Electrophotographic method of generating electrostatic images on two sides of an insulating foil

Electrostatic charge images of identical shape but opposite sign are generated on both sides of a transparent, highly insulating foil. Subsequently, pigment is deposited on both sides of the foil by means of oppositely charged developers. The optical density of an electrophotographic image on a transparent insulating foil is increased, as compared to densities achieved in the past, for a given surface charge density by establishing a charge exchange between one side of the foil and an electrode. On the other side of the foil a charge image is generated and the foil. The electrode are separated from each other prior to development.

BACKGROUND OF THE INVENTION 
The invention relates to an electrophotographic method where electrostatic 
charge images of identical shape but opposite sign are generated on both 
sides of a transparent, highly insulating foil, pigment being deposited on 
both sides of the foil by means of oppositely charged developers. 
Electrophotography utilizes the local variation of the conductivity of a 
flat photosemiconductor in reaction to light for generating images 
(Ullmans Encyklopadie der technischen Chemie, 3rd edition, volume 14 
(Munich-Berlin 1963) page 678). Electroradiography is a special kind of 
electrophotography. While electrophotography utilizes light rays for the 
recording, electroradiography utlizes X-rays or other directly ionizing 
rays. (German Offenlegungsschrift No. 26 41 067.) Ionography is another 
special kind of electrophotography for recording X-ray images. In 
ionography, a latent image of the radiogram is formed as a distribution of 
the electric charge on an insulating surface rather than another selenium 
or photoconductor. The latent image is generated by collecting ions on the 
surface of an insulating foil which is suspended in front of an electrode 
of an ionization chamber. These ions are formed by radiation in a layer of 
a suitable gas which fills the space adjoining the foil. The latent image 
generated by the electric charge pattern can be made visible (developed) 
in various ways which are customarily used in electrophotography, (German 
Offenlegungsschrift No. 24 31 036 which corresponds to U.S. Pat. No. 
3,963,924.) 
The ionographic method described in U.S. Pat. No. 3,963,924 German 
Offenlegungsschrift No. 24 31 036 utilizes ionizing radiation which passes 
through an object to be imaged and which subsequently passes into an 
ionization chamber. The ionization chamber contains a layer of a gas, at 
least some atoms of which have a high absorption coefficient for X-rays. 
The gas layer is bounded by a pair of electrodes which sustain an electric 
field in the chamber. The ions produced in the gas layer are collected on 
the surface of a transparent insulating foil. In a modified version of 
this method, the foil is centrally arranged in the ionization chamber so 
that positive ions are collected on one side and an equal charge of 
negative ions is collected on the other side, the ions of opposite charge 
keeping each other in position as a result of their force of attraction, 
the net load on the foil being almost zero. It is important that the foil 
is held exactly in such a position that the opposite charges obtained on 
both sides of the foil are equal. The correct position is usually situated 
in the vicinity of the geometrical center of the gas layer. Both surfaces 
of a foil thus charged can be developed by means of some known method, for 
example, development by powder or liquid or by introduced or deposited 
substances with optically active properties. 
Direct absorption of X-rays in a gas in the vicinity of the recording layer 
produces pairs of ions which are separated by an applied electric field, 
so that ions of the same charge polarity are collected on the recording 
layer. In the ionization chamber shown in FIG. 8 of the German 
Offenlegungsschrift 24 31 036, a number of charge pairs are formed by 
irradiation. After the irradiation is completed, negative charges are 
present on one side of the foil and positive charges are present on the 
other side of the foil. The number of such charge pairs amounts to half 
the number of charge pairs originally formed, because the positive 
partners of the charge pairs formed on one side of the foil proceed to the 
cathode, while the negative partners of the charge pairs formed on the 
other side of the foil proceed to the anode and are lost as far as the 
recording process is concerned. For the sake of comparison it is assumed 
that the method known from German Offenlegungsschrift No. 24 31 036 
produces an optical density amounting to 1 on a single foil. This 
assumption wil be described further below. 
SUMMARY OF THE INVENTION 
An object of the invention is to increase the optical density of 
electrophotographic images on a single, transparent, highly insulating 
foil at a given surface charge density. 
To this end, the method according to the invention is characterized in that 
there is a charge exchange between one side of the foil and an electrode. 
At the same time a charge image is generated on the other side of the 
foil. The foil and the electrode are then separated from each other prior 
to development. 
For making the charge image, the method according to the invention can 
utilize all known methods and devices, for example, the methods and 
devices described above. When use is made of a transparent, highly 
insulating foil, a charge exchange occurs between one side of the foil and 
an electrode, and a charge image is formed on the other side of the foil. 
For example, when real negative electric charges are present on the free 
foil surface, the associated charges of opposite polarity, i.e. real 
positive charges, are formed on the other side of the foil, that is to say 
on the electrode side. 
Preferably, but not necessarily, the electrode is connected to the foil to 
be charged, i.e. it is in intimate contact therewith. The electrode can 
also be formed by a corona discharge. 
