Liquid crystal cell array and method for driving the same

Abstract: A first transparent substrate (2) has formed thereon a plurality of sequentially arranged first transparent electrodes (9₁, 9₂, ...) and a plurality of individual cell drive lines (20₁, 20₂, ...) each connected to one of them. A second transparent substrate (3) has formed thereon at least one transparent electrode (10) opposite the first transparent electrodes and first and second common drive lines (21a, 21b) each connected to one side of the second transparent electrode. The first and second transparent substrates are disposed opposite with a gap defined there­ between and the gap is filled with liquid crystal (16) to form a liquid crystal cell array. Voltages of different magnitudes are applied to the first and second common drive lines to simultaneously form substantially the same voltage gradient in the second transparent electrode in all of cell windows, and at the same time, voltages corresponding to pixel signals are ap­ plied to the individual cell drive lines, respectively, thereby controlling the aperture ratios of all the cell windows. In driving a cell array using a two-frequency drive liquid crystal, in an operation mode for closing all the cell windows a low-frequency voltage is applied to the liquid crystal, and in an operation mode for diaphragming at least one of the cell windows, different high- or low-­ frequency voltages are applied to the first and second com­ mon drive lines, forming a high- or low-frequency voltage gradient in each cell window.

Description: BACKGROUND OF THE INVENTION The present invention relates to a liquid crystal cell array which can be applied to a liquid crystal light shutter capable of modulating the light-transmitting (or bright) area by the operation of individual liquid crystal cells. The invention also pertains to a method for driving such a liquid crystal cell array. Heretofore, liquid crystal has been applied not only to character and image displays but also to a light shutter which controls the transmission therethrough of light. In the light shutter the transmission of light through the liquid crystal is usually subjected to ON-OFF control on a binary basis. The document JP-A-60-143315, for example, discloses a liquid-crystal optical switch array having a liquid crystal layer sandwiched between a first and a second one of opposing substrates. The first substrate carries a plurality of individual transparent electrodes arranged in a row and each connected to a respective cell drive line. On the second substrate one stripe-shaped transparent common electrode is formed onto which a metal film having good conductivity is vapor-deposited and then etched to expose the underlying transparent electrode in window regions respectively registered with the transparent electrodes on the first substrate. The vapor-deposited film serves as a light shield on the one hand and as a common electrode on the other hand, this common electrode surrounding each window portion of the transparent electrode thereby ensuring that there is no voltage gradient along either side of those window portions. The present inventors produced a nematic liquid crystal cell with a metal electrode deposited on either side of a transparent electrode, and proposed a driving method therefor according to which a potential gradient is produced in the transparent electrode by applying different voltages to the metal electrodes to form a light-transmitting and a light-intercepting areas (This operation will hereinafter be referred to as the diaphragming operation) and the ratio between these areas can freely be varied by controlling the potential gradient (Japanese Patent Application No. 181,912/86). The principle of the driving method will be described with reference to Fig. 1. Fig. 1 is an exploded perspective view, partly cut away, of a liquid crystal cell 11 which is used for forming the above-mentioned liquid crystal light shutter. The liquid crystal cell 11 is made up of a spacer 4 and a pair of opposed transparent substrates 2 and 3 with the spacer 4 interposed therebetween and contains liquid crystal in the space defined by the two transparent substrates 2 and 3 and the spacer 4. The one transparent substrate 2 (hereinafter referred to also as the first substrate) has a transparent electrode 9 deposited on the inner surface thereof and a metal electrode 7 deposited on one marginal portion of the transparent electrode 9. The other transparent substrate 3 (hereinafter referred to also as the second substrate) also has a transparent electrode 8 deposited on the inner surface thereof and metal electrodes 5 and 6 deposited in parallel on both marginal portions of the transparent electrode 8. A pair of opposed polarizing plates (not shown) are disposed with such a liquid crystal cell sandwiched therebetween, forming a light shutter. The region over which light can be transmitted through the transparent electrodes 2 and 3 of the liquid crystal cell 11 is called a cell window. In general, the transparent electrodes 8 and 9 are formed by transparent conductive films of the indium oxide or tin oxide series and the metal electrodes 5 to 7 are formed by vapor deposition of aluminum, nickel or chromium, but these electrodes are not limited specifically to them and may be formed of any materials so long as they are lower in resistance than the transparent electrodes 8 and 9 and capable of intercepting light. The following description of the prior art will be given of the case where the liquid crystal sealed in the cell 11 is ferroelectric liquid crystal. Since the ferroelectric liquid crystal changes its orientation from one to the other upon inversion of the polarity of the applied voltage, it is possible to determine the direction of polarization of the polarizing plates relative to the liquid crystal cell 11 so that it permits or inhibits the passage therethrough of light in response to a desired one of the polarities of the applied voltage. Next, the basic operation of the liquid crystal cell 11 of the above arrangement will be described with regard to the drawings. The inner marginal edges (hereinafter referred to as electrode edges) of the metal electrodes 5 and 6 on the second substrate 3 in Fig. 1 are indicated by a and b, respectively. When voltages V₁ and V₂ are applied to the metal electrodes 5 and 6, respectively, if the voltages V₁ and V₂ are not equal, then current will flow through the transparent electrode 8 across the metal electrodes 5 and 6. At this time, the transparent electrode 8 serves as a resistor, and accordingly, a substantially linear potential gradient develops in the transparent electrode 8 between the electrode edges a and b. This is shown in Fig. 2A, in which the solid line (i) indicates the potential gradient developed between the electrode edges a and b. On the other hand, when a voltage V₃ is fed to the metal electrode 7 of the first substrate 2, the potential on the transparent electrode 9 will become constant over the area of its surface as indicated by the solid line (ii) in Fig. 2A. As a result of this, a voltage corresponding to the difference between the solid lines (i) and (ii), that is, a voltage corresponding to the potential difference is provided between the two transparent electrodes 8 and 9 and this voltage is applied to the liquid crystal sandwiched between them. Now, the direction of the polarizing plates is predetermined so that light is intercepted when the potential of the transparent electrode 8 is higher than the potential of the transparent electrode 9. When the voltages V₁, V₂ and V₃ are applied to the metal electrodes 5, 6 and 7 with the relationships V₁ > V₃ > V₂, there are formed, on one side of the intersecting point K of the solid lines (i) and (ii), an area Ka where the potential of the second substrate 3 is higher than the potential of the first substrate 2 and, on the other side, an area Kb where the potential of the second substrate 3 is lower than the potential of the first substrate 2. In this instance, only the area Ka transmits therethrough light but the area Kb intercepts it. Fig. 2B shows the front elevation of the cell window in this state. In Fig. 2B reference numeral 41 indicates the light-transmitting area of the cell window and 42 the light-intercepting area. The operation (or the state) of the liquid crystal by (or in) which light is permitted to pass through a portion of the cell window as described above will hereinafter be referred to as the "diaphragming operation (or state)" and the ratio of the light-transmitting area to the total area of the cell window as the "aperture ratio". Further, the liquid crystal cell which is capable of controlling the aperture ratio of the cell window by the voltage gradient as mentioned above will hereinafter be called the gradient voltage drive liquid crystal cell. The aperture ratio of the cell window can be changed by suitably selecting the voltages V₁, V₂ and V₃ in Fig. 2A, for example. When the voltage V₃ on the side of the first substrate 2, that is, the voltage which is applied to the transparent electrode 9, is decreased as indicated by the solid line (ii) in Fig. 2C, the intersecting point K of the solid lines (i) and (ii) shifts toward the electrode edge b, increasing the light-transmitting area 41 and decreasing the light-intercepting area 42. Conversely, when the voltage V₃ is increased in Fig. 2A, the light-transmitting area 41 decreases and the light-intercepting area 42 increases. It is apparent that the aperture ratio can also be varied by changing the values of the voltages V₁ and/or V₂ while keeping the voltage V₃ at a fixed value. Such a diaphragming operation of the cell window is not limited according to the type of the cell used but can be achieved for a birefringence controlling type liquid crystal cell, a TN type liquid crystal cell, a guest-host type liquid crystal cell, a two-frequency driving type liquid crystal cell, etc. as well as the ferroelectric liquid crystal cell. In a light shutter in which a number of such gradient voltage drive liquid crystal cells 11 as shown in Fig. 1 are simply arrayed, it is necessary to provide wiring on the second substrate 3 for sending at least two kinds of drive voltage signals to each liquid crystal cell. This constitutes an obstacle to high density packaging of cell arrays. Moreover, at least two drivers must be provided for each gradient voltage drive liquid crystal cell; this also enlarges the scale of the light shutter and raises its cost accordingly. SUMMARY OF THE INVENTION It is an object of the present invention to provide a gradient voltage drive type liquid crystal cell array which does not require a large number of driving lines, and accordingly, can be driven with a small number of drivers. Another object of the present invention is to provide a driving method suitable for such a liquid crystal cell array. These objects are achieved with a liquid crystal cell array and a method of driving it as claimed in claims 1 and 10, respectively. Preferred embodiments of the invention are subject-matter of dependent claims. The liquid crystal cell array of the present invention includes: a first transparent substrate which has a plurality of first transparent electrodes and a plurality of individual cell drive lines for individually driving the first transparent electrodes; a second transparent substrate which has at least one second transparent electrode disposed opposite the first transparent electrodes and common side electrodes provided on both sides of the second transparent electrode, the one common side electrode being connected to a first common drive line and the other common side electrode being connected to a second common drive line; and liquid crystal sealed in the space defined by the first and second transparent substrates. A plurality of cell windows of the liquid crystal cell array are defined by the respective opposed first and second transparent electrodes and the liquid crystal interposed therebetween. According to the drive method for such a liquid crystal cell array, voltages of different magnitudes are applied to the first and second common drive lines to simultaneously develop substantially equal voltage gradients in the regions of the second transparent electrode constituting all the cell windows, and at the same time, voltages corresponding to individual pixel signals are applied to the individual cell drive lines, thereby controlling the aperture ratios of the respective cell windows. In the case where the liquid crystal cell array of the present invention employs two-frequency driving type liquid crystal cells, low- or high-frequency voltages of different magnitudes are applied to the first and second common drive lines connected to the both sides of the second transparent electrode, respectively, to thereby produce low- or high-frequency voltage gradients in the second transparent electrodes. At the same time, voltages corresponding to individual pixel signals are applied to the individual cell drive lines connected to the first transparent electrodes, respectively. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded perspective view of a conventional gradient voltage driving type liquid crystal cell; Figs. 2A, 2B, 2C and 2D are diagrams, for explaining the principle of operation of the liquid crystal cell depicted in Fig. 1; Figs. 3A and 3B are diagrams schematically showing the structures of first and second transparent substrates forming the liquid crystal cell array of the present invention; Figs. 4A and 4B are a longitudinal-sectional view taken on the broken line 1Va-1Va and a cross-sectional view taken on the broken line 1Vb-1Vb in Fig. 3B; Fig. 4B is a sectional view, corresponding to Fig. 4A, illustrating another embodiment of the liquid crystal cell array of the present invention; Fig. 5 is a graph showing a drive voltage frequency versus dielectric anisotropy characteristic of a two-frequency drive type liquid crystal; Fig. 6 is a diagram schematically showing the structure of a light shutter employing the liquid crystal cell array of the present invention; Fig. 7 is a diagram, for explaining the principle of the drive method of the present invention; Fig. 8 is a diagram schematically showing the liquid cell array structure, for explaining various embodiments of the drive method of the present invention; Fig. 9 is a waveform diagram, for explaining an embodiment of the drive method of the present invention; Fig. 10 is a waveform diagram, for explaining another embodiment of the drive method of the present invention; Fig. 11 is a waveform diagram, for explaining another embodiment of the drive method of the present invention; Fig. 12 is a waveform diagram, for explaining another embodiment of the drive method of the present invention; Fig. 13 is a waveform diagram, for explaining another embodiment of the drive method of the present invention; Fig. 14 is a waveform diagram, for explaining another embodiment of the drive method of the present invention; Fig. 15 is a waveform diagram, for explaining in detail the waveform diagram shown in Fig. 14; Figs. 16A and 16B are voltage distribution diagrams, for explaining the principle of operation of another embodiment of the drive method of the present invention; Figs. 17A and 17B are voltage distribution diagrams, for explaining the principle of operation of another embodiment of the drive method of the present invention; Fig. 18 is a waveform diagram, for explaining still another embodiment of the drive method of the present invention; Fig. 19 is a waveform diagram, for explaining a modification of the drive method described with regard to Fig. 18; Fig. 20 is a waveform diagram, for explaining yet another embodiment of the drive method of the present invention; Fig. 21 is a waveform diagram, for explaining a modification of the drive method described in respect of Fig. 20; Fig. 22 is a waveform diagram having rewritten low-frequency voltage waveforms depicted in Fig. 19; Fig. 23 is a diagram showing waveforms which occur in the case of a combined use of the drive methods described in connection with Figs. 22 and 13; Fig. 24 is a waveform diagram, for explaining another drive method of the present invention; Fig. 25 is a block diagram illustrating a signal generator for generating waveforms C a and C b depicted in Fig. 23; Fig. 26 is a timing chart showing various signal waveforms occurring in the signal generator depicted in Fig. 25; Fig. 27 is a circuit diagram illustrating a signal converter for generating waveforms D₁, D₂, ... in Fig. 23; and Fig. 28 is a timing chart, for explaining the operation of the circuit shown in Fig. 27. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3A is a plan view showing a first transparent substrate 2 of the liquid crystal cell array of the present invention, a plurality of first transparent electrodes 9₁, 9₂, ... formed on the first transparent substrate 2 and individual cell drive lines 20₁, 20₂, ... connected to the first transparent electrodes. Fig. 3B is a plan view showing a second transparent substrate 3 of the liquid cell array, a plurality of second transparent electrodes 10₁, 10₂, ... formed on the second transparent substrate 3 and first and second common drive lines 21a and 21b for applying voltages to both sides of the transparent electrodes at the same time. The first transparent electrodes 9₁, 9₂, ... substantially square in shape are arranged side by side on the first substrate 2 at regular intervals, and the individual cell drive lines 20₁, 20₂, ... made of metal are connected to the first transparent electrodes 9₁, 9₂, ... at one marginal edge thereof. In this example, the drive lines 20₁, 20₂, ... extend at right angles to the direction of arrangement of the first transparent electrodes 9₁, 9₂, ... and in opposite directions alternately. On the second transparent substrate 3 the second transparent electrodes 10₁, 10₂, ..., also substantially square in shape, are arranged side by side at regular intervals so that they are opposed to the first transparent electrodes 9₁, 9₂, ..., respectively. On the second transparent substrate 3 there are further provided the first common drive line 21a which is made of metal and extends along marginal edges of the second transparent electrodes 10₁, 10₂, ... on the same side, for interconnecting them, and the second common drive line 21b which is also made of metal and extends along marginal edges of the second transparent electrodes 10₁, 10₂, ... on the opposite side from the first common drive line 21a, for interconnecting them. These metal drive lines 20₁, 20₂, ... and 21a, 21b are formed so that they also serve as metal electrodes of the transparent electrodes 9₁, 9₂, ... and 10₁, 10₂, ... Moreover, there are formed on the second transparent substrate 3 light shielding layers 12₁, 12₂, ... for shielding from light those areas between adjacent second transparent electrodes. The light shielding layers 12₁, 12₂, ... are formed in the following manner: At first, gelatin containing a black dye is coated about 1 µm thick over the entire area of the top surface of the second transparent substrate 3 including the first and second common drive lines 21a and 21b and the second transparent electrodes 10₁, 10₂, ... and is then selectively removed by photoetching into a desired pattern to form windows 23₁, 23₂, ... on the transparent electrodes 10₁, 10₂, ... (Fig. 4A). The first and second transparent substrates 2 and 3 shown in Figs. 3A and 3B are assembled together in parallel, adjacent but spaced relation so that the first and second transparent electrodes 9₁, 9₂, ... and 10₁, 10₂, ... are opposed to each other across a gap defined therebetween by a spacer (not shown) disposed between the first and second substrates 2 and 3. The gap defined between the first and second transparent substrates 2 and 3 thus assembled is filled with liquid crystal 16. Figs. 4A and 4B are a longitudinal-sectional view and a cross-sectional view of the gradient voltage driving type liquid crystal cell array 11 thus constructed, showing the sections along the broken lines 1Va-1Va and 1Vb-1Vb in Fig. 3B, respectively. While in the above embodiment the substantially square, spaced-apart second transparent electrodes 10₁, 10₂, ... are formed on the second transparent substrate 3, it is also possible to define the second transparent electrodes 10₁, 10₂, ... by forming a stripe of transparent electrode 10 on the second transparent substrate 3 and then forming, on the electrode 10, light shielding layers 12₁, 12₂, ... in the same pattern as shown in Fig. 3B. Fig. 4C is a sectional view, corresponding to Fig. 4A, of the liquid crystal cell array in the above case. The formation of a mask pattern for obtaining such a stripe of transparent electrode 10 is easier than the formation of a mask pattern for providing the spaced-apart transparent electrodes 10₁, 10₂, ..., and the former affords the reduction of size accuracy and positioning accuracy of the light shielding layers 12₁, 12₂, ... which are formed on the stripe-shaped transparent electrode 10. Consequently, the production yield rate of the liquid crystal cell array can be improved. In order to drive the gradient voltage driving type liquid crystal cell array 11 of the present invention, it is necessary to apply voltage across each of the second transparent electrodes 10₁, 10₂, ... to create a voltage gradient. When the applied voltage is constant, a decrease in the electric resistance of each second transparent electrode will cause an increase in the amount of heat generated therein, and it is therefore preferable that the second transparent electrodes 10₁, 10₂, ... be high in electric resistance. However, the electric resistance needs to be sufficiently lower than the impedance of the liquid crystal 16 sealed in the cell 11. From the practical point of view, the electric resistance of the second transparent electrodes 10₁, 10₂, ... may preferably be in the range of about 10⁸ to 10⁵ Ω/sq. On the other hand, the electric resistance of each of the first transparent electrodes 9₁, 9₂, ... is selected sufficiently lower than the resistance of the second transparent electrode, approximately 50 Ω/sq in practice so that the voltage applied to each first transparent electrode is uniformly distributed over the entire area thereof. As noted previously, ferroelectric liquid crystal, TN liquid crystal, two-frequency drive liquid crystal, etc. are usable for the gradient voltage drive type liquid crystal cell array of the present invention shown in Figs. 3A, 3B, 4A and 4B. In the case of the ferroelectric liquid crystal, however, if each drive signal is accompanied by a reverse polarity signal for charge cancellation use so as to protect the liquid crystal from deterioration, then the orientation of ferrodielectric liquid crystal molecules will be reversed during the application of the reverse polarity signal. Accordingly, the light source for irradiating the light shutter must be kept OFF in this while. A special device is needed for such control of the light source, and this also poses a problem of cost. The TN liquid crystal responds to voltage application relatively quickly in 0.1 to 10 milliseconds but its response speed to the removal of the applied voltage is as low as tens to hundreds of milliseconds. Consequently, the TN liquid crystal is not suitable for high-speed driving of the light shutter. On the other hand, the two-frequency drive liquid crystal is voltage-controlled birefringence (ECB) type liquid crystal, and, for example, as shown in Fig. 5, it exhibits a positive dielectric anisotropy when the frequency of the applied voltage is lower than a point P and exhibits a negative dielectric anisotropy when the frequency is higher than the point P. The point P of such a transition will hereinafter be referred to as the transition point. Frequency higher than the point P and frequency lower than the point P will hereinafter be referred to simply as high-frequency and low-frequency, respectively. The application of low-frequency voltage will provide homeotropic alignment (vertical alignment) of the liquid crystal molecules and the application of high-frequency voltage will provide homogeneous alignment (horizontal alignment) of the molecules. In addition, the response time of this liquid crystal to switching between high and low frequency voltages can be as short as hundreds of microseconds, though depending on the magnitude of the applied voltage. This two-frequency drive liquid crystal is disclosed in, for example, Bak, Ko and Labes, "Fast decay in a twisted nematic