EP0608556A1 - Méthode et système pour améliorer le vitesse de réponse électro-optique d'une valve optique à cristaux liquides - Google Patents

Méthode et système pour améliorer le vitesse de réponse électro-optique d'une valve optique à cristaux liquides Download PDF

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Publication number
EP0608556A1
EP0608556A1 EP93120725A EP93120725A EP0608556A1 EP 0608556 A1 EP0608556 A1 EP 0608556A1 EP 93120725 A EP93120725 A EP 93120725A EP 93120725 A EP93120725 A EP 93120725A EP 0608556 A1 EP0608556 A1 EP 0608556A1
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EP
European Patent Office
Prior art keywords
signal
video signal
lclv
compensating
liquid crystal
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.)
Withdrawn
Application number
EP93120725A
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German (de)
English (en)
Inventor
John H. Erdmann
Anna M. Lackner
David J. Margerum
Peter C. Baron
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Raytheon Co
Original Assignee
Hughes Aircraft Co
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Filing date
Publication date
Application filed by Hughes Aircraft Co filed Critical Hughes Aircraft Co
Publication of EP0608556A1 publication Critical patent/EP0608556A1/fr
Withdrawn legal-status Critical Current

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    • 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
    • G09G3/02—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes by tracing or scanning a light beam on a screen
    • 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
    • G09G2310/0264—Details of driving circuits
    • G09G2310/027—Details of drivers for data electrodes, the drivers handling digital grey scale data, e.g. use of D/A converters
    • 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
    • G09G2310/0264—Details of driving circuits
    • G09G2310/0289—Details of voltage level shifters arranged for use in a driving circuit
    • 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
    • G09G2320/0252—Improving the response speed

