JPWO2009096293A1 - Direct backlight unit - Google Patents

Abstract

The direct type backlight device 1 includes a reflecting plate 20, a plurality of linear light sources 10 arranged substantially parallel to each other, direct light from the linear light source, and reflections emitted from the linear light source and reflected by the reflecting plate. A light diffusing plate 30 that enters light from the light incident surface 32 and emits light from the light emitting surface 34 is provided. The average distance a (mm) between the centers of adjacent linear light sources and the average distance b (mm) between the centers of the linear light sources and the light incident surface satisfy a predetermined relationship. A plurality of first linear prisms having a plurality of inclined surfaces and extending in a direction substantially parallel to the linear light source are formed on the light exit surface, and each of the first linear prisms is configured. Among these slopes, the slope angle θ1 (degrees) of the slope A1 having the largest area and the total light transmittance T (%) of the material constituting the light diffusion plate satisfy a predetermined relationship.

Description

  The present invention relates to a direct-type backlight device, and more particularly to a direct-type backlight device that can reduce luminance unevenness on a light emitting surface and can reduce the thickness and the number of parts of the direct-type backlight device.

  Conventionally, as a backlight device for a liquid crystal display device, for example, a reflecting plate, a plurality of linear light sources arranged substantially in parallel, direct light from these linear light sources and reflected light from the reflecting plate are used. A direct type backlight device that includes a light diffusing plate that is incident from a light incident surface and is emitted from a light emitting surface to be a light emitting surface in this order is widely used. As such a direct type backlight device, the distance between the reflection surface of the reflection sheet and the light incident surface of the light diffusing plate is about 18 to 22 mm, and the distance b between the center of the linear light source and the light incident surface of the light diffusing plate is b. Is generally about 15 mm, the distance a between the centers of adjacent linear light sources is about 20 mm, and a / b is about 1.3.

  On the light emitting surface of such a direct type backlight device, high luminance can be easily obtained, while the luminance at the portion directly above the linear light source on the light emitting surface (the portion where the linear light source is projected vertically onto the light diffusion plate). The brightness tends to decrease with increasing distance from the portion directly above, and periodic brightness unevenness may occur on the light emitting surface. For this reason, conventionally, a striped pattern or a dot-shaped light amount correction pattern is printed on the light diffusion plate to reduce the amount of light irradiated on the portion directly above the linear light source, thereby reducing the portion between the linear light sources (adjacent to each other). A technique is disclosed in which the luminance unevenness of the light-emitting surface is reduced by relatively increasing the amount of light irradiated to the portion where the intermediate position of the linear light source is projected perpendicularly to the light diffusing plate (Patent Document 1: Japanese Patent Laid-Open No. Hei. 6-273760).

  For example, as shown in Japanese Patent Application No. 2006-211276, the present inventors have formed a prism array including a plurality of linear prisms having the same triangular cross section on the light exit surface of a light diffusing plate. There has been proposed a direct type backlight device that further improves luminance unevenness by setting the position of the image of the linear light source appearing on the exit surface within a predetermined range. In this direct type backlight device, a / b is about 2.5.

  In recent years, there has been a demand for thinner liquid crystal display devices and fewer parts. For this reason, it is also required to reduce the thickness of the direct type backlight device itself and reduce the number of parts. That is, a mode in which a is increased, b is decreased, or both are required. Specifically, an aspect that satisfies a / b> 3 is desired.

  Thus, when the direct type backlight device is thinned or the number of linear light sources is reduced, the distance between the linear light source and the light diffusion plate is reduced, or the interval between the linear light sources is increased. . For this reason, the light traveling from the linear light source to the portion between the linear light sources has a large incident angle on the light incident surface of the light diffusing plate, increases its Fresnel reflectivity, and increases the projected area. Therefore, there is a problem that the luminance unevenness on the light emitting surface becomes more remarkable. For this reason, as shown in Patent Document 1, the luminance unevenness cannot be sufficiently improved only by the method of printing the light amount correction pattern on the predetermined portion of the light diffusion plate. Further, even in an aspect in which the image of the linear light source on the light exit surface appears at a specific position, when a / b is increased, the luminance unevenness is not necessarily improved sufficiently.

  An object of the present invention is to provide a direct type backlight device that can reduce luminance unevenness on a light emitting surface and can reduce the thickness and / or the number of linear light sources used.

  The present inventors examined a direct type backlight device when a / b is large. When a / b is large, the incident angle to the position corresponding to the middle of the linear light source (angle with respect to the normal line of the light diffusing plate) increases, and the transmission distance of the light beam inside the light diffusing plate increases. At this time, if the total light transmittance inside the light diffusing plate is low, the utilization efficiency of incident light to a position corresponding to the middle of the linear light source is reduced, and the luminance in the middle of the linear light source is reduced. obtain. For this reason, when a / b becomes large, it turned out that the total light transmittance T of the material which forms a light diffusing plate needs to be enlarged. Therefore, when the present inventors examined in more detail, the inclination angle of the linear prism on the light exit surface is within a specific range, and the total light transmittance of the material constituting the light diffusion plate is between a / b. It has been found that the luminance unevenness can be sufficiently reduced by having a specific relationship to the above, and the present invention has been completed.

  The direct type backlight device of the present invention includes a reflector, a plurality of linear light sources arranged substantially parallel to each other, direct light from the linear light source, and the linear light source that is emitted from the reflective light source. A direct-type backlight device that includes a light diffusing plate that enters reflected light reflected by a plate from a light incident surface and emits from the light emitting surface in this order, and has an average distance between the centers of adjacent linear light sources. When a (mm) and the average distance between the center of the linear light source and the light incident surface is b (mm), the relationship of a / b> 3 is satisfied, and the light emitting surface includes A plurality of first linear prisms extending in a direction substantially parallel to the linear light source and having a plurality of inclined surfaces and having a polygonal cross section are formed. Of the plurality of inclined surfaces constituting each first linear prism, The inclination angle of the slope A1 with the largest area is θ1 (degrees), and the front The total light transmittance of the material constituting the light diffusing plate upon the T (%), a 30 ≦ θ1 ≦ 45, and satisfy the relationship T ≧ 4 × (a / b) +60.

  Here, the numerical value a may be a constant numerical value or a variable numerical value, but is a value within a numerical range described below (usually 10 to 100 mm, preferably 10 to 50 mm). When a is variable, a preferable total light transmittance may be calculated using an average value of a.

