Electrophotography – Image formation – Development
Reexamination Certificate
1999-08-23
2001-08-21
Lee, Susan S. Y. (Department: 2852)
Electrophotography
Image formation
Development
C399S296000
Reexamination Certificate
active
06278855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the electrophotographic printing of a print image having a plurality of colors on a carrier.
2. Description of the Related Art
U.S. Pat. No. 4,078,929 discloses a method for electrophotographic printing with two colors. This method also bears the name “Tri-Level Method”. In this method, a print image contains at least one first image element of a first color and at least one third image element with a third color. A second image element of the print image has a background color of the image carrier, so that no toner particles are to be applied.
The first image element is allocated to a first surface element of a photo conductor layer. The photo conductor layer and an electrode layer carrying a predetermined reference potential are contained in a light-sensitive layer system. The reference potential is usually zero potential. Likewise, the second image element is allocated to a second surface element or region and the third image element is allocated to a third surface element of the photo conductor layer. What is disadvantageous in the tri-level method is that only toner particles of two colors can be applied in a print image. A color mixing with the assistance of three primary colors, accordingly, is precluded without repeating the process. A plurality of printing events are possible for color mixing, whereby a printing event contains the steps of charging, image-wise illumination and developing.
European Published Application EP 0 606 141 A2 discloses a printer that can be employed for the electrophotographic printing of a print image with a number of colors. The maximum plurality of colors with one illumination step, however, is limited to three colors. For example, it is not possible to print additional, decorative colors such as, for example, gold or silver with exact registration in addition to the three primary colors. Given a printer according to the European Published Application, moreover, a layer system that is composed of four layers is employed. Added outlay is needed in order to manufacture this layer system since four layers instead of two layers are to be arranged on top of one another.
Further, two light-sensitive layers contained in the layer system must be illuminated with two light beams that respectively have different wavelengths. In a first version of the print according to the European Published Application, the photoconductor is conducted past an illumination station for the image-wise illuminating twice. As a result thereof, the printing speed is reduced to half. An illumination station must offer two different light beams in both passes. The outlay for the illumination is therefore doubled, for example in view of generating the illumination signals and in view of the demands made of the optical system with respect to the imaging errors.
In a second version of the European Published Application, the light-sensitive layer system is conducted past two illumination locations in one pass. However, two illumination beams must still be offered at each of these illumination locations. The demands made of the imaging precision of the optical system employed are more stringent in the second version. Thus, TE light waves (transversal-electromagnetic) are employed for the first illumination station and TM light waves (transversal-magnetic) are employed in the second illumination station for the different wavelengths of the two illuminating beams.
U.S. Pat. No. 5,155,541 discloses a printing method wherein the tri-level method has been further-developed for printing at least three colors.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for the electrophotographic printing of a print image with at least three colors that allows a printing with high printing quality and that can be implemented on a simply constructed printer.
This and other objects of the invention are achieved by a method for the electrophotographic printing of a print image with a plurality of colors on a carrier, wherein a layer system is charged to a starting potential; at least three different potentials, namely a first, third and fourth potential, are generated on the layer system by image-wise illuminating, the third potential has a value that is greater in terms of amount than the fourth potential and the first potential has a value that is greater in terms of amount than the third potential; in a developing step with color particles of a first color and first polarity, these color particles are applied onto locations having the first potential; in a developing step with color particles of a fourth color of the other polarity, these color particles are applied onto locations having the fourth potential; the third potential is lowered in terms of amount by uniform illuminating to a value below the momentary value of the fourth potential that is present after the developing step with color particles of the fourth color; and in a developing step with color particles of a third color of the other polarity, these color particles are deposited at locations having the lowered, third potential; the potential on surface elements already covered with color particles is increased in terms of amount at least once before the uniform illumination. Compared to the known method of the aforementioned Published Application EP 0 606 141 A2, picture elements with at least three different colors of the color particles applied in the developing steps are contained in the print image in the invention. Despite foregoing subtractive color mixing in a printing procedure given the method of the invention, additive color mixing can be produced by color particles arranged in juxtaposition. In the method of the invention, an image-wise illumination step with only a single light frequency and light of one polarization type is required, so that a simple image-wise illumination is implemented. The light-sensitive layer system can be simply constructed in the method of the invention. Compared to the European Patent document EP 0 606 141 A2, a photoconductor layer and an intermediate layer can be eliminated when printing at least three colors. A subtractive color mixing can ensue in that a further print image is printed with optimally exact registration in a following printing event. The further print image is thereby generated on the same photoconductor after removal of the first print image or is generated on a further photoconductor.
The invention is based on the perception that the printing quality given a multiple illumination for printing a print image decreases since, due to unavoidable imaging errors and a positioning of the layer system affected by tolerances, it cannot be assured that both illumination steps lead to a positional exact positioning of the picture elements of the print image. Only one image-wise illumination step is therefore implemented in the invention. Moreover, imaging errors are minimized in the invention in that the light-sensitive layer system employed contains only one electrode layer carrying a predetermined reference potential and one photoconductor layer that, for example, are mechanically and electrically connected in large-area fashion.
After the application of charged color particles of the respective color onto at least one of the surface elements, the potential on this surface element is increased in terms of amount (made more positive from a negative value) in the invention. The light source with the uniform light distribution thus need only emit a lower optical energy since the potentials have to be lowered by only a smaller amount. As a result thereof, the contrast range which has been predetermined by the initial potential can be well-utilized.
In a first exemplary embodiment of the invention, the print image contains at least one first image element of a first color, at least one second image element with the color of the carrier, at least one third image element of a third color and at least one fourth image element of a fourth color. An addi
Maess Volkhard
Schleusener Martin
Lee Susan S. Y.
Oce Printing Systems GmbH
Schiff & Hardin & Waite
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