JP2023113017A - Light diffusion sheet, backlight unit, liquid crystal display, and information instrument - Google Patents

Abstract

To provide a light diffusion sheet that can improve in-plane luminance uniformity.SOLUTION: A light diffusion sheet 43 has a first surface 21a to be a light emission surface, and a second surface 21b to be a light incident surface. The first surface 21a is provided with a plurality of recesses 22 with a substantially reverse polygonal pyramid shape. The second surface 21b has an arithmetic average roughness of 1.0 μm or more and 3.0 μm or less. The light diffusion sheet 43 has an internal haze of 1.5% or less.SELECTED DRAWING: Figure 5

Description

The present disclosure relates to a light diffusion sheet, backlight unit, liquid crystal display device, and information equipment.

2. Description of the Related Art In recent years, liquid crystal display devices (hereinafter also referred to as liquid crystal displays) have been widely used as display devices for various information devices such as smartphones and tablet terminals. As a backlight for a liquid crystal display, a direct type in which a light source is arranged on the back surface of the liquid crystal panel or an edge light type in which a light source is arranged near the side surface of the liquid crystal panel is mainly used.

When a direct type backlight is used, a light diffusion sheet is used to diffuse the light from a light source such as an LED (Light Emitting Diode) to improve the uniformity of brightness and chromaticity over the entire screen ( For example, see Patent Document 1).

The light diffusion sheet utilizes the diffusion caused by providing an uneven shape on the light exit surface and the diffusion caused by dispersing fine particles having a different refractive index from the base material in the sheet base material, to make the light incident surface Diffuse the light incident from the

In thin displays such as laptop computers and tablet terminals, for example, a sheet having an inverted pyramid-shaped concave portion formed on the light exit surface and an embossed light incident surface is used as the light diffusion sheet.

JP 2011-129277 A

However, since the light source is placed directly under the display screen in the direct type backlight, if the distance from the light source to the light diffusion sheet and the thickness of the light diffusion sheet are reduced as the display becomes thinner, the light diffusion The sheet makes it difficult to diffuse the light sufficiently. As a result, there arises a problem that luminance uniformity within the screen (in-plane luminance uniformity) deteriorates.

An object of the present disclosure is to provide a light diffusion sheet capable of improving in-plane luminance uniformity.

In order to achieve the above object, a light diffusion sheet according to the present disclosure is a light diffusion sheet having a first surface serving as a light exit surface and a second surface serving as a light incidence surface, wherein the first surface is provided with a plurality of substantially inverted polygonal pyramid-shaped recesses, the second surface has an arithmetic mean roughness of 1.0 μm or more and 3.0 μm or less, and an internal haze of 1.5% or less.

According to the light diffusion sheet according to the present disclosure, the arithmetic mean roughness of the second surface, which is the light incident surface, is 3.0 μm or less and the internal haze is 1.5% or less, so that the light incident from the second surface The light reaches the uneven first surface without substantially diffusing inside the sheet. As a result, high-brightness light that has traveled straight from the light source toward the light diffusion sheet can be uniformly diffused by the concave portions of the first surface, so that the image of the light source can be eliminated and the in-plane brightness uniformity can be improved. can. Moreover, since the arithmetic mean roughness of the second surface, which serves as the light incident surface, is 1.0 μm or more, it is possible to suppress a decrease in luminance. Therefore, it is possible to cope with further thinning and reduction in the number of light sources.

In the light diffusion sheet according to the present disclosure, the plurality of concave portions may be formed in a substantially inverted quadrangular pyramid shape. With this configuration, the light coming straight from the light source can be uniformly diffused by the first surface.

In the light diffusion sheet according to the present disclosure, the apex angles of the plurality of concave portions may be 80° or more and 100° or less. With this configuration, the light coming straight from the light source can be uniformly diffused by the first surface.

A backlight unit according to the present disclosure is a backlight unit that is incorporated in a liquid crystal display device and guides light emitted from a plurality of light sources to a display screen side, wherein between the display screen and the plurality of light sources, The light diffusion sheet according to the present disclosure described above is provided, and the light diffusion sheet is arranged with the second surface facing the plurality of light sources.

