Programmable liquid-crystal TV spatial light modulator: modified drive electronics to improve device performancefor spatial-light-modulation operation J. Aiken, B. Bates, M. G. Catney, and P. C. Miller Liquid crystal television (LCTV) continues to play a useful role as a spatial light modulator in the development and evaluation of systems for optical image processing. We outline new addressing electronics developed for a commercially available LCTV that permit writing to individual pixels at an improved display up-date rate and allow the input video signal to cover a much greater transmittance range of the TV display for black and white pixels. We illustrate this by measuring the diffraction efficiency for gratings written onto the display. For vertical gratings written along the displaycolumns the diffraction efficiency is increased significantly, but there is no improvement for horizontal gratings. Some merits of the modified LCTV modulator for optical processing applications are considered briefly. Key words: Liquid-crystal display, spatial light modulator. 1. Introduction In recent years there has been considerable interest in the use of modified pocket-sized liquid-crystal televisions (LCTV's) as spatial light modulators (SLM's) in a variety of optical image-processing appli- cations. A key feature of these devices is that they are inexpensive. Although their performance in terms of optical quality, space-bandwidth product, and pixel contrast is quite limited compared with that of com- mercial SLM's, they have proved to be invaluable for demonstrating the feasibility of several new and important image-processing techniques." In our own applications, we developed a system that employs four LCTV's in a combined optical logic gate-joint transform correlator arrangement, which has used speckle metrology to demonstrate real-time (video rate) display of object motion. The principles of operation and the useful perfor- mance characteristics of the liquid-crystal modulator have been discussed in some detail both for conven- P. C. Miller is with the Department of Electrical and Electronic Engineering, Queen's University of Belfast, Belfast BT7 NN, Northern Ireland, UK. The other authors are with the Department of Pure and AppliedPhysics, Queen's University of Belfast, Belfast BT7 INN, Northern Ireland, UK. Received 10 September 1990. 0003-6935/91/324605-05$05.00/0. C 1991 Optical Society of America. tional amplitude modulation and for the bipolar phase 7 modes of operation. This study addresses the problem of the display pixel transmittance and con- trast. As was shown by Boreman and Raudenbush, 5 the pixel transmittance (T) and the modulation depth (defined as M = Tm - Tmin/Tmax+ Tmin) are func- tions of the brightness-control voltage applied across the screen. Although a useful maximum value of M 0.68 was obtained for an optimum brightness-control voltage (V 1 ) setting, the actual value for the maxi- mum screen transmittance is quite small(Tin = 0.5%; Tm. 2.6%). As Boreman and Raudenbush have pointed out, one of the limitations of the LCTV under dynamic operation is that, with the video drive cir- cuitry, the range of video levels from black to white is insufficient to cover the full transmittance range of the modulator at any one brightness setting. We have made modifications to the display circuitry that per- mit a much wider transmittance range of the modula- tor at the optimum brightness setting and conse- quently provide a potentially more useful performance of the LCTV for certain optical processing applica- tions. I. Liquid-Crystal Television Drive Modifications The LCTV that we modified is the Casio TV 21. The liquid-crystal (LC) display is a twisted nematic type employing an electric field to modulate the polariza- 10 November 1991 / Vol. 30, No. 32 / APPLIED OPTICS 4605
Transcript
Programmable liquid-crystal TV spatial lightmodulator: modified drive electronics to improvedevice performance for spatial-light-modulationoperation
J. Aiken, B. Bates, M. G. Catney, and P. C. Miller
Liquid crystal television (LCTV) continues to play a useful role as a spatial light modulator in thedevelopment and evaluation of systems for optical image processing. We outline new addressingelectronics developed for a commercially available LCTV that permit writing to individual pixels at animproved display up-date rate and allow the input video signal to cover a much greater transmittancerange of the TV display for black and white pixels. We illustrate this by measuring the diffractionefficiency for gratings written onto the display. For vertical gratings written along the display columns thediffraction efficiency is increased significantly, but there is no improvement for horizontal gratings. Somemerits of the modified LCTV modulator for optical processing applications are considered briefly.
