US3853553A - Method for image transfer using persistent internal polarization - Google Patents

Abstract

1. A photosensitive plate comprising a photoconductive layer having persistent internal polarization characteristics is interposed between an electrically insulating layer and an electrically conductive layer and used in a method for forming a latent electrostatic image on the photosensitive plate by first exposing the image onto the photoconductive layer and simultaneously applying a DC. corona discharge to the photoconductive layer. An AC. corona discharge is then applied to the dark patterns of the latent electrostatic image and the photosensitive plate is then uniformly irradiated to enhance the contrast of the latent electrostatic image.

Description

lliiite d @taites Eaterit [191 lf'asiiie et all.

[ METHOD FOR IMAGE TRANSFER USING PERSHSTENT HNTERNAL PULARIZATIION [75] lnventors: Tsuyoslii Yiasuie, lzumiotsu-shi,

Osaka-fu; llfiunilti Seino, Amagaski-shi, l-iyogo-ken, both of Japan [73] Assignee: Minolta Camera lfialhusliiki llfiaisha,

Osaka, Japan 22 Filed: Nov.2 l,]1969 21 Appl.No.:379,4lfl7 [30] Foreign Application Priority Data UNITED STATES PATENTS 3,438,706 4/1969 Tanaka etaL 355/11 [111 meassa [4 1 Dec. 10,1974

Primary Examiner-Charles E. Van Horn Attorney, Agent, or Firm-Wats0n, Cole, Grindle & Watson EXEMPLARY CLAIM A photosensitive plate comprising a photoconductive layer having persistent internal polarization characteristics is interposed between an electrically insulating layer and an electrically conductive layer and used in a method for forming a latent electrostatic image on the photosensitive plate by first exposing the image onto the photoconductive layer and simultaneously applying a DC. corona discharge to the photoconductive layer. An AC. corona discharge is then applied to the dark patterns of the latent electrostatic image and the photosensitive plate is then uniformly irradiated to enhance the contrast of the latent electrostatic image.

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ATTORNEY PAIENIEL mu: 1 0 I974 mm w 3 INVENTOR ATTORNEY METHOD FOR IMAGE TRANSFER USING PERSISTENT INTERNAL POLARIZATION BACKGROUND OF THE INVENTION In an electronic photographing method of the capacitance image type in the prior art, a negative image corresponding to the dark portion of an original picture and a positive image corresponding to the light portion of an original image can not be obtained without fogging. Such a drawback is caused by an electrostatic latent image which does not come to zero electric potential or residual electric potential.

The present invention provides a method for electronic photographing for removing such residual electric potential thoroughly and obtaining a picture free of ground contamination.

SUMMARY OF THE INVENTION The present invention relates to a method for image transfer which comprises a first step for making use of a photosensitive plate composed of a photoconductive layer having persistent internal polarization characteristics interposed between an electrically insulating layer and a conductive layer. A light image irradiation is applied to the photosensitive plate simultaneously with a DC corona discharge to form persistent internal polarization in the photoconductive layer corresponding to the light portion of the light image so as to form an electrostatic image due to the difference of surface electric potential produced in accordance with the light and dark patterns of the light image on the surface of the electrically insulating layer. In the second step a AC corona discharge is applied to the dark pattern so that an electric charge remains only on the surface of the electrically insulating layer corresponding to the light portion of the light image. In the third step the whole surface of the photosensitive plate is uniformly irradiated, thereby bringing the surface electric potential of the dark portion of the light image to zero electric potential and the surface electric potential of the light portion of the light image to a higher electric potential.

The primary object of the present invention is to provide a method of forming an image on a photosensitive plate by an electrostatic latent image which brings the surface electric potential of the dark portion of a light image to zero electric potential and' holds the surface electric potential of the light portion of the light image at a higher electric potential.

The second object of the present invention is to provide a negative picture free of ground contamination.

The third object of the present invention is to providea method for electronic photographing which eliminates the need for uniformly charging a photosensitive plate prior to exposure of the image thereon.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 2A-2G are illustrations showing another electronic photographing method well known in the prior art, wherein FIG. 2A shows an overall uniform charging as a first step thereof and FIG. 2D shows the surface electric potential thereof. FIG. 2B shows the forming of an electrostatic image in the second step and FIG. 2E shows the corresponding surface electric potential; FIG. 2C shows an overall irradiation as the third step and FIG. 2F shows the surface electric potential thereof; and FIG. 2G shows the time change in the surface electric potential in the respective steps.

FIG. 3 is a partial enlarged section of a photosensitive plate put to use in the present invention.

