U.S. patent number RE45,918 [Application Number 13/721,099] was granted by the patent office on 2016-03-08 for optical scanning apparatus, optical writing apparatus, and image forming apparatus.
This patent grant is currently assigned to RICOH COMPANY, LIMITED. The grantee listed for this patent is Ricoh Company, Limited. Invention is credited to Kenichiroh Saisho, Kohji Sakai.
United States Patent |
RE45,918 |
Saisho , et al. |
March 8, 2016 |
Optical scanning apparatus, optical writing apparatus, and image
forming apparatus
Abstract
F-theta lenses, included in scanning lens systems, are arranged
on a main scanning plane facing an optical deflector and
substantially linearly symmetrically on the main scanning plane
with reference to a rotational center of the optical deflector.
Each f-theta lens has a no-power portion in the main scanning
direction. Synchronization-detecting light passes through the
no-power portion of the f-theta lens, thus enabling reduction in
color shift due to temperature variation in an image forming
apparatus without increasing the cost and complexity in controlling
color shift.
Inventors: |
Saisho; Kenichiroh (Kanagawa,
JP), Sakai; Kohji (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Ricoh Company, Limited |
Ohta-ku, Tokyo |
N/A |
JP |
|
|
Assignee: |
RICOH COMPANY, LIMITED (Tokyo,
JP)
|
Family
ID: |
38478625 |
Appl.
No.: |
13/721,099 |
Filed: |
December 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
11714116 |
Mar 6, 2007 |
7876486 |
Jan 25, 2011 |
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Foreign Application Priority Data
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Mar 8, 2006 [JP] |
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2006-062983 |
Mar 14, 2006 [JP] |
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2006-069460 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
26/125 (20130101) |
Current International
Class: |
G02B
26/08 (20060101); G02B 26/12 (20060101) |
Field of
Search: |
;359/204.1-204.4,205.1,206.1 ;347/233-235,243-244,258-261,250
;250/234-236 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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03-081720 |
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Apr 1991 |
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JP |
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03-131817 |
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Jun 1991 |
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JP |
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04-175718 |
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Jun 1992 |
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JP |
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05-046005 |
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Feb 1993 |
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JP |
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05-289008 |
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Nov 1993 |
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JP |
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08-297256 |
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Nov 1996 |
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JP |
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09-184976 |
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Jul 1997 |
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JP |
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10-010445 |
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Jan 1998 |
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JP |
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10-197823 |
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Jul 1998 |
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JP |
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11-311749 |
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Nov 1999 |
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JP |
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2000-121983 |
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Apr 2000 |
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JP |
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2000-214405 |
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Aug 2000 |
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JP |
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2002-090672 |
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Mar 2002 |
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JP |
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2002-098921 |
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Apr 2002 |
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JP |
|
3293345 |
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Apr 2002 |
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JP |
|
2002-131664 |
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May 2002 |
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JP |
|
2003-185952 |
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Jul 2003 |
|
JP |
|
2004-21171 |
|
Jan 2004 |
|
JP |
|
2005-099336 |
|
Apr 2005 |
|
JP |
|
Primary Examiner: Phan; James
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Claims
What is claimed is:
1. An optical scanning apparatus comprising: 2n (where n.gtoreq.1)
light sources.[., each light source including m (where m.gtoreq.1)
light emitting units.].; a .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit.]. .Iadd.units
.Iaddend.that .[.receives m.times.n.]. .Iadd.receive .Iaddend.light
beams from the light sources.[., scans a scanning surface by the
light beams emitted by 2n light sources, and determines a write
timing for writing to the scanning surface, the light beams being
substantially symmetrical with respect to a sub-scanning
cross-section that includes a rotational axis of an optical
deflector.].; .Iadd.an optical deflector that deflects the light
beams from the light sources; and .Iaddend. a first scanning lens
system and a second scanning lens system .[.arranged on either side
of the optical deflector and causing m.]. .Iadd.that cause the
.Iaddend.light beams to perform imaging on .[.the.]. respective
.[.scanning surfaces.]. .Iadd.photosensitive members.Iaddend.,
.Iadd.the first scanning lens system including at least a first
scanning lens and the second scanning lens system including at
least a second scanning lens, .Iaddend.wherein .[.synchronization
detection by.]. the .Iadd.plurality of .Iaddend.synchronization
detecting .[.unit is performed at one end of each scan line.].
.Iadd.units are configured to perform synchronization
detection.Iaddend., .[.and.]. the first scanning lens .Iadd.is a
scanning lens that the light beams deflected by the deflector first
enter in the first scanning lens .Iaddend.system and the second
scanning lens .[.system.]. .Iadd.is a scanning lens that the light
beams deflected by the deflector first enter in the second scanning
lens system, the first scanning lens and the second scanning lens
.Iaddend.are arranged substantially symmetrically with reference to
a line .[.in a main scanning direction orthogonal to the.].
.Iadd.that passes through a .Iaddend.rotational axis of the optical
deflector .Iadd.and is substantially parallel to a longitudinal
direction of the first and second scanning lenses.Iaddend., and the
.Iadd.first and second .Iaddend.scanning lens .[.system.].
.Iadd.systems .Iaddend..[.includes.]. .Iadd.include .Iaddend.a
no-power portion in the main scanning direction, and the light
beams guided to the .Iadd.plurality of .Iaddend.synchronization
detecting .[.unit.]. .Iadd.units .Iaddend.pass through the no-power
portion.
2. The optical scanning apparatus according to claim 1, wherein a
magnitude of a spot position shift of .[.the.].
synchronization-detecting light in the main scanning direction on
an imaging surface or its equivalent due to temperature variation
detected by the .Iadd.plurality of .Iaddend.synchronization
detecting .[.unit.]. .Iadd.units .Iaddend.is 5 .mu.m/.degree. C. or
less.
3. The optical scanning apparatus according to claim 1, wherein an
inter-surface deviation of a surface tilt of the optical deflector
is kept to 200 seconds or less.
4. The optical scanning apparatus according to claim 1, wherein
.[.a.]. synchronization optical .[.system.]. .Iadd.systems
.Iaddend.of the .Iadd.plurality of .Iaddend.synchronization
detecting .[.unit.]. .Iadd.units .Iaddend.that .[.converges the.].
.Iadd.converge .Iaddend.synchronization-detecting light into
.[.a.]. light-receiving .[.unit corrects.]. .Iadd.units correct
.Iaddend.the effect of a surface tilt of the optical deflector.
5. The optical scanning apparatus according to claim 1, wherein
from among a first set of scanning lenses L1, L2, and so on up to q
(where j=1, 2, 3 . . . ) forming the first scanning lens system in
the sequence of the nearest to the farthest from the optical
deflector on one side of the optical deflector and a second set of
scanning lenses L'1, L'2, and so on up to Lj (where j=1, 2, 3, . .
. ), any pair of scanning lenses Lj and Lj have an identical
geometry.
6. An image forming apparatus comprising.Iadd.: an exposing unit; a
developing unit; a transfer unit; a fixing unit; and .Iaddend. the
optical scanning apparatus according to claim 1.
7. The optical scanning apparatus according to claim 1, wherein the
first scanning lens system and the second scanning lens system are
arranged facing each other on either side of the optical
deflector.
