726 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 6 , JUNE 1981

Optical Properties of Electrophoretic Image Displays BEVERLY FITZHENRY-RITZ

Abstract-Those variables determining the optical properties of one type of ejectrophoreticj-mage display (EPID) are discussed in this paper. The working medium in the EPID with which this paper is concerned is a colloidal suspension of submicron pigment particles in a medium of matching specific gravity and contrasting color. The colloidal suspen- sion is contained between two transparent electrodes. By changing the polarity of the applied voltage, the charged pigment particles may be moved to one or the other electrode. The effects of suspension com- position, and the physical and chemical properties of the suspension components are discussed.

T I. INTRODUCTION

HIS PAPER discusses the work that has been done to optimize the optical properties of one type of electro-

phoretic image display (EPID) [ l ] , [2]. Information is dis- played in this EPID by the electrophoretic movement of charged submicron pigment particles that are colloidally dis- persed in a medium of matching specific gravity and con- trasting color which is contained between two transparent electrodes (see Fig. 1). By changing the polarity of the volt- age applied to the display electrodes, the pigment particles, which are negatively charged in this case, may be moved to either electrode. When the pigment is moved to the front electrode, the viewer sees the color of the pigment; when the pigment is moved to the rear electrode, the contrasting color of the dyed medium is seen.

For electrophoresis to be possible, the pigment must be charged. In this EPID suspension this is accomplished via re- action of the pigment with a polymeric hydrocarbon. The product of this reaction is a negatively charged species [3]. The polymer also serves a second function of providing a steric barrier around the suspended particles, thereby pre- venting them from coming into contact with the neighboring pigment particles.

-

11. OPTICAL MEASUREMENTS A . Definition of Terms

The amount of light backscattered from an EPID when the pigment is moved to the front electrode (brightness) is ex- pressed relative to the amount of light backscattered from an external standard. Two types of standards have been used for optical measurements, the first being a plaque of BaS04 [4], a white highly reflecting pigment. The second standard is

Manuscript received September 2, 1980; revised February 2, 1981. The author was with Philips Laboratories, Briarcliff Manor, NY

10510. She is now with the Raychem Corporation, Menlo Park, CA 94025.

E L E C T R O P H O R E T I C !MAGE D I S P L A Y C E L L C R O S S - S E C T I O N OF AN

I

I I

r

-TRANSPARENT ELECTRODE

-+ELECTRODE SEGMENT z SUBSTRATE

! I CHARGED P IGMENT PARTICLES i-) 1 DYED SUSPENDING

MEDIUM

---+-ELECTRODE SEGMENT

- SPACER / SEAL

-GLASS PLATE

Fig. 1. Cross section of EPID cell.

prepared from a slurry of Digrylide Yellow (DAY) pigment and a polymeric charging agent (both of which are used in these EPID suspensions) which are mixed in a minimal amount of solvent. This slurry is contained between the same trans- parent electrodes used in the EPID. Brightness values quoted in this paper are expressed relative to BaS04 unless otherwise noted.

The defining equation for brightness in this paper is

Light level of cell Light level of standard

% Brightness = x 100.

The contrast of an EPID is defined in this paper as

Light level-Dark level Dark level

Contrast =

In the work reported in this paper, suspensions had either of two color combinations: red/yellow or black/green. The red or black states are defined as “dark level” in the above equa- tion, and the yellow or green states are defined as “light level.” To avoid confusion, the color combinations are indicated throughout this paper. The contrast co,uld also be expressed

0018-9383/8l/0600-0726$00.75 0 1981 IEEE

FITZHENRY-RITZ: EPID OPTICAL PROPERTIES 1 2 1

W A V E L E N G T H Angs t roms)

Fig. 2. Spectral response of photopic filter.

in terms of the CIE color difference [SI,’ but this system is not used in this paper.

Two types of optical filters may be used in measuring the contrast and brightness. A photopic measurement utilizes a filter having a spectrum comparable to that of the human eye (see Fig. 2 ) . It has a full width at half maximum of 1000 A and a X,, of 5500 A. Narrow-band measurements may be made using a filter having a X,,, and bandwidth as desired.

