DE1764966A1 - Method and circuit arrangement for rapidly adjusting the scattering capacity of a cell containing a nematic liquid - Google Patents

Description

tk.-lnq. Emsi Sommwhld Dr. Diefer r. Bezofd

Alunchen 23, Dunanfifr. 6th

6649-63 / Dr.v.B / E

RCA 59310/59425

Ü.S.Ser.Nob.667.857 / 667.856

Piled: September Ik, 1967

Radio Corporation of America New York N.Y. (V.St.A.)

Method and circuit arrangement for rapidly adjusting the scattering power of a cell containing a nematic liquid

The present invention relates to methods and circuit arrangements for quickly adjusting the Faithfulness of a cell containing a nematic liquid to which electrical pulses are repeatedly applied.

Nematic liquids ^ which because of the

The prevailing arrangement of the molecules is also called "liquid" Crystals "are relatively transparent in the unexcited state" in light, while in the excited state Scatter state of light. The light scattering is through a

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Turbulence in the excited area of the liquid causes, as will be explained in more detail, and can therefore as "dynamic scattering".

The dynamic scattering power of such

Nematic liquids or liquid crystals can be used in flat surfaces that work with reflection, absorption or transmission Playback devices, optical shutters, etc. are used. In applications of such liquids, where the liquid or certain areas of it are excited by a series of pulses of relatively short duration, e.g. video pulses, that is, have to be brought from the transparent state to the light-scattering state, there may be a number of these Pulses required to achieve the full scattering power and thus the full brightening of the liquid. This is particularly disadvantageous when reproducing television images, since the leading edge of moving images is smeared Objects leads.

The present invention is therefore based on the object of an element of a nematic liquid or a liquid crystal to switch on quickly, i.e. such an element quickly from the transparent to the light-scattering Bring state.

This object is achieved according to the invention

solved in that the internal resistance of the element of the nematic liquid or one containing the liquid

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Cell before the repeated impulses are applied is enlarged.

The method according to the invention can

also serve to increase the amount of scattering power of the liquid to increase.

In particular, the time until

Adjustment of the scattering power of a nematic liquid to the value determined by the amplitude of the pulses reduced by the fact that current carriers are removed from the liquid and thereby the internal resistance of the liquid is increased prior to the application of the excitation pulses.

Further training and developments of the

Method according to the invention and circuit arrangements to carry out the method are in the subclaims marked.

The invention is explained in more detail below with reference to the drawing, in which: f

1 shows a schematic representation of a nematic liquid in the non-excited state;

2 shows a schematic representation of a nematic liquid in the excited state;

Fig. 3 is a circuit diagram of a known one

Arrangement for exciting a nematic liquid, which by its equivalent circuit diagram is shown;

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Fig. 4 is a graphic representation of the

Time course of the voltage prevailing in the circuit according to FIG. 3 at the nematic liquid after a number of excitation pulses were applied;

Fig. 5 is a graphic representation of the

Temporal progression of the light scattering capacity of the liquid in the circuit of FIG. 3;

6 and 10 show a graphic representation

the time course of signals and the scattering power for the circuit according to FIG. 3 and circuits to be described according to the invention shown in Figures 8, 9 and 11;

Fig. 7 is a graphic representation of the

Voltage applied to the nematic liquid in the circuit of FIG. 3 at different times during the operation of the Circuit prevails;

8, 9 and 11 are circuit diagrams of circuits according to the invention; and

Fig. 12 is a graphic representation of the

Course of signals to which reference is made in the explanation of the circuit shown in FIG.

In a nematic liquid like this

The species of interest are the molecules of the liquid in the

ORIGINAL INSPECTED

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Direction suitable temperature range is arranged as shown in FIG. Unlike ordinary Liquids in which the molecules have an arbitrary or statistical orientation are nematic Liquid small groups of molecules aligned with respect to each other. These groups are said to be referred to as "domains" will. The orientation of the domains in relation to each other is arbitrary and because of the number of molecules in a domain is relatively small, the liquid looks relatively transparent.

