JPH1195014A - Conversing sheet for back light - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost conversing sheet for the back light of a liquid crystal display having high conversing efficiency. SOLUTION: A conversing sheet in which plural equilateral triangles holding center-of-gravity points in common are arranged with their respective sides parallel and in which unit equilateral triangle Fresnel plates 41 which are constituted to reduce the variations of similar ratio relating to the innermost equilateral triangle in response to the distances of the center-of-gravity points are two-dimensionally and regularly arranged in the close-packed condition on a transparent base is transfer prepared by an ultraviolet curing method. Since spaces between adjacent sides of the equilateral triangles within the Fresnel plate 41 are designed to be reduced gradually in response to the distances from the center-of-gravity points, a light diffraction angle gets larger toward its outer circumferencial part to exhibit a conversing effect similar to a convex lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a light-collecting sheet used for a backlight of a liquid crystal display panel.

[0002]

2. Description of the Related Art In recent years, the technical progress of a liquid crystal display device, particularly a direct-view type liquid crystal display device using a color display device has been remarkable, and many displays having a display quality not inferior to that of a CRT have been developed. In addition, the screen size is increasing, and 12.1
Inch type is becoming popular, lighter weight, narrower frame,
Demands for larger screens, higher brightness, and power saving have been increasing. As a key device for achieving these demands, it is essential to improve the performance of the backlight.

A conventional backlight for a liquid crystal panel display employs an edge light system in many cases and has a structure shown in FIG. 11 is a fluorescent lamp (light source), 12 is a reflector, 13 is a wedge-shaped light guide plate, 14 is a light scattering plate, 15 is a reflector, 16 is a diffusion sheet, and 17 is a prism sheet.
A general method of manufacturing the light guide plate 13 as a main part of the backlight is to first mold an acrylic resin by an injection molding method,
Titanium oxide (Ti) which is a light diffusing material by screen printing on the bottom
The light scattering plate 14 is formed by circular or square dots using O ₂ ) or the like. The dots on the bottom are
As mentioned above, to spread the light evenly,
This is for increasing the brightness, and the density is coarser as the distance from the light source increases, and the density increases as the distance from the light source increases. Irregular dots may be used instead of dots using a light diffusing material. The diffusion sheet 16 scatters light to make it uniform. The prism sheet 17 placed on the diffusion sheet has a thickness of 150 to 230 μm and an apex angle of 90 to 110.
゜, triangular prisms having a pitch of 30 to 110 μm are two-dimensionally arranged, and their function is to collect scattered light and illuminate the LCD panel with a directional light flux.
This prism sheet is one of the key devices for determining the viewing angle of display and the screen brightness.

However, as shown in FIG. 2 (a), the prism sheet 21 made of a triangular prism has excellent light condensing properties in the direction perpendicular to the groove of the prism portion, that is, in the X direction in the figure, It does not have the function of condensing light rays in the parallel direction, that is, the Y direction, and cannot meet the demand for higher brightness.
As a result, at present, as shown in FIG. 2 (b), two triangular prism prism sheets 21 and 22 are arranged so that the longitudinal directions of the triangular prisms are orthogonal to each other, and the light condensing property in the XY directions is enhanced, thereby achieving high brightness. In general, a method for achieving this is adopted. However, the effect is not sufficiently obtained as expected due to light loss due to reflection at the interface with the air layer due to the use of two prism sheets. Further, there is a problem that using two prism sheets is not preferable in terms of cost.

Further, in a conventional triangular prism sheet, prisms are periodically formed at a pitch of about 30 to 110 μm, and an ultraviolet curing method using a precision mold or an injection molding method is used to manufacture the prism. Is done. However, the manufacture of the mold requires an ultra-precision cutting technique for machining with submicron accuracy, and the mold requires a considerable amount of manufacturing time because it is large with a side of 20 to 30 cm. These factors have increased the unit price of the prism sheet. Specifically, the cost of the conventionally used triangular prism sheet currently accounts for more than half of the cost of the entire backlight (FIG. 1).

[0005]

SUMMARY OF THE INVENTION The present invention sufficiently addresses the above-mentioned problems of the prior art, such as reduction in thickness, increase in screen size, increase in brightness, and reduction in power consumption. To provide a light condensing sheet.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and comprises a collection of two triangular prism sheets generally used for a backlight in a conventional liquid crystal panel display. The optical function is integrated by using a light-condensing effect utilizing the light diffraction of a Fresnel plate arranged two-dimensionally closest. By integrating two prism sheets into one Fresnel plate, the number of interfaces is reduced, and the reflection loss is reduced. As a result, the cost can be reduced.

Another factor that has made it possible to reduce the cost is related to the method of manufacturing the condensing prism sheet of the present invention. The present invention uses a photolithography technique and a plating technique to manufacture a mold instead of the conventional ultra-precision cutting technique for manufacturing a mold, and then uses the mold by an ultraviolet curing method or an injection molding method. This is basically similar to the method of manufacturing an optical disc such as a compact disc.

The light-collecting sheet for a backlight according to the present invention is composed of many unit Fresnel plates. The function of the unit Fresnel plate is a light-collecting function found in a Fresnel zone plate widely used in an overhead projector.
That is, it is similar to the lens action. The operation of the present invention will be described with reference to FIG. When light having a wavelength λ enters a plurality of straight lines arranged in parallel with a period d, that is, a periodic grating, the emitted light is diffracted, and the course changes by an angle 2θ that satisfies the following equation. In a diffraction grating having a period d 'with 2d sin θ = λ d ′ <d, θ ′ becomes θ ′> θ, and the diffraction angle increases. In the present invention, since d is designed to gradually decrease from the center of gravity of the unit Fresnel plate toward the outer periphery, the diffraction angle gradually increases toward the outer periphery. That is, it has a light condensing action similar to that of the convex lens.

The manufacturing method of the light-condensing sheet of the present invention will be described step by step. A number of equilateral triangles having a constant line width and sharing the center of gravity are arranged so that the sides of the equilateral triangle are parallel, and the amount of change in similarity ratio with the innermost equilateral triangle depends on the distance from the center of gravity. A unit original triangular Fresnel plate configured so as to decrease is drawn two-dimensionally and regularly on a transparent substrate in the form of a glass plate by photolithography to produce a mask original. The length of one side of an equilateral triangular Fresnel plate is 100-200 μm, the line width of many equilateral triangles that make up the equilateral triangular Fresnel plate is 1-2 μm, and the pitch of parallel sides of adjacent equilateral triangles is 0.4 μm minimum and 5 μm maximum.
Is a preferable value for obtaining a light collecting effect by a light diffraction effect.

Next, a photoresist having a thickness of 1-3 μm is
Spin coating on a 5mm thick, 30cm square glass plate, overlaying a mask in which the equilateral triangular Fresnel plate obtained in the above process is two-dimensionally regularly arranged on a transparent substrate, exposed, and developed Produce a photoresist master. The photoresist used at this time can be either a negative type or a positive type. For example, when a negative resist is used, an equilateral triangular Fresnel plate group formed by a convex straight line with the unexposed resist remaining is manufactured. Next, several chromium thin films are formed on the photoresist master surface on which the convex pattern is formed, for example, by sputtering.
Deposits 100 nm film to make the photoresist and glass surface conductive. A nickel plating layer having a thickness of about 0.5 mm is formed by electrolytic plating. Next, an iron plate having a thickness of about 5 mm is adhered to the nickel plating layer as a support via an adhesive. The nickel plating layer including the support and the photoresist master are peeled off, and the photoresist remaining on the nickel plating layer is removed with an organic solvent, thereby completing the master mold. As a matter of course, when a positive type resist is used, a nickel-plated master is obtained in which the concavo-convex pattern in the above-described process is reversed.

A mold comprising the nickel plating layer including the support obtained in the above step is used as a master mold, and a nickel plating layer is formed on the master mold through the same steps as the above steps. Can also be produced in multiple pieces. In this case, although the master mold and the mother mold have inverted concavities and convexities, the optical diffraction effects of the condensing sheets obtained by transferring the two are equivalent, and their condensing functions are also the same. When mass-producing a light-collecting sheet, the latter mother mold is generally used. The reason for this is that a plurality of mother dies can be used instead of a plurality of prepared mother dies in order to prevent mold deterioration in the manufacturing process, for example, generation of scratches.

The light-collecting sheet according to the present invention is manufactured by using the mold manufactured in the above steps. As a manufacturing method, a method of injection molding a synthetic resin, a method of transferring an uneven shape of a master by transferring an ultraviolet-curable resin composition into a mold, and then irradiating and curing ultraviolet rays, a resin plate and There is a method of transferring the surface shape of the mold by heating and pressing the mold and the like. These manufacturing methods have characteristics such as mass productivity and the quality of the obtained condensing sheet in terms of optical quality (for example, birefringence), and the manufacturing method is selected according to the situation. Here, an example using an ultraviolet curing method is shown, and the manufacturing process thereof will be described below.

In the mold master with support obtained in the above-mentioned step, the surface on which the Fresnel plate is formed faces upward, and the ultraviolet-curable resin composition is injected into the mold. Then on this
Adhere a film of transparent polycarbonate, acrylic, polyester, vinyl chloride, etc. having a thickness of about 30-150 μm, and further place a pressure glass plate about 5 mm thick on top of this, and then use an ultraviolet exposure device from above. The ultraviolet curable resin composition is cured. The pressurizing glass plate is removed, and the light-collecting sheet of the present invention is completed by being peeled off from the mold together with the transparent substrate.

The UV-curable resin composition that can be used in the present invention includes polyester-acrylate, epoxy acrylate and urethane acrylate. These resins may be used alone or in combination, but may be appropriately selected in consideration of curability, optical characteristics (transparency, birefringence, etc.) of the obtained molded product, mechanical strength, and the like. it can.

[0015]

FIG. 4 is a diagram illustrating the structure of a light-collecting sheet for a backlight according to the present invention. FIG. 4A is a view of a part of the sheet as viewed from above, and shows a unit regular triangular Fresnel plate 41.
Are two-dimensionally densely arranged. FIG. 4B is an enlarged view of the unit regular triangular Fresnel plate 41,
FIG. 4 (c) is a cross-sectional structural view along the straight line in FIG. 4 (b), and each side of the triangle constituting the equilateral triangular Fresnel plate actually has a line width. Further, as described above, the distance between the adjacent sides decreases from the inner peripheral portion toward the outer peripheral portion.
With this configuration, the closer to the outer peripheral portion of the unit regular triangular Fresnel plate, the larger the diffraction angle. That is, the unit regular triangular Fresnel plate exhibits a light-collecting effect similar to a convex lens. The aggregate of the unit regular triangular Fresnel plates is the backlight condensing sheet of the present invention, and efficiently illuminates the LCD panel with a directional light beam.

[0016]

The present invention will be described below in detail with reference to examples. For the backlight consisting of a group of equilateral triangular Fresnel plates by transferring the mold obtained through the steps of original mask production, master production using photoresist, and nickel plating master production with the acrylic ultraviolet curing resin composition described above. A light collecting sheet was manufactured. A 100 μm thick polyester sheet was used as the base film. 365nm as main wavelength, 2
It is irradiated with ultraviolet rays having a power density of 0-30 mW / cm ² for 30 seconds, the outer dimensions are 20 × 25 cm, and the unit regular triangular Fresnel plate is composed of similar triangles sharing 14 centroid points, The length L of one side of the triangle (n = 1) is 100 μm, and that of the innermost equilateral triangle (n = 14) is L = 8 μm.Between both equilateral triangles, n and the length of one side L (μm) Is L = -0.472 * n ² +100.
472 The equilateral triangles determined by the relationship
And a convex straight line having a height of 1 μm. That is, in the unit regular triangular Fresnel plate, one side of the regular triangle is 100, 98.6, 96.2, 92.9, 88.7, 83.5, 77.3, 70 in order from the outer periphery.
It consists of 14 equilateral triangles sharing the center of gravity and parallel to each other: 3, 62.2, 53.3, 43.4, 32.5, 20.7, 8.0 μm. The distance between parallel sides of similar triangles in a unit regular triangular Fresnel plate is 0.4, 0.7, 0.95, 1.
2, 1.5, 1.8, 2.0, 2.3, 2.6, 2.9, 3.15, 3.4, 3.7μm
The linear density increases as approaching the outer periphery. In the backlight condensing sheet of this example, approximately 11.56 million unit regular triangular Fresnel plates in which the unit regular triangular Fresnel plates were regularly and densely arranged could be obtained.

The performance of the light-collecting sheet prepared in the above steps was evaluated as follows. The light-condensing sheet is placed on the diffusion sheet with the surface on which the equilateral triangular Fresnel plate is formed facing up, and the measurement angle is changed along the normal line including the straight line ab in FIG. Was measured for surface luminance. The result is shown by a solid line in FIG. In the figure, the vertical direction is 0
°. In FIG. 4 (a), when the measurement angle was changed along the normal plane including the straight line cd, the result almost coincided with the result measured along the normal plane including the straight line ab. For comparison, when the two prisms shown in FIG. 2 were used, they were normalized by measured values in the vertical direction of a normal plane including the x-axis, and are shown by dotted lines.

The luminance immediately above the diffusion sheet was 1250 cd / m ² . In the case of the condensing sheet of the present invention, the luminance in the vertical direction was 2405 cd / m ² , whereas when using the two-prism (FIG. 2B), it was 1880 cd / m ² . . From the above results, the viewing angle characteristics of both were almost the same, but the brightness was improved by 28%.

[0019]

The backlight condensing sheet of the present invention is
A large number of equilateral triangles formed of concave or convex straight lines having a constant line width and a cross-sectional shape having a height of 1-3 μm have a common center of gravity, and the sides of the equilateral triangle are parallel. It is arranged so that the amount of change in similarity ratio with the innermost equilateral triangle gradually decreases according to the distance from the center of gravity, and the unitary triangular Fresnel plate exhibits a light-collecting effect similar to a convex lens, and Since the triangular Fresnel plates are two-dimensionally and regularly arranged on the transparent substrate in a close-packed manner, one backlight condensing sheet of the present invention replaces two conventional triangular prism prism arrays with a conventional condensing sheet for backlight. Can be provided. The function, that is, the light-condensing effect was equal to or greater than that of the conventional two triangular prism prism array condensing sheets. According to the present invention, the effect of improving luminance can be obtained as described above, and the manufacturing process is simpler than that of a conventional prismatic triangular prism, and as a result, the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a structural sectional view showing a conventional backlight.

FIG. 2 (a) is a diagram for explaining the light converging effect when one conventional prism sheet is used. (b) is a diagram for explaining the light-collecting effect when two conventional prism sheets are used.

FIG. 3 is a diagram illustrating the operation of the present invention.

FIG. 4 is a structural explanatory view of a light-collecting sheet for a backlight according to the present invention. (a) is a diagram in which a part of the sheet is viewed from the upper surface, and shows a state in which unit regular triangular Fresnel plates are two-dimensionally densely arranged. (b) is an enlarged view of a unit regular triangular Fresnel plate. (c) is a sectional structural view along a straight line in (b).

FIG. 5 is an example of a luminance distribution measurement result of the backlight condensing sheet of the present invention.

FIG. 6 is an enlarged view of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 fluorescent lamp (light source) 2 reflector 3 wedge-shaped light guide plate 4 light scattering plate 5 reflector 6 diffusion sheet 7 prism sheet 21 first prism sheet 22 second prism sheet 41 unit regular triangular Fresnel plate

Claims (1)

[Claims] 1. An n (n> 10) equilateral triangles formed by concave or convex straight lines having a constant line width and a cross-sectional shape of a height of 1 to 3 μm is a common center of gravity with the outermost equilateral triangle. Points, and each side of the equilateral triangle is arranged to be parallel.The similarity ratio between the n-th equilateral triangle and the outermost equilateral triangle (n = 1) is given by a function of n and increases by n A unitary triangular Fresnel plate composed of a large number of equilateral triangle groups configured to monotonically decrease with the above arrangement is two-dimensionally and regularly arranged in a close-packed manner on a transparent sheet. Sheet.