Optical Pattern Processing Utilizing Nematic Liquid Crystals

Seiji Inokuchi, Yasuji Morita, and Yoshifumi Sakurai

This paper describes a new type of optical processing based on the dynamic scattering effect in liquidcrystals. It is possible to realize a binary spatial filter with nematic liquid crystals and to control theoptical modes by voltage signals. A two-dimensional Walsh function is used for extracting propertiesof the input patterns. The realization of an 8 X 8 Walsh function filter with nematic liquid crystals isdemonstrated. The contrast ratio is over 30 to 1. The variation in transparency is within d43%.Experimental results show that it is possible to extract the properties of handwritten characters by de-tecting the intensity of light transmitted through both the input pattern and the liquid crystal filter.

Introduction

There are two important steps in a basic patternrecognition system. The processor extracts the prop-erties of an input pattern, and the decision networkprovides the desired classification. This paper isrestricted to the processor. Electronic processing isusually utilized for character recognition. It is one-dimensional and requires a high speed electronic com-puter. Optical processing is desirable for a visiblepattern. In the process of signal conversion in anoptical system, the multiplication by a weightingfunction can be replaced by a spatial filter and theintegration can be carried out by a convex lens. Butmaterials whose optical transmittance can easily becontrolled have not been available. It is the purposeof this paper to describe an optical processor based onthe dynamic scattering effect in nematic liquid crystals.Optical transmittance of a nematic liquid crystalsandwiched between transparent electrodes can be con-trolled by an applied voltage. It becomes possible torealize a two-dimensional spatial filter whose modesare controlled by external electric signals. Thispaper has two main objectives: one concerned with theapplication of Walsh functions to an optical processorand another about the realization of a spatial filterwith nematic liquid crystals.

Theory

For pattern recognition, it is necessary to carry outone of the transformations that extract certain proper-ties of an input pattern. A set of orthogonal functionsis suitable for these transformations. Sine and

The authors are with the Faculty of Engineering Science,Osaka University, Toyonaka, Osaka, Japan.

Received 9 August 1971.

cosine functions are typical and permit the representa-tion of a large class of functions by superposition. Inorder to apply the Fourier transformation to anincoherent optical processing system, it is necessarythat the transmittance of a spatial filter be a trigono-metrical function. It is very difficult to generate aspatial filter by continuously varying the optical density.The binary spatial filter based on Walsh functions issuitable for optical processing.', 2 (In Ref. 2, Deckeruses the term Hadamard transform. A Walsh transformis equivalent to a Hadamard transform. The Walshfunction is defined by the ordered form of Hadamardmatrix in which the sequence of each row is larger thanthat of the preceding row.4 ) Walsh functions aredefined by the difference equations:

wal(2p + qx) = (-1)(p/2 + q){wal[p,2(x + 4)]+ (-1)P+wal[p,2(x -where q = 0 or 1, p =0, 1, 2,. . ., wal(O,x) = 1, and(p/2 + q) means the largest integer smaller than orequal to (p/2 + q). This set of functions is orthogonaland complete.1 But it is impossible to carry out themultiplication by -1 in an incoherent optical system.The liquid crystal cell can function as a binary multi-plier by + 1 and 0, as will be mentioned later. It isnecessary to modify the Walsh functions by replacing-1 with 0. These are designated as the modified Walshfunctions. Figure 1 shows the two-dimensional modifiedWalsh function filters, where the white (transparent)portions represent +1 and the black (opaque) ones 0.A two-dimensional Walsh function is defined as theproduct of a row and a column, each of which is a one-dimensional Walsh function. On the other hand, a two-dimensional modified Walsh function is defined as theexclusive or of a row and a column, each of which is amodified Walsh function. Figure 2, in which themode (3, 4) is shown as an example, shows that everyelement is defined by exclusive or of the correspondingrow and column.

October 1972 / Vol. 11, No. 10 / APPLIED OPTICS 2223

'ue 1 2 3 4 5 6 7

2 <OaN

4 =8&K&H2

6 _ > §R N M m

Fig. 1. Two-dimensional Walsh function filter.

> :. >. > :> > >a 0 0 0 0 0 0 0

- 0 - - n !a0 -

OR 0E 0R O *_ _ 0_

0

1

This type of property detector makes the processingsensitive to the over-all characteristics of the inputpattern rather than to an individual point,3' 4 andsensitive to vertical and horizontal spatial frequenciesof the input pattern. Thus, it is suitable for detectingthe property of Chinese characters, which are generallymade up of vertical and horizontal line segments.

