JP3106050B2 - 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. 9 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 in FIG. 9 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. 8 shows a cross section perpendicular to both the inclined surfaces of the prism of such a prism sheet. The incident light beam has a component a ( Hereinafter, this is referred to as primary transmitted light), a component b (hereinafter referred to as “return light”) which is reflected on the prism slope once and then reflected again on the other slope and returned to the incident side, and is reflected once on the prism slope and then on the other slope. 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.

The second problem is the existence of useless secondary transmitted light. If this component can be reduced, further improvement in efficiency can be expected. For this purpose USP 2,474,3
As pointed out in 17

[0009]

(2) 180２-4 (90 ゜ -θc) / m, (m is an integer of 3 or more) (2)

[0010] It is conceivable to select the above. 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 that gradually decreases in a direction beyond a predetermined viewing angle with an increase in the viewing angle, and to provide the secondary transmitted light as described above. An object of the present invention is to develop an optical sheet having a high-efficiency luminance increasing effect in which generation of light is suppressed.

[0012]

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, at the joint between the curved surface and the joint between the right and left curved surface and the plane, each tangent surface and plane intersect at an angle of less than 180 ° , and
Take the z-axis parallel to the normal of the light sheet, and generate the generatrix of the convex curved surface
Take the y-axis parallel to and the x-axis in the column direction of the forming units
So that the shape of the forming unit satisfies the following expression (1).
It is characterized by being formed .

[0013]

[0014]

(Equation 3)

Here, φ = 0.8θc to 1.2θc, θc = sin
^-1 (1 / n), n represents the refractive index of the forming unit. And p = 0.8
T to 1.5T, T represents the width of the forming unit.

Further , a transparent sheet according to a second aspect of the present invention, wherein a plurality of partial polygonal prism surfaces formed by symmetrically connecting four planes on one surface are formed side by side in a direction perpendicular to the generatrix of the polygonal prism surfaces. And adjacent faces of the polygonal prism faces are 180
゜ A brightness-enhancing transparent sheet characterized by having a shape intersecting at a smaller angle.

[0017]

The operation of the present invention will be described below with reference to the drawings. FIG. 2 is an xz sectional view of the transparent sheet according to claim 2 of the present invention,
Equation (1), which is the most characteristic of φ = θc and p = T,
9 illustrates a case suitable for explaining the lens design of the invention . Here, the plane between OR and PS is a plane, and its function is shown in FIG.
This is the same as the slope of the prism sheet. The slope between OR
Assuming that A, the relationship between OR and PS is expressed by the following formula.
Is done. B indicates the value of the point R on the x-axis.

[0018]

## EQU4 ## z = Ax (0 ≦ x ≦ B: between ORs) z = −A (x−T) (TB−x ≦ T: between PSs)

On the other hand, between RQ and SQ is a curved surface introduced by the present invention. In the conventional prism sheet of FIG. 8, most of the secondary transmitted light is emitted from near the apex angle of the prism, so in order to prevent this, the shape near the apex angle was changed and reflected on the opposite slope. All light should be totally reflected at this portion. FIG. 2
In the transparent sheet described above, a convex curved surface is formed between RQ and SQ near the apex angle, and light reaching this portion from the opposing slope is always incident at an angle of total reflection angle θc = sin ⁻¹ (1 / n) or more. It is made to enter the part. Since the ray incident at the smallest incident angle when viewed from each point between RQ is the reflected light from point P, the angle between the straight line connecting each point between P and RQ and the normal of the surface at the same point is If it is selected to be equal to θc, all the reflected light from other points on the right slope will be reflected between RQ. At this time, if the point O is taken as the origin, the coordinates of the point P are (T, 0), and the condition that the curved surface RQ satisfies is

[0020]

(Equation 5)

## EQU1 ## In practice, the form of RQ is determined by numerically integrating this equation under the boundary condition that coincides with the plane portion OR at the point R. Similarly , between the SQs, the curved surface SQ is satisfied.
The conditions are as follows, and the shape becomes symmetrical about x = T / 2. Thus, one shape unit ORQS
The whole of P is represented by the equation (1).

