GB1601449A

GB1601449A - Liquid crystal cells

Info

Publication number: GB1601449A
Application number: GB407577A
Authority: GB
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1977-01-05
Filing date: 1977-02-01
Publication date: 1981-10-28

Description

(54) IMPROVEMENTS RELATING TO LIQUID CRYSTAL CELLS (71) We, BRITISH AEROSPACE PUBLIC LIMITED COMPANY, a British Company organised under British Aerospace (Nominated Company) Order 1980 and British Aerospace (Appointed Day) Order 1980, of 100 Pall Mall, London, SW1Y 5HR, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to twisted nematic liquid crystal cells and to video signal display apparatus comprising such cells.

In one kind of twisted nematic liquid crystal cell, known as a transmission cell, two spaced transparent plates, of glass for example, contain between them liquid crystal material of which the molecules can be considered to be elongate and to lie in layers parallel to the plates. In each layer the molecules are aligned in the same direction and this direction varies progressively from layer to layer. This provides the cell with the ability to rotate the direction of polarisation of polarised light passing through the cell.

By the application of a suitable electric field to the cell a torque may be induced to act on the molecules to align them perpendicular to the two plates and hence reduce the polarisaton rotating effect.

Whether the molecules tend to align themselves perpendicular to or parallel to the voltage gradient of the applied field determines the sign of a parameter known as the dielectric anisotropy of the liquid crystal material. If the molecules tend to become parallel to the voltage gradient, the dielectric anisotropy is positive and vice versa. Also this parameter is dependent upon the frequency of the applied field and there can be associated with a liquid crystal material a critical frequency at which the parameter changes sign.It is known to use this effect to produce a forced turn-off of the cell, i.e. following the application of a low frequency field between the plates of a cell having a positive dielectric anisotropy at low frequencies and thereby causing the molecules to align themselves perpendicular to the plates, to apply a signal burst of abovecritical frequency to which the molecules respond by moving back to their original parallel alignments more quickly than they would if the high frequency burst were not applied.

According to one aspect of the present invention, there is provided a liquid crystal device including a twisted nematic liquid crystal cell having associated therewith a critical frequency of an electric field applied to the cell, at which frequency the dielectric anisotropy of the liquid crystal material undergoes a sign reversal, the device further including control means for applying an electric field to the liquid crystal material and thereby to exert a torque upon the molecules of the material and produce an optical response in the cell, the control means comprising pulse generating means for supplying to the cell first and second pulse signals which are so coherently synchronised that when both signals are supplied to the cell, they combine to have a frequency spectrum different from the simple sum of the respective spectra of the two signals and the frequency spectrum of the first signal being so arranged about said critical frequency that this signal alone produces substantially no nett torque upon said molecules.

Preferably, the first and second pulse signals are dipolar signals.

The second pulse signal may be such that, alone, it produces an electric field having maximum effect on the liquid crystal molecules while the combination of the first and second pulse signals reduces that effect, or the second pulse signal may be such that alone it produces an electric field having minimum effect on the molecules while the combination increases that effect, or the second pulse signal alone may produce an intermediate effect on the molecules while the pulse generating means is controllable so that the combination of the pulse signals supplied thereby is able to reduce and increase that intermediate effect as desired.

The liquid crystal cell is preferably a matrix a dressed cell of which respective portions are addressable by selecting combinations of respective signal receiving members from a first and a second set of such members with which the cell is provided, the pulse generating means being operable to apply the first and second pulse signals to the first and second sets respectively. The first and second sets of signal receiving members may be in the form of respective crossed sets of electrodes on respective opposite faces of the cell.

According to a second aspect of the invention, there is provided display apparatus for visually representing a video signal, the apparatus comprising a liquid crystal device having a matrix addressed ccii as described above, and illuminating means for directing distributed light towards the cell of the liquid crystal device, said pulse gene rate ing means being responsive to raster scanning synchronisation signal supply means to apply said second pulse signal in sequence to the signal receiving members of the second set thereof thereby to address, in sequence, respective parallel linear regions of the cell, and being responsive to picture signal supply means to apply said first pulse signal to the signal receiving members of the first set thereof to modulate the optical response of respective portions of the cell arranged along each said linear region when it is addressed by application thereto of said second pulse signal.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 shows diagrammatically the construction of a matrix addressed liquid crystal cell, Figures 2 and 3 show examples of pulse signals for controlling the Figure 1 cell, and Figure 4 is a simplified diagram of video signal display apparatus.

