JPS63109422A - Image projector - Google Patents

Abstract

PURPOSE:To obtain rapid responsibility corresponding to a moving picture and to easily display also a half tone by using a light valve obtained by combining a light transmitting thin film with ferroelectric liquid crystal. CONSTITUTION:An optical image is written in a liquid crystal light valve 2 having a light transmitting layer 22 and a ferroelectric liquid crystal layer 27 between transparent electrodes 24. Since the ferroelectric liquid crystal has extremely rapid responsibility, the problem of a response speed can be settled and a half tone to be the problem of the ferroelectric liquid crystal can be also settled by writing an optical image on the light transmitting layer 22 by a CRT 1 or the like. In addition, a transmitted light switching means 8 for switching the color of transmitted light periodically is arranged on the passage route of light projected from a projecting light source 5 to write the monochromatic image of a corresponding color signal synchronously with the switching of a color on the light transmitting layer 22, so that a full color image can be displayed only by one light valve 2.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an image projector capable of projecting high resolution, high brightness images onto a large screen.

2. Description of the Related Art A conventionally popular image projector is a type that directly projects images from a cathode ray tube, but in this case, the brightness is low because the cathode ray tube is a light emitting type.

An image projector using a light-receiving display element uses a nematic liquid crystal with positive dielectric anisotropy as a light valve (for example, Baird, Blair et al.; Applied Physics Letters, Vol. 22, No. 3, 90). Pages (T, D,
Beard, W., P., Bleha,
S.

Y, Wong: Appl, Phys, Lett
, Vo122. Il&lL3゜P2O)). FIG. 6 is a diagram showing the overall configuration. It consists of the aforementioned light valve, a xenon arc lamp, a polarizing optical system, and a cathode ray tube. FIG. 7 is a sectional view of a liquid crystal light valve, and the principle of operation of the projector will be explained using this. When an AC bias voltage is applied to the cell through the transparent electrode, when the cathode ray tube is not emitting light, most of the voltage is applied to the photoconductive layer due to the high impedance of the photoconductive layer, so the liquid crystal remains in its initial orientation and does not move. . On the other hand, as the brightness of the cathode ray tube increases (as the photoconductive layer receives the light, its impedance decreases, the AC bias voltage applied to the liquid crystal layer increases, and the liquid crystal molecules gradually turn in the direction of the electric field from the intermediate layer). At this time, the polarized light incident from the light source side is affected by the birefringence effect.

The light-shielding layer and the dielectric reflection plate are provided to separate the projected light and the photoconductive layer, and the dielectric reflection layer also has the function of eliminating direct current caused by the superpower of the photoconduction layer.

In addition to the conventional example described above, the same liquid crystal as above is used, but by providing a thin film transistor in each pixel on a substrate with electrodes arranged in a matrix, high contrast images can be created using the twisted nematic effect or guest-host effect. There is a report that a prototype projector was made by projecting light from the back (for example, Morozumi, Sonehara et al.: Nis-I-D 86 Digest, p. 375 (Moro
zumi, S onehara: S ID86
DIGEST, P 375)).

In the conventional examples described above, including the light-emitting type, in order to perform full-color display, three cathode ray tubes or light valves are used to display images of each color, and the images are optically combined.

Next, we will discuss ferroelectric liquid crystals, which have not been used for image projectors, but have recently attracted attention due to their unprecedented high-speed response (for example, Hideo Takesei, Atsuo Habayo et al.;
"Industrial Materials", Volume 31, No. 10, Page 22). Ferroelectric liquid crystals have a dipole moment in the direction perpendicular to the long axis of the molecules, and when thinned, they exhibit spontaneous polarization.

Furthermore, chiral smectic liquid crystals exhibiting ferroelectricity exhibit a layered structure, and the molecular axis is always inclined at a constant angle θ with respect to the layer normal. FIG. 8 shows the principle of a conventional ferroelectric liquid crystal display method, and is a plan view seen from above the substrate.

92 is a liquid crystal molecule tilted ±θ degrees with respect to the layer normal, 93 is one θ
The liquid crystal molecules are tilted, 94 is the dipole moment toward the front of the page, 95 is the dipole moment toward the back of the page, and 96 is the direction of the two polarizing plates. FIG. 8 (Al is the general state when no voltage is applied, and the molecules are parallel to the substrate and tilted by ±θ degrees.

Figure 8(b) shows the state of the molecules when a negative voltage is applied from the front to the back of the paper. ±θ
Since it takes either of the tilted states, it is possible to represent light and dark by using the birefringence effect or dichroism. However, microscopically, it is not possible to have two states, so in order to produce intermediate tones, it is necessary to use the mottled state shown in Fig. 8(b) to (Fig. 8(al) obtained when transitioning to C1. It is considered.

