JP2001155520A - Flat light source, and back light optical system and display using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a bright flat light source and a back light optical system by improving the efficiency of a light guide plate. SOLUTION: A diffraction grating 7 of a pitch <=0.5 μm is formed on at least a part of the upper surface 5 of the light transmission plate 1 and by using the characteristic of a dense diffraction grating which efficiently diffracts specific straight polarization, light emitted from the plate 1 is evenly linear polarized. Further, a phase difference film, a light diffusion pattern, etc., are formed on the slant 8 of the plate 1 to give a function of rotating polarization, and polarized light entering the slant 8 from the grating 7 is rotated to increase the polarization components of high diffraction efficiency. Since the whole light inside the plate can be evenly linear polarized finally, a light quantity which is lost at a polarizing plate conventionally can be utilized completely and the efficiency can be doubled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display for modulating the polarization state of linearly polarized light, such as a liquid crystal panel, for display.
And a planar light source and a backlight optical system used for the same.

[0002]

2. Description of the Related Art A liquid crystal display has a structure in which polarizing plates are provided on a light incident side and a light emitting side of an element (liquid crystal panel) having a liquid crystal layer sandwiched between a pair of substrates made of glass or the like. The display is performed by modulating the polarization state of the linearly polarized light incident on the liquid crystal layer by the liquid crystal layer. Among the liquid crystal displays, a transmissive type has a backlight unit for irradiating light from the back of the liquid crystal panel.

In recent years, in order to improve the efficiency of this backlight unit, a method of aligning the polarization direction of light emitted from the backlight unit to linearly polarized light and doubling the transmittance of a polarizing plate attached to a liquid crystal panel has been proposed. Has been proposed.

For example, Japanese Patent Laid-Open Publication No. Hei 10-78581 discloses a backlight in which a polarizing beam splitter 21 is provided between a cold cathode tube 3 and a light guide plate 1 as shown in FIG. Hereinafter, the operation of this backlight will be described. The polarization beam splitter 21 has a function of separating incident light into transmitted light and reflected light whose polarization directions are orthogonal to each other. The light emitted from the cold-cathode tube 3 (light 21a) is non-polarized light (randomly polarized light) having no fixed polarization direction. When this light enters the polarization beam splitter 21, the S-polarized light (light 21c) in a direction perpendicular to the paper surface becomes The reflected P-polarized light (light 21 b) in the direction parallel to the paper surface is transmitted and enters the light guide plate 1. The light beam 21c reflected by the polarization beam splitter 21 is reflected by a reflection plate 22 provided below the polarization beam splitter 21, reflected again by the polarization beam splitter 21 while maintaining the polarization direction, and returned to the reflector 4 (light beam 21d). ). The light reflected by the surface of the reflector 4 is scattered and reflected, so that the polarization direction is not maintained, and the light becomes a light (ray 21e) in which S-polarized light and P-polarized light are mixed. When this light ray 21e enters the polarization beam splitter 21, P
The polarized light enters the light guide plate 1, and the S-polarized light is reflected again. This series of operations is repeated, and finally all the light enters the light guide plate 1 as P-polarized light.

As described above, when the polarizing beam splitter 21 is used, the light incident on the light guide plate 1 is linearly polarized, so that the light emitted from the light guide plate 1 can also be linearly polarized. Linearly polarized light. Therefore, the polarized light component conventionally cut off by the polarizing plate is eliminated, and the light use efficiency can be improved.

[0006]

However, generally, a light diffusion pattern for diffusing light is provided on the slope or upper surface of the light guide plate. The light diffusion pattern has a function of diffusing a light reflection direction or a light transmission direction, and a function of rotating the polarization direction to reduce the polarization purity of the linearly polarized light that has entered. Therefore, even if linearly polarized light is incident on the light guide plate as described above, light emitted from the light guide plate toward the liquid crystal panel is randomly polarized by the function of the light diffusion pattern. And
Since this random polarized light has a component that is linearly polarized by a polarizing plate attached to the liquid crystal panel and cut, the light use efficiency cannot be improved.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems of the prior art, and the light emitted from the light guide plate without impairing the utilization efficiency of light from a cold cathode fluorescent lamp which is a random polarized light source. To provide a planar light source capable of improving the efficiency of use of backlight by linearly polarizing the light, and a bright backlight optical system and display using the same.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies and researches to solve the above-mentioned problems, and as a result, by providing a high-density diffraction grating on the upper surface of the light guide plate, the light utilization efficiency has been improved. It has been found that an improved planar light source and a backlight optical system and display using the same can be obtained.

