IVI LLC EXHIBIT 2011
`XILINX V. IVI LLC
`IPR Case 2013-00029
`
`

`

`5,170,271
`Page 2
`
`Mackniok et 31.. “High Resolution Displays Using
`OTHER PUBLICATIONS
`NCAP Liquid Crystals“, SPIE. vol. 1080, Liquid Crys-
`G. P. Montgomery: "Polymer—dispersed liquid crystal
`ta] Chemistry. Physics and Applications (1989), pp.
`films for light control applications", SPIE, vol. 1030,
`169473.
`1989, pp. 242—249.
`Takizawa et al.. “Transmission Mode Spatial Light
`Afonin et al., “Optionally Controllable Transparencies
`Based on Structures Consisting of a Photoconductor Modulator Using 3 31151020 Crystal and Polymer-Dis-
`andaPolymcr—Encapsulated Nematic Liquid Crystal",
`parsed Liquid—Crystal Layers”. Appl. Phys. Lett.
`Sov. Tech. Leit. 14(1), Jan. 1933. pp. 56, 58.
`56(11}, 12 Mar. 1990, pp. 999—1001.
`
`

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`US. Patent
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`Dec. 8, 1992
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`Dec. 3, 1992
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`US. Patent
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`Dec. 3, 1992
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`US. Patent
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`Dec. 8, 1992
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`Sheet 4 of 5
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`1
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`5,170,271
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`SHAPED VOLTAGE PULSE METHOD FOR
`OPERATING A POLYMER DISPERSED LIQUID
`CRYSTAL CELL, AND LIGHT VALVE
`EMPLOYING THE SAME
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`This invention relates to methods for operating a
`dynamic response polymer dispersed liquid crystal
`(PDLC) cell to increase its transmissivity and reSpon-
`sivity. and to PDLC light valve systems employing
`such methods.
`2. Description of the Related Art
`Photoactivated and charge-coupled device {CCD}
`addressed liquid crystal light valves (LCLVs) are well
`known, and are described for example in Margerum et
`al., “Reversible Ultraviolet Imaging With Liquid Crys'
`tals“, Appl. Phys. Letters. Vol. 17. No. 2. 15 July 3970,
`pages 51—53. Efron et al.. “The Silicon Liquid-Crystal
`Light Valve"..1oumnl ofApplied Physics, Vol. 57, No. 4,
`15 February 1935, pages 1356—68. Efron et al.. "A Sub-
`rnicron Metal Grid Mirror Liquid Crystal Light Valve
`for Optical Processing Applications”, SPIE. Vol. 1151.
`1989. pages 591—606. and Sterling et al., “Video~Rate
`LCLV Using an Amorphous Silicon Photoconductor“.
`SID 90 Digest. Vol. 21, paper 17A.2, 16 May 1990. They
`use various types of nematic liquid crystal
`layers to
`modulate a readout light beam, which may be used in a
`transmissive or reflective mode of operation, depending
`upon the design of the LCLV and the input signal.
`Photoactivated LCLVs are often addressed with an
`input image to be amplified, such as that presented by
`the phosphor screen of a cathode ray tube or a scanning
`laser beam. particularly for dynamic modulation of a
`readout beam.
`Nematic LCLVs operate by modulating the spatial
`orientation ofthe liquid crystals in a cell, in accordance
`with the input signal pattern. This often requires the use
`of polarizers to obtain a corresponding modulation of
`the readout beam. The polarizers, however, reduce the
`total
`light
`throughput. Also. alignment
`layers are
`needed on each side of the cell for surface alignment of
`the liquid crystals, thus adding to the expense of the
`device. The response time of the nematic liquid crystal
`to changes in the voltage across the cell may also be
`somewhat limited.
`More recently, polymer dispersed liquid crystal
`(PDLC) films have been reported, including the use of
`such films in photoactive LCLVs and in active matrix
`LCLV projectiou displays. Unlike most nematic liquid
`crystal cells which modulate the optical polarization in
`response to an applied voltage, PDLCs scatter light and
`become transparent with an applied voltage. They have
`several advantages over nematic liquid crystal devices.
