CA1041643A - Imaging system - Google Patents

Abstract

IMAGING SYSTEM

ABSTRACT OF THE DISCLOSURE
An imaging system wherein an imaging member com-prises a layer of an imaging material adjacent a substantially transparent member comprising a substantially transparent sub-strate having an index of refraction which is the same or ap-proximately the same as that of the imaging material and a substantially transparent conductive layer having an optical thickness which is very small compared to the wavelength(s) of the illumination used to view the imaging member and an index of refraction which is different than that of the substrate material and the imaging material. Images may be formed by the application of any of many suitable stimuli to the imaging material including, for example, thermal energy, electro-magnetic radiation, magnetic fields, electrical fields and physical forces such as shear.

Description

~L0~L lL6~3 BACKGROUND OF THE INVENTION
This invention relates generally to an imaging system and more particularly to an imaging system wherein an imaging member includes a substantially transparent member com-prising a substantially transparent substrate carrying a thinsubstantially transparent conductive layer.
There are known in the art many different types of imaging and display systems including, for example, electro-phoretic, electroluminescent, photoelectrophoretlc, ferroelectric, and liquid crystal. In such systems it is known to form images by the application of various stimuli to the imaging materials.
In one preferred embodiment a layer of imaging material is ar-ranged adjacent a substantially transparent electrode and images - are formed by steps including app:lying an electrical field across the imaging layer. In a well known embodiment a layer of imaging material is arranged between a pair of planar full frame electrodes one of which may include a photoconductive in-sulating layer. In many instances the images ~ormed in these types of imaging and display members are made up of areas which scatter light and those which do not scatter light. Depending, inter alia, upon the particular electrode system the images may be read out in transmission or reflection~ Moreover, the images may typically be viewed directly by an observer or may be used in other ways such as, ~or example, where the 25 image is projected onto means adapted to make a hard copy re- ~-production thereof. ~
Imaging and display members of this type are ~, capable of providing excellent images; however some difficulty may be encountered in reading out the images which may adversely affect device performance. For example, when the reflection readout mode is utilized the contrast of an image perceived by ', ~
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1(~4~L~43 a viewer i9 typically limited by spurious front surface re-flections. Such limitations may in some instances render direct xeadout virtually impossible and may dictate the use of image enhancement means such as polarizers which has hereto-S fore been the case with some liquid crys al imaging members.
~he polarizers typically exploit the birefringence of the liquid crystal materials and typically provide greatly in-creased contrast. However, the necessity of employing polarizers to read out the image is not a completely satisfactory expedient becaus~, inter alia, they typically require off-axis optics and thus complicate the imaging system. Additionally, polarizers generally cause relatively large light losses which is un-desirable. It would ~e highly desirable to minimize any loss in image contrast caused by light reflections when an imaging or di~play member is read out in transmission or reflection.

SUMMARY OF THE I~VE~TIO~
In accordance with one aspect of this invention there is provided an imaging method comprising the steps of ;~ (a) providing an imaging member comprising a layer of an lmaging material having an index of refraction ni `
between first and second electrodes, said first electrode com-prising a substantially transparent substrate having an index of refraction nS carrying a substantially transparent conductive-layer having an index of refraction nc, said conductive layer being adjacent said imaging material layer, wherein ni/ ns is in the range of from about 0.7 to about 1~3 and nc is diffPrent than ns or ni;
(b) forming an image in said imaging layer; and :' :

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10~6~3 (c) viewing said image with readout illumination :~
which passes through at least said first electrode, wherein the ?
optical path length of said readout illumination in said sub-stantially transparent conductive layer is about one-fourth of the shortest wavelength of said readout illumination or less In accordance with another aspect of this invention . : -there is provided an imaging member comprising first and second ;;:
electrodes arranged on opposite sides of a layer of imaging materiai having an index of refraction ni, said first electrode comprising a substantially transparent substrate having an index of refraction , ns carryi.ng a substantially transparent conductive Iayer having an~
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index of refraction nc and a thickness of about 200 angstroms or less, said conductive'layer being adjacent said imaging layer, wherein ni/nS is in the range of from about 0.7 to about 1~3 and, . .
wherein nc is dif~erent than ni or ns. 1.'! '~',',~' By way of supplemental explanation, in accordance with an aspect of the present invention there is provided ''.
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6~3 imaginy system comprising an imaging member including a layer of an imaging material adjacent a substantially transparent member comprising a substantially transparent substrate having an index of refraction which i5 the same or approximately the same as that of the imaging material carrying a substantially transparent conductive layer having an optical thickness which is very small compared to the wavelengthts) of illumination used to view the images formed in the member and an index of refraction which is greater or smaller than those of the imaging material and substrate material. The substantially trans-parent member is arranged with the conductive layer adjacent the layer of imaging material. When light is incident upon the sub-stantially transparent member, light reflections from the substrate-conductive layer interface and the conductive layer-imaging layer interface, respectively, partially or substantially completely extinguish each other.
The imaging layer may comprise any material the light scattering and/or the light absorption properties of which may be ;~; changed in imagewise configuration. By the term "light scaktering"
is meant any phenomenon involving the absorption and reemission of photons in approximately equal numbers. This definition is intended ; . .
to include, for example, specular reflection, phenomena involving wavelength conversion, conversion of state of polarization, etc.

By the term "absorption" is meant the absorption of incident photons and subsequent reemission of a substantially smaller number or essen-tially none, the energy being converted to some other form typically kinetic energy of the atoms, etc. This definition is intended to include, for example, wavelength dependent absorption coefficients such as are involved in colored images. The images formed may con-stitute differences in scattering properties, differences in absorp-tion properties or combinations thereof. Hence, it should be under-stood that the present invention may be used with virtually any types of images formed in any suitable imaging material.

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BRIEF DESCRIPTION OF THE DRAWI~GS

For a better understanding of the invention reference is made to the following detailed description of various pre-ferred embodiments thereof, taken in conjunction with the accompanying drawings wherein:
Fig. 1 is a partially schematic, cross-sectional view oE an embodiment of an imaging member according to the invention;
Fig. ~A illustrates the light reflection Erom an ~10 air-substrate intexface;
Fig. 2B illustrates the light reflections which occur when a conventional one layer anti-reElection coating is arranged on the surface of a substrate;
- ~ Fig. 3 is a partially schematic perspective view j~ -of an imaging member according to the invention wherein the desired image is defined by the shape oE an electrode;
Fig. 4 illustrates an imaging system wherein an imaging member i~ Lmaged by an electron beam address system; ~-~ ;~ Fig. 5 is an exploded isometric view of an -20~ imaging member including an X-Y electrical address system~
Fig. 6 i5 a partially schematic, cross-sectional view o~ an embodiment o an imaging member which is imaged by -a thermal image projection addres~ system; -Fig. 7 is a partially schematic, cross sectional view o~ a display member a¢cording to the invention;
Fig. 8 i~ a graphical plot showing percent reflec-tion from the substrate-conductive layer interface and the , conductive layer-imaging layer interface o~ a typical imaging ~ member according to the invention as a function of the conductive~

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Fig. 9 is a graphical plot showing percent reflec~
tance from the substrate-conductive layer interface and the conductive layer-imaging layer interface of a typical imaging member according to the invention as a function of the wave-length.