  The total light transmittance is a value measured with a 2 mm-thick plate smooth on both sides based on Japanese Industrial Standard JIS K7361-1. It can be determined using a turbidity meter, for example, NDH2000 manufactured by Nippon Denshoku Industries Co., Ltd. The inclination angle of the prism is an arithmetic average inclination obtained based on Japanese Industrial Standard JIS B0601-1994, and can be obtained using an ultra-deep microscope, for example, VK-9500 manufactured by Keyence Corporation.

  According to the present invention, the inclination angle of the linear prism formed on the light exit surface is set to a specific range, and a specific relationship is established between the total light transmittance of the material forming the light diffusion plate and a / b. Since it was made to do in this way, the light utilization efficiency in the intermediate part of the linear light source where luminance becomes the lowest can be improved. For this reason, the luminance unevenness of the light emitting surface can be further reduced.

  Further, since the cross section of the linear prism (cross section perpendicular to the longitudinal direction) is polygonal, a light diffusion plate provided with such a linear prism can be relatively formed by injection molding, extrusion molding, or the like, which will be described later. Easy to mold. In addition, the light diffusing plate used in the present invention is not particularly limited in its manufacturing method, and examples thereof include those formed by the following method.

  That is, as a forming method, for example, a method of forming a prism array composed of a plurality of linear prisms on the surface of a flat light diffusion plate may be used, or a flat plate portion (light It may be a method of integrally forming the prism rows at the same time as the formation of the diffusion plate base.

  Examples of the method of forming the prism array on the surface of the flat light diffusion plate include a method of cutting the surface of the flat light diffusion plate, a prism sheet having a desired shape on the flat light diffusion plate, etc. A method of laminating or pasting a sheet having a concavo-convex structure, applying a photocurable resin or a thermosetting resin to the surface of a flat light diffusing plate, transferring a desired shape to the coating film with a roll or a mold, Examples thereof include a method of curing the coating film in a state, and an embossing method in which the surface of the flat light diffusion plate is pressed with a roll or a stamp having a desired shape.

  In addition, as a method of integrally forming the prism row simultaneously with the formation of the light diffusing plate base, a casting method using a casting mold that can form a desired prism row, or a mold that can form a desired prism row is used. The injection molding method used can be mentioned. As described above, the injection molding method and the casting method have a simple process because the prism row can be formed simultaneously with the formation of the light diffusion plate base. The casting method can be performed in a mold capable of forming a plate, or can be performed continuously while pouring a raw material between two continuous belts and moving the belt. In the injection molding method, in order to increase the shape transfer rate, it is preferable to raise the mold temperature at the time of injecting the resin and rapidly cool the mold during cooling. Moreover, you may apply the injection compression molding method which expands a type | mold when injecting resin and closes a type | mold after that.

  In the direct type backlight device, the tilt angle θ1 satisfies the relationship of 32.5 ≦ θ1 ≦ 42.5, more preferably 35 ≦ θ1 ≦ 40, and the total light transmittance T is T ≧ 4 ×. It is preferable to satisfy (a / b) +65. According to such a configuration, the luminance unevenness of the light emitting surface can be further reduced.

  In the direct-type backlight device, it is preferable that the linear prism has an isosceles triangle cross section. With such a configuration, the light diffusing plate can be easily designed and manufactured.

  In the direct type backlight device, it is preferable that a cross section of the linear prism is symmetric with respect to an axis parallel to the thickness direction of the light diffusion plate. According to such a configuration, it is relatively easy to mold, and even when the direct-type backlight device is observed from an oblique direction orthogonal to the linear light source, uneven luminance can be suppressed, The viewing angle in the direction orthogonal to the linear light source can be made symmetric.

  In the direct type backlight device, it is preferable that a prism sheet is disposed at a position farther from the linear light source than the light diffusion plate. According to such a configuration, the luminance unevenness of the light emitting surface can be further reduced.

  Further, in the direct type backlight device, a plurality of second linear prisms having a polygonal cross section having a plurality of inclined surfaces and extending in a direction substantially parallel to the linear light source are formed on the light incident surface. When the inclination angle of the slope A2 having the maximum area among the slopes constituting each second linear prism is θ2 (degrees), the relationship of 12.5 ≦ θ2 ≦ 27.5 is satisfied. It can be.

  According to the present invention, the inclination angle of the linear prism formed on the light exit surface is set to a specific range, and a specific relationship is established between the total light transmittance of the material forming the light diffusion plate and a / b. Since it was made to do in this way, the light utilization efficiency in the intermediate part of the linear light source where luminance becomes the lowest can be improved. For this reason, the luminance unevenness of the light emitting surface can be further reduced.

FIG. 1 is a longitudinal sectional view schematically showing a direct type backlight device according to the first embodiment. FIG. 2 is a cross-sectional view for specifically explaining the surface shape of the light diffusing plate used in the second embodiment. FIG. 3 is a diagram for specifically explaining the shape of a linear prism having a polygonal cross section according to the third embodiment. FIG. 4 is a diagram for specifically explaining the shape of a linear prism having a polygonal cross section different from that in FIG. 3. FIG. 5 is a graph for showing a good range of the total light transmittance T with respect to a / b.

<First Embodiment>
Hereinafter, a direct backlight device according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view schematically showing a direct type backlight device according to this embodiment. As shown in FIG. 1, the direct type backlight device 1 includes a reflecting plate 20, a plurality of linear light sources 10 arranged substantially parallel to each other, direct light from the linear light source 10, and the linear light source 10. The light diffusing plate 30 that is incident on the light incident surface 32 and is emitted from the light exit surface 34 is provided in this order.

  In the present specification, unless otherwise specified, the “up” and “down” directions are “up” in a state where the direct type backlight device is placed so that its light emission surface is horizontally upward. And the “down” direction, which corresponds to the up and down directions in each drawing.

  The linear light source 10 is a light source formed in a straight tube shape (linear shape), and a straight tube cold cathode tube (CCFL) is preferably used from the viewpoint of luminance uniformity. The linear light source 10 is not limited to a cold cathode tube, but, for example, an external electrode fluorescent tube (EEFL), a xenon lamp, a xenon mercury lamp, a hot cathode tube, and a light emitting diode (LED) are arranged in a straight line. A thing which combined LED and a light guide, etc. can also be used. The outer diameter of the linear light source 10 is usually 2.0 to 20.0 mm, preferably 3.0 to 10.0 mm. Moreover, the internal diameter r of the linear light source 10 is 1.0-19.0 mm normally, Preferably it is 2.0-9.0 mm. By setting it as such a diameter, it can contribute to thickness reduction of a direct type backlight apparatus.

  As the shape of the linear light source 10, in addition to a linear one, a U-shaped one in which two substantially parallel straight tubes are connected by a substantially semicircular connecting tube, or three substantially parallel straight tubes are used. N-shaped pipes connected to one using two semicircular connecting pipes, four straight pipes substantially parallel, one using three semicircular connecting pipes And a W-shaped one connected to the.

  The number of the linear light sources 10 is not particularly limited. For example, when the direct type backlight device 1 is used for a 32-inch liquid crystal display device, the number of linear light sources is an even number such as 16, 14, 12, 8, 4, 2 or the like. , Can be an odd number. When the linear light source is U-shaped, N-shaped, or W-shaped as described above, the number of the linear light sources is counted according to the number of straight tubes.

  The average distance a between the centers of the adjacent linear light sources 10 is usually 10 to 100 mm, and preferably 10 to 50 mm. By setting the average distance in the above range, there is an advantage that the power consumption of the direct type backlight device can be reduced, the assembly of the device is facilitated, and the luminance unevenness of the light emitting surface can be suppressed. Here, the distance between the centers of adjacent linear light sources is substantially constant from the viewpoint of the luminance uniformity of the direct type backlight device (the ratio of the maximum luminance and the minimum luminance in the light emitting surface of the direct type backlight device). (It is preferable that the average distance is within a range of ± 5%.) However, the distance may not be constant, may be random, or increases continuously or stepwise toward a specific location or You may give the regularity which becomes small. Here, the specific location is, for example, a central location including a line connecting one long side of a rectangular light diffusing plate or the central locations of opposing short sides.

The average distance b (mm) between the center of the linear light source 10 and the light incident surface 32 of the light diffusing plate 30 may be designed in consideration of the thickness and luminance uniformity of the direct type backlight device. It can be 2 mm to 13 mm, and is preferably 3 mm to 10 mm. By setting the average distance b within the above range, luminance unevenness can be reduced, and a decrease in the luminous efficiency of the lamp can be prevented, and the direct type backlight device can be made thinner. In the present embodiment, the plurality of linear light sources 10 are arranged such that the average distance b (mm) from the light incident surface 32 is substantially constant for all the linear light sources. Note that “almost constant” means that the maximum value of the average distance b (mm) / the minimum value of the average distance b (mm) ≦ 1.3.
However, a plurality of linear light sources may be arranged such that some linear light sources are closer to the light incident surface 32 than other linear light sources. For example, it may be random or may have regularity that becomes larger or smaller as it goes to a specific location. Here, the specific location is, for example, a central location including a long side of a rectangular light diffusing plate or a line connecting the central locations of opposing short sides.

  The reflection plate 20 is a plate material that reflects light emitted from the linear light source 10. As a material of the reflecting plate 20, a resin colored in white or silver, a metal, or the like can be used, and a resin is preferable from the viewpoint of weight reduction. Moreover, it is preferable that the color of the reflecting plate 20 is white from the viewpoint of reducing luminance unevenness. However, in order to balance the brightness and the brightness uniformity to a high degree, the reflector 20 may be made of a mixture of white and silver.

  Moreover, the area | region located between several linear light sources in the said reflecting plate may be provided with the protrusion part which protrudes in the light diffusion plate side and extends along the longitudinal direction of several linear light sources. At this time, it is preferable that the protrusion is provided at a substantially middle position between adjacent linear light sources. Furthermore, the cross-sectional shape in the short direction of the protrusion is not particularly limited, but isosceles triangle, isosceles trapezoid, circular cut shape, elliptical shape cut by a line segment parallel to the short axis, and elliptical shape is long. Examples include a shape cut by a line segment parallel to the axis, a shape in which downward convex curves are connected so as to be a line object, and a shape in which upward convex curves are connected in line symmetry. The apex portions of these shapes may be pointed or rounded. From the viewpoint of brightness uniformity and ease of production, a triangular shape is preferable. Moreover, it is preferable that the cross-sectional shape of the protrusion is line symmetric with respect to a line segment perpendicular to the thickness direction of the light diffusion plate. By setting it as such a structure, the brightness nonuniformity in the light-projection surface of a light diffusing plate can be suppressed. The protrusions may be continuous in a bowl shape or intermittent as a series of pits, but are preferably continuous because the luminance uniformity can be further improved. The protrusions may be installed by coating a metal frame with protrusions in white or silver, attaching a white or silver reflective sheet to the metal frame with protrusions, white or silver flat For example, a method of bending the reflecting sheet and placing it on a flat metal frame, a method of molding a white or silver resin using a mold having a predetermined shape, and the like can be mentioned.

  The light diffusion plate 30 is a plate material that diffuses and emits incident light. As the material of the light diffusing plate 30, glass, a mixture of two or more resins that are difficult to mix, a material in which a light diffusing agent is dispersed in a transparent resin, one kind of transparent resin, and the like can be used. Among these, a resin is preferable because it is lightweight and easy to mold, and one kind of transparent resin is preferable from the viewpoint that luminance can be easily improved. Moreover, you may use what disperse | distributed the light-diffusion agent to transparent resin at the point which adjustment of a total light transmittance and haze is easy.

  The transparent resin is a resin having a total light transmittance of 70% or more measured with a 2 mm-thick plate smooth on both sides based on JIS K7361-1, for example, polyethylene, propylene-ethylene copolymer, Polypropylene, polystyrene, copolymer of aromatic vinyl monomer and (meth) acrylic acid alkyl ester having a lower alkyl group, polyethylene terephthalate, terephthalic acid-ethylene glycol-cyclohexanedimethanol copolymer, polycarbonate, acrylic resin, And a resin having an alicyclic structure. In addition, (meth) acrylic acid is acrylic acid and methacrylic acid.

  Among these, as the transparent resin, polycarbonate (polystyrene), a copolymer of an aromatic vinyl monomer containing 10% or more of an aromatic vinyl monomer and a (meth) acrylic acid alkyl ester having a lower alkyl group ( The content ratio of the monomer is a mass ratio, and the upper limit of the content ratio of the aromatic vinyl monomer is not particularly limited, but can be 90%), and a resin having an alicyclic structure, etc. A resin having a water absorption rate of 0% or more and 0.25% or less is preferable in that a large light diffusion plate with little warpage can be obtained because deformation due to moisture absorption is small.

  A resin having an alicyclic structure is more preferable because it has good fluidity and can efficiently produce a large light diffusion plate. A mixture of a resin having an alicyclic structure and a light diffusing agent has both high permeability and high diffusibility required for a light diffusing plate, and has good chromaticity, so that it can be suitably used.

  The resin having an alicyclic structure is a resin having an alicyclic structure in the main chain and / or side chain. From the viewpoint of mechanical strength, heat resistance, etc., a resin containing an alicyclic structure in the main chain is particularly preferred. Examples of the alicyclic structure include a saturated cyclic hydrocarbon (cycloalkane) structure and an unsaturated cyclic hydrocarbon (cycloalkene, cycloalkyne) structure. From the viewpoint of mechanical strength, heat resistance and the like, a cycloalkane structure and a cycloalkene structure are preferable, and among them, a cycloalkane structure is most preferable. When the number of carbon atoms constituting the alicyclic structure is usually in the range of 4 to 30, preferably 5 to 20, more preferably 5 to 15, the mechanical strength, heat resistance and light diffusion plate Formability characteristics are highly balanced and suitable.

  The proportion of the repeating unit having an alicyclic structure in the resin having an alicyclic structure may be appropriately selected according to the purpose of use, but is usually 50% by weight or more, preferably 70% by weight or more, more preferably 90%. % By weight or more. The upper limit of this ratio can be 100% by weight. When the ratio of the repeating unit having an alicyclic structure is too small, the heat resistance is lowered, which is not preferable. In addition, repeating units other than the repeating unit which has an alicyclic structure in resin which has an alicyclic structure are suitably selected according to the intended purpose.

  Specific examples of the resin having an alicyclic structure include (1) a ring-opening polymer of a norbornene monomer and a ring-opening copolymer of the norbornene monomer and other monomers capable of ring-opening copolymerization Norbornene polymers such as hydrogenated products, addition polymers of norbornene monomers, and addition copolymers of norbornene monomers with other monomers copolymerizable therewith; (2) Monocyclic olefin polymer and hydrogenated product thereof; (3) Cyclic conjugated diene polymer and hydrogenated product thereof; (4) Polymer of vinyl alicyclic hydrocarbon monomer and vinyl alicyclic hydrocarbon Copolymers of monomers and other monomers copolymerizable therewith, as well as hydrogenated products thereof, aromatic ring hydrogenated products of vinyl aromatic monomers, and vinyl aromatic monomers. Copolymerization of the monomer and other monomers copolymerizable therewith Vinyl alicyclic hydrocarbon polymers such as hydrogenated products of the body of the aromatic ring; and the like.

  Among these, from the viewpoints of heat resistance, mechanical strength, and the like, norbornene polymers and vinyl alicyclic hydrocarbon polymers are preferred, and ring-opening polymer hydrogenated products of norbornene monomers, norbornene monomers, and Hydrogenation of ring-opening copolymer with other monomers capable of ring-opening copolymerization, hydrogenation of aromatic ring of polymer of vinyl aromatic monomer, and copolymerization with vinyl aromatic monomer and this More preferred are hydrogenated aromatic rings of copolymers with other possible monomers.

  The light diffusing agent is a particle having a property of diffusing light, and can be roughly classified into an inorganic filler and an organic filler. Examples of the inorganic filler include silica, aluminum hydroxide, aluminum oxide, titanium oxide, zinc oxide, barium sulfate, magnesium silicate, and a mixture thereof. Examples of the organic filler include acrylic resin, polyurethane, polyvinyl chloride, polystyrene resin, polyacrylonitrile, polyamide, polysiloxane resin, melamine resin, and benzoguanamine resin. Among these, as the organic filler, fine particles composed of polystyrene resin, polysiloxane resin, and cross-linked products thereof are preferable in terms of high dispersibility, high heat resistance, and no coloration (yellowing) during molding. Among these, fine particles made of a cross-linked product of polysiloxane resin are more preferable from the viewpoint of more excellent heat resistance.

  Examples of the shape of the light diffusing agent include a spherical shape, a cubic shape, a needle shape, a rod shape, a spindle shape, a plate shape, a scale shape, and a fiber shape. Among these, the light diffusing direction can be exemplified. Spherical shape is preferable in that it can be squarely. The light diffusing agent is used in a state of being uniformly dispersed in the transparent resin.

The ratio of the light diffusing agent to be dispersed in the transparent resin can be appropriately selected according to the thickness of the light diffusing plate, the interval between the linear light sources, and the like, but the light is transmitted so that the total light transmittance T of the dispersion satisfies the following formula 1. It is preferable to adjust the content of the diffusing agent, and it is more preferable to adjust the content of the light diffusing agent so as to satisfy Equation 2 below. By setting the total light transmittance within the following preferable range, the luminance and the luminance uniformity can be further improved.
T ≧ 4 × (a / b) +60 (Formula 1)
T ≧ 4 × (a / b) +65 (Formula 2)

  The total light transmittance is a value measured with a 2 mm-thick plate smooth on both sides based on JIS K7361-1. Here, from the above formulas 1 and 2, the numerical range of a / b is 3 <a / b ≦ 10, and preferably 3 <a / b ≦ 8.75.

  The material constituting the light diffusion plate used in the present invention preferably has a glass transition point (Tg) of 90 ° C. or higher, more preferably 100 ° C. or higher. Although the upper limit of a glass transition point is not specifically limited, It can be 250 degreeC.

  The thickness of the light diffusing plate 30 is preferably 0.4 to 5.0 mm, and more preferably 0.8 to 4.0 mm. By setting the thickness of the light diffusing plate within the above preferable range, it is possible to suppress bending due to its own weight and to facilitate the molding. The average distance b between the center of the linear light source 10 and the light incident surface 32 is usually 2.0 to 13.0 mm, and preferably 3.0 to 10.0 mm. Moreover, the average distance of the center of the linear light source 10 and the reflecting plate 20 is 1.5-5.0 mm normally, and it is preferable that it is 2.0-4.0 mm. The size of the light diffusing plate is 17 inches (212 mm x 376 mm) to 100 inches (1245 mm x 2214 mm), preferably 32 inches (398 mm x 708 mm wide) to 65 inches (809 mm x 1439 mm). ) Can be preferably used.

