JPH11231311A - Reflection display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the contrast by suppressing unnecessary reflection of external light with respect to a reflection display device. SOLUTION: A panel 0 is provided with a transparent first substrate 1 placed on the incidence side of external light, a second substrate 2 which joined with the firs substrate 1 with a prescribed gap between them and is placed on the opposite side, a liquid crystal layer 3 held in this gap, and electrodes 5 and 6 which apply a voltage to this layer. A light diffusion layer 7, a quarter-wave plate 8, and a polarizing plate 9 are laminated in order from the bottom on the first substrate 1 of the panel 0. The polarizing plate 9 converts external light to linearly polarized light, and this light is made incident on the first substrate 1 side of the panel 0. The light diffusion layer 7 diffusively emits external light reflected on the second substrate 2 side of the panel 0. A part of external light made incident through the polarizing plate 9 is unnecessarily reflected by the light diffusion layer 7, the quarter-wave plate 8 cooperates with the polarizing plate 9 to intercept the external light unnecessarily reflected by the light diffusion layer 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection type display device for displaying images by using external light such as illumination light emitted from an indoor illumination lamp or natural light entering from a window in a room. More specifically, the present invention relates to a technique for preventing a light source such as an illumination lamp from being reflected on a screen of a reflective display device, and for preventing a contrast from falling below a sheet due to unnecessary reflection.

[0002]

2. Description of the Related Art A display device using a liquid crystal or the like as an electro-optical material has a flat panel shape, and is characterized by light weight and thinness and low power consumption. For this reason, it is actively developed as a display of a portable device. The liquid crystal is not of a self-luminous type and selectively transmits and blocks external light to display an image. Such passive display devices are classified into a transmission type and a reflection type according to an illumination system. In a transmissive display device, a panel holding, for example, a liquid crystal as an electro-optical material is formed between a pair of transparent substrates, and a backlight is arranged on the back surface, and an image is observed from the front of the panel. In the case of the transmission type, a backlight is indispensable, and for example, a cold cathode tube or the like is used. When viewed as a whole display, the backlight consumes most of the power and is not suitable for portable devices. On the other hand, in the reflection type, while a reflection plate is arranged on the back of the panel, indoor illumination light or the like is incident from the front, and an image is similarly observed from the front by using the reflected light. Unlike the transmissive type, which does not use a dedicated backlight, the reflective type requires relatively low power consumption and is suitable for a display of a portable device.

[0003]

By the way, a display device using a liquid crystal as an electro-optical material has a large viewing angle dependency.
The display contrast changes significantly depending on the angle at which the observer views the screen (viewing angle). In order to alleviate this viewing angle dependency, a structure in which a light diffusing plate is arranged on the front surface of a panel facing an observer has been conventionally proposed. For example, a structure in which a light diffusing plate is mounted on the front surface of a transmission type liquid crystal display device is disclosed in
No. 17793. The light diffusing plate can diffuse the light source light emitted from the backlight to reduce the viewing angle dependency. However, the light diffusion plate has a problem that backscattering of external light occurs, which causes a reduction in display contrast. This backscattering refers to a phenomenon in which external light incident on the screen from the observer side is scattered and reflected by the light diffusion plate and scattered rearward where the observer is located. In order to suppress backscattering, it is necessary to devise measures such as darkening the indoor lighting lamps and applying blinds to windows to block external light. In the case of the transmissive type, even if the illumination is darkened or external light is blocked, there is no problem in displaying the screen itself because the backlight is provided.