According to the backlight unit according to the present disclosure, since the light diffusion sheet according to the present disclosure described above is provided, the in-plane luminance uniformity can be improved, so that it is possible to further reduce the thickness and reduce the number of light sources. can.

In the backlight unit according to the present disclosure, the plurality of light sources may be arranged on a reflective sheet provided on the opposite side of the display screen when viewed from the light diffusion sheet. By doing so, the light is further diffused by multiple reflections between the light diffusion sheet and the reflection sheet, so that the in-plane luminance uniformity is further improved.

In the backlight unit according to the present disclosure, a plurality of the light diffusion sheets may be laminated and arranged between the display screen and the plurality of light sources. With this configuration, the first surface of each light diffusion sheet repeatedly diffuses the light traveling straight from the light source, thereby further improving the in-plane luminance uniformity.

In the backlight unit according to the present disclosure, a distance between the plurality of light sources and the light diffusion sheet may be 10 mm or less. In this way, deterioration of in-plane luminance uniformity can be suppressed by the diffusion performance of the light diffusion sheet according to the present disclosure described above.

A liquid crystal display device according to the present disclosure includes the aforementioned backlight unit according to the present disclosure and a liquid crystal display panel.

According to the liquid crystal display device according to the present disclosure, since the backlight unit according to the present disclosure described above is provided, in-plane luminance uniformity can be improved. can.

An information device according to the present disclosure includes the aforementioned liquid crystal display device according to the present disclosure.

According to the information equipment according to the present disclosure, since the liquid crystal display device according to the present disclosure described above is provided, in-plane luminance uniformity can be improved. .

According to the present disclosure, it is possible to provide a light diffusion sheet capable of improving in-plane luminance uniformity.

1 is a cross-sectional view of a liquid crystal display device according to an embodiment; FIG. 4 is a cross-sectional view of the backlight unit according to the embodiment; FIG. 1 is a cross-sectional view of a light diffusion sheet according to an embodiment; FIG. FIG. 4 is a cross-sectional view of a light diffusion sheet according to a comparative example; FIG. 5 is a diagram showing the evaluation results of the in-plane luminance uniformity of the light diffusion sheets of Examples 1 to 8 and Comparative Examples 1 and 2; FIG. 2 is a diagram showing evaluation results of luminance of light diffusion sheets of Examples 1 to 8 and Comparative Examples 1 and 2;

(embodiment)
Hereinafter, a light diffusion sheet, a backlight unit, a liquid crystal display device, and an information device according to embodiments will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments, and can be arbitrarily changed within the scope of the technical ideas of the present disclosure.

FIG. 1 is an example of a cross-sectional view of a liquid crystal display device according to this embodiment, FIG. 2 is an example of a cross-sectional view of a backlight unit according to this embodiment, and FIG. It is an example of sectional drawing of a diffusion sheet.

As shown in FIG. 1, the liquid crystal display device 50 includes a liquid crystal display panel 5, a first polarizing plate 6 attached to the lower surface of the liquid crystal display panel 5, and a second polarizing plate attached to the upper surface of the liquid crystal display panel 5. 7 and a backlight unit 40 provided on the back side of the liquid crystal display panel 5 with the first polarizing plate 6 interposed therebetween. The liquid crystal display panel 5 includes a TFT substrate 1 and a CF substrate 2 facing each other, a liquid crystal layer 3 provided between the TFT substrate 1 and the CF substrate 2, and the TFT substrate 1 and the CF substrate 2. A frame-shaped sealing material (not shown) is provided to seal the liquid crystal layer 3 between them.

The shape of the display screen 50a of the liquid crystal display device 50 viewed from the front (upper side in FIG. 1) is, in principle, rectangular or square, but is not limited thereto, and may be a rectangular shape with rounded corners, an elliptical shape, a circular shape, or the like. Any shape such as a trapezoid or an automobile instrument panel (instrument panel) may be used.

