US3457070A

US3457070A - Electrophotography

Info

Publication number: US3457070A
Application number: US471606A
Authority: US
Inventors: Yoshiyuki Watanabe; Koichi Kinoshita
Original assignee: Matsuragawa Electric Co Ltd
Current assignee: Matsuragawa Electric Co Ltd
Priority date: 1964-07-25
Filing date: 1965-07-13
Publication date: 1969-07-22
Anticipated expiration: 1986-07-22
Also published as: DE1497164B2; GB1120123A; FR1454439A; NL6509608A; DE1497164A1; BE667299A; DE1497164C3

Description

y 1969, YOSHIYUKI WATANABE ET AL 3,457,070

ELECTROPHOTOGRAPHY Filed July 13, 1965 2 Sheets-Sheet 1 FZ a mvrsmons Ham/Y m WATA/YABE By Kale/41 KIA/05m TA AT TORNEYS.

United States Patent US. Cl. 961.4 18 Claims ABSTRACT OF THE DISCLOSURE A method of electrophotography in which an electrostatic latent image is provided on an element made up of a thin electrically insulating transparent layer that is bonded throughout its entire area to a photosensitive layer, the element having a conductive electrode on the side of the photosensitive layer opposite the transparent insulating layer. The latent image is formed on tthe transparent insulating layer by first subjecting the element to an electrical field to deposit charges of one polarity on the element, thereafter illuminating the element with a light image while subjecting the element to an electrical field to the deposit charges of a polarity opposite to the first polarity on the insulating layer, stopping the illumination of the element with the image and removing the electrical field. The latent image is characterized by not being erased by subsequent exposeure to ambient light.

The present invention relates to improvements in electrophotography according to which a latent image impressed as an electrical charge pattern on a photosensitive plate is developed by applying finely divided particles to the charged areas.

The present invention provides important advantages over known systems for electrophotography. The known systems of electrophotography may be categorized into the following four types in accordance with the basic principle underlying each such type.

The first involves the so-called xerography or electrofax processes in which a surface charge initially induced on the surface of a sensitive element is locally and selectively released under irradiation by light. Such systems are described in US. Patents Nos. 2,221,776 and 2,727,808.

The second involves a process such as that described in United States Patent 2,825,814 wherein a layer of photoconductive material is used as a photoelectric conversion element to obtain an induced electrostatic charge pattern on a specially prepared recording paper.

The third involves a process, such as that disclosed in Japanese Patent 3,017 of 1959, wherein an internal polarization is induced in a non-photosensitive recording plate of special construction to accomplish copying, utilizing residual internal polarization effects.

The fourth involves the so-called persistent internal polarization process, wherein a latent image on a phosphor material induced by persistent internal polarization of such material exposed to light is utilized to effect copying.

Probably the most widely used of these four processes is Xerography, now widley commercialized. Electrophotographic processes employing xerography principles, however, require certain basic and exacting limitations on the materials used as photosensitive elements, as the electric current flowing through the photosensitive element is used to induce required charge patterns. According to patent specifications which teach this process, it is a requirement of the process that a photosensitive ele- 'ice ment be used having an ohmic resistance that is higher in darkness, and that the electrostatic charges induced on the surface of the photosensitive element be locally and selectively retained by making use of this characteristic of the photosensitive element. Additionally, the process, as an optical system, requires an element of high photosensitivity, and the surface of the photosensitive element itself must exhibit photoconductivity. Thus, the process requires integration of certain conditions which are essentially contradictory to each other to produce a practically operable system. The requirement that the ohmic resistance of the surface must be high in a dark environment, plus the requirement that the photosensitivity of the material must also be high, rigorously limits the selection of materials which can be used for the element. As a practical matter, therefore, while a number of technical difficulties connected with the manufacture of suitable photosensitive elements were overcome, by treating ZnO, or using spattered Se films to retain the mechanical strength of the element, low sensitivity remains as one of the serious problems. Another difficulty is the fact that the photosensitive element must be kept in the dark after the completion of the exposure operation until the developing operation is completed, since the latent image will be lost if exposed to light.

The second system makes use of a phenomenon in which localized and selective irradiation by light induces transfer of electric charges from the photosensitive material to the recording plate when such photosensitive material is spaced from the recording plate by a narrow air gap of from about 2 to 10 microns in thickness. It is noteworthy that this narrow air gap operates as a specific type of resistance component, and that the transfer of electrical charges does not occur until the electric field intensity selectively impressed across this resistance attains a certain critical value; however, when such field intensity does reach this critical value, the migration of charges takes place with abrupt intensity. The process makes use of this phenomenon in an effort to obtain a higher resolution of the copy. Conversely, this phenomenon is obviously essential in this particular process to maintain a latent image induced on the recording plate. One of the major technical obstacles associated with this process is the necessity of maintaining such as critical narrow air gap between the photosensitive element and the recording plate.

The third system attempts to make use of the residual internal polarization. In this process, the recording plate is made so that prolonged retention of residual internal polarization is possible by mixing a specially selected material in the recording plate; this enables retention of internal polarization of the recording plate that is induced by a rise of electric field intensity in areas selectively distributed over the recording plate, which is in turn induced by the selectively localized decrease of field intensity impressed over the photosensitive element at areas thereof exposed to light. Since this process makes use of the internal polarization, it differs substantially from the above-described two processes, and particularly does not require the narrow air gap of the second process. This process, therefore, is less difficult to practice as a practical matter.

It must be noted, however, that only a limited selection of materials is available to provide the desired retention of the residual internal polarization, which is the most important feature of this process. Furthermore, in actual practice, only a very short retention of internal polarization can be achieved, and the process requires numerous technical refinements.

In contrast to the third process described above, which essentially attempts to make use of induced internal polarization in a non-photosensitive material, the fourth process, i.e., the so-called persistent internal polarization (PIP) process, makes use of a phenomenon known as persistent internal polarization induced within the photosensitive element. By using a phosphor material and controlling its deep trap level, it is possible to induce a latent image on the photosensitive plate and to retain such image in the dark. The PIP process not only requires that electrical charges be trapped at the trap level in the areas exposed to light, but also requires that a certain amount of resistance be present in areas exposed to light. It appears, therefore, that photosensitive materials used for the PIP process are essentially limited to phosphor materials, thus indicating a major obstacle for any attempt to improve the light sensitivity of the elements used in the PIP process. For example, CdSwhich is one of the photosensitive materials preferably used in the present invention cannot be used for the PIP process because it causes a high photosensitivity of areas exposed to light, which in turn decreases localized distribution of electric field over the sensitive layer and increases the trapping of electric charges at the trap level. Consequently, the electric po tential diiference between the areas exposed to light and the areas not exposed to light can be so little that an image-producing charge pattern will not result. The characteristics described above are generally common to all highly photosensitive materials, which indicates that highspeed copying by the PIP process has major limitations.

One of the objects of the present invention is to pro vide an improved electrophotographic process and apparatus for eliminating basic defects of electrophotographic copying processes with which the art is now familiar, particularly those defects of the known processes relating to the lack of photosensitivity, low strength of latent image and poor persistence of latent image.

Another object of this invention is to provide electrophotographic copying processes by which intense latent images can be formed by relatively simple operations.

Still another object of this invention is to provide a novel photosensitive element using photoconductive materials of high sensitivity that heretofore have been unusable in the field of electrophotographic copying, as well as certain methods whereby to determine the operative characteristics of such photosensitive elements.

Still another object of the present invention is to provide certain control methods whereby light images are converted into electrophotographic latent images by using such a novel photosensitive element.

