OPTICAL MODULATION DEVICE AND DRIVING METHOD THEREFOR

Abstract: An optical modulation device comprises a first substrate having thereon a first conductor film, a second substrate having thereon a second conductor film disposed opposite to the first conductor film, and an optical modulation material disposed between the first and second substrates. The optical modulation device is driven by forming a potential gradient along the extension of the first conductor film or both the first and second conductor films, and applying an information signal to the first or second conductor film.

Description: The present invention relates to an optical modulation apparatus for a display panel, more specifically a display panel using a liquid crystal material, especially a ferroelectric liquid crystal, showing at least two stable states, particularly a liquid crystal optical device adapted to gradational or tonal display. In the conventional liquid crystal television panel of the active matrix driving system, thin film transistors (TFTs) are arranged in matrix corresponding to respective pixels. When a gate-on pulse is applied to a TFT to turn on the source-drain channel, a picture image signal is applied to the source and stored in a capacitor. A liquid crystal (e.g., TN (twisted nematic) liquid crystal) is driven by the stored image signal and a gradational display is effected by voltage modulation of pixels. In the document US-A-4 367 924 there is disclosed an optical modulation apparatus which is comprising a first substrate having thereon a first conductor film in the form of stripes and a second substrate having thereon a second conductor film in the form of stripes which are disposed opposite to the first conductor film stripes. Between the two substrates, there is disposed an optical modulation material so as to form pixels which are defined by the intersections of the first and second conductor film stripes. By means of a voltage application means it is possible to apply voltages to the first and second conductor film stripes, which voltages are adapted to produce the respective modulation state of the pixels. However, it is difficult to control gradation modulation especially in the case of a very great number of pixels, that is to say in the case of a high pixel density. Therefore, it is the object of the present invention to improve an optical modulation apparatus of the above referenced type in such a way that gradation can be optimally controlled even in the case of a high pixel density. According to the present invention there is provided an optical modulation apparatus, comprising [a] an optical modulation device comprising [a1] a first substrate having thereon a first conductor film, [a2] a second substrate having thereon a second conductor film which is disposed opposite to said first conductor film, and [a3] an optical modulation material being disposed between said first and second substrate so as to form a pixel which is defined by the intersection of said first and second conductor film; and [b] voltage application means for applying voltages to said first and second conductor films; characterized in that said voltage application means comprises [c] first means for applying a voltage corresponding to an information signal to said first conductor film, [d] and second means for applying a voltage to said second conductor film in such a way that there is developed a potential gradient along said second conductor film, wherein the potential difference between said information signal voltage and a selectable part of said potential gradient is exceeding an inversion threshold voltage of said modulation material. Hence, according to the invention, the voltage application means comprises first means for applying a voltage to the first conductor film corresponding to an information signal. In combination with a second means, which is applying a voltage to the second conductor film in such a way that there is developed a potential gradient along the second conductor film wherein the potential difference between the information signal voltage and a selectable part of the potential gradient is exceeding an inversion threshold voltage of the modulation material, it is possible to easily produce differently sized parts of the respective pixels which have the one or the other state of the modulation material. The respective size of each part is proportional to the gradation of the pixel. By these measures it is possible to excellently control gradation even at a high pixel density. These features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. Figures 1 and 2 are schematic perspective views illustrating the operation principle of a ferroelectric liquid crystal device used in the present invention; Figure 3 is a partial perspective view of one side of a substrate used in the present invention; Figures 4A and 4B are explanatory views for schematically illustrating potential gradients used in the present invention; Figure 5 is a schematic sectional view