JPS6248813B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of displaying numbers, characters, images, etc. using liquid crystal. Liquid crystal display devices have many advantages, such as low voltage drive, low power consumption, large area and high brightness, relatively low cost, and mass production. These advantages make it suitable as a display device, and it is used in many practical applications such as calculators and watches, and recently, development into televisions and character displays has also made great progress. However, it has drawbacks such as not very fast response speed, narrow operating temperature range, and narrow viewing angle. FIG. 1 shows a cross section of a liquid crystal cell used in a liquid crystal display device. In this liquid crystal cell, a pair of glasses 1 each having a transparent conductive film 2 formed thereon are bonded together with a spacer 3 interposed therebetween, with a gap formed between the glasses 1, and this gap is filled with liquid crystal 4. Conventionally, this liquid crystal cell is filled with a smectic liquid crystal that changes from a smectic phase to a nematic phase to a liquid phase as the temperature rises, and is normally maintained in a state exhibiting a smectic phase. In a liquid crystal display device using such a liquid crystal, the following method has been used to display an image. To write an image, a laser beam is scanned over the surface of the liquid crystal cell to increase the temperature of the irradiated portion of the liquid crystal until it enters the liquid phase. laser·
The part irradiated with the beam has a high temperature and is in a liquid phase, but the very peripheral part remains in a nematic phase, and the part outside of that does not experience a temperature rise and remains in a smectic phase. This situation is shown in FIG.
In the figure, 6 indicates a region that has become a liquid phase due to laser beam irradiation, 5 indicates a nematic phase, and 4 indicates a smectic phase. laser·
In the beam scanning operation, when the laser beam moves to another part, the irradiated part tries to return to its original smectic phase by cooling. At this time, all the liquid crystal molecules whose alignments were spatially different from the center to the liquid phase, nematic phase, and smectate phase try to return to the smectate phase. It remains in a disordered state (this is called a honeycomb structure) and becomes opaque, and an image is written on it. The light transmittance -
The temperature characteristics are schematically shown in Figure 3.
Point a in the figure shows the state before laser beam irradiation, point c shows the state at the time of laser beam irradiation, and point a' shows the state after laser beam irradiation. The phase change of liquid crystal explained above is the third phase change.
It changes as shown by the arrow in the figure. That is, a→
It changes from c to a' and becomes opaque. When erasing a written image on the entire surface, a high frequency electric field is applied to the liquid crystal via the transparent conductive film of the liquid crystal cell shown in FIG. The applied high-frequency electric field causes the liquid crystal molecules to become homeotropically aligned and become transparent. Furthermore, when erasing only a part of the written image, a weaker electric field is applied to the liquid crystal than the electric field applied when erasing the entire image. At this time, the written image is not erased. In this state, the area to be erased is irradiated with a laser beam to raise the temperature of the liquid crystal until it exhibits a nematic phase, and then when the laser beam irradiation is stopped, the irradiated area becomes transparent and erased. In this way, in the conventional display method using liquid crystal,
(b) Still image display, (b) full erasing, and (c) partial erasing are possible. However, (a) the writing operation requires a temperature rise to cause a phase change from smectic phase to nematic phase to liquid phase, which limits the writing speed; (b) video display is not possible; and (c) The drawback is that still images and videos cannot be displayed simultaneously. In order to eliminate the drawbacks of conventional display methods using liquid crystals, the inventors invented something called "image display method using liquid crystals" (Japanese Patent Application Laid-Open No. 114143/1983). In this display system, the liquid crystal used is a mixed liquid crystal of n-type nematic liquid crystal and cholesteric liquid crystal that changes from nematic phase to liquid phase. In such a mixed liquid crystal, the liquid crystal molecular arrangement is twisted within a temperature range that is approximately the same as the temperature range of the nematic phase exhibited by the nematic liquid phase before mixing. Therefore, the nematic phase becomes a cholesteric phase. (For convenience of explanation, the expression nematic phase before mixing is sometimes used.) In the present invention, the expression cholesteric phase is used. The same applies to mixed liquid crystals below. In order to display an image using this display method using mixed liquid crystal, the liquid crystal cell is kept at a temperature at which the liquid crystal exhibits a cholesteric phase. The liquid crystal is irradiated with a laser beam while a high-frequency electric field is applied. Before the laser beam irradiation, the liquid crystal in the area irradiated with the laser beam
During laser beam irradiation and when laser beam irradiation is stopped, the phase changes from cholesteric phase (low temperature) to cholesteric phase (high temperature) to cholesteric phase (low temperature). The mixed liquid crystal used here exhibits a change in optical properties, being transparent at the low temperature side of the cholesteric phase and opaque at the high temperature side. Therefore, the liquid crystal changes from transparent to opaque (when irradiated) by scanning the laser beam.
