Title: Optical scanning device
Abstract: An optical scanning device for scanning a plurality of light beams on scanned surfaces, such as photosensitive drums, includes a light source, a front optical system, a deflector (e.g., a rotating polygon mirror) that scans the light beams in a main scanning direction, and a rear optical system for directing the light beams toward the scanned surfaces so that two of the light beams are parallel in a sub-scanning direction that is orthogonal to the main scanning direction and two of the light beams diverge in the sub-scanning direction in the rear optical system. The front optical system includes collimating and converging optical systems. The rear optical system includes cylindrical lens parts that are oppositely inclined relative to the optical axis in a plane that includes the sub-scanning direction so as to correct curvatures of the scanning lines.
Patent Number: 6,914,705 Issued on 07/05/2005 to Nakai
| Inventors:
|
Nakai; Yoko (Fuchu, JP)
|
| Assignee:
|
Fujinon Corporation (Saitama, JP)
|
| Appl. No.:
|
606887 |
| Filed:
|
June 27, 2003 |
Foreign Application Priority Data
| Jun 28, 2002[JP] | 2002-190429 |
| Current U.S. Class: |
359/204; 359/205; 359/207; 359/216; 347/243; 347/244 |
| Intern'l Class: |
G02B 026/08 |
| Field of Search: |
359/204-207
347/243-244
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Phan; James
Attorney, Agent or Firm: Arnold International, Henry; Jon W., Arnold; Bruce Y.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is related in subject matter to commonly assigned U.S. application
Ser. No. 10/397,301, filed Mar. 27, 2003, entitled "Optical Scanning System".
Claims
1. An optical scanning device for scanning a plurality of light beams on one
or more scanned surfaces, the optical scanning device comprising:
a light source for emitting said plurality of light beams;
a front optical system for receiving said plurality of light beams from the light
source along an optical axis that extends within the optical scanning device;
a deflector for receiving said plurality of light beams from said front optical
system and for deflecting said plurality of light beams in a first direction so
that said plurality of light beams scan in said first direction;
a rear optical system for receiving said plurality of light beams from said deflector
and for directing said plurality of light beams toward the one or more scanned
surfaces so that said plurality of light beams are separated farther in a second
direction that is orthogonal to said first direction within said rear optical system
than at said deflector;
wherein
two light beams of said plurality of light beams are deflected by said deflector,
and said two light beams travel parallel to one another in a plane that includes
said second direction before said two light beams enter said rear optical system;
and
other light beams of said plurality of light beams are deflected by said deflector
and said other light beams progressively separate from one another in said plane
before said other light beams enter said rear optical system.
2. The optical scanning device of claim 1, wherein:
said front optical system includes, in order from the light source side, a first
optical system for collimating each of said plurality of light beams and a second
optical system for converging each of said plurality of light beams at least in
said second direction.
3. The optical scanning device of claim 1, wherein:
said rear optical system includes, in order from the light source side, a third
optical system for converging in said first direction each of said plurality of
light beams deflected from said deflector, a separation optical system for separating
said plurality of light beams in said second direction after they pass through
said third optical system, and a fourth optical system for converging each of said
plurality of light beams at least in the second direction after they pass through
said separation optical system.
4. The optical scanning device of claim 2, wherein:
said rear optical system includes, in order from the light source side, a third
optical system for converging in said first direction each of said plurality of
light beams deflected from said deflector, a separation optical system for separating
said plurality of light beams in said second direction after they pass through
said third optical system, and a fourth optical system for converging each of said
plurality of light beams at least in the second direction after they pass through
said separation optical system.
5. The optical scanning device of claim 4, wherein said third optical system
includes at least one cylindrical lens having refractive power at least in said
first direction.
6. The optical scanning device of claim 5, wherein said at least one cylindrical
lens includes, in order from the light source side, a first cylindrical lens having
negative refractive power in said first direction and a second cylindrical lens
having positive refractive power in said first direction.
7. The optical scanning device of claim 6, wherein said other light beams intersect
in said plane between said second optical system and said deflector.
8. The optical scanning device of claim 7, wherein said other light beams enter
said rear optical system at positions in said plane that are outside said two light beams.
9. The optical scanning device of claim 8, wherein:
said deflector includes plural reflecting surfaces that deflect said plurality
of light beams; and
said two light beams are reflected by said plural reflecting surfaces at right
angles in said plane and enter said third optical system at right angles in said
plane.
10. The optical scanning device of claim 8, wherein:
said plurality of light beams includes, in order in said second direction, first,
second, third, and fourth light beams that are deflected by said deflector so that
said second and third light beams are parallel in said plane;
said second cylindrical lens includes first and second lens parts that are adjacent
to one another in said second direction;
the first lens part includes a first light incident surface that is inclined
at an angle in said plane so that the sum of the absolute values of the angles
that the first and second light beams make with said first light incident surface
is equal to the sum of the absolute values of the angles that the first and second
light beams would make with said first light incident surface if said first light
incident surface were perpendicular to the center optical axis of said third optical
system; and
the second lens part includes a second light incident surface that is inclined
at an angle in said plane so that the sum of the absolute values of the angles
that the third and fourth light beams make with said second light incident surface
is equal to the sum of the absolute values of the angles that the third and fourth
light beams would make with said second light incident surface if said second light
incident surface were perpendicular to the center optical axis of said third optical
system.
