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WO2001016928A1 - Reduction of effects caused by imbalanced driving of liquid crystal cells - Google Patents

Reduction of effects caused by imbalanced driving of liquid crystal cells Download PDF

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Publication number
WO2001016928A1
WO2001016928A1 PCT/US2000/023645 US0023645W WO0116928A1 WO 2001016928 A1 WO2001016928 A1 WO 2001016928A1 US 0023645 W US0023645 W US 0023645W WO 0116928 A1 WO0116928 A1 WO 0116928A1
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WO
WIPO (PCT)
Prior art keywords
liquid crystal
time
cell
fields
given
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/US2000/023645
Other languages
French (fr)
Inventor
Jiuzhi Xue
Lianhua Ji
Mark A. Handschy
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Displaytech Inc
Original Assignee
Displaytech Inc
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Filing date
Publication date
Application filed by Displaytech Inc filed Critical Displaytech Inc
Priority to AU73352/00A priority Critical patent/AU7335200A/en
Publication of WO2001016928A1 publication Critical patent/WO2001016928A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3648—Control of matrices with row and column drivers using an active matrix
    • G09G3/3651—Control of matrices with row and column drivers using an active matrix using multistable liquid crystals, e.g. ferroelectric liquid crystals
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
    • G09G2310/0243—Details of the generation of driving signals
    • G09G2310/0251—Precharge or discharge of pixel before applying new pixel voltage
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/06—Details of flat display driving waveforms
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/06—Details of flat display driving waveforms
    • G09G2310/061—Details of flat display driving waveforms for resetting or blanking
    • G09G2310/063—Waveforms for resetting the whole screen at once
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0204—Compensation of DC component across the pixels in flat panels
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0257—Reduction of after-image effects
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3614—Control of polarity reversal in general
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3648—Control of matrices with row and column drivers using an active matrix
    • G09G3/3655—Details of drivers for counter electrodes, e.g. common electrodes for pixel capacitors or supplementary storage capacitors

Definitions

  • the present invention relates generally to liquid crystal devices and more specifically to schemes for d ⁇ ving a liquid crystal cell, such as a ferroelectric liquid crystal cell without requi ⁇ ng DC-balancing of the liquid crystal cell
  • a non-DC-balanced drive scheme for liquid crystal device
  • liquid crystal mate ⁇ al such as a ferroelect ⁇ c liquid crystal (FLC) mate ⁇ al
  • FLC ferroelect ⁇ c liquid crystal
  • This light modulation of liquid crystal mate ⁇ al is accomplished by establishing and maintaining elect ⁇ c fields across the liquid crystal mate ⁇ al in a controlled way m order to switch the light modulating characte ⁇ stics of the mate ⁇ al
  • an electric field is established m one direction across the FLC mate ⁇ al in order to produce a first light modulating state, for example an ON state
  • An electric field is established in the opposite direction across the FLC material in order to produce a second light modulating state, for example an OFF state
  • balancing the electric fields or DC balancing refers to balancing the time integral of the electric fields
  • the electric fields are balanced when the cumulative time integral of the elect ⁇ c field (hit is present in one direction across the liquid crystal material is substantially equal to the cumulative time integral of the elect ⁇ c field that is present in the opposite direction during a predetermined amount of lime during the operation of the spatial light modulator
  • Another way of slating this is that the electric fields are balanced when the average of the product of the applied voltage and the amount of time that field is present averages to substantially zero during a predetermined amount of time during the operation of the spatial light modulator
  • the image produced by the SLM du ⁇ ng the lime in which the frame is inverted for purposes of DC balancing may not typically be viewed If the system were viewed during the inverted lime without correcting for the inversion of the image, the image would be degraded In the case in which the image is inverted at a frequency faster than the critical flicker rate of the human eye, the overall image would be completely washed out and all of ( he pixels would appear to be half on In the case in which the image is inverted at a frequency slower than the critical flicker rate of the human eye, the viewer would see the image switching between the positive image and the inverted image Neither of these sitti.it ions would provide a usable display In one approach to solving this problem, the light source used to illuminate the SLM is switched off or directed away from the SLM du ⁇ ng the time when the frame is inverted However, this approach substantially limits (he brightness and efficiency of the system In the case where the magnitude of the electric field du ⁇ ng the DC-balancing and
  • compensator cells have been proposed for SLMs
  • copending United States Patent Application Serial Number 09/251 ,627 entitled COMPENSATOR ARRANGEMENTS FOR A CONTINUOUSLY VIEWABLE, DC-HELD BALANCED, REFLECTIVE, FERROELECTRIC LIQUID CRYSTAL DISPLAY SYSTEM, attorney docket number DIS P01 1CIP, which application i s incorporated herein by reference, discloses several approaches lo providing display systems that include compensator cells These compensator cells are intended to correct for the frame inversion during the time when the FLC pixel is being operated in its inverted stale, thereby allowing the display to be substantially continuously viewable
  • these compensator cell arrangements appear to work well, they increase the complexity and cost of the display system by requiring the use of a compensator cell and in many cases other additional components
  • the pixels are defined as the intersection of a column electrode with a row electrode
  • the column electrodes are formed as long, narrow, and parallel conductors that run entirely across the display with each column electrode being the width of one pixel
  • the row electrodes are long, narrow, and parallel conductors that run entirely across the display in a direction perpendicular to the column electrodes w ⁇ (h each row electrode being the height of one pixel
  • These electrodes typically consist of transparent Indium-Tin Oxide, and this mate ⁇ al is deposited directly onto the inner surfaces of two glass substrates
  • the column electrodes are put on one substrate, while the row electrodes are put on the second substrate
  • the substrates are then assembled to have the FLC layer between them
  • the FLC mate ⁇ al comprising a pixel is forced to one of two electro-optic states (ON or OFF in the display) by the application of an elect ⁇ c field
  • the image data are written to the display a row at a time, and all the rows are written, usually sequentially, du ⁇ ng each image frame Any given row is selected for writing by applying a particular voltage to the associated row electrode Meanwhile, the image data for each pixel in the selected row are applied to each associated column electrode as a particular voltage The difference between these two voltages provides the electric field needed to switch each specific FLC pixel After a short lime the next row is selected and the image data are written to it with the approp ⁇ ate pixel voltages applied to the columns Typically, voltages greater than 10V magnitude are applied to the electrodes, since only such high voltages can cause (he FLC to switch in the very small fraction of the frame lime du ⁇ ng which the image data are actually applied to any one row
  • the present invention applies to active matrix displays that maintain a selected electric field at all times This means that the active mat ⁇ x methods and apparatus of the present invention could not make use of the p ⁇ or art passive matrix drive waveforms
  • the present invention discloses novel methods for solving or reducing the above described image sticking problems caused by unbalanced electric fields without requiring DC-balancing. These novel methods improve the effectiveness of the display system without increasing the complexilyof the system, as would be the case if DC-balancing were required.
  • the method uses input image data to control how the cell is operated.
  • the method includes the step of applying image producing electric fields to Ihe cell during a first portion of the given period of time.
  • the image producing electric fields depend in a predetermined way upon the input image data.
  • the method further includes the step of applying additional electric fields to the cell during a second portion of the given period of time.
  • the image producing electric fields and the additional electric fields are such that the cumulative time integral of the electric fields that are present in one direction across the liquid crystal material is not substantially equal to the cumulative time integral of the electric fields that are present in the opposite direction during the given period of time during the operation of liquid crystal cell.
  • the additional electric fields are electric fields configured to reduce the amount of image sticking caused by the image producing electric fields compared to the amount of image sticking that would occur if only the image producing electric fields were applied to the cell during Ihe given period of time.
  • the image data is divided into frame image data corresponding to individual frames of image data.
  • the given period of time is a frame time associated with one frame of image data.
  • the method is a method of operating the liquid crystal cell for a plurality of frame times at a certain frame rate.
  • the liquid crystal cell is a ferroelectric liquid crystal cell including ferroelectric liquid crystal material.
  • the ferroelectric liquid crystal cell is a ferroelectric liquid crystal spatial light modulator for modulating light directed into the spatial light modulator.