Liquid Crystals

Several display devices have been reported whichutilize the dynamic scattering effect in nematic liquidcrystals. 5 -7 A nematic liquid crystal layer severaltens of microns thick, sandwiched between two electro-conductive glasses, is transparent under zero appliedvoltage. When a voltage above V is applied, itbecomes turbulent and scatters the incident light.Figure 3 shows the intensity of transmitted lightmeasured for a cell size of 1 cm X 1 cm and a layerthickness of 20 um at a distance 10 cm from the cell.This behavior does not depend on the polarity of theapplied voltage. Figure 4 shows the optical off/oncontrast ratio for a layer thickness of 20 Atm at thesaturation level as a function of the distance from thecell to the photodetector. As the nematic liquidcrystal scatters the incident light not only in the back-ward direction but also forward, the contrast ratio

1.0

1 ( OV)

1 ( OV)

0 (50V)

0 (50V)

1 ( OV)

t,

cc)aa11

0.8

0.6

0.4

0.2

1 ( OV)

00 (50V)

0 (50V)

Fig. 2. Filter logic of modified Walsh function.

layer thickness: 20 urm

0 20 40 60

Applied voltage (V)

Fig. 3. Transmitted light intensity vs applied voltage.

40 -

The Walsh-Fourier transform for a spatially limitedfunction f(x,y) is given by:

F(,u, v) =f |o #f(x,y)wa1(u,x)wal(Y~y)dxdy,

where f(x,y) = 0 for x < 0, x _ 1, y < 0, and y > . Thevalues of the modified Walsh-Fourier transform aregiven by:

Fm(u,4,) = [F(0,0) + F(ii,v)]/2.

0

a

0

Photographic film30 _.

20 -

10 -

o

Liquid crystal cell(20 m thickness)

5 10

Distance ( cm )

Fig. 4. Contrast ratio vs distance.

2224 APPLIED OPTICS / Vol. 11, No. 10 / October 1972

15. X |

Liquid crystal filter.

Optical System

Figure 5 shows the liquid crystal filter that generatesthe spatial modes of the two-dimensional modifiedWalsh function. The nematic material TN-1 producedfor a display cell, whose mesophase exists from 100Cto 550C, is sandwiched between two sheets of NESA-coated glass. Each glass plate has eight arrays ofstrip electrodes insulated by narrow nonconductivebands. One of these sheets has eight arrays in thevertical direction and the other is horizontal. Theseelectrodes were made by blowing heated air containingSnCl2 vapor against ultraflat glass, which was paintedbeforehand to form insulating bands 0.3 mm wide.A transparency of over 85% and a surface resistivityof less than 200 Q2/cm 2 were obtained. It is desired thatthe liquid crystal layer between the two sheets of glassbe thin and uniform. Not only for this purpose butalso for providing insulation between the two sheets,SiO 2 of about 10 m thickness was evaporated invacuum on the narrow insulating bands. 50 V and 0 Vare applied to the row and the column, corresponding to0 and 1, respectively, as shown in Fig. 2. Uniformityof optical property was satisfied over the whole surface.The variation in transparency was within a 3% for all64 elements. The contrast ratio of transparent stateto opaque was almost constant over all the elements.Figure 6 shows the filter whose mode is (4,4). Rowelectrodes are controlled by a one-dimensional Walshfunction generator and the column electrodes byanother. Figure 7 shows its logic diagram.

Fig. 6. Liquid crystal filter.

increases as the photodetector is moved away from theliquid crystal cell. On the other hand, the contrastratio of light through absorbing materials such as aphotographic film does not increase as strongly asthrough the liquid crystal. It is possible to obtain acontrast ratio of over 30 to 1. A binary spatial filtercan be realized by corresponding the transparent stateand the scattering one (opaque state) to 1 and 0,respectively. Suppose that for the voltage applied tothe electrodes, the low voltage (zero volts) and thehigh one (saturation voltage, for example, 50 V in thispaper) correspond to 1 and 0, respectively. Then thefunction of the liquid crystal filter is represented bythe exclusive or of the signals on the two electrodes.Through the utilization of exclusive or function such asthe modified Walsh functions as described above, allelements are addressed at a time by applying a voltagesignal only for the row and the column.

I I .

Reset Triggerpulse pulse

I - X., Yi

X2 , Y2

X3, Y3X4, Y4

_ X5s Y5

X6, Y6

X7 , Y7

X8 , Y 8

Flip flop

Exclusive or

Fig. 7. Logic diagram of Walsh function generator.

Liquid crystalCollimator filter

Light Photosource det cto

Monochromatic Inputfilter pattern

Fig. 8. Optical configuration.

October 1972 / Vol. 11, No. 10 / APPLIED OPTICS 2225

Glass

1-4 mmNESA 11111""" -11- , ,,_/// o , SiO2 (Insulator)(SnO2) . .: . . -.