Figure 1 shows the construction of a matrix addressed liquid-crystal cell. It comprises two glass plates 1 and 2 carrying arrays 3 and 4 respectively of parallel strip electrodes. When assembled into a cell the plates are arranged so that the strips are orthogonal to each other thereby defining a matrix of elemental regions in a liquid crystal material confined within a spacer 11 sandwiched between the plates. Signals applied to a selected pair of orthogonal electrodes will produce a potential difference and create an electric field at their point of intersection. In nematic liquid-crystals of relatively low resistivity (ca 107 ohm.cm) the applied electric field can excite dynamic scattering, an effect associated with the passage of a small current through the material.For nematics of high resistivity (1010 ohm.cm) various field effects are possible depending on the equilibrium alignment which has been induced by wall treatment, the sign of the dielectric anisotropy, he, of the liquid crystal and the direction of the applied field. In a twisted nematic liquid crystal cell, the electrode bearing front and back walls 1 and 2 of the cell are treated in such a way that the liquid-crystal molecules all lie in the plane of these walls, those near one wall lying in a different direction from those near the other opposite wall, and across the thickness of the cell there is a continuous twist of molecule directions within the plane of the cell.

The angle of twist is typically 90" and this has the effect of rotating through 90" the plane of incoming linearly polarized light.

When a suitable electric field is applied through the electrodes, i.e. in a direction normal to the cell walls, the molecules align in this direction and the twisted arrangement and optical rotary power are destroyed. Thus, by placing the cell between crossed polarizers (not shown) a region of the cell may be switched from clear (off) to dark (on) by application of an electric field to that region via the appropriate pair of electrodes from arrays 3 and 4. Other field effects are possible with different types of twisted nematic liquid crystal cell.

As mentioned earlier, the liquid crystal molecules align themselves parallel to the voltage gradient of the applied field if the dielectric anisotropy of the material is positive and this dielectric anisotropy may change sign at a particular critical frequency of the applied field. Because of this frequency dependence of the dielectric anisotropy, it is found that when a train of pulses is used to produce the field across the cell, the optical response of the cell is strongly dependent on pulse width. As is well known, application of Fourier's Theorem to the analysis of pulse waveforms shows that such waveforms are made up of an extensive spectrum of components at different frequencies, the content of the spectrum being dependent primarily upon pulse width and secondly upon pulse shape.

In one embodiment of the present invention, with a matrix addressed twisted nematic liquid-crystal cell such as that shown in Figure 1, the row electrodes 4 of the matrix are addressed individually by separate row pulses, three of which are shown at (a), (b) and (c) in Figure 2, and each of which comprises a dipolar pulse of half-width n.

Meanwhile, each column electrode receives the pulse train shown at (d) in the figure.

This column pulse train comprises a series of pairs of narrow pulses each of width m and separated by a time exactly 2n. Waveforms (e) and (f) show the effect of the combination of these address pulses at the elemental regions formed by row 1 - column 1 and row 2 - column 1 respectively. When row and column pulses coincide at an addressed elemental region, pulse width addition occurs to produce dipolar pulses of half width n + m. The pulse trains are made such that the induced response of the optical parameter at an element of the cell is variable from zero to maximum by means of combined pulses having a range of from 2n to 2(n + mmaX) It follows, therefore, that both the row and column pulses alone must have no nett effect on the liquid crystal material.This is done by ensuring that each train of pulses possess frequency spectrums distributed about the critical frequency so as to give a balanced effect, that is the torque exerted by field components tending to align the liquid crystal molecules parallel to the direction of the field is balanced by the torque exerted by the field components tending to align the molecules perpendicular to the field. Meanwhile, since the row and column pulse trains are synchronised to one another as shown, they combine to produce pulse width addition, i.e. they combine to have an overall frequency spectrum, different to the simple sum of the two component spectra, including lower frequency components not balanced by above critical frequency components and hence producing a cell response.