`including the elimination of surface alignment layers
`and polarizers. and a faster reSponse time. However,
`while PDLCs exhibit a rapid response to a shift in ap-
`plied voltage between fully OFF and fully 0N voltage
`levels, their response to gray scale levels (levels not
`fully on or fully off) is quite slow.
`LCLVs are generally Operated with an alternating
`current applied voltage, since the liquid crystals tend to
`deteriorate under a DC voltage. There have been sev-
`eral reports of the response of PDLC-type films to
`Square wave type voltage pulses. including the use of
`such films in photoactivated LCLVs and in active ma-
`trix LCLV projection displays. In Afonin et a1., “Opti-
`
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`cally Controllable Transparencies Based on Structures
`Consisting of a Photoconductor and a Polymer-Encap-
`sulated Nematic Liquid Crystal". Sov. Tech. Phys. Lett.
`Vol. 14, No. 56. January 1938. pages 56-58, a PDLC-
`type film was photoactivated with a ZnSe photocon-
`ductor. The photoactivated rise and decay times (with a
`constant bias voltage in typical LCLV operation) were
`5—10 ms on-time and 1.5—3 seconds off-time; thus, the
`frame time (on-time plus off-time) is very slow com»
`pared to a dynamic television image frame time of less
`than 33 ms. The response of the PDLC-type film layer
`to a square voltage pulse was much faster. with rise and
`decay times of less than 1 ms and 15 ms. respectively.
`but such a pulse shape and response time is not attain-
`able with this photoconductor.
`In Macknick et al., "High Resolution Displays Using
`NCAP Liquid Crystals“, Liquid Crystal Chemistry.
`Physics. and Applications. SHE. Vol. 1080, January
`1989. pages 169—173, fast response PDLC-type films
`were reported with a square pulse input signal of 5.3 ms.
`About 50% transmissiou was reached during the 5.3 ms
`pulse. and the decay time at the end of the pulse was
`about 2 ms; full voltage activation of the film was not
`shown.
`In Takizawa et al.. “Transmission Mode Spatial Light
`Modulator Using a custom Crystal and Polymer-Dis-
`persed Liquid Crystal Layers". Appl. Phys. Lett. Vol.
`56. No. 11, March 1990. pages 999—1001. fast photoacti~
`vated PDLC film response of 10 ms ON and 36 ms OFF
`was reported for a 60 ms square pulse of bright white
`activating light with a 30 volt bias across a photocon-
`ductor/PDLC cell. A hysteresis loop was reported
`when the cell was scanned with increasing and decreas-
`ing writing light intensities. but the loops were said to
`disappear when pulsed write light was incident on the
`device. Response times were reported and discussed
`only for square wave intensity writing light pulses.
`In Kunigita et al., “A Full-Color Projection TV
`Using LC/Polymer Composite Light Valves", SID
`International Symposium Digest. May 3990. pages
`221—230, a low voltage PDLC-type film was used in an
`active matrix display with a poly-Si thin film transistor
`and a storage capacitor for each pixel. Three active
`matrix cells were used for red. blue and green channels
`of full color projection TV. The response time of the
`PDLC-type film to a square wave voltage pulse was
`given for a full on-tirne ol'35 ms and a decay } time onS
`ms. The use of a storage capacitor at each pixel was
`necessary to obtain square wave voltage pulses used in
`this display.
`In Lauer et al., "A Frame-Sequential Color-TV Pro-
`jection Display", SID International Symposium Digest.
`May 1990. pages 534-537, a PDLC active matrix dis-
`play was made with CdSe thin film transistors. The time
`response characteristics were fast enough for sequential
`three-color filtering effects at 50 Hz (6.67 ms for each
`color). PDLC response times were reported only for 50
`volt square wave pulses of 5 ms, with the PDLC reach—
`ing a transient 60% transmission level in 5 ms of on-
`time. and decaying in about 2 ms. giving a relatively
`low light throughput in the frame time. Full projection
`light illumination was reported as having a large effect
`on the thin film transistor off-state current.