DESCRIPTIO~ OF T~IE PREFERRED EMBODI~ TS
Referring now to Fig. 1 there is seen in partially schematic, cross-sectional view an electrooptic imaging member, generally designated 10, wherein a substantially transparent substrate 12 and a relatively thin substantially transparent layer 14 comprise a substantially transparent electrode. For ease of discussion this type of electrode will be referred to hereinafter as the "anti-reflection1e~l~ectrode". Adjacent the anti-reflection electrode is a layer of imaging material which is adjacent oEtional photoconduct:ive insulating layer 18. The imaging member also includes a second substantially tranæparent electrcde comprising substantially transparent substrat~ 20 and a substantially transparent conductive layer 22. It should be noted that the bottom electrode could also comprise an anti-reflection electrode if it is so desired. The imaging memberpreferably also includes an optional conventional anti-ref~e~ on coating (not shown) on the free surface of substrate 12.
The imaging and display members provided according 'co the invention are preferably read out in reflection and acco~dingl~ imaging member 10 is illustrated as being read out in this mode. It should be noted however that transmissive readout may also be used. Moreover, although in the particularly preferred ~mbodiment shown in Fig. 1 the electrodes are both full frame electrodes it should be recognized that any electxode system capable of providing an imagewi~e el~ectrical field across ,r, ~ ,. , : .
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~',',; ' ' ' ' ' ' ' 4~1 imaging layer 16 or erasing an image formed by other means may be utilized as will be described in detail hereinafter.
An imaging member which is to be read out in reflection requires a mirror positioned behind the member or should include a highly reflecting surface. In the embodiment shown in Fig. 1 the photoconductive insulating layer 18 may provide the light reflecting surface since there are known many photoconductive materials which have a smooth surface when deposited on a flat surface thus giving them relatively high reflectance properties, e.g., from about 10~/o to about 50%.
Alternatively the bottom electrode may comprise a highly light reflecting material. The image formed in imaging member 10 is read out with illumination which propagates downwardly from above the member. It is seen that a number of specular re- ;
flections occur in such a reflective type imaging member. The - reflection from the air-substrate interface (Rl) can be re-duced greatly or substantially completely eliminated through the use of conventional commercially available anti-reflection coatings. For ease of discussion it will be assumed that a single layer anti-reflection coating is employed which makes the light reflected by thi: interface equal in amplitude and out of phase by 7r radians (or any odd multiple) thereby causing destructive interference to occur. The manner in which this result is effected by single layer anti-reflection coatings is -25 illustrated in Fig. 2. In Fig. 2A there is seen a layer of a typical substrate material 24. The reflection from the air-substrate interface (R) is given by the expression (n - n ) where nl is the index of refraction of air and n2 is the index 4~
of refraction of the substrate material. In the instance where substrate 24 compriseis gla s (n ~- 1.5) it is ~een that R equals approximately 4% since air has an index of refraction of 1.
Fig. 2B illustrates the e~bodiment whierein a single layer anti-reflection coating 26 is deposited over substrate 24. Anti-reflection coating 26 may typically comprise a dielsctric material such as, for example~ magnesium fluoride which typically has an index of refraction of a~out 1.38 at 550 nm~ It should be noted here that mu~ti-layered anti-reflection coatings are also available and may be used with similar results. As illustrated R' is the reflection from the air-anti-reflection coating inter-face and R" is the reflection from the anti~reflection coating-substrate interface. The phase difference (~ between R' and R"
is gi~en by ~ 2~o .
where QO is the optical path difference a~i A is the wavelength of ~he incident light. In this instance QO = 2nd where d is the thickness of anti-reflection coating 26 and n is its index of refraction. Thus, it is seen that ~f the thickness of layer 26 is equal to ~/4n then ~ = 7T radians and R" is effectively suppressed. Commercially available anti-reflection coatings typically reduce R" to about 0.25%. It is clear from equation (l) however that a single layer anti-reflection coating will ~unction optimally only over a narrow band of wavelengths.
Furthermore a dispersion in n will aggravate the situation.
Of course, multi-layered commercial anti-reflection coatings function optimally over a much wider range of wavelengths.
Returning now to Fig. 1 it will be recognized that just as Rl could be greatly suppressed or substantially completely ,- . , , , , : , a3 eliminated through the expedient of an anti-reflection coating then in a similar manner the reflection from the substrate-conductive layer interface (R2) could also be dealt with by depositing another anti-reflection coating between substrate 12 and conductive layer 14~ However, the reflection from the _ conductive layer 14-ima~ ng layer 16 interface (R3) ~ n not be suppressed in all cases~by the use of an anti-reflection coating `~
at that interface because for many of the phenomena exploited to form and/or erase images in electrooptic imaging members such as, for example, dynamic scattering in ~ematic liquid crystalline devices and the current-induced Grandjean to focal-cQnic texture transformation in optically negative li~uid crystalline devices, a flow of current between the conductive layer 14 and the imaging material layer 16 must be present in order for the de-sired effect ~Q occur. Of ccurse, a dielectric layer interposed between conductive layer 14 and imaging layer 16 would greatly retard current flow between these layer~ and wculd render it di~ficult to obtain the desired effect.
R2 and R3 are sub~tantially eliminated according to the imaging system of the present invention by exploiting the fact that when light reflects from an interface which goes from a lower to a higher index of refraction a 1~0 degree phæ e shift occurs whereas the converse is not true, that is, no phase shift occurs when the reflection is from an interface which goes from a higher to a lower index of refraction. Of course, if R2 and R3 are 180 degrees out of phase and are of e~ual amplitudes then destructive interference will occur. It can be seen from Fig. 1 that the typical ima~ing member of the invention illustrated therein has a desirable arrangement of indices of refraction for a 180 degree phase difference between R2 and R3. As noted previously substrate 12 typically comprise~

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ylass which has an index of refraction of about loS~ Conductive layer 14 typically comprises a layer of a metal oxide such as tin oxide or indium oxide which have indices of refraction of about 2Ø
In the case where layer 16 comprises liquid crystalline material it typically has an index of refraction of about 1.5. It should be noted here that the index of refraction of the material comprising conductive layer 14 is only required to be different than the indices of refraction of the materials comprising substrate 12 and imaging layer 16, it may be greater or smaller and in both cases R2and R3 will be about 180 degrees out of phase. For example an imaging member may have a substrate comprising strontium titanate which has an index of refraction of about 2.5, a tin oxide conductive layer and an imaging layer comprising ferroelectric material with an index of refraction of about 2.5.
Equal amplitudes for R2 and R3 ar~ obtained by making the ~ n (difference in the indices of refraction) between the mater-` ial comprising conductive layer 14 and the materials comprising the layers above and below it the same or substantially the same. Where the ~ n values are substantially different then R2 and R3 will have ~ different amplitude and wlll not completely cancel each other even though they may be perfectly out of phase. It will be understood that R2 and R3 will have equal amplitude when the materials compris-ing layers 12 and 16 have substantially equal indices of refraction because the amplitude for R2 is given by the expression (ns - nC)2

