JP3106055B2 - Brightness improvement sheet for surface light source - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for improving the front luminance of a backlight unit used in a liquid crystal display device or the like by optical means.

[0002]

2. Description of the Related Art In recent years, power consumption is large in products such as a portable notebook personal computer equipped with a color liquid crystal display device, a portable liquid crystal TV using a color liquid crystal panel, and a battery-integrated liquid crystal TV, which are driven by a battery. The liquid crystal display device is an obstacle to extending the battery driving time. Above all, the percentage of power consumption of the backlight used for this is large, and keeping it low is an important goal in extending battery drive time and increasing the practical value of the above products.

At this time, if the brightness of the backlight is reduced in order to suppress power consumption, it is not preferable because the display becomes difficult to see. Therefore, in order to suppress power consumption without sacrificing luminance, it is desired to improve the optical efficiency of the backlight.
A method of placing a prism sheet 1 having a prism array 2 formed on one side as shown in FIG. 6 on a light emitting surface 4 of a backlight 3 is as follows.
Currently in practical use. The increase in front luminance due to the prism sheet is caused by the following mechanism.

A part of the light from the surface light source is refracted and transmitted by the prism sheet, and the rest is reflected and returned to the surface light source. An edge light type surface light source such as 3 shown in FIG. 6 generally has relatively low front luminance and high directivity with high luminance when viewed from an oblique direction. As a result, the directional characteristics are improved. Also, the reflected light from the prism sheet 1 is diffusely reflected by the diffusion sheet 4 on the light emitting surface of the surface light source, and increases the luminance of the light emitting surface.

[0005]

FIG. 5 shows a cross section perpendicular to both slopes of a prism of such a prism sheet, and an incident light beam has a component a ( Hereinafter, referred to as primary transmitted light), a component b (hereinafter referred to as “return light”) which is reflected once on the prism inclined surface and then reflected again on the other inclined surface and returned to the incident side. Component c that passes through and exits before (hereinafter referred to as secondary transmitted light)
(Multiple-reflected components also exist depending on the selection of the prism apex angle, but the proportion is usually small). At this time, the component a is a light beam that is actually used including light emitted in the front direction, that is, the observation direction, and the component b is diffusely reflected by the diffusion sheet on the light emitting surface of the surface light source, and is effective to increase the luminance of the light emitting surface. It is a ray. On the other hand, the component c is a light ray that is emitted at a wide angle outside the effective viewing angle of the liquid crystal panel and is useless.

As a result, the light from the prism sheet is viewed at a viewing angle of about ± 40 ° from the front in the direction perpendicular to the ridge line of the prism (when the apex angle is 90 ° to 100 ° and the refractive index is about 1.5 to 1.59).
Brightness (primary transmitted light) is emitted in the width of, and when the viewing angle is further increased, the brightness sharply decreases, and once it becomes almost zero, the brightness increases again at a larger viewing angle (secondary transmitted light). . As a result, the brightness is increased by narrowing the angle range of the emitted light beam.

The problem here is, first, the directivity in which the luminance sharply decreases at a viewing angle of about 40 ° or more. In recent years, the directivity of liquid crystal panels has been improved, and a liquid crystal panel exhibiting a sufficiently practical contrast at a viewing angle of 40 ° or more even in a narrow direction has been developed. Such a liquid crystal panel has a gradual decrease in luminance with an increase in viewing angle,
It is desired to exhibit a certain wide directivity.

[0008] The second problem is the existence of secondary transmitted light that is emitted on the wide angle side. If this component can be reduced, further improvement in efficiency can be expected. For this purpose USP 2,47
As pointed out in 4,317, the vertex angle of the prism

[0009]

(Equation 2) It is conceivable to select As a result, the secondary transmitted light becomes zero and the efficiency as a whole is improved, but the first problem still remains.

An object of the present invention is to exhibit the highest luminance in the front direction, to have a luminance distribution which gradually decreases in a direction exceeding a predetermined viewing angle with an increase in the viewing angle, and to provide the above-mentioned secondary transmitted light. An object of the present invention is to develop an optical sheet having a high-efficiency luminance increasing effect that suppresses escape of light in a wide angle direction.

[0011]

According to the first aspect of the present invention, the luminance direction
The upper transparent sheet has, on one surface, a number of shape units in which two convex curved surfaces whose generatrix is parallel to each other and a plane connected to each of them are symmetrically coupled to each other, are formed side by side in a direction perpendicular to the generatrix. In the transparent sheet, the tangent surfaces intersect at an angle of less than 180 ° at the joint between the curved surfaces, and at the joint between the curved surfaces and the planes, the tangent surfaces coincide with the planes (the inclination is smaller). Continuous) and the normal of the transparent sheet
Take the z-axis parallel to and the y-axis parallel to the generatrix of the convex curved surface.
When the x-axis is taken in the column direction of the forming unit, the forming unit
Is formed so as to satisfy the following expression (1).
It is a brightness enhancement transparent sheet characterized by the above-mentioned.