induced by frequency switching", Journal of Applied Physics, Vol. 46, No. 1, January 1975. It is therefore considered that the two-frequency drive liquid crystal is suitable for use in the light shutter which is required to be driven at high speed. The principle of driving the gradient voltage drive liquid crystal cell array of the present invention, in the case of employing the two-frequency drive liquid crystal, will be described first in connection with a single liquid crystal cell. Fig. 6 schematically illustrates, in section, a light shutter 110 formed by a gradient voltage drive liquid crystal cell 11, and this sectional view corresponds to that depicted in Fig. 4B. On the inner surface of the first transparent substrate 2 there are provided the first transparent electrode 9 and the metal electrode 20 connected thereto, and on the inner surface of the second transparent substrate 3 there are provided the second transparent electrode 10 and the metal electrodes 21a and 21b connected to its both marginal sides. The first and second transparent substrates 2 and 3 are assembled together in parallel, adjacent but spaced relation, with the spacer 4 sandwiched therebetween. A pair of polarizing plates 17a and 17b are disposed on both sides of the liquid crystal cell 11, with their planes of polarization held at right angles to each other. Disposed behind the polarizing plate 17a is a light source 18. The electrodes 20, 21a and 21b are connected to AC voltage generators 13, 14 and 15, respectively. Any of these AC voltage generators 13, 14 and 15 can generate an arbitrary voltage at an arbitrary frequency and in an arbitrary phase. Let it be assumed here that the AC voltage generators 14 and 15 are generating AC voltage at the same frequency and in the same phase. The operation starts with closing the light shutter 110 by grounding the AC voltage generators 13 (V₃ = 0) and causing the AC voltage generators 14 and 15 to yield low-frequency voltages V₁(L) and V₂(L) lower than the transition point P shown in Fig. 5. Consequently, the liquid crystal molecules assume the homeotropic alignment and linearly polarized light having passed through the polarizing plate 17a passes through the liquid crystal cell 11, but it is intercepted by the polarizing plate 17b set in the state of cross-nicol. In other words, the light shutter 110 is in the closed state, i.e. in the light intercepting state. Conversely, even if the AC voltage generator 13 generates a low-frequency voltage and the AC voltage generators 14 and 15 are grounded, the light shutter 110 similarly intercepts light. Next, the light shutter 110 is opened with a desired aperture ratio. By causing the AC voltage generator 13 to produce the low-frequency voltage V₃(L) and the AC voltage generators 14 and 15 to produce high-frequency voltages V₁(H) and V₂(H) higher than the transition point P, with relationships V₁(H) ≧ V₃(L) ≧ V₂(H), there will be performed a diaphragming operation similar to that described previously with respect to Figs. 2A and 2B. As shown in the voltage distribution diagram of Fig. 7, a gradient by the high-frequency voltage which decreases from the electrode 21a toward the electrode 21b, as indicated by the broken line (i), is created relative to the low-frequency voltage V₃(L) indicated by the solid line (ii). In this instance, the region where the low-frequency voltage V₃(L) is lower than the high-frequency voltage becomes the light transmitting region 41 and the region where the low-frequency voltage V₃(L) is higher than the high-frequency voltage becomes a light intercepting region 42. By changing the magnitude of the low-frequency voltage V₃(L) which is created by the AC voltage generator 13, the point of intersection of the solid line (ii) with the broken line (i) is shifted, that is, the area of the light transmitting region 41 can freely be changed. By increasing the low-frequency voltage V₃(L) so that V₃ ≧ V₁(H), the light shutter 110 is fully closed, whereas, by decreasing the low-frequency voltage V₃(L) so that V₂(H) ≧ V₃, the light shutter 110 is fully opened. Although in the above example the low-frequency voltage V₃(L) is applied to the electrode 20 and the high-frequency voltages V₁(H) and V₂(H) are applied to the electrodes 21a and 21b, respectively, the diaphragming operation is performed even in the case where a high-frequency voltage V₃(H) is applied to the electrode 20, low-frequency voltages V₁(L) and V₂(L) are applied to the electrodes 21a and 21b, and the voltages are controlled in the range of V₁(L) ≧ V₃(H) ≧ V₂(L) so that the relationship between the low- and high-frequency voltages is reversed in Fig. 7. The former method will hereinafter be referred to as the high-frequency gradient drive method and the latter method as the low-frequency gradient drive method. In the manner described above, the light shutter 110 can be driven to quickly perform the diaphragming operation at desired timing to provide the light transmitting region 41 of a desired area. In the following, some embodiments of the two-frequency drive method according to the present invention will be described using the liquid crystal cell arrays shown in Figs. 3A, 3B, 4A and 4C. To facilitate an easy understanding of the relationships between an array of liquid crystal cells and the drive signals which are applied thereto, there is shown in Fig. 8 the liquid crystal cell array in a symbolic form. In Fig. 8, reference numerals 23₁, 23₂ and 23₃ indicate regions which permits the passage therethrough of light, that is, cell windows defined in the first transparent electrodes 9₁, 9₂, ... and the opposing second transparent electrodes 10₁, 10₂, ... by the edges of the drive lines 21a and 21b and the light shielding layers 12₁, 12₂, ... The opening, closing and diaphragming operations of the cell windows 23₁, 23₂ and 23₃ are controlled by signals D₁, D₂, D₃ and C a , C b which are applied to the individual cell drive lines 20₁, 20₂, 20₃ and the common drive lines 21a, 21b, respectively. [Drive Method-1] The high-frequency gradient drive method will be described first. In Fig. 9, C a and C b are voltage waveforms of common drive signals which are provided to the common drive lines 21a and 21b in Fig. 8; D₁, D₂ and D₃ show, by way of example, voltage waveforms of pixel signals which are provided to the individual cell drive lines 20₁, 20₂ and 20₃ in Fig. 8; and F shows a sequence of modes of operation of the liquid cell array. In an initial close mode period S₁ during which the cell windows 23₁, 23₂ and 23₃ all remain closed, low-frequency voltage waveforms C a and C b of the same wave height are input in an in-phase relation, whereas the voltage waveforms D₁, D₂ and D₃ are held at zero volt. In the next diaphragm mode period I₁ during which at least one of the cell windows 23₁, 23₂ and 23₃ is placed in its diaphragmed state, a high-frequency, high voltage V a and a high-frequency, low voltage V b are applied as the waveforms C a and C b , respectively. The high-frequency voltages are each indicated by a block with a cross marked therein. Further, low-frequency voltages V₁, V₂ and V₃ each corresponding to a pixel signal are applied as the waveforms D₁, D₂ and D₃, respectively. In this example, the low-frequency voltage V₃ is zero and the values of the low-frequency voltages V₁ and V₂ are intermediate between the high-frequency voltages V a and V b . In the period I₁ the high-frequency voltage gradient is developed in all of the cell windows 23₁, 23₂ and 23₃. In this instance, the cell window 23₃ is not supplied with the low-frequency voltage V₃, and hence permits the passage therethrough of light over the entire area thereof (i.e. fully opened). On the other hand, in the cell windows 23₁ and 23₂ supplied over the entire window areas thereof with the low frequency voltages V₁ and V₂, respectively, such a voltage distribution as shown in Fig. 7 is provided because of the high-frequency voltage gradient. Consequently, the cell windows 23₁ and 23₂ are placed in the diaphragmed state. Since the low-frequency voltage V₁ of the waveform D₁ is higher than the low-frequency voltage V₂ of the waveform D₂, the light transmitting region 41 of the cell window 23₁ is smaller in area than the light transmitting region 41 of the cell window 23₂. When the low-frequency voltage V₁ of the waveform D₁ becomes higher than the high-frequency voltage V a of the waveform C a , the cell window 23₁ intercepts light over the entire area thereof (i.e. fully closed). Next, when the liquid cell array enters a close mode S₂, the cell windows 23₁, 23₂ and 23₃ are all switched to the light intercepting state, for they are supplied with the low-frequency voltages of the waveforms C a and C b alone as in the initial close mode S₁. Also in this case, the switching from the diaphragmed state to the light intercepting state is performed at high speed. Next, the low-frequency gradient drive method will be described. In Fig. 10, C a and C b are voltage waveforms of common drive signals which are applied to the common drive lines 21a and 21b in Fig. 8; D₁, D₂ and D₃ show, by way of example, voltage waveforms of pixel signals which are applied to the individual cell drive lines 20₁, 20₂ and 20₃ in Fig. 8; and F shows a sequence of modes of operation of the liquid cell array. In the initial close mode period S₁ during which the cell windows 23₁, 23₂ and 23₃ are all in the closed state, low-frequency voltages of the waveforms C a and C b are applied in the same phase and at the same wave height, whereas the voltage waveforms D₁, D₂ and D₃ are held at zero volt. In the next diaphragm mode period I₁ in which at least one of the cell windows 23₁, 23₂ and 23₃ is placed in the diaphragmed state, a low-frequency, high voltage V a and a low-frequency, low voltage V b are applied as the waveforms C a and C b , respectively. Further, high-frequency voltages V₁, V₂ and V₃ each corresponding to a pixel signal are applied as the waveforms D₁, D₂ and D₃, respectively. In this example, however, the high-frequency voltage V₃ is zero and the wave heights of the high-frequency voltages V₁ and V₂ are both selected intermediate between the low-frequency voltages V a and V b . In the period I₁ the low-frequency voltage gradient is developed in every cell window, but the cell window 23₃ is supplied with the low-frequency voltages V a and V b alone, and hence inhibits the passage therethrough of light over the entire area thereof (i.e. fully closed). On the other hand, in the cell windows 23₁ and 23₂ supplied uniformly over the entire window areas thereof with the high-frequency voltages V₁ and V₂ of the waveforms D₁ and D₂ there is provided the low-frequency voltage gradient, and accordingly, the cell windows 23₁ and 23₂ are placed in the diaphragmed state. Since the high-frequency voltage of the waveform D₁ is higher than the high-frequency voltage V₂ of the waveform D₂, the light transmitting region 41 of the cell window 23₁ is larger in area than the light transmitting region 41 of the cell window 23₂. When the high-frequency voltage V₁ of the waveform D₁ exceeds the low-frequency voltage V a of the waveform C a at that time, the cell window 23₁ permits the passage therethrough of light over the entire area thereof (i.e. fully opened). Next, when the liquid crystal cell array enters the close mode S₂ after the diaphragm mode I₁, the cell windows 23₁, 23₂ and 23₃ are all switched to the light intercepting state, for they are supplied with the low-frequency voltages of the waveforms V a and V b alone as in the initial close mode period S₁. Also in this instance, the switching from the diaphragmed state to the light intercepting state is performed at high speed. While in the above the aperture ratio of each cell window during the diaphragming operation is controlled by controlling the wave height of each applied voltage, it is also possible to change the aperture ratio by changing the pulse width (i.e. the duty cycle) of the high-frequency voltage while keeping the applied voltage constant. In other words, the control of the diaphragming operation depends on the effective value of the applied voltage. A pulse width control method is therefore applicable to the drive methods described above. The method of changing the area of the light transmitting region by controlling