of an optical modulation device according to the present invention; Figures 6 and Figures 7A - 7E show examples of signals used in the present invention, and Figures 8A - 8D show schematic sketches showing bright-to-dark gradational states of a pixel obtained correspondingly; Figure 9, Figures 10AA - 10AE and Figures 10BA - 10BE show other examples of pulse signal waveforms used in the present invention; Figure 11 is a schematic perspective view of another liquid crystal otpical device used in the present invention; Figures 12, 13 and 14 are schematic plan views showing other examples of matrix electrode arrangement used in the present invention. As an optical modulation material used in the driving method according to the present invention, a material which shows a first optically stable state (e.g., assumed to form a "bright" state) and a second optically stable state (e.g., assumed to form a "dark" state) depending on an electric field applied thereto, i.e., one showing at least two stable states in response to an electric field, particularly a liquid crystal showing such a property, may be used. Preferable ferroelectric liquid crystals showing at least two stable states, particularly bistability, which can be used in the driving method according to the present invention are chiral smectic liquid crystals having ferroelectricity, among which liquid crystals showing chiral smectic C phase (SmC*), H phase (SmH*), I phase (SmI*), F phase (SmF*) or G phase (SmG*) are suitable. These ferroelectric liquid crystals are described in, e.g., "LE JOURNAL DE PHYSIQUE LETTRE" 36 (L-69), 1975 "Ferroelectric Liquid Crystals"; "Applied Physics Letters" 36 (11) 1980, "Submicro Second Bistable Electrooptic Switching in Liquid Crystals", "Kotai Butsuri (Solid State Physics)" 16 (141), 1981 "Liquid Crystal", etc. Ferroelectric liquid crystals disclosed in these publications may be used in the present invention. More particularly, examples of ferroelectric liquid crystal compound usable in the method according to the present invention include decyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (DOBAMBC), hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate (HOBACPC), 4-o-(2-methyl) butylresorcylidene-4'-octylaniline (MBRA 8), etc. When a device is constituted using these materials, the device may be supported with a block of copper, etc., in which a heater is embedded in order to realize a temperature condition where the liquid crystal compounds assume an SmC*, SmH*, SmI*, SmF* or SmG* phase. Referring to Figure 1, there is schematically shown an example of a ferroelectric liquid crystal cell for explanation of the operation thereof. Reference numerals 11a and 11b denote substrates (glass plates) on which a transparent electrode of, e.g., In₂O₃, SnO₂, ITO (Indium-Tin-Oxide), etc., is disposed, respectively. A liquid crystal of, e.g., an SmC*-phase in which liquid crystal molecular layers 12 are oriented perpendicular to surfaces of the glass plates is hermetically disposed therebetween. Full lines 13 show liquid crystal molecules. Each liquid crystal molecule 13 has a dipole moment (P ⊥ ) 14 in a direction perpendicular to the axis thereof. When a voltage higher than a certain threshold level is applied between electrodes formed on the base plates 11a and 11b, a helical structure of the liquid crystal molecule 13 is unwound or released to change the alignment direction of respective liquid crystal molecules 13 so that the dipole moments (P ⊥ ) 14 are all directed in the direction of the electric field. The liquid crystal molecules 13 have an elongates shape and show refractive anisotropy between the long axis and the short axis thereof. Accordingly, it is easily understood that when, for instance, polarizers arranged in a cross nicol relationship, i.e., with their polarizing directions crossing each other, are disposed on the upper and the lower surfaces of the glass plates, the liquid crystal cell thus arranged functions as a liquid crystal optical modulation device, of which optical characteristics vary depending upon the polarity of an applied voltage. Further, when the thickness of the liquid crystal cell (the thickness of the ferroelectric liquid crystal layer) is sufficiently thin (e.g., 1 µ), the helical structure of the liquid crystal molecules is unwound to provide a non-helical structure even in the absence of an electric field whereby the dipole moment assumes either of the two states, i.e., Pa in an upper direction 23a or Pb in a lower direction 24a as shown in Figure 2. When electric field Ea or Eb higher than a certain threshold level and different from each other in polarity as shown in Figure 2 is applied to a cell having the above-mentioned characteristics, the dipole moment is directed either in the upper direction 24a or in the lower direction 24b depending on the vector of the electric field Ea or Eb. In correspondence with this, the liquid crystal molecules are oriented in either of a first stable state 23a (bright state) and a second stable