However, in a reflective display device, if the illumination is reduced or external light is blocked in order to suppress backscattering, an image cannot be displayed. Therefore, in the reflective display device, in order to prevent a decrease in display contrast caused by backscattering, it is an issue to be solved to suppress the backscattering itself. In the reflective display device, a light diffusing plate is indispensable in order to prevent the reflection of an external illumination lamp in addition to improving the viewing angle dependency. In the reflection type, since the reflection direction of the external illumination light matches the viewing angle direction at which the observer visually recognizes the display, if the reflection is present, the display becomes extremely difficult to see. In the reflection type, a structure has been proposed in which a reflection type light diffusion plate is provided on the back side instead of providing a transmission type light diffusion plate on the front surface of the panel.
If the reflection type light diffusion plate is used, there is no possibility that the display contrast is reduced due to the above-mentioned back scattering. However, incorporating a reflection type light diffusion plate into a panel is not preferable because it leads to an increase in cost.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems of the prior art, the following measures have been taken. That is, the reflective display device according to the present invention has a panel, a light diffusion layer, a quarter-wave plate, and a polarizing plate as basic components. The panel is a transparent first substrate located on the incident side of external light, a second substrate joined to the first substrate via a predetermined gap and located on the reflection side, and electro-optics such as liquid crystal held in the gap. A material and an electrode for applying a voltage to the electro-optic material. As a feature, a light diffusion layer, a quarter-wave plate, and a polarizing plate are sequentially stacked on the first substrate from the bottom. The polarizing plate converts external light into linearly polarized light and enters the panel on the first substrate side.
The light diffusion layer diffusely emits external light reflected on the second substrate side of the panel, and unnecessarily reflects part of the external light incident through the polarizing plate (backscattering). The quarter-wave plate cooperates with the polarizing plate to block external light (backscattering component) unnecessarily reflected by the light diffusion layer. Preferably, the panel uses a liquid crystal layer functioning as a quarter-wave plate according to a voltage application state as an electro-optical material. For example, the liquid crystal layer is made of a nematic liquid crystal having a positive dielectric anisotropy and twisted orientation, functions as a quarter-wave plate when no voltage is applied, and loses the function of a quarter-wave plate when voltage is applied.

According to the present invention, a polarizing plate, a quarter-wave plate and a light diffusing layer are stacked on the front surface of a panel facing an observer. In particular, the light diffusing layer is inserted between the stack of polarizing plates and quarter-wave plates and the panel. The lamination of the polarizing plate and the quarter-wave plate can block the backscatter generated in the light diffusion layer, which leads to an improvement in the contrast of the reflective display device. In particular, if a panel is used in which the lamination of a polarizing plate and a quarter-wave plate is indispensable on the principle of operation, the light diffusion layer is simply placed between the lamination of the polarizing plate and the quarter-wave plate and the panel. Backscattering can be suppressed only by mounting. As such a panel, there is a normally white mode TN-ECB using a liquid crystal layer functioning as a quarter-wave plate in a state where no voltage is applied as an electro-optical material.

[0007]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a basic configuration of a reflective display device according to the present invention.
As shown in the figure, the reflective display device has a panel 0, a light diffusion layer 7, a quarter-wave plate 8, and a polarizing plate 9 as basic components.
And The panel 0 is composed of a transparent first substrate 1 located on the outside light incident side and a second substrate 2 joined to the first substrate 1 via a predetermined gap and located on the reflection side. A liquid crystal layer 3 is held in the gap between the substrates 1 and 2 as an electro-optical material. However, the present invention is not limited to this, and other electro-optical materials can be used instead of the liquid crystal. An electrode for applying a voltage to the liquid crystal layer 3 is formed on the panel 0. Specifically, a transparent electrode 5 made of a transparent conductive film such as ITO is formed on the inner surface of a first substrate 1 made of glass or the like, while a metal deposited film made of a metal such as aluminum is formed on the inner surface of the second substrate 2. The reflection electrode 6 is formed. The liquid crystal layer 3 is sandwiched between the transparent electrode 5 and the reflective electrode 6 from above and below. As a feature of the present invention, a light diffusion layer 7, a quarter-wave plate 8, and a polarizing plate 9 are sequentially stacked on the first substrate 1 of the panel 0 from below.
The polarizing plate 9 converts external light into linearly polarized light and enters the first substrate 1. The light diffusion layer 7 diffusely emits external light reflected by the reflection electrode 6 on the second substrate 2 side. Thereby, the viewing angle dependency caused by the liquid crystal layer 3 can be reduced. The light diffusion layer 7 is basically of a light transmission type, but unnecessarily reflects a part of the external light incident through the polarizing plate 9 to generate a so-called backscattering component. The quarter-wave plate 8 cooperates with the polarizing plate 9 to block external light unnecessarily reflected by the light diffusion layer 7. As a result, backscattering is suppressed, and a decrease in display contrast can be prevented.

