U.S. patent application number 11/059613 was filed with the patent office on 2005-08-25 for optical scanning apparatus.
Invention is credited to Kudo, Genichiro.
Application Number | 20050185236 11/059613 |
Document ID | / |
Family ID | 34857933 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050185236 |
Kind Code |
A1 |
Kudo, Genichiro |
August 25, 2005 |
Optical scanning apparatus
Abstract
The present invention has as its object to provide a compact
optical scanning apparatus of high performance. So, in the present
invention, an optical scanning apparatus comprised of two or more
reflecting mirrors is of a construction in which the optical
characteristics of the mirrors are made different from each other,
and an unevenness in light quantity on a surface to be scanned is
corrected.
Inventors: |
Kudo, Genichiro;
(Utsunomiya-shi, JP) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
3 WORLD FINANCIