JPH0827453B2 - Ferroelectric liquid crystal color electro-optical device - Google Patents

Description

The present invention relates to a ferroelectric liquid crystal color electro-optical device.
In particular, the present invention relates to a driving method of a ferroelectric liquid crystal color electro-optical device which provides a color display by additive color mixing using a ferroelectric liquid crystal.

[Outline of Invention]

According to the present invention, additive color mixture is generated by irradiating the ferroelectric liquid crystal element with light of different colors successively from a light source installed on the back surface of the ferroelectric liquid crystal element to emit light of different colors. In the ferroelectric liquid crystal color electro-optical device, after rewriting the screen of the ferroelectric liquid crystal element, a screen holding period is provided by utilizing its memory property, and light is emitted from the light source during the screen holding period. This makes it possible to obtain a color display with high brightness.

[Conventional technology]

It has been announced that a liquid crystal cell is used as a shutter and a light emitting element (for example, LED, CRT, etc.) is installed behind the shutter to realize color display by a phenomenon of successive additive mixing. For example, Philip B announced at Eurodisplay '84
os, Thomas Buzak, Rolf Vatne et al. 7-9 "4A Full-Colo
r Field-Sequential Color Display '' (1984/9 / 18-2
0) and SID'85, Hasebe, Kobayashi et al.
Can be cited as a prior document.

However, there is no invention disclosed about a specific driving method when the same method is applied to a ferroelectric liquid crystal display element. Therefore, first, a conventional method of driving a ferroelectric liquid crystal will be described. Ferroelectric liquid crystal cell (hereinafter simply referred to as liquid crystal cell) and its driving circuit for electrically switching the bistable states of molecules such as chiral smectic liquid crystal (hereinafter referred to as SmC ^* ) to each other (hereinafter referred to as switching) Is disclosed in JP-A-61-94026. FIG. 2 shows a perspective view of a conventional liquid crystal cell. 1-1 is a pair of substrates which are arranged to face each other. Reference numeral 2-2 is a uniaxial or random horizontal alignment film provided on the plane inside the substrate. 3 is an alignment film 2-2
Have the SmC ^* originally helical layer structure is narrow or the SmC ^* film liquid crystal molecules as shown in FIG When a thin film sandwiched between the alignment film is horizontally aligned in layers by.

However, when the SmC ^* thin film 3 is viewed from above, the molecular axis is tilted by 0 with respect to the normal line 4 of the layer. There are two positions, a first stable state 5 and a second stable state 6. by the way
The SmC ^* molecule has a spontaneous polarization 7 in a direction orthogonal to the molecular axis. The direction of the spontaneous polarization 7 is perpendicular to the substrate 1 and has the opposite polarity between the bistable states. Therefore, stable states can be switched to each other by applying a pulse of a predetermined polarity. Reference numeral 8-8 is a pair of polarizing plates having polarization axes orthogonal to each other, and is an optical conversion member that distinguishes the first stable state and the second stable state of liquid crystal molecules as light passing and light blocking by birefringence. . Reference numerals 9 and 10 are electrodes arranged to face each other and apply a pulse to SmC ^* .

FIG. 3 shows the electrode structure. Reference numeral 9 is a scanning electrode and 10 is a signal electrode. A matrix is composed of both electrodes and well-known time division driving is performed.

FIG. 4 (a) is an example of a drive waveform applied to one of the matrix pixels shown in FIG. First, during the first selection period, one cycle of an AC pulse having a _peak value V _op and a pulse width T equal to or higher than the threshold value is applied. Assuming now that the polarity of the first-half pulse P ₁ is in the direction of switching the liquid crystal molecules from the first stable state to the second stable state, the polarity of the _second- half pulse P ₂ is the reverse switching (from the second stable state To the first stable state). As a result, the switching of P ₁ pulse is not maintained and the switching by P ₂ pulse is always effective. Next, during the non-selection period, an AC pulse having a peak value equal to or lower than the threshold value is applied, and the first stable state obtained previously is stored. Hereinafter, the period up to this point is referred to as a first frame.

Next, in the second frame comprising P _3, P ₄ pulses, P _3, P ₄ wave height of the pulse is equal to or less than a threshold value and in comparison with the first frame phase is inverted. Therefore, in the first frame, P ₂
The first stable state written by the pulse is retained.

