Display device

Abstract: A display device comprises a display panel (11) having a ferroelectric liquid crystal, information electrodes and scan electrodes for changing orientations of the ferroelectric liquid crystal in the display panel, a memory for storing display data, a scan electrode driver (12), a data electrode driver (13) a display controller (CPU), a power supply means (15) and a switch (1) for turning on and off the supply of power from said power supply. Display erasing means (26) are provided for erasing display data stored in the display panel in response to the actuation of the switch (1).

Description: The present invention relates to a display device for displaying data, and more particularly to a display device having a property of retaining its display state after the removal of electric field, for example one having a ferroelectric liquid crystal panel. The use of a bistable liquid crystal element has been proposed by Clark and Lagerwall (JP-A-56107216/1981 and U.S. Patent US-A-4,367,924). Ferroelectric liquid crystal having chiral smectic C phase (Sm C *) or H phase (Sm H *) is usually used as the bistable liquid crystal. This liquid crystal has bistable state to an electric field, including a first optically stable state (first orientation state) and a second optically stable state (second orientation state). Accordingly, unlike an optical modulation element used in a TN type liquid crystal, the liquid crystal is oriented in the first optically stable state for one electric field vector, and the liquid crystal is oriented in the second optically stable state for the other electric field vector. The liquid crystal of this type quickly responds to the applied electric field to assume one of the two stable states and maintains the state when the electric field is removed. Many of the problems involved in the TN type element are essentially resolved by making use of the above property. In the display device which uses the TN type element, the TN type element has no memory function and hence the content of display is not stored in the display panel. Accordingly, no special means for erasing the display content is necessary from a stand-point of security of confidential information. On the other hand, in the display panel which uses the bistable ferroelectric liquid crystal, the display content is stored in the display panel. In a transmission type display device which allows observation of the display content by illumination of a back light, the stored display content is not recognised when the back light is turned off, but when the back light is turned on, the stored display content appears. This raises a problem in confidential information security. The display device defined by the appended claims is provided as a solution. In the display device defined it is arranged that the display contents of the display device are to be erased in response to the turn-off of a switch. It is also arranged that the power to the scan and information line drivers of the display device is turned off following erasure. As a result of the above arrangement, the display contents are not maintained when the device is switched off. Accordingly, information displayed during use is erased and is not displayed following turn-off and is not compromised. It is acknowledged that GB-A-2157471 discloses a display device having a liquid crystal panel and which is arranged to initiate the display on the panel following turn-on of a power switch. In the accompanying drawings: Fig. 1 shows a preferred embodiment of the present invention; Figs. 2 and 3 show perspective views of ferroelectric liquid crystal elements that can be used in the embodiment; Fig. 4 shows a waveform of a display content erase voltage that can be used in the embodiment; Figs. 5A and 5B show projections of a director of a chiral smectic layer in a uniform orientation; and Figs. 6A and 6B show projections of a director of a chiral smectic layer in a twist orientation. A preferred embodiment of the present invention will be described hereinbelow and reference will be made to the drawings listed above. The following description is given by way of example only. A chiral smectic liquid crystal having ferroelectric properties is particularly suitable as a liquid crystal material used in the present invention. Specifically, chiral smectic C phase (Sm C *), chiral smectic G phase (Sm G *), chiral smectic F phase (Sm F *), chiral smectic I phase (Sm I *) or chiral smectic H phase (Sm H *) liquid crystal may be used. Details of the ferroelectric liquid crystal are described in "Ferroelectric Liquid Crystals" LE JOURNAL DE PHYSIQUE LETTRES 1975, NO. 36(L-69), "Submicro Second Bistable Electro-optic Switching in Liquid Crystals" Applied Physics Letters, 1980, No. 36 (11), and "Liquid Crystals" Solid-State Physics of Japan, 1981, NO. 16(141). The present invention may use the ferroelectric liquid crystals disclosed in those articles. Specific examples of the ferroelectric liquid crystal compound are decyloxybenzylidene-p'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-p'-amino-2-chloropropylcinnamate (HOBACPC), and 4-o-(2-methyl)-butylresorcylidene-4'-octylaniline (MBRA 8). The ferroelectric liquid crystal which exhibits cholesteric phase at a temperature higher than that of chiral smecticphase liquid crystal is most preferable. For example, biphenylester liquid crystal which exhibits a phase transition temperature described in the embodiment may be used. When the element is