the essential voltage application time, as mentioned above, permits easy control by digital circuitry because it utilizes voltage of a fixed wave height. On this account, drive circuitry used is simpler than in the case of changing the aperture ratio by voltage control. [Drive Method-2] In any of the drive methods described above with respect to Figs. 9 and 10, any of the bipolar voltage waveforms D₁, D₂ and D₃ of pixel signals is required to be a waveform which is a combination of signals of three levels, i.e. 0 volt, a positive voltage and a negative voltage. However, the three-level signal poses a problem as it necessitates the use of a more complicated drive circuit than is required for a two-level signal which is a combination of a positive signal and 0 volt or positive and negative signals. This disadvantage is not particularly serious when the number of drive circuits used is small, but its influence is great when the liquid cell array has hundreds of cell windows or more. Then, a description will be given, with reference to Fig. 11, of a diaphragm control using a unipolar signals which affords economization of the drive circuitry. In Fig. 11, C a and C b are voltage waveforms which are applied to the common drive lines 21a and 21b in Fig. 8; D₁, D₂ and D₃ show, by way of example, unipolar voltage waveforms which are applied to the individual cell drive lines 20₁, 20₂ and 20₃ in Fig. 8; and F shows a sequence of modes of operation of the liquid cell array depicted in Fig. 8. In the initial close mode period S₁ in which the cell windows 23₁, 23₂ and 23₃ are all in the closed state, low-frequency voltages of bipolar waveforms C a and C b are applied in the same phase, but the voltage waveforms D₁, D₂ and D₃ are held at zero volt. In the next diaphragm mode period I₁ in which at least one cell window 23₁ is diaphragmed, the cell window 23₂ is fully closed and the cell window 23₃ is fully opened. As in the case of Fig. 9, bipolar high-frequency voltages V a and V b of different values are applied as the voltage waveforms C a and C b , forming the high-frequency voltage gradient. Further, unipolar low-frequency voltages V₁ and V₂ are applied as the voltage waveforms D₁ and D₂. In this example, V a > V₁ > V b . On the other hand, the voltage V₂ is selected higher than or equal to the voltage V a . As a result of this, in the period I₁ the cell window 23₁ is placed in the diaphragmed state, the cell window 23₂ is placed in the light intercepting state because the low-frequency voltage V₂ is higher than the high-frequency voltage V a , and the cell window 23₃ is placed in the light transmitting state because all the signals input thereto are high-frequency. In the next close mode period S₂ in which the cell windows 23₁, 23₂ and 23₃ are closed, they are all switched to the light intercepting state at high speed as in the case of Fig. 9. [Drive Method-3] Incidentally, since the drive methods described above in connection with Figs. 9 and 10 utilize the bipolar voltage waveforms D₁, D₂ and D₃ as pixel signals, the voltage waveforms which are applied to the liquid crystal in each cell of the liquid crystal cell array constitute an AC voltage with no DC component when they are averaged, and consequently, there is no fear of deteriorating the liquid crystal. In the case of using such unipolar voltage waveforms D₁, D₂ and D₃ as described above with regard to Fig. 11, however, the avarage value of the voltage which is applied to the liquid crystal in the liquid crystal cell deviates from zero and assumes a particular one of a positive and a negative value for each diaphragming operation, i.e. the voltage has a DC component. As a result of this, the liquid crystal may be deteriorated and its lifetime may become shorter than in the case of driving the liquid cell by such DC component-free voltage waveforms as described previously in respect of Figs. 9 and 10. In addition, the area of the light transmitting region 41 varies instably during high-speed operation. This problem can be solved by a suitable selection of the voltage waveform which is applied to the liquid crystal. Fig. 12 shows the voltage waveforms C a and C b which are provided to the common drive lines 21a and 21b in Fig. 8, a unipolar voltage waveform D₁ which is provided to the individual cell drive line 20₁, and composite voltage waveforms C a -D₁ and C b -D₁ which are applied across the liquid crystal at the marginal portions of the cell window 23₁ near the common drive lines 21a and 21b. In Fig. 12, F shows a sequence of modes of operation of the cell array. The composite waveforms C a -D₁ and C b -D₁ in the diaphragm mode period I₁ are indicated by the combination of broken line and solid line so as to distinguish the polarity deviations (DC components) from the high-frequency voltages V a (H) and V b (H). In the periods S₁ and S₂ the voltage waveforms C a and C b are fixed at low-frequency voltages equal to each other and the composite voltage waveforms C a -D₁ and C b -D₁ which are provided to the liquid crystal of the cell window 23₁ assume the same waveforms as those of the voltages C a and C b . Consequently, the cell window 23₁ is in the light intercepting state. In the period I₁ the voltage waveforms C a and C b are high-frequency as in the case of Fig. 11 and a unipolar (positive in this example) intermediate voltage V₁₁ is provided as the voltage waveform D₁ for performing the diaphragming operation. In this example V a (H) > V₁₁ > V b (H). A period indicated by J₁ is a DC component compensate mode period, in which an identical unipolar voltage V a is applied to either of the common drive lines 21a and 21b, and the voltage waveform D₁ is provided as a unipolar voltage V₁₂ which is of the same polarity (i.e. positive) as the unipolar voltage V₁₁ in the preceding period I₁ and has a fixed time width. In this example, the voltages V a , V₁₁ and V₁₂ are selected so that V a - V₁₂ = V₁₁ and, therefore, the time width of the period J₁ is equal to the period I₁. With such a method, the effective voltages of the composite waveforms C a -D₁ and C b -D₁, which are provided to the liquid crystal of the cell window 23₁ at its marginal portions near the common drive lines 21a and 21b, are symmetric in the periods I₁ and J₁ as indicated by the solid lines, and consequently, the DC component in the diaphragming mode period I₁ is compensated for in the period J₁. Since the DC component caused by the unipolar drive voltage V₁₁ in the period V₁₁ is uniform over the entire area of the cell window 23₁, the DC component can be compensated for over the entire area of the cell window 23₁. Although in the above the voltages V a and V₁₂ for compensating for the DC component of the waveforms C a , C b and D₁ are described to be set in the same period J₁ with a view to facilitating an easy understanding of the description, the same compensating effect could be obtained by providing the compensating voltages V a and V₁₂ in different time slots. In this case, it is necessary only that the product of the effective voltage (V₁₁) applied to the liquid crystal in the period I₁ and its duration (t₁) be equal in absolute value to and reverse in sign from the product of the voltage (V a - V₁₂) applied to the liquid crystal in the period J₁ and its duration (t₂). Even if the voltages V₁₁ and (V a - V₁₂) and the durations t₁ and t₂ are respectively different from each other, the same effect as mentioned above can be obtained so long as the voltage-time products are equal to each other. [Drive Method-4] According to the above-described drive method-3, it is necessary to provide the compensating period J in the time slot of each of the voltage waveforms C a , C b and D₁ as shown in Fig. 12 and apply the compensating signals in that period so as to eliminate the DC component of the voltages applied to the liquid crystal. This introduces the necessity of decreasing the rate of supply of pixel signals, that is, D₁, D₂ and D₃, to the liquid crystal cell array accordingly. Next, a description will be given, with reference to Figs. 8 and 13, of a liquid crystal cell array drive method which eliminates the DC component of applied voltages caused by use of unipolar pixel signals, without the necessity of independently providing the compensating period. The drive method, which employs waveforms shown in Fig. 13, is the high-frequency voltage gradient drive method, and the aperture ratios of the cell windows 23₁, 23₂ and 23₃ can arbitrarily be varied by controlling the timing of the unipolar pixel signals D₁, D₂ and D₃ which are provided to the individual cell drive lines 20₁, 20₂ and 20₃. Fig. 13 shows, however, an example of driving only the cell window 23₁. Common drive voltage waveforms C a and C b are provided to the common drive lines 21a and 21b. The voltage waveforms C a and C b in each of the periods I₁ and I₂ are equivalent to waveforms which are obtained by dividing the corresponding high-frequency voltage waveforms of Fig. 9 in each of the periods I₁ and I₂ into former and latter halves of the same length and by biasing one of them, the former half in this example, with a voltage equal to the voltage V₁ (which is selected equal to the voltage V a , in this example) of the unipolar individual cell drive signal waveform D₁ in the same polarity. Further, high-frequency voltages V a (H) and V b (H) in the voltage waveforms C a and C b and the voltage V₁ (= V a ) are predetermined fixed values which satisfy the relationship V a ≧ V a (H) > V b (H). In Fig. 13, S₁, S₂ and S₃ are close mode periods in which the cell windows 23₁, 23₂, ... are all closed and these cell windows are not supplied with the pixel signal voltage waveform D₁ but supplied with only the same low-frequency voltage waveforms C a and C b via the common drive lines 21a and 21b. The periods I₁, I₂ and I₃ are diaphragm mode periods in which the aperture ratios of the cell windows 23₁, 23₂, ... are controlled as desired. As shown in the period I₁, the cell drive pixel signal (D₁) is generated at the timing of the latter half of the afore-mentioned high-frequency voltage waveform divided into two. A composite voltage waveform which is applied to the liquid crystal near the common drive line 21a and a composite voltage waveform which is applied to the liquid crystal near the common drive line 21b are depicted as voltage waveforms C a -D₁ and C b -D₁ in the period I₁. The composite voltage waveforms in the period I₁ take such a form that the high-frequency voltage waveforms of the voltages V a (H) and V b (H) are biased by the low-frequency voltage waveform of the voltage V a , and the voltages are selected so that V a ≧ V a (H) > V b (H). Consequently, the cell window 23₁ is fully closed. On the other hand, in the period I₂ the pixel signal D₁ is produced at the timing of the former half of the afore-mentioned high-frequency voltage waveform split into two. In consequence, the composite voltage waveforms which are provided to the liquid crystal of the cell window 23₁ are only high-frequency voltages as shown in the period I₂ of the voltage waveforms C a -D₁ and C b -D₁, so that the cell window 23₁ is fully opened. In the period I₃ the timing for generating the cell drive pixel signal D₁ is selected at any desired location in a range covering both the first and second halves of the high-frequency voltage waveform split into two. As a result, the composite voltage waveforms C a -D₁ and C b -D₁ are each a combination of periods T₁ and T₃ in which the high-frequency voltage waveform is biased by the low-frequency voltage and periods T₂ and T₄ in which they are not biased by the low-frequency voltage, as shown in the period I₃. Consequently, the mean effective voltage which is applied to the liquid crystal of the cell window 23₁ near the common drive line 21a in the period I₃ is mainly a relatively large high-frequency voltage, opening the cell window 23₁. On the other hand, the mean effective voltage which is applied to the liquid crystal of the cell window 23₁ near the common drive line 21b in the period I₃ is mainly the low-frequency voltage, closing the cell window 23₁. In the cell window 23₁ this effective high-frequency voltage gradually decreases from the common drive line 21a toward the line 21b, and on the other side of a certain boundary line, the effective