Liquid crystal element of optical writing type

Abstract: A liquid crystal element of optical writing type includes a pair of transparent substrates respectively having electrodes formed on the overall surfaces thereof. The liquid crystal element also includes a photoconductive layer formed on one of the transparent substrates and having a back-to-back diode structure, which impedance is to be changed depending on an amount of incident light. The liquid crystal element further includes a mirror layer for reflecting light entered from the other side of a writing side of the photoconductive layer, and a liquid crystal layer disposed between the pair of transparent substrates.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element of optical writing type, and more particularly to the liquid crystal element of optical writing type which is preferably adapted to a projection type liquid crystal display apparatus arranged to project an image onto a light-reflective screen, a sensor for an image scanner, and a wavelength converting element for converting near infrared light into visible light, for example. 2. Description of the Related Art The representative related art about the liquid crystal element of optical writing type known by the present inventors will be described in this item. The liquid crystal element of optical writing type is composed of glass substrates on both of the outer sides, transparent electrodes formed on the inner sides of the respective glass substrates, a photoconductive layer formed on one of the transparent electrodes, a dielectric mirror layer formed on the photoconductive layer, orientation films formed respectively on the dielectric mirror layer and the other transparent electrode, and a liquid crystal layer disposed between the orientation films. The photoconductive layer is made of hydrogenated amorphous silicon (referred to as a-Si:H), which layer is formed by means of a plasma CVD method using silane gas and hydrogen gas as raw materials. The dielectric mirror is formed of multi-layered films composed of silicon or silicon oxide by means of a sputtering method. The orientation film is formed on a polyimide film by means of a spin coating technique. The liquid crystal layer is made of nematic liquid crystal. This kind of liquid crystal element of optical writing type is allowed to have such operation modes as a twisted nematic (TN) mode, a hybrid field effect (HFE) mode, a guest host (GH) mode, and a phase transition mode. In operation, an alternating voltage is applied between the transparent electrodes. When a ray of light is emitted from a CRT (Cathode Ray Tube) to one of the glass substrates, the photoconductive layer serves to lower its impedance at the light-incident area (bright condition) so that the applied voltage is strong enough to drive the liquid crystal layer, and keeps its impedance at the other area where no light is incident (dark condition) so that the applied voltage is not strong enough to drive the liquid crystal layer. The contrast between the bright condition and the dark condition results in forming an image. The foregoing related art uses the a-Si:H layer as its photoconductive layer. The a-Si:H layer has the similar magnitude of dark conductivity as the conductivity of the liquid crystal. It means that the photoconductive layer has the similar magnitude of the impedance as the liquid crystal layer so that a certain amount of voltage is applied to the liquid crystal layer in the dark condition. When a ray of light is incident onto this photoconductive layer, that is, the light-incident area of the photoconductive layer becomes the bright condition, the photoconductive layer lowers its impedance so that the voltage applied to the liquid crystal layer becomes strong enough to drive the liquid crystal at the corresponding portion. However, in this structure, since the impedance of the photoconductive layer is in the similar magnitude as that of the liquid crystal layer as stated above, the ratio of a voltage applied to the liquid crystal layer between the dark condition and the bright condition (on/off voltage ratio) is too small to obtain a high contrast image. To overcome this shortcoming, the photoconductive layer (a-Si:H layer) may be composed to have Schottky structure or diode structure. The use of such a structure leads to the reduction of the voltage applied to the liquid crystal layer in the dark condition where the impedance of the photoconductive layer is made high since it is reverse-biased. It results in enlarging an on/off voltage ratio of the liquid crystal layer between the bright condition and the dark condition. However, this structure has the drawback of applying d.c. voltage component to the liquid crystal. This application of the d.c. voltage to the liquid crystal cell often results in decomposing the liquid crystal material itself and causing attraction of ion components contained in the liquid crystal to the glass substrate, thereby causing disorder of the molecular orientation and degradation of the characteristic of the liquid crystal. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a liquid crystal element of optical writing type which is capable of restricting the application of the d.c. voltage component to the liquid crystal layer and at the same time, forming a high contrast image. The liquid crystal element of the present invention is defined by claim 1. The features of the preamble of claim 1 are known from WO 87/00642. An image display apparatus utilizing the liquid crystal element is defined by claim 12. In operation, when a ray of light is incident to the liquid crystal element of optical writing type, the photoconductive layer serves to change its impedance depending on the amount of the incident light. Hence, since a voltage is applied between the electrodes of the liquid crystal element, the voltage can be variably applied to the liquid crystal layer between the transparent substrates depending on the change of the impedance of the photoconductive layer. This results in changing the orientation state of the liquid crystal to form an image. That is, since the photoconductive layer has the back-to-back diode structure, the impedance on the reverse-biased portion of the diode structure is higher than that of the liquid crystal layer, in the dark condition. This means that the voltage is hardly applied to the liquid crystal layer in this dark condition. On the other hand, in the bright condition, the photoconductive layer lowers its impedance by the photoelectric effect, so that the impedance of the photoconductive