  • the ferroelectric liquid crystal material of the spatial light modulator is divided into a plurality of individually controllable pixels.
  • the step of applying image producing electric fields to the cell includes the step of applying image producing electric fields to each of the individually controllable pixels during the first portion of the given pe ⁇ od of time This causes Ihe individually controllable pixels to form a desired light modulating pattern for modulating light directed into the spatial light modulator
  • the immediately above described spatial light modulator may be used as part of an overall display system that includes an illuminator for directing light into the spatial light modulator
  • the method includes the step of causing the illuminator not to direct light into the spatial light modulator during the second portion of the given period of time during which the additional electric fields are being applied to Ihe spatial light modulator
  • the ferroelectric liquid crystal material includes a top and a bottom surface The top and bottom surfaces of the liquid crystal material are approximately coplanar
  • the spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material
  • the spatial light modulator also includes a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material Each of Ihe plurality of pixel electrodes is associated with, and capable of controlling, one of the plurality of pixels
  • the method includes the step of setting all of the pixel electrodes to the same elect ⁇ c potential Additionally, an elect ⁇ c potential having a varying magnitude and polarity is applied to the top electrode of the spatial light modulator for the second portion of the given period of time
  • the method includes the step of individually setting each pixel electrode to an electric potential related in a predetermined way to at least one of the elect ⁇ c fields applied to that pixel during the first portion of the given pe ⁇ od of time Furthermore, an elect ⁇ c potential having a varying magnitude and polarity is applied to the top electrode of the spatial light modulator for the second portion of the given pe ⁇ od of time
  • the method includes the step of providing an open circuit to each of the pixel electrodes so as to float the electric potential of each of the pixel electrodes
  • this embodiment further includes the step of applying an electric potential having a varying magnitude and pola ⁇ ty to the top electrode of the spatial light modulator for the second portion of the given pe ⁇ od of time
  • the method includes the step of holding the top electrode of the spatial light modulator at a constant electric potential
  • an electric potential having a varying magnitude and polarity is applied to all of the pixel electrodes for the second portion of the given period of time
  • the second portion of the given period of time is less than or equal to about twenty percent of the duration of Ihe given period of time
  • the maximum magnitude of the additional electric fields applied to the cell during the second portion of the given pe ⁇ od of time is preferably substantially greater than Ihe maximum magnitude of the elect ⁇ c fields applied to the cell du ⁇ ng the first portion of the given pe ⁇ od of time
  • the maximum magnitudes of the additional electric fields applied to the cell du ⁇ ng the second portion of the given period of lime are preferably in the range of about one to twenty times the maximum magnitudes of the image producing electric fields applied to the cell during the first portion of the given period of lime
  • the magnitude of the additional electric fields that are applied to the cell during the second portion of the given period of time decrease in magnitude during Ihe second portion of the given period of lime
  • the additional electric fields that are applied to the cell during the second portion of the given period of time may be applied at an increasing frequency du ⁇ ng the second portion of the given period of time
  • Figure 1 is a diagrammatic perspective view of an exemplary FLC SLM based display system which may be operated using the methods of the present invention
  • Figure 2A is a diagrammatic perspective view of the FLC SLM of the display system of Figure I
  • Figure 2B is a diagrammatic cross sectional view of the FLC SLM of Figure 2A
  • Figure 2C is a diagrammatic illustration showing the operation of one of the pixels of the FLC
  • FIG. 3 is a flow diagram illustrating the various steps of a method of operating a liquid crystal cell in accordance with the invention
  • Figure 4 is a graph illustrating a first embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention during a given time pe ⁇ od
  • Figure 5 is a graph illustrating a second embodiment of the elect ⁇ c field voltages used to operate a liquid crystal cell in accordance with the invention during a given time pe ⁇ od
  • Figure 6 is a graph illustrating a third embodiment of Ihe electric field voltages used to operate liquid crystal cell in accordance with the invention during a given time pe ⁇ od
  • Figure 7 is a graph illustrating a fourth embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention du ⁇ ng a given time pe ⁇ od
  • Figure 8 is a graph illustrating a fifth embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention during a given lime period
  • Figure 9 is a graph illustrating a sixth embodiment of the electric field voltages used to opera t e a liquid crystal cell in accordance with the invention during a given time period
  • An invention is desc ⁇ bed herein for providing a method of operating a liquid crystal cell during a given period of lime without requiring DC-balancing of Ihe cell
  • numerous specific details are set forth in order to provide a thorough understanding of the present invention
  • the present invention may be embodied in a wide variety of specific configurations
  • well known liquid crystal cell manufacturing processes and known methods of controlling liquid crystal cells using various electrical circuits will not be desc ⁇ bed in detail herein so as not to unnecessarily obscure the present invention
  • the method of the present invention may be used with a wide va ⁇ ety of types of liquid crystal cells that may be used in a wide variety of specific applications
  • the method of the present invention will be described with reference to a ferroelectric liquid crystal display system such as those desc ⁇ bed in the above referenced United States patents 5,748,164 and 5,808,800
  • the methods of the present invention will be desc ⁇ bed with reference to these specific types of ferroelectric liquid crystal display systems, it should be understood that Ihe methods of the present invention are not limited to these types of systems
  • the novel methods disclosed herein may be utilized to operate all types of liquid crystal cells including a wide variety of types of ferroelect ⁇ c liquid crystal cells and nematic liquid crystal cells
  • the present invention is not limited to display systems but instead would equally apply to any systems that use liquid crystal cells that may exhibit problems as a result of image-sticking caused by unbalanced elect ⁇ c fields passed through the cell
  • the display system includes a ferroelectric liquid crystal VLSI (FLC/VLSI) spatial light modulator 12
  • Display system 10 also includes a data w ⁇ ting arrangement 14 for controlling FLC/VLSI spatial light modulator 12 and a video or digitized image source 1 which creates or provides, as an input to data w ⁇ ting arrangement 14, digitized images or input image data
  • Display system 10 further includes an illumination arrangement generally designated by reference numeral 16 for illuminating spatial light modulator 12 and an appropriately designed readily available lens 18 for producing a viewable image of the SLM FLC VLSI spatial light modulator 12 includes an array of individually addressable pixels, not shown in Figure 1, designed to be switched by data writing arrangement 14 between ON (light) and OFF (dark) states
  • Illumination arrangement 16 includes a light source 20 that may be switchably controlled by data writing arrangement 14, a collimatm
  • either unpola ⁇ zed or polarized light that is generated by light source 20 in the form of light rays 26 is collected by collimatmg a ⁇ angement 22 and directed into polarizer/analyzer 24
  • the pola ⁇ zer/analyzer 24 cjuses light of a particular polarization state, for example S polarized light, to be directed into FLC/VLSI spatial light modulator 12 while any light of Ihe opposite polarization state, for example P polarized light is lost
  • the polarized light directed into FLC/VLSI spatial light modulator 12 is reflected back lo polarizer/analyzer 24 by the individual pixels of the spatial light modulator
  • the light's polarization state is either maintained (for example S-pola ⁇ zed) or changed (for example P-pola ⁇ zed) depending on the ON/OFF state of the individual pixels of FLC/VLSI spatial light modulator 12
  • the pixels which are in the ON state the pixels which are in the ON state
  • FLC/VLSI spatial light modulator 12 includes a thin layer of ferroelectric liquid crystal (FLC) 38, a silicon VLSI circuitry backplane 40, a glass window 42 and a transparent electrode 44 FLC layer 38 is confined between VLSI circuitry backplane 40 and a glass window 42 Glass window 42 is coated on its inner side with transparent electrode layer 44 which, in this case, is a layer of indium tin oxide (ITO)
  • VLSI backplane 40 includes an array of aluminum pads, one of which is indicated at 46 Aluminum pads 46 are positioned on the upper surface of VLSI backplane 40 Each pad has a reflective top surface 48, best shown in Figure 2C, which is designed to reflect light directed into the spatial light modulator back out of the spatial light modulator Each of the aluminum pads 46 making up the array of aluminum pads also acts as an electrode controlled by data writing arrangement 14
  • the display system uses input image data to control how the cell is operated
  • the method includes the step of applying image producing electric fields to the cell during a first portion of a given pe ⁇ od of time T as indicated by block 102
  • image-producing elect ⁇ c fields depend in a predetermined way upon the input image data provided by the display system