state 23b (dark state). When the above-mentioned ferroelectric liquid crystal is used as an optical modulation element, it is possible to obtain two advantages. First is that the response speed is quite fast. Second is that the orientation of the liquid crystal shows bistability. The second advantage will be further explained, e.g., with reference to Figure 2. When the electric field Ea is applied to the liquid crystal molecules, they are oriented to the first stable state 23a. This state is stably retained even if the electric field is removed. On the other hand, when the electric field Eb of which direction is opposite to that of the electric field Ea is applied thereto, the liquid crystal molecules are oriented to the second stable state 23b, whereby the directions of molecules are changed. This state is also stably retained even if the electric field is removed. Further, as long as the magnitude of the electric field Ea or Eb being applied is not above a certain threshold value, the liquid crystal molecules are placed in the respective orientation states. In order to effectively realize high response speed and bistability, it is preferable that the thickness of the cell is as thin as possible and generally 0.5 to 20 µ, particularly 1 to 5 µ. A liquid crystal-electrooptical device having a matrix electrode structure in which the ferroelectric liquid crystal of this kind is used is proposed, e.g., in the specification of U.S. Patent No. 4367924 by Clark and Lagerwall. Hereinbelow, an embodiment of the display device used in the present invention will be explained with reference to Figure 3. Referring to Figure 3, on one substrate 31 are disposed a display conductor film 32 and transmission electrodes 33 (33a, 33b, 33c, ...) of low resistive metal films which are laminated on the display conductor film 31 in parallel with each other and with equal spacings. Opposite the substrate 31, the other substrate (not shown) is disposed having a counter conductor film (counter electrode) 34 thereon so as to define pixels, e.g., one denoted by A . The above mentioned optical modulation material is sandwiched between the display conductor film 32 and the counter electrode 34. In the liquid crystal optical device constructed in the manner described above, a potential gradient may be provided by applying a scanning voltage to a transmission electrode 33, thereby to cause a voltage gradient in electric field between the display conductor film 32 and a counter electrode 34. In this instance, when a transmission electrode 33b, for example, is supplied with a first reference potential Va and adjacent transmission electrodes 33a and 33c are supplied with a second reference potential (e.g., 0 volt), a potential gradient of Va is provided in a length ℓ₁ between the transmission electrodes 33b and 33a and in a length ℓ₂ between 33b and 33c along the extension of the conductor film 32. At this time, when the inversion threshold voltage Vth is assumed to be Va, and an information signal of -Vb is applied to the counter electrode 34, a potential difference Va + Vb exceeding the inversion threshold voltage Vth is applied to the ferroelectric liquid crystal corresponding to lengths m₁ and m₂ along the extension of the conductor film 32, so that the region corresponding to the m₁ + m₂ may be inverted, e.g., from the bright state to the dark state. Accordingly, a gradation may be displayed by applying a value Vb of signal selected corresponding to the gradation to individual pixels. In this instance, it is possible to modulate the magnitude of the voltage signal -Vb applied to the counter electrode 34 in the above described manner, alternatively to modulate the pulse duration thereof, or to modulate the number of pulses thereof, thereby to effect the control of the resultant gradation. Next, a specific example of an optical modulation which is preferred for providing the above mentioned gradational characteristic will be explained with reference to Figure 3. Referring to Figure 3, a 3000 Å-thick transparent conductor film of SnO₂ (tin oxide) is formed by sputtering as a display conductor film 32 on a glass substrate 31. Then, a 1000 Å-thick Al film is formed by vacuum evaporation on the SnO₂ film and then patterned to form a plurality of transmission electrodes 33 in the form of stripes. For example, the transmission electrodes may be formed with a spacing of 230 µm and a width of 20 µm. On the other hand, counter electrodes 34 of ITO (indium tin oxide) film are formed on a counter substrate. On each of the thus prepared two substrates, an about 500 Å-thick polyvinyl alcohol film is formed as a liquid crystal orientation film and subjected to a rubbing treatment. In a preferred embodiment according to the present invention, the sheet resistivity (according to ASTM D 257) of the display conductor film 32 may preferably on the order of 10⁴ Ω/□ (ohm per square) - 10⁷ Ω/□ and may be on the order of 10³ - 10⁹ Ω/□ as a tolerable range. An example of a film having such a resistivity