change the alignment state of the liquid crystal layer 3. This adjusts the transmittance of light incident from the backlight unit 40 through the first polarizer 6. The light with the adjusted transmittance is emitted through the second polarizer 7 to display an image.

[0047] The liquid crystal display device 50 of this embodiment is used as a display device to be incorporated into various information devices (for example, in-vehicle devices such as car navigation systems, personal computers, mobile phones, portable information terminals such as laptops and tablets, portable game machines, copy machines, ticket vending machines, automated teller machines, etc.).

[0048] The TFT substrate 1 includes, for example, a plurality of TFTs arranged in a matrix on a glass substrate, an interlayer insulating film covering each TFT, a plurality of pixel electrodes arranged in a matrix on the interlayer insulating film and connected to each of the plurality of TFTs, and an alignment film covering each pixel electrode. The CF substrate 2 includes, for example, a black matrix arranged in a grid pattern on the glass substrate, color filters including red, green, and blue layers respectively arranged between each grid of the black matrix, a common electrode covering the black matrix and the color filters, and an alignment film covering the common electrode. The liquid crystal layer 3 is made of a nematic liquid crystal material containing liquid crystal molecules with electro-optical properties. The first polarizer 6 and the second polarizer 7 include, for example, a polarizer layer having a unidirectional polarization axis and a pair of protective layers sandwiching the polarizer layer.

[0049] 2 , the backlight unit 40 of the present embodiment mainly includes a plurality of light sources 42 and a laminated light diffusion sheet 100 provided above the plurality of light sources 42. The laminated light diffusion sheet 100 has a structure in which a first light diffusion sheet 101 as a lower layer and a second light diffusion sheet 102 as an upper layer are bonded together with an adhesive member 111. The first light diffusion sheet 101 and the second light diffusion sheet 102 are bonded together by adhering or applying, for example, an OCA (Optical Clear Adhesive) film, ink containing an ultraviolet curable resin, ink containing an infrared absorbing agent, or the like to the bonding surfaces of the first light diffusion sheet 101 or the second light diffusion sheet 102.

[0050] In the example shown in Fig. 2, the multiple light sources 42 are blue light sources and are arranged two-dimensionally on the reflective sheet 41. A wavelength selective sheet 43 and a color conversion sheet 44 are arranged between the multiple light sources 42 and the laminated light diffusing sheet 100. The wavelength selective sheet 43 is arranged below the color conversion sheet 44. The wavelength selective sheet 43 selectively transmits light having the emission wavelength of the light source 42 and reflects light having other wavelengths. The color conversion sheet 44 converts the color of the light emitted by the light source 42.

[0051] To improve brightness, a brightness enhancement sheet 47 is disposed on the upper side of the laminated light diffusing sheet 100. The type of brightness enhancement sheet 47 is not particularly limited as long as it can increase the brightness of light emitted from the light source 42. In the example shown in Fig. 2, the brightness enhancement sheet 47 is formed by a first prism sheet 45 in the lower layer and a second prism sheet 46 in the upper layer, which are disposed with their prism extension directions perpendicular to each other.

[0052] Although not shown, a polarizing sheet may be provided above the brightness enhancing sheet 47. 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.

[0053] In this disclosure, the term "light diffusion sheet" encompasses a plate-shaped "light diffusion plate" and a film-shaped "light diffusion film." Furthermore, the term "optical sheet" refers to a sheet having optical functions such as diffusion, light collection, refraction, and reflection, and includes "light diffusion sheets," "reflective sheets," "brightness enhancement sheets," and the like.

[0054] Each component of the backlight unit 40 will be described below.

[0055] [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.

[0056] [Light Source] The type of light source 42 is not particularly limited, but may be, for example, an LED element or a laser element. LED elements may also be used from the viewpoint of cost, productivity, and the like. A lens may be attached to the LED element serving as the light source 42 to adjust the light output angle characteristics of the LED element. The light source 42 may have a rectangular shape in a plan view, in which 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, more preferably 1 mm or less). The number of light sources 42 is also not particularly limited, but when multiple light sources 42 are dispersed, it is preferable to arrange them regularly on the reflective sheet 41. "Regularly arranged" means arranging the light sources 42 according to a certain rule, for example, arranging the light sources 42 at equal intervals. 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.

[0057] In this embodiment, a plurality of light sources 42 made up of LED elements are arranged in a two-dimensional array at regular intervals, as shown in Fig. 3. In other words, the plurality of light sources 42 are arranged along two directions that are perpendicular to each other.

[0058] When a blue light source is used as the light source 42, the blue light source may emit light such that x<0.24 and y<0.18 in the CIE 1931 chromaticity coordinates, for example. When a white light source is used as the light source 42, the white light source may be composed of an LED element having a peak wavelength in the blue region, an LED element having a peak wavelength in the green region, and an LED element having a peak wavelength in the red region, and may emit light such that 0.24<x<0.42 and 0.18<y<0.48 in the CIE 1931 chromaticity coordinates, for example. When a white light source is used as the light source 42, the wavelength selection sheet 43 and the color conversion sheet 44 do not need to be provided.

[0059] [Wavelength Selective Sheet and Color Conversion Sheet] The wavelength selective sheet 43 selectively transmits light having the emission wavelength of the light source 42 (e.g., blue light) and reflects light having other wavelengths. The color conversion sheet 44 converts light from the light source 42 (e.g., blue light) into light having a peak wavelength of a desired color (e.g., green or red). The color conversion sheet 44 converts, for example, blue light with a wavelength of 450 nm into green light with a wavelength of 540 nm and red light with a wavelength of 650 nm. In this case, if a light source 42 emitting blue light with a wavelength of 450 nm is used, the color conversion sheet 44 partially converts the blue light into green light and red light, so that the light transmitted through the color conversion sheet 44 becomes white light. The color conversion sheet 44 may be, for example, a quantum dot (QD) sheet or a fluorescent sheet. Because the wavelength selective sheet 43 is disposed below the color conversion sheet 44, light whose wavelength has been changed by the color conversion sheet 44 can only travel above the color conversion sheet 44.

[0060] In the example shown in Figure 2, the wavelength selection sheet 43 and the color conversion sheet 44 are arranged between the light source 42 and the laminated light diffusion sheet 100, but instead, they may be arranged between the laminated light diffusion sheet 100 and the brightness enhancement sheet 47.

[0061] [Brightness Enhancement Sheet] The first prism sheet 45 and the second prism sheet 46 constituting the brightness enhancement sheet 47 are, for example, films in which a plurality of grooves having an isosceles triangular cross section are formed adjacent to each other, and the apex angle of the prisms sandwiched between a pair of adjacent grooves is approximately 90°. Here, the grooves formed in the first prism sheet 45 and the grooves formed in the second prism sheet 46 are arranged so as to be perpendicular to each other. The first prism sheet 45 and the second prism sheet 46 may be integrally formed. For example, the first prism sheet 45 and the second prism sheet 46 may be formed by forming a prism shape on a polyethylene terephthalate (PET) film using a UV-curable acrylic resin.

[0062] <Laminated Light Diffusion Sheet> [Configuration of Light Diffusion Sheet] The laminated light diffusion sheet 100 has a first light diffusion sheet 101 and a second light diffusion sheet 102 having the same structure. The thickness of the first light diffusion sheet 101 and the second light diffusion sheet 102 is, for example, 30 μm or more and 1000 μm or less. The first light diffusion sheet 101 and the second light diffusion sheet 102 each have a base layer 21. The base layer 21 is formed, for example, using polycarbonate as a base material (matrix resin) and preferably does not contain a diffusing agent, but may contain, for example, about 10 parts by mass or less of a diffusing agent per 100 parts by mass of the base material. Any known material can be used as the diffusing agent.

[0063] A plurality of recesses 22 are provided on the first surface 101a of the first light diffusion sheet 101 and the first surface 102a of the second light diffusion sheet 102 (in the example shown in FIG. 2, the first surfaces 101a and 102a are light output surfaces). The plurality of recesses 22 are formed in the shape of a substantially inverted polygonal pyramid or a substantially inverted truncated polygonal pyramid. In this example, the plurality of recesses 22 are formed in the shape of a substantially inverted regular square pyramid. The plurality of recesses 22 have substantially the same shape within the range of processing error. Adjacent recesses 22 are separated by ridge lines 23.

[0064] Specifically, as shown in FIG. 4 , recesses 22 having a substantially inverted quadrangular pyramid shape (inverted pyramid shape) are arranged in a two-dimensional matrix on the first surface 101a of the first light diffusion sheet 101 and the first surface 102a of the second light diffusion sheet 102. In other words, the recesses 22 are arranged along two mutually perpendicular directions. Adjacent recesses 22 are separated by ridge lines 23. The ridge lines 23 extend along the two directions in which the recesses 22 are arranged. The centers of the recesses 22 (the apexes of the inverted pyramids) are the deepest parts of the recesses 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 recesses 22, the recesses 22 may be arranged without gaps on the first surface 101a (102a) or at predetermined intervals. Furthermore, some of the recesses 22 may be arranged randomly, as long as the light diffusion effect is not impaired.

[0065] In this embodiment, in both the first light diffusion sheet 101 and the second light diffusion sheet 102, the recesses 22 are provided in the base layer 21. In other words, the sheet portion where the recesses 22 are provided (recessed portion) and the other sheet portion (base portion) are integrally formed from the same material. The first light diffusion sheet 101 and the second light diffusion sheet 102 are each integrally formed by transferring the shape of the recesses using, for example, a method such as pressing a molten or softened sheet material with two rolls or in a press. This prevents the recessed portion from peeling off from the base portion during reliability tests, etc.

[0066] The arrangement pitch of the recesses 22 is set to, for example, approximately 20 μm or more. The arrangement pitch of the recesses is basically proportional to the thickness of the light diffusion sheet on which the recesses are provided. When a thin light diffusion sheet is bonded to another light diffusion sheet, the rigidity of the thin light diffusion sheet is increased, making it easier to handle during assembly of the backlight unit, etc. Even in the case of a thick light diffusion sheet, when it is bonded to another light diffusion sheet, the number of parts required during assembly of the backlight unit, etc., is reduced, thereby reducing the assembly effort. From the above perspectives, in the first light diffusion sheet 101 and the second light diffusion sheet 102, the arrangement pitch of the recesses 22 is set to 30 μm or more and 500 μm or less, preferably 50 μm or more and 300 μm or less, and more preferably 50 μm or more and 200 μm or less.

[0067] The angle formed by the wall surface of the recess 22 (the slope of the approximately inverted polygonal pyramid or the approximately inverted polygonal truncated pyramid) and the sheet surface of the first light diffusion sheet 101 and the second light diffusion sheet 102 (a 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, and 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, and more preferably 70 degrees or more and 86 degrees or less.