More specifically, the planar light source of the present invention comprises at least a light source and a light guide plate, wherein the light from the light source is incident from an end face of the light guide plate and emitted from the upper surface of the light guide plate. At least a part of the upper surface of the light guide plate has a diffraction grating having a pitch of 0.5 μm or less, and the lower surface of the light guide plate is a slope inclined with respect to the upper surface, and the polarized light is rotated on the slope. And a light diffusion pattern for rotating polarized light, thereby achieving the above object.

It is preferable that a scattering plate is provided on the front side of the diffraction grating, or that the diffraction grating has a function of scattering light.

A backlight optical system according to the present invention includes the planar light source according to the present invention, thereby achieving the above object.

In the display according to the present invention, the planar light source according to the present invention is disposed on the back side of the display panel, with the upper surface being disposed on the display panel side, thereby achieving the above object.

The operation of the present invention will be described below.

In the present invention, since at least a part of the upper surface of the light guide plate has a diffraction grating, light in the light guide plate is emitted from the upper surface to the outside according to the diffraction conditions. When the pitch of the diffraction grating is equal to or less than the wavelength of the light incident on the diffraction grating, the diffraction efficiency changes depending on the polarization direction of the light incident on the diffraction grating. In the present invention, the pitch of the diffraction grating is 0.5 μm.
Since it is less than m, the S-polarized light has higher diffraction efficiency and is emitted from the light guide plate, and the P-polarized light has higher reflectivity and is reflected to continue to guide the light inside the light guide plate. Therefore, the emitted light can be aligned with a specific linearly polarized light (S-polarized light).

Further, since a phase difference film, a light diffusion pattern, or the like is provided on the slope of the light guide plate to provide a function of rotating polarized light, P light reflected from the diffraction grating provided on the upper surface of the light guide plate and incident on the slope is provided. The polarization is rotated, and an S-polarized component having high diffraction efficiency can be given. When this light is incident on the diffraction grating on the upper surface, the S-polarized light component is emitted from the light guide plate, and the P-polarized light component is reflected.
It is possible to emit the light with the polarization being the same. Therefore, it is possible to use all the light amount that has been conventionally lost in the polarizing plate.

By providing a scattering plate on the front side of the diffraction grating or using a diffraction grating having a light scattering function such as a scattering hologram, it is possible to reduce the coloring property and improve the whiteness of the backlight light. It is possible.

In the present invention, P-polarized light means that the signal direction of the light is in a plane including the incident light and the normal direction, and S-polarized light means that the signal direction of the light is the normal direction to the incident light. It means something that lies in a plane perpendicular to the plane that contains it.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing a schematic structure of a liquid crystal display according to an embodiment of the present invention. On the opposite side (back side) of the liquid crystal unit 11 from the viewer 5, a planar light source is disposed via a polarizing plate 12. This planar light source is of a sidelight type, and includes a cold cathode tube 3 and a reflector 4 on an incident end side 2 of a light guide plate 1. A high-density diffraction grating 7 is provided on the upper surface (the surface on the observer 6 side) 5 of the light guide plate 1.
Having. Note that a high-density diffraction grating refers to a diffraction grating having a small pitch Λ. This high-density diffraction grating
It can be formed by a general photolithography method.

In the conventional light guide plate, since the inclined surface and the upper surface are tapered, light incident on the light guide plate deviates from the condition of total reflection while being repeatedly reflected on the inclined surface and the upper surface, and exits the light guide plate. On the other hand, in the present invention, since the diffraction grating 7 is provided on the upper surface 5, the light incident on the light guide plate 1 is emitted from the upper surface 5 to the outside according to the diffraction condition. The diffraction conditions,
It is shown in the following equation (1).

N1 · sin θ1 + sin θ2 = λ / Λ (1) where N1 is a refractive index of the light guide plate, θ1 is an incident angle at which light enters the diffraction grating in the light guide plate, θ2 is a diffraction angle, and λ is light. Is the pitch of the diffraction grating.