As shown in the figure, the light diffusion layer 7 is made of, for example, a transparent optical film in which fine particles 7a are dispersed in a resin 7b. For example, the light diffusion layer 7 is formed by combining fine particles 7a made of polymer beads with a resin (matrix polymer) 7 having a different refractive index.
b is a transparent film dispersed and fixed in the substrate b, and is bonded to the front surface of the first substrate 1 made of glass or the like via an adhesive or the like. The refractive index of the fine particles 7a is in the range of 1.0 to 1.9. Its particle size is about 1 to 10 μm. On the other hand, the resin (matrix polymer) 7b is made of an acrylic resin and has a refractive index of 1.5. By making the refractive indexes of the fine particles 7a and the resin 7b greatly different, the light diffusion layer 7 can exhibit excellent scattering ability. That is, the external light reflected by the reflective electrode 6 is diffusely transmitted toward the observer, and the viewing angle dependence of the liquid crystal layer 3 can be reduced. However, external light partially reflected by the light diffusion layer 7 becomes a backscattering component, which may cause a reduction in display contrast.

The quarter-wave plate 8 is made of, for example, a uniaxially stretched polymer film, and gives a phase difference of a quarter wavelength between ordinary light and extraordinary light. The optical axis (one-axis anisotropic axis) of the quarter-wave plate 8 is 45 ° with respect to the polarization axis (transmission axis) of the polarizing plate 9.
It is arranged so as to form an angle. Thereby, it is possible to block the backscattering component by the light diffusion layer 7 from the observer side. That is, the external light becomes linearly polarized light when transmitted through the polarizing plate 9. When this linearly polarized light passes through the quarter-wave plate 8, it becomes circularly polarized light. Part of the circularly polarized light is unnecessarily reflected by the light diffusion layer 7 to generate a backscattered component. Unnecessarily reflected circularly polarized light returns to linearly polarized light when passing through the quarter-wave plate 8 in reverse.
However, the polarization axis at the time of emission is rotated by 90 ° from the polarization axis at the time of incidence. As a result, the output linearly polarized light becomes orthogonal to the transmission axis of the polarizing plate 9 and is all absorbed. Therefore, the light diffusion layer 7
The backscattering component due to is not leaked to the observer side.

In this embodiment, the panel 0 uses the liquid crystal layer 3 functioning as a quarter-wave plate according to the voltage application state as an electro-optical material. In other words, panel 0
Display using the birefringence of the liquid crystal layer 3, and employ a so-called ECB mode. However, the present invention is not limited to this, and may adopt a guest host mode or a twisted nematic mode. The liquid crystal layer 3 is composed of a nematic liquid crystal 3m having a positive dielectric anisotropy and a twist alignment. That is, in the present embodiment, a twisted nematic (TN) liquid crystal 3m is used, and a so-called TN-EC
This is the B mode. However, the present invention is not limited to this, and includes a normal ECB mode. The liquid crystal layer 3 functions as a quarter-wave plate when no voltage is applied, and cooperates with the polarizing plate 9 and the quarter-wave plate 8 to transmit external light reflected by the reflective electrode 6 to realize a white display. I do. When a voltage is applied, the liquid crystal layer 3 loses the function of a quarter-wave plate and blocks external light reflected by the reflective electrode 6 to display black. That is, in the present embodiment, the panel 0 is in the normally white mode. However, the present invention is not limited to this. The panel 0 in the normally white mode and the ECB mode uses the polarizing plate 9 and the quarter-wave plate 8 as essential components in terms of the operation principle. By using the polarizing plate 9 and the quarter-wave plate 8 as they are, it is possible to block the back scattering by the light diffusion layer 7. Specific parameters of panel 0 are listed below. Optical anisotropy Δ of 3m nematic liquid crystal
n is about 0.7, and the thickness of the liquid crystal layer 3 is about 3 μm. Therefore, the calculated retardation of the liquid crystal layer 3 Δ
n · d is 0.2 to 0.25 μm. Twist angle of twisted nematic liquid crystal 3m is 60 ° -70 °
It is. By twisting the nematic liquid crystal 3m, the effective retardation of the liquid crystal layer 3 is 0.15 μm.
m (150 nm), which is almost the center wavelength λ of external light.
(About 600 nm), and the liquid crystal layer 3 functions as a quarter-wave plate.