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

The liquid crystal display device 50 of the present embodiment can be used for various information devices (for example, in-vehicle devices such as car navigation systems, personal computers, mobile phones, personal digital assistants, portable game machines, copiers, ticket vending machines, automated teller machines, etc.). ) is used as a display device incorporated in

The TFT substrate 1 includes, for example, a plurality of TFTs provided in a matrix on a glass substrate, an interlayer insulating film provided so as to cover each TFT, and a plurality of TFTs provided in a matrix on the interlayer insulating film. and an alignment film provided to cover each pixel electrode. The CF substrate 2 includes, for example, a black matrix provided in a grid pattern on a glass substrate, a color filter including a red layer, a green layer, and a blue layer provided between the grids of the black matrix, and a black matrix and a color filter. A common electrode is provided to cover the filter, and an alignment film is provided to cover the common electrode. The liquid crystal layer 3 is made of a nematic liquid crystal material or the like containing liquid crystal molecules having electro-optical properties. The first polarizing plate 6 and the second polarizing plate 7 each include, for example, a polarizer layer having a unidirectional polarization axis and a pair of protective layers provided to sandwich the polarizer layer.

As shown in FIG. 2, the backlight unit 40 includes a reflective sheet 41, a plurality of light sources 42 arranged two-dimensionally on the reflective sheet 41, and a light diffusion sheet 43 provided above the plurality of light sources 42. , a first prism sheet 44 and a second prism sheet 45 provided in this order above the light diffusion sheet 43 , and a polarizing sheet 46 provided above the second prism sheet 45 .

FIG. 2 illustrates a case where two layers of the light diffusion sheet 43 having the same structure are laminated and provided in the backlight unit 40, but the light diffusion sheet 43 may be used as a single layer, or as a three-layered light diffusion sheet. It may be used by laminating more than one layer.

The reflective sheet 41 is composed of, for example, a white polyethylene terephthalate resin film, a silver-deposited film, or the like.

Although the type of the light source 42 is not particularly limited, it may be, for example, an LED element, a laser element, or the like, and an LED element may be used from the viewpoint of cost, productivity, and the like. The light source 42 may have a rectangular shape when viewed from above, in which case the length of one side is 10 μm or more (preferably 50 μm or more) and 20 mm or less (preferably 10 mm or less, more preferably 5 mm or less). There may be. When LEDs are used as the light source 42, a plurality of LED chips may be arranged on the reflective sheet 41 at regular intervals. In addition, a lens may be attached to the LED in order to adjust the light emission angle characteristics of the LED serving as the light source 42 .

The light diffusion sheet 43 has a base material layer 21, as shown in FIGS. The base material layer 21 is configured using, for example, clear polycarbonate as a base material (matrix resin). Base material layer 21 does not substantially contain a diffusing agent. The light diffusion sheet 43 (base material layer 21) has a first surface 21a that serves as a light emitting surface and a second surface 21b that serves as a light incident surface. That is, the light diffusion sheet 43 is arranged with the second surface 21 b facing the light source 42 .

On the first surface 21 a of the light diffusing sheet 43 , a plurality of concave portions 22 having a substantially inverted polygonal pyramid shape, for example, a substantially inverted quadrangular pyramid shape (inverted pyramid shape) are arranged two-dimensionally. On the other hand, the arithmetic average roughness of the second surface 21b of the light diffusion sheet 43 is 3.0 μm or less.

The internal haze of the light diffusion sheet 43 (base material layer 21) is 1.5% or less. The term "internal haze" means haze excluding the surface haze caused by the surface shape (specifically, the concave portions 22 of the first surface 21a) among the total haze.

The apex angle θ of the recesses 22 is 80° or more and 100° or less, for example, 90°, and the arrangement pitch p of the recesses 22 is, for example, about 100 μm. Here, the apex angle θ of the concave portion 22 is a plane (longitudinal section) perpendicular to the second surface 21b (horizontal plane) of the light diffusion sheet 43, passing through the vertex of the inverted polygonal pyramid and facing the vertex. The angle formed by the cross-sectional lines of the slopes in the cross section that appears when a pair of slopes are cut perpendicularly across. The arrangement pitch p of the recesses 22 is the horizontal distance (distance along the direction parallel to the second surface 21b) between the vertices of the inverted polygonal pyramids in each of the adjacent recesses 22 .