When using a nematic

Liquid in display devices, closures and the like. The liquid between two in Fig. 2 is shown schematically shown conductive elements 10 and 12 and placed in the liquid at a field that is greater than the threshold field for dynamic scattering injects a current. The electric field causes a number of the domains shown in Fig. 1 with respect to each other, so that the domains are relatively large will. The ion current in the liquid is from a migration of negative ions from the negatively charged electrode element 10 accompanies the positively charged electrode element 12. Presumably encounter the negatives wandering through the liquid Ions and possibly also other ions present in the liquid together with the large domains or

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disturb these in some other way, so that the large domain areas move continuously. This movement is indicated schematically in Fig. 2 by arrows I ^ and 15. The visible effect of these domain movements is that the liquid now scatters light. Through the scattering you can Contrast ratios of more than 10: 1 can be achieved. In other words, the brightness is thin Liquid layer with incident light (which is normally unpolarized) when it is in a turbulent state Domains (according to FIG. 2) more than ten times that of the unexcited liquid (FIG. 1).

In practice, a nematic liquid display device has two flat elements with a thin layer of liquid between them. One of the elements can be transparent and the other being reflective. To excite the liquid to be able to, it can be connected to row and column conductors, which are designed as transparent conductors can and allow selectable areas of the liquid to be excited.

A simplified equivalent circuit diagram for a

a display element containing nematic liquid from a resistor 16 to which a capacitance 18 is connected in parallel. The liquid contained in a cell Z. is excited by a brief electrical pulse 20.

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This impulse can occur when playing back television pictures have a duration of 0.06 ms, which corresponds to a line duration (US standard). It is assumed that a whole line the information is simultaneously entered into the display devicei || cann. Such an operating mode, so the Simultaneous excitation of an entire line and not individual elements of the line one after the other is preferred, as for the charge of the capacitance 18 of the cell Z is then available for a longer period of time. It is also important that the capacitance 18 holds its charge for a reasonable period of time so that the state of dynamic scattering can adjust. To ensure such charge storage, a diode 21 is provided which prevents that the capacitance 18 is discharged by the pulse generator (not shown) delivering the pulse 20. 'The capacity 18 must therefore discharge itself through the liquid, that is, through the resistor 16 of the equivalent circuit diagram.

The voltage actually prevailing at the cell Zv after a number has been created by excitation pulses 20 on it is shown in Fig. K. At time t ₀ , pulse 20 is applied. As a result, the capacity is charged, which then discharges exponentially until there is practically no charge left at time t ^.

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The temporal course of the scattering power

of cell Z is shown in FIG. Until the maximum scattering power is reached, a period of time from t 1 to t is required, which is approximately 1 to 10 ms and depends on various parameters, such as the temperature, the field strength and the material used. At time t *, when the voltage on the cell has completely decayed. the scattering power is still of considerable value, since the mechanical time constant, ie the period of time which the domains need to return from their excited state shown in FIG. 2 to their de-excited state shown in FIG. 1, is relatively long.

In the practical operation of the in Fig. 3

The circuit shown shows that an initially dark (transparent) cell has a relatively large number of excitation pulses 20 is required until the liquid shows the scattering characteristic shown in FIG. This effect is shown in FIG. 6. The excitation pulses shown in the top curve have a constant amplitude, which is greater than the threshold voltage required for dynamic dispersion when used for television purposes ;. The pulses are video pulses, for example, and their amplitude corresponds to the video information contained in a special Element is to be reproduced during successive excitations of this element. In this case

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the duration of an excitation pulse would be approximately Ο, Οβ ms amount and the repetition frequency; would be about 30 Hz (US standard) .