→It exhibits a change of transparency (after irradiation) and does not have a memory effect that leaves opacity behind. In other words, a moving image display is created in which an image can be displayed only while the laser beam is being irradiated. This phase change of liquid crystal is called
As shown in the figure. This display method has the advantage that it is not necessary to raise the temperature of the liquid crystal until it becomes a liquid phase, and the writing speed can be improved. Furthermore, this display method can also perform full-screen erasure and still image display in the same way as the conventional display method using liquid crystals described above. That is, this display method can perform (1) still image display, (2) full erasure, and (3) moving image display. But(1)
(2) It is not possible to display still images and moving images simultaneously on one screen. SUMMARY OF THE INVENTION An object of the present invention is to provide a display method that eliminates the drawbacks of conventional liquid crystal display methods, can simultaneously display still images and moving images on one screen, and has a high writing speed. The liquid crystal display method of the present invention uses a mixed liquid crystal in which cholesteric liquid crystal is mixed with a p-type smectic liquid crystal that changes from a smectic phase to a nematic phase to a liquid phase as the temperature rises, and this mixed liquid crystal (hereinafter abbreviated as liquid crystal) is used. After writing a still image by irradiating the liquid crystal with a laser beam, irradiating the liquid crystal with a laser beam while applying a weak electric field to the extent that the still image is not erased, or performing the process in the reverse order; At the same time as a still image is written, a still image and a moving image are displayed on the same screen by applying a weak electric field as described above and irradiating a laser beam only to the liquid crystal in the area other than the area where the still image is displayed. Has a process,
A full-scale erasing process in which an electric field stronger than that applied during image writing is applied to the liquid crystal on which an image has been written to erase all written images, or a weaker electric field is applied to the liquid crystal on which an image has been written, similar to that used when writing a moving image. This method includes a partial erasing process in which only the laser beam irradiated area is erased by applying a laser beam and irradiating the laser beam. In the present invention, when the liquid crystal is scanned with a laser beam to cause a temperature change, the optical characteristics and phase changes of the liquid crystal become as shown in FIG. That is, smectic phase (before laser beam irradiation, shown as point a in Figure 5) → cholesteric phase (during laser beam irradiation, shown as point b in Figure 5) → smectic phase (after laser beam irradiation, shown as point b in Figure 5) (indicated by a' in the figure), the part irradiated by the laser beam remains opaque, and a still image is written. Further, when a high frequency electric field is applied to the liquid crystal and a laser beam is irradiated to change the temperature, the liquid crystal undergoes a phase change as shown in FIG. In other words, the smectic phase (before laser beam irradiation, a in the figure)
) → cholesteric phase (during laser beam irradiation, shown as point b in the figure) → smectic phase (after laser beam irradiation, shown as point a in the figure). The optical characteristics at this time change from transparent to opaque to transparent, and the opaque portion becomes transparent when the laser beam irradiation is stopped, resulting in a moving image display. Therefore, in order to simultaneously display a still image and a moving image on one screen, the two methods described above may be performed simultaneously (in this case, an electric field is applied only to the portion where the moving image is to be displayed). Alternatively, the above-described moving image may be written after writing a still image, or the still image may be written after writing a moving image. The electric field strength applied at this time is such that the applied voltage V satisfies V ₂ <V < V ₁ as can be seen from FIG. 7, which will be described later. FIG. 7 is a diagram showing the electro-optic effect of the liquid crystal in the present invention. The curve shows the characteristics of the liquid crystal at a temperature without laser beam irradiation, and the curve shows the irradiated area when the laser beam is irradiated. It shows the characteristics of liquid crystal at a temperature of . here
V ₁ V ₂ is a voltage that provides the electric field strength necessary for the liquid crystal to change from an opaque state to a transparent state at each temperature. Therefore, the voltage V applied to the liquid crystal
If V ₂ <V < V ₁ , a moving image can be created without affecting still images. When erasing an image, the entire image can be erased by applying a voltage higher than V ₁ to the liquid crystal, as can be seen from FIG. Also, V ₂ <V<
If a laser beam is scanned while a voltage _V1 is applied, only the portion irradiated with the laser beam can be erased, resulting in partial erasure. In the present invention, all operations can be performed by changing the smectic phase and the cholesteric phase, and as there is no need to raise the temperature to the liquid phase as seen in conventional methods, the laser beam power required for writing can be reduced. . Also, if the power is the same, it can be written faster. In the present invention, since a mixed liquid crystal of a p-type smectic liquid crystal and a cholesteric liquid crystal is used, the electric field required for full erasure and partial erasure, that is, the voltage shown in FIG. V ₁ V ₂
It also has the effect of requiring a lower value. This is because the molecular arrangement of p-type liquid crystal and cholesteric liquid crystal is different, and when it is mixed into cholesteric liquid crystal or p-type smectic liquid crystal, the liquid crystal molecules respond more easily to the applied electric field than when only p-type smectic liquid crystal is used. It is. Note that the same operation as described above can be achieved by using a laser beam and increasing the temperature to the liquid phase as in the conventional method. The effects of the present invention are shown in the table below in comparison with conventional ones.

