Temperature compensation of ferro-electric liquid crystal displays

The invention provides an addressing scheme with temperature compensation for temperature induced changes in liquid crystal material switching parameters. Temperature compensation is provided by measuring liquid crystal temperature, and varying the length of strobe waveforms accordingly. A ferroelectric liquid crystal cell is addressed by row and column electrodes forming an x,y matrix of display elements. A strobe waveform is applied to each row in sequence whilst appropriate data waveforms are applied to all the column electrodes. At each display element, the material receives an addressing waveform to switch it to one of its two switched states depending upon the polarity of the addressing waveform. The data waveforms are, e.g., alternating positive and negative pulses of period 2 ts. The strobe waveform has a zero for one time period ts followed by a unipolar voltage pulse of significant duration, e.g., equal to or greater than 0.25 ts or more. This may result in an overlapping of addressing in adjacent rows, e.g., the end of a strobe pulse on one row overlaps with the beginning of a strobe pulse on the next row. The display elements may be switched into one of their two states by one of two strobe pulses of opposite polarity. Alternatively, a blanking pulse may switch all elements to one state and a strobe used to switch selected elements to the other state.

BACKGROUND OF THE INVENTION 
Temperature compensation of ferro-electric liquid crystal displays. 
This invention relates to the temperature compensation of multiplex 
addressed ferro-electric liquid crystal displays. Such displays use a 
tilted chiral smectic C, I, or F liquid crystal material. 
DISCUSSION OF PRIOR ART 
Liquid crystal devices commonly comprise a thin layer of a liquid crystal 
material contained between two glass slides. Optically transparent 
electrodes are formed on the inner surface of both slides. When an 
electric voltage is applied to these electrodes the resulting electric 
field changes the molecular alignment of the liquid crystal molecules. The 
changes in molecular alignment are readily observable and form the basis 
for many types of liquid crystal display devices. 
In one type of ferro electric liquid crystal device, surface stabilised 
ferro electric liquid crystal devices (SSFLC--N. A. Clark & S. T. 
Lagerwall, App Phys Lett 36(11) 1980 pp 899-901), the molecules switch 
between two different alignment directions depending on the polarity of an 
applied electric field. These devices have a bistability and remain in one 
of the two switched states until switched to the other switched state. 
This allows the multiplex addressing of quite large displays. 
One common multiplex display has display elements, ie pixels, arranged in 
an x, y matrix format for the display of e.g., alpha numeric characters. 
The matrix format is provided by forming the electrodes on one slide as a 
series of column electrodes, and the electrodes on the other slide as a 
series of row electrodes. The intersections between each column and row 
form addressable elements or pixels. Other matrix layout are known, e.g, 
polar co-ordinate (r-g), and seven bar numeric displays. 
There are many different multiplex addressing schemes. A common feature is 
application of a waveform, called a strobe waveform to each row or line in 
sequence. Coincidentially with the strobe applied at each row, appropriate 
one of two waveforms, called data waveforms are applied to all column 
electrodes for one period of the data waveform, frequently called the line 
address time. The differences between the different schemes lies in the 
shape of the strobe and data voltage waveforms. 
European Patent Application 0,306,203 describes one multiplex addressing 
scheme for ferro electric liquid crystal displays. In this application the 
strobe is a unipolar pulse of alternating polarity, and the two data 
waveforms are rectangular waves of opposite sign. The strobe pulse width 
is one half the data waveform period. The combination of the strobe and 
the appropriate one of the data voltages provides a switching of the 
liquid crystal material. 
GB 2,262,831, WO-A-92/02925, describes another addressing scheme in which a 
strobe waveform is first a zero for one time slot followed by a dc pulse 
of length greater than one time slot, eg two time slots or more. Data 
waveforms are alternating pulses of +/- data voltages Vd of pulse length 
one time slot. Line address time is twice the time slot length. The effect 
of this is that there is an overlapping of addressing time between 
different rows. Extending the time length of the strobe pulse means an 
overlapping of addressing in successive row electrodes. Such overlapping 
effectively increases the width of the switching pulse whilst not 
affecting the other waveforms and thus reduces the total time taken to 
address a complete display whilst maintaining a good contrast ratio 
between elements in the two different switched states. 
Other addressing schemes are described in GB 2,146,473-A; GB-2,173,336A; 
GB-2,173,337-A; GB-2,173,629-A; WO 89/05025; Harada et al 1985 S. I. D 
Digest Paper 8.4 pp 131-134; and Lagerwall et al 1985 IEEE, IDRC pp 
213-221; Proc 1988 IEEE, IDRC p 98-101 Fast Addressing for Ferro Electric 
LC Display Panels, P Maltese et al. 
The liquid crystal material may be switched between its two states by two 
strobe pulses of opposite sign, in conjunction with a data waveform. 