may be an SnO₂ film as described above formed by sputtering in an argon stream. Such a method of sputtering in an argon stream is disclosed, for example, by D.B. Fraser and H.D. Cook, "Highly Conductive Transparent Films of Sputtered In₂ - xSn x O 3-y ", Journal of the Electrochemical Society Solid-State Science and Technology, Vol. 119, No. 10 (published in 1972). By adjusting the resistivity of the display conductor film 32 in the above described manner, the above described gradational display may be applicable to a wide variety of optical modulation materials, and moreover the power consumption can be minimized when a potential gradient is provided in the above mentioned manner. The above condition is first of all effective to suppress the heat generation due to a current passing through the display electrode at the time of forming a potential gradient for gradational display. More specifically, the temperature increase ΔT of the electrode may be calculated by the following equation as a simple theoretical calculation where a heat dissipation to the glass substrate, etc.: wherein the respective symbols denote the following: V: Va - V E (potential difference between transmission electrodes), R: resistance between transmission electrodes, t: application period for the above V, C: heat capacity of a display electrode (between transmission electrodes), and v: volume of the display electrodes between transmission electrodes. Herein, when one pixel is noted, and if V is, e.g., 10 volts, t is 100 µsec., C is 2 - 3 J/cm³·°K, and v is 230 µm x 230 µm x 3000 Å as exemplified above, then ΔT is calculated to be on the order of 250000/R (°K). Even if the heat dissipation or conduction to the glass substrate, etc., is considered here, if the R (equal to the sheet resistivity as described above) is on the order of several Ω to several tens of Ω, the temperature increase amounts to a considerable value. It is possible that the temperature increase exerts an ill effect on the optical or physical properties of an optical modulation material disposed in the neighborhood. Especially, the effect is further pronounced when the voltage is increased, the writing period for a pixel is increased, or even if the writing period becomes shorter, the pixel area is decreased further. Moreover, in a case where the voltage V is volts, if R is 100 Ω for example, then the power consumption per pixel becomes 1 W, which amounts to a large value of 1 KW when a large number of such pixels are arranged in a matrix and, e.g., 1000 pixels are provided with a gradation at a time. Accordingly, as a result of consideration on the above matter, we have resolved the above problem by setting the sheet resistivity of the display electrode to above 10³ Ω/□ . Further, in order to alleviate the above effect due to temperature increase, make possible the selection of an optical modulation material from a wide scope and smoothly reduce the writing period and pixel area, the sheet resistivity of 10⁴ Ω/□ or more may preferably be selected. The upper limit of the sheet resistivity may preferably be on the order of 10⁹ Ω/□ or below, further preferably on the order of 10⁷ Ω/□ or below. The reason for this may be considered as follows. If the dielectric constant of an optical modulation material used is assumed to be about 5, a typical value for a ferroelectric liquid crystal, the layer thickness thereof be 1 µm, and the pixel area be 230 µm square, then the capacitance of the liquid crystal layer becomes about 2.5 pF (2.5x10⁻¹² F) per pixel. Here, if it is required to drive the above liquid crystal for example in 100 µsec., it is preferred that the product CR of the resistance R of the display electrode and the capacitance C of the liquid crystal layer be sufficiently below the above mentioned 100 µsec. More specifically, when the above values are taken as typical, if R = 10⁷ Ω/□ , it becomes 2.5x10⁻⁵ sec (25 µsec), so that optimization is possible if a lower R is used. However, if a slower driving is possible (e.g., 1 msec - 10 msec) or a thicker liquid crystal layer providing a smaller capacitance is possible, then a resistivity on the order of 10⁹ Ω/□ may be acceptable. By the above, optimum conditions for the display electrode for providing a gradation have been explained. On the other hand, the above mentioned transmission electrode may be sufficient if it can transmit a voltage signal to the display electrode from a terminal connected to a power supply. Thus, a transmission electrode of a sufficiently low sheet resistivity, e.g., around 1 Ω/□ or below, may be used. Further, as an example of the counter electrode, one of ITO of about 20 Ω/□ may suitably be used as described above. As a specific example of practice, two substrates prepared in the above described manner were disposed opposite to each other with a spacing of about 1 µm to form a cell, into which a ferroelectric liquid crystal (a composition consisting mainly of p-n-octyloxybenzoic acid-p'-(2-methylbutyloxy)phenylester and p-n-nonyloxybenzoic