Optical sheet, backlight unit, liquid crystal display device, and information apparatus

Abstract

To provide an optical sheet which can suppress generation of display screen glare.SOLUTION: An optical sheet 5 has irregularities in a first surface. The glare contrast of the first surface measured in accordance with JIS C 1006:2019 is not larger than 4%. The arithmetic average roughness Ra of the first surface measured in accordance with JIS B 0601:2001 (the evaluation length is set at 290 μm) is not larger than 0.6 μm. The peak count RPc of the first surface measured in accordance with JIS B 0601:2001 (the evaluation length is set at 290 μm) is at least 16.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] In recent years, as a display device for various information devices such as smartphones and tablet terminals, a liquid crystal display device (hereinafter sometimes referred to as an LCD (liquid crystal display)) has been widely used. As a backlight unit (hereinafter sometimes referred to as a BLU) of the LCD, a direct type method in which a light source is disposed behind a liquid crystal panel or an edge light type method in which a light source is disposed near the side surface of the liquid crystal panel has become mainstream.

[0003] A conventional edge light type BLU 101 shown in FIG. 12 includes a light source 102, a rectangular plate-shaped light guide plate 103 disposed along an end portion of the light source 102, a plurality of optical sheets 104 disposed on the front surface side of the light guide plate 103, and a reflection sheet 105 disposed on the back surface side of the light guide plate 103. The light guide plate 103 is generally made of a synthetic resin, and polycarbonate, an acrylic resin, etc. are used as main components of the light guide plate 103. As the light source 102, an LED (light emitting diode), a cold cathode tube, etc. are used, but LEDs are widely used at present from the viewpoints of miniaturization and energy saving. As the optical sheet 104, (1) a lower light diffusion sheet 106 that is superposed on the front surface side of the light guide plate 103 and mainly has a light diffusion function, (2) a prism sheet 107 that is superposed on the front surface side of the lower light diffusion sheet 106 and has a refraction function in the normal direction side, and (3) an upper light diffusion sheet 108 that is superposed on the front surface side of the prism sheet 107 and suppresses luminance unevenness caused by the shape of the prism portion of the prism sheet 107 by slightly diffusing light rays is used (see Patent Document 1). As the upper light diffusion sheet 108, generally, a sheet including a base material layer and a light diffusion layer laminated on the front surface side of the base material layer and having a resin matrix and resin beads is used.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-77448 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] In the display screen of an LCD, due to the interference between the optical sheet used for the BLU and the liquid crystal panel, a phenomenon occurs that deteriorates the display quality called "moiré" (also called "ripple", "glare", "sparkle", "fine brightness unevenness", etc.). It is known that moiré is caused by the interference between the surface unevenness of the upper light diffusion sheet constituting the uppermost surface of the BLU and the arrangement of the cells (pixels) of the liquid crystal panel. In addition, with the recent progress in high definition of liquid crystal panels, the arrangement pitch of the cells has become smaller, making moiré more likely to occur. The same problem is not limited to the display screen of an LCD, and has also been pointed out in various other display devices.

[0006] Therefore, an object of the present disclosure is to provide an optical sheet capable of suppressing the occurrence of moiré in the display screen. [Means for Solving the Problems]

[0007] In order to achieve the above object, as a result of intensive studies by the inventors of the present application, it has been found that in an LCD, by making the particle diameter of the diffusing agent contained in the light diffusion layer of the upper light diffusion sheet smaller than about 7 μm and making the particle diameter as uniform as possible, moiré can be reduced as compared with conventional products. Also, even when the particle diameter of the diffusing agent (resin beads) is made small, in a state where an aggregated region of the beads and a sparse region where the beads hardly exist (a state called "island"), the surface unevenness (surface roughness, etc.) of the upper light diffusion sheet becomes large, and it has been found that moiré is not sufficiently reduced.

[0008] Therefore, the inventor of the present application prepared an optical sheet without aggregation (island) of beads, that is, an optical sheet in which beads are uniformly arranged, by spreading finer resin beads of about 2 μm or less. When this optical sheet was applied to the upper light diffusion sheet of an LCD (BLU), the flicker could be stably reduced. Specifically, the flicker contrast measured in accordance with JIS C 1006:2019 could be suppressed to about 4% or less. Furthermore, it was found that an optical sheet having an uneven shape that can stably obtain a flicker reduction effect is not limited to one formed by dispersing resin beads in a resin matrix, and can also be formed using a mold, a laser printer, or the like.

[0009] The optical sheet according to the present disclosure is based on the above findings. Specifically, it is an optical sheet having an uneven shape on at least the first surface, and the flicker contrast measured in accordance with JIS C 1006:2019 of the first surface is 4% or less. In the optical sheet according to the present disclosure, the haze value may be 40% or more and 70% or less.

[0010] In the optical sheet according to the present disclosure, the arithmetic mean roughness Ra measured in accordance with JIS B 0601:2001 (however, the evaluation length is set to 290 μm) of the first surface is 0.6 μm or less, and the peak count RPc measured in accordance with JIS B 0601:2001 (however, the evaluation length is set to 290 μm) of the first surface may be 16 or more. By doing so, the first surface of the optical sheet has a shape in which minute unevenness is provided densely and uniformly. Therefore, when the optical sheet according to the present disclosure is used as a surface material for various display devices, the occurrence of flicker on the display screen can be stably suppressed.

[0011] In the optical sheet according to the present disclosure, the arithmetic mean roughness Ra may be 0.5 μm or less, and the peak count RPc may be 18 or more. By doing so, the first surface of the optical sheet has a shape in which minute irregularities are provided more densely and uniformly. Therefore, when the optical sheet according to the present disclosure is used as a surface material for various display devices, the occurrence of flicker on the display screen can be further suppressed. Specifically, the flicker contrast can be suppressed to about 3% or less.

[0012] In the optical sheet according to the present disclosure, the average interval Sm of the irregularities measured in accordance with JIS B 0601:1994 (however, the evaluation length is set to 290 μm) of the first surface may be 20 μm or less, more preferably 15 μm or less. By doing so, the first surface of the optical sheet has a shape in which minute irregularities are provided more densely and uniformly. Therefore, when the optical sheet according to the present disclosure is used as a surface material for various display devices, the occurrence of flicker on the display screen can be further suppressed.

[0013] The optical sheet according to the present disclosure includes a base material layer and a light diffusion layer formed on the base material layer and having the first surface, and the uneven shape may be formed by dispersing resin beads in a resin matrix that becomes the light diffusion layer. By doing so, by uniformly dispersing resin beads having a small particle size in the resin matrix, a shape in which minute irregularities are provided densely and uniformly can be provided on the first surface of the optical sheet. In this case, when the refractive index difference between the resin matrix and the resin beads is 0.05 or less, the influence of light diffusion at the interface between the resin matrix and the resin beads can be suppressed. In addition, when uniformly dispersing resin beads having a small particle size in the resin matrix, by mixing a small amount of resin beads having a slightly larger particle size (for example, a particle size of about 5 μm) that does not cause flicker, the scratch resistance and haze value of the optical sheet according to the present disclosure can be improved.

[0014] The optical sheet according to the present disclosure may be disposed on the surface side of the prism sheet in the backlight unit of the liquid crystal display device. Thus, when the optical sheet according to the present disclosure is applied to the upper light diffusion sheet of the LCD (BLU), it is possible to suppress the occurrence of flickering on the display screen due to the interference between the surface unevenness of the upper light diffusion sheet and the arrangement of the cells of the liquid crystal panel. Further, it is possible to suppress the luminance unevenness caused by the shape of the prism portion of the prism sheet disposed on the back surface side of the upper light diffusion sheet.

[0015] The backlight unit according to the present disclosure includes a light source, a prism sheet into which light from the light source is introduced, and an optical sheet formed on the light emitting surface side of the prism sheet, and the optical sheet is the optical sheet according to the above-described present disclosure.