Alternatively, a blanking pulse may be used to switch the material into 
one state, and a single strobe pulse used with an appropriate data pulse 
to selectively switch back pixels to the other state. Periodically the 
sign of the blanking and the strobe pulses are alternated to maintain a 
net zero d.c. value. 
These blanking pulses are normally greater in amplitude and length of 
application than the strobe pulses so that the material switches 
irrespective of which of the two data waveforms is applied to any one 
intersection. Blanking pulses may be applied on a line by line basis ahead 
of the strobe, or the whole display may be blanked at one time, or a group 
of lines may be simultaneously blanked. 
One known blanking scheme uses blanking pulse of equal voltage (V) time (t) 
product Vt, but opposite polarity, to the strobe pulse Vt product. The 
blanking pulse has an amplitude of half and a time of application of twice 
that of the strobe pulse. These values ensure the blanking and strobe have 
a net zero d.c. value without periodic reversal of polarity. 
Another known scheme with a blanking pulse is described in EP 0,378.293. 
This uses a conventional d.c. balanced strobe pulse (of equal periods of 
opposite polarity) with a similar d.c. balanced blanking pulse (of equal 
periods of opposite polarity) in which the width of the blanking pulse may 
be several times that of the strobe pulse. Such a scheme has a net zero 
d.c. value without periodic reversal of polarity of blanking and strobe 
waveforms. 
The feature of d.c. balance is particularly important in projection 
displays since if it is desired to switch the gap between pixels to one 
optical state then periodic reversal of polarities is not permissible. 
One problem with existing displays is the variation of device parameters 
with temperature; this limits the temperature range over which a device 
may be used. To overcome this problem it is common to change the drive 
parameters. Typically this will involve the row or (strobe) voltage Vs, 
the column (data) voltage Vd, and the line address time. This is described 
in EP-A-0,285,402 and EP-A-0,303,343. Another technique is to introduce a 
variable voltage level into a strobe prepulse whose purpose is to modify 
the liquid crystal material operating parameters so that it continues to 
operate over a wide temperature range, eg about 20.degree. C., without 
need for compensation of Vs, Vd or line address time. This is described in 
GB 2.232.802. WO-A-92/02925. 
SUMMARY OF THE INVENTION 
According to this invention, the problem of temperature compensation is 
solved by varying the time length of a strobe pulse, whilst maintaining 
the same time between application of strobe to successively addressed rows 
(ie the data waveform period or line address time), in accordance with 
changes in liquid crystal material temperature. 
According to this invention a method of temperature compensating a 
multiplex addressed ferro electric liquid crystal matrix display comprises 
the steps of providing a liquid crystal cell with cell walls enclosing a 
layer of ferroelectric liquid crystal material; 
providing a first set of electrodes on one cell wall and a second set of 
electrodes on the other cell wall, the electrodes forming by their 
intersections a matrix of addressable elements: 
addressing sequentially each electrode individually in the first set of 
electrodes, such addressing being either by application of a strobe 
waveform of pulses of positive and negative values, or by application of a 
blanking pulse followed by a strobe pulse and arranged to maintain a net 
zero d.c. value, 
applying one of two data waveforms to each electrode in the second set of 
electrodes synchronised with the strobe waveform, both data waveforms 
comprising pulses of positive and negative values each pulse lasting a 
time period of one time slot (ts) with one data waveform the inverse of 
the other data waveform, 
Characterised by: 
the temperature of the liquid crystal material, 
varying the time length of the strobe waveform in accordance with the 
measured liquid crystal temperature whilst maintaining the same time 
between application of strobe waveform to successively addressed 
electrodes in the second set of electrodes and maintaining the same time 
periods (ts) in the data waveform. 
whereby temperature induced changes in the liquid crystal material 
parameters are compensated. 
The time between application of strobe waveform to successive rows is the 
data waveform period, eg may be 2 ts or 4 ts depending upon the type of 
addressing scheme. Often the data waveform period is referred to as the 
line addressing time, which when multiplied by the number of lines in a 
display, gives a frame time. 
The strobe waveform may be in two parts, a first part which may be a zero 
in the first period, ts, immediately followed by a second part namely a 
non zero voltage (main) pulse for a significant portion of ts or greater 
than ts, eg (0.25, 0.5. 1.0, 1.5, 2.0, 2.5, 3.0 or more) x.ts. The second 
part of the strobe waveform lasts sufficiently long to provide ( in 
combination with the first part of the strobe waveforms and the data 
waveforms) switching of the liquid crystal material. For example the 
second part of the strobe waveform may be about 0.25 ts upwards, typically 
0.5 ts upwards. The length of the second part of the strobe waveform may 
be continuously variable, or variable in steps of eg 0.5 ts or 1.ts. 
Additionally, the strobe waveform may have a non zero voltage in the first 
ts period of the same or different polarity to the remainder of the strobe 
to provide additional temperature compensation. 
The liquid crystal material may be switched between its two states by 
coincidence of a strobe pulse and an appropriate data waveform. 