2 (ns ~ nC)2 (where n5 is the index of refraction of the substrate material and c is the index of refraction of the conductive layer material) and the amplitude for R3 ~assuming negligible absorption of light by the conductive layer) is given by the expression (nc ~ ni)2 tl - R2 (nC + ni) where ni i~ the index of refraction of the imaging material (the intensity of the light reaching the respective interfaces differs by the amplitude of R2). The condition for R2 = R3 is (ns ~ nc) rnC - ni)21 r (ns ~ nC) __ = 1 -(ns + nc)2 l (nc + ni)2 (ns ~ nc)2 Therefore, in order to obtain equal amplitude for R2 and R3 the indices of refraction for the materials comprising layers 12 and ' 16 should be slightly different. In practice the index of refrac-tion of the imaging material should typically be about 0~006 less ; than that of the substrate material in the case~where the conductive layer is substantially transparent. In the case where there is any appreciable absorption of light by the conductive layer the condi-tlons on the indices of refraction of the three materials must be adjusted accordingly in order to obtain R2 ~ R3.
~; Mence, it is particularly preferred that the mater-ials comprising layers~l2 and 16 have substantially the same indices of refraction. However it should be recognized that satisfactory :
results may be obtained according to the invention where there is ~; ~ ; some greater difference between the respective indices of refrac-tion. Generally the indices of refraction of the substrate mater-ial and the imaging material may have a relationship to each other in the`range of from about 0.7 to about 1.3 and preferably from :: ~
about 0.9 to about 1.1.
Since the phase of the light ray which produces R3 is changed somewhat in traversing through the member, that is, a phase lag is introduced into this ray due to the thickness of conductive layer 14 it is also necessary to make this , contribution to the phase difference between R2 and R3 very small in comparison to the reflection phase difference ~.

The necessary conditions to obtain the deqired result may be derived from equation (1). Since the phase lag introduced in R3 must be very small in comparison to the phase difference between R2 and R3 due to the reflection phase shift ~ then 27~
~T ~ ~ (2) In the case of conductive layex 14 the optical path length = 2 nd and therefore 1 >~ 4~d (3) Generally the optical pathlength of R3 in the conductive layer 14 should typically be less than 1/4 ~ and preferably about 1/lO ~ or smaller. An example of this requirement is that for : light in the visible region of the spectrum, since conductive layer 14 typically comprises a material havin~ an index of re-fraction of about 2.0, the thickness of conductive layer 14 must be substantially thinner than 600~. In the case of infrared light the-layer typically must be substantially thinner than 6000A for light of the order of 5 microns depend~n~,~ of course, on the details of the dispersion in the index of refraction.
For ultraviolet light, layer 14 typically must be substantially thinner than 200A for light in the 2000~ regime ayain dependent on the details of the dispersion in the index of refraction.
From the foregoing it will be appreciated that R2 and R3 are substantially eliminated according to the present invention by controlling the thickness of conductive layer 14 and by selecting materials which have the same or substantially the ~ame, indices of refraction for su~strate 12 and imaging layer 16. Since the phase shift upon reflection of R2 is independent of wavelength this advantageous technique is substantially in-., . . , , , - - ,, .

dependent of wavelength. E~uation (3) defines the criterion for essentially complete destructive interference. Since equation (3) is an ine~uality a~ opposed to an equality any value of d which ~atisfies the equation ful~ lls the desired S condition. Consequently even relatively large fluctuations in d within the same layer compatible with equation (3) will not compr3mise performance in the above-described mode. This characteristic îs completely unique since all other presently known commercial anti-xeflection films suffer in performance in proportion to variations in thickness. It will also be noted that the present in~ention makes it possible to make maximum !,use of the reflection at the imaging layer 16-photoconductor layer 18 interface (R4) which is about 15% when lay~r 16 com-prises an optically negative liquid crystalline material in the ; 15 Grandjean texture state and layer 18 comprises a typical phob~
l~~ conductive material having an i~dex of refraation of about 3Ø

3!; ~Hence, it is seen that R4 lar~ely determines the optical effi-ciency of such a device.
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~ Where conductive layer 16 comprises an approximately 3 ~
lOO~thick layer ~f indium oxide the layer can have a surface resistivity of about 1 kohm/s~ua~e. Generally layer 16 should typically have sufficient lateral conductivity so as not to significantly compromise device operation. Of course, the re~uLsite lateral conductivity in any particular instance will ; 25 be dependent, inter alia, upon the type of imaging material uti.lized in layer 16.
In the embodiment illustrated in Fig~ 1 substrate 12 may comprise any suitable substantially transparent material such as, for e~ample, glass or clear plastic materials. Con-ductive layer 14 may comprise any suitable conductive materialwhich i5 at least substantially transparent to the readout ''''~ ' `, `. ;' ' ', . ' '` :