8. An optical scanning apparatus, comprising: 2n (where n.gtoreq.1)
light sources.[., each light source including m (where m.gtoreq.1)
light emitting units.].; a .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit.]. .Iadd.units
.Iaddend.that .[.receives m.times.n.]. .Iadd.receive .Iaddend.light
beams from the light sources.[., scans a scanning surface by the
light beams emitted by 2n light sources, and determines a write
timing for writing to the scanning surface, the light beams being
substantially symmetrical with respect to a sub-scanning
cross-section that includes a rotational axis of an optical
deflector.].; .[.and.]. .Iadd.an optical deflector that deflects
the light beams from the light sources; and .Iaddend. a first
scanning lens system and a second scanning lens system .[.arranged
on either side of the optical deflector and causing m.]. .Iadd.that
cause the .Iaddend.light beams to perform imaging on .[.the.].
respective .[.scanning surfaces.]. .Iadd.photosensitive
members.Iaddend., .Iadd.the first scanning lens system including at
least a first scanning lens and the second scanning lens system
including at least a second scanning lens, .Iaddend.wherein
.[.synchronization detection by.]. the .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit is performed at one end
of each scan line.]. .Iadd.units are configured to perform
synchronization detection.Iaddend., the first scanning lens
.[.system.]. .Iadd.is a scanning lens that the light beams
deflected by the deflector first enter in the first scanning lens
system .Iaddend.and the second scanning lens .[.system.]. .Iadd.is
a scanning lens that the light beams deflected by the deflector
first enter in the second scanning lens system, the first scanning
lens and the second scanning lens .Iaddend.are arranged
substantially symmetrically with reference to a line .[.in a main
scanning direction orthogonal to the.]. .Iadd.that passes through a
.Iaddend.rotational axis of the optical deflector .Iadd.and is
substantially parallel to a longitudinal direction of the first and
second scanning lenses.Iaddend., and the
.[.synchronization-detecting.]. .Iadd.synchronization detecting
units are configured to receive the .Iaddend.light .[.does.].
.Iadd.beams such that the light beams do .Iaddend.not pass through
the .[.scanning lens system.]. .Iadd.first and second scanning
lenses.Iaddend..
9. The optical scanning apparatus according to claim 8, wherein the
first scanning lens system and the second scanning lens system are
arranged facing each other on either side of the optical
deflector.
10. An optical scanning apparatus, comprising: 2n (where
n.gtoreq.1) light sources.[., each light source including m (where
m.gtoreq.1) light emitting units.].; a .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit.]. .Iadd.units
.Iaddend.that .[.receives m.times.n.]. .Iadd.receive .Iaddend.light
beams from the light sources.[., scans a scanning surface by the
light beams emitted by 2n light sources, and determines a write
timing for writing to the scanning surface, the light beams being
substantially symmetrical with respect to a sub-scanning
cross-section that includes a rotational axis of an optical
deflector.].; .[.and.]. .Iadd.an optical deflector that deflects
the light beams from the light sources; and .Iaddend. a first
scanning lens system and a second scanning lens system .[.arranged
on either side of the optical deflector and causing m.]. .Iadd.that
cause the .Iaddend.light beams to perform imaging on .[.the.].
respective .[.scanning surfaces.]. .Iadd.photosensitive
members.Iaddend., .Iadd.the first scanning lens system including at
least a first scanning lens and the second scanning lens system
including at least a second scanning lens, .Iaddend.wherein
.[.synchronization detection by.]. the .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit is performed at one end
of each scan line.]. .Iadd.units are configured to perform
synchronization detection.Iaddend., the first scanning lens
.Iadd.is a scanning lens that the light beams deflected by the
deflector first enter in the first scanning lens .Iaddend.system
and the second scanning lens .[.system.]. .Iadd.is a scanning lens
that the light beams deflected by the deflector first enter in the
second scanning lens system, the first scanning lens and the second
scanning lens .Iaddend.are arranged substantially symmetrically
with reference to a line .[.in a main scanning direction orthogonal
to the.]. .Iadd.that passes through a .Iaddend.rotational axis of
the optical deflector .Iadd.and is substantially parallel to a
longitudinal direction of the first and second scanning
lenses.Iaddend., and an opening is provided in .Iadd.each of
.Iaddend.the .Iadd.first and second .Iaddend.scanning lens
.[.system.]. .Iadd.systems.Iaddend., and .[.the
synchronization-detecting.]. .Iadd.synchronization detecting
.Iaddend.light passes through the opening.
11. The optical scanning apparatus according to claim 10, wherein
the first scanning lens system and the second scanning lens system
are arranged facing each other on either side of the optical
deflector.
12. An optical writing apparatus comprising: a plurality of optical
scanning apparatuses, each optical scanning apparatus including, 2n
(where n.gtoreq.1) light sources.[., each light source including m
(where m.gtoreq.1) light emitting units.].; a .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit.]. .Iadd.units
.Iaddend.that .[.receives m.times.n.]. .Iadd.receive .Iaddend.light
beams from the light sources.[., scans a scanning surface by the
light beams emitted by 2n light sources, and determines a write
timing for writing to the scanning surface, the light beams being
substantially symmetrical with respect to a sub-scanning
cross-section that includes a rotational axis of an optical
deflector.].; .[.and.]. .Iadd.an optical deflector that deflects
the light beams from the light sources; and .Iaddend. a first
scanning lens system and a second scanning lens system .[.arranged
facing each other on either side of optical deflector and causing
m.]. .Iadd.that cause the .Iaddend.light beams to perform imaging
on .[.the.]. respective .[.scanning surfaces.].
.Iadd.photosensitive members.Iaddend., .Iadd.the first scanning
lens system including at least a first scanning lens and the second
scanning lens system including at least a second scanning lens,
.Iaddend. wherein .[.synchronization detection by.]. the
.Iadd.plurality of .Iaddend.synchronization detecting .[.unit is
performed at one end of each scan line.]. .Iadd.units are
configured to perform synchronization detection.Iaddend., and the
first scanning lens .Iadd.is a scanning lens that the light beams
deflected by the deflector first enter in the first scanning lens
.Iaddend.system and the second scanning lens .[.system.]. .Iadd.is
a scanning lens that the light beams deflected by the deflector
first enter in the second scanning lens system, the first scanning
lens and the second scanning lens .Iaddend.are arranged
substantially symmetrically with reference to a line .[.in a main
scanning direction orthogonal to the.]. .Iadd.that passes through a
.Iaddend.rotational axis of the optical deflector .Iadd.and is
substantially parallel to a longitudinal direction of the first and
second scanning lenses.Iaddend., wherein
.[.synchronization-detecting.]. .Iadd.synchronization detecting
.Iaddend.light of only one optical scanning apparatus is detected,
and write timings of the other optical scanning apparatuses
.[.is.]. .Iadd.are .Iaddend.electrically estimated based on
detection signals of detected .[.synchronization-detecting.].
.Iadd.synchronization detecting .Iaddend.light.
13. The optical writing apparatus according to claim 12, wherein
the optical deflectors of the optical scanning apparatuses have a
common rotational axis.
14. The optical writing apparatus according to claim 12, wherein
the light beams from the plurality of levels are incident on the
same phase plane of the optical deflector at different incidence
angles in the main scanning direction.
15. The optical writing apparatus according to claim 12, wherein
.[.the.]. synchronization optical .[.system includes.].
.Iadd.systems include .Iaddend.a synchronization mirror that guides
the light beam towards the .Iadd.plurality of
.Iaddend.synchronization detecting .[.unit.]. .Iadd.units.Iaddend.,
and the synchronization .[.mirror.]. .Iadd.mirrors .Iaddend.and the
.Iadd.plurality of .Iaddend.synchronization detecting .[.unit.].
.Iadd.units .Iaddend.are coupled within the sub-scanning
cross-section.
16. The optical writing apparatus according to claim 12, wherein
the light beams from .[.the.]. .Iadd.a .Iaddend.plurality of levels
incident on .[.the.]. .Iadd.a .Iaddend.same phase plane of the
optical deflector are deflected in a main scanning direction by
different reflecting points in the main scanning direction.
.Iadd.17. An image forming apparatus comprising; an exposing unit;
a developing unit; a transfer unit; a fixing unit; and the optical
scanning apparatus according to claim 8..Iaddend.
.Iadd.18. An image forming apparatus comprising; an exposing unit;
a developing unit; a transfer unit; a fixing unit; and the optical
scanning apparatus according to claim 10..Iaddend.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
The present document incorporates by reference the entire contents
of Japanese priority document, 2006-062983 filed in Japan on Mar.
