LIQUID CRYSTAL DISPLAY APPARATUS

Abstract: A liquid crystal display apparatus in which an image to be displayed is written in a liquid crystal cell (20) by irradiating it with a laser beam and in which the image thus written is enlarged and projected on a screen (48) by optical projection means (41 to 47) and wherein the amount of energy supplied to a spot in the liquid crystal cell (20) in which an image is to be produced is increased when writing is not and will not be done in immediately preceding and succeeding spots, so that the image writing in the liquid crystal cell (20) can be completed within a shorter time.

Description: This invention relates to liquid crystal display apparatus in which writing into a liquid crystal cell is done with a laser beam. Previously, a liquid crystal display apparatus has been proposed in which the characters or the display images are written into the liquid crystal cell by irradiating the cell with a laser beam, and the image is then enlarged and displayed on a screen by optical projection means comprising a light source, a lens and a filter. In such a liquid crystal display apparatus, image writing in the liquid crystal cell is achieved using the following principle. The laser beam is irradiated on the liquid crystal cell for converting the laser beam energy into thermal energy. The heat thus generated is used to heat the liquid crystal to cause a phase transition in the heated spot of the cell. As the irradiation by the laser beam is terminated, the liquid crystal tends to revert to its original phase. However, the liquid crystal spot that has undergone a phase transition cools rapidly, so the disrupted state of the molecular disposition of the liquid crystal caused by the phase transition is maintained and the light scattering centre may remain in the liquid crystal spot. This causes the image to be stored or written into the spot. More specifically, image writing can be achieved by turning the laser beam on and off in a controlled manner with simultaneous scanning for sequentially writing picture elements, spots, blocks or pixels that are the minimum size unit of the image. The laser beam can be scanned by so-called raster scanning or vector scanning. In raster scanning, the laser beam is scanned linearly from, for example, the left side towards the right side of the liquid crystal cell, and when the beam reaches the right side of the cell it is promptly returned to the left side but at a position slightly below the starting point for the preceding scanning line. The beam then again resumes linear scanning towards the right side of the cell, and this sequence is repeated vertically from the top towards the bottom of the cell. In vector scanning, the laser beam is scanned randomly. Such image writing is disclosed in our European patent specification EP-A1-0-087 477, in our West German patent specification DE-A1-34 15 638, in UK patent specification GB-A-1 507 182, and in IBM Journal of Research and Development, Vol. 26, no 2, March 1982, pages 177 to 185. It should be noted that when the image is written into the liquid crystal cell by the described raster scanning, the state of the image writing in a given cell spot corresponding to one pixel depends substantially on whether the writing has also been performed or is to be performed in the immediately preceding and succeeding spots on the same scanning line. That is, when writing is also accomplished in the two adjacent spots to the particular spot being considered, stronger writing is obtained for the same amount of energy supplied to the intermediate spot. That is, the product of the laser output multiplied by the writing time necessary for writing in a spot corresponding to each pixel is greater in the case in which the writing is not performed in either of the immediately adjacent spots. This is because the heat received by the immediately adjacent spots will be transmitted to the intermediate spot in which writing is to be effected, thus facilitating writing in the intermediate spot. It will be noted that the scanning speed of the laser beam depends on the writing time per spot, which is usually set to a constant value. In other words, for an equivalent writing quality more energy is required in the case where writing has not and will not occur in the immediately adjacent spots than for the case where writing occurs in one or both of the immediately adjacent spots. Thus, it has been usual to set the scanning speed of the laser beam by taking as a reference the case of the greatest difficulty of writing, which is the case in which neither of the immediately adjacent spots is written in. However, this results in a prolonged writing time for each liquid crystal cell. On the other hand, when the scanning speed of the laser beam is set by taking as a reference the case of the greatest ease of writing, that is, the case in which writing is performed in one or both of the immediately adjacent spots, the writing time per liquid crystal cell is reduced. However, for this case, optimum writing is not achieved, resulting in a lower display quality. For this case, optimum writing can possibly