layer becomes much lower than that of the liquid crystal layer. Thus, most of the voltage applied between the electrodes is in fact applied to the liquid crystal layer, thereby changing the orientation state of the liquid crystal. Accordingly, the voltage applied to the liquid crystal layer keeps a sufficiently large on/off voltage ratio for the bright and dark conditions, and further, the applied voltage hardly provides a difference due to the polarity in case that the alternating voltage is applied between the electrodes. This results in sufficiently bringing about the optical change of the liquid crystal layer, thereby making it possible to form a high contrast image and effectively restrict the application of the d.c. voltage component to the liquid crystal layer. According to the present invention, the photoconductive layer in the back-to-back diode structure is constructed to include a junction with a wider optical band gap at the image-writing side than that at the other side. For this purpose, the photoconductive layer may be composed of materials wherein the optical gap formed on the image-writing light side, by use of hetero junction for example, is wider than the optical gap formed on the other side. Thus, the writing light can be entered so deeply that the current-voltage characteristic of the back-to-back diode structure is made symmetric with respect to both of the polarities, resulting in applying a symmetric alternating voltage to the liquid crystal layer. To obtain such a symmetric current-voltage characteristic with respect to both polarities, in the back-to-back diode structure, the image-writing light side may be made from, for example, hydrogenated amorphous silicon carbide having a wider optical gap than hydrogenated amorphous silicon, while the other side is made from hydrogenated amorphous silicon. Accordingly, in case that the symmetric alternating electric voltage is applied between the electrodes, substantially no d.c. voltage component is applied to the liquid crystal layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a liquid crystal element of optical writing type (liquid crystal light valve) as a preferred embodiment according to the present invention; Fig. 2, composed of Figs. 2a to 2c, are charts each showing a voltage waveform applied to a liquid crystal layer of the liquid crystal element of Fig. 1 when it is operated with an alternating electric voltage; Fig. 3 is a graph showing a current-voltage characteristic of a photoconductive layer of the liquid crystal element of Fig. 1; Fig. 4 is a schematic view of a liquid crystal display system of projection type in which the liquid crystal element of Fig. 1 is utilized; and Fig. 5 is a diagram of an optical system for writing the liquid crystal element of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples useful for an understanding of the present invention and preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. The liquid crystal light valve as the preferred embodiment includes a photoconductive layer formed in a back-to-back diode structure, in which the diodes are connected in series with reverse polarity, in principle. This kind of photoconductive layer may be made from amorphous materials or crystalline materials. EXAMPLE 1 Fig. 1 is a sectional view showing a liquid crystal light valve 1, which includes a pair of glass substrates 2a and 2b having reflection-preventing films 1a, 1b respectively formed thereon. Transparent electrodes 3a and 3b are disposed on the substrates 2a and 2b, respectively. Each of the transparent electrodes 3a and 3b has a multiple layer structure including layers of ITO (indium tin oxide) transparent conductive films and SnO₂ (tin oxide) transparent conductive films, and is formed by means of a sputtering method. Alternatively, each of the transparent electrodes 3a and 3b may be composed of a single layer of ITO or SnO₂. The transparent electrode 3b may be composed of a metallic semi-transparent film made from Mo, Al, Ta and so forth. A photoconductive layer 4 is disposed on the transparent electrode 3b. The photoconductive layer 4 is made from hydrogenated amorphous silicon (a-Si:H) and formed in a back-to-back diode structure. As shown in an expanded portion of Fig. 1, the photoconductive layer 4 has a five-layer structure composed of films 41 to 45. Concretely, the photoconductive layer 4 is composed of a p-type a-Si:H film 41, an i-type a-Si:H film 42, an n-type a-Si:H film 43, an i-type a-Si:H film 44, and a p-type a-Si:H film 45. The formation of the photoconductive layer 4 is implemented by the following process. The p-type a-Si:H film 41 is formed under the conditions of a substrate temperature of 250°C and a pressure of 0.2 Torr by a plasma CVD method using the mixture of 100 sccm of silane (SiH₄) gas, 200 sccm of hydrogen (H₂) gas and 0.2 sccm of diborane (B₂H₆) gas serving as doping gas. The thickness of the p-type a-Si:H film 41 is preferably in the range of 100 to 500 Å (Angstrom). The most preferable thickness is about 200 Å. The i-type a-Si:H film 42 is formed on the p-type a-Si:H film 41. The i-type a-Si:H film 42 is formed by using the mixture of 100 sccm of silane (SiH₄) gas and 200 sccm of hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 42 is preferably in the range of 0.4 to 2.5 µm. The most preferable thickness is about 0.8 µm. Then, the n-type a-Si:H film 43 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 200 sccm of hydrogen (H₂) gas and 0.1 sccm of phosphine (PH₃) gas serving as doping gas. The thickness of the n-type a-Si:H film 43 is preferably in the range of 100 to 500 Å. The most preferable thickness is about 350 Å. Then, the i-type a-Si:H film 44 is formed by using the mixture of 100 sccm of silane (SiH₄) gas and 200 sccm of hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.9 µm. The p-type a-Si:H film 45 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 200 sccm of hydrogen (H₂) gas and 0.2 sccm of diborane (B₂H₆) gas serving as doping gas. The thickness of the p-type a-Si:H film 45 is preferably in the range of 100 to 700 Å. The most preferable thickness is about 250 Å. In forming the films, the preferable temperature is 200 °C to 300 °C but room temperature is also allowable. The glass substrate 2b is made, for example, of 7059 glass manufactured by the Corning Company. However, it goes without saying that the glass substrate may be made of another kind of glass. Further, the mixing ratios of the gases are not limited to the foregoing values. The mixing ratios of the gases can be selected so as to obtain a desired