The middle curve in Pig. β, which indicates the scattering power of the cell, shows that in a special case 15 excitation pulses were required in order to allow dynamic scattering to occur in the circuit according to FIG. In addition, the dynamic scattering only reaches its final amplitude after more than 20 M excitation pulses. The mentioned pulse numbers refer of course only to a special embodiment and the delay in the excitation of the scattering power depends on various parameters, e.g. 3. the temperature, the material of the liquid, the dimensions of the cell, etc., so that in other cases more or fewer excitation pulses may be required until the state of equilibrium of the scattering power is reached. FIG. 6 also shows that the curve representing the scatter is shifted with respect to the zero axis, which means that the time between the later occurring excitation pulses is insufficient to let the scattering power of the cell fall back to zero. The reason for this offset has already been briefly mentioned above, namely that the relaxation time constant for the turbulence in the liquid is relatively long, i.e. longer than the time interval between two successive pulses.

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The reason for the above mentioned long

The switch-on or energization time of a cell containing a nematic liquid or of a v / id-delivery element operating with such a liquid is presumably due to the fact that the resistor 16 is in fact non-linear. Initially, that is, when the first excitation pulse occurs, this resistor has a relatively small resistance value, so that the capacitance 18 discharges again relatively quickly, as shown by the solid curve 30 in FIG. At time t, that is, after the period of time that is required to allow the scattering power to increase to its maximum value, the voltage V ₀ on the cell is very low, in particular lower than the threshold voltage of the cell required for dynamic scattering. There is therefore no scattering, as the second curve in FIG. 6 also shows.

The relatively low initial

The resistance of the cell is much lower than the resistance that the cell ultimately assumes. One in every detail a satisfactory explanation for this is still pending, but the low initial distance probably has its own Cause in charge carriers that are present in the liquid. This can be free ions or impurities, which act as conductive particles or act as other charge carriers, the nature of which is not entirely clear. If the ne-

ORIGIN

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matic liquid for the first time or after a long time Pause is excited, i.e. when it is subjected to electrical impulses, these charge carriers migrate, the negative ones and can be positive, through the liquid to the electrodes, so in Fig. 2 to the electrodes 10 and 12. These Migration of charge carriers through the liquid then results in the relatively low resistance of the liquid. A relatively low resistance is in this context -

already a specific resistance in the order of magnitude of

10 ohm-cm in contrast to the high terminal resistance of the cell, which is on the order of 10 ohm-cm. These numerical values are also only examples for a special cell _3, since cells of different dimensions, cells made of different materials and cells with different parameters can have different, higher or lower resistance values.

By continuously applying pulses to the cell, the aforementioned free current carriers become gradually withdrawn from the liquid and they reach the positive and negative electrodes 10 and 12 in, respectively FIG. 2. The internal resistance, which is represented by the resistor 16 in FIG. 3, increases. With increasing resistance also changes the discharge time constant of the cell and thus the shape of the exponential discharge curve, as shown in FIG the voltage on a specific cell after applying

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a shows a pulses, while curve 32 shows the voltage after about 10 to 15 consecutive pulses and curve J> k shows the voltage after about 25 consecutive pulses. As the cell resistance increases, the voltage that is still on the cell at time t increases. In particular, it increases from an initial value V via a mean value V to an end value V. The threshold voltage for the dynamic scatter is somewhere V ₀ and V and as soon as it is exceeded, the liquid begins

ability to spread dynamically like the second curve in Fig. 6 shows.

The present method can be carried out by means of

a circuit according to FIG. 8 or FIG. 9 can be carried out. In each of these figures, only one cell Z ₃ containing a nematic liquid is shown; of course, a large number of such cells with the corresponding circuits can be arranged in a matrix which forms a display device.

In the circuit of FIG. 8, the

Cell Z is biased by a bias voltage source 42, shown schematically as a battery, to a voltage which can be significantly lower than the threshold voltage. Good results are obtained with a bias which is of the order of 25 % of the threshold voltage for dynamic scattering. Between the bias source

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42 and cell Z can have a normally mute diode 44 be switched on, which prevents that from an excitation pulse source 46 generated excitation pulses are short-circuited to ground via the bias voltage source. In appropriate Thus, a diode connected between the cell Z and the excitation pulse source 46 prevents a short circuit of the bias voltage.

