CA2005403C - Display device - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A ferroelectric liquid crystal device has a first state (TX1) of maximum transmission, a second state (TX2) of minimum transmission and a value of voltage pulse width (tg) and voltage pulse height (VS) sufficient for a switching pulse to switch the cell from the first state (TX1) to the second state (TX2) or vice verse. A method of controlling the transmission of electromagnetic radiation through the ferroelectric liquid crystal device comprises the step of applying, for a time period greater than said value of pulse width (tS), a plurality of consecutive controlling pulses of one polarity. Each controlling pulse is itself of insufficient pulse height and pulse width to switch the cell from the first state (TX1) to the second state (T12) or vice versa.

Description

2005~03 DISPLAY DEVICE

This invention relates to a method of addressing a ferroelectric llquid crystal device lPLCD), in particular to a method of controlling the transmission of electromagnetlc radiation through such a device. This method is particularly, though not exclusively, intended for addressing such a device used as an optical shutter. It is snvisaged that such a method could be used to control the transmission through a FLCD of electromagnetic radiation of other wavelengths e.g. infra-red and ultra-violet radiation as well as optical radiation.
Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawlngs of which:
Figure 1 shows a typical electro-optic characteristic for a ferroelectric liquid crystal material;
Figures 2, 3 and 4 each show a graph of voltage applied to a ferroelectric liquid crystal layer against time and a graph of optical transmission of that liquid crystal layer over the ~ame time for known addressing schemes;
Figure 5 is a schematic representation of an optical shutter including a ferroelectric liquid crystal cell;
Figure 6 is a cross-section of the ferroelectric liquid crystal cell of Figure 5;
Figures 7 and 8 each show a graph of voltage applied to the ~hutter of Pigure 5 against time and a graph of optical transmission of that shutter over the same time for addressing schemes provided in accordance with the present invention;

q :, : , - . .
' ~ .
: .

: 2 : 200~03 F~gure 9 shows a graph of voltage applled to tho ~hutter o~
Figure 5 against time for a further dddressing scheme provldod in accordance with the present invention~
Figure 10 shows a graph of optical transmission of the shutter of Pigure 5 over time for an addressing scheme similar to that shown ln Figure 9;
Figures lla and llb show respectively a graph of optlcal transmission ovet time for a shutter used in a camera system and a graph of voltage applied to the shutter in an addresslng scheme provided in accordance with the present invention;
and Figure 12 shows schematically a circuit for addressing the shutter of Figure 5 by an addressing scheme provlded ln accordance with the present lnvention.
Ferroelectric liguid crystal materials have a DC voltage response. An ~LCD containing such a material between polarizers can be switched from a light transmissive state to a non-transmissive state and vice versa by an applied voltage of sufficient magnitude and pulse width, the state into which it is switched being dependent upon the polarity of the applied voltage. A variety of voltage waveforms can be used but a waveform with a step function, e.g. a square wave pulse, is preferred for a minimum rise and fall time (fast response).
Pigure 1 shows an electro-optic characteristic, i.e. a plot of pulse height Vs against pulse width ts f a monopolar pulse wave (see inset - ~igure 1) to produce switching from a light transmissive state to a non-transmissive state or vice versa for a layer of a typical ferroelectric liguid crystal material, such as SCE13 ~supplied by BDH Ltd., Poole, U~). The layer was l.S ~m thick and the temperature was 25 C.
Flgure 2 shows a graph of voltage applied to a ferroelectric liquid crystal layer agalnst time and a graph of optlcal transmission of that liquid crystal layer over the same tlme. Monopolar pulses of sufficient pulse height Vs and pulse width ts to switch the liquid crystal layer between a first state TXl of maximum optical transmission and a second state ~X2 of minimum optical transmi~sion are applied. The 5' ~
.. . . .
S' 20~- 403 ideal optlcal transml3sion curve i8 shown ln dotted lines - tho llguid crystal ls latched ln the ~lrst or second state until a pu~se of the polarity reguired to switch it lnto the other state is applled. Lowever, ln a practical embodlment some relaxatlon of the latched ~tates usually occurs wlthln a period of lOts and the separatlon of the monopolar pulBe9 i6 greater than this. The contlnuous curve of Figure 2 shows this relaxation whlch reduces the contrast ratio, an undesieable effect for a llght shutter.
A variety of addressing schemes have been tried to avoid the problem of relaxatlon. In one scheme, as shown in Figure 3, the device is switched between the first and second states TXl, TX2 by a continuously applied AC sguare wave voltage.
The AC square wave voltage pulses are of sufficient height Vs an~ pulse width ts to swltch between the first and second states. The applied voltage Vs prevents relaxation occurring and malntains the liguid crystal cell in the ~xl or TX2 state, ensuring that the contrast remains high. However the alignment of the llguid crystal layer in the device can ea~lly be damaged in an irreverslble manner when alternating electrlc flelds above a critical value are applied. Alignment damage to the liguid crystal layer reduces the contrast ratio of the shutter and tends to increase the response time of the materlal. Por many materials, the critical value is typically of the order of lOV/ ~m - well below that usually required to reallse the maximum switching speed.
In an alternative scheme, as shown in Figure 4, a high freguency background AC slgnal of voltage magnitude VAc is applled to stablllse the states TXl and TX2. When VAc has a flnlte value Va, there 1~ stab1llsation whereas when VAc - 0, relaxation occurs. Unfortunately the value of the flelds necessary for AC stabilisation can depend on a varlety of parameters such as cell thicknes9, preparation of the alignment layer materlal and physlcal properties of the liquid crystal materlal, such as its dlelectric anisotropy e.g. as disclosed by T.Umeda et al : InflUence8 of Alignment Materials and LC Layer Thlckness on AC Pield - Stabilization Phenomena of Ferroelectric r '~;