`In each of the above papers, the PDLC response is
`described with respect
`to an idealized step-voltage
`change, or to a quare wave pulse.
`
`

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`3
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`SUMMARY OF THE INVENTION
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`5,170,271
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`In view of the limitations ofthe approaches described
`above. the present invention seeks to provide a method
`of operating a PDLC cell to provide a higher optical
`throughput and better gray scale response than has
`previously been obtained at comparable current levels.
`while retaining or even improving upon the fast re-
`sponse of PDLC to a voltage change. The invention
`also describes a photoactivated LCLV system that uti-
`lizes this method.
`
`In accordance with the invention. shaped voltage
`pulses are applied to a PDLC cell such that the cell
`achieves a higher optical throughput. good gray scale
`operation. and a rapid response time. The shape refers
`to the instantaneous rms (rms per cycle) envelope of the
`applied AC voltage pulse or the instantaneous DC volt»
`age of the applied DC pulse. The applied voltage is
`referenced to a time frame during which a readout from
`the PDLC is desired. The voltage level is initially raised
`to a level substantially in excess of the PDLC‘s thresh-
`old voltage; this initial voltage level is applied for a
`substantially shorter period than the duration of the
`time frame. It is then gradually reduced within the time
`frame to a level less than the threshold voltage. The
`initial voltage is applied for a substantially shorter per-
`iod of time than the time over which the voltage is
`being reduced; the applied voltage waveform is prefera-
`bly shaped so that this reduction occurs exponentially.
`Shaped pulse signals of this type can be obtained, for
`example, from raster scan inputs from a CRT activating
`a photosubstrate in which the photoactivation of each
`spot decays fully within the frame time ofthe full raster
`scan. or from an active matrix in which the charge on
`each PDLC picture element (pixel) decays below the
`threshold voltage within the frame time. in about 3 to 5
`RC time constants of the PDLC film.
`The voltage waveform takes advantage of a hystere-
`sis effect
`in the PDLC transmission versus voltage
`curves that has previously been considered a nuisance
`because of the difference in transmission values on volt-
`age rise and fall curves, and because of the long times
`taken to reach steady state transmission levels at inter-
`mediate voltages from either the voltage rise or fall
`curves. The voltage waveform of this invention always
`starts below the threshold voltage necessary to change
`the transmission of the PDLC. This shaped waveform
`begins with a steep initial increase to a voltage that is
`substantially higher than the minimum steady state volt-
`age needed to obtain the desired PDLC transmission
`level, maintains this initial voltage for only a short per-
`iod that is less than the time required for the PDLC to
`reach a steady state transmission. and then decreases
`more gradually back down to below the threshold volt-
`age before the end of the frame time. The PDLC re-
`aponds quickly to the large initial voltage increase
`which driVes it toward a higher transmission hysteresis
`state. The transmission changes slowly during the grad-
`ual decrease of the voltage waveform, permitting a
`relatively high integrated transmission. The PDLC
`transmission drops quickly as the voltage decreases to
`below the threshold level so that by the end of the frame
`time the PDLC returns to its initial state and is ready to
`respond to a new pulse (of the same or different voltage
`level} without any memory effect from the prior pulse.
`This provides a rapid response to both gray scale and
`fully 0N levels. with a time-integrated voltage that
`need be no greater than a square wave that produces a
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`lower throughput and a much slower gray scale re-
`sponse.
`To minimize electrochemical deterioration of the
`PDLC, the voltage is preferably applied so that there is
`no overall net DC current through the PDLC film. For
`example. in a photoactivated amorphous silicon (a-Si:I-I)
`LCLV the applied voltage can be an AC signal. with a
`periodicity much shorter than that of the frame time.
`whose pulse envelope establishes the voltage wave-
`form. In an' active matrix display the applied voltage
`can be a series of shaped DC pulses of alternating polar-
`ity. each of which establishes the voltage waveform.