The above is the waveform to be applied to the pixel to be written in the first stable state, but for the pixel to be written in the second stable state, the peak value of the P ₁ and P ₂ pulses is below the threshold value, while P ₃ , The peak value of P ₄ pulse should be above the threshold. That is, one pixel is formed by scanning for two frames due to the driving method in which the pixels to be brought into the first stable state are written in the first frame and the pixels to be brought into the second stable state are written in the second frame. . Therefore, there is a time lag between the two, and after two frames have been scanned, the first stable state is displayed longer than the second stable state by one frame time. By the way, FIG. 4 (b) shows the transmitted light characteristics when the waveform of FIG. 4 (a) is applied. The transmitted light is ΔI due to the AC pulse in the non-selected period.
Only constantly changing. This causes a reduction in contrast.

[Problems to be solved by the invention]

Now, it is assumed that one of the different colors of the planar light emitting device arranged on the back surface emits light only during the two frames as shown in FIG. 5, and the first stable state is the light blocking state (hereinafter, black). The second stable state is referred to as a light transmission state (hereinafter referred to as white). Here, in FIG. 3, the top row, that is, the first row and the bottom row, that is, m of the cells arranged in a matrix of m rows and n columns are shown.
Consider the pixel in the row. In both cases, the previous state was black and this 2
If it is a pixel that should be inverted to white between frames, black is retained in the first frame and it will be inverted to white in the second frame, but by the end of the second frame, the uppermost pixel will be white. Is (2m-1) T, while
The pixel at the bottom is T. By the way, T of SmC ^* is 200-300
Since it is μsee, the transmitted luminescent color cannot be recognized in such a time, and there is a problem that a desired color display cannot be obtained in the lowermost pixel.

[Means for solving problems]

The present invention aims to solve the problems of the conventional technology, and after writing screen information to a ferroelectric liquid crystal element, a period for holding the written screen is provided by utilizing its memory property, The flat light emitting element continuously emits light during the period of holding the screen.

[Embodiment] FIG. 1 shows the drive timing of the ferroelectric liquid crystal element and the light emission timing of the planar light emitting element. In FIG. 1, pixels for which red light emission is desired are written in white in the second frame of the two frames of time A. On the other hand, the red light source of the planar light emitting element is time A
During that period, light emission is continued.

After that, a screen holding signal (hereinafter referred to as an OPEN signal) is sent to the drive circuit of the ferroelectric liquid crystal element to hold the screen for a desired time. Red emission is continued during the holding period A '. Similarly, the pixels that are desired to emit blue light are written in white in the second frame of the two frames of time B, during which blue light emission is continued, the OPEN signal is sent, and the blue display temporary surface is held B'C blue. Continue to emit light. For green light emission, a similar procedure is performed, and successive additive color mixing is performed by repeating the cycle of A, A ', B, B', C, C '. As pointed out as the problem of the conventional example, the lowermost pixel transmits light for T time only, but in the present invention, the holding period A ′,
By providing B'and C ', the transmission time can be lengthened. The time of the holding periods A ', B', C'can be arbitrarily set by the pulse width of the OPEN signal. Therefore, if the time A '= B' = C 'is lengthened, the amount of transmitted light increases, so that a color display with high brightness can be obtained.
The hue can be changed by selecting the relationship of A '≠ B' ≠ C '.

By the way, a method of holding the screen by the OPEN signal will be described next.

FIG. 6 shows an example of a waveform applied to the matrix electrode when white writing is performed by line-sequential driving using the 1/5 bias averaging method. φ _X is the selection signal electrode waveform, the first half V _S ,
_Second half V _DD is applied. _X is a non-selection signal electrode waveform, and the first half V ₂ and the second half V ₃ are applied. φ _Y is a waveform applied to the selective scan electrode, and the first half V _DD and the second half V ₅ are applied. _Y is a waveform applied to the non-selected scan electrodes, and the first half V ₄ is applied to the second half V ₁ . This is shown below in the output truth table of the signal electrode (hereinafter referred to as segment) and the output psychological value table of the scan electrode (hereinafter referred to as common).

Here, DATA is a video signal input to the drive circuit, H indicates segment selection, and L indicates segment non-selection. FLM
Is a scanning signal input to the drive circuit, H is common selection,
L indicates common non-selection. DF is an alternating signal input to the drive circuit, and H is the first half of the basic unit of the applied voltage waveform,
L also shows the latter half. For example, in the segment output truth table, when DATA is H, that is, the first half V ₅ is applied to the second half V _{DD in the} selected segment. This is φ _X in FIG.