Λ the operation of the in Fig 8flargestellten. Formwork are tung current carriers Z are present approximately in the liquid of the cell, drawn to the electrodes of the cell, so that the charges then via the supply lines 49, 51 of the cell connected to the connected to the liquid-connected electrodes. When an excitation pulse occurs, the number of charge carriers present in the liquid is therefore significantly smaller than without a bias voltage, so that the cell has a high internal resistance when the excitation pulse source 46 supplies an excitation pulse to the cell. As a result, the cell is "brightened" almost immediately and relatively strongly, ie it immediately assumes a relatively high scattering power.

The actual course of the scattering power that can be achieved with the present invention is in the third curve in Fig. 6 shown. The improvement over the second curve is obvious. as additional advantage is achieved with the method according to

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According to the invention, the scattering power, i.e. the brightening, of the liquid for a given pulse amplitude when using the invention, in particular the circuit according to FIG. 8 or FIG. 9, is significantly greater than in the known case when using the circuit according to FIG. 3. The maximum scattering power that can be achieved with the circuit according to FIG. 3 is _{denoted by S M1} in FIG. 6, while the maximum scattering power that can be achieved with the invention is denoted _{by S ^ 2.} When normalized to the same zero axis, the result is that S _M? is much larger than S _M1

Fig. 9 shows a second circuit for

Implementation of the method according to the invention. Here, the cell Z containing a nematic liquid is excited by the simultaneous application of a positive pulse from a pulse source 50 and a negative pulse from a pulse source 52. The pulse source 50 can supply an output voltage which is negative with respect to ground in the pauses between the pulses running in the positive direction, while the pulse source 52 can deliver an output voltage which corresponds to ground potential in the pauses between the negative pulses. The bias voltage source 54 shown as a battery is coupled to the cell Z via a series diode 56 and two further diodes 58 and 60. As in FIG. 8, the bias voltage can be, for example, 25% of the threshold voltage required for dynamic scattering.

ORIGINAL INSPECTED

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Another method of increasing the

The internal resistance of a cell containing a nematic liquid and a circuit arrangement for carrying out this method will now be described with reference to FIGS. 10 to 12. This embodiment is also explained in connection with the reproduction of television pictures and it is assumed that the cells or elements are excited line by line. In contrast to the previous Ausführungsbeispxelen, * in which the elements or cells of the matrix in the idle state, no pulses are supplied, are in the now to be described circuitry, such as may be similar to in FIG. 3, the elements or cells in each pulses in continuous Sequence fed. Each of these pulses has an amplitude that is slightly smaller than the threshold voltage at which the element in question begins to dynamically scatter. These pulses 40 are shown in Figure 6C. The pulses 40 have a duration of 0.06 ms and a repetition frequency of about 30 Hz. If the threshold value for the dynamic dispersion of the cell is 40 volts, the pulses 40 in FIG. IOC can have an amplitude of, for example, 35 volts .

When a cell is to be switched on,

the amplitude of the pulses applied to this cell is increased. The increased pulses 4l can have an amplitude of 100 volts, for example. When applying such a pulse If the amplitude is increased, the cell is immediately excited, i.e. it begins to scatter, as curve D in Fig. 10 shows, in

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which the time course of the scattering power is plotted. It can be seen that this was caused by the first pulse 41 The scattering power is quite large, it is close to the maximum value of the scattering power that can be expected. The scattering power is increased by the subsequent increased pulses 4l slightly increased until it finally reaches its maximum value.

Pig. 1OB shows the time course of the

Scattering power in the application of excitation pulses of constant amplitude (Pig. 10A), as in the prior art is equivalent to.