illumination in the la~er thickness range described above.
Typical suitable transparent conductive layers include con-tinuously conductive coatings of conductors such as indium, tin oxide, thin layers of tin, aluminum, chromium or other suitable conductors. These substantially transparent conductive coatings are typically evaporated or sputtered onto the more insulating transparent support materialO
The bottom electrode may comprise any suitable material and may be opaque or transparent. Where a substantially transparent electrode is employed support layer 20 and conductive layer 22 can be any of the materials described above. NESA
glass, a tin oxide coated glass manufactured by the Pittsburgh Plate Glass Company is a commercially available example of a typical transparent conductive layer coated over a transparent 15~ substrate. It is again noted that the bottom electrode in the imaging member illustrated in Fig. 1 may also be an anti-reflection ~ electrode.
; Imaging layer 16 may comprise any of many different lmaging materials. Generally, imaging layer 16 may comprise any material wherein there can be ~ormed an image which com-~ prises differences in the Iight scattering and/or light absorbing ; properties of the material. Various liquid crystalline materials may be used in layer 16 including any optically negative liquid crystalline materials or compositions, nematic liquid crystal-line materials including the structural arrangement commonly referred to "twisted nematics" and smectic liquid crystalline materials. It should be noted that optically negative liquid crystalline materials or compositions include~, for example, cholesteric liquid crystalline materials, mixtures of choles-teric and nematic liquid crystalline materials, mixtures ofcholesteric and smectic liquid crystalline materials, ~ Jrr~de I~R,rk :' ~,.,,, ,.. , .. , .. , .. ,, , ; ,........ :
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mix-tures of nematic liquid crystalline material and suitable optically active non-mesomorphic materials, mixtures of choles-teric liquid crystalline materials and suitable optically active non-mesomorphic materials, etc. Typical liquid crystal imaging ~ ;
systems which are capable of forming images with the desired characteristics and which therefore may be utilized in the advantageous system of the present invention include, for -~
example; texture transfoxmations in optically negative liquid crystalline materials such as from the Grandjean to the focal~
conic (see, for example, U. S. Patent 3,642,348, J. Wysocki et al, issued February 15, 1972) or from the focal-conic to ~he ~;
Grandjean (see, for example, U.S. Patent 3,680,950, W. Haas et al, issued August 1, 1972); the optically negative to optically positive phase transition in optically negative liquid crystal~
line materials which are initially in a light scattering ~
condition (see, for example, U. S. Patent 3,652,148, J. Wysocki ~:
: et al, issued August 28, 1972); texture transformations in smectic liquid crystalline materials: dynamic scattering in nematic liquid crystalline materials; dynamic scattering in 11 2Q nematic liquid crystalline materials initially in the homo~
geneous texture; dynamic scattering in initially homeotropically ~:
aligned nematic liquid crystalline materials includi~g those where the homoeotropic alignment is caused by surface treatment with materials such as lecithin applied to the surface o~ a substrate upon which a layer of nematic liquid crystalline material is applied (see, for example, U. S. Patent 3,597,043, ":
J. Dryer, issued August 3, 1971) and those where the homeotropic : :
alignment is fostered by additives which cause the composition ~ :
to adopt the homeotropically aligned state when a thin film of 3~ the composition is deposited on a substrate (see, for example, U. S. Patent 3,803,050, W. Haas et al, issued April 19, 1974;
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the structural ~rrangement known as twisted nematics (see Applied Phys. Letters, Vol. 18, No. 4, Feb. 15, 1971, pp.
126-128), etc. -It should be noted here that although in many ~ -S of the preferred embodiments of the invention the images are formed by applying an imagewise electrical field across the imaging layer, images which exhibit the desired characteristic may be formed by the application of other stimuli to the imaging layer. Therefore, it should be recognized that the advantageous technique for minimizing loss of image contrast ~ ~-when an imaged member is read out is essentially independent of how the image has been created. For example, an imaging member comprising a layer of an optically negative liquid crystalline material intially provided in the Grandjean ~clear) texture state may be thermally imaged by imagewise applying thermal enexgy such as from a laser so as to heat ~-image portions of the imaging layer above the isotropic trans- ~-ition temperature of the material and then allowed to cool to ~;
some temperature in the mesomorphic temperature range of the material whereby the image areas typically assume the focal-conic (light scattering) textures ~see, for example, W. Haas et al V. S. Patents 3,666,947 and 3,666,948 each issued May 30, 1972).
The image may then be erased by applying an electrical field to --;.
place the imaging layer uniformly in the Grandjean texture state.
Imaging may also be effected through the use of various other stimuli such as, for example, shear, electromagnetic radiation and magnetic fields as is known in the liquid crystal art.

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... , ~. '''"'.' : '' . :' '' ~ , Imaging layer 16 may comprise an electrophoretic suspension comprising imaging particle.s in an electrically insu-lating liquid which may be a different color than the particles.
Such imaging layers could be used, for example, in an embodiment wherein a photoconductive layer is present in a display device or in a display device which includes an electrical X-Y matrix address system. Photoelectrophoretic imaging suspensions compris-ing electrically photosensitive pigment particles in an electric-ally insulating liquid may be used in layer 16 (see, for example, U. S. Patent 3,6~7,256, M. Silverberg, issued September 21, 1971).
Another type of photoelectrophoretic imaging suspension comprises electrically photosensitive pigment particles and inert particles in an electrically insulating liquid (see, for example, U. S. ~- ;
Patent 3,772,013, J. Wells, issued Novemeber 13, 1973). Where imaging layer 16 comprises a photoelectrophoretic imaging suspen-sion typically the device is exposed to imagewise activating electromagnetic radiation to which the photosensitive particles are responsive and hence a photoconductive layer is not required.
The electrically photosensitive particles may be the same or different colors and the electrically insulating liquid-may be a diferent color than some or all of the imaging particles. Hence, monochromatic or polychromatic images may be formed and the images may be on a clear background or on a differently colored background, etc. Another imaging system which may be used to form images which may be used in the present imaging system is described in U. Sn Patent 3,850,627, J. Wells et al, issued November 26,1974. The imaging system described in aforementioned patent 3,850,627 can employ an imaging member such as is illus-trated in Fig. 1 wherein the photoconductive layer has a thick-ness of up to about 5 microns and the imaging material comprisesa suspension of finely divided particles in an electrically insul-ating liquid. In operation an electrical ield is applied across , ~0~ 3 the imaging layer and the photoconductive layer is exposed to an imagewise pattern of activating electromagnetic radiation. ~-Imaging layer 16 may comprise a ferroelectric material, electroluminescent material, electrochemical material ~ or an electrofluorescent dye solution such as is disclosed in ; _EE Transactions on Electron Devices, Vol. ED-20, No. 11, November, 1973, pp. 1028-32. The thickness of layer 16 is depen-dent, inter alia, on the type of material which forms the layer.
Generally, layer 16 has a thickness in the range of from about 0.5 micron to about 100 microns or more~ In a preferred embodi-ment of the invention wherein the imaging layer comprises optical- ^-ly negative liquid crystalline material and images are formed by the texture transformation system as is described in U. S. Pat~nt 3,642,348, J. Wysocki et al, issued February 15, 1972, the imaging layer is optimally about 10 microns in thickness. Many ~
materials of the types useful in imaging layer 16 are known in `-~ ;
the art and a broad variety of these materials are listed in the -patents and articles referenced above. Accordingly any extensive discussion of materials is not required here. -Any typical suitable photoconductive insulating material may b~ used for optional layer 18. Typical suitable . ~ , photconductive insulating materials include, for example, selen-i ium, poly-n-vinylcarbazole (PV~), poly-n-vinylcarbazole doped j with sensitizers such as Brilliant green dye and 2 r 4,7-trinitro-9-fluorenone (TNF); cadmium sulfide, cadmium selenide, zinc oxide, anthracene and tellurium. Additionally, photoconductive layer 16 may comprise a finely ground photoconductive insulating material dispersed in a high resistance electrical binder such as is dis-closed in U. S. Patent 3,121,006, Middleton et al, issued ~-February 1964 or an inorganic photoconductive insulating material such as is disclosed in U. S. Patent 3,121,007, Middleton et al, issued February 1964 or an organic photoconductor such as phtha-locyanine in a binder. Generally, any photoconductive insulating A ~ 18A~

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~04~ 3 material or composition may be used for layer 18.
The thickness of the photoconductive layer 18 is typically in the range from about 0.1 microns to about 200 microns or more, the thickness of the layer in any particular instance depends, :