[Table] An example of a liquid crystal display device to which the display method of the present invention is applied is shown in FIG. In this liquid crystal display device, a liquid crystal cell 11 is filled with a mixed liquid crystal using 4-4' cyano octyl biphenyl (COB) as a p-type smectic liquid crystal and cholesteric nonanoate (CN) as a cholesteric liquid crystal. The mixing ratio of this mixed liquid crystal was COB:CN=10:1 (weight ratio). When the ambient temperature is high, instead of a mixed liquid crystal of COB and CN, the weight ratio of 4-4′-cyanooctyloxybihenyl (COOB) and CN, which has a high smectic←→cholesteric phase transition temperature, is used.
A 10:1 mixed liquid crystal is used. Furthermore, a mixed liquid crystal of COB, COOB, and CN can be used depending on the ambient temperature. With this mixing ratio, operation is possible even when the cholesteric content ranges from a few percent to more than 20%. A heater 12 is installed in the liquid crystal cell, and the heater is controlled by a temperature controller 21 so that the display device operates normally even when the ambient temperature changes. A projection light source 15 is located behind the liquid crystal cell (on the side where the heater 12 of the liquid crystal cell is installed).
has been installed. The image written on the liquid crystal cell 11 is projected onto a screen 18 installed in front of the liquid crystal cell 11 by this projection light source. A condenser lens 14 is installed between the liquid crystal cell 11 and the projection light source so that the liquid crystal cell 11 is effectively irradiated with light from the projection light source. Furthermore, a cold filter 13 is installed between the condenser lens 14 and the liquid crystal cell to prevent the temperature of the liquid crystal cell from rising due to the light from the projection light source. A projection lens 17 is installed in front of the liquid crystal cell 11,
An image is clearly projected onto the screen 18. The laser beam for writing or erasing an image on the liquid crystal cell 11 may be any beam that can locally raise the temperature, and is emitted from, for example, a YAG or semiconductor laser 22 (hereinafter referred to as YAG laser), and is emitted from a modulator 23. , Slit 19, Y
modulation through the axis deflector 24 and the X-axis deflector 25;
The light is deflected, focused by the writing lens 28, and irradiated onto the liquid crystal cell via the dichroic mirror 16. The modulator 23, Y-axis deflector 24, X-axis deflector 25, etc. are connected to a signal source 27 via a driving amplifier 26, and are controlled by a signal from the signal source 27. In the liquid crystal display device having such a configuration, the YAG laser 22, the signal source 27, and the AC power source 20 perform the above-described writing and erasing operations, and when the projection light source 15 projects the image onto the screen 18, an image is displayed. FIG. 9 shows an example of an image projected onto the screen. FIG. 9a shows a still image 30, and FIG. 9b shows the still image 30 completely erased. FIG. 9c is a diagram in which a is partially erased. 9d, e, and f show the still image 31 and the moving image 32 displayed simultaneously, and d is 0 seconds, that is, the still image 3
1. Shows the moment when the video 32 is displayed simultaneously, e shows 0.2 seconds after d, and f shows 2/3 seconds after d. As explained above, according to the present invention, it is possible to simultaneously display still images and moving images on the same screen at high speed.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a liquid crystal cell used in a liquid crystal display device, and FIG. 2 is an enlarged view of the vicinity of the irradiated part of the liquid crystal cell irradiated with a laser beam. Figure 3 shows the thermo-optic effect of liquid crystal in the conventional display system.
The figure shows the electro-thermo-optic effect in a conventional display method. FIG. 5, FIG. 6, and FIG. 7 respectively show the thermo-optic effect, electro-thermo-optic effect, and electro-optic effect of the liquid crystal in the present invention. FIG. 8 shows a liquid crystal display device embodying the display method of the present invention, and FIG. 9 shows an example of an image displayed by the liquid crystal display device of FIG. In the figure, 1 is a glass plate, 2 is a transparent conductive film, 3 is a spacer, 4 is a smectic liquid crystal, 5 is a nematic phase, 6 is a liquid phase, 11 is a liquid crystal cell, 15 is a projection light source, 20 is an AC power source, and 22 is 27 indicates a signal source, YAG or semiconductor laser.

Claims (1)

[Claims] 1 A still image writing process in which a laser beam is irradiated onto a mixed liquid crystal consisting of a P-type smectic liquid crystal and a cholesteric liquid crystal, and a laser beam is applied to the whole or a portion of the mixed liquid crystal while applying a weak electric field to the extent that the still image is not erased. A video writing process in which a still image and a video are irradiated are simultaneously performed, and a still image and a video are displayed together on the same screen, or a video or a still image is written by either the still image writing process or the video writing process. There is a writing step in which a still image and a moving image are displayed together on the same screen by performing the other remaining step afterward, and writing is performed by applying an electric field stronger than the electric field applied when writing a moving image to the mixed liquid crystal. A full-scale erasing process that erases all the still images in the area, or a part where a weak electric field similar to that used when writing the moving image is applied to the mixed liquid crystal, and a laser beam is irradiated to erase the still image only in the area irradiated with the laser beam. A display method using a liquid crystal characterized by having an erasing step.