In order to achieve a proper arrangement of the frequency spectrum of the column pulse train about the critical frequency, additional complexity could be introduced therein, for example as shown at g in Figure 2. Here, there are again pairs of dipolar signal pulses synchronised to the row pulses so as to modify the width thereof but, since these pulses are wider than those shown at d, the consequential lower frequency components of each pair of these pulses alone are balanced by following each with groups of much narrower pulses, the number of these narrower pulses in each group being appropriate to give the correct balance considering the width of the preceding pair of signal pulses.

Instead of being as described above, the row addressing pulses could be such that, alone, they produce a maximum response of the cell, i.e. the frequency spectrum and amplitude of these pulses could produce a maximum torque on the liquid crystal molecules. Meanwhile, the column pulses are still such that alone they produce no nett torque but, in combination with the row pulses, so affect the frequency spectrum of the combined signal that the molecules respond less to the combined signal than to the row pulses alone. This effect can be achieved as shown in Figure 3.At a in Figure 3, the column pulses are arranged to in effect reduce the widths of the row pulses, instead of adding to the widths as in Figure 2, whereby the frequency spectrum of the combined signal is to altered in relation to that of the row pulses alone, that its effect is like that of a signal nearer in frequency to the critical frequency, and hence less effective, than the row signal. The same effect is achieved at b and c in Figure 3 by so arranging the column pulses that, in combination with the row pulses, a complex pulse is produced. Here again, the column pulses alone produce no nett torque on the liquid crystal molecules but, because they are coherently synchronised with the row pulses, the frequency spectrum of the combined pulses is such that these pulses produce less cell response than the row pulses alone.

Further methods of cell response control are also possible employing, for instance, simultaneous addition and subtraction wherein the original row pulse is arranged to initially activate the liquid crystal cell to approximately half maximum response, addition of the column pulses will then lengthen the combined pulse to increase cell response towards maximum, and subtraction of the column pulses will shorten the combined pulse to reduce cell response towards minimum.

The total number N of columns in the matrix that can be address in this way is limited by the formula: N = Td/Ta where, Td, is the decay time of the optical response, and Ta is the minimum address time. The line time Tl that is the time between column pulses intended for elements on successive rows, is 1/Nth of the frame time Tf, that is the time between pulses on any one row. There is therefore a limitation imposed by the details of the variation of response with pulse width and the variation with pulse repetition period T.

The row pulses, being much less frequent than the column pulses, must be much longer, yet the two in combination must raise the response from around threshold to as high a value as possible. However, pulse widths are not additive in there effect like voltage. Many small voltage pulses deli vered quickly have the combined effect of a large voltage pulse, but many narrow pulses in quick succession do not have the effect of a wide pulse.

In practice, it has been found that the response at a single element can be varied from zero to full for N up to 150 elements without exciting any response at other elements along this same column. This can be achieved with frame periods at least as low as 40mS which implies a line period of around 270is. Even at this rapid rate, response can be kept sub-threshold for pulse widths 2m up to 701us. At this rate, operating voltage would be about 150V and row pulse width 2n = 200cos. By moving to a frame period of 200ms at which decay of response and hence flicker is just perceptible, a voltage of 100V can be used with 2n = 500us. The response can also be effected by pulse shaping.For example, the voltage threshold can be raised by using very sharp pulses and lowered by using pulses with rounded edges. This simplifies the matching of row and column voltages.

The liquid crystal material used in this example has a critical or cross-over frequency of approximately 5KHz. Whilst this frequency is variable as a function of ambient temperature, it is found that there is sufficient latitude in setting up the row and column pulses to give sub-threshold response, and that the system is not excessively temperature sensitive. The frame rate, number of lines, voltage, pulse widths, width of grey scale and operating temperature range are all complexly linked together.