low-frequency voltage is predominant. The position of this boundary line can freely be selected by continuously changing the timing position of the pixel signal D₁ from the position of the first half to the latter half of the two-split high-frequency voltage waveform. In this way, the diaphragming operation of controlling the aperture ratio of the cell window 23₁ is performed in the period I₃. [Drive Method-5] Referring now to Figs. 8, 14 and 15, another liquid crystal cell array drive method using a different unipolar pixel signal will be described in connection with one cell window 23₁. This method utilizes the low-frequency voltage gradient method. The common voltage waveforms C a and C b which are supplied to the common drive lines 21a and 21b are composed of low-frequency voltage waveforms (indicated by the thick lines in Fig. 14) and a unipolar high-frequency voltage V(H) of a fixed wave height (indicated by a block marked with a cross therein in the diaphragm mode periods I₁, I₂ and I₃) superimposed thereon. On the other hand, the voltage waveforms of pixel signals which are applied to the individual cell drive lines 20₁, 20₂, ... are each the same unipolar high-frequency voltage waveform as the high-frequency voltage V(H) superimposed on the voltage waveforms C a and C b , as typically indicated by D₁. In this example, V (H) is a voltage waveform delayed by a phase angle of 180° relative to the high-frequency voltage waveform V(H). In the diaphragm mode periods I₁, I₂ and I₃ the cell window 23₁ is placed in the opened, closed and medium-diaphragmed states, respectively. The operations therefor will be described with reference to Fig. 15. In Fig. 15 there are not shown the voltage waveform C b and the composite waveform C b -D₁ of the voltage waveform C b and the individual cell drive voltage waveform D₁. In the diaphragm mode period I₁ in Fig. 15 the cell window 23₁ is opened. The voltage waveform D₁ of the individual cell drive signal, that is, a pixel signal, is 180° out of phase with the high-frequency voltage waveform V(H) superimposed on the voltage waveform C a which is provided to the common drive line 21a. Consequently, a composite waveform of the voltage waveforms C a and D₁ which are applied to the liquid crystal of the cell window 23₁ near the common drive line 21a takes a voltage waveform in which the high-frequency component is emphasized as shown in C a -D₁, and the cell window 23₁ is fully opened. The composite waveform C a -D₁ is free from a DC component. The period I₂ in Fig. 15 is a period in which to close the cell window 23₁ in the diaphragm mode, and the pixel signal voltage waveform D₁ is in phase with the high-frequency voltage waveform V(H) superimposed on the voltage waveform C a . Consequently, the composite waveform C a -D₁ which is applied to the liquid crystal becomes a low-frequency voltage waveform of no DC component, fully closing the cell window 23₁. The next period I₃ is a period in which to place the cell window 23₁ in the medium-diaphragmed state by the diaphragm mode of operation. The pixel signal voltage waveform D₁ is composed of the high-frequency voltage waveform V(H) provided in the middle of the period I₃ and two high-frequency voltage waveforms V (H) 180° out of phase with the waveform V(H) and provided before and after it. A composite waveform of such a pixel signal voltage waveform D₁ and the voltage waveform C a contains a period in which high-frequency components are emphasized and a period in which low-frequency components are emphasized, as indicated by C a -D₁, and the duty ratio between them is in agreement with the duty ratio between the voltage waveforms V(H) and V (H) of the voltage waveform D₁. By the composite voltage waveform C a -D₁ and a composite voltage waveform C b -D₁ (not shown) a low-frequency voltage gradient is formed in the liquid crystal of the cell window 23₁ from the common drive line 21a toward 21b, providing in the plane of the cell window 23₁ a region where the effective voltage of the superimposed high-frequency voltage is predominant and a region where the effective voltage of the low-frequency voltage is predominant. The former is a light transmitting region and the latter a light intercepting region. The area ratio between these regions is dependent on the above-mentioned duty ratio. According to the low-frequency voltage gradient drive method described above, the composite voltage waveform which is provided to the liquid crystal is not one-sided in polarity as shown by the waveform C a -D₁ although the cell window is driven by the unipolar pixel signal. [Drive Method-6] As described above in conjunction with the liquid crystal cell array drive methods 1 to 5, in either of the high-frequency voltage gradient drive method and the low-frequency voltage gradient drive method the liquid crystal assumes the homeotropic or homogeneous alignment depending on the high-low relation between the effective voltages of the high- and low-frequency voltages which are provided to the liquid crystal. It has been found, however, that when the cell window is driven to permit the passage therethrough of light over the entire area thereof by use of the drive waveforms of the high-frequency voltage gradient drive method or the low-frequency voltage gradient drive method shown in Fig. 9 or 10, the light transmitting state is obtained at high speed in the region where the difference between the high- and low-frequency voltages is large and at low speed in the region where the difference is small, and consequently, exposure of a light-sensitive member is larger in the quantity of light in the former than in the latter, failing to achieve uniform exposure over the entire area of the cell window. On this account, in the case of employing the drive waveforms shown in Fig. 9, a bright image is formed in the region of the cell window near the electrode 21a where the voltage waveform C a is applied and a somewhat dark image is formed in the region near the electrode 21b where the voltage waveform C b is applied. In the case of using the drive waveforms depicted in Fig. 10, an image formed in the region where the voltage C a is applied is darker than in the region where the voltage C b is applied. In the case of recording a white-black image such as a character or the like, the above-mentioned nonuniform exposure over the entire area of the cell window poses problems of thinning of the character and fogging of the white background, leading the formation of an image of poor quality. Next, a description will be given of drive method which provides uniform exposure over the entire area of the cell window in the case of fully opening the window. Figs. 16A, 16B and 17A, 17B are diagrams, similar to Fig. 7, showing voltage distributions in the diaphragm mode period I in the case of the high- and low-frequency voltage gradient methods, respectively. Fig. 16B shows the voltage distribution for transmitting light through the cell window over the entire area thereof in the case of the high-frequency voltage gradient drive method. By virtue of the high-frequency voltage gradients indicated by the broken lines (i) and the low-frequency voltage (ii) set to the ground potential, the region near the common drive line 21a is fast in rising up to the light transmitting state but the region near the common drive line 21b is slow. To compensate this response distribution, drive waveforms are used which develop first a voltage distribution for compensation, shown in Fig. 16A, and then forms the voltage distribution depicted in Fig. 16B. According to this method, in the state shown in Fig. 16A the applied voltage in the region near the drive line 21b is higher than the applied voltage in the region near the drive line 21a, and consequently, switching from the light intercepting state in the period S to the light transmitting state becomes faster toward the common drive line 21b. Then, when the voltage distribution of Fig. 16B is developed after the above voltage distribution, the region near the common drive line 21a rises up quickly, for the applied voltage thereto is higher than that of the region near the common drive line 21b. However, the region near the drive line 21b has already risen up by virtue of the compensating voltage distribution of Fig. 16A, and by adjusting the voltages to be applied and the time of their application for providing the voltage distributions of Figs. 16A and 16B, the amounts of exposure of a light-sensitive member in the regions on the both sides 21a and 21b become substantially equal, forming uniformly exposed picture elements. Fig. 17B shows the voltage distribution for transmitting light through the cell window over the entire area thereof by the low-frequency voltage gradient method. By virtue of the low-frequency voltage gradient indicated by the solid line (ii) and the high-frequency voltage indicated by the broken line (i), the region near the common drive line 21b is fast in rising up to the light transmitting state but the region near the common drive line 21a is slow. To compensate such a response distribution, drive waveforms are used which form first a compensating voltage distribution shown in Fig. 17A and then the voltage distribution depicted in Fig. 17B, as in the case of Fig. 16. With this method, in the compensating voltage distribution shown in Fig. 17A, since the difference between the high- and low-frequency voltages is greater on the side 21a than on the side 21b, switching from the light intercepting state in the all cell close mode period S to the light transmitting state in the diaphragm mode period I becomes faster toward the side 21a. Then, producing the voltage distribution of Fig. 17B, the region near the drive line 21b rises up quickly, for the voltage difference is smaller in this region than in the region near the drive line 21a. However, the region near the side 21a has already risen up by virtue of the voltage distribution of Fig. 17A as is the case with Fig. 16. By controlling the voltage distributions of Figs. 17A and 17B and the voltage application durations, the quantities of light exposure to a photographic material in the region on the both sides 21a and 21b become substantially equal, forming a uniformly exposed picture element. Some examples of drive methods which combine these driving conditions and the diaphragming operations explained previously will be described using the liquid crystal cell array schematically shown in Fig. 8 and various drive voltage waveforms shown in Figs. 18 to 21. Fig. 18 shows various drive voltage waveforms for use in the application of the afore-mentioned response compensating method to the high-frequency voltage gradient drive method. In Fig. 18, C a and C b show common gradient forming voltage waveforms which are fed to the common drive lines 21a and 21b, respectively, D₁ , D₂ and D₃ examples of pixel signal waveforms which are fed to the individual cell drive lines 20₁, 20₂ and 20₃, respectively, and F a sequence of modes of operation. In Fig. 18, low-frequency voltages are each indicated by a broken line showing the base voltage of 0 V in each rectangular block, and high-frequency voltages are each marked with a cross in each block. In the close mode period S₁ in which the cell windows 23₁, 23₂ and 23₃ are all closed, the low-frequency voltages D₁, D₂ and D₃ are applied in the same phase, whereas the voltages of the waveforms C a and C b are held zero. The next period P₁ is a response distribution compensating period in which a compensating voltage distribution corresponding to Fig. 16A is formed for uniformly transmitting light through the cell window over the entire area thereof. In this example, the voltage waveform C a is zero volt and a high-frequency voltage is applied as the voltage waveform C b . At this time, the waveform D₂ is a signal for making the entire area of the cell window light-transmitting and is zero volt. These voltage waveforms C a , C b and D₂ are used to form the compensating voltage distribution shown in Fig. 16A. On the other hand, the voltage waveforms D₁ and D₃ are signals for placing the cell windows 23₁ and 23₃ in the medium-diaphragmed state and the closed state, respectively, and the same low-frequency voltages as in the period S₁ are applied. Under such conditions, the cell windows 23₁ and 23₃ remain in the light intercepting state (that is, the closed state) during the period P₁. In the next period I₁, the voltage waveform C a becomes a high-frequency voltage higher than the voltage waveform C b , forming the high-frequency voltage gradient shown in Fig. 16B. The voltage waveform D₁ forms, together with the voltage waveforms C a and C b , such a voltage distribution as shown in Fig. 7, diaphragming the cell window 23₁. The voltage waveform D₃ maintains the cell window 23₃ in the light intercepting state. Fig. 16A corresponding to the period P₁ and Fig. 16B corresponding to the period I₁ differ from each other in the magnitude of high-frequency voltage applied to the common drive line 21b, but in Fig. 18 the signals C b in the periods P₁ and I₁ are shown to be of the same voltage for convenience of driving. It does not matter whether they are of the same voltage or not. By the combined use of the sets of voltage waveforms described above, it is possible to place a desired cell window in the diaphragmed state, the light transmitting state with uniform exposure over the entire area of the window and the light intercepting state in the high-frequency voltage gradient drive method. Fig. 19 shows, by way of example, simplification of the pixel signal voltage waveforms D₁, D₂ and D₃ by applying low-frequency voltages as the signals C a and C b instead of using the low-frequency voltage waveforms D₁, D₂ and D₃ in the periods S₁, S₂, ... in Fig. 18. Also in this case, the composite voltage waveform which is applied to the liquid crystal becomes exactly the same as in Fig. 18, though not shown. Fig. 20 shows, by way of example, drive waveforms which include the response distribution compensating period in the case of the low-frequency voltage gradient drive method described above in respect of Figs. 17A and 17B. In the period S₁ in which the cell windows 23₁, 23₂ and 23₃ (see Fig. 8) are all held in the closed state, low-frequency voltage waveforms C a and C b are provided to the common drive lines 21a and 21b in the same phase. The voltage waveforms D₁, D₂ and D₃ are held at zero volt. In the next period P₁ a voltage distribution corresponding to Fig. 17A is provided for making, for example, the cell window 23₂ transmit light uniformly over the entire area thereof. In this instance, the voltage waveform C a is a low-frequency voltage lower than the voltage waveform C b which is a low-frequency voltage lower than in the period S₁. The voltage waveform D₂ in the period P₁ is a compensating signal for making the cell window 23₂ transmit light uniformly over the entire area thereof in the next period I₁, and this signal is a high-frequency voltage. The voltage waveforms D₁ and D₃ in the period P₁ are signals for placing the cell windows 23₁ and 23₃ in the light intercepting state, respectively, and they are zero volt as in the period S₁. In the next subsequent period I₁ the voltage waveform C a becomes a low-frequency voltage higher than the voltage waveform C b , forming the low-frequency voltage gradient in Fig. 17B. Further, the high-frequency voltage waveform D₁ is a signal for placing the cell window 23₁ in the medium-diaphragmed state and the voltage waveform D₃ a signal for maintaining the cell window 23₃ in the closed state. By combining the drive voltage waveforms described above, it is possible to place a desired cell window in the light transmitting state with uniform exposure, the diaphragmed state and the light intercepting state also in the low-frequency voltage gradient method. Fig. 21 shows the case where the low-frequency voltage provided as the signals C a and C b in the periods S₁, S₂, ... in Fig. 20 are applied as the signals D₁, D₂ and D₃. In this instance, the composite voltage waveform which is applied across the liquid crystal is the same as in Fig. 20 and produces the same effect as mentioned above. [Drive Method-7] In the examples described above in connection with Figs. 16A to 21, the aperture ratio is controlled by controlling the wave height of the voltage waveform. It is also possible to vary the aperture ratio by changing the high-frequency pulse width (i.e. the duty) while holding the voltage constant. The method of changing the aperture ratio by controlling the duration of effective voltage application permits easy control by a digital circuit in combination with the use of a fixed voltage. This method is therefore advantageous in that the drive circuit structure used is simpler than is required in the method of changing the aperture ratio by voltage control. Moreover, the examples of Figs. 16A to 21 have been described to control the diaphragming operation by the bipolar signal, but a unipolar signal can be employed, in which case the drive circuit used is further simplified. The drive method which involves the compensation of the response distribution (the periods P₁, P₂, ...) shown in Fig. 19 can also be so modified as to use unipolar signals D₁, D₂, ... and compensate for the DC component. Next, a description will be given of this modified method. Fig. 22 shows mere revised versions of the drive waveforms of Fig. 19 for better understanding of the low-frequency voltage waveforms. To reduce the scale of the drive circuitry used, the pixel signal waveforms D₁, D₂, ... in Fig. 22 are changed to unipolar signal waveforms of a fixed wave height as shown in Fig. 23, and the common drive voltage waveforms C a and C b in Fig. 22 are also changed in the same manner as in the case of Fig. 13 to compensate for the DC component which would result from the employment of the unipolar signal waveforms. Fig. 23 shows these drive voltage. waveforms. The voltage waveforms C a and C b in the periods P₁ and I₁ in Fig. 23 are equivalent to those which are obtained by extracting and polarity-inverting the negative portion of the low-frequency voltage waveform D₃ used in Fig. 22 for closing a cell window and biasing the voltage waveforms C a and C b in Fig. 22 with the polarity-inverted waveform portion. This applies to the periods P₂ and I₂. On the other hand, the voltage waveforms C a and C b in the periods S₁ in Fig. 23 are obtained by removing the negative waveform portions of the waveforms C a and C b in the corresponding period in Fig. 22. The removed negative waveforms are inverted in polarity and combined with the voltage waveforms D₁, D₂, ... in the same period, providing the voltage waveforms D₁, D₂, ... in the period S₁ in Fig. 23. The periods S₁, S₂, ... are close mode periods in which to close all the cell windows 23₁, 23₂ and 23₃. The unipolar low-frequency voltages C a and C b which are identical with each other are fed to the common drive lines 21a and 21b, and low-frequency voltages which are 180° out of phase with the voltages C a and C b but of the same polarity are fed to the individual cell drive lines 20₁, 20₂ and 20₃. The periods I₁, I₂, ... are diaphragm mode periods, in which high-frequency voltages biased by such a low-frequency voltage as explained above are fed as the common drive signals C a and C b to the common drive lines 21a and 21b, forming a high-frequency voltage gradient which gradually decreases from the common drive line 21a toward the drive line 21b. On the other hand, the unipolar low-frequency individual cell drive signals D₁, D₂, ... each have, in the first half of each cycle, a waveform of a duty ratio corresponding to a pixel signal and, in the second half of each cycle, a waveform obtained by reversing the duty ratio in the first half cycle. As a result, the effective voltage of the low-frequency component superimposed on the high-frequency component of the composite voltage waveform that is provided to the liquid crystal varies with pixel signals. In the period I₁ in Fig. 23, the cell window 23₂ which is supplied with the waveform D₂ is fully opened because the low-frequency component of the effective voltage applied to the liquid crystal is zero, and the cell window 23₃ which is supplied with the waveform D₃ is fully closed because the low-frequency component of the effective voltage applied to the liquid crystal is equal to or larger than the high-frequency component (the high-frequencies f H1 and f H2 and the low-frequency voltage f L being selected accordingly). In the cell window 23₁ which is supplied with the waveform D₁ the low-frequency component of the effective voltage applied to the liquid crystal becomes intermediate between the low-frequency components in the cases of the waveforms D₂ and D₃, and consequently, the cell window 23₁ is placed in the medium-diaphragmed state. In the compensation mode periods P₁, P₂, ... the high-frequency voltage superimposed on the low-frequency voltage is fed to the common drive line 21b alone, forming a high-frequency voltage gradient which gradually decreases from the common drive line 21b toward the drive line 21a. This high-frequency voltage gradient compensates the response distributions in the cell windows 23₁ and 23₂ in the diaphragm mode periods I l , I₂, ... With the use of the drive waveforms shown in Fig. 23, the response distribution in each cell window can be compensated, and further, composite voltages which are provided to the liquid crystal in each cell window, for example, such composite voltages as C a -D₁ and C b -D₁, are free from the DC component in all of the periods S, P and I. In addition, the voltage waveform D of each individual cell drive signal is unipolar, and hence can be generated by simple-structured digital circuits. [Other Drive Methods] The foregoing has described some examples of the two-frequency drive method for driving the liquid crystal cell array of the present invention. As noted previously, ferroelectric liquid crystal can be used for the liquid crystal cell array of the present invention. Also in the case of employing the ferroelectric liquid crystal, each cell of the cell array operates on the same principle as described previously with respect to Figs. 2A to 2D. The driving of the liquid crystal cell array of the present invention such as schematically shown in Fig. 8, for instance, is far easier than in the case of independently driving each of a plurality of conventional liquid crystal cells (shown in Fig. 1) merely assembled together. In addition, there is no need of providing at least two voltage generators of different frequencies in each drive circuit which is employed for the drive methods 1 to 7. Fig. 24 is a waveform diagram, for explaining a drive method for use in the case of employing the ferroelectric liquid crystal in the liquid crystal cell array of the present invention schematically depicted in Fig. 8. In Fig. 24 there are shown drive waveforms for only the cell windows 23₁ and 23₂. In the period S₁ in which all the cell windows are closed, the same low voltage V L is fed to both of the first and second common drive lines C a and C b , and consequently, no voltage gradient is formed in the cell windows 23₁, 23₂ and 23₃. At this time, the individual cell drive lines 20₁, 20₂ and 20₃ are not supplied with voltage and are held at zero volt. As a result, the voltage which is applied to liquid crystal of the cell windows 23₁, 23₂ and 23₃ is only the same voltage V L which is uniform over the entire area of each cell window, as indicated by composite voltage waveforms C a -D₁, C b -D₁, C a -D₂, C b -D₂. The polarizing directions of the two polarizing plates, disposed on the opposite sides of the liquid cell array have been preset so that light passing through the cell windows 23₁, 23₂ and 23₃ is intercepted in this state. In the period I₁ in which the individual cell windows are diaphragmed as desired, the cell window 23₁ is placed in the medium-diaphragmed state and the cell window 23₂ in the closed state in this example. To perform this, a positive voltage V₁ which is lower than a voltage V H and corresponds to a pixel signal and zero volt are applied to the individual cell drive lines 20₁ and 20₂, respectively. In consequence, the composite voltage which is applied across the liquid crystal of the cell window 23₁ gradually varies from a relatively small positive value to a relatively large negative value in the direction from the drive line 21a toward the drive line 21b, as shown by composite voltage waveforms C a -D₁ and C b -D₁. The region supplied with the positive voltage intercepts