be achieved by increasing the laser output. However, since the laser unit usually employed for writing is a semiconductor laser which is used at a constant laser output of 30 mW, which is the presently available maximum power, it is not in that case feasible to increase the output of the laser unit. According to the present invention there is provided a liquid crystal display apparatus in which an image is written into a liquid crystal cell by scanning the cell with a laser beam which is controlled by a driving circuit in response to an image signal for sequentially writing pixels; characterized by: decision means which, being connected to said laser driving circuit, determines the amount of energy to be used for writing each pixel, said amount of energy being dependent on whether adjacent pixels have been or are to be written when forming said image. Thus in embodiments of the invention, writing is effected in a spot on the liquid crystal corresponding to a pixel, based on the information as to whether writing has been or is to be effected at least in either of the two immediately adjacent spots, and the amount of energy supplied to the spot is increased if writing is not to occur in either of those immediately adjacent spots. If it has been decided that writing is to be effected in a desired spot corresponding to a pixel but not in either of the two spots adjacent to the desired spot, the amount of energy supplied to the spot in which writing is to be effected is increased such that the optimum writing can be achieved, although the scanning speed of the laser beam is set by taking as a reference the case in which writing is effected in one or both of the immediately adjacent spots. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a block diagram showing an embodiment of liquid crystal display apparatus according to the present invention; Figure 2 is a sectional view showing the structure of the liquid crystal cell in the embodiment of Figure 1; and Figure 3 is a schematic plan view showing a spot on the liquid crystal cell. Figure 1 shows an embodiment of liquid crystal display apparatus wherein the image data D for each spot corresponding to a pixel of the image to be written is supplied to a shift register which has four stages Fn-1, Fn, Fn+1 and Fn+2 and which is driven by clock signals . The stage Fn of the shift register 1 is coupled to a monostable multivibrator 2 and is also coupled to an AND circuit 3. The stages Fn-1, Fn+1 and Fn+2 of ther shift register 1 are connected through inverters 4, 5 and 6 to the AND circuit 3, and the output of the AND circuit 3 is connected to a monostable multivibrator 7 which acts as a means for enhancing the energy supplied to a spot Sn which will be later described. The shift register 1, the inverters 4, 5 and 6 and the AND circuit 3 comprise the discrimination or decision determining means. The multivibrator 2 produces output write pulses PA which pulses have a pulse width or duration corresponding to one pixel or spot. The multivibrator 7 produces as write pulses PB, pulses which have pulse width equal to, for example, 1.4 times that of the write pulses PA. The outputs of the multivibrators 2 and 7 are connected through an OR circuit 8 to a laser driving circuit 9 which drives a writing laser diode 10. The laser diode 10 is a GaAlAs semiconductor laser which has an output of, for example, 30 mW, and produces near infrared light having a wavelength equal to 780 nm. In the present embodiment, the laser diode 10 is used with a constant maximum output of 30 mW. Horizontal address data AX are supplied to a digital to analog (D/A) converter 11 which is connected to a scanner driving circuit 12, which in turn is connected to a horizontal scanner 13. Vertical address data AY are supplied to a D/A converter 14 which is connected to a scanner driving circuit 15, which is connected to a vertical scanner 16. The laser beam emitted from the laser diode 10 impinges upon a galvano-mirror 17 which is driven by the horizontal and vertical scanners 13 and 16, so as to deflect the laser beam horizontally and vertically for scanning. The laser beam from the galvano-mirror 17 passes to a prism 19 through a lens 18, and is deflected at right angles by the prism 19, and falls on a liquid crystal cell 20 which has an area of 16 mm times 16 mm. Thus, 2,000 pixels can be written horizontally and vertically so that a total of 2,000 times 2,000 pixels can be written into the liquid crystal cell 20. Figure 2 illustrates in cross-section the structure of the liquid crystal cell 20 which comprises a liquid crystal layer 21, a pair of orientation coating layers 22 and 23 which are mounted on respective sides of the liquid crystal layer 21, and a glass plate 25 on which a transparent electrically conductive coating 24 such as an ITO film is applied by vacuum deposition. A second glass plate 28 has a cold filter coating 26 for reflecting the laser beam, on which a transparent electrically conductive coating 27 is applied by vacuum deposition, and is mounted in sandwich form as illustrated in Figure 2 with the glass plate 25. The glass plate 25 has its