In this embodiment, the light diffusing sheet 43 has a one-layer structure of the base material layer 21 having an uneven shape (recesses 22) on the first surface 21a. However, instead of this, the light diffusing sheet 43 may be configured with a two-layer structure of a substrate layer having flat surfaces on both sides and a layer having an uneven surface on one surface, or may have an uneven surface on one surface. It may be configured with a structure of three or more layers including layers.

In addition, in the present embodiment, the recesses 22 having an inverted pyramid shape (substantially inverted square pyramid shape) are arranged two-dimensionally to provide an uneven shape on the first surface 21a. Alternatively, the recesses 22 may be arranged randomly to the extent that the effects of the present invention are not lost.

In the present disclosure, in consideration of the fact that it is difficult to form a geometrically strict inverted polygonal pyramid recess using a normal shape transfer technique, the term “substantially inverted polygonal pyramid” is used, but “substantially "Inverted polygonal pyramid" shall include shapes that can be regarded as true or substantially inverted polygonal pyramids. Further, the word "substantially" means that it can be approximated, and for example, "substantially inverted quadrangular pyramid" means a shape that can be approximated to an inverted quadrangular pyramid. For example, an "inverted polygonal truncated pyramid" with a flat top is also included in the "substantially inverted polygonal pyramid" if the top area is small to the extent that the effects of the present invention are not lost. Further, the "substantially inverted polygonal pyramid" also includes a shape deformed from the "inverted polygonal pyramid" within the range of unavoidable variations in shape due to the processing accuracy in industrial production.

Moreover, as the "inverted polygonal pyramid" shape of the concave portion 22, a triangular pyramid, a square pyramid, or a hexagonal pyramid that can be two-dimensionally arranged without gaps is preferable. An inverted quadrangular pyramid may be selected as the "inverted polygonal pyramid" in consideration of the accuracy of the surface cutting work of the mold (metal roll) used in the manufacturing process such as extrusion molding and injection molding when providing the recess 22. . When the recesses 22 are regularly arranged two-dimensionally, the recesses 22 may be provided without gaps on the first surface 21a, or may be provided at predetermined intervals.

The first prism sheet 44 and the second prism sheet 45 are formed, for example, so that a plurality of grooves having an isosceles triangular cross section are adjacent to each other, and the apex angle of the prism sandwiched between a pair of adjacent grooves is It is a film formed at an angle of about 90°. Each groove formed in the first prism sheet 44 and each groove formed in the second prism sheet 45 are arranged so as to be orthogonal to each other. The first prism sheet 44 and the second prism sheet 45 may be integrally formed. As the first prism sheet 44 and the second prism sheet 45, for example, PET (polyethylene terephthalate) film may be formed into a prism shape using a UV curable acrylic resin.

As the polarizing sheet 46, for example, DBEF series manufactured by 3M may be used. The polarizing sheet 46 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.

According to the light diffusion sheet 43 of the present embodiment described above, the first surface 21a, which is the light exit surface, is provided with a plurality of concave portions 22 having a substantially inverted polygonal pyramid shape, and the second surface 21b, which is the light incident surface, is provided. has an arithmetic mean roughness of 3.0 μm or less and an internal haze of 1.5% or less. Therefore, the light incident from the second surface 21b reaches the first surface 21a, which is an uneven surface, without being substantially diffused inside the light diffusion sheet 43 (base material layer 21). Therefore, the high-intensity light traveling straight from the light source 42 toward the light diffusion sheet 43 can be uniformly diffused by the concave portions 22 of the first surface 21a, thereby eliminating the image of the light source 42 on the display screen 50a. In-plane luminance uniformity can be improved. Therefore, it is possible to cope with further thinning and reduction in the number of light sources.

FIG. 4 shows a cross-sectional configuration of a light diffusion sheet 43A of a comparative example in which uneven shapes are provided on the second surface 21b by embossing. 4, the same components as those of the light diffusion sheet 43 of the present embodiment shown in FIG. 3 are denoted by the same reference numerals. In the light diffusion sheet 43A of the comparative example, since the light coming straight from the light source 42 is randomly diffused by the second surface 21b, the light cannot be uniformly diffused by the concave portions 22 of the first surface 21a. In other words, the degree to which the image of the light source 42 is eliminated differs depending on the position of the light source 42 on the first surface 21a. As a result, there arises a problem that the in-plane luminance uniformity deteriorates.

