JPH11249126A - Liquid crystal optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress reflectance in a dark state in a visible light area and to perform a high contrast display by optimizing the characteristics of a liquid crystal optical element by making a phase difference plate to plural-sheets constitution. SOLUTION: A first phase difference plate 4a is provided with wavelength dispersion almost same as a parallel orientation ECB liquid crystal layer 3 to visible incident light, the absolute value of retardation is made almost equal, a code is made opposite and the sum of the retardation of the first phase difference plate 4a and the parallel ECB liquid crystal layer 3 is made within the range of -λ/20 to λ/20. Also, a second phase difference plate 4b is the retardation of the size of λ/4-λ/20 to λ/4+λ/20 to while defining a wavelength as λat an incident angle within the range of -70 degree to 70 degree on the basis of the normal direction of the second phase difference plate 4b to the visible incident light. By applying such an optical element, a phase difference film is designed by taking the orientation state of liquid crystal and the wavelength dispersion into consideration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical element capable of electrically controlling the light intensity of reflected light by using liquid crystal to enable color display, and a liquid crystal display device using the optical element.

[0002]

2. Description of the Related Art Conventionally, as an optical element for controlling reflected light using liquid crystal, a diffuse reflection plate having fine irregularities is added to a liquid crystal optical element using two polarizing plates and a liquid crystal material having a twisted structure. Liquid crystal optical elements having such a structure have been used. FIG. 2 shows a cross section of a conventionally used liquid crystal optical element. In the figure, a glass substrate 1b on which a transparent electrode (ITO) 2b is adhered and a liquid crystal layer 3 made of a liquid crystal material having a twisted structure sandwiched between a glass substrate 1c on which a transparent electrode (ITO) 2c is adhered, A pair of polarizing plates 5 is sandwiched, and a diffuse reflection plate 6 is attached to one side. Then, the light incident from the other polarizing plate side is reflected by the diffuse reflection plate 6, and in the process, the light reflected by the liquid crystal of the liquid crystal layer 3 is controlled.

In addition, a liquid crystal optical element having a structure in which one of the above-mentioned pair of transparent electrodes is used as a mirror electrode and a diffusion plate is provided on the surface side of the element, instead of attaching the above-mentioned diffuse reflection plate, has been developed. ing. A reflection type full color liquid crystal display device which applies such a liquid crystal optical element and colors white light from the surface with a color filter to display a full color image has been developed.

FIG. 8 is a sectional view of the reflection type full color liquid crystal display device. This reflective liquid crystal display device uses glass 1
b, 1c, color filter 10, transparent conductive film (IT
O) 2b, liquid crystal 3, and mirror electrode 2a, which is also an electrode also serving as a mirror reflector, are sandwiched in layers as layers, and a diffusion plate 11 is provided on the surface side. Then, the structure is such that a full-color image can be obtained by selectively reflecting white incident light from the front side by the present liquid crystal display device. Here, for simplification of description, description of a polarizing plate, a phase difference plate, and description of a driving circuit are omitted in the drawing.

FIG. 9A shows an example of a detailed structure of a liquid crystal cell applied to such a reflection type full color liquid crystal display device. This reflection type liquid crystal cell has a diffusion plate 11 from above.
Polarizing plate 5, retardation plate 12, glass 1b, transparent conductive film (IT
O) It has a laminated structure of 2b, liquid crystal 15, mirror electrode 2a, and glass 1c. Although illustration of the color filters is omitted in FIG. 9, colorization can be easily achieved by adding a color filter as shown in FIG.

Here, FIG. 9A shows a parallel orientation ECB (el
FIG. 9B shows a bend-aligned liquid crystal cell shown in FIG.
A HAN (hybrid aligned nematic) alignment liquid crystal cell shown in (c) can be similarly applied. In addition,
Instead of the mirror electrode 2a shown in FIGS. 8 and 9, a metal electrode serving also as a diffuse reflection plate may be used, or
A diffuse reflection plate, a transparent flattening film, and a transparent electrode may be used. When such a diffuse reflection plate is used, the diffusion plate 11 can be omitted.

[0007]

However, the optical element described above cannot provide sufficient brightness and contrast in a wide viewing angle range. The present invention relates to a liquid crystal optical element to which a liquid crystal cell of any of a parallel alignment ECB, a bend alignment, and a HAN alignment is applied, in which the brightness and contrast are optimized, and the liquid crystal optical element having high reflectance, high resolution, and high contrast is provided. It is an object to provide an element and to realize a liquid crystal display device to which the liquid crystal optical element is applied.

