Facsimile and static presentation processing – Static presentation processing – Size – resolution – or scale control
Reexamination Certificate
2001-01-12
2004-02-17
Grant, II, Jerome (Department: 2622)
Facsimile and static presentation processing
Static presentation processing
Size, resolution, or scale control
C358S451000
Reexamination Certificate
active
06693723
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to laser printers and more particularly, this invention relates to a laser printer having selectable printing resolutions.
BACKGROUND OF THE INVENTION
Within the last decade, laser printers have become the standard against which all other printers are measured in the personal computing industry. Understandably, a large amount of research has been recently undertaken to increase their versatility and print quality. This has resulted in the development of personal laser printers having resolutions in excess of 1200 DPI (dots-per-inch).
FIG. 1
of the attached drawings shows a representational diagram of a basic laser printer configuration, to which the reader is referred to aid in the following explanation of the electrophotographic process for recording and registering an image on paper. In general terms, a computer, or similar device, sends a series of codes representing an image to the input port
27
of the laser printer. The laser printer converts the codes to a series of binary signals, each of which generally represents one dot of the thousands of dots which together form the image. The binary signals are used to pulse the beam of a laser such that the binary pattern is represented or transmitted by turning off and on the laser. This pattern is then recorded on a light sensitive drum which transfers the pattern onto a printing medium, such as paper, in the form of toner or a similar indelible substance. In general terms, an image forming device of this nature has two main components, a raster image processor for processing the image and a print engine to fix the image to media.
As can be seen in
FIG. 1
, once the data has been transmitted to the laser printer it is analyzed in Formatter
11
, here consisting of a microprocessor
23
and related memory
24
and buffer
12
. Formatter
11
parses out the printer control commands from the text and graphics, and manipulates the text and graphics in accordance with the printer control commands and user input control codes from user input and display panel
25
. Once the page has been formatted, it is transmitted to the buffer
12
. Data is then sent to a circuit which drives the laser
13
.
The data is used to modulate a light beam produced by laser
13
. The modulation of the beam is accomplished by laser driver-controller
26
. The laser beam is then reflected off of a multifaceted, spinning mirror
14
. Here, the multifaceted mirror is shown as a six sided polyhedron, however as few as two facets are common. As each facet of the mirror
14
spins through the light beam, it reflects, or “scans”, the beam across the side of a photoconductive drum
15
. The photoconductive drum
15
is rotated about its axis such that it advances just enough that each successive scan of the light beam is recorded immediately after the previous scan directly on the photoconductive drum
15
. In this manner, each strip of data from the buffer
12
is recorded on the photoconductive drum
15
, as a line one after the other to reproduce the page on the drum.
The laser beam actually discharges the area on the photoconductive drum
15
it irradiates. The photoconductive drum
15
is first charged using a high voltage primary corona wire, shown at
16
, to have a negative polarity at its surface. Because of the special photoconductive material which covers the drum, the laser beam effectively discharges any areas which it irradiates. This process creates a “latent” electrostatic image on the drum. This portion of the drum then comes into close proximity to the developing roller
17
which rotates counter clockwise, or opposite to the photoconductive drum
15
. The developing roller
17
transfers the toner from the toner bath to the photoconductive drum
15
. Here, a dry toner is used which consists of fine thermoplastic particles impregnated with a ferromagnetic material such as iron. The developer roller
17
uses the negative pole of an internal magnet to attract the toner. Triboelectric charging results in a negative charge to the particles themselves. The developer roller
17
is electrically biased so as to repel the charged toner to the image areas. In this manner, the toner is transferred to the photoconductive drum
15
and forms a pattern thereon which duplicates the image.
The toner is transferred from the photoconductive drum
15
to the printing medium, e g. paper
18
, using an electrostatic process. Media drive system
28
advances the sheet of media through the printer. Here a second corona wire, transfer corona
19
, is used to impart a relatively strong positive charge to the back side of the paper
18
as it passes by the photoconductive drum
15
. The high positive charge attracts the negatively charged toner and pulls it from the drum, maintaining the same pattern. The toner is then fused to the paper
18
by passing both the toner and paper through a pair of hot fusing rollers
20
.
The photoconductive drum
15
usually has a circumference which is less than the length of most paper. Hence, the drum must rotate several times to print a full sheet. The drum is cleaned with cleaning blade
21
, completely discharged by discharge lamps
22
and recharged by corona
16
.
The following discussion concerns references, of which the inventors are aware, showing technologies and improvements related to the instant invention. U.S. Pat. No. 4,578,689 issued March, 1986 to SPENCER ET AL. for a Dual Mode Laser Printer teaches a laser printer having two modes for printing at different resolutions. The first is a high speed/low resolution mode and the second is a low speed/high resolution mode. The disclosure claims that the printer is capable of printing up to 16 pages per minute in low resolution mode, i.e. less than 400 DPI (dots-per-inch), and at approximately 4 pages per minute in high resolution mode, i.e. above 500 DPI. The printer accomplishes this by changing the speed of the paper drive and photoconductive drum using stepper motors, adjusting the heat produced by the fuser, adjusting the corona current in the electrophotographic process, controlling the laser output power and controlling the number of facets of the rotating mirror which are actually used to scan the photoconductive drum. The speed of the rotating mirror is kept constant regardless of the resolution selected. When reducing both the speed of the paper and the photoconductive drum, only every other, or fewer, facets of the rotating mirror are used, thus allowing enough time to elapse to permit the information to be accumulated without increasing the data transfer rate to the printer.
U.S. Pat. No. 4,700,201 issued October, 1987 to SATO for a Dot Corrected Laser teaches an image enhancement technique which varies the size of the dot produced dependant upon whether the dot is relatively isolated, e.g. not completely surrounded by other dots immediately adjacent to it, or in a densely populated area of the image. If a particular dot is determined to be isolated, the invention will increase the size of the dot by varying the duty cycle of the laser. In a positive exposure system, the “on time” of the laser is increased while in a negative exposure system the “off time” of the laser is increased. The invention uses either a modified driver circuit or an acousto-optic modulator to control the duty cycle of the irradiation to vary the spot or dot sized produced.
U.S. Pat. No. 4,717,925 issued January, 1988 to SHIBATA ET AL. for an Optical Scanner Without Extra Convergent Lens teaches a scanner error correction device which is similar in concept to that described in U.S. Pat. No. 4,613,877 discussed above. Additionally, this patent teaches an adjustable intensity laser beam using a photodetector, cooperating driver controller and laser driver to continuously adjust the intensity of the beam. As the data video rate increases, the intensity of the beam is increased to provide a uniform spot or dot size regardless of the shortened duty cycle.
U.S. Pat. No. 4,734,715 issued March, 1988 to SHIRAISHI for a Variable Light Beam
Keithley Douglas G.
Rasmussen Brent D.
Baca Anthony J
Grant II Jerome
Hewlett-Packard Development Company LP.
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