The relative merits of photopic and narrow-band measure- ments depend upon experimental requirements. The photopic filter imitates the spectral range and sensitivity of the human eye. However, measurements with a photopic filter do not distinguish colors as the human eye does.

This is illustrated by measuring contrast between colors from opposite ends of the photopic range, that is, red and blue. The

The CEI color difference defines coordinates on a color space by

L* = 116 (Y /Yo) - 16 U* = so0 (x/x0)1/3 - ( z / z O V b* = 200 ( Y / Y ~ ) I / ~ - (z/z0)1/3

where X, Y , and Z are the tristimulus values of the color, and XO, YO, and 20 are the tristimulus values of the Bas04 standard normalized to YO = 100. The total color difference, E& between the color of the cell when the pigment is displaced to the front electrode and its color when the pigment is displaced to the rear electrode is defined as

U 0 SPECTRA P R I T C H A R D MODEL 1980 PHOTOMETER

@ G A M M A S C I E N T I F I C M O D E L 2 2 0 S T A N D A R D L A M P S O U R C E

@ C E L L H O L D E R

@ BLACK MATTE FINISH C A R D B O A R D B A C K G R O U N D

Fig. 3. Alignment of optical setup.

human eye sees a sharp contrast in these colors. However, when red and blue color plates were measured using a photopic filter, a contrast of 0.454 was measured. ‘This is caused by the fact that by selecting two colors such as A and B in Fig. 2 which correspond to nearly equal relative luminosities on the curve, the intensities of light reaching the photometer through a photopic filter are almost equal. The photometer measures differences in light intensity, not differences in wavelength.

A contrast of 3.83 is measured for points A and B on the curve in Fig. 2 with a narrow-band filter having a X,,, of 4800 A and a bandwidth of 50 8. The colors red and blue do not fall on equal relative luminosities in the spectral re- sponse curve of this filter, so the photometer measures a dif- ference in the colors.

In an EPID colors are used which do not have equal relative luminosities in the curve in Fig. 2 . Therefore, optical prop- erties reported in this paper are measured with a photopic filter unless otherwise noted.

B. Equipment and Procedure The experimental setup used for the optical measurements

reported in this paper is shown in Fig. 3 . The EPID cells and the calibration standards are held in a fixed position by a holder which maintains the front face of either cell or standard at an equal distance from the photometer. The cell or stan- dard is illuminated with a standard lamp source [6]. The backscattered light from the illuminated cell is measured using a Spectra Pritchard Model 1980 photometer. The cell is positioned so that the specular reflection off the face of the glass cell is directed away from the photometer. The cell is switched with a 1-Hz squarewave of the desired voltage for 15-20 s to insure the even distribution of the pigment in the cell. Subsequently the negatively charged pigment is moved to the front electrode (which is the anode in this case) by applying a dc voltage. Under constant illumination the light backscattered from the cell is measured. This is called the “light level,” and it has units of foot-lamberts.

The cell i s then switched again for 15-20 s to insure the even distribution of the pigment. Now the rear electrode is made the anode, and with the negatively charged pigment on that electrode, the light backscattered from the illuminated cell is measured. This is called the “dark level,” and it has units of foot-lamberts.

728 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 6 , JUNE 1981

TABLE I PRECISION OF OPTICAL MEASUREMENTS OF Bas04 STANDARD

I. Phsopic Light Level X = 6.10 fL

20 = 0.241 fL n = 5 0

11. NaEow-Band Filter Light Level X = 1.73 X lo-’ fL

n = l l 2a = 1.04 X 10-4 fL

The simple test cells which are measured and whose optical properties are reported in this paper are 1” X 1” cells in which the electrodes are spaced 50 pm apart. After measurement of each of these test cells is completed, each of the standards is measured under constant illumination.

C. Precision The precision with which the BaS04 standard has been

measured using various filters and the method described above is summarized in Table I. Considering the additional factors involved in measuring a cell, 20 of 3 percent-5 per- cent is expected for the values of contrast and brightness quoted in this paper.

