JPH0442654B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal display device, and more particularly to a transmission type liquid crystal display device using ferroelectric liquid crystal.

[Summary of the Disclosure] This specification and drawings describe writing of "dark" or "bright" to pixels of a selected scanning signal line in a transmissive liquid crystal display device using ferroelectric liquid crystal.
This is done before the period when the amount of light from the illumination light source is at its minimum to prevent flickering in selected and non-selected pixels.

[Prior Art] In recent years, emphasis has been placed on the development of ferroelectric liquid crystal elements in place of nematic liquid crystals, which have been widely used in the past. Ferroelectric liquid crystal elements can be made to have bistability by changing the cell construction method, so it is expected that high time division liquid crystal display elements will be realized. However, unlike conventional TN liquid crystal devices, the response is the product of spontaneous polarization and electric field, and the response threshold is
It is determined by both the electric field strength and application time. Bistability of ferroelectric liquid crystal devices has been proposed, for example, by Clark and Ragaval in US Pat.
It can be granted by doing so. Such cells are surface-stabilized ferroelectric liquid crystal cells (SSFLC).
known as cell).

FIG. 4 schematically depicts an example of an SSFLC cell.

11 and 11' are In ₂ O ₃ , SnO ₂ or ITO (Indium-
It is a substrate (glass plate) coated with a transparent electrode such as Tin-Oxide, and a SnC phase liquid crystal with a liquid crystal molecular layer 12 oriented perpendicular to the glass surface is sealed between the substrates (glass plates). A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 is
Dipole moment P⊥ in the direction perpendicular to the molecule
It has 24.

Due to the interfacial effect of the substrate surface, the helical structure of the liquid crystal molecules unravels (non-helical structure), and its dipole moment P or P' becomes upward 24p or downward 24'.
It takes either state p′. When an electric field E or E' with a different polarity above a certain threshold value is applied to such a cell for a predetermined period of time, the dipole moment will change depending on the electric field E or E'.
The direction changes to upward 24p or downward 24'p' in response to the electric field vector of E', and accordingly, the liquid crystal molecules are aligned in either the first alignment state 23 or the second alignment state 23'.

The advantage of using such a ferroelectric liquid crystal as an optical modulation element is that the response speed is extremely fast and the orientation of the liquid crystal molecules has a bistable state. When the voltage is applied, the liquid crystal molecules are aligned in the first alignment state 23, but this state is stable even when the electric field is turned off. Furthermore, when applying an electric field E′ in the opposite direction, the liquid crystal molecules
The molecules are oriented to the orientation state 23' and change their orientation, but they remain in this state even when the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, the respective orientation state is still maintained.

[Problems to be Solved by the Invention] Figure 5 shows the state of switching between such bistable states, and shows the time change (horizontal axis) of the amount of light transmitted through the liquid crystal cell (vertical axis). FIG. Above and below the liquid crystal cell, a pair of polarizing plates are arranged in a crossed nicol relationship with each other so that, for example, the polarization axis coincides with the alignment direction of the liquid crystal molecules in the first stable state. The first stable state is displayed as a "dark level" and the second stable state is displayed as a "light level." In this case, switching between bistable states can be effected by single pulses of mutually opposite polarity above a threshold value.
Figure 5 a shows the state of switching from "dark" to "bright", and b shows the state of switching from "bright" to "dark"; in both cases, the state changes from dark level to
It can be seen that the light level cannot be easily transferred from the first stable state to the second stable state, and that it temporarily overshoots and then settles down to the appropriate stable state.

Possible causes of such an overshoot are as follows. In other words, in a stable state, the liquid crystal molecules are twisted within the liquid crystal molecular layer 12, so the apparent tilt angle (true tilt angle) is smaller than the tilt angle when the liquid crystal molecules are parallel within the layer. This is because the upper tilt angle responds as an apparent tilt angle that has decreased. In other words, it is presumed that optical overshoot occurs because the liquid crystal molecules, which are at the true tilt angle while the electric field is applied, shift to a stable twisted state as soon as the electric field is cut off. This phenomenon is caused by the matrix configuration of scanning signal lines and information signal lines.
In an SSFLC cell, when writing is performed using line sequential scanning, this means that overshoot occurs in all selected pixels, which causes screen flickering from a viewing angle, resulting in an extremely difficult to see display. In particular, the present inventors have confirmed through psychological experiments that such screen flickering is particularly noticeable when switching from a "bright" state to a "dark" state.