`More complex. unsymmetrical. voltage formats can also
`be applied, such as are used in MOS-silicon LCLVs.
`CCD-LCLVs. Schottky—LCLVs. p~i-n photodiode-
`LCLVs, etc.. as long as the desired shaped pulse voltage
`waveform envelope is obtained. The invention also
`encompasses an LCLV system which includes an opti-
`cal input means that. together with the photoconductor,
`is selected to collectively produce the desired voltage
`waveform across the PDLC within the light valve.
`These and other features and advantages of the inven-
`tion will be apparent to those skilled in the art from the
`following detailed description. taken together with the
`accompanying drawings, in which:
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a graph of hysteresis curves for two PDLC
`films;
`FIGS. 2a, 26 and 2c are graphs showing the resporISe
`of a PDLC film to step voltage shifts between fully ON
`and fully OFF levels, fully OFF and nominal half-on
`levels, and fully ON and nominal half-off levels, respec~
`tively;
`FIGS. 3-6 are graphs of applied AC voltage wave-
`forms and PDLC responses in accordance with the
`invention;
`FIGS. '7 and 8 are graphs of a PDLC response respec-
`tively to an AC square wave and to an AC voltage
`waveform that is shaped in accordance with the inven-
`tion. both voltage signals having approximately the
`same integrated area;
`FIG. 9 is a graph of a PDLC response to alternating
`positive and negative DC voltage waveforms in accor-
`dance with the invention; and
`FIG. 10 is an illustrative sectional view of an
`LCLV/CRT system that can be used to implement the
`invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The present invention takes advantage of a hysteresis
`effect in PDLC films that previously was considered to
`be a disadvantage in obtaining gray scale and fast re-
`sponse for displays. Two sets of hysteresis curves show-
`ing percent transmission through the PDLC, plotted as
`a function of RMS voltage across the PDLC, are given
`in FIG. 1. The right hand set of curves 2 were obtained
`with BDH-EQ/NOA65 PDLC. while the left hand set 4
`were obtained with HRbPDSD/NOAtSS. The rise
`curves 2a and 40 were obtained by ramping a 100 Hz
`voltage signal up from zero to a fully ON level of 100
`volts over a period of 75 seconds. while the fall curves
`2b and 4b were obtained by ramping the voltage back
`down to zero over another 75 second interval.
`As shown in FIG. 1 each type of PDLC exhibits a
`threshold voltage bEIOw which it
`is nomtransmissive.
`This threshold voltage is about 25 volts for curves 2,
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`5
`and about 6 volts for curves 4. Above these thresholds
`levels, the hysteresis fall curves 2b,4b are shifted to the
`left from the rise curves 20,40. Thus, for any particular
`voltage above the threshold level and below the fully
`ON level,
`there is a higher degree of transmission
`through the PDLC on the fall curve than on the rise
`curve. The invention makes beneficial use of this phe-
`nomenon by driving the PDLC cell with a shaped volt-
`age waveform that forces the majority of the transmis-
`sion period toward the fall curve, and thus produces a
`substantial
`increase in the total optical
`transmission
`through the PDLC.
`FIGS. 20—2-6 illustrate the problem of achieving good
`gray scale response with PDLC films, using square
`wave pulse envelopes of 100 Hz AC signals for 100 ms.
`The fully ON and fully OFF PDLC response times can
`be quite fast, as shown in FIG. 2a. The PDLC in this
`case was switched between zero volts and a fully ON
`level of 70 volts (rms) with a 100 msec square wave
`pulse. The turn-off time along curve segment 6 was
`about 7 msec, while the turn-on time along curve 8 was
`about 1 msec.