Definitions

  • This invention relates to liquid crystal light valves (LCLVs), and more particularly to a method and associated system for increasing the rate at which the liquid crystal responds to an electrical input signal.
  • LCLVs liquid crystal light valves
  • Light valves which employ liquid crystals as an electro-optic medium are used to spatially modulate an optical readout beam, under the control of an input drive signal pattern. They can be used to greatly amplify the input pattern by controlling a readout beam of much greater intensity, to convert spatially modulated incoherent radiation to a coherent readout laser beam with a similar spatial modulation, for optical data processing, wavelength conversion, or for other purposes that involve the conversion of an input signal pattern to a corresponding spatial modulation of a separate readout beam.
  • LCLV cadmium sulfide
  • a principal limitation of the CdS-based light valve is its slow response time, yielding image-after effects.
  • a second generation silicon-photoconductor-based LCLV was next developed which retained the advantages of the CdS-based light valve, but had a considerably faster response time.
  • the silicon-based device is described in an article by Efron et al., "The Silicon-Liquid Crystal Light Valve", Journal of Applied Physics , 57(4), pages 1356-68 (1985). This article also summarizes some of the prior light valve efforts.
  • the silicon-based LCLV While improving the response time of the photoconductor, the silicon-based LCLV still exhibits a slower than desirable speed of operation due to the finite time required for the liquid crystals to respond to changes in the applied voltage across the liquid crystal layer.
  • the liquid crystal electro-optic response time increases approximately with the square of the thickness of the liquid crystal cell resident in the light valve, and becomes particularly noticeable when the cell thickness exceeds 4 microns.
  • One is that the light throughput decreases dramatically when the liquid crystal cell thickness is reduced below 4 microns.
  • the switching ratio (effective voltage) required to drive a liquid crystal cell the device is switched between two intermediate (gray scale) transmission levels.
  • the fastest response times generally occur when the LCLV is driven to fully on or to fully off from any other transmission level.
  • the response times for switching between all possible gray scale levels are important in order to achieve high quality video-rate performance; changes in light throughput levels should occur in not more than two frames at 60Hz operation, or the video will appear fuzzy. While this switching speed can normally be achieved with a liquid crystal cell thickness of less than 4 microns, a poor video image will result for the majority of switching situations for this liquid crystal at thicknesses greater than 4 microns because the liquid crystal response will most often require more than two frames. Thus, a tradeoff has been required in the past between the high quality video imaging associated with a fast liquid crystal response time, and the increased light throughput and low switching ratio associated with thicker liquid crystal layers that also exhibit slower response times.
  • the present invention seeks to provide a method and associated LCLV system that significantly increases the response rate of the liquid crystal layer to changes in a gray scale video input for liquid crystal layers thicker than 4 microns, and yet preserves their desirable light throughput and switching ratio characteristics.
  • the compensating signal in imposed for only the first frame following a transition between different video levels.
  • the compensating signal is selected as a function of the video signal's absolute magnitude. It can also be selected by delaying each frame of the video signal, determining the difference if any between the video signals for the current frame and for the previous frame, and selecting the compensating signal as a function of this difference as well as of the absolute signal value.
  • the desired compensating signals for different frame-to-frame video signal differentials are stored in a lookup table, from which they are retrieved when the absolute video signal values and/or the differential between successive frames has been determined.
  • the drive signal is normally provided as a pixelized signal that actuates respective regions of the unpixelized LCLV, and is modified by the electronic compensating signals on a pixel-by-pixel basis.
  • the compensating signals preferably boost the drive signal substantially to its maximum level for transitions from a lower to a higher drive level, and lower the drive signal substantially to its minimum level for transitions from a higher to a lower level drive.
  • the invention is applicable to photoactivated LCLVs by using the modified drive signal to modulate an optical input to the LCLV; to active matrix LCLVs by applying the drive signal directly to the active matrix and also to other types of LCLVs. It is particularly useful for liquid crystal layers with a thickness in excess of 4 microns, or with slow liquid crystal materials, and when a constant bias voltage must be applied to the entire liquid crystal layer.
  • FIG. 2 A diagram of the invention as applied to a photoactivated LCLV 6 is given in FIG. 2.
  • the LCLV 6 includes a transparent input substrate 8, generally glass, upon which is formed a transparent back electrode layer 10 such as indium tin oxide (ITO) or P++ semiconductor, and a layer of photoconductor material 12 such as silicon and cadmium sulfide.
  • a transparent input substrate 8 generally glass
  • a transparent back electrode layer 10 such as indium tin oxide (ITO) or P++ semiconductor
  • a layer of photoconductor material 12 such as silicon and cadmium sulfide.
  • a liquid crystal layer 18 is sandwiched between alignment layers 20a and 20b on the readout side of the mirror, with a counter-electrode layer 22 and a front transparent substrate 24 formed in turn on the readout side of the liquid crystal cell.
  • An AC voltage source 26 is connected across the back electrode 10 and counter-electrode 22 to establish a bias that sets an operating point for the liquid crystal.
  • an input image 28 from an optical source such as cathode ray tube (CRT) 30, a scanning laser or the like is applied to the input side of the LCLV, while a linearly polarized readout beam 32 is transmitted through the liquid crystal cell 18 and reflected back from the mirror 16 through a cross polarizer (not shown).