In the light guide plate 1, light propagates at an angle larger than the total reflection angle. However, the smaller the angle of incidence on the diffraction grating 7, the higher the diffraction efficiency is obtained. The light having the highest efficiency to be emitted to the outside of the light guide plate is light near the total reflection angle. Therefore, it is assumed that θ1 is the total reflection angle. At this time, for example, if N1 is 1.5 and λ is 0.5 μm, と す る becomes 0.5 in order to make the diffraction angle θ2 0.

If the incident angle θ1 of the light incident on the diffraction grating 7 is equal to or larger than the total reflection angle (referred to as θ0), diffracted light and reflected light are generated, and the incident angle θ1 is smaller than the total reflection angle (θ0). Generates diffracted light, reflected light, and transmitted light. In any case, the diffraction efficiency is determined by the specifications (pitch, refractive index, shape, and depth) of the diffraction grating.
Since all of the light incident on the light guide plate 1 satisfies the condition of total reflection, it is assumed that only diffracted light and reflected light are generated.

Here, when the pitch の of the diffraction grating 7 is smaller than the wavelength, the polarization direction of the incident light affects the diffraction efficiency, as shown in FIG. Here, when the incident light is P-polarized light, the diffraction efficiency is low and the reflectance is high. On the other hand, when the incident light is S-polarized light, the diffraction efficiency is high and the reflectance is low. For this reason, each time the light guided inside the light guide plate 1 enters the diffraction grating 7, the S-polarized light is emitted to the outside by diffraction, and the P-polarized light continues to be guided inside by the reflection. In this light guide plate 1,
When there is no device having a polarization rotation function other than the diffraction grating, the inside of the light guide plate 1 is filled with P-polarized light.

Therefore, in the present invention, the inclined surface 8 of the light guide plate 1 is provided with a polarization rotating function. For example, as shown in FIG. 3A, a retardation film 9 is provided on the slope 8 of the light guide plate 1. Note that a retardation film refers to a film having different properties in a phase change which is received for each of two linearly polarized lights whose polarization directions are orthogonal to each other when light passes through the film. Organic compounds are well known as a medium having primary or secondary or higher optical anisotropy, and the retardation film 9 can be formed by, for example, a method such as spin coating, vapor deposition, or sputtering. When formed on such a slope 8 of the retardation film 9, as shown in FIG. 3A, a phase difference is given to the P-polarized light reflected from the diffraction grating 7 on the upper surface 5 of the light guide plate 1 and incident on the slope 8. To rotate the polarization. When this light is incident on the diffraction grating 7 on the upper surface 5, the P-polarized light component is totally reflected and the S-polarized light component is emitted to the outside, so that the S-polarized light component of the emitted light to the outside can be increased.

Alternatively, as shown in FIG. 3B, by providing a light diffusion pattern 10 on the inclined surface 8 of the light guide plate 1, the polarization can be rotated in a pseudo manner. The light reflected by the light diffusion pattern 10 is scattered so that the light is bent in a direction other than the regular reflection, so that the polarization is rotated. For example, a light diffusion pattern having a hemispherical shape, a triangular pyramid shape, a quadrangular pyramid shape, a quadrangular prism shape, a random unevenness, or the like can be given. The operation when the light diffusion pattern 10 is formed on the slope 8 is the same as the case where the above-described retardation film 9 is provided.

As described above, when the polarization rotation function is given to the inclined surface 8 of the light guide plate 1, the P-polarized reflected light from the diffraction grating 7 is rotated on the inclined surface 8 to become light having S-polarized light and P-polarized light components. When entering the diffraction grating 7 on the upper surface 5, S-polarized light is emitted and P-polarized light is reflected. By repeating this operation, finally, all the light becomes S-polarized light and the light guide plate 1
It is emitted outside. For this reason, the light emitted from the conventional light guide plate is light of random polarization, whereas the light guide plate of the present invention can be adjusted to linearly polarized light. Therefore, conventionally, the polarizing plate 1 provided on the light guide plate 1 side of the liquid crystal panel 11 shown in FIG.
Where the amount of light attenuated by half when passing through 2,
Since the light can be almost transmitted, the light use efficiency can be doubled.