8, 2006 and 2006-069460 filed in Japan on Mar. 14, 2006.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as
a multi function peripheral or a plotter that includes at least two
of the following, namely, an optical scanning apparatus that scans
a scanning surface, an optical writing apparatus that includes the
optical scanning apparatus, a copying machine that includes the
optical writing apparatus, a printer, and a fax machine.
2. Description of the Related Art
Optical scanning apparatuses are widely known in relation to
digital copying apparatuses and laser printers. Optical scanning
apparatuses employ scanning optical system, which involves focusing
optical beams deflected by an optical deflector to form a beam spot
on a scanning surface.
In an optical writing apparatus or an image forming apparatus that
includes an optical scanning apparatus, there is a tendency for the
optical performance to deteriorate due to reduced tolerance of the
optical element, expansion/contraction of the optical element due
to temperature variation, etc. A stable optical performance is thus
desirable alongside a small beam spot diameter.
A stable optical performance can be attained by including an
adjustment mechanism in the optical element. However, this method
is neither cost-effective nor space-effective.
Stability particularly is sought with regard to the position of the
beam spot of different colors in a main scanning direction. A
multi-color image clearly shows degradation if the beam spots of
all the colors do not form at the same spot.
The position of the beam spot in the main scanning direction can be
stabilized, by adjusting the write timing. Synchronization
detection method is well known as a method for electrically
adjusting a write-start timing and involves providing a photo
sensor in all parts except those used for writing.
In synchronization detection method, photoreception can be
performed at one end of a scan line or both ends of the scan line.
The latter method can be expected to considerably reduce
non-coincidence of the beam spots in the main scanning direction as
a standard for the write start timing is set at both ends of the
scan line. However, in a color image forming apparatus producing
color images, two photo sensors would be required per scanning
optical system (for each color), increasing the number of
components in terms of the photo sensors as well as electrical
control boards for the photo sensors.
A technology thus is sought to enable adjustment of the write start
timing using as few photo receptors as possible.
Again stability is desirable in the light beam used in detecting
synchronization (hereinafter, "synchronization-detecting light") as
it is taken as a standard for the write start timing. However, even
the synchronization-detecting light that reaches the photo sensor
is affected by the deterioration of the optical performance of the
scanning lens due to reduced tolerance, temperature variation, and
the like.
In the optical scanning apparatus disclosed in Japanese Patent
Application Laid-open No. 2002-98921, a notch is provided at an end
of the scanning lens for the passage of the
synchronization-detecting light, so that the
synchronization-detecting light does not pass through the scanning
lens and be adversely affected by the expansion and contraction of
the scanning lens.
In the optical scanning apparatus disclosed in Japanese Patent
Application Laid-open No. 3293345, a rib is provided for the
passage of the synchronization-detecting light, again so that the
synchronization-detecting light does not pass through the scanning
lens and be adversely affected by the expansion and contraction of
the scanning lens.
In an apparatus with optical writing as its principal
functionality, the synchronous optical systems need to be placed
where they will not interfere with the parts performing optical
writing. This puts a constraint on the freedom in designing,
especially with increasing demand for low-cost and compact optical
scanning apparatus. Particularly, this constraint necessitates the
length of the synchronous optical system path to be increased,
leading to less than ideal conditions for photoreception.
A method is disclosed in Japanese Patent Application Laid-open No.
H3-81720 wherein a synchronization mirror which forms a
synchronization optical system and a synchronization detecting unit
are coupled within a sub-scanning cross-section.
The synchronization-detecting light only reduces the shift of the
beam spots of each color individually in the main scanning
direction and by no means addresses the shift of the beam spots
between different colors.
To obtain a high quality image, merely reducing the shift of the
beam spot of each color individually in the main scanning direction
is not enough, but it is also necessary for the optical scanning
apparatus and the optical writing apparatus to be able to reduce
the shift in the beam spots of different colors (hereinafter,
"color shift") in the main scanning direction, and not allow color
shift due to temperature variation.
The layout constraint encountered in a quest to make the scanning
optical systems compact cannot be bypassed only in the method
disclosed in Japanese Patent Application Laid-open No.
H3-81720.
SUMMARY OF THE INVENTION
It is an object of the present invention to at least partially
solve the problems in the conventional technology.
According to an aspect of the present invention, an optical
scanning apparatus includes 2n (where n.gtoreq.1) light sources,
each light source including m (where m.gtoreq.1) light emitting
units; a synchronization detecting unit that receives m.times.n
light beams from the light sources, scans a scanning surface by the
light beams emitted by 2n light sources, and determines a write
timing for writing to the scanning surface, the light beams being
substantially symmetrical with respect to a sub-scanning
cross-section that includes a rotational axis of an optical
deflector; and a first scanning lens system and a second scanning
lens system arranged facing each other on either side of optical
deflector and causing m light beams to perform imaging on the
respective scanning surfaces, wherein synchronization detection by
the synchronization detecting unit is performed at one end of each
scan line, and the first scanning lens system and the second
scanning lens system are arranged substantially symmetrically with
reference to a line in a main scanning direction orthogonal to the
rotational axis of the optical deflector.
According to another aspect of the present invention, an optical
scanning apparatus includes an optical deflector that deflects
light beams emitted by a light source in a main scanning direction;
a scanning lens system that focuses the light beams deflected by
the optical deflector on a scanning surface as a beam spot; a
synchronization detecting unit that determines a write timing for
writing to the scanning surface; and a synchronization optical
system that includes at least one converging unit that converges
the light beams on the synchronization detecting unit, wherein the
converging unit that contributes the most to a sub-scanning
position shift by the synchronization detecting unit and the
synchronization detecting unit are coupled within a sub-scanning
cross-section and a condition D>Z/2 is satisfied, where D is a
beam diameter at a light-receiving unit of the synchronization
detecting unit, and Z is a light-receiving zone of the
light-receiving unit in a sub-scanning direction.
According to still another aspect of the present invention, an
optical writing apparatus includes a plurality of optical scanning
apparatuses, each optical scanning apparatus including, 2n (where
n.gtoreq.1) light sources, each light source including m (where
m.gtoreq.1) light emitting units; a synchronization detecting unit
that receives m.times.n light beams from the light sources, scans a
scanning surface by the light beams emitted by 2n light sources,
and determines a write timing for writing to the scanning surface,
the light beams being substantially symmetrical with respect to a
sub-scanning cross-section that includes a rotational axis of an
optical deflector; and a first scanning lens system and a second
scanning lens system arranged facing each other on either side of
optical deflector and causing m light beams to perform imaging on
the respective scanning surfaces, wherein synchronization detection
by the synchronization detecting unit is performed at one end of
each scan line, and the first scanning lens system and the second
scanning lens system are arranged substantially symmetrically with
reference to a line in a main scanning direction orthogonal to the
rotational axis of the optical deflector, wherein
synchronization-detecting light of only one optical scanning
apparatus is detected, and write timings of the other optical
scanning apparatuses is electrically estimated based on detection
signals of detected synchronization-detecting light.
According to still another aspect of the present invention, an
image forming apparatus includes the above optical scanning
apparatus.