111. SUSPENSIONS A. Physical Properties of Materials

I ) Chemical Structure: The organic pigment used in the EPID suspensions with which this paper is concerned is Digyl- ide yellow (DAY) [7]. This pigment was chosen because of its low specific gravity, insolubility in the solvent system, and its ability to be easily charged [3].

There are eight distinct chemical structures in this class of benzidine-type diazo pigments. Sometimes the optical prop- erties of these pigments are varied by addition of resin and/or BaS04. The effects of resin are discussed in Section 111-C.

The chemical structure of DAY presently used in EPID sus- pensions discussed in this paper is the AAOT (Pigment Yel- low 14, #21095), which results from the coupling of one mole of 3,3’-dichlorobenzidine with two moles of ace toge t -e - - toluidide. This structure was chosen because of the superior brightness of the pigment as compared to another form of DAY called the AAA form (Pigment Yellow 12, #210901), which results from the reaction of one mole of 3,3’-dichloro- benzidine with two moles ofaceto-acetanilide.

Table I1 shows the difference in contrast between the two structures. A suspension made with the AAOT structure has more than 23 times the contrast of the average AAA suspen- sion, all other components and concentrations in the sus- pension being kept constant. A similar trend is seen in the brightness. Note that the supplier, purity, and presence or absence of BaS04 do not have significant effects on the op- tical properties. It is possible that because of the change in chemical structure, the pigment may have a different refrac- tive index or particle size. The effects of changes in these two parameters are discussed later in this paper.

The chemical structure of the charging agent affects its ability to interact successfully with the pigment to provide

TABLE I1 OPTICAL PROPERTIES OF AAOT AND AAA FORMS OF DAY

Constrast Brightness

S truc ture Suppl i er Pur i ty Addi t ives L-D/D % Bas0 -

AAOT AC Extracted Bas0 /res in 4 .72 12 .7

AAA sun Crude Resin 0 .214 4 .87

AAA Inmont Crude Resin 0 .129 6 .68

7iAA Sun Extracted Resin 0 .163 8 .54

A m AC Crude BaSO / r e s i n 0 .306 6 .31

Suspension Composition

29 mg/rnl pigment

14 .5 mg/ml charging agent

7 mg/ml black dye

L = green

D = black

both charge and steric stability by the means used in these particular suspensions [3].

The chemical structure of the dye affects the extent to which it adsorbs on the surface of the pigment. This is dis- cussed in Section 111-D on Dye Adsorption.

A change in the chemical structure of the dye also changes its extinction coefficient. A lower extinction coefficient re- sults in a reduction in absorption of the light entering the cell and passing through the dye layer to the pigment layer when it is on the back electrode. This results in a higher dark level, hence poorer contrast.

2) Refractive Index: In general the best scattering of light by a particle occurs when there is the maximum possible dif- ference in the refractive indices of the particle and the sur- rounding medium. It is, therefore, desirable to have the highest possible refractive index for the pigment used in this display. However, other considerations in the EPID system discussed in this paper make this impractical.

First, the specific gravity of a compound usually increases with the refractive index. For example, the rutile form of TiOz, an excellent scatterer, has a refractive index of 2.903, but its specific gravity is 4.26. Because the EPID suspensions discussed in this paper set the requirement that the pigment must be density matched with the solvent to prevent gravita- tional settling, such pigments cannot be used in these suspen- sions unless their specific gravity is reduced by encapsulation in a low density polymer. This process has negative effects on the optical properties of the suspension, primarily due to the difficulty of encapsulating single particles.

The polarity of a surface also increases as the refractive in- dex increases. If used in an EPID, these polar pigments would be suspended in a dyed medium. The dyes are also polar be- cause of the inherent dipole moment associated with organic chromophores. Thus a strong van der Waals attractive force would exist between polar pigment and dye, and extensive adsorption of the dye on the surface of the pigment would occur. The effect of this phenomenon on the optical prop- erties of an EPID suspension is discussed in Section 111-D.

FITZHENRY-RITZ: EPID OPTICAL PROPERTIES 729

1 loo

B D l A R Y L l D E

Y E LLOW

- 90 - 80

- 70 s

-60 I) m

-50 r n m n

- 4 0 d 0 z

730

W A V E L E N G T H ( A n g s t r o m s )

Fig. 4. Reflectance spectra of DAY pigment and Bas04 standard.