  • the method further includes Ihe step of applying additional elect ⁇ c fields to the cell during a second portion of Ihe given period of lime T
  • the method further includes the step of causing the illuminator not to direct light into the spatial light modulator or otherwise blocking the light from passing through the basis during the second portion of the given period of time T This is indicated by block 106 This prevents the display system from being viewable during
  • the combination of Ihe image producing elect ⁇ c fields and Ihe additional electric fields are not necessarily DC-balanced That is, the cumulative time integral of Ihe elect ⁇ c fields that are present in one direction across the liquid crystal mate ⁇ al is not necessanly equal to the cumulative time integral of the electric fields that are present in the opposite direction du ⁇ ng the given period of time that includes both the image producing electric fields and the additional electric fields
  • the additional electric fields are electric fields that are specifically configured to reduce the amount of image sticking caused by the image producing electric fields That is, there is reduced image sticking compared to Ihe amount of image sticking that would occur if only Ihe image producing electric fields were applied to the cell du ⁇ ng the given period of time
  • the additional elect ⁇ c fields may take on a wide variety of specific configurations and still remain within the scope of the invention
  • the purpose of these additional electric fields is to remove, or drive back into the liquid crystal mate ⁇ al, any built up ions that may be collected near or be sticking along one of the surfaces of the liquid crystal mate ⁇ al as a result of the image producing electric fields
  • this has typically been achieved by DC-balancing the liquid crystal cell which typically requires that the image producing electric fields be inverted and directed through the liquid crystal cell to counter act any biases created by the image producing electric fields
  • FIG. 4 is a graph illustrating the voltages of the various electric fields applied to the liquid crystal cell during the given time period T
  • Time pe ⁇ od T is divided into two portions Tl and T2
  • lime period T may correspond to one image frame for a display system
  • time period T may correspond to one of three different color subframes that in turn make up an overall image frame
  • the image producing elect ⁇ cal fields take the form of either positive or negative 2 5VDC elect ⁇ c fields applied to the cell These voltages are applied du ⁇ ng the first portion of the lime pe ⁇ od indicated by Tl and are illustrated by stepped line 108 in Figure 4 Each of these steps may co ⁇ espond lo one of several subframes that provide binary control of the gray scale of the liquid crystal cell as described in detail in the above referenced United States Patent 5,748,164
  • the liquid crystal cell is switched on and off in a manner that modulates light directed into the cell in a desired manner du ⁇ ng the time period Tl as is well known in the art
  • the given pe ⁇ od of time T also includes a second portion of time T2 during which additional elect ⁇ c fields are applied to the liquid crystal cell in order to reduce or eliminate the image-sticking problem
  • the additional electric fields of this embodiment take the form of a relatively high alternating voltage waveform as indicated by wavetorm line 1 10
  • the wavetorm line 1 10 the wavetorm line 1 10
  • I I maximum voltage of the alternating waveform 1 10 used during time T2 is about one to twenty times ( I e 2 5 to 50VDC) the maximum voltage (l e 2 5VDC) of the elect ⁇ c fields used to normally switch the liquid crystal cell between its on and off states during lime Tl
  • alternating waveform oscillates from its maximum positive to its maximum negative voltage one to several times within the time period T2 As mentioned above, light is not directed into the liquid crystal cell during time T2 thereby preventing any degradation of the desired image by the optical effects caused by waveform 1 10
  • the alternating waveform is described as being a waveform having a maximum voltage about one to twenty times that of the voltage used to switch the cell between its on and off state, this is not a requirement Instead, the voltage may be a wide variety of voltages however it appears as though voltages in the range of about 1-20 times the normal switching voltage are most effective Also, it has been found, that for some cu ⁇ ently available liquid crystal cells, voltages substantially greater than about twenty times the normal switching voltage may potentially cause new forms of damage or other problems to the cell
  • display systems of Ihe type being desc ⁇ bed typically are operated at a certain frame rate, for example 60 frames per second At this frame rate, each frame, which corresponds to the time period T, lasts approximately 16 67 milliseconds Since the time period T2 during which the additional elect ⁇ c fields are applied to the cell preferably lasts no more than about twenty percent of the time pe ⁇ od T, time period T2 last no more than about 3 3 milliseconds Therefore, in order to have alternating waveform 1 10 oscillate one to several times within time T2, alternating waveform 1 10 would have a frequency of up to about 1000 hertz
  • FIG. 5 7 illustrate three alternative waveforms that may be used during time T2
  • the magnitude of the additional electric fields that are applied to the cell during the second portion T2 of the given pe ⁇ od of lime T decrease in magnitude during the time T2 as indicated by wave form 1 12
  • the additional electric fields that are applied to the cell during time T2 may be applied at an increasing frequency during time T2 as indicated by waveform 1 14
  • the additional electric fields that are applied to the cell du ⁇ ng time T2 may be of a polarity, magnitude, and frequency that at least in part are dependent upon the electric fields applied to the cell during the first portion Tl of the given period of time.
  • the additional electric fields of the present invention have been described as being located at Ihe end of each time period T, this is not a requirement of the invention. Instead, the additional electric fields can be applied at any desired time during the operation of the system. For example, as illustrated in Figure 8, a single pulse, designed in accordance with the invention to reduce the image sticking problem, may be applied at the end of each of several subframes as indicated by waveforms 120 and 122.
  • a single pulse designed in accordance with the invention to reduce the image sticking problem, may be applied at the end of each of several subframes as indicated by waveforms 120 and 122.
  • the light source is not directed the display for normal viewing while waveforms 120 and 122 are applied to the cell.
  • additional electric fields have been illustrated in Figures 5-7 as including a waveform that alternates from positive to negative several times at the end of each time period T, this is not a requirement of the invention. Instead, as illustrated in Figure 9, single pulses such as those indicated by waveforms 124 and 126 may be applied at the end of each time period T. As also illustrated in Figure 9, these waveforms may vary from positive to negative from time period to time period. All of these various configurations would equally fall within the scope of the invention so long as these additional electric fields reduce the image sticking problem.
  • the methods of the present invention may be implemented in a wide variety of manners.
  • the step 104 ( Figure 3) of applying additional electric fields to the cell may be accomplished in the following manner.
  • all of the pixel electrodes 46 are set to the same electric potential.
  • an electric potential having a varying magnitude and polarity is applied to top electrode 44 of Ihe spatial light modulator for the second portion of the given period of time T2.
  • each pixel electrode 46 may be set to an electric potential related in a predetermined way to at least one of the electric fields applied to that pixel during the first portion Tl of the given period of time T.
  • an electric potential having a varying magnitude and polarity is applied to top electrode 44 of the spatial light modulator for the second portion T2 of the given period of time T.
  • an open circuit may be provided to each of the pixel electrodes 46 so as to float the electric potential of each of Ihe pixel electrodes.
  • this embodiment further includes the step of applying an electric potential having a varying magnitude and polarity to top electrode 44 of the spatial light modulator for the second portion of the given period of time.
  • top electrode 44 of the spatial light modulator may be held at a constant electric potential. In this version, an electric potential having a varying magnitude and polarity is then applied to all of the pixel electrodes 46 for the second portion of the given period of lime T2.

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Abstract

A method of operating a liquid crystal cell during a given period of time without requiring DC-balancing of the cell includes the step of applying image producing electric fields to the cell during a first portion of the given period of time. The method uses input image data to control how the cell is operated and the image producing electric fields depend in a predetermined way upon the input image data. The method further includes the step of applying additional electric fields to the cell during a second portion of the given period of time. In this method, the image producing electric fields and the additional electric fields are such that the cumulative time integral of the electric fields that are present in one direction across the liquid crystal material is not substantially equal to the cumulative time integral of the electric fields that are present in the opposite direction during the given period of time during the operation of liquid crystal cell. Also, the additional electric fields are electric fields configured to reduce the amount of image sticking caused by the image producing electric fields compared to the amount of image sticking that would occur if only the image producing electric fields were applied to the cell during the given period of time.