[0068] The second surface 101b of the first light diffusion sheet 101 and the second surface 102b of the second light diffusion sheet 102 may be flat (mirror-finished), or may be matte to improve diffusion. In the example shown in Fig. 2, the second surface 101b of the first light diffusion sheet 101 and the second surface 102b of the second light diffusion sheet 102 are arranged so as to serve as light incident surfaces. However, instead, the second surface 101b of the first light diffusion sheet 101 and the second surface 102b of the second light diffusion sheet 102 may be arranged so as to serve as light exit surfaces. Alternatively, one of the second surface 101b of the first light diffusion sheet 101 and the second surface 102b of the second light diffusion sheet 102 may be arranged so as to serve as a light incident surface and the other as a light exit surface.

[0069] In the laminated light diffusion sheet 100 of this embodiment, the first light diffusion sheet 101 and the second light diffusion sheet 102 having the same structure are bonded together. However, instead, the first light diffusion sheet 101 and the second light diffusion sheet 102 having different structures may be bonded together. For example, the dimensions, shape, and arrangement pitch of the recesses 22 provided in the second light diffusion sheet 102 may be different from the dimensions, shape, and arrangement pitch of the recesses 22 provided in the first light diffusion sheet 101. Alternatively, the surface shape of the second surface 102b of the second light diffusion sheet 102 may be different from the surface shape of the second surface 101b of the first light diffusion sheet 101. Alternatively, the second light diffusion sheet 102 may not have recesses 22.

[0070] [Bonding of Light Diffusion Sheets] In the laminated light diffusion sheet 100 of this embodiment, the first surface 101a (concave pyramid-forming surface) of the first light diffusion sheet 101 is bonded to the second surface 102b (matte surface) of the second light diffusion sheet 102. The peel strength of the bonding area between the first light diffusion sheet 101 and the second light diffusion sheet 102 is 5.0 g weight / 25 mm or more, preferably 10.0 g weight / 25 mm or more, more preferably 100 g weight / 25 mm or more, and even more preferably 140 g weight / 25 mm or more.

[0071] The bonding portion of the first light diffusion sheet 101 to the second light diffusion sheet 102 may be the entire first surface 101 a or the peripheral portion of the first surface 101 a (the edge portion excluding the active area directly below the display screen 50 a). Alternatively, a plurality of dot-like or linear regions on the first surface 101 a of the first light diffusion sheet 101 may be bonded to the second surface 102 b of the second light diffusion sheet 102.

[0072] The first light diffusion sheet 101 and the second light diffusion sheet 102 may be bonded together using an OCA film that serves as the adhesive member 111. In this case, an OCA film may be attached to the entire surface of the second surface 102b of the second light diffusion sheet 102, and then the first surface 101a of the first light diffusion sheet 101 may be bonded to the OCA film.

[0073] The first light diffusion sheet 101 and the second light diffusion sheet 102 may be bonded together by applying an ink containing an ultraviolet-curable resin (UV curable ink) to serve as the adhesive member 111 to the second surface 102b of the second light diffusion sheet 102, and then contacting the first surface 101a of the first light diffusion sheet 101 with the second surface 102b of the second light diffusion sheet 102 and irradiating them with ultraviolet light. In this case, the UV curable ink may be applied to the entire surface of the second surface 102b of the second light diffusion sheet 102, or the UV curable ink may be applied to multiple dotted or linear regions on the second surface 102b of the second light diffusion sheet 102. The dotted or linear regions to which the UV curable ink is applied may have a predetermined area and shape, and may be arranged regularly or randomly. Alternatively, the UV curable ink may be applied to the peripheral portions along the four edges of the second surface 102b of the second light diffusion sheet 102.

[0074] The first light diffusion sheet 101 and the second light diffusion sheet 102 may be bonded together by applying ink containing an infrared absorbing agent to the four corners (corners of the peripheral portion) of the second surface 102b of the second light diffusion sheet 102, and then abutting the first surface 101a of the first light diffusion sheet 101 against the second surface 102b of the second light diffusion sheet 102 and irradiating them with an infrared laser. In this case, the base material layers 21 at the four corners of the first light diffusion sheet 101 and the second light diffusion sheet 102 are heat-welded to each other to form adhesive members 111. The infrared absorbing agent may remain in the adhesive member 111.

[0075] When the first light diffusion sheet 101 and the second light diffusion sheet 102 are bonded together, the adhesive material or part of the base layer 21 of the second light diffusion sheet 102 may get into the recess 22 on the first surface 101a of the first light diffusion sheet 101.

[0076] In contrast, a feature of the laminated light diffusion sheet 100 of this embodiment is that, where V0 is the volume of the recess 22 and Va is the volume of the void remaining in the recess 22, the first light diffusion sheet 101 is bonded to the first surface 101a (the active area directly below the display screen 50a excluding the peripheral portion) so that Va / V0 is 40% or more, preferably 50% or more. This makes it possible to suppress the reduction in brightness and brightness uniformity caused by bonding the first light diffusion sheet 101 and the second light diffusion sheet 102 to a practical level. For example, when only the peripheral portion of the first surface 101a of the first light diffusion sheet 101 is bonded to the second light diffusion sheet 102, Va / V0 reaches the upper limit of 100%.

[0077] In the laminated light diffusion sheet 100 of this embodiment, the first light diffusion sheet 101 and the second light diffusion sheet 102 are bonded together with their respective first surfaces 101a, 102a serving as light-emitting surfaces, i.e., the first light diffusion sheet 101 and the second light diffusion sheet 102 facing the same direction. In this case, it is preferable that Va / V0 is 55% or greater. Furthermore, by bonding the first light diffusion sheet 101 and the second light diffusion sheet 102 facing the same direction, bonding is easier than when the first light diffusion sheet 101 and the second light diffusion sheet 102 are bonded together facing different directions, for example, when the first surface 101a of the first light diffusion sheet 101 is bonded to the first surface 102a of the second light diffusion sheet 102.

[0078] The second light diffusion sheet 102 may be a light diffusion sheet of a different type from the first light diffusion sheet 101, for example, a light diffusion sheet having irregularities on the surface thereof made of resin beads or the like. In this case, when bonding the light diffusion sheet to the first surface 101a of the first light diffusion sheet 101, it is important to ensure that the Va / V0 is 40% or more by preventing the recesses 22 on the first surface 101a from being filled with adhesive material or resin beads.

[0079] <Variations of the Laminated Light Diffusion Sheet> In the configuration example of the backlight unit 40 shown in FIG. 2 , the laminated light diffusion sheet 100 is formed of two light diffusion sheets, namely, the first light diffusion sheet 101 and the second light diffusion sheet 102. Alternatively, as shown in the modified example shown in FIG. 5 , the laminated light diffusion sheet 100 may be formed of three light diffusion sheets, namely, the first light diffusion sheet 101, the second light diffusion sheet 102, and the third light diffusion sheet 103. In this case, the third light diffusion sheet 103 may have the same structure as the first light diffusion sheet 101 and the second light diffusion sheet 102. That is, the first surface 103a of the third light diffusion sheet 103 (the light-emitting surface in the example shown in FIG. 5 ) may be provided with a plurality of recesses 22 formed in the shape of a substantially inverted square pyramid. The second surface 103b of the third light diffusion sheet 103 may be a matte surface. 5 , the laminated light diffusion sheet 100 has a structure in which a first light diffusion sheet 101 as a lower layer and a second light diffusion sheet 102 as a middle layer are bonded together with an adhesive member 111, and the second light diffusion sheet 102 as a middle layer and a third light diffusion sheet 103 as an upper layer are bonded together with an adhesive member 112. The second light diffusion sheet 102 and the third light diffusion sheet 103 are bonded together by adhering or applying, for example, an OCA film, an ink containing an ultraviolet curable resin, or an ink containing an infrared absorbing agent to the bonding surface of the second light diffusion sheet 102 or the third light diffusion sheet 103, similar to the bonding of the first light diffusion sheet 101 and the second light diffusion sheet 102 in the embodiment shown in FIG.

[0080] Although the laminated light diffusion sheet 100 of this modified example is made up of three light diffusion sheets, the laminated light diffusion sheet 100 may be made up of four or more light diffusion sheets.

[0081] In addition, in the laminated light diffusion sheet 100 of this modified example, the second surfaces 101b to 103b of the first to third light diffusion sheets 101 to 103 are arranged to serve as light incident surfaces, but instead, the second surfaces 101b to 103b of the first to third light diffusion sheets 101 to 103 may be arranged to serve as light exit surfaces. Alternatively, one or two of the second surfaces 101b to 103b of the first to third light diffusion sheets 101 to 103 may be arranged to serve as light incident surfaces, and the others may be arranged to serve as light exit surfaces.

[0082] Furthermore, in the laminated light diffusion sheet 100 of this embodiment, the first to third light diffusion sheets 101 to 103 having the same structure are bonded together. However, instead, the first to third light diffusion sheets 101 to 103 having different structures may be bonded together. Alternatively, only one of the first to third light diffusion sheets 101 to 103 may have a structure different from the other two. For example, the dimensions, shape, and arrangement pitch of the recesses 22 provided in the second light diffusion sheet 102 and / or the third light diffusion sheet 103 may be different from the dimensions, shape, and arrangement pitch of the recesses 22 provided in the first light diffusion sheet 101. Alternatively, the surface shape of the second surface 102b of the second light diffusion sheet 102 and / or the surface shape of the second surface 103b of the third light diffusion sheet 103 may be different from the surface shape of the second surface 101b of the first light diffusion sheet 101. Alternatively, the recesses 22 may not be provided in the second light diffusion sheet 102 and / or the third light diffusion sheet 103 .

[0083] <Variations of light diffusion sheets> In the examples shown in Figures 2 and 5, multiple recesses 22 are formed on the first surfaces 101a to 103a of the first to third light diffusion sheets 101 to 103, but in addition to this, multiple other recesses similar to the recesses 22 may also be formed on the second surfaces 101b to 103b of the first to third light diffusion sheets 101 to 103.

[0084] The plurality of recesses 22 may be formed in the shape of a substantially inverted polygonal pyramid or a substantially inverted polygonal truncated pyramid. The plurality of recesses 22 may be regularly arranged two-dimensionally. As the "inverted polygonal pyramid (trapezoid)", a triangular pyramid (trapezoid), a square pyramid (trapezoid), or a hexagonal pyramid (trapezoid) is preferred, as they can be arranged two-dimensionally without gaps. A mold (metal roll) is used in a manufacturing process such as extrusion molding or injection molding to form the recesses 22, and in consideration of the accuracy of the cutting work on the surface of this mold (metal roll), an inverted square pyramid (trapezoid) may be selected as the "inverted polygonal pyramid (trapezoid)".

