Note: Descriptions are shown in the official language in which they were submitted.
~l~g~7
The invention herein is concerned with a method
and apparatus for effecting image reversal auring toning
electrophotographic film.
The fiela of toning has become important in
recent years in connection with the increasing use of
electrostatic techniques for reproducing drawings and
documents and even more recently in the imaging of
scenes directly, as the equivalent of conventional
silver halide photography carried out under varying
conditions of light ana at high speeds.
Electrostatic imaging involves charging a
photoconductive member in darkness and illuminating
the charged surface with a light pattern to be reproauced.
The charge is selectively dissipated by the light pattern,
remaining at those surface increments of the photoconductive
member which are not illuminated and being removed from
those illuminated surface increments. The degree of
dissipation is directly related to the degree of
illumination to produce a latent charge image of the
pattern on the surface of the photoconductive ~ember
which slowly fades as the charge continues to leak off
from the charged increments.
Without detailing the theory of operation of
.,
this technology, it would be noted in passing that the
ability of a photoconductive member to retain the latent
image is made up of surface charged increments side by
7~
side having different degrees of charge depending upon
the exposure of each to the light pattern and which is
a function of its electrical anisotropy. A comminuted
powder of carbon, resin or the like called toner is
dusted, rubbed or flowed onto the surface of the photo-
conductive member before the latent image has materially
deteriorated. Toner particles carry charge of polarity
opposite to the polarity of the charges which remain
on the surface of the electrophotographic member.
Accordingly, the toner particles adhere to the surface
in proportion to the degree of charge remaining on the
charged increments of the surface after exposure to
convert the latent image into a visible image.
Minute residual potentials tend to remain on
the photoconductive surface after exposure and are not
related to the latent image carried thereby. This is
one cause of so-called background fogging. Also sign-
ificant as a cause of background fogging are so-called
floating toner particles which precipitate upon evaporation
of the suspending medium for the toner.
; Ordinarily the latent image charge pattern is
representative of the scene, that is areas exposed to
greater intensity light would carry the least charge and
; areas exposed to the least intensity light would carry
the higher charge and thus, when toned, would be the
most attractive to toner particles.
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Thus what would appear "black" on the original
would appear black on the copy and hence be a "positive
print"and vice versa~ It is often desirable to reverse
the image to print a negative, again using photographic
- 5 terminology, that is to reverse the grey scale of the
"print" It has been discovered that this image reversal
can be effected while background fogging likewise can
be eliminated.
Accordingly the invention provides a method
for toning the latent image on a charged electrophoto-
graphic member to effect image reversal, where said
member has an ohmic layer and a photoconductive coating
and the latent image is formed by selective charge
distribution in the photoconductive coating and having
an incremental surface charge effect selectively to
attract toner particles of one polarity related to the
charge acceptance characteristic of the coating for
each surface increment in inverse relation to the amount
. of radiation received by that surface increment, said
image reversal method comprising: bringing a conductive
plate into juxtaposed parallel relation with the surface
of the coating to define a liquid toner retaining volume,
applying a high d.c. bias voltage between the plate and
ohmic layer of a polarity opposite to said one polarity
and of a value and relationship such as to reverse the
polarity of the surface increments to an extent greatest
at the surface increments which received the most radia-
`:
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tion during exposure but without substantially changing
the overall relative charge relationship of the surface
at remaining increments, and further effectively to
neutralize the surface charge potential at increments
which received the least radiation during exposure,
introducing into said volume liquid toner into said
volume,said toner formed of particles of polarity
opposite to said one polarity and the dispersant medium,
retaining the liquid toner in the volume for a period of
time which will substantially fully tone the latent image
while at the same time maintaining the applied bias
for at least the majority of said period while also
maintaining the positional relationship of said plate
and coating, and thereafter removing the liquid remaining
in said volume after said period and z.pplying a clearing d.c.
bias between the plate and ohmic layer by changing the
polarity of the Qaid high d.c. bias such that its polar-
ity and the voltage relationship between the plate and
the ohmic layer favor attraction of particles to said
plate.