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~ ter alia, largely upon the spatial frequency of the information to be recorded and upon the sensitivity to the imaging radiation. Photo-conductive layer 18 may be formed on conductive layer 22 by any of the many methods which are well known to those skilled in the art includ-ing, for example, vacuum evaporation, dip coating from a solution, etc.
In operation of the imaging member 10 an electrical field isestablished across imaging layer 16 and photoconductive layer 18 by means of voltage applied from power source 21 to opposite ends of which are connected conductive layers 14 and 22 and the member is exposed to an imagewise pattern of activating radiation to which the photocon-ductive material which comprises layer 18 is sensitive thereby forming an image having the above-described characteristics. The imagewise pattern thus created across imaging layer 16 may form an image therein ` comprising clear transparent areas and light scattering areas. It should be noted here that imaging layer 16 may initially uniformly appear clear and transparent in which case light scattering image areas may be created; or the layer may initially uniformly appear light scattering and clear transparent image areas may be created. Thus, it is apparent that the images formed in layer 16 may comprise clear, transparent image areas on a light scattering background or light scattering image areas on a clear transparent background. Moreover, the images may be of one color on a differently colored bacXground.
power source 21 may be A.C., D.C. or combinations thereof. It should be also noted that although the imagewise illumination is shown being projected upon imaging member 10 from the bottom it may be projected thereon from above. However, where photoconductive layer 18 is present in the member the imagewise illumination must be able to reach this layer. Accordingly, in the embodiment illustrated in Fig.l if exposure were effected from above, layer 16 would have to be optically trans-30 parent to the imagewise illumination. The image formed in imagingmember 10 may be read out with ambient light or a separate readout light source (not shown) may be provided.
In ~ig. 3 there is shown an embodiment of an imaging member wherein the de~ired image is defined by the shape of an electrode and consequently by the shape of the corresponding electrical field. The imaging member comprises anti-reflection electrode comprising transparent substrate 12 and substantially transparent conductive layer 14. Sub~tantially transparent substrate 20 is separated from the anti-reflection electrode by spacer gasket 28 having a void area 30 filled with imaging material and comprising substantially the entire area of spacer gasket 28~ The desired image is defined by the shape of the substantially transparent conductive coating 32 which is affixed to the inner surface of transparent substrate 20 only in the desired image configuration. It is noted that the anti-reflection electrode ~om~rises transparent su~strate 12 with substantially transparent conductive coating 14 upon the entire inner surface of the electrode. A very thin, or substant~ lly transpæ ent, conductor 34 is necessary in this embodiment to electrically connect the elsctrode in the desired image configuration to the external circuit which comprises potential source 21. In operation thi~ embodiment will produce an electrical field only in areas where there are parallel elactrodes, i.e., between the electrode in the desired image configuration and the anti-reflection electrode. The imagewise electrode may have an opague substrate where it is desired to observe the imaged me~ber in reflection from the anti reflection electrode side of the member or a mirror could be positioned ad~acent to the outer surface of substrate 20 o~ the imagewise electrode. Again, it is noted that the imaged member may be read out with ambient light or ~y means o~ a readout light source.
~he spacer member 28 in Fig. 3 which separated the electrodes and con~ ins the imaging layer between the electrodes is typically chemically inert, transparent, sub-stantially insulating and has appropriate dielectric character-istics. Materials suitable for use as insulating spacers in---~0--,....... . ... .... . .
.. .

6~3 clude cellulose acetate, cellulose triacetate, callulose acetate butyrate, polyurethane elastomers, polyethylene, polypropylene, polyesters, polystyrene, polycarbonates, poly-vinylfluoride, polytetrafluoroethylene, polyethylene terephthal ate and mixtures thereof.
In Fig. 4 another preferred embodiment of the advantageous imaging system is illustrated wherein an electron beam address system is provided for the generation of an imagewise field across the imaging layer. In Fig. 4 the electron beam address system is within vacuum tube 35 and the address system itself comprises electron gun 36, accelerator-38 and deflecto~ 40 wh~ch are provided with electrical leads throu~h vacuum tube 35 so that suita~le electrical circuitry ma~ be connected therewith to operate the electron beam imaging system. The imaging member in conjunction with the electron beam address system comprises an anti-reflection electrode comprising transparent substrate 12 and substantially transparent conductive coating 14 (which is grounded) affixed thereto.
Light reflecting electrically insulating layer 42 is positioned over imaging layer 16. The impingement of electrons from electron gun 36 upon layer 42 creates a momentary field when taken in conjunction with grounded conducti~e layer 14. The momentary field across imaging layer 16 creates the image.
Another çmbodiment of the electron beam address system is a configuration wherein the electric field created by the electron beam is transmitted through a thin layer which is substantially insulating in the lateral directions parallel with the plane of the layer but is substantially conductive in the direction perpendicular to the plane of the layer (i.e., a pin tube). This embodiment permits the imaging layer and anti-reflection electrode to be outside the vacuum sys~ m.

,. , . , , , , , "

~i~4~6~L3 For transient di~plays using this embodiment of the electron beam address system, the ~ace plate is substantially insulating in all directions.
It will also be appreciated that the electron beam address system may be used in conjunction with an elec-troded liquid crystalline imaging member wherein the sum of the fields created by the electrode system and the electron beam address system is sufficient to create a total field of strength sufficient to shift the pitch of the liquid crystalline material the necessary amount. Similarly, any suitable com-bination of address syst~ms including any of the ~her s~stems disclosed herein and others may be combined in the same manner to produce the desired result.
; The imaged member is shown being viewed in reflection by an observsr 44 with illumination provided by light source 46. It should be noted that in the imaging system illustrated in Fig. 4 the imaged member may be read out in transmission.
This embodiment would re~uire a readout light source located inside vacuum tube 35. However the in-tu~e source of illumination .,~
would have to be so placed as not to interfere with the electron beam which created the image on the face of the tube. Moreover, layer 42 would have to be transparent to the readout illumination.
Alternatively, where imaging layer 16 is self-supporting then layer 42 is not needed. Imaging layers comprising liquid crystalline material or electroluminescent material are par-t~ ularly preferred for use in the imaging system shown in Fig. 4.
In F.ig. 5 an electrical X-Y matrix address system su~table for imaging an imaging member provided according to the invention is illustrated in exploded isometric view. The imaging layer is placed in void area 30 within the transparent , ,, .
i~ , ~ , , , L69~3 and substantially insulating spacer gasket 28. The imaging layer and the spacer are sandwi~ ed between a pair of trans-parent substrates 12 upon which strips of substankially trans-parent conductive material 48 are coated. The substantially transparent electrodes are oriented so that conductive strips 48a and 48b cross each other in an X-Y matrix or grid. It should be noted that one or both of the electrodes ma~ be anti-reflection electrodes, that is, conductive strips 48a and/or 48b may be very thin according to the invention. Each conductive strip in each set of paralle~l strips 48a and 48b is electrically connected to a c rcuit sytem 50 which is suitable for sequential operation. Through selection system 50 and external circuit 52 which includes a source of potential 21 an electrical field suitable to effect imaging can be created across selected points or a selected sequence of points.
It will be understood that substantially transparent conductive strips may vary in width from a very fine, wire-like configuration to any desired strip width. In addition one substrate may be opaque where the imaging system is to be observed in reflection.
The imaging member shown in Fig~ 5 lends itself particularly well to transmissive readout and in many instances i~ ~s préfeEred to utilize this type of readout mode with this type of an imaging memberO
In Fig. 6 an imaging member is shown being imaged by a thermal image projection address system. This imaging system may be used where the imaging material comprise~ liguid crystalline material for example. Here the imaging member comprises anti-reflection electrode comprising transparent substrate 12 and substantially transparent conductive coating 14 and a second electrode comprising transparent substrate 20 and substantially transparent conductive coating 22. Of ." , . . . .. . . . . . ............................ ..
.
,~. . , .
",, course, the second electrode may also be an anti-reflection electrode where it is so desired. The electrodes are separated by spacer member 28 which enclose~ a layer of liquid crystalline imaging material, for example, within said area 30. In this S embodiment of the inventive system a source of a thermal image 54, shown here as a heat source in the desired image configu-ration, is shown in posit~ion with conventional means 56 for focu ing and projecting a thermal and optical image~ The thermal image 54 appears in the liquid crystalline film in the areas where the liquid cxystalline material is heated into the temperature range required for the particular effect keing ex-ploited such as, for example, the optically negative~optically positive phase transition or the Grandjean to focal-conic texture transformation, while at the same time the imaging memher is biased by external circuit 21 so that the field across the liquid crystalline film is sufficient to cause the desired effect when the imaged areas of the film reach the temperature at which the effect will occur. It is again noted that the image areas may be transparent and clear and the background-~areas light scattering or the reverse may be the case~ The imaged member may be read out in reflection in which case ~he rear electrode comprises preferably a highly light reflecting material; or it may be read out in transmi~sion.
It will be understood that the thermal imaging system may be used without projection means 56 if the thermal im~ e is suf-ficiently defined and very close to the imaging member ~ self.
It is again noted that some imaging efects may be carried out with heat alone wihhout the necessity for biasing the member.
Fox example, as previously discussed, it is known that the Grandjean to focal-conic texture transformation in optically , .. . . . .

