JP7286269B2 - backlight unit - Google Patents

Description

The present invention relates to a light diffusion plate and a backlight unit. More specifically, the present invention relates to a light diffuser plate having high light transmittance and light diffusing properties and improving the luminance and uniformity of a liquid crystal display, and a backlight unit including the light diffuser plate.

2. Description of the Related Art Light diffusing plates are used in lighting fixtures, liquid crystal display devices, and the like to uniformly diffuse light from a light source. A backlight system in which a liquid crystal layer is illuminated from behind to emit light is widely used in liquid crystal display devices, and a backlight unit is provided on the lower surface side of the liquid crystal layer. As backlight units, there are known an edge light system in which a light source is arranged at the end of a rectangular light guide plate and a direct backlight system in which a light source is directly below a liquid crystal display. A backlight unit is generally equipped with a light diffusing plate and an optical sheet such as a prism sheet on the surface side of the light diffusing plate. In particular, in the case of a direct type backlight of a liquid crystal display, there is a possibility that the light source can be seen through.

As a light diffusing plate, a light diffusing plate using polycarbonate as a forming material is known. Polycarbonate resin has a wide range of applications as a thermoplastic resin with excellent impact resistance, heat resistance, and transparency.In addition, it is lighter than inorganic glass and has excellent productivity, so it provides light diffusion. By doing so, it can be suitably used for applications requiring light diffusibility, such as lighting covers, lighting signboards, transmissive screens, various displays, and light diffusion sheets for liquid crystal display devices. Inorganic compounds such as glass, silica, and aluminum hydroxide have been added to impart light diffusibility to polycarbonate resins, but they have low thermal stability during molding and extrusion, and have poor impact strength. Mechanical strength may decrease. As a method for improving such drawbacks, it is known to use organic fine polymer particles having a large difference in refractive index as a diffusing agent.

The light diffusion plate of Patent Document 1 is made from a composition in which 0.05 to 0.5 parts by weight of inorganic or organic fine particles having a particle size of 50 μm or less and an average particle size of 5 to 20 μm are blended with 100 parts by weight of a polycarbonate resin. It is a polycarbonate resin high transmittance light diffusion plate having a total light transmittance of 85% or more and a diffuse transmittance of 10 to 30%. It is said that this light diffusion plate has excellent surface smoothness, and can produce clear images without iridescent luster even when used on large screens, but further investigation is required for light diffusion.

Patent Document 2 discloses a film having a thickness of 30 to 300 μm formed from a composition comprising 100 parts by weight of a polycarbonate resin and 1 to 20 parts by weight of inorganic and organic particles having an average particle size of 1 to 25 μm. A highly transmissive light diffusion film made of polycarbonate resin having a transmittance of 92% or more and a haze of 60% or more is described. However, there is no mention of optical properties such as total light transmittance and haze when the thickness exceeds 300 μm. In addition, if this light diffusion film is used in a direct type backlight unit, the light diffusion and hiding properties are insufficient, and the light source may be seen through.

The light diffusion plate of Patent Document 3 contains a total of 100 parts by weight of 99.7 to 80% by weight of polycarbonate resin, 0.3 to 20% by weight of transparent fine particles having an average particle size of 1 to 30 μm, and 0.0005 to 0.0005 of a fluorescent brightening agent. A light diffusing plate for a direct type backlight made of polycarbonate resin and having a thickness of 0.5 to 3.0 mm, which is formed from a resin composition containing 1 part by weight, and whose brightness is improved by using a fluorescent brightening agent. Therefore, the heat stability is insufficient, and discoloration may occur during molding and recycling.

In Patent Document 4, 0.01 to 2 parts by weight of a polyorganosiloxane having a phenyl group in at least a side chain and having a viscosity of 1 to 1000 cSt at 25 ° C. is added to 100 parts by weight of an aromatic polycarbonate resin. and 0.05 to 2 parts by weight of silicone diffusion fine particles having a weight average diameter of 0.7 to 5 μm, and a light diffusing member using this resin composition. there is This light diffusing resin composition is said to have both high light diffusibility and high total light transmittance at the same time, but the total light transmittance is 60% or less, and regarding compatibility between light transmittance and light diffusivity need further study.

Patent Documents 1 to 4 describe the light diffusibility and light transmittance of the light diffusion plate or light diffusion film alone, but when used in combination with the prism sheet, the light transmittance and light Diffusion is not mentioned. In other words, no evaluation or study has been made on the light diffusion plate when combined with an optical sheet such as a prism sheet, corresponding to the performance of an actual backlight unit.