The above and other objects, features, advantages and technical and
industrial significance of this invention will be better understood
by reading the following detailed description of presently
preferred embodiments of the invention, when considered in
connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph for explaining a main scanning position
shift;
FIG. 2 is a schematic of an optical scanning apparatus according to
a first embodiment of the present invention having a linearly
symmetrical arrangement;
FIG. 3 is a schematic of an optical scanning apparatus having a
rotationally symmetrical arrangement;
FIG. 4 is a graph for explaining the main scanning position shift
in the linearly symmetrical arrangement;
FIG. 5 is a graph for explaining the main scanning position shift
in the rotationally symmetrical arrangement;
FIG. 6 is a graph for explaining the main scanning position shift
when the arrangement is linearly symmetrical and a
synchronization-detecting-light passage portion has no power;
FIG. 7 is a graph for explaining the main scanning position shift
when the arrangement is rotationally symmetrical and the
synchronization-detecting-light passage portion has no power;
FIG. 8 is a side view of an optical writing apparatus wherein
optical scanning apparatuses are arranged at two levels;
FIG. 9 is a schematic wherein incidence angles of light beams of an
upper level and a lower level in a main scanning direction are
different;
FIG. 10 is a schematic wherein reflecting point of the light beams
of the upper level and the level are different;
FIG. 11A is a schematic of the conventional horizontal incidence
method;
FIG. 11B is a schematic of a grazing-incidence method;
FIG. 12 is a schematic of a full-color image forming apparatus;
FIG. 13 is an oblique view of the principal parts of an optical
scanning apparatus according to embodiment 2-1;
FIG. 14 is a graph showing a relation between a sub-scanning
position shift of synchronization-detecting light incident on a
synchronization detecting unit and a diameter of a beam spot;
and
FIG. 15 is a schematic of a single-color image forming
apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Exemplary embodiments of the present invention are described below
with reference to the accompanying drawings.
Four combinations are possible with regard to the portion of a
scanning lens through which the synchronization-detecting light
passes (hereinafter, "synchronization-detecting-light passage
portion") and arrangement of scanning optical systems, based on the
presence or absence of power in the synchronization-detecting-light
passage portion and linearly symmetrical or rotationally
symmetrical arrangement of the scanning optical systems. The four
combinations are given below in Table 1.
TABLE-US-00001 TABLE 1 Presence/absence of power in
Synchronization-detecting-light passage portion/ Linearly
Rotationally Arrangement of scanning optical systems symmetrical
symmetrical Power present A B Power absent C D
Each of the combinations A to D is considered under an assumption
that temperature variation on, either side of an optical deflector
is uniform (hereinafter, "uniform temperature variation").
In FIG. 2 and FIG. 3, image height h is mapped upward, and
.DELTA..times. represents the position shift in a main scanning
direction (hereinafter, "main scanning position shift") due to
temperature variation. When performing synchronization detection on
both sides, there are methods for detecting a scanning-start end
and a scanning-stop end. As the scanning-stop end is considered as
a scanning-start end for scanning by the next surface of the
optical deflector, the argument is carried forth by focusing on
sensing of the scanning-start end.
For the sake of simplicity, it is assumed that the rise in the
temperature in an optical scanning apparatus according to the
present embodiment is uniform. In other words, it is assumed that
the thermal expansion of the scanning lens on either side of the
optical deflector is uniform.
The color shift produced due to the deformation of the scanning
lenses due to temperature variation is assessed in each of the four
cases A to D.
FIG. 2 is a schematic of an optical layout when the scanning
optical systems are arranged linearly symmetrically (Case A and
Case C in Table 1). In FIG. 2, the reference numerals 1 and 1'
represent light sources, 2 represents an optical deflector (such as
a polygon mirror), 3 and 3' represent f-theta lenses which are the
scanning lenses that form a scanning lens system, 4 and 4'
represent synchronization detecting units, and 6 and 6' represent
scanning surfaces.
The f-theta lenses 3 and 3' are arranged on the main scanning
direction facing the optical deflector 2, and substantially
linearly symmetrically on a main scanning plane with reference to a
rotational center 7 of the optical reflector 2.
As the f-theta lenses 3 and 3' are arranged substantially linearly
symmetrically, the main scanning position shift in one scanning
optical system can be taken as .DELTA..times.1 on a positive side
of the image height h, and the main scanning position shift in the
other scanning optical system can be taken as .DELTA..times.2 on a
negative side of the image height h.
Between Case A and Case C shown in Table 1, the light beam reaching
the synchronization detecting units 4 and 4' may or may not be
affected by the temperature variation.
FIG. 3 is a schematic of an optical layout when the scanning
optical systems are arranged rotationally symmetrically (Case B and
Case D in Table 1). As the scanning lenses 3 and 3' are arranged
rotationally symmetrically, on a write-end side, the main scanning
position shift in the scanning optical system can be taken as
.DELTA..times.1, and on a write-start side, the main scanning
position shift in the scanning optical system can be taken as
.DELTA..times.2.
The difference between Case B and Case D shown in Table 1 is that
in Case B the light beam reaching the synchronization detecting
units 4 and 4' is affected by temperature variation and Case D, the
light beam reaching the synchronization detecting units 4 and 4' is
not affected by temperature variation.
As shown in FIG. 1, the main scanning position shift can be
described in terms of the image height h. In FIG. 1 (as in other
drawings), an arrow S represents a scanning direction, and the
portion of the scan line represented by a dashed line m represents
a non-emitting portion of the light source.
In Case A, the synchronization-detecting light passing through the
scanning optical system produces a position shift of
.DELTA..times.1 in one scanning optical system and .DELTA..times.2
in the opposite direction in the other scanning optical system.
Consequently, the main scanning position shift caused by the
scanning optical systems on the two sides resembles the graph shown
in FIG. 4, which is essentially the graph shown in FIG. 1 shifted
upward by .DELTA..times.1 and downward by .DELTA..times.2.
The color shift is the difference between the graph shown in FIG. 1
and the graph shown in FIG. 4, and is thus
.DELTA..times.1+.DELTA..times.2.
In Case B, a position shift of .DELTA..times.1 is produced in the
synchronization-detecting light passing through both the scanning
optical systems but in the opposite directions. Consequently, the
main scanning position shift caused by the scanning optical systems
on the two sides resembles the graph shown in FIG. 5, which is
essentially the graph shown in FIG. 1 shifted upward and downward
by .DELTA..times.1. The color shift is the difference between the
graph shown in FIG. 1 and the graph shown in FIG. 4, and is thus
.DELTA..times.1+.DELTA..times.2.
In Case A and Case B, the synchronization-detecting light is
affected by the deformation of the scanning lens. Consequently,
even if the scanning optical systems are arranged symmetrically
with respect to the optical deflector 2, color shift of a magnitude
of the sum of the main scanning position shift at the two ends of
the scan line (.DELTA..times.1+.DELTA..times.2) is bound to occur.
This shows that mere symmetrical arrangement of the scanning
optical systems is not adequate to reduce color shift.
In Case C, temperature variation does not cause position shift in
the synchronization-detecting light in either scanning optical
system. Consequently, the main scanning position shift caused by
the scanning optical systems on the two sides resembles the graph
shown in FIG. 6, which is essentially similar to the graph shown in
FIG. 1. Thus, there is virtually no color shift in this case.
In Case D too, temperature variation does not cause position shift
in the synchronization-detecting light in either scanning optical
system.
In Case D, the main scanning position shift caused by the scanning
optical systems on the two sides will be the reverse of one
another, and consequently resemble the graph shown in FIG. 7, which
is essentially the graph shown in FIG. 1, only reversed.
In Case D, a color shift of the magnitude of
.DELTA..times.1-.DELTA..times.2 will be produced.
It can be surmised that even in Case D where the
synchronization-detecting light is not affected by temperature
variation, a perfectly symmetrical main scanning position shift
does not occur at zero image height h (that is, at h=0).
Thus, it can be concluded theoretically that Case C is the only
case where color shift is minimized by a combination of arrangement
of the scanning optical systems in an opposing scanning method and
absence of power in the synchronization-detecting-light passage
portion.
In actuality, temperature variation is seldom uniform within the
optical scanning apparatus. Hence, let us assume that, of the two
scanning optical systems, temperature variation occurs only in one.
It stands to reason in terms of FIGS. 4 to 7 that the main scanning
position shift of one of the scanning optical systems will be
zero.
Therefore, in Cases A to D, the color shift would be of the
magnitude of at least .DELTA..times.1 or .DELTA..times.2.
