JPH0643499A - Liquid crystal photorefractive element - Google Patents

Abstract

(57) [Abstract] [Purpose] To prevent the displayed image of the liquid crystal photorefractive element from becoming whitish when a low voltage is applied. [Structure] A liquid crystal photorefractive element utilizing the birefringence phenomenon of liquid crystal is characterized in that a liquid crystal 3 is mixed with an isotropic coloring agent 4. The colorant 4 absorbs scattered light due to light scattering of the liquid crystal 3 and prevents the liquid crystal 3 from becoming whitish. Further, since it is isotropic, the color tone does not change even if the orientation of the liquid crystal 3 changes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal photorefractive element which can be used for a liquid crystal lens, an ECB mode LCD and the like and which utilizes the birefringence phenomenon of liquid crystal.

[0002]

2. Description of the Related Art As a liquid crystal photorefractive element, a variable focus liquid crystal lens having a variable focal length utilizing the electro-optical effect of liquid crystal has been known. For example, JP-A-60-50
In Japanese Patent Laid-Open No. 510-510, a concave lens-shaped transparent substrate and a flat plate-shaped transparent substrate are opposed to each other, a transparent conductive film is formed on the inner surface of each of the opposing transparent substrates, and a space defined by the pair of transparent substrates is formed. A varifocal liquid crystal lens is known in which liquid crystal molecules with positive dielectric anisotropy are filled with a field effect liquid crystal and liquid crystal molecules are oriented in one direction so as to be parallel to a substrate.

When a voltage above the threshold value is applied to this liquid crystal lens, each liquid crystal molecule changes its major axis direction to the voltage application direction. Then, by controlling the applied voltage, the direction of the long axis of the liquid crystal molecules can be continuously changed from the position parallel to the substrate to the position perpendicular to the substrate. Therefore, by controlling the applied voltage, the apparent refractive index of the liquid crystal lens can be continuously changed from a value for extraordinary light to a value for ordinary light.

In order to make it possible to control even the amount of transmitted light, JP-A-62-169116 discloses a variable-focus liquid crystal in which a dichroic dye having anisotropy in absorption of visible light is dissolved in liquid crystal. A lens is disclosed. With this configuration, the molecular arrangement of the dichroic dye can be continuously changed in accordance with the change in the orientation of the liquid crystal molecules. Therefore, the amount of transmitted light can be automatically adjusted in conjunction with the focus adjustment of the liquid crystal lens.

[0005]

However, it has been found that when the above-mentioned liquid crystal lens is operated, the displayed image looks whitish. The cause of this phenomenon has not been fully clarified yet, but it is particularly likely to occur at low voltage driving, so that when the voltage before and after the alignment force due to the electric field strength of the liquid crystal molecules exceeds the alignment force due to the alignment film is applied. It is presumed that the fluctuation phenomenon of liquid crystal molecules becomes large and the display image becomes whitish due to the light scattering phenomenon by the liquid crystal molecules.

The present invention has been made in view of such circumstances, and an object thereof is to prevent the display image of the liquid crystal photorefractive element from becoming whitish.

[0007]

A liquid crystal photorefractive element of the present invention for solving the above-mentioned problems is a liquid crystal photorefractive element utilizing a birefringence phenomenon of liquid crystal, in which an isotropic coloring agent is mixed with the liquid crystal. Characterize. As the isotropic colorant, either a pigment or a dye can be used. The reason why the color is isotropic is not preferable because, for example, when a dichroic dye is used, the change in the light transmittance due to the voltage application becomes large and the displayed image looks uncomfortable.

Although the boundary between the dichroic dye and the isotropic dye is unclear, the isotropic dye according to the present invention has a dichroic ratio (D _r ) of positive absorption anisotropy. 1 if the value is (D _p ).
~ 1.7, with negative absorption anisotropy (D _N ).
If so, it means 1 to 1.85. When the dichroic ratio is within this range, the change in light transmittance and haze value is small, and it can be put to practical use.

