US 3687661 A
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COLOR ELECTROPHOTOGRAPHIC PROCESS Filed Nov 5, 1970 2 Sheets-Sheet 1' SEIJI MATSUMOTO BY I ATTORNEY Aug.29, 1972 MA$AM|H| SATO ETAL I 3,687,661
7 COLOR ELECTROPHOTOGRAPHIC PROCESS Filed Nov 5, 1970 2 Sheets-Sheet 2 l l I l I7: I i
V i v5 l I V 13 l I United States Patent 01 fice 3,687,661 Patented Aug. 29, 1972 US. Cl. 961.2 7 Claims ABSTRACT OF THE DISCLOSURE A color reproducing process is herein disclosed in which a series of color toner images are sequentially developed in superposition upon the surface of a photoconductive plate. Between each development step, the plate is imaged by charging the plate to a potential of a first polarity in both the previously developed and non-developed regions. The charge accepted in the previously developed regions is then partially neutralized by applying thereto a second charge having a polarity opposite to that of the initial charge whereby the original charge in the previously developed regions is reduced to a level substantially equal to the potential in the non-developed region. The now uniformly charged plate is exposed to a light image containing additional input scene information relating to the next image to be developed.
BACKGROUND OF THE INVENTION Field of the invention This invention relates to an electrophotographic process and, in particular, to a xerographic color process in which the colorants are placed in superposition upon a photo conductive plate to produce a color copy of a multi-color original.
Description of the prior art conventionally, in the xerographic art, a photoconductive plate is first sensitized by applying a uniform charge potential to the surface thereof. The sensitized plate is then exposed to a light image of an original to be reproduced to selectively dissipate the charge thereon whereby a latent electrostatic image is recorded on the plate con taining the original input scene information. The latent image is developed or made visible by applying oppositely charged electroscopic marking particles, i.e. toner, to the plate where the toner particles are attracted into imaged areas in proportion to the density of the charge present. That is, images of the original input scene information that are recorded on the plate as regions of relatively high charge density are developed as images of high toner density while those original images that are recorded as regions of relatively low charge density are developed as images of comparatively lower toner density.
It has been found that the basic xerographic process, as described above, can be adapted to reproduce multicolor originals by the application of known subtractive color copying techniques. In one such color process, the original subject matter is initially separated into three primary color components and each of the color components processed in sequence upon a photoconductive plate so as to formulate three complementary toner images thereon. The colorants act alone or in concert, as when superimposed one upon the other, to recreate the original color input scene information. This superimposed development technique is advantageous in color copying because it can be conveniently automated and also because it elminates many registration problems normally encountered in this type of color copying. However, it has heretofore been difiicult to accurately record image information on the regions of the photoconductive plate that have previously been developed. The amount of charge accepted by the plate in the previously developed regions will generally differ from that accepted in the nondeveloped regions. The electrostatic image recorded during exposure will therefore contain erroneous input scene information resulting in a considerable reduction in the quality of the color copy produced.
SUMMARY OF THE INVENTION It is therefore an object of the present invention to improve color xerography.
A further object of the present invention is to improve superimposed development in xerography to facilitate the production of high quality color copy.
Another object of this invention is to produce a uniformity of charge over the entire surface of a previously developed xerographic plate.
These and other objects of the present invention are attained by means of a xerographic color process wherein a multi-color original is separated into primary color components and each color component processed in sequence upon the surface of a photoconductive plate toproduce complementary toner images thereon that are superimposed one upon the other in a manner to faithfully reproduce the original. Between each development step, the plate surface is initially charged to a potential of a first polarity and the initial charge then neutralized in the previously developed regions to reduce the charge density to a level substantially equal to that of the charge found in the non-developed regions. The now uniformly charged plate is then exposed to a light image relating to the next color component to be developed to accurately record color input scene information thereon.
BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention as Well as other objects and further features thereof, reference is had to the following detailed description of the present invention to be read in connection with the accompanying drawings wherein:
FIG. 1 illustrates the condition of two images that have been ideally formulated one upon the other;
FIG. 2 depicts the condition of a superimposed image as a result of the differences in charge acceptance found in a previously developed region and non-developed region on the plate;
FIG. 3 is a graphic illustration showing the charge acceptance curves for previously developed and for nondeveloped regions on the plate;
FIG. 4 also is a graphic illustration showing typical light decay characteristic curves for previously developed and non-developed regions on the plate;
FIG. 5 is a graphic illustration showing the principles of the present invention for reducing the charge potential in developed regions on the plate surface to a level substantially equal to the potential in the non-developed regions;
FIG. 6 is a graphic illustration embodying the teachings of the present invention in which the charge potential in the previously developed region is reduced after the sensitized plate has been imaged.
DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention relates generally to a xerographic color process wherein colorants, e.g., toners, are combined in a subtractive color system to reproduce the hues found in the original to be copied. In this particular process, a multi-color original is broken down into the three primary color components of red, green and blue using conventional filtering or color separating techniques and each component is then xerographically processed to formulate toner images of cyan, magenta and yellow. In order to eliminate registration problems and the like, the toner images are herein developed in superposition upon a single photoconductive plate. In the practice of the present invention, a light image of the red color component is initially employed to expose the sensitized surface of a photocnductive plate, and the latent electrostatic image created then developed using a cyan toner. The plate is resensitized, exposed to a light image of the green color component and developed with a magenta toner. Finally the steps are repeated for the blue color component and the plate developed using a yellow toner. Ideally, as in all substractive color systems, each of the colorants laid down acts alone and/or in concert with the other colorants to selectively absorb light energy so that when the composite toner image is viewed under white light it will faithfully reflect the original input scene. It should be clear, therefore, that it is extremely important to regulate the amount, or the density, of each colorant applied to the plate during the development steps in order to preserve the color integrity of the original.
Initially, the plate is uniformly charged by means of a corona generator similar to that disclosed by Vyverberg in US. Pat. 2,836,725 and the charged plate then exposed to a red color component in a conventional manner. By use of known development techniques, such as electroded cascade development, magnetic brush development or the like, the cyan toner image is accurately recorded on the plate surface whereby the density of the toner images produced substantially corresponds to the density of the original input image.
As illustrated in FIG. 1, the first developed cyan image 13 is formulated directly on the surface of a xerographic plate consisting of a photoconductive coating 11 placed upon a conductive backing 12. The second, or magenta, image is superposed over the first cyan image. Ideally, the magenta image should accurately record the input scene information provided by the second or green color component in both the previously developed and nondeveloped region on the plate. For instance, when an image of uniform input density is recorded on the plate that partially overlies a previously developed cyan region and partially overlies a non-developed plate region, the resultant developed magenta toner image should take the form of image 14 shown in FIG. 1. The density of that portion 14 of the image overlying the previously developed cyan region should be substantially equal to the density of the portion 14" overlying the photoconductive plate surface 11.
In practice, it has been found that subsequently developed images of this nature do not, in fact, attain a uniformity of density approaching that of the ideal case described above. As illustrated in FIG. 2, that portion of the image 14 overlying a previously developed toner region on the plate surface will attract more toner particles than are attracted into region 14" overlying previously non-developed regions. As can be seen, depositing an inordinate amount of magenta toner over a previously developed-cyan image would result in erroneous color information being recorded on the plate surface. It has been found that eflect of this phenomena becomes more pronounced as the densities of the images involved increase.
The reason for this phenomena can be best explained in conjunction with the charge acceptance characteristic curves shown in FIG. 3. These curves typify the amount of charge accepted in both the developed and non-developed regions on the photoconductive plate when charged by means of a corona generator similar to that disclosed in the previously mentioned Vyverberg patent.
The typical charge acceptance curve for the bare, or nondeveloped region on the plate is represented by curve 18. It will be noted that this curve rises rapidly to a relatively high voltage and then levels ed as the voltage approaches the maximum acceptance potential for the plate, that is, the potential level at which the plate will accept no further charge. Similarly, curve 17 typically illustrating the charge acceptance in the previously developed region on the plate closely approximates that of the bare plate curve but will remain at a higher potential throughout the entire charging period, t to t The shaded area be tween the two curves represents the potential diiference between the charge accepted in the previously developed and the non-developed regions on the plate. This potential diflierence has been found to range anywhere from a few volts to several hundred volts depending on the systems parameters and materials involved.
Although the phenomena is not fully understood at this time, it is believed that both the photoconductive plate and a toner image supported thereon are capable of accepting and storing a charge potential. As a consequence, two distinct charges are thought to exist in close proximity in and about in the previously developed regions, the resultant charge in and about these developed regions being greater in magnitude than the charge found in the nonimaged or bare plate regions. Regardless of the exact mechanism involved, if left uncorrected, the excessive voltage produced in and about the previously developed regions leads to improper recording and development of the next subsequent image to be processed.
The steps of the present invention leading to the removal or neutralization of this unwanted residual charge potential in the previously developed regions will be explained in reference to FIG. 5. In the instant process, the previously developed plate surface is charged in a conventional manner as noted above from time t to t Here again, the charge on the bare plate surface is represented by curve 18. The plate is initially charged to a point V wherein the bare plate potential approaches the maximum acceptance potential. During the charging period, the charge deposited in the previously developed regions will typically follow that of the bare plate curve, however this charge will be at a slightly higher potential V Following the initial charging step, the plate is again charged only this time to a potential of an opposite polarity. A corona generator similar to that disclosed in the previously mentioned Vyverberg patent can be also used for this purpose. The opposite charge is applied for a period extending from time t, to which is sufficient to remove or neutralize the residual charge found in the previously developed regions. It will be noted from the curves shown in FIG. 5 that during this neutralization period a portion of the charge retained on the bare plate surface is also neutralized. However, because of the nature of the mechanism involved, the excessive charge retained in the previously developed regions is dissipated long before the plate potential is reduced to a level below which development can no longer be maintained. The charge potential in both the previously developed and non-developed regions is thus brought to substantially the same potential V prior to imaging the plate preparatory to the next development step. Care should be taken during this neutralization period to obtain a substantial uniformity of charge in both the imaged and non-imaged areas and to avoid producing local variations in the charge potential which could effect subsequent imaging and developing steps. To this end, a corona generator is employed capable of depositing a charge upon the once developed plate surface at a rate that is substantially independent of the potential found upon the receiving surface.