In other words, when the temperature variation is not uniform, the
combination of arrangement of the scanning optical systems and
absence of power in the synchronization-detecting-light passage
portion yields poor results.
Based on the study described above, the scanning optical systems in
the present embodiment are of the type represented by Case C of
Table 1 wherein the combination of the arrangement of the scanning
optical systems and presence or absence of power in the
synchronization-detection-light passage portion of the scanning
lens is used for minimizing color shift due to temperature
variation, assuming that the temperature variation is uniform on
either side of the optical deflector 2.
Thus, the optical scanning apparatus according to the present
embodiment has the configuration shown in FIG. 2. That is, the
optical scanning apparatus is of an opposing scanning type having
two scanning optical systems that focus the light beam deflected by
the optical deflector 2 on the scanning surfaces 6 and 6' as beam
spots. The salient feature of optical scanning apparatus is
described below.
The optical scanning apparatus according to the present embodiment
includes 2n (where n.gtoreq.1) light sources, each light source
having m (where m.gtoreq.1) light-emitting units, synchronization
detecting units, and scanning lens systems. The synchronization
detecting units receive the incident m.times.n light beams of the
slight sources which are symmetrical with respect to a sub-scanning
cross-sectional surface that include a rotational axis of the
optical deflector 2, scan the scanning surface by the light beams,
and determine a write timing for writing on the scanning surface.
The scanning lens systems focus the light beams on the scanning
surfaces facing the optical deflector 2.
Synchronization detection in the optical scanning apparatus
according to the present embodiment is performed at only one end of
each scan line, and the scanning lens systems are arranged
substantially linearly symmetrically with respect to a main
scanning direction and orthogonal to the rotational axis of the
optical deflector 2. The shift due to temperature variation in the
beam spot position of the synchronization-detecting light in the
main scanning direction on the scanning surfaces or their
equivalent in the optical scanning apparatus according to the
present embodiment is of the magnitude of 5 .mu.m/.degree. C. or
less.
The magnitude of 5 .mu.m/.degree. C. or less is not an unambiguous
result for Case C of Table 1 and is only a possibility.
The scanning optical systems facing the center of the optical
deflector 2 are substantially linearly symmetrical with respect to
the rotational center of the optical deflector 2 in the main
scanning direction orthogonal to a rotational axis of the optical
deflector 2, and hence the arrangement is termed "linearly
symmetrical arrangement".
The color shift can be minimized of the magnitude of the beam spot
position shift due to temperature variation that occurs as a matter
of course in an optical writing apparatus is 5 .mu.m/.degree. C. or
less.
Even though the actual temperature variation in an optical writing
apparatus is seldom uniform, the closed space of the optical
writing apparatus does not allow for too much variation in
temperature distribution, enabling realization of the favorable
result specified for Case C.
In the optical scanning apparatus according to the present
embodiment, it is preferable for a no-power portion of the scanning
lenses 3 and 3' and the synchronization optical systems that focus
the synchronization-detecting light of the synchronization
detecting units 4 and 4' into light-receiving units to have power
to correct the surface-tilt of the optical deflector 2 in the
sub-scanning direction.
Further, in the optical scanning apparatus according to the present
embodiment, it is preferable to keep the inter-surface deviation in
the reflecting-surface-tilt of the optical deflector to 200 seconds
or less to minimize the position shift in the sub-scanning
direction when the synchronization-detecting light reaches the
light-receiving unit and stabilize determination of write timing.
In other words, the write position shift for each surface of the
optical deflector is minimized in the optical scanning apparatus
according to the present embodiment.
In the present embodiment, two scanning optical systems are
arranged substantially symmetrically with respect to the
sub-scanning cross-sectional surface (line 7 shown in FIG. 2)
passing through rotational axis of the optical deflector 2.
Consequently, the light beams are scanned in opposite directions on
the scanning surfaces 6 and 6' located on either side of the
rotating optical deflector 2. However, scanning is performed
substantially linearly symmetrically on either side of the optical
deflector 2.
In other words, scanning lens system components required on both
sides for producing satisfactory beam spots on the scanning
surfaces 6 and 6' have an identical geometry, reducing the
manufacturing cost.
In the example shown in FIG. 2, the scanning lens system is
represented by the f-theta lenses 3 and 3'. Other scanning lenses
can equally be used in a substantially linearly symmetrical
arrangement to configure the scanning lens system.
If L1, L2, and so on up to Lj (where j=1, 2, 3, . . . ) are
scanning lenses forming the scanning lens system on one side of the
optical deflector 2 in the sequence of the nearest to the farthest
from the optical deflector 2, and L'1, L'2 and so on up to L'j
(where j=1, 2, 3, . . . ) are scanning lenses on the other side of
the optical deflector in the sequence of the nearest to the
farthest from the optical deflector 2, the geometry of the lens at
any position on either side of the optical deflector 2, such as the
lenses Lj and L'j, are identical. This minimizes the cost of
manufacturing.
In other words, the number of lenses, and the number of processes
for manufacturing the optical scanning apparatus is minimized.
In the optical writing apparatus which includes a plurality of
optical scanning apparatuses having the configuration described
above, synchronization-detecting light from only one of the optical
scanning apparatus can be detected, and the write timings of the
other optical scanning apparatus can be electrically estimated
based on the detected synchronization-detecting light signal.
There is enhanced reliability of synchronization detection in the
optical scanning apparatus according to the present invention
because of the stability of the synchronization-detecting light
against the main scanning direction position shift. This enables
the write timings of all the optical scanning apparatuses to be
determined fairly accurately even if they are electrically
estimated based on the synchronization-detection light signal of
one of the optical scanning apparatus.
By this method (hereinafter, "delay method"), the number of
light-receiving elements (synchronization detecting units) can be
reduced, cutting down the cost of the writing optical system.
Embodiment 1-3 is described below with reference to FIG. 8.
In this embodiment, an optical writing apparatus 10 includes a
plurality of optical scanning apparatuses arranged at two different
levels, an upper level and a lower level.
The reference numeral 5 denotes a folded mirror and the reference
numeral 6 denotes the scanning surface in the form of a
photosensitive drum.
The rotational axis of the optical deflector 2 is common to both
the levels. Consequently, use of just a single optical deflector 2,
which is relatively expensive to manufacture, enables the cost of
the optical writing apparatus 10 to be kept down.
Further, as the phases of the optical deflector 2 rotate around a
common axis at both the levels, write timing can be estimated more
accruing when synchronization detection is performed using the
delay method.
The optical writing apparatus 10 shown in FIG. 8 can be configured
in such a way that the light beams from the upper level and the
lower level are incident on the optical deflector 2 at different
incidence angles (that is, the light beams are caused to form an
intersection angle), as shown in FIG. 9.
In FIG. 9, the reference numeral H denotes a coupling lens, the
reference numeral 12 denotes an aperture, and the reference numeral
13 denotes a cylindrical lens.
Thus, the space between the two levels of the optical scanning
apparatus can be reduced up to the extent of the gap between the
light sources of the upper level and the lower level, achieving
compactness.
The formation of an intersection angle between the light beams from
the light sources of the upper and lower levels can produce
difference in the optical characteristics of the light beams and
cause the optical deflector 2 to deflect the light beams at
different angles. In particular, one of the
synchronization-detecting light beams near the deflection angle
tends to get eclipsed.
This eclipsing of the light beam can be reduced by slightly
adjusting the incidence point (by compensation of the difference in
the maximum write widths of the upper level and lower level), as
shown in FIG. 10, preventing the difference in the optical
characteristics of the light beams of the scanning optical systems
at the two levels.
The optical writing apparatus according to the preceding
embodiments, the light beam incident on a deflective reflection
surface of the optical deflector 2 is parallel to the normal
dropped from the deflective reflection surface. The present
invention can equally be applied to an optical writing apparatus in
which the light beam incident on the deflective reflection surface
of the optical deflector 2 can form an angle with the normal
dropped from the deflective reflection surface (Embodiment 1-6).