In recent years there has been considerable interestin the use of modified pocket-sized liquid-crystaltelevisions (LCTV's) as spatial light modulators(SLM's) in a variety of optical image-processing appli-cations. A key feature of these devices is that they areinexpensive. Although their performance in terms ofoptical quality, space-bandwidth product, and pixelcontrast is quite limited compared with that of com-mercial SLM's, they have proved to be invaluable fordemonstrating the feasibility of several new andimportant image-processing techniques." In our ownapplications, we developed a system that employsfour LCTV's in a combined optical logic gate-jointtransform correlator arrangement, which has usedspeckle metrology to demonstrate real-time (videorate) display of object motion.
The principles of operation and the useful perfor-mance characteristics of the liquid-crystal modulatorhave been discussed in some detail both for conven-
P. C. Miller is with the Department of Electrical and ElectronicEngineering, Queen's University of Belfast, Belfast BT7 NN,Northern Ireland, UK. The other authors are with the Departmentof Pure and Applied Physics, Queen's University of Belfast, BelfastBT7 INN, Northern Ireland, UK.
Received 10 September 1990.0003-6935/91/324605-05$05.00/0.C 1991 Optical Society of America.
tional amplitude modulation and for the bipolarphase7 modes of operation. This study addresses theproblem of the display pixel transmittance and con-trast. As was shown by Boreman and Raudenbush,5the pixel transmittance (T) and the modulation depth(defined as M = Tm - Tmin/Tmax + Tmin) are func-tions of the brightness-control voltage applied acrossthe screen. Although a useful maximum value of M 0.68 was obtained for an optimum brightness-controlvoltage (V1) setting, the actual value for the maxi-mum screen transmittance is quite small(Tin = 0.5%;Tm. 2.6%). As Boreman and Raudenbush havepointed out, one of the limitations of the LCTV underdynamic operation is that, with the video drive cir-cuitry, the range of video levels from black to white isinsufficient to cover the full transmittance range ofthe modulator at any one brightness setting. We havemade modifications to the display circuitry that per-mit a much wider transmittance range of the modula-tor at the optimum brightness setting and conse-quently provide a potentially more useful performanceof the LCTV for certain optical processing applica-tions.
I. Liquid-Crystal Television Drive ModificationsThe LCTV that we modified is the Casio TV 21. Theliquid-crystal (LC) display is a twisted nematic typeemploying an electric field to modulate the polariza-
tion of the incident light.6 The display is 40 mm wideby 30 mm high, and there are 110 rows (lines) eachwith 139 pixels. Each pixel is 260 jim x 260 Am,with an interpixel region around each pixel 14 [umwide. To display video information from a computeror a TV camera, the video signal must be made tomodulate a rf carrier, which is then fed into the TV.When the device is used in this way, it is evident thatits contrast is low and images do not have sharplydefined edges. These problems are associated with theelectronics driving the display.
The objective of our program was to develop sepa-rate addressing electronics that permit writing toindividual pixels and allow the video signal to cover awider transmittance range of the TV display. Inprinciple, this could be achieved by removing the LCdisplay from the TV and by developing independentdrive electronics. However, since there are 110-rowand 139-column connections, this approach wouldrequire a great deal of additional, complicated cir-cuitry. In the block diagram shown in Fig. 1 the fourdisplay integrated circuits (IC's) are already perma-nently connected to the LC display. By using theseIC 's it is still possible to select particular pixels and tovary the electrical parameters that control the displaypixel transmittance. This approach obviates the need
Fig. 1. Block diagram of the unmodified LCTV. In the presentstudy the important elements are the LC display together with therow and column IC's, since the remaining components mainlyprocess the transmitted TV signals. V1-V5 are five referencevoltages that are used by the row and column IC's to set the pixelvoltages. The voltage ratios are fixed in the standard LCTV
electronics. In the modified circuitry they are made variable tooptimize the pixel transmittance. A/D, analog to digital.
for numerous additional connections to the display.Connections to the IC's themselves, although thecircuits are surface-mounted components, are morestraightforward and with care can be made by usingfine, enameled copper wires connected to a multipinconnector.