(Equation 6)

As described above, in the transparent sheet having the cross-sectional shape shown in FIG.
The light becomes return light as shown in C, and no secondary transmitted light is emitted from the curved surface RQS. Some secondary transmitted light is emitted from the slopes OR and PS, but if the B in equation (1) is properly selected, most of the secondary transmitted light hits the adjacent shape unit and is absorbed, and a part of it is returned light. Join. As a result, light emitted at a wide angle as shown in FIG.

The plane portions OR, P of the shape unit of the transparent sheet
Since the function of S is equivalent to that of the slope of the conventional prism sheet, the primary transmitted light from here exhibits the same directivity as that of the conventional prism sheet, but the slope of the curved surface RQS is gentler than that of the flat portion and the surface is curved. Because of this, 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. .

In the above, the case where φ = θc and p = T in the equation (1) has been described. However, the characteristics obtained by adjusting the values of φ and p can be slightly changed. For example, if φ> θc or p <T, the curved column surface RQ of the forming unit
The slope of S becomes uniformly gentle, and the viewing angle is further widened.
Therefore, it is particularly effective when a wide viewing angle is required.
In this case as well, no secondary transmitted light is generated, but the increase rate of the front luminance is reduced as compared with the case where φ = θc and p = T by widening the viewing angle. If the rate of increase in the front luminance is too low, the merit of the transparent sheet is diminished. Therefore, it is preferable that φ be up to about 1.2θc and p be up to about 0.8T at least.

On the other hand, if φ <θc or p> T, the viewing angle is narrowed, and it can be expected that the rate of increase of the front luminance is further increased. In this case, secondary transmitted light is generated. However, if the decrease in φ or the increase in p is slight, no major problem occurs. However, if p is set too large or φ is set too small, it is not preferable because the secondary transmitted light increases and the front luminance decreases. Preferably, φ is at least about 0.8θc and p is at most about 1.5T.

In the above-mentioned transparent sheet, A in equation (1), that is, the inclination of the plane portions OR and PS determines the viewing angle range where the luminance is the highest. When this viewing angle is ω, ω is given by the following equation. expressed.

[0027]

(Equation 7)

Refractive index 1.59 (polycarbonate), 1.49
FIG. 4 shows the relationship between γ and ω in the case of (polymethyl methacrylate). If γ is reduced, ω is increased and the viewing angle is widened, but if it is too small, the effect of improving luminance is undesirably weak. Further, it is not practically preferable to make the viewing angle too narrow, and as a result, it is preferable to select γ within the range of 35 ° to 55 °, and most preferably, γ is in the range of 40 ° to 50 °. As described above, although the brightness of the transparent sheet of the present invention decreases at an angle exceeding ω, the curved surface R
The degree of the decrease is gradual due to the primary transmitted light from the QS, and the practical viewing angle is wider than ω.

B in the expression (1) determines the ratio between the plane portion and the curved surface portion in the forming unit. If B is too large, it becomes close to a conventional prism sheet and the effect of the present invention is weakened, which is not preferable. If B is too small, the viewing angle is improved. It is practically preferable to select B in the range of 0.2T to 0.35T.

As described above, the transparent sheet having a row of units of the cross-sectional shape represented by the expression (1) on one side can improve the directivity characteristic showing a gradual change in luminance and the efficiency improvement by suppressing the secondary transmitted light. It can be seen that they can be realized simultaneously. However, this effect is not necessarily limited to the case where the cross-sectional shapes of the curved surfaces RQ and SQ are represented by the formula (1). Although there is a difference in the magnitude of the effect, if the shape is a convex curved surface and the absolute value of the inclination is smaller than the plane portions OR and PS, other function forms (cylindrical surface, quadratic curved surface) Figure 2)
(1) and an effect of relaxing a change in angle of luminance.

Further, even if RQ and SQ are planes as described in claim 3, the same effect is obtained if the absolute value of the inclination is smaller than OR and PS. FIG. 3 is a diagram in which the portion between RQ and SQ in FIG. 2 is replaced by a plane. In the conventional prism shape shown by a dotted line in FIG. In the same manner, in the conventional prism shape, a part of the secondary transmitted light going to a wide angle like C 'is bent more greatly on the plane RQ like C, and is returned to the next forming unit. It enters and a part of it becomes return light. RQ,
If the inclination between the SQs is further reduced, the secondary transmitted light can be made zero. However, the luminance angle distribution in these cases is not as gradual as the transparent sheet of the first and second aspects, but a relatively narrow angular distribution created by the primary transmitted light from the plane portions OR and PS and the first order between RQ and SQ. A characteristic that changes stepwise by superimposition of a relatively wide angle distribution created by transmitted light is obtained.

