JP2789672B2 - Liquid crystal display - Google Patents

Description

The present invention relates to a liquid crystal display in which an optical image is recorded on a liquid crystal layer by thermal writing using a semiconductor laser beam, and the optical image is projected and displayed by visible light from a light source. The present invention relates to a device, particularly to improvement of light-to-heat conversion efficiency and prevention of light deterioration of a light-to-heat conversion dye contained in the liquid crystal layer.

[Summary of the Invention]

The present invention relates to a liquid crystal display device which records an optical image on a liquid crystal layer by thermal writing using a semiconductor laser beam and projects and displays the optical image with visible light from a light source. By specifying the molecular structure of the system dye and appropriately selecting the wavelength of the projection light, it is possible to improve the light-heat exchange efficiency, prevent the light-heat conversion dye from being deteriorated by light, and extend the operating life of the liquid crystal display device. Things.

[Conventional technology]

In the field of liquid crystal display, an optical image is formed by irradiating a liquid crystal cell with laser light modulated by a character signal or video signal based on information to be displayed, and irradiating the liquid crystal cell with visible light to form the optical image. For example, there has been proposed a liquid crystal display device of a type in which is projected on a screen.

The principle of display in this type of liquid crystal display device is that first, when the light energy absorbed by the liquid crystal cell is converted into heat energy at the irradiated part of the laser beam, the liquid crystal is selectively heated at the irradiated part and a phase transition occurs. The subsequent quenching preserves the disorder of the alignment state of the liquid crystal molecules caused by this phase transition and forms an optical scattering center, resulting in a difference in optical characteristics between the non-irradiated part and this being recognized as an optical image. It is based on things. As the phase transition usable here, a phase transition between a cholesteric liquid crystal and an isotropic phase in a cholesteric liquid crystal, or a phase transition between a smectic A phase and an isotropic phase in a smectic liquid crystal have been proposed. Usually, the initial state or the erased state looks transparent, and the recorded state looks opaque due to strong scattering of light. The opaque structure can be returned to a transparent structure by applying an AC electric field higher than the cutoff frequency for dynamic scattering if the liquid crystal is a cholesteric liquid crystal, or annealing after the entire heating if the liquid crystal is a smectic liquid crystal. Can be. That is, under normal use conditions, opaque tissue is stably stored, so that information can be stored. When an external electric field or heat is applied thereto, the information is erased.

In order to apply such a liquid crystal display device to a computer terminal device or the like, it is necessary to rewrite displayed information at a practical speed, and for that purpose, it is necessary to increase the photothermal conversion efficiency in the liquid crystal cell. Is required. As a method for improving the light-to-heat conversion efficiency, for example, a method in which a metal thin film such as an aluminum vapor-deposited film is placed in contact with the liquid crystal layer, or a light-to-heat conversion dye having a large absorption in the wavelength region of laser light used for writing is used. And the like have been proposed. In order to obtain practically high light-to-heat conversion efficiency, the latter method is considered to be advantageous. Also,
In recent years, semiconductor lasers have also been used as a light source of laser light used for writing, because the device can be downsized.

For the reasons described above, the photothermal conversion dye added to the liquid crystal layer of this type of liquid crystal display device has an absorption band of sufficient intensity in the near infrared region that is the wavelength of the semiconductor laser light,
Characteristics such as high transmittance in the visible light range and compatibility with liquid crystal are required. For example, Japanese Unexamined Patent Publication No.
JP-A-197485 and JP-A-61-228418 disclose 1,3
-Squarylium-based dyes having a diazulenylcyclobutanedione skeleton (trade name: NK-27, manufactured by Japan Photosensitive Dye Laboratories)
It is commercially available as 72. ) Is disclosed.

[Problems to be solved by the invention]

The squarylium-based dye described above dissolves in about 0.35% by weight of cyanobiphenyl-based smectic liquid crystal, and exhibits light absorption at around 780 nm when dissolved in the liquid crystal. However, since the actual amount of the dye added is selected to be lower than the above-mentioned value in order to avoid precipitation in the liquid crystal,
The light energy of the laser beam cannot be sufficiently absorbed, and the writing speed cannot be increased.