Description

REDUCTION OF EFFECTS CAUSED BY INBALANCED DR IVING OF L IQUID CRYSTAL CELLS
BACKGROIXND OF THE INVENTION
The present invention relates generally to liquid crystal devices and more specifically to schemes for dπving a liquid crystal cell, such as a ferroelectric liquid crystal cell without requiπng DC-balancing of the liquid crystal cell In one aspect there is disclosed a non-DC-balanced drive scheme for liquid crystal device
In the field of image generators and especially those using spatial light modulators (SLMs), it is well known that stationary and moving images, either monochrome or color, may be sampled and both color-separated and gray-scale separated pixel by pixel These pixelated separations may be digitized forming digitized images that correspond to the given images These digitized images are used by devices in this field to create visual images that can be used for a direct visual display, a projected display, a pπnter device, or for dπving other devices that use visual images as their input
One such novel image generator is disclosed in United States Patent 5,748,164, entitled ACTIVE MATRLX LIQUID CRYSTAL IMAGE GENERATOR, and issued May 5, 1998, which patent is incorporated herein by reference An image generator of this type is further descπbed in United States Patent 5,808,800, entitled OPTICS ARRANGEMENTS INCLUDING LIGHT SOURCE ARRANGEMENTS FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued September 15, 1998, which patent is also incorporated herein by reference
As descπbed m detail in the above recited United States Patents 5,748,164 and 5,808,800, the inventions disclosed contemplate the use of a liquid crystal mateπal, such as a ferroelectπc liquid crystal (FLC) mateπal, as a preferred light modulating medium for the spatial light modulator of the disclosed inventions This light modulation of liquid crystal mateπal is accomplished by establishing and maintaining electπc fields across the liquid crystal mateπal in a controlled way m order to switch the light modulating characteπstics of the mateπal As an example, in the case of an FLC material, an electric field is established m one direction across the FLC mateπal in order to produce a first light modulating state, for example an ON state An electric field is established in the opposite direction across the FLC material in order to produce a second light modulating state, for example an OFF state
Because currently available liquid crystal materials manufactured using currently available manufacturing processes are not completely insulating, and because currently available assembly processes for manufacturing liquid crystal SLMs may introduce cintanunants into the SLM assembly, this formation of electπc fields across the liquid crystal mateπal may cause leakage current to flow through the liquid crystal material while the electric fields are applied to the mateπal If these electπc fields are not balanced, the unbalanced fields (or the unbalanced leakage current) are believed to cause the degradation of the electro-optic characteristic of the liquid crystal mateπal, thereby dramatically reducing the effectiveness and useful life of the material as a light modulating medium
Throughout this specification, balancing the electric fields or DC balancing refers to balancing the time integral of the electric fields In other words, the electric fields are balanced when the cumulative time integral of the electπc field (hit is present in one direction across the liquid crystal material is substantially equal to the cumulative time integral of the electπc field that is present in the opposite direction during a predetermined amount of lime during the operation of the spatial light modulator Another way of slating this is that the electric fields are balanced when the average of the product of the applied voltage and the amount of time that field is present averages to substantially zero during a predetermined amount of time during the operation of the spatial light modulator
The presence of unbalanced fields across the light modulating medium tends to polarize or bias the light modulating medium if the electπc fields are not balanced over time When the electric fields are not balanced, it is believed that the net electric field in one direction causes ionic charges to migrate through the light modulating medium and build up or slick on the sides of the light modulating medium This sticking or build up of ionic charges tends to interfere with the electπc fields subsequently applied to the light modulating medium and therefore interfere with the operation of the spatial light modulator This interference typically results in image slicking (hat interferes with the proper operation of the display system For purposes of this specification, image sticking is defined as unwanted image interference during a given frame that is caused by latent electrical effects caused by previous image frames Traditionally, this problem of image sticking is avoided or reduced by DC balancing the driving electric field applied to the FLC material As mentioned above, in the case of FLC mjieπals, the materials are switched to one state (I e ON) by applying a particular voltage through the material (i e +2 5 VDC) and switched to the other state (1 e OFF) by applying a different voltage through the mateπal (i e -2 5 VDC) Because FLC materials respond differently to positive and negative voltages, it is difficult to DC balance them in situations where it is desired to vary the ratio of ON time to OFF time arbitrarily Therefore, DC-field balancing for FLC SLMs is most often accomplished by displaying a frame of image data for a certain period of time Then, a frame of the inverse image data is displayed for an equal peπod of time in order to obtain an average DC field of zero for each pixel making up the SLMs
In the case of an active matrix image generating system or display, the image produced by the SLM duπng the lime in which the frame is inverted for purposes of DC balancing may not typically be viewed If the system were viewed during the inverted lime without correcting for the inversion of the image, the image would be degraded In the case in which the image is inverted at a frequency faster than the critical flicker rate of the human eye, the overall image would be completely washed out and all of (he pixels would appear to be half on In the case in which the image is inverted at a frequency slower than the critical flicker rate of the human eye, the viewer would see the image switching between the positive image and the inverted image Neither of these sitti.it ions would provide a usable display In one approach to solving this problem, the light source used to illuminate the SLM is switched off or directed away from the SLM duπng the time when the frame is inverted However, this approach substantially limits (he brightness and efficiency of the system In the case where the magnitude of the electric field duπng the DC-balancing and the time when the frame is inverted is equal to the magnitude of the electπc field and the time when the frame is viewed the light from a given light source may only be utilized a maximum of 50% of the time
In order to overcome this problem of not being able to view the system during the DC-balancing frame inversion time, compensator cells have been proposed for SLMs For Example, copending United States Patent Application Serial Number 09/251 ,627, entitled COMPENSATOR ARRANGEMENTS FOR A CONTINUOUSLY VIEWABLE, DC-HELD BALANCED, REFLECTIVE, FERROELECTRIC LIQUID CRYSTAL DISPLAY SYSTEM, attorney docket number DIS P01 1CIP, which application is incorporated herein by reference, discloses several approaches lo providing display systems that include compensator cells These compensator cells are intended to correct for the frame inversion during the time when the FLC pixel is being operated in its inverted stale, thereby allowing the display to be substantially continuously viewable Although these compensator cell arrangements appear to work well, they increase the complexity and cost of the display system by requiring the use of a compensator cell and in many cases other additional components
Much of the earliest work with FLC displays also encountered the DC balance problem and a class of solutions was found The early work dealt with passive matrix displays, because the unique properties of FLCs were expected to enable much larger displays having many more rows and columns of pixels than were then allowed using passive matrix nematic displays There is a large amount of patent and scientific literature associated with passive matrix FLC displays However, United States Patent 4,709,995 issued to Kuπbayashi is typical of the approach to DC balance taken in almost all such work
In a passive matrix FLC display, the pixels are defined as the intersection of a column electrode with a row electrode The column electrodes are formed as long, narrow, and parallel conductors that run entirely across the display with each column electrode being the width of one pixel Likewise, the row electrodes are long, narrow, and parallel conductors that run entirely across the display in a direction perpendicular to the column electrodes wι(h each row electrode being the height of one pixel These electrodes typically consist of transparent Indium-Tin Oxide, and this mateπal is deposited directly onto the inner surfaces of two glass substrates The column electrodes are put on one substrate, while the row electrodes are put on the second substrate The substrates are then assembled to have the FLC layer between them
There are no active transistors or other components in a passive matrix display The FLC mateπal comprising a pixel is forced to one of two electro-optic states (ON or OFF in the display) by the application of an electπc field In the passive matrix display, the image data are written to the display a row at a time, and all the rows are written, usually sequentially, duπng each image frame Any given row is selected for writing by applying a particular voltage to the associated row electrode Meanwhile, the image data for each pixel in the selected row are applied to each associated column electrode as a particular voltage The difference between these two voltages provides the electric field needed to switch each specific FLC pixel After a short lime the next row is selected and the image data are written to it with the appropπate pixel voltages applied to the columns Typically, voltages greater than 10V magnitude are applied to the electrodes, since only such high voltages can cause (he FLC to switch in the very small fraction of the frame lime duπng which the image data are actually applied to any one row