[0085] In this disclosure, taking into consideration the difficulty of forming a recess having a geometrically strict inverted polygonal pyramid or inverted polygonal truncated pyramid shape using ordinary shape transfer technology, the terms "approximately inverted polygonal pyramid" or "approximately inverted polygonal truncated pyramid" are used, but it goes without saying that these terms also include shapes that can be regarded as true or substantially inverted polygonal pyramids or inverted polygonal truncated pyramids. Furthermore, "approximately" means that something can be approximated; for example, "approximately square pyramid" refers to a shape that can be approximated to a square pyramid. Furthermore, shapes that are deformed from an "inverted polygonal pyramid" or "inverted polygonal truncated pyramid" within the range of unavoidable shape variations due to processing accuracy in industrial production are also included in the "approximately inverted polygonal pyramid" or "approximately inverted polygonal pyramid" shapes.

[0086] When multiple recesses 22 are regularly arranged two-dimensionally, the multiple recesses 22 may be provided without gaps across the entire surface of the first to third light diffusion sheets 101 to 103, or flat portions of a predetermined width may be provided between the recesses 22.

[0087] The first to third light diffusion sheets 101 to 103 may be configured with a substrate layer 21 that does not contain a diffusing agent, for example, a substrate layer 21 made of clear polycarbonate. When a diffusing agent is contained in the substrate layer 21, the material of the diffusing agent is not particularly limited, but inorganic particles such as silica, titanium oxide, aluminum hydroxide, barium sulfate, etc., and organic particles such as acrylic, acrylonitrile, silicone, polystyrene, polyamide, etc., may be used. From the viewpoint of 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). From the viewpoint of the reflection and refraction effects of the substantially inverted polygonal pyramid shape and the light diffusion effect of the diffusing agent, the first to third light diffusion sheets 101 to 103 preferably do not contain a diffusing agent. However, the content of the diffusing agent may be, for example, 0.1 parts by mass or more (preferably 0.3 parts by mass or more) to 10 parts by mass or less (preferably 8 parts by mass or less) per 100 parts by mass of the material (matrix) constituting the substrate layer 21. The difference between the refractive index of the diffusing agent and the refractive index of the matrix of the substrate layer 21 may be 0.01 or more, preferably 0.03 or more, more preferably 0.05 or more, even more preferably 0.1 or more, and most preferably 0.15 or more. If the difference between the refractive index of the diffusing agent and the refractive index of the matrix of the substrate layer 21 is less than 0.01, the diffusion effect of the diffusing agent will be insufficient.

[0088] The resin that forms the matrix of the base material layer 21 is not particularly limited as long as it is a material that transmits light, but for example, acrylic, polystyrene, styrene acrylic, polycarbonate, MS (methyl methacrylate-styrene copolymer) resin, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyimide, etc. may be used.

[0089] The thickness of the first to third light diffusion sheets 101 to 103 is not particularly limited, but considering the effect of lamination, it may be, for example, 1 mm or less (preferably 800 μm or less, more preferably 500 μm or less, and even more preferably 300 μm or less) and 30 μm or more (preferably 50 μm or more, and more preferably 70 μm or more). If the thickness of the first to third light diffusion sheets 101 to 103 exceeds 1 mm, it becomes difficult to achieve a thin liquid crystal display. On the other hand, if the thickness of the first to third light diffusion sheets 101 to 103 is less than 30 μm, it becomes difficult to form the film while forming an uneven shape.

[0090] <Method for Manufacturing Light Diffusion Sheets> A method for manufacturing the first to third light diffusion sheets 101 to 103 will be described below. The method for manufacturing the first to third light diffusion sheets 101 to 103 is not particularly limited. For example, extrusion molding, compression molding, transfer methods using UV-curable resins or thermosetting resins, injection molding, etc. may be used. However, it is preferable to use extrusion molding, compression molding, injection molding, etc., which enable integral molding using the same material. When the first to third light diffusion sheets 101 to 103 are extrusion-molded, for example, the line speed may be set to preferably 2 m / min to 50 m / min (more preferably 3 m / min to 40 m / min) and the compression line pressure may be set to preferably 50 kgf / cm to 1000 kgf / cm (more preferably 100 kgf / cm to 800 kgf / cm, and even more preferably 200 kgf / cm to 500 kgf / cm). If the line speed exceeds 50 m / min, the resin for the light diffusion sheet is not sufficiently cooled, and the shape transfer rate is likely to decrease, while if the line speed is below 2 m / min, productivity is likely to decrease and the shape transfer rate also decreases. Furthermore, if the compression linear pressure exceeds 1000 kgf / cm, the mechanical strength of the manufacturing equipment may be exceeded, while if the compression linear pressure is below 50 kgf / cm, the shape transfer rate is likely to decrease.

[0091] The procedure for producing a single-layer light diffusion sheet having a textured surface using an extrusion molding method is as follows. First, pellet-shaped plastic particles (to which a diffusing agent may be added) are introduced into a single-screw extruder, melted, and kneaded while being heated. The molten resin extruded through a T-die is then sandwiched between two metal rolls and cooled, and then conveyed using a guide roll. In the case of a relatively thin film, the resin is wound into a roll using a winder and then cut to a predetermined size. In the case of a relatively thick sheet, the resin is cut into individual flat sheets using a sheet cutter, thereby producing a light diffusion sheet. Here, by sandwiching the molten resin using metal rolls having a surface that is the inverse of the desired textured shape, the inverse shape of the roll surface is transferred to the resin, allowing the desired textured shape to be formed on the surface of the light diffusion sheet. Furthermore, since the shape transferred to the resin is not necessarily 100% the shape of the roll surface, the shape of the roll surface may be designed by calculating backwards from the degree of transfer.

[0092] When using an extrusion molding method to manufacture a two-layer light diffusion sheet having an uneven surface, for example, pellet-shaped plastic particles required to form each layer are fed into each of two single-screw extruders, and then the same procedure as described above is carried out for each layer, and the resulting layers are laminated.

[0093] Alternatively, a two-layer light diffusion sheet having an uneven surface may be produced as follows. First, pellet-shaped plastic particles required for forming each layer are charged into each of two single-screw extruders, and melted and kneaded while heated. The molten resins for each layer are then charged into a T-die and laminated within the T-die. The laminated molten resin extruded through the T-die is sandwiched between two metal rolls and cooled. The laminated molten resin is then transported using guide rolls and wound into a roll and then cut, or it may be formed into a sheet and cut into individual flat plates using a sheet cutter, thereby producing a two-layer light diffusion sheet having an uneven surface.

[0094] <Shape of ridge lines separating recesses in light diffusion sheet> Further features of the first to third light diffusion sheets 101 to 103, specifically the features of the shape of the ridge lines 23 separating the recesses 22, will be described below with reference to Figures 6 to 15, using the first light diffusion sheet 101 as an example.

[0095] As shown in Fig. 6, the first surface 101a of the first light diffusion sheet 101 is provided with a plurality of recesses 22 formed, for example, in the shape of a substantially inverted square pyramid. The plurality of recesses 22 may be formed in the shape of a substantially inverted truncated square pyramid. The center 22a of each 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), which are perpendicular to each other. Adjacent recesses 22 are separated by ridge lines 23. The ridge lines 23 extend along the X direction and the Y direction.

[0096] In the first light diffusion sheet 101, the ridge lines 23 may have a recessed shape between the intersection points 23a with respect to the straight lines Lx and Ly connecting the intersection points 23a of the ridge lines 23. The maximum height difference d between the straight lines Lx and Ly connecting the intersection points 23a and the ridge lines 23 may be 1 μm or more and 30 μm or less, preferably 1.5 μm or more and 20 μm or less, and more preferably 2.5 μm or more and 10 μm or less.

[0097] In the first light diffusion sheet 101, the ridge lines 23 may have a concave shape between all of the intersection points 23a of the ridge lines, or the ridge lines 23 may not have a concave shape between some of the intersection points 23a.

[0098] Fig. 7 shows an example of the shape of a ridge line 23 extending in the X direction along line Ax-Bx 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 a ridge line 23 extending in the Y direction along line Ay-By in Fig. 6 when viewed from a direction parallel to the sheet surface and perpendicular to the Y direction. As shown in Fig. 7, the ridge line 23 may have a recessed shape between intersections 23a of the ridge lines 23 in the X direction with respect to a straight line Lx connecting the intersections 23a of the ridge lines 23. In this case, if the arrangement pitch of the recesses 22 in the X direction is Px, the ridge line 23 extending in the X direction has its lowest point 23b at a position Px / 2 (half the pitch) from the intersection 23a, for example, and the distance (maximum height difference) from the straight line Lx to the lowest point 23b is dx. 8 , the ridge lines 23 may have a recessed shape between the intersection points 23 a with respect to the straight line Ly connecting the intersection points 23 a of the ridge lines 23 in the Y direction. In this case, the arrangement pitch of the recesses 22 in the Y direction is Py, and the ridge lines 23 extending in the Y direction have their lowest points 23 b at a position Py / 2 (half the pitch) from the intersection points 23 a, for example, and the distance (maximum height difference) from the straight line Ly to the lowest point 23 b is dy.

[0099] When the recesses 22 are formed into an inverted square pyramid, the arrangement pitch Px of the recesses 22 in the X direction is equal to the distance (horizontal distance) between the intersections 23 a in the X direction, and the arrangement pitch Py of the recesses 22 in the Y direction is equal to the distance (horizontal distance) between the intersections 23 a in the Y direction.

[0100] The maximum height difference d may be set to an average value of the maximum height difference dx in the X direction and the maximum height difference dy in the Y direction, and the maximum height difference d may be set to 1 μm or more and 30 μm or less, preferably 1.5 μm or more and 20 μm or less, and more preferably 2.5 μm or more and 10 μm or less.

[0101] The recessed shape of the ridge line 23 between the intersection points 23 a is not particularly limited, but for example, as shown in Fig. 9, the ridge line 23 may be recessed between the intersection points 23 a in a substantially arc shape (Fig. 9A), a substantially parabolic shape (Fig. 9B), a substantially triangular shape (Fig. 9C), or a substantially trapezoidal shape (Fig. 9D) with respect to the straight line L connecting the intersection points 23 a of the ridge line 23. Furthermore, the ridge line 23 may have a recessed shape that is substantially symmetrical with respect to the straight line L connecting the intersection points 23 a of the ridge line 23, with the lowest point of the ridge line 23 as the center, between the intersection points 23 a.

[0102] A feature of the first light diffusion sheet 101 is that, if the arrangement pitch of the recesses 22 is P and the dimension occupied by the curved portions of the tops of the ridge lines 23 in the arrangement direction of the recesses 22 is Wr, the ratio Wr / P must be 0.25 (25%) or less, preferably 0.2 (20%) or less, and more preferably 0.15 (15%) or less. However, to avoid a decrease in abrasion resistance, the ratio Wr / P is set to 0.005 (0.5%) or more, preferably 0.01 (1.0%) or more.