transparent conductive coating 24 disposed on the orientation coating layer 22 of the liquid crystal layer 21, and the second glass plate 28 has its transparent conductive coating 27 disposed on the other orientation layer 23 of the liquid crystal layer 21. The laser beam is received from the left side of the glass plate 25 as illustrated in Figure 2. The liquid crystal layer 21 comprises a liquid crystal having a smectic A phase with an addition as dyestuff of 1,3-di(1,4-dimenthyl-7-isopropyl azulenyl)-cyclobutenediillium which is described in our copending Japanese Patent Application No. 58/73322. Addition of the dyestuff results in a more efficient absorption and transmission of the laser beam to the liquid crystal. The cyanobiphenyl type liquid crystal consisting of a mixture of, for example, 40 mol. percent of octylcyanobiphenyl and 60 mol. percent of decylcyanobiphenyl may be used as the liquid crystal. Such liquid crystal changes in its crystal state and undergoes a phase transition through the smectic A phase, nematic phase and isotropic phase with changes in the temperature, and thus on the absorption of the laser beam. For example, the first transition temperature from the smectic A phase to the nematic phase is at 44.8 o C, and the second phase transition temperature from the nematic to the isotropic phase is at 45.2 o C. As shown in Figure 1, the liquid crystal cell 20 is held by a temperature control circuit 31 at a bias temperature of 43.3 o C, which is lower for example by 1.5 o C than the first phase transition temperature of the liquid crystal which is 44.8 o C. Therefore, in the normal state, that is in the preparatory state for writing the display image, the liquid crystal of the liquid crystal layer 21 is held in the smectic phase A. An erasure signal E is supplied to an erasure circuit 32 which applies an AC field across the transparent conductive coatings 24 and 27 of the liquid crystal cell 20, so as to erase previously written images. The display image written into the liquid crystal cell 20 is projected by the following optical projection means. The projection light beam from a halogen lamp 41, which may be a 150 watt lamp, passes to a mirror 45 through an infrared filter 42, condenser lens 43 and an ultraviolet filter 44, and is reflected by the mirror 45 so as to irradiate the liquid crystal cell 20 through a lens 46 from the same side as the glass plate 28. The light beam which passes through the liquid crystal cell 20 reaches a screen 48 through the prism 19 and a projection lens 47. In this manner, the image written in the liquid crystal cell 20 is enlarged and is projected onto the screen 48. Figure 3 illustrates the method of writing the image in the liquid crystal display device. It is assumed that the scanning direction of the laser beam is as indicated by the arrow, and writing is at the spot indicated by Sn. When the image data D corresponding to the spot Sn is "0", that is, when the contents of the stage Fn of the shift register 1 is "0", no pulse outputs are supplied from the multivibrators 2 or 7 and no pixel writing occurs at the spot Sn. In this case, it is not necessary to consider the contents of the other stages Fn-1, Fn+1 and Fn+2 or the shift register 1. When the image data D corresponding to the spot Sn is "1", that is, when the contents of the stage Fn of the shift register 1 is "1", a writing pulse or writing pulses are supplied from both of the multivibrators 2 and 7 or from only the multivibrator 2 for pixel writing of the spot Sn. Specifically, the contents of the other stages Fn-1, Fn+1 and Fn+2 of the shift register 1 are determined, and when the contents of one or more of the other stages is "1", that is, when pixel writing has been or is to be accomplished not only in the spot Sn but in at least one of the spots Sn-1, Sn+1 and Sn+2, a writing pulse PA having a pulse width corresponding to one pixel or spot is produced at the output of the multivibrator 2 and is supplied to the laser driving circuit 9 through the OR circuit 8. The laser driving circuit 9 causes the laser diode 10 to be turned on for a period corresponding to the duration of the writing pulse PA for writing the pixel in the spot Sn in the liquid crystal cell 20. On the other hand, when the contents of the stages Fn-1, Fn+1 and Fn+2 of the shift register 1 are all Zero, that is, when the pixels have not been and will not be written in any of the spots Sn-1, Sn+1 and Sn+2, but must be written in the spot Sn, a write pulse PA will be supplied from the multivibrator 2, and the write pulse PB having a pulse width equal to for example 1.4 times that of the write pulse PA is produced at the output of the multivibrator 7 and is supplied to the laser driving circuit 9 through the OR circuit 8. The pulse PB is greater than the pulse PA in pulse width. Due to the operation of the laser driving circuit 9, the laser diode 20 will be turned on during the time interval corresponding to the pulse width of the write pulse PB for writing the pixel in the spot Sn of the liquid crystal cell 20. Thus, in the present case, the time interval of the laser beam irradiation will be longer