After residual or excessive charge has been neutralized, the plate is exposed from time t to time (FIG. 5) in a conventional manner to record a latent electrostatic image of the next color component thereon. The electrostatic latent image is then developed in superimposed position over the prevoiusly developed image or images. In this case, a second magenta image is placed over the existing cyan image. The steps as herein described are then repeated for the blue component which is finally developed over the other two images using a yellow toner so as to formulate a multi-color copy of the original.
Another embodiment of the present invention is graphically illustrated in FIG. 6. Here, the developed plate is charged from time t to time t in the manner described above. At time 1 however, the plate is exposed to a light image of the next color component to be recorded on the plate. This step is accomplished between times t and t in FIG. 6 and, for explanatory reasons, illustrates the effects of exposing the plate to an original image containing little or no input information, as for example background information. The bare plate potential, as depicted by curve 18, is reduced to approximately zero potential level. However, the previously developed regions retain a residual charge thereon after exposure as illustrated by the curve 17. It should be made clear, however, that if the plate were exposed to other than background information, the bare plate potential would remain at a relatively high level. Consequently, previously developed regions, retaining an excessive or residual charge thereon, would be at a correspondingly higher potential. After the exposure step is completed, the plate is then treated with a neutralizing or opposite charge between times 1 and t wherein the potential in the previously developed and non-developed regions are substantially equalized.
While this invention has been described with reference to the structure disclosed herein, it is not necessarily confined to the details set forth and this application is intended to cover any modifictaions or changes that may come within the scope of the following claims.
What is claimed is: 1. In an electrophotographic reproducing process of the type wherein a series of images are sequentially developed in superposition upon a photoconductive member, the method of recording an electrostatic latent image between each development step including initially charging the previously developed photoconductive member to a potential of a first polarity,
applying a neutralizing charge to the photoconductive surface to equalize the initial charge retained upon the developed photoconductive member in both the developed and non-developed regions, said equalized charged potential being of a magnitude sufiicient to support further development, and
exposing the uniformly charged member to a light image to record a latent electrostatic image thereon containing input scene information relating to new subject matter to be developed.
2. In an electrophotographic color reproducing process of the type wherein a series of color images are sequentially developed in superposition upon a photoconductive member, a method of recording an electrostatic image between each image development step including initially charging the previously developed photoconductive member to a potential having a first polarity,
applying a charge having a second polarity to the previously developed regions upon the photoconductive member to reduce the charge in said developed regions to a level substantially equal to the charge in the non-developed regions, said equalized charged potential being of a magnitude sufficient to support further development, and
exposing the charged member to a light image to record a latent electrostatic image thereon containing input scene information relating to the next image to be developed.
3. The method of claim 2 wherein the image bearing photoconductive member is initially charged by means of a corona generator.
4. The method of claim 3 wherein said initial charge is reduced in the previously developed regions by applying corona of a polarity opposite that of said first polarity to the developed region on said member.
5. In an electrostatic color reproducing process of the type wherein a series of color images are developed sequentially in superposition upon a photoconductive member, the method of recording an electrostatic latent image between each developing step including charging the previously developed photoconductive member to a potential of a first polarity,
exposing the charged member to a light image to record a latent electrostatic image thereon containing input scene information relating to the next toner image to be developed,
partially neutralizing the charged retained in the previously developed regions upon said photoconductive member to eliminate the excessive charge retained in said regions after exposure wherein a uniform charge capable of supporting further development is retained on said photoconductive member.
6. The method of claim 5 whereby the previously developed photoconductive member is initially charged by means of a corona generator.
7. The method of claim 6 whereby said initial charge is partially neutralized after exposure by applying corona of a polarity opposite that of the polarity of said initial gharge to the previously developed regions on said mem- References Cited UNITED STATES PATENTS 3,337,340 8/1967 Matkan 96-1 R 3,060,020 10/1962 Greig 96-1.2
3,060,019 10/1962 Johnson 96-].2
3,576,624 4/1971 Matkan 96-1 R FOREIGN PATENTS 1,082,912 9/1967 Great Britain 96-1 CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.