The four display IC's are in two groups, and theydrive the display in a time-multiplexed mode. Onegroup selects which row of pixels is being given newdata. The other group provides data in parallel for theselected row, using the transparent column elec-trodes. All the rows from first to last are sequentiallyselected and supplied with the relevant data to updateand refresh the display pixels. This type of addressingis known as line at a time. An examination of thesignals that are normally applied to the row andcolumn IC's has enabled us to develop circuitry forwriting directly to individual pixels of the LC display.In brief the signals involved are
1. A data clock (3.125-MHz, two phase): This is thehighest-frequency signal applied to the IC's, and it isused to clock the data sequentially into the columndrive IC's. Since there are 139 pixels on each row, 139cycles of this signal clock one row of data into thecolumn IC's. In our circuit design all the othercontrol signals are derived from this clock to ensuresynchronization.
2. A data signal: This is a parallel 3-bit binary-coded signal representing the eight pixel transmit-tance levels ranging from black to white. Since we areusing only black-and-white (binary) images, the 3-bitsignal is replaced by a single-bit signal, and only twobinary codes are employed; for example, 1 = blackand 0 = white. In a normal TV the 3-bit data comefrom an analog-to-digital converter (see Fig. 1).
3. Chip select: These signals select which of thecolumn drive IC's receive the data.
4. Row select: This takes 2.5 times the normal TVline time (64 Rs) and is used sequentially to selectrows from the top to the bottom of the display when anew scan has begun.
5. Frame synchronization: This normally occurs at50 Hz, indicating that a new scan of the display hasbegun.
6. Voltages V1-V5: Five reference voltages used bythe row and column IC's to set the voltage across eachpixel according to the binary data code for each pixel.The ratio of these voltages is fixed by a potentialdivider network supplied by the brightness control(Fig. 2). The brightness-control voltage is variablefrom -4 to -16 V for the present TV. The data inbinary form are input sequentially to IC's 3 and 4,which are also fed with the reference voltages V1-V5.The chip-select signal determines which IC (3 or 4) isto receive data from the common data bus; IC 3receives data for columns 1-70 and IC 4 for columns71-139. Depending on the value of the binary code,
Fig. 2. Potential divider network that provides the referencevoltages V1-V5 supplied to the row and column IC's. The resistorsare fixed in the standard TV but are variable in the modifiedcircuitry to permit easier determination of optimum operatingvoltages for binary operation.
the appropriate combination of reference voltages ismade available to the appropriate column electrode.
As we have stated, the LC display and the drive IC'swere removed from the TV and fitted with connectorsto the new drive circuitry. However, if required, theoriginal TV electronics could be reconnected to thedisplay to permit a comparison of performance. Thecircuitry developed provides all the signals describedabove. Also, it was possible to reduce the frame timeto permit some 73 complete images to be displayedper second. Each of the five reference voltages (Vi-V5) was made variable by using potentiometers in thedivider network. Adjustment of these voltages pro-vides a greater range in pixel transmittance, whichcan be used to provide better contrast on the LCdisplay.
The new LC display controller developed for thisstudy is shown schematically in Fig. 3. It includes a16-k bit dual-port RAM IC used to store the data forthe image to be displayed. The memory IC is arrangedso that each display pixel has a corresponding bit inmemory. A single bit per pixel is sufficient since thedisplayed images contain only black or white pixels (apixel is black if its corresponding memory bit is set to1 and white if set to 0). These data are fed to thecolumn IC's synchronized with the timing signals toproduce the required display image. Using a dual-portRAM allows the LC display circuitry uninterruptedaccess to the display data while the data source, in thepresent case a microcomputer, updates the memoryas necessary.