~ . .
':

: 4 : 20~03 Liquld Crystals (Japanese Journal of Applied Physics Vol. 27.
No. 7. July l9B8, pages 1115-1121) and T. Nagata et al :
Physical Properties of Perroelectric Llquid Crystals and AC
Stabilizatlon Effect ~JapAnese Journal of Applied Physics Vol.
27. No. 7. July 1988, pages 1122-1125). With many liquid crystal materials, AC stabilisation is not very successful.
Often large AC fields are required which are about or greater than the critical value which will produce alignment damage to the liquid crystal layer and reduce the contrast ratio.
GB 21~5725A tSeikosha) discloses a method of driving an electro-optical display device (such as an FLCD) for producing a display consisting of display elements and which comprises first and second sets of electrodes, the electrodes of one set crossing those of the other. A selection signal is sequentially applied to the first set of electrodes while a non-selection signal is applied to each of the first set of electrodes to which the selection signal is not applied. In the methods described, when a non-selection signal is applied to a first electrode defining a display element, the resultant waveform across that display element is a substantially true pulsed AC
waveform. In two embodiments, this substantially true pu~sed AC
waveform comprises two pulses of one polarity having a reduced duration half or less than half of the duration of the switching pulse followed by two pulses of the same reduced duration but of the other polarity. The provision of a substantially time pulsed AC waveform engures that the substantially transparent electrodes do not become blackened, the liquid crystal material does not deteriorate and double colour pigment does not become discoloured, even after driving for a long time. The AC
waveform provided durlng non-selection also provides good contrast.
US 4508429 ~Nagae et al) dislose a PLC display in which two light transmitting states, i.e. a bright state and a dark state, can be established. Bach of these states is defined by the average brightness brought about by pulse voltage trains of a respective polarity. Each pulse in the pulse voltage trains shown is of the same pulse height which is accordingly 2~0~03 sufficient to swltch the FLC display from one defined light teansmittlng state to the other and vice versa. ~owever, a problem wlth this drivlng method is that, unle~s the duratlon of the bright display state 19 equal to that of the dark dlsplay state, the voltage VLc applied to the FLC will lnclude a DC
component. ~S 4S08429 dislo~es that 'It is well ~nown that when a DC component is applled to a liquid crystal element durlng the drlving thereof, the deterioration of the element is accelerated because of an electrochemical reaction, thereby resultlng ln a reduced llfe.' It is an object of the present invention to provide an lmproved method of addressing a ferroelectric liguid crystal device.
According to the present invention there is provided a method of controlling the transmission of electromagnetic radiation through a ferroelectric liquid crystal device having a first state of maximum transmission, a second state or mlni~um transmission and a value of voltage pulse width and voltage pulse height sufficient for a switching pulse to swltch the cell from said first state to said second state, the method comprislng the step of applying, for a tlme period greater than sald value of voltage pulse width, a plurality of consecutive controlllng pulses of one polarity to control the transmlsslon of the cell wherein each controlling pulse is of insufficlent pulse heigh~ and pul~e width to switch the cell from said first state to sald second state or vice versa.
Por the avoldance of doubt, it 1~ hereby state that the term 'pulse' as used herelnafter is in the sense of a non-zero voltage excursion which need not have a constant voltage magnltude but ls of one polarity.
A scheme accordlng to the present lnventlon permlt~
quasl-analogue control of the transmisslon of electromagnetic radiatlon through a ferroelectric liquid crystal devlce. In particular, it is possible to use high frequency pulses of a magnitude less than that which would cause alignment damage.