This method is called the grazing-incidence method.
In the existing optical scanning apparatuses that use a horizontal
opposing scanning method, as shown in FIG. 11A, polygon mirrors 40
are required to be provided at two levels to obtain a distance Z
required for separating the light beams directed towards different
scanning surfaces. It is also possible to align the polygon mirrors
40 at a single level. However, this will increase the thickness of
the polygon mirror portion, adversely affecting the performance
speed and the cost.
In the grazing-incidence optical systems used in the present
invention, as shown in FIG. 11B, a pair of light beams from a
plurality of light sources that form different angles in the
sub-scanning direction with respect to the normal are made incident
on two opposing deflective reflection surfaces of the same optical
deflector 2. Consequently, by limiting provision of the optical
deflector 2 to just one level and further by reducing the thickness
of the polygon mirror portion in the sub-scanning direction, the
rotational inertia of the optical deflector 2 can be reduced,
shortening the startup time.
The advantage of the grazing-incidence method is that the
manufacturing cost of the optical writing apparatus can be kept
down as providing the optical deflector 2 at one level enables
forming of a full-color image.
To implement the grazing-incidence method, the surface of the
scanning lenses needs to be modified appropriately as, otherwise
the scan line tends to bend and wavefront aberration occurs.
However, the effects can be put to practical use when assembling,
cutting down the cost.
Embodiment 1-7 (full color image forming apparatus) is described
below with reference to FIG. 12.
In a full color image forming apparatus according to the present
embodiment, such as a laser color printer, photosensitive drums 20Y
(yellow), 20M (magenta), 20C (cyan), and 20K (black) are arranged
serially pressed against an intermediate transfer belt 21, which
stretched tightly over rollers 102a, 102b, and 102c.
Arranged around the photosensitive drum 20Y in counter-clockwise
direction are a not shown charging unit, an optical scanning
apparatus 105 that functions as an exposing unit common to all the
photosensitive drums, a developing unit 106Y, a not shown primary
transfer roller provided on the underside of the intermediate
transfer belt 21, a not shown cleaning unit, a not shown
neutralizing unit, etc. Identical components are arranged around
each of the photosensitive drums 20M, 20C, and 20K.
An electrostatic latent image is formed on each of the
photosensitive drums 20Y, 20M, 20C, and 20K by light beams L1, L2,
L3, and L4, respectively, based on each color image data. The
latent images on the photosensitive drums 20Y, 20M, 20C, and 20K
are converted to visible images by the respective developing units
106Y, 106M, 106C, and 106K.
A toner image of each color is superposed sequentially on the
intermediate transfer belt 21 and transferred. A secondary transfer
roller 102d batch-transfers the superposed image onto a transfer
sheet (recording medium) supplied at a predetermined timing from a
sheet feeder 111. A not shown cleaning unit cleans the intermediate
transfer belt 21 after the transfer of the superposed image to the
transfer sheet. The transfer sheet is conveyed to a fixing device
114, which fixes the color image by application of heat and
pressure.
The transfer sheet, after being passed through the fixing device
114, is transported substantially vertically in the main body of
the apparatus to be ejected into a discharge tray 110 located at
the uppermost portion of the apparatus.
Excellent reproduction of images can be realized by using the
optical scanning apparatus according to the first embodiment as the
optical scanning apparatus 105.
Various materials such as silver salt film can be used as
photosensitive image-bearing member. The latent image is formed by
optical scanning and is converted into a visual image by the normal
silver salt photographic process. Such image forming devices can be
implemented as optical photo-engraving devices or optical imaging
devices that produce CT scan images, etc.
Another photosensitive image-bearing member could be a full color
medium in the form of a full color printing paper, which directly
produces a visible color image by the heat energy of the beam spot
produced by optical scanning.
Alternatively, a photoconductive photosensitive member such as a
zinc oxide sheet can be used as a photosensitive image-bearing
member. Another option is a reusable selenium photosensitive member
or organic semiconductor member in drum or belt form.
When a photoconductive photosensitive member is used as the
image-bearing member, an electrostatic latent image is formed by
the uniform charging of the photosensitive member and optical
scanning by the optical scanning apparatus. The electrostatic
latent image is converted to a visible toner image by developing.
Fixing of the toner image takes place directly in the case where
the photosensitive member is a zinc oxide sheet. If the
photosensitive member is of the reusable type, the toner image is
first transferred to a recording medium in sheet form such as a
transfer sheet or an overhead projector (OHP) sheet (plastic sheet
for overhead projector), before the image is fixed.
The toner image may either be directly transferred from the
photosensitive member onto the recording medium (direct transfer
method) or first may be transferred onto an intermediate transfer
medium and therefrom to the recording medium (intermediate transfer
method).
Such image forming devices can be implemented as optical printers
or plotters, digital copiers, etc.
The optical scanning apparatus according to the present invention
achieves the effect of reduced color shift by virtue of being
configured by combining a linearly symmetrical arrangement of the
scanning optical systems and a condition, which does not allow the
synchronization-detecting light beams to be affected by temperature
variation.
The scanning optical systems for each color would need as far as
possible to be made close to each in terms of their form to realize
an optical scanning apparatus that performs well in spite of
temperature variation. In other words, even if main scanning
position shift occurs for each color, the color shift will not be
discernible, as all the colors would show the same magnitude of
main scanning position shift.
The main scanning position shift caused by temperature variation is
explained below in detail. In the scanning optical system, the
light beams deflected by the optical deflector are
refracted/condensed by the scanning lenses and scan the
sub-scanning surface at a constant speed. When the scanning speed
is ideal, a proportional relation is established between a rotation
angle .theta. of the optical deflection and the beam spot position
on the sub-scanning surface.
In other words, ideal beam spot position (hereinafter, "ideal image
height") is determined by the angle of the deflective reflection
surface of the optical deflector 2. A shift from the ideal image
height caused due to any factor is defined as the main scanning
position shift. From this it can be easily imagined that the main
scanning position shift can become so large that the optical line
passing through the scanning lenses recedes far from the optical
axis.
The rotation axis of the optical deflector 2 is different from the
position of the deflective reflection surface an asymmetrical
scanning with respect to the optical axis of the scanning lenses
takes place. Consequently, the main scanning position shift of the
image height on the either side of the optical deflector 2 will be
different, yielding an asymmetrical form shown in FIG. 1 when the
main scanning position shift is plotted in terms of image height
h.
The following methods can be used for preventing only the
synchronization-detecting light beams from being affected by
temperature variation. The effects due to the present invention can
be garnered if an appropriate method is selected in accordance with
the scanning optical system.
(1) Designing a synchronization optical system such that the
synchronization-detecting light does not pass through the scanning
lenses at all.
(2) Providing an air layer (an opening) in the
synchronization-detecting-light passage portion in the scanning
lens to prevent refraction by the scanning lens.
(3) Configuring the synchronization-detecting-light passage portion
such that it is in the form a horizontal plate in the main scanning
direction so that even if deformation occurs in the scanning lens,
the synchronization-detecting light is not affected.
Method (1) is most effective for stabilization of the
synchronization-detecting light. However, achieving compactness and
slimness of the optical scanning apparatus necessitates having to
place the beam-shaping optical systems, the
synchronization-detecting-light passage portion, and the scanning
lens very close to the optical deflector, which is not desirable.
Thus, there is a constraint on the quest for an ever more compact
and slim optical scanning apparatus by way of the closest the
various optical components can be placed to the optical
deflector.
The problem faced in Method (1) can be avoided by implementation of
Methods (2) and (3). However, in these methods, the scanning lens
has to be subjected to a secondary processing, increasing the
cost.
The synchronization-detecting light passes through the no-power
portion of the scanning lens in all the three Methods (1) to
(3).