To solve this problem, in the light diffusion sheet 43, the arithmetic average roughness of the second surface 21b, which is the light incident surface, is set to 3.0 μm or less. From the viewpoint of improving the in-plane luminance uniformity, the arithmetic average roughness of the second surface 21b of the light diffusion sheet 43 is preferably 0.5 μm or less, more preferably 0.3 μm or less. It is more preferably 0.1 μm or less, and even more preferably 0.05 μm or less. On the other hand, from the viewpoint of suppressing a decrease in luminance, the arithmetic average roughness of the second surface 21b of the light diffusion sheet 43 is preferably 0.1 μm or more, more preferably 0.5 μm or more. It is more preferably 0 μm or more.

Further, the problem that the in-plane luminance uniformity deteriorates also occurs when fine particles (diffusing agent) having a refractive index different from that of the base layer 21 are dispersed in the base layer 21 to diffuse light. That is, in the light diffusion sheet 43 of the present embodiment, the content of the diffusing agent, that is, the internal haze, is preferably as small as possible. Specifically, the internal haze of the light diffusion sheet 43 is preferably 5% or less, more preferably 3% or less, more preferably 1.5% or less, and 1.0% or less. is even more preferable.

In the light diffusion sheet 43 of the present embodiment, when the concave portions 22 are formed in a substantially inverted quadrangular pyramid shape, the light coming straight from the light source 42 can be uniformly diffused on the first surface 21a.

In the light diffusion sheet 43 of the present embodiment, when the apex angle of the concave portions 22 is 80° or more and 100° or less, light coming straight from the light source 42 can be uniformly diffused on the first surface 21a.

The backlight unit 40 of this embodiment is incorporated in the liquid crystal display device 50, and guides light emitted from the plurality of light sources 42 to the display screen 50a side. In the backlight unit 40 , the light diffusion sheet 43 of the present embodiment is arranged between the display screen 50 a and the light source 42 with the second surface 21 b facing the light source 42 . Therefore, the light diffusing sheet 43 can improve the in-plane luminance uniformity, so that it is possible to further reduce the thickness and reduce the number of light sources.

In the backlight unit 40 of this embodiment, the light source 42 may be arranged on the reflective sheet 41 provided on the opposite side of the display screen 50 a when viewed from the light diffusion sheet 43 . By doing so, the light is further diffused by multiple reflections between the light diffusion sheet 43 and the reflection sheet 41, so that the in-plane luminance uniformity is further improved.

In the backlight unit 40 of the present embodiment, a plurality of light diffusion sheets 43 may be laminated and arranged between the display screen 50 a and the light source 42 . With this configuration, the light coming straight from the light source 42 is repeatedly diffused by the first surface 21a of each light diffusion sheet 43, so that the in-plane luminance uniformity is further improved.

In the backlight unit 40 of the present embodiment, when the distance between the light source 42 and the light diffusion sheet 43 is 10 mm or less, the diffusion performance of the light diffusion sheet 43 suppresses the deterioration of the in-plane luminance uniformity more than before. can do.

The liquid crystal display device 50 of this embodiment includes the backlight unit 40 of this embodiment and the liquid crystal display panel 5 . Therefore, the backlight unit 40 can improve the in-plane luminance uniformity, so that it is possible to further reduce the thickness and reduce the number of light sources. Similar effects can be obtained in information equipment (personal computers, mobile phones, etc.) in which the liquid crystal display device 50 of the present embodiment is incorporated.

In this embodiment, the number of light sources 42 to be arranged is not particularly limited. Arranging regularly means arranging with a certain rule, and for example, the case where the light sources 42 are arranged at regular intervals corresponds to this. When the light sources 42 are arranged at regular 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.

Further, in the present embodiment, the light diffusion sheet 43 (base layer 21) may contain a diffusing agent and other additives as long as the effects of the present invention are not lost. Additives that can be contained are not particularly limited, but may be, for example, inorganic particles such as silica, titanium oxide, aluminum hydroxide, barium sulfate, etc.; They may be organic particles.