characteristic. A light-screening layer 5 is disposed on the photoconductive layer 4. The light-screening layer 5 is formed by means of a spin coating technique using carbon dispersed type black acrylic resin. The thickness of the light-screening layer 5 is about 1 µm. This light-screening layer 5 may employ an inorganic material such as praseodymium manganese oxide (PrMnO₃) and cermet. Then, a dielectric mirror layer 6 is disposed on the light-screening layer 5. The dielectric mirror layer 6 has a multiple layer structure including silicon films and silicon dioxide films and is formed by means of an electron beam vaporization technique. The dielectric mirror layer 6 may employ as raw materials zirconia oxide (ZrO₂) and magnesium fluoride (MgF₂). Orientation films 7a and 7b are disposed on the transparent electrode 3a and the dielectric mirror layer 6 respectively. Each of the orientation films 7a and 7b is made of polyimide film which is formed by means of a spin coating technique and is applied with a molecular orientation process by means of a rubbing technique. The glass substrates 2a and 2b are attached together through spacer 8. A liquid crystal layer 9 is disposed between the orientation films 7a and 7b. The liquid crystal layer 9 is formed by pouring and sealing a composite nematic liquid crystal. The thickness of the cell of the liquid crystal is about 6 µm. In the operation of the liquid crystal light valve 1, a hybrid electric field mode is used as the operation mode of the light valve 1. An a.c. power source 10 serves to apply a pure alternating voltage between the transparent electrodes 3a and 3b included in the liquid crystal light valve 1. When a ray of light 11 for writing is incident to the liquid crystal light valve 1 from the side of the glass substrate 2b, the photoconductive layer 4 serves to lower its impedance in an area where the light 11 is incident (bright condition) so that the alternating voltage is applied by the power source 10 to the liquid crystal layer 9. On the area where no light is incident (dark condition), on the other hand, the photoconductive layer 4 serves to keep its impedance high at the junction portion where the alternating electric voltage is to be reversely biased i.e. its impedance is kept higher than the impedance of the liquid crystal at the other portion by 1 or 2 digits, so that only a lower voltage than a threshold voltage is applied to the liquid crystal layer 9. The difference of the impedances between the bright condition and the dark condition results in forming an image. Fig. 2 show voltage waveforms to be applied to the liquid crystal in the bright condition and the dark condition. As shown, Fig. 2a denotes a voltage applied between the electrodes 3a and 3b, Fig. 2b denotes a voltage applied to the liquid crystal in the bright condition, and Fig. 2c denotes a voltage applied to the liquid crystal in the dark condition. The graphs indicate that the positive and negative voltages are applied to the liquid crystal in the substantially same manner in the bright condition and the dark condition without applying d.c. voltage components. In operation, when the ray of the light 11 is incident to the liquid crystal light valve 1, the photoconductive layer 4 serves to change its impedance depending on the amount of the incident light 11. Since the photoconductive layer 4 has the back-to-back diode structure, the impedance on the reverse-biased portion of the diode structure is much higher than that of the liquid crystal layer 9, in the dark condition. This means the voltage is hardly applied to the liquid crystal layer 9 in the dark condition, In the bright condition, the photoconductive layer 4 lowers its impedance so that most of the voltage applied between the electrodes 3a, 3b is applied to the liquid crystal layer 9, thereby changing the orientation state of the liquid crystal. Hence, the voltage applied to the liquid crystal keeps a sufficiently large on/off voltage ratio for the bright and dark conditions and the applied voltage hardly provides a difference due to the polarity of the alternating voltage. This results in sufficiently bringing about the optical change of the liquid crystal layer 9, thereby making it possible to form a high contrast image and effectively preventing the application of d.c. voltage component to the liquid crystal layer 9. Fig. 4 shows a liquid crystal display system of projection type in which the liquid crystal light valve 1 is utilized. In Fig. 4, a CRT 61 is used to write an image on the liquid crystal light valve 1. For projecting the image written on the liquid crystal light valve 1, the lamp 62 emits a ray of light to the liquid crystal light valve 1 through the lens 63 and the polarization beam splitter 64. The incident ray of light from the lamp 62 is reflected on the dielectric mirror 6 through the liquid crystal layer 9. In case the reflected light is passed through the portion of the liquid crystal layer 9 where the orientation state is changed, the reflected light changes its polarizing direction by the electro-optical effect of that portion of the liquid crystal, so that the reflected light is allowed to pass through the polarization beam splitter 64. The reflected light is expanded through the lens 65, resulting in allowing the image written on the liquid crystal light valve 1 to be projected onto the screen 66. The light-screening layer 5 is provided because it is difficult to completely reflect the white light which is emitted from the lamp 62 by the dielectric mirror 6 and which may influence the photoconductive layer 4. However, if the incident light is completely reflected off by the dielectric mirror 6, the light-screening layer 5 is unnecessary. For example, in case of using a coherent beam source such as a laser in place of the lamp 62, the light screening layer 5 can be omitted in the present embodiment. In place of the CRT, it is possible to use another means for writing an image, such as a plasma display, a LED display or a laser device. The present invention is characterized by the nature of the photoconductive layer 4. The following description will concern mainly the photoconductive layer 4. EXAMPLE 2 In this example, the photoconductive layer 4 included in the liquid crystal light valve 1 shown in Fig. 1 is formed in a back-to-back diode structure made from hydrogenated amorphous silicon. The photoconductive layer 4 is composed of a p-type a-Si:H film 41, an i-type a-Si:H film 42, an n-type a-Si:H film 43, an i-type a-Si:H film 44, and a p-type a-Si:H film 45. The formation of the photoconductive layer 4 is implemented by the following process. The p-type a-Si:H film 41 is formed by a plasma CVD method using silane (SiH₄) gas, hydrogen (H₂) gas and doping gas (B₂H₆). The thickness of the p-type a-Si:H film 41 is preferably in the range of 100 to 500 Å. The most preferable thickness is about 200 Å. The i-type a-Si:H film 42 is formed on the p-type a-Si:H film 41 by using silane (SiH₄) gas and hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 42 is preferably in the range of 0. 