JP2532408B2 - Image projector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image projector capable of projecting an image of high resolution and high brightness on a large screen.

2. Description of the Related Art The most popular image projector in the past is a type in which an image of a cathode ray tube is directly projected, but in this case, the luminance 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 having a positive dielectric anisotropy for a light valve (for example, Baird, Blair et al .; Applied Physics Letter, Vol. 22, No. 3, 90). Page [TD Beard, WP
Bleha, SYWong: Appl.Phys.Lett.Vo122, No.3, P90]).
FIG. 6 is a diagram showing the overall structure. It is composed of the light valve, the xenon arc lamp, the polarization optical system and the cathode ray tube. FIG. 7 is a sectional view of the liquid crystal light valve.
The operating principle of the projector will be described using this. When an AC bias voltage is applied to the cell via the transparent electrode, when the cathode ray tube is not emitting light, the voltage is almost applied to the photoconductive layer because the impedance of the photoconductive layer is high, and the liquid crystal remains in the initial alignment. . On the other hand, when the brightness of the cathode ray tube increases, the photoconductive layer receives the light and the impedance decreases, the AC bias voltage applied to the liquid crystal layer increases, and the liquid crystal molecules gradually start to face the electric field direction from the intermediate layer. . At this time, the polarized light entering from the light source side is subjected to the birefringence effect. The light shielding layer and the dielectric reflection plate are provided to separate the projected light and the photoconductive layer from each other, and the dielectric reflection layer also has a function of eliminating a direct current due to super power of the photoconductive layer.

In addition to the above-mentioned conventional example, the same liquid crystal as above is used, but by providing a thin film transistor in each pixel on a substrate having a matrix of electrodes, a high-contrast image is formed by twisting nematic effect or guest-host effect. It has been reported that a projector was realized by illuminating it from the back and prototyped it (for example, Morozumi, Sonehara et al .: S-Id 86 digest, page 375 [Morozumi, So
nehara: SID86 DIGEST, P 375]).

In the above-described conventional example, in order to perform full-color display including the light-emitting type, a method of displaying images of respective colors by three cathode ray tubes or light valves and optically combining them is adopted.

Next, although there is no example used for an image projector, in recent years,
A description will be given of a ferroelectric liquid crystal that has been attracting attention due to its unprecedented high-speed response (for example, Hideo Takezoe, Atsushi Fukuda,
"Industrial materials", Vol. 31, No. 10, p. 22). Ferroelectric liquid crystal has a dipole moment in a direction perpendicular to the long axis of the molecule, and becomes spontaneously polarized by thinning it. Further, the chiral smectic liquid crystal exhibiting ferroelectricity has a layered structure, and the molecular alignment axis is always inclined by 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 a liquid crystal molecule tilted −θ degrees, 94 is a dipole moment in the front direction of the paper, 95 is a dipole moment in the back direction of the paper, and 96 is two polarizing plates. Direction. FIG. 8 (a) shows a general state in which no voltage is applied, and the molecules are mixed in a state parallel to the substrate and inclined by ± θ degrees. FIG. 8 (b) shows the state of the molecules when a negative voltage is applied from the front side to the back side of the paper, and FIG. 8 (c) shows the state of the molecules when a positive voltage is applied. In this way, since the molecule is tilted by ± θ degrees depending on the direction in which the voltage is applied, it is possible to express lightness or darkness by utilizing the birefringence effect or dichroism. However, microscopically, the two states cannot be taken, and therefore a mottled state such as that shown in FIG. 8 (a) obtained at the time of shifting from FIG. 8 (b) to (c) is used for producing a halftone. Is being considered.

Problems to be Solved by the Invention In a conventional light valve using a mannequin liquid crystal, even if the response speed of the liquid crystal is fast, it is about 100 msec. Therefore, when a moving image is displayed, a sharp image quality cannot be obtained in a rapidly changing image.

Therefore, it is possible to use a ferroelectric liquid crystal,
Since the ferroelectric liquid crystal uses the display principle as described above, it is very difficult to display halftone uniformly.

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

Means for Solving the Problems In order to solve the above problems, the image projector of the present invention is
At least a liquid crystal light valve having a photoconductive layer and a ferroelectric liquid crystal layer between electrodes, means for applying an AC voltage to the liquid crystal light valve, means for writing an optical image in the liquid crystal light valve, and a projection light source, The means for writing the optical image is configured to have a modulating means for modulating the decay time at the intersection. Ferroelectric liquid crystals are very fast, so
The problem of the response speed is solved, and the problem of the half tone of the ferroelectric liquid crystal is solved by writing an optical image in the photoconductive layer by, for example, a cathode ray tube.