Alternatively the material may be switched into one of its states by a 
blanking pulse and subsequently selected pixels switched back to the other 
state by coincidence of a strobe pulse and an appropriate data waveform. 
The blanking pulse may be in one or two parts. For a two part blanking 
pulse the first part is of opposite polarity to the second; the two parts 
of the blanking pulse are arranged to have a voltage time product Vt that 
combines with the Vt product of the single strobe to give a net zero d.c. 
value. 
According to this invention a temperature compensated multiplex addressed 
liquid crystal display comprises: 
a liquid crystal cell formed by a layer of liquid crystal material 
contained between two cell walls, the liquid crystal material being a 
tilted chiral smectic material, the cell walls carrying electrodes formed 
as a first series of electrodes on one wall and a second series of 
electrodes on the other cell walls, the electrodes being arranged to form 
collectively a matrix of addressable intersections, at least one of the 
cell walls being surface treated to provide surface alignment to liquid 
crystal molecules along a single direction; 
means for generating a strobe waveform comprising dc pulses of positive and 
negative values; 
driver circuits for applying the strobe waveform in sequence to each 
electrode in the first set of electrodes; 
means for generating two sets of data waveforms of equal amplitude and 
frequency but opposite sign, each data waveform comprising dc pulses of 
positive and negative values lasting a time period of one time slot (ts); 
driver circuits for applying the data waveforms to the second set of 
electrodes: 
and means for controlling the order of data waveforms so that a desired 
display pattern is obtained and an overall net zero de level; 
Characterised by: 
means for measuring the temperature of the liquid crystal material: 
means for varying the length of at least one pulse in the strobe waveform, 
relative to the period of the data waveforms without changing the data 
waveform time periods (ts), in accordance with the measured liquid crystal 
temperature to compensate for changes in liquid crystal material 
parameters with temperature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The display 1 shown in FIGS. 1, 2 comprises two glass walls 2, 3 spaced 
about 1-6 .mu.m apart by a spacer ring 4 and/or distributed spacers. 
Electrode structures 5, 6 of transparent tin oxide or indium tine oxide 
(ITO) are formed on the inner face of both walls. These electrodes are 
shown as row and column forming an X, Y matrix but may be of other forms. 
For example, radial and curved shape for an r, .theta. display, or of 
segments form for a digital seven bar display. A layer 7 of liquid crystal 
material is contained between the walls 2, 3 and spacer ring 4. 
Polarisers 8, 9 are arranged in front of and behind the cell 1. Row 10 and 
column 11 drivers apply voltage signals to the cell. Two sets of waveforms 
are generated for supplying the row and column drivers 10, 11. A strobe 
wave form generator 12 supplies row waveforms, and a data waveform 
generator 13 supplies ON and OFF waveforms to the column drivers 11. 
Overall control of timing and display format is controlled by a control 
logic unit 14. Temperature of the liquid crystal layer 7 is measured by a 
thermocouple 15 whose output is fed to the strobe generator 12. The 
thermocouple 15 output may be direct to the generator or via a 
proportioning element 16 e.g. a programmed ROM chip to vary one part of 
the strobe pulse and or data waveform. 
Prior to assembly the cell walls are surface treated in a known manner, 
e.g. by applying a thin layer of polyimide or polyamide, drying and, where 
appropriate, curing and buffing with a cloth (e.g. rayon) in a single 
direction, R1, R2. Alternatively a thin layer of e.g. silicon monoxide may 
be evaporated at an oblique angle. These treatments provide a surface 
alignment for the liquid crystal molecules. The alignment/rubbing 
directions R1, R2 may be parallel or anti parallel. When suitable 
unidirectional voltages are applied the molecules director align along one 
of two directions D1, D2 depending on polarity of the voltage. Ideally the 
angle between D1, D2 is about 45.degree.. In the absence of an applied 
electric field the molecules adopt an intermediate alignment direction 
between R1, R2 and the directions D1, D2. 
The device may operate in a transmissive or reflective mode. In the former 
light passing through the device e.g. from a tungsten bulb is selectively 
transmitted or blocked to form the desired display. In the reflective mode 
a mirror is placed behind the second polariser 9 to reflect ambient light 
back through the cell 1 and two polarisers. By making the mirror partly 
reflecting the device may be operated both in a transmissive and 
reflective mode. 
Pleochroic dyes may be added to the material 7. In this case only one 
polariser is needed and the layer thickness may be 4-10 .mu.m. Some 
suitable mixtures are given below. 
Liquid crystal material at an intersection of a row and column electrode is 
switched by application of an addressing voltage. This addressing voltage 
is obtained by the combination of applying a strobe waveform Vs to the row 
electrode, and a data waveform Vd to the column electrode. ie: 
EQU Vr=Vs-Vd 
where 
Vr=instantaneous value of addressing waveform 
Vs=instantaneous value of strobe waveform, and 
Vd=instantaneous value of data waveform 
Chiral tilted smectic materials switch on the product of voltage and time. 