JP-A-05-257002 Japanese Patent Application Laid-Open No. 2003-240921 Japanese Patent Application Laid-Open No. 2004-29091 JP-A-2006-63121

The inventors of the present invention have found that when a light diffusion plate having excellent light diffusion and light transmission properties is used in combination with a prism sheet, the excellent light diffusion and light transmission properties are not necessarily exhibited. and made intensive studies on a light diffusing plate that exhibits the most excellent light diffusibility and light transmittance when combined with a prism sheet. That is, the present invention provides a light diffusion plate that exhibits excellent light diffusion and light transmission properties when used in combination with a prism sheet, and a backlight unit that can improve the brightness and uniformity of a liquid crystal display. The task is to provide

As a result of intensive studies by the present inventors in order to solve the above problems, it was found that light transmittance and light diffusibility can be improved by adding a specific amount of a silicone-based light diffusing agent having a specific particle size to a polycarbonate resin. In particular, when used in combination with a prism sheet in a direct type backlight unit, the present inventors have found a light diffusion plate that exhibits excellent light transmittance and light diffusion, thereby completing the present invention.

One aspect of the present invention, which has been made to solve the above problems, has a resin matrix and a diffusing agent dispersed in the resin matrix, the resin matrix is an aromatic polycarbonate, and the diffusing agent is a silicone-based It is a diffusing agent, and the average particle diameter D of the silicone-based diffusing agent is 0.8 μm or more and 12 μm or less, and the content of the silicone-based diffusing agent with respect to 100 parts by mass of the resin matrix is the average particle diameter of the silicone-based diffusing agent. D _OM mass parts or more in the following formula (1) represented by D, D _OL mass parts or less in the following formula (2), a total light transmittance of 60% or more, and a thickness of 0.5 mm or more It is a light diffusion plate having a thickness of 1.8 mm or less.
_DOM = 0.0625 x D + 0.06 (1)
D _OL =0.20×D+1.70 (2)

The light diffusing plate is a light diffusing plate in which an aromatic polycarbonate and a silicone-based diffusing agent are mixed. In the light diffusion plate, the average particle diameter D of the silicone-based diffusing agent is within the above range, and the content of the silicone-based diffusing agent with respect to 100 parts by mass of the aromatic polycarbonate is the value of the above formula (1) or more, and the above formula (2 ), the light diffusibility is good. The light diffusing plate has a total light transmittance equal to or higher than the above lower limit and a thickness within the above range, and thus exhibits a high light transmittance. The light diffusion plate can exhibit particularly excellent optical properties when used in combination with a prism sheet.

The light diffusion plate preferably has a surface roughness (Ra) of 1 μm or more and 6 μm or less. Thereby, the light diffusibility of the light diffusion plate can be improved.

The light diffusion plate preferably has a back surface roughness (Ra) of 0.01 μm or more and 0.05 μm or less. Thereby, the light diffusibility of the light diffusion plate can be further improved.

The light diffuser plate is preferably formed by an extrusion molding method. Thereby, the said light diffusion plate can be obtained easily.

Another aspect of the present invention, which has been made to solve the above problems, is a backlight unit including the light diffusion plate and a prism sheet having a plurality of prisms extending in one direction on its surface. Since the backlight unit includes a prism sheet and a light diffusion plate that exhibits excellent optical properties in combination with the prism sheet, it is possible to improve the brightness and uniformity of the liquid crystal display.

The backlight unit preferably further includes a prism sheet having a plurality of prisms extending in a direction perpendicular to the one direction. Thereby, the brightness and uniformity of the liquid crystal display can be further improved.

As described above, the light diffusing plate of the present invention has excellent light diffusibility and light transmittance, and when used in combination with a prism sheet, can exhibit particularly excellent optical properties. Also, the backlight unit of the present invention can improve the brightness and uniformity of the liquid crystal display.

FIG. 4 is a schematic side view showing two prism sheets used in Examples of the present invention; 2 is a schematic top view of the two prism sheets of FIG. 1; FIG. FIG. 4 is a schematic side view showing an embodiment in which the light diffusibility of the light diffusion plate according to the present invention was measured;

Embodiments of the light diffusing plate and the backlight unit of the present invention will be described in detail below. In the description, "front surface" indicates the surface from which light is emitted, and "back surface" indicates the opposite surface.