FIGS. 4A and 4B show apparatus for forming an electrostatic image in the first step in accordance with the method of the present invention, wherein FIG. 4B shows the surface electric potential thereof.

FIGS. 5A and 5B show the correction obtained in the second step of the method, wherein FIG. 5B shows the surface electric potential at that time.

FIGS. 6A and 6B show an overall uniform irradiation in the third step of the method, wherein FIG. 6B is the surface electric potential at that time.

FIGS. 7A-7C show respectively an equivalent circuit for the three steps of the method.

FIG. 8 shows a change of the surface electric potential in accordance with the present invention.

FIG. 9 is a transverse section of an example of a device for carrying out the electronic photographing method in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In order to help in understanding the present invention, before the preferred embodiment is described the electronic photographing type of the capacitance image method well known in the prior art is explained and the difference between them is discussed.

FIGS. llA-llE and FIGS ZA-ZG are respectively an illustration of electronic photographing methods well known in the prior art. The method shown in FIGS. llA-llE comprises two steps, and a negative picture of the original picture is obtained therefrom. FIG. IA shows the first step, wherein to a high electrically insulating layer of a three-layered-p'hotosensitive plate comprising a high electrically insulating layer, a photo conductive layer, and a conductive layer, is applied a light image irradiation comprising light and dark patterns and simultaneously therewith a DC corona discharge. An electrostatic image due to the difference of surface electric potential is produced in accordance with the light and dark patterns shown in FIG. 1D. Then as shown in FIG. 118 by irradiating uniformly the whole surface of the photosensitive plate and reducing the surface electric potential of the dark portion of the light image the pattern of surface electric potential is reversed. This fact means that the electric potential on point d is reduced to point (1 in FIG. IC, and as seen from the drawing the difference of surface electric potential on the light and dark portions is increased and the surface electric potential shown in FIG. IE is obtained. However. according to this method an overall uniform irradiation is given leaving the electric charge corresponding to the dark portion of the light image just as it is, so that even though the surface electric potential on that portion reduces to point (1 it does not reach zero potential. Therefore, after development there is a fog corresponding to electric potential d and a clearly visible image can not be obtained.

And, another method well known in the prior art and shown in FIGS. 2A-2G is an electrostatic image forming method comprising three steps. That is, as shown in FIG. 2A first an overall uniform charge is applied to the surface of the insulating layer of a photosensitive plate, and then as shown in FIG. 28 a light image irradiation and simultaneously therewith an AC corona discharge is applied, and subsequently as shown in FIG. 2C an overall uniform irradiation step is carried out. According to this method, unless electric charge of inverse polarity is trapped uniformly in the boundary surface between the photoconductive layer and the electrically insulating layer, or the interior of the photoconductive layer by the overall uniform charging step it is impossible to carry out the light image forming process. Therefore, as shown in FIG. 2G by the overall uniform charging step the surface electric potential is increased uniformly as shown by Vp to get the surface electric potential as shown in FIG. 2D, and then the light image forming step in the second process the surface electric potential in the dark portion of the light image reduces as shown by curve V in FIG. 2 to zero electric potential. On the other hand the surface electric potential in the light portion of the light image descends slowly as shown curve V to zero electric potential taking a longer time than in curve V because the resistance in the photoconductive layer gets smaller.

Therefore, when the surface electric potential in the dark portion of the light image reaches zero electric potential, the light portion of the light image does not reach zero electric potential but has the surface electric potential as shown in FIG. 2E. Especially, when the lightness of the light portion of the light image is relatively weak, a portion of the negative electric charge trapped in the overall uniform charging process is not dischargedthoroughly onto the photoconductive layer and remains there, and even though the surface electric potential reaches zero electric potential, when the overall irradiation process is applied the surface electric potential ascends as shown in FIG. 2F and produces a fog.

On the contrary, in the present invention the above mentioned fog is eliminated and the photosensitive plate used forms photoconductive layer 2 having persistent internal polarization characteristics by making use of a spray onto high electrically insulating layer l as shown in FIG. 3.

In this case, in order to improve the bond between photoconductive layer 2; and the other layer as occasion demands, a little quantity of a resin binder can be added. And, onto photoconductive layer 2 conductive layer 3 is bonded.

High electrically insulating layer l is preferred to be highly resistant to be able to hold an electric charge and have transmissibility for permitting the light ray of the light image to transmit to photoconductive layer 2, and accordingly polyester resin, polycarbonate resin, cellulose acetate resin, fluorine resin, and the like are most suitable.