[0103] In the present disclosure, when the recesses 22 are arranged without any gaps as shown in Fig. 6, the "curved portion of the top of the ridge line 23" is considered to be the "boundary between the recesses 22". When the recesses 22 are arranged with a gap between them, the "flat portion intentionally arranged between the recesses 22" is considered to be the "boundary between the recesses 22".

[0104] Fig. 10 shows an example of a cross-sectional configuration of the first light diffusion sheet 101 taken along the Cx-Dx line in Fig. 6, and Fig. 11 shows an example of a cross-sectional configuration of the first light diffusion sheet 101 taken along the Cy-Dy line in Fig. 6. Specifically, Fig. 10 shows a cross-sectional configuration of the first light diffusion sheet 101 taken along a plane that passes through the centers 22a of the recesses 22 adjacent to each other in the X direction and the midpoints between the intersections 23a of the ridge lines 23 located between the recesses 22, and is perpendicular to the sheet surface. Fig. 11 shows a cross-sectional configuration of the first light diffusion sheet 101 taken along a plane that passes through the centers 22a of the recesses 22 adjacent to each other in the Y direction and the midpoints between the intersections 23a of the ridge lines 23 located between the recesses 22, and is perpendicular to the sheet surface.

[0105] In the cross-sectional configuration shown in Figure 10, the distance (horizontal distance) between the centers 22a of adjacent recesses 22 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) at the top of the ridge line 23 in the X direction is Wrx. The dimensions occupied by the straight portions of the wall surfaces (slope of the inverted square pyramid) of adjacent recesses 22 across the ridge line 23 in the X direction are Wsx1 and Wsx2. The angle formed by the wall surfaces (slope of the inverted square pyramid) of the recesses 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 intersections 23a) of the ridge line 23 (ridge line 23 extending in the Y direction) is Hx.

[0106] In the cross-sectional configuration shown in Figure 11, the distance (horizontal distance) between the centers 22a of adjacent recesses 22 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) at the top of the ridge line 23 in the Y direction is Wry. The dimensions occupied by the straight portions of the wall surfaces (slope of the inverted square pyramid) of adjacent recesses 22 across the ridge line 23 in the Y direction are Wsy1 and Wsy2. The angle formed by the wall surfaces (slope of the inverted square pyramid) of the recesses 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 intersections 23a) of the ridge line 23 (ridge line 23 extending in the X direction) is Hy.

[0107] When the recess 22 is formed into an inverted pyramid, the average value of the arrangement pitch Px and the arrangement pitch Py is defined as P, and the average value of the dimension Wrx and the dimension Wry is defined as Wr, and the ratio Wr / P must be set to 0.25 (25%) or less, preferably 0.2 (20%) or less, and more preferably 0.15 (15%) or less.

[0108] FIG. 12 shows an example of the results of measuring the shape and dimensions of the X-direction ridgelines 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 ridgelines 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. Note that the maximum distances (maximum height differences) dx and dy between the ridgelines 23 and the straight lines Lx and Ly connecting the intersections 23a of the ridgelines 23 were measured by taking the maximum values ​​of the lengths of perpendicular lines drawn from points on the ridgelines 23 perpendicular to the straight lines Lx and Ly. Furthermore, the arrangement pitches Px and Py were measured by determining the "horizontal distances between the intersections 23a" in the X and Y directions as Px and Py. Thus, the arrangement pitches Px and Py can be easily and accurately determined by measuring the "horizontal distances between the intersections 23a."

[0109] <Features of the embodiment (including modified examples)> As described above, the laminated light diffusion sheet 100 of the present embodiment includes a first light diffusion sheet 101 having a first surface 101a on which a plurality of recesses 22 formed in the shape of a substantially inverted polygonal pyramid or a substantially inverted polygonal truncated pyramid are arranged, and a second light diffusion sheet 102 bonded to the first surface 101a of the first light diffusion sheet 101. The peel strength of the bonding portion between the first light diffusion sheet 101 and the second light diffusion sheet 102 is 5.0 g weight / 25 mm or more, and where V0 is the volume of the recesses 22 and Va is the volume of voids remaining in the recesses 22, Va / V0 is 40% or more and 100% or less in a region of the first surface 101a of the first light diffusion sheet 101 excluding the peripheral edge portion.

[0110] According to the laminated light diffusion sheet 100 of this embodiment, the first light diffusion sheet 101 and the second light diffusion sheet 102 are bonded together with a peel strength of 5.0 g weight / 25 mm or more, which improves the handleability of the light diffusion sheets 101 and 102 when assembling the backlight unit 40. Furthermore, on the first surface 101a (the active area directly below the display screen 50a excluding the peripheral portion) of the first light diffusion sheet 101 on which the recesses 22 formed in the shape of a substantially inverted polygonal pyramid or a substantially inverted polygonal truncated pyramid are arranged, the ratio (volume Va of voids remaining in the recesses 22) / (volume V0 of the recesses 22) is 40% or more, which suppresses deterioration in brightness and brightness uniformity due to bonding.

[0111] Furthermore, in a laminated light diffusion sheet having a Va / V0 of 40% or more, if the peel strength at the bonding point between the first light diffusion sheet and the second light diffusion sheet is 100 g weight / 25 mm or more, preferably 140 g weight / 25 mm or more, the handleability of each light diffusion sheet 101 and 102 when assembling the backlight unit 40 can be further improved.

[0112] In the laminated light-diffusing sheet 100 of this embodiment, if the peel strength measured at 85° C. is 10 g weight / 25 mm or more, the laminated light-diffusing sheet 100 can have sufficient heat resistance.

[0113] In the laminated light-diffusing sheet 100 of this embodiment, if the peel strength measured at 85° C. is 100 g / 25 mm or more, the laminated light-diffusing sheet 100 can have excellent heat resistance.

[0114] In the laminated light-diffusing sheet 100 of this embodiment, if the peel strength measured at 85° C. is 140 g / 25 mm or more, a laminated light-diffusing sheet with superior heat resistance can be obtained.

[0115] In the laminated light diffusion sheet 100 of this embodiment, the bonding locations of the light diffusion sheets 101 and 102 may be the entire surface, the peripheral portion, or a plurality of dotted or linear regions on the first surface 101a of the first light diffusion sheet 101. That is, in the laminated light diffusion sheet 100 of this embodiment, the bonding locations are not particularly limited as long as the peel strength between the first light diffusion sheet 101 and the second light diffusion sheet 102 is 5.0 g weight / 25 mm or more and the ratio (volume Va of voids remaining in the recesses 22) / (volume V0 of the recesses 22) is 40% or more for the recesses 22 arranged on the first surface 101a of the first light diffusion sheet 101.

[0116] In the laminated light diffusion sheet 100 of this embodiment, the recesses 22 are formed in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid, and the thickness of the first light diffusion sheet 101 is 30 μm or more and 1000 μm or less. The portion of the first light diffusion sheet 101 where the recesses 22 are provided and the other portions thereof may be integrally molded from the same material. Forming the recesses 22 in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid improves the cutting accuracy of the mold (metal roll) used in the manufacturing process. When the thickness of the first light diffusion sheet 101 is 30 μm or more, improved luminance uniformity is easily achieved. However, when the thickness of the first light diffusion sheet 101 is less than 30 μm, the recesses 22 are likely to penetrate the sheet, making stable processing difficult. When the thickness of the first light diffusion sheet 101 is 1000 μm or less, the backlight unit 40, i.e., the liquid crystal display device 50, can be made thinner. In general, an optical sheet having a thickness of more than 500 μm has sufficient rigidity even when used alone, making it easy to handle during assembly of a backlight unit without having to be laminated with other optical sheets. If the portion of first light diffusing sheet 101 where recesses 22 are provided (recessed portion) and the other portion (substrate portion) are integrally molded from the same material, peeling of the recessed portion from the substrate portion can be prevented during reliability tests, etc.

[0117] For example, in a light diffusion sheet in which a UV-curable resin is shaped and laminated onto a substrate film to form a concave portion, the concave portion may peel off from the substrate film during reliability tests, etc. On the other hand, a light diffusion sheet in which the substrate portion and the concave portion are integrally molded, such as an extrusion molded product or a compression molded product, can prevent the concave portion from peeling off from the substrate portion during reliability tests, etc. Furthermore, in general, in an optical sheet bonded to another optical sheet, stress tends to concentrate at the bonding points due to the adhesive effect associated with the bonding. However, if the substrate portion and the concave portion are not integrally molded, there is also the problem that the substrate portion and the concave portion are more likely to peel off due to the difference in the linear expansion coefficients of each portion.

[0118] In the laminated light diffusing sheet 100 of this embodiment, when the recesses 22 of the first light diffusing sheet 101 are formed in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid, a plurality of recesses 22 formed in the shape of an approximately inverted square pyramid or an approximately inverted square truncated pyramid may be arranged on the first surface 102a of the second light diffusing sheet 102, the second surface 102b of the second light diffusing sheet 102 may be a flat or matte surface, and the first surface 101a of the first light diffusing sheet 101 and the second surface 102b of the second light diffusing sheet 102 may be bonded together. In this way, a laminated light diffusing sheet 100 that can exhibit an excellent effect of improving brightness uniformity can be obtained relatively easily.

[0119] In the laminated light-diffusing sheet 100 of this embodiment, when Va / V0 is 50% or more, the deterioration of brightness and brightness uniformity caused by lamination can be further suppressed.

[0120] In the laminated light diffusion sheet 100 of this embodiment, the first light diffusion sheet 101 and the second light diffusion sheet 102 may be bonded together using an OCA (optical transparent adhesive), which simplifies the bonding process.

[0121] In the laminated light diffusion sheet 100 of this embodiment, the first light diffusion sheet 101 and the second light diffusion sheet 102 may be bonded together using ink containing an ultraviolet-curable resin. In this way, the thickness of the adhesive layer bonding the first light diffusion sheet 101 and the second light diffusion sheet 102 together can be changed depending on the type of ink. However, compared to bonding using OCA, this method requires an additional step of curing the ink by irradiating it with ultraviolet (UV) light or the like.

[0122] In the laminated light diffusion sheet 100 of this embodiment, the first light diffusion sheet 101 and the second light diffusion sheet 102 may be bonded together by laser welding using an infrared absorbing agent. In this way, sufficient peel strength can be obtained even when only the four corners of the sheets are bonded together. However, compared to bonding by OCA, this method requires an additional step of welding by irradiating an infrared laser.