As a result of the separation of the foil and the electrode from each other 
prior to development, according to the invention, the charges which are 
present on the electrode side of the foil are also used for making the 
charge image visible. When the charges on both sides of the foil are 
developed by depositing pigment on both sides of the foil by means of 
oppositely charged developers, an advantage is achieved over known methods 
in that an image with an optical density 2 is formed on the foil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the device shown in FIG. 1 a charge image which corresponds to an object 
4 is generated, by means of radiation 3, on a transparent, highly 
insulating foil 1. The back side of foil 1 is provided with an 
electrically conductive layer, electrode 2. The radiation generates charge 
carriers in a photoconductive layer 5. The photoconductive layer 5 is 
connected on one side to an electrode. The other side of layer 5 contacts, 
via a gas gap 7, the foil 1. The electrodes 2 and 6 are interconnected via 
a voltage source 8. Electrode 2 comprises, for example, a liquid layer, 
consisting of glycerine with an ionogenic addition, or a conductive solid 
substance. 
As denoted by plus and minus signs in FIG. 1, real negative electric 
charges are present on the free surface of the foil. The associated 
charges of opposite polarity are present on the opposite side of the foil 
with electrode 2. 
After generating the charge image, the electrode 2 on the back side of the 
foil 1 is removed. When glycerine with an ionogenic addition is used, the 
electrode is removed by rinsing first with water and subsequently with 
isopropanol. Water and isopropanol residues are removed by drying. As has 
already been stated, other electrode materials can alternatively be used. 
When the electrode is removed, however, care must be taken so that no 
additional charges are generated by friction. Cleaning must be performed 
without mechanical loading. The unavoidable transverse conductivity, i.e. 
electrical conductivity in the direction of the foil surface, is of no 
importance, because all image charges are rigidly retained by the charges 
on the dry side of the foil. However, simultaneous contacting of an 
electrically conductive medium by both foil sides must always be 
prevented. After removal of the electrode, both foil sides carry real 
electric charges. 
FIG. 2 corresponds to FIG. 1, however in FIG. 2 the voltage source is 
coupled to the foil via a corona gas discharge 9. This device directly 
produces a charge image which consists of real charges on both sides of 
the foil. 
In the device shown in FIG. 1, the electrode 2 must be 
radiation-transparent. FIG. 3 shows a device where this need not be the 
case. The foil 1 is situated on the side of the device which need not be 
radiation transparent. The foil 1 is arranged on a metal carrier plate 10. 
Between the carrier plate and the foil there is provided a liquid 
intermediate layer 2 which serves to form a homogeneous conductive 
connection between the foil and the carrier plate which can be readily 
interrupted. After the formation of the charge image in the device shown 
in FIG. 3, the foil 1 must be separated from the carrier plate 10 and the 
intermediate layer 2 must be removed therefrom. 
After generating the charge image in the devices shown in the FIGS. 1, 2 or 
3 and after separating the foil from the electrode, both surfaces of the 
highly insulating transparent foil carry the same number of real charges 
of opposite sign which represent an image. 
A device for developing these charge images is shown in FIG. 4. Opposite 
the charge images there are arranged developing electrodes 11a and 11b. 
The developing chambers 12a and 12b contain developer suspenions with 
oppositely charged pigment particles. During development, pigment is 
deposited on both sides of the foil 1. The symbols D.sub.1 * and D.sub.2 * 
will be described below. 
In order to clarify the invention, the already described state of the art 
is also shown in the drawing. As shown in German Offenlegungsschrift No. 
24 31 036 (FIG. 8), FIG. 5 herein shows an ionization chamber 15 which is 
bounded by electrodes 13 and 14 and in which an ionizable gas is present. 
A foil 1 is arranged in the center of the chamber. FIG. 5 also shows four 
charge carrier pairs which have been formed by radiation. For each charge 
pair, one negative or positive partner of the pair proceeds to an 
electrode and is lost to the process. In the device shown in FIG. 5, only 
the two negative charges on the top side of the foil and only the two 
positive charges on the back side of the foil can be developed. As has 
already been stated, this results in a density amounting to 1. 
For better comparison with FIG. 5, FIG. 6 shows a simplified modication of 
the device shown in FIG. 1. In FIG. 6, the reference numeral 2 again 
denotes a liquid of low conductivity, for example, alcohol or glycerine 
with ionogenic addition. The reference numeral 16 denotes an X-ray 
transparent, conductive carrier plate, for example of graphite or 
beryllium. As in FIG. 5, four charge carrier pairs are formed. At the end 
of the exposure, four negative charges are present on the foil 1. The four 
positive partners disappear in the photoconductive layer 5. If the image 
foil 1 in this condition is brought into contact with a developer in a 
device as shown in FIG. 7, without the foil being detached from the 
electrode, a density amounting to 1 is obtained again. 
FIG. 7 shows a customary device for liquid development of a charge image. 
Therein, a developing electrode 11 is arranged opposite the charge image. 
The developing electrode and the back electrode 2 of the foil 1 are 
brought into electrically conductive contact. The space 12 between the 
developing electrode and the foil surface is filled with a liquid 
developer. The symbol D.sub.2 will be described below. 