[0008]

The present invention focuses on the fact that a phase difference plate greatly contributes to the characteristics of a liquid crystal optical element, and has been made by analyzing the characteristics of the phase difference plate in detail. This is based on the finding that the characteristics of the liquid crystal optical element are optimized by forming a plurality of retardation plates each composed of one sheet.

That is, when the present inventors use a birefringent liquid crystal, the retardation of the liquid crystal (the product of the birefringence and the thickness: Δn · d, and the thickness is expressed as a multiple of the wavelength λ) Has a wavelength dependence, and the wavelength dependence changes depending on the incident angle of light. Therefore, in order to realize a high contrast, all wavelengths in the visible region are required regardless of the incident angle of light. They have found that it is necessary to design and stack a retardation plate so that light is in a dark state.

Therefore, the detailed configuration of the retardation plate necessary for this purpose was studied. In general, the retardation of the liquid crystal changes depending on the wavelength λ of light, the incident angle θ, and the voltage V applied to the liquid crystal. Therefore, if λ ₀ is the optimum wavelength at the time of design, the retardation R _lc of the liquid crystal can be expressed by the following equation. R _lc (λ, V, θ) = R _lc (λ = λ ₀ , θ = 0, V) A (θ, V) B (λ) (1) On the other hand, the retardation R _Film of the retardation plate is the wavelength of light. , Since it depends on the angle of incidence. R _Film (λ, θ) = R _Film (λ = λ ₀ , θ = 0) K (θ) L (λ) (2) Here, from equation (2), two types having equal wavelength dependence or angle dependence are obtained. Can be realized by one kind of retardation plate as represented by the following equation.

When the wavelength dependence is the same, the following is obtained.
You. R_Film1(λ = λ₀, θ = 0) K₁(θ) L (λ) + R_Film2(λ = λ₀, θ = 0) K_Two(θ) L λ) = ｛R_Film1(λ = λ₀, θ = 0) K₁(θ) + R_Film2(λ = λ₀, θ = 0) K_Two(θ)｝ L (λ) (3) On the other hand, when the viewing angle characteristics are the same, it is expressed by the following equation. R_Film1(λ = λ₀, θ = 0) K (θ) L₁(λ) + R_Film2(λ = λ₀, θ = 0) K (θ) L _Two (λ) = ｛R_Film1(λ = λ₀, θ = 0) L₁(λ) + R_Film2(λ = λ₀, θ = 0) L_Two(λ)｝ K (θ) (4) Here, a reflective liquid composed of one polarizing plate, retardation plate, and liquid crystal
The reflectance of the crystal element can be expressed by the following equation. R_reflectance= Cos^Two((2π | R_lc-R_film|) / Λ) (5) From Eqs. (1) and (5), darkness at all wavelengths and incident angles was obtained.
The characteristics of the retardation plate required to obtain the state depend on θ and λ.
Instead, the following equation may be satisfied. R_Film= R_lc (λ = λ₀, θ = 0, V) A (θ, V) B (λ) −λ / 4 (6) where the first term on the right side has the same wavelength dispersion as the liquid crystal, and
Even if the viewing angle θ is changed, the absolute value is the same and negative retardation
FIG. The second time, the incident angle of light θ
A retardation plate that satisfies the 1/4 wavelength condition regardless of the wavelength λ is displayed.
doing.

Since these two retardation plates have different viewing angle dependence and wavelength dependence, two types of retardation plates cannot be realized as one retardation plate. Therefore,
The high contrast can be realized only by independently designing and laminating the two phase difference plates. In the present invention, for the first time, a suitable appropriate range is found for the characteristics of the two phase difference plates. It was.

That is, the present invention provides a substrate, a reflector formed on the substrate and having an electrode function, a parallel alignment ECB liquid crystal layer adhered on the reflection plate, and a parallel alignment ECB liquid crystal layer formed on the parallel alignment ECB liquid crystal layer. A glass substrate having a transparent electrode formed on the side of the parallel alignment ECB liquid crystal layer, a first phase difference plate mounted on the glass substrate, and a glass substrate mounted on the first phase difference plate. A liquid crystal optical element comprising: a second retardation plate; and a polarizing plate mounted on the second retardation plate, wherein the first retardation plate is configured to detect the incident light with respect to visible light. It has substantially the same wavelength dispersion as the parallel alignment ECB liquid crystal layer, the absolute value of the retardation is substantially equal, and the sign is reversed, and the sum of the retardation of the first retardation plate and the parallel alignment ECB liquid crystal layer is- λ / 2
0 to λ / 20, wherein the second retardation plate is in a range from −70 degrees to 70 degrees with respect to visible incident light with respect to a normal direction of the second retardation plate. At an incident angle in the range of degrees, the wavelength is λ, and λ / 4−λ / 20 to λ / 4 +
This problem has been solved by a liquid crystal optical element characterized in that the retardation has a magnitude of λ / 20.