When the amount of the colorant is too small, it cannot be prevented from becoming whitish at the time of driving at a low voltage. When the amount of the colorant is too large, the light transmittance becomes small and the display as a liquid crystal photorefractive element is hindered. There is an optimum range depending on the type of colorant.

[0010]

In the liquid crystal photorefractive element of the present invention, even if light is scattered in the liquid crystal molecules due to fluctuations in the liquid crystal molecules, the scattered light is absorbed by the colorant due to the coexistence of the colorant. It is prevented from being visually recognized from. This prevents the displayed image from becoming whitish.

Since the colorant is isotropic, the colorant hardly changes even if the orientation of the liquid crystal molecules changes. Therefore, even if the refractive index of the liquid crystal photorefractive element is changed, the color tone visually recognized from the outside hardly changes, so that there is no discomfort.

[0012]

EXAMPLES The present invention will be specifically described below with reference to examples. The present embodiment applies the present invention to a variable focus liquid crystal lens. (Embodiment) The variable focus liquid crystal lens of the present embodiment shown in FIGS. 1 and 2 is enclosed by a flat glass substrate 1, a Fresnel lens substrate 2 arranged facing the flat glass substrate 1, and both. The liquid crystal 3 and the dye 4 are mainly formed.

The flat glass substrate 1 is made of soda lime glass, and the Fresnel lens substrate 2 is made of polycarbonate. On the inner surfaces of the substrates 1 and 2 facing each other, a transparent conductive film 10 made of an ITO film,
20 are formed. Lead wires (not shown) are connected to the transparent conductive films 10 and 20, respectively. Then, on the surface of the transparent conductive film 10 formed on the surface of the flat glass substrate 1, polyimide (“RN722” manufactured by Nissan Chemical Co., Ltd.) having a thickness of about 700 Å is further provided with an insulating film 11 formed of a SiO ₂ film. Horizontal alignment treatment film 1
2 is formed. On the surface of the transparent conductive film 20 formed on the Fresnel lens substrate 2, a horizontal alignment treatment film 2 made of polyvinyl alcohol and having a thickness of about 1000Å is further formed.
1 is formed.

The peripheral edges of both substrates 1 and 2 are sealed with a sealant 5 made of an epoxy adhesive, and a hermetically sealed space is formed between the substrates 1 and 2. And in the enclosed space, P
A type nematic liquid crystal 3 and an isotropic dye (mixture of azo and anthraquinone dyes) 4 are enclosed. The mixing concentration of the isotropic dye 4 in the liquid crystal 3 is 2% by weight. Also,
The dichroic ratio (D _r ) of the isotropic dye 4 is 1.5.

In this variable focus lens, the transparent conductive film 1 is used.
When an AC voltage is applied between 0 and 20, the horizontal alignment film 12,
The liquid crystal 3 that has been aligned along 21 changes its alignment state continuously from a position parallel to the plate-shaped glass substrate 1 (FIG. 1) to a vertical position (FIG. 2) according to the magnitude of the applied voltage. Therefore, the apparent refractive index of the varifocal liquid crystal lens continuously changes, and the focal length can continuously change from a value for extraordinary light to a value for ordinary light. On the other hand, the isotropic dye 4 exists regardless of the change in the alignment direction of the liquid crystal 3, and absorbs scattered light to prevent the liquid crystal 3 from appearing whitish. (Evaluation) An AC voltage was applied to the variable focus liquid crystal lens in the range of 0 to 10 V, and the haze value was measured. Also,
A variable focus liquid crystal lens having the same configuration as that of the example except that the isotropic dye 4 was not used was used as a comparative example, and the haze value was measured in the same manner. Since the variable focus liquid crystal lens of the example has a lower transmittance than the variable focus liquid crystal lens of the comparative example due to the addition of the dye, the variable focus liquid crystal lens of the example naturally has a lower haze value, and is directly compared. That is not valid. Therefore, the haze value of the varifocal liquid crystal lens of the example is proportionally converted so that the haze value is the same when no voltage is applied, and the converted value and the raw data of the varifocal liquid crystal lens of the comparative example are shown. 3 shows.