W A V E L E N G T H (Angstroms)

Fig. 5 . Visible absorption spectra of dyes used in EPID suspensions.

One may get around these problems in several ways. In the suspensions discussed in this paper a compromise has been made by choosing a pigment with a low specific gravity and as high a refractive index as possible. To get a low specific grav- ity without microencapsulation, one is restricted' to organic pigments. It is possible, however, to achieve high refractive indices in organic compounds; The refractive index as mea- sured by the Becke line method [8]' is approximately 2.1 l in DAY [9] , .an unusually high figure. This 'value is not unex- pected, however, since it was measured near a strong absorp- tion region [ 101 3.

2This method had to be used because of the extremely small particle size, and it can give only approxeate values for materials of such high index of refraction.

3Explained by Kramers-Kronig relationship.

3) Visible Spectra: To achieve the best optical properties in an EPID one chooses a dye which has minimum transmis- sion where the pigment has maximum reflection, so that the dye can - hide the pigment when it is on the rear electrode. This ideal situation has been approached with the compounds used in the suspensions described herein. Fig. 4 shows that the reflectance of DAY starts to increase around 5000 A and peaks at 5850 A. In Fig. 5 the transmission spectra of three typical dyes used in these EPID suspensions are shown. In the range 5400-6200 A, where the pigment rkflectance is at a maximum, the dyes exhibit transition from minimum to maxi- mum transmission. The shift of the transition in the dye visible spectrum to a longer wavelength would be beneficial.

A computer model has been developed [ l 11 to predict the theoretical b.rightness of the EPID suspensions discussed in this paper. The model is based on theoretical scattering at a single

730 IEEE TRANSACTIONS ON ELECTRQN DEVICES, VOL. ED-28, NO. 6 , JUNE 1981

O : NON- ADSORBING DYE 0 2 0 : A D S O R B I N G D Y E

I I I I I I

0 2 4 6 8 10 12 14

DRY DYE CONCENTION (mg/ml)

Fig. 6. Theoretical brightness versus dye concentration.

wavelength by a single spherical particle of given refractive in- dex, specific gravity, and radius, suspended in a medium of given refractive index and specific gravity. The model extrap- olates from a single particle to a multiple particle system by making simplifying assumptions such as exclusion of interpar- ticle diffraction. The model includes the absorptivity of the dye in the system, making it possible to predict the effect of a change in the visible spectrum of the dye.

The differences at a single wavelength in the visible spectra of the three dyes shown in Fig. 5 produce very little differ- ence in the theoretical brightness of the suspensions, as shown in Fig. 6 [ l l ] .

B. Component Concentrations Changes in the concentrations of any of the three major

components of an EPID produce a change in the optical properties of the suspensions. The variations may be caused by either physical changes (such as simply a larger pigment mass backscattering more light) or by chemical changes (such as reactions between charging agents and pigment producing hydrodynamic instabilities which lead to poor packing of the pigment at the electrode [12]). Several experiments to test these effects are discussed in this section.

1) Variation of Dye Concentration: As the concentration of dye is increased, while the concentrations of all other components are kept constant, the contrast improves, but the brightness deteriorates (see Figs. 7 and 8). This is caused by the following: increase the dye concentration and there is more dye in the space between the cathode and the pigment

3 5 I 301

2 9 r n g / m l RESINATED P IGMENT 14.5rnglml CHARGING AGENT V A R I A B L E D Y E CONC.

- 2 . 5 - \ 0

J - I- lrl a LT +

2.0 -

5 1 5 - V

0 , 2 3 4 5 6 7 I I I I I I

DRY DYE CONCENTRATION (mg/ml)

Fig. 7. Photopic contrast versus dye concentration.

1 4 5 m g l m l CHARGING AGENT 29rnglrnl R E S I N A T E D P I G M E N T

VARIABLE DYE CONC.