Furthermore, since the squarylium-based dye causes light deterioration due to light used for projecting an optical image, there is a serious drawback that the operation life of the liquid crystal cell and thus the liquid crystal display device cannot be sufficiently extended.

Therefore, an object of the present invention is to provide a liquid crystal display device having a sufficiently high photothermal conversion efficiency and a long operating life.

[Means for solving the problem]

The liquid crystal display device according to the present invention is proposed to achieve the above object, and records an optical image on a liquid crystal layer by thermal writing using a semiconductor laser beam, and the optical image is irradiated with light from a light source. A liquid crystal display device for projection display, wherein the liquid crystal layer has the following general formula as a photothermal conversion dye: Wherein at least one of the squarylium-based dyes (I), (II), and (III) is represented, and the light from the light source has a wavelength longer than 360 nm.

The squarylium-based dyes have a structural isomerism relationship with each other, and all have an absorption maximum at 810 to 830 nm, and can be used alone in theory. Among them, the squarylium dye (III) is difficult to isolate, and is practically treated as a mixture with the squarylium dye (II). These dyes are added at a concentration within a range that can be dissolved in the liquid crystal constituting the liquid crystal layer and that does not precipitate even at room temperature cooling, but when a plurality of isomers are used in any combination, Is advantageous in that it can dissolve an amount close to the total solubility of each isomer, so that the amount of energy absorbed by laser light can be increased as compared with the case where it is used alone. In addition, when a plurality of isomers are used as a mixture, the resistance to ultraviolet light which causes photodegradation is improved as compared with the case where the isomers are used alone.

The squarylium-based dye described above is used by dissolving it in a conventionally known smectic liquid crystal or cholesteric liquid crystal.

A liquid crystal cell in which a liquid crystal composition containing such a squarylium dye is sandwiched between suitable substrates as a liquid crystal layer. In the liquid crystal display device of the present invention, an optical image recorded on the liquid crystal layer is projected and displayed by irradiating the liquid crystal cell with light from a light source. Here, it has been found that repeated irradiation or irradiation for a long time causes photodeterioration of the squarylium-based dye and shortens the operating life of the liquid crystal display device. The present inventors have investigated the cause, the squarylium-based dye has a slight absorption peak also in the ultraviolet wavelength range, that the light in the wavelength range of the projection light is included as a component It has been clarified that this component is absorbed by dye molecules and causes photodegradation. Since the highest intensity of these absorption peaks exists at 360 nm or less, in the present invention, light obtained by removing a wavelength component of 360 nm or less from light of a general-purpose white light source described later is used as projection light. However, since the squarylium dye has a rather strong absorption peak near 400 nm, in order to prevent light in this wavelength band from being absorbed by the dye molecules, a wavelength component of 420 nm or less is further removed and projected. If the light is substantially only a visible light component, light degradation of the dye can be more effectively suppressed.

As a light source of the projection light, a general-purpose white light source such as a xenon lamp, a metal halide lamp, and a halogen lamp is used. Therefore, as described above, in order to limit the wavelength of the projection light, in the optical system of the liquid crystal display device, light having a wavelength of 360 nm or less, more preferably 420 nm or less, is provided on the optical path until the light from the light source reaches the liquid crystal layer. What is necessary is just to provide the means which cuts off. The simplest means is to insert an optical filter having desired wavelength transmission characteristics. Alternatively, the same wavelength is obtained by, for example, mixing an ultraviolet absorbing substance into a condenser lens or a dichroic mirror, which is a constituent member of the optical system, and a substrate of the liquid crystal cell on the side where the projection light is incident. Transmission characteristics may be provided.
By the way, the light source of the projection light usually contains near-infrared wavelength components (so-called heat rays), and when this is absorbed by the liquid crystal cell, uniform heating occurs and masks local heating by laser light. Would. Therefore, it is desirable to provide the heat ray blocking means at an appropriate position on the optical path of the projection light and before the semiconductor laser light enters the liquid crystal cell. As such means, it is easiest to insert a heat ray cutoff filter having a desired wavelength transmission characteristic. However, a condenser lens, a dichroic mirror, or the like may have the same wavelength transmission characteristic.