The basic principle of the light guide plate according to the present invention has been described above. Next, a method for equalizing the luminance of an actual light guide plate will be described.

In the present invention, the diffraction grating plays the role of emitting the light in the light guide plate to the outside. In order to equalize the luminance, a diffraction grating is formed on the entire upper surface of the light guide plate and diffraction at each location is performed. Methods to optimize efficiency are possible. However, in order to control the diffraction index of the diffraction grating for each location, high manufacturing accuracy is required over the entire surface of the light guide plate, and the production cost increases. Therefore, the diffraction grating is not provided on the entire upper surface as in the conventional light diffusion pattern, but is partially formed in an island shape as shown in FIGS. 4 (a) and 4 (b). It is effective to optimize the arrangement and the size distribution of the diffusion patterns 13 to achieve uniform luminance. In this method, even if the diffraction efficiency is not set to a desired value for each location, if the diffraction efficiency has reproducibility, the light emission amount can be controlled by the area ratio of the diffraction grating to the light guide plate area. It is possible to easily realize luminance equalization. Although FIG. 4 shows a state in which the island-like diffraction gratings 13 are arranged on the upper surface 5 when the retardation film 9 is formed on the inclined surface 8 of the light guide plate 1, the light diffusion pattern may be formed on the inclined surface 8. The same is true.

By the way, as shown in the above equation (1), the diffraction angle θ2 from the diffraction grating to the outside of the light guide plate is a function of the wavelength, and the diffraction angle differs depending on the wavelength. For this reason, depending on the observer outside the light guide plate, the upper surface of the light guide plate is colored like a rainbow, and the whiteness required for the liquid crystal backlight unit is impaired.

In order to alleviate the coloring property, there is a method of disposing a scattering plate on the viewer side of the diffraction grating. Each color incident on the scattering plate is scattered and overlaps with each other to whiten the color. Alternatively, it is not necessary to additionally provide a scattering plate by adding such a function of the scattering plate to the diffraction grating. As shown in FIG. 5, a hologram (scattering hologram) 14 in which scattered light is recorded can be used as the diffraction grating having the function of the scattering plate. In addition,
FIG. 5 shows a case where the retardation film 9 is formed on the inclined surface 8 of the light guide plate 1. However, the same applies to a case where a light diffusion pattern is formed on the inclined surface 8, and the scattering hologram 14 is provided in an island shape. Needless to say, it is good.

The scattering hologram is obtained by broadening the spectrum of the pitch of the diffraction grating, and the most typical manufacturing method is a two-beam interference exposure method. This will be described with reference to FIG. What is the two-beam interference exposure method?
A hologram exposure method that records interference fringes between object light and reference light. A laser 15 is used as a light source.
Ar laser, He that can obtain wavelengths from green to ultraviolet light
A -Cd laser or the like is suitable. This laser light is applied to mirror 1
The light is split into two light beams by using the light 6a, and the coherence is enhanced by the spatial filter 17, and interference exposure is performed on the exposure dry plate 18. At this time, a diffusing plate 19 is provided in one of the light beams to generate scattered light to be used as object light. Thereafter, the exposure dry plate 18 is subjected to a development process and a fixing process to obtain a scattering hologram. The influence of external vibration can be removed by setting an optical system on the vibration isolation table 20. In FIG. 6, 16b, 16c and 16d are also mirrors. The scattering hologram produced in this manner may be used as it is, and the hologram is used as a master for the 2P method (Photo Poly).
mer) or the like.

According to the planar light source of this embodiment configured as described above, light having a uniform polarization direction can be applied to the polarizing plate 12, so that the light use efficiency of the backlight optical system and the display can be increased. And a bright display can be obtained with low power consumption.

In the above embodiment, the material of the light guide plate is not particularly limited. However, for example, polymethyl methacrylate (PMMA) can be used because of the advantage of the manufacturing method that it can be manufactured by injection molding and the optical characteristics such as transparency and refractive index. ), A copolymer of methyl methacrylate with another acrylate or methacrylate, or a transparent resin such as polycarbonate. In addition, it is preferable to use a cycloolefin polymer having a smaller specific gravity than PMMA (cycloolefin polymer 1.0 with respect to PMMA 1.2) because the weight can be reduced. In addition, cycloolefin polymers have better fluidity during injection molding than PMMA, so they are suitable for thin molding and have a high refractive index (PMM
A1.49 to cycloolefin polymer 1.5
3) Therefore, the effect of light confinement is great, and it is suitable for reducing the thickness of the light guide plate. Furthermore, since the cycloolefin polymer has low hygroscopicity, it has the advantage that the shape is less deformed even under high humidity conditions.