[Backlight unit]
A backlight unit that is an embodiment of the present invention mainly includes a light source, a light diffusion plate, and a prism sheet. The backlight unit is particularly suitable for use in liquid crystal displays. In this embodiment, a direct type backlight unit will be described.

<Light source>
The light source is arranged on the rear surface of the light diffusion plate and emits light toward this rear surface. The light source is not particularly limited, and for example, a light emitting diode (LED) can be used. It is preferable that one or a plurality of light emitting diodes are arranged so as to illuminate the entire rear surface of the light diffusion plate.

<Prism sheet>
The prism sheet is arranged so that its back surface faces the front surface of the light diffusion plate. The back surface of the prism sheet is close to or in contact with the front surface of the light diffusion plate. Since the prism sheet is required to transmit light, it is preferably made mainly of a transparent, particularly colorless and transparent synthetic resin. The prism sheet has a plurality of prisms extending in one direction on its surface. The prism sheet improves the light output performance of the backlight unit by refracting the light emitted from the light source and transmitted through the light diffusion plate by the prisms. The plurality of prisms may be provided on the back surface.

The prisms are convex portions arranged in parallel rows on the surface of the prism sheet. The cross-sectional shape of the prism is not particularly limited, but may be semicircular, semielliptical, equilateral triangular, isosceles triangular, or the like. Although the height from the base to the vertex of the prism is not particularly limited, it can be, for example, 5 μm or more and 20 μm or less. The average ridge line interval (average pitch) between a prism and a prism adjacent to this prism is not particularly limited, but can be, for example, 10 μm or more and 50 μm or less.

It is preferable that the backlight unit further includes a prism sheet having a plurality of prisms extending in a direction orthogonal to the one direction on its surface. For example, two identical prism sheets may be used, and the two prism sheets may be arranged such that the prisms of one prism sheet are perpendicular to the prisms of the other prism sheet. By providing a plurality of prisms and prisms perpendicular to the prisms, the light output performance of the backlight unit can be further improved.

<Light diffusion plate>
The light diffusion plate is arranged between the light source and the prism sheet. The light diffusion plate has a plate shape with a substantially uniform thickness. The light diffusion plate has a resin matrix and a diffusing agent dispersed in the resin matrix. When incident light from the rear surface is transmitted to the front surface of the light diffusion plate, the light diffusion plate diffuses the transmitted light with a diffusing agent in the resin matrix to make the emitted light uniform. The light diffusion plate is one aspect of the present invention.

The lower limit of the average thickness of the light diffusion plate is 0.5 mm, preferably 0.7 mm, more preferably 0.8 mm. On the other hand, the upper limit of the average thickness of the light diffusion plate is 1.8 mm, preferably 1.5 mm, more preferably 1.2 mm. If the average thickness of the light diffusing plate is less than the above lower limit, the light diffusing plate cannot obtain sufficient light diffusibility and hiding property, and the light source may be seen through. In addition, the mechanical strength is lowered, and cracks may occur during processing, use, and the like. On the other hand, if the average thickness of the light diffusing plate exceeds the above upper limit, the total light transmittance may decrease, and the size of equipment employing the light diffusing plate may increase.

The light diffusion plate has a lower limit of 60% for total light transmittance. The lower limit of the total light transmittance is preferably 62%, more preferably 65%. The total light transmittance is determined by the light transmittance of the resin matrix, the average particle size and addition amount of the silicone-based diffusing agent, and other additives. If the total light transmittance is less than the above lower limit, the amount of light transmitted is reduced, the loss of the amount of light output is increased, and the brightness of the liquid crystal display surface may not be sufficiently obtained. The total light transmittance means a value measured according to JIS-K7105:1981.

The surface roughness (Ra) of the surface of the light diffusion plate is preferably 1 μm or more and 6 μm or less. The lower limit of the surface roughness (Ra) of the surface is more preferably 2 μm, still more preferably 2.8 μm. On the other hand, the upper limit of the surface roughness (Ra) of the surface is more preferably 5 μm, more preferably 4.2 μm. By setting the surface roughness (Ra) of the surface of the light diffusing plate within the above range, the light diffusibility can be improved. The surface roughness Ra means a value measured with a cutoff (λc) of 2.5 mm and an evaluation length (l) of 12.5 mm according to JIS-B0601:2001.