acid-p'-(2-methylbutyloxy)phenyl-ester) was injected. The size of the pixel A constituted by a superposed portion of a display conductor film 32 and a counter electrode 34 was made 230 µm x 230 µm, where the width of the pixel A was made ℓ₁/2 + ℓ₂/2. On both sides of the liquid crystal cell thus prepared, a pair of polarizers were disposed in the form of cross nicols, and the optical characteristics were observed. Figure 5 schematically illustrates a method of applying electric signals to a liquid crystal cell which includes a substrate 31, a display electrode 32, transmission electrodes 33a, 33b and 33c, a counter electrode 34, a ferroelectric liquid crystal 41 which is preferably a chiral smectic liquid crystal under a bistability condition, and a counter substrate 42 by means of driver circuits 43 and 44 for supplying signals. Figure 6 and 7 show electric signals applied to the liquid crystal cell. More specifically, Figure 6 shows a waveform of SIGNAL(b) generated by the driver circuit 44 shown in Figure 5, and Figures 7A - 7E show waveforms of SIGNAL (a) generated by the driver circuit 43 shown in Figure 5. Now, a 200 µsec-pulse of -12 V is applied as a signal(b) to all the transmission electrodes 33a, 33b, 33c, ..., and a 200 µsec-pulse (called "erasure pulse") is preliminarily applied in phase with a counter electrode 34 in an erasure step. Then, the liquid crystal 41 is switched to the first stable state to provide a bright state (as a pair of polarisers are arranged in cross nicols in such a manner). Incidentally, for convenience of explanation, the liquid crystal used herein is assumed to have a switching or inversion threshold of ±15 V - ±16 V. From this state, in phase with the pulse shown in Figure 6 applied to the transmission electrode 33b as a signal (b) while the transmission electrodes 33a and 33c are placed at a reference potential (0 in this example), various pulses as shown in Figures 7A - 7E are respectively applied to the pixel A. The resultant optical states of the pixel A are shown in Figures 8A - 8D. More specifically, when the pulse voltage applied to the counter electrode is -2 V as shown in Figure 7A, no change at all occurs from the bright state 71 as shown in Figure 8A. When a pulse voltage of -4 V as shown in Figure 7B is applied, a portion of the liquid crystal in a close proximity of the transmission electrode 33b is switched to a dark state 72 as shown in Figure 8B because of an electric field exceeding the threshold of the liquid crystal. When the applied voltage is further increased to -6 V (Figure 7C) or -8 V (Figure 7D), the dark state area 72 is enlarged as representatively shown in Figure 8C because the area of exceeding the switching threshold is enlarged. When the applied voltage is -10 V (Figure 7E), the pixel A is wholly changed to the dark state as shown in Figure 8D. In this way, a gradational image may be formed. The optical state changes as shown in Figures 8A - 8D may also be obtained when various pulses (a) with different pulse durations as shown in Figures 10AA - 10AE are applied in phase with a triangular wave signal (b) as shown in Figure 9. More specifically, in this instance, a gradational display may be effected by applying the signal shown in Figure 9B to the transmission electrode 33b while applying the pulses shown in Figures 10AA - 10AE to the counter electrode 34 corresponding to given gradation data in phase with the signal applied to the transmission electrode 33b. Further, it would be readily understood that a similar effect is obtained by applying various pulses (a) of different phases as shown in Figures 10BA - 10BE in synchronism with the triangular wave signal (b). In this invention, the transmission electrodes 33a, 33b, ... may be composed of a metal such as silver, copper, gold or chromium, or a transparent electrode such as low-resistivity ITO instead of aluminum (Al) used in the above example. The sheet resistivity may preferably be 10² Ω/□ or below. The sheet resistivity of the electrodes may be adjusted to an appropriate value, for example, by controlling the film thickness. In the above, a method of providing a gradation has been explained. However, it is of course possible to provide a two-value display without expressing a gradation by particularly selecting two signal levels. In this case, the signal shown in Figure 7E, or Figure 10AE or 10BE may be used as an inversion signal. Figure 11 is a partial schematic view of a specific embodiment of a liquid crystal panel adapted to application of the gradational display system to matrix driving. The display panel shown in Figure 11 comprises a plurality of stripe conductor films 101 (101a, 101b, 101c, ...) disposed on a glass substrate 31, and transmission electrodes 102 (102a, 102b, 102c, ...) and 103 disposed on both lateral sides along the longitudinal direction of each stripe conductor film 101. Opposite to the substrate 31, there is disposed a counter substrate (not shown) on which counter electrodes 104 (104a, 104b, ...) are