: 6 : 200a~03 Preferably the method further comprlses the step of applying a switching pulse of sufficient pulse height and pulse width to switch the device from said first state to said second state or vlce versa. In this way, the switching pulse can be used to switch at high speed in a digital fashion between the first and second states while the controlling pulses can be used to control the transmission of electromagnetic radiation through the device once it is in the first or second state.
In an advantageous embodiment, the step of applyins said switching pulse is followed by the step of applying a plurality of consecutive controlling pulses of the same polarity as said switching pulse whereby the cell is maintained in one of said first or said second states. A cell addressed by such a method has a high contrast ratio and the quick response produced by the switching pulse.
An optical shutter may be driven by an addressing scheme ln which the steps of applying a switching pulse of one polarity and a plurality of consecutive controlling pulses of the same polarity as said switching pulse is followed by the steps of applying a switching pulse of the other polarity and a plurality of consecutive controlling pulses of that other polarity. The period for which pulses of one polarity are applied may be equal to the period for which pulses of the other polarity are applied, resulting in the optical shutter being in the states of maximum and minimum transmission for equal periods of time and in a DC compensated waveform.
Alternatively, the optical shutter may bç driven by an addressing scheme in whlch the period for which pulses of one polarity are applled i8 not equal to the period for whlch pulses of the other polarlty are applled and 80 the optlcal ~hutter ls ln the states of maximum and mlnimum transmission for unequal perlods of time. The lnventor has surprisingly found that the present inventlon can provide an addressing scheme in which the problems of degradation of alignment due to DC electrolytic ;`' ' ,,.
.