[0123] The backlight unit 40 of this embodiment is incorporated into a liquid crystal display device 50, guides light emitted from a light source 41 toward a display screen 50a, and includes the laminated light diffusion sheet 100 of this embodiment described above between the display screen 50a and the light source 41. Therefore, when assembling the backlight unit 40 using a plurality of light diffusion sheets 101 and 102, it is possible to improve the handleability of the light diffusion sheets 101 and 102 while suppressing a decrease in brightness and brightness uniformity.

[0124] The backlight unit 40 of this embodiment may further include a color conversion sheet 44 between the display screen 50a and the light source 41, which converts the wavelength of light emitted from the light source 41. In this way, there is no need to use an expensive white light source as the light source 41, thereby reducing the cost of the backlight unit 40. The position of the color conversion sheet 44 is not particularly limited as long as it is between the display screen 50a and the light source 41. For example, the color conversion sheet 44 may be placed between the light source 41 and the laminated light diffusing sheet 100, or between the display screen 50a and the laminated light diffusing sheet 100.

[0125] In the backlight unit 40 of this embodiment, the content of the light diffusing agent in the first light diffusing sheet 101 is not particularly limited as long as the light diffusing effect due to reflection and refraction at the recesses 22 formed in the shape of an approximately inverted polygonal pyramid or an approximately inverted polygonal truncated pyramid is not impaired. For example, the light diffusing agent may be contained in an amount of 0 to 10 parts by mass per 100 parts by mass of the matrix resin.

[0126] The liquid crystal display device 50 of this embodiment includes the backlight unit 40 of this embodiment described above and a liquid crystal display panel 5. Therefore, when assembling the backlight unit 40 using a plurality of light diffusion sheets 101 and 102, it is possible to suppress a decrease in brightness and brightness uniformity while improving the handleability of the light diffusion sheets 101 and 102. The same effect can be obtained in information equipment including the liquid crystal display device 50 of this embodiment.

[0127] In addition, it is preferable that the concave-shaped portion and the base material portion of the first light diffusion sheet 101 are molded from the same material so that interfacial peeling does not occur between the concave-shaped portion and the base material portion during reliability tests, etc., and it is particularly preferable that the concave-shaped portion and the base material portion are molded integrally by a method such as extrusion molding or compression molding.

[0128] In the first light diffusion sheet 101, the arrangement pitch of the recesses 22 is preferably 30 μm or more and 1000 μm or less, and the width of the boundary between adjacent recesses 22 is preferably 25% or less of the arrangement pitch. Specifically, to reduce the area ratio of the boundary, the width of the boundary is set to 25% or less, preferably 20% or less, and more preferably 15% or less of the arrangement pitch. However, to avoid a decrease in abrasion resistance, the width of the boundary is set to 0.5% or more, preferably 1.0% or more of the arrangement pitch. In the present disclosure, the "boundary between recesses 22" means "the width of the flat portion intentionally arranged between the recesses 22" when the recesses 22 are arranged with a gap between them, and means "the width of the curved portion at the top of the ridge line 23 separating the recesses 22" when the recesses 22 are arranged without gaps.

[0129] In the first light diffusion sheet 101, when the arrangement pitch is 1000 μm or less, an increase in the thickness of the first light diffusion sheet 101 can be suppressed, and the backlight unit 40 can be made thinner.

[0130] In the first light diffusing sheet 101, the width of the boundary portion is the width of the curved portion at the apex of the ridge line 23 that separates the recesses 22. In other words, when the recesses 22 are arranged without any gaps, brightness uniformity can be improved compared to when the recesses 22 are arranged with spaces between them. In this case, the recesses 22 are formed in a substantially inverted square pyramid or a substantially inverted square pyramid truncated shape, the ridge line 23 extends in the first direction and the second direction, the arrangement pitch is the average value of the first arrangement pitch of the recesses 22 in the first direction and the second arrangement pitch of the recesses 22 in the second direction, and the width of the boundary portion may be the average value of the width occupied by the curved portion at the apex of the ridge line 23 in the first direction and the width occupied by the curved portion at the apex of the ridge line 23 in the second direction. This makes it easy to form the recesses 22.

[0131] In the first light diffusion sheet 101, if the angle formed by the wall surface of the recess 22 and the sheet surface of the first light diffusion sheet 101 is 40 degrees or more and 65 degrees or less, the recess 22 can achieve a sufficient effect of improving brightness uniformity.

[0132] In the first light diffusion sheet 101, the recesses 22 are provided only on the first surface 101a, and the second surface 101b of the first light diffusion sheet 101 is a flat or matte surface, so that the effect of improving brightness uniformity can be obtained while suppressing wear and damage on the second surface 101b.

[0133] In the first light diffusion sheet 101, the ridge lines 23 (opening edges of the recesses 22) that define the recesses 22 are prone to wear and damage. However, the recessed shape of the ridge lines 23 between their intersections 23a reduces wear and damage even when the sheet is used in conjunction with other optical sheets or other light diffusion sheets. Furthermore, the dimension Wr of the curved portion of the apex of the ridge lines 23 in the arrangement direction of the recesses 22 is limited to 25% or less of the arrangement pitch P of the recesses. This allows the apex of the ridge lines 23 to maintain a steep shape, so that even if the ridge lines 23 are recessed between the intersections 23a, brightness uniformity is less likely to decrease. Furthermore, setting the maximum height difference d between the ridge lines 23 and the straight line connecting the intersections 23a to 1 μm or more improves scratch resistance, while setting the maximum height difference d to 30 μm or less reduces brightness uniformity. In particular, when the maximum height difference d is 1.5 μm or more and 20 μm or less, both the scratch resistance and the brightness uniformity can be further improved, and when the maximum height difference d is 2.5 μm or more and 10 μm or less, both the scratch resistance and the brightness uniformity can be further improved.

[0134] Examples Hereinafter, examples will be described together with comparative examples and reference examples.

[0135] <Light diffusion sheet used> The configuration and optical characteristics of a sample of a light diffusion sheet having inverted pyramidal recesses formed therein (hereinafter referred to as "light diffusion sheet #1") used in the examples, comparative examples, and reference examples are shown in Table 1. 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 between the recesses."

[0136]

[0137] The light diffusion sheet #1 used in the Examples, Comparative Examples, and Reference Examples was prepared as follows. First, an aromatic polycarbonate resin having a melt mass flow rate of 15 g / 10 min measured in accordance with ISO 1133 was fed into an extruder, melt-kneaded, and then extruded from a T-die. Thereafter, one of the two metal rolls was used as a casting roll, with a surface having the shape shown in FIGS. 16A and 16B ((B) is a cross-sectional view of (A) taken along the X-Y line) (a regular square pyramid shape with a height of 50 μm, a square base with a side length (pitch) of 100 μm, and a 90-degree apex angle). The other roll was used as a pressing roll, with a random matte surface (surface roughness Ra = 2.1 μm). The molten resin extruded from the T-die was sandwiched between the two rolls and cooled while the shape was transferred. As a result, a single-layer light diffusion sheet #1 having a thickness of 90 μm was produced by extrusion molding.

[0138] As shown in Table 1, the light diffusion sheet #1 is a single-layer polycarbonate sheet that does not contain a diffusing agent and has a pyramidal depression (inverted square pyramid) on one surface (first side) with a height (depth) H of 43 μm, determined depending on the height of the regular square pyramid on the roll, and the other surface (second side) is a matte surface with a surface roughness Ra = 1.4 μm. In addition, the pitch P (the length of one side of the base of the square pyramid), the apex angle, and the ridge width Wr and Wr / P of the inverted square pyramid are 100 μm, 90 degrees, 16 μm, and 16%, respectively, and the light transmittance, light reflectance, and haze at a wavelength of 450 nm are 94%, 12%, and 94%, respectively.

[0139] <Measurement of Recess Arrangement Pitch, Apex Angle, Ridge Line Width, and Inverted Square Pyramid Height> The shapes of the recesses (inverted square pyramids) formed on the surface of light diffusion sheet #1 shown in Table 1 were measured using a laser microscope. Specifically, 50 mm square test pieces were cut out from light diffusion sheet #1 obtained by extrusion molding, and the arrangement pitches Px and Py (the horizontal distances between intersections 23a in the X and Y directions) of the recesses 22 shown in Figures 7 and 8 were measured, and the arrangement pitch P was calculated as the average value. In addition, angles θx and θy (the angles between the wall surfaces of the recesses 22 (the slopes of the inverted square pyramids) and the sheet surface in the X and Y directions) shown in Figures 10 and 11 were measured, and the apex angle (apex angle = 180 degrees - (θx + θy)) was calculated based on the average value. In addition, dimensions Wrx and Wry (the dimensions occupied by the curved portions of the apexes of the ridge lines 23 in the X and Y directions) shown in Figures 10 and 11 were measured, and the ridge line width Wr was calculated as the average value. The ratio Wr / P of the width Wr to the arrangement pitch P is expressed in %, and the heights Hx and Hy (heights from the center 22a of the recess 22 to the apex of the ridge 23) shown in Figures 10 and 11 were measured, and the height H of the inverted pyramid was calculated as the average value.

[0140] <Calculation of V0 (Volume of Inverted Square Pyramidal Recesses)> V0 (Volume of Inverted Square Pyramidal Recesses) can be calculated from the shape measurement results of the recesses described above, specifically, the height H of the inverted square pyramid and the arrangement pitch P of the recesses. Specifically, while observing the inverted square pyramidal (pyramidal) recesses with a microscope, the recesses are cut parallel to one of the ridges of the recesses and passing through the bottoms of the recesses (the apexes of the pyramids), and H and P are measured for the cross section, thereby enabling more accurate calculation of V0. Furthermore, for a laminated light diffusion sheet, two bonded light diffusion sheets are peeled off to separate the individual light diffusion sheets, and then the bonded surfaces of the sheets are observed, and H and P are measured for the pyramidal shapes of the recesses, thereby enabling more accurate calculation of V0.

[0141] <Measurement of Surface Roughness (Ra) of Light Diffusion Sheet> The surface roughness of the matte surface of the light diffusion sheet #1 shown in Table 1 was measured using a test piece cut into a 50 mm square using an SJ-210 manufactured by Mitutoyo Corporation in accordance with JIS B 0601.

[0142] <Measurement of Optical Properties> The light transmittance and light reflectance at a wavelength of 450 nm of the light diffusion sheet #1 shown in Table 1 were measured using a test piece cut into a 50 mm square with a V-670 manufactured by JASCO Corporation, and the haze was measured in accordance with JIS K 7361: 2000 with an HZ-2 manufactured by Suga Test Instruments Co., Ltd. The optical properties were measured using the surface having the recessed portion formed into an inverted square pyramid as the light incident surface.