  Next, the outer shape of the light diffusing plate 30 will be described. As shown in FIG. 1, in the light diffusing plate 30 of the present embodiment, a plurality of linear prisms 36 (corresponding to the first linear prism) having an isosceles triangular shape with the same cross section are provided on the light emitting surface 34. It is formed on the entire surface side by side. Each linear prism 36 includes two inclined surfaces having the same inclination angle θ1. In addition, since a cross section is an isosceles triangle, both these slopes correspond to slope A1, respectively.

  Next, a description will be given of the optical superiority when the inclination angle θ1 of these slopes is about. When the inclination angle θ1 of the linear prism 36 is in the vicinity of 0 (degrees), almost all of the light beams travel straight and emit light when passing through the diffuser at the position directly above the linear light source. On the surface, the portion directly above the linear light source becomes a bright portion, and in the middle, a dark portion, resulting in uneven brightness. On the other hand, when the inclination angle θ1 of the linear prism is around 45 (degrees), as shown by Snell's law, the light is reflected on the slope of the linear prism at the position directly above the linear light source and then into the backlight device. Since it recurs, it is difficult to emit light in the normal direction of the light diffusing plate. That is, the amount of light emitted from the intermediate position of the linear light source can be adjusted by adjusting the inclination angle of the linear prism according to the configuration of the backlight device, and the amount of light emitted from the position directly above the linear light source. Thus, it is possible to obtain a backlight device with uniform luminance and a uniform luminance. As a result of the study by the present inventors, by setting the inclination angle θ1 (degree) of the linear prism to 30 ≦ θ1 ≦ 45, the linear prism returns to the inside of the backlight device at a position immediately above the linear light source. It has been found that the amount of light and the amount of light emitted to the outside of the light diffusing plate can be suitably balanced at the intermediate position of the linear light source, and uneven brightness can be reduced.

  The inclination angle of the prism is an arithmetic average inclination obtained based on Japanese Industrial Standard JIS B0601-1994, and can be obtained with an ultradeep microscope VK-9500 or the like.

  In the light diffusing plate 30 of this embodiment, a plurality of linear prisms having an isosceles triangular shape having the same cross section are arranged on almost the entire surface. By providing a plurality of linear prisms having the same shape as described above, the light diffusing plate can be easily designed and manufactured. However, the shapes of these linear prisms may be appropriately different depending on the position in the light diffusion plate.

  Here, the isosceles triangle shape includes those in which the difference between the inclination angles of the two inclined surfaces constituting the linear prism is within 2 degrees. Further, a linear prism having the same isosceles triangle shape in cross section includes an inclination angle of one inclined surface X1 of a certain linear prism X and an inclined surface Y1 corresponding to the one inclined surface X1 of the linear prism Y to be compared. Between the inclination angle of the other slope X2 of the linear prism X and the inclination angle of the slope Y2 corresponding to the other slope X2 of the linear prism Y to be compared. The case where the difference is within 2 degrees is also included. In addition, the cross-sectional triangle shape includes a mode in which the apex portion of the isosceles triangle is curved.

  The height of the linear prism is usually Ra (max), where Ra (max) is the maximum value of the centerline average roughness Ra measured along various directions on the light exit surface of the light diffusion plate. Is 1 to 1,000 μm, preferably Ra (max) is 2 to 500 μm, and more preferably Ra (max) is 3 to 100 μm. Moreover, the width dimension of the linear prism is usually 10 to 500 μm, preferably 20 to 400 μm, and more preferably 30 to 300 μm.

<Second Embodiment>
The direct type backlight device 2 of the present embodiment is different from the first embodiment only in the outer shape of the light diffusion plate. For this reason, in this embodiment, only a different point is mainly demonstrated and other description is simplified. In addition, the thing with the same code | symbol shows the same or equivalent component.

FIG. 2 is a cross-sectional view for specifically explaining the surface shape of the light diffusing plate 130.
As shown in FIG. 2, in the light diffusing plate 130 of the present embodiment, in addition to the light emitting surface, the light incident surface 132 also covers the entire surface of the linear prism 136 (second prism) having an isosceles triangle shape. A prism array 138 having a plurality of linear prisms) is formed. Of the two slopes constituting each linear prism 136, the slope angle of the slope A2 having the largest area is θ2 (degrees), and the slope angle θ2 is in the range of 12.5 ≦ θ2 ≦ 27.5. By setting the inclination angle θ2 in the above range, the amount of light incident on the inside of the light diffusing plate can be suitably increased at the intermediate position of the linear light source, and the luminance unevenness reducing effect further superior to that of the first embodiment can be achieved. Obtainable.

<Third Embodiment>
The direct type backlight device 3 of the present embodiment is different from the first embodiment only in the cross-sectional shape of the linear prism. FIG. 3 is a diagram specifically explaining the shape of a linear prism having a polygonal cross section. As shown in FIG. 3, the linear prism includes slopes A31 and A32 having an inclination angle θa and slopes B31 and B32 having an inclination angle θb. At this time, the areas of the slopes having the same inclination angle need to coincide with each other within ± 5% with respect to the left and right average values. That is, the area SA31 of the slope A31 and the area SA32 of the slope A32 are ((SA31 + SA32) / 2) × 0.95 or more and ((SA31 + SA32) / 2) × 1.05 or less, respectively.

  As a modification of the linear prism shown in FIG. 3, the linear prism shown in FIG. 4 can be given. FIG. 4 is a diagram for specifically explaining the shape of a linear prism having a polygonal cross section different from that in FIG. 3. As shown in FIG. 4, the main prism includes slopes A41 and A42 having an inclination angle θa and slopes B41 and B42 having an inclination angle θb. Since this linear prism has the same number of surfaces with the same inclination angle in one linear prism, it can be considered that the optical function is the same as the linear prism shown in FIG. it can.

  According to the present embodiment, substantially the same effects as those of the first embodiment can be obtained, and a plurality of inclination angles are provided in the linear prism. The luminance unevenness of the surface can be further reduced.

  In the light diffusing plate of the present embodiment, the linear prism may be formed on the entire surface, or may have a different shape depending on the position in the light diffusing plate.