`
`However, the dynamic response of the PDLC film
`was found to be strongly influenced by the hysteresis
`effect when voltages were switched to intermediate
`gray scale levels, below the fully 0N voltage. FIG. 2b
`shows the results of switching the voltage from zero
`volts to a gray scale 18 volt level, while FIG. 2: shows
`switching from a fully ON 70 volt level to a gray scale
`13 volt level. As a reference (not shown), a long-term
`activation of several minutes at 13 volts resulted in 50%
`transmission. However, as illustrated in FIG. 2b,
`the
`transmission level increased to Only about 30% with an
`18 volt square wave pulse that commenced at time zero
`and lasted for 100 msec. The result when the voltage
`was reduced from 1'0 volts to )8 volts with a 100 msec
`square pulse is shown in FIG. 2c—the final transmission
`level was about 60%. All PDLC-type films that were
`tested showed this type of hysteresis effect, which on
`the surface would appear to be a serious deterrent to
`obtaining rapid response displays with reproducible
`gray scale.
`The invention overcomes this problem in a manner
`that not only achieves good gray scale operation, but
`also substantially increases the optical throughput. As
`mentioned above, a shaped waveform that causes an
`appreciable part of the PDLC transmission to take place
`along a hysteresis fall curve at a higher transmission
`level is used, rather than a square wave. In addition,
`during the initial portion of the applied waveform the
`PDLC is overdriven by using an initial voltage level
`that is substantially higher than the voltage level that
`would produce the desired transmission level in steady»
`state operation. However,
`this initial voltage level is
`rapidly reduced from its initial high level so that the
`PDLC peaks at about the desired level of transmission.
`The voltage is reduced, preferably at an exponential
`decay rate, causing the PDLC to exhibit a relatively
`high level of optical transmission along a hysteresis fall
`curve for as long as the applied voltage is above the
`PDLC‘s transmission threshold.
`
`It is important that the applied voltage be brought
`down to a level below the PDLC‘s transmission thresh-
`old voltage before the end of each time frame so that the
`PDLC transmission returns to the initial bias level dur-
`ing the frame time. In an LCLV, the time frame is estab-
`lished by the scanning periodicity of the input signal On
`each pixel, such as from a CRT scan of a photoactivated
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`6
`LCLV or an activating voltage in an active matrix
`LCLV. Starting each time frame from a voltage level
`below the threshold ensures that the liquid crystal Oper-
`ates reproducibly for a given signal during each frame.
`Several experiments have demonstrated the advan-
`tages realized with the specially shaped waveform. In
`these demonstrations a PDLC sample was prepared by
`photopolymerization of a
`1:1 mixture of Norland
`NAObS monomer/initiator and BDH-ET liquid crystal
`in a transmission mode test cell formed with indium tin
`oxide (ITO) coated glass separated by a 0.5 mil spacing.
`An ultraviolet cure was performed with a 300 Watt
`mercury lamp {8 mW/cm2 at 365 rim} for three minutes.
`resulting in liquid crystal droplet sizes estimated at be-
`tween } and 2 microns. The cells were read out with a
`
`green HeNe laser beam in examples 1—4 and 6. and a red
`HeNe laser beam in examples 5 and 7.
`EXAMPLE 1
`
`A voltage waveform representing the product of a
`shaped pulse signal and a bias sine wave signal was
`applied to the PDLC cell, and repeated every 25 msec.
`The bias voltage level was established slightly below
`the PDLC's transmission threshold. The peak voltage
`level of the shaped pulse was 25 volts while the bias
`level was 1.5 volts. representing a 16:] amplitude ratio
`that was higher than presently available LCLV switch-
`ing ratios. Bias voltage frequencies of l. 3 and 5 KHz
`were tested. and showed no significant change in light
`throughput or response times.
`The PDLC optical response with a 5 KHz bias signal
`is shown in FIG. 3. The shaped voltage waveform can
`he considered as the instantaneous rms values from the
`envelope 10 of the alternating polarity 5 RH: cycles.
`The applied voltage rose rapidly to its peak level 12,
`and then decayed approximately exponentially to the
`bias level 14. A maximum transmission level 16 of about
`70% was achieved for about
`1 msec, and slowly de-
`creased along a generally exponential curve until the
`voltage neared the bias level, at which point the trans-
`missivity leveled off in region 18 at about a 4% level;
`this was retained until the end of the frame.