disposed. Further, a ferroelectric liquid crystal is disposed between the stripe electrode films 101 and the counter electrodes 104. In this embodiment, prior to writing, the transmission electrodes 102 and 103 are placed at the same potential level so as to apply a uniform electric field across the whole or a part of the pixels formed at the intersections of the stripe conductor films 101 and the stripe counter electrodes 104, thereby to bring the whole or the part of the pixels to either one of the bright and dark states at a time; or prior to writing for each writing line, the whole or a part of the pixels on the writing line are brought to either one of the bright and dark states. Then, a pulse as shown in Figure 6 or 9 is sequentially applied as a scanning signal to individual transmission electrodes on one side while transmission electrodes on the other side are placed at a reference potential level (e.g., 0 volt), whereby a potential gradient is sequentially provided to the stripe conductor films 101 between the transmission electrodes 102 and 103. In this instance, the scanning selection signal is preferably be made a voltage pulse equal to or somewhat lower than the inversion threshold voltage of the ferroelectric liquid crystal. On the other hand, voltage signals as shown in Figures 7A - 7E or Figures 10AA - 10AE (or 10BA - 10BE) corresponding to give gradation data are applied to respective stripe counter electrodes 104 in phase with the scanning selection signal applied to the transmission electrodes 102, whereby the pixels on a scanned line are written into a gradational state. By conducting the above writing operation line-sequentially, one picture frame with gradation can be formed. In this case, a pixel corresponding to the above mentioned pixel A is formed at an intersection of stripe electrodes 101 and 104, and the inverted region of the liquid crystal is caused to spread from either one prescribed of the transmission electrodes 102 and 103. Further, in the present invention, it is also possible to provide a picture frame with gradations by applying a scanning pulse signal as shown in Figure 6 or Figure 9 sequentially to the stripe electrodes 104, and in phase with the scanning signal, applying voltage signals as shown in Figures 7 or Figures 10 corresponding to given gradation data to transmission electrodes on one side, e.g., 102, while the other transmission electrodes are connected to a reference potential level. Figure 12 shows a matrix electrode arrangement according to another embodiment of the present invention. A liquid crystal optical device represented by the electrode arrangement shown in Figure 12 comprises a plurality of stripe conductor films 111 disposed on one substrate, and a plurality of stripe conductor films 112 disposed opposite to and intersecting with the stripe conductor films 111 on the other substrate by the medium of a ferroelectric liquid crystal. Further, to both lateral sides of the stripe conductor films 111 and 112, transmission electrodes 113 and 114, and 115 and 116, of low resistivity, are respectively connected. Individual terminals S₁, S₂, ... S₇, ... of the transmission electrodes 113 are connected to a scanning signal generator circuit (not shown), and individual terminals I₁, I₂, ... I₆, ... of the transmission electrodes 115 on the other side are connected to an information signal generator circuit (not shown) and terminals G₁₁, G₁₂, ... G₁₆ ..., and G₂₂, ... G₂₇, ... are connected to a reference potential supply (e.g., at 0 volt). As a result, according to this embodiment, at respective pixels after an erasure step, a potential gradient is developed along the extension of a scanning signal side conductor film and also a potential gradient is developed along the extension of an information signal side conductor film, so that a potential difference with a gradient generated by the combination of the potential gradients on both sides is applied to the ferroelectric liquid crystal at a pixel, whereby display of a picture with multi-level gradation becomes possible. In this instance, when a potential gradient is provided between transmission electrodes for supplying information signals and a case of driving a picture frame with, e.g., 1000 x 1000 pixels is considered, a power consumed in display electrodes between transmission electrodes for supplying information signals amounts to those required at pixels in a number of 1000 x (the number of selected pixels). As a result, the power consumed becomes considerable unless the display electrodes have an appropriately low resistivity. Accordingly, it is most preferable to provide a potential gradient between transmission electrodes for supplying a scanning signal as is in the embodiment shown in Figure 11 or one shown in Figure 13 explained hereinafter. In the structures shown in Figures 11 and 12, the stripe conductor films 11 are provided separately for respective lines. According to the present invention, the stripe conductor film may preferably