The surface roughness (Ra) of the rear surface of the light diffusion plate is preferably 0.01 μm or more and 0.05 μm or less. The lower limit of the surface roughness (Ra) of the back surface is more preferably 0.015 μm, still more preferably 0.02 μm. On the other hand, the upper limit of the surface roughness (Ra) of the back surface is more preferably 0.04 μm, and still more preferably 0.035 μm. By setting the surface roughness (Ra) of the back surface of the light diffusing plate within the above range, the light diffusibility can be further improved. In addition, when a film, optical sheet, etc. is attached to the light diffusion plate in order to impart functions such as light reflectivity and light selective transmission to the backlight unit, or a functional coating layer, vapor deposition layer, etc. When laminated on a light diffusing plate, the back surface of the light diffusing plate has a surface roughness (Ra) within the above range, so that the adhesiveness to the film, coating layer, or the like can be improved.

In addition, the surface of the light diffusing plate may be subjected to uneven processing such as embossing, V-groove processing, ridged processing, or the like by a known method in order to increase the light diffusibility. By performing these processes, the light diffusibility can be further improved.

<Resin matrix>
The resin matrix forming the light diffusion plate is aromatic polycarbonate. That is, the content of the polycarbonate-based resin with respect to the resin matrix is 100% by mass. Since the light diffusion plate employs a polycarbonate-based resin as a resin matrix, it is excellent in transparency, impact resistance, flame retardancy, and dimensional stability.

As the polycarbonate resin, for example, one obtained by reacting a dihydric phenol and a carbonate precursor by an interfacial polymerization method or a melting method can be used. Examples of dihydric phenols include 2,2-bis(4-hydroxyphenyl)propane called bisphenol A, 1,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2-bis(3-methyl-4-hydroxyphenyl)propane, 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)sulfide, bis(4-hydroxy phenyl) sulfone and the like, among which bisphenol A is preferred. These dihydric phenols can be used alone or in combination of two or more.

Carbonate precursors include, for example, carbonyl halides, carbonate esters, haloformates, and the like. Specific examples include phosgene, diphenyl carbonate, dihaloformate of dihydric phenol, and the like.

When the dihydric phenol and the carbonate precursor are reacted by an interfacial polymerization method or a melting method to produce a polycarbonate resin, a molecular weight modifier, a catalyst, or the like can be used. Furthermore, the polycarbonate resin may contain additives such as release agents such as esters and partial esters of polyhydric alcohols and fatty acids, heat stabilizers such as phosphites, phosphates and phosphonates, benzotriazoles, acetophenone A system, an ultraviolet absorber such as a salicylic acid ester, an antistatic agent, a coloring agent, a whitening agent, a flame retardant, and the like may be added. Further, the polycarbonate resin may be a branched polycarbonate resin obtained by copolymerizing a trifunctional or higher polyfunctional aromatic compound, or a polyester carbonate resin obtained by copolymerizing an aromatic or aliphatic bifunctional carboxylic acid. It is good also as a mixture which mixed 2 or more types of obtained polycarbonate resin.

As for the lower limit of the molecular weight of the polycarbonate resin, the viscosity-average molecular weight (M) is preferably 10,000 or more, more preferably 15,000 or more. On the other hand, the upper limit of the molecular weight of the polycarbonate resin is preferably 100,000 or less, more preferably 35,000 or less. A polycarbonate resin having such a viscosity-average molecular weight can provide sufficient strength and good melt fluidity during molding. The “viscosity-average molecular weight” is the specific viscosity (η _sp ) obtained from a solution of 0.7 g of a polycarbonate resin dissolved in 100 mL of methylene chloride at 20° C. and calculated by the following formulas (3) and (4). be.
η _sp /c=[η]+0.45×[η] ² c (3)
[η]=1.23×10 ⁻⁴ M ^0.83 (4)
However, [η] is the intrinsic viscosity. Moreover, "c" is a polymer concentration of 0.7 (g/100 ml).

<Diffusion agent>
The diffusing agent dispersed in the resin matrix is a transparent or translucent substantially spherical particle, and has a function of diffusing light transmitted through the resin matrix. The diffusing agent of the light diffusing plate is a silicone-based diffusing agent. The silicone-based diffusing agent is spherical fine particles having siloxane bonds, such as silica, silicone resin, silicone rubber, silicone composite powder which is spherical powder obtained by coating the surface of spherical silicone rubber powder with silicone resin, or a combination thereof. Used. Among them, silicone composite powder is preferred. Since the light diffusion plate employs a silicone-based diffusion agent as a diffusion agent, it is excellent in transparency and light diffusion.