In the present invention, problems like those discussed above are avoided or solved. Desired contact of a transparent electrode and the resistance surface of the photosensitive layer can be achieved by a simple application of pressure. It is immaterial according to the invention whether there is an air gap that makes it diflicult to precipitate a release of surface charges, or whether the transparent electrode and the insulating layer of the element are so close that charges forming the latent image can easily migrate into the transparent electrode. In any case, the image-inducing and image-retaining mechanism of the present invention can operate exactly in the same manner.

The present invention relates to the formation of a latent image, defined by electrical charge patterns corresponding to the light image, directly on a thin layer of material having high electrical insulation properties and comprising an integral portion of a photosensitive element. The latent image may be developed by applying finely divided particles of any suitable dry developing material to the charge patterns and transferring the resulting visible image to another surface, such as the surface of a recording paper sheet. Use of a special recording paper is not required. The latent image does not scatter or disappear even when the element containing such latent image is subsequently exposed to light.

The foregoing and other objects, advantages and fealllIG of th pr sent in ention will b app e t om the .4 following description of several embodiments of the invention in connection with the accompanying drawings in which:

FIGURE 1 is an example of a photosensitive element used in the present invention, with a section thereof broken away to show its construction;

FIGURE 2 is a schematic side view of apparatus for forming a charged latent image on the photosensitive element, according to this invention;

FIGURE 3 is a graphic illustration of an example of relative values of impressed voltage and illumination by light;

FIGURE 4 graphically illustrates changes in relation to time of electrical charge potential on the surface of a photosensitive element used in this invention;

FIGURE 5 diagrammatically illustrates means, and an operation, for developing the latent image on the photosensitive element by application of dry developer particles;

FIGURE 6 diagrammatically illustrates means and operations for transferring the latent image to a sheet of paper and fixing it on the paper;

FIGURES 7, 8 and 9 are, respectively, a representation of an equivalent circuit and characteristic curves illustrating the operation of the photosensitive element embodying the invention;

FIGURE 10 is a schematic drawing illustrating an example of the electrical potential distribution in a photosensitive element of the invention;

FIGURE 11 is another example of relative values of impressed voltage and illumination of light;

FIGURES 12A and 12B are graphic illustrations of still another example of the relationship to time of electrical charge potential on the surface of a photosensitive element in this invention;

FIGURE 13 is another example of a photosensitive element embodying this invention with a section thereof broken away to show its construction;

FIGURE 14 is a graphic illustration of another example of relative values of impressed voltage and illumination of light, for a photosensitive element intended for a particular use.

Example 1 A photosensitive element 1 embodying the invention, shown in FIGURE 1, is of laminated construction and comprises a photosensitive layer 2, a thin layer of a material of high electrical resistivity (hereinafter sometimes referred to as an insulating layer) 3 and an electrode 4. The photosensitive layer 2 is a layer preferably of about micron thickness formed by bonding crystals of copper (Cu)-activated cadmium sulfide (CdS) and having grain size of approximately 10 microns, with cellulose acetate as the adhesive bonding agent. The thin insulating layer 3 is preferably a 12.5 micron thick layer of, for example, solid, transparent polyester syntheic resin material; the insulating layer 3 is laminated to one of the surfaces of the photosensitive layer 2 by a polyester synthetic resin adhesive transparent to light. An electrode 4 of a thin layer of flexible, electrically conductive material, such as aluminum, copper or silver sheet or foil, is firmly laminated by an electrically conductive adhesive, for example, an acrylic resin adhesive containing silver particles, to the side of the photosensitive layer 2 opposite the side thereof laminated to the insulating layer 3. Since all layers comprising the element are flexible, the integral element 1 of this embodiment itself retains substantial flexibility.

FIGURE 2 is a schematic view of an electrophotographic system embodying the present invention for inducing a latent image or charge pattern corresponding to a light image impressed on the element 1. An electrode 5 of glass that is transparent to light and electrically conductive, such, for example, as NESA glass made by orning Glass Works of Corning, NY is di p h a conductive surface 5a in contact with the surface of the insulating layer 3 of the element 1. A direct current (DC) supply source 6 impresses an electric potential across the transparent electrode 5 between its conductive surface 511 and the electrode 4 of element 1. A switching means 7 is connected between the current source 6 and element 1 to connect or disconnect the electric current, and to change the polarity of the electric potential so impressed across the transparent electrode 5 and the electrode 4. Preferably, the element 1 is resiliently pressed against the transparent electrode 5 so that substantially the entire surface of the insulating layer 3 is in contact with the electrically conductive surface 5a of the transparent electrode 5. An optical system 8 projects a light image of an object 9 to be reproduced against the surface of the element 1 through the transparent electrode 5. Separate light sources 10 can be provided to illuminate the object 9. It is understood that the device shown in FIGURE 2 represents but one of many possible devices by which the process of the present invention can be carried out.

The manner in which a latent image is produced in the element 1 is graphically illustrated in FIGURE 3 in which the ordinate in the upper part of the figure is the electric potential impressed between the transparent electrode 5 and the electrode layer 4 of the element 1, and the ordinate in the lower part of the figure represents the light projected on element 1. The abscissae represent time in each case. As shown in FIGURE 3, during passage of time from t to t an electric potential is applied across the transparent electrode 5 and the electrode layer 4 in such a manner that the polarity of electrode 5 is positive with respect to the layer 4. The element 1 is not at this time exposed to light. At time t the polarity of the electric potential applied is reversed so that the polarity at the transparent electrode 5 is negative with respect to layer 4; this negative potential is maintained from time t to time t during which time the element 1 is exposed to a light image as shown in the lower part of FIGURE 3. At time t the electrical potential is removed and exposure to the light image is stopped. Thereafter, the transparent electrode 5 is removed from the element 1. The surface of the insulating layer 3 of the element 1 at this time contains an electrical charge pattern forming a latent image which corresponds to the incident light image to which the element 1 was exposed during the period t to t After the removal of the transparent electrode 5 from the element 1 as above described, the latent image so formed demonstrates almost no attenuation even under a subsequent exposure to light, nor is there any diffusion of charges comprising such latent image.

In an actual example, the time intervals t to t and t to t were both 0.1 second, and the intensity of the light image projected against the element 1 was 20 lux. Under such exposure conditions, when the voltage supplied by the DC supply source '6 was 2,000 volts, the surface areas of the insulating layer 3 exposed to light were charged with electrical potential of l,500 volts, whereas the electrical charge potential of the areas not exposed to light remained substantially at zero level.

The polarity of the surface charge of the element 1 thus induced by impression of an electric field is the same as that of the transparent electrode 5, which indicates that the latent image formed on the element 1 is not formed by internal polarization, per se, of the element, but substantially by electrostatic charging of the surface of the insulating layer of the element 1.

FIGURE 4- graphically illustrates the changes of electrical potential in the element 1 with respect to time in this embodiment of the invention. The symbols t t and t represent respectively the same time sequences described at t t and t in connection with FIGURE 3. Curve a depicts changes in electrical charge potential on the upper surface of the element 1 during the interval t through 2 The broken line a connected to curve a indicates that continued impression of the electrical potential having the same polarity will not further increase the surface charge potential of the element 1. Curve b represents the change on the surface electrical potential of the element 1 at areas where the incident light does not strike the element surface during the time interval represented by t through t Curve 0 represents the changes in the surface electrical potential of the element 1 at areas where incident light strikes the element surface during the time interval t through tg.