<Modification>
The present invention is not limited to the above embodiments.
In the above embodiment, the cross-sectional shape of the linear prism is an isosceles triangle or pentagon. Can do. Moreover, although the inclination angle of the inclined surface of the linear prism is the same shape on the left and right with the thickness direction of the light diffusing plate as an axis, it is not limited to this. However, the angle between the left and right slopes is preferably the same, and more preferably a left-right symmetric shape, because the design is easy and luminance unevenness does not occur when viewed from the left or right.

  In each of the embodiments, the linear prisms are formed so that the skirt portions thereof are continuous. However, the present invention is not limited to this, and the linear prisms are spaced apart from each other, that is, between the adjacent linear prisms. It can also be set as the aspect in which a part (flat surface) exists. At this time, the width dimension of the smooth portion may be all uniform, or may change continuously or stepwise as the distance from the linear light source increases.

  In the present invention, the surface of each linear prism may be smoothed or roughened. When the surface is roughened, the direction of emission can be more varied within an appropriate range, and the releasability from the mold during molding can be improved. From the above viewpoint, the arithmetic average height Ra on the surface of the linear prism is preferably 0.01 μm or more and 3 μm or less, more preferably 0.02 μm or more and 2 μm or less, and 0.05 μm or more and 1 μm or less. Further preferred. Further, the roughening of the surface of such a linear prism may be on the entire surface of the structure of the linear prism, or a part thereof, or on the entire surface of one linear prism. Or part of it.

  Further, in the direct type backlight device, it is preferable that an optical sheet is disposed on the light emitting side of the light diffusion plate, that is, at a position farther from the linear light source than the light diffusion plate. There may be one optical sheet or a plurality of optical sheets. The optical sheet preferably includes one or more prism sheets. By using a sheet having a prism having an isosceles triangle cross section, the amount of light returning to the inside of the direct type backlight device at the position directly above the linear light source and the direct type backlight at the intermediate position of the linear light source The amount of light emitted to the outside of the apparatus can be suitably balanced, and uneven brightness can be further reduced.

  The optical sheet may include a diffusion sheet. As the diffusion sheet, a sheet obtained by applying a diffusion agent dispersed in a binder on a transparent sheet can be used.

  In addition, the optical sheet may include one or more reflective polarizers. The reflective polarizer is preferably provided at a position farther from the linear light source than the light exit surface side, that is, the light diffusion plate. As the reflective polarizer, a reflective polarizer using the difference in reflectance of the polarization component depending on the Brewster angle (for example, the one described in JP-T-6-508449); using selective reflection characteristics by cholesteric liquid crystal Reflective polarizer; specifically, a laminate of a film made of cholesteric liquid crystal and a quarter-wave plate (for example, one described in JP-A-3-45906); a fine metal linear pattern is applied Reflective polarizers (for example, those described in JP-A-2-308106); at least two kinds of polymer films are laminated, and the reflective polarization using the anisotropy of the reflectance due to the refractive index anisotropy Child (for example, those described in Japanese Patent Publication No. 9-506837); having a sea-island structure formed of at least two kinds of polymers in a polymer film, and anisotropy of reflectance due to refractive index anisotropy Profit Reflective polarizer used (for example, those described in US Pat. No. 5,825,543); particles are dispersed in a polymer film, and anisotropy of reflectance due to refractive index anisotropy is utilized Reflective polarizers (for example, those described in JP-A-11-509014); reflection utilizing the anisotropy of reflectance based on the scattering ability difference depending on the size in which inorganic particles are dispersed in a polymer film Type polarizers (for example, those described in JP-A-9-297204) can be used.

  The direct backlight device of the present invention includes, for example, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a hybrid alignment nematic (HAN) mode, a vertical alignment (VA) mode, a multi-domain vertical alignment (MVA) mode, It can be suitably used for a liquid crystal display device in each display mode such as an in-plane switching (IPS) mode and an optically compensated birefringence (OCB) mode.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

(Production Example 1: Light Diffusing Plate Pellets P1)
99.8 parts of a resin having an alicyclic structure as a transparent resin (Nippon Zeon Co., Ltd., ZEONOR 1060R, water absorption 0.01%) and a cross-linked product of a polysiloxane polymer having an average particle diameter of 2 μm as a light diffusing agent The mixture was mixed with 0.2 parts of fine particles, kneaded with a twin-screw extruder, extruded into a strand, and cut with a pelletizer to produce a light diffusion plate pellet P1. From this light diffusion plate pellet P1, a 100 mm × 50 mm test plate with a smooth thickness of 2 mm on both sides was molded using an injection molding machine (clamping force 1000 kN). The total light transmittance and haze of the test plate were measured using an integrating sphere type color difference turbidimeter according to JIS K7361-1 and JIS K7136. The total light transmittance was 89.6%.

(Production Example 2: Light Pellet Pellets P2)
99.85 parts of the resin having the alicyclic structure (Nippon Zeon Co., Ltd., ZEONOR 1060R) as a transparent resin and 0.15 part of fine particles made of a crosslinked product of the polysiloxane polymer as a light diffusing agent are mixed. Then, the mixture was kneaded with a twin-screw extruder, extruded into a strand shape, and cut with a pelletizer to produce a light diffusion plate pellet P2. From this light diffusion plate pellet P2, a 100 mm × 50 mm test plate with a smooth thickness of 2 mm on both sides was molded using the injection molding machine. When the total light transmittance and haze of the test plate were measured in the same manner as described above, the total light transmittance was 91.5%.

(Production Example 3: Light Diffusing Plate Pellets P3)
99.6 parts of resin having the alicyclic structure as a transparent resin (Nippon Zeon Co., Ltd., ZEONOR 1060R) and 0.4 part of fine particles composed of a crosslinked product of the polysiloxane polymer as a light diffusing agent are mixed. Then, the mixture was kneaded with a twin-screw extruder, extruded into a strand shape, and cut with a pelletizer to produce a light diffusion plate pellet P3. From this light diffusion plate pellet P3, a test plate of 100 mm × 50 mm having a smooth thickness of 2 mm on both sides was molded using the injection molding machine. When the total light transmittance and haze of the test plate were measured in the same manner as described above, the total light transmittance was 81.9%.