It is necessary to DC balance the electric field applied to any passive matrix FLC pixel, addition to switching the pixel into the proper stale The generic method for accomplishing this in passive matπx displays is to first apply a field which would switch the pixel lo the opposite state from the one that is wanted After the false initial field, the field that will put the pixel into the desired state is then applied This pulse-pair switching approach is accomplished by applying a succession of electrical pulses to the row and column electrodes associated with any one row duπng the time it is being written The succession of pulses are arranged for each pixel so that the integral of Ihe applied field over the row time becomes zero, and this result must be true for both the ON and Ihe OFF slates
Duπng most of each image frame, any given row is not selected, so that Ihe data appeaπng on the column electrodes is almost always associated with the pixels of some other row This circumstance requires that the FLC in the pixel be bistable Bistability means that 1) Ihe FLC must maintain the proper electro-optic state for one entire frame interval even though the electric field which selected that state is no longer present and 2) the FLC must maintain Ihe proper electro optic state despite the fact that voltages directed to other rows are constantly appearing on the column electrodes and these will try to perturb any given pixel from its proper state
Much of ihe pπor art associated with passive matrix displays, including United States Patent 4,709,995, constitutes Ihe disclosure of particular sequences of voltage pulses to the row and column electrodes, which sequences are especially suited to operate the pixels of passive matrix displays of various designs All of the known methods and apparatus regarding passive matrix displays require FLC bistability These methods and apparatus will not make a successful display if they are applied to a FLC material that is not bistable Also, all of the passive matrix pπor art concerns methods or apparatus that provide approximately DC balanced operation
To use an FLC mateπal that is not bistable requires that the electric field that selects the electro- optic state must be present throughout the entire frame time A passive matπx display and the associated methods of operation cannot accomplish such continuous application of the electric field The present invention applies to active matrix displays that maintain a selected electric field at all times This means that the active matπx methods and apparatus of the present invention could not make use of the pπor art passive matrix drive waveforms The present invention discloses novel methods for solving or reducing the above described image sticking problems caused by unbalanced electric fields without requiring DC-balancing. These novel methods improve the effectiveness of the display system without increasing the complexilyof the system, as would be the case if DC-balancing were required. SUMMARY OF THE INVENTION
As will be described in more detail hereinafter, a method of operating a liquid crystal cell during a given period of time without requiring complete or exact DC-balancing of the cell is disclosed. Applicants have discovered through practical tests of FLC displays, that the deleterious effects usually associated with non DC-balanced electric fields can be reduced or eliminated altogether even though the electrical fields applied to FLC devices of the present invention are not exactly DC-balanced. The method and apparatus which achieve this result and which constitute the invention are diametrically opposed to prior art methods and apparatus and defy the conventional wisdom that FLC displays must be DC-balanced.
In one embodiment, the method uses input image data to control how the cell is operated. The method includes the step of applying image producing electric fields to Ihe cell during a first portion of the given period of time. The image producing electric fields depend in a predetermined way upon the input image data. The method further includes the step of applying additional electric fields to the cell during a second portion of the given period of time. In accordance with one aspect of the invention, the image producing electric fields and the additional electric fields are such that the cumulative time integral of the electric fields that are present in one direction across the liquid crystal material is not substantially equal to the cumulative time integral of the electric fields that are present in the opposite direction during the given period of time during the operation of liquid crystal cell. Also, the additional electric fields are electric fields configured to reduce the amount of image sticking caused by the image producing electric fields compared to the amount of image sticking that would occur if only the image producing electric fields were applied to the cell during Ihe given period of time.
In another embodiment, the image data is divided into frame image data corresponding to individual frames of image data. The given period of time is a frame time associated with one frame of image data. Also, the method is a method of operating the liquid crystal cell for a plurality of frame times at a certain frame rate.
In another embodiment, the liquid crystal cell is a ferroelectric liquid crystal cell including ferroelectric liquid crystal material. The ferroelectric liquid crystal cell is a ferroelectric liquid crystal spatial light modulator for modulating light directed into the spatial light modulator. The ferroelectric liquid crystal material of the spatial light modulator is divided into a plurality of individually controllable pixels. In this embodiment, the step of applying image producing electric fields to the cell includes the step of applying image producing electric fields to each of the individually controllable pixels during the first portion of the given peπod of time This causes Ihe individually controllable pixels to form a desired light modulating pattern for modulating light directed into the spatial light modulator
The immediately above described spatial light modulator may be used as part of an overall display system that includes an illuminator for directing light into the spatial light modulator In this case, the method includes the step of causing the illuminator not to direct light into the spatial light modulator during the second portion of the given period of time during which the additional electric fields are being applied to Ihe spatial light modulator
In another embodiment, the ferroelectric liquid crystal material includes a top and a bottom surface The top and bottom surfaces of the liquid crystal material are approximately coplanar The spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material The spatial light modulator also includes a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material Each of Ihe plurality of pixel electrodes is associated with, and capable of controlling, one of the plurality of pixels
In one embodiment of a method associated with Ihe immediately above described spatial light modulator, the method includes the step of setting all of the pixel electrodes to the same electπc potential Additionally, an electπc potential having a varying magnitude and polarity is applied to the top electrode of the spatial light modulator for the second portion of the given period of time
In a second embodiment associated with this configuration, the method includes the step of individually setting each pixel electrode to an electric potential related in a predetermined way to at least one of the electπc fields applied to that pixel during the first portion of the given peπod of time Furthermore, an electπc potential having a varying magnitude and polarity is applied to the top electrode of the spatial light modulator for the second portion of the given peπod of time
In a third embodiment, the method includes the step of providing an open circuit to each of the pixel electrodes so as to float the electric potential of each of the pixel electrodes Again, this embodiment further includes the step of applying an electric potential having a varying magnitude and polaπty to the top electrode of the spatial light modulator for the second portion of the given peπod of time
In a fourth embodiment of this configuration, the method includes the step of holding the top electrode of the spatial light modulator at a constant electric potential In this version, an electric potential having a varying magnitude and polarity is applied to all of the pixel electrodes for the second portion of the given period of time
In a preferred embodiment of Ihe invention, the second portion of the given period of time is less than or equal to about twenty percent of the duration of Ihe given period of time Also, the maximum magnitude of the additional electric fields applied to the cell during the second portion of the given peπod of time is preferably substantially greater than Ihe maximum magnitude of the electπc fields applied to the cell duπng the first portion of the given peπod of time For example, the maximum magnitudes of the additional electric fields applied to the cell duπng the second portion of the given period of lime are preferably in the range of about one to twenty times the maximum magnitudes of the image producing electric fields applied to the cell during the first portion of the given period of lime In another embodiment of the invention the magnitude of the additional electric fields that are applied to the cell during the second portion of the given period of time decrease in magnitude during Ihe second portion of the given period of lime Alternatively, the additional electric fields that are applied to the cell during the second portion of the given period of time may be applied at an increasing frequency duπng the second portion of the given period of time In still another variation, the additional electπc fields that are applied to the cell during the second portion of the given period of time may be of a polarity, magnitude, and frequency that at least in part are dependent upon Ihe electric fields applied to the cell duπng the first portion of the given period of time
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention may best be understood by reference lo the following descπption of the presently preferred embodiments together with the accompanying drawings in which