When impression of the electrical potential is continued beyond t curve b continues to drop downward at a change rate which is substantially smaller than the change rate of drop observed during the time interval t through t as shown by broken line b thus approaching, but gradually, curve 0; and curve 0, as indicated by broken line c, shows no appreciable drop even when impression of the electrical potential is continued beyond t Line G represents the electrical potential difference at time t of the element 1 between the areas exposed to incident light and the areas not exposed to incident light; this also represents the latent image intensity defined by the electrical charge pattern on the element 1 at time 1 The electrical charge pattern induced as a latent image on the element 1 can readily be developed to a visible image by applying a finely divided charged dry developer of a suitable known type that is electrostatically attractable to the exposed or non-exposed areas to produce a visible deposited image. For example, as a negatively charged toner, carbon black impregnated styrene resin may be used; and as a positively charged toner, a thermoplastic, methylated paraflinic resin, essentially composed by polymers of pinene, such as Piccopale resin manufactured by Pennsylvania Industrial Chemical Corp. of Clairton, Pa., impregnated with oil black, may be used. In either case, powdered iron may be used as carriers for the toner. Particularly because of the high potential difference between the areas charged and the areas substantially devoid of charges, the visible deposited image will have a fine resolution and will provide excellent reproduction of the latent image on the element. In the examples given, the negatively charged toner will adhere to the areas that were not exposed to light, while the positively charged toner will adhere to the areas that were exposed to light. This can be accomplished, for example, as shown in FIGURE 5, by placing the charged photosensitive element 1 with its charged surface facing down over a container 21 containing the toner particles admixed in a suitable carrier 22 that are moved by agitator 23 into contact with the element to form an image 24. If necessary or desired, undesired particles can be removed by conventional means, as by blowing air. As shown in FIGURE 6, the visible image can then be readily transferred to a sheet 25 of paper or the like by pressing the paper against the surface of the element 1 carrying the visible image to transfer the particles forming the image to the paper by light pressure applied by an electrode 26, preferably a roller electrode, electrically connected to backing electrode 4 of element 1. The paper can then be passed to a processor 27 that sets the image on the paper, as by fusing it in the conventional manner by known means.

There is offered here a theoretical explanation regarding the formation of the latent image and the reasons why processes embodying the present invention operate effectively while utilizing highly photosensitive materials as the sensitive element. It is to be understood, however, that the theoretical explanation offered herein merely represents one of the possible elucidations of the phonemena utilized in the present invention and any different between the explanation herein offered and those which may hereafter be offered will not in any way alter or modify the substance of the present invention.

FIGURE 7 depicts an equivalent circuit, representing the light sensitive element 1 when an electric potential E is impressed on the element, in which Z represents the impedance component of the transparent insulating layer 3 comprising the surface of the element 1, and Z represents the impedance component of the light sensitive layer 2. A DC field impressed on

layers

2 and 3 from outside of the layers is essentially electrostatic in nature. Such DC field will be distributed according to the impedance present, because the light sensitive element of the present invention exhibits extremely high resistivity and current flow is'minimal. In the element 1 used in Example 1, the polyester resin film constituting the thin insulating layer 3 had a high unit area resistivity of approximately l.25 ohms per square centimeter and a dielectric constant of 3.1. The light sensitive layer 2 exhibited, after having been stored in the dark for a prolonged period of time, a unit area resistivity of about 1x 10 ohms, which reduced to approximately 1X 10 ohms when exposed to light having intensity of 20 lux. The dielectric constant of layer 2, determined by a method generally used, was 2.4 in the dark, which increased to as high as 30 or more when exposed to light having intensity of about 20 lux.

Accordingly, when an electric potential of about 2,000 volts is impressed across the element, the potential which theoretically impressed on the insulating layer 3 in the darkness should be about 14% of the impressed voltage, which should increase up to about 66% when the element is exposed to 20 lux light. In practice, however, a large amount of field intensity is lost across the adhesive layer and the lamination surfaces, which act as resistances. The charge potential induced over the surface of the insulating layer 3 depends upon the intensity of the electric field impressed across the element. When the impressed voltage is high, the charge potential induced on the insulating layer surface is high; and conversely, when the impressed voltage is lower, the potential at the insulating layer surface is also lower.

Although the potential distribution at each layer comprising the light sensitive element is as discussed above, the electric potential appearing on the surface of the insulating layer 3 of the element exhibits peculiar characteristic curves shown in FIGURE 8, which illustrates the changes in the surface potential of the insulating layer 3 of the element 1 with respect to time when an electric field is impressed on the element. Curves D and D show the changes exhibited when respectively positive and negative potential is applied to the transparent electrode 5 while the element 1 is in the dark. Curves L and L show the changes exhibited when respectively positive and negative potential is applied to the transparent electrode 5 while element 1 is exposed to light. As apparent from these curves, each curve converges to a specific value when impression of the electric potential is continued over a length of time. However, in practice, the respective values to which said curves converge do not coincide with the changes in the dielectric constant and impedance referred to in the discussion of FIGURE 7 and as between L curves and D curves, the surface potential levels shown by the curves illustrating those of the unexposed element (D curves) does not appear proportionate to the difference of the potential between the exposed and the unexposed element as may be theoretically derived from the changes of impedance and dielectric constant of the sensitive element, and appears somewhat higher than the expected value.

component as described above, it would be more realistic I to consider that the layer functions essentially as a ca pacitive component, Furthermore, although the light sensitive layer 2 in its unexposed state exhibits a very high resistance', it would function as a capacitive component also. Since one end of each such capacitive component is directly connected to an electrode, its charging time should be extremely short. In FIGURE 8 the length of the time required to have the surface potential saturated to a specific value is shown as T, which in terms of the photosensitive element described in Example 1 was approximately 0.1 second. However, when the charging time is computed, assuming that pure capacitive components alone were involved, the results indicate that the saturation should be reached in much shorter time than the actual saturation time given above. The fact that it takes a longer period of time for saturation, coupled with the fact that the level of the electric potential of the unexposed element is not determined by the impedance and dielectric constant of the element 1, appears to suggest occurrence of unique potential changes within the light sensitive layer 2.

We have investigated into possible reason why the preexposed element would exhibit such a potential level which does not conform to the impedance and the dielectric constant of the element.

The following consideration is offered to explain this phenomenon. It is known that the sensitivity of photoconductive materials is maintained by adding suitable activators. It is also known that the Cu-activated CdS employed in the photosensitive layer 2 of Example 1 has a large number of trap levels at the depth of about 1.4 electron-volts (ev.) and 0.03 ev. from the bottom of the conduction band. The fact that CdS has two such trap levels, one shallow and another deep, suggests an important basis for considering the photoelectric current response to the incidence of light manifested in a photoconductive system employing CdS, and it bears close relevancy to the present invention as explained below.

When a photoconductive material is exposed to illumination of light, while impressing a DC potential, a certain persistence characteristic is observed, i.e., the photoelectric current will continue to flow for a length of time even after the excitation is removed. In case of CdSzCu, such persistence characteristic is particularly prolonged, as confirmed by experimental results. It is also clear that this characteristic is due to Cu added as the activator to CdS crystal for the purpose of increasing, upon exposure to light, the density of charged carriers in the conduction band within CdS crystal which participate in photoconductive effect.

Such higher level of potential observed on the unexposed element is obviously undesirable in electrocopying, because the phenomenon only contributes toward the lowering of the potential difference between the areas of the element exposed to light and those not exposed to light, which means that the signal-to-noise ratio of the element leaves much to be desired.