[Action]

The squarylium-based dye used in the present invention has a large absorption band in the wavelength region of a semiconductor laser, and thus is useful as a photothermal conversion dye for thermal writing. Since the solubility of the three types of structural isomers of the above-mentioned dyes in liquid crystals is almost independent, especially when these are used in any combination, the solubility of the dye as a whole is substantially given as the sum of the solubilities of the respective isomers. As a result, the amount of energy absorbed by the laser beam can be increased.

Further, in the present invention, the wavelength of light from a light source used for projection display is limited to a wavelength longer than 360 nm. This makes it possible to suppress the light absorption in the ultraviolet wavelength region that causes the squarylium-based dye to undergo photodeterioration, thereby extending the operating life of the liquid crystal display device.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described based on experimental results.

First, a configuration example of a liquid crystal display device according to the present invention will be schematically described with reference to FIG.

This liquid crystal display device is roughly divided into a liquid crystal cell, a recording optical system,
It is composed of a projection optical system.

The liquid crystal cell (1) has a spacer (2) having a predetermined thickness.
A liquid crystal layer (4) is sandwiched between two glass substrates (3) which are arranged to face each other. The liquid crystal layer (4) is made of a composition in which the above-mentioned squarylium-based dye is dissolved in liquid crystal.

The recording optical system includes a semiconductor laser (5) serving as a recording light source, a beam shape correcting unit (6) for correcting a beam shape of laser light from the semiconductor laser (5) into a circle, and a beam shape correcting unit ( 6) A pair of galvanometer mirrors (7) and (7) for scanning the laser beam having passed through in the XY direction at a constant angular velocity, and the parallel beams scanned at a constant angular velocity by the galvanomirrors (7) and (7). F-θ lens (8) for scanning the laser beam at a constant linear velocity on its focal plane, that is, on the surface of the liquid crystal cell (1), and the optical path of the laser light passing through the f-θ lens (8). A dichroic mirror (9) or the like is turned by 90 ° so that it can be incident on a liquid crystal cell (1) placed on the optical axis of a projection optical system described later, and the light of a projection light source (10) described later travels straight. Mainly composed.

One projection optical system includes a white projection light source (10) such as a xenon arc lamp, a condenser lens (11) for concentrating light from the projection light source (10) in a desired direction to increase illuminance, and A heat ray cutoff filter (12) for absorbing near-infrared components contained in light from the light source (10) to prevent the liquid crystal cell (1) from being entirely heated; a liquid crystal layer (4) having a predetermined transmission wavelength characteristic; ), An optical filter (13) for preventing photodegradation of the squarylium dye contained in the liquid crystal cell (1), and a projection lens () for enlarging and projecting an optical image recorded in the liquid crystal cell (1) at a subsequent stage of the liquid crystal cell (1) 14) and a screen (15) on which an optical image is projected.

According to the liquid crystal display device having such a configuration, the semiconductor laser (5) is turned on and off and the galvanomirrors (7) and (7) are driven in accordance with a character signal or a video signal based on information to be displayed. As a result, selective light irradiation is performed, and an optical image is recorded on the liquid crystal layer (1).
This optical image is projected on a screen (15) by light from a projection light source (10), and this light has a near-infrared component by a heat ray blocking filter (12) and an optical filter (1).
After the ultraviolet component or the short-wavelength visible light component is removed by 3), the light enters the liquid crystal cell (1), so that the liquid crystal layer (4) is not uniformly heated, and the liquid crystal layer (4) is not heated. It does not induce photodeterioration of the squarylium-based dye contained in the compound.

Next, the photodegradation of the squarylium dye when the transmission characteristics of the optical filter (13) were changed in the above-described liquid crystal display device was examined by an accelerated test.

Experiments i to iv First, the liquid crystal layer (4) of the liquid crystal cell (1) was composed of a cyanobiphenyl-based cholesteric liquid crystal containing 0.185% of squarylium dye (I), squarylium dyes (II) and (I).
It was formed by injecting a liquid crystal composition in which the mixture of II) was dissolved at 0.265%.