: 7 : 2 0 0 ~ ~ a 3 effects can be alleviated wlthout the need to en~ure that the waveform is DC compensated overall.
Figure 5 shows an optlcal shutter 2 in front of a llght ~ource shown schematlcally at 4. The optical shutter 2 i9 ghown in an exploded view and comprlses a ferroelectric liquid crystal cell 6 on either -qida of which is a polarizer 3, 9. The polarizers are usually crossed. The shutter 2 has a first state TXl of maximum optlcal transmlqsion and a second state TX2 of minimum optical transmission. Application of a voltage pulse of sufficient pulse height Vs and pulse width ts and of the correct polarity switche~ the shutter 2 from the first state to ~he second state or viee versa.
Figure 6 shows the ferroelectric liquid crystal cell 6 of Figure 5 in greater detall. The cell 6 consists of two glass plates 11, lla each coated with a transparent conducting electrode 12, 12a formed of indium tin oxide and an alignment layer 13, 13a, typically of nylon or polyimide, rubbed unidirectionally. Insulating layers 14, 14a and 15-, 15a can be used respectively to separate the glass substrate 11, lla from the electrode 12, 12a and the electrode 12, 12a from the alignment layer 13, 13a. The two glass plates 11, lla are spaced 1.5 ~m apart and are sealed around the perimeter with an adhesive edge seal 16 which holds the glass plates together.
The indium tin oxide is patterned to define a single active element which can be directly driven by an applied voltage. A
ferroelectric liquid crystal material 17, such as SCE13 ~supplied by BDH Ltd., Poole~ UR) is sandwiched between the two glass plates 11, lla.
Figure 7 shows an addressing scheme provided in accordance with the present invention which can be used to address the shutter of Figure 5 and maintain a high contrast ratio. The seheme is a waveform comprising single high voltage switching pulses 20 ~ollowed by a series of consecutive low voltage pulses 22 of the same polarity and a separation and pulse width typically the same as the pulse width of the switching pulse 20. ~he switching pulses have a pulse height Vs and a pulse 2~ )3 s ~ ~

wldth ts AUch that the rhutter c~n be switched trom the flr~t state to the second state or vlce versa in tbe minimum tlme posslble. Once the shutter has been sw1tched lnto the flrst or the second state, ln the absence of any applled voltage lt would tend to relax as mentioned herelnbefore. The low voltage pulse~
22 control the optlcal transmlsslon of the shutter by continually driving the devlce back into the flr~t or second state before any slgnificant relaxatlon can occur and 80 are effective as latching pul~es. These low voltage pulses 22 each have a pulse helght VL Vb and pulse width tL which individually are insufficient to sw~tch the shutter from the first state to the second state or vice versa. As the latching pulses 22 prevent or at lea~t reduce any relaxation of the first and second states, they ensure that the contrast ratio of the shutter remains as high as possible.
Because the ferroelectrlc liguid crystal has a DC respon8e, the use of discrete latching pulses 22 can result in optical noise (i.e. the optical transmission TX will try to follow the instantaneous value of the applied voltage). This problem can be alleviated by keeping the pulse height-pulse width product for each latching pulse 22 to a minimum.
~ he use of a plurality of low voltage latching pulses of one polarity can cause DC electrolytlc effects wlthin the liquid crystal material, which can lead to alignment damage to the liquid crystal layer. Such effects can be reduced by using latching pulses of pulse-wldths simllar to or smaller than the pulse wldth ts f the swltching pulse. It is believed that this improvement i8 due to the use of pulses of low pulse width, reducing the time during which charge can accumulate at the surfaces of the liquid crystal layer and allowlng time between pulses for any accumulated charge to disperse before any lrreverslble distortlon occurs ln the alignment of the liquid crystal layer.
The pulse height used for the latching pulses 18 cho~en to minimlse the relaxatlon process without degradation of the allgnment due to AC flelds or any DC electrolytic effects. For , ~, , ,~ - ' ' , ' ' ' .
.

2l0rj~03 ~ome llquid crystAl mlxture~, lf the pulse holghta and pUl8e wldths are carefully chosen, sequence~ of latchlng pul~es of the same polarity lasting a few seconds aan be achleved wlthout causing DC allgnment damage.
In one example, a shutter comprising a l.5)~m thick cell containing the liguid crystbl material SCE13 (supplled by BD~ Ltd., Poole, UR) was operated at a temperature of 25C and a frequency of switchlng of 0.5~z. The switching pulsçs were of pulse height 50V and pul~e width about 15 ~. The latching pulses were of pulse height 5V with a pulse width and separation of about 15 ~8.
Figure 8 illustrates the use of controlling pulses 24 in waveforms to control the optical transmission of the shutter.
Switching pulses 26 of pulse height Vs and pulse width ts can be used to switch the shutter from the state TXl to the state TX2 and vice versa in the minimum time possible. Pulses of varying helghts can be used to control the rate of change of optical transmisslon though it i5 envisaged that there is a minimum pulse height for a pulse below which the effect i~
negligible. Pulses of different polarities can be used to increase and decrease the optical transmission.
~ he pulse heights and pulse widths should be chosen to avoid or at least alleviate potential alignment damage to the liquid crystal layer by DC or AC effects. Por example, the controlling pulse magnitude should be kept below the critical value for AC damage, typically about lOV/~m, though a few isolated controlling pul~es can be similar in pulse height magnitude to that of the switching pulse. In particular, seguences of pulses of alternating polarity with a pulse height magnitude greater than the critical value should be kept to a minimum as this can cause AC alignment damage effects. The pulse width of the controlling pulses should be kept similar or smaller than the pulse width ts of the ~witching pulse, as deflned by the electro-optlc characterlstlc of the liguid crystal material, e.g. as shown ln Pigure 1. The risk of DC
electrolytlc damage to the allgnment increases as the pulse 20(~54()3 ~ 10 ~