light and the region supplied with the negative voltage transmits light therethrough. On the other hand, the composite voltage which is applied across the liquid crystal of the cell window 23₂ gradually varies from a relatively large positive value to a relatively small positive value in the direction from the drive line 21a toward the drive line 21b, as shown by composite voltage waveforms C a -D₂ and C b -D₂, and the cell window 23₂ is closed since the applied voltage thereto is positive over the entire area thereof. In the diaphragm mode period I₂ the cell window 23₁ is placed in the closed state and the cell window 23₂ in the medium-diaphragmed state in a similar manner. If the voltage V₁ which is fed to the individual drive line 20₁ in the period I₁ is selected equal to or higher than the voltage V H , then the composite voltage C a -D₁ will become zero or negative, fully opening the cell window 23₂. Where the liquid crystal used has a property of monostable molecular alignment, even if the afore-mentioned low voltage V L in Fig. 24 is set to zero, the same operation as mentioned above can be performed by arranging the two polarizing plates in a manner to intercept light in the stable alignment state of the liquid crystal. [Specific Operative Example of Drive Circuit] The foregoing description has been given of various drive methods suitable for the liquid crystal cell array of the present invention. Next, a description will be given of a specific operative example of the drive circuit for implementing the drive method-7 described previously with regard to Fig. 23, which is one of the most practical drive methods. Fig. 25 is a block diagram of a common drive signal generating apparatus for generating the voltage waveforms C a and C b shown in Fig. 23. The apparatus comprises a clock generator 50, a signal generator 51, inverters 52₁ and 52₂, analog switches 53₁ to 53₄ and adders 54 a and 54 b . The analog switches 53₂ and 53₄ are ON-OFF controlled by gate signals G₁ and G₂ from the signal generator 51 so that they pass therethrough AC signals f H1 and f H2 from the signal generator 51 to the adders 54 a and 54 b , respectively. The signal generator 51 generates, in synchronism with a system clock SCK from the clock generator 50, two bipolar high-frequency signals f H1 and f H2 of different amplitudes, a unipolar low-frequency signal B and the two gate signals G₁ and G₂ as shown in Fig. 26. The adder 54 a performs an in-phase addition of a zero potential fed via the switch 53₁ or the high-frequency signal f H1 via the switch 53₂ and the signal B and provides the added output as a common drive signal C a . The adder 54 b similarly performs an in-phase addition of a zero potential fed thereto via the switch 53₃ or the high-frequency signal f H2 via the switch 53₄ and the signal B and provides the added output as a common drive signal C b . In this way, the voltage waveforms C a and C b in Fig. 23 are produced. Fig. 27 illustrates a signal converter for converting input image data to pixel signals D₁, D₂, ... which are individual cell drive voltage waveforms shown in Fig. 23. A control signal generator 60 generates, in synchronism with the system clock SCK from the clock generator 50 in Fig. 25, such control signals CL , CK, CLEAR, ACT and CRE as shown in Fig. 28. Successive n image data for one line each of, for example, parallel 6 bits are sequentially input into n stages of parallel 6-bit shift registers 61. The n image data thus loaded in the shift registers 61 are simultaneously latched by the latch signal CL into n 6-bit latches 62₁ to 62 n , and at the same time, n image data of the next line are sequentially input into the shift registers 61. The n image data latched in the latches 62₁ to 62 n are respectively provided to inverters 63₁ to 63 n , from which they are output in inverted or uninverted form according to the logical levels of the control signals ACT and CRE such as depicted in Fig. 28. For example, when the control signal ACT is H-level and the control signal CRE is L-level, the image data is not inverted and the inverters 63₁ to 63 n output the input 6-bit data in uninverted form. When the control signal ACT is L-level and the signal CRE is H-level, the image data is inverted and each inverter outputs the 6-bit image data with the logic of each bit inverted. The 6-bit signals from the inverters 63₁ to 63 n are applied to a pulse width converter 64, outputting therefrom pulses of widths determined by the 6-bit signals. The pulse width converter 64 counts pulses of the signal CK input thereto by the numbers equal to the values of the 6-bit data input from the inverters 63₁ to 63 n , respectively, and generates pulses of durations equal to the respective count values in one cycle specified by the signal CLEAR. The signal CLEAR is produced, for instance, every 2⁶ (i.e. 64) CK pulses and its period is preset to 1/2 the period with which the image data are latched in the latches 62₁ to 62 n . As indicated by the signals ACT and CRE in Fig. 28, the inverters 63₁ to 63 n each repeat the non-inversion and inversion every 64 pulses CK. For example, as indicated by the waveform D₁ in Fig. 23, pulses D₁₁, D ₁₁, D₁₂, D ₁₂, D₁₃, D ₁₃, D₁₄, D ₁₄ are sequentially derived, by the pulse width converter 64, from 6-bit image data D₁₁, D₁₂, D₁₃ and D₁₄ which are applied to the latch 62₁ one after another. Thus, the voltage waveform D₁ is produced. In the compensation mode period P₁ the image data D₁₁ and D₁₂ are both "111111", and in the diaphragm mode period I₁ the image data D₁₃ and D₁₄ can take any arbitrary values. Next, experimental examples will be described. [Experimental Example 1] The liquid crystal cell array of the structure shown in Figs. 3A and 3B was produced as follows: The first transparent electrodes 9₁, 9₂, 9₃, ... forming cell windows were each formed by an indium oxide film having a sheet resistance of 50 Ω/sq, the second transparent electrodes 10₁, 10₂, 10₃, ... disposed opposite the first transparent electrodes were each formed by a tin oxide film having a sheet resistance 100 KΩ/sq, and the drive lines 20₁, 20₂, ... and 21a and 21b were formed by vapor deposition of aluminum. Each cell window was made in a size of 250 µm². The spacer 4 made of polyester and 6 µm thick was sandwiched between the first and second transparent substrates 2 and 3, and the gap defined by the spacer 4 between the first and second transparent substrates was filled with liquid crystal 16 for two-frequency drive. The polarizing plates were mounted on the top of the first transparent substrate 2 and the underside of the second transparent substrate 3, respectively, so that their directions of polarization would intersect each other in the low-frequency drive. The drive lines 20₁, 20₂, ... and 21a and 21b were connected to independent drive circuits. The waveforms shown in Fig. 9 were used as drive waveforms, and the respective waveforms D₁, D₂, ... for application to the individual cell drive lines 20₁, 20₂, ... were controlled in accordance with pixel signals. The frequencies of the waveforms C a , C b and D₁, D₂, ... in Fig. 9 were selected as follows: The frequencies of the low- and high-frequency voltages for the waveforms C a and C b were 2 and 100 KHz, respectively, and the wave height of the low-frequency voltage was ±25 V and the wave height of the high-frequency voltage was ±25 V for C a and ±5 V for C b . The frequency of the low-frequency voltage for the waveforms D₁, D₂ and D₃ in Fig. 9 was 4 KHz and its wave height was 0 to ±25 V. The cell windows 23₁, 23₂, ... were controlled under such frequency conditions. As a result, the cell windows were diaphragmed with desired aperture ratios and their operation characteristics did not markedly change even by repeating the operation 10⁷ times. [Experimental Example 2] The same cell array as used in Example 1 was employed. The waveforms shown in Fig. 10 were used for driving the cell array. The voltage waveforms D₁, D₂, ... were controlled in accordance with pixel signals. The frequencies of the voltage waveforms C a , C b and D₁, D₂, ... used were selected as follows: The frequency of the low-frequency voltage for the waveforms C a and C b was 2 KHz and its wave height was ±25 V. The wave height of the low-frequency voltage in the diaphragm mode periods I₁ and I₂ was ±25 V for C a and ±5 V for C b . The frequency of the high-frequency voltage for the waveforms D₁, D₂, ... was 100 KHz and its wave height was 0 to ±25 V. The cell windows 23₁, 23₂, ... were controlled under such frequency conditions. As a result, the cell windows were diaphragmed with desired aperture ratios and their operation characteristics did not markedly change even after repeating the operation 10⁷ times. [Experimental Example 3] The same cell array as in Experimental Example 1 was used. The waveforms shown in Fig. 11 were used for driving the cell array. The voltage waveforms D₁, D₂, ... were controlled in accordance with pixel signals. The frequencies of the voltage waveforms C a and C b were the same as those in Experimental Example 1. The high-frequency voltages of the waveforms D₁, D₂, ... for individually controlling the cell windows 23₁, 23₂, ... were unipolar voltages and their wave heights ranged from 0 to 25 V. As a result, the cell windows were diaphragmed with desired aperture ratios, but when the cycle period of operation was in excess of about 200 times per second, the area of the light transmitting region 41 of each cell window during the diaphragming operation varied and its high-speed operation was unstable. [Experimental Example 4] The same cell array as in Experimental Example 1 was used. The waveforms shown in Fig. 12 were used for driving the cell array and their concrete frequencies and voltages were the same as those in Experimental Example 3, but in this example, waveforms for compensating for DC component in the period J were additionally employed. As a result, each cell window was diaphragmed with a desired aperture ratio, and even when the cycle period of operation was 400 times per second, the cell window operated stably with substantially no variation in the area of its light transmitting region during diaphragming. Furthermore, the operation characteristics of the cell windows did not markedly change even after repeating the operation 10⁷ times. [Experimental Example 5] The liquid crystal cell array of the structure shown in Figs. 3A and 3B was produced as follows: The first transparent electrodes 9₁, 9₂, ... were each formed by an indium oxide film having a sheet resistance of 50 Ω/sq, the second transparent electrodes 10₁, 10₂, ... were each formed by a tin oxide film having a sheet resistance of 100 KΩ/sq, and the drive lines 20₁, 20₂, ... and 21a and 21b were formed of nickel by electroless plating. Each cell window was 250 µm². Glass beads were interposed as a spacer between the first and second transparent substrates 2 and 3 to define therebetween a gap, which was filled with liquid crystal for two-frequency drive. The polarizing plates were mounted on the top of the first transparent substrate 2 and the underside of the second transparent substrate 3, respectively, with their directions of polarization crossed at right angles to each other. The drive lines 20₁, 20₂, ... and 21a and 21b were connected to independent drive circuits. The waveforms shown in Fig. 13 were employed for driving the liquid crystal cell array. The waveforms D₁, D₂, ... (only the waveform D₁ being shown in Fig. 3) to be applied to the individual cell drive lines 20₁, 20₂, ... were controlled in accordance with pixel signals corresponding thereto. The frequencies of the waveforms C a , C b and D₁, D₂, ... used were selected as follows: The frequency of the low-frequency voltage used for C a and C b in the period I₁ was 1 KHz and a bipolar high-frequency voltage 100 KHz was superimposed on the low-frequency voltage. The wave height of the low-frequency voltage was set to ±25 V and the wave height of the high-frequency voltage ±20 V for C a and ±5 V for C b . As the low-frequency voltage signal for each of the waveforms D₁, D₂, ... in Fig. 13, a +25 V pulse with a 0.5 ms width was used and its rise-up timing was controlled within the range of each of the periods I₁, I₂ and I₃ of a 1 ms time slot. The cell windows 23₁, 23₂, ... were