In the present embodiment, the resin that forms the matrix of the base layer 21 is not particularly limited as long as it is made of a material that allows light to pass through. Polymerization) resin, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyimide, and the like.

In addition, in the present embodiment, the thickness of the light diffusion sheet 43 is not particularly limited. may be When the thickness of the light diffusion sheet 43 exceeds 3 mm, it becomes difficult to achieve thinning of the liquid crystal display. On the other hand, when the thickness of the light diffusing sheet 43 is less than 0.1 mm, it becomes difficult to exhibit the aforementioned effect of improving the brightness uniformity. The light diffusion sheet 43 may be film-like or plate-like.

Moreover, in the present embodiment, the method of manufacturing the light diffusion sheet 43 is not particularly limited, but for example, an extrusion molding method, an injection molding method, or the like may be used.

The procedure for producing a single-layer light diffusion sheet having an uneven surface by extrusion molding is as follows. First, pellet-shaped plastic particles (which may contain a diffusing agent) are put into a single-screw extruder, melted and kneaded while being heated. After that, the molten resin extruded by the T-die is sandwiched between two metal rolls, cooled, transported using guide rolls, and cut into flat plates by a sheet cutter to produce a light diffusion sheet. . Here, by sandwiching the molten resin using metal rolls having a surface that is the reverse of the desired uneven shape, the reverse shape of the roll surface is transferred to the resin. can be shaped into In addition, since the shape transferred to the resin does not always correspond to the shape of the roll surface that is 100% transferred, the shape of the roll surface may be designed by calculating backward from the degree of transfer.

When manufacturing a light diffusion sheet with a two-layer structure having an uneven surface using an extrusion molding method, for example, pellet-shaped plastic particles necessary for forming each layer are supplied to each of two single-screw extruders. After charging, the same procedure as described above is performed for each layer, and each sheet thus produced may be laminated.

Alternatively, as described below, a diffusion sheet having a two-layer structure having an uneven surface may be produced. First, pellet-like plastic particles necessary for forming each layer are put into each of two single-screw extruders, melted and kneaded while being heated. After that, the molten resin for each layer is put into one T-die, laminated in the T-die, and the laminated molten resin extruded by the T-die is sandwiched between two metal rolls and cooled. After that, the laminated molten resin may be conveyed using guide rolls and cut into flat plates with a sheet cutter to produce a diffusion sheet having a two-layer structure having an uneven surface.

Further, the light diffusion sheet 43 may be manufactured as follows by shape transfer using UV (ultraviolet). First, a roll having an inverted shape of the uneven shape to be transferred is filled with an uncured UV curable resin, and the substrate is pressed against the resin. Next, in a state in which the roll filled with the ultraviolet curable resin and the substrate are integrated, the resin is cured by irradiating ultraviolet rays. Next, the sheet on which the concavo-convex shape has been shape-transferred by the resin is separated from the roll. Finally, the sheet is again irradiated with ultraviolet rays to completely harden the resin, thereby producing a light diffusion sheet having an uneven surface.

Further, in this embodiment, as the backlight unit 40, a direct type backlight unit in which a plurality of light sources 42 are dispersedly arranged on the back side of the display screen 50a of the liquid crystal display device 50 is used. Therefore, in order to miniaturize the liquid crystal display device 50, it is necessary to reduce the distance between the light source 42 and the light diffusion sheet 43. FIG. However, when this distance is reduced, a phenomenon (brightness unevenness) is likely to occur in which the brightness of the display screen 50a in the portion located on the area between the light sources 42 dispersedly arranged is lower than that in the other portions.

In contrast, using the light diffusion sheet 43 of the present embodiment is useful for suppressing luminance unevenness. In particular, in anticipation of future thinning of small and medium-sized liquid crystal displays, the distance between the light source and the light diffusion sheet is set to 15 mm or less, preferably 10 mm or less, more preferably 5 mm or less, further preferably 2 mm or less, and ultimately 0 mm. In this case, the usefulness of the present invention is considered to be even more pronounced.

(Examples and Comparative Examples)
Examples and comparative examples are described below.