We have discovered that the foregoing disadvantage can be overcome by applying an electric field of the inverse polarity to the element immediately prior to the exposure of the element to light signal, which forms an important and essential element of the present invention, and We have further discovered that by taking this step, any effect from the prior history of light irradiation on the element can effectively be eliminated, with a result that the level of the electric potential of the unexposed areas of the element can be held at substantially the value of the socalled dark current, while the potential level of the exposed areas of the element remains virtually the same with or without such pre-exposure impression of the inverse polarity, thus enabling one to obtain an optimum signal-tonoise ratio in forming a latent image on the photosensitive element. Such effect of the pre-exposure impression of the inverse field will become readily apparent from the following explanations.

When excitation by light is removed while continuing to impress a DC potential across a photosensitive layer 2 of CdS:Cu, electrons will be present in the conduction band for a considerable length of time, and the impedance of the crystals rises but gradually. If in the course of such gradual rise of impedance, when the polarity of the DC field impressed on the layer is abruptly reversed, sudden increase of impedance will occur. FIGURE 9 illustrates this change in terms of current flow. The potential impressed in FIGURE 9 has the same polarity during the time intervals respectively designated as t to t and then beyond t During the time interval shown at t to t the polarity is reversed. Excitation by light is performed during the interval shown as t to t A decrease of current fiow commences immediately after the removal of excitation, and would follow the curve indicated by the broken line except for reversal of the polarity t When the polarity of the field is reversed at t there is an instantaneous substantial flow of current, which decreases rapidly and settles down to a flow of small amount. When the polarity is again reversed at i back to the original polarity, an instantaneous substantial flow of sudden current is again observed, which attenuates rapidly in a short period of time and reaches the value of the so-called dark current.

This phenomenon is observed more markedly when either both or one of the surfaces of the sensitive layer 2 is coated with a thin layer of highly resistive material. When only one of the surfaces of the sensitive layer is coated with the insulating layer, however, the changes observed will not be symmetrical depending upon the polarity of the potential impressed. The type of current fluctuation described above are more markedly observed in a photoconductive layer 2 comprising particulate photoconductive material crystals bonded together into a thin film by electrically insulating adhesive, rather than a layer formed of monocrystals or spattered films. This phenomenon is important as it controls the impedance of the sensitive layer.

Utilizing the embodiment described in Example 1, We offer hereunder the theoretical explanation of occurrences in the element 1 at each of the stages of the latent image formation steps already described in reference to FIGURE 3 and FIGURE 4. Some explanation of the image retention mechanism of the element 1, particularly with respect to the important feature of the present invention, namely, that the latent image formed on the surface of the insulating layer 3 will not be destroyed after the removal of the transparent electrode 5 by any subsequent illumination of the element 1 by light, will also be made.

During the time interval of t to t in FIGURE 4, the residual effect of the light to which the CdS crystals of layer 2 may have been previously exposed is predominant, and the density of electrons within the conduction band or in the trap levels, which can easily liberate electrons into the conduction band, is quite high. By applying a DC current having a definite polarity to the sensitive layer 2, the conduction electrons readily drift toward the positive pole and thus form internal polarization. The polarization of the sensitive layer 2 thus effected, as shown in FIGURE 10, is in the direction opposite to the polarity of the electric field externally applied, and the charge distribution within the sensitive layer 2 will be as shown in the figure. At this time because of their polarized distribution, electrons in the conduction band recombine at a low rate and do not decrease rapidly. Even though the field distribution as defined by the intrinsic impedance and dielectric constant appears to require that most of the field is impressed on the impedance component Z of the sensitive layer 2, and only a small part thereof on the impedance component Z of the insulating layer 3, because of the polarized charges induced within the sensitive layer, there will occur a corresponding increase in the intensity of the field distributed to the impedance component Z of the insulating layer, which accounts for a higher charge potential which appears on the surface of the insulating layer as shown in FIGURE 4.

This may also be explained by saying that there will be an apparent reduction of impedance component Z; or increase of dielectric constant of the photosensitive layer 2 because of its prior history of exposure. What is of particular importance is the fact that the internal polarization of the sensitive layer is not the only agent which maintains the field equilibrium, but accumulation of charges having opposite polarity on the reverse side of the insulating layer as well as on the boundary surfaces of photosensitive crystals and bonding agent also contributes in the maintenance of this balance.

With respect to time interval t to 1 of FIGURE 4, the occurrences in the areas not exposed to light will be discussed first. In this time interval, conduction electrons still remaining in the conduction band or conduction electrons among the charged carriers distributed by internal polarization liberated from the trap levels due to field emission induced by the field change, migrate within the conduction band toward the newly set up field in the direction opposite to the direction of drift observed in the time interval t to t There will be a rise in the density of recombining, and the probability of recombination of electrons (which was low during the time interval t to t increases suddenly, thus sharply reducing the number of electrons participating in conduction. No additional polarization results, so that immediately after the impression of reverse voltage, there will be an abrupt impression of electric field in the insulating layer 3 of polarity opposite that of the field impressed during the tiume interval of t to t as shown by curve b in FIG- URE 4.

When a certain point is reached, there will be a decrease in the rate of field build-up and it continues but at a markedly slow rate, and as the polarization set up during the interval of t to t becomes depolarized by heat or other causes, polarization in a new direction proceeds, but only gradually, thereby slowly building up the surface charge. This means that the phenomenon is equally explicable as a rapid apparent increase in the impedance component Z of the sensitive layer.

The areas exposed to light during the time interval of f to t will now be discussed. The polarization which has been induced during the time interval of t to I is completely depolarized by the incident light. The impedance component Z of the sensitive layer 2 drops, and the dielectric constant increases as a result, which also increases the intensity of the field impressed on the insulating layer. This in turn induces a rise in the charge potential at the surface of the insulating layer. The polarized carriers in the sensitive layer will become trapped according to the polarity of newly impressed field, and at the same time, there will be induced within the sensitive layer and the insulating layer certain internal charging, which occurs completely independently of the trapping of charged carriers.

Both at the areas unexposed to incident light and at the areas exposed to incident light, the internal polarization and internal charges induced will operate to maintain the equilibrium with the charges on the surface of the insulating layer 3. Therefore, even when the electric field is removed as the next step of operation, this equilibrium does not go out of balance. This is one of the important features of this invention. In other words, if it was assumed that the retention of such charge balance of the photosensitive element is not possible, then when the field is removed the surface charges would cancel out across the transparent electrode 5 with which the surface of the insulating layer 3 is closely in contact, and the areas exposed and unexposed would settle into the same potential level. Actually, when the transparent electrode 5 and the insulating layer 3 are in contact, the absence of internal charges of opposite polarity will result in the homogenizing of the surface charges. Of course, the internal field induced by the trapped carriers tends to disappear with passage of time, which causes changes in internal charge structure, bringing about corresponding changes in the surface charges present on the photosensitive element.

As a next step, the transparent electrode is removed from the light sensitive element. As discussed, there is an internal distribution of charges within the element which maintains an equilibrium with the surface charges, so that even if transparent electrode 5 is removed unevenly from the element, any capacitive changes induced by such uneven peeling will hardly cause redistribution of charge pattern, and the latent image remains unperturbed. Since surface resistivity of the insulating layer of the element is selected to be quite high, it prevents surface diffusion of such charges.