L643 `;
negative liquid cry~stalline materials may be caused by the application of thermal energy.
Fig. 7 illustrates another embodiment of a display~
cell which may be utilized according to the system of the inven~
tion. The display cell comprises an anti-reflection electrode comprising transparent substrate 12 and substantially trans-parent conductive coating 14 and a second electrode comprising transparent substrate 20 and substantially transparent conductive ~;
coating 22. Imaging layer 16 is contained between optional spacer member 28 where necessary. The display cell shown in Fig. 7 is particularly well adapted for use with photoelectrophoretic ~ ;~
suspensions comprising electrically photosensitive pigment `~
particles in an electrically insulating liquid such as are disclosed is U. S. Patents 3,384,565, V. Tulagin, issued May 21, 19~8 and 3,384,566, H. Clark, issued May 21, 1968~ The photoelectrophoretic display cell may be used to provide monochromatic or polychromatic displays depending upon the imaging suspension. The insulating liquid may be the same or a different color than some or all of the imaging particles.
~o In operation an imagewise pattern of activating electromagnetic ~-radiation is projected upon the display cell and an electrical : i. :
field is applied across the suspension layer. Depending, inter alia, upon the polarity of the applied potential the pigment particles will be deposited upon the surface of at least~one electrode in imagewise configuration. In a preferred mode of operation at least a substantlal portion of the pigment particles are initially caused to form a substantially unïform layer on the surface of one of the electrodes, subsequently imagewise activating radiation is projected on the cell and an electrical field is applied across the suspension layer thereby causing the ; imaging particles to be repelled in the light struck areas from : .

;. :.: . , .',, ;, ;, . , ; ' ,", ' ............... - : ' ,: . , - ;. . . . . .. .

6~;~
the surface of the electrode to which they are initially attracted and become attached to the surface of the other electrodes. V.S. Patent 3,772,013, J. B. Wells, issued November 13, 1973 discloses photoelectrophoretic imaging suspensions including electrically photosensitive pi~ment particles and inert particles in an electrically insulating liquid and these types of suspensions may also be used in a display cell.
The advantageous results provided by the anti-reflection electrode according to the present invention are illustrated by experiments conducted with a preferred embodiment of an imaging member. A layer of an approximately 80% by ~-weight N-(p-methoxybenzylidene)-p-butylaniline : 20% choles~eryl oleyl carbonate optically negative liquid crystalline compo-sition having an index of refraction of about 1.5 was formed ~`
on a glass substrate having an index of refraction of about 1.5.
I~ contact with the surface of the liquid crystal layer was placed the indium oxide layer of an electrode comprising a conductive layer of indium oxide (n = 2.0) residing on a glass substrate ~n = 1.5). 4880g light from a Spectra-Physics Argon Ion Laser was directed upon the member and a photodetector was located in a position to intercept a beam of reflected light which included reflections from the glass-indium oxide interface and the indium oxide-liquid crystal layer interface. Measure-ments were made with various thicknesses of indium oxide. In the great majority of prior art electrodes of this type the conductive coating thickness is about 2000~; however, some ~-commercially available electrodes have conductive coating thicknesses of about 400R. The thicknesses of the indium oxide layers were measured by interferometry. Fig. 8 illustrates the percent reflectance of the combined reflections from the glass-indium oxide interface and the indium oxide-liquid crystal layer interface as a function of the indium oxide I6~3 - ~7_ layer thic~ness. It is seen that these reflections are signifi-cantly decreased when the indium oxide layer thickness is in the vicinity of 200A or below.
Measurements were made of efficiency and contrast ratio with imaging members of the type described with respect to Fig.8, both with and without conventional anti-reflection coatings (AR).
Measurements were made with the imaging layer uniformly in the clear state and then uniformly in th.e light scattering state. The con-trast ratio values shown in Table I represent the intensity of the light reflected by the imaging material in the clear state relative to the intensity of the ligh.t reflected by the imaging material in the light scattering state. The efficiency values shown in Table I represent the intensity of the light reflected by the imaging material in the clear state relative to the intensity of the incident light. The prior art "thick transparent electrode refers to a conductive indium oxide coating thickness of about 400A
on a glass substrate and the "thin" transparent electrode refers to a conductive indium oxide coating of about 150A on a glass sub-strate. The conventional anti-reflection coatings were multiple layer coatings available from Optical Coating Laboratories, Inc., Santa RoSa, California.
T~BLE I
Efficiency Contrast Ratio A) No AR coating on 16%2.5 : 1 glass and "thick"
transparent electrode.

~) AR coating on 12% 4 : 1 glass and "thick"
transparent electrode.

C) No AR coating on 14% 2.7 : 1 glass and "thin"
transparent electrode.
D) AR coating on 10% 10 : 1 glass and "thin"
transparent electrode.

It is seen that a slight improvement in contrast ratio over a prior art electrode is obtained with the anti- ' reflection elec~rode of the invention when neither is treated ,. . . .. . .. . . . .

", .. . . .

