Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
1999-02-23
2001-02-27
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S237000
Reexamination Certificate
active
06195109
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrophotographic printer where a surface of a photoconductor is illuminated by a fixed print head having a plurality of light-emitting elements.
DESCRIPTION OF THE RELATED ART
An electrophotographic printer is a known conventional apparatus where a surface of a photoconductor is illuminated by a non-movable print head having a plurality of light-emitting elements. Such types of print head include an LED head and LCS (liquid crystal shutter) head. The print head has non-movable part as the name implies and a short light path, simplifying its optical system as well as lending itself to miniaturizing the print head.
FIG. 13
illustrates a part of a conventional printer. An LED head
3
is constructed of an LED array
4
, drive unit
5
, and optical system, not shown. The LED array
4
has a plurality of LEDs
7
. The drive unit
5
has a plurality of drive circuits
8
each of which drives a corresponding LED
7
. The optical system directs the light emitted from the LEDs
7
to form the images of the light on the photoconductor. The drive circuits
8
are serially connected so that the image data is serially transferred therethrough. The image data is input to an endmost drive circuit (leftmost) of a row of the drive circuits and sequentially transferred to another endmost circuit (right most) on the other end of the row. When all the image data for one line has been transferred to the drive circuits
8
, the image data is latched and the corresponding LEDs
7
are energized to emit light.
Recent remarkable improvement of performance of personal computers has brought about the need for printer output of as high a quality as photographs. The LED head
3
used in the conventional electrophotographic printers are driven in such a way that all the LEDs
7
are energized simultaneously and therefore are capable of representing only an image consisting of dots expressed in binary values. Therefore, when an image is to be printed with gradation, pseudo gradation such as dither method and error diffusion method are used. The dither reduces the resolution of the image. The error diffusion method suffers from an inherent problem of a striped pattern. Thus, these methods impose limitations on high quality print out.
Electrophotographic printers using conventional LED heads suffer from a problem that simultaneously energizing all the LEDs requires a large current that causes noise. The noise in turn causes deteriorated print quality.
SUMMARY OF THE INVENTION
An object of the invention is to provide an electrophotographic printer which prints images with gradation while maintaining high resolution.
Another object of the invention is to provide an electrophotographic printer where energization of the lightemitting elements is started at shifted timings so that a total current required for driving the light-emitting elements is as small as possible.
An electrophotographic printer has a print head which illuminates a photoconductor to form an electrostatic latent image thereon. The print head has a plurality of light-emitting elements. Each light emitting element corresponds to a pixel of the image and emits light when driven. A plurality of drive circuits are provided. Each drive circuit drives a corresponding one of the plurality of light-emitting elements in accordance with a value of the pixel. A controller controls the plurality of drive circuits to start driving corresponding light-emitting elements in a predetermined order.
The value of the pixel describes, for example, a length of time during which a corresponding light-emitting element is energized to emit light.
The electrophotographic printer may include a memory and a correction section. The memory stores correction values for corresponding light-emitting elements. Using the correction values, the correction section changes the lengths of time to accommodate variations of light-emitting characteristics of the light-emitting elements.
The drive circuits are connected to a common data bus and the controller outputs values of pixels to the drive circuits over the common data bus.
The drive circuits are connected to a common address bus. Each drive circuit has a corresponding address and an address memory which stores the corresponding address. The controller outputs an address over the common address bus, the address specifying a corresponding drive circuit. Each drive circuit receives the value of the pixel only when the address stored in the address memory coincides with the address outputted from the controller.
The drive circuits are serially connected such that a timing signal is sequentially transferred through the drive circuits from a preceding drive circuit to a following drive circuit upon a clock. Each drive circuit starts driving a corresponding light-emitting element upon the timing signal supplied thereto.
The drive circuit has a counter that counts the length of time described by the value of the pixel. The drive circuit starts driving the corresponding light-emitting element when the counter is loaded with the length of time and stops driving when the counter has counted to a predetermined.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 5657069 (1997-08-01), Kelly et al.
patent: 3-41864 (1991-02-01), None
Oki Data Corporation
Pendegrass Joan
Rabin & Champagne, P.C.
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