Hereinafter, the operation of the reflection type display device shown in FIG. 1 will be described in detail with reference to FIGS. FIG. 2 schematically shows the operation when no voltage is applied. As described above, the present reflective display device has a structure in which the polarizing plate 9, the quarter-wave plate 8, the light diffusing layer 7, the liquid crystal layer 3, and the reflective electrode 6 are arranged in this order from the viewer side. ing. Polarizing plate 9
Is represented by 9p. The optical axis 8d of the quarter-wave plate 8 intersects the transmission axis 9p at an angle of 45 °.
The nematic liquid crystal 3m included in the liquid crystal layer 3 is twist-oriented with no voltage applied, and the liquid crystal layer 3 functions as a quarter-wave plate. In the present embodiment, the alignment direction of the nematic liquid crystal 3m on the first substrate side is parallel to the transmission axis 9p of the polarizing plate 9. The nematic liquid crystal 3m is the first
The substrate is twist-oriented from the substrate toward the second substrate, and the twist angle is 60 ° to 70 ° as described above.

When the incident light 11 passes through the polarizing plate 9, it is converted into linearly polarized light. The polarization direction is parallel to the transmission axis 9p, and will be referred to as parallel linear polarization hereinafter. This parallel linearly polarized light 12 is converted into circularly polarized light 13 when passing through the quarter-wave plate 8. Most of the circularly polarized light 13 diffuses through the light diffusion layer 7 and reaches the liquid crystal layer 3. However, part of the circularly polarized light 13 is diffusely reflected by the light diffusion layer 7 and becomes a backscattering component 17. The circularly polarized light 13 transmitted through the light diffusion layer 7 toward the front is converted into linearly polarized light by the liquid crystal layer 3 functioning as a quarter-wave plate. However, the polarization direction of the linearly polarized light is orthogonal to the parallel linearly polarized light 12 and is hereinafter referred to as orthogonal linearly polarized light. The orthogonal linearly polarized light 14 reflected by the reflection electrode 6 becomes circularly polarized light when passing through the liquid crystal layer 3 in the opposite direction. Further, the circularly polarized light that has diffusely passed through the light diffusion layer 7 is converted by the quarter-wave plate 8 into parallel linearly polarized light 15. The parallel linearly polarized light 15 passes through the polarizing plate 9 as it is, and becomes the outgoing light 16 and reaches the observer. Therefore, a white display is obtained. When no voltage is applied, the liquid crystal layer 3 functions as a quarter-wave plate, and when combined with the original quarter-wave plate 8, the display panel functions as a half-wave plate as a whole. In the case of the reflection type, since external light reciprocates on the display panel, the display panel functions equivalently as a one-wavelength plate after all. Since the one-wavelength plate has substantially no optical effect, the external light is reflected as it is to the observer, and a white display is made.

On the other hand, the backscattering component (circularly polarized light) 17 reflected by the light diffusion layer 7 becomes orthogonal linearly polarized light by the quarter-wave plate 8. Since this is perpendicular to the transmission axis 9p of the polarizing plate 9, it cannot pass through, and is intercepted and absorbed, so that the backscattered component 17 does not reach the observer. The linearly polarized light rotates the polarization direction by 90 ° every time the linearly polarized light reciprocates through the quarter-wave plate. Backscattering component 17
Is only reciprocated through the quarter-wave plate 8, the polarization direction is rotated by 90 °. On the other hand, most of the external light passes through the quarter-wave plate 8 and the liquid crystal layer 3 reciprocally, so that the polarization direction is rotated by 90 ° × 2 = 180 °. This difference makes it possible to separate and remove only unnecessary backscattering components from the external light components that contribute to display.