~Q~ 3 with a conventional anti-reflection coating. This is 80 because the reflection from the air-substrate interface wh~h is attacked by an anti-reflection coating is large compared to the combined reflections from the substrate-conductive coating interface and the conductive coatin~-imaging layer interface which are attacked by the anti-reflection electrode according to the invention. However, when a conventional anti-reflection coating is used with both the prior art electrode and the anti-reflection electrode it is seen that a significant increase in ~ contrast ratio is obtained. It shculd be recognized that the efficiency of the members, whr h i~ a measure of how efficiently the readout illumination is used and in the reflection readout mode consequently constitutes approxima~ ly the percentage of all the light reflected, becomes smaller in approximately direct proportion to the percentage of the light reflections extinguished by the conventional anti-reflection coating and/or the anti~
reflection electrode.
Fig. 9 illustrates percent reflectance for the combined reflections from the glass-indium oxide layer inter-face and the indium oxide layer-liquid crystal layer inter~ace as a function of wavelength. The indium oxide layer in the member used in this experiment was about 150~. The results shown in Figs. 8 and 9 are essentially independent of angle of incidence since re~raction limits the angle of incidence in the indium oxide layer to about 30.
Although the invention has been described with relation to various preferred embodiments thereof it is not intended to be ~imited thereto but rather those skilled in the art will recognize that variations and modifications may be made therein which are within the spirit of the invention and the scope of the claims.

, .
,

Claims (84)

WHAT IS CLAIMED IS:

1. An imaging method comprising the steps of (a) providing an imaging member comprising a layer of an imaging material having an index of refraction ni between first and second electrodes, said first electrode com-prising a substantially transparent substrate having an index of refraction ns carrying a substantially transparent conductive layer having an index of refraction nc, said conductive layer being adjacent said imaging material layer, wherein ni/ ns is in the range of from about 0.7 to about 1.3 and nc is different than ns or ni;
(b) forming an image in said imaging layer; and (c) viewing said image with readout illumination which passes through at least said first electrode, wherein the optical path length of said readout illumination in said sub-stantially transparent conductive layer is about one-fourth of the shortest wavelength of said readout illumination or less.

2. The method as defined in Claim 1 wherein ni/ns is in the range of from about 0.9 to about 1.1.

3. The method as defined in Claim 1 wherein ni and ns are substantially equal.

4. The method as defined in Claim 3 wherein ni is about 0.006 less than ns.

5. The method as defined in Claim 3 wherein the optical pathlength of said readout illumination in said sub-stantially transparent conductive layer is about one tenth of the shortest wavelength of said readout illumination or less.

6. The method as defined in Claim 5 wherein said imaging member further includes an anti-reflection coating on the surface of said first electrode substrate opposite that carrying said substantially transparent conductive layer.

7. The method as defined in Claim 6 wherein said anti-reflection coating is a multiple layer coating.

8. The method as defined in Claim 7 wherein said second electrode comprises opaque material.

9. The method as defined in Claim 7 wherein said second electrode is substantially transparent.

10. The method as defined in Claim 7 wherein said imaging layer comprises a suspension of electrically photo-sensitive pigment particles and inert particles in an electri-cally insulating liquid and step (b) comprises establishing an electrical field between said electrodes and exposing said suspension to an imagewise pattern of activating electromagnetic radiation whereby an image comprising substantially completely inert particles is formed at one of said electrodes.

11. The method as defined in Claim 10 wherein said electrically insulating liquid has a different color than said inert particles.

12. The method as defined in Claim 7 wherein said imaging layer comprises a suspension of electrically photo-sensitive pigment particles in an electrically insulating liquid and step (b) comprises establishing an electrical field between said electrodes and exposing said suspension to an imagewise pattern of activating electromagnetic radiation.

13. The method as defined in Claim 7 wherein said imaging layer comprises a suspension of at least two differently colored electrically photosensitive pigment particles in an electrically insulating liquid and step (b) comprises establishing an electrical field between said electrodes and exposing said suspension to an imagewise pattern of activating electromagnetic radiation.

14. The method as defined in Claim 7 wherein said second electrode includes a photoconductive insulating layer adjacent said imaging material layer, and wherein nc is greater than either of ns and ni.

15. The method as defined in Claim 14 wherein said imaging member is viewed in reflection.

16. The method as defined in Claim 15 wherein said imaging material comprises liquid crystalline material.

17. The method as defined in Claim 16 wherein said imaging layer has a thickness in the range of from about 0.5 micron to about 100 microns.

18. The method as defined in Claim 17 wherein said imaging material comprises nematic liquid crystalline material.

19. The method as defined in Claim 18 wherein said nematic liquid crystalline material is in the homeotropic texture state.

20. The method as defined in Claim 17 wherein said imaging material comprises smectic liquid crystalline material.

21. The method as defined in Claim 17 wherein said imaging material comprises optically negative liquid crystalline material.

22. The method as defined in Claim 21 wherein said optically negative liquid crystalline material is provided sub-stantially uniformly in the Grandjean texture state and step (b) comprises establishing an electrical field between said elec-trodes and exposing said photoconductive layer to an imagewise pattern of activating electromagnetic radiation whereby there is formed across said imaging layer an imagewise electrical field in the Grandjean to focal-conic texture transformation electrical field strength range of said optically negative liquid crystalline material.

23. The method as defined in Claim 22 and further including the step (d) of erasing said image by applying across said imaging layer an A.C. electrical field having a frequency which is sufficient to suppress ion flow within the liquid crystal-line material.

24. The method as defined in Claim 23 wherein steps (b)-(d) are repeated at least one additional time.

25. The method as defined in Claim 24 wherein said imaging layer has a thickness of about 10 microns.

26. The method as defined in claim 21 wherein said optically negative liquid crystalline material is provided sub-stantially uniformly in the focal-conic texture state and step (b) comprises establishing an A.C. electrical field having a frequency which is sufficient to suppress ion flow within the liquid crystal-line material between said electrodes and exposing said photocon-ductive layer to an imagewise pattern of activating electromagnetic radiation whereby there is formed across said imaging layer an imagewise electrical field in the focal-conic to Grandjean texture transformation electrical field strength range of said optically negative liquid crystalline material.

27. The method as defined in Claim 26 and further including the step (d) of erasing said image by applying an elec-trical field across said imaging layer.

28. The method as defined in Claim 27 wherein steps (b)-(d) are repeated at least one additional time.

29. The method as defined in Claim 21 wherein said optically negative crystalline material is provided substantially uniformly in the focal-conic texture state and step (b) comprises establishing an electrical field between said electrodes and ex-posing said photoconductive layer to an imagewise pattern of activating electromagnetic radiation whereby there is formed across said imaging layer an imagewise electrical field in the optically negative-optically positive phase transition electrical field strength range of said optically negative liquid crystalline material.

30. The method as defined in Claim 29 and further including the step (d) of erasing said image.

31. The method as defined in Claim 30 wherein steps (b)-(d) are repeated at least one additional time.

32. The method as defined in Claim 14 wherein said imaging material comprises imaging particles in an elec-trically insulating liquid and step (b) comprises establishing an electrical field between said electrodes and exposing said photoconductive insulating layer to an imagewise pattern of activating electromagnetic radiation.

33. The method as defined in Claim 32 wherein said electrically insulating liquid has a different color than at least some of said imaging particles.

34. The method as defined in Claim 32 wherein said imaging particles comprise electrically photosensitive pigment particles and said suspension further includes inert particles and wherein said photo conductive insulating layer has a thick-ness of up to about 5 microns.