Further, means for periodically switching the color of the transmitted light is provided on the passage of the 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 color switching. Thus, a single light valve can display a full-color image.

Action 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 higher than the threshold voltage of the ferroelectric liquid crystal panel, spontaneous polarization occurs in the direction of the electric field. Are aligned, and if small, the state immediately before scanning is retained. Depending on the alignment treatment such as rubbing, and the condition such as cell thickness, the same molecular arrangement is always returned when no voltage is applied.Therefore, if the optical system is set so that the display becomes dark when the above-mentioned molecular arrangement is used, for example, a light image is displayed in a cathode ray tube. When written by, the afterglow of the optical image disappears immediately before scanning, and therefore the picture element also becomes dark. Therefore, binary display can be performed microscopically depending on the brightness of the light image. Further, if the diameter of the light emitting point is increased when the luminance signal is increased, the area in which the liquid crystal element is in the ON state is increased, so that the halftone can be easily displayed. Further, the time when the brightness of the light emitting point is equal to or higher than the brightness sufficient for turning on the cell depends on the initial brightness at the time of scanning. If the ferroelectric liquid crystal panel has bistability, a sufficiently large panel voltage resets the entire surface of the panel, and then an optical image is written and a pulse voltage is applied to write the image in the liquid crystal layer. Even if it disappears, the projector remains. The means for writing the light image is not limited to the cathode ray tube as long as the diameter of the light emitting point or the decay time can be modulated.

Ferroelectric liquid crystals show a very fast response time of several microseconds to several hundred microseconds, so they can sufficiently follow up even if they are scanned at a high speed of 180 Hz or higher. Since the cathode ray tube can also scan at 180 Hz, one cathode ray tube and a ferroelectric liquid crystal light valve display the image data of each primary color sequentially, and if the colors of the projected light are switched, an image of three primary colors is displayed at 60 Hz. Therefore, the observer can see a full-color image.

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

FIG. 1 is an overall view of the image projector of this embodiment. The color of the projected light is to rotate the disc directly with a driving motor at 60Hz, which is a disk with three color films as shown in Fig. 2 (a) and (b) at the position where the light of the xenon lamp is focused by the aiming lens. It was switched by. Synchronization was performed so that light would hit the partition between the color filters during the vertical blanking period of the cathode ray tube, and the filter configuration was determined.

FIG. 3 is a sectional view of the liquid crystal light valve in FIG. The photoconductive layer is a thin film of CdS having a thickness of about 5 μm, the light-shielding layer is CdTe, and the dielectric reflecting layer is ZnS and MgF ₂ . For the liquid crystal layer, a mixture of ester-based ferroelectric liquid crystals is used, and a gap of 2.2 μm is maintained by a spacer. The insulating film of SiO ₂ is for preventing a short circuit. The liquid crystal is aligned by rubbing a polyimide alignment film, but when the rubbing is performed under a considerably strong pressure, the alignment shown in FIG. 4 is exhibited. That is, when a negative voltage is applied, the molecule is oriented in the rubbing direction as shown in FIG. 4 (a), and at a positive voltage, it is deviated from the rubbing axis by twice the molecular tilt angle θ as shown in FIG. 4 (b). At this time, it was possible to set so that when the electric field was cut off, the molecules always faced the rubbing direction, and became darker when the brightness of the cathode ray tube increased. In this panel, so that the transmitted light is not colored,
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. FIG. 5 (a) shows the emission luminance of a certain pixel on the cathode ray tube, FIG. 5 (b) shows the voltage waveform applied to the liquid crystal light valve, and FIG. 5 (c) shows the intensity of the output projection light. It is a figure. The applied voltage waveform is asymmetric, but since the product of the voltage of the positive and negative pulses and the pulse width is equal, the liquid crystal does not deteriorate. The positive pulse makes the image brighter, and the negative pulse makes the pulse width narrower, so that the change in the luminance is small. When a rectangular wave with a duty ratio of 50% is used, it becomes dark with a negative pulse, so the waveform of FIG. 5 has a larger maximum luminance. Also, if the rubbing pressure is weakened or the inorganic liquid crystal is obliquely vapor-deposited to orient the ferroelectric liquid crystal, when the liquid crystal layer has a thickness of about 2 μm to 10 μm, the molecules twist in the upper and lower substrate directions when no voltage is applied. It has a structure. In a thick cell with a thickness of 5 μm or more, twisted liquid crystal molecules serve as waveguides, and the linearly polarized light that has been incident is emitted in the same state along the waveguides, so the picture element becomes dark, and a voltage is applied and molecules enter. The pixels become brighter when they are aligned in one direction, which is deviated from the polarization direction of light.
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 actually performing the display experiment by the above method, it was confirmed that any of the liquid crystal switches can achieve full color high quality display with a maximum contrast ratio of 30 and a gradation number of 32 or more.