11. The optical scanning device of claim 8, wherein:
said plurality of light beams includes, in order in said second direction, first,
second, third, and fourth light beams that are deflected by said deflector so that
said second and third light beams are parallel in said plane;
said second cylindrical lens includes first and second lens parts that are adjacent
to one another in said second direction;
the first lens part includes a first light incident surface that is inclined
at an angle in said plane so that the first light beam produces a straight scanning
line; and
the second lens part includes a second light incident surface that is inclined
at an angle in said plane so that the fourth light beam produces a straight scanning
line.
12. The optical scanning device of claim 4, wherein said fourth optical system
includes a cylindrical mirror that has refractive power at least in said second direction.
13. The optical scanning device of claim 5, wherein said fourth optical system
includes a cylindrical mirror that has refractive power at least in said second direction.
14. The optical scanning device of claim 6, wherein said fourth optical system
includes a cylindrical mirror that has refractive power at least in said second direction.
15. The optical scanning device of claim 1, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
16. The optical scanning device of claim 2, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
17. The optical scanning device of claim 4, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
18. The optical scanning device of claim 5, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
19. The optical scanning device of claim 6, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
20. The optical scanning device of claim 7, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
21. An optical scanning device for scanning a plurality of light beams on one
or more scanned surfaces, the optical scanning device comprising:
a light source for emitting said plurality of light beams;
a front optical system for receiving said plurality of light beams from the light
source along an optical axis that extends within the optical scanning device;
a deflector for receiving said plurality of light beams from said front optical
system and for deflecting said plurality of light beams in a first direction so
that said plurality of light beams scan in said first direction;
a rear optical system for receiving said plurality of light beams from said deflector
and for directing said plurality of light beams toward the one or more scanned
surfaces so that said plurality of light beams are separated farther in a second
direction that is orthogonal to said first direction within said rear optical system
than at said deflector;
wherein
two light beams of said plurality of light beams are deflected by said deflector
so that said two light beams travel parallel to one another in a plane that includes
said second direction between said deflector and said rear optical system;
other light beams of said plurality of light beams that are deflected by said
deflector enter said rear optical system so that other light beams progressively
separate from one another in said plane;
said front optical system includes, in order from the light source side, a first
optical system for collimating each of said plurality of light beams and a second
optical system for converging each of said plurality of light beams at least in
said second direction;
said rear optical system includes, in order from the light source side, a third
optical system for converging in said first direction each of said plurality of
light beams deflected from said deflector, a separation optical system for separating
said plurality of light beams in said second direction after they pass through
said third optical system, and a fourth optical system for converging each of said
plurality of light beams at least in the second direction after they pass through
said separation optical system;
said third optical system includes at least one cylindrical lens having refractive
power at least in said first direction;
said at least one cylindrical lens includes, in order from the light source side,
a first cylindrical lens having negative refractive power in said first direction
and a second cylindrical lens having positive refractive power in said first direction;
and
said other light beams intersect in said plane between said second optical system
and said deflector.
22. The optical scanning device of claim 21, wherein said other light beams enter
said rear optical system at positions in said plane that are outside said two light beams.
23. The optical scanning device of claim 22, wherein:
said deflector includes plural reflecting surfaces that deflect said plurality
of light beams; and
said two light beams are reflected by said plural reflecting surfaces at right
angles in said plane and enter said third optical system at right angles in said
plane.
24. The optical scanning device of claim 22, wherein:
said plurality of light beams includes, in order in said second direction, first,
second, third, and fourth light beams that are deflected by said deflector so that
said second and third light beams are parallel in said plane;
said second cylindrical lens includes first and second lens parts that are adjacent
to one another in said second direction;
the first lens part includes a first light incident surface that is inclined
at an angle in said plane so that the sum of the absolute values of the angles
that the first and second light beams make with said first light incident surface
is equal to the sum of the absolute values of the angles that the first and second
light beams would make with said first light incident surface if said first light
incident surface were perpendicular to the center optical axis of said third optical
system; and
the second lens part includes a second light incident surface that is inclined
at an angle in said plane so that the sum of the absolute values of the angles
that the third and fourth light beams make with said second light incident surface
is equal to the sum of the absolute values of the angles that the third and fourth
light beams would make with said second light incident surface if said second light
incident surface were perpendicular to the center optical axis of said third optical
system.
25. The optical scanning device of claim 22, wherein:
said plurality of light beams includes, in order in said second direction, first,
second, third, and fourth light beams that are deflected by said deflector so that
said second and third light beams are parallel in said plane;
said second cylindrical lens includes first and second lens parts that are adjacent
to one another in said second direction;
the first lens part includes a first light incident surface that is inclined
at an angle in said plane so that the first light beam produces a straight scanning
line; and
the second lens part includes a second light incident surface that is inclined
at an angle in said plane so that the fourth light beam produces a straight scanning
line.
26. The optical scanning device of claim 21, and further including a plurality
of scanned surfaces wherein each scanned surface is on a different one of a plurality
of photosensitive drums, and each of said plurality of light beams scans one of
said plurality of scanned surfaces.
