Optical scanner

Details Optical scanner Optical scanner

2003-06-16

2004-08-03

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

High distortion lens

C359S206100, C359S784000, C359S216100, C250S234000

Reexamination Certificate

active

06771429

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanner reflecting/deflecting a light beam such as a laser beam for scanning an object.
2. Description of the Background Art
In general, a two-dimensional image apparatus such as a laser printer or a scanner is mounted with an optical scanner precisely scanning an object with a laser beam. This type of optical scanner reflects/deflects the laser beam with a light deflector such as a galvanometer mirror or a polygon mirror for scanning an objective surface of a photosensitive drum or the like. While the light deflector rotates at an equiangular velocity, the laser beam must scan the objective surface at a uniform rate. Therefore, the optical scanner employs an f-&thgr; (ef-theta) lens as an optical system letting the laser beam reflected/deflected by the light deflector scan the objective surface at a uniform rate. The f-&thgr; lens is an optical system having a distortion characteristic satisfying y=f&ohgr;(f: focal distance, &ohgr;: half angle of view) in relation to an ideal image height y.
FIGS. 11 and 12
show a conventional optical scanner mounted with an f-&thgr; lens
104
.
FIG. 11
is a schematic block diagram of the optical scanner developed along a Y-Z plane, and
FIG. 12
is a longitudinal sectional view developing the optical scanner shown in
FIG. 11
along an optical axis. Referring to
FIGS. 11 and 12
, numeral
100
denotes a light source (semiconductor laser), numeral
101
denotes a collimator lens, numeral
102
denotes a cylindrical lens, numeral
103
denotes a polygon mirror, numeral
104
denotes the f-&thgr; lens, numeral
105
denotes an anamorphic lens and numeral
106
denotes an objective surface. Directions X, Y and Z shown in
FIGS. 11 and 12
are perpendicular to each other.
The light source
100
oscillates a laser beam
107
directly modulated by a driving circuit (not shown). This laser beam
107
is parallelized by the collimator lens
101
and converged by the cylindrical lens
102
for forming a linear image on a reflecting surface
103
r
of the polygon mirror
103
. The polygon mirror
103
rotates about a rotational axis
103
c
by tens of thousands of revolutions per minute and the f-&thgr; lens
104
is an optical system converting equiangular velocity motion of incident light from the reflecting surface
103
r
to uniform motion, whereby a light beam reflected by the reflecting surface
103
r
of the polygon mirror
103
is deflected at an equilateral velocity and scans the objective surface
106
in the direction Y. The anamorphic lens
105
converges light incident from the f-&thgr; lens
104
perpendicularly (direction X) to a primary scanning direction (direction Y) for forming an image on the objective surface
106
.
As shown in
FIG. 11
, the light beam scans the objective surface
106
over a scanning line length W, and hence the f-&thgr; lens
104
must have a wide total angle &thgr; of view. Further, the size of an image has recently been so increased that an optical scanner having a large scanning line length W is required. Assuming that f represents the focal distance of the f-&thgr; lens
104
at the working wavelength for the light beam, the following relational expression holds:
W=f&thgr;
When the scanning line length W is enlarged while keeping the total angle &thgr; of view constant, therefore, the focal distance f of the f-&thgr; lens
104
is increased. In order to enlarge the scanning line length W while keeping the focal distance f of the f-&thgr; lens
104
constant, on the other hand, the total angle &thgr; of view must be increased. In this case, the aperture of the f-&thgr; lens
104
is so increased that it is difficult to precisely work the f-&thgr; lens
104
and correct optical aberration values thereof, to readily increase the cost for the f-&thgr; lens
104
.
Compactification of the optical scanner has also been required in recent years. As shown in
FIG. 13
, an f-&thgr; lens
104
built in the optical scanner is formed by three groups of lenses, i.e., a first lens
111
having negative refracting power, a second lens
112
having positive refracting power an a third lens
113
having positive refracting power. Between the total length L (face-to-face distance between an entrance-side curved surface
111
i
of the first lens
111
and an exit-side curved surface
113
e
of the third lens
113
) of the f-&thgr;) lens
104
and a focal distance f, the following relational expression holds:
0.100≦L/f≦0.108
Hence, the total length L exceeds 0.100×f. An f-&thgr; lens having optical performance not deteriorated also when the total length L is further reduced has recently been required.
SUMMARY OF THE INVENTION
The present invention is directed to an optical scanner reflecting/deflecting a light beam such as a laser beam for scanning an object.
According to the present invention, the optical scanner comprises a light deflector periodically reflecting a light beam emitted from a light source to periodically deflect said light beam and an imaging optical system having such a distortion characteristic that the product of a focal distance and a half angle of view defines an ideal image height for imaging the light beam deflected by the light deflector on an objective surface, and the imaging optical system comprises a first lens having negative refracting power, a second lens having positive refracting power and a third lens having positive refracting power successively from an entrance side for the light beam to satisfy the following expressions (1) and (2):
L
f
<
0.100
(
1
)
0.10
≤
r1
r3
≤
0.26
(
2
)
where L represents the length between a plane of incidence of the first lens and a plane of exit of the third lens along an optical axis direction and f represents the composite focal distance of the first lens, the second lens and the third lens in the above expression (1) while r1 represents the radius of curvature of a refracting interface on the entrance side for the light beam in the first lens and r3 represents the radius of curvature of a refracting interface on the entrance side for the light beam in the second lens in the above expression (2).
A compact imaging optical system can be formed with a total length L smaller as compared with a focal distance f by satisfying the above expression (1), thereby implementing a compact optical scanner. Further, the imaging optical system can properly correct bending of a meridional image surface by satisfying the above expression (2). According to the present invention, both conditions of the above expressions (1) and (2) are compatible with each other, whereby a compact optical scanner having high optical performance can be manufactured.
Preferably, the first lens, the second lens and the third lens are made of an optical material satisfying the following expression (4) on the basis of a partial Abbe's number &ngr; defined in the following expression (3):
υ
=
N
A
-
1
N
MIN
-
N
MAX
(
3
)
1.40
≤
υ
p
⁢
 
⁢
s
υ
n
⁢
 
⁢
g
≤
1.70
(
4
)
where N
A
represents a refractive index with respect to the central wavelength of a working wave range of the light beam, N
MIN
represents a refractive index with respect to the lower limit of the working wave range of the light beam and N
MAX
represents a refractive index with respect to the upper limit of the working wave range of the light beam in the above expression (3) while &ngr;
ps
represents the partial Abbe's number of the second lens and the third lens and &ngr;
ng
represents the partial Abbe's number of the first lens in the above expression (4).
An imaging optical system capable of correcting on-axis chromatic aberration within tolerance can be implemented by satisfying the above expression (4).
More preferably, the imaging optical system satisfies the following expression (5):
0.26
≤
&LeftBracketingBar;
f1
&RightBracketingBar;
f
≤
0.33
(
5
)
where f1 represents the focal distance of the first lens in the above e

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