[0016] According to the backlight unit according to the present disclosure, since the optical sheet according to the above-described present disclosure is applied to the upper light diffusion sheet, it is possible to suppress the occurrence of flickering on the display screen due to the interference between the surface unevenness of the upper light diffusion sheet and the arrangement of the cells of the liquid crystal panel. Further, it is possible to suppress the luminance unevenness caused by the shape of the prism portion of the prism sheet disposed on the back surface side of the upper light diffusion sheet.

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

[0018] According to the liquid crystal display device according to the present disclosure, since the above-described backlight unit according to the present disclosure is provided, it is possible to suppress the occurrence of flickering on the display screen.

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

[0020] According to the information device according to the present disclosure, since the above-described liquid crystal display device according to the present disclosure is provided, it is possible to suppress the occurrence of flickering on the display screen.

Effects of the Invention

[0021] According to the present disclosure, an optical sheet capable of suppressing the occurrence of flicker on a display screen can be provided.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Modes for Carrying Out the Invention

[0023] (Embodiment) Hereinafter, an optical sheet, a backlight unit, a liquid crystal display device, and an information device according to embodiments of the present disclosure will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments and can be arbitrarily changed within the scope of the technical idea of the present disclosure. In the present disclosure, the "front side" means the viewer side in the liquid crystal display device, and the "back side" means the opposite side thereof.

[0024] <Backlight unit> FIG. 1 is a schematic cross-sectional view showing a backlight unit according to the present embodiment, and FIG. 2 is a schematic cross-sectional view showing the arrangement state of the upper light diffusion sheet and the prism sheet of the backlight unit shown in FIG. 1.

[0025] The backlight unit of the liquid crystal display device shown in FIG. 1 includes a prism sheet 4 and an upper light diffusion sheet 5 disposed on the front side of the prism sheet 4. More specifically, the backlight unit shown in FIG. 1 is an edge-lit type backlight unit, and includes a light guide sheet 1 that guides light rays incident from an end face to the front side, a light source 2 that irradiates light rays toward the end face of the light guide sheet 1, a lower light diffusion sheet 3 that is superimposed on the front side of the light guide sheet 1, a prism sheet 4 that is disposed on the front side of the lower light diffusion sheet 3, an upper light diffusion sheet 5 that is superimposed on the front side of the prism sheet 4, and a reflection sheet 6 that is disposed on the back side of the light guide sheet 1.

[0026] The lower light diffusion sheet 3 diffuses the light rays incident from the back side and condenses them toward the normal direction side (that is, condenses and diffuses them). The prism sheet 4 refracts the light rays incident from the back side toward the normal direction side. The upper light diffusion sheet 5 slightly diffuses the light rays incident from the back side to suppress the luminance unevenness caused by the shape of the prism portion of the prism sheet 4 and the like. Here, as will be described later, the upper light diffusion sheet 5 of the present embodiment suppresses the occurrence of flicker caused by interference with the cell arrangement of a liquid crystal panel (not shown) disposed on the front side of the upper light diffusion sheet 5. The reflection sheet 6 reflects the light rays emitted from the back side of the light guide sheet 1 to the front side and makes them incident on the light guide sheet 1 again.

[0027] <Upper light diffusion sheet> As shown in FIGS. 1 and 2, the upper light diffusion sheet 5 is disposed on the front surface side of the prism sheet 4, and in this embodiment, it is particularly superposed directly on the surface of the prism sheet 4 (without passing through other sheets or the like). The upper light diffusion sheet 5 includes a base material layer 11 and a light diffusion layer 12 laminated on the front surface side of the base material layer 11. The upper light diffusion sheet 5 is configured as a two-layer structure of the base material layer 11 and the light diffusion layer 12.

[0028] Since the base material layer 11 needs to transmit light rays, it is formed mainly of a transparent (for example, colorless and transparent) synthetic resin. The main component of the base material layer 11 is not particularly limited, and for example, polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, weather-resistant vinyl chloride, etc. may be used as the main component of the base material layer 11. Incidentally, the "main component" refers to the component with the largest content, for example, a component with a content of 50% by mass or more.

[0029] As the lower limit of the average thickness of the base material layer 11, about 10 μm is preferable, about 35 μm is more preferable, and about 50 μm is even more preferable. On the other hand, as the upper limit of the average thickness of the base material layer 11, about 500 μm is preferable, about 250 μm is more preferable, and about 188 μm is even more preferable. If the average thickness of the base material layer 11 is less than the lower limit, there is a risk of curling when the light diffusion layer 12 is formed by coating. Conversely, if the average thickness of the base material layer 11 exceeds the upper limit, there is a risk that the brightness of the liquid crystal display device will decrease and that it may not meet the requirement for thinning of the liquid crystal display device. Incidentally, the "average thickness" refers to the average value of the thicknesses of any 10 points.

[0030] The light diffusion layer 12 constitutes the outermost surface of the upper light diffusion sheet 5. The light diffusion layer 12 has a resin matrix 13 and resin beads 14 dispersed in the resin matrix 13. The light diffusion layer 12 contains the resin beads 14 dispersed with substantially equal density. The resin beads 14 are surrounded by the resin matrix 13. By dispersing the resin beads 14 in the resin matrix 13, minute irregularities are formed on the surface of the light diffusion layer 12, and the light diffusion layer 12 diffuses light rays to the outside due to the irregularities. In the present embodiment, the glare contrast measured in accordance with JIS C 1006:2019 on the irregular surface of the light diffusion layer 12 (the outermost surface of the upper light diffusion sheet 5) is 4% or less.

[0031] The lower limit of the average thickness of the light diffusion layer 12 is, for example, about 1 μm, and more preferably about 2 μm. On the other hand, the upper limit of the average thickness of the light diffusion layer 12 is, for example, about 7 μm, more preferably about 5 μm, and even more preferably about 4 μm. If the average thickness of the light diffusion layer 12 is less than the lower limit, the resin beads 14 cannot be reliably fixed by the resin matrix 13, and the resin beads 14 may fall off from the light diffusion layer 12. Conversely, if the average thickness of the light diffusion layer 12 exceeds the upper limit, it becomes difficult to form minute and high-density irregularities on the surface of the light diffusion layer 12. As a result, there is a possibility that the occurrence of glare caused by interference with the cell arrangement of the liquid crystal panel disposed on the surface side of the upper light diffusion sheet 5 cannot be sufficiently suppressed.

[0032] Since the resin matrix 13 needs to transmit light rays, it is formed mainly of a transparent (especially colorless and transparent) synthetic resin. As the synthetic resin, for example, a thermosetting resin, an active energy ray-curable resin, etc. can be used. As the thermosetting resin, for example, an epoxy resin, a silicone resin, a phenolic resin, a urea resin, an unsaturated polyester resin, a melamine resin, an alkyd resin, a polyimide resin, an acrylic resin, an amide-functional copolymer, a urethane resin, etc. can be used. As the active energy ray-curable resin, for example, an ultraviolet-curable resin that crosslinks and cures by irradiating ultraviolet rays, an electron beam-curable resin that crosslinks and cures by irradiating electron beams, etc. can be used, and they can be appropriately selected from polymerizable monomers and polymerizable oligomers and used. In order to improve the adhesion to the base material layer 11 and prevent the resin beads 14 from falling off from the light diffusion layer 12, as the active energy ray-curable resin, for example, an acrylic-based, urethane-based or acrylic-urethane-based ultraviolet-curable resin may be used.

[0033] In addition, the resin matrix 13 may contain additives in addition to the aforementioned synthetic resin. As the additives, for example, silicone-based additives, fluorine-based additives, antistatic agents, etc. can be used. The content of the additives in terms of solid content with respect to 100 parts by mass of the synthetic resin component in the resin matrix 13 may be, for example, 0.05 parts by mass or more and 5 parts by mass or less.

[0034] The resin beads 14 are resin particles having the property of transmitting and diffusing light rays. The resin beads 14 are formed mainly of a transparent, especially colorless and transparent synthetic resin. As the main component of the resin beads 14, for example, an acrylic resin, an acrylonitrile resin, a polyurethane, a polyvinyl chloride, a polystyrene, a polyamide, a polyacrylonitrile, etc. can be used. The shape of the resin beads 14 is not particularly limited, and may be, for example, spherical, cubic, needle-shaped, rod-shaped, spindle-shaped, plate-shaped, scaly, fibrous, etc., but particularly, a spherical shape having excellent light diffusibility is preferable.