4 to 2. 5 µm. The most preferable thickness is about 0. 5 µm. Then, the n-type a-Si:H film 43 is formed by using silane (SiH₄) gas, hydrogen (H₂) gas and doping gas (PH₃). The thickness of the n-type a-Si:H film 43 is preferably in the range of 100 to 500 Å. The most preferable thickness is about 350 Å. Then, the i-type a-Si:H film 44 is formed by using silane (SiH₄) gas and hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.6 µm. The p-type a-Si:H film 45 is formed by using silane (SiH₄) gas, hydrogen (H₂) gas and doping gas (B₂H₆). The thickness of the p-type a-Si:H film 45 is preferably in the range of 100 to 700 Å. The most preferable thickness is about 250 Å. The light-screening layer 5 is formed on the photoconductive layer 4 by using carbon dispersed type black acrylic resin by means of a spin coating technique, so that the thickness of the light-screening layer 5 is about 1 µm. Then the dielectric mirror 6 is formed on the light-screening layer 5 so as to have multiple layer structure including films of silicon oxide and titanium oxide by means of an electron beam vaporization technique. Next, the orientation films 7a and 7b are formed on the dielectric mirror 6 and the transparent electrode 3a respectively. The orientation films 7a and 7b are made of silicon oxide and obliquely deposited by means of the electron beam vaporization technique. The glass substrates 2a and 2b are attached to each other through the spacer 8. The liquid crystal layer 9 is formed between the orientation films 7a and 7b by pouring and sealing a ferroelectric liquid crystal CS-1014 manufactured by the Chisso Company. The thickness of the cell is about 2 µm. Fig.5 is a diagram showing an optical writing system which can be utilized in place of the CRT 61 of Fig. 4. The laser beam emitted from a laser 31 is scanned through a polygon mirror 32 and a galvano mirror 33 controlled by a control circuit 34 so that an image is written on the liquid crystal light valve 1. The liquid crystal light valve 1 is,driven by a power source 35 operated in synchronization with the control circuit 34. The polarity of the voltage applied to the liquid crystal light valve 1 in writing an image is opposite to the polarity of the voltage applied in erasing an image. The embodiment 2 has thinner i-type a-Si:H layer 42 and i-type a-Si:H layer 44 than the embodiment 1 in order to suit the ferroelectric liquid crystal. EXAMPLE 3 In this example, the photoconductive layer 4 included in the liquid crystal light valve 1 shown in Fig. 1 is formed in a back-to-back diode structure and is made from hydrogenated amorphous silicon. This photoconductive layer 4 is composed of an n-type a-Si:H film 41, an i-type a-Si:H film 42, a p-type a-Si:H film 43, an i-type a-Si:H film 44, and an n-type a-Si:H film 45. The formation of the photoconductive layer 4 is implemented by the following process. The n-type a-Si:H film 41 is formed under the conditions of a substrate temperature of 300°C and a pressure of 0.15 Torr by a plasma CVD method using the mixture of 80 sccm of silane (SiH₄) gas, 240 sccm of hydrogen (H₂) gas and 0.1 sccm of doping gas (PH₃). The thickness of the n-type a-Si:H film 41 is preferably in the range of 100 to 500 Å. The most preferable thickness is about 250 Å. The i-type a-Si:H film 42 is formed on the n-type a-Si:H film 41. The i-type a-Si:H film 42 is formed by using the mixture of 80 sccm of silane (SiH₄) gas and 240 sccm of hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 42 is preferably in the range of 0.4 to 2.5 µm. The most preferable thickness is about 0.8 µm. Then, the p-type a-Si:H film 43 is formed by using the mixture of 80 sccm of silane (SiH₄) gas, 240 sccm of hydrogen (H₂) gas and 0.15 sccm of doping gas (B₂H₆). The thickness of the p-type a-Si:H film 43 is preferably in the range of 100 to 500 Å. The most preferable thickness is about 200 Å. Then, the i-type a-Si:H film 44 is formed by using the mixture of 80 sccm of silane (SiH₄) gas and 240 sccm of hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.9 µm. The n-type a-Si:H film 45 is formed by using the mixture of 80 sccm of silane (SiH₄) gas, 240 sccm of hydrogen (H₂) gas and 0.1 sccm of doping gas (PH₃). The thickness of the i-type a-Si:H film 45 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 250 Å. It goes without saying that the mixing ratios of gases are not limited to the foregoing values and the mixing ratios of gases may be selected so as to obtain a desired characteristic. The other structure of the example 3 is the same as that of the example 1. As described above, the foregoing examples 1 to 3 have the photoconductive layer 4 made from the a-Si:H material. According to the characteristic of the a-Si:H material, the incident light tends to be attenuated along the passage of the light through the photoconductive layer 4. Thus, the application of the alternating voltage causes the voltage-current characteristic to be slightly different between the + and - polarities of the voltage. When an alternating voltage is applied between the electrodes 3a and 3b, however, the d.c. voltage component applied to the liquid crystal is negligible, because the photoconductive layer 4 has much smaller impedance than the liquid crystal layer 9 by about 2 to 3 digits. In accordance with the invention, such a shortcoming as for the voltage-current characteristic is overcome by forming the photoconductive layer 4 to have a larger optical band gap on the writing side than that on the other side. The following embodiment employs the photoconductive layer 4 to satisfy this condition. EMBODIMENT 4 In this embodiment, the photoconductive layer 4 is formed in a back-to-back diode structure including a hetero junction made from hydrogenated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous silicon (a-Si:H). The photoconductive layer 4 is composed of a p-type a-SiC:H film 41, an i-type a-SiC:H film 42, an n-type a-SiC:H film 43, an i-type a-Si:H film 44, and a p-type a-Si:H film 45. The formation of the photoconductive layer 4 is implemented by the following process. The p-type a-SiC:H film 41 