  • the input image 28 produces a corresponding spatial voltage distribution across the liquid crystal cell, altering the localized alignment of the liquid crystals in accordance with the applied voltage pattern. This results in a spatial modulation of the readout beam 32, permitting a transfer of information from the input image 28 to the readout beam.
  • the CRT 30 or other optical device provides the input image 28 on a pixelized basis.
  • the LCLV input surface may be considered to be divided into a corresponding pixel array, typically on the order of 500 ⁇ 500 or 1,000 ⁇ 1,000 pixels, or more.
  • Each separate LCLV pixel is provided with an individual illumination from the CRT 30, which may be the same as or different from the illumination of adjacent pixels, and may change at a different rate from adjacent pixels.
  • the resistance of each pixel in the photoconductor layer 12 varies negatively with its degree of illumination by the CRT, such that voltage in shifted from the photoconductor layer 12 to the liquid crystal cell 18 at each pixel in response to an increase in illumination.
  • the additional voltage across the LCLV increases the light throughput for that pixel, and thereby increases the intensity of the processed readout beam 32 for the same pixel.
  • the CRT 30 is typically driven by a video signal at a 60 frame per second rate, with each successive pair of frames corresponding to the image at a particular time.
  • the video signal presented at a video input terminal 34 is modified to alter the output image 28 from the CRT so as to force a faster response from the liquid crystal in the light valve.
  • a modification circuit 36 is provided that receives its input from the video terminal 34, modifies it to enhance the liquid crystal response time, and provides the modified video signal as a drive for the LCLV 6 via the CRT 30.
  • the invention is also applicable to other types of LCLVs, such as the active matrix device 38 illustrated in FIG. 3.
  • the active matrix LCLV has several elements in common with the photoactivated LCLV of FIG. 2, and these are indicated by the same reference numerals.
  • the back electrode, photoconductor layer and light blocking layer of the photoactivated device are replaced by a conductive grid 40 that divides the back face of the LCLV 38 into a pixel matrix with an activation transistor for each pixel.
  • a video signal is applied directly to the grid 40 over a bus 42 that includes either a separate input line for each pixel, or more preferably input lines for each row and column in the matrix; with the latter arrangement the application of signals to the row and column lines is corrdinated with the activation of the pixels in a known manner to produce a scanning of the video signal across the LCLV.
  • a drive signal modification circuit 36' is also used to modify the video drive signal for the active matrix device.
  • the drive for other types of LCLVs such as transmission-mode photoactivated and active matrix devices, CCD (charge coupled device) and electron-beam driven devices, can be similarly modified to improve their liquid crystal response times.
  • the invention operates by imposing a compensating signal onto the initial portion of a new video signal level, following a switch in gray scale levels.
  • the compensating signal is provided on a pixel-by-pixel basis, with each pixel treated independent of the others.
  • the video level for a particular pixel will be held substantially constant for at least several video frames. It has been found that a significant increase in the liquid crystal response rate, sufficient to achieve a full response within the desired two frames in most cases, can be achieved by applying the compensating signal for only the first frame following a transition in gray scale video levels.
  • the invention is not limited to a single frame compensating signal, and may if desired be implemented with an compensating signal that extends for more than a single frame.
  • the compensating signal is preferably equal to the maximum, fully on signal level for gray scale increases in the video input, and equal to the fully off, minimum signal level for reductions in the gray scale video input. While such compensating signal levels can be conveniently realized, the invention is not limited to any particular compensating signal magnitude, and if desired different modified video signal levels could be used for different magnitudes of gray scale transitions by appropriate programming of the lookup table (52). In addition, although the liquid crystal response to a transition between a fully on or fully off level and a gray scale level is normally faster than the transition between two gray scale levels (even if the gray scale-to-gray scale differential is smaller), the invention could also be applied if desired to transitions between gray scale and fully on or fully off levels.
  • FIG. 4 is a block diagram that illustrates one embodiment of a modification circuit that can be used to modify the input video signal at terminal 34 as called for by the invention. Numerous alternate circuits could be designed to provide electronic compensating functions; that illustrated in FIG. 4 can be implemented on the DIGIMAX® image processing system supplied by Datacube Corporation.
  • the video input at terminal 34 is in analog format, and is converted to a digital signal by analog-to-digital converter 44; an eight-bit conversion will generally be sufficient.
  • Each video frame is stored for one frame interval in a frame delay store 46.
  • a frame subtract element 48 When the next video frame arrives it is subtracted from the first frame held in store 46 by a frame subtract element 48 on a pixel-by-pixel basis; at the same time the second frame is loaded into a second frame delay store 50 to set up the subsequent cycle.
  • the output of the frame subtract element 48 is fed to a lookup table 52, which outputs a desired modification for the video signal. This output is a function of the polarity, and if desired also the size, of the pixel-by-pixel frame difference signal from the frame subtract element 48.
  • the lookup table output signal is then added in a frame adder 54 on a pixel-by-pixel basis to the second frame signal held in the frame delay 50 to generate the desired compensating video signal that, after conversion back to analog format in a digital-to-analog converter 56, is applied to the CRT 30 (for a photoactivated LCLV), or directly to the LCLV 38 (for an active matrix device).
  • This analog signal provides the extra drive that is used to significantly reduce the fall and rise times of the liquid crystals.