[0035] The particle size of the resin beads 14 is preferably as small as possible while being equal to or greater than the wavelength of the light rays irradiated by the light source 2 (hereinafter sometimes referred to as the light source wavelength). The upper limit of the average particle size of the resin beads 14 may be, for example, about 2 μm, more preferably about 1 μm. If the average particle size of the resin beads 14 is less than the light source wavelength, the unevenness on the surface of the light diffusion layer 12 becomes too small, resulting in insufficient light diffusion properties, and there is a risk that the occurrence of luminance unevenness caused by the shape of the prism portion of the prism sheet 4 cannot be sufficiently suppressed. Conversely, if the average particle size of the resin beads 14 exceeds the above upper limit, a large number of relatively large unevennesses are formed on the surface of the light diffusion layer 12, and there is a risk that the occurrence of glare caused by interference with the cell arrangement of the liquid crystal panel cannot be sufficiently suppressed.

[0036] As the lower limit of the refractive index of the resin beads 14, for example, 1.46 is preferable, and 1.48 is more preferable. On the other hand, as the upper limit of the refractive index of the resin beads 14, for example, 1.60 is preferable, and 1.59 is more preferable. By setting the refractive index of the resin beads 14 within the above range, the refractive index difference from the resin matrix 13 can be appropriately adjusted (for example, to 0.05 or less), and thereby, it becomes easier to suppress the luminance unevenness caused by the shape of the ridge prism portion of the prism sheet 4. Note that the "refractive index" refers to the refractive index at a light wavelength of 589.3 nm (sodium D line).

[0037] The lower limit of the arithmetic mean roughness Ra of the surface of the light diffusion layer 12 (i.e., the surface of the upper light diffusion sheet 5) may be, for example, about 0.1 μm, more preferably about 0.2 μm, and even more preferably about 0.3 μm. On the other hand, the upper limit of the arithmetic mean roughness Ra of the surface of the light diffusion layer 12 is preferably about 0.6 μm, and more preferably about 0.5 μm. If the arithmetic mean roughness Ra of the surface of the light diffusion layer 12 is less than the lower limit, the unevenness on the surface of the light diffusion layer 12 becomes too small, resulting in insufficient light diffusion, and there is a possibility that the luminance unevenness caused by the shape of the ridge prism portion of the prism sheet 4 cannot be sufficiently suppressed. Conversely, if the arithmetic mean roughness Ra of the surface of the light diffusion layer 12 exceeds the upper limit, a relatively large number of large unevenness are formed on the surface of the light diffusion layer 12, and there is a possibility that the occurrence of glare caused by interference with the cell arrangement of the liquid crystal panel cannot be sufficiently suppressed. Note that the "arithmetic mean roughness Ra" means the "average value of the absolute value deviations from the average line". Specifically, in accordance with JIS B 0601:2001 (ISO 4287:1997), there is no cut-off, and the evaluation length is 290 μm.

[0038] The lower limit of the peak count RPc of the surface of the light diffusion layer 12 (that is, the surface of the upper light diffusion sheet 5) may be, for example, about 10, more preferably about 16, and even more preferably about 18. On the other hand, the upper limit of the peak count RPc of the surface of the light diffusion layer 12 is preferably about 300, more preferably about 250, and even more preferably about 200. If the peak count RPc of the surface of the light diffusion layer 12 is less than the lower limit, a large number of relatively large irregularities are formed on the surface of the light diffusion layer 12, and there is a risk that the occurrence of glare caused by interference with the cell arrangement of the liquid crystal panel cannot be sufficiently suppressed. Conversely, if the peak count RPc of the surface of the light diffusion layer 12 exceeds the upper limit, the irregularities on the surface of the light diffusion layer 12 become too small and the light diffusion property becomes insufficient, and there is a risk that the luminance unevenness caused by the shape of the ridge prism portion of the prism sheet 4 cannot be sufficiently suppressed. Incidentally, the "peak count RPc" means the "number of peaks per evaluation length". Specifically, in accordance with JIS B 0601:2001 (ISO 4287:1997), the evaluation length is set to 290 μm, the dead band width on both sides of the average line is set to 0%, and the number of peaks (peak count 2) when one peak is defined as the section from a point below the average line to above the average line and then back below the average line again.

[0039] The lower limit of the average interval Sm of the irregularities on the surface of the light diffusion layer 12 (that is, the surface of the upper light diffusion sheet 5) may be, for example, about 0.5 μm, more preferably about 1.0 μm, and even more preferably about 1.5 μm. On the other hand, as the upper limit of the average interval Sm of the irregularities on the surface of the light diffusion layer 12, for example, about 25 μm is preferable, about 20 μm is more preferable, and about 15 μm is even more preferable. If the average interval Sm of the irregularities on the surface of the light diffusion layer 12 is less than the lower limit, the irregularities on the surface of the light diffusion layer 12 become too small, resulting in insufficient light diffusion properties, and there is a possibility that the luminance unevenness caused by the shape of the ridge prism portion of the prism sheet 4 cannot be sufficiently suppressed. Conversely, if the average interval Sm of the irregularities in the light diffusion layer 12 exceeds the upper limit, it becomes difficult to form minute and high-density irregularities on the surface of the light diffusion layer 12, and there is a possibility that the occurrence of flicker caused by the interference with the cell arrangement of the liquid crystal panel disposed on the surface side of the upper light diffusion sheet 5 cannot be sufficiently suppressed. Note that the "average interval Sm of the irregularities" means the "arithmetic mean value of the intervals of the irregularities (the distance between one peak and the adjacent valley) in the evaluation length". Specifically, in accordance with JIS B 0601:1994 (ISO 468-1982, ISO 3274-1975, ISO 4287 / 1-1984, ISO 4287 / 2-1984, ISO 4288 / 1985), there is no cut-off, and the evaluation length is 290 μm.

[0040] The lower limit of the coating amount (in terms of solid content) of the light diffusion layer 12 is, for example, 1 g / m 2 , more preferably 2 g / m 2 and may be. On the other hand, the upper limit of the coating amount of the light diffusion layer 12 is, for example, 15 g / m 2 , more preferably 10 g / m 2 and may be. If the coating amount of the light diffusion layer 12 is less than the lower limit, the resin beads 14 cannot be reliably fixed by the resin matrix 13, and there is a possibility that the resin beads 14 will fall off from the light diffusion layer 12. Conversely, if the coating amount of the light diffusion layer 12 exceeds the upper limit, it becomes difficult to form minute and high-density irregularities on the surface of the light diffusion layer 12. As a result, there is a possibility that the occurrence of flicker caused by the interference with the cell arrangement of the liquid crystal panel disposed on the surface side of the upper light diffusion sheet 5 cannot be sufficiently suppressed.

[0041] The lower limit of the content of the resin matrix 13 in the light diffusing layer 12 may be, for example, about 10% by mass, more preferably about 20% by mass. On the other hand, the upper limit of the content of the resin matrix 13 in the light diffusing layer 12 may be, for example, about 80% by mass, more preferably about 75% by mass. If the content of the resin matrix 13 is less than the lower limit, the light diffusibility of the light diffusing layer 12 may become too high and the brightness of the liquid crystal display device may not be sufficiently high. Conversely, if the content of the resin matrix 13 exceeds the upper limit, the number of resin beads 14 in the light diffusing layer 12 may be insufficient, making it difficult to form minute and high-density irregularities on the surface of the light diffusing layer 12, and there is a possibility that the occurrence of flicker caused by interference with the cell arrangement of the liquid crystal panel disposed on the surface side of the upper light diffusing sheet 5 cannot be sufficiently suppressed.