EFFECTS OF THE INVENTION The image projector of the present invention uses a light valve in which a photoconductive thin film and a ferroelectric liquid crystal are combined, so that a high-speed response capable of supporting moving images can be obtained, and the problem of the ferroelectric liquid crystal panel can be obtained. The halftone that was a point can be easily displayed. Furthermore, by utilizing the high-speed response of the ferroelectric liquid crystal,
By switching the color of the projected light with one set of light valves, a full-color high-quality display can be projected with a very simple structure.

[Brief description of drawings]

FIG. 1 is a perspective view of an image projector of the present invention, and FIG. 2 (a).
(B) is a side view and a plan view of a means for switching the color of the projection light of the image projector, FIG. 3 is a sectional view of a liquid crystal light valve of the image projector, and FIGS. 4 (a) and (b) are the sectional views. 5A, 5B, and 5C are schematic diagrams showing the orientation of liquid crystal molecules in the light valve, FIG. 5A is a voltage waveform diagram applied to the light valve, and FIG. 6 is a perspective view showing a configuration of a conventional image projector. FIG. 7 is a sectional view of the liquid crystal light valve in FIG. 6, and FIGS. 8A, 8B and 8C are plan views showing the display principle of a conventional ferroelectric liquid crystal panel. 1 ... Cathode ray tube, 2 ... Liquid crystal light valve, 3 ... Polarization beam splitting prism, 4 ... Collimator, 5 ... Light source,
6 ... Projection lens, 7 ... Screen, 8 ... Transmitted light switching means.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Go Uemura 1006 Kadoma, Kadoma-shi Matsushita Electric Industrial Co., Ltd. (72) Inventor Hiroyuki Onishi 1006 Kadoma, Kadoma-shi Matsushita Electric Industrial Co., Ltd. (72) Inventor Shuji Ohba 1006 Kadoma, Kadoma, Kadoma-shi, Matsushita Electric Industrial Co., Ltd. (56) References JP-A-59-216126 (JP, A) JP-A-56-27198 (JP, A) JP-A-61-221727 (JP, 221727) A) Japanese Patent Publication Sho 56-38956 (JP, B1)

Claims (6)

(57) [Claims] 1. A liquid crystal light valve having a photoconductive layer and a ferroelectric liquid crystal layer between electrodes, a means for applying an AC voltage to the liquid crystal light valve, a means for writing an optical image in the liquid crystal light valve, and a projection light source. And a modulation means for modulating the decay time of the light emitting point of the means for writing the optical image, and the signal of the means for applying the AC voltage is positive / negative asymmetric and the time average of the positive and negative voltages becomes zero. An image projector characterized by being an electric signal. 2. The image projector according to claim 1, wherein all the liquid crystal molecules of the ferroelectric liquid crystal are aligned in a substantially constant direction when no voltage is applied. 3. The image projector according to claim 1, wherein the liquid crystal molecules of the ferroelectric liquid crystal have a structure in which the upper and lower substrates are twisted when a voltage is not applied. 4. A means for periodically switching the color of the transmitted light is provided on the passage of the light emitted from the projection light source, and the color corresponding to the color on the photoconductive layer is synchronized with the color switching. The image projector according to any one of claims (1) to (3), wherein a monochrome image of a signal is written in a liquid crystal light valve having a ferroelectric liquid crystal layer. 5. The image projector according to any one of claims (1) to (4), wherein the ferroelectric liquid crystal layer has bistability. 6. The product of the anisotropy of the refractive index of the ferroelectric liquid crystal and the thickness of the liquid crystal layer is about 0.25, and any one of claims (1) to (5). The image projector according to 1.