[0143] <Preparation of Laminated Light Diffusing Sheet and Measurement of Va / V0 (Spatial Volume Ratio)> Two light diffusing sheets #1 cut to a size of 300 mm long x 200 mm wide were laminated together using the method described below to prepare laminated light diffusing sheets for each of the Examples, Comparative Examples, and Reference Examples. The spatial volume ratio (Va / V0) at the bonding surface of the two light diffusing sheets #1 constituting the laminated light diffusing sheet was measured as follows. V0 (the volume of the inverted square pyramid-shaped recesses) can be calculated from the shape measurement results of the recesses described above, specifically, the height H of the inverted square pyramids and the arrangement pitch P of the recesses. Va is the volume of the voids (spaces free of sheet substrates and adhesive) remaining within the recesses in the laminated light diffusing sheet after lamination. The spatial volume ratio (Va / V0) may be calculated as an average value for a randomly selected plurality of recesses.

[0144] In Example 1, two light diffusion sheets #1 were bonded together using an OCA, MHM-FWD25 manufactured by Nichiei Shinka Co., Ltd. The bonding method involved first bonding a 25 μm-thick OCA to the matte surface of the first light diffusion sheet #1, and then bonding the second light diffusion sheet #1 by bringing the surface with the inverted pyramidal recesses into contact with the surface of the first light diffusion sheet #1 to which the OCA was bonded.

[0145] In this case, as shown in Figure 17, the OCA layer (adhesive member 111) is approximately horizontal with respect to the sheet surface of the second light diffusion sheet #1 (first light diffusion sheet 101), so the shape of the voids 121 remaining in the recesses (recesses 22) also resembles a regular square pyramid. That is, the shape of the voids 121 is approximately similar to the inverted square pyramid shape of the recesses before lamination. Therefore, after laminating the two light diffusion sheets #1, the laminated light diffusion sheet is cut with a microtome along two planes (i.e., the X cross section and the Y cross section perpendicular to the X direction and the Y direction shown in Figure 6) that pass through the center (deepest part) of the recesses of the inverted square pyramid shape and are perpendicular to the ridge line of the inverted square pyramid. The shapes of the X cross section and the Y cross section are observed with a laser microscope. The heights hax and hay of the voids 121 in the recesses where there is no OCA layer in the X cross section and the Y cross section are measured, and the average value ha of hax and hay is calculated. Next, using the formula for calculating the volume of a square pyramid, the spatial volume ratio Va / V0 (%) = (ha / H) 3 17 and 18, the same elements as those in the laminated light diffusing sheet 100 of the embodiment shown in FIGS. 2, 4, etc. are denoted by the same reference numerals.

[0146] In Examples 2 to 4 and Comparative Example 1, two light diffusion sheets #1 were bonded together using ink containing a UV-curable resin. A UV-curable acrylic urethane-based light-transmitting ink was used as the ink containing the UV-curable resin, and screen printing was used for printing. After applying the ink containing the UV-curable resin, the two light diffusion sheets #1 were sandwiched and pressed together with rubber rolls while the bonding surfaces of the two sheets were overlapped, and then the ink was cured by ultraviolet light irradiation.

[0147] Specifically, in Examples 2 and 3, ink containing a UV-curable resin was printed on the entire matte surface of a first light diffusion sheet #1, with a thickness of 10 μm in Example 2 and 20 μm in Example 3. Thereafter, the surface of a second light diffusion sheet #1 having inverted square pyramidal recesses was bonded to the ink-printed surface of the first light diffusion sheet #1. In Comparative Example 1, ink containing a UV-curable resin was printed on the entire surface of the surface of a second light diffusion sheet #1 having inverted square pyramidal recesses so that the recesses were almost 100% filled. Thereafter, the matte surface of the first light diffusion sheet #1 was bonded to the ink-printed surface (the surface having the inverted square pyramidal recesses) of the second light diffusion sheet #1. In Example 4, a first light diffusion sheet #1 cut to a size of 300 mm x 200 mm was printed with ink containing the aforementioned UV-curable resin at a thickness of 10 μm along the four edges of the matte surface in a range of 1.7 mm from the four edges toward the center of the sheet, with a width of 1.7 mm. Thereafter, the surface of the second light diffusion sheet #1 having the inverted pyramidal recesses was bonded to the ink-printed surface of the first light diffusion sheet #1. In Examples 2 to 4 and Comparative Example 1, after the two light diffusion sheets #1 were bonded together, ultraviolet light was irradiated to cure the ink containing the UV-curable resin, and then the optical properties and peel strength measurements described below were performed.

[0148] In Examples 2 and 3, as shown in FIG. 19 , a phenomenon was observed in which the UV-curable resin (adhesive member 111) was adsorbed near the ridge line (ridge line 23) of the inverted square pyramid-shaped recess (recess 22). This adsorption phenomenon is thought to be caused by surface tension. As a result, the shape of the void 121 remaining in the recess 22 was different from that of a regular square pyramid. Therefore, as described below, the void 121 was divided into shape parts whose volume could be calculated, and the volume of the void 121 was calculated from the sum of the volumes of the shape parts. That is, after two light diffusion sheets #1 were laminated together, the laminated light diffusion sheet was cut with a microtome along two planes (i.e., the X and Y cross sections perpendicular to the X and Y directions shown in FIG. 6) that passed through the center (deepest part) of the recesses in the shape of an inverted square pyramid and were perpendicular to the ridges of the inverted square pyramids. The shapes of the X and Y cross sections were observed with a laser microscope. The heights ha1x and ha1y of the height range of the recesses not filled with UV-curable resin were measured in the X and Y cross sections, and the average value ha1 of ha1x and ha1y was calculated. This gave the volume A (triangular cross section) of the square pyramid portion of height ha1. Furthermore, the heights ha2x and ha1y and widths (the base lengths of the square pyramid with height ha1) wx and wy of the height range of the recesses partially filled with UV-curable resin in the X and Y cross sections were measured, and the average value ha2 of ha2x and ha2y was calculated. This gave a volume B (square cross section) of the rectangular parallelepiped portion with height ha2 and widths wx and wy. Next, the spatial volume ratio Va / V0 (%) = (volume A + volume B) / V0 × 100 was calculated. Figure 20 is a photograph showing the cross-sectional structure of the laminated light diffusion sheet of Example 2. In Figures 19 and 20, the same elements as those in the laminated light diffusion sheet 100 of the embodiment shown in Figures 2 and 4 are denoted by the same reference numerals.

[0149] In Comparative Example 1, Va = 0, so the spatial volume ratio Va / V0(%) = 0(%), and in Example 4, no printing ink is present in the active area (area where optical properties are measured) excluding the peripheral edge of the sheet, so Va = V0, and therefore Va / V0(%) = 100(%).

[0150] In Example 5, two light diffusion sheets #1 cut to dimensions of 300 mm x 200 mm were bonded together at their four corners using an infrared absorbing ink prepared by mixing 75 mass % of a UV curable resin (INKJET-CLC-K-01) manufactured by Natoco Corporation and 25 mass % of a solar radiation shielding dispersion (YMW-D20) manufactured by Sumitomo Metal Mining Co., Ltd. Heat welding was performed by irradiating an infrared laser using a semiconductor laser (L13920-511(M)) manufactured by Hamamatsu Photonics KK

[0151] Specifically, the above-mentioned infrared-absorbing ink was printed in a grid pattern on the four corners (10 mm square portions) of the matte surface of the first light diffusion sheet #1. The dots were circular dots with a diameter of 50 μm, and the center-to-center distance between two adjacent dots was 100 μm. Then, the second light diffusion sheet #1 was placed over the first light diffusion sheet #1 so that the surface with the inverted pyramidal recesses was in contact with the ink-printed surface of the first light diffusion sheet #1. While the two light diffusion sheets #1 were held down by glass plates from above and below, an infrared laser (output 50 W, spot diameter 1.6 mm) was irradiated for 500 milliseconds to bond the two light diffusion sheets #1 together.

[0152] In Example 5, there is no printing ink in the active area (area where optical properties are measured) excluding the peripheral edge of the sheet, so Va=V0, and therefore Va / V0(%)=100(%).

[0153] In Reference Example 1, the two light diffusion sheets #1 were not bonded together, but were stacked so that the matte surface of the first light diffusion sheet #1 was in contact with the surface of the second light diffusion sheet #1 having the inverted pyramidal recesses. Therefore, in Reference Example 1, Va = V0, and therefore Va / V0 (%) = 100 (%).

[0154] In Examples 6 to 10, as in Example 1, two light diffusion sheets #1 were bonded together using OCA. In detail, in Example 6, a 50 μm thick OCA was attached to the matte surface of the first light diffusion sheet #1 using ARONTACK MF-29 manufactured by Toagosei Co., Ltd., in Example 7, a 25 μm thick OCA was attached to the matte surface of the first light diffusion sheet #1 using ARONTACK MF-29 manufactured by Toagosei Co., Ltd., in Example 8, a 50 μm thick OCA was attached to the matte surface of the first light diffusion sheet #1 using ARONTACK MF-25 manufactured by Toagosei Co., Ltd., in Example 9, a 25 μm thick OCA was attached to the matte surface of the first light diffusion sheet #1 using ARONTACK MF-25 manufactured by Toagosei Co., Ltd., and in Example 10, a 50 μm thick OCA was attached to the matte surface of the first light diffusion sheet #1 using MHM-FWD50 manufactured by Nichiei Shinka Co., Ltd. In Examples 6 to 9, two light diffusion sheets #1 were attached together while being heated to a temperature of 60°C.

[0155] In Examples 11 to 13, two light diffusion sheets #1 were bonded together using OCA, as in Example 7. Specifically, a 25 μm-thick OCA was bonded to the matte surface of the first light diffusion sheet #1 using Arontack MF-29 manufactured by Toagosei Co., Ltd. Next, the two light diffusion sheets #1 were bonded together at room temperature of 23° C. without heating in Example 11, while being heated to 30° C. in Example 12, and while being heated to 40° C. in Example 13.

[0156] <Peel Strength Measurement> The peel strength of the bonded portion of the two light diffusion sheets #1 constituting the laminated light diffusion sheet (Examples 1 to 13, Comparative Example 1) after bonding was measured as follows. A test piece 25 mm wide and 150 mm long was cut from each laminated light diffusion sheet sample, and the two light diffusion sheets #1 were peeled away from each other in a 180-degree direction at a peel speed of 1000 mm / min using an IMADA peel tester (IPTS-5N). In Example 4, as described above, a UV-curable resin-containing ink was printed in a 1.7 mm line along each of the four edges of the first light diffusion sheet #1, and the sheets were bonded together. A 25 mm wide test piece was cut out in a direction perpendicular to the ink line, and the peel strength was measured. In Example 5, a 25 mm wide test piece was cut out so as to include the entire spot portion welded with an infrared laser, and the peel strength was measured.