[0042] The lower limit of the content of the resin beads 14 in the light diffusing layer 12 may be, for example, about 20% by mass, more preferably about 25% by mass. On the other hand, the upper limit of the content of the resin beads 14 in the light diffusing layer 12 may be, for example, about 90% by mass, more preferably about 80% by mass. If the content of the resin beads 14 in the light diffusing layer 12 is less than the lower limit, it becomes difficult to form minute and high-density irregularities on the surface of the light diffusing layer 12, and there is a possibility that the occurrence of flicker caused by interference with the cell arrangement of the liquid crystal panel disposed on the surface side of the upper light diffusing sheet 5 cannot be sufficiently suppressed. Conversely, if the content of the resin beads 14 in the light diffusing layer 12 exceeds the upper limit, the light diffusibility of the light diffusing layer 12 may become too high and the brightness of the liquid crystal display device may not be sufficiently high.

[0043] The lower limit of the haze value of the upper light diffusing sheet 5 may be, for example, about 10%, more preferably about 30%, and even more preferably about 40%. On the other hand, the upper limit of the haze value of the upper light diffusing sheet 5 may be about 90%, more preferably about 70%. If the haze value of the upper light diffusing sheet 5 is less than the lower limit, there is a possibility that the luminance unevenness caused by the shape of the ridge prism portion of the prism sheet 4 cannot be sufficiently suppressed. Conversely, if the haze value of the upper light diffusing sheet 5 exceeds the upper limit, the luminance of the liquid crystal display device may become insufficient. Note that the "haze value" refers to a value measured in accordance with JIS K 7136:2000.

[0044] <Manufacturing method of upper light diffusing sheet> The manufacturing method of the upper light diffusing sheet 5 is not particularly limited. For example, it may include a step of forming a sheet body constituting the base material layer 11 (hereinafter referred to as the base material layer forming step), and a step of laminating the light diffusing layer 12 on one surface side of the sheet body (hereinafter referred to as the light diffusing layer laminating step).

[0045] The base material layer forming step is not particularly limited. For example, a method of extruding a molten thermoplastic resin from a T-die and then stretching the extruded body in the layer longitudinal direction and the layer width direction to form a sheet body can be used. As a well-known extrusion molding method using a T-die, for example, a polishing roll method or a chill roll method can be used. Also, as a method of stretching the sheet body, for example, a tubular film biaxial stretching method or a flat film biaxial stretching method can be used.

[0046] The light diffusion layer lamination process may include, for example, a step of preparing a coating liquid containing a resin matrix 13 and resin beads 14 (hereinafter referred to as the preparation step), a step of applying the coating liquid prepared in the preparation step to one surface side of a sheet body (hereinafter referred to as the application step), and a step of drying and curing the coating liquid applied in the application step (hereinafter referred to as the curing step). In the preparation step, a coating liquid containing an active energy ray curable resin as a main component of the resin matrix 13 and containing resin beads 14 may be prepared. In the method for manufacturing the above-mentioned light diffusion sheet, when an active energy ray curable resin is used as the main component of the resin matrix 13, after applying the coating liquid in the application step, the active energy ray curable resin can be relatively quickly cured by irradiating, for example, ultraviolet rays in the curing step. Further, in the preparation step of the method for manufacturing the above-mentioned light diffusion sheet, by preparing a coating liquid containing many resin beads with a small particle size and slightly containing resin beads with a large particle size as the resin beads 14, the generation of glare caused by interference with the cell arrangement of the liquid crystal panel can be suppressed by the resin beads with a small particle size, and sticking to the liquid crystal panel can be prevented by the resin beads with a large particle size.

[0047] In addition, the method for manufacturing the above-mentioned light diffusion sheet 5 may further include a surface treatment step of performing corona discharge treatment, ozone treatment, low-temperature plasma treatment, glow discharge treatment, oxidation treatment, primer coat treatment, undercoat treatment, anchor coat treatment, etc. on the surface of the sheet body on the side where the light diffusion layer is laminated before the light diffusion layer lamination process.

[0048] <Prism sheet> Since the prism sheet 4 needs to transmit light rays, it is formed mainly of a transparent (for example, colorless and transparent) synthetic resin. The prism sheet 4 has a base material layer 15 and a row of protrusions composed of a plurality of ridge prism parts 16 laminated on the surface of the base material layer 15. The ridge prism parts 16 are laminated in a stripe shape on the surface of the base material layer 15. The ridge prism part 16 is a triangular prism whose back surface is in contact with the surface of the base material layer 15.

[0049] The lower limit of the thickness of the prism sheet 4 (the height from the back surface of the base material layer 15 to the apex of the ridge prism portion 16) may be, for example, about 50 μm, more preferably about 100 μm. On the other hand, the upper limit of the thickness of the prism sheet 4 may be about 200 μm, more preferably about 180 μm. The lower limit of the pitch p (see FIG. 2) of the ridge prism portions 16 in the prism sheet 4 may be, for example, about 20 μm, more preferably about 30 μm. On the other hand, the upper limit of the pitch p of the ridge prism portions 16 in the prism sheet 4 may be, for example, about 100 μm, more preferably about 60 μm. The apex angle of the ridge prism portion 16 may be, for example, 85° or more and 95° or less. The lower limit of the refractive index of the ridge prism portion 16 may be, for example, 1.5, more preferably 1.55. On the other hand, the upper limit of the refractive index of the ridge prism portion 16 may be, for example, 1.7.

[0050] Note that the backlight unit of the present embodiment shown in FIG. 1 is not limited to having only one prism sheet 4, and may further have another prism sheet superimposed on the prism sheet 4. In this case, it is preferable that the ridge lines of the plurality of ridge prism portions 16 of the prism sheet 4 and the ridge lines of the plurality of ridge prism portions of the other prism sheet are orthogonal to each other. By doing so, the light rays incident from the lower light diffusion sheet 3 can be refracted toward the normal direction side by one prism sheet, and the light rays emitted from the prism sheet can be refracted by the other prism sheet so as to travel substantially perpendicular to the back surface of the upper light diffusion sheet 5. The forming material, thickness, pitch of the ridge prism portion, apex angle of the ridge prism portion, and refractive index of the ridge prism portion of the other prism sheet may be the same as those of the prism sheet 4.

[0051] <Light guide sheet> The light guide sheet 1 is a sheet-shaped optical member that propagates the light rays emitted from the light source 2 inside while emitting them from the surface. The light guide sheet 1 may be formed in a substantially wedge-shaped cross-section or may be formed in a substantially flat plate shape. Since the light guide sheet 1 needs to have light transmissibility, it is formed mainly of a transparent (for example, colorless and transparent) resin. The main component of the light guide sheet 1 is not particularly limited, but may be a synthetic resin such as polycarbonate that is excellent in transparency, strength, etc., or an acrylic resin that is excellent in transparency, scratch resistance, etc. Polycarbonate is excellent in transparency and has a high refractive index. Therefore, when the main component of the light guide sheet 1 is polycarbonate, total reflection is likely to occur at the interface with the air layer (the layer formed in the gap between the light guide sheet 1 and the lower light diffusion sheet 3 and the layer formed in the gap between the light guide sheet 1 and the reflection sheet 6), so that the light rays can be efficiently propagated. In addition, since polycarbonate has heat resistance, deterioration due to the heat generation of the light source 2 is unlikely to occur.

[0052] <Light source> The light source 2 is disposed such that the irradiation surface faces (or abuts on) the end surface of the light guide sheet 1. As the light source 2, various ones can be used. For example, a light emitting diode (LED) can be used. Specifically, as the light source 2, one in which a plurality of LEDs are disposed along the end surface of the light guide sheet 1 can be used.

[0053] <Lower light diffusion sheet> The bottom light diffusion sheet 3 has a base material layer 17, a light diffusion layer 18 disposed on the front surface side of the base material layer 17, and an anti-sticking layer 19 disposed on the back surface side of the base material layer 17. The base material layer 17 of the bottom light diffusion sheet 3 may have the same configuration as the base material layer 11 of the above-described top light diffusion sheet 5. The light diffusion layer 18 of the bottom light diffusion sheet 3 has a light diffusing agent and its binder. The light diffusing agent is particles having the property of diffusing light rays and is roughly classified into an inorganic filler and an organic filler. As the inorganic filler, for example, silica, aluminum hydroxide, aluminum oxide, zinc oxide, barium sulfide, magnesium silicate, or a mixture thereof can be used. As the organic filler, for example, an acrylic resin, acrylonitrile resin, polyurethane, polyvinyl chloride, polystyrene, polyamide, polyacrylonitrile, etc. can be used. The shape of the light diffusing agent is not particularly limited and may be, for example, spherical, cubic, needle-shaped, rod-shaped, spindle-shaped, plate-shaped, scaly, fibrous, etc., but a spherical shape is preferable as it has excellent light diffusion properties.