Accordingly, after the removal of the transparent electrode, it is not necessary for the element 1 to have any internal mechanism to retain the latent image induced on the surface of the insulating layer. Any subsequent exposure to light of the element will not affect the charge distribution on the surface 'of the element. The subsequent development operations can be handled in light. Furthermore, the latent image on element 1 will be retained semi-permanently, until such time as the element is again exposed to a new electric field. What is particularly noteworthy in the photosensitive element 1 is the fact that the charges of opposite polarity internally present in the element do not extend their influence externally because of the backing electrode 4 integrally comprising the sensitive element. The presence of the backing electrode additionally suggests a number of interesting technical possibilities.

As described above, the latent image defined by the charge pattern present on the surface of the insulating layer 3 interacts with the internal charges and shows hardly any attenuation. After a simple exposure to light to depolarize the internal polarization which may have been present from the prior use of the element, when an electric field is impressed across the element, the entire process as described above commencing with t to and thereafter can be repeated. Accordingly, even when charges are present on the surface of element 1, such charges will not affect in any way the subsequent formation of a fresh latent image on the element. The element will be subjected to certain mechanical stimuli, but such stimuli do not extend their effect to the sensitive layer, and the effect will be limited to the thin insulating layer, which indicates that the possibility of fatigue or mechanical breakdown of the photosensitive layer is nil.

From the foregoing it is apparent that the charge distribution within the photosensitive layer of the element causes changes in the effective field distributed on the surface of the insulating layer and, without the provision of any air gap between the element and the transparent electrode or without special conditioning of the insulating layer, induces an electrostatic charge pattern forming a latent image of the pattern of incident light on the surface of the insulating layer.

Particularly noteworthy is the fact that the contacting of the transparent electrode and the photosensitive element does not require special skill or arrangement. It has been found that after the electrostatic charges have been induced according to the procedure described above, when the transparent electrode 5 and the electrode 4 laminated to the photosensitive element are short-circuited before removing the transparent electrode from the element, the latent image is destroyed. Such destruction of the image occurs as a result of a sudden drop of the surface potential of the element to zero level, and indicates that the electric charges present at the surface of the element are dispersed through the short-circuiting. In this case, the internal charge potential of the element which interacted to retain the equilibrium with the surface potential will also be destroyed. Considering this in terms of the polarity of the latent image and the polarity of the electric field impressed, it appears that the latent image is formed electrostatically, and that space at t e contacting surfaces of the transparent electrode and the insulating layer will not prevent transfer of charged carriers between the contacting electrode and the surface of the element. As a practical matter, any space that may be present between the transparent electrode 5 and the surface of the insulating layer 4 of the element 1 does not affect the image formation, and it has been ex perimentally established that it is sufficient that the transparent electrode be in contact with the element with appreciable mechanical pressure. Conversely, image formation as described above will not be substantially adversely affected even if space may exist between the transparent electrode and the element.

The electric charges within the sensitive layer act as the retainer and controller of the charges distributed over the insulating layer surface, as indicated below.

After electrostatic charges corresponding to the light image have been induced on the insulating layer 3 of the element 1, when light is irradiated before the transparent electrode 5 is removed from the element but without short-circuiting the electrode 4 of the element and the transparent electrode 5, the potential difference between the exposed areas and unexposed areas will disappear, although the average potential over the entire surface drops only slightly, so it is not possible to develop the image. This indicates that the latent image formation mechanism will become lost as a result of the equalization of charge distribution induced by exposure to light. Thereafter, when the transparent electrode and the electrode of the element are short-circuited, the surface potential, like in the previous case, drops completely to zero level.

As indicated above, the construction of the photosensitive element and the control method thereof in the present invention make possible the formation of a stable latent image while utilizing high sensitivity photoconductive materials which have not heretofore been used because their characteristics, manifested because they are highly sensitive to light, made them unfit for prior electrophotographic processes.

As discussed in Example 1, factors which contribute in latent image formation are the internal polarization and the charge distributions on the reverse side of the insulating layer or in the sensitive layer having a polarity which is the reverse of that of the charge pattern present on the surface of the insulating layer which defines the latent image. The internal polarization, as already indicated, contributes in inducing field distribution at the time the latent image is formed and in the image retention thereafter until and only until the transparent electrode is removed from the surface of the photosensitive element. The internal polarization may disappear at any time after the removal of the transparent electrode, without in any way. affecting the latent image formed on the surface of the insulating layer. In other words, the polarization need not be as deep in the trap level as would be necessitated when persistent internal polarization is desired, and this forms an important feature of the present invention as it eliminates the difficulties of selecting photosensitive materials suitable for the persistent internal polarization processes. Put in another way, generally, a material which exhibits persistent internal polarization effects would have a low photosensitivity and low impedance change characteristics except those resulting from the internal polarization effect. On the other hand, in the present invention materials of high photosensitivity and high impedance change characteristics may be used, thereby obtaining a very fast electrophotographic plate. However, the use of materials which exhibits persistent internal polarization effects are not precluded from the present invention, as shown in the following example.

Example 2 ZnCds in powder form, silver (Ag) activated and having a grain size of about 5 microns, was molded into a thin layer 2 of about 50 micron thickness using cellulose acetate as the bonding agent. To the reverse side of the photosensitive layer thus formed an electrode 4 made from thin aluminum film, and on the obverse side an insulating layer made from 12.5 micron thick polyester resin film, were respectively laminated by suitable adhesive. This constituted the photosensitive element 1. The transparent electrode 5 and the DC current source 6 were substantially the same as those of Example 1.

During the image formation process, as shown in FIGURE 11, the element 1 was illuminated substantially uniformly over the entire surface by light during the time interval t to t while at the same time a voltage was impressed across the two electrodes such that the polarity of the potential was positive at the transparent electrode 5. This step was taken to create an internal polarization of the entire surface of the element 1 in a single uniform direction, thereby preparing the plate for subsequent illumination by image. Thereafter, the polarity was reversed and during the time interval 1 to t the element was exposed to a light image. The prior illumination of the element was the only step which was materially different from those employed in Example 1. The preexposure illumination lasted for 2 seconds, and the light image intensity at the time of the exposure of the element was 20 luxes. When the externally impressed voltage was 2,000 volts, there was a surface charge of +200 volts observed on the areas not exposed to light, and a charge of 400 volts at the areas exposed to light, if subsequently treated by another illumination by light, as hereunder discussed.

FIGURE 12-A illustrates the changes in the surface potential at each step of the operation described, and FIGURE 12-B illustrates the changes in the surface potential when the element was not exposed to light at the time the initial voltage was impressed across the element. As it is clear from a comparison of FIGURES 12-A and 12-B, the potential difference G between the exposed and unexposed areas, which determines the intensity of the latent image, is much greater in FIG- URE 12-A, which represents the operation in which the element was initially exposed to light when the field voltage was impressed. A unique characteristic of this embodiment is the fact that the surface charge potential registered appears quite low even after the element is separated from the transparent electrode, if the element is not thereafter exposed to light. In the example given, whereas the surface potential at the exposed areas of the element measured prior to such post-exposure illumination by light was found to be -200 volts, the same was measured at -400 volts after the element was so illuminated by light.

This phenomenon apparently arises because the internal polarization operates as an effective image formation and image retention mechanism in the element used in Example 2, and such internal polarization is primarily formed by electrons at the deeper trap levels that cause persistent internal polarization which will become depolarized by the post-exposure illumination by light. In other words, even after the transparent electrode is removed after the formation of latent image, the persistent internal polarization effects continue to persist and thus balances with the surface charge potential until the element is again illuminated by light. When the element is so illuminated by light after the removal of the transparent electrode, and persistent internal polarization killed, the surface charge effect alone would be extracted, and thus the apparent net surface charge would become intensified. However, when a phosphor material is used for photosensitive element as in Example 2, since the impedance fluctuation of such material is quite low, it appears that the formation of internal polarization is the primary factor contributing to the formation of a latent image and that, accordingly, a system having high light sensitivity will not result. However, such an element will be useful in recording images of radiant energy other than the visible light.