Fig. 11 shows a matrix of cells Z.

in a circuit according to this embodiment of the invention. For the sake of simplicity, each row and each column of the matrix contain only two elements or cells Z, in the In practice, of course, there are considerably more such cells or elements available. The diodes 21 serve two purposes, namely, they isolate the various elements from one another and allow each element to store Z charge. In each Grid interval, i.e. approximately every 30 ms, a line of Elements addressed by the fact that the output voltage of a line pulse source Rl or R2 for a line interval of 0.06 ms is increased from -50 volts to +35 volts. Whenever a row of elements Z is addressed, all column pulse generators Cl, C2 supply output pulses whose output

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plitude corresponds to the brightness of the video information to be reproduced by the relevant elements Z. If the cell in question is to remain dark, for example, the associated column pulse generator supplies the output voltage 0 volts, while the row pulse generator supplies an output voltage of 35 volts. The relevant diode 21 then conducts and a voltage of 35 volts appears on the associated element Z. As long as the column pulse generators C1 ₃ C2 do not supply any output signals, the internal resistance of the cells is kept high by the uninterrupted application of such pulses of relatively low amplitude. This amplitude corresponds to the pulses 40 in FIG. IOC, the return current path of these pulses leads to ground via the relatively small internal impedance of the pulse generators C1 and C2. However, since the amplitude of the pulses 40 is below the threshold voltage for dynamic scattering, the cells in question are not switched on, ie they do not scatter light.

When a specific cell in a row

is to be brightened, supplies the associated salt pulse generator Zl or Z2 a negative direction voltage pulse of such amplitude. that the difference between the row and column pulse voltages exceeds the threshold voltage for the dynamic spread of the cell (which in the present example is assumed to be 40 volts will). In Pig. A column pulse 60 is shown in FIG.

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When this pulse 60, which can have an amplitude of +50 volts, for example, meets the in positive Directional pulse 62 from the line pulse generator occurs at the cell in question 85 volts. This is over the threshold voltage at which the cell begins to scatter and the cell heals itself immediately, i.e. it takes on a scattering power whose amount depends on the amplitude of the output pulse of the column pulse generator.

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Claims (9)

Fatenta claims Method for quickly adjusting the scattering power of a nematic liquid Cell to which electrical impulses are repeatedly applied to a value determined by the amplitude of the impulses, characterized in that the internal resistance of the cell prior to the application of the pulses is considerable is increased. 2. The method according to claim 1, characterized. that the number of free Current carrier in the nematic liquid is considerably reduced prior to the application of the repeated pulses. 3. The method according to claim 2, characterized in that the nematic liquid a bias voltage is applied, the value of which is significantly smaller than the threshold voltage at which the liquid is dynamic begins to scatter. 4. The method according to claim 3, characterized in that an uninterrupted bias voltage DC voltage applied 209836/0186 5. The method according to claim 3 »characterized in that as a bias a continuous pulse train is applied. 6. Circuit arrangement for implementation of the method of claim 1 with a cell which is a nematic Contains liquid, which dynamically scatters when a voltage exceeding a threshold voltage is applied, and with an arrangement by which the cell receives excitation pulses are supplied, plural marriage exceed the threshold voltage, indicated by a circuit (42, 44; 54, 56; Rl, R2, 21), through which the cell (Z) a voltage can be supplied which has the same polarity as the excitation pulses but a significantly lower amplitude than the threshold voltage for dynamic scatter. 7. Circuit arrangement according to claim 6, characterized in that the bias voltage source (42, 54) is a DC voltage source. 8. Circuit arrangement according to claim 6, characterized in that the bias source (Rl, R2) is a pulse source which is continuous Provides pulses with an amplitude below the threshold voltage for dynamic scattering. 209836/0186 9. Circuit arrangement according to claim 8, characterized by an arrangement by means of which additional pulses (60) can be fed to the cell (50) are whose amplitude can assume such values that the threshold voltage for the dynamic spread of the cell is exceeded. 209836/0186 Blank page