When applying the optical scanning apparatus according to the
present invention to a multi-beam method, all the components from
line imaging optical system to the scanning optical system can be
made common to a plurality of light beams that are coupled. This
enables the optical scanning apparatus to be configured from line
imaging optical system onwards as if for a single-beam method.
Consequently, a multi-beam optical scanning apparatus can be
realized that is stable against mechanical variations.
As compared to the single beam method, the same writing speed can
be realized by fewer turns of the optical deflector in the
multi-beam method. Thus, less power is consumed for driving the
optical deflector, resulting in energy saving.
The light source used in the multi-beam method can use a laser
diode (LD) array method or a beam compositing method. If a gap of
10 .mu.m or more is kept between the light-emitting units in the LD
array light source, adverse thermal and electrical effect of
adjacent light-emitting units can be effectively prevented, and an
ideal multi-beam optical scanning can be performed.
In the grazing-incidence method, the optical scanning apparatus has
the optical deflectors on just a single level. In other words, by
using the grazing-incidence method, the height of the deflecting
unit (polygon mirror) (the height in the sub-scanning direction)
can be reduced, and as the surface area in contact with the
atmosphere is small, increase in power consumption due to windage
loss can be prevented, resulting in less power consumption by the
optical writing apparatus.
FIG. 13 is a schematic of the principal parts of the optical
scanning apparatus according to a second embodiment of the present
invention. The optical scanning apparatus according to the second
embodiment employs the single beam method. A light source 1 in the
form of a semiconductor laser device emits a light beam, which is
dispersive, and which enters the coupling lens 11 and is coupled
into the optical system disposed subsequent to the coupling
lens.
The light beam emerging from the coupling lens 11 is weakly
dispersive, and passes through the 12, undergoes beam shaping where
the aperture 12 permits only the central portion of the light beam
to pass through, cutting off the peripheral portion of the light
beam. The light beam exiting the aperture 12 enters the cylindrical
lens 13, which is a linear imaging optical system. The cylindrical
lens 13, which has no power in the main scanning direction and a
positive power in the sub-scanning direction, converges the light
beam entering it in the form of a linear image, which is oblong in
the main scanning direction in the vicinity of a deflective
reflection surface of the optical deflector 2. The optical
deflector 2 functions as an optical deflector.
The deflective reflection surfaces of the optical deflector 2,
which is spinning at a constant speed, deflect the light beam
equiangularly and at uniform speed. The light beam thus deflected
by the optical deflector 2 passes through a single f-theta lens 3,
which forms a scanning optical system. The optical path of the
light beam exiting the f-theta lens 3 is bent by a folded mirror 5
and the light beam is focused as a beam spot on a photoconductive
photosensitive member 25, which is the scanning surface. In this
way, the photoconductive photosensitive member 25, that is, the
scanning surface, is optically scanned.
Prior to optically scanning the photoconductive photosensitive
member 25 of sheet-type, the deflected light beam passes through
the no-power portion of the f-theta lens 3, is reflected by a
synchronization mirror 4a, which functions as a converging unit,
and is again converged in the main scanning direction by a
synchronization lens 4b, which functions as a converging unit, into
a synchronization detecting unit 4c. The write start timing is
determined based on the output from the synchronization detecting
unit 4c. The synchronization mirror 4a and the synchronization lens
4b together form the synchronization optical system.
The term scanning optical system refers to an optical system that
converges the light beam deflected by the optical deflector 2
functioning as a beam deflector, as a beam spot on the scanning
surface. In the present embodiment, the scanning optical system
includes a single f-theta lens 3.
The term "spot diameter of beam spot" used in the specification is
defined by an intensity 1/e.sup.2 of a line spread function of the
light intensity distribution in the beam spot on the scanning
surface.
If the light intensity distribution, f(Y,Z) to be determined from a
main scanning direction coordinate Y and a sub-scanning direction
coordinate Z, taking the central coordinates of the beam spot
formed on the scanning surface as the reference, the line spread
function in Z direction LSZ is defined by, LSZ(Z)=.intg.f(Y,Z)dY
(The integration of the entire width of the beam spot in Y
direction is performed).
The line spread function in Y direction LSY is defined by,
LSY(Y)=.intg.f(Y,Z)dZ (The integration of the entire width of the
beam spot in Z direction is performed).
The line spread functions LSZ(Z) and LSY(Y) have substantially
Gauss distribution shape. The spot diameters in Y direction and Z
direction in that area are considered where the line spread
functions LSZ(Z) and LSY(Y) have a maximum value of 1/e.sup.2 or
greater.
The spot diameter defined by the line spread functions can be
easily measured by optically scanning at a uniform speed the beam
spot along a slit and integrating the amount of light received by a
light detector that receives the light that comes out of the slit.
Apparatuses for measuring the spot diameter are available in the
market.
In the second embodiment, the optical surface (converging unit)
that contributes the most towards a sub-scanning position shift in
the synchronization detecting unit 4c is the synchronization mirror
4a.
Accordingly, in the second embodiment, the power of the
synchronization lens 4b in the sub-scanning direction is such that
the synchronization mirror 4a and the synchronization detecting
unit 4c are coupled within the sub-scanning cross-section, and in
the main scanning direction is such that the synchronization lens
4b can converge the light beam into the synchronization detecting
unit 4c.
Thus, the state of convergence of the light beam within the
sub-scanning cross-section of the synchronization detecting unit 4c
is determined based on the positional relation between the optical
elements (converging units) of the synchronization optical
system.
In the second embodiment, if the power of the synchronization lens
4b is set so as to satisfy a condition: D>Z/2 (1) where D is the
beam diameter at the light-receiving unit of the synchronization
detecting unit 4c, and Z is the light-receiving zone of the
light-receiving unit in the sub-scanning direction, the
synchronization light intensity in the light-receiving zone of the
synchronization detecting unit 4c will resemble the Gaussian
distribution shown by the solid line in FIG. 14.
A major part of the sub-scanning position shift due to error in the
optical elements in the scanning optical system gets cancelled out
due to the coupling relation of the synchronization mirror 4a and
the synchronization detecting unit 4c. However, an insignificant
amount of sub-scanning position shift occurs due to error of the
optical elements that are not included in the coupling
relation.
As a result, the Gaussian distribution shifts in the sub-scanning
direction as shown by the dashed line with the light intensity
distribution restricted to the hatched portion in FIG. 14. If the
condition in Non-equality (1) is satisfied, the beam spot diameter
in the sub-scanning direction is large enough to cover the
light-receiving zone. Thus, even if there is a residual
sub-scanning position shift, not much of the light received is
lost.
The beam spot position shift occurring in the synchronization
detecting unit is described below. Synchronization detection is
performed when a light beam scanned in the main scanning direction
passes through a photodiode (synchronization detecting unit).
Therefore, the synchronization optical system in general converges
the light beam into the synchronization detecting unit within the
main scanning cross-section.
As the timing of light emission from the light source can be
electrically determined based on the light reception timing, even
if there is a main scanning position shift, synchronization
detection is not affected.
However, in case of detection of a sub-scanning position shift,
either the synchronization-detecting light does not enter the
synchronization detecting unit or, even if the
synchronization-detecting light does enter the synchronization
detecting unit, the amount of light is not adequate enough for the
synchronization detecting unit to convert into electrical signals.
Failure of synchronization detection leads to faulty determination
of light emission timing, resulting in poor optical writing.
The increasing demand for more and more compact scanning optical
system necessitates increasing the length of the optical path the
synchronization optical system. However, increasing the length of
the optical path has the effect of amplifying the position shift
due to the optical surfaces in the optical system, and the tilt of
the optical surface and the sub-scanning position shift in such a
case is too significant to be ignored.
Therefore, in the second embodiment, the optical surface that
contributes largely to sub-scanning position shift is selected to
be coupled with the synchronization detecting unit within the
sub-scanning cross-section to realize an optical system, that
includes all the components from the light source to the
synchronization detecting unit, that causes minimum sub-scanning
position shift.