wldth lncreases to, e.g., a value of sevecal ts. It should also be noted that wlth some materlal~ having a i'a~t swltchlng response, a reverse polarlty pulse could swltch the devlce completely from one state to the other when thls is not requlred.
For most ferroelectric llguid crystal addre~slng schemes (either multiplexlng or direct-drive) lt 19 usual to arrange for the pulse sequence over the full driving cycle to be DC
compensated i.e the sum of the pulse height pulse width product for the positive polarity pulses equals that of the negative polarity pulses. However, the inventor has surpri~ingly found that providing the appropriate measures described previously are taken to prevent degradation of alignment due to AC fields and DC electrolytic effects, it is possible to drive the device with an a~ymmetric waveform such as shown in Pigure 9, in which pulses of one polarity are applied for a period Tl and then pulses of the other polarity are applied for a period T2 (T ~ T2), resulting in asymmetric optical shutter transmission, i.e. an optical response with a mark-to-space ratio of Tl to T2. Figure 10 shows an optical response for a shutter addressed by the scheme of Figure 9 in which the mark-to-space ratio is 10~ sing the same example and driving conditions as described previously - 1.5J m thick cell containing liquid crystal material SCE13 at 25 C etc -mark-to-space ratios up to lO:l (or the inverse 1:10) can be achieved with no cell alignment degradation.
One application of an optical shutter with a mark-to-space ratio not equal to one is in a high-speed camera shutter. As the state of minimum optical transmission (non-transmissive or dark state) of a ferroelectric liguid crystal still allows some light to be transmitted, a mechanical camera shutter is used in combination with the liquid crystal optical shutter to prevent slow exposure of the photographic film. Pigure lla shows the optical transmlssion TX f the liguid crystal optical shutter over time for an exposure of the film whilst Figure llb shows ~not to the same time scale) the voltage waveforms used to produced this ei'fect.

... .

2()~)5~()3 s 11:

While the mechanical shutter ~8 shut, the ~tate o~ the liguld crystal optlcal shutter 18 not 1mportant ana can be unspecifled. Just prior to the opening of the mechanlcal shuttee, the liquid crystal optlcal shutter is swltched to the dark state TX2. When the mechanical shutter i5 opened at tlme tl, the liguid crystal optical shutter is being ~aintained ln the dark state TX2 by latching pulses 27, pulse height VL, pulse width tL f one polarity. At the required time t2, a switching pulse 28 of the other polarlty ls applled to switch the liquid crystal optical shutter into the state TXl of maxlmum transmlssion (light state) and 80 expose the film.
During the exposure time, latching pulse~ 29a of the same polarity as the switching pulse may be applied, if necessary ~as shown) to maintain the shutter in the TXl state. At the end of the exposure, time t3, the liguid crystal optical shutter is switched back to the dark state TX2 by a switching pulse 29 and latching pulses 29a are applled to mainta1n the liguid crystal optical shutter in the dark state until the mechanical shutter is closed at time t4. The voltage applied to the liquid crystal optical shutter can then be removed. The esposure time ~t3-t2~ will depend upon the switching speed of the liquid crystal, the light transmitted through the liquid crystal optical shutter and the speed of the film.
~sing commercial available high speed photographlc film, acceptable results were achieved with such a camera shutter system us~ng the liguid crystal mixture SCB13 at 25 C in a l.S pm thick cell w1th an exposure time It3-t2) of 20~L8 and a total dark stage ~t4-tl) of 20ms. In this respect, it is to be noted that the waveform applled to the liquid crystal materlal for the camera system is a 'slngle-shot' waveform, l.e.
the waveform 18 not belng continually repeated or cycled.
Accordlngly, a mark-to-space ratio well in excess of the previously mentioned 10:1 (1000:1 in this example~ i8 permitted as any cell alignment degradation due to DC electrolytic effects wlll occur over a considerably longer time scale than the shutter time of a high speed camera. The contrast ratio of the 2005~03 ~ 12 llquld crystal optlcal shutter, the light transmitt~d by th-llguld crystal ln the dark state and the speed of the fll~ wlll llmlt the maximum mark-to-space ratio.
A suitable circult for generatlng waveforms to address the S shutter of Plgure 5 19 shown schematically in Pigure 12. The required waveform ls generated by a computer programme loaded into a computer 30 ~e.g. a Hewlett-Packard 9000/300) which determines the relative pulse heights at each of a number of time slots of the waveform produced by an arbltrary waveform generator 32 (eg a Wavetek Model 275 12M8z programmeable arbitrary function generator). The arbitrary waveform generator 32 is able to generate voltages ln the range ~ lOV. The output of the arbitrary waveform generator 32 is fed to a voltage ampllfler 34, capable of generating voltages in the range ~ 80V, to generate the eequired waveform across the ferroelectric llguid crystal cell 6.
A variety of modifications to the embodiments de~cribed herein and within the scope of the present invention will be apparent to those skilled ln the art.

:
~ ' -.

Claims (9)

1. A method of controlling the transmission of electromagnetic radiation through a ferroelectric liquid crystal shutter comprising at least one liquid crystal cell having a first state of maximum and a second state of minimum transmission, the cell being switchable between the first and second states by the application of a switching pulse having a value of voltage pulse width and voltage pulse height which, in combination, are sufficient to switch the cell, the method comprising applying a first switching pulse of one polarity to switch the cell to one of the first or second states and then applying a first plurality of consecutive controlling pulses of the same polarity as that of said first switching pulse for a time period greater than the pulse width of said first switching pulse, each controlling pulse having a pulse height and pulse width which, in combination, are insufficient to switch the cell between the two states, the controlling pulses serving to control the transmission of the cell in said one of the first and second states by counteracting any relaxation of the cell in said one of the first and second states.

2. A method according to claim 1 further comprising applying a further plurality of consecutive controlling pulses, the further plurality of controlling pulses being of opposite polarity to the first plurality of controlling pulses for controlling the transmission of the cell between said one of the first and second states and the other of said one of the first and second states.

3. A method according to claim 1 comprising applying a further switching pulse, of opposite polarity to the first switching pulse, followed by a plurality of consecutive controlling pulses of the same polarity as the further switching pulse.

4. A method according to claim 2 comprising applying a further switching pulse, of opposite polarity to the first switching pulse, after the further plurality of consecutive controlling pulses, the further switching pulse being followed by a plurality of consecutive controlling pulses of the same polarity as the further switching pulse.

5. A method according to claim 2 wherein the first and further pluralities of controlling pulses are applied to the cell for a substantially equal period of time.

6. A method according to claim 1 wherein the first plurality of controlling pulses have a pulse width substantially equal to the pulse width of the first switching pulse.

7. A method according to claim 2 wherein the further plurality of controlling pulses have a pulse width substantially equal to the pulse width of the first switching pulse.

8. A method according to claim 1 wherein the first plurality of controlling pulses have a mark space ratio 1:1.

9. A method according to claim 2 wherein the further plurality of controlling pulses have a mark space ratio of 1:1.