Problems to be Solved by the Invention In light valves using conventional manetic liquid crystals, the response speed of the liquid crystal is only about 100 m5ec at best, so when displaying a moving image, a sharp image quality cannot be obtained with rapidly changing images.

Therefore, it is possible to use ferroelectric liquid crystal, but since ferroelectric liquid crystal uses the above-mentioned display principle, it is very difficult to display halftones uniformly.

Furthermore, in order to obtain a color display with a conventional image projector, extremely fine alignment of the light valve and the cathode ray tube is required in order to synthesize a three-color image.

Means for Solving the Problems In order to solve the above problems, the image projector of the present invention has a structure in which a light image is written on a liquid crystal light valve having a photoconductive layer and a ferroelectric liquid crystal layer between transparent electrodes. ing. Since ferroelectric liquid crystals are very fast, the first problem, response speed, can be solved, and the problem with ferroelectric liquid crystals, which is halftone, can be solved by applying a light image to the photoconductive layer using, for example, a cathode ray tube. Solved by writing.

Furthermore, a means for switching the color of the transmitted light at intervals is provided on the path of light emitted from the projection light source, and a monochrome image of the corresponding color signal is written on the photoconductive layer in synchronization with the switching of the color. This allows full-color images to be displayed with a single light bulb.

Effect The impedance of the photoconductive layer changes according to the written optical image, and the voltage applied to the liquid crystal changes, so when the applied voltage is greater than the dark value voltage of the ferroelectric liquid crystal panel, it will spontaneously move in the direction of the electric field. If the polarization is uniform and small, the state immediately before scanning is maintained. Depending on orientation processing such as rubbing and conditions such as cell thickness, the molecular arrangement will always return to the same when no voltage is applied, so if the optical system is set so that the display becomes dark when the molecular arrangement is In the case of writing with a tube, immediately before scanning, the afterglow of the light image is gone, so the picture elements are also dark. Therefore, microscopic binary display can be performed depending on the brightness of the optical image.

Furthermore, if the diameter of the light emitting point is made larger when the luminance signal increases, the area where the liquid crystal element is in the on state increases, so that halftones can also be easily displayed. Further, since the time during which the luminance of the light emitting point is greater than or equal to the luminance sufficient to turn on the cell depends on the initial luminance during scanning, halftones can be expressed by modulating the time during which the light emitting point becomes on. In addition, if a ferroelectric liquid crystal panel has bistability, the optical image can be written by resetting the entire surface of the panel with a sufficiently large pulse voltage and writing the image on the liquid crystal layer by applying a pulse voltage. Even if it disappears, the projected image remains. The means for writing an optical image is not limited to a cathode ray tube as long as the diameter or decay time of the light emitting point can be modulated.

Since ferroelectric liquid crystal exhibits a very fast response of several microseconds to several hundred microseconds, it can sufficiently follow even high-speed scanning of 180 Hz or more. Cathode ray tubes can also scan at 180 Hz, so by sequentially displaying image data for each primary color with a single cathode ray tube and ferroelectric liquid crystal light valve, and switching the color of the projected light, an image of the three primary colors can be created at 601)z. The viewer will see a full-color image.

Embodiment Hereinafter, an image projector according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is an overall view of the image projector of this embodiment.

The color of the projected light can be determined by directly rotating a disc with three color films as shown in Figure 2 (al (b)) at the position where the light from the xenon lamp is focused by the aiming lens and rotating the drive motor at 60Hz. We decided to synchronize the color filters so that the light would hit the partitions between the color filters during the cathode ray tube's vertical retrace period, and also decided on the filter configuration.

FIG. 3 is a sectional view of the liquid crystal light valve in FIG. 1.