(Production Example 4: Light Diffusing Plate Pellets P4)
99.7 parts of a resin having an alicyclic structure (Nippon Zeon Co., Ltd., ZEONOR 1060R) as a transparent resin and 0.3 part of fine particles composed of a crosslinked product of the polysiloxane polymer as a light diffusing agent are mixed. Then, the mixture was kneaded with a twin-screw extruder, extruded into a strand shape, and cut with a pelletizer to produce a light diffusion plate pellet P4. From this light diffusion plate pellet P4, a test plate having a thickness of 2 mm and a smooth surface of 100 mm × 50 mm was formed using the injection molding machine. When the total light transmittance and haze of the test plate were measured in the same manner as described above, the total light transmittance was 85.8%.

(Production Example 5: Light diffusion plate pellet P5)
99.3 parts of resin having the alicyclic structure (Nippon Zeon Co., Ltd., ZEONOR 1060R) as a transparent resin and 0.7 part of fine particles made of a crosslinked product of the polysiloxane polymer as a light diffusing agent are mixed. Then, the mixture was kneaded with a twin-screw extruder, extruded into a strand shape, and cut with a pelletizer to produce a light diffusion plate pellet P5. From this light diffusion plate pellet P5, a 100 mm × 50 mm test plate with a smooth thickness of 2 mm on both sides was molded using the injection molding machine. When the total light transmittance and haze of this test plate were measured in the same manner as described above, the total light transmittance was 69.6%.

(Production Example 6: Light diffusion plate pellet P6)
99.5 parts of resin having the alicyclic structure (Nippon Zeon Co., Ltd., ZEONOR 1060R) as a transparent resin and 0.5 part of fine particles composed of a crosslinked product of the polysiloxane polymer as a light diffusing agent are mixed. Then, the mixture was kneaded with a twin screw extruder, extruded into a strand shape, and cut with a pelletizer to produce a light diffusion plate pellet P6. From this light diffusion plate pellet P6, a test plate having a smooth thickness of 2 mm and a thickness of 2 mm and a size of 100 mm × 50 mm was formed using the injection molding machine. When the total light transmittance and haze of this test plate were measured in the same manner as described above, the total light transmittance was 77.9%.

<Example 1>
(a reflector)
A reflective plate (“E6SV”, manufactured by Toray Industries, Inc.) was attached to the inner surface of an aluminum case having an inner dimension of a long side of 729 mm, a short side of 404 mm, and a depth of 10 mm, thereby creating a reflector.

(Light diffusion plate)
A mold part having a predetermined shape is molded using a light diffusion plate pellet P1 as a raw material by using an injection molding machine (clamping force 9,810 KN), cylinder temperature 290 ° C., holding pressure 50 MPa, holding time 3 seconds, mold temperature 90 A light diffusion plate was molded under the condition of ° C. The obtained light diffusing plate was a rectangular flat plate having a thickness of 2 mm and 730 mm × 405 mm. On one surface of the light diffusing plate, a predetermined pattern of a concavo-convex structure is formed in which isosceles triangular linear prisms having an inclination angle of 35 degrees (corresponding to an apex angle of 110 degrees) are arranged substantially in parallel at a pitch of 70 μm. It was. The other surface of the light diffusion plate was smooth.

  Sixteen cold cathode tubes having an outer diameter of 3 mm were attached in parallel to the inner dimension long side direction of the reflector. The distance a between the centers of the cold cathode tubes was 24 mm, and the distance from the reflector to the center of the cold cathode tubes was 3.5 mm. The vicinity of the electrode part was fixed with a silicone sealant, and an inverter was attached. Next, the surface on which the pattern shown in Table 1 was formed was directed to the hot cathode tube side, and the light diffusing plate was placed on a reflecting plate made of an aluminum case with a reflecting sheet attached. At this time, the distance b between the center of the cold cathode tube and the light incident surface of the light diffusing plate was 6.5 mm.

  In this direct type backlight device, a / b is 3.7, and the value on the right side of Equation 1 is 4 × (a / b) +60 is 74.8.

  Further, on the light exit surface side of the light diffusion plate, a diffusion sheet (Kimoto Co., “188GM3”), a prism sheet (Sumitomo 3M Co., “BEFIII-10T”), a diffusion sheet (equivalent to an optical sheet), Kimoto Co., “188GM3”) and polarizing plate (manufactured by Sanlitz) were installed in this order.

Next, a tube current of 5 mA was applied to the obtained direct type backlight device to light it. Using a two-dimensional color distribution measuring device, the luminance in the front direction of 100 points was measured at equal intervals along the long side direction of the case on the center line in the short direction of the aluminum case. The central brightness measurement was 5490 cd / m ² . Also, according to the following formulas 3 and 4, the average luminance value (front luminance) LA and the luminance unevenness LU in the front direction were obtained. The luminance unevenness was 0.8%. The results are shown in Table 1.
Luminance average value LA = (L1 + L2) / 2 (Formula 3)
Luminance unevenness LU = ((L1-L2) / LA) × 100 (Equation 4)
L1: Average brightness maximum value just above a plurality of installed hot cathode tubes L2: Average minimum value sandwiched between maximum values Note that brightness unevenness is an index indicating brightness uniformity, and brightness unevenness When it is bad, the figure increases.

<Example 2>
(a reflector)
A reflecting plate (“E6SV”, manufactured by Toray Industries, Inc.) was attached to the inner surface of an aluminum case having an inner dimension of a long side of 729 mm, a short side of 404 mm, and a depth of 8 mm, thereby creating a reflecting plate.

(Light diffusion plate)
A light diffusing plate was molded under the same conditions as in Example 1 by using an injection molding machine (clamping force 9,810 KN) as a raw material and using a light diffusing plate pellet P2 as a raw material. The obtained light diffusing plate was a rectangular flat plate having a thickness of 2 mm and 730 mm × 405 mm. On one surface of the light diffusing plate, a predetermined pattern of a concavo-convex structure is formed, in which isosceles triangular linear prisms with an inclination angle of 40 degrees (corresponding to an apex angle of 100 degrees) are arranged almost in parallel at a pitch of 70 μm. It was. The other surface of the light diffusion plate was smooth.

  Sixteen cold cathode tubes having an outer diameter of 3 mm were attached in parallel to the inner dimension long side direction of the reflector. The distance a between the centers of the cold cathode tubes was 25 mm, and the distance from the reflector to the center of the cold cathode tubes was 2.5 mm. The vicinity of the electrode part was fixed with a silicone sealant, and an inverter was attached. Next, the surface on which the pattern shown in Table 1 was formed was directed to the hot cathode tube side, and the light diffusing plate was placed on a reflecting plate made of an aluminum case with a reflecting sheet attached. At this time, the distance b between the center of the cold cathode tube and the light incident surface of the light diffusing plate was 5.5 mm.