Even though photoconductive layer 2 is much larger in leakage current as compared with a photosemiconductor used in the conventional electronic photograph such as an electrofax and a zero graphine it is good if the density of the trapping center is relatively large. For example, an inorganic photosemiconductor such as Cds, Zns, Zno, ZnCds, CdTe, CdSe, Se, SeTe, SeSb, SeS, etc. and an organic photosemiconductor such as polyvinylcarbazole derivative thereof, etc. and other mixtures thereof can be used and moreover an electric charge bearing body produced by a light irradiation need not move the whole thickness of the photo-semiconductor layer as in a photo-semiconductor in the photosensitive body of a conventional electronic camera. It is possible to improve its sensitivity more than times as compared with the photosensitive plate in a conventional electric camera.

Conductive layer 3 needs to be transparent or phototransmissible when light image irradiation is directed from the side of the conductive layer and a copper iodide transparent thin layer, a nesaglass, a phototrans missible metal evaporation thin film, or the like is used. However, when light image irradiation is not directed from the side of this layer, a metal conductive body, a sheet of hygroscopic paper or the like can be used.

A photosensitive plate used in the present invention is formed fundamentally as described above,'however, by selecting a material for photoconductive layer 2 and taking the increase or decrease of electric charge from conductive layer 3 into consideration interposing an electrically insulating layer between a photoconductive layer 2 and a conductive layer 3 can be used as well.

FIGS. 4A, 48, 5A, 58, 6A and 6B illustrate respective steps for forming an elestrostatic image on the photosensitive plate, that is, the light image forming step, the correction step, and the overall uniform irradiation step, and the charge patterns on the photosensitive plate and the surface electric potentials of the photosensitive plate in each respective step as illustrated. And, first as shown in FIG. 4A the light image irradiation and simultaneously charging onto the electrically insulating layer of the photosensitive plate by a DC c0- rona discharge are applied so as to form on the surface of electrically insulating layer an electrostatic image due to the difference of the surface electric potential produced in accordance with light and dark patterns of the light image. That is, nowprovided positive charging by a DC corona discharge is applied, on dark portion 4 shown in FIG. 4A, in the inside of photoconductive layer 2 polarization due to movement of an electric charge is very little, and the surface electric potential of electrically insulating layer 1 ascends as the charging time increases as shown by curve V 0) in FIG. 8. On the other hand, on the light portion 5 internal polarization due to the electric charge takes place and more electric charge than the dark portion 4 is charged on the surface of electrically insulating layer ll, however, by the function of the electric field due to persistent internal polarization formed in photoconductive layer 2 the electric potential thereof gets lower than the surface electric potential on the dark portion 4 and increases as the charging time increases as shown by curve V,,, (t) in FIG. 8. Therefore, on the surface of electrically insulating layer l at the time when the light image forming step is finished surface electric potential difference [V ,"(t)V ,'-(t)] is produced corresponding to light and dark patterns of the light image so as to form an electrostatic image due to the surface electric potential as shown in FIG. 4B.

The charging polarity in the process can be either positive or negative however, it is preferable to be negative when the material of photoconduc tive layer 2 is a N-type photosemiconductor and to be positive when it is a P-type photosemiconductor.

Next, in the correction step, as shown in FIG. 5A an AC corona discharge is applied in a dark place and thereby on the dark portion A of the light image polarization does not take place in photoconductive layer 2, so that positive electric charge on the surface of electrically insulating layer I decreases as the AC corona discharging time increases to be neutralized fully as shown FIG. 5B and thereby the surface electric potential of electrically insulating layer ll decreases as shown by curve V;,,"(t) in FIG. 8 to zero electric potential.

Onthe other hand, on the light portion 5 polarization remains in the interior of photo-conductive layer 2, so that positive electric charge on the surface of electrically insulating layer ll decreases as the AC corona discharging time increases, however, as shown in FIG. 53, by the function of the electric field due to persistent internal polarization formed in photoconductive layer 2 the positive electric charge is not neutral ized fully and remains to neutralize the function of the electric field and thereby the surface electric potential of electrically insulating layer I decreases as shown by curve V (t) in FIG. to zero electric potential.

Therefore, on the surface of electrically insulating layer 1 after the correction process is finished only the light portion of the light image is charged to a positive charge and its surface electric potential is zero as shown in FIG. 5B.

In the correction process, as the AC corona discharging time increases the surface electric potential of the light portion and dark portion of the light image reaches zero electric potential while decreasing its difference [V (t)V (t)], so that the operational efficiency of neutralization of the surface electric potential is good and even for a light image wherein the lightness of the light portion is different the AC corona discharging time can be always invariable.