Description
BACKGROUND OF THE INVENTION
Optical scanning devices are conventionally used to form images in laser
beam printers and similar devices. An optical scanning device emits a light beam,
conventionally a laser beam, that scans as a light spot along a scanned surface
where photosensitive material is present. More precisely, the optical scanning
device includes a collimator lens to collimate a light beam emitted from a light
source, such as a semiconductor laser device, and then uses an optical deflector
such as a high-speed rotating polygon mirror in order to deflect the collimated
light beam onto a scanned surface, such as a photosensitive drum surface.
Multi-beam scanners that simultaneously use plural light beams to scan
are in development for laser printers, including color laser printers. A multi-beam
scanner uses an optical system to guide plural light beams emitted from a light
source to a shared polygon mirror. The polygon mirror is rotated to reflect the
plural light beams to different points on scanned surfaces. In order to guide the
plural light beams to multiple scanned surfaces from a shared polygon mirror, the
plural light beams have to be separated from one another. Therefore, a separation
optical system such as a splitting mirror is provided between the polygon mirror
and the scanned surfaces.
Recent demand for higher printing speed requires higher scanning speed, which
in turn requires higher rotation rates of the polygon mirror. To achieve higher
rotation rates, the polygon mirror must be made smaller and lighter. For that to
occur, it is important that the polygon mirror have a small thickness in the sub-scanning
direction that is generally parallel to the axis of rotation of the polygon mirror
and that is orthogonal to the main scanning direction in which rotation of the
polygon mirror causes the laser beams to scan.
The multi-beam scanner includes plural light beams arranged in the sub-scanning
direction. Therefore, the polygon mirror has a larger thickness in the sub-scanning
direction than that of a single beam scanner. Consequently, the polygon mirror
is heavier and less compact. It is understood that the plurality of light beams
may be spaced closer together in order to reduce the thickness of the polygon mirror
in the sub-scanning direction. However, a limitation is imposed on this close spacing
by the fact that the plurality of light beams must be separable on the image side
of the polygon mirror. Thus, it is difficult in conventional multi-beam scanners
to make the polygon mirror thin in the sub-scanning direction as required to increase
printing speed.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to an optical scanning device in which plural light
beams are spaced so as to be separable on the image side of an optical deflector
and the optical deflector has a small thickness in the sub-scanning direction so
that high speed printing can be realized in such an optical scanning device, such
as a laser printer.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description
given below and the accompanying drawings, which are given by way of illustration
only and thus are not limitative of the present invention, wherein:
FIG. 1 shows a plan view of the basic components of the optical scanning device
of a preferred embodiment of the invention in the plane that includes the main
scanning direction;
FIG. 2 shows a cross-sectional view along line II-II of FIG. 1;
FIG. 3 shows an enlarged cross-sectional view in the sub-scanning direction
of a portion of the optical scanning device of FIG. 1;
FIG. 4 shows an enlarged cross-sectional view in the sub-scanning direction
of a portion of the optical scanning device of a comparative example to that of
FIG. 3;
FIGS. 5A-5B show enlarged cross-sectional views in the main scanning direction
of two modifications of a portion of the optical scanning device of FIG. 1;
FIGS. 6A-6B, 7A-7B, and 8A-8B show enlarged side
views in the same direction as FIG. 2 of three alternative embodiments of a rear
portion of the optical scanning device of the present invention;
FIG. 9A shows a simplified enlarged view of the light incident surface of a
lens part shown in FIG. 6A;
FIG. 9B shows a simplified enlarged view of a portion of the light incident
surface of a lens part corresponding to light incidence surfaces shown in FIGS.
7A and 8A;
FIGS. 10A-10F are intensity contour maps of different light spots at the scanned
plane using the optical system of FIG. 6A;
FIGS. 11A-11F are intensity contour maps of different light spots at the scanned
plane corresponding to the optical systems shown in FIGS. 7A-7B and 8A-8B;
FIG. 12 shows a perspective view of a cylindrical lens of FIG. 2 with ray tracings
to show how the cylindrical lens creates a curved line image;
FIG. 13 shows a perspective view of a cylindrical mirror of FIG. 2 with ray
tracings to show how the cylindrical mirror creates a curved line image;
FIGS. 14A-14C show simplified enlarged cross-sectional views of the light incident
surface of the cylindrical lens of FIG. 12, but with the cylindrical lens inclined
at different angles;
FIGS. 15A-15B show simplified enlarged cross-sectional views of the light incident
surface of the cylindrical lens of FIG. 12, but with the cylindrical lens inclined
at angles different from those of FIGS. 14A-14C; and
FIGS. 16A-16C show cross-sectional views of a cover glass at various inclinations
for changing the curvature of a scanning line.
DETAILED DESCRIPTION
The present invention will now be described in terms of preferred embodiments
of the invention with reference to the attached drawings. First, a preferred embodiment
will be described with reference to FIGS. 1 and 2. FIG. 1 shows a plan view of
the basic components of the optical scanning device of a preferred embodiment of
the invention in the plane that includes the main scanning direction. FIG. 2 shows
a cross-sectional view along line II-II of FIG. 1, which is a plane that includes
the sub-scanning direction. The splitting mirror
8 and folding mirrors
9A
to
9D (that will be described later) in FIG. 2 are omitted in FIG. 1, and
the folded optical paths are shown as straightened in FIG.
1.