_l -- .4--Liquid crystal0.3 mGls

Glass

Fig. 5.

Fig. 9. Experimental result.

'j- 0 1 2 3 4 5 6 7v- 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6

Filter mode

The optical configuration is shown in Fig. 8. Aconventional tungsten lamp with a small filament wasused as a light source. A parallel light beam generatedby a monochromatic filter and a collimating lenssystem was transmitted through the input patterntransparency and the liquid crystal filter. Thetransmitted light is integrated spatially by a convexlens, and its intensity is measured by a photomultipliertube.

Results and Discussion

Optical processing of this type is available for inputpatterns written on a transparent sheet. As anexample, asterisks in Fig. 9 show experimental valuesfor the input pattern of a handwritten Chinese char-acter. The horizontal axis indicates the filter mode(up,), and the vertical axis shows normalized coefficientsof the transforms F(4,v)/F(O,0), where F(A,v) is ob-tained from the measured value Fm(,ii) as F(,u) =21'(,)-F(0,0). For comparison, the calculated

value obtained for the same input pattern by a digitalcomputer is also shown in Fig. 9. It is found that theexperimental value agrees satisfactorily with thecalculated one.

Concerning the liquid crystal filter, there are severalproblems in time response, contrast, and resolution.Time response of the dynamic scattering effect innematic materials depends mainly upon layer thicknessand temperature. Generally speaking, the decay timeof a liquid crystal is from three to ten times as long asthe rise time. It was experimentally confirmed thatthe decay time was remarkably shortened by decreasingthe thickness of the liquid crystal layer. Figure 10shows the delay time, rise time, and decay time,whose definitions are shown in the figure. Their depen-dence on temperature is not as great as is the depen-dence on thickness. It is possible to realize a filterwith a total response time of 3 msec. The contrastratio of the liquid crystal cell increases as the photo-electric tube is moved away from the filter. Non-conducting bands covering about 10% of the available

2226 APPLIED OPTICS / Vol. 11, No. 10 / October 1972

003

Ca

40 t, :delay time ( )

90%-- tr :rise time (td decay time o ) /

30 tt

Driving pulse

20

E-.

10

0I~~~~ °

00 2 4 6 8 10 12

Thickness C pm

Fig. 10. Response time.

filter area cause the contrast ratio to be decreased. Itcan be improved by making the insulating bandsnarrower. Nowadays, by etching techniques it ispossible to make insulating bands 0.05 mm in width.But an increased contrast ratio is not indispensable,because the error due to a finite contrast ratio can becompensated for by a simple calculation. Moreeffort should be spent to minimize the variation intransparency and in contrast ratio over the wholesurface. To increase the resolution, it is requiredthat the size of electrodes be decreased. The spreadof scattering to transparency in a liquid crystal medium

is negligible because it is of the order of the layerthickness. Considering the width of the insulatingbands, it is possible to make electrodes 0.5 mm wide.As the electrodes become narrower, the number ofarrays can be increased. In electronic calculationsincreasing the number of arrays enormously augmentsthe computing time. But in optical devices the pro-cessing time for one filter mode does not change.

ConclusionWe have demonstrated a new type of optical pro-

cessor based on the dynamic scattering effect in nematicliquid crystals. Experimental results indicate that itis possible to extract the properties of handwrittencharacters, in this case the coefficients of Walsh-Fourier transform, through the use of a liquid crystalspatial filter consisting of 8 X elements. A contrastratio of over 30 to 1 was obtained, and the variation intransparency was within 3%. Time response wasnot satisfactory but can be improved by selecting thematerial and adjusting the operating conditions, forexample, operating temperature and layer thickness.We believe that this type of processing can serve asone of the processors in an optical system.

The authors thank H. Matsumura of Nippon Cal-culating Machine Co. for supplying the liquid crystalmaterials. Thanks are also due to T. Shiho for hisassistance in the experiment.

References1. H. F. Harmuth, Transmission of Information by Orthogonal

Functions (Springer-Verlag, Berlin, 1969).2. J. A. Decker, Jr., Appl. Opt. 9, 1392 (1970).3. A. N. Mucciadri and E. E. Gose, IEEE Trans. EC-15, 257

(1966).4. W. K. Pratt et al., Proc. IEEE 57, 58 (1969).5. G. H. Heilmeier et al., Proc. IEEE 56, 1162 (1968).6. J. A. Van Raalte, Proc. IEEE 56, 2146 (1968).7. R. A. Soref, Appl. Opt. 9, 1323 (1970).

Richard G. Schulze of Honeywell Corporate ResearchCenter measures the spectral response of a gallium phos-phide photocell being developed under Air Force sponsor-ship as a star tracker.

October 1972 / Vol. 11, No. 10 / APPLIED OPTICS 2227