This characteristic is shown in FIG. 5. Voltage time products above the 
curve will switch a material; below the curve is a non-switching regime. 
Note, the switching characteristic is independent of the sign of the 
voltage; i.e. the material switches for either a positive or a negative 
voltage of a given amplitude. The direction to which the material switches 
is dependent on the polarity of voltage. 
Two curves are shown in FIG. 5 because the switching characteristic depends 
upon the shape of the addressing voltage pulse combination. The upper 
curve is obtained when the addressing voltage is immediately preceded by a 
small prepulse of opposite sign; e.g. a small negative pulse followed by a 
larger positive pulse. The material behaves the same on application of a 
small positive pulse followed by a large negative pulse. This upper curve 
usually exhibits a turn round or a minimum response time at one voltage. 
The small prepulse may be termed a leading pulse (Lp) and the larger 
addressing pulse a trailing pulse (Tp). The upper curve applies for a 
negative value of the ratio Lp/Tp. 
The lower curve is obtained when the addressing voltage is immediately 
preceded by a small pre-pulse of the same sign; i.e. a small positive 
pulse followed by a larger positive pulse. The same applies for a small 
negative pulse followed by a large negative pulse. The lower curve has a 
positive Lp/Tp ratio. This lower curve has a different shape to that of 
the upper curve; for some materials it may not have a minimum value of a 
voltage time curve. 
The difference in shape between the two curves allows a device to be 
operated without ambiguity over quite a wide range of time values. This is 
obtained by operating a device in a regime between the two curves e.g. as 
shown in hatched lines. Intersections required to be switched are 
addressed by an addressing voltage having a shape where the lower curve 
applies and where the voltage and pulse width lie above the curve. 
Intersections not requiring to be switched either receive an addressing 
voltage having the shape where the upper curve applies, and where the 
voltage and pulse width lie below the curve, or only receive a data 
waveform voltage. This is described in more detail below. 
FIG. 11 shows strobe, data, and addressing waveforms of one embodiment of 
the present invention where strobe pulses applied to one row extend into 
the addressing of the following row. The strobe waveform is first a zero 
for a time period ts followed by +3 for twice ts. This is applied to each 
row in sequence, i.e. one time frame period. The second part of the strobe 
is a zero for one ts period followed by +3 for twice ts. Again this is 
applied to each row in sequence for one time frame period. Complete 
addressing of a display takes two time frame periods. The values of +3, -3 
are units of voltage given for the purpose of illustration, actual values 
are given later for specific materials. 
Data waveforms are arbitrarily defined as data ON and data OFF, or D1, and 
D2. Data ON has first a value of +1 for a first time period of ts followed 
by a -1 for a time period ts. This is repeated; i.e. data ON is an 
alternating signal of amplitude 1 and period 2 ts. Data OFF is similar but 
has an inital value of -1 followed by +1 ; i.e. the inverse of data ON. 
The first part of the data waveform, e.g. for data ON the value of +1 for 
a time period ts, is coincident with the first part of the strobe 
waveform, i.e. zero for time period ts. 
The addressing waveform is the sum of strobe and data. The combination of a 
positive strobe pulse and data ON is: -1, 4, 2, 1, -1, 1 etc. The value 4 
immediately preceded by -1 ensures the material switch characteristics are 
governed by the upper curve of FIG. 5. The combination of a negative 
strobe pulse and data ON is: -1, -2, -4, 1, -1, 1 etc. The combination of 
smaller pulses of the same sign as the large (-4) pulse ensures the 
material switch characteristics are governed by the lower curve in FIG. 5. 
Similarly a positive strobe pulse and data OFF combine to give: 1, 2, 4, 
-1, 1 etc; and a negative strobe pulse and data OFF combine to give: 1, 
-4, -2, -1, 1, -1 etc. 
When not receiving a strobe pulse each row receives a zero voltage. Each 
column receives either data ON or data OFF throughout. The effect is that 
all intersections receive an alternating signal, caused by the data 
waveforms, when not being addressed. This provides an a.c. bias to each 
intersection and helps maintain material in its switched state. Larger 
amounts of a.c. bias lead to improved contrast by the known a.c. 
stabilisation described in Proc 4th IDRC 1984, pp 217-220. 
Further a.c. bias may be provided, e.g. from a 50 KHz source, direct onto 
those rows not receiving a strobe pulse. 
Alternative extended strobe waveforms are shown in FIGS. 10, 12, 13. In 
FIG. 10 the strobe is first a zero for 1 ts and +3 for 1.5 ts, followed by 
its inverse. In FIG. 12 the strobe is first a zero for 1.times.tsh and 3 
for 3.times.ts, followed by its inverse. In FIG. 13 the strobe waveforms 
is first a zero for 1.times.ts and 3 for 4.times.ts, followed by its 
inverse. 
