JP4381492B2 - Liquid crystal optical element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical element that enables liquid crystal display by electrically controlling the light intensity of reflected light using liquid crystal, and a liquid crystal display device using the optical element.
[0002]
[Prior art]
Conventionally, as an optical element for controlling reflected light using liquid crystal, a liquid crystal optical element having a structure in which a liquid crystal optical element using a liquid crystal material having two polarizing plates and a twisted structure is added with a diffuse reflection plate having a fine uneven shape Devices have been used.
FIG. 2 shows a cross section of a conventionally used liquid crystal optical element. In the figure, a liquid crystal layer 3 made of a liquid crystal material having a twisted structure sandwiched between a glass substrate 1b to which a transparent electrode (ITO) 2b is attached and a glass substrate 1c to which a transparent electrode (ITO) 2c is attached. It is sandwiched between a pair of polarizing plates 5, and a diffuse reflector 6 is attached to one side. Then, the light incident from the other polarizing plate side is reflected by the diffusive reflecting plate 6, and in the process, the reflected light by the liquid crystal of the liquid crystal layer 3 is controlled.
[0003]
In addition, a liquid crystal optical element having a structure in which one of the pair of transparent electrodes is a mirror electrode and a diffusion plate is provided on the surface side of the element has been developed instead of attaching the above-described diffuse reflector.
A reflection type full-color liquid crystal display device has been developed that applies such a liquid crystal optical element and colors white light from the surface with a color filter to display a full-color image.
[0004]
FIG. 8 shows a cross-sectional view of the reflective full-color liquid crystal display device. In this reflection type liquid crystal display device, a color filter 10, a transparent conductive film (ITO) 2b, a liquid crystal 3 and a mirror electrode 2a serving as a mirror reflector are sandwiched between glass 1b and 1c, respectively. A diffusion plate 11 is provided on the surface side. A white color incident light from the surface side is selectively reflected by the present liquid crystal display device so that a full color image can be obtained. Here, in this drawing, for the sake of simplicity of explanation, the description of the polarizing plate, the phase difference plate, the description of the drive circuit, and the like are omitted.
[0005]
An example of a detailed structure of a liquid crystal cell applied to such a reflection type full color liquid crystal display device is shown in FIG. This reflection type liquid crystal cell has a laminated structure of a diffusion plate 11, a polarizing plate 5, a retardation plate 12, glass 1b, a transparent conductive film (ITO) 2b, a liquid crystal 15, a mirror electrode 2a, and glass 1c from above. . However, although the description of the color filter is omitted in FIG. 9, it can be easily colored by adding a color filter as shown in FIG.
[0006]
Here, FIG. 9A shows a parallel aligned ECB (electrically controlled birefringence) liquid crystal cell, but a bend aligned liquid crystal cell shown in FIG. 9B and a HAN (hybrid aligned nematic) aligned liquid crystal shown in FIG. 9C. A cell or the like can be similarly applied.
In addition, instead of the mirror electrode 2a shown in FIG. 8 and FIG. 9, a metal electrode that also serves as a diffuse reflector may be used, or a diffuse reflector, a transparent flattening film, and a transparent electrode may be used. Also good. When such a diffuse reflector is used, the diffuser 11 can be omitted.
[0007]
[Problems to be solved by the invention]
However, the optical element described above cannot obtain sufficient brightness and contrast in a wide viewing angle range.
The present invention is a liquid crystal optical element using a liquid crystal cell of parallel alignment ECB, bend alignment, or HAN alignment, optimizing its brightness and contrast, and having high reflectivity, high resolution, and high contrast. 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]
[Means for Solving the Problems]
The present invention has been made by paying attention to the fact that the retardation plate greatly contributes to the characteristics of the liquid crystal optical element, and has been made by analyzing the characteristics of the retardation plate in detail. This is based on the finding that the characteristics of the liquid crystal optical element are best by using a plurality of plates.
[0009]
That is, when the present inventors use a birefringent liquid crystal, the retardation of the liquid crystal (product of birefringence and thickness: Δn · d, thickness may be expressed as a multiple of the wavelength λ. ) Has a wavelength dependency, and this wavelength dependency changes depending on the incident angle of light. Therefore, in order to achieve high contrast, all light in the visible region is dark regardless of the incident angle of light. It has been found that it is necessary to design and laminate a retardation plate so as to be in a state.