is formed under the conditions of a substrate temperature of 300°C and a pressure of 0.4 Torr by a plasma CVD method using the mixture of 100 sccm of silane (SiH₄) gas, 250 sccm of hydrogen (H₂) gas, 60 sccm of methane (CH₄) gas, and 0.3 sccm of diborane (B₂H₆) gas serving as doping gas. The thickness of the p-type a-SiC:H film 41 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 300 Å. The i-type a-SiC:H film 42 is formed on the p-type a-SiC:H film 41. The i-type a-SiC:H film 42 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 250 sccm of hydrogen (H₂) gas and 20 sccm of methane (CH₄) gas. The thickness of the i-type a-SiC:H film 42 is preferably in the range of 0.4 to 2.0 µm. The most preferable thickness is about 0.5 µm. Then, the n-type a-SiC:H film 43 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 250 sccm of hydrogen (H₂) gas, 20 sccm of methane (CH₄) gas, and 0.4 sccm of phosphine (PH₃) gas serving as doping gas. The thickness of the n-type a-SiC:H film 43 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 400 Å. Then, the i-type a-Si:H film 44 is formed by using the mixture of 100 sccm of silane (SiH₄) gas and 200 sccm of hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.5 µm. The p-type a-Si:H film 45 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 200 sccm of hydrogen (H₂) gas and 0.2 sccm of diborane (B₂H₆) gas serving as doping gas. The thickness of the p-type a-Si:H film 45 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 300 Å. In forming the films, the preferable temperature is 200 °C to 350 °C but room temperature is also allowable. The glass substrate 2b may be made of 7059 glass manufactured by the Corning Company. However, it goes without saying that the glass substrate may be made of another kind of glass. Further, the mixing ratios of gases are not limited to the foregoing values. The mixing ratios of gases may be selected so as to obtain a desired characteristic. In this embodiment, the hetero junction is formed between the n-type a-SiC:H film 43 and the i-type a-Si:H film 44. The photoconductive layer 4 in the next embodiment to be described below, is formed in a back-to-back diode structure including the hetero junction between the hydrogenated amorphous silicon carbide (a-SiC:H) and the hydrogenated amorphous silicon (a-Si:H). EMBODIMENT 5 In this embodiment, the photoconductive layer 4 is composed of an n-type a-SiC:H film 41, an i-type a-SiC:H film 42, a p-type a-SiC:H film 43, an i-type a-Si:H film 44, and an n-type a-Si:H film 45. The hetero junction is formed between the p-type a-SiC:H film 43 and the i-type a-Si:H film 44. As a method of forming the films, the n-type a-SiC:H film 41 is formed under the conditions of a substrate temperature of 250°C and a pressure of 0.3 Torr by a plasma CVD method using the mixture of 50 sccm of silane (SiH₄) gas, 300 sccm of hydrogen (H₂) gas, 20 sccm of methane (CH₄) gas and 0.1 sccm of doping gas (PH₃). The thickness of the n-type a-SiC:H film 41 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 400 Å. The i-type a-SiC:H film 42 is formed on the n-type a-SiC:H film 41. The i-type a-SiC:H film 42 is formed by using the mixture of 50 sccm of silane (SiH₄), 300 sccm of hydrogen (H₂) gas and 10 sccm of methane (CH₄) gas. The thickness of the i-type a-SiC:H film 42 is preferably in the range of 0.4 to 2.0 µm. The most preferable thickness is about 0.5 µm. Then, the p-type a-SiC:H film 43 is formed by using the mixture of 50 sccm of silane (SiH₄) gas, 300 sccm of hydrogen (H₂) gas, 10 sccm of methane (CH₄) gas and 0.1 sccm of doping (B₂H₆) gas. The thickness of the p-type a-SiC:H film 43 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 250 Å. Then, the i-type a-Si:H film 44 is formed by using the mixture of 80 sccm of silane (SiH₄) gas and 240 sccm of hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.6 µm. The n-type a-Si:H film 45 is formed by using the mixture of 80 sccm of silane (SiH₄) gas, 240 sccm of hydrogen (H₂) gas and 0.1 sccm of doping (PH₃) gas. The thickness of the n-type a-Si:H film 45 is preferably in the range of 100 to 700 Å . The most preferable thickness is about 300 Å. It goes without saying that the mixing ratios of gases are not limited to the foregoing values and the mixing ratios of gases may be selected so as to obtain a desired characteristic. The other structure of the embodiment 5 is the same as that of the example 1. In the conditions for forming the films 41 to 45 of the photoconductive layer 4, the mixing ratios of gases are just required to be selected for obtaining a desired characteristic Thus, the following embodiment leaves out the illustration of the mixing ratios. EMBODIMENT 6 In this embodiment, the photoconductive layer 4 is composed of an n-type a-SiC:H film 41, an i-type a-SiC:H film 42, a p-type a-Si:H film 43, an i-type a-Si:H film 44, and an n-type a-Si:H film 45. The hetero junction is formed between the i-type a-SiC:H film 42 and the p-type a-Si:H film 43. As a method for forming the films, the n-type a-SiC:H film 41 is formed by a plasma CVD method using the mixture of silane (SiH₄) gas, hydrogen (H₂) gas, methane (CH₄) gas, and doping gas (PH₃). The thickness of the n-type a-SiC:H film 41 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 400 Å. The i-type a-SiC:H film 42 is formed on the n-type a-SiC:H film 41 by using the mixture of silane (SiH)₄ gas, hydrogen (H₂) gas and methane (CH₄) gas. The thickness of the i-type a-SiC:H film 42 is preferably in the range of 0.4 to 2.0 µm. The most preferable thickness is about 0.5 µm. Then, the p-type a-Si:H film 43 is formed by using the mixture of silane (SiH₄) gas, hydrogen (H₂) gas and doping gas (B₂H₆). The thickness of the p-type a-Si:H film 43 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 250 Å. Then, the i-type a-Si:H film 44 is formed by using the mixture of silane (SiH₄) gas and hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0. 