FIG. 8 shows strobe, data, and resultant addressing waveforms where the 
strobe does not intrude into the next raw addressing time. As shown the 
strobe is a zero for 1 ts followed by +3 for 1 ts. The inverse is applied 
in the following field time. In this example the strobe and data waveforms 
have the same period of 2 ts. Resultant waveforms for the four different 
combination of strobe and data waveforms are shown. Switching occurs when 
a larger pulse is preceded by a smaller pulse of the same polarity. 
FIG. 9 shows strobe, data, and resultant addressing waveforms where the 
non-zero voltage part of a strobe waveform is less than a single time slot 
1.ts. The strobe waveform is a zero for 1.ts, then +3 for 0.5 ts, followed 
by zero for the remainder of ts. Resultant waveforms for the four 
different combination of strobe and data waveforms are shown. As in FIG. 
8, switching occurs when a larger pulse is preceded by a smaller pulse of 
the same polarity. 
An alternative to using two strobe pulses of opposite polarity is to blank 
all pixels to one state, then selectively switch with strobe pulses to the 
other state. This may require periodic polarity inverting to maintain net 
zero dc. 
FIG. 15 shows a single blanking pulse of amplitude 4 applied for 4 ts. This 
switches all the intersections to one switched state. A strobe (as in FIG. 
11) is then used to switch selected intersections to the other switched 
state. Periodically the sign of the blanking and strobe are reversed to 
maintain overall net zero d.c. voltages. The use of a blanking pulse and 
single strobe can be applied to all the schemes of FIGS. 8-14. An 
advantage of blanking and strobe systems is that the whole display can be 
addressed in a single field time period. 
An alternative blanking scheme is shown in FIG. 16 where the blanking pulse 
is in two parts. The first part is +3 for 4 ts immediately followed by -3 
for 6 ts forming the second part. These two pulses are dc balanced with a 
single strobe of +3 for 2 ts 
The blanking pulse may precede the strobe pulse by a variable amount but 
there is an optimum position for response time, contrast and visible 
flicker in the display. This is typically with a blanking pulse starting 
six lines ahead of the strobe pulse but is dependent upon material 
parameters and the detail of the multiplex scheme. 
FIGS. 19, 20 show the waveforms involved in addressing a 4.times.4 matrix 
array showing information as shown in FIG. 18. Solid circles are 
arbitrarily shown as ON electrode intersections, i.e. display elements, 
unmarked intersections are OFF. The addressing scheme is that used in FIG. 
11. 
The positive strobe pulse is applied to each row 1 to 4 in turn; this 
comprises the first field. After the last row is addressed by the positive 
strobe pulse the negative strobe pulse is applied to each row 1 to 4 in 
turn and comprises the second field. Note there is an overlap between 
rows. For example the third ts period for row 1 occurs at the same as the 
first ts period of row 2. This overlap is more noticable for displays 
using the strobe waveforms shown in FIGS. 12, 13. 
The data waveform data ON applied to column 1 remains constant because each 
intersection in column is always ON. Similarly for column 2 the data 
waveform is data OFF and remains constant because all intersections in 
column 2 are OFF. For column 3 the data waveform is data OFF whilst rows 1 
and 2 are addressed, changing to data ON whilst row 3 is addressed, then 
changing back to data OFF whilst row 4 is addressed. This means that 
column 3 receives data OFF for 4.times.ts, data ON for 2.times.ts, data 
OFF for 2.times.ts, a period of one field time, the time taken for the 
positive strobe pulse to address every row. Similarly for column 4 the 
data waveform is data OFF for 2 ts, data ON for 2 ts, data OFF for 2 ts, 
and data ON for 2 ts. This is repeated for a further field period whilst 
the negative strobe pulse is applied. Two field periods are required to 
provide one frame period and completely address the display. The above is 
repeated until a new display pattern is needed. 
Resulting addressing waveforms are shown in FIG. 20. For intersection row 1 
column 1 (R1,C1) the material does not switch during the first field 
period because the material switching follows the upper curve of FIG. 5, 
and time and applied voltage level are made to lie below the switching 
curve. Instead the material switches during the second field period where 
the material switches because of the lower voltage/time requirements shown 
by the lower curve of FIG. 5. A similar reasoning applies to intersection 
R1,C2 where the material switches during the first field period. 
For intersection R3,C3 the material switches during the second field period 
because the time/voltage applied during the first field period does not 
reach the higher value required by the upper curve of FIG. 5. Intersection 
R4,C4 switches at the end of the second field period whilst a negative 
strobe pulse is being applied. 
When the display of FIGS. 1, 2 is in use the temperature of the liquid 
crystal material may change; this results in a change of the switching 
characteristics. Small temperature changes can be compensated for by small 
changes in the amplitude and sign of the first pulse in the strobe as 
shown in FIG. 14. Also the value of ts may be varied to provide some 
temperature compensation. Larger temperature changes are compensated for 
by varying the length of the strobe waveform as shown in FIG. 17. 