[0054] The lower limit of the average particle diameter of the light diffusing agent used in the light diffusion layer 18 may be, for example, about 8 μm, more preferably about 10 μm. On the other hand, the upper limit of the average particle diameter of the light diffusing agent may be, for example, about 50 μm, more preferably about 20 μm, and even more preferably about 15 μm. If the average particle diameter of the light diffusing agent is less than the lower limit, the unevenness on the surface of the light diffusion layer 18 becomes small, and there is a possibility that the required light diffusion property of the bottom light diffusion sheet 3 may not be satisfied. Conversely, if the average particle diameter of the light diffusing agent exceeds the upper limit, the thickness of the bottom light diffusion sheet 3 may increase, and uniform diffusion may become difficult.

[0055] The lower limit of the binder content in the light diffusion layer 18 may be, for example, about 15% by mass, more preferably about 30% by mass. On the other hand, the upper limit of the binder content in the light diffusion layer 18 may be, for example, about 48% by mass, more preferably about 45% by mass. If the binder content is less than the lower limit, there is a possibility that the light diffusing agent may not be securely fixed by the binder. Conversely, if the binder content exceeds the upper limit, there is a possibility that the light diffusion property may become insufficient.

[0056] The lower limit of the content of the light diffusing agent in the light diffusing layer 18 may be, for example, about 52% by mass, more preferably about 55% by mass. On the other hand, the upper limit of the content of the light diffusing agent in the light diffusing layer 18 may be, for example, about 85% by mass, more preferably about 70% by mass. If the content of the light diffusing agent is less than the lower limit, there is a risk that the light diffusing property may be insufficient. Conversely, if the content of the light diffusing agent exceeds the upper limit, there is a risk that the light diffusing agent may not be securely fixed by the binder.

[0057] The lower limit of the arithmetic mean roughness Ra of the surface of the light diffusing layer 18 may be, for example, about 1.1 μm, more preferably about 1.3 μm, and even more preferably about 1.4 μm. On the other hand, the upper limit of the arithmetic mean roughness Ra of the surface of the light diffusing layer 18 may be, for example, about 5 μm, more preferably about 3 μm, and even more preferably about 2 μm. If the arithmetic mean roughness Ra of the surface of the light diffusing layer 18 is less than the lower limit, there is a risk that the light diffusing property may be insufficient. Conversely, if the arithmetic mean roughness Ra of the surface of the light diffusing layer 18 exceeds the upper limit, there is a risk that the light transmittance may decrease and the brightness of the liquid crystal display device may be insufficient.

[0058] The anti-sticking layer 19 is formed by dispersing resin beads in a resin matrix. These resin beads are scatteredly disposed on the back surface side of the base material layer 17. The anti-sticking layer 19 has a plurality of convex portions formed due to the resin beads and flat portions where the resin beads do not exist by the scattered arrangement of the resin beads. The anti-sticking layer 19 comes into scattered contact with the light guide sheet 1 disposed on the back surface side at the plurality of convex portions and does not come into contact with the entire back surface, thereby preventing sticking and suppressing uneven brightness of the liquid crystal display device.

[0059] The lower limit of the haze value of the lower light diffusion sheet 3 may be, for example, about 80%, more preferably about 85%, and even more preferably about 90%. If the haze value of the lower light diffusion sheet 3 is less than the lower limit, the light diffusibility may be insufficient. Incidentally, the upper limit of the haze value of the lower light diffusion sheet 3 may be, for example, about 95%.

[0060] <Reflection sheet> As the reflection sheet 6, for example, a white sheet in which a filler is dispersed and contained in a base resin such as polyester, or a mirror sheet in which a metal such as aluminum or silver is vapor-deposited on the surface of a film formed from polyester or the like, and having enhanced specular reflectivity can be used.

[0061] <Effects of the embodiment> According to the present embodiment described above, for the surface of the upper light diffusion sheet 5 (light diffusion layer 12) having the concavo-convex shape, the glare contrast measured in accordance with JIS C 1006:2019 is 4% or less. Here, on the concavo-convex surface of the light diffusion layer 12, the arithmetic mean roughness Ra and the peak count RPc measured in accordance with JIS B 0601:2001 (however, the evaluation length is set to 290 μm) may be 0.6 μm or less and 16 or more, respectively. That is, the surface of the upper light diffusion sheet 5 has a shape in which minute concavo-convexities are provided densely and uniformly. Therefore, it is possible to suppress the occurrence of unevenness on the display screen due to the interference between the surface unevenness of the upper light diffusion sheet 5 and the cell arrangement of the liquid crystal panel.

[0062] On the other hand, when the surface of the upper light diffusion sheet 5 does not have a shape in which minute concavo-convexities are provided densely and uniformly, as in the backlight unit according to the comparative example shown in FIG. 3, it is not possible to sufficiently suppress the occurrence of unevenness on the display screen due to the interference between the surface unevenness of the upper light diffusion sheet 5 and the cell arrangement of the liquid crystal panel.

[0063] In addition, regarding the surface of the upper light-diffusing sheet 5 of the present embodiment, when the arithmetic mean roughness Ra and the peak count RPc are 0.5 μm or less and 18 or more, respectively, the surface of the upper light-diffusing sheet 5 has a shape in which minute irregularities are provided more densely and uniformly. Therefore, it is possible to further suppress the occurrence of flicker on the display screen due to the interference between the surface irregularities of the upper light-diffusing sheet 5 and the cell arrangement of the liquid crystal panel. Specifically, the flicker contrast can be suppressed to about 3% or less.

[0064] In addition, regarding the surface of the upper light-diffusing sheet 5 of the present embodiment, when the average interval Sm of the irregularities measured in accordance with JIS B 0601:1994 (however, the evaluation length is set to 290 μm) is 20 μm or less, more preferably 15 μm or less, the surface of the upper light-diffusing sheet 5 has a shape in which minute irregularities are provided more densely and uniformly. Therefore, it is possible to further suppress the occurrence of flicker on the display screen due to the interference between the surface irregularities of the upper light-diffusing sheet 5 and the cell arrangement of the liquid crystal panel.

[0065] In addition, the upper light-diffusing sheet 5 of the present embodiment includes a base material layer 11 and a light-diffusing layer 12 formed on the base material layer 11. By dispersing resin beads 14 in a resin matrix 13 that becomes the light-diffusing layer 12, an uneven shape is formed. Therefore, by uniformly dispersing the resin beads 14 with a small particle size in the resin matrix 13, it is possible to provide a shape in which minute irregularities are provided densely and uniformly on the surface of the upper light-diffusing sheet 5 (light-diffusing layer 12). In this case, when the refractive index difference between the resin matrix 13 and the resin beads 14 is 0.05 or less, it is possible to suppress the influence of light diffusion at the interface between the resin matrix 13 and the resin beads 14. In addition, when uniformly dispersing the resin beads 14 with a small particle size in the resin matrix 13, by mixing a small amount of resin beads 14 having a slightly larger particle size (for example, a particle size of about 5 μm) that does not cause flicker, the scratch resistance and haze value of the upper light-diffusing sheet 5 can be improved.

[0066] In addition, in the backlight unit of the present embodiment shown in FIG. 1, the upper light diffusion sheet 5 is formed on the light-emitting surface side of the prism sheet 4. In other words, the prism sheet 4 is disposed on the back surface side of the upper light diffusion sheet 5. Therefore, the luminance unevenness caused by the shape of the protruding prism portion 16 of the prism sheet 4 can be suppressed by the upper light diffusion sheet 5.