However, there is a degree of flexibility in these parameters which makes the system very suitable for displays of high degree of complexity. The natural decay time of around 200ms is not far from acceptable for T.V. application. Forced turn-off at the end of each frame by applying a burst of high frequency pulses may improve the temporal response characteristics. It is envisaged that minor adjustments to the composition of the liquid-crystal will result in optimization for particular applications. It should be noted that in the addressing method described, the critical or cross-over frequency alone is not used, so, undesirable dynamic scattering effects are not observed. Rather a balance between frequencies at which AE is positive and those at which it is negative is invoked.

In a T.V. reproduction system shown in Figure 4 a matrix addressed liquid crystal cell 20 is positioned in front of a distributed light source (not shown) and is addressed by signals derived from the video ouptut 21 from a t.v. camera 22 and the scanning control signals 23.

The system operates by ordering scanning control signals 23 through a master period generator 24 and line counter 25 to sequentially enable the outputs of a row generator 26 to correspondingly gate invariable dipolar pulses of suitable amplitude and with, such that the pulses alone produce no nett optical response, from pulse shaping circuit 27 to sequentially address the rows in the matrix of cell 20. Simultaneously the video output signals 21 from camera 22 are temporarily stored in storage unit 28 to control a bank 29 of parallel pulse width modulators which control column generators 30 so that all of the columns of the matrix of cell 20 are simultaneously addressed with pulses which vary in accordance with the level of the video output thus representing picture brightness.The frequency spectrum of the column pulses alone is such as to produce no nett optical effect until combined with the row pulses. Thus the final result is to vary the optical parameter in each element of the matrix cell 20 in accordance with the required picture brightness and thereby modulate the light from the light source behind the cell to produce the required picture.

WHAT WE CLAIM IS: 1. A liquid crystal device including a twisted nematic liquid crystal cell having associated therewith a critical frequency of an electric field applied to the cell, at which frequency the dielectric anisotrophy of the liquid crystal material undergoes a sign reversal, the device further including control means for applying an electric field to the liquid crystal material and thereby to exert a torque upon the molecules of the material and produce an optical response in the cell, the control means comprising pulse generating means for supplying to the cell first and second pulse signals which are so coherently synchronised that when both signals are supplied to the cell, they combine to have a frequency spectrum different from the simple sum of the respective spectra of the two signals and the frequency spectrum of the first signal being so arranged about said critical frequency that this signal alone produces substantially no nett torque upon said molecules.

2. A liquid crystal device according to claim 1 wherein the generated pulses are dipolar.

3. A liquid crystal device according to claim 1 or 2, wherein the second pulse signal is such that, alone it produces an electric field having maximum effect on the liquid crystal molecules while the combination of the first and second pulse signals reduces that effect.