However, the coupling relation mainly relaxes the angle shift of
the synchronization-detecting light incident on the synchronization
detecting unit within the sub-scanning cross-section and does not
improve the shift of the synchronization-detecting light in the
sub-scanning direction. Therefore, if there is error in the entire
optical surface, residual sub-scanning position shift will
definitely occur.
In such a case, it would be preferable that in addition to the
coupling relation, the following condition be satisfied: D>Z/2
(1) where D is the beam spot diameter at the light-receiving unit
of the synchronization detecting unit of the synchronization
optical system, and Z is the light-receiving zone of the
light-receiving unit in the sub-scanning direction.
If the diameter of the beam spot in the sub-scanning direction is
greater than the radius of the light-receiving zone Z/2, adequate
amount of light becomes incident on the photo diode, irrespective
of the position shift.
Thus, due to the synergetic effect of the coupling relation and an
adequately large beam spot diameter in the sub-scanning direction,
an image forming apparatus in which synchronization optical system
is used is sturdier against external shocks during the shipping
process, even if minor variations in mass production are taken into
account.
The optical system formed from the synchronization optical system
and the synchronization detecting unit 4c, which form a coupling
relation, should preferably be a reducing system within the
sub-scanning cross-section. In the form of a reducing system, the
synchronization optical system reduces the error before the
converging unit and before being incident on the synchronization
detecting unit 4c, thus more strongly preventing a sub-scanning
position shift.
The synchronization optical system can include the synchronization
mirror 4a to guide the light beam into the synchronization
detecting unit 4c. As the synchronization mirror 4a contributes
greatly to the sub-scanning position shift, the contribution can be
reduced by coupling the synchronization mirror 4a and the
synchronization detecting unit 4c. Further, effect of reduced
sub-scanning position shift can be further improved by enabling the
optical system formed by the synchronization optical system and the
synchronization detecting unit 4c to function as a reducing
system.
The f-theta lens 3 of the scanning optical system that includes the
synchronization optical system can include a no-power portion 30 in
the main scanning direction. The synchronization-detecting light
passing through the no-power portion 30 is not affected by
deformation of the f-theta lens due to temperature variation and
maintains an unvarying main scanning position.
The surface tilt of the optical deflector 2 also contributes
largely to sub-scanning position shift of the synchronization
detecting unit 4c. Therefore, in the optical scanning apparatus
according to the present embodiment, the inter-surface deviation of
the reflecting-surface-tilt of the optical deflector 2 is limited
to 200 seconds or less to further reduce the sub-scanning position
shift when the synchronization-detecting light reaches the
light-receiving unit and to stabilize the determination of the
write timing.
However, the cost involved in realizing high precision in
inter-surface deviation is very high. In addition, there are
physical constraints. Therefore, it is preferable for the no-power
portion 30 to have power so as to optically correct the surface
tilt of the optical deflector 2 in the sub-scanning direction.
If the synchronization optical system does not include the
synchronization mirror 4a, the adverse effect of the surface tilt
of the optical deflector 2 can be reduced by coupling the
reflecting point of the optical deflector 2 and the synchronization
detecting unit 4c using the no-power portion 30 and the
synchronization lens 4b.
However, if the synchronization optical system includes the
synchronization mirror 4a, the adverse effect of the surface tilt
of the optical deflector 2 and the synchronization mirror 4a can be
reduced by coupling the reflecting point of the optical deflector 2
and the synchronization detecting unit 4c as well as reflecting
point of the synchronization mirror 4a and the synchronization
detecting unit 4c.
It is preferable to use an optical scanning apparatus that includes
an assembly of a plurality of scanning optical systems (optical
scanning apparatuses). The assembly of the scanning optical systems
can be in the form of a common optical deflector 2 and a plurality
of scanning optical systems arranged facing each other on either
side of the optical deflector 2 (opposing scanning method).
A synchronization detecting unit common to all the scanning optical
systems can detect the synchronization-detecting light of the
plurality of scanning optical systems of the optical scanning
apparatus.
By using just a single synchronization detecting unit, the cost can
be kept down, as photodiodes are generally very expensive.
Corresponding to the reduced number of photodiode, the number of
substrate also can be reduced, increasing the flexibility of layout
and enabling realization of a compact optical scanning
apparatus.
The write timings of the scanning optical systems whose light beams
are not guided into the only synchronization detecting unit are
electrically estimated based on the detected
synchronization-detecting light signal.
Adoption of this method of electrically detecting the write timings
of the scanning optical systems, called the delay method, enables
cost reduction and realization of a compact writing optical system
as the number of photodiodes is reduced.
The image forming apparatus according to embodiment 2-2 is
described below with reference to FIG. 15.
An image forming apparatus 1000 according to the present
embodiment, such as a laser printer, includes a cylindrical
drum-type photosensitive image-bearing member 1110. Around the
image-bearing member 1110 are arranged a charging roller 1121, a
developing device 1131, a transfer roller 1141, and a cleaning
device 1151. A corona charger is used as a charging unit.
An optical scanning apparatus 1171 that performs optical scanning
by a laser beam LB carries out exposure for optical writing between
the charging roller 1121 and the developing device 1131.
In FIG. 15, the reference numeral 1161 denotes a fixing device, the
reference numeral 1181 denotes a cassette, the reference numeral
1191 denotes a pair of resist rollers, the reference numeral 1201
denotes a feed roller, the reference numeral 1211 denotes a
conveyance route, the reference numeral 1221 denotes a pair of
ejection rollers, the reference numeral 1231 denotes a tray, and
the reference symbol P denotes a sheet-type recording medium in the
form of a transfer sheet.
When performing image formation, the image-bearing member 1110
turns clockwise at a constant speed. The charging roller 1121
uniformly charges the surface of the image-bearing member 1110. The
laser beam LB of the optical scanning apparatus 1171 exposes
surface of the image-bearing member 1110 to form thereon an
electrostatic latent image. The latent image is a negative latent
image and the image portion is exposed.
The developing device 1131 performs a reversal development on the
latent image and forms a visible toner image on the image-bearing
member 1110. The cassette 1181 containing the transfer sheet P is
removably engaged into the main frame of the image forming
apparatus 1000. When the cassette 1181 is in an engaged state, the
feed roller 1201 feeds the leading edge of the topmost transfer
sheet P between the pair of resist rollers 1191. The resist rollers
1191 passes the transfer sheet along to a transfer region of the
image-bearing member 1110, timing it so that the transfer sheet
reaches the transfer region when the toner image on the
image-bearing member 1110 reaches a transfer position.
At the transfer region, the transfer roller 1141 performs an
electrostatic transfer to transfer the toner image from the
image-bearing member 1110 onto the transfer sheet P. The transfer
sheet P carrying the toner image thereon is conveyed to the fixing
device 1161, which fixes the toner image. The transfer sheet P is
then conveyed through the conveyance route 1211 and ejected out on
to the tray 1231 by the pair of ejection rollers 1221.
After the transfer of the toner image to the transfer sheet P, the
cleaning device 1151 cleans the surface of the image-bearing member
1110 to remove residual toner or paper particles.
Excellent image forming can be realized if the optical scanning
apparatus according to the first embodiment is used as the optical
scanning apparatus 1171.
The second embodiment can be expanded to a full color image forming
apparatus such as the one shown in FIG. 12.
In other words, excellent image forming can be realized by using
the optical scanning apparatus 1171 of embodiment 2-1 in place of
the optical scanning apparatus 105.
According to an aspect of the present invention, color shift can be
reduced by combining a linearly symmetrical arrangement of scanning
optical systems and a condition in which synchronization-detecting
light is not affected by temperature variation.
Although the invention has been described with respect to a
specific embodiment for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art that fairly fall within the
basic teaching herein set forth.
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