An object of the present invention is to provide a ferroelectric liquid crystal device that is easier to view by minimizing the flickering of the screen described above.

[Means for Solving the Problems] and [Operation] The present invention provides: a scanning signal lines and information signal lines that intersect at intervals, a matrix of pixels formed at the intersections of the scanning signal lines, and the scanning signal lines and the information signal lines. a liquid crystal panel having a ferroelectric liquid crystal disposed between the information signal line and oriented in one of the orientation states and the other orientation state depending on the polarity of the applied voltage, b. a light source that changes periodically between a dark state and a dark state and is arranged to illuminate the liquid crystal panel from the back; c. sequentially scans and selects the scanning signal lines and applies a scanning selection signal to the information signal line; , by applying an information signal consisting of bipolar pulses with the same absolute value of the applied voltage and the same application time according to the information in synchronization with the scanning selection signal, to the pixel on the scanning signal line selected for scanning. Driving means that performs writing and applies a scan non-selection signal to the scan signal line that is not selected for scanning so that the pixel on the scan signal line that is not selected for scan maintains the written state of the pixel. , and d performing writing by applying the information signal so that the minimum dark state of the light emission amount of the light source coincides with the center of the application period of the information signal synchronized with the scanning selection signal; When the amount of light emitted from the light source is in a bright state, the amount of light emitted from the light source is synchronized with the output of the scan selection signal and the information signal from the driving means so that writing is not performed to pixels on the scanning signal line selected for scanning. This is a ferroelectric liquid crystal device having synchronization means for Therefore, flickering due to transient overshoot at the selected pixel can be made less noticeable.

[Example] Hereinafter, an example of the present invention will be described with reference to FIGS. 1 to 3.

FIG. 2 is a schematic diagram of a liquid crystal element 31 having a matrix electrode structure with a ferroelectric liquid crystal sandwiched between them. In the figure, S ₁ to _{S 5} are scanning signal lines to which scanning selection signals are sequentially applied, and I ₁ to _{I 5} are information signal lines to which information signals are applied. Also, A indicates the "dark" state of the pixel, and C indicates the "bright" state of the pixel.

FIG. 1 is a time chart of voltage waveforms when driving the liquid crystal element. In the figure, T indicates the time change in the light intensity of the illumination light source, and here a fluorescent lamp whose light intensity changes at a frequency of about 100 Hz was used. This frequency can be arbitrarily selected as long as it is such that the human eye cannot perceive a change in the amount of light due to the afterimage phenomenon. Further, S ₁ to S ₃ and I ₁ to _{I 2} respectively indicate an example of the voltage waveforms applied to the scanning signal line and the information signal line shown in FIG.
In an optical arrangement in which "dark" is written with a negative voltage and "bright" is written with a positive voltage, the display form shown in FIG. 234 is obtained. Further, A and C indicate voltage waveforms applied to the pixels indicated by A and C in FIG.

In this embodiment, a driving method is used in which pixels on a selected scanning line are once brought into a "bright" state and then a "dark" or "bright" selection signal is applied. In Figure 1, the signal applied to the scanning signal line has a peak value of
It consists of two pulses: 3Vo and -2Vo. First, when a pulse of 3Vo is applied to the scanning signal line and the information signal line is OV, the pixel becomes a "bright" state regardless of its previous history. Next, connect the scanning signal line to -
When a pulse of 2Vo is applied, an information signal consisting of two pulses with peak values of Vo and -Vo is applied to the information signal line. The selection of "dark" or "bright" is given by the first pulse of the information signal consisting of two pulses, the second pulse being provided to prevent crosstalk that would occur if the same information signal was given repeatedly. be. For example, in pixel A, the voltage difference between the upper and lower electrodes becomes - before timing _T1 .
2Vo−Vo=3Vo, and the pixel becomes "dark."
On the other hand, in the pixel C, the voltage difference becomes -2Vo-(-Vo)=-Vo before timing _T3 , and the pixel maintains the "bright" state. The pulse widths of the scanning signal and the information signal are the same To, and the threshold value Vth of the liquid crystal is set as follows for the pulse width To.