6 µm. The n-type a-Si:H film 45 is formed by using the mixture of silane (SiH₄) gas, hydrogen (H₂) gas and doping gas (PH₃). The thickness of the n-type a-Si:H film 45 is preferably in the range of 100 to 700 Å. The most preferable thickness is about 300 Å. EMBODIMENT 7 The photoconductive layer 4 is composed of an n-type a-SiC:H film 41, an i-type a-SiC:H film 42, a p-type a-Si:H film 43, an i-type a-Si:H film 44, and an n-type a-Si:H film 45. The hetero junction is formed between the i-type a-SiC:H film 42 and the p-type a-Si:H film 43. As a method for forming the films, the n-type a-SiC:H film 41 is formed by a reactive sputtering method. Namely,the n-type a-SiC:H film 41 is formed by sputtering argon (Ar) to a silicon target in the introduced reactive gases such as hydrogen (H₂) gas, methane (CH₄) gas, and phosphine (PH₃) gas serving as doping gas. The thickness of the n-type a-SiC:H film 41 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 400 Å. The i-type a-SiC:H film 42 is formed on the n-type a-SiC:H film 41 by a reactive sputtering method. Namely, the i-type a-SiC:H film 42 is formed by sputtering argon (Ar) to a silicon target in the reactive gases such as hydrogen (H₂) gas and methane (CH₄) gas. The thickness of the i-type a-SiC:H film 42 is preferably in the range of 0.4 to 2.0 µm. The most preferable thickness is about 0.5 µm. Then, the p-type a-Si:H film 43 is formed by a plasma CVD method using the mixture of silane (SiH₄) gas, hydrogen (H₂) gas and diborane (B₂H₆) gas. The thickness of the p-type a-Si:H film 43 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 250 Å. Then, the i-type a-Si:H film 44 is formed by a plasma CVD method using the mixture of silane (SiH₄) gas and hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.6 µm. The n-type a-Si:H film 45 is formed by using the mixture of silane (SiH₄) gas, hydrogen (H₂) gas and doping gas (PH₃). The thickness of the n-type a-Si:H film 45 is preferably in the range of 100 to 700 Å. The most preferable thickness is about 300 Å. The other structure of the embodiment 7 is the same as that of the example 1. EMBODIMENT 8 The photoconductive layer 4 is composed of an n-type a-SiC:H film 41, an i-type a-Si:H film 42, a p-type a-Si:H film 43, an i-type a-Si:H film 44, and an n-type a-Si:H film 45. The hetero junction is formed between the n-type a-SiC:H film 41 and the i-type a-Si:H film 42. As a method for forming the films, the n-type a-SiC:H film 41 is formed by a plasma CVD method using the mixture of disilane (Si₂H₆) gas, hydrogen (H₂) gas, ethylene (C₂H₄) gas, and doping gas (PH₃). The thickness of the n-type a-SiC:H film 41 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 400 Å. The i-type a-Si:H film 42 is formed on the n-type a-SiC:H film 41 by using the mixture of disilane (Si₂H₆) gas and hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 42 is preferably in the range of 0.3 to 2.5 µm. The most preferable thickness is about 0.5 µm. Then, the p-type a-Si:H film 43 is formed by using the mixture of disilane (Si₂H₆) gas, hydrogen (H₂) gas and doping gas (B₂H₆). The thickness of the p-type a-Si:H film 43 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 250 Å. Then, the i-type a-Si:H film 44 is formed by using the mixture of disilane (Si₂H₆) gas and hydrogen (H₂) gas. The thickness of the i-type a-Si:H film 44 is preferably in the range of 0. 4 to 3. 0 µm. The most preferable thickness is about 0. 6 µm. The n-type a-Si:H film 45 is formed by using the mixture of disilane (Si₂H₆) gas, hydrogen (H₂) gas and doping gas (PH₃). The thickness of the n-type a-Si:H film 45 is preferably in the range of 100 to 700 Å. The most preferable thickness is about 300 Å. The other structure of the embodiment 8 is the same as that of the example 1. In the foregoing embodiments 5 to 8, the formation of the hetero junction at respective locations, are explained. Likewise, in the embodiment 4, the hetero junction may be shifted to the writing side further than the location between the films 43 and 44. To form a larger optical band gap on the writing side than the other side in the photoconductive layer 4, it is possible to use the characteristic that the hydrogenated amorphous silicon carbide has a larger optical band gap than the hydrogenated amorphous silicon like the embodiments 4 to 8, as well as the characteristic that the hydrogenated amorphous silicon germanium has a smaller optical band gap than the hydrogenated amorphous silicon. Thus, in the foregoing examples and embodiments, at least one of the films 43, 44, 45 may be formed from the hydrogenated amorphous silicon germanium so as to utilize this latter characteristic. More concretely, in the examples 1 and 2, at least one of the n-type a-Si:H film 43, the i-type a-Si:H film 44 and the p-type a-Si:H film 45 may be made from the amorphous silicon germanium, for example. For making the film, the mixed gas of the silane (SiH₄) or the disilane (Si₄H₆) and the germane (GeH₄ or Ge₂H₆) may be used. Further, the combination of this method with the embodiments 4 to 8 may be allowed. As the material providing a wider band gap, hydrogenated amorphous silicon nitride (a-SiN:H), the a-SiN:H with carbon added thereto, and hydrogenated amorphous silicon oxide (a-SiO:H) may be utilized in addition to the hydrogenated amorphous silicon carbide (a-SiC:H). As the materials providing a smaller band gap, hydrogenated amorphous silicon tin (a-SiSn:H) may be utilized in addition to the hydrogenated amorphous silicon germanium (a-SiGe:H). Here, the combination of these materials enables formation of a larger optical band gap at the writing side in the photoconductive layer 4. The above-mentioned embodiments employ the hetero junction for providing a wider optical band gap on the writing side than the other side in the photoconductive layer 4. Instead, a homo junction may be used with a changed adjusting ratio of the materials, for example, a changed carbon amount. The following embodiment 9 employs this homo junction method. EMBODIMENT 9 In this embodiment, the photoconductive layer 4 is formed in a back-to-back diode structure made from the hydrogenated amorphous silicon carbide (a-SiC:H). The photoconductive layer 4 is composed of a p-type a-SiC:H film 41, an i-type a-SiC:H film 42, an n-type a-SiC:H film 43, an i-type a-SiC:H film 44, and a p-type a-SiC:H film 45. As a method for forming the films, the p-type a-SiC:H film 41 is formed under the conditions of a substrate temperature of 300°C and a pressure of 0.3 Torr by a plasma CVD method using the mixture of 100 sccm of silane (SiH₄) gas, 200 sccm of hydrogen (H₂) gas, 120 sccm of methane (CH₄) gas and 0.5 sccm of doping gas (B₂H₆). The thickness of the p-type a-SiC:H film 41 is preferably in the range of 100 to 500 Å. The most preferable thickness is about 200 Å. The i-type a-SiC:H film 42 is formed on the p-type a-SiC:H film 41 by using the mixture of 100 sccm of silane (SiH₄) gas, 100 sccm of methane (CH₄) gas, and 200 sccm of hydrogen (H₂) gas. The thickness of the i-type a-SiC:H film 42 is preferably in the range of 0.3 to 2.5 µm. The most preferable thickness is about 0.6 µm. Then, the n-type a-SiC:H film 43 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 100 sccm of methane (CH₄) gas, 