controlled under such conditions and could be diaphragmed with desired aperture ratios. [Experimental Example 6] The liquid crystal cell array used was the same as in Experimental Example 1. The waveforms shown in Fig. 14 were used for driving the cell array. The waveforms D₁, D₂, ... (only the waveform D₁ being shown in Fig. 14) to be applied to the individual cell drive lines 20₁, 20₂, ... were controlled in accordance with pixel signals corresponding thereto. The waveforms C a , C b and D₁, D₂, ... in Fig. 14 were selected as follows: The frequency of the low-frequency voltage for C a and C b was set to 1 KHz and a unipolar +20 V high-frequency voltage of a frequency 100 KHz was superimposed on the low-frequency voltage. The wave height of the low-frequency voltage in the periods I₁, I₂, ... was set to ±25 V for C a and ±5 V for C b . A +20 V, 100 KHz high-frequency voltage was used for D₁, D₂, ... The cell windows 23₁, 23₂, ... were controlled by applying thereto the high-frequency voltage in phase or 180° out of phase with the low-frequency voltage signals C a and C b . As a result, the cell windows 23₁, 23₂, ... could be diaphragmed with desired aperture ratios. [Experimental Example 7] The liquid crystal cell array used was the same as in Experimental Example 1. The waveforms shown in Fig. 18 were employed for driving the liquid crystal cell array. The waveforms D₁, D₂, ... to be applied to the individual cell drive lines 20₁, 20₂, ... were controlled in accordance with pixel signals corresponding thereto. The frequencies of the waveforms C a , C b and D₁, D₂, ... were selected as follows: The frequency of the low-frequency voltage for the waveforms C a and C b was set to 1.3 KHz and the frequency of the high-frequency voltage therefor was set to 150 KHz. The wave height of the low-frequency voltage was set to ±30 V and the wave height of the high-frequency voltage was set to ±30 V for C a and ±15 V for C b . As a result, the cell windows 23₁, 23₂, ... could be diaphragmed with desired aperture ratios and placed in the light transmitting state with uniform exposure over the entire area of each cell. [Experimental Example 8] The liquid crystal cell array used was the same as in Experimental Example 1. The waveforms shown in Fig. 20 were employed for driving the liquid crystal cell array. The waveforms D₁, D₂, ... to be applied to the individual cell drive lines 20₁, 20₂, ... were controlled in accordance with pixel signals corresponding thereto. The frequencies of the waveforms C a , C b and D₁, D₂, ... were selected as follows: The frequency of the low-frequency voltage for the waveforms C a and C b was set to 1.5 KHz, and its wave height was set to ±30 V in the periods S₁, S₂, ..., to ±15 V for C a and ±25 V for C b in the periods P₁ and P₂ and to ±25 V for C a and ±10 V for C b in the periods I₁, I₂, .... The frequency of the high-frequency voltage for the waveforms D₁, D₂, ... in Fig. 20 was set to 150 KHz and its wave height in the range of 0 to ±30 V. As a result, the cell windows 23₁, 23₂, ... could be diaphragmed with desired aperture ratios and place in the light transmitting state with uniform exposure over the entire area of each cell. [Experimental Example 9] The same liquid crystal cell array as in Experimental Example 1 was used. The waveforms shown in Fig. 23 were employed for driving the liquid crystal cell array. The voltage waveforms D₁, D₂, ... to be applied to the individual cell drive lines 20₁, 20₂, ... were controlled in accordance with pixel signals corresponding thereto. The frequencies of the waveforms C a , C b and D₁, D₂, ... were selected as follows: The low-frequency voltage for the waveforms C a and C b was 2 KHz in frequency and unipolar and its wave height was set to 30 V. This low-frequency voltage is the low-frequency voltage B referred to previously in connection with Figs. 25 and 26. The frequencies of the high-frequency voltages f H1 and f H2 were set to 150 KHz and the wave height of the former was set to ±25 V and the wave height of the latter ±10 V. The signals C a and C b were produced by the circuit depicted in Fig. 25. The low-frequency voltage for the waveforms D₁, D₂, ... was 30 V in wave height and unipolar. As a result, the cell windows 23₁, 23₂, ... could be diaphragmed with desired aperture ratios. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims: A liquid crystal cell array comprising: a first transparent substrate (2); a second transparent substrate (3) disposed in parallel and opposite to the first transparent substrate such that the inner surface of the first transparent substrate faces the inner surface of the second transparent substrate; a plurality of first transparent electrodes (9₁, 9₂, 9₃, ...) arranged side by side in a line on the inner surface of the first transparent substrate (2); a plurality of individual cell drive lines (20₁, 20₂, 20₃, ...) formed on the inner surface of the first transparent substrate (2) and each connected to a respective one of the plurality of first transparent electrodes (9₁, 9₂, 9₃, ...); a second transparent electrode (10) formed on the inner surface of the second transparent substrate (3) in an opposed relation to the first transparent electrodes (9₁, 9₂, 9₃, ...); first and second common side electrodes formed along and contacting two opposite sides, respectively, of the second transparent electrode (10), said sides substantially parallel to the direction of arrangement of said first transparent electrodes (9₁, 9₂, 9₃, ...); a first common drive line (21a) formed on the inner surface of the second transparent substrate (3) and connected to the first common side electrode; a second common drive line (21b) formed on the inner surface of the second transparent substrate (3) and connected to the second common side electrode; and a liquid crystal (16) sealed between the first and second transparent substrates (2, 3); wherein regions where the first transparent electrodes (9₁, 9₂, 9₃ ...) and the second transparent electrode (10) overlap define a plurality of cell windows (23). The cell array of claim 1, wherein the plurality of first transparent electrodes (9₁, 9₂, 9₃, ...) are aligned at regular intervals. The cell array of claim 1, wherein the first and second common side electrodes and the first and second common drive lines (21a, 21b) are made of metal, the first common side electrode and the first common drive line (21a) and the second common side electrode and the second drive line (21b) being formed as one body, respectively. The cell array of claim 1, 2 or 3, wherein the second transparent electrode (10) comprises a plurality of substantially square shaped electrodes (10₁,..., 10₅,...) arranged in a line and spaced apart from one another, said two opposite sides of the second transparent electrode being substantially parallel to said line. The cell array of claim 1, 2 or 3, wherein the second transparent electrode (10) is a single stripe-shaped transparent electrode, portions of which being covered by a light-shielding material (12) such that between those portions a plurality of cell windows is defined at regular intervals in the lengthwise direction of the stripe-shaped transparent electrode (10). The cell array according to any one of the preceding claims , wherein the second transparent electrode (10) has a sheet resistance higher than those of the first transparent electrodes. The cell array of claim 6, wherein the sheet resistance of the second transparent electrode is in the range of 10⁸ to 10⁵ Ω/sq. The cell array according to any one of the preceding claims , wherein the liquid crystal is a ferroelectric liquid crystal. The cell array according to any one of claims 1 to 7, wherein the liquid crystal is one having a dielectric anisotropy of a first sign at frequencies below a characteristic transition frequency f c and the opposite second sign at frequencies above said transition frequency. A method of driving a liquid crystal cell array as defined in any one of the preceding claims, wherein, in a diaphragm mode for arbitrarily controlling, in accordance with respective pixel signals, the aperture ratio of each cell window, a first and a second common drive voltage different from each other are applied to the first and second common drive lines, respectively, so as to form a voltage gradient in the second transparent electrode, and at the same time cell drive voltages correponding to the pixel signals are applied to the cell drive lines, respectively. The method of claim 10 for driving a liquid crystal cell array as defined in 8, wherein, in a close mode for closing all the cell windows, said first and second common drive voltages are low voltages of the same polarity while all of said cell drive voltages are zero. The method of claim 10 for driving a liquid crystal cell array as defined in 9, wherein, in a close mode for closing all the cell windows, said first and second common drive voltages and said cell drive voltages are such as to result in a lower-frequency voltage having a frequency lower than said transition frequency being applied across the second transparent electrode and each of the first transparent electrodes, and in the diaphragm mode said first and second common drive voltages have a first frequency while each cell drive voltage has a second frequency, one of said first and second frequencies being higher, higher-frequency, and the other lower, lower-frequency, than said transition frequency. The method of claim 12, wherein a diaphragm mode period is preceded by a compensation mode period compensating for a nonuniform response distribution within a cell window, the amplitude ratio between said first and second common drive voltages in the compensation mode period being reverse compared to that during the diaphragm mode period. The method of claim 13, wherein the common drive voltages are higher-frequency voltages of which in the compensation mode period the smaller one is zero. The method of claim 13, wherein the lower-frequency voltage in the close mode is applied to each individual cell drive line. The method of claim 13, wherein the lower-frequency voltage in the close mode is applied to the first and the second common drive lines. The method of claim 12 or 13, wherein said cell drive voltages applied in the diaphragm mode are unipolar lower-frequency voltages. The method of claim 17, wherein the higher-frequency first and second common drive voltages are bipolar and the diaphragm mode is preceded or followed by a compensation period compensating for a DC component of the voltage applied to the liquid crystal in the diaphragm mode. The method of claim 17, wherein the higher-frequency first and second common drive voltages are each composed of a unipolar lower-frequency voltage component to which a bipolar higher-frequency voltage component is superimposed with the amplitude of the superimposed higher-frequency voltage component of the second common drive voltage being different from that of the first common drive voltage, and each cell drive voltage has the same fixed amplitude and a timing relative to said lower-frequency voltage component corresponding to the respective pixel signal. The method of claim 13 where the common drive voltages are higher-frequency voltages, wherein in all operation mode periods a first unipolar lower-frequency voltage is applied as a bias voltage to the first and the second common drive line, in the close mode period, a second unipolar lower-frequency voltage having the same frequency as but being phase shifted by 180° with respect to the first unipolar lower-frequency voltage is applied to each individual cell drive line, and in the diaphragm mode period the second unipolar lower-frequency voltage which is applied to each individual cell drive line has, in the first half of each of its cycles, a waveform of a duty ratio corrsponding to a respective pixel signal and, in the second half, a waveform of a duty ratio reverse from that in the frist half.

Inventors: MASUMORI TADAAKI C O NIPPON TE, OOBA YUJI C O NIPPON TELEGRAPH, SUGIHARA SHIGEO C O NIPPON TEL, MASUMORI, TADAAKI C/O NIPPON TELEGRAPH AND, OOBA, YUJI C/O NIPPON TELEGRAPH AND, SUGIHARA, SHIGEO C/O NIPPON TELEGRAPH AND
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1988
Main classification: G02F1
IPC codes: G02F1, G09G3