  • the lookup table 52 may store modification signals based upon the pixel-by-pixel differences between successive frames, the absolute value of the second frame pixels, or both.
  • the second frame signal can also be fed via line 58 to the lookup table, which is then programmed to accept both inputs and to generate the optimum compensation signal.
  • the system is programmed to generate a compensating signal equal to 100% of the fully on level for frame-to-frame increase in the gray scale level, and a fully off 0% signal for frame-to-frame reduction in the gray scale level.
  • the table will obtain the polarity of the gray scale change from the frame subtract element 48, and the absolute value of the second frame from line 58.
  • the modification signal to be added to the delayed second frame is then selected from the lookup table as that signal value necessary to boost the second frame level to 100% (for an increase in gray scale level), or to reduce it to 0% (for a reduction in gray scale level). If there is a concern that such a compensating will be too large in the case of a relatively small gray scale differential, smaller modifications may be programmed into the lookup table for such cases. If such an approach is taken, the compensating value will also generally be a function of the difference between the second frame level and either 100% (for a gray scale increase) or 0% (for a gray scale reduction).
  • FIG. 5 Several gray scale transitions and possible compensating signals that can be generated for each are illustrated in FIG. 5. Assume that the video signal is initially at a 40% gray scale level (line 60), and that it increases during a subsequent frame to 70% (line 62). For the first frame (or additional frames if desired) following the transition, a compensating signal 64 at a 100% video level is applied. After the compensating signal has terminated, the particular pixel being observed reverts to the 70% level until the next change in video level, which is illustrated as a drop to the 50% level (line 66). The liquid crystal response to this reduction in the gray scale level is hastened by applying a new compensating signal 68 down to the 0% level during the next succeeding frame.
  • the video signal is then assumed to remain at 50% for several frames, after which it increases to the 60% level (line 70). If it is believed that a 100% compensating signal will produce an overshoot in the liquid crystal response to greater than 60%, a somewhat less than fully on compensating signal 72 can be employed for this case.
  • the video signal is next shown as dropping down to 20% (line 74), with another 0% signal 76 during the first frame at this level. Finally, another 10% reduction down to the 10% level (line 78) is illustrated. Although this reduction equates to the same absolute magnitude of gray scale differential as the prior transition from 50% to 60%, for which a reduced compensating signal 72 was assumed to be employed, the new gray scale level of 10% is close enough to the 0% level that a full 0% signal 80 can still be employed.
  • FIGs. 6a and 6b The results obtained with a 5.3 micron test cell that employed a negative dielectric anisotropy, low viscosity liquid crystal, for an increase in the normalized average light throughout from 25% to 75% and then back down to 25% without the input signal modification of the invention, are illustrated in FIGs. 6a and 6b; corresponding results obtained with the addition of the compensating signal are illustrated in FIGs. 7a and 7b.
  • the test cell was addressed with pulses at a 60Hz repetition rate.
  • the unmodified drive pulses are shown in FIG. 6a, while the resulting light throughput that corresponds to the liquid crystal response is shown in FIG. 6b.
  • RMS peak voltage pulses 82 were initially applied to produce the 25% light throughput, and then increased to a 7.245 RMS peak voltage level 84.
  • the liquid crystal response along curve 86 required 7 frames, or about 117ms, to complete--this would produce a poor gray scale quality.
  • the operation was reversed, with the voltage peaks reduced from the 75% level 84 back to the 25% level 82, another significantly long response time along curve 88 was required for the liquid crystal to react.
  • Liquid crystal cells with three different thicknesses were used to demonstrate the improvement in response time for thicknesses greater than 4 microns.
  • the cells were formed between a pair of 1.25cm thick optical flats, with 600 Angstrom thick indium tin oxide electrodes, a liquid crystal alignment layer and SiO x spacer pads used to maintain the cell gap thickness. Voltage waveforms were applied to the test cells to simulate the output of a photoactivated light valve. Cell thicknesses of 3.6, 4.4 and 5.3 microns were tested, with bias voltages of 3.45, 3.40 and 3.15 volts RMS respectively at 10kHz.
  • the peak-to-bias voltage ratio for the 3.6 micron cell was 1.85 for 25% light throughput, 2.19 for 50%, 2.66 for 75% and 4.50 for 100%; with the 3.6 micron cell the peak-to-bias voltage ratio was 1.75 at 25% light throughput, 1.97 at 50%, 2.21 at 75% and 2.90 at 100%; for the 5.3 micron cell it was 1.87 for 25% light throughput, 2.09 for 50%, 2.30 for 75% and 2.75 for 100%.
  • the decay times ranged from 4.5ms for the 4.4 micron cell at 25% light throughput, to 12ms for the 3.6 micron cell at 100%.
  • FIG. 8 compares the gray scale response times for the three cell thicknesses, in terms of the number of voltage level transitions that resulted in a full liquid crystal response in not more than two frames, with and without the compensating signal. Little improvement was gained by the compensating signal for the 3.6 micron cell, which was already quite fast. However, the response times of the 4.4 and 5.3 micron cells were improved by a factor of about 3. Whereas two or fewer pulses were required to complete the liquid crystal response in only about 25% of the voltage transitions without the compensating signals, 75% or more of the responses were completed in two pulses or less when the compensating signals were added. In each case the voltage transitions were between the various combinations of 0%, 25%, 50%, 75% and 100% light throughput.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Liquid Crystal Display Device Control (AREA)
  • Liquid Crystal (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)
EP93120725A 1992-12-23 1993-12-22 Méthode et système pour améliorer le vitesse de réponse électro-optique d'une valve optique à cristaux liquides Withdrawn EP0608556A1 (fr)