The invention further provides apparatus for
imaging by electrostatic means which comprises: means
for charging an electrophotographic member, means for
exposing said member to radiation to achieve a latent
image on a surface of said member, means forming a
toning chamber with said member and including a conductive
plate parallel to said surface and relatively close to
~1~97~7
said surface, means for introducing liquid having negatively
charged toner particles into said chamber and permitting
same to remain for a period of time required to achieve
toning of said latent image, means operating in the said
period of time for biasing the member relative to the
plate with a d.c. bias of a voltage sufficiently high at
the surface to reverse the polarity of the charges
distributed to form the latent image so that the areas of
said member which received the highest radiation will have
the highest and opposite polarity charge potential without
changing the relative charge relationship making up the
latent image, means for running the liquid toner out of
said chamber, and means for moving said member out of its
relationship with said plate.
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.
- The preferred embodiments of this invention
will now be described, by way of example, with reference
to the drawings accompanying this specification in which:
Figure 1 is a diagrammatic sectional view
through a fragment of an electrophotographic film in
relationship to apparatus for carrying out the method;
Figure 2 is a view similar to that of Figure 1
but showing a subsequent step of the method;
Figure 3 is a chart used in explaining the
method of the invention and the operation of the appa-
, ratus thereof;
Figure 4 is a view similar to that of Figure 2
but showing the "clearing" step of the method of the
invention;
; 15 Figure 5 is a view similar t:o that of Figure 4
showing the last step of the method of the invention;
Figures 6 and 7 are fragmentary sectional views
through apparatus embodying the invention; and
Figure 8 is a circuit diagram of a switching
' 20 arrangement for use with the apparatus of the invention.
.
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Ini-tially the invention and its principles will
be explained in connection ~ith the diagrams o~ Figures 1
to 5 inclusive, showing the conditions existing in an
apparatus such as a camera while the various steps of
the method are carried out.
In Figure 1 there is shown an electrophotographic
member 10 comprising a substrate 12 formed of a sheet of
organic polymer material that is transparent and clean,
such as polyethylene terphthalate (~Iylar*- DuPont) about
.005 inch in thickness carrying an ohmic layer 14 and a
. photoconductive coating 16 deposited onto the ohmic layer
14. The ohmic layer in one example is indium-tin oxide
about 300 to 500 ~ngstroms thick, while the coa-ting 16 is
crystalline cadmium sulfide, of preferred hi.ghly ordered
. 15 ~rientation, vertically relative to the surface on which
deposited usiny a r.f. sputtering method during which a bias
voltage is maintained, the thickness being about 2000 to
4000 Angstroms.
The pho-toconductive member 16, through its sur- :~ace 18,may be presumed to have been charged to a suit-
able voltage and maintained in darkness to provide nega-
tive charges evenly distributed at or near the surface
(the cadmium sulfide being n-type semi-conductor mate-
rial) and e~posed to a projected liyht pattern to produce
. 25 a latent charge image on the surface. It is now desired
to tone the hatent image to render it visible. The appa-
ratus includes a conductive plate 20 o~ any suitab]e
* Trademark E.I. DuPont
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7~7f
metal that is arranged parallel to and spaced from the
surface 18 of the photoconductive coating 16. Some kind
of framing border around the film can be provided to dam
i the liquid toner so that there is a toner chamber 22
formed within the border and between the plate 2~ and
the coating 16. Other means are feasible.
A typical spacing between the plate 20 and the
surface 18 to form the vertical thickness of the chamber
22 is 005" to .20". The plate 20 can be described as an
image intensification plate for reasons which presently
will appear.
Just after the latent image has been produced
but before the toner is flooded into the chamber 22, a
high positive bias potential is applied between the ohmic
layer 14 and the plate 20. This biasing potential in the
range of +20 to +30 volts, is achieved by connecting the
ohmic layer 14 through a conductor 24 to the positive side
of a potentlal source 26 and the plate 20 by way of the
conductor 28 to the negative terminal of the source 26.
In the case illustrated, this latter connection is
through ground, shown symbolically.
The effect of the bias is most strongly felt
close to the ohmic layer 14 and furthest from ground and,
when the bias has been established the liquid toner is
run into the chamber 22. Figure 1 illustrates conditions
just before the toner has flooded the chamber 22, the
toner being shown at 30 in Figure 2 and not at all in
Figure 1. The toner particles are negatively charged.