The optical scanning device of FIG. 1 can be used, for example, in a color laser
printer. As shown in FIG. 1, four light beams L
1 to L
4 are guided
to a scanned surface
7 to form light spots that move on the scanned surface
7 in a first direction at a fixed speed for optical scanning. The direction
in which the light spots move is the main scanning direction x (that defines an
x-axis) and the direction orthogonal to it (orthogonal to the plane of FIG. 1)
is the sub-scanning direction y (that defines a y-axis). The scanned surface
7
moves in the sub-scanning direction y.
The optical scanning device includes a light source
1 that emits plural
light beams, a front optical system PRE provided in the optical path of the light
beams L
1 to L
4 from the light source
1, a polygon mirror
4
which is used as an optical deflector that is positioned at the rear of the front
optical system and shared by the light beams L
1 to L
4, and a rear
optical system PST provided between the polygon mirror
4 and the scanned
surface
7. In the description which follows, "front" refers to the light
source side of the polygon mirror
4 and "rear" refers to the side after
reflection from the polygon mirror
4.
The front optical system PRE includes a first optical system
2 for collimating
the plural light beams from the light source
1 and a second optical system
3 for converging each of the collimated light beams in the sub-scanning
direction y that is orthogonal to the first direction. The polygon mirror
4
serves to collectively deflect the light beams L
1 to L
4 from the
front optical system PRE so as to scan the beams in a first direction that is parallel
to the x-axis. The rear optical system PST serves at least to separate from one
another, in a second direction that is parallel to the y-axis, the light beams
L
1 to L
4 that have been deflected by the polygon mirror
4
before they are incident on the scanned surface
7.
The light source
1 of the optical scanning device includes four light
sources
11 to
14, arranged along a first direction x
1 that
corresponds to a main scanning direction, that emit the light beams L
1 to
L
4, respectively. The two inner light sources
12 and
13 emit
the light beams L
2 and L
3 so that they are parallel to each other
in a plane that includes a second direction y
1 that may correspond to the
sub-scanning direction y. On the other hand, the two outer light sources
11
and
14 emit the light beams L
1 and L
4 so that they approach
each other in a plane that includes the second direction y
1. Each of the
light sources
11 to
14 includes a semiconductor laser element and
emits, for example, a light beam having a wavelength of 780 nm. The light sources
are controlled by a control part (not shown in the drawings) so as to independently
emit the light beams L
1 to L
4. The light beams L
1 to L
4
are merely exemplary light beams of the embodiment of FIG.
1.
Collimator lenses
21 to
24 are provided on the exit side
of the light sources
11 to
14 to form a first optical system
2
of the front optical system PRE. Collimator lenses
21 to
24 of the
first optical system
2 collimate the diverging light beams L
1 to
L
4 from the light sources
11 to
14, respectively.
A second optical system
3 of the front optical system PRE is provided
after
the collimator lenses
21 to
24. The second optical system
3
includes four cylindrical lenses
31 to
34. The cylindrical lenses
31 to
34 each have positive refractive power in the second direction
for converging each of the four light beams L
1 to L
4 in the second
direction. Each of the cylindrical lenses
31 to
34 can be formed
of a single lens element or plural lens elements.
The polygon mirror
4 provided after the cylindrical lenses
31 to
34 is shared by the plural light beams L
1 to L
4. The polygon
mirror
4 deflects the plural light beams transmitted by the second optical
system
3 to change their directions of travel along the first direction,
that is, the main scanning direction. More precisely, the polygon mirror
4
is, for example, a hexagonal mirror made as a hexagonal column with six facets,
each facet carrying a reflecting surface
41. The plural light beams from
the second optical system
3 strike the reflecting surfaces
41 with
separations from one another in the second direction, that is, the sub-scanning
direction. The polygon mirror
4 is rotated at a fixed speed (for example,
5000 to 20,000 revolutions per minute) in the direction indicated by the arrow
R in FIG. 1 around the rotation axis orthogonal to the plane of FIG.
1.
The polygon mirror
4 is rotated to change the angles of incidence and thus
the reflecting angles of the light beams L
1 to L
4 so as to change
the direction of travel of the light beams L
1 to L
4 along the first
direction, that is, along the main scanning direction.
The polygon mirror
4 is followed by a third optical system
5 that
includes a first rear cylindrical lens
51 and a second rear cylindrical
lens
52. The third optical system
5 serves as an f·θ lens
to regulate the scanning speed on the scanned surface
7 so as to be constant.
The first rear cylindrical lens
51 has a negative refractive power in the
first direction and the second rear cylindrical lens
52 has a positive refractive
power in the first direction. The third optical system
5 has an overall
positive refractive power in the first direction to converge the light beams L
1
to L
4 from the polygon mirror
4 in the first direction, that is,
in the main scanning direction.
Referring to FIG. 2, the second rear cylindrical lens
52 includes
a first lens part
52A and a second lens part
52B that are adjacent
to each other in the second direction. The first and second lens parts
52A
and
52B are inclined at different angles corresponding to the incident angles
of the light beams L
1 to L
4, or are not inclined, as will be described later.
As shown in FIG. 2, the third optical system
5 is followed by a separation
optical system
9 that includes a splitting mirror
8 and folding mirrors
9A to
9D. The splitting mirror
8 separates the light beams
L
1 and L
2 from the light beams L
3 and L
4, which all
come from the third optical system
5. The splitting mirror
8 is formed,
for example, as a rectangular column extending in the first direction, which is
orthogonal to the plane of FIG.