The lower limit of the average particle diameter D of the silicone-based diffusing agent is 0.8 μm, preferably 0.9 μm, more preferably 1 μm. On the other hand, the upper limit of the average particle size of the silicone-based diffusing agent is 12 μm, preferably 8 μm, more preferably 6 μm. If the average particle size of the silicone-based diffusing agent is less than the above lower limit, sufficient light diffusibility may not be obtained. On the other hand, if the average particle size of the silicone-based diffusing agent exceeds the above upper limit, the transparency of the light diffusing plate may be lowered and the transmitted light in the resin matrix may not be uniformly diffused.

The content of the silicone-based diffusing agent with respect to 100 parts by mass of the resin matrix depends on the average particle diameter D (μm) of the silicone-based diffusing agent. Specifically, the lower limit of the content is preferably the value of _DOM in the following formula (1), and preferably the value of _DOM in the following formula (5). On the other hand, the upper limit of the content is the value of _DOL in the following formula (2), and preferably the value of _DOLB in the following formula (6). If the content is less than the lower limit, sufficient light diffusibility may not be obtained. On the other hand, if the content exceeds the upper limit, the light transmittance of the light diffuser plate may be reduced, the mechanical strength of the light diffuser plate may be insufficient, and sufficient light diffusibility may not be obtained. There is a possibility that it will not. That is, an increase in the content of the silicone-based diffusing agent does not increase the light diffusibility, and a light diffusion plate exhibiting excellent light diffusivity can be obtained by setting the content in the above range. .
_DOM = 0.0625 x D + 0.06 (1)
D _OL =0.20×D+1.70 (2)
D _OMB = 0.0625 x D + 0.275 (5)
D _OLB =0.20×D+0.70 (6)

[Method for manufacturing light diffusion plate]
The method for manufacturing the light diffusion plate is not particularly limited, but for example, a manufacturing method including a step of mixing a resin material and a diffusing agent and a step of molding the mixture can be adopted.

The method for molding the light diffusion plate is not particularly limited, and known methods for molding thermoplastic resins can be used. For example, from the viewpoint of productivity, injection molding, injection compression molding, and extrusion molding from a pelletized resin composition etc. can be mentioned. Alternatively, vacuum forming, air pressure forming, or the like may be employed from an extruded sheet-like molded product. Among them, the extrusion method is preferable from the viewpoint of productivity and manufacturing cost.

[advantage]
In the light diffusion plate, the average particle size of the silicone-based diffusing agent is within the above range, and the content of the silicone-based diffusing agent with respect to 100 parts by mass of the aromatic polycarbonate is within the above range, so it exhibits excellent light diffusing properties. while being excellent in light transmittance. In particular, when used in combination with a prism sheet in a backlight unit, excellent light diffusibility and light transmittance are exhibited.

In addition, since the backlight unit includes the light diffusion plate and the prism sheet, it is possible to improve the luminance and uniformity of various displays, and it is particularly suitable for use as a backlight unit for large-sized liquid crystal televisions.

[Other embodiments]
It should be considered that the embodiments disclosed this time are illustrative in all respects and not restrictive. The scope of the present invention is not limited to the configuration of the above-described embodiment, but is indicated by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims. be.

Although the direct type backlight unit has been described in the above embodiment, the light diffusion plate can also be used in an edge light system.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to the following examples.

[Light diffusion plate]
Example no. 1 to No. 23, Comparative Example No. 1 to No. 17 light diffusers were produced. The procedure for making the light diffusion plate is as follows. First, to 100 parts by mass of an aromatic polycarbonate, a mixture obtained by adding a silicone-based diffusing agent at a content shown in Table 1 is charged into a vented single-screw extruder with a screw diameter of 40 mm, and the charged mixture is melted at 250 ° C. After kneading while heating, pellets were obtained using a strand cutter. The pellets were dried at 120° C. for 6 to 8 hours using a hot air dryer to obtain raw material pellets. These raw material pellets were extruded into a plate at 250° C. using a vented single-screw extruder with a screw diameter of 60 mm by the T-die method. After cooling the extruded plate-shaped kneaded material between matte-surface rolls and mirror-surface rolls adjusted to 130° C., it was further cooled while being conveyed by the mirror-surface rolls. The extrusion rate and take-up speed were adjusted so that the average thickness of the light diffusion plate was the thickness shown in Table 1.