[0067] <Modification example of the upper light diffusion sheet> The upper light diffusion sheet 25 of the present modification example shown in FIG. 4 can be applied to the backlight unit shown in FIG. 1 instead of the upper light diffusion sheet 5 of the above embodiment. The upper light diffusion sheet 25 diffuses the light rays incident from the back surface side to some extent to suppress the luminance unevenness caused by the shape of the ridge prism portion 16 of the prism sheet 4, and suppresses the occurrence of flicker caused by interference with the cell arrangement of a liquid crystal panel (not shown) disposed on the front surface side of the upper light diffusion sheet 25. The upper light diffusion sheet 25 includes a base material layer 11, a light diffusion layer 12 laminated on the front surface side of the base material layer 11, and an anti-sticking layer 26 laminated on the back surface side of the base material layer 11. The upper light diffusion sheet 25 is configured as a three-layer structure including the base material layer 11, the light diffusion layer 12, and the anti-sticking layer 26. The base material layer 11 and the light diffusion layer 12 of the upper light diffusion sheet 25 may have the same configuration as the base material layer 11 and the light diffusion layer 12 of the upper light diffusion sheet 5 of the above embodiment.

[0068] The anti-sticking layer 26 constitutes the lowermost surface of the upper light diffusing sheet 25. Since the anti-sticking layer 26 needs to transmit light rays, it is formed mainly of a transparent (for example, colorless and transparent) synthetic resin. The anti-sticking layer 26 is configured in a film shape with a flat back surface and a substantially uniform thickness. The anti-sticking layer 26 is configured to partially contact the top of the ridge prism portion 16 of the prism sheet 4 disposed on the back surface side of the upper light diffusing sheet 25, thereby preventing sticking to the prism sheet 4. As the main component of the anti-sticking layer 26, for example, polycarbonate, acrylic resin, polyethylene terephthalate, polyethylene naphthalate, polystyrene, methyl (meth)acrylate-styrene copolymer, polyolefin, cycloolefin polymer, cycloolefin copolymer, cellulose acetate, weather-resistant vinyl chloride, active energy ray curable resin, etc. can be used. In particular, when an acrylic resin is used as the main component of the anti-sticking layer 26, the strength of the back surface of the upper light diffusing sheet 25 can be increased, and it is easier to prevent damage to the back surface.

[0069] The lower limit of the average thickness of the anti-sticking layer 26 may be, for example, about 1 μm, more preferably about 2 μm. On the other hand, the upper limit of the average thickness of the anti-sticking layer 26 may be, for example, about 10 μm, more preferably about 8 μm. If the average thickness of the anti-sticking layer 26 is less than the lower limit, there is a possibility that damage to the back surface of the upper light diffusing sheet 25 cannot be reliably prevented. Conversely, if the average thickness of the anti-sticking layer 26 exceeds the upper limit, the brightness of the liquid crystal display device may decrease.

[0070] The upper limit of the arithmetic mean roughness Ra of the back surface of the anti-sticking layer 26 may be, for example, about 0.04 μm, more preferably about 0.035 μm, and even more preferably about 0.03 μm. If the arithmetic mean roughness Ra of the back surface of the anti-sticking layer 26 exceeds the upper limit, the ridge prism portion 16 of the prism sheet 4 may be damaged due to contact with the anti-sticking layer 26. Incidentally, the lower limit of the arithmetic mean roughness Ra of the back surface of the anti-sticking layer 26 is not particularly limited, but may be, for example, 0.01 μm.

[0071] The manufacturing method of the upper light diffusion sheet 25 may include, for example, a step of forming a sheet body constituting the base material layer 11 (hereinafter referred to as the base material layer forming step), a step of laminating the light diffusion layer 12 on one surface side of the sheet body (hereinafter referred to as the light diffusion layer laminating step), and a step of laminating the anti-sticking layer 26 on the other surface side of the sheet body constituting the base material layer 11 (hereinafter referred to as the anti-sticking layer laminating step). As the anti-sticking layer laminating step, for example, a method of forming the anti-sticking layer 26 simultaneously with the sheet body constituting the base material layer 11 by a co-extrusion method, a method of laminating the anti-sticking layer 26 by coating on the other surface side of the sheet body, etc. can be used.

[0072] Incidentally, the base material layer forming step in the manufacturing method of the upper light diffusion sheet 25 may be performed simultaneously with the anti-sticking layer laminating step by the co-extrusion method as described above, or may be performed separately from the anti-sticking layer laminating step. When the base material layer forming step and the anti-sticking layer forming step are performed separately, the base material layer forming step can be performed in the same manner as the base material layer forming step of the upper light diffusion sheet 5 in the above embodiment. Also, the light diffusion layer laminating step in the manufacturing method of the upper light diffusion sheet 25 can be performed in the same manner as the light diffusion layer laminating step of the upper light diffusion sheet 5 in the above embodiment.

[0073] According to the upper light diffusing sheet 25 of this modified example described above, in addition to the same effects as the upper light diffusing sheet 5 of the above embodiment, the following effects can be obtained. That is, since the anti-sticking layer 26 is laminated on the back side of the base material layer 11, while suppressing the luminance unevenness caused by the shape of the ridge prism portion 16 of the prism sheet 4, etc., the anti-sticking property with the prism sheet 4 and the anti-scratching property of the back surface of the upper light diffusing sheet 25 can be improved.

[0074] <Liquid crystal display device> The liquid crystal display device shown in FIG. 5 includes a light guide sheet 1 that guides light rays incident from an end face to the front surface side, a light source 2 that irradiates light rays toward the end face of the light guide sheet 1, a lower light diffusing sheet 3 that is superimposed on the front surface side of the light guide sheet 1, a prism sheet 4 that is disposed on the front surface side of the lower light diffusing sheet 3, an upper light diffusing sheet 5 that is superimposed on the front surface side of the prism sheet 4, a reflection sheet 6 that is disposed on the back surface side of the light guide sheet 1, and a liquid crystal panel 31 that is superimposed on the front surface side of the upper light diffusing sheet 5. That is, the liquid crystal display module shown in FIG. 5 has a configuration in which the liquid crystal panel 31 is disposed on the front surface side of the upper light diffusing sheet 5 in the backlight unit of the above embodiment shown in FIG. 1.

[0075] The liquid crystal panel 31 is disposed directly on the surface of the upper light diffusing sheet 5 (without passing through other sheets, etc.). The liquid crystal panel 31 has a front surface side polarizing plate 32 and a back surface side polarizing plate 33 that are disposed substantially parallel to each other and at a predetermined interval, and a liquid crystal cell 34 that is disposed therebetween. The front surface side polarizing plate 32 and the back surface side polarizing plate 33 are each composed of, for example, a polarizer such as an iodine-based polarizer, a dye-based polarizer, a polyene-based polarizer, etc., and a pair of transparent protective films disposed on both sides thereof. The transmission axis directions of the front surface side polarizing plate 32 and the back surface side polarizing plate 33 are orthogonal to each other.

[0076] The liquid crystal cell 34 has a function of controlling the amount of transmitted light, and various known types are adopted. The liquid crystal cell 34 is generally a laminated structure composed of a substrate, a color filter, a counter electrode, a liquid crystal layer, a pixel electrode, a substrate, etc. A transparent conductive film such as ITO is used for this pixel electrode. As the display mode of the liquid crystal cell 34, for example, TN (Twisted Nematic), VA (Virtical Alignment), IPS (In-Place Switching), FLC (Ferroelectric Liquid Crystal), AFLC (Anti-ferroelectric Liquid Crystal), OCB (Optically Compensatory Bend), STN (Supper Twisted Nematic), HAN (Hybrid Aligned Nematic), etc. can be used. The pixel pitch of the liquid crystal panel 31 (the pixel pitch of the liquid crystal cell) may be, for example, about 25 μm or less.

[0077] According to the liquid crystal display device shown in FIG. 5, since the upper light diffusion sheet 5 of the above embodiment is provided, it is possible to suppress the luminance unevenness caused by the shape of the ridge prism portion 16 of the prism sheet 4, etc. Further, since the upper light diffusion sheet 5 is disposed on the back surface side of the liquid crystal panel 31, it is possible to suppress the occurrence of flicker caused by the interference between the unevenness formed on the surface of the light diffusion layer 12 of the upper light diffusion sheet 5 and the cell arrangement of the liquid crystal panel 31.

