JP2006119411A - Light guide plate and liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light guide plate capable of decreasing loss of light caused by a liquid crystal panel using polarized light, further increasing the use efficiency of light made incident from a light source, and emitting intense light, and to provide a display device using the light guide plate as a backlight. <P>SOLUTION: The light guide plate which makes diffracted light containing a large quantity of a linearly polarized light component in a specified direction exit when the light made incident from the light source exits to a liquid crystal panel side, is employed. The light guide plate is disposed beneath a liquid crystal panel comprising at least a liquid crystal cell having alignment of liquid crystal molecules twisted in a predetermined angle, and an upper polarizing plate and a lower polarizing plate having transmission axes coincident with the alignment of liquid crystal molecules and sandwiching the liquid crystal cell between them. The light guide plate is disposed in such a manner that when the light made incident from the light source on the inside of the guide plate from the light guide plate exits to the liquid crystal panel side, diffracted light including a large quantity of a linearly polarized light component coincident with the transmission axis of the lower polarized plate is made to exit from the light guide plate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light guide plate that guides light from a light source, and a liquid crystal display device that uses light emitted from the light guide plate as illumination light.

A display device using a liquid crystal panel (hereinafter also referred to as a liquid crystal display device or a display device using an LCD, and these terms are used in combination) can be designed to be lightweight and compact. Widely used in TV and TV.
This type of display device consists of a plurality of pixels arranged in a matrix, and each pixel in the matrix includes a light valve that allows light to pass through the display in response to a control signal. In order to illuminate the display, a backlight is disposed on the back of the display.

Some backlights use a planar light guide plate.
FIG. 1 is a conceptual diagram showing a general configuration of a display device to which such a backlight is applied.
That is, when random light R such as white light is irradiated from a light source 10 such as an LED or CCFL (cold cathode ray tube), the light R is guided along the light guide direction F in the light guide plate 12. To be lighted. In the middle of the light guide, light (emitted light) I leaked to the outside of the light guide plate 12 is diffused by the diffusion film 14 and then enters the liquid crystal panel through the prism sheet 16.

  The diffusion film 14 is used for diffusing the light I and equally illuminating the entire surface of the liquid crystal panel. The prism sheet 16 is used to control the incident light path so that the emitted light I enters the liquid crystal panel substantially perpendicularly.

  With such a configuration, the luminance of the liquid crystal panel can be increased to some extent, but the utilization rate of the light R from the light source 10 is not so high. This is because only about 60 to 70% of the light R from the light source 10 is used as the light I for illumination of the liquid crystal panel, and about 30 to 40% is guided to the terminal portion 11 of the light guide plate 12, This is because the light L leaks as it is.

As another factor that the utilization rate of the light R is not high, there is a phenomenon of “polarization” used by the liquid crystal panel.
The liquid crystal panel includes a liquid crystal cell 18, and an upper polarizing plate 19 a and a lower polarizing plate 19 b that sandwich the liquid crystal cell 18.
The liquid crystal cell 18 is formed by sandwiching a liquid crystal material between two glass substrates on which a transparent electrode film is formed. As the liquid crystal substance, the alignment direction of the liquid crystal molecules in contact with one glass substrate of the liquid crystal cell 18 and the alignment direction of the liquid crystal molecules in contact with the other substrate are shifted by 90 degrees by the alignment treatment, and as the plane in which the molecules are aligned is advanced. Many of them have a twisted nematic (TN) structure in which the molecular axis is twisted little by little and finally shifted to 90 degrees.

  In the liquid crystal cell 18 having such a TN structure, since light rotates along the direction of the molecular axis, it is possible to obtain a display with high contrast by selecting predetermined polarized light using a polarizing plate. it can. This type of liquid crystal display device is often used to display numbers and simple characters because it has a long response time but can be manufactured at low cost.

  Further, a liquid crystal cell having a structure in which the molecular axis of the liquid crystal cell 18 having the TN structure is further twisted to set a twist angle of 90 degrees or more is often used. A liquid crystal cell having such a structure is called a super twisted nematic (STN). As a result, when the twist angle is around 240 degrees, the change from the arrangement state to the non-arrangement state due to the voltage becomes steep and a sharp image can be obtained.