[0012]

[0013]

(Equation 3)

The operation of the present invention will be described below with reference to the drawings. Figure 1 is the invention xz brightness enhancing transparency sheet according to claim 1
Ru sectional view der. Here, the plane between OR and PS is a plane, and its function is the same as that of the slope of the prism sheet in FIG. OR
Assuming that the slope between the two is A, the relationship between OR and PS is represented by the following equation. B indicates the value of the point R on the z-axis.

[0015]

(Equation 4)

On the other hand , between PQ and SQ is a curved surface introduced by the present invention. In the conventional prism sheet of FIG. 5, a part of the secondary transmitted light enters the adjacent prism, enters the sheet again, and a part is added to the return light and reused.
Others take various optical paths, and some of them are effectively used in addition to the primary transmitted light, so that they are not necessarily wasted. On the other hand , most of the secondary transmitted light emitted from near the apex angle of the prism is wide-angled without being blocked by the adjacent prism.
It is emitted and becomes lossy light. This changing the shape of the vicinity of the apex angle by changing the emission angle in order not to generate, transparent sheet of the present invention to that as secondary transmission light emitted to the outside is absorbed all next to the prism from this portion It is. For this reason , in the transparent sheet of FIG. 1, between RQ and SQ in the vicinity of the apex angle is a convex curved surface, so that the light reaching this portion from the opposing slope is always incident on the slope of the adjacent forming unit. It is. Since the light ray incident at the smallest incident angle when viewed from each point between RQ is the reflected light from point P, the light ray that goes out through each point between P and RQ as shown by the dotted line in FIG. If the inclination at each point is selected so that is converged to the position of the vertex Q 'of the adjacent forming unit, all the reflected light from other points on the right slope will be reflected by the adjacent forming unit as shown in FIGS. The light is incident on the slope or totally reflected as B to be returned light. That is , RQ
Should be a lens shape that connects the image of the point P to the position of Q ′. Here, assuming that the point O is the origin and the coordinates of the point P are (T, 0) and the coordinates of the point Q are (0.5T, h) , the specific conditions that the curved surface RQ should satisfy are:

[0017]

(Equation 5) Becomes Similarly , between the SQs, the condition that the curved surface SQ satisfies
The condition is as follows, and the shape is symmetrical about x = T / 2. Thus, one shape unit ORQSP
Is expressed by equation (1).

(Equation 6)

Further, in the present invention, since the curved cylindrical surface RQ is connected to the plane portion OR at the point R so that the inclination is not discontinuous, the secondary transmitted light refracted by OR is more than the secondary transmitted light refracted by RQ. It will be bent inward (seat side),
As a result, all the secondary transmitted light emitted from the slope ORQ is taken in the adjacent forming unit. That is, the condition that the coordinates and the inclination of the curved surface RQ and the plane portion OR coincide at the point R is required in addition to the expression (1).

As described above, in the transparent sheet having the cross-sectional shape shown in FIG. 1, all the light is primarily transmitted light such as A, return light such as B, or absorbed by adjacent forming units such as C and D. The secondary transmitted light does not appear on the wide angle side.

The method for obtaining the shape of the forming unit according to claim 2 using equation (1) will be specifically described. Since the shape of the forming unit is symmetric with respect to the point Q, the shape of the left half QRO is determined. The solution of the formula (1) is determined by the height h of the forming unit and the refractive index n. If the material to be used is specified, n
Is determined. Therefore, the only remaining parameter is h. First, the value of h is appropriately selected, and starting from the point Q (0.5T, h), a numerical calculation (numerical integration) is performed in the negative direction of x in accordance with the equation (1) to determine the curve QR. Coordinates
Based on (x, z) and dz / dx

[0021]

(Equation 7) If a point is obtained, this point becomes R. Then, if R and the origin O are connected by a straight line, the shape of the QRO is determined. The value of equation (3) is equal to A in equation (1) and can be adjusted by changing h. The preferred value of A is determined as follows.

The functions of the plane portions OR and PS of the shape unit are the same as those of the inclined surface of the conventional prism sheet, and the inclination determines the range of the viewing angle at which the highest luminance is obtained. When this viewing angle is ω, ω is represented by the following equation.

[0023]

(Equation 8) For example, refractive index 1.59 (polycarbonate), 1.4
FIG. 3 shows the relationship between γ and ω in the case of 9 (polymethyl methacrylate). Here, it can be considered that ω is specified from the required performance of the backlight. If ω = 35 °, γ = 45 ° for polycarbonate and γ = 58 ° for polymethyl methacrylate.

That is, the value of the angle γ (inclination A) is determined from the required specification of the viewing angle, and h is adjusted so that A becomes a required value to determine the entire shape. FIG. 4 shows a design example when ω = 35 °. The solid line is polycarbonate, and the dotted line is polymethyl methacrylate.