constructed by using one of those materials, the element may be supported by a copper block having a heater embedded therein in order to keep the element at a temperature at which the liquid crystal compound exhibits a desired phase. Fig. 2 shows a cell to explain the operation of the ferroelectric liquid crystal. The Sm C * phase is assumed as the desired phase. Numerals 31 and 31' denote substrates (glass plates) covered by transparent electrodes made of thin films such as In 2 O 3 , SnO 2 or ITO (indium-tin oxide), and Sm C * phase liquid crystal which is oriented such that a liquid crystal molecule layer 32 is normal to the glass plate is filled therebetween. Thick lines 33 represent the liquid crystal molecules which form a continuous spiral structure in parallel with the substrate plane. An angle between a center axis 35 of the spiral structure and an axis of the liquid crystal molecules 33 is represented by H ○. The liquid crystal molecules 33 each have a bipolar moment (P⊥) 34 orthogonally to the molecule. When a voltage higher than a predetermined threshold is applied between the substrates 31 and 31', the spiral structure of the liquid crystal molecules 33 is released and the liquid crystal molecules 33 may be reoriented so that all the bipolar moments (P⊥) 34 are oriented along the electric field. The liquid crystal molecule 33 is of elongated shape and a refractive index along a major axis and a refractive index along a minor axis are different. Thus, when polarizers which are cross-nicol to each other are placed on the opposite sides of the glass plate, a liquid crystal optical element whose optical characteristic changes depending on a polarity of applied voltage is provided. The liquid crystal cell preferably used in the liquid crystal optical element of the present invention may be very thin (for example, 10 µm or less). As the liquid crystal layer is thinned, the spiral structure of the liquid crystal molecules is released even under non-application of the electric field as shown in Fig. 3, and the bipolar moment P or P' is oriented either upward (44) or downwards (44'). One half of an angle between the molecule axis of the liquid crystal molecule 43 and a direction 43 is called a tilt angle (H ○) which is equal to one half of an apex angle of a cone of the spiral structure. Electric field E or E' of different polarity, which is higher than a predetermined threshold is applied to such a cell by voltage application means 41 or 41' as shown in Fig. 3. Thus, the bipolar moment is reoriented upwards 44 or downwards 44' in accordance with the electric field vector of the electric field E or E', and the liquid crystal molecules are oriented in either the first stable state 43 or the second stable state 43'. There are two advantages in utilizing the ferroelectric liquid crystal optical element described above. First, the response speed is very fast, and secondly, the orientation of the liquid crystal molecule is bistable. The second avantage is explained with reference to Fig. 3. When the electric field E is applied, the liquid crystal molecule is oriented in the first stable state 43 which is stable even after the electric field is removed. When the electric field E' of the opposite polarity is applied, the liquid crystal molecule is oriented in the second stable state 43' which is also stable even after the electric field is removed. The cell is preferably as thin as possible in order to effectively attain the fast response speed and the bistability. Fig. 1 shows a circuit configuration of an embodiment of the display device of the present invention. Numeral 11 denotes a ferroelectric liquid crystal display panel, numeral 12 denotes a scan line driver, numeral 13 denotes an information line driver, numeral 14 denotes a controller, and numeral 16 denotes a back light arranged on a back side of the display panel 11. In the display device shown in Fig. 1, when an operator turns off a main switch 1 to terminate the display, a signal is generated by a display content erase signal generator 26 and it is supplied to the controller 14. The scan line driver 12 and the information line driver 13 are enabled by a control signal from the controller 14 so that an erase signal is supplied to the scan line driver 12 and erase data is supplied to the information line driver 13, from the controller 14. The signal supplied from the scan line driver 12 when the display content is to be erased may be identical to the scan signal used for writing. A signal to orient the ferroelectric liquid crystal to one stable state (white) is simultaneously applied to the information lines in synchronism with the output signal of the scan line driver 12. Fig. 4 shows the signals VS 1 , VS 2 ,... produced by the scan line driver 12 and the signals VI 1 , VI 2 , ... produced by the information line driver 13. In the present invention, instead of those signals, a 2V 0 pulse may be simultaneously applied to the scan lines, and a -V 0 pulse may be applied to the information lines in synchronism therewith. After the display content has been erased, the scan line driver 12 supplies an end signal to the controller 14 which send a power-off signal to a power controller 15 so that the power is turned off. In the present embodiment, when the display content is to be observed by the illumination of the back light 16 arranged behind the display panel 11, the display content erase signal is supplied to the controller 14 which controls the turn-off of the back light 16. When