200 sccm of hydrogen (H₂) gas and 0.5 sccm of doping gas (PH₃). The thickness of the n-type a-SiC:H film 43 is preferably in the range of 100 to 600 Å. The most preferable thickness is about 300 Å. Then, the i-type a-SiC:H film 44 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 50 sccm of methane (CH₄) gas and 200 sccm of hydrogen (H₂) gas. The thickness of the i-type a-SiC:H film 44 is preferably in the range of 0.4 to 3.0 µm. The most preferable thickness is about 0.7 µm. The p-type a-SiC:H film 45 is formed by using the mixture of 100 sccm of silane (SiH₄) gas, 50 sccm of methane (CH₄) gas, 200 sccm of hydrogen (H₂) gas and 0.5 sccm of doping gas (B₂H₆). The thickness of the p-type a-SiC:H film 45 is preferably in the range of 100 to 700 Å. The most preferable thickness is about 300 Å. As stated above, the amount of the used methane gas is different between the films 41 and 42, and between the films 43 and 44. More methane gas results in providing more carbon contained in the film, thereby making an optical band gap larger. It goes without saying that in forming the films, the mixing ratios of the gases are not limited to the foregoing values and the mixing ratios of gases may be selected so as to obtain a desired characteristic. To provide the large optical band gap on the writing side, at least one film is required to contain a greater amount of carbon on the writing side of the film. Like the carbon, it is just necessary to adjust the amount of the other materials whose optical band gap is to be adjusted for the purpose of providing the larger optical band gap on the writing side than on the other side. Further, the homo junction is just required to apply to at least one of the junctions between the adjacent ones of the films 41 to 44. Next, an example using a crystalline photoconductive layer 4 will be described below. EXAMPLE 10 In this example, the photoconductive layer 4 is formed in a back-to-back diode structure made from polycrystalline silicon (poly-Si). The photoconductive layer 4 has three-layer structure composed of films 41, 43, and 45. Concretely, the photoconductive layer 4 is composed of an n-type poly-Si film 41, a p-type poly-Si film 43, and an n-type poly-Si film 45. As a method for forming the films, the n-type poly-Si film 41 is formed by a decompression heat CVD method using silane (SiH₄) gas and doping gas (PH₃). The temperature for forming the film is preferably in the range of 600 to 650°C. The most preferable temperature is about 620°C. Hence, the glass substrate 2b is preferably made of a heat resistant material, for example, quartz glass. The thickness of the film 41 is preferably in the range of 0.2 to 2.6 µm. The most preferable thickness is about 0.5 µm. Then, the p-type poly-Si film 43 is formed by using silane (SiH₄) gas and doping gas (B₂H₆). The thickness of the film 43 is preferably in the range of 0.3 to 3.2 µm. The most preferable thickness is about 1.2 µm. Next, the n-type poly-Si film 45 is formed by using silane gas (SiH₄) and doping gas (PH₃). The thickness of the film 45 is preferably in the range of 0.1 to 2.0 µm. The most preferable thickness is about 0.6 µm. For implementing the doping to the poly-Si material, it is possible to use an ion plantation technique for doping after forming the film in place of the introduction of the doping gas while forming the film. EXAMPLE 11 In this example, the photoconductive layer 4 is formed in a back-to-back diode structure made from polycrystalline silicon (poly-Si). The photoconductive layer 4 is composed of three films, i.e. a p-type poly-Si film 41, an n-type poly-Si film 43, and a p-type poly-Si film 45. As a method for forming the films, the p-type poly-Si film 41 is formed by a decompression heat CVD method using silane (SiH₄) gas and doping gas (B₂H₆). The temperature for forming the film is preferably in the range of 600 to 650°C. The most preferable temperature is about 620°C. The thickness of the film 41 is preferably in the range of 0.3 to 3.0 µm. The most preferable thickness is about 0.4 µm. Then, the n-type poly-Si film 43 is formed by using silane (SiH₄) gas and doping gas (PH₃). The proper thickness of the film 43 is in the range of 0.4 to 3.0 µm. The most preferable thickness is about 1.0 µm. Next, the p-type poly-Si film 45 is formed by using silane (SiH₄) gas and doping gas (B₂H₆). The thickness of the film 45 is preferably in the range of 0.2 to 2.0 µm. The most preferable thickness is about 0.5 µm. For implementing the doping to the poly-Si material, it is possible to use an ion plantation technique for doping after forming the film in place of the introduction of the doping gas while forming the film. EXAMPLE 12 In this example, the photoconductive layer 4 is formed in a back-to-back diode structure made from polycrystalline silicon (poly-Si). The photoconductive layer 4 has five-layer structure composed of films 41, 42, 43, 44 and 45. Concretely, the photoconductive layer 4 is composed of a p-type poly-Si film 41, an i-type poly-Si film 42, an n-type poly-Si film 43, an i-type poly-Si film 44, and a p-type poly-Si film 45. As a method for forming the films, the p-type poly-Si film 41 is formed by a decompression heat CVD method using silane (SiH₄) gas and doping gas (B₂H₆). The temperature for forming the film is preferably in the range of 600 to 650°C. The most preferable temperature is about 620°C. The thickness of the film 41 is preferably in the range of 0.05 to 1.0 µm. The most preferable thickness is about 0.1 µm. Then, the i-type poly-Si film 42 is formed by using silane (SiH₄) gas. The thickness of the film 42 is preferably in the range of 0.3 to 2.2 µm. The most preferable thickness is about 0. 