The photoconductive layer is a thin film of about 5 μm made of CdS, the light shielding layer is made of CdTe, and the dielectric reflective layer is made of ZnS and M g F 2 . A mixture of ester-based ferroelectric liquid crystals is used for the liquid crystal layer, and a gap of 2.2 μm is maintained with a spacer. The SiO□ insulating film is for preventing short circuits. The liquid crystal is oriented by rubbing a polyimide alignment film, but when the rubbing is performed under fairly strong pressure, the alignment shown in FIG. 4 is obtained. That is, when a negative voltage is applied, the molecules are oriented in the rubbing direction as shown in Figure 4 (al).
At a positive voltage, the molecule deviates from the rubbing axis by twice the inclination angle θ, as shown in Fig. 4). At this time, we were able to set the molecules so that when the electric field was turned off, the molecules always faced the rubbing direction, and that they became darker as the brightness of the cathode ray tube increased. In this panel, the thickness was set so that the product of the thickness d of the liquid crystal layer and the refractive index anisotropy Δn was 0.25 so that the transmitted light was not colored. Figure 5 (a
l is the luminance of a certain picture element on the cathode ray tube, Figure 5 (b) is the voltage waveform applied to the liquid crystal light valve, Figure 5 (C1
is a diagram showing the intensity of output projection light. Although the applied voltage waveform is asymmetrical, the product of the voltage and pulse width of the positive and negative pulses is equal, so the liquid crystal does not deteriorate. Negative pulses make it brighter, and positive pulses have narrower pulse widths, so there is little change in brightness. If a rectangular wave with a duty of 100% is used, it will become dark due to negative pulses, so the waveform shown in FIG. 5 can achieve a higher maximum brightness. In addition, if the rubbing pressure is weakened or an inorganic material is obliquely deposited to align the ferroelectric liquid crystal, if the liquid crystal layer has a thickness of about 2 μm to 10 μm, the molecules will be twisted in the direction of the upper and lower substrates when no voltage is applied. The structure was adopted. In a thick cell of 5 μm or more, twisted liquid crystal molecules act as a waveguide, and the incident linearly polarized light is emitted in the same state along the waveguide, so the pixel becomes dark and a voltage is applied, causing the incident molecules to When the light is aligned in one direction that is different from the polarization direction of the light, the picture element becomes brighter. Thus, it can be seen that sufficient contrast can be obtained even with a ferroelectric liquid crystal switch having a twisted structure. As a result of actual display experiments using the method described above, it was confirmed that full-color, high-quality display with a maximum contrast ratio of 30 and the number of gradations of 32 or more is possible with any of the liquid crystal switches.

Effects of the Invention By using a light valve that combines a photoconductive thin film and a ferroelectric liquid crystal, the image projector of the present invention achieves high-speed response that can handle moving images. Halftones that used to be dots can also be easily displayed. Furthermore, by utilizing the high-speed response of ferroelectric liquid crystal,
By switching the color of the projected light using a set of light valves, a full-color, high-quality display can be projected with a very simple configuration.

[Brief explanation of the drawing]

Fig. 1 is a perspective view of the image projector of the present invention, Fig. 2(a)
(bl is a side view and a plan view of means for switching the color of the projection light of the image projector, FIG. 3 is a sectional view of the liquid crystal light valve of the image projector, FIG. 4(a) (bl is the light valve) Schematic diagram showing the orientation of liquid crystal molecules in 5th
(al (bl (cl) is a voltage waveform diagram applied to the light valve, FIG. 6 is a perspective view showing the configuration of a conventional image projector, FIG. 7 is a sectional view of the liquid crystal light valve in FIG. 6, Figure 8 (al (b) (C) is a plan view showing the display principle of a conventional ferroelectric liquid crystal panel. 1... Cathode ray tube, 2... Liquid crystal light valve, 3 ...Polarizing beam splitting prism, 4...
... Collimator, 5 ... Light source, 6 ...
...Projection lens, 7...Screen, 8...
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Claims (10)

[Claims] (1) An image projector comprising at least a projection light source and means for writing an optical image onto a liquid crystal light valve having a photoconductive layer and a ferroelectric liquid crystal layer between transparent electrodes. (2) The image projector according to claim (1), wherein the optical image writing means is a cathode ray tube. (3) The image projector according to claim 1 or 2, wherein all molecules of the ferroelectric liquid crystal are aligned in a substantially constant direction when no voltage is applied. (4) The ferroelectric liquid crystal has a structure in which molecules are twisted between the upper and lower substrates when no voltage is applied. Image projector. (5) The image projector according to claim 1 or 2, wherein the ferroelectric liquid crystal layer has bistability. (6) Means for periodically switching the color of the transmitted light is provided on the passage path of the light emitted from the projection light source, and in synchronization with the switching of the color, a corresponding color signal is displayed on the photoconductive layer. An image projector according to any one of claims (1) to (5), characterized in that it writes a monochrome image. (7) The image projector according to claim (6), wherein the color switching occurs at a cycle of 30 hertz or more. (8) Any one of claims (1) to (7), characterized in that the product of the refractive index anisotropy of the ferroelectric liquid crystal and the thickness of the liquid crystal layer is approximately 0.25. The image projector described in . (9) The image projector according to any one of items (1) to (8), wherein the liquid crystal light valve is provided with means for applying an alternating current voltage. (10) The means for applying an alternating voltage to the liquid crystal light valve is configured to generate an electrical signal in which the width of positive and negative pulses is different, but the time average of the voltage is zero. The image projector according to scope (9).