[0157] <Measurement of Peel Strength at 85°C> The peel strength at 85°C of the bonded portion of the two light diffusion sheets #1 constituting the laminated light diffusion sheet (Examples 6 to 13) after bonding was measured as follows: A test piece 25 mm wide and 100 mm long was cut from each laminated light diffusion sheet sample, and the test piece was held in an atmosphere at 85°C for 10 minutes. After that, the peel strength was measured by using a tensile tester (Autograph AGX-V) manufactured by Shimadzu Corporation while peeling the two light diffusion sheets #1 from each other in a 180-degree direction at a peel rate of 50 mm / min in an atmosphere at 85°C.

[0158] <Heat Cycle Test> A heat cycle test (reliability test) was performed on the laminated light diffusion sheets (Examples 6 to 13). In the heat cycle test, a sample of each laminated light diffusion sheet was placed inside a low-temperature thermostatic chamber (PL-1KP) manufactured by ESPEC Corporation, and then the sample was first cooled to -40°C, then held at -40°C for 1 hour, and then the temperature inside the low-temperature thermostatic chamber was raised to 85°C over 40 minutes, and then held at 85°C for 1 hour, and then the temperature inside the low-temperature thermostatic chamber was cooled to -40°C over 40 minutes, and then held at -40°C for 1 hour, and then the temperature inside the low-temperature thermostatic chamber was again raised to 85°C over 40 minutes. This cooling and heating heat cycle was repeated 96 times.

[0159] <Measurement of luminance and luminance uniformity> In the examples, reference examples, and comparative examples, the luminance and luminance uniformity were measured by using the backlight unit 40 configuration shown in Fig. 2, in which the laminated light diffusion sheet 100 formed by bonding two light diffusion sheets #1 was arranged so that the surfaces having the inverted quadrangular pyramid-shaped recesses 22 (first surfaces 101a and 102a) served as light-emitting surfaces (facing the luminance enhancing sheet 47). As the plurality of light sources 42, an LED array was used in which the light sources 42, which were LEDs, were arranged vertically and horizontally in a matrix of 20 light sources 42 by 20 light sources 42 by 20 light sources 42 by reflective sheet 41, as shown in Fig. 21. Specifically, an LED array was used in which blue LEDs (XPGDRY-L1-0000-00501) manufactured by Cree were arranged at a pitch of 3.5 mm x 4.5 mm to serve as light sources 42, and a wavelength selection sheet 43, a color conversion sheet 44, a sample to be evaluated (the laminated light diffusion sheet 100), and a brightness enhancement sheet 47 (two prism sheets 45 and 46 with prism extension directions perpendicular to each other) were placed on top of the arrayed light sources (LEDs) 42, and the brightness and brightness uniformity were measured.

[0160] In measuring the luminance uniformity, first, using the LED array shown in FIG. 21, two-dimensional luminance distribution was measured on the surface of the luminance enhancement sheet 47, the top layer in the sheet stacking configuration of the backlight unit 40 described above, using an SR-5000WS manufactured by Topcon Technohouse Corporation. Then, the average and standard deviation were calculated for the measured luminance values ​​of all 65,536 pixels (256 points x 256 points) within an area R of 12 vertical x 9 horizontal light sources 42 (LEDs) in the LED array shown in FIG. 21. This average value was used to evaluate the luminance, and the luminance uniformity was calculated using the average and standard deviation of the luminance as follows: luminance uniformity = (average luminance (cd / m 2 )) ÷ (Standard deviation of luminance (cd / m 2 The higher the luminance uniformity value calculated in this way, the more uniform the luminance.

[0161] <Evaluation of luminance and luminance uniformity and overall evaluation> The luminance and luminance uniformity of the Examples, Comparative Examples, and Reference Examples were evaluated using the relative luminance (%) and relative luminance uniformity (%) according to the following criteria: The relative luminance (%) and relative luminance uniformity (%) were calculated as relative values, with the value of Reference Example 1 being set to 100%, by dividing the values ​​of luminance and luminance uniformity measured in each Example and Comparative Example by the values ​​of luminance and luminance uniformity measured in Reference Example 1 (two light diffusion sheets #1 stacked without being bonded together).

[0162] Excellent: Relative value is 90% or more (excellent laminated light diffusion sheet) Good: Relative value is 80% or more but less than 90% (practical laminated light diffusion sheet) Poor: Relative value is less than 80% (laminated light diffusion sheet with significant degradation in optical performance due to lamination).

[0163] The overall evaluation of the Examples, Comparative Examples, and Reference Examples was carried out according to the following criteria.

[0164] Excellent: Peel strength is 5g weight / 25mm or more, and both brightness and brightness uniformity are excellent. Good: Peel strength is 5g weight / 25mm or more, and both brightness and brightness uniformity are good, or one is good and the other is excellent. Poor: Peel strength is less than 5g weight / 25mm, or at least one of brightness and brightness uniformity is poor.

[0165] <Evaluation of Examples 1 to 5, Comparative Example 1, and Reference Example 1> In Examples 1 to 5, Comparative Example 1, and Reference Example 1, as described above, laminated light diffusion sheets were constructed using two light diffusion sheets #1 shown in Table 1, which were cut into 200 mm x 300 mm rectangles, and the relationship between the spatial volume ratio (Va / V0) and optical properties (measured values ​​of luminance and luminance uniformity, etc.) was evaluated. The results are shown in Table 2, along with the dimensions, physical properties, lamination method, peel strength, and other characteristics of each laminated light diffusion sheet.

[0166]

[0167] As shown in Table 2, in Examples 1 to 5, the laminated light diffusion sheet was made by laminating two light diffusion sheets #1, each having inverted pyramidal recesses arranged at a pitch of 100 μm. The sheets were laminated so that the spatial volume ratio (Va / V0) was 40% or more, which maintained the effectiveness of the voids within the recesses. As a result, brightness was not impaired and brightness uniformity was good. In particular, when the spatial volume ratio (Va / V0) was 50% or more (preferably 55% or more), excellent brightness and brightness uniformity were obtained.

[0168] On the other hand, in Comparative Example 1, the inverted pyramidal recesses are filled with ink having a higher refractive index than air so that the spatial volume ratio (Va / V0) is less than 40%, and therefore the light diffusion effect of the inverted pyramidal recesses is lost, resulting in a laminated light diffusion sheet with low brightness uniformity.

[0169] As can be seen from the evaluation results of Reference Example 1, when the two light diffusion sheets #1 are not bonded together, the spatial volume ratio (Va / V0) is 100%, and there is no adverse effect on the light diffusion effect of the inverted pyramidal recesses, so good brightness and brightness uniformity are obtained. However, since the two light diffusion sheets #1 are not bonded together (i.e., the peel strength is 0), handling during assembly of the backlight unit becomes difficult.

[0170] <Evaluation of Examples 6 to 10> For Examples 6 to 10, the relationship between the spatial volume ratio (Va / V0) and optical properties (measured values ​​of luminance and luminance uniformity, etc.) was evaluated in the same manner as in Examples 1 to 5. Furthermore, as additional evaluations, measurements of peel strength at 85°C and heat cycle tests were performed. The results are shown in Table 3.

[0171]

[0172] As shown in Table 3, in Examples 6 to 10, in the laminated light diffusion sheet formed by bonding two light diffusion sheets #1, each having an inverted pyramidal recess arranged at a pitch of 100 μm, the sheets were bonded together so that the spatial volume ratio (Va / V0) was 40% or more. As a result, the effectiveness of the voids within the recesses was maintained, and as a result, brightness was not impaired and brightness uniformity was good.

[0173] Furthermore, in Examples 6 to 9, the peel strength at 85°C was high at 100 g weight / 25 mm or more, and when the appearance of the laminated light-diffusing sheet was checked before and after the heat cycle test, no bubbles were observed even after the heat cycle test at -40°C to 85°C, indicating that the laminated light-diffusing sheet had good heat resistance. On the other hand, in Example 10, the peel strength at 85°C was 4 g weight / 25 mm, and bubbles were observed to have appeared in the 11th cycle of the heat cycle test, and the bubbles had expanded significantly by the time 96 cycles were completed.

[0174] <Evaluation of Examples 11 to 13> For Examples 11 to 13, the relationship between the spatial volume ratio (Va / V0) and optical properties (measured values ​​of luminance and luminance uniformity, etc.) was evaluated in the same manner as in Examples 6 to 10. Furthermore, as additional evaluations, measurements of peel strength at 85°C and heat cycle tests were performed. The results are shown in Table 4.

[0175]

[0176] As shown in Table 4, in Examples 11 to 13, in the laminated light diffusion sheet formed by bonding two light diffusion sheets #1, each having an inverted pyramidal-shaped depression arranged at a pitch of 100 μm, the sheets were bonded together so that the spatial volume ratio (Va / V0) was 40% or more. As a result, the effectiveness of the voids within the depressions was maintained, and as a result, brightness was not impaired and brightness uniformity was good.

[0177] Furthermore, as shown in Table 4, in Examples 11 to 13, even when the peel strength at 85° C. was 10 g weight / 25 mm, 20 g weight / 25 mm, or 40 g weight / 25 mm, respectively, no bubbles were observed after the heat cycle test at −40° C. to 85° C. Examples 11 to 13 show that when the peel strength at 85° C. is 10 g weight / 25 mm or more, a laminated light-diffusing sheet can be obtained that does not impair brightness, has good brightness uniformity, and has sufficient heat resistance.

[0178] (Other Embodiments) Although embodiments of the present disclosure (including modifications and examples; the same applies hereinafter) have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the disclosure. In other words, the descriptions of the above-described embodiments are merely exemplary in nature and are not intended to limit the present disclosure, its applications, or its uses.

[0179] For example, the configuration (layer structure, materials, etc.) of the laminated light diffusion sheet is not limited to the configuration of the laminated light diffusion sheet 100 of the above-described embodiment, as long as it includes a first light diffusion sheet having a first surface on which a plurality of recesses formed in the shape of an approximately inverted polygonal pyramid or an approximately inverted polygonal pyramid trapezoid are arranged, and a second light diffusion sheet bonded to the first surface of the first light diffusion sheet, and the peel strength at the bonded location is 5.0 g weight / 25 mm or more, and the spatial volume ratio (Va / V0) is 40% or more.

[0180] Furthermore, the configuration of a backlight to which a laminated light diffusion sheet is applied and a liquid crystal display device equipped with such a backlight are not limited to the configuration of the backlight unit 40 or liquid crystal display device 50 of the above-described embodiment, as long as they include a laminated light diffusion sheet that includes "a first light diffusion sheet having a plurality of recesses formed in the shape of an approximately inverted polygonal pyramid or an approximately inverted polygonal truncated pyramid arranged on its first surface, and a second light diffusion sheet bonded to the first surface of the first light diffusion sheet, and the peel strength of the bonded portion is 5.0 g weight / 25 mm or more and the spatial volume ratio (Va / V0) is 40% or more."