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In Figure 2, the toner has been flooded into
the chamber 22 and is contained therein. Such contain-
ment could be based upon surface tension, for example.
The effect of the bias is not obvious unless one
considers the nature of the phenomena which occur. Where
charge e~ists on the surface 18 (or slightly below), the ;-
bias potential will reduce the surface potential almost
to zero. Where charge has been dissipatea due to the
effects of light photons affecting the increments and
causing recombination of electrons and there is no sur-
face potential or very little, the charge potential will
be raised to a high positive polarity. The bias renders
these low or zero charge increments strongly positive,
and since the toner particles are negatively charged,
they are attracted to those areas.
One may consider that the entire base line of
the charge-carrying surface is changed in polarity with-
out in any way changing the gradients or relative charge
differences between increments so that the relative
amounts of toner particles attracted by and adhered to
the several increments are not affected. Since the in-
crements which would otherwise be at zero or slightly
negative surface potential are now highly positive nega-
tive, toner particles are attracted to these latter in-
crements instead of being repulsed. Those areas which
carried high negative charges, would be at zero or only
slightly neyative and would not attract any negatively
" .
~. .
charged particles. The positive bias applied for image
reversal is slightly less than the maximum negative
charge of that comprising the latent image.
In Figure 3, the surface is shown at 40 and 42
aligned with the columns that represent adjacent incre-
ments. The upper surface 40 represents the surface
attraction or repulsion of negative toner particles with
no bias, called "normal". The lower surface 42 repre-
sents the surface attraction or repulsion of the negative
toner particles when there is a positive bias. The
areas across the chart designated A to H inclusive
represent 8 increments of the surface 18. The lower
portion of the chart shows the normal base line 44
which is presumed to be zero potential and the base
line 46 as a broken line a voltage below the normal base
line by a voltage representing the bias.
The lower bars are an attempt to illustrate
graphically the total charge of the respective incre-
ments represented by the latent image. Only the solid
line bars are shown and change in their values is a
relative concept related to the base line to which
they are referred.
The small charged circles above the surface 18
- represent quantities of toner particles which will be
attracted or repelled. The net charge of each increment
is represented by the polarity sign adjacent the surface
lines 18 at each increment.
7~ 7
Assume that the level of charge voltage of the
surface 18 without bias for the various increments rela-
tive to the normal base line 44 is at 51 to 5~ respective-
ly for the increments A to H. Obviously, the surface at
40 will in all such cases be either at a net voltage of a
negative polarity or no polarity where the charge has
totally been dissipated. Thus, at A, B, D, E, F, G and H
the polarity is shown by a negative sign at the surface
40, the increment C having no polarity sign. The vol-
tage levels are of aifferent values. Levels 52, 54 and
58 are quite negative, representing very little light
having reached the increments; levels 55 and 57 are
quite small but not zero; level 53 is at zero potential
having been fully discharged; and the level 56 is at a
moderate negative potential. All of these relate to the
base line 44.
; Now consider the attraction of the negatively
charged particles. It is seen that the number of small
circles with plus signs shown is a rough measure of
actual number of degree of attraction for the particles.
Above surface 40, at A one circle is shown with an arrow
pointed upward, indicating repulsion. An increment B
there are four such circles with their arrows pointed
upward; at increment D there are three circles, etc.
All increments repulse particles except for the incre-
! ment C which shows a circle with a minus sign but no
arrow. This represents a condition where there is no
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arrow. This represents a condition where there is no
charge, but where there will be toner particles float-
ing in the dispersant. The increments E and G have
little surface voltage so they probably represent some
totally blank background and repulse toner only slightly.
In the increment C, toner particles will fall
out of the dispersant when it evaporates to color this
increment as well as the others.
~ow consider the addition of the bias voltage,
which lowers the base line to 46. This in effect raises
the surface voltage of all increments by a substantial
positive amount. The net result of this is that all of
the levels 51 through 58 are raised by the positive vol-
tage 60. Effective surface voltages 52, 53, and 58 will
be at most only slightly positive, zero or slightly nega-
tive. The charge relationships in the latent image will
remain the same, even after the aadition of the bias
voltage. The small circles in these incremental spaces
are pointed downward or upward, indicating extent of
attraction of tonex particles to the surface 42. As for
the levels 51, 54, 55, 56, and 57, these are above the
base line 46 and hence have strong net positive charges.
