Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2003-06-19
2004-10-05
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S259000, C347S260000
Reexamination Certificate
active
06801239
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, etc., using an electrophotographic method, and more particularly, it relates to such an image forming apparatus having a plurality of photosensitive members.
2. Description of the Related Art
FIGS. 5 through 7
show one example of an image forming apparatus of the so-called tandem type having a plurality of photosensitive members.
FIG. 5
is an image forming apparatus for printing color images, which has independent image bearing members (hereinafter referred to as photosensitive drums) for colors of yellow, magenta, cyanogen and black, respectively.
Each photosensitive drum is formed of a conductor with a photosensitive layer coated thereon, and serves to form an electrostatic latent image thereon with a laser beam projected from a scanning optical unit.
A scanning optical unit
21
irradiates laser beams to the photosensitive drums based on the image information sent by an unillustrated image reading device, a personal computer, etc. A plurality of development units
22
serve to form toner images on the corresponding photosensitive drums, respectively, with a toner which is friction electrified or charged. An intermediate transfer belt
23
conveys the toner images on the photosensitive drums to a sheet of transfer paper. A sheet cassette
24
stores a multitude of sheets of transfer paper on which toner images are to be formed. A fixing unit
25
serves to make the toner images thus transferred onto the transfer sheet to be adsorbed to the transfer sheet through application of heat. A discharge tray
26
serves to receive the image-fixed transfer sheet discharged thereto. A plurality of cleaners
27
clean or remove the toner remaining on the corresponding photosensitive drums, respectively.
The formation of images is performed in such a manner that electrostatic latent images are formed on the photosensitive drums electrically charged by corresponding chargers by irradiating laser beams emitted based on the image information from the scanning optical unit
21
onto the photosensitive drums, respectively. Thereafter, toner images are formed on the photosensitive drums, respectively, by attaching the toner friction electrified or charged in the development unit
22
to the electrostatic latent images.
The toner images are then transferred from the photosensitive drums onto the intermediate transfer belt
23
. By retransferring the toner images to a transfer sheet conveyed from the sheet cassette
24
arranged at a lower portion of the main body of the apparatus, an image is formed on the transfer sheet.
The toner of the image transferred onto the transfer sheet is fixed by means of the fixing unit
25
, and the transfer sheet is then discharged and loaded onto the discharge tray
26
.
FIG. 6
shows an image forming unit of FIG.
5
. Since the image forming unit is bilaterally symmetrical, symbols in this figure are attached on one side alone, in this figure the scanning optical unit
21
forms electrostatic latent images on the photosensitive drums, respectively, by means of laser beams which pass through a rotary polygon mirror in the form of a polygon mirror
28
, which acts to perform deflection scanning of the laser beams emitted based on image information, through imaging elements in the form of f&thgr; lenses
29
,
30
for forming or focusing spot images on the photosensitive drums while scanning the laser beams at a uniform speed, through a plurality of reflection members in the form of return mirrors
31
a
-
31
d
, which reflect the laser beams in prescribed directions, respectively, and through a dustproof glass
32
for protecting the scanning optical unit
21
from dust.
Unlike a conventional scanning optical system in which laser beams are irradiated onto photosensitive drums from locations away therefrom, the scanning optical unit
21
is now coming to be arranged at a position near the photosensitive drums in accordance with the main body of the image forming apparatus being made compact. Thus, a system is adopted in which one polygon motor unit is used to irradiate a plurality of (e.g., four) photosensitive drums, as shown in
FIG. 6
, with two scanning groups being provided for irradiating a plurality of laser beams to opposed sides of the polygon mirror
28
.
In addition, a plurality of return mirrors are used to serve the purpose of achieving the compactification of the scanning optical unit. There is also used a mold lens which is composed of two lenses stuck together or which is molded integrally with two optical paths, so that laser beams with two different optical paths are caused to form or focus images on the photosensitive drums, respectively.
In the parallel optical system in which a plurality of laser beams are scanned in parallel with one another from this polygon mirror
28
, a deflection surface or place is required for each of the optical paths, and hence a thick polygon mirror or a polygon mirror of a two stage configuration has to be employed.
In contrast to the above-mentioned optical system using the thick or two stage polygon mirror, however, there is another optical system using a thin polygon mirror
33
, as shown in
FIG. 7
, which is capable of reducing the thickness of the entire optical system.
In this system, respective laser beams are made to enter and exit the polygon mirror at different angles, respectively, and the laser beams to be irradiated to the respective photosensitive drums are separated from each other at a location at which a predetermined interval between the laser beams is obtained.
The laser beams, after having been deflection scanned by means of the polygon mirror
33
, pass through common f&thgr; lenses
35
,
36
, and then proceed by way of two return mirrors
34
a
,
34
c
as well as one concave mirror
34
b
, or by way of two return mirrors
34
d
,
34
f
as well as one concave mirror
34
e
, so that they are irradiated to the photosensitive drums, respectively.
Moreover, with such a laser beam separation configuration, one of the laser beams, being deflection scanned at a lower side of
FIG. 7
, is reflected in an upward direction in this figure by the return mirror
34
d
disposed in an optical path toward an inner photosensitive drum, in such a manner that it crosses the other laser beam which is being deflection scanned at an upper side of FIG.
7
. Thereafter, the one laser beam is reflected and irradiated to the inner photosensitive drum by means of the plurality of return mirrors
34
e
,
34
f
disposed at an upper portion of an optical casing.
In the case of the oblique incidence optical system of
FIG. 7
, assuming that the f&thgr; lenses have refractive power in the main scanning direction, the same operation as in the case of parallel optical system of
FIG. 6
is performed. Therefore, the oblique incidence optical system can be arranged just like the parallel optical system of
FIG. 6
, but with respect to the sub-scanning direction, laser beams enter the f&thgr; lenses obliquely or at angles relative to the optical axes of the f&thgr; lenses, and hence it is fundamentally difficult for the oblique incidence optical system to ensure the performance of converging or focusing laser beams on the photosensitive drums. Thus, the concave mirrors
34
b
,
34
e
are inevitably added for converging the laser beams into the sub-scanning direction after separation of the respective laser beams. (Also, even when a third focusing lens with refractive power in the sub-scanning direction is added instead of these concave mirrors, a similar operation is performed.)
On the other hand, in the case of the oblique incidence optical system of
FIG. 7
, it is necessary to arrange a total of four imaging optical elements in one scanning group at locations after the polygon mirror. Thus, for another or second oblique incidence optical system with a reduced number of optical elements, a proposal has been made that the second imaging lens of the two f&thgr; lenses is arran
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