From FIG. 3, it is shown that the variable focus lens of the comparative example has a large variation in the haze value and a large difference in the degree of the light scattering phenomenon. However, in the varifocal lens of the example, the fluctuation of the haze value is smaller than that of the comparative example, which shows that the light scattering phenomenon is prevented. Reference Example Various dyes having different dichroic ratios were selected as liquid crystal mixed dyes to prepare liquid crystal lenses having the same configuration as in the example, and the transmittance at each applied voltage was measured for each.

When the absorption anisotropy of the dichroic dye is different, the change in the transmittance is reversed, and it is difficult to compare the two simultaneously. That is, as shown in FIG. 4, the absorption anisotropy in positive dye (D _P), the darkest state in which no voltage is applied. In the case of a dye (D _N ) having a negative absorption anisotropy, the state in which no voltage is applied is the brightest. Therefore, in the case of a dye having a positive absorption anisotropy (D _P ), the relative transmittance is calculated by setting the transmittance when a voltage of 10 V is applied to 100, and in the case of a dye having a negative absorption anisotropy (D _N ). The relative transmittance was calculated assuming that the transmittance when no voltage was applied was 100.

Then, for each dye, the degree of reduction in relative transmittance (ΔT in FIG. 4) between 0 and 10 V was calculated, and the relationship with the dichroic ratio is shown in FIG. If reducing the width of the relative transmission rate ([Delta] T) and 10% or less, since it is usable as a liquid crystal lens, the value of the dichroic ratio than 5, the D _P 1 to 1.8,
It can be seen that D _N is preferably 1 to 1.85. next,
For each of the above liquid crystal lenses, the haze value when driven with various voltages was measured, and the difference between the maximum haze value and the haze value when no voltage was applied (increase haze value ΔH) was calculated,
The relationship with the dichroic ratio is shown in FIG. Increased haze value (ΔH)
It is known that the smaller the value, the better, but if it is less than 10, it is known that there is less discomfort due to light scattering. Therefore, the value of the dichroic ratio is 1 to 1.7 for D _P and D _{N according to} FIG.
Then, it is understood that 1 to 1.85 is preferable. This range is within the range of the relative transmittance reduction (ΔT), and if the dichroic ratio of the dye is within this range, the relative transmittance reduction range (ΔT) and the increase haze value of the obtained liquid crystal lens are increased. It can be seen that both (ΔH) are satisfied.

That is, any dye having a dichroic ratio in the above range can be used as the isotropic colorant in the present invention.

[0020]

According to the liquid crystal photorefractive element of the present invention, the display image is prevented from becoming whitish at the time of driving at a low voltage and the color tone is not changed, so that the display quality is improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a state before a voltage is applied to a variable focus liquid crystal lens according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a state of a varifocal liquid crystal lens according to an embodiment of the present invention when a voltage is applied.

FIG. 3 is a graph showing the relationship between applied voltage and haze value.

FIG. 4 is a graph showing the relationship between applied voltage and relative transmittance.

FIG. 5: Dichroic ratio of dye and reduction range of relative transmittance (ΔT)
It is a graph which shows the relationship of.

FIG. 6 is a graph showing the relationship between the dichroic ratio of the dye and the increased haze value (ΔH).

[Explanation of symbols]

1: Flat glass substrate 2: Fresnel lens substrate
3: Liquid crystal 4: Isotropic dye (isotropic colorant) 5: Sealant

Claims (1)

[Claims] 1. A liquid crystal photorefractive element utilizing the birefringence phenomenon of liquid crystal, wherein the liquid crystal is mixed with an isotropic coloring agent.