Ill. Measured Pixel Transmittance
The transmittance of a single black or white pixel as afunction of the brightness-control voltage (VE) isshown in Fig. 4. The measurements were made with aHe-Ne laser beam, which was focused to a spot size of
70 jim at the center of a single pixel. Intensitymeasurements were made with a photodiode placed
ROW
SIGNAL
BUS
Fig. 3. Block diagram of the new LC display controller. The threemain sections are the LC display (LCD) with drive IC's (as in thestandard LCTV); the new LC display control circuitry, whichprovides the signals discussed in Section II; and the microcom-puter, which provides the display input data in digital form.
immediately behind the display. These results pro-vide a direct comparison of pixel transmittance prop-erties obtained with the original TV drive electronicsand those obtained with the optimum settings usingthe new drive circuitry. In these measurements thewhole screen of the display was set to be either allblack or all white, and the results shown were ob-tained with the original, intact LCTV polarizers.Variations in transmittance from one pixel to anotherwere not significant.
Using the new drive circuitry, we examined thepixel transmittance at different settings of the bright-ness-control voltage, and we systematically adjustedthe individual supply voltages V1-V5 to determinethe transmittance range for black and white pixels(i.e., Tmin and Tm9. The optimum performance occursat a brightness-control voltage VB -15.5 V, and themost sensitive individual voltage for increasing thetransmittance dynamic range is V4, which is in-creased significantly from the setting employed in thestandard LCTV electronics. Figure 4 shows the varia-tion of Tm. and Tmin with V obtained with thevoltages V1-V5 optimized. It is evident that at theoptimum operating condition (VE = -15.5 V) there isa useful reduction in the black-pixel transmittanceand a dramatic increase in the white-pixel transmit-tance compared with that obtained with the originalLCTV drive electronics. Transmittance characteris-
Pixel transmittance characteristics of the LC display
obtained with the original TV drive electronics (top) and with theoptimized new drive circuitry (bottom). The transmittances areshown as a function of the brightness-control voltage (V). Thesquares and triangles give the transmittance for the white andblack pixels, respectively. (Note: there is a contrast reversal whenthe original TV drive electronics are used at VB = 13.5 V and atV, = -11 V in the case of the new drive circuitry.) In both cases
optimum contrast occurs near V = -15.5 V. With the newcircuitry there is a dramatic increase in the range of transmittancebetween black and white pixels near the optimum operatingvoltage.
tics for the optimum operating conditions at Tm =
33% and Tmin = 1% (M 0.94) for the new drivecircuitry, compared with Tm., = 3.5% and Ti. = 1.7%(M 0.35) for the unmodified LCTV.
IV. Liquid-Crystal Television Diffraction Efficiency
We carried out some simple tests to determine thediffraction efficiency of the TV when various gratingswere written onto the display. For these tests thedisplay was mounted in a liquid gate with its originalpolarizers and illuminated with a collimated He-Nelaser beam 25 mm in diameter, and the diffractionpattern intensity was recorded in the focal plane ofthe transform lens. Both vertical (i.e., along thecolumn direction) and horizontal gratings of spatialwidths 1 pixel on/i off and 2 pixels on/2 off were
Table 1. Measured Values of the Flrst-Order/dc DiffractionEfficiency Riatio'
Grating type Optimum Standard
Vertical, 2-pixels period 0.166 0.017
Vertical, 4-pixels period 0.165 0.013
Horizontal, 2-pixels period 0.017 0.026
Horizontal, 4-pixels period 0.015 0.025
'For vertical (along column direction) and horizontal gratingsobtained using the optimum and standard voltage settings.
written onto the LC display, using computer gener-ated signals.
The ratio of the measured first-order to zero-order(dc) diffracted intensity is summarized in Table I fordifferent operating conditions. For these measure-ments we have used only the modified drive circuitryin which the display pixels are addressed individuallyand precisely. The results in Table I compare diffrac-tion efficiencies obtained with the optimum pixelvoltage settings (pixel transmittances as given inSection III) and the standard voltage settings, thelatter corresponding to the voltages V1-V5 that areused in commercial TV. When we use the originalLCTV drive electronics, precise registration of thescreen pixels is not possible for computer-generatedvideo signals, which are fed into the LCTV on a rfcarrier. Consequently in this case the contrast andthe edge sharpness of gratings that are displayed onthe screen are visually poorer than those obtained byusing the new addressing electronics.