Regarding the light guide plate 12, when the liquid crystal cell 18 has the TN structure or the STN structure as described above, twisting occurs when light is transmitted, so the lower polarizing plate 19b, the liquid crystal cell 18, and the upper polarizing plate 19a. As the light passes through in order, the light emission luminance decreases.
For example, when an STN structure liquid crystal cell having a twist angle of 240 degrees is used, the transmission axis 100 of the lower polarizing plate 19b travels straight through the light from the LED 10 as shown in FIG. The direction is set so as to be inclined by 60 ° with respect to the X axis.
On the other hand, the light I emitted upward by the light guide plate 12 travels along the Y-axis orthogonal to the X-axis, so that it is difficult for the light I to pass through the lower polarizing plate 19b with the transmission axis 100 inclined.

On the other hand, as a means for emitting light guided through the light guide plate 12 along the light guide direction F from the light source 10 with total reflection to the outside of the light guide plate 12 everywhere, the upper surface of the light guide plate 12. In addition, an injection coupler (light extraction port) is formed on the lower surface.
There are various types of injection couplers, such as a dot pattern by screen printing, a concavo-convex structure, and a prism structure, and proposals for adopting a relief type diffraction grating as an injection coupler are known. (For example, see Patent Documents 1 and 2)

Japanese Patent Laid-Open No. 7-248496 JP 2000-305474 A

When a relief type diffraction grating is formed in parallel in the light guide direction F as the emission coupler (grating fringes extend in a direction perpendicular to the paper surface), the diffraction efficiency of the diffracted emission light I is shown in FIG. As shown in FIG. 3B, S-polarized light (vibrates in a direction perpendicular to the paper surface) is emitted, rather than for the TM mode in which P-polarized light (vibrates in the direction of the white arrow in the paper surface) is emitted. It is higher for the TE mode.
Since the diffraction grating has a structure with grooves in one direction, there may be a significant difference in diffraction efficiency depending on the polarization state of the incident light. The groove direction of the diffraction grating is perpendicular to the vibration direction of the electric field vector. In the S-polarized light (TM wave), a large fluctuation in diffraction efficiency is observed, and in the P-polarized light (TE wave) in which the groove direction of the diffraction grating and the vibration direction of the electric field vector are parallel, there is no large fluctuation as in the S-polarized light. It is known.
That is, when the diffraction grating is employed as the exit coupler, the linearly polarized light component included in the exit light has many components having a vibration direction equal to the groove direction of the diffraction grating (the direction in which the grating fringes extend).

  FIG. 6 is a graph showing the diffraction efficiency of the first-order diffracted reflected light that is diffracted by the diffraction grating (injection coupler) and emitted from the incident light that propagates with total reflection in the light guide plate.

The horizontal axis represents the incident angle of the light R with respect to the inner surface of the light guide, and the vertical axis represents the diffraction efficiency.
This graph shows a blazed relief type diffraction grating of 2000 lines / mm as the diffraction grating 4, 1.5 as the refractive index of the light guide, and 500 nm as the wavelength of the incident light R. Is a result of calculation for each of the TE mode in which P is diffracted and the TM mode in which P-polarized light of the light R is diffracted.

  Αc is a critical angle. As shown in FIG. 3, since the diffraction efficiency of the TE mode is several times that of the TM mode, S-polarized light is emitted several times more than the P-polarized light from the light guide plate.

As described above, the light irradiated from the backlight unit passes through the lower polarizing plate 19 b of the liquid crystal cell 18 and is irradiated into the liquid crystal cell 18. And it advances while being twisted along the molecular axis of the liquid crystal whose direction is changed by the application of voltage. And it cuts by the upper side polarizing plate 19a, or permeate | transmits and lets it pass upwards.
In this way, the light transmitted through the lower polarizing plate 19b is limited to the light irradiated at an angle corresponding to the transmission axis 100 of the lower polarizing plate 19b, and therefore, against the transmission axis 100 of the lower polarizing plate 19b. Since the light is blocked, the loss increases.
The present invention has been made in view of such circumstances, and the loss of light caused by the use of polarized light by the liquid crystal panel is reduced, the utilization efficiency of light incident from the light source is further increased, and strong light is emitted. An object of the present invention is to provide a light guide plate capable of emitting light and a display device using the light guide plate as a backlight.

  In order to achieve the above object, the present invention takes the following measures.