than in the case in which the pixels are written in at least one of the spots Sn-1, Sn+1 and Sn+2, so that the energy applied to the spot Sn is increased during writing. The condition that the pixel is not written in the spot Sn+2 as well is required because any pixel written in the spot Sn is likely to affect the adjoining spot Sn+1 to cause the spot Sn+1 to collapse. When the pulse width of the write pulse PB is set to the usual value, it is not necessary to take the spot Sn+2 into consideration. Therefore, even if the scanning speed of the laser beam is adjusted by taking as a reference the case of the greatest ease of image writing of the spot Sn, that is the case in which writing is accomplished in at least one of the spots Sn-1, Sn+1 and Sn+2 also, optimum image writing can be achieved in the spot Sn in the case of the utmost difficulty in image writing of the spot Sn, that is, the case in which image writing is not accomplished in any of the spots Sn-1, Sn+1 and Sn+2. Therefore, the time interval of image writing into the liquid crystal cell 20 can be reduced without lowering the display quality. More specifically, the writing time per pixel can be reduced in this embodiment for example to 2.5 microseconds from a writing time of say 4.3 microseconds. Therefore, with the blanking time for horizontal scanning of the laser beam equal to 1 millisecond the time necessary for writing, for example, 2,000 x 2,000 pixels can be reduced to 12.0 seconds from 19.2 seconds, and this results in a reduction of 7.2 seconds in the writing time while maintaining the same quality of display. It will be noted that since the laser diode 10 is employed in the present embodiment at the constant maximum output of 30 mW, the amount of energy to be applied to the spot Sn into which the image writing is to be accomplished is controlled by changing the time of the irradiation of the laser beam. However, when there is a sufficient laser output available from the laser source, the laser output can be changed so as to control the amount of energy applied to the spot. In addition, the present invention can also be applied to image writing by vector scanning.

Claims: A liquid crystal display apparatus in which an image is written into a liquid crystal cell (20) by scanning the cell (20) with a laser beam which is controlled by a driving circuit (9) in response to an image signal (D) for sequentially writing pixels; characterized by: decision means (1, 3 to 6) which, being connected to said laser driving circuit (9), determines the amount of energy to be used for writing each pixel, said amount of energy being dependent on whether or not pixels adjacent to the pixel to be written have been or are to be written when forming said image. A liquid crystal display apparatus according to claim 1 wherein said liquid crystal cell (20) has orientation layers (22, 23) provided on respective sides of a liquid crystal layer (21), and the resulting composite sheet is sandwiched between a glass plate (25) having a transparent conductive layer (24) deposited thereon by evaporation and a second glass plate (28) having a cold filter coating (26) for reflecting the laser beam and a transparent conductive layer (27) deposited thereon by evaporation. A liquid crystal display apparatus according to claim 2 wherein the liquid crystal layer (21) comprises a liquid crystal having a smectic A phase which is supplemented by a dyestuff material. A liquid crystal apparatus according to claim 1 wherein said decision means (1, 3 to 6) includes a shift register (1), an inverter (4 to 6) and an AND circuit (3). A liquid crystal display apparatus according to claim 1 wherein said decision means (1, 3 to 6) takes into consideration that writing will not occur on the one pixel immediately preceding and the two pixels immediately succeeding each pixel to be written. A liquid crystal display apparatus according to claim 1 wherein said decision means (1, 3 to 6) determines the time of irradiation of the laser beam on each pixel to be written so as to increase the amount of the energy supplied to said each pixel. A liquid crystal display apparatus according to any one of the preceding claims wherein said decision means (1, 3 to 6) comprises a multistage shift register (1) which receives incoming write information, a first monostable multivibrator (2) with a first time period connected to one stage of said shift register (1), at least two inverters (4 to 6) connected to other stages of said shift register (1) adjacent to said one stage, an AND gate (3) connected to said one stage and said at least two invertors (4 to 6), and a second monostable multivibrator (7) with a second time period connected to said AND gate (3), said laser driving circuit (9) being connected to said first and second multivibrators (2, 7). A liquid crystal display apparatus according to claim 7 including an OR gate (8) connected between said first and second monostable multivibrators (2, 7) and said laser driving circuit (9).

Inventors: SHIONOYA, TOSHIO C/O PATENTS DIVISION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: G02F1
IPC codes: G09G3, C09K19, G02F1