[0010]
Therefore, the detailed configuration of the retardation plate necessary for this purpose was studied.
In general, the retardation of a liquid crystal changes depending on the wavelength λ of light, the incident angle θ, and the applied voltage V applied to the liquid crystal. Accordingly, when λ ₀ is the optimum wavelength at the time of design, the retardation R _lc of the liquid crystal can be expressed as follows:
R _lc (λ, V, θ) = R _lc (λ = λ ₀ , θ = 0, V) A (θ, V) B (λ) (1)
On the other hand, retardation R _Film of the retardation plate depends on the wavelength of light and the incident angle, and can be expressed as the following equation.
R _Film (λ, θ) = R _Film (λ = λ ₀ , θ = 0) K (θ) L (λ) (2)
Here, from the equation (2), two types of retardation plates having the same wavelength dependency or angle dependency can be realized by one type of retardation plate as represented by the following equation.
[0011]
When the wavelength dependence is the same, it is as follows.
R _Film1 (λ = λ ₀ , θ = 0) K ₁ (θ) L (λ) + R _Film2 (λ = λ ₀ , θ = 0) K ₂ (θ) L ( λ) = {R _Film1 (λ = λ ₀ , θ = 0) K ₁ (θ) + R _Film2 (λ = λ ₀ , θ = 0) K ₂ (θ)} L (λ) (3)
On the other hand, when the viewing angle characteristics are the same, the following expression is given.
R _Film1 (λ = λ ₀ , θ = 0) K (θ) L ₁ (λ) + R _Film2 (λ = λ ₀ , θ = 0) K (θ) L ₂ (λ) = {R _Film1 (λ = λ ₀ , θ = 0) L ₁ (λ) + R _Film2 (λ = λ ₀ , θ = 0) L ₂ (λ)} K (θ) (4)
Here, the reflectance of a reflective liquid crystal element composed of one polarizing plate, a retardation plate, and liquid crystal can be expressed by the following equation.
R _reflectance = cos ² ((2π | R _lc −R _film |) / λ) (5)
From the equations (1) and (5), the characteristics of the retardation plate necessary for obtaining a dark state at all wavelengths and incident angles may satisfy the following equations regardless of θ and λ.
R _Film = R _lc (λ = λ ₀ , θ = 0, V) A (θ, V) B (λ) −λ / 4 (6)
Here, the first term on the right side represents a retardation plate having the same wavelength dispersion as that of the liquid crystal and retardation having the same absolute value and the opposite sign even when the viewing angle θ is changed. The second term represents a retardation plate that does not depend on the incident angle θ and the wavelength λ of light but satisfies the 1/4 wavelength condition.
[0012]
Since the two retardation plates are different in both viewing angle dependency and wavelength dependency, two types of retardation plates cannot be realized as one retardation plate.
Therefore, it is possible to achieve high contrast only by designing and laminating the two retardation plates independently, and for the first time in the present invention, a suitable appropriate range for the characteristics of the two retardation plates. Was found.
[0013]
That is, the present invention is mounted on a substrate, a reflective plate formed on the substrate and having an electrode function, a parallel alignment ECB liquid crystal layer deposited on the reflection plate, and the parallel alignment ECB liquid crystal layer. A glass substrate having a transparent electrode formed on the parallel alignment ECB liquid crystal layer side, a first retardation plate placed on the glass substrate, and a second plate placed on the first retardation plate. And a polarizing plate mounted on the second retardation plate, wherein the first retardation plate is parallel-aligned ECB with respect to visible incident light. It has substantially the same wavelength dispersion as the liquid crystal layer, the absolute value of retardation is substantially equal, the sign is reversed, and the sum of retardations of the first retardation plate and the parallel-aligned ECB liquid crystal layer is −λ / 20 In the range of ~ λ / 20, wherein the second retardation plate is in front of visible incident light. Retardation with a wavelength of λ / 4−λ / 20 to λ / 4 + λ / 20 at an incident angle within a range of −70 ° to 70 ° with respect to the normal direction of the second phase difference plate The above-described problems have been solved by a liquid crystal optical element characterized by the above.