This is indicated by the plus signs adjacent the surface
42 in these incremental areas. As a result, the high
positive potential of the surface will attract the nega-
tive toner.
As a result of the bias as described, the incre-
mental areas which are intended to provide blank back-
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grounds are quite clean since the toner particles are not
attracted to these areas, all without affecting the rela-
tive amount of toner adhered to the photoconductive coat-
ing in other areas of the film.
Continuing with a description of the method of
the invention, the condition in Figure 2 is that the sur-
face 18 is attracting toner particles where there is net
charge of a voltage that is slightly more negative than the
amount of the positive bias afforded by the source 26, all
other increments having a smaller negative charge. This
is because the positive bias is slightly less than the
maximum negative charge of that comprising the latent image.
; This condition is permitted to obtain for a
period of time which is considered sufficient to tone
the image. In a typical case the imaging time may be
about 100 milliseconds. In other instances the time may
be somewhat longer. The toner bias is applied to the film
and the image intensification plate 20 as soon as possible
before toning. This could be a condition that is brought
about when the image intensification plate is moved into
position over the film 10 so that conceivably the bias
may be applied by such movement. The exposure should be
complete at this time. In the case of high speed film
it is desirable that there be no hiatus between the
termination of exposure and the commencement of the toning
period. Suitable mechanism can be devised to accomplish
this which at the time will apply the requirea bias to
effect proper toning.
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,, .
The toning period for such high speed film can
- be of the order of one half to one second. Other films
may require longer toning periods. The bias referred to
stays on during this period, as a result of which the
incremental areas at zero charge potential attract no
toner particles.
At the end of the toning period, as measured by
the parameters of the film and the characteristics of the
toner, the bias voltage is reversed in polarity and re-
duced to a low level. This condition is indicated in
Figure 4. Here particles of toner from the body of toner
30 have been deposited on the surface 18 at incremental
areas which were biased positively. These area are in-
dicated at 64. The toner dispersant still carries par-
ticles floating therein which have not been attracted to
the zero potential areas. There is still some danger that
there will be a deposit of them in blank areas of the image
'"
after toning. Accordingly, the bias voltage is reversed
with the resulting effect being that all the negatively
charged toner particles are attracted preferentially to
the bottom surface 66 of plate 20 and some will adhere to
it. As a result the dispersant above the surface 18 is
cleared of floating toner particlesO
The voltage of the particle-clearing bias is of
the opposite polarity to the bias previously applied by
the source 26. It remains in place while the now clear
dispersant is decanted or drawn out of the chamber 22.
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This is shown in Figure 5 where the adhered toner particles
are shown at 68 on the bottom surface 66 of the plate 20,
the dispersant of toner 30 now being drawn off as indicated
by the arrow 70. This can be effected by vacuum, pressure
S capillary action or a combination thereof Foe example,
the damming means could be highly absorbent of the disper-
sant so that it literally sucks the dispersant into itself
when the plate 20 is suitably moved to break the surface
tension. The liquid film of dispersant which may remain
on the surface 18 will have no particles in it, hence
when such film evaporates, there will be no "fall-out"
of particles onto the background areas which are intended
to be blank. The time for the clearing bias, as it might
be called depends upon the time to eliminate the toner
dispersant which could be of the order of one second.
The image intensification plate 20 is normally a
permanent part of the camera or other imaging device with
which the invPntion is associated. The accumulated toner
particles 68 on its lower surface are readily wiped off
! 20 from time to time but the accumulation is so slight that
this need not be done for substantial periods of time.
Mechanism can readily be devised to do so when film mem-
bers are placed in position or when cartridges of film
are introduced into the apparatus.
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. .
The practical application of the invention to a
suitable reproducing apparatus or camera can take a large
variety of forms. In Figure 6 the apparatus 80 illustrated
can be considered part of a larger structure comprising
a camera or other imaging device. An electrophotographic
member 10 is shown, the various layers or coatings 12, 14
and 16 thereof not being shown for simplication. Member 10
is mounted to a suitable framing structure 82 whose framing
side parts are shown in section at 84 and 86 and in elevation
at 88. As shown in Figure 7, the framing structure 82
cooperates with the image intensification plate 20 when
the latter is in position directly over the active photo-
..
conductive surface 18 of the film member 10 to dam the
liquid toner 30, thus forming the cha~er 22 mentioned
above in connection with the description of Figures 1, 2,
4 and 5.