Figure 1 is a diagrammatic perspective view of an exemplary FLC SLM based display system which may be operated using the methods of the present invention
Figure 2A is a diagrammatic perspective view of the FLC SLM of the display system of Figure I
Figure 2B is a diagrammatic cross sectional view of the FLC SLM of Figure 2A Figure 2C is a diagrammatic illustration showing the operation of one of the pixels of the FLC
SLM of Figure 2A
Figure 3 is a flow diagram illustrating the various steps of a method of operating a liquid crystal cell in accordance with the invention
Figure 4 is a graph illustrating a first embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention during a given time peπod
Figure 5 is a graph illustrating a second embodiment of the electπc field voltages used to operate a liquid crystal cell in accordance with the invention during a given time peπod
Figure 6 is a graph illustrating a third embodiment of Ihe electric field voltages used to operate liquid crystal cell in accordance with the invention during a given time peπod
Figure 7 is a graph illustrating a fourth embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention duπng a given time peπod Figure 8 is a graph illustrating a fifth embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention during a given lime period
Figure 9 is a graph illustrating a sixth embodiment of the electric field voltages used to operate a liquid crystal cell in accordance with the invention during a given time period
DETAILFD DFSCRIPTION OF THE PREFERRED EMBODIMENTS
An invention is descπbed herein for providing a method of operating a liquid crystal cell during a given period of lime without requiring DC-balancing of Ihe cell In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention However, it will be obvious to one skilled in the art that the present invention may be embodied in a wide variety of specific configurations Also, well known liquid crystal cell manufacturing processes and known methods of controlling liquid crystal cells using various electrical circuits will not be descπbed in detail herein so as not to unnecessarily obscure the present invention
The method of the present invention may be used with a wide vaπety of types of liquid crystal cells that may be used in a wide variety of specific applications However, for purposes of an example, the method of the present invention will be described with reference to a ferroelectric liquid crystal display system such as those descπbed in the above referenced United States patents 5,748,164 and 5,808,800 Although the methods of the present invention will be descπbed with reference to these specific types of ferroelectric liquid crystal display systems, it should be understood that Ihe methods of the present invention are not limited to these types of systems Instead, the novel methods disclosed herein may be utilized to operate all types of liquid crystal cells including a wide variety of types of ferroelectπc liquid crystal cells and nematic liquid crystal cells Also, the present invention is not limited to display systems but instead would equally apply to any systems that use liquid crystal cells that may exhibit problems as a result of image-sticking caused by unbalanced electπc fields passed through the cell
Refemng initially to Figure 1, an exemplary miniature display system generally designated by reference numeral 10 will be described As is described in detail in the above referenced patents 5,748,164 and 5,808,800, the display system includes a ferroelectric liquid crystal VLSI (FLC/VLSI) spatial light modulator 12 Display system 10 also includes a data wπting arrangement 14 for controlling FLC/VLSI spatial light modulator 12 and a video or digitized image source 1 which creates or provides, as an input to data wπting arrangement 14, digitized images or input image data Display system 10 further includes an illumination arrangement generally designated by reference numeral 16 for illuminating spatial light modulator 12 and an appropriately designed readily available lens 18 for producing a viewable image of the SLM FLC VLSI spatial light modulator 12 includes an array of individually addressable pixels, not shown in Figure 1, designed to be switched by data writing arrangement 14 between ON (light) and OFF (dark) states Illumination arrangement 16 includes a light source 20 that may be switchably controlled by data writing arrangement 14, a collimatmg arrangement 22, and a polaπzer/analyzer 24
In a system such as display system 1 , either unpolaπzed or polarized light that is generated by light source 20 in the form of light rays 26 is collected by collimatmg aπangement 22 and directed into polarizer/analyzer 24 The polaπzer/analyzer 24 cjuses light of a particular polarization state, for example S polarized light, to be directed into FLC/VLSI spatial light modulator 12 while any light of Ihe opposite polarization state, for example P polarized light is lost The polarized light directed into FLC/VLSI spatial light modulator 12 is reflected back lo polarizer/analyzer 24 by the individual pixels of the spatial light modulator As the light passes through the pixels, the light's polarization state is either maintained (for example S-polaπzed) or changed (for example P-polaπzed) depending on the ON/OFF state of the individual pixels of FLC/VLSI spatial light modulator 12 For the pixels which are in the ON state, the polarization of the light is changed by the FLC which allows the light to pass through polarizer/analyzer 24 into lens 18 presenting a bright pixel in the array of pixels to a viewer of the display For Ihe pixels which are in the OFF state, the light's polaπzation is maintained, causing the polaπzer/analyzer 24 to direct the light back up toward the light source or away from lens 18, thereby presenting a dark pixel to the viewer
Referring now to Figures 2A-C, the FLC/VLSI spatial light modulator will be descπbed in a little more detail In this particular example of a display system, FLC/VLSI spatial light modulator 12 includes a thin layer of ferroelectric liquid crystal (FLC) 38, a silicon VLSI circuitry backplane 40, a glass window 42 and a transparent electrode 44 FLC layer 38 is confined between VLSI circuitry backplane 40 and a glass window 42 Glass window 42 is coated on its inner side with transparent electrode layer 44 which, in this case, is a layer of indium tin oxide (ITO) VLSI backplane 40 includes an array of aluminum pads, one of which is indicated at 46 Aluminum pads 46 are positioned on the upper surface of VLSI backplane 40 Each pad has a reflective top surface 48, best shown in Figure 2C, which is designed to reflect light directed into the spatial light modulator back out of the spatial light modulator Each of the aluminum pads 46 making up the array of aluminum pads also acts as an electrode controlled by data writing arrangement 14 as mentioned above These aluminum pad electrodes 46 and ITO electrode 44 positioned on the opposite side of FLC layer 38 are used to form electπc fields through FLC layer 38 and divide FLC layer 38 into individually controllable FLC pixels which correspond to the positions of aluminum pads 46
In Ihe case of a display system such as display system 10 descπbed above, the image data is typically divided into frame image data corresponding to individual frames of image data Therefore, there is a given peπod of time that is equal to a frame time associated with one frame of image data These individual frames of image data are successively presented on the display system to produce an overall display image The given period of time descπbed above, which is equal to a frame time in this example, will be referred to throughout this description as the lime T Now that Ihe structure and operation of an exemplary display system has been briefly described, several examples of methods of operating a liquid crystal cell in accordance with the invention will be described Each of these methods provides for the reduction or elimination of the image-sticking problems described above without requiring overall DC balancing of the liquid crystal cell
Refemng now to Figure 3, a first embodiment of a method in accordance with the invention which may be used to operate a display system such as display system 10 will be described As mentioned above, the display system uses input image data to control how the cell is operated In this embodiment, the method includes the step of applying image producing electric fields to the cell during a first portion of a given peπod of time T as indicated by block 102 These image-producing electπc fields depend in a predetermined way upon the input image data provided by the display system As indicated in block 104 of Figure 3, the method further includes Ihe step of applying additional electπc fields to the cell during a second portion of Ihe given period of lime T Additionally, in the case in which the display system includes an illuminator for directing light into the spatial light modulator, the method further includes the step of causing the illuminator not to direct light into the spatial light modulator or otherwise blocking the light from passing through the basis during the second portion of the given period of time T This is indicated by block 106 This prevents the display system from being viewable during the time that the additional electπc fields are being applied to the spatial light modulator
In accordance with the invention, the combination of Ihe image producing electπc fields and Ihe additional electric fields are not necessarily DC-balanced That is, the cumulative time integral of Ihe electπc fields that are present in one direction across the liquid crystal mateπal is not necessanly equal to the cumulative time integral of the electric fields that are present in the opposite direction duπng the given period of time that includes both the image producing electric fields and the additional electric fields Also, in accordance with the invention, the additional electric fields are electric fields that are specifically configured to reduce the amount of image sticking caused by the image producing electric fields That is, there is reduced image sticking compared to Ihe amount of image sticking that would occur if only Ihe image producing electric fields were applied to the cell duπng the given period of time