[0014]
And it discovered that the said subject could be solved similarly even if the said parallel alignment ECB liquid crystal layer was either a HAN alignment liquid crystal layer or a bend alignment liquid crystal layer.
Moreover, in the above, it discovered that it was suitable for the said reflecting plate to be a specular reflecting plate, and to set it as the structure which further loaded the diffuser plate on the said polarizing plate. On the other hand, it has been found that the reflection plate is a diffuse reflection plate having fine irregularities, and a transparent flattening film that absorbs the irregularities may be formed on the diffusion reflection plate.
[0015]
Here, when a diffusive reflector is used as the reflector, the parallel alignment ECB liquid crystal layer is most preferably a bend alignment liquid crystal layer.
In the liquid crystal display device using the liquid crystal optical element as described above, the matrix reflector includes an RGB color filter for performing full color display, and the reflector has an electrode function. It has been found that it is preferable to apply different voltages to the RGB pixels so as to achieve the same gradation between the transparent electrode and the transparent electrode.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Here, the retardation characteristics of the two retardation plates were examined.
FIG. 3 shows the relationship between the retardation of the first retardation plate and the liquid crystal layer on the horizontal axis and the dark state reflectance at that time on the vertical axis.
FIG. 4 shows the retardation of the second retardation plate and the change in the reflectance in the dark state at that time.
[0017]
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 100%. Here, the contrast ratio between the dark state and the bright state is defined as follows.
Contrast ratio = (brightness reflectance / darkness reflectance)
Therefore, if the minimum allowable contrast ratio value is 10, the dark state reflectance needs to be 10% or less.
[0018]
Here, when the contrast ratio of the minimum of each wavelength due to the error in retardation and 10, respectively, Li Polygonum sum of Shon is ± lambda / 20 of the former of the first retardation plate and the liquid crystal layer, the latter of the second retardation It can be seen that the retardation of the plate should be within ± λ / 20 with respect to λ / 4.
Therefore, for incident light in the visible light range where the wavelength λ of light is between 400 nm and 700 nm, the first retardation plate has a sum of the retardation of the liquid crystal layer ±± regardless of the incident angle in the optical axis direction of the liquid crystal. The second retardation plate may be designed to have a retardation with a magnitude of λ / 4−λ / 20 to λ / 4 + λ / 20, respectively. High contrast of 10 or more can be realized.
[0019]
【Example】
Next, the realization of an actual liquid crystal display device was examined using the liquid crystal optical element of the present invention.
In the case of a liquid crystal optical element using birefringence, when each pixel of RGB is driven with the same voltage-reflectance characteristics, the reflectivity varies depending on the wavelength dependence in a state other than the dark state due to the wavelength dispersion of the refractive index. In addition, this wavelength dependency changes depending on the incident angle of light.
[0020]
Therefore, when an achromatic color is to be displayed, it is necessary to apply different voltages so that the saturation is always reduced with respect to the voltage-reflectance characteristics that are different between the RGB pixels. At this time, in order to prevent the hue from changing even if the viewing angle is changed, the viewing angle characteristics of the RGB pixels need to be the same. However, as shown in FIG. Viewing angle characteristics can be made the same by making the maximum reflectance equal for each gradation.
[0021]
FIG. 5 shows the viewing angle characteristics of the reflectance of each RGB pixel. As shown in the figure, the reflectance of each pixel depends on the maximum reflectance (R = 100%, R = 75%, R = 50%, R = 25%) in the front direction, and hardly depends on the wavelength. Therefore, the viewing angle characteristics between the RGB pixels can be equalized by making the maximum reflectance of the RGB pixels the same.
[0022]
In the liquid crystal optical element of the present invention, high contrast display and wide viewing angle display are possible by using the above principle.
Based on the knowledge described above, a reflective liquid crystal optical element of the present invention was prototyped.
The measurement result of the wavelength-reflectance characteristic in the dark state is shown in FIG. From this result, it was confirmed that the reflectance in the dark state can be made substantially zero in the visible light region by using the liquid crystal optical element of the present invention.
[0023]
Further, FIG. 7 shows the viewing angle characteristics of the RGB pixels when the voltages are controlled so as to correct different voltage-reflectance characteristics of the RGB pixels.
From this result, it is confirmed that the viewing angle characteristics of the RGB pixels are almost the same, the change in hue due to the change in viewing angle is suppressed, and a liquid crystal display device capable of wide viewing angle display can be realized. did.