The primary transmitted light from the plane portion of the transparent sheet forming unit of the present invention has the same directivity as that of the conventional prism sheet, but the inclination of the curved surface RQS is more gentle and curved than the plane portion. The primary transmitted light from this portion is wider than that of the plane portions OR and PS and has directivity in which the luminance gradually changes in angle. As a result, the present transparent sheet does not show a sudden drop in luminance even beyond the viewing angle of the conventional prism sheet, and exhibits directivity that gradually approaches zero while making a shoulder in the luminance-angle curve. .

As described above, a transparent sheet having a row of units of the cross-sectional shape represented by the formula (1) on one side is capable of suppressing directional characteristics showing a gradual change in luminance and suppressing transmitted light emitted at a useless wide angle. It can be understood that the efficiency improvement can be realized simultaneously. However, this effect is due to the curved surface RQ, S
It is not necessarily limited to the case where the cross-sectional shape of Q is represented by the expression (1). Although the magnitude of the effect is different, other functional forms (cylindrical surface, quadratic cylindrical surface, etc.) work as shown in Fig. 1 if the shape is a convex curved surface that smoothly connects from the plane part. It has the effect of relaxing the angle change of brightness and brightness (claim 1).

FIG. 2 is a perspective view of the transparent sheet according to the first and second aspects of the present invention. Actual sheet thickness is 0.1m
The pitch T of the unit is about 30 μm to about 0.5 mm. This transparent sheet is used in place of the prism sheet 1 in FIG. Further, when two transparent sheets of the present invention are used so as to be orthogonal to each other, the front luminance can be further improved.

[0028]

EXAMPLE T = 50 .mu.m based on the solid line in FIG. 4 was used to fabricate a mold having grooves similar in cross section to that shown in FIG.
The transparent sheet of claim 2 was manufactured by hot pressing on the polycarbonate transparent plate. Further, this transparent sheet was superimposed on the light emitting surface of the edge light type surface light source, and the angular distribution of luminance was measured. The result is shown in FIG. At this time, the front luminance increase rate was 1.54 times.

As a comparative example, a mold was prepared in which prisms having apex angles of 90 ° and 112 ° were arranged at a pitch of 50 μm, and the thickness was 2 mm.
The conventional prism sheet was manufactured by hot pressing on a polycarbonate transparent plate. Here, the apex angle 112 ° is obtained by setting m = 3 in the equation (2). This transparent sheet was superimposed on the light emitting surface of the same surface light source as before, and the angular distribution of luminance was measured. The results are shown by a solid line and a dotted line in FIG. At this time, the front luminance increase rate was 1.52 times when the apex angle was 90 °, and 1.46 times when the apex angle was 112 °.

As is apparent from a comparison between FIG. 7 and FIG.
The transparent sheet of the present invention has a feature that is not present in the conventional prism sheet, such that the brightness gradually decreases with an increase in the viewing angle while having a front luminance increase rate comparable to that of the conventional prism sheet, and a liquid crystal with a wide viewing angle. It can be seen that directivity that makes full use of the performance of the panel is realized.

[0031]

According to the transparent sheet of the present invention, the directional characteristics are improved by dividing the two slopes of the conventional prism sheet into a flat surface and a curved surface smoothly connected to the flat surface, without impairing the front luminance improving effect. It is possible to realize a backlight having directivity that can be used for a liquid crystal panel having a wide viewing angle.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an operation of a transparent sheet of the present invention.

FIG. 2 is a perspective view of the transparent sheet of the present invention.

FIG. 3 is a graph showing a relationship between the inclination of a slope of a prism sheet and a viewing angle.

FIG. 4 is a design example of a forming unit formed on one surface of the transparent sheet of the present invention.

FIG. 5 is a cross-sectional view illustrating the operation of a conventional prism sheet.

FIG. 6 is a perspective view showing a form in which a conventional prism sheet or the transparent sheet of the present invention is used.

FIG. 7 is a graph showing an angle change of luminance of a backlight using the transparent sheet of the example.

FIG. 8 is a graph showing a change in angle of luminance of a backlight using a prism sheet according to a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Prism sheet 2 ... Prism row 3 ... Backlight 4 ... Diffusion film 5 ... Cold cathode ray tube 6 ... Reflection film 7 ... Light guide 8 ... Transparent sheet Forming units on the surface 9 ... The transparent sheet of the present invention

Claims (1)

(57) [Claims] 1. A shape unit in which two convex curved surfaces whose generatrix is parallel to each other and a plane connected to each of them are symmetrically connected to each other on one surface, and a number of shape units are formed side by side in a direction perpendicular to the generatrix. a transparent sheet, both contact surfaces at the junction of the both songs cylindrical surface is intersect at an angle smaller than 180 °, the coupling portion of each surface and each plane, the respective contact surfaces coincides with the plane Tomo
And the z-axis parallel to the normal of the transparent sheet
Take the y-axis parallel to the generatrix, and the x-axis
When taken, the shape of the forming unit satisfies the following formula (1)
The brightness enhancement transparent sheet characterized by being formed as follows . (Equation 1)