The light guide plate according to the present invention comprises:
When light incident inward from the light source is emitted toward the liquid crystal panel, diffracted light containing a large amount of linearly polarized light components in a specific direction is emitted.

  The light guide plate includes at least a liquid crystal cell having a liquid crystal molecular arrangement twisted at a predetermined angle, and an upper polarizing plate and a lower polarizing plate having a transmission axis that matches the liquid crystal molecular arrangement and sandwiching the liquid crystal cell. When the light guide plate is placed below the liquid crystal panel, the light guide plate emits a large amount of linearly polarized light components that coincide with the transmission axis of the lower polarizing plate when the light incident from the light source is emitted to the liquid crystal panel side. Arrange to emit light.

  According to the light guide plate and the liquid crystal display device of the present invention, the diffraction grating, which is the exit coupler on the surface of the light guide plate, matches the transmission axis of the polarizing plate provided corresponding to the twist angle of a specific liquid crystal molecule, The light emitted from the light source can reach the liquid crystal cell surface without being attenuated by the polarizing plate, and the utilization efficiency of light in the backlight is further improved.

Hereinafter, a liquid crystal display device according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 4 is a perspective view showing an outline of the main part (backlight part) of the liquid crystal display device according to the embodiment of the present invention.

The liquid crystal display device includes a liquid crystal panel (not shown) and a backlight unit 20.
The liquid crystal panel has the same configuration as that shown in FIG. 1 according to the prior art, and includes a liquid crystal cell 18 and an upper polarizing plate 19a and a lower polarizing plate 19b sandwiching the liquid crystal cell 18.
The liquid crystal cell 18 is formed by sandwiching a liquid crystal material between two glass substrates on which a transparent electrode film is formed. As the liquid crystal substance, those having a TN structure or an STN structure are employed.

As is well known, the liquid crystal cell 18 is provided with an upper transparent electrode, a lower transparent electrode, an upper alignment film, and a lower alignment film on the inner side of an upper glass substrate and a lower glass substrate that are disposed relative to each other. It is sealed at a certain interval and has a structure in which a liquid crystal substance is sealed.
As the liquid crystal material, STN having a twist angle of 240 ° for liquid crystal molecules is used.
Further, the upper polarizing plate 19a and the lower polarizing plate 19b are integrally formed on the outer sides of the upper glass substrate and the lower glass substrate of the liquid crystal cell 18, respectively, and conform to the twist angle of the liquid crystal material as shown in FIG. A transmission axis 100 is provided.

The backlight unit 20 includes at least a light source 10 such as an LED or a CCFL (cold cathode ray tube) and a light guide plate 12, and further includes the diffusion film 14 and the prism sheet 16 shown in FIG. 1 as necessary. Is.
In the embodiment shown in FIG. 4, the LED 10 and the light guide plate 12 are configured. The LED 10 is installed at a substantially central portion of one side surface of the light guide plate 12, and the light emitting surface faces the side surface. .

The light guide plate 12 is formed in a plate shape with a transparent acrylic material having substantially the same area as the liquid crystal panel, and a diffraction grating 3 serving as an emission coupler is formed below (back side) of the light guide plate 12.
In the present invention, the diffraction grating 3 is formed so that the grating fringes extend at an angle of the transmission axis 100 so as to coincide with the transmission axis 100 of the lower polarizing plate 19b of the liquid crystal panel.

  Further, in the side light emitting type (edge light type) backlight structure as in the present embodiment, the amount of light near the LED 10 is large, so the amount of emitted light (diffracted light amount) is kept low, and the amount of light decreases as the distance from the LED 10 increases. Therefore, it is necessary to increase the amount of emitted light (diffracted light amount) and to uniformly illuminate the entire surface of the light guide plate 12.

For this reason, appropriate design changes such as making the diffraction grating near the side surface (LED 10) of the light guide plate 12 into a grating shape with low diffraction efficiency and making the diffraction grating far from the LED 10 into a grating shape with high diffraction efficiency are performed. Is called.
A typical grating shape with high diffraction efficiency is a blazed (sawtooth) grating whose cross-sectional shape is not symmetrical, such as rectangular or sinusoidal.
Further, the same control is possible by controlling the depth of the diffraction grating. The depth of the grating is shallower near the LED 10 and is formed deeper as the distance from the LED 10 is increased. The light can be made uniform.