I I I I I I 2 3 4 5 6 7

DRY DYE CONCENTRATION img / m l )

Fig. 8. Photopic brightness versus dye concentration.

layer on the anode (assuming the pigment is negatively charged, as it is in these suspensions), and the increased extinction by the dye produces a much lower dark level. However, at the same

FITZHENRY-RITZ: EPID OPTICAL PROPERTIES

7

I 0

0

0

0 0

2 9 r n g / m l R E S I N A T E D P I G M E N T 14.5 mglrnl CHARGING AGENT

0 : D Y E NS I X = D Y E NE 2

0

g o 0

X

X

0 I I I I I I I I I I 12 13 14 15 16 17 18 19

73 1

B R I G H T N E S S ( % B a S O , 1

Fig. 9. Photopic contrast versus brightness for variable dye concen- trations.

time, more dye in the interstitial spaces between the pigment particles reduces the brightness of the pigment by absorbing some of the light backscattered by the pigment. The result is a loss in brightness with increasing dye concentration.

The object is to get the highest possible value of both con- trast and brightness. Obviously only one point corresponds to the maximum possible values of both. However, a plot of contrast versus brightness shows the region of optimum values. Fig. 9 is such a graph using the same data as appear in Figs. 7 and 8. One can choose an optimum favoring either contrast or brightness, depending upon which one feels is more im- portant. This region of compromise would be those Points lying the greatest distance from both the ordinate and the abcissa. This corresponds to the area in the square in Fig. 9.

2 ) Variation of Pigment Concentration: The effects of a change in pigment concentration on the optical properties of the EPID suspensions discussed in this paper are fairly predictable. Figs. 10 and 11 show plots of contrast and brightness, respectively, versus pigment concentration for suspensions made with the DAY pigment discussed previously.

One anticipates that brightness should increase as the pig- ment concentration increases. But there should be a point of diminishing t-bturns where additional layers of pigment con- tribute less to the amount of backscattered light (a pigment layer thickness of approximately 10 pm). Fig. 11 supports this conclusion.

At low pigment concentrations the light level is poor, but the dark level is excellent. Since the method of calculating contrast emphasizes the dark level, contrast is fairly good. As the thickness of the pigment layer is increased, the light level improves. (This was seen in the brightness results in

Fig. 11.) However, the dark level deteriorates, because the thicker pigment layer is harder to hide. Consequently, con- trast is poorer, as shown in Fig. 10.

3) Variation of Charging Agent Concentration: A variation in the concentration of charging agent has an effect on the colloidal stability of the suspension. A less stable suspension exhibits agglomeration of the pigment particles and an uneven dispersion ,of these particles in the medium. If the pigment layer which is packed against the electrode is uneven, resulting in open spaces, the optical properties obviously suffer. Ag- glomeration also produces a change in the size of the scattering centers, an effect which is discussed in the following section.

C. Pigment Particle Size One important factor affecting the efficiency with which

particles scatter light is their size, Theory [13] predicts that the maximum scattering for a single spherical particle uccurs at particle radii around 1250-1500 A, depending on the wave- length of light interacting with the particle. Extrapolation to a multiple particle system such as an EPID is mathematically insoluble without simplifying assumptions such as spherical particles, and exclusion of interparticle diffraction. Even with these assumptions, a computer model [ l l ] produces theoretical results in close agreement with experimental ob- servations discussed below. Fig. 12 shows the computed theoretical monochromatic (5600 A) brightness of suspen- sions made with DAY pigment as a function of pigment par- ticle radius. Note that the maximum brightness is predicted for a particle radius of 1550 A.

The relationship between particle size and optical properties is dramatically illustrated in two forms of the AAOT structure

732 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 6 , JUNE 1981

PIGMENT C O N C E N T R A T I O N ( w t / W I )

Fig. 10. Photopic contrast versus pigment concentration.

1 9 . 5 1

16 0 I I 1 I I I 1.0 I 5 2.0 2.5 3.0 3 . 5

PIGMENT CONCENTRATION ( w l / w l % 1

Fig. 11. Photopic brightness versus pigment concentration.

P A R T I C L E RADIUS(Microns1

Fig. 12. Theoretical brightness of suspensions made with DAY pigment versus pigment particle size.