be composed of SnO₂, etc., providing a high resistivity of 10³ Ω/□ than ITO of a low resistivity. Now, a process for effectively and finely etching an SnO₂ film is briefly described. More specifically as an etching process for an SnO₂ film, it has been known to etching the SnO₂ film with a paste of zinc powder mixed with water or with a dilute hydrochloric acid in some cases, a preferred fine etching process is one wherein an SnO₂ film is etched by reacting the SnO₂ with BF₃ (boron trifluoride) plasma to separate the SnO₂. Figure 13 shows an electrode arrangement according to another embodiment. An liquid crystal optical device represented by Figure 13 comprises a plurality of transmission electrodes 122 disposed in parallel with each other on one conductor film 122 respectively connected to terminals S₁, S₂, ... S₇, ... which in turn are connected to a scanning signal generator circuit (not shown). Intersecting with these transmission electrodes 122, electrodes 123 of plural stripe conductor films 123 are oppositely disposed, and a ferroelectric liquid crystal is disposed between the conductor film 121 and the stripe electrodes 123. The terminals I₁, I₂, ... I₅, ... are respectively connected to an information signal generator circuit (not shown). As a result, after an erasure step in this embodiment, a scanning signal is sequentially applied to terminals S₁, S₂, ... S₇, ... of the liquid crystal optical device while connecting the remaining terminals not supplied with the scanning signal to a reference potential supply, thereby to provide a potential gradient. On the other hand, gradation signals are supplied to the stripe electrodes 123 in phase with the scanning signal, whereby a gradational picture may be formed. Herein, it is possible to apply the scanning signal first to, e.g., the odd-numbered terminals S₁, S₃, S₅ ... S 2n-1 sequentially, and then to the even-numbered terminals S₂, S₄, S₆, ... S 2n sequentially. In the foregoing explanation, the reference potential level has been mainly referred to as 0 volt, whereas the reference potential level may be raised to some extent. In this case, the resultant potential gradient for providing a gradation becomes moderate as the potential difference Va - V E becomes smaller, but the absolute value of the potential level provided to the information signal can be decreased. On the contrary, it is also possible to enlarge the gradation range by lowering the reference potential level to increase Va - V E . Further, in the present invention, it is also possible to effect gradational driving by sequentially applying a scanning signal to the stripe electrodes, and in phase with the scanning signal, applying gradation signals to the odd-numbered (or even-numbered) transmission electrodes while connecting the even-numbered (or odd-numbered) transmission electrodes to a reference potential supply. Figure 14 shows a matrix electrode arrangement according to still another embodiment of the present invention. A liquid crystal optical device represented by Figure 13 comprises conductor films 141 on one substrate, a plurality of conductor films 142 disposed on the other substrate disposed opposite to and intersecting with the conductor films 141, and a ferroelectric liquid crystal disposed between the conductor films 141 and 142. Further, low-resistivity transmission electrodes 143 and 144 are disposed on the conductor films 141 and 142. In this embodiment, terminals S₁, S₂, ... S₇ ... of the transmission electrodes are respectively connected to a scanning signal generator circuit (not shown), and terminals I₁, I₂, ... I₆, ... of the transmission electrodes on the other side are respectively connected to an information signal generator circuit (not shown). As a result, according to this embodiment, at respective pixels after an erasure step, a potential gradient is developed along the extension of a scanning signal side conductor film and also a potential gradient is developed along the extension of an information signal side conductor film, so that a potential difference with a gradient generated by the combination of the potential gradients on both sides is applied to the ferroelectric liquid crystal at a pixel, whereby display of a picture with multi-level gradation becomes possible. In the above, the present invention has been explained with reference to a device using a ferroelectric liquid crystal, particularly a ferroelectric liquid crystal having at least two stable states, as a most preferable example, whereas the present invention may also be applied to another liquid crystal such as a twisted-nematic liquid crystal or a guest-host liquid crystal or further to an optical modulation material other than a liquid crystal. Thus, according to the present invention, a gradational display may be effected by forming in the extension of at least one of a pair of electrodes constituting a pixel, and applying a gradation signal modulated with respect to voltage, pulse duration or number of pulses as an input signal.