~he image intensification plate 20 is arranged to
move relative to the film 10 to cover the exposed or framed
portion of the film surface 18 (i.e., that area which is
not blocked by the framing side parts of the framing
~tructure 82). This movement can be either a sliding
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7~ 7
movement as indicated by the broken line fragment 20' in
Figure 6 moving to the left following the arrows; it could
be effected by a rotation bringing the plate 20 into posi-
tion; it could be achieved by a translated movement; or
by a combination of any of these. In Figure 7 the means
for moving the image intensification plate 20 into position
is symbolically illustrated by the linkage mechanism 90
connected thereto. The electrical lead 28 is shown in
Figure 7 along with its companion lead 24, the latter
being connected to the ohmic layer 14 which is not de-
tailed in Figure 7.
In Figure 6, the particular embodiment 80 includes
a pod or bag 92 of liquid toner 30 that is adhered to the
upper surface of the framing side part 84. The bag can be
made out of any flexible synthetic resin that is not
affected by the hydrocarbon dispersant of the toner it
carries. For example, polymerized resins of the Mylar
(DuPont) type would be suitable. There is a weakened
lip at 94 which overhangs a slight groove 96 provided in
the side framing part 84 where it faces the chamber 22.
When the metal image intensification plate 20 is pushed
into position to enclose the chamber 22 by the mechanism
90, the leading edge lO0 pinches the lip 94 and ruptures
the bag so that the liquid toner 30 floods into the cham-
ber 22, now closed by the plate 20. The ruptured bag is
shown at 92' in Figure 7.
The bag 92 may be a very small one attached to
each film frame 82 or it can be a supply in the form of a
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1097~7
perforated, normally enclosed article which is "milked"
or drawn upon by the mechanism which operates the plate 20
It is feasible to have the toner in a crushable member
where it is encapsulated as an assembly or respective dry
and iiquid particles to be mixed when crushed and released.
Many variations will occur to-those skilled in this art.
For example, plate 20 may be foraminous and the toner
expressed through it.
The inner surfaces of the framiny sides may be
treated with a material such as shown at 104 that slowly
dissolves in the dispersant or retards the absorbing of
the toner by the framing structure ~2 for a time suffi-
cient to enable the toning to take place. It has been
found that even where the material of the framing struc-
ture 82 is highly absorbent, as for example, made out of
a very absorbent and/or porous cellulose or paper, if the
transverse dimension of the chamber 22 is of the order of
abou-t .4mm the surface tension of the toner 30 itself will
xetain the body of toner in the chamber unti] surface
tension is relieved by removal of the plate from the
chamber. At this point, the dispersan-t remaining will
quickly be absorbed into the side parts ~4, 86 and 88.
This is a capillary action phenomenon.
. . .
The sliding movement of the plate 20 ls helped
by providing a layer 106 of*Teflon TM or other "s]ippery"
material on its bottom side. Since the particles of
toner which are attracted by the action of the counter-
* Trademark E.I. DuPont
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bias will adhere to the bottom surface of the plate 20
the presence of a non~wettable surface material makes
them easy to remove. This can be done, as mentioned,
either periodically by any suitable means or even by the
bringing into position of the next electrophotographic
member 10 and its frame82. The leading edge of the
frame 82 or the carrier for the frame, shown at 108 in
Figure 7, can be arranged to wipe the bottom surface of
the plate free of toner particles from the previous
toning operation as the two are moved relative to one
another. This is shown at 110 in Figure 6 where the
toner particles were removed by an exterior edge of the
frame 82. The amount of toner particles is so little
that the accumulation from one toning is not even
noticeable.
The carrier 108 is moved relative to the plate
20 by any suitable mechanism such as shown at 112 as an
alternate to or in addition to the mechanism 90.