FIG. 1 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.

[0033]

Example: For a refractive index of 1.59 (polycarbonate),
The shape of the forming unit of claim 2 when φ = θc, p = T, A = 1 (tilt 45 °), and B = T / 4 is shown by a solid line in FIG. This can be easily calculated by numerical integration from equation (1). Based on this, a mold was prepared in which grooves having a sectional shape similar to that of FIG. 1 were arranged at T = 50 μm, and hot-pressed on a polycarbonate transparent plate having a thickness of 2 mm to produce a transparent sheet according to claim 2. 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 by a solid line in FIG. At this time, the front luminance increase rate was 1.55 times.

Also, as shown in the dotted line of FIG. 5, a die having a cross-sectional shape in which each vertex is connected by a straight line, similarly having T = 50 μm, and arranging grooves having a cross-sectional shape similar to that of FIG. The transparent sheet of claim 3 was manufactured by hot pressing on a polycarbonate transparent plate. The result of measuring the angular distribution of luminance of this transparent sheet in the same manner as above is shown by a dotted line 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 had a thickness of 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 result is shown by a solid line and a dotted line in FIG. 7, respectively. 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. 6 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. Further, as can be seen from the comparison between the solid line and the dotted line in FIG. 6, in order to realize a smoother luminance distribution, the transparent sheet according to claim 2 having a curved cylindrical surface is more preferable. Claim 3 is superior, and one of them may be selected depending on whether performance or cost is more important.

[0037]

According to the transparent sheet of the present invention, both slopes of the conventional prism sheet are divided into two, and the surface forming the apex angle is a convex curved surface having a gentle slope (claims 1 and 2) or By changing to a flat surface (claim 3), the directivity characteristics are improved, and it is possible to realize a backlight having directivity usable for a liquid crystal panel having a wide viewing angle without impairing the front luminance improving effect.

[Brief description of the drawings]

FIG. 1 is a perspective view of a transparent sheet of the present invention.

FIG. 2 is a cross-sectional view illustrating the operation of the transparent sheet of the present invention.

FIG. 3 is a cross-sectional view illustrating the operation of the transparent sheet of the present invention.

FIG. 4 is a graph showing a relationship between the inclination of a slope of a prism sheet and a viewing angle.

FIG. 5 is a cross-sectional view of a design example of a forming unit formed on one surface of the transparent sheet of the present invention.

FIG. 6 is a graph showing an angle change of luminance of a backlight using the transparent sheet of the example.

FIG. 7 is a graph showing a change in angle of luminance of a backlight using a prism sheet according to a comparative example.

FIG. 8 is a cross-sectional view illustrating the operation of a conventional prism sheet.

FIG. 9 is a perspective view showing a form in which a conventional prism sheet is used.

[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 (2)

(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, the coupling portion of the coupling portion and the curved surface and the plane of the both songs cylindrical surface, with each contact surface and the plane intersect at an angle smaller than 180 °, the transparent sheet
Take the z-axis parallel to the normal of, and parallel to the generatrix of the convex curved surface
When taking the y-axis and taking the x-axis in the column direction of the forming units,
The shape of the unit is formed so as to satisfy the following expression (1).
A transparent sheet with improved brightness . (Equation 1) (Where φ = 0.8θc ~ 1.2θc, θc = sin-1 (1 / n), n is
Represents the refractive index of the forming unit. p = 0.8T ~ 1.5T, T is formed
Indicates the width of the unit. ) 2. A transparent sheet in which a plurality of partial polygonal prism surfaces in which four planes are symmetrically connected to one another are formed side by side in a direction perpendicular to the generatrix of the polygonal prism surface. A brightness-enhancing transparent sheet, characterized in that adjacent surfaces of the prism surfaces intersect at an angle of less than 180 °.