In Examples and Comparative Examples, a light diffusion sheet having a substrate layer with a thickness of 130 μm and a base material of clear polycarbonate was used. In both the examples and the comparative examples, a plurality of approximately inverted quadrangular pyramid-shaped (inverted pyramid-shaped) depressions having a vertical angle of 90° were two-dimensionally arranged at a pitch of 100 μm on the first surface (light emission surface) of the light diffusion sheet. did. As examples, four types of light diffusion sheets having second surfaces (light incident surfaces) processed to have arithmetic average roughness Ra of 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm were used. prepared. As a comparative example, a light diffusion sheet having a second surface (light incident surface) processed to have an arithmetic mean roughness Ra of 3.4 μm was prepared.

A method for manufacturing the light diffusion sheet of the example is as follows. First, a pellet-shaped base material resin (plastic resin) was made into a resin film by an extruder. After that, a roll with a convex pyramid shape is used as one of the two metal rolls, and a mirror surface roll is used as the other roll. , and a single-layer light diffusion sheet having a mirror surface on the other side.

A method for manufacturing the light diffusion sheet of the comparative example is as follows. First, a pellet-shaped base material resin (plastic resin) was made into a resin film by an extruder. After that, one of the two metal rolls is a roll having a convex pyramid shape on the surface, and the other roll is an embossing roll having a random mat shape, and both rolls are pressed against the resin film. A single-layer light diffusion sheet having an inverted pyramid shape on one side and an embossed shape on the other side was fabricated. The difference in roughness of the embossed surface was controlled by the roughness of the embossing roll surface.

The surface roughness (arithmetic mean roughness Ra) of the light diffusion sheets of Examples and Comparative Examples was measured using SJ-210 manufactured by Mitutoyo Co., Ltd., in accordance with JIS B 0601-1994, at a measurement speed of 0.5 mm/s. , the measurement distance was set to 4 mm, and the cutoff value λc was set to 0.8 mm.

The internal haze and total light transmittance of the light diffusion sheet of Example were 0.6% and 90.8%, respectively. The internal haze and total light transmittance were measured by filling the depressions (inverted pyramids) on the first surface of the light diffusion sheet with a UV curable resin (the same resin as the matrix resin of the light diffusion sheet). A resin having the same refractive index as the matrix resin of the light diffusion sheet was used as the UV curable resin. The internal haze and total light transmittance were measured according to JIS K 7136 using a haze meter HZ-2 manufactured by Suga Test Instruments Co., Ltd.

The in-plane luminance uniformity of the light diffusion sheets of Examples and Comparative Examples was evaluated as follows. First, on a blue LED array arranged at a pitch of 2.8 mm, two or three light diffusion sheets of Examples (four types) and Comparative Example are laminated and arranged, and two prism sheets are placed thereon. Place a transparent glass plate on top of it to suppress the floating of the sheets, and use a two-dimensional color luminance meter UA-200 manufactured by Topcon Technohouse Co., Ltd. Vertically upward (from the LED array to the glass plate direction) was measured. Next, the obtained two-dimensional luminance distribution image is corrected for variations in the emission intensity of individual LEDs, and filtered to suppress bright and dark point noise caused by foreign matter. Average values and standard deviations were calculated for the brightness of the pixels. Finally, the “in-plane luminance uniformity” was defined as “luminance average value/luminance standard deviation” to calculate the in-plane luminance uniformity of the light diffusion sheets of Examples and Comparative Examples.

Table 1 shows the evaluation results of luminance and in-plane luminance uniformity of the light diffusion sheets of Examples and Comparative Examples. The luminance shown in Table 1 is a relative luminance with the luminance (average value) of the comparative example having the same number of superimposed sheets set to 1.

In Table 1, Examples 1 to 4 have the second surface (light incident surface) processed so that the arithmetic mean roughness Ra is 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm, respectively. This is the result of evaluating luminance and in-plane luminance uniformity by stacking two light diffusion sheets of the above-described example. In addition, in Comparative Example 1, two light diffusion sheets of the above Comparative Example having the second surface processed so as to have an arithmetic mean roughness Ra of 3.4 μm were superimposed, and luminance and in-plane luminance uniformity were measured. This is the result of evaluation. Further, Examples 5 to 8 have the second surface processed so that the arithmetic average roughness Ra is 2.6 μm, 1.8 μm, 1.2 μm, and 0.03 μm, respectively. This is the result of evaluating luminance and in-plane luminance uniformity by stacking three sheets. In addition, in Comparative Example 2, three light diffusion sheets of the above Comparative Example having the second surface processed so as to have an arithmetic mean roughness Ra of 3.4 μm were stacked, and luminance and in-plane luminance uniformity were measured. This is the result of evaluation.