35. The method as defined in Claim 1 wherein the optical pathlength of said readout illumination in said sub-stantially transparent conductive layer is about one-tenth of the shortest wavelength of said readout illumination or less.

36. The method as defined in Claim 1 wherein nc is greater than either of ns and ni.

37. The method as defined in Claim 1 wherein said imaging member further includes an anti-reflection coating on the surface of said first electrode substrate opposite that carrying said substantially transparent conductive layer.

38. The method as defined in Claim 37 wherein said anti-reflection coating is a multiple layer coating.

39. The method as defined in Claim 1 wherein said second electrode is substantially transparent.

40. The method as defined in Claim 1 wherein said second electrode comprises opaque material.

41. The method as defined in Claim 1 wherein said second electrode includes a photoconductive insulating layer adjacent said imaging material layer.

42. The method as defined in Claim 1 wherein said imaging member is viewed in reflection.

43. The method as defined in Claim 1 wherein said imaging layer comprises a suspension of electrically photo-sensitive pigment particles in an electrically insulating liquid and step (b) comprises establishing an electrical field between said electrodes and exposing said suspension to an imagewise pattern of activating electromagnetic radiation.

44. The method as defined in claim 43 wherein said suspension includes at least two differently colored electrically photosensitive pigment particles.

45. The method as defined in Claim 1 wherein said imaging layer comprises electrically photosensitive pigment particles and inert particles in an electrically insulating liquid and step (b) comprises establishing an electrical field between said electrodes and exposing said suspension to an image-wise pattern of activating electromagnetic radiation whereby an image comprised substantially completely of inert particles is formed at one of said electrodes.

46. The method as defined in Claim 1 wherein said imaging material comprises liquid crystalline material.

47. The method as defined in Claim 46 wherein said imaging material comprises smectic liquid crystalline material.

48. The method as defined in Claim 46 wherein said imaging material comprises nematic liquid crystalline material.

49. The method as defined in claim 48 wherein said imaging layer comprises nematic liquid crystalline material in the homeotropic texture state.

50. The method as defined in Claim 46 wherein said imaging material comprises optically negative liquid crystalline material.

51. The method as defined in Claim 50 wherein nc is greater than either of ns and ni, wherein said second electrode further includes a photoconductive insulating layer adjacent said imaging layer, wherein said optically negative liquid crystalline material is initially provided substantially uniformly in the Grandjean texture state and step (b) comprises establishing an electrical field between said electrodes and exposing said photo-conductive layer to an imagewise pattern of activating electro-magnetic radiation whereby there is formed across said imaging layer an imagewise electrical field in the Grandjean to focal-conic texture transformation electrical field strength range of said optically negative liquid crystalline material.

52. The method as defined in Claim 51 and further including the step (d) of erasing said image by applying across said imaging layer an A.C. electrical field having a frequency which is sufficient to prevent ion flow in said liquid crystal-line material.

53. The method as defined in Claim 52 wherein steps (b)-(d) are repeated at least one additional time.

54. The method as defined in Claim 53 wherein said imaging layer has a thickness in the range of from about 0.5 to about 100 microns

55. The method as defined in Claim 53 wherein said imaging layer has a thickness of about 10 microns.

56. The method as defined in Claim 1 wherein said.
imaging layer has a thickness in the range of from about 0.5 to about 100 microns.

57. The method as defined in Claim 1 wherein said second electrode is in a desired image configuration.

58. The method as defined in Claim 1 wherein said first and second electrodes comprise an electrical X-Y matrix address system and step (b) comprises applying an electrical field across selected areas of said imaging layer simultaneously or sequentially.

59. The method as defined in claim 1 wherein said second electrode further includes a photoconductive insulating layer adjacent said imaging layer and wherein step (b) comprise establishing an electric field between said electrodes and expos-ing said photoconductive insulating layer to an imagewise pattern.
of activating electromagnetic radiation.

60. The method as defined in Claim 1 wherein step (b) comprises applying an electrical field across said imaging layer.

61. The method as defined in claim 60 wherein said imaging member is viewed in reflection.

62. An imaging member comprising first and second electrodes arranged on opposite sides of a layer of imaging material having an index of refraction ni, said first electrode comprising a substantially transparent substrate having an index of refraction ns carrying a substantially transparent conductive layer having an index of refraction nc and a thickness of about 200 angstroms or less, said conductive layer being adjacent said imaging layer, wherein ni/ns is in the range of from about 0.7 to about 1.3 and wherein nc is different than ni or ns.

63. The imaging member as defined in Claim 62 wherein nc is greater than either of ni and ns.

64. The imaging member as defined in Claim 62 wherein ni/ns is in the range of from about 0.9 to about 1.1.

65. The imaging member as defined in Claim 62 wherein ni and ns are substantially equal.

66. The imaging member as defined in Claim 62 wherein ni is about 0.006 less than ns.

67. The imaging member as defined in Claim 62 and further including an anti-reflection coating on the surface of said first electrode opposite that carrying said conductive layer.

68. The imaging member as defined in claim 67 wherein said anti-reflection coating is a multiple layer coating.

69. The imaging member as defined in Claim 62 wherein said imaging layer comprises a suspension of imaging particles in an electrically insulating liquid.

70. The imaging member as defined in Claim 69 wherein said liquid has a different color than at least some of said imaging particles.

71. The imaging member as defined in Claim 62 wherein said imaging layer has a thickness in the range of from about 0.5 to about 100 microns.

72. The imaging member as defined in Claim 62 wherein said imaging layer comprises liquid crystalline material.

73. The imaging member as defined in Claim 72 wherein said imaging material comprises smectic liquid crystal-line material.

74. The imaging member as defined in Claim 72 wherein said imaging material comprises nematic liquid crystalline material.

75. The imaging member as defined in Claim 72 wherein said imaging material comprises a nematic liquid crystalline material substantially uniformly in the homeotropic texture state.

76. The imaging member as defined in Claim 72 wherein said imaging material comprises optically negative liquid crystalline material.

77. The imaging member as defined in Claim 76 wherein said optically negative liquid crystalline material comprises a mixture of cholesteric and nematic liquid crystal-line materials.

78. The imaging member as defined in Claim 76 wherein said imaging material layer has a thickness in the range of from about 0.5 micron to about 100 microns.

79. The method as defined in Claim 76 wherein said imaging material layer has a thickness of about 10 microns.

80. The imaging member as defined in Claim 62 wherein said second electrode further includes a photoconductive insulating layer adjacent said imaging material layer.

81. The imaging member as defined in Claim 80 wherein said imaging layer comprises optically negative liquid crystalline material and has a thickness in the range of from about 0.5 to about 100 microns, said member further includes an anti-reflection coating on the surface of said first electrode substrate opposite that carrying said conductive layer and ni and ns are substantially equal.

82. The imaging member as defined in Claim 81 wherein ni about 0.006 less than ns.

83. The imaging member as defined in Claim 62 wherein said second electrode is shaped in image configuration.

84. The imaging member as defined in Claim 62 wherein said first and second electrodes comprise an electrical X-Y matrix address system.