[0078] Further, when the liquid crystal display device shown in FIG. 5 is used as a display device for various information devices such as a smartphone or a tablet terminal, it is possible to suppress the occurrence of flicker on the display screen of the various information devices.

[0079] (Example) Hereinafter, the present disclosure will be described in more detail by way of examples, but the present disclosure is not limited to these examples.

[0080] <Example 1> On the surface of a base material layer with an average thickness of 75 μm mainly composed of polyethylene terephthalate, a light diffusion layer in which resin beads are dispersed in a resin matrix mainly composed of an ultraviolet curable resin was laminated to produce the optical sheet (upper light diffusion sheet) of Example 1. As the resin beads, a mixture of first beads with a large average particle size (average particle size of 5 μm) and second beads with a smaller average particle size than the first beads (average particle size of 0.8 μm) at a ratio of 1:55 (mass ratio) was used. The laminated amount of the light diffusion layer was 3.5 g / m 2 , the content of the resin matrix in the light diffusion layer was 31.17% by mass, and the average thickness of the light diffusion layer was 3.5 μm. FIG. 6 is a micrograph of the surface of the upper light diffusion sheet of Example 1 magnified 1000 times.

[0081] <Example 2> On the surface of a base material layer with an average thickness of 125 μm mainly composed of polyethylene terephthalate, a light diffusion layer in which resin beads are dispersed in a resin matrix mainly composed of a thermosetting resin was laminated to produce the optical sheet (upper light diffusion sheet) of Example 2. As the resin beads, a mixture of first beads with a large average particle size (average particle size of 5 μm), second beads with a smaller average particle size than the first beads (average particle size of 2 μm), and third beads with a smaller average particle size than the first beads and a larger average particle size than the second beads (average particle size of 3 μm) at a ratio of 2:2:1 (mass ratio) was used. The laminated amount of the light diffusion layer was 3.5 g / m 2 , the content of the resin matrix in the light diffusion layer was 51.93% by mass, and the average thickness of the light diffusion layer was 3.5 μm. FIG. 7 is a micrograph of the surface of the upper light diffusion sheet of Example 2 magnified 1000 times.

[0082] <Example 3> On the surface of a base material layer with a main component of polyethylene terephthalate and an average thickness of 125 μm, a light diffusion layer in which resin beads are dispersed in a resin matrix mainly composed of a thermosetting resin was laminated to produce the optical sheet (upper light diffusion sheet) of Example 3. As the resin beads, the first beads with a large average particle size (average particle size of 5 μm), the second beads with an average particle size smaller than that of the first beads (average particle size of 2 μm), and the third beads with an average particle size smaller than that of the first beads and larger than that of the second beads (average particle size of 3 μm) were mixed at a ratio of 2:2:1 (mass ratio). The laminated amount of the light diffusion layer was 3.5 g / m 2 , the content of the resin matrix in the light diffusion layer was 49.07% by mass, and the average thickness of the light diffusion layer was 3.5 μm. Fig. 8 is a micrograph of the surface of the upper light diffusion sheet of Example 3 magnified 1000 times.

[0083] <Example 4> On the surface of a base material layer with a main component of polyethylene terephthalate and an average thickness of 125 μm, a light diffusion layer in which resin beads are dispersed in a resin matrix mainly composed of a thermosetting resin was laminated to produce the optical sheet (upper light diffusion sheet) of Example 3. As the resin beads, the first beads with a large average particle size (average particle size of 7 μm), the second beads with an average particle size smaller than that of the first beads (average particle size of 2 μm), and the third beads with an average particle size smaller than that of the first beads and larger than that of the second beads (average particle size of 3 μm) were mixed at a ratio of 2:2:1 (mass ratio). The laminated amount of the light diffusion layer was 3.5 g / m 2 , the content of the resin matrix in the light diffusion layer was 50.02% by mass, and the average thickness of the light diffusion layer was 3.5 μm.

[0084] <Example 5> On the surface of a base material layer similar to that of Example 2, a light diffusion layer in which resin beads are dispersed in a resin matrix mainly composed of an ultraviolet curable resin was laminated to produce the upper light diffusion sheet of Example 5. As the resin beads, the first beads with a large average particle size (average particle size of 7 μm), the second beads with an average particle size smaller than that of the first beads (average particle size of 2 μm), and the third beads with an average particle size smaller than that of the first beads and larger than that of the second beads (average particle size of 3 μm) were mixed at a ratio of 2:2:1 (mass ratio). The lamination amount of the light diffusion layer was 2.0 g / m 2 , the content of the resin matrix in the light diffusion layer was 70.02% by mass, and the average thickness of the light diffusion layer was 2.0 μm. FIG. 9 is a microscopic photograph of the surface of the upper light diffusion sheet of Example 5 magnified 1000 times.

[0085] <Comparative Example 1> On the surface of a base material layer similar to that of Example 2, a light diffusion layer in which resin beads are dispersed in a resin matrix mainly composed of an ultraviolet curable resin was laminated to produce the upper light diffusion sheet of Comparative Example 1. As the resin beads, the first beads with a large average particle size (average particle size of 7 μm) and the second beads with an average particle size smaller than that of the first beads (average particle size of 2 μm) were mixed at a ratio of 1.5:1 (mass ratio). The lamination amount of the light diffusion layer was 2.0 g / m 2 , the content of the resin matrix in the light diffusion layer was 77.09% by mass, and the average thickness of the light diffusion layer was 2.0 μm. FIG. 10 is a microscopic photograph of the surface of the upper light diffusion sheet of Comparative Example 1 magnified 1000 times.

[0086] <Comparative Example 2> On the surface of a base material layer with an average thickness of 100 μm mainly composed of polyethylene terephthalate, a light diffusion layer in which acrylic resin beads with an average particle size of 8 μm are dispersed in a resin matrix mainly composed of an ultraviolet curable resin was laminated to produce the upper light diffusion sheet of Comparative Example 2. The lamination amount of the light diffusion layer was 5.0 g / m 2The content fraction of the resin matrix in the light diffusion layer was 76.19% by mass, and the average thickness of the light diffusion layer was 5.0 μm. Fig. 11 is a micrograph of the surface of the upper light diffusion sheet of Comparative Example 2 magnified 1000 times.

[0087] <Evaluation Results of Examples> The upper light diffusion sheets of Examples 1 to 5 and Comparative Examples 1 and 2 were incorporated between the prism sheet and the liquid crystal panel of the edge-light type backlight unit of the liquid crystal display device, and the degree of glare was measured according to JIS C 1006:2019. The measurement results are shown in Table 1. The measured value is the "glare contrast (unit: %)" of JIS C 1006:2019, and the measuring device used was the one with the specifications of "Measuring Device B" in Appendix A of JIS C 1006:2019 (SMS-1000 manufactured by DM&S). Specifically, in a dark room under constant temperature and humidity (23°C, 50% RH), the upper light diffusion sheet was placed with the light diffusion layer on the upper side, sandwiching the Pixel pattern matrix (latest version 2019: thickness 2 mm, 254 ppi) attached to the SMS-1000 above a light source with a Green film (LEE Color Filter (model: 124 Dark Green) manufactured by LEE Filters). The degree of glare was measured in the Sparkle mode. A 50 mm objective lens was used for the imaging camera, the distance from the light source to the objective lens was set to approximately 240 mm (Pixel Ratio of SMS-1000 was 6.0), and the exposure time was set to approximately 20 ms. The image filtering process was performed using the application attached to the SMS-1000.

[0088] Also, the haze values of the upper light diffusion sheets of Examples 1 to 5 and Comparative Examples 1 and 2 were measured according to JIS K 7136:2000 using a haze meter HZ-2 manufactured by Suga Test Instruments Co., Ltd. The measurement results are shown in Table 1.

[0089] Also, the "arithmetic mean roughness Ra" of the surface of the upper light diffusion sheets of Examples 1 to 5 and Comparative Examples 1 and 2 was measured in accordance with JIS B 0601:2001 without cut-off and with an evaluation length of 290 μm. The measurement results are shown in Table 1. Incidentally, the measurement of the "arithmetic mean roughness Ra", which is an index of line roughness, was performed on three samples (N1, N2, N3), and Table 1 shows the measured values of each sample and their average values.