For example, if the pigment particles are positively charged while the foil 
surface is negatively charged, as shown in FIG. 7, pigment is deposited on 
the foil surface at the areas of negative charge. At the same time, 
however, the charge carrier distribution in the back electrode 2 of the 
foil which consists of a current to the developing electrode 11 also 
changes and causes equalization of the charge carrier distribution in the 
rear electrode 2. 
It can be established that the known method utilizes only the transport of 
the charged pigment particles to the foil surface for making the charge 
image visible, while all other charge carrier currents are not used. 
However, if the charged foil 1 is detached from the electrode 2 as denoted 
by an arrow in FIG. 8, the associated four positive charges adhere, due to 
the electrostatic force of attraction. The positive charges are located 
exactly opposite the negative charges on the rear of the image foil. The 
foil then accommodates four negative and four positive charges. These can 
be developed to produce a density amounting to 2. 
In order to demonstrate that a density amounting to 2 is obtained by means 
of the method according to the invention, three experiments (a, b and c) 
were carried out. These experiments will be successively described. 
As has already been described, in the device shown in FIG. 4 pigment is 
deposited on both sides of the foil 1 during development. This development 
corresponds to the experiment c yet to be described. The optical density 
D* then obtained has an additive composition D*=D*.sub.1 +D*.sub.2 (see 
the symbols in FIG. 4). 
As will be separately demonstrated hereinafter, the experiments reveal that 
D*.sub.1 and D*.sub.2 (experiment c) are identical to the optical 
densities D.sub.1 and D.sub.2 obtained when the same charge images on the 
two foil surfaces are separately developed by means of a device as shown 
in FIG. 7 (experiments a and b). 
Instead of using a device as shown in FIG. 7, for example, for the negative 
surface charges (experiment a) the device shown in FIG. 4 was modified as 
follows in order to obtain the device shown in FIG. 7. 
The foil surface carrying the positive charges is provided with an 
electrode which itself is conductively connected to the developing 
electrode 11b. The pigment particles deposited on the free surface produce 
the optical density D.sub.2, i.e. the same value as the value to be 
assigned to the negative charges during development in accordance with 
FIG. 4 (D*.sub.2). After deposition (according to FIG. 7), the capacitor 
device has been completely or substantially completely discharged. This 
means that no further charges can be deposited by a subsequent method, 
unless a new charge pattern is impressed. 
The deposition shown in FIG. 4, however, results in a higher optical 
density. For example, if the two developers used are equally sensitive, a 
factor of two times the optical density is achieved. 
For all three experiments a polyethylene terephthalate foil is charged to a 
surface potential of -400 Volts by means of the device of FIG. 1, which 
means that the initial surface charge density is always the same. Two 
different developers are used, one with positively charged pigment and the 
other with negatively charged pigment, contained in the upper part and the 
lower part, respectively, of the developing chamber shown in FIG. 4. 
Experiment (a) The upper part of the developing chamber according to FIG. 4 
is used in this experiment. The lower side of the foil, carrying the 
positive charges, is provided with an electrode. A conductive connection 
is made from this electrode to the developing electrode 11b. After 
development with the positively charged developer, the optical density is 
measured: D.sub.2 =0.82. 
Experiment (b) The lower part of the developing chamber according to FIG. 4 
is used and the procedure is otherwise according to experiment (a). The 
optical density is then measured: D.sub.1 =0.65. 
Experiment (c) Both developing chambers according to FIG. 4 are used. The 
optical density is measured: D=1.42. 
Taking into account the measuring accuracy , D* is additively composed of 
D.sub.1 and D.sub.2. When the pigment of the negatively charged developer 
is removed from one side of the foil, the subsequent measurement of the 
optical density produces 
EQU D*.sub.2 =0.78. 
The same is applicable to the developer with positively charged pigment 
removed. 
D*.sub.1 =0.65 is obtained. 
The following is applicable within the accuracy of the above measurements. 
EQU D*.sub.1 =D.sub.1, D*.sub.2 =D.sub.2. 
FIG. 9 shows a developing device which comprises two developing tanks 17a 
and 17b, for example of polymethacrylate, in which two developing 
electrodes 11a and 11b, for example gauze with a mesh width of 0.5 mm, are 
arranged so that their distances from the surfaces of the charged foil 1 
amount to from 0.1 to 5 mm, preferably from 0.5 to 1 mm. They are 
conductively connected to contacts 18a and 18b which are accessable from 
the outside. As desired, these contacts may be short-circuited during 
development or may be connected to a voltage source 8 in order to increase 
the image contrast, that is in order to compensate for any background 
charges. The siphon vessels 19a and 19b contain developers of opposite 
polarity. Via tubes 20a and 20b, these vessels are connected to the 
developing spaces 12a and 12b in the developing tanks 17a and 17b, tanks 
17a and 17b can be filled with developer up to riser pipes 21a and 21b. 
After development, the developing spaces are emptied by lowering the 
vessels 19a and 19b. The contacts 18a and 18b are then disconnected from 
each other or from the voltage source 8, the tank halves 17a and 17b are 
separated from each other, and the developed foil 1, is removed.