As shown in FIG. 17 the strobe pulse is first zero for its, followed by 
n.ts where n is a number greater than about 0.25 ts and is varied with 
measured temperature as shown in FIGS. 6, 7. The sign of the strobe pulse 
is alternated in successive frames to achieve net zero dc. The value of n 
may be a number of system clock pulse times, each much smaller than ts, to 
give a smooth change of strobe pulse length. Alternatively n may be 
adjusted in steps of eg 0.5 ts or an integer number of ts values. 
FIGS. 8-13 above show how the display may be addressed with strobe pulses 
of different lengths. FIGS. 6, 7 show how the length of the strobe pulse 
needs to be changed to compensate for temperature for one specific 
material. The material used for FIGS. 6, 7 was Merck ZLI 5014-000 in a 1.8 
.mu.m thick layer. For FIG. 6 the strobe voltage was 50 volts, data 
voltage 10 volts, data period (2.ts) 60 .mu.s. For FIG. 7 the strobe was 
40 volts, data 10 volts, and data period 100 .mu.s. In the FIGS. 6, 7 the 
vertical axis shows the length of the second part of the strobe waveform, 
and the horizontal axis material temperature. As shown in FIG. 6, 
temperature compensation is obtainable from just below 15.degree. C. to 
over 45.degree. C. In FIG. 7 the temperature compensation is obtained from 
below 5.degree. C. to over 35.degree. C. 
Tables 1, 2 show ranges of temperature compensation for the material of 
FIGS. 6, 7 with different drive conditions. By way of comparison, details 
are also given of operating temperature range with no compensation and 
temperature compensation range obtained by varying the length of the data 
period 2.ts. Varying strobe waveform can provide temperature compensation 
of a range of greater than 30.degree. C., whereas varying the length of ts 
provides temperature compensation over a range of 25.degree. C. 
TABLE 1 
______________________________________ 
Material Merck ZLI 5014-000. in a 1.8 .mu.m thick layer. 
Vs = 50v, Vd = +/- 10v, data waveform period = 60 .mu.s. 
Data 
Temp range .DELTA.T 
waveform change of 2X 
Drive Scheme 
(.degree.C.) 
(.degree.C.) 
Temp Range .degree.C. 
.DELTA.T 
______________________________________ 
FIG. 8 24-45.5 21.5 24-49 25 
FIG. 11 17-31.5 14.5 17-36.5 19.5 
FIG. 12 14-25 11 14-31.5 17.5 
______________________________________ 
TABLE 2 
______________________________________ 
Material Merck ZLI 5014-000, in a 1.8 .mu.m thick layer. 
Vs = 40v, Vd = +/- 10v, data waveform period = 100 .mu.s. 
Temp range 
.DELTA.T 
Data waveform change of 2X 
Drive Scheme 
(.degree.C.) 
(.degree.C.) 
Temp Range .degree.C. 
.DELTA.T 
______________________________________ 
FIG. 8 18-36.5 18.5 18-42.5 24.5 
FIG. 11 7-24 17 7-32 25 
FIG. 12 &lt;5-14 9 &lt;5-32 18 approx 
______________________________________ 
Thus as the liquid crystal material 7 temperature varies, the length of the 
strobe waveform is varied eg from that shown in FIG. 9, to that of FIG. 
13, of length 5 ts, or longer. Circuitry for extending the strobe pulse 
length without changing line address time from 2 ts, is shown in FIGS. 3, 
4. 
FIG. 3 shows a part of FIG. 1 to an enlarged scale, only row electrodes and 
drivers are illustrated for simplicity. Rows R1 to R256 are connected to 
driver circuits IC1 to IC8; eg integrated circuits HV60 (obtainable from 
Supertex USA). Outputs 1-32 of IC1 are connected to rows R1, R9, R17 . . . 
R248. Similarly outputs 1-32 of IC2 are connected to rows R2, R10, R18, . 
. . R249, etc for all the ICs. A control logic has row waveform input, a 
temperature input from sensor 15, and clock phase and enable control 
outputs to a bus line connecting all the IC1-8. 
A strobe waveform is clocked down each row in turn; first a zero voltage 
then a pulse of appropriate polarity for the longest pulse extension which 
might be required, eg 5 ts. The length of this pulse depends upon the 
sensed temperature. When each strobe pulse has been applied to a given 
row, the strobe amplitude value remains until the control logic signals an 
enabling signal which terminates the strobe at that row. All rows are 
addressed in the first field, then readdressed in a second field with the 
strobe polarity reversed; two fields of addressing make up a single frame 
addressing time. In this embodiment each IC is addressed in turn to give 
an output at its output 1. This is repeated for successive IC outputs 
2-32. 