FIG. 5 is an explanatory diagram showing an improved example of the present embodiment, and illustrates an example in which the grid arrangement density is changed instead of the cross-sectional shape and depth of the grid in order to uniformly illuminate the entire surface of the light guide plate 12. ing.
The diffraction grating 3 on the side close to the LED 10 is arranged in a sparse state, whereas the diffraction grating 3 on the side far from the LED 10 is arranged in a dense state.

The present invention is not limited to the above-described embodiment, and various modifications are possible within a scope that does not depart from the gist of the present invention, and the modifications illustrated below are included in the present application.
a) The diffraction grating 3 which is an injection coupler is arranged not on the back surface of the light guide plate 12 but on the front surface (liquid crystal panel side) or on both the back surface and the front surface.
b) It is assumed that the arrangement of the diffraction gratings 3 is not two-dimensionally arranged as shown in FIG. 4 (uniform on the entire surface) or FIG.
c) Change other than the cross-sectional shape, depth, and arrangement density of the diffraction grating 3. For example, the lattice pitch is one of the changing parameters, and is changed gradually or stepwise according to the distance from the LED 10.
Due to the regional change of the grating pitch, the coloration of the emitted light, which appears to change the color of the visible light into a rainbow color due to the chromatic dispersion (spectroscopy) of the diffracted light, is eliminated by color mixing, and achromatic (white) ) Light can be obtained.
d) A light source other than LED (for example, CCFL) is adopted.
e) The direction of the transmission axis 100 of the lower polarizing plate is not inclined in the screen and is orthogonal to the light guide direction by the light guide plate 12.

The conceptual diagram which shows the general structure of the display apparatus to which a backlight is applied. Explanatory drawing which shows that when the liquid crystal cell of the STN structure which has a specific twist angle is used, it has set so that the transmission axis of a lower polarizing plate may become the direction which inclined. FIG. 3A is an explanatory diagram showing the diffraction efficiency of the diffracted exit light I when a relief type diffraction grating is formed in parallel in the light guide direction F as an exit coupler, and FIG. (B) is explanatory drawing about S polarized light. The perspective view which shows the principal part (backlight part) outline | summary of the liquid crystal display device which concerns on one Embodiment of this invention. Explanatory drawing which shows the example of improvement of this embodiment. The graph which shows the diffraction efficiency of the primary diffraction reflected light which the incident light which propagates in a light-guide plate diffracts by a diffraction grating, and inject | emits.

Explanation of symbols

10 Light source such as LED and CCFL (Cold Cathode Ray Tube) 12 Light guide plate 14 Diffusion film 16 Prism sheet 18 Liquid crystal cell 19a Upper polarizer 19b Lower polarizer 100 Transmission axis R Random light such as white light I Light guide plate 12 Leaked to the outside (emission light)

Claims (3)

  A light guide plate that emits diffracted light containing a large amount of linearly polarized light components in a specific direction when light incident inward from a light source is emitted toward the liquid crystal panel. Below the liquid crystal panel comprising at least a liquid crystal cell having a liquid crystal molecular arrangement twisted at a predetermined angle, and an upper polarizing plate and a lower polarizing plate having a transmission axis coincident with the liquid crystal molecular arrangement and sandwiching the liquid crystal cell In the light guide plate arranged in
The light guide plate is configured to emit diffracted light including a large amount of linearly polarized light components that coincide with the transmission axis of the lower polarizing plate when light incident on the inside from a light source is emitted to the liquid crystal panel side. A light guide plate. A liquid crystal cell having a liquid crystal molecular arrangement twisted at a predetermined angle, an upper polarizing plate and a lower polarizing plate having a transmission axis that matches the liquid crystal molecular arrangement and sandwiching the liquid crystal cell, and below the lower polarizing plate In a liquid crystal display device comprising a disposed light guide plate and a backlight light source disposed on a side surface of the light guide plate,
The light guide plate includes a diffraction grating for emitting light incident inward from the light source to the liquid crystal panel as diffracted light on at least one of its bottom surface / top surface,
2. The liquid crystal display device according to claim 1, wherein the diffraction grating is mostly a diffraction grating having a grating stripe extending in a direction coinciding with the transmission axis of the lower polarizing plate.

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