4. A liquid crystal device according to claims 1 or 2, wherein the second pulse signal is such that, alone, it produces an electric field having minimum effect on the liquid crystal molecules while the combination of the first and second pulse signals increases that effect.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. vered quickly have the combined effect of a large voltage pulse, but many narrow pulses in quick succession do not have the effect of a wide pulse. In practice, it has been found that the response at a single element can be varied from zero to full for N up to 150 elements without exciting any response at other elements along this same column. This can be achieved with frame periods at least as low as 40mS which implies a line period of around 270is. Even at this rapid rate, response can be kept sub-threshold for pulse widths 2m up to 701us. At this rate, operating voltage would be about 150V and row pulse width 2n = 200cos. By moving to a frame period of 200ms at which decay of response and hence flicker is just perceptible, a voltage of 100V can be used with 2n = 500us. The response can also be effected by pulse shaping.For example, the voltage threshold can be raised by using very sharp pulses and lowered by using pulses with rounded edges. This simplifies the matching of row and column voltages. The liquid crystal material used in this example has a critical or cross-over frequency of approximately 5KHz. Whilst this frequency is variable as a function of ambient temperature, it is found that there is sufficient latitude in setting up the row and column pulses to give sub-threshold response, and that the system is not excessively temperature sensitive. The frame rate, number of lines, voltage, pulse widths, width of grey scale and operating temperature range are all complexly linked together. However, there is a degree of flexibility in these parameters which makes the system very suitable for displays of high degree of complexity. The natural decay time of around 200ms is not far from acceptable for T.V. application. Forced turn-off at the end of each frame by applying a burst of high frequency pulses may improve the temporal response characteristics. It is envisaged that minor adjustments to the composition of the liquid-crystal will result in optimization for particular applications. It should be noted that in the addressing method described, the critical or cross-over frequency alone is not used, so, undesirable dynamic scattering effects are not observed. Rather a balance between frequencies at which AE is positive and those at which it is negative is invoked. In a T.V. reproduction system shown in Figure 4 a matrix addressed liquid crystal cell 20 is positioned in front of a distributed light source (not shown) and is addressed by signals derived from the video ouptut 21 from a t.v. camera 22 and the scanning control signals 23. The system operates by ordering scanning control signals 23 through a master period generator 24 and line counter 25 to sequentially enable the outputs of a row generator 26 to correspondingly gate invariable dipolar pulses of suitable amplitude and with, such that the pulses alone produce no nett optical response, from pulse shaping circuit 27 to sequentially address the rows in the matrix of cell 20. Simultaneously the video output signals 21 from camera 22 are temporarily stored in storage unit 28 to control a bank 29 of parallel pulse width modulators which control column generators 30 so that all of the columns of the matrix of cell 20 are simultaneously addressed with pulses which vary in accordance with the level of the video output thus representing picture brightness.The frequency spectrum of the column pulses alone is such as to produce no nett optical effect until combined with the row pulses. Thus the final result is to vary the optical parameter in each element of the matrix cell 20 in accordance with the required picture brightness and thereby modulate the light from the light source behind the cell to produce the required picture. WHAT WE CLAIM IS:

1. A liquid crystal device including a twisted nematic liquid crystal cell having associated therewith a critical frequency of an electric field applied to the cell, at which frequency the dielectric anisotrophy of the liquid crystal material undergoes a sign reversal, the device further including control means for applying an electric field to the liquid crystal material and thereby to exert a torque upon the molecules of the material and produce an optical response in the cell, the control means comprising pulse generating means for supplying to the cell first and second pulse signals which are so coherently synchronised that when both signals are supplied to the cell, they combine to have a frequency spectrum different from the simple sum of the respective spectra of the two signals and the frequency spectrum of the first signal being so arranged about said critical frequency that this signal alone produces substantially no nett torque upon said molecules.

5. A liquid crystal device according to

claim 1 or 2, wherein the second pulse signal is such that, alone, it produces an electric field having an intermediate effect on the liquid crystal molecules and the pulse generating means is controllable so that the combination of the first and second pulse signals supplied thereby is operative to increase and decrease the effect.

6. A liquid crystal device according to any preceding claim wherein the liquid crystal cell is of the light transmitting type and is positioned between crossed polarising light filters.

7. A liquid crystal device according to any preceding claim, wherein said cell is a matrix addressed cell of which respective portions are addressable by selecting combinations of respective signal receiving members from a first and a second set of such members with which the cell is provided. and wherein the pulse generating means is operable to apply said first pulse signals to the first set of signal receiving members and the second pulse signals to the second set of signal receiving members.

8. A liquid crystal device according to claim 7, wherein said first set of signal receiving members comprises electrodes arranged on one face of the cell and the second set of signal receiving members comprises electrodes arranged on the opposite face of the cell and in intersecting relationship with the electrodes arranged on said one face of the cell.

9. Display apparatus for visually representing a video signal, the apparatus comprising a liquid crystal device according to claim 7 or 8, and illuminating means for directing distributed light towards the cell of the liquid crystal device, said pulse generating means being responsive to raster scanning synchronisation signal supply means to apply said second pulse signal in sequence to the signal receiving members of the second set thereof thereby to address, in sequence, respective parallel linear regions of the cell, and being responsive to picture signal supply means to apply said first pulse signal to the signal receiving members of the first set thereof to modulate the optical response of respective portions of the cell arranged along each said linear region when it is addressed by application thereto of said second pulse signal.

10. Display apparatus for visually representing a video signal, the apparatus being substantially as hereinbefore described with reference to the accompanying drawings.

11. A liquid crystal device substantially as described herein with reference to the accompanying drawings.