<Aromatic Polycarbonate>
Mitsubishi Gas Chemical Co., Ltd. "Iupilon S-3000" (melt mass flow rate: 15 g / 10 minutes)
<Silicone Diffusing Agent>
Shin-Etsu Chemical Co., Ltd. "X-52-7030" (Silicone composite powder, average particle size: 0.8 μm)
Shin-Etsu Chemical Co., Ltd. "KMP-605" (silicone composite powder, average particle size: 2.0 μm)
Shin-Etsu Chemical Co., Ltd. "KMP-600" (silicone composite powder, average particle size: 5.0 μm)
Shin-Etsu Chemical Co., Ltd. "KMP-601" (silicone composite powder, average particle size: 12.0 μm)
Shin-Etsu Chemical Co., Ltd. "X-52-854" (silicone composite powder, average particle size: 0.7 μm)
Shin-Etsu Chemical Co., Ltd. "KMP-602" (Silicone composite powder, average particle size: 30 μm)

[measurement]
Each measurement was performed on the prepared light diffusion plate and the combination of the light diffusion plate and the prism sheet using the following measuring instruments.

<Light source>
A CREE light-emitting diode “XP-G3 Royal Blue” was used as a light source.
<Brightness measurement camera>
It was measured using "RISA-COLOR/ONE" manufactured by Hyland Co., Ltd.
<Thickness>
It was measured using a dial microgauge manufactured by Mitutoyo Corporation.
<Surface roughness (Ra)>
The surface roughness (Ra) was measured according to JIS B-601 using SJ-210 manufactured by Mitutoyo Corporation.
<Total light transmittance and haze>
Total light transmittance and haze were measured using V-670 manufactured by JASCO Corporation, and HZ-2 manufactured by Suga Test Instruments Co., Ltd. for haze. The haze was measured according to JIS-K7361:2000.

[evaluation]
<Total light transmittance>
The evaluation of the total light transmittance of the light diffusion plate is as follows: "A" for a total light transmittance of 65% or more, "B" for a total light transmittance of 60% or more and less than 65%, and "B" for a total light transmittance of less than 60%. "C".

<Light diffusion>
As shown in FIG. 1, a plurality of prisms having an isosceles triangular cross-sectional shape, an average height of 12.5 μm, an apex angle (a) of 90°, and an average ridge interval (P) of 25 μm are provided on the surface, Two prism sheets having a total thickness (t) of 270 μm were prepared as a first prism sheet 1 and a second prism sheet 2 . As shown in FIG. 2, the prisms of each prism sheet were arranged so as to be perpendicular to each other, and as shown in FIG. A reflective polarizing film 4 (Dual Brightness Enhancement Film: DBEF) was further laminated on the surface side of the first prism sheet 1 to form a laminate 5, and the light diffusivity of this laminate 5 was measured. The light source 6 is placed at a position with a distance (D) of 4 mm from the center of the back surface of the laminate 5 in the normal direction, and at a position with a distance (H) of 140 mm from the center of the surface of the laminate 5 in the normal direction. A luminance measurement camera 7 was arranged to measure the luminance distribution of the light emitted from the laminate 5 two-dimensionally. Then, from the measured luminance distribution, a one-dimensional cross-sectional luminance distribution is obtained in the diameter direction of a circle including the central position showing the maximum luminance and centered on the light source 6, and the full width at half maximum is obtained from the obtained one-dimensional cross-sectional luminance distribution. Calculated. Evaluation of the diffusibility of the light diffusion plate, the calculated full width at half maximum (mm) value is 9.5 mm or more is "A", the value of the full width at half maximum is 9 mm or more and less than 9.5 mm is "B", the full width at half maximum A value of less than 9 mm was designated as "C".

<Comprehensive evaluation>
The overall evaluation of the light diffusion plate is "A" when both the total light transmittance and light diffusion are evaluated as A, and one of the total light transmittance and light diffusion is A and the other is B. "B" indicates that both total light transmittance and light diffusivity are B, and "C" indicates that at least one of total light transmittance and light diffusivity is C.

Example no. 1 to No. The results of Comparative Example No. 23 are shown in Table 1. 1 to No. 17 results are shown in Table 2.