2. The longitudinal direction of the rectangular
column is parallel to the longitudinal direction of the second rear cylindrical
lens
52. Two facets of the splitting mirror
8 face the second rear
cylindrical lens
52 and are orthogonal to each other so as to form reflecting
surfaces that are inclined by ±45° in relation to the direction in which
the light beams L
1 to L
4 proceed, that is, the center line direction
of the four light beams after they pass through the second rear cylindrical lens
52.
The light beams L
1 to L
4 strike the splitting mirror
8 in
a vertical row as shown in FIG.
2. As shown in FIG. 2, the top two light
beams L
1 and L
2 are reflected upward and the light beams L
3
and L
4 are reflected downward. After being reflected by the splitting mirror
8, the light beam L
1 strikes the folding mirror
9A and the
light beam L
2 strikes the folding mirror
9B. Furthermore, the light
beam L
3 strikes the folding mirror
9C and the light beam L
4
strikes the folding mirror
9D.
The folding mirrors
9A to
9D are followed by a fourth optical system
6 that includes, for example, cylindrical mirrors
6A to
6D.
The fourth optical system serves to converge each of the light beams L
1
to L
4 from the folding mirrors
9A to
9D mainly in the second
direction. Here, it is preferred that the fourth optical system
6 that includes
cylindrical mirrors
6A to
6D have a positive power at least in the
second direction. The cylindrical mirror
6A reflects the light beam L
1
from the reflecting mirror
9A to converge it in the second direction and
the cylindrical mirror
6B reflects the light beam L
2 from the reflecting
mirror
9B to converge it in the second direction. Similarly, the cylindrical
mirror
6C reflects the light beam L
3 from the reflecting mirror
9C
to converge it in the second direction, and the cylindrical mirror
6D reflects
the light beam L
4 from the reflecting mirror
9D to converge it in
the second direction.
The fourth optical system
6 is followed by cover glasses
10A to
10D at positions corresponding to the cylindrical mirrors
6A to
6D,
respectively. The cover glass
10A transmits the light beam L
1 from
the cylindrical mirror
6A to correct its scanning line curvature on the
scanned surfaces
7. The cover glass
10B transmits the light beam
L
2 from the cylindrical mirror
6B to correct its scanning line curvature
on the scanned surfaces
7. Similarly, the cover glasses
10C and
10D
transmit the light beams L
3 and L
4 from the cylindrical mirrors
6C
and
6D, respectively, to correct their scanning line curvature on the scanned
surfaces
7. The scanning line curvature and the correction of the curvatures
will be described later.
The cover glasses
10A to
10D are followed by the scanned surfaces
7. The scanned surfaces
7 are, for example, layers of photosensitive
material, such as selenium, formed on four photosensitive drums
7A to
7D
which are oriented with their cylindrical axes parallel to one another. The photosensitive
drums
7A to
7D include scanned surfaces
71 to
74, respectively.
Each of the scanned surfaces
71 to
74 is scanned by one of the light
beams L
1 to L
4.
The operation of the optical scanning device having the above structure will
be described below, with reference to FIGS. 1 and 2. Upon receipt of an image forming
start command by an external device, such as a computer, the polygon mirror
4
starts rotating. Then, the photosensitive drums
7A to
7D are rotated
and four light sources of the light source
1 are modulated and activated
to emit the light beams L
1 to L
4 based on input image information.
After passing through the collimator lenses
21 to
24, respectively,
each of the light beams L
1 to L
4 is at least approximately collimated.
The collimated light beams L
1 to L
4 are focused to line images in
the second direction, the sub-scanning direction, near the reflecting surface
41
of the polygon mirror
4 by the refractive power in the second direction
of the cylindrical lenses
31 to
34. Here, the line images are linear
in the first direction, that is, the main scanning direction.
The light beams L
1 to L
4, reflected by the polygon mirror
4,
pass through the first rear cylindrical lens
51 and the second rear cylindrical
lens
52 so as to converge in the first direction. Then, they are reflected
sequentially by the splitting mirror
8, reflecting mirrors
9A to
9D, and cylindrical mirrors
6A to
6D to form images on the
scanned surfaces
71 to
74, respectively. After they form images in
the second direction near the reflecting surface
41 of the polygon mirror
4, the light beams L
1 to L
4 proceed while diverging in the
second direction. However, individually the light beams L
1 to L
4
converge in the second direction by the positive power of the cylindrical mirrors
6A to
6D and, finally, form circular spots on the scanned surfaces
71 to
74.
In this way, the surfaces of the photosensitive drums
7A to
7D
are
exposed and electrostatic latent images are formed thereon based on different color
image data. Different color toners having the opposite charge to the electrostatic
latent image are deposited on the image region of the photosensitive drums
7A
to
7D to transfer the images to recording paper. This is followed by the
fixing process in which the color images are fixed on the recording paper.
The optical effects of the optical scanning device of the present invention will
now be described with reference to FIGS. 3,
4, and
6-
13, as
well as FIG. 5 that will be described with regard to modifications of the optical
scanning device. First, the optical effects of the second optical scanning system
3 will be compared to a comparative embodiment.