[0090] Also, the "peak count RPc" of the surface of the upper light diffusion sheets of Examples 1 to 5 and Comparative Examples 1 and 2 was measured in accordance with JIS B 0601:2001. The measurement results are shown in Table 1. Incidentally, in the measurement of the "peak count RPc", which is an index of line roughness, the evaluation length was set to 290 μm, the dead band width on both sides of the average line was set to 0%, and the number of peaks (peaks) was counted when one peak was defined as the point that first rises above the average line from a point below the average line and then drops below the average line again. The measurement was performed on three samples (N1, N2, N3), and Table 1 shows the measured values of each sample and their average values.

[0091] Also, the "average spacing Sm of unevenness" of the surface of the upper light diffusion sheets of Examples 1 to 5 and Comparative Examples 1 and 2 was measured in accordance with JIS B 0601:1994 without cut-off and with an evaluation length of 290 μm. The measurement results are shown in Table 1. Incidentally, the measurement of the "average spacing Sm of unevenness", which is an index of line roughness, was performed on three samples (N1, N2, N3), and Table 1 shows the measured values of each sample and their average values.

[0092] The measurements of the above line roughness indices, namely, the "arithmetic mean roughness Ra", "peak count RPc", and "average spacing Sm of unevenness", were performed using a laser microscope VX-K100 manufactured by KEYENCE CORPORATION. Specifically, while setting the magnification to 1000 times, the pitch to 0.20, and the dead band width to 0, laser imaging of the target surface was performed while performing automatic tilt correction in the line roughness mode, and the captured image was analyzed using a dedicated analysis application for VK-X100.

[0093]

Table 1

[0094] As shown in Table 1, for Examples 1 to 5, it was possible to suppress the flicker contrast measured in accordance with JIS C 1006:2019 to about 4% or less. Further, regarding the surface of the upper light diffusion sheet, when the arithmetic mean roughness Ra and the peak count RPc are 0.6 μm or less and 16 or more, respectively, as in Examples 1 to 4, the flicker contrast can be made lower than 4%. Furthermore, when the arithmetic mean roughness Ra, the peak count RPc, and the average interval Sm of unevenness are 0.5 μm or less, 18 or more, and 20 μm or less, respectively, as in Examples 1 to 3, the occurrence of flicker on the display screen due to the interference between the surface unevenness of the upper light diffusion sheet and the cell arrangement of the liquid crystal panel can be sufficiently suppressed. Specifically, the flicker contrast can be suppressed to about 3% or less. On the other hand, in Comparative Examples 1 and 2, the flicker contrast exceeds 5%. Specifically, as in Comparative Examples 1 and 2, when the arithmetic mean roughness Ra, the peak count RPc, or the average interval Sm of unevenness is outside the above-described range, the occurrence of flicker on the display screen cannot be sufficiently suppressed.

[0095] (Other Embodiments) As described above, embodiments (including modifications and examples. The same applies hereinafter) of the present disclosure have been described, but the present disclosure is not limited to the above-described embodiments only, and various changes are possible within the scope of the disclosure. That is, the description of the above-described embodiments is merely illustrative in nature and is not intended to limit the present disclosure, its applications, or its uses.

[0096] Specifically, the upper light diffusion sheet according to the present disclosure may be a two-layer structure of a base material layer and a light diffusion layer, or a three-layer structure of a base material layer, a light diffusion layer, and an anti-sticking layer, as in the above-described embodiments, or may have another layer between the base material layer and the light diffusion layer, or between the base material layer and the anti-sticking layer.

[0097] In addition to the above-described embodiment, the backlight unit according to the present disclosure can be implemented in various modified and improved forms. For example, the backlight unit may include, on the surface side of the light guide sheet, an optical sheet other than the upper light diffusion sheet, the prism sheet, and the lower light diffusion sheet. Further, the backlight unit is not limited to an edge-lit type backlight unit, and may be, for example, a direct-lit type backlight unit in which a diffusion plate and a light source are disposed on the back surface side of the lower light diffusion sheet. Furthermore, the specific configurations of the prism sheet, the light diffusion sheet, the light guide sheet, the light source, and the reflection sheet in the backlight unit are not particularly limited, and various configurations can be adopted.

[0098] In addition, the backlight unit according to the present disclosure can be used in a wide range of applications such as relatively large display devices such as personal computers and liquid crystal televisions, mobile phone terminals such as smartphones, and portable information terminals such as tablet terminals.

[0099] In the above-described embodiment, resin beads are dispersed in a resin matrix that becomes a light diffusion layer in an optical sheet including a base material layer and a light diffusion layer formed on the base material layer, to form an uneven shape on the surface of the optical sheet. However, instead of this, for example, a mold obtained by transferring the uneven shape formed by dispersing resin beads in the resin matrix may be used, or an uneven shape may be formed on the surface of the optical sheet by using a laser printer device that stores the uneven shape.

[0100] In the above-described embodiment, the case of suppressing the occurrence of flicker on the display screen due to the interference between the surface unevenness of the upper light diffusion sheet of the backlight unit and the cell arrangement of the liquid crystal panel is exemplified. However, the present disclosure is not limited thereto, and by applying an optical sheet having the same configuration as the upper light diffusion sheet of the present disclosure to, for example, a display such as a CRT or a flat panel display, or a surface material such as a touch panel used for these displays, the occurrence of flicker on the display screen may be suppressed.

Description of Reference Numerals

[0101] 1 Light guide sheet 2 Light source 3 Lower light diffusion sheet 4 Prism sheet 5 Upper light diffusion sheet 6 Reflective sheet 11 Base material layer 12 Light diffusion layer 13 Resin matrix 14 Resin beads 15 Base material layer 16 Ridge prism part 17 Base material layer 18 Light diffusion layer 19 Anti-sticking layer 25 Upper light diffusion sheet 26 Anti-sticking layer 31 Liquid crystal panel 32 Front side polarizer 33 Back side polarizer 34 Liquid crystal cell

Claims

1. An optical sheet having a concave-convex shape on at least a first surface, The glare contrast of the first surface measured in accordance with JIS C 1006:2019 is 4% or less; The first surface has an arithmetic average roughness Ra of 0.6 μm or less, as measured in accordance with JIS B 0601:2001 (with the evaluation length set to 290 μm); The peak count RPc of the first surface measured in accordance with JIS B 0601:2001 (with the evaluation length set to 290 μm) is 16 or more, the optical sheet includes a base layer and a light diffusion layer formed on the base layer and having the first surface, The uneven shape is formed by dispersing resin beads in a resin matrix that becomes the light diffusion layer, The resin beads include first beads having an average particle size of 5 μm or more and second beads having an average particle size of 2 μm or less, The content by mass of the second beads in the light diffusion layer is equal to or greater than the content by mass of the first beads. Optical sheet.

2. The haze value is 40% or more and 70% or less. The optical sheet according to claim 1 .

3. The arithmetic average roughness Ra is 0.5 μm or less, The peak count RPc is 18 or more. The optical sheet according to claim 1 .

4. The average spacing Sm of the irregularities on the first surface measured in accordance with JIS B 0601:1994 (with the evaluation length set to 290 μm) is 20 μm or less. The optical sheet according to any one of claims 1 to 3.

5. The refractive index difference between the resin matrix and the resin beads is 0.05 or less. The optical sheet according to any one of claims 1 to 4.

6. It is disposed on the front surface side of a prism sheet in a backlight unit of a liquid crystal display device. The optical sheet according to any one of claims 1 to 5.

7. A light source; a prism sheet into which light from the light source is introduced; an optical sheet formed on the light output surface side of the prism sheet, The optical sheet is an optical sheet according to any one of claims 1 to 6. A backlight unit for a liquid crystal display device.

8. A backlight unit according to claim 7; A liquid crystal display panel. LCD display device.

9. A liquid crystal display device comprising the liquid crystal display device according to claim 8. Information equipment.

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