`
`EXAMPLE 2
`
`The PDLC sample was tested with a shaped pulse
`and a 5 KB: bias signal combination, refreshed at a 60
`Hz frame rate. The results are shown in FIG. 4. In one
`
`case a very fast-rising 0.1 msec pulse signal 20 with a
`peak level of 25 volts and a bias level of 4.3 volts was
`used. while in a second case the pulsed signal 22 had the
`same 25 volt peak voltage, a bias voltage of 2.6 volts,
`and a slower 1.5 msec rise time. The PDLC optical
`response to the voltage waveforms within envelopes 20
`and 22 is indicated by curves 24 and 26, respectively.
`The higher bias voltage of signal 20 did not appear to
`effect the PDLC’s off-state transmission or the resulting
`contrast.
`Transmission curve 24 had a faster rise time than
`curve 26, corresponding to the faster rise time of its
`voltage signal 2|], but a lower peak transmission level
`corresponding to the more rapid termination of its peak
`voltage. Both transmission curves 24 and 26 exhibited a
`fairly high PDLC transmission level until the voltage
`dropped to the bias value, resulting in a higher optical
`throughput than a fast response twisted nematic cell.
`
`

`

`7
`EXAMPLE 3
`
`5,170,271
`
`The same PDLC cell was tested with a shaped pulse
`and a 5 KHz bias signal combination, refreshed at a 60
`Hz frame rate. An initial 90 volt peak signal was expo»
`nentially decayed down to zero over about half the
`frame period. This resulted in an initial peak transmis-
`sion of 91%, which gradually fell to a minimum trans-
`mission of 8% by the end of the frame. The voltage
`envelope 28 and transmission curve 30 are shown in
`FIG. 5.
`
`EXAMPLE 4
`
`The same conditions were employed as in Example 3,
`but the initial peak voltage was lowered from 90 volts to
`60 volts. as indicated by voltage envelope 32 in FIG. 6.
`The resulting optical transmission curve 34 exhibited a
`maximum transmission of 82% and a minimum of 6%.
`In addition to the change in the maximum and minimum
`transmission levels. the shape of the transmission decay
`curve was also changed, with the transmission decaying
`more rapidly for the lower initial voltage of Example 4.
`This demonstrated that higher initial voltages (corre-
`sponding for example to higher input light levels in a
`photoconductive LCLV) would result in higher bright-
`ness from the PDLC cell, making it possible to achieve
`quality gray scale operation.
`EXAMPLE 5
`
`A similar PDLC cell made with 0.14 mil spacing was
`subjected to one square voltage pulse and one shaped
`voltage pulse for comparison. using 10 KHZ AC. Both
`rms signals had the same integrated area (amplitude x
`pulse width) switching ratio of 1.5. and bias voltage
`level of 12.11 volts.
`
`The Optical response 36 to a 7 msec IOng. 4L] volt
`amplitude square wave pulse 38 is shoWn in FIG. 7.
`This signal partially activated the PDLC film. reaching
`a 31% maximum transmission level at the end of the 7
`msec pulse. and rapidly decayed to a transmission level
`of less than 5%. The resulting total light throughput
`(LTP) was 8.1% for the 30 Hz frame time.
`The optical response 40 of the same PDLC film to a
`shaped pulse 42 with a 60.0 volt peak and a 7 msec
`decay is shown in FIG. 8. The voltage area of shaped
`pulse 42 was equal to that of square wave pulse 33. A
`much faster rise time of 0.83 msec was experienced with
`the shaped pulse. in contrast to the square wave pulse
`whose rise time occupied the entire 7 msec. The maxi-
`mum light transmission for the shaped pulse was 69%,
`and its total light throughput for the 30 Hz frame time
`was 19.5%. This example demonstrated the improve-
`ments in rise time and light throughput from a shaped
`pulse signal with a rapid rise and a gradual decay.