As will be descπbed in more detail hereinafter, the additional electπc fields may take on a wide variety of specific configurations and still remain within the scope of the invention The purpose of these additional electric fields is to remove, or drive back into the liquid crystal mateπal, any built up ions that may be collected near or be sticking along one of the surfaces of the liquid crystal mateπal as a result of the image producing electric fields As described above, in Ihe past, this has typically been achieved by DC-balancing the liquid crystal cell which typically requires that the image producing electric fields be inverted and directed through the liquid crystal cell to counter act any biases created by the image producing electric fields However, as also mentioned above, this means that, if the same magnitude electπc fields are used during the time DC-balancing is being performed, the display may not be viewed duπng half of the overall time without the use of some type of a compensator cell In a preferred embodiment of the present invention, the second portion of Ihe given peπod of time duππg which the additional electπc fields are directed through the liquid crystal cell is substantially shorter in duration than Ihe tirst portion ot the given period of lime during which the image producing electric fields are directed through the liquid crystal cell This shorter second portion of the given period of time T insures that the illumination arrangement is more efficiently utilized than would be the case if a conventional DC balanced system that switched off the illumination arrangement for half of the time were utilized Using some of the specific approaches of the present invention as described immediately below, it has been found that using a second portion of ihe given period of time that is less than or equal to about twenty percent of the duration of the given period of time T produces a substantial reduction in the image-sticking problem while providing a substantial improvement in the efficiency of the use of the illumination arrangement
Now that the basic steps of the method of the present invention have been described, several specific examples will be descπbed for illustrative purposes Although only a few specific examples of methods of operating a liquid crystal cell in accordance with the invention will be described in detail, it should be understood that the invention would equally apply lo a wide variety of specific methods This is the case so long as the additional electric fields are configured to reduce the image-sticking problems described above
Referring now to Figure 4, a first specific embodiment of a method of operating a liquid crystal cell will be descπbed For the following examples, it will be assumed that the method is being used to operate a display system such as display system 10 that includes spatial light modulator 12 Figure 4 is a graph illustrating the voltages of the various electric fields applied to the liquid crystal cell during the given time period T Time peπod T is divided into two portions Tl and T2 In a simple example, lime period T may correspond to one image frame for a display system Alternatively, for a color display, time period T may correspond to one of three different color subframes that in turn make up an overall image frame
In the embodiment being described, the image producing electπcal fields take the form of either positive or negative 2 5VDC electπc fields applied to the cell These voltages are applied duπng the first portion of the lime peπod indicated by Tl and are illustrated by stepped line 108 in Figure 4 Each of these steps may coπespond lo one of several subframes that provide binary control of the gray scale of the liquid crystal cell as described in detail in the above referenced United States Patent 5,748,164 The liquid crystal cell is switched on and off in a manner that modulates light directed into the cell in a desired manner duπng the time period Tl as is well known in the art However, in accordance with the invention, the given peπod of time T also includes a second portion of time T2 during which additional electπc fields are applied to the liquid crystal cell in order to reduce or eliminate the image-sticking problem
As illustrated in Figure 4, the additional electric fields of this embodiment take the form of a relatively high alternating voltage waveform as indicated by wavetorm line 1 10 In this case, the
I I maximum voltage of the alternating waveform 1 10 used during time T2 is about one to twenty times (I e 2 5 to 50VDC) the maximum voltage (l e 2 5VDC) of the electπc fields used to normally switch the liquid crystal cell between its on and off states during lime Tl Also, in this embodiment, alternating waveform oscillates from its maximum positive to its maximum negative voltage one to several times within the time period T2 As mentioned above, light is not directed into the liquid crystal cell during time T2 thereby preventing any degradation of the desired image by the optical effects caused by waveform 1 10
Although the alternating waveform is described as being a waveform having a maximum voltage about one to twenty times that of the voltage used to switch the cell between its on and off state, this is not a requirement Instead, the voltage may be a wide variety of voltages however it appears as though voltages in the range of about 1-20 times the normal switching voltage are most effective Also, it has been found, that for some cuπently available liquid crystal cells, voltages substantially greater than about twenty times the normal switching voltage may potentially cause new forms of damage or other problems to the cell
As mentioned above, display systems of Ihe type being descπbed typically are operated at a certain frame rate, for example 60 frames per second At this frame rate, each frame, which corresponds to the time period T, lasts approximately 16 67 milliseconds Since the time period T2 during which the additional electπc fields are applied to the cell preferably lasts no more than about twenty percent of the time peπod T, time period T2 last no more than about 3 3 milliseconds Therefore, in order to have alternating waveform 1 10 oscillate one to several times within time T2, alternating waveform 1 10 would have a frequency of up to about 1000 hertz
In accordance with the invention, it has been found that applying alternating waveform 1 10 to the liquid crystal cell as described above substantially reduces or eliminates the image-sticking problems described above in the background of the invention This is the case even though the electπc fields that are applied to the cell during the overall time period T are not DC balanced That is, this approach eliminates the need to invert the input image data and direct the electric fields associated with the inverse input data through Ihe liquid crystal material in order to DC-balance the liquid crystal material
Although the alternating waveform descπbed above has been illustrated as having a substantially uniform amplitude and frequency throughout time T2, this is not a requirement of the invention Instead both the amplitude and the frequency may vary during the time T2 Figures 5 7 illustrate three alternative waveforms that may be used during time T2 In the example illustrated in Figure 5, the magnitude of the additional electric fields that are applied to the cell during the second portion T2 of the given peπod of lime T decrease in magnitude during the time T2 as indicated by wave form 1 12 Alternatively, as illustrated in Figure 6, the additional electric fields that are applied to the cell during time T2 may be applied at an increasing frequency during time T2 as indicated by waveform 1 14 In still another vaπation, the additional electric fields that are applied to the cell duπng time T2 may be of a polarity, magnitude, and frequency that at least in part are dependent upon the electric fields applied to the cell during the first portion Tl of the given period of time. This is illustrated in Figure 7 in which the electric fields applied to the cell during time T2, as indicated by alternating waveform 1 16, are biased toward ihe positive because, in this specific example, the cell is switched to the off stale during the entire time Tl as indicated by line 1 18. This approach has the effect of at least partially DC-balancing the electric fields used to operate the liquid crystal cell.
Although the additional electric fields of the present invention have been described as being located at Ihe end of each time period T, this is not a requirement of the invention. Instead, the additional electric fields can be applied at any desired time during the operation of the system. For example, as illustrated in Figure 8, a single pulse, designed in accordance with the invention to reduce the image sticking problem, may be applied at the end of each of several subframes as indicated by waveforms 120 and 122. Of course, as mentioned above, in Ihe case of a display, the light source is not directed the display for normal viewing while waveforms 120 and 122 are applied to the cell.
Furthermore, although the additional electric fields have been illustrated in Figures 5-7 as including a waveform that alternates from positive to negative several times at the end of each time period T, this is not a requirement of the invention. Instead, as illustrated in Figure 9, single pulses such as those indicated by waveforms 124 and 126 may be applied at the end of each time period T. As also illustrated in Figure 9, these waveforms may vary from positive to negative from time period to time period. All of these various configurations would equally fall within the scope of the invention so long as these additional electric fields reduce the image sticking problem.
Referring back to Figures 2A-C, the methods of the present invention may be implemented in a wide variety of manners. For example, the step 104 (Figure 3) of applying additional electric fields to the cell may be accomplished in the following manner. First, all of the pixel electrodes 46 are set to the same electric potential. Then, an electric potential having a varying magnitude and polarity is applied to top electrode 44 of Ihe spatial light modulator for the second portion of the given period of time T2. Alternatively, each pixel electrode 46 may be set to an electric potential related in a predetermined way to at least one of the electric fields applied to that pixel during the first portion Tl of the given period of time T. With the pixel electrodes all set, an electric potential having a varying magnitude and polarity is applied to top electrode 44 of the spatial light modulator for the second portion T2 of the given period of time T.