The practical construction of the apparatus of
2~ the invention has involved film 10 and framing members
82 of relatively small size. The film is of the size
; which is known as "110", having approximately the exposed
dimensions of 12 by 16 mm with the framing member 82
being of the order of 18 by 25 mm. A typical framing
member of paperboard would be about 1.5 to 2 mm thicX.
Ce]lulosic members might be somewhat thicker, but must
have sufficient rigidity to support the electrophoto-
:~ -20-
,'
graphic film during use and thereafter to enable display,
enlargement, etc. Larger film members can be toned by
the invention.
The plate 20 and film member 10 are generally
illustrated as blocXs. Obviously some structure is re-
quired ko enable these members to be moved relative to
one another without inadvertently removing the connec-
tions which have been established. Likewise, the connec-
tion to the film member 10 must be such that it is auto-
matically established when the film member is changed.
This is within the skill of those in this art.
A d.c. source 120 is shown as a battery connected
to a resistor which serves as a voltage divider 122. The
desired voltage values are chosen by simple experiment
and are established as the terminals 26 and 62, providing
a positive voltage source in one case and a negative
voltage source in the other case. The center 124 between
these two terminals is at zero or ground potential. It
is expected that the voltage drop between the terminal
62 and ground 124 and between the terminal 26 and ground
will be substantially equal, albeit of different polar-
ities~
A double pole throw switch is shown at 126 with
its central terminals 128 and 130 connectea respectively
by the leads 28 and 24, respectively, to the plate 20
and the film 10. The terminal contacts 132 and 134 are
connected by suitable electrical leads 136 and 138,
respectively, to ground 124. The terrninal 140 is connected
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by the lead 142 to the positive voltage tap 62 while the
terminal 144 is connected by the lead 146 to the negative
voltage tap 126.
This arrangement is intended for a method and
apparatus wherein the photoconductive coating is of n-
type with the latent image comprised of negatively
charged areas. The toner particles are required to have
negative charges and will be attracted to positively
charged increments of the film 10 subsequent to the
application of the positive, image reversal bias. To
effect the bias, the switch 126 is thrown to the top con-
tacts 132 and 144. This connects the ohmic layer 14 of
the film 10 to the negative voltage tap 26 while at the
same time connecting the plate 20 to ground 124. When
clearing is desired, the bias polarity is reversed by
throwing the switch 126 to the bottom contacts 140 and
134. This connects the ohmic layer 14 to ground 124 and
the plate 20 to the positive voltage tap 62. The bias
voltage range of the image reversal bias extends from +10
20 to +40 volts, with the preferred range of +10 to +30 volts,
which is considered high when compared with imaging bias
applied in accord with the method of Canadian Patent
; 1,057,138.
In a practical device, the switching can be done
electronically and automatically. The polarities can be
readily changed for use with p-type photoconductive materials~
As a summary of the procedure which occurs dur-
ing the process of making a`complete toned film the
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following is submitted:
1. The film 10 is charged and exposed to a pro-
jected image by any suitable apparatus.
2. Immediately that the exposure is completed,
the image intensification plate 20 is
moved into position relative to the sur-
face of the film.
3. Then the plate 20 is in position, the image
reversal bias is applied, although it
can be applied all the time.
4. Toner 30 is run into the chamber 22 while
the image reversal bias is still being
applied. Figure 2 shows this.
The timing of the above four steps depends upon
the characteristics of the film and the practical aspects
o moving physical things in the performance of the de-
scribed functions. Assuming a high speed film, ideally
the instant that the exposure is complete the image rever-
sal bias and toning can start. In a practical considera-
tion, there is a finite time required to move the plate 20
into position and introduce the toner. On this account,
the image reversal bias can be applied just before the
toning begins~
5. The toning period is completed with the
image reversal bias in place, but just
before the liquid toner is removed from
the chamber 22 the bias potential is
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reversed in polarity. This is shown in
Figure 4.
6~ With the reversal of polarity of the bias
potential, the toner 30 is removed from
the chamber 22, This is shown in Figure
; 5. Thereafter the process is complete
and the toned film member may be dis-
pensed from the camera. The clearing
bias may be removed at any convenient
time after the liquid has been drawn off
because it ceases acting on floating par-
ticles as soon as the chamber 22 is empty.
As a practical matter there are practically
, no remaining particles in the toner 30
so that it is clear dispersant.
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