Note: Descriptions are shown in the official language in which they were submitted.
~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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(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~
. ,.
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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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.
~, . . . .
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~
'~! 30 layer thickness; and
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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
--7--
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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 )
1 2
1 2
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
,- . , , , , : ,
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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~
--10--
, . . .
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 ~.
-12-
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.
~:
~ 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
-14-
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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
-15-
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~LV4~43 ~ :;
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;
electric field effects in
-16-
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~0~L~6~3 :
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.
-17-
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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
18-
,
~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~
, - . - ;, . . . ~. .
, ... . . . . .. . .
~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, :
' '
-18B-
...... . . ..
;, , , . -:"
, : ~ , , : :.: ~ :
~ 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.
-21-
,. , . , , , , , "
~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
-22-
, ,, .
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
." , . . . .. . . . . . ............................ ..
.
,~. . , .
",,
6913
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
-24-
,
.. . . . .
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
-25- -
: .
;. :.: . , .',, ;, ;, . , ; ' ,", ' ............... - : '
,: . , - ;. . . . . .. .
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
-26-
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.
-28-
, .
,