Vo<Vth<3Vo or -Vo<-Vth<-3Vo In the embodiment of the present invention, as shown in FIG. 1, the first pulse of the scanning signal is The key is to time it so that it is applied. In this case, the timing is set so that the light amount T of the light source becomes minimum at an intermediate time (T ₁ , T ₂ , . . . ) after the pulse of the information signal.

FIG. 3 is a schematic configuration diagram of a liquid crystal display device. To synchronize the scanning signal with changes in the light intensity of the illumination light source,
The clock pulses from the illumination light source 3 may be input to the clock pulse generator 4, and the clock pulses synchronized with the clock pulse generator 4 may be input to the liquid crystal drive circuit 5 at predetermined timing.

[Effects of the Invention] As described above, according to the present invention, in addition to being able to prevent flickering of a selected pixel during writing to a liquid crystal element, it also has the following effects.

In other words, as mentioned above, it has bistability.
In SSFLC cells, switching between bistable states does not occur for electric fields below the threshold, and the liquid crystal molecules remain in their original stable state. However, this does not mean that the liquid crystal molecules do not respond at all to the electric field E below the threshold value. That is, although no transition occurs between the two stable states, there is a sufficient response around the equilibrium point of one of the stable states, which is optically detected as a temporary change in the amount of transmitted light. Such a phenomenon becomes a particular problem when writing is performed by line sequential scanning in an SSFLC cell having a matrix configuration of scanning signal lines and information signal lines. Therefore, since matrix driving using line sequential scanning is performed, information signals are also applied to pixels on non-selected scanning lines, so liquid crystal molecules respond in all pixels on the information signal lines to which the information signals are applied. I will do it. Of course, the information signal is set to a voltage below the threshold value, so the liquid crystal molecules do not change their orientation, but as mentioned above, they respond to the voltage, so from a viewing angle this appears as flickering on the screen. be exposed. Such flickering is a particular problem when rewriting a part of the screen, that is, when scanning only a part of the scanning lines, because it affects the entire screen, but according to the present invention, it is possible to Since all signals are applied around the time when the amount of light from the back light source is at its minimum, it is possible to suppress such flickering in non-selected pixels.

[Brief explanation of drawings]

1 to 3 are diagrams showing examples, FIG. 4 is a schematic diagram of a cell, and FIG. 5 is a diagram showing an optical response state of a liquid crystal cell. _S1 to _S5 ...Scanning signal line, _I2 to _I5 ...Information signal line, 1...Liquid crystal panel, 2...Back light source, 3...
Power supply for rear light source, 4...Clock pulse generator,
5...Liquid crystal drive circuit.

Claims (1)

[Scope of Claims] 1a A scanning signal line and an information signal line that intersect with each other at intervals, a matrix of pixels formed at the intersections thereof, and a voltage source arranged between the scanning signal line and the information signal line. a liquid crystal panel having a ferroelectric liquid crystal that is oriented in either one of the orientation states and the other orientation state depending on the polarity of the voltage; b. the amount of emitted light changes periodically between a bright state and a dark state; a light source arranged to illuminate the liquid crystal panel from the back; c. sequentially scanning and selecting the scanning signal lines to apply a scanning selection signal; and applying information to the information signal line in synchronization with the scanning selection signal; By applying an information signal consisting of bipolar pulses with the same absolute value of the applied voltage and the same application time in accordance with For pixels on the line, driving means applies a scanning non-selection signal to the scanning signal line which is not selected for scanning so that the pixel maintains its written state; Writing is performed by applying the information signal so that the state and the center of the application period of the information signal synchronized with the scan selection signal coincide, and when the amount of light emitted from the light source is in the bright state, the scan selection is performed. The invention further comprises a synchronizing means for synchronizing the amount of light emitted from the light source and the output of the scanning selection signal and information signal from the driving means so as not to write to the pixels on the scanning signal line. Dielectric liquid crystal device.