Next, as shown in FIG. 6A the whole surface is irradiated uniformly, and then on dark portion 4 by the correction step positive electric charge is not already charged onto the surface of electrically insulating layer ll so that there is no change at all as shown in FIG, 6B and its surface electric potential is zero. However, on the other side, on dark portion 5, as shown in FIG. 6A, the persistent internal polarization formed in photoconductive layer 2 by the light image forming step is dissociated by the overall uniform irradiation and the surface electric potential of the electrically insulating layer attains a very large electric potential as shown in FIG. 613.

Therefore, by the overall uniform irradiation the surface electric potential of electrically insulating layer ll gets higher only on light portion 5 of the light image as shown in FIG. dB to reverse the electric potential in the light image forming process and its potential difference increases.

And, the surface electric potential obtained by the overall uniform irradiation is fixed in accordance with the discharging time in the correction step, and when the overall uniform irradiation is applied, on dark portion A the surface electric potential of electrically insulating layer ll decreases fromcurve V,,,"(t) shown in FIG. b to the electric potential shown by curve V U), and as the AC corona discharging time increases the rate of decrease of the surface electric potential [V ,"(t)V ,,(t)] is reduced to coincide with zero electric potential. On light portion 5 the surface electric potential of electrically insulating layer 1 ascends from curve V t) shown in FIG. 8 to the electric potential shown by curve V (t) which is slower than V and as theAC corona discharging time the increase rate of increase of the surface electric potential [V (t)V (t)] to attain maximum at zero electric potential induced by the AC corona discharging. Therefore, by the overall uniform irradiation when the surface electric potential reaches zero electric potential in the correction step, the surface electric potential of dark portion 4 brings about no fog at zero electric potential and the difference of the surface electric potential between light portion 5 and dark portion 4 [V (t)V (t)] reaches maximum so as to form on the surface of electrically insulating layer 1 an electrostatic image the contrast of which is very large and can be developed in a light place. The development can be carried out in the same manner as in the conventional electronic photographing methods and by fixing as it is or after transcribing it is possible to obtain a reversal visible image which is free of fog and which has a high contrast.

FIGS. 7A, 7B and 7C show respectively an equivalent circuit for each respective step for forming the electrostatic image, wherein on the assumption that resistance Rp of photoconductive layer 2 of the photosensitive plate is zero on the light portion. and infinity on the dark portion it is shown that by the surface electric potential and the persistent internal polarization formed in photoconductive layer 2 in the electric field, namely, a polarization charge which is trapped in the deep trapping center and not dissociated an electrostatic image is formed. In practice, however, according to the dark attenuation of persistent internal polarization formed in photoconductive layer 2 on the light portion of a light image, dark polarization on the dark portion, and the constitution of photoconductive layer 2 there is a pour-in of electric charge from conductive layer 3 and it seems that more complicated phonomena take part therein.

And, it is an important feature of the present invention that in the correction step polarization must be kept up in photoconductive layer 2 and for photoconductive layer 2 a layer having persistent internal polarization characteristics has been selected.

Next, in the photosensitive plate and device embodied in accordance with the present. invention, the photosensitive plate is formed by a Cds-epoxide resin group that is, for electrically insulating layer la polyester film of 1541 in thickness is put to use which is coated 1001.1. or so in thickness as photoconductive layer 2 with a mixture of Cds powder and epoxide resin at ratio 5:1 and adhered onto aluminum plate 3.

- The photosensitive plate is fed by means of a feeding device, as shown diagrammatically in FIG. 9, to the DC corona discharging device 6 and a device for irradiating the light image. DC corona discharging device 6 is provided with a window of a conductive glass plate on the top and impressed with an electric potential of '6000V, and simultaneously by irradiating a light image through the glass plate from the device for irradiating the light image to give an exposure of i0 Lux. Sec. it is possible to obtain an electric potential of -l 200V or so on the dark portion and -l000V or so on the light portion on the surface of high electrically insulating layer 1. Further, the photosensitive plate is given in a darkroom a discharge by means of corona discharge device 7 impressed with an AC voltage of SOOOV and then by giving a uniform irradiation onto the whole surface of the photosensitive plate by means of light source 8 or by taking out the photosensitive plate to a light place, a surface electric potential of V on the dark portion of the light image and 6000V or so on the light portion of the light image are produced on the surface of electrically insulating layer 1 and it is possible to obtain an electrostatic image of high contrast.

When this electrostatic image is developed with a toner having a positive polarity by the magnetic brush method, no toner adheres to the dark portion thereof, therefore, it is possible to obtain a reversed or negative visible image free of so-called ground contamination.

It is possible to obtain a clear reversed image of high contrast thoroughly and-easily which is fully free of ground contamination which has been impossible to remove perfectly in the prior art, and accordingly the present invention is very advantageous in an enlargement process for film images as well as general photographing and reproduction.