The parallel-aligned ECB liquid crystal layer has an HA
It has been found that the above problem can be similarly solved by using either the N-oriented liquid crystal layer or the bend-oriented liquid crystal layer. Further, in the above, the reflector is a specular reflector,
It has been found that a configuration in which a diffusion plate is further loaded on the polarizing plate is preferable. On the other hand, they have found that the reflector may be a diffuse reflector having fine irregularities, and a transparent flattening film for absorbing irregularities may be formed on the diffuse reflector.

Here, when a diffuse reflector is used as the reflector, it is most preferable that the parallel alignment ECB liquid crystal layer is a bend alignment liquid crystal layer. In the liquid crystal display device using the liquid crystal optical element as described above, the liquid crystal display device includes an RGB color filter for performing full-color display, and the reflector is a matrix reflector having an electrode function. And the transparent electrode, the R
It has been found that it is preferable to apply different voltages to the respective pixels of GB so that the same gradation is obtained.

[0016]

Here, the retardation characteristics of the two retardation plates were examined. FIG. 3 shows the relationship between the sum of the retardations of the first retardation plate and the liquid crystal layer on the horizontal axis and the reflectance in the dark state at that time on the vertical axis.
FIG. 4 shows the retardation of the second retardation plate and the change in reflectance in the dark state at that time.

In FIGS. 3 and 4, the reflectance in the dark state on the vertical axis indicates the reflectance in the dark state when the light state of the optical element of the present invention is set to 100%. Here, the contrast ratio between the dark state and the bright state is defined as follows. Contrast ratio = (reflectance in bright state / reflectance in dark state) Therefore, the lowest acceptable contrast ratio value is 10
Then, the reflectance in the dark state needs to be 10% or less.

Assuming that the lowest contrast ratio of each wavelength due to the retardation error is 10 respectively, the sum of the retardation of the first retardation plate and the liquid crystal layer is ± 1.
λ / 20, the latter retardation of the second retardation plate is λ / 4
It can be seen that it is only necessary to be within ± λ / 20. Therefore, for the incident light in the visible light range where the wavelength λ of the light is between 400 nm and 700 nm, the first retardation plate does not depend on the incident angle in the optical axis direction of the liquid crystal and the sum with the retardation of the liquid crystal layer is ± λ / 20, and the second retardation plate is designed so as to have a retardation of a magnitude of λ / 4−λ / 20 to λ / 4 + λ / 20. High contrast of 10 or more can be realized.

[0019]

EXAMPLE Next, the realization of an actual liquid crystal display using the liquid crystal optical element of the present invention was examined. In the case of a liquid crystal optical element using birefringence, if each of the RGB pixels is driven with the same voltage-reflectance characteristics, the reflectance differs due to wavelength dependence in a state other than the dark state due to the wavelength dispersion of the refractive index. In addition, the wavelength dependency changes depending on the incident angle of light.

Therefore, when an achromatic display is to be performed, it is necessary to apply different voltages so that the saturation is always small for the different voltage-reflectance characteristics between the RGB pixels. At this time, in order to prevent a change in hue from occurring even when the viewing angle is changed, it is necessary to make the viewing angle characteristics of the RGB pixels the same, but as shown in FIG. The viewing angle characteristics can be made the same by making the maximum reflectance equal for each gradation.

FIG. 5 shows the viewing angle characteristics of the reflectance of each of the RGB pixels. As shown in the figure, the reflectance of each pixel is the maximum reflectance in the front direction (R = 100%, R = 75%, R = 50%, R = 100%).
25%) and hardly depend on wavelength. Therefore, the viewing angle characteristics between the RGB pixels can be made equal by making the maximum reflectance of each pixel of RGB the same.

In the liquid crystal optical element of the present invention, high contrast display and wide viewing angle display can be realized by utilizing the above principle. Based on the findings described above, a reflective liquid crystal optical element of the present invention was prototyped. FIG. 6 shows the measurement results of the wavelength-reflectance characteristics in the dark state. From these results, it was found that the reflectance in the dark state was almost zero in the visible light region by using the liquid crystal optical element of the present invention.
I confirmed that I can do it.

Further, when driving by controlling the voltage so as to correct different voltage-reflectance characteristics of each of the RGB pixels,
FIG. 7 shows the viewing angle characteristics of the GB pixels. From this result, RG
It has been confirmed that the viewing angle characteristics of the B pixels are almost the same, the change in hue accompanying the change in viewing angle is suppressed, and a liquid crystal display device capable of wide viewing angle display can be realized.