data is to be displayed on the display device, a video RAM address data is sent to the controller 14 which produces the control signal to enable the scan line driver 12 and the information line driver 13. The controller 14 decodes the video RAM address data and sends a scan line address signal and a display data to the scan line driver 12 and the information line driver 13, respectively. The above ferroelectric liquid crystal element is more easily attained in the twist orientation in which the liquid crystal molecules are twisted from the upper substrate to the lower substrate in the molecular layer as shown in Figs. 6A and 6B than in the uniform orientation in which the liquid crystal molecules are arranged in parallel in the liquid crystal molecule layer as shown in figs. 5A and 5B. When the liquid crystal molecules are in the twist orientation, the apparent tilt angle between the liquid crystal molecule axes in the first orientation and second orientation is small, resulting in the reduction of contrast and transmitted light as well as overshoot in the response of the liquid crystal molecule when the orientation is switched. This causes fluctuation of the transmitted light due to flicker of the display image. Accordingly, the display device having the liquid crystal molecules uniformly oriented is preferable. Figs. 6A and 6B show a director or C director 71 cut in a plane of a smectic layer of a bistable liquid crystal cell when the spiral structure is released, and an array of self-polarisations 72. A top circle (which corresponds to a projection of the liquid crystal cone onto the smectic layer) shows a state near the upper substrate, and a bottom circle shows a state near the lower substrate. In Fig. 6A, an average self-polarisation 73b is oriented downward, and in Fig. 6B, an average self-polarisation 73a is oriented upward. Accordingly, switching takes place between the state of Fig. 6A and the state of Fig. 6B depending on the electric field. Figs. 5A and 5B show arrays of C directors when there is no twist along the direction of thickness of the liquid crystal cell, that is, in an ideal state. For generalisation purpose, the liquid crystal molecules are shown as somewhat tilted to the substrate plane. The direction of the self-polarisation is upward in figs. 5A and downward in Fig. 5B. The uniform orientation shown in Figs. 5A and 5B is attained applying to the ferroelectric liquid crystal in the twist orientation shown in Figs. 6A and 6B an A.C. voltage higher than a threshold voltage (10 - 500 V) at a frequency of higher than 0.1 Hz, preferably 10 Hz - 5 KHz. The uniform orientation of the ferroelectric liquid crystal element may also be attained by the A.C. voltage applied to the display panel when the display content is to be erased. In this case, the display content erase voltage may be an A.C. voltage of 10 V - 500 V at a frequency of higher than 0.1 Hz.

Claims: A display device comprising: a liquid crystal display panel (11) wherein the orientation of the liquid crystal molecules is stable even after the removal of electric field; control means (12-14) having a display controller (14), a scan line driver (12) and an information line driver (13) for controlling the display of said display panel (11); a switch (1) for turning on and off a supply of power from a power supply to said control means (12-14); display content erase means (14,26) for erasing, in response to the turn-off of said switch (1), display data stored in said display panel (11) by said control means (12-14); and a power controller (15) for controlling the power supplied to said scan line driver (12) and said information line driver (13), arranged to turn off the power after the display content has been erased in response to the turn-off of said switch (1). A display device according to claim 1, including a backlight (16) arranged behind said display panel (11); and, wherein said control means (12-14) is adapted to control the turn-off of said backlight (16). A display device according to either preceding claim, wherein said display content erase means (14,26) is operable to control said scan line driver (12) and said information line driver (13) to erase the contents of said display panel (11) line sequentially. A display device according to either of claims 1 or 2, wherein said display content erase means (14,26) is operable to control said scan line driver (12) and said information line driver (13) to erase simultaneously the whole content of said display panel (11). A display device according to either of claims 1 or 2, wherein said display content erase means (14,26) is operable to control said information line driver (12) to supply an erase voltage simultaneously to the information lines of said display panel (11). A display device according to any preceding claim wherein said display controller (14) is adapted to control said power controller (15) to turn-off the power supplied therefrom once the display content has been erased. A display device according to any preceding claim, wherein said display panel (11) is a ferroelectric liquid crystal display panel. A display device according to claim 7, wherein said display panel (11) is one sandwiching a chiral smectic liquid crystal of a uniform orientation between a pair of substrates.

Inventors: INOUE HIROSHI, KANNO HIDEO, MIZUTOME ATSUSHI, NETSU HIROSHI, INOUE, HIROSHI, KANNO, HIDEO, MIZUTOME, ATSUSHI, NETSU, HIROSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: G09G3
IPC codes: G09G3