  In this direct type backlight device, a / b is 4.5, and the value on the right side of Equation 1 is 4 × (a / b) +60 is 78.2.

  Further, on the light emitting surface side of the light diffusion plate, two diffusion sheets (Kimoto Co., “188GM3”), prism sheets (Sumitomo 3M Co., “BEFIII-10T”), diffusion, each corresponding to an optical sheet, A sheet (manufactured by Kimoto Co., “188GM3”) and a polarizing plate (manufactured by Sanlitz) were installed in this order.

Next, the same evaluation as in Example 1 was performed. The measured value of luminance at the center was 5560 cd / m ² and the luminance unevenness was 0.7%. The results are shown in Table 1.

<Example 3>
A direct type backlight device was prepared and evaluated in the same manner as in Example 2 except that the light diffusion plate pellet P3 was used instead of the light diffusion plate pellet P2. The results are shown in Table 1.

<Example 4>
Using ZEONOR 1060R (total light transmittance 97.0%) containing no light diffusing agent instead of the light diffusing plate pellet P2, the distance a between the centers of the cold cathode tubes is 24 mm, and from the reflector to the center of the cold cathode tubes A direct-type backlight device was prepared in the same manner as in Example 2 except that the distance b was 3.5 mm and the distance b between the center of the cold cathode tube and the light incident surface of the light diffusion plate was 4.5 mm.
The obtained direct type backlight device was evaluated in the same manner as Example 1 in terms of luminance and luminance unevenness. The results are shown in Table 1.

<Example 5>
As in Example 1, except that three diffusion sheets (Kimoto Co., “188GM3”) and polarizing plates (Sanlitz Co., Ltd.) were installed in this order on the light exit surface side of the light diffusion plate. A direct type backlight device was created.
The obtained direct type backlight device was evaluated in the same manner as Example 1 in terms of luminance and luminance unevenness. The results are shown in Table 2.

<Example 6>
A direct type backlight device was prepared and evaluated in the same manner as in Example 4 except that instead of ZEONOR 1060R containing no light diffusing agent, pellet P4 for light diffusing plate was used. The results are shown in Table 2.

<Example 7>
On the light incident surface of the light diffusing plate, a prism having a triangular cross section with an inclination angle of 22.5 degrees (vertical angle of 135 degrees) and a base of 70 μm is formed on the entire surface with no gap in the flat part (the flat part exists) In the same manner as in Example 4, a direct type backlight device was prepared except that the triangular base corner portions adjacent to each other were in contact with each other.
The obtained direct type backlight device was evaluated in the same manner as Example 1 in terms of luminance and luminance unevenness. The results are shown in Table 2.

<Example 8>
The cross section of the prism on the light exit surface of the light diffusing plate has a polygonal shape as shown in FIG. 2 having an inclination angle of 35 degrees and 30 degrees (the projection length on the prism base of the hypotenuse of each angle of the cross section is the same). Other than this, a direct type backlight device was prepared and evaluated in the same manner as in Example 1. The results are shown in Table 2.

<Comparative Example 1>
A direct type backlight device was prepared and evaluated in the same manner as in Example 1 except that the prism on the light exit surface of the light diffusing plate was set at an inclination angle of 50 degrees (vertical angle 80 degrees). The results are shown in Table 3.

<Comparative Example 2>
A direct type backlight device was prepared and evaluated in the same manner as in Example 1 except that the light diffusion plate pellet P5 was used. The results are shown in Table 3.

<Comparative Example 3>
Similar to Example 7, except that the prism of the light exit surface of the light diffusing plate has an inclination angle of 50 degrees (vertical angle 80 degrees) and the prism of the light exit surface has an inclination angle of 37.5 degrees (apex angle 105 degrees). A direct type backlight device was created and evaluated. The results are shown in Table 3.

<Comparative example 4>
A direct type backlight device was prepared and evaluated in the same manner as in Example 7 except that the light diffusion plate pellet P6 was used. The results are shown in Table 3.

  Regarding Examples 1 to 7 and Comparative Examples 2 and 4, with respect to the transmittance T of a / b, examples with good luminance unevenness are indicated by circles, and comparative examples with poor luminance unevenness are indicated by crosses. Plotted in 5. As a result, it can be seen that the results are satisfactory within the range satisfying Equation 1.

  From the comparison between Example 1 and Comparative Example 1 and the comparison between Example 6 and Comparative Example 4, it can be seen that good results can be obtained by setting the inclination angle of the prism to a predetermined value. Further, comparing Example 1 and Example 5, it can be seen that better results are obtained when the prism sheet is provided.

Claims (4)

A reflecting plate, a plurality of linear light sources arranged substantially parallel to each other, direct light from the linear light source, and reflected light emitted from the linear light source and reflected by the reflecting plate is a light incident surface A direct-type backlight device comprising a light diffusing plate that is incident from and exits from a light exit surface in this order,
When the average distance between the centers of adjacent linear light sources is a (mm) and the average distance between the center of the linear light sources and the light incident surface is b (mm), a / b> 3 Meet relationships,
A plurality of first linear prisms extending in a direction substantially parallel to the linear light source and having a plurality of inclined surfaces and having a polygonal cross section are formed on the light emitting surface,
Of the plurality of inclined surfaces constituting each first linear prism, the inclination angle of the inclined surface A1 having the maximum area is θ1 (degrees), and the total light transmittance of the material constituting the light diffusion plate is T (%), A direct type backlight device satisfying the relationship of 30 ≦ θ1 ≦ 45 and T ≧ 4 × (a / b) +60. In the direct type backlight device according to claim 1,
The linear prism is a direct type backlight device having an isosceles triangle shape in cross section. In the direct type backlight device according to claim 1,
A direct type backlight device in which a prism sheet is disposed at a position farther from the linear light source than the light diffusion plate. In the direct type backlight device according to claim 1,
On the light incident surface, a plurality of second linear prisms having a polygonal cross section are formed, extending in a direction substantially parallel to the linear light source and having a plurality of inclined surfaces,
A direct type satisfying the relationship of 12.5 ≦ θ2 ≦ 27.5 when the inclination angle of the inclined surface A2 having the largest area among the inclined surfaces constituting each second linear prism is θ2 (degrees). Backlight device.

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