The light guide plate of the present invention can be manufactured by injection molding or compression molding using the above-mentioned resin, and at the same time, the above-mentioned light diffusion pattern can be formed on the light guide plate. In addition, a light diffusion pattern can be formed by printing after molding the light guide plate.

In the planar light source of the present invention, a combination of a cold cathode tube and a reflector can be used as the light source device.

[0037]

As described in detail above, according to the present invention,
Since a diffraction grating having a pitch of 0.5 μm or less is provided on at least a part of the upper surface of the light guide plate, only light in a specific polarization direction can be emitted to the outside of the light guide plate. In addition, because the function of rotating polarized light is given to the slope of the light guide plate,
The polarization direction of the light inside the light guide plate can be converted into a polarization direction with high diffraction efficiency, and all the light inside the light guide plate can be emitted to the outside while being aligned in a specific polarization direction. Therefore, light use efficiency can be improved.

By forming a retardation film on the inclined surface of the light guide plate using a simple method such as spin coating, vapor deposition, and sputtering, a function of rotating polarized light can be provided. It can be manufactured and has excellent mass productivity.

Alternatively, when the light guide plate is formed by injection molding, compression formation, or the like, a function of rotating polarized light can be provided by forming a light diffusion pattern on the slope, so that it can be manufactured at low cost. Excellent mass productivity.

By using the planar light source of the present invention, it is possible to use the entire amount of light that has been lost in the polarizing plate in the past, so that the light use efficiency of the backlight optical system and the display is improved and low power consumption is achieved. Bright display can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a schematic configuration of a liquid crystal display according to an embodiment of the present invention.

FIG. 2 is a diagram showing polarization dependence of diffraction efficiency characteristics in a diffraction grating.

3A is a cross-sectional view showing a configuration in which a retardation film is provided to give a rotating function to a slope of a light guide plate, and FIG. 3B is a cross-sectional view showing a diffusion pattern for giving a rotation function to the slope of the light guide plate; It is sectional drawing which shows the structure provided with.

4A is a cross-sectional view showing a configuration in which a diffraction grating is provided in an island shape on the upper surface of a light guide plate, and FIG. 4B is a plan view thereof.

FIG. 5 is a cross-sectional view showing a configuration in which a scattering hologram is provided on an upper surface of a light guide plate.

FIG. 6 is a diagram for explaining an example of a method for producing a scattering hologram.

FIG. 7 is a cross-sectional view showing a schematic configuration of a liquid crystal display using a conventional planar light source.

[Explanation of symbols]

 Reference Signs List 1 light guide plate 2 entrance end 3 cold cathode tube 4 reflector 5 upper surface of light guide plate 6 observer 7 high-density diffraction grating 8 slope of light guide plate 9 retardation film 10 light diffusion pattern 11 liquid crystal panel 12 polarizing plate 13 island diffraction grating 14 Scattering hologram 15 laser 16a, 16b, 16c, 16d mirror 17 spatial filter 18 exposure dry plate 19 diffusion plate 20 anti-vibration table 21 polarization beam splitter 22 reflection plate

Claims (4)

[Claims] 1. A planar light source comprising at least a light source and a light guide plate, wherein light from the light source is made incident on an end face of the light guide plate and emitted from the upper surface of the light guide plate, wherein at least the upper surface of the light guide plate is provided. Partly, the pitch is 0.5
μm or less, a lower surface of the light guide plate is a slope inclined with respect to the upper surface, and the slope is a retardation film for rotating polarized light or a light diffusion pattern for rotating polarized light. A planar light source having: 2. The planar light source according to claim 1, wherein a scattering plate is provided on the front side of the diffraction grating, or the diffraction grating has a function of scattering light. 3. A backlight optical system comprising the planar light source according to claim 1. 4. A display in which the planar light source according to claim 1 or 2 is arranged on the back side of the display panel, with the upper surface side being arranged on the display panel side.