`EXAMPLE 6
`
`To compare the shaped pulse mode operation of the
`PDLC film with twisted nematic liquid crystals,
`the
`same liquid crystal as in Examples 1—5 was tested in a
`90° twisted nematic cell of 4.8 micron thickness. This
`liquid crystal thickness corresponded to the 10 micron
`thick PDLC film of Examples 1—5. which contained
`about 50% liquid crystal by volume. The cell was fabri-
`cated with 90“ twisted surface parallel alignment be-
`tween medium angle deposition/shallow angle deposi~
`tion 510; coated conductive electrodes. Transmission
`measurements were performed with the same Optical
`setup used for the PDLC samples. a green HeNe laser.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`8
`and two parallel polarizers inserted into the system.
`Shaped pulses of S KHz AC voltages with 25 msec
`repetition rates, as in Example I. were used. The maxi-
`mum rms pulse amplitudes were considerably less than
`in Example 1, since a substantial portion of the applied
`voltage in a PDLC cell is dropped across the polymer,
`rather than the liquid crystals themselves. Steady-state
`levels of only about 2 volts are usually required to turn
`this type of liquid crystal cell fully on.
`The results of this test are summarized in the table
`below. The PDLC test cells in FIG. 3 exhibited higher
`contrast ratios and optical efficiency as compared to the
`twisted nematic cell. which can be attributed to the
`PDLC film‘s fast rise time and the effect of hysteresis in
`slowing down the optical decay. The test results indi-
`cated that the twisted nematic mode response time was
`too slow for a 25 Hz frame rate with an exponential
`decay pulse mode operation, and that the transmissiOn
`decay within each time frame was too slow to obtain
`contrasts above 5:1 even with a signal ratio of 400:1.
`The high residual transmission at the end of each frame
`would also interfere with gray scale changes in a dy—
`namic display. With the same shaped driving pulse. the
`PDLC film of Example 1 was operated between 70%
`maximum transmission and 4% minimum transmission.
`for a transmission ratio of 17.5 and a switching ratio of
`about 17:1.
`
`
`Bias
`Applied
`Swilching
`Tampa"
`
`Voltage
`Voltage
`Ratio
`9? Tm.”
`9‘2 Tm}.
`Tm"
`0.3
`4.7
`15.7
`34.7
`60.0
`1.4
`0.2
`4.5
`32.5
`46.0
`26.0
`1.8
`0.6
`10.0
`16.1
`97.0
`42.6
`2.3
`1.35
`12.0
`3.9
`94.0
`56.8
`1.6
`0.03
`12.0
`‘00
`94.0
`22.0
`4.3
`
`
`EXAMPLE 7
`
`A voltage waveform of a pulse shape signal of alter-
`nating positive and negative DC pulses. each with a 16.7
`ms frame time was applied to the PDLC cell of Exam-
`ple 5. This waveform corresponds to that obtained by
`placing an instantaneous (submillisecond) charge on the
`PDLC and having it leak off within a frame time by
`conduction through the PDLC due to its resistivity
`value of 7.5 x 109 ohm-cm (corresponding to 8 RC time
`constant of 3.3 ms), and then in the next frame placing
`the opposite polarity charge on the PDLC and having it
`leak off. The results are shown in FIG. 9. In each frame
`time. the optical response of the PDLC quickly reached
`a high transmission level within a millisecond, decayed
`relatively slowly until the DC pulse had decayed by
`more than an RC time constant. and then dropped back
`to its off-state transmission before the end of the 16.7 ms
`
`frame time. The positive and negative DC shaped pulses
`gave the same optical response. This resulted in an
`integrated light throughput which was 37.4% of the
`total incident light.
`This invention may be implemented in a relatively
`simple active matrix raster-scan display system in which
`the pixel circuitry requires neither a storage capacitor
`nor a reset voltage to obtain the type of shaped pulse
`signal waveform described in Example 7 and shown in
`FIG. 9. A relatively low resistivity PDLC film, such as
`the 7.5X109 ohm-cm film in Example 7, provides the
`shaped pulse waveform needed for good optical
`throughput from the fast response on-time and inte-
`
`

`

`5,1?0,2?1
`
`9
`grated transmission effect from the exponential decay of
`the signal in each frame time at the 60 Hz frame rate.