[0024]

By applying the liquid crystal optical element of the present invention, it is possible to design a retardation film in consideration of the alignment state and wavelength dispersion of the liquid crystal, and suppress the reflectance in the dark state in the visible light region. As a result, a reflective liquid crystal optical element capable of high-contrast display can be realized.

Further, since the optical element can be driven by controlling the voltage so as to correct the voltage-reflectance characteristics different between RGB pixels, it is possible to suppress a change in hue due to a change in viewing angle. Thus, a liquid crystal display device having a wide viewing angle characteristic can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view of a reflective liquid crystal optical element of the present invention.

FIG. 2 is a sectional view of a conventional reflective liquid crystal optical element.

FIG. 3 is a graph showing the relationship between the sum of the retardations of the retardation film 1 and the liquid crystal layer and the reflectance in the dark state for the retardation film 1 used in the reflective liquid crystal optical element of the present invention.

FIG. 4 is a graph showing a relationship between retardation and reflectance in a dark state for a retardation plate 2 used in a reflective liquid crystal optical element of the present invention.

FIG. 5 is a diagram illustrating a reflection type liquid crystal optical element according to the present invention;
9 is a graph illustrating adjustment of viewing angle characteristics with respect to respective reflectances of respective pixels so as to be equal.

FIG. 6 is a graph comparing wavelength-reflectance characteristics in a dark state between the present invention example and the conventional example.

FIG. 7 is a graph showing a viewing angle characteristic with respect to a reflectance of each of RGB pixels in the reflection type liquid crystal optical element of the present invention.

FIG. 8 is a cross-sectional view showing the structure of a conventional reflective full-color liquid crystal display device.

FIGS. 9A and 9B are schematic diagrams showing a liquid crystal structure of a reflection type liquid crystal display device, wherein FIG. 9A shows a parallel alignment ECB cell, FIG. 9B shows a bend alignment liquid crystal cell, and FIG. 9C shows a HAN alignment liquid crystal cell. .

[Explanation of symbols]

 Reference Signs List 1a substrate 1b, 1c glass substrate 2a mirror electrode 2b, 2c transparent conductive film (ITO) 3 liquid crystal layer 4a first retardation plate 4b second retardation plate 5 polarizing plate 6 diffuse reflection plate 10 micro color filter 11 diffusion plate 12 Phase difference plate 15 Liquid crystal

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takahiro Ishinabe 33 Ones Village 1-103, 1-1-1, Nishikicho, Aoba-ku, Sendai, Miyagi

Claims (5)

[Claims] 1. A substrate, a reflector formed on the substrate and having an electrode function, and a parallel alignment EC attached on the reflector.
A B liquid crystal layer, a glass substrate mounted on the parallel alignment ECB liquid crystal layer and having a transparent electrode formed on the side of the parallel alignment ECB liquid crystal layer, and a first retardation plate mounted on the glass substrate. ,
A liquid crystal optical element comprising a second retardation plate mounted on the first retardation plate and a polarizing plate mounted on the second retardation plate, wherein the first The retardation plate has substantially the same wavelength dispersion as the parallel alignment ECB liquid crystal layer for visible incident light, the absolute values of retardation are substantially equal, and the signs are reversed, and the wavelength of the incident light is λ, The sum of the retardation of the first retardation plate and the retardation of the parallel alignment ECB liquid crystal layer is in the range of -λ / 20 to λ / 20, and the second retardation plate has A liquid crystal optical element characterized in that the retardation of the light is λ / 4−λ / 20 to λ / 4 + λ / 20. 2. The liquid crystal optical element according to claim 1, wherein said parallel alignment ECB liquid crystal layer is one of a HAN alignment liquid crystal layer and a bend alignment liquid crystal layer. 3. The liquid crystal optical element according to claim 1, wherein the reflection plate is a specular reflection plate, and a diffusion plate is further mounted on the polarizing plate. 4. The reflector according to claim 1, wherein the reflector is a diffuse reflector having fine irregularities, and a transparent flattening film for absorbing the irregularities is formed on the diffuse reflector. A liquid crystal optical element according to any one of the above. 5. A liquid crystal display device using a liquid crystal optical element according to claim 1, further comprising an RGB color filter for performing full color display, wherein said reflection plate has an electrode function. A matrix reflector, wherein the RG is provided between the matrix reflector and the transparent electrode.
A liquid crystal display device wherein different voltages are applied to the respective B pixels so that the same gradation is obtained.