FIG. 3 shows an enlarged cross-sectional view in the sub-scanning direction
of a portion of the optical system of FIG.
1. FIG. 4 shows an enlarged cross-sectional
view in the sub-scanning direction of a portion of the optical system of a comparative
example to that of FIG.
3. Light emitted from the light source
1
and passing through the collimator lenses
21 to
24 pass through the
cylindrical lenses
31 to
34, respectively, as light beams L
1
to L
4 before they reach the reflecting surface
41 of the polygon
mirror
4. After reflection by the reflecting surface
41, the light
beams L
1 to L
4 enter the first rear cylindrical lens
51. FIG.
3 shows the cylindrical lenses
31-
34 in overlapping relationship
in the first direction orthogonal to the plane of FIG.
3. In practice, the
cylindrical lenses
31 to
34 are shifted slightly in the vertical
direction as shown in FIG.
3. In particular, the light beams L
2 and
L
3 pass through the cylindrical lenses
32 and
33 at their centers.
As shown in FIG. 3, the collimated light beams L
1 to L
4 as they
exit the collimator lenses
21 to
24 (not shown in FIG. 3) are in
the order of L
4, L
2, L
3, and L
1 from top to bottom
and enter the cylindrical lenses
31 to
34. The cylindrical lenses
31 to
34 each have positive refractive power in the second direction.
Therefore, the transmitted light beams L
1 to L
4 form images in the
second direction near the reflecting surface
41 with their diameters reduced
in the second direction. FIG. 3 does not show the widths of the light beams. Among
the light beams L
1 to L
4 that have passed through the cylindrical
lenses
31 to
34, the light beams L
2 and L
3 travel parallel
to each other before they reach the reflecting surface
41. On the other
hand, the light beams L
1 and L
4 intersect each other before they
reach the reflecting surface
41 of the polygon mirror
4. It is preferred
that the parallel light beams L
2 and L
3 strike the reflecting surface
41 at a right angle. This eliminates the possibility of the following third
optical system
5 causing curvature in the scanning lines of the light beams
L
2 and L
3 on the scanned surfaces
72 and
73, as will
be described later.
Among the light beams L
1 to L
4 that have formed images in the
second direction near the reflecting surface
41 of the polygon mirror
4,
the light beams L
2 and L
3 travel parallel to each other in the plane
including the second direction (that is, the plane perpendicular to the first direction)
and enter the first rear cylindrical lens
51 of the rear optical system
PST. The light beams L
1 and L
4 enter the first rear cylindrical lens
51 at angles that allow them to separate from each other in the plane including
the second direction. Here, the light beams L
1 and L
4 enter the first
rear cylindrical lens
51 at the positions outside the light beams L
2
and L
3. Assuming the light beams L
1 and L
2 make an angle of
γ
1 and the light beams L
3 and L
4 make an angle of γ
2,
it is preferred that γ
1=γ
2.
A comparative embodiment of the optical scanning device will now be described
with
reference to FIG.
4. In the comparative embodiment shown in FIG. 4, light
beams L
11 to L
14 emitted by the light source
1 pass through
the collimator lenses
21 to
24 (not shown in FIG. 4) so as to become
collimated light and then pass through a single cylindrical lens
131 having
positive refractive power in the second direction before they reach a reflecting
surface
141.
After passing through the collima tor lenses
21 to
24 (not shown
in FIG.
4), the collimated light beams L
11 to L
14 enter the
cylindrical lens
131 at specified angles. They are aligned in the second
direction in the order of L
11, L
12, L
13, and L
14 from
the top in FIG.
4. Because the cylindrical lens
131 has positive
refractive power, the light beams L
11 to L
14 that have passed through
the cylindrical lens
131 travel in parallel and with their diameters reduced
to form images in the second direction near the reflecting surface
41. Here,
the light beams L
11 to L
14 strike the reflecting surface
141
at a right angle with their spacings maintained and parallel to each other so as
to avoid curved scanning lines being formed on the scanned surface
7 at
a later stage. The light beams L
11 to L
14 reflected by the reflecting
surface
141 maintain their spacings and enter the following separation optical
system with the same spacings.
In the comparative embodiment of FIG. 4, the spacings of the light beams L
11
to L
14 are determined so that they are separable by the following separation
optical system. Assuming that d
1 is the minimum distance for which the separation
optical system is able to separate light beams, d
1 is also the acceptable
minimum spacing of the light beams L
11 to L
14 at the reflecting surface
141. Accordingly, the reflecting surface
141 has to have a width
141D of (3×d
1) or larger in the second direction.
Conversely, in the optical scanning device of the present invention,
as shown in FIG. 3, among the light beams L
1 to L
4 that have passed
through the cylindrical lenses
31 to
34, light beams L
2 and
L
3 are maintained parallel to each other when they strike the reflecting
surface
41 and then reach the first rear cylindrical lens
51 and
the light beams L
1 and L
4 intersect and gradually separate from each
other in the second direction before they reach the reflecting surface
41
and rear cylindrical lens
51. Thus, the spacing d
23 between the parallel
light beams L
2 and L
3 can be as small as the minimum distance d
1
and the spacing d
12 between the light beams L
1 and L
2 and
the spacing d
34 between the light beams L
3 and L
4 can be smaller
than the minimum spacing d
1 on the reflecting surface
41 of the polygon
mirror
4. Among the light beams L
1 to L
4 reflected by the
reflecting surface
41 of the polygon mirror
4, the light beams L
1
and L
4 reach the separation optical system
9 with their spacings
further increased in the second direction. Therefore, the spacing d
12 between
the light beams L
1 and L
2 and the spacing d
34 between the
light beams L
3 and L
4 become larger than the minimum spacing d
1
when these light beams enter the separation optical system, although they are smaller
than the minimum spacing d
1 on the reflecting surface
41 of the polygon
mirror
4.