Claims: An optical modulation apparatus, comprising: [a] an optical modulation device (Fig.1, 2) comprising [a1] a first substrate (42) having thereon a first conductor film (34), [a2] a second substrate (31) having thereon a second conductor film (32) which is disposed opposite to said first conductor film (34), and [a3] an optical modulation material (41) being disposed between said first (42) and second substrate (31) so as to form a pixel (A) which is defined by the intersection of said first (34) and second conductor film (32); and [b] voltage application means (43, 44) for applying voltages to said first and second conductor films (34, 32); characterized in that said voltage application means comprises [c] first means (43) for applying a voltage (V b ) corresponding to an information signal to said first conductor film (34), [d] and second means (44) for applying a voltage to said second conductor film (32) in such a way that there is developed a potential gradient (Fig.4A, 4B) along said second conductor film (32), wherein the potential difference between said information signal voltage (V b ) and a selectable part of said potential gradient is exceeding an inversion threshold voltage (V th ) of said modulation material. An optical modulation apparatus according to claim 1, characterized in that there are at least two transmission electrodes (33) connected to said second conductor film (32), said second means (44) applying voltage signals (Va, VE) to said transmission electrodes which are developing a potential gradient (Fig.4A, 4B) along said second conductor film (32) between said transmission electrodes (33). An optical modulation apparatus according to claim 1, characterized in that said first (34) and second conductor films (32) each are provided in a plurality so as to form a matrix pixel structure, each second conductor film (32) being connected to at least one respective transmission electrode (33), wherein said second means (44) is sequentially applying a scanning signal to said transmission electrodes (33) and said first means (43) is applying information signals to said first conductor film (34). An optical modulation apparatus according to claim 3, characterized in that said second conductor film (32) is disposed in the form of stripes (101), each lateral side of each stripe (101) being connected to one transmission electrode (102, 103), wherein said second means (44) is supplying to one transmission electrode (102) said scanning signal and to the respective other transmission electrode (103) a reference potential. An optical modulation apparatus according to claim 3 or 4, characterized in that said first conductor film (34) is disposed in the form of stripes (104), wherein said first means (43) is supplying said information signals to said stripes (104) in phase with said scanning signal. An optical modulation apparatus according to claim 5, characterized in that said first conductor films (34) at each lateral side thereof are connected to one signal electrode (I, G), wherein said first means (43) - in phase with said scanning signal - is supplying to one signal electrode (I) said information signals and to the respective other signal electrode (G) a reference potential. An optical modulation apparatus according to one of the preceding claims, characterized in that said information signals are corresponding to given gradation data. An optical modulation apparatus according to claim 7, characterized in that said information signals corresponding to given gradation data are voltage-modulated pulse signals. An optical modulation apparatus according to claim 7, characterized in that said information signals corresponding to given gradation data are pulse-duration-modulated pulse signals. An optical modulation apparatus according to claim 7, characterized in that said information signals corresponding to given gradation data are pulse-number-modulated pulse signals.

Inventors: ENARI, MASAHIKO, KANEKO, SHUZO, KITAYAMA, HIROYUKI, KUNO, MITSUTOSHI, MOURI, AKIHIRO, TAKAHASHI, TOHRU, TOYONO, TSUTOMU
Applicants: CANON KABUSHIKI KAISHA
Application year: 1986
Main classification: G09G3
IPC codes: G02F1, H04N3, G09G3