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US99586092A 1992-12-23 1992-12-23
US995860 1992-12-23

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EP0608556A1 true EP0608556A1 (fr) 1994-08-03

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EP93120725A Withdrawn EP0608556A1 (fr) 1992-12-23 1993-12-22 Méthode et système pour améliorer le vitesse de réponse électro-optique d'une valve optique à cristaux liquides

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EP (1) EP0608556A1 (fr)
JP (1) JPH06282247A (fr)
KR (1) KR970004244B1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0707304A3 (fr) * 1994-10-06 1996-08-07 Matsushita Electric Ind Co Ltd Procédé de commande pour un modulateur spatial de lumière et système d'affichage par projection

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
TW513598B (en) * 2000-03-29 2002-12-11 Sharp Kk Liquid crystal display device
JP2003172915A (ja) * 2001-09-26 2003-06-20 Sharp Corp 液晶表示装置

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0187533A2 (fr) * 1984-12-29 1986-07-16 Sony Corporation Dispositif d'affichage à cristal liquide
JPH05100208A (ja) * 1991-10-09 1993-04-23 Canon Inc 液晶ライトバルブ装置

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0187533A2 (fr) * 1984-12-29 1986-07-16 Sony Corporation Dispositif d'affichage à cristal liquide
JPH05100208A (ja) * 1991-10-09 1993-04-23 Canon Inc 液晶ライトバルブ装置

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
DUANE HAVEN: "Electron-beam addressed liquid-crystal light-valve.", CONFERENCE RECORDS OF 1982 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, 19 October 1982 (1982-10-19), NEW YORK, N.Y., U.S.A., pages 72 - 75, XP001373738 *
J. TRIAS ET AL.: "A 1075-line video-rate laser-addressed liquid-crystal light-valve projection display.", PROCEEDINGS OF THE SID., vol. 29, no. 4, 1988, LOS ANGELES US, pages 275 - 277 *
PATENT ABSTRACTS OF JAPAN vol. 17, no. 448 (P - 1594) 17 August 1993 (1993-08-17) *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0707304A3 (fr) * 1994-10-06 1996-08-07 Matsushita Electric Ind Co Ltd Procédé de commande pour un modulateur spatial de lumière et système d'affichage par projection
US5731797A (en) * 1994-10-06 1998-03-24 Matsushita Electric Industrial Co., Ltd. Driving method for spatial light modulator and projection display system

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JPH06282247A (ja) 1994-10-07
KR970004244B1 (ko) 1997-03-26
KR940015593A (ko) 1994-07-21

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