Looking at both truth tables, the V _DD potential is commonly used in the segment and common. Therefore, the DATA signal input to the segment driver is forced to H by logic control,
Set the DF signal to L. Then V _{DD for} all segments
Is applied. FLM input to common driver at the same time
The signal is forced to H by logic control, and the DF signal is set to H. Then V _DD is applied to all segments.
This will force both electrodes to the V _DD potential.

FIG. 7 is a drive circuit for forcibly connecting the matrix electrode to V _DD described above, and will be described together with the time chart of FIG. Reference numeral 11 is a segment driver for driving a segment of the LC panel 12. Required potential level V _{DD to} V
Input ₅ and φ _X based on DATA and DF signals
And _X are combined and output. CL ₂ is a high speed clock for converting serial DATA to parallel DATA, and CL ₁ is a clock for controlling the line sequential timing. CL ₁ latches parallel DATA and outputs it in parallel. Reference numeral 13 is a common driver that drives the common of the LC panel 12. Required potential level
V _{DD to} V ₅ are input, and φ _Y and _Y are combined and output based on the FLM and DF signals. CL ₁ is a clock that controls the line sequential timing. Now, assume that the OPEN signal has changed from L to H. The OPEN signal is a signal that controls the drive circuit to shift from the write operation to the memory operation. When the signal OPEN becomes H, DATA input to the segment driver 11 via the OR gate 14 is forcibly set to H. Further, DF is forced to be L via the inverter 15 and the AND gate 16. Therefore, as is clear from the segment output truth table, all the segments of the LC panel 12 are at the V _DD level. Also, when the OPEN signal goes high, OR gate 17
The FLM input to the common driver 13 via is forced to H. Also, DF is forced to H through the OR gate 18. Therefore, as is clear from the common output truth table, LC
All panel 12 commons are at V _DD level. As a result, the segment and common are held at the same V _DD potential and the memory state is maintained.

This memory state is different from the state of FIG. 4 (b),
Since the deflection of the transmitted light of ΔI does not occur, the irradiated light amount can be transmitted without deterioration of the contrast.

〔The invention's effect〕

As described above, according to the present invention, it is possible to arbitrarily adjust the time during which the light emitting source of each color passes through the ferroelectric liquid crystal element by providing the screen holding period. There is an effect that it is possible to obtain a color display whose hue can be adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the drive timing of SmC ^* and the light emission timing of a planar light emitting device of the present invention in succession, FIG. 2 is a perspective view of a conventional liquid crystal cell, and FIG. Fig. 4 shows the electrode layout of the conventional liquid crystal cell, Fig. 4 shows the drive waveform and its transmitted light characteristics of the conventional liquid crystal cell, and Fig. 5 shows the drive timing of the conventional SmC ^* and the light emission timing of the planar light emitting device. Figures that have been represented over time,
FIG. 6 is a waveform diagram applied to the segment and common, and FIG.
FIG. 8 is an LC panel drive circuit diagram, and FIG. 8 is a time chart of the drive circuit shown in FIG. 11 …… Segment driver 12 …… LC panel 13 …… Common driver 14,15,16,17,18 …… Logic circuit

Claims (3)

[Claims] 1. A ferroelectric liquid crystal element composed of a ferroelectric liquid crystal thin film sandwiched between two transparent members, and a plurality of or a single unit capable of continuously irradiating the display surface with lights of different colors. In a ferroelectric liquid crystal color electro-optical device comprising a flat light source consisting of one light source, after writing screen information to be transmitted corresponding to the light source of each color in the ferroelectric liquid crystal, at any time, A ferroelectric liquid crystal color electro-optical device, which holds a written screen, and the light sources of the respective colors continue to emit light during a period of writing screen information and during a period of holding the written screen. 2. A light emission time of a light source of each color is set to be the same for each color by sending a screen holding signal to a drive circuit of the ferroelectric liquid crystal element, or the light emission time is set arbitrarily.
The ferroelectric liquid crystal color electro-optical device according to claim 1, wherein the light emission time of the light source of each color is set differently for each color and set arbitrarily. 3. A ferroelectric liquid crystal color electro-optical device according to claim 1, wherein no voltage is applied to the ferroelectric liquid crystal while the written screen is held. .