FITZHENRY-RITZ: EPID OPTICAL PROPERTIES 133

Fig. 13. TEM of resinated DAY pigment.

Fig. 14. TEM of nonresinated DAY pigment.

of DAY pigment. The standard form in which this material is an average radius of 1215 a. A TEM photograph of this resin- available is one in which the particles are coated with resin. ated pigment appears in Fig. 13. For nonresinated pigment This is achieved by quenching the final coupling reaction of the particles have an average radius of 445 a. A TEM of this the synthesis in a solution of natural or synthetic wood resin. pigment appears in Fig. 14. The improvement in optical properties which results is not due A comparison of the brightness of suspensions made with to the resin, but rather to the production of particles having resinated and nonresinated pigment appears in Table III.

734. IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 6 , JUNE 1981

TABLE 111

PIGMENT COMPARISON OF BRIGHTNESS FOR RESINATED AND NONRESINATED DAY

W e Conc. B r i g h t n e s s ( % BaS04)

Dye ( m s / m l ) Non-Res ina ted Res ina ted -

None 0 35.3% 40.9%

Sudan Red-4BA 2.7 10.2% 15.7%

O i l S o l u b l e Deep Black - B B 2.0 8.07% 11.6%

Sudan Black - BB 2.0 9.60% 15. 0%

Sudan Black - 7B 4.5 9.89% 13.9%

Suspension compos i t ion

29 mg/ml pigment

1 4 . 5 mg/ml c h a r g i n g a g e n t

With many different kinds of dye and even without dye, the resinated ,pigment is significantly brighter. It is noteworthy, however, that the differences predicted by the theoretical model (Fig. 12) are far greater than those actually observed. This illustrates the limitations on the theoretical model im- posed by the use of simplifying assumptions such as discussed earlier.

D. Dye Adsorption [14] The scattering power of the pigment when on the front

electrode can be adversely affected if some portion of the dye from the medium adsorbs onto the surface of the pigment. When this occurs, the amount of light backscattered from the pigment layer (brightness) is substantially lower than if all the dye remains in the solution. This effect is due to the reduced intensity of the internal reflections in the pigment caused by the absorption of the light by the dye adsorbed on the surface. The ability of the dye to absorb this light is substantially in- creased when the dye is in close proximity by being adsorbed on the pigment surface.

The amount of one type of dye adsorbed on the pigment surface has been determined quantitatively. This was ac- complished by allowing a known amount of the dye to inter- act with the pigment surface, removing the pigment con- trifugally, and optically determining the concentration of the dye remaining in solution using a laser. Results of this analysis appear in Fig. 15. It can be seen that the degree of adsorption increases as a function of dye concentration. The curve approaches a plateau, where the surface of the pigment is saturated, and no more dye can be adsorbed.

Primarily dye adsorption occurs due to van der Waals inter- actions between the polar sites on the dye and on the pigment surface [ 151. When the molecules approach each other closely enough, the n bonds’ in the two systems overlap. Since both the dye and the pigment have extended n systems and many polar sites, they are capable of extensive adsorption.

There is no way to eliminate this type of interaction, since the polar sites and extended n system are required for the color of the materials. However, the extent to which the interaction occurs can be reduced by setting up a steric barrier

I - E I I I

s i 0 J / I

z l / 1

D Y E C O N C E N T R A T I O N ( r n g / r n l )

Fig. 15. Extent of dye adsorption versus concentration.

- 4 0 -

a 0 3 5 -

-1

I- 30- u) 4 (L 5 2 5 - 0 V

. -

20 -

15-

NON-ADSORBINGDYE

5 -

1 1 1 2 4 6 8 I O 12 14 16 18

B R I G H T N E S S ( % B o S 0 4 )

Fig. 16. Photopic contrast versus brightness for adsorbing and non- adsorbing dyes.

which prevents close approach of the dye molecules to the pigment surface and thereby reduces the overlap of the n sys- tem. This steric barrier is usually a large hydrocarbon moiety. The presence of this large hydrocarbon group also makes the dye more soluble in the solvent, which is also a hydrocarbon in these suspensions discussed here, thereby making it en- ergetically less favorable for the dye to leave the solvent and adsorb on the pigment surface.