8 µm. Then, the n-type poly-Si film 43 is formed by using silane (SiH₄) gas and doping gas (PH₃). The thickness of the film 43 is preferably in the range of 0.04 to 0.8 µm. The most preferable thickness is about 0.1 µm. Next, the i-type poly-Si film 44 is formed by using silane (SiH₄) gas. The thickness is preferably in the range of 0.3 to 2.5 µm. The most preferable thickness is about 0.9 µm. The p-type poly-Si film 45 is formed by using silane (SiH₄) gas and doping gas (B₂H₆). The thickness of the film 45 is preferably in the range of 0.05 to 1.0 µm. The most preferable thickness is about 0.1 µm. The other structure of the embodiment 12 is the same as that of the example 1. For implementing the doping to the poly-Si material, it is possible to use an ion plantation technique for doping after forming the film in place of the introduction of the doping gas while forming the film. The different respect of the present example 12 from the example 11 is the addition of the i-type poly-Si film for the purpose of improving a photoconductive effect. The example 10 may also be adapted to have the i-type poly-Si film. The foregoing embodiments have employed silicon as a material of the photoconductive layer. However, it goes without saying that the photoconductive layer 4 can be composed of germanium in place of silicon if a smaller optical band gap is required. In this case, germane and digermane may be used in place of silane and disilane. As a method of forming the photoconductive layer, a sputtering method, a heat CVD method, an optical CVD method and an ECR plasma method may be utilized in addition to the plasma CVD method. As a gas, Si₂H₆ may be used in place of SiH₄ and so also GeH₄ and GeH₆. Further, it is possible to use C₂H₆, C₃H₈, C₄H₁₀, C₂H₄, and C₂H₂ in addition to CH₄. The foregoing embodiments have employed silicon and germanium as main materials for forming the films. However, the present invention may employ a III-V family compound semiconductor system material such as GaAs, AlGaAs, or InP in place of silicon and germanium. As a liquid crystal display mode, it is possible to use a twisted nematic mode, an electric field inducing birefringence mode, a dynamic scattering mode, a guest host mode, and a phase transition effect mode in addition to the hybrid electric field effect mode employed in the foregoing embodiments, if the nematic liquid crystal is used. If the smectic liquid crystal is used, it is possible to use a birefringence mode, a guest host mode, and a light scattering mode. In addition, ferroelectric liquid crystal, polymeric liquid crystal, and a polymer dispersed liquid crystal film may also be used. Many widely different embodiments of the present invention may be constructed without departing from the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

Claims: A liquid crystal element (1) comprising: a pair of transparent substrates (2a,2b); a liquid crystal layer (9) disposed between said pair of transparent substrates (2a,2b); said pair of substrates (2a,2b) having electrodes (3a,3b) on their respective major surfaces which are nearer to said liquid crystal layer (9); a photoconductive layer (4) formed on one (2a) of said transparent substrates and having a back-to-back diode structure (41 to 45;41,43,45), said photoconductive layer being arranged such that its impedance is changed in dependence upon incident image-writing light (11) which enters the element (1) through said one transparent substrate (2a); and a mirror layer (6) arranged to reflect image-projecting light which enters said liquid crystal layer (9) through the other (2b) of said transparent substrates; characterised in that: said photoconductive layer (4) includes a junction having a wider optical band gap at the side on which said image-writing light (11) is incident than the optical band gap at the other side. A liquid crystal element according to claim 1, wherein said photoconductive layer (4) comprises: a first layer (41) of a p-type hydrogenated amorphous semiconductor, a second layer (42) of an i-type hydrogenated amorphous semiconductor, a third layer (43) of an n-type hydrogenated amorphous semiconductor, a fourth layer (44) of an i-type hydrogenated amorphous semiconductor, and a fifth layer (45) of a p-type hydrogenated amorphous semiconductor, said five layers being laminated in this order. A liquid crystal element according to claim 1, wherein said photoconductive layer (4) includes: a first layer (41) of an n-type hydrogenated amorphous semiconductor, a second layer (42) of an i-type hydrogenated amorphous semiconductor, a third layer (43) of a p-type hydrogenated amorphous semiconductor, a fourth layer (44) of an i-type hydrogenated amorphous semiconductor, and a fifth layer (45) of an n-type hydrogenated amorphous semiconductor, said five layers being laminated in this order. A liquid crystal element according to claim 2 or claim 3, wherein said five layers (41 to 45) are made of silicon, or germanium, as a main material. A liquid crystal element according to any of claims 2 to 4, wherein said junction comprises a hetero-junction formed between said first and second layers (41,42), between said second and third layers (42,43), between said third and fourth layers (43,44), or between said fourth and fifth layers (44,45). A liquid crystal element according to claim 5, wherein the pair of layers composing the hetero-junction comprises a layer of hydrogenated amorphous silicon carbide and a layer of hydrogenated amorphous silicon. A liquid crystal element according to claim 5, wherein one of the layers composing said hetero-junction comprises hydrogenated amorphous silicon nitride, hydrogenated amorphous silicon nitride with carbon added thereto, hydrogenated amorphous silicon oxide, hydrogenated amorphous silicon tin, or hydrogenated amorphous silicon germanium. A liquid crystal element according to claim 1, wherein said photoconductive layer (4) comprises silicon, germanium, or Group III-V compound semiconductor, as a main material. A liquid crystal element according to claim 1, wherein said junction comprises a hetero-junction or a homo-junction. A liquid crystal element according to claim 1, further comprising: a pair of reflection-preventing films (1a,1b) formed on the respective major surfaces of said pair of substrates (2a,2b) which are farther from said liquid crystal layer (9); a light-screening layer (5) formed between said mirror layer (6) and said photoconductive layer (4); and a pair of orientation films (7a,7b) formed on respective sides of said liquid crystal layer (9). A liquid crystal element according to claim 1, wherein said liquid crystal layer comprises nematic liquid crystal, smectic liquid crystal, polymeric liquid crystal, or polymer dispersed liquid crystal film. An image display apparatus comprising: a liquid crystal element (1) according to any preceding claim; means (61;31 to 34) for applying image-writing light (11) to the element through said one transparent substrate (2a); means (10) for applying an alternating voltage across said electrodes (3a,3b) of the element; and means (62,63,64) for projecting image-projecting light onto the element through the other substrate (2b) thereof, to be selectively reflected by said mirror layer (6) in accordance with conductivity variation of said photoconductive layer (4).

Inventors: HATANO AKITSUGU, KIMURA NAOFUMI, HATANO, AKITSUGU, KIMURA, NAOFUMI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: G02F1
IPC codes: G02F1