An important difference between the processes of Example 1 and Example 2 involves charges at the boundary surface regions of the light sensitive crystals and bonding agent which are related to the changes in the intrinsic impedance of the light sensitive crystals.

In case of Example 1, during the interval of t to t (FIGURES 3 and 4), the impedance of crystals present in the photosensitive layer unexposed to light is quite high, so that it is probable that, at the boundary surfaces of the light sensitive crystals, electric charges will build up which will correspond to the polarity of the externally applied electric field. On the other hand, during the interval of t to in the areas exposed to light, the resistivity of the light sensitive crystals greatly decreases, which indicates that such electrostatic charges would hardly build up at the surface regions of the crystals exposed to light. Experimentally, when the surface charge of the areas unexposed to light during the time interval of t to t is measured after the removal of the transparent electrode 5, but without subsequently exposing the element to light, the measurement will give the value of approximately 10() volts; on the other hand, when the element is exposed to light after the removal of the transparent electrode and then the surface charges of the unexposed area measured, the value obtained is in the neighborhood of 0 volt, suggesting that the post-exposure illumination of the element (after the removal of the transparent electrode 5) will bring the surface charges of the unexposed areas of the element to zero level. The values of surface charges observed during the time interval t to t i.e., during the time interval wherein a field having the polarity opposite to that applied during the exposure, do not appreciably differ as between previously exposed areas and unexposed areas. Additionally, as shown by curves L and L in FIGURE 8, the value of the surface charge of the exposed area, as measured after the exposure of the element followed by removal of the transparent electrode 5, is exactly the same whether or not the element is exposed to light after the removal of the transparent electrode. This suggests that in case of Example 1 wherein highly sensitive photoconductive crystals in particulate form are used, no charge build-up at the boundary surfaces of the photosensitive crystals where they are in contact with the bonding agent will occur in the areas exposed to light, and such charge build-up on the surface of the boundary regions will occur only in the areas unexposed to light during the time interval t to t In Example 2, however, the situation is different, and it is apparent that the rate of electron trap will become higher in the boundary surface regions because conduction electron density becomes higher there even at the areas exposed to light, and the boundary surface regions maintain a high resistivity and do not permit free migration of charged carriers at the surface of the crystals. As a result, in Example 2, in which phosphor crystals in powder form are used as the photosensitive material, the charge build-up may occur at the boundary surfaces of the photosensitive crystals where they are in contact with the bonding agent, irrespective of whether the crystals have been exposed to light. In this case, the surface charges of the element are not counterbalanced merely by reverse charges and the internal polarization within the light sensitive crystals, but it should be clear from the foregoing discussion that it also involves the charges built up at the boundary surface regions of the light sensitive crystals and the bonding medium. Such charges built up in the boundary surface regions have a reverse polarity with respect to the polarity of the internal polarization and operate to negate the internal polarization field induced within the crystals, and thus the total effect of such charges present at the boundary surface regions is detrimental to obtaining the optimum signal-to-noise ratio.

The foregoing comparison of the elements used in Example 1 (using a highly photoconductivesubstance) and Example 2 (using a phosphor) will make clear that the present invention does not rely solely upon the persistent internal polarization principles. Because the internal polarization principles fail to explain the relatively large fluctuation of the surface charge observed on the unexposed areas of the element when the element, after the removal of the transparent electrode, is illuminated by light, despite the fact that the areas not exposed to light during the time interval of t to t would have the least amount of photoconductive electrons and would have, accordingly, the least opportunity to form internal polarization, we believe that, as discussed above, the unexposed areas of the element described in Example 1 possess on the surface of the photoconductive crystals electric charges having such polarity as will be dictated by the externally applied electric field, which will disappear through recombination of electrons as a result of the lowering of the resistivity of the crystals induced by light to which the element is exposed after the removal of the transparent electrode.

In inducing a latent image on the element utilized in Example 1, when the polarity of the voltage impressed is entirely reversed, i.e., when a voltage is impressed with the polarity of the transparent electrode negative during the time interval t to t, and then positive during the time interval t to t in FIGURES 3 and 4, both the photosensitivity of the system and the clarity of the latent image decrease. There will also be marked deterioration of the latent image when exposed to excess light. This appears to be due to the unsymmetrical construction of the photosensitive element, and appears to be attributable to a type of rectification effect exhibited by the element; this characteristic is not observed when a phosphor is used as in Example 2.

The elements 11 in Examples 3 and 4 below are more symmetrical in construction than the element 1 used in Example 1, and they remove the disadvantages resulting from the asymmetrical construction utilized in Example 1. Examples 3 and 4 also prove (as a result of insertion of another insulating layer, sandwiching the light sensitive layer) that the working principles of the element of the present invention do not depend on the transfer of charges carriers between the light sensitive layer and the electrical field externally applied for effective latent image formation, but that such formation of latent image on the surface of the insulating layer depends solely on the occurrences of certain phenomena, as described above, within the light sensitive layer, which distinguishes the invention from other dry electrophotographic methods with which the art is now familiar.

Example 3 The only difference in the construction of the light sensi- Example 1 is, as shown in FIGURE 13, that another insulating layer 10, having the material characteristics and dimensions equal to those of the insulating layer 3, was interposed between the sensitive layer 2 and the electrode 4, so the light sensitive layer 2 had an insulating layer on each side. Insulating layer was also integrally laminated to the element. Latent image formation was successfully carried out on this element 11 by the steps exactly as described in Example 1. The photosensitivity of the element 11 was the same as the element described in Example 1, and an exposure for 0.1 second to a light image of luxes obtained a latent image of usable strength. The exposure to light image, and the time sequency of field impression were the same as described in Example 1. When the voltage externally applied was 2,000 volts, as in Example 1, the charge potential of the element at the area exposed to light was 1,200 volts, whereas the charge potential at the unexposed areas was subsentially zero. While there was thus a slight drop in efficiency, this was compensated by an advantage not demonstrated by the element used in Example 1. The signal-to-noise ratio of the latent image was considerably improved compared with Example 1, which indicates elimination of the .socalled noise factor due to the highly unsymmetrical structure of the element or of the low energy input appearing as latent image. Particularly, the fact that noises due to unsymmetrical nature of the element are essentially eliminated has an important practical significance.

Example 4 In this example, the same photosentive element 11 described in Example 3 and shown in FIGURE 13 was used. As shown in FIGURE 14, the steps up to t starting from t were the same as those employed in Example 1. During the time interval to t without illuminating the element with a light image, the electric field was removed, the transparent electrode 5 temporarily separated from the element 11 and the transparent electrode 5 was again contacted with element 11. Beginning at time t and continuing until time t the element was illuminated with a light image, while an electric potential was impressed having a polarity the reverse of that of the potential impressed during the time interval t to t During the next time interval, t to t the light image was absent and only the electric potential having the same polarity as the field impressed during the interval t to t was impressed; at point t the element was separated from the transparent electrode. The resulting latent image on the element 11 had almost the same intensity and clarity as that obtained by Example 1.

The latent image produced in each of Examples 2 to 4, inclusive, can be developed into a visible image and transferred to a sheet of paper, or the like, by the means and methods discussed above in connection with Example 1.