What is claimed is:

1. An electronic photographing method for a photosensitive plate of the type wherein a photoconductive layer having a persistent internal polarization characteristic is interposed between an electrically insulating layer and an electrically conductive layer, comprising the steps of:

1. exposing an original image onto said photoconductive layer and simultaneously charging said electrically insulating layer by a DC. corona discharge to form an image by persistent internal polarization of said photoconductive layer according to the light portion of said original image and an electrostatic image on said electrically insulating layer by the difference of surface electric potential produced in accordance with the light and dark patterns of said original image,

2.. applying an AC. corona discharge to the dark pattern of said electrically insulating layer thereby leaving an electric charge only on said electrically insulating layer corresponding to the light portion of the light image, and

3. uniformly irradiating said photosensitive plate to reduce the surface electric potential of the dark pattern of electrically insulating layer to zero potential and the surface electric potential of said light pattern of said electrically insulating layer to a high potential.

2. A method as claimed in claim 1, wherein said conductive layer has photo-transmissible characteristics, and said photoconductive layer is exposed with said conductive layer facing said original image.

3. A method as claimed in claim 1, wherein said photoconductive layer is composed of a P-type photosemiconductor.

4. A method as claimed in claim 1, wherein said electrically insulating layer has photo-transmissible characteristics, and said photoconductive layer is exposed with said electrically insulating layer facing said original image.

5. A method as claimed in claim 4, wherein said photoconductive layer is composed of an N-type photosemiconductor.

=l I l l

Claims (10)

1. AN ELECTRONIC PHOTOGRAPHING METHOD FOR A PHOTOSENSITIVE PLATE OF THE TYPE WHEREIN A PHOTOCONDUCTIVE LAYER HAVING A PERSISTENT INTERNAL POLARIZATION CHARACTERISTIC IS INTERPOSED BETWEEN AN ELECTRICALLY INSULATING LAYER AND AN ELECTRICALLY CONDUCTIVE LAYER, COMPRISING THE STEP OF:

1. EXPOSING AN ORIGINAL IMAGE ONTO SAID PHOTOCONDUCTIVE LAYER AND SIMULTANEOUSLY CHARGING SAID ELECTRICALLY INSULATING LAYER BY THE DIFFERENCE OF SURFACE ELECTRIC POBY PERSISTENT INTERNAL POLARIZATION OF SAID PHOTOCONDUCTIVE LAYER ACCORDING TO THE LIGHT PORTION OF SAID ORIGINAL IMAGE AND AN ELECTROSTATIC IMAGE ON SAID ELECTRICALLY INSULATING LAYER BY THE DIFFERENCE OF SURFACE ELECTRIC POTENTIAL PRODUCED IN ACCORDANCE WITH THE LIGHT AND DARK PATTERNS OF SAID ORIGINAL IMAGES,

2. APPLYING AN AC. CORONA DISCHARGE TO THE DARK PATTERN OF SAID ELECTRICALLY INSULATING LAYER THEREBY LEAVING AN ELECTRIC CHARGE ONLY ON SAID ELECTRICALLY INSULATING LAYER CORRESPONDNG TO THE LIGHT PORTION OF THE LIGHT IMAGE, AND

2. A method as claimed in claim 1, wherein said conductive layer has photo-transmissible characteristics, and said photoconductive layer is exposed with said conductive layer facing said original image.

2. applying an AC. corona discharge to the dark pattern of said electrically insulating layer thereby leaving an electric charge only on said electrically insulating layer corresponding to the light portion of the light image, and

3. uniformly irradiating said photosensitive plate to reduce the surface electric potential of the dark pattern of electrically insulating layer to zero potential and the surface electric potential of said light pattern of said electrically insulating layer to a high potential.

3. A method as claimed in claim 1, wherein said photoconductive layer is composed of a P-type photosemiconductor.

3. UNIFORMLY IRRADIATING SAID PHOTOSENSITIVE PLATE TO REDUCE THE SURFACE ELECTRIC POTENTIAL OF THE DARK PATTERN OF ELECTRICALLY INSULTAING LAYER TO ZERO POTENTIAL AND THE SURFACE ELECTRIC POTENTIAL OF SAID LIGHT PATTERN OF SAID ELECTRICALLY INSULATING LAYER TO A HIGH POTENTIAL.

4. A method as claimed in claim 1, wherein said electrically insulating layer has photo-transmissible characteristics, and said photoconductive layer is exposed with said electrically insulating layer facing said original image.

5. A method as claimed in claim 4, wherein said photoconductive layer is composed of an N-type photosemiconductor.

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