Example no. 1 to No. In the light diffusion plate No. 23, the average particle diameter D of the silicone-based diffusing agent is in the range of 0.8 μm or more and 12 μm or less, and the content of the silicone-based diffusing agent with respect to 100 parts by mass of the resin matrix is the above formula (1). It is in the range of not less than D _OM mass parts and not more than D _OL mass parts in the above formula (2). Therefore, No. 1 to No. The light diffusing plate No. 23 has a total light transmittance of 60% or more and a full width at half maximum of 9.0 mm or more, and is a light diffusing plate excellent in light transmittance and light diffusion.

Comparative example no. 1 to No. In No. 12, the average particle diameter D of the silicone-based diffusing agent is in the range of 0.8 μm or more and 12 μm or less, but the content of the silicone-based diffusing agent with respect to 100 parts by mass of the resin matrix is _DOM mass parts of the above formula (1). Since it is less than or exceeds the _DOL mass part of the above formula (2), the full width at half maximum is less than 9 mm and the light diffusibility is insufficient. Also, Comparative Example No. In No. 2, the total light transmittance is less than 60% and the light transmittance is also insufficient. Comparative example no. 13 to NO. No. 16 is a _diffusing agent having an average particle diameter D of less than 0.8 μm or more than 12 μm. As described above, although it is equal to or less than the _DOL mass part of the above formula (2), the full width at half maximum is less than 9 mm, and the light diffusibility is insufficient. Also, Comparative Example No. In No. 17, since the thickness of the light diffusion plate is 0.4 mm, the full width at half maximum is less than 9 mm, and the light diffusion is insufficient.

INDUSTRIAL APPLICABILITY The light diffusing plate and backlight unit of the present invention have high light diffusibility and light transmittance, and therefore can be suitably used for large-sized liquid crystal displays and the like.

REFERENCE SIGNS LIST 1 first prism sheet 2 second prism sheet 3 light diffusion plate 4 reflective polarizing film 5 laminate 6 light source 7 luminance measurement camera

Claims (2)

having a resin matrix and a diffusing agent dispersed in the resin matrix,
The resin matrix is an aromatic polycarbonate,
The diffusing agent is a silicone-based diffusing agent,
The average particle diameter D of the silicone-based diffusing agent is 0.8 μm or more and 5.0 μm or less,
The content of the silicone-based diffusing agent with respect to 100 parts by mass of the resin matrix is 0.40 parts by mass or more and 1.00 parts by mass or less, and the following expressed using the average particle diameter D [μm] of the silicone-based diffusing agent not less than the _DOMB mass part of the formula (5) and not more than the _DOLB mass part of the following formula (6),
Total light transmittance is 60% or more,
The surface roughness (Ra) of the surface is 3.028 μm or more and 4.131 μm or less,
The surface roughness (Ra) of the back surface is 0.021 μm or more and 0.033 μm or less,
a light diffusion plate having a thickness of 0.5 mm or more and 1.8 mm or less;
a first prism sheet having a plurality of prisms extending in one direction on its surface;
a second prism sheet having, on its surface, a plurality of prisms extending in a direction orthogonal to the one direction ;
a light source that emits light toward the back surface of the light diffusion plate;
with
the plurality of prisms are convex portions arranged in parallel rows on the surface of the first prism sheet and the surface of the second prism sheet;
The height from the base to the vertex of the convex portion is 5 μm or more and 20 μm or less,
The average ridge line spacing of the convex portions is 10 μm or more and 50 μm or less,
The second prism sheet and the first prism sheet are laminated on the surface of the light diffusion plate, and a reflective polarizing film is laminated on the surface side to form a laminate, and the normal direction from the center of the back surface of this laminate A light source for measurement is placed at a position at a distance of 4 mm, and a luminance measurement camera is placed at a position at a distance of 140 mm in the normal direction from the center of the surface of the laminate, and the luminance distribution of the light emitted from the laminate is measured in two. dimension, obtain a one-dimensional cross-sectional luminance distribution in the diameter direction of a circle including the central position showing the maximum luminance from the obtained luminance distribution and centering on the light source for measurement , and obtain the one-dimensional cross-sectional luminance A direct type backlight unit having a full width at half maximum calculated from the distribution of 9.0 mm or more.
D _OMB = 0.0625 x D + 0.275 (5)
D _OLB =0.20×D+0.70 (6) 2. The backlight unit according to claim 1, wherein the aromatic polycarbonate resin has a viscosity average molecular weight of 10,000 or more and 100,000 or less.

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