FIG. 3 schematically shows an operation state of the reflective display device shown in FIG. 1 when a voltage is applied. Parts corresponding to the voltage non-applied state shown in FIG. 2 are denoted by corresponding reference numerals to facilitate understanding. In a voltage applied state, the nematic liquid crystal 3m having a positive dielectric anisotropy rises in response to the voltage and shifts to vertical alignment. As a result, the liquid crystal layer 3 loses its function as a quarter-wave plate. The incident light 11 becomes parallel linearly polarized light 12 after passing through the polarizing plate 9. The parallel linearly polarized light 12 is converted by the quarter-wave plate 8 into circularly polarized light 13. A part of the circularly polarized light 13 becomes a backscattering component 17 and again passes through the quarter-wave plate 8 in the reverse direction to become orthogonal linearly polarized light. Since this cannot pass through the polarizing plate 9, it is blocked. That is, in the present invention, the backscattering component 17 is always cut off regardless of the voltage application state. On the other hand, most of the circularly polarized light 13 that has diffusely passed through the light diffusion layer 7 passes through the liquid crystal layer 3 as it is, reaches the reflective electrode 6, and maintains a circularly polarized state. The circularly polarized light 14 a reflected by the reflection electrode 6 passes through the light diffusion layer 7 and reaches the quarter-wave plate 8. Here, the circularly polarized light 14a is converted into orthogonal linearly polarized light 15a. Since the orthogonal linearly polarized light 15a cannot pass through the polarizing plate 9, a black display is obtained. As described above, in the present invention, since the backscattering component 17 due to the light diffusion layer 7 is absorbed by the quarter-wave plate 8 and the polarizing plate 9, it does not contribute to display. In particular, light leakage of the backscattering component during black display is eliminated, and the contrast can be significantly improved.

FIG. 4 is a schematic sectional view showing a reference example of the reflection type display device. Parts corresponding to those of the reflection type display device according to the present invention shown in FIG. 1 are denoted by corresponding reference numerals to facilitate understanding. The difference is that, in this reference example, the light diffusion layer 7 is mounted on the polarizing plate 9 as usual. The light diffusion layer 7 diffusely transmits most of the incident light 11 and guides the forward scattering component 18 to the panel 0 side. At this time, the light diffusion layer 7 diffusely reflects a part of the incident light 11 to generate a backscattering component 17. Unlike the present invention, in this reference example, since the backscattering component 17 cannot be blocked at all, the display contrast is reduced. In particular, the sinking during black display is not sufficient, and the contrast becomes weak.

[0016]

As described above, according to the present invention,
In a reflective display device, an unnecessary backscatter component generated in the light diffusion layer is blocked by stacking a light diffusion layer, a quarter-wave plate, and a polarizing plate on the panel facing the viewer side in order from the bottom. It becomes possible to do. As a result, light leakage particularly during black display can be suppressed, and the contrast can be significantly improved. Since a transmission type light diffusion plate can be used without lowering the contrast, it is not necessary to incorporate a reflection type light diffusion plate into the panel as in the conventional case, and the manufacturing cost can be reduced. In addition, it becomes possible to use a diffusion film having a relatively large backscattering component as the light diffusion layer, and the material selection range of the light diffusion layer is widened.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an embodiment of a reflective display device according to the present invention.

FIG. 2 is a schematic diagram showing an operation of the reflective display device shown in FIG. 1 when no voltage is applied.

FIG. 3 is a schematic diagram showing an operation when a voltage is applied to the reflective display device shown in FIG. 1;

FIG. 4 is a cross-sectional view illustrating a reference example of the reflective display device.

[Explanation of symbols]

0 panel, 1 first substrate, 2 second substrate, 3 liquid crystal layer, 5 transparent electrode, 6 reflective electrode, 7 light diffusion layer , 8 ... quarter wave plate, 9
···Polarizer

Claims (3)

[Claims] 1. A transparent first substrate located on an incident side of external light, a second substrate joined to the first substrate via a predetermined gap and located on a reflection side, and electro-optic held in the gap. A reflective display device comprising: a panel having a substance and an electrode for applying a voltage to the electro-optical substance; a light diffusion layer, a quarter-wave plate, and a polarizing plate sequentially stacked on the first substrate from below. Wherein the polarizing plate converts external light into linearly polarized light and enters the first substrate side, and the light diffusion layer diffusely emits external light reflected on the second substrate side and the polarizing plate Unnecessarily reflects a part of the external light incident through the light-transmitting layer, and the quarter-wave plate cooperates with the polarizing plate to block the external light unnecessary reflected by the light diffusion layer. Reflective display device. 2. The reflection type display device according to claim 1, wherein the panel uses a liquid crystal layer functioning as a quarter-wave plate according to a voltage application state as an electro-optical material. 3. The liquid crystal layer is composed of a nematic liquid crystal having a positive dielectric anisotropy and twisted orientation, functions as a quarter-wave plate when no voltage is applied, and functions as a quarter-wave plate when voltage is applied. 3. The reflective display device according to claim 2, wherein