A different arrangement is shown in FIG. 4 which has the same components 
but differently connected to that of FIG. 3. In this FIG. 4 embodiment the 
output 1-32 of IC1 connect to rows R1 to R32, and outputs 1-32 of IC2 
connect to rows R33 to R64 etc. An advantage of this arrangement is a 
reduced number of crossover of connecting leads. The rows are addressed 
non consecutively, ie rows R1, R33, R65 . . . R225, R2, R34, R66, . . . 
R226, R3, R35, R67, . . . R227, etc. 
In addition to varying the strobe pulse lengths, the amplitude and sign of 
the strobe prepulse. FIG. 14, and amplitude values of Vs and Vd may also 
be varied to provide temperature compensation. Further, the length of the 
time slots ts may also be varied. Variation is ts may improve contrast 
ratio between the two switched states as shown in FIG. 21. In this FIG. 21 
contrast ratio dependence on period of applied ac square wave at +/-10 v 
data voltage are shown at five temperatures, 5.degree. C., 15.degree. C., 
25.degree. C., 35.degree. C., and 45.degree. C. for Merck ZLI 5014-000 in 
a 1.8 .mu.m thick layer. To obtain the curves shown the device was 
switched between its two optical states with a monopolar strobe pulse of 
alternate polarites and sufficient voltage-time product and an ac square 
wave was superimposed to simulate the column waveforms of a multiplex 
drive scheme. 
In nematic and ferroelectric liquid crystal devices it is known, eg GB 
2,262,831, to reduce peak row and column voltages required at the driver 
circuits by applying additional voltage reduction waveforms (VRW) to both 
row and column electrodes. These VRWs combine at each addressed element to 
give the same resultant voltage as displays not using VRWs. Such VRWs may 
be applied to the waveforms of FIGS. 8-17 above. 
The addressing schemes shown above with reference to FIGS. 8 to 17 involve 
variations on two time slot addressing; the data waveforms are pulses of 
alternating +/-Vd applied for one time slot. The principle of the present 
invention may also be applied to known addressing schemes which use a 
different number of time slots. 
FIG. 22 shows a known addressing scheme, and FIGS. 23, 24 show how this can 
be modified by the present invention. 
In FIG. 22 the strobe waveform is of four time slots (4 ts) long. In the 
first ts the voltage is zero, then Vs for 3 ts during a first field time. 
In the second field time the voltages are reversed. The data waveforms 
are: Data 1 +Vd for 1 ts, then -Vd for 2 ts, and +Vs for 1 ts; Data 2 is 
the inverse. Resultant waveforms are shown. A non switch resultant of 
positive strobe and Data 1 is: -Vd, +Vs+Vd, +Vs+Vd, +Vs-Vd in successive 
time slots. A switching resultant of negative strobe and Data i is: -Vd, 
-(Vs-Vd), -(Vs-Vd), -(Vs+Vd) in successive time slots. Switching and non 
switching resultants are shown for Data 2, and are the inverse of the 
above. 
FIG. 23 shows how the strobe of FIG. 22 may be ex tended by maintaining the 
voltage Vs for a further 2 ts. Successive rows are addressed after each 
data waveform period as in FIG. 22. This may result in different data 1 
and data 2 waveforms being applied in any sequence to a particular column 
due to the required pattern of display. Resultant waveforms are shown and 
for the first 4 ts are as for FIG. 22. The dotted lines during the fifth 
and sixth time slots allow for the fact that the data waveforms at a 
particular pixel may change as the next row is being addressed. 
FIG. 24 shows a strobe waveform extended by maintaining Vs for a further 4 
ts. Resultant waveforms are shown far both switching and non switching 
waveforms. As with FIG. 23 dotted lines show possible variation of 
resultant due to the different pattern of data waveform which may be 
applied during addressing of the next row. 
The effect of the addressing schemes of FIGS. 22-24 on switching 
characteristics are shown in FIG. 25. The material used was Merck 
ZLI-5014-000, Vd=10 v, at a temperature of 25.degree. C. 
FIG. 26 shows another known addressing scheme, and FIGS. 27, 28 show how 
this can be modified by the present invention. 
As shown in FIG. 26 a strobe waveform is a +Vs for 1 ts immediately 
followed by -Vs for 1 ts. This is used in a first field time, and its 
inverse used in a second field time. Data 1 waveform is +Vd for 1 ts and 
-Vd for the next 1 ts. Data 2 is the inverse of data 1. A non switching 
resultant waveform is Vs-Vd for its followed by -(Vs-Vd) for 1 ts. A 
switching resultant waveform is Vd+Vs for its followed by -(Vs+Vd) for 
its. Both non switching and switching is also shown by the inverse of the 
above. 
FIG. 27 shows the strobe pulse of FIG. 26 extended in time. The first and 
second pulse are extended to occupy 2 ts. This requires that the first 
strobe pulse is applied before the relevant data waveform, ie the strobe 
is started its ahead of normal whilst a previous row is being addressed. 