Suspensions prepared with these “nonadsorbing” dyes show considerable improvement in their optical properties. Fig. 16 shows that for a given brightness, a higher contrast results from the use of nonadsorbing dyes.

Using the computer model described earlier [ 11 1 , it is pos- sible to demonstrate that a reduction in adsorption is the primary reason for the improved optical properties. The computer model takes into account differences in the visible spectra (absorption coefficients CY) of the dyes. It predicts that, without adsorption, these differences in absorption coefficient cause only small differences in the predicted monochromatic brightness of suspensions (see Fig. 17). The

FITZHENRY-RITZ: EPID OPTICAL PROPERTIES

40r 010 = A D S O R B I N G D Y E S

OPEN SYMBOLS: THEORETICAL VALUES SOLID SYMBOLS : M E A S U R E D V A L U E S

0 ; N O N - A D S O R B I N G D Y E

30 -

25 - - > 0

I 20- v) v) W z 5 1 5 -

a ae

2 m K

1 I I 1 I 8 I 0 2 4 6 8 10 I 2

D R Y DYE C O N C E N T R A T I O N ( m g / m l )

Fig. 17. Theoretical and experimental photopic brightness curves.

experimental values show that suspensions deviate from the theoretical curve because of dye adsorbing on the surface of the pigment. It is noteworthy, however, that nonadsorbing dyes fall a considerable distance from the theoretical curve.

IV. SUMMARY AND CONCLUSIONS The effects of several parameters on the optical properties

of the EPID have been discussed in this paper. However, the optical properties are not the only consideration in the de- velopment of a display. Some applications require suspensions having very low charge carrier density. Suspensions having long lifetimes are required in other applications. The final use of the device determines the construction design of the

1 3 5

cell and the type of suspensions which will be used to fill it. Many possibilities exist, both for the cell construction and for the suspension composition. This paper has discussed one particular type of suspension among the multitude which are possible. Given the constraints of the final application on the cell and suspension structure, this paper has shown what sort of optical properties the resulting device can be expected to have.

ACKNOWLEDGMENT The author extends thanks to K. Ritz and W. Stacy for TEM

analyses, A. L. Dalisa (now of Exxon Enterprises) for laser data, to I. Monahan and R. White for excellent technical sup- port, and to A. L. Dalisa, R. Liebert, P. Murau, J. Nadan, R. Seymour (now of CUNY), B. Singer, W. Smith, and F. Zernike (now of Perkin-Elmer Corporation) for helpful technical discussions.

REFERENCES A. L. Dalisa, IEEE Trans. Electron Devices, vol. 24, p. 827,1977. J. Lewis, “Electrophoretic displays,” in Nonemissive Electrooptic Displays, A. R. Kmetz and F. K. von Willisen, Eds. New York: Plenum, 1975. B. Fitzhenry, Appl. Spectroscopy, vol. 33, p. 107, 1979. Available from Eastman-Kodak, Rochester, NY. H. Pauli,J. Opt. Soc. Amer., vol. 66, p. 866, 1976. Gamma Scientific Model #220. L. Shapiro, “Diarylide yellow and orange pigments,” in Pigment Handbook, T. C. Patton, Ed. New York: Wiley, 1973, Vol. I, pp. 555ff. N. H. Hartshorne and S . Stuart, Crystals and the Polarizing Microscope. New York: American Elsevier, 1970, pp. 259ff. Private communication, Ernest F. Fullam, Mar. 24, 1978. See, for example, M. Garbuny, Optical Physics. New York: Academic, 1965, pp. 288ff. Private communication, B. Singer, Philips Laboratories. P. Murau and B. Singer, J. Appl. Phys., vol. 49, p. 820,1978. V. C. Van der Hulst, Light Scattering by Small Particles. New York: Wiley, 1957. B. Fitzhenry, Appl. Opt., vol. 18, p. 3332, 1979. P. Rys and H. Zollinger, Fundamentals of the Chemistry and Ap- plication of Dyes. New York: Wiley-Interscience, 1972, pp. 160ff.