`Similarly, a PDLC film with a resistivity of 1.5x1010
`ohm-cm provides fast
`response and good optical
`throughput when activated at a 30 Hz frame rate. Ac-
`tive matrix displays using nematic LCs (not in PDLCs)
`require much higher resistivity values (greater than
`1011 ohm-cm) to obtain good light throughput without
`storage capacitors in the pixel circuitry when operated
`at 30 Hz and 60 Hz frame rates.
`The invention may be implemented in an LCLV
`system such as that shown in FIG. 10. An LCLV #4 is
`addressed by a cathode ray tube 4-6. The LCLV in-
`cludes a plasma enhanced chemical vapor deposition
`deposited a-Si:H photoconductor 48 with a transparent
`ITO electrode 50 and a fiberoptic face plate 52 on one
`side. and a CdTe light-blocking layer 54 on the other
`side. After the light-blocking layer comes a dielectric
`mirror 56. a PDLC film 58 surrounded by a spacer ring
`60. an ITO counterselectrode 62, and a quartz readout
`window 64. An audio frequency power supply 66 is
`COnnected across the two electrodes 50 and 62.
`The CRT 46 directs an electron beam 68 across a
`phosphor screen 70. which is positioned to illuminate
`the liberoptic face plate 52 with radiation emitted from
`the locations on the screen that are struck by the elec~
`tron beam. Controlled scanning of the beam across the
`phosphor screen thus forms an input image to the light
`valve.
`The input image is transmitted through the fiberoptic
`face plate 52 and transparent electrode 50 to the photo-
`conductor layer 48. The impedance of the photocon-
`doctor layer is lowered in proportion to the intensity of
`the incident light. resulting in a spatially varying impe-
`dance pattern. This causes a corresponding increase in
`the voltage dropped across the liquid crystal layer 58 in
`a spatially varying pattern that matches the input image.
`This pattern modulates a readout beam 72 that is di-
`rected through the liquid crystal. reflects off of dielec-
`tric mirror 56 and exits back through the liquid crystal.
`The input and output beams are thus optically isolated,
`giving the light valve a capability of accepting a low»
`intensity light
`image and converting it
`into a much
`higher output image. The light blocking layer 54 pre-
`vents the readout beam from interfering with the photo-
`conductor layer 48. For a higher contrast reflective
`mode PDLC-LCLV display. a wedge-shape front glass
`surface 64 is used so that the front surface reflection of
`the readout beam 72 (whether at normal incidence or at
`a small off-normal angle of incidence as indicated in
`FIG. 10) is reflected out of the optical system used to
`collect the main readout beam 72 that is reflected by the
`dichroic mirrors.
`The shape of the voltage waveform across the liquid
`crystal will be a combined function of the phosphors
`used in the CRT. and the type of photoconductor in the
`light valve. These should be selected through empirical
`determinations to obtain the desired waveform. For
`
`example, a suitable combination of phosphor and photo-
`c0nductor for the voltage waveforms in Example 2 is
`obtained by using the fast response a-Si:I-I photosub-
`strate (with greater than 60 Hz frame rate) character-
`ized by Sterling et al.. (cited above) activated with a
`medium persistence red CRT phosphor such as PZZR
`(about 2 ms decay to 10%). Another example is to use
`relatively thin (5 pm thick) boron-doped a-Si:H films
`such as those described by Ashley and Davis, “Amor-
`phous Silicon Photoconductor in a Liquid Crystal Spa-
`
`10
`tial Light Modulator". Applied Optics, Vol. 26. No. 2. 15
`January 1987, pages 241-246,
`in conjunction with a
`medium-short persistence green CRT phosphor such as
`P3] (about 33 its to 10%} to obtain voltage waveforms
`such as those in Example I. This voltage waveform on
`the PDLC will provide frame rates of about 40 Hz,
`which is considerably greater than the 10 Hz frame rate
`Ashley and Davis observed using step pulse reSponse
`from optically chopped white light with the nematic
`liquid crystal EDI-I44. Altema