As a result, in the present invention, the minimum width
41D (=d
12+d
23+d
34=d
1+(d
12+d
34))
in the second direction of the reflecting surface
41 can be smaller than
the minimum width
141D (=3×d
1). This allows the polygon mirror
4 to have reduced thickness in the second direction. In particular, the
polygon mirror
4 can have the most reduced thickness when the light beams
L
1 and L
2 intersect each other on the reflecting surface
41
and so do the light beams L
3 and L
4. This is because the spacing
d
12 between the light beams L
1 and L
2 and the spacing d
34
between the light beams L
3 and L
4 become zero and, therefore, the
minimum width
41D can be nearly equal to the spacing d
23 between
the light beams L
2 and L
3.
The optical effects of the third optical system
5 will be described next
with reference to FIGS. 6A-6B to
12. FIGS. 6A-6B,
7A-
7B, and
8A-
8B show enlarged side views in the same direction as FIG. 2 of
three alternative embodiments of a rear portion of the optical system of the present
invention, including the third optical system
5 in the plane including the
second direction. FIGS. 6A-6B show side views where the lens parts
52A and
52B forming the second rear cylindrical lens
52 of the third optical
system
5 are not inclined. FIGS. 7A-7B and
8A-
8B show side
views where the lens parts
52A and
52B are inclined by specified
angles. Furthermore, FIGS. 6A,
7A, and
8A show the state before the
scanning line curvature is corrected and FIGS. 6B,
7B, and
8B show
the state after the scanning line curvature is corrected. In the figures, the light
beams L
1 and L
2 and the scanning lines S
1 and S
2 produced
by them on the scanned surface
7 are shown. In the figures, only elements
relating to the top two light beams L
1 and L
2 are shown and elements
relating to the bottom two light beams L
3 and L
4 are omitted. The
figures also neglect the refraction the light beams undergo when they pass through
the rear cylindrical lens. FIG. 9A shows a simplified enlarged view of the light
incident surface of a lens part shown in FIG. 6A, specifically near the incident
surface
53A of the second rear cylindrical lens
52. FIG. 9B shows
a simplified enlarged view of a portion of the light incident surface of a lens
part corresponding to light incidence surfaces shown in FIGS. 7A and 8A, specifically
near the incident surface
53A of the second rear cylindrical lens
52.
FIGS. 6A and 9A show cases in which the light beams L
1 and L
2
from the reflecting surface
41 enter the first and second rear cylindrical
lenses
51 and
52 with their spacings increasing. The first and second
rear cylindrical lenses
51 and
52 do not have refractive power in
the second direction. Therefore, the light beam L
1 that enters them obliquely
exits the cylindrical lenses parallel to its incident direction. The light beam
L
2 enters at a right angle, passes through, and exits the cylindrical lenses
without being refracted. It is assumed that the angles of incidence in the second
direction of the light beams L
1 and L
2 onto the surface
53A
of the second cylindrical lens
52 at the center in the first direction are
α
1 and α
2, respectively, as measured from the surface
normal. In this example, α
2=0.
The light beams L
1 and L
2 that pass through the second rear cylindrical
lens
52 are twisted (so-called skewed beams) except for the components that
pass through the second cylindrical lens at the center in the first direction.
The degrees of twist vary depending on the incident points. Specifically, the components
that pass through the second rear cylindrical lens
52 near the center in
the first direction are less twisted and the components that pass through it near
the periphery in the first direction are more twisted. In addition, the twist is
more pronounced when the incident angle α
1 in the second direction
is larger in absolute value. The twisted light beams result in distorted spots
on the scanned surface
7, which will be described later. In this example,
distortion is not observed in the light spot of the light beam L
2 because
its incident angle α
2 in the second direction is zero. Distortion
is not observed in the light spot of the light beam L
3, either, because
its incident angle α
3 in the second direction is zero. However, distortion
is observed in the light spot of the light beam L
4 because its incident
angle α
4 in the second direction is not zero.
In the example of FIG. 6A, of the resultant scanning lines S
1 and S
2
on the scanned surface, the scanning line S
1 is raised in the middle and
the scanning line S
2 is straight, as will be described in detail later.
On the other hand, in the examples of FIGS. 7A,
8A, and
9B, the first
lens part
52A is inclined by an angle of θ and the second lens part
52B is inclined by an angle of -;θ. The angle θ is larger in
FIG. 8A than in FIG.
7A. The angle θ in FIG. 7A produces scanning
lines S
1 and S
2 that are curved oppositely in direction but equal
in magnitude. The increased angle θ in FIG. 8A produces the linear scanning
line S
1. The relationship between the inclination angle θ and the
scanning line curvature will be described in detail later.