The second strobe pulse extends after the relevant data waveform has 
ceased and the next row is being addressed. A pixel that does not switch 
receives +Vs+Vd or Vs-Vd, +Vs-Vd, -(Vs-Vd), -(Vs+Vd) or -(Vs-Vd) in 
successive time slots. The reason for alternatives, shown in dotted, is 
possible different data waveform applied during the previous and the next 
addressed row. A pixel that switches receives -(Vs+Vd) or -(Vs-Vd), 
-(Vs+Vd), +Vs+Vd, +Vs+Vd or +(Vs-Vd) in successive time slots. The inverse 
of these two resultants are also non switching or switching respectively. 
FIG. 28 shows another modification of FIG. 26. In this the strobe is 
extended by 1.5 ts in both the positive and negative pulses. As shown the 
first pulse extends into the addressing of the previous row by 0.5 ts, 
whilst the second pulse extends 0.5 ts into the addressing time of the 
next row. A non switching resultant waveform is +Vd or -Vd for 0.5 ts, 
Vs+Vd or Vs-Vd for 0.5 ts, Vs-Vd for 1 ts, -(Vs-Vd) for 1 ts, -(Vs+Vd) or 
-(Vs-Vd) for 0.5 ts, +Vd or -Vd for 0.5 ts. A switching resultant waveform 
is +Vd or -Vd for 0.5 ts, -(Vs+vd) or -(Vs-Vd) for 0.5 ts, -(Vs+Vd) for 
its, +Vs+Vd for its, +Vs+Vd or +Vs-Vd for 0.5 ts, and +Vd or -Vd for 0.5 
ts. The inverse of these two resultants also does not or does switch as 
illustrated. 
Suitable liquid crystal materials are: 
Merck catalogue reference number SCE 8 (available from Merck Ltd Poole, 
England) which has a Ps of about 5 nC/square cm at 30.degree. C., a 
dielectric anisotropy of about -2.0, and a phase sequence of: Sc 
59.degree. C. Sa 79.degree. C. N 98.degree. C. 
Mixture A which contains 5% racemic dopant and 3% chiral dopant in the 
host; 
Mixture B which contains 9.5% racemic dopant and 3.5% chiral dopant in the 
host. 
______________________________________ 
Host 
______________________________________ 
##STR1## 37% by weight 
##STR2## 41% 
##STR3## 14% 
##STR4## 8 
Dopant (both racemic and chiral) 
##STR5## 
______________________________________ 
The * denotes chirality, without it the material is racemic. 
Another suitable mixture is Merck catalogue reference number ZLI-5014-000 
(available from Merck, Poole, England) which has a spontaneous 
polarisation coefficient (Ps) of -2.8 nC/cm.sup.2 at 20.degree. C., a 
dielectric anisotropy of -0.7, and phase sequence of -10.degree. C. Sc 
64.degree. C. Sa 68.degree. C. N 70.degree. C. I. 
Both mixtures A, B have a Ps of about 7 nC/square cm at 30.degree. C. and a 
dielectric anisotropy of about -2.3. 
Mixture A has the phase sequence Sc 100.degree. C. Sa 111.degree. C. N 
136.degree. C. 
Mixture B has the phase sequence Sc 87.degree. C. 118.degree. C. N 
132.degree. C. 
Device operating parameters and contrast ratios for some of the addressing 
schemes shown above are as follows: 
Material SCE8 in a 1.8 .mu.m thick layer at 25.degree. C. 
TABLE 3 
______________________________________ 
addressing scheme of FIG. 11 
Vs Vd ts CR 
______________________________________ 
50 5 36-53 8-7 
50 7.5 46-115 45-15 
40 10 46-88 77-21.5 
50 10 57-140 71-9.5 
______________________________________ 
TABLE 4 
______________________________________ 
addressing scheme of FIG. 12 
______________________________________ 
50 7.5 40-73 26-11 
40 10 34-57 64-23 
50 10 47-100 67-17 
______________________________________ 
TABLE 5 
______________________________________ 
addressing scheme of FIG. 8 
______________________________________ 
50 5 65-450 23-3 
50 7.5 75-480 65-2.2 
40 10 95-345 49-2.7 
50 10 83-370 63-2.3 
______________________________________ 
Mixture B in a layer 1.7 .mu.m thick at 30.degree. C. 
TABLE 6 
______________________________________ 
addressing scheme of FIG. 11 
Vs Vd ts CR (at lowest ts) 
______________________________________ 
50 10 22-78 51 
50 7.5 17-82 33 
40 10 16-47 56 
______________________________________ 
TABLE 7 
______________________________________ 
addressing scheme of FIG. 12 
______________________________________ 
50 10 20-68 51 
50 7.5 14-62 24 
40 10 13-36 53 
40 7.5 10-37 7.2 
45 7.5 10-42 10 
______________________________________