As apparent from the above examples the characteristics of the photosensitive materials used are an important feature of the invention. Generally speaking, when photosensitive material in powder form is bonded together in a thin layer by electrically insulating adhesive, and either one or both surfaces of the layer is covered with thin layer of electrically insulating material, a sudden change of resistance will occur when an electric field impressed across such element is reversed. This effect is more pronounced when an activating material is present in the photosensitive material to activate its light sensitivity. Work on this invention has shown that various kinds'of photoconductive materials and phosphors may be used as the photosensitive material according to the present invention; examples are CdS, ZnS, ZnO, CdSe, PbS, ZnSe, ZnTe, CdTe, and the like, preferably actuated by known materials for actuating photosensitivity, such as copper. Preferably these materials are sufficiently finely divided to permit them to be formed into a photosensitive layer of 200 microns or less in thickness, in which the particles are held by a bonding agent. Such construction is preferable because it promotes sudden decrease of photoconductive electron density on reversal of polarity and induction of internal polarization.

In such a construction, the selection of a bonding agent should be based on the following considerations. The relative impedances and dielectric constants of the sensitive layer and the insulating layer must be considered. As it is essential to confine the phenomenon occurring in the photosensitive material in each particulate unit of such material in order to obtain good image retention and higher resolution, it is preferable to select the bonding agent so that the particles of photosensitive material are essentially uniformly separated and dispersed throughout the layer 2.

,F or these purposes, the selected bonding agent should have a specific volume resistivity of at least 10 ohm-centimeters and be transparent to the incident light used.

The thickness of the sensitive layer should also be considered in selecting the bonding agent for desired strength and mechanical flexibility of the layer, as well as for the absorption of incident light.

On the basis of the resistivity of the sensitive film exhibited in the dark as well as in exposure to light, a suitable insulating layer material, which should be transparent to the incident light and which should allow most of the field to be impressed upon the sensitive layer when the sensitive layer is unexposed and most of the field to be impressed upon the insulating layer when the sensitive layer is exposed to light, is selected and bonded to the sensitive layer by a suitable adhesive transparent to the incident light and having high electrical resistance.

Such insulating layer may be bonded to one side of the sensitive layer, in which case the opposite side of the sensitive layer should be bonded by a suitable adhesive to an electrically conductive electrode, or such insulating layer may be bonded to both sides of the sensitive layer, in which case the backing electrode should be bonded to one of the outside surfaces of such insulating layers sandwiching the sensitive layer.

The insulating layer should preferably be selected from a group of materials having a volume resistivity of ohm-cm. or more and also having surface resistivity of 10 ohm-cm. or more. The surface-to-surface resistance per unit area of such insulating layer should be 10 ohms per square centimeter or more, and dielectric constant thereof should be such that when considered in terms of the dielectric constant of the sensitive layer, in a perfect darkness and having no prior history of exposure to light, the intensity of electric field impressed to the insulating layer in that condition should be at least equal to or less than that of the field which would be distributed to the sensitive layer. Another consideration relating to the se lection of the insulating layer material is that when the dielectric constant of the photosensitive layer is increased as a result either of sudden increase of conduction electrons within the sensitive layer or at the trap levels induced by exposure to light, the electric field impressed upon the insulating layer should be markedly greater than that impressed upon the sensitive layer.

The light sensitive material made into a thin layer, as a whole, and the insulating layer should satisfy the following requirements: First, as the rate of light absorption determines the sensitivity of the layer, light absorption should be high, but a layer excessively thick with respect to the incident light requires that a higher external voltage be impressed on the element, and such excess thickness also operates detrimentally to the formation of charge on the surface of the element; accordingly, the sensitive layer should not be thicker than necessary. Secondly, the insulating layer should be selected so that the impedance and dielectric constant thereof maintain discrete relationship to the impedance and dielectric constant of the sensitive layer. This is essential in obtaining an element of high efficiency. Thirdly, because a charge distribution which is induced on the reverse side of the insulating layer works advantageously for image retention, a better result is obtained if the material from which the bonding agent for sensitive layer is made is different from the material from which the insulating layer is made. The selection of different materials for the bonding agent and the insulating layer is preferred also for the purpose of setting the relative impedance of the two materials in the manner previously explained, and this offers an easier method of constructing a satisfactory photosensitive element.

Particular attention must be paid in bonding the light sensitive layer to the insulating layer. If the bonding achieved happens to be uneven or if the thickness of the adhesive is not uniform, it will constitute a cause for local irregularity of the field distribution over the element regardless of whether or not a satisfactory contact is achieved between the transparent electrode and the element. When the sensitive layer 2 and the insulating layer 3 are not integrally laminated but are only closely in contact with one another, while in principle the element 1 will perform the necessary function, it has been found that integral lamination of the sensitive layer and the insulating layer is preferred, particularly when the area of the element is large. Integral construction of the element is also preferred to prevent any mechanical stimulus from intervening with the latent image formation of the element.

In the foregoing specification, whenever the word light is used, it should be understood to mean any radiant energy in the form of electromagnetic waves having the wavelength substantially that of gamma rays up to and including infra-red rays of not more than 4 micron wavelength.

Modifications of the invention other than those indi cated above will be apparent to those skilled in the art.

It is intended that the patent shall cover, by suitable expression in the appended claims, whatever features of patentable novelty reside in the invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention of excluding such equivalents of the invention described or of portions thereof as fall within the purview of the claims.

We claim:

1. A method of forming an electrostatic latent image comprising varying electric charges on an insulating surface, which method comprises providing an element made up of a thin electrically insulating transparent layer that is bonded to a thin layer comprising finely divided particles of a material that develops electrical conductivity when exposed to light, which particles are bonded into such thin layer by a solid bonding agent that is electrically non-conductive but transparent to light, which thin layer of particles is in inductive proximity to another electrode on the side thereof opposite the side to which said insulating layer is bonded, positioning a removable light-transparent electrically conductive electrode in contact with said insulating transparent layer, impressing a direct current voltage across said transparent electrode and said other electrode, than reversing the polarity of said direct current voltage While substantially simultaneously illuminating through said transparent electrode the surface of said transparent insulating layer with a light image, extinguishing said illumination by said light image, ceasing impression of said direct current voltage and removing said transparent electrode from said surface of said transparent insulating layer while not permitting light to illuminate said layer of particles, thereby creating on the surface of said transparent insulating layer a charge pattern substantially defining a latent image representing the light image that was illuminated.

2. A method of electrophotography which comprises providing an element composed of a photosensitive material in particulate form bonded into a film not more than 200 microns thick by means of a high polymer bonding agent having a specific volume resisitivity of at least 10 ohm-centimeters and transparent to light thereby providing a photosensitive layer, a thin insulating layer not more than 50 microns thick composed of a material selected from a group of materials being transparent to light and possessing a highly insulating property and having a specific volume resistivity of at least 10 ohm-centimeters and a specific surface resistivity of at least 10 ohms per square centimeter, said thin insulating layer having a surface-to-surface resistivity per unit area of at least 10 ohms per square centimeter and being bonded to said photosensitive layer and a electrode composed of an electrically conductive material bonded to the surface of said photosensitive layer opposite said insulating film, disposing a removable transparent electrode in surface-to-surface contact with said thin insulating film, impressing a direct current voltage across said transparent electrode and said electrode bonded to said photosensitive element While not permitting light to illuminate said photosensitive element, then reversing the polarity of said direct current voltage: so impressed while substantially at the same time illuminating the surface of said photosensitive element by a light image, then extinguishing said illumination of said light image and removing the impression of said voltage at a time not substantially prior to extinguishing said illumination, and removing said transparent electrode from said surface of said photosensitive element while not permitting light to illuminate said photosensitive element, thereby inducing on the surface of said thin insulating layer a charge pattern substantially defining a latent image being characterized by not being erased by subsequent exposure to ambient light.