As shown in FIG. 9B, it is assumed that the angle of incidence in the incident
plane including the second direction of the first and second light beams L
1
and L
2 (the angles between the light beams L
1 and L
2 and the
normal line to the incident surface
53A of the first lens part
52A),
are β
1 and β
2 respectively, when the first lens part
52A is inclined by an angle of θ. It is preferred that the first lens
part
52A is inclined so that the absolute value total of the angle of incidence
|β
1|+|β
2| of the first and second light beams L
1
and L
2 is equal to the absolute value total of the angle of incidence |α
1+|α
2|
when the first lens part
52A is not inclined (FIG.
6A). In other
words, the inclination angle θ is preferably determined to satisfy the following condition:
Thus, the inclination angle θ should satisfy the following condition:
Likewise, it is assumed that the angle of incidence in the incident plane
including the second direction of the third and fourth light beams L
3 and
L
4 (not shown in FIG.
9B), that is, the angles between the light
beams L
3 and L
4 and the normal line to the incident surface
53B
of the second lens part
52B, are β
3 and β
4, respectively,
(not shown) when the second lens part
52B is inclined by an angle of -;θ.
It is preferred that the second lens part
52B is inclined so that the absolute
value total of the angles of incidence |β
3|+|β
4| of the
third and fourth light beams L
3 and L
4 is equal to the absolute value
total of the angles of incidence |α
3|+|α
4| when the second
lens part
52B is not inclined. In other words, the inclination angle -;θ
is preferably determined to satisfy the following condition:
Thus, the inclination angle -;θ should satisfy the following condition:
With the first and second lens parts
52A and
52B inclined by the
angles described above, the distortion in shape of the light spots formed by the
light beams L
1 and L
2 on the scanned surface
7 is reduced.
As described above, the light beams L
2 and L
3 produce light spots
with no distortion, but the light beams L
1 and L
4 produce distorted
light spots in the example shown in FIG.
6A. All the light beams L
1
to L
4 produce distorted light spots in the example shown in FIG. 7A, but
to a lesser degree compared to the light beams L
1 and L
4 in FIG.
6A. The light beam L
1 produces a light spot with much less distortion
in the example shown in FIG.
8A.
FIGS. 10A-10F are intensity contour maps of different light spots at the scanned
plane of scanned surface
7 with the optical system of FIG. 6A where the
first lens part
52A is inclined by 0°. FIGS. 11A-11F are intensity
contour maps of different light spots at the scanned plane of scanned surface
7
with the optical system of FIG. 7A where the first lens part
52A is inclined
by 1.0°. FIGS. 10A to
10C and FIGS. 11A to
11C show the case
in which the incident angle is 0.0°. FIGS. 10D to
10F and FIGS. 11D
to
11F show the case in which the incident angle is 1.5°. FIGS. 10B
and 10E and FIGS. 11B and 11E represent the light intensities in contour within
the light spots produced by the light beams that pass through the second cylindrical
lens
52 at the center in the first direction. FIGS. 10A,
10C,
10D,
and
10F and FIGS. 11A,
11C,
11D, and
11F represent
the light intensities in contour within the light spots produced by the light beam
that pass through the second cylindrical lens
52 at either periphery in
the first direction.
When the first lens part
52A is not inclined, as shown in FIG. 10A, the
light intensities in contour within the light spot produced by the light beam that
passes through the second cylindrical lens
52 at the center in the first
direction show no differences between the incident angels of 0.0° (FIG. 10B)
and 1.5° (FIG.
10E). No distortions are observed. However, the cross-section
is larger for an angle of incidence of 1.5° than at an angle of incidence
of 0.0°. On the other hand, the light intensities in contour within the light
spots produced by the light beams that pass through either periphery demonstrate
more distortion with the incident angle of 1.5° (FIGS. 10D and 10F) than with
the incident angle of 0.0° (FIGS.
10A and
10C).
When the first lens part
52A is inclined by 1.0°, as shown in FIGS.
11A to
11F, the light intensities in contour within the light spot produced
by the light beams that pass through the second cylindrical lens
52 at either
periphery in the first direction demonstrate less distortion compared with those
in FIGS. 10A to
10F. The light intensities in contour within the light spots
are less distorted in FIGS. 11D and 11F compared with those in FIGS. 10D and 10F.
The light intensities in contour within the light spots produced by the light beams
that pass through the second cylindrical lens
52 at the center in the first
direction show no differences between the angle of incidence 0.0° (FIG. 11B)
and 1.5° (FIG.
11E). No distortions are observed.
According to the results described above, it is understood that with the
first lens part
52A of the second cylindrical lens
52 being inclined,
the light spot on the scanned surface
7, particularly the light spot produced
by light beams that passes through the rear second cylindrical lens
52 at
either periphery, are significantly improved in shape. Likewise, with the second
lens part
52B being inclined, the light spot produced by the light beams
L
3 and L
4 on the scanned surface
7, particularly the light
spots produced by light beams that pass through the second cylindrical lens
52
at either periphery, are significantly improved in shape.
The scanning line curvatures that occur mainly when the light beams pass through
the third optical system
5 and correction thereof are described below. How
the scanning line curvatures occur will be described with reference to FIGS. 6A,
6B,
8A,
8B,
12, and
13. FIGS. 12 and 13 are