The absorption spectrum measured with the squarylium dye dissolved in the liquid crystal in this manner is shown by curve A in FIG. Although this dye has a large absorption peak in the infrared wavelength region, the absorption in the visible light region is extremely low, and further, around 360 nm and 400 nm in the ultraviolet wavelength region, and in the visible light region.
It can be seen that it has a small absorption peak near 490 nm.

Therefore, an optical filter having a wavelength transmission characteristic for sequentially removing small absorption peaks existing in the ultraviolet wavelength region and the short wavelength visible light region is inserted into the above-described projection optical system, and irradiated with light from a xenon arc lamp. Then, the time when the absorbance of the squarylium dye was reduced by half was measured, and an attempt was made to characterize the wavelength band involved in photodegradation.

The optical filters used here were L-39, L-40, Y-
45, O-54 (all manufactured by Toshiba Corporation). Wavelength transmission characteristics when these are combined with the heat ray cutoff filter IRA-25S are shown by curves i to iv in FIG. Curve i is a combination of L-39 and IRA-25S, and curve ii is a combination of L-40.
The combination with IRA-25S, the curve iii represents the combination of Y-45 and IRA-25S, and the curve iv represents the combination of O-54 and IRA-25S. The numbers i to iv of the curves correspond to the experimental numbers i to iV. As is apparent from FIG.
According to the combination of -39 or L-40 and IRA-25S, the measured absorption spectrum of the liquid crystal composition is obtained by removing the absorption peak in the infrared wavelength region and the absorption peak near 360 nm from the curve A. According to the combination of Y-45 and IRA-25S, an absorption peak near 400 nm is further removed in addition to the above-mentioned absorption peaks.

For comparison, the same measurement was performed for the case where only the infrared cutoff filter IRA-25S was used. Table 1 shows the results.

Here, the reason why the xenon arc lamp was condensed in Experiments i to iv is that the irradiation light intensity was insufficient with parallel light, the photodegradation of the dye did not progress easily, and the experiment time was extremely prolonged. . Here, comparing the product of the irradiation light intensity and the half-life as a measure of the irradiation light amount, when the infrared cutoff filter and the optical filter are used together, the squarylium is significantly larger than when only the infrared cutoff filter is used. It is clear that the light fastness of the dye is improved and the life of the liquid crystal display device can be extended. The improvement of the light resistance by the combined use of the optical filter is about 60 times that when L-39 or L-40 is used (Experiment i and Experiment ii) and not used (comparison), and Y-45 or O-54 is used. In this case (experiment iii and experiment iv), it reached 3000 times that of the non-use. Therefore, it can be seen that the photodeterioration is remarkably suppressed by preventing the light near 360 nm from being absorbed by the squarylium dye, and the suppression effect is further increased by not absorbing light near 400 nm. Further, even when O-54 was used, the light fastness was not improved as compared with the case where Y-45 was used, indicating that the light absorption around 490 nm was not involved in the photodegradation of the squarylium dye. .

〔The invention's effect〕

As is clear from the above description, according to the present invention, a squarylium dye having excellent light-to-heat conversion efficiency is used, and a wavelength component that causes photodeterioration of the squarylium dye is removed from projection light. Thus, a liquid crystal display device capable of high-speed response and having long-term reliability is provided.

[Brief description of the drawings]

FIG. 1 is a spectrum diagram showing the absorption spectrum of a liquid crystal composition mixed with a squarylium dye and the wavelength transmission characteristics of various filters. FIG. 2 is a schematic diagram showing one configuration example of a liquid crystal display device. 1 Liquid crystal cell 5 Semiconductor laser 10 Projection light source 12 Heat blocking filter 13 Optical filter 15 Screen

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/13

Claims (1)

(57) [Claims] 1. A liquid crystal display device in which an optical image is recorded on a liquid crystal layer by thermal writing using a semiconductor laser beam, and the optical image is projected and displayed by light from a light source. formula A liquid crystal display device comprising at least one of the squarylium-based dyes (I), (II), and (III) represented by the formula: wherein light from the light source has a wavelength longer than 360 nm.