[0024]
【The invention's effect】
By applying the liquid crystal optical element of the present invention, it becomes possible to design a retardation film in consideration of the alignment state and wavelength dispersion of the liquid crystal, and the reflectance in the dark state can be suppressed in the visible light region. A reflective liquid crystal optical element capable of contrast display can be realized.
[0025]
In addition, since this optical element can be driven by controlling the voltage so as to correct different voltage-reflectance characteristics between RGB pixels, it becomes possible to suppress a change in hue due to a change in viewing angle. A liquid crystal display device having angular characteristics can be realized.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a reflective liquid crystal optical element of the present invention.
FIG. 2 is a cross-sectional view of a conventional reflective liquid crystal optical element.
FIG. 3 is a graph showing the relationship between the sum of retardation of the retardation plate 1 and the liquid crystal layer and the reflectance in the dark state of the retardation plate 1 used in the reflective liquid crystal optical element of the present invention.
FIG. 4 is a graph showing the relationship between retardation and dark state reflectance of the retardation plate 2 used in the reflective liquid crystal optical element of the present invention.
FIG. 5 is a graph illustrating the adjustment of the reflection-type liquid crystal optical element of the present invention so that viewing angle characteristics with respect to respective reflectances of RGB pixels are made equal.
FIG. 6 is a graph comparing wavelength-reflectance characteristics in the dark state of the present invention example and a conventional example.
FIG. 7 is a graph showing viewing angle characteristics with respect to respective reflectances of RGB pixels in the reflective liquid crystal optical element of the present invention.
FIG. 8 is a cross-sectional view showing a structure of a conventional reflective full-color liquid crystal display device.
FIG. 9 is a schematic diagram showing a liquid crystal structure of a reflective liquid crystal display device, where (a) shows a parallel alignment ECB cell, (b) shows a bend alignment liquid crystal cell, and (c) shows each structure of a HAN alignment liquid crystal cell. .
[Explanation of symbols]
1a board
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 reflector
10 Micro color filter
11 Diffuser
12 Retardation plate
15 LCD

Claims (5)

A substrate, a reflector formed on the substrate and having an electrode function, a parallel alignment ECB liquid crystal layer deposited on the reflector, and the parallel alignment ECB liquid crystal mounted on the parallel alignment ECB liquid crystal layer A glass substrate having a transparent electrode formed on the layer side; a first retardation plate placed on the glass substrate; a second retardation plate placed on the first retardation plate; A liquid crystal optical element comprising a polarizing plate mounted on the second retardation plate, wherein the first retardation plate has substantially the same wavelength as the parallel-aligned ECB liquid crystal layer with respect to visible incident light. It has dispersion, the absolute value of retardation is approximately equal, the sign is reversed, the wavelength of incident light is λ, and the first retardation plate is parallel to the first retardation plate regardless of the incident angle in the optical axis direction of the liquid crystal The sum of retardations with the aligned ECB liquid crystal layer is in the range of -λ / 20 to λ / 20, Serial second retardation plate, to visible incident light, the incident angle in a range from -70 degrees to 70 degrees relative to the normal direction of the second retardation plate, the retardation lambda / 4 A liquid crystal optical element having a size of -λ / 20 to λ / 4 + λ / 20.  2. The liquid crystal optical element according to claim 1, wherein the parallel alignment ECB liquid crystal layer is either a HAN alignment liquid crystal layer or a bend alignment liquid crystal layer.  The liquid crystal optical element according to claim 1, wherein the reflection plate is a specular reflection plate, and a diffusion plate is further loaded on the polarizing plate.  The said reflecting plate is a diffused reflecting plate with fine unevenness | corrugation, The transparent planarization film | membrane which absorbs an unevenness | corrugation is formed on this diffused reflecting plate, Either of Claim 1 or 2 characterized by the above-mentioned. Liquid crystal optical element. 5. A liquid crystal display device having RGB color filters for performing full color display and using the liquid crystal optical element according to claim 1, wherein the reflector is a matrix reflector having an electrode function. The liquid crystal display device is characterized in that different voltages are applied to the RGB pixels to achieve the same gradation between the matrix reflector and the transparent electrode.

Images