Table I shows that, for vertical gratings writtenonto the display, the optimized pixel voltage settingslead to a significant improvement in the diffractionefficiency as measured in terms of the first-order/dcintensity ratio. The experiment was simulated usinga conventional fast-Fourier-transform program, andcalculations show that the intensity of the first-orderdiffraction peak is increased by a factor of 50 for theoptimized pixel voltages relative to that with stan-dard voltage settings. This increase arises from thegreater transmittance range of the black and whitepixels, namely, Tm,, = 33% and Tnin = 1% for theoptimized voltage settings compared with Tm. ,3.5% and Tmi. = 1.4% for standard pixel voltages.There is, however, an increase in the dc intensitycontribution when the display is used with the opti-mized voltage settings. Taking account also of theinterpixel contribution, we increased the dc intensityby a factor of 3.7, giving an overall improvement inthe first-order/dc intensity ratio of 13. This result isin broad agreement with the experimental data.
When horizontal gratings are written onto thedisplay, however, as Table I shows, the resultingdiffraction efficiency is poor. The explanation for thislies in the increased voltage applied to activatedpixels. Since each pixel has electrical characteristicssimilar to those of a capacitor, cross talk betweenpixels in the same column results in a bias voltageexisting on the column electrode. In the case of a
vertical grating, the signal applied to each columnelectrode is dc; hence no cross talk occurs betweenadjacent column pixels, as they are all at the samevoltage. For a horizontal grating, however, a squarewave is applied to the column electrodes to producealternate black and white pixels, between which crosstalk occurs. The effect of this is the creation of a biasvoltage that effectively reduces the voltage rangeapplied to the pixels such that the transmittance of awhite pixel becomes smaller, while that of a blackpixel is increased. Since this bias voltage is propor-tional to the applied pixel voltage, the reduction incontrast becomes significant only when the latter hasa large dynamic range. Our experiments have shownthat, when signals with a large dynamic range areused, the extent of the contrast loss depends on theratio of black to white pixels within a column; theworst case occurs when a single black or white pixel iscontained within a column in which all the others arewhite or black, respectively.
V. Conclusion
We have developed a prototype display control systemto drive the liquid crystal display of a commerciallyavailable Casio TV 21 LCTV. Replacing the originalLCTV drive electronics has resulted in a number ofimprovements with regard to the operation of thedevice as a SLM. The major improvement in thisrespect is the ability to write directly to individualpixels, thus making the device fully programmable. Asecond improvement is an increase in the displayupdate rate from 50 to 78 Hz, reducing the amount ofLC relaxation between successive frames. Third, theincrease in voltage range applied to the pixels resultsin a much greater transmittance range between theblack and white pixels. As was discussed in SectionIV, however, the improvement in the dynamic con-trast ratio is much smaller than that expected on thebasis of the pixel transmittance measurements, ex-cept for the special case of the vertical gratingswritten onto the display. However, the overall perfor-mance as a SLM is still significantly better than thatof the original device.
The display control system that we have developedhas converted the LCTV into a low-cost programma-ble binary SLM. Although it is inferior in terms ofcontrast and display update rate to other, moreexpensive SLM's, such as the Semetex Sight-mod, webelieve that this system provides a practical low-cost
alternative for those systems for which several de-vices are required. Possible applications include thewriting of binary-phase-only filters9 onto the devicefor pattern recognition purposes and the opticalimplementation of a crossbar switch'0 based on themultiplication of a vector with a binary matrix array.This crossbar switch could have a significant effect indigital optical computing and telecommunicationsand in binary matrix-matrix operations" such asmultiplication and Boolean logic, which are used tosolve many problems in computer science such asexpert systems.'2
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