[0181] In addition, in the above-described embodiment, when the first light diffusion sheet 101 and the second light diffusion sheet 102 are bonded together using an OCA (optically transparent adhesive), an OCA film is attached to the bonding surfaces, but instead, OCA may be applied to the bonding surfaces.

[0182] The present inventors have discovered a phenomenon in which, when a light diffusion sheet having a plurality of substantially inverted polygonal pyramidal recesses arranged therein is laminated to another light diffusion sheet, the proportion of voids remaining in each recess (spatial volume ratio Va / V0) varies, resulting in mottled patterns appearing on the display screen of a liquid crystal display device, resulting in reduced brightness and brightness uniformity. Furthermore, the present inventors have discovered that, when a light diffusion sheet having a plurality of substantially inverted polygonal pyramidal recesses arranged therein is laminated to another light diffusion sheet, the occurrence of mottled patterns can be suppressed if the other light diffusion sheet also has a plurality of other substantially inverted polygonal pyramidal recesses arranged in a direction different from the direction of arrangement of the other recesses.

[0183] 22 shows an example of the arrangement direction of the recesses of the first light diffusion sheet 101 and the second light diffusion sheet 102 constituting the laminated light diffusion sheet 100 in the backlight unit 40 shown in FIG. 2. FIG. 22 shows a portion of each of the first light diffusion sheet 101 and the second light diffusion sheet 102. As shown in FIG. 22, the arrangement direction D1 of the inverted pyramid-shaped recesses 22 of the first light diffusion sheet 101 (i.e., the direction D1 in which the ridge lines 23 defining the recesses 22 of the first light diffusion sheet 101 extend) is different from the arrangement direction D2 of the inverted pyramid-shaped recesses 22 of the second light diffusion sheet 102 (i.e., the direction D2 in which the ridge lines 23 defining the recesses 22 of the second light diffusion sheet 102 extend). The ridge lines 23 defining the recesses 22 of the first light diffusion sheet 101 extend in the X direction and the Y direction, which are perpendicular to each other, and the arrangement direction D1 is the X direction.

[0184] In the actual manufacturing process, a sheet base material that will become the first light diffusion sheet 101 and a sheet base material that will become the second light diffusion sheet 102 are bonded together so that the arrangement directions of the recesses 22 of each sheet are different, and then the laminated light diffusion sheet 100 is formed by cutting it to a size suitable for the backlight unit 40. Therefore, the edges of the first light diffusion sheet 101 and the second light diffusion sheet 102 that make up the laminated light diffusion sheet 100 are aligned.

[0185] Table 5 shows the results of visually checking whether or not a mottled pattern was present when two light diffusion sheets #1 in Example 1 were bonded together while changing the angle formed by the arrangement directions of the recesses of each sheet.

[0186]

[0187] As shown in Table 5, it was found that when the arrangement direction D1 of the recesses 22 of the first light diffusion sheet 101 is different from the arrangement direction D2 of the recesses 22 of the second light diffusion sheet 102, it is possible to suppress the occurrence of mottled patterns and the reduction in brightness and brightness uniformity, even if there is variation in the spatial volume ratio Va / V0 of each recess 22 of the first light diffusion sheet 101. Specifically, it was found that, in order to suppress the reduction in brightness and brightness uniformity, the difference between the arrangement direction D1 of the recesses 22 of the first light diffusion sheet 101 and the arrangement direction D2 of the other recesses 22 of the second light diffusion sheet 102 is preferably 5° to 85°, more preferably 8° to 82°, and even more preferably 15° to 75°. Furthermore, in a configuration in which the arrangement direction D1 of the recesses 22 of the first light diffusion sheet 101 is different from the arrangement direction D2 of the recesses 22 of the second light diffusion sheet 102, it was possible to obtain practically sufficient brightness and brightness uniformity even if the spatial volume ratio (Va / V0) of the recesses 22 of the first light diffusion sheet 101 was less than 40%.

[0188] The shapes of the recesses 22 of the first light diffusion sheet 101 and the recesses 22 of the second light diffusion sheet 102 are not limited to the substantially inverted square pyramid, but may be substantially inverted square pyramid truncated, substantially inverted triangular pyramid or substantially inverted triangular pyramid truncated, or substantially inverted hexagonal pyramid or substantially inverted hexagonal pyramid truncated, etc. In this way, it becomes easy to arrange the recesses 22 of the first light diffusion sheet 101 and the recesses 22 of the second light diffusion sheet 102 regularly in two dimensions.

[0189] REFERENCE SIGNS LIST 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 layer 22 Recess 22a Center 23 Ridge line 23a Intersection 23b Lowest point 40 Backlight unit 41 Reflective sheet 42 Light source 43 Wavelength selection sheet 44 Color conversion sheet 45 First prism sheet 46 Second prism sheet 47 Brightness enhancement sheet 50 Liquid crystal display device 50a Display screen 100 Laminated light diffusion sheet 101 First light diffusion sheet 101a First surface 101b Second surface 102 Second light diffusion sheet 102a First surface 102b Second surface 103 Third light diffusion sheet 103a First surface 103b Second surface 111 Adhesive member 112 Adhesive member 121 void

Claims

1. A laminated light diffusion sheet comprising a first light diffusion sheet having a first surface on which a plurality of recesses each having an approximately inverted polygonal pyramid shape or an approximately inverted polygonal pyramid trapezoid shape are arranged, and a second light diffusion sheet bonded to the first surface of the first light diffusion sheet, wherein the peel strength of the bonding point between the first light diffusion sheet and the second light diffusion sheet is 5.0 g force / 25 mm or more, and where the volume of the recess is V0 and the volume of the void remaining in the recess is Va, Va / V0 is 40% or more and 100% or less in the region excluding the peripheral portion of the first surface of the first light diffusion sheet.

2. The laminated light-diffusing sheet according to claim 1, wherein the peel strength measured at 85° C. is 10 g / 25 mm or more.

3. The laminated light-diffusing sheet according to claim 1, wherein the peel strength measured at 85° C. is 100 g / 25 mm or more.

4. The laminated light-diffusing sheet according to claim 1, wherein the bonding area is the entire surface of the first surface of the first light-diffusing sheet, the peripheral portion, or a plurality of dot-like or linear areas.

5. A laminated light-diffusing sheet as described in claim 1, wherein the recess is formed in the shape of an approximately inverted quadrangular pyramid or an approximately inverted truncated quadrangular pyramid, the thickness of the first light-diffusing sheet is 30 μm or more and 1000 μm or less, and the portion of the first light-diffusing sheet in which the recess is provided and other portions are integrally molded from the same material.

6. A laminated light diffusion sheet as described in claim 5, wherein one surface of the second light diffusion sheet has a plurality of other recesses formed in the shape of an approximately inverted quadrangular pyramid or an approximately inverted truncated quadrangular pyramid arranged thereon, the other surface of the second light diffusion sheet is a flat or matte surface, and the first surface of the first light diffusion sheet and the other surface of the second light diffusion sheet are bonded together.

7. A laminated light-diffusing sheet as described in claim 1, wherein a plurality of other recesses each formed in the shape of an approximately inverted polygonal pyramid or an approximately inverted polygonal truncated pyramid are arranged on one surface of the second light-diffusing sheet, and the arrangement direction of the recesses is different from the arrangement direction of the other recesses.

8. The laminated light-diffusing sheet according to claim 7, wherein the recess and the other recess are formed in the shape of an approximately inverted square pyramid or an approximately inverted truncated square pyramid, an approximately inverted triangular pyramid or an approximately inverted truncated triangular pyramid, or an approximately inverted hexagonal pyramid or an approximately inverted truncated hexagonal pyramid.

9. The laminated light-diffusing sheet according to claim 7, wherein the arrangement direction of the recesses differs from the arrangement direction of the other recesses by 8° or more.

10. The laminated light-diffusing sheet according to claim 1, wherein Va / V0 is 50% or more.

11. The laminated light diffusion sheet according to claim 1, wherein the first light diffusion sheet and the second light diffusion sheet are bonded together with an OCA (optically transparent adhesive).

12. The laminated light diffusion sheet according to claim 1, wherein the first light diffusion sheet and the second light diffusion sheet are bonded together by ink containing an ultraviolet curable resin.

13. The laminated light-diffusing sheet according to claim 1, wherein the first light-diffusing sheet and the second light-diffusing sheet are bonded together by laser welding using an infrared absorbing agent.

14. A laminated light diffusion sheet comprising a first light diffusion sheet having a first surface on which a plurality of recesses each having an approximate inverted polygonal pyramid shape or an approximate inverted polygonal truncated pyramid shape are arranged, and a second light diffusion sheet bonded to the first surface of the first light diffusion sheet, wherein the peel strength of the bonding point between the first light diffusion sheet and the second light diffusion sheet is 5.0 g force / 25 mm or more, and a plurality of other recesses each having an approximate inverted polygonal pyramid shape or an approximate inverted polygonal truncated pyramid shape are arranged on one surface of the second light diffusion sheet, and the arrangement direction of the recesses is different from the arrangement direction of the other recesses.

15. The laminated light-diffusing sheet according to claim 14, wherein the recess and the other recess are formed in the shape of an approximately inverted square pyramid or an approximately inverted truncated square pyramid, an approximately inverted triangular pyramid or an approximately inverted truncated triangular pyramid, or an approximately inverted hexagonal pyramid or an approximately inverted truncated hexagonal pyramid.

16. The laminated light-diffusing sheet according to claim 14, wherein the arrangement direction of the recesses differs from the arrangement direction of the other recesses by 8° or more.

17. A backlight unit that is incorporated in a liquid crystal display device and directs light emitted from a light source toward a display screen, the backlight unit comprising a laminated light diffusing sheet according to any one of claims 1 to 16 between the display screen and the light source.

18. The backlight unit according to claim 17, further comprising a color conversion sheet between the display screen and the light source, the color conversion sheet converting the wavelength of light emitted from the light source.

19. The backlight unit according to claim 18, wherein the color conversion sheet is disposed between the light source and the laminated light diffusion sheet, or between the display screen and the laminated light diffusion sheet.

20. The backlight unit according to claim 17, wherein the first light diffusion sheet contains 0 parts by mass or more and 10 parts by mass or less of a light diffusion agent relative to 100 parts by mass of a matrix resin.

21. A liquid crystal display device comprising: the backlight unit according to claim 17; and a liquid crystal display panel.

22. An information device comprising the liquid crystal display device according to claim 21.

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