3. A method of electrophotography which comprises providing an element composed of a photosensitive material in particulate form bonded into a film not more than 200 microns thick by means of a high polymer bonding agent having a specific volume resistivity of at least ohm-centimeters and transparent to light thereby providing a photosensitive layer, a thin insulating layer having not more than 50 microns in thickness of a material selected from a group of materials being transparent to light and possessing highly insulating property and having a specific volume resistivity of at least 10 ohms per square centimeter, said thin insulating layer having a surface-to-surface resistivity per unit area of at least 10 ohms per square centimeter and being bonded to said photosensitive layer, said materials of said photosensitive layer and said thin insulating layer being selected such that the consumption of electricity by an electric field externally impressed upon said two layers in said photosensitive layer at the time said photosensitive layer is illuminated by light falling thereon is not greater than the consumption of electricity by said field so impressed externally in said thin insulating layer, and an electrode composed of an electrically conductive material bonded to the surface of said photosensitive layer opposite said insulating layer, disposing a removable transparent electrode in surface-to-surface contact with said thin insulating layer, impressing a direct current voltage across said transparent electrode and said electrode integrally laminated to said photosensitive element while not permitting light to illuminate said photosensitive element, then reversing the polarity of said direct current voltage so impreseed while substantially at the same time illuminating the surface of said photosensitive element by a light image through said transparent electrode, then extinguishing said illumination of said light image and removing the impression of said voltage at a time not prior to extinguishing said illumination, then removing said transparent electrode from said surface of said photosensitive element while not permitting light to illuminate said photosensitive element, thereby inducing on the surface of said thin insulating layer a charge pattern substantially defining a latent image representing said light image so illuminated, said latent image being characterized by not being erased by ambient light.

4. A method of electrophotography which comprises providing an element composed of a photoconductive or phosphor material in particulate form bonded into a film not more than 200 microns thick by means of a high polymer bonding agent having a specific volume resistivity of at least 10 ohm-centimeters and transparent to light thereby providing a photosensitive layer, a first thin insulating layer not more than 50 microns thick composed of a material selected from a group of materials being transparent to light and possessing highly insulatmg property and having a specific volume resistivity of at least 10 ohm-centimeters and a specific surface resistivity of at least 10 ohms per square centimeter prepared such that the surface-to-surface resistivity of said layer is at least 10 ohms per square centimeter and being bonded to said photosensitive layer, a second thin insulating layer composed of a material selected from a group of materials possessing highly insulating property and having a specific volume resistivity of at least 10 ohm-centimeters and a specific surface resistivity of at least 10 ohms per square centimeter prepared such that the surface-to-surface resistivity per unit area of said layer is at least 10 ohms per square centimeter bonded to the other surface of said photosensitive layer, and an electrode composed of an electrically conductive material bonded to the surface of the second insulating layer thereby providing a photosensitive element, disposing a removable transparent electrode in surface-to-surface contact with said first thin insulating layer, impressing a direct current voltage across said transparent electrode and said electrode integrally laminated to said photosensitive element while not permitting light to illuminate said photosensitive element, then reversing the polarity of said direct current voltage so impressed while substantially at the same time illuminating the surface of said photosensitive element by a light image through said transparent electrode, then extinguishing said illumination by said light image and removing the impression of said voltage at a time not substantially prior to extinguishing said illumination, then removing said transparent electrode from said surface of the first insulating layer while not permitting light to illuminate said photosensitive element, thereby inducing on the surface of said first thin insulating layer of said photosensitive element a charge pattern substantially defining a latent image representing said light image, said latent image being characterized by not being erased by subsequent exposure to ambient light.

5. The method of forming an electrostatic latent image embodying varying electric charges on a surface composed of insulating material which comprises providing an element made up of a thin electrically insulating layer directly bonded through its entire area to a photosensitive layer, subjecting said element to an electric field between an electrode on the same side of said element as said insulating layer and an electrode on the side of said element opposite said insulating layer to deposit charges of a first polarity on the element, thereafter illuminating the element with a light image while subjecting the element to an electric field between an electrode on the same side of said element as said insulating layer and an electrode on the side of said element opposite said insulating layer to deposit on the element charges of a polarity opposite to the first polarity, stopping the illumination of the element with the image and removing the electric field without shortcircuiting the said electrodes between which the lastmentioned field was created, said latent image being characterized by not being erased by subsequent exposure to ambient light.

6. A method according to claim 5 in which the photosensitive element is not illuminated during the time that the charges of the first polarity are deposited on the element.

7. A method according to claim 5 wherein the photosensitive element is subjected to substantially uniform illumination during the time that the charges of the first polarity are deposited on said element.

'8. A method according to claim 5 in which the insulating layer is transparent.

9. A method according to claim 8 in which the element has a conductive electrode bonded on the side of the photosensitive layer opposite the transparent insulating layer.

10. A method according to claim 9 in which another insulating layer is interposed between the photosensitive layer and the conductive electrode.

11. The method of forming an electrostatic latent image embodying varying electric charges on a surface composed of insulating material which comprises providing an element made up of a thin electrically insulating layer directly bonded through its entire area to a photosensitive layer, subjecting said element to an electric field between an electrode on the same side of said element as said insulating layer and an electrode on the side of said element opposite said insulating layer to deposit charges of a first polarity on the element, thereafter illuminating the element with a light image while subjecting the element to an electric field between an electrode on the same side of said element as said insulating layer and an electrode on the side of said element opposite said insulating layer to deposit on the element charges of a polarity opposite to the first polarity, stopping the illumination of the element with the image, removing the electric field without short-circuiting the said electrodes between which the last-mentioned field was created and thereafter subjecting said element to substantially uniform illumination thereby enhancing said latent image, said latent image being characterized by not being erased by subsequent exposure to ambient light.

12. A method according to claim 11 wherein the photosensitive element is subjected to substantially uniform illumination during the time that the charges of the first polarity are deposited on said element.

13. A method according to claim 11 including the additional step of developing the latent image by the application of a toner thereto.

14. A method according to claim 13 in which the developed image is subsequently transferred to a sheet of paper or the like.

15. A method according to claim '11 in which the photosensitive element is not illuminated during the time that the charges of the first polarity are deposited on the element.

References Cited UNITED STATES PATENTS 2,741,959 4/1956 Rheinfrank et al. 96-1.4 2,853,383 9/1958 Keck 96-1 2,901,348 8/1959 Dessauer et al 96-1.5 2,912,592 11/ 1959 Mayer.

3,196,011 7/1965 Gunther et al. 96-1.1 3,268,331 8/1966 Harper 961 3,308,233 3/1967 Button et a1 961 3,347,669 10/1967 Kalman 96-1 NORMAN G. TORCHIN, Primary Examiner C. E. VAN HORN, Assistant Examiner U.S. C1. X.R.

27 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 457, 070 Dated July 22, 1969 Inventor(s) Yoshiyuki Watanabe and Koichi Kinoshita It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- In the heading the name of the Assignee reading Matsuragawa Electric Company, Ltd.

should read Katsuragawa Electric Company, Ltd.

SIGNED AND SEALED OCT 2 1 I969 Attest:

mum E. 'SOHUYLEB, m. Attesting Officer Gomissioner of Patmtfl