In a third variation, an open circuit may be provided to each of the pixel electrodes 46 so as to float the electric potential of each of Ihe pixel electrodes. Again, this embodiment further includes the step of applying an electric potential having a varying magnitude and polarity to top electrode 44 of the spatial light modulator for the second portion of the given period of time. And finally, in a fourth example, top electrode 44 of the spatial light modulator may be held at a constant electric potential. In this version, an electric potential having a varying magnitude and polarity is then applied to all of the pixel electrodes 46 for the second portion of the given period of lime T2. Although only a few specific embodiments have been described for how the various electric fields may be applied to the liquid crystal cell, it should be understood that a wide variety of approaches may be used and still fall within the scope of ihe invention Also, although only a few specific examples of waveforms that may be used have been descπbed, it should be understood that the invention is not limited to these specific examples Instead, a wide vaπety of waveforms or electric field configurations may be used so long as the additional electric fields applied to the cell reduce Ihe problem of image sticking
Furthermore, although the above described embodiments have been describe with the various components having particular respective orientations, il should be understood that the present invention may take on a wide vaπety of specific configurations with the various components being located in a wide vaπety of positions and mutual oπentations and still remain within the scope of the present invention For example, although the methods of the present invention have been described with reference to a specific type of ferroelectric liquid crystal display system, the invention is not limited to this type of display system or to display systems in general Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims

Claims

WHAT IS CLAIMED
1 A method of operating a liquid crystal cell duπng a given peπod of time the method using input image data to control how the cell is operated, the method comprising the steps of applying image producing electπc fields to the cell duπng a first portion of the given peπod of time, the image producing electπc fields depending in a predetermined way upon the input image data, and applying additional electπc fields to the cell duπng a second portion of the given peπod of time, the image producing electπc fields and the additional electπc fields being such that the cumulative time integral of the electπc fields that are present in one direction across the liquid crystal mateπal is not substantially equal to the cumulative time integral of the electπc fields that are present m the opposite direction dunng the given peπod of time duπng the operation of liquid crystal cell, the additional electπc fields being electπc fields configured to reduce the amount of image sticking caused by the image producing electπc fields compared to the amount of image sticking that would occur if only the image producing electπc fields were applied to the cell duπng the given peπod of time
2 A method according to claim 1 wherein the image data is divided into frame image data corresponding to individual frames of image data, the given peπod of time is a frame time associated with one frame of image data, and the method is a method of operating the liquid crystal cell for a plurality of frame times at a certain frame rate
3 A method according to claim 1 wherein the liquid crystal cell is a ferroelectπc liquid crystal cell including ferroelectπc liquid crystal mateπal
4 A method according to claim 3 wherein the ferroelectnc liquid crystal cell is a ferroelectπc liquid crystal spatial light modulator for modulating light directed into the spatial light modulator, the ferroelectπc liquid crystal mateπal of the spatial light modulator is divided into a plurality of individually controllable pixels, and the step of applying image producing electπc fields to the cell includes the step of applying image producing electπc fields to each of the individually controllable pixels duπng the first portion of the given peπod of time, thereby causing the individually controllable pixels to form a desired light modulating pattern for modulating light directed into the spatial light modulator
5 A method according to claim 4 wherein the spatial light modulator is part of an overall display system that includes an illuminator for directing light into the spatial light modulator, and the method includes the step of causing the illuminator not to direct light into the spatial light modulator during the second portion of the given period of time during which the additional electric fields are being applied to the spatial light modulator.
6. A method according to claim 4 wherein; the ferroelectric liquid crystal material includes a top and a bottom surface, the top and bottom surfaces of the liquid crystal material being approximately coplanar; the ferroelectric liquid crystal spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material and a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material, each of the plurality of pixel electrodes being associated with, and capable of controlling, one of the plurality of pixels; and the step of applying the additional electric fields to the cell for the second portion of the given period of time includes the steps of (i) setting all of the pixel electrodes to the same electric potential and (ii) applying an electric potential having a varying magnitude and polarity to the top electrode of the spatial light modulator for the second portion of the given period of time.
7. A method according to claim 4 wherein; the ferroelectric liquid crystal material includes a top and a bottom surface, the top and bottom surfaces of the liquid crystal material being approximately coplanar; the ferroelectric liquid crystal spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material and a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material, each of the plurality of pixel electrodes being associated with, and capable of controlling, one of the plurality of pixels; and the step of applying the additional electric fields to the cell for the second portion of the given period of time includes the steps of (i) individually setting each pixel electrode to an electric potential related in a predetermined way to at least one of the electric fields applied to that pixel during the first portion of the given period of time during which the image producing electric fields are applied to each of the individually controllable pixels and (ii) applying an electric potential having a varying magnitude and polarity to the top electrode of the spatial light modulator for the second portion of the given period of time.
8. A method according to claim 4 wherein; the ferroelectric liquid crystal material includes a top and a bottom surface, the top and bottom surfaces of the liquid crystal material being approximately coplanar; the ferroelectric liquid crystal spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material and a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material, each of the plurality of pixel electrodes being associated with, and capable of controlling, one of the plurality of pixels; and the step of applying the additional electric fields to the cell for the second portion of the given period of time includes the steps of (i) providing an open circuit to each of the pixel electrodes so as to float the electric potential of each of the pixel electrodes and (ii) applying an electric potential having a varying magnitude and polaπty to the top electrode of the spatial light modulator for the second portion of the given peπod of time
9 A method according to claim 4 wherein, the ferroelectπc liquid crystal mateπal includes a top and a bottom surface, the top and bottom surfaces of the liquid crystal mateπal being approximately coplanar, the ferroelectπc liquid crystal spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectπc liquid crystal mateπal and a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectπc liquid crystal mateπal, each of the plurality of pixel electrodes being associated with, and capable of controlling, one of the plurality of pixels, and the step of applying the additional electπc fields to the cell for the second portion of the given peπod of time includes the steps of (l) holding the top electrode of the spatial light modulator at a constant electπc potential and (π) applying an electπc potential having a varying magnitude and polaπty to all of the pixel electrodes for the second portion of the given peπod of time.
10 A method according to claim 1 wherein the second portion of the given peπod of time is less than or equal to about twenty percent of the duration of the given peπod of time.
11 A method according to claim 1 wherein the maximum magnitude of the additional electπc fields applied to the cell duπng the second portion of the given peπod of time is substantially greater than the maximum magnitude of the electπc fields applied to the cell duπng the first portion of the given peπod of time
12 A method according to claim 11 wherein the maximum magnitudes of the additional electπc fields applied to the cell duπng the second portion of the given peπod of time are the range of about one to twenty times the maximum magnitudes of the image producing electπc fields applied to the cell duπng the first portion of the given peπod of time
13 A method according to claim 1 wherein the magnitude of the additional electπc fields that are applied to the cell duπng the second portion of the given peπod of time decrease in magnitude duπng the second portion of the given peπod of time
14 A method according to claim 1 wherein the additional electπc fields that are applied to the cell duπng the second portion of the given peπod of time increase m frequency duπng the second portion of the given peπod of time
15 A method according to claim 1 wherein the additional electπc fields that are applied to the cell duπng the second portion of the given peπod of time are of a polaπty, magnitude, and frequency that at least in part are dependent upon the electπc fields applied to the cell duπng the first portion of the given peπod of time
16. The method of any one of the preceding claims wherein DC balancing of the cell is not required.
PCT/US2000/023645 1999-09-01 2000-08-29 Reduction of effects caused by imbalanced driving of liquid crystal cells Ceased WO2001016928A1 (en)

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Citations (5)

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Publication number Priority date Publication date Assignee Title
EP0373786A2 (en) * 1988-12-14 1990-06-20 THORN EMI plc Display device
WO1996006422A1 (en) * 1994-08-23 1996-02-29 Philips Electronics N.V. Display device
WO1997031359A2 (en) * 1996-02-22 1997-08-28 Philips Electronics N.V. Display device
WO1998027537A1 (en) * 1996-12-19 1998-06-25 Colorado Microdisplay, Inc. Display system which applies reference voltage to pixel electrodes before display of new image
US5933202A (en) * 1995-09-28 1999-08-03 Nec Corporation Liquid crystal display device having an alternating common electrode voltage

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0373786A2 (en) * 1988-12-14 1990-06-20 THORN EMI plc Display device
WO1996006422A1 (en) * 1994-08-23 1996-02-29 Philips Electronics N.V. Display device
US5933202A (en) * 1995-09-28 1999-08-03 Nec Corporation Liquid crystal display device having an alternating common electrode voltage
WO1997031359A2 (en) * 1996-02-22 1997-08-28 Philips Electronics N.V. Display device
WO1998027537A1 (en) * 1996-12-19 1998-06-25 Colorado Microdisplay, Inc. Display system which applies reference voltage to pixel electrodes before display of new image

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