FIG. 5 shows the relationship between the surface roughness (arithmetic mean roughness) Ra of the light incident surface of the light diffusion sheet and the in-plane luminance uniformity in Examples 1 to 8 and Comparative Examples 1 and 2, respectively. Further, FIG. 6 shows the relationship between the surface roughness (arithmetic mean roughness) Ra of the light incident surface of the light diffusion sheet and the luminance in Examples 1 to 8 and Comparative Examples 1 and 2, respectively.

As shown in Table 1 and FIG. 5, regardless of the number of stacked light diffusion sheets, as the surface roughness Ra of the light incident surface decreased, the in-plane luminance uniformity improved. In particular, when the light incident surface had the smallest surface roughness Ra of 0.03 μm (mirror surface) (Examples 4 and 8), the in-plane luminance uniformity was maximized for each number of layers. Further, when the surface roughness Ra of the light incident surface was 3.0 μm or less, the deterioration of the in-plane luminance uniformity compared to the mirror surface was suppressed for each number of stacked layers.

On the other hand, as shown in Table 1 and FIG. 6, when the light incident surface had the smallest surface roughness Ra of 0.03 μm (mirror surface) (Examples 4 and 8), the luminance decreased with each number of layers. It was observed. Moreover, when the surface roughness Ra of the light incident surface was 1.0 μm or more, no decrease in luminance was observed for each number of stacked layers.

From the results shown in Table 1, FIG. 5, and FIG. It was found that the conditions of luminance and in-plane luminance uniformity could be satisfied. For example, in a product that requires improvement of in-plane luminance uniformity and suppression of luminance decrease, the surface roughness Ra of the light incident surface of the light diffusion sheet may be set to 1.0 μm or more and 3.0 μm or less.

Although the embodiments (including examples; hereinafter the same) of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the disclosure. That is, the foregoing description of the embodiments is merely illustrative in nature and is not intended to limit the present disclosure, its applications or uses.

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 material layer 21a first surface 21b second surface 22 concave portion 40 backlight unit 41 reflective sheet 42 light source 43 light diffusion sheet 44 first prism sheet 45 second prism sheet 46 polarizing sheet 50 liquid crystal display device 50a display screen

Claims (9)

A light diffusion sheet having a first surface serving as a light emitting surface and a second surface serving as a light incident surface,
The first surface is provided with a plurality of substantially inverted polygonal pyramid-shaped recesses,
The arithmetic mean roughness of the second surface is 1.0 μm or more and 3.0 μm or less,
A light diffusion sheet having an internal haze of 1.5% or less. 2. The light diffusion sheet according to claim 1, wherein the plurality of concave portions are formed in a substantially inverted quadrangular pyramid shape. 3. The light diffusion sheet according to claim 1, wherein the apex angles of the plurality of concave portions are 80[deg.] or more and 100[deg.] or less. A backlight unit that is incorporated in a liquid crystal display device and guides light emitted from a plurality of light sources to a display screen side,
The light diffusion sheet according to any one of claims 1 to 3 is provided between the display screen and the plurality of light sources,
The light diffusion sheet is a backlight unit arranged with the second surface facing the plurality of light sources. 5. The backlight unit according to claim 4, wherein the plurality of light sources are arranged on a reflective sheet provided on the opposite side of the display screen when viewed from the light diffusion sheet. 6. The backlight unit according to claim 4, wherein a plurality of said light diffusion sheets are laminated and arranged between said display screen and said plurality of light sources. 7. The backlight unit according to claim 4, wherein the distance between the plurality of light sources and the light diffusion sheet is 10 mm or less. a backlight unit according to any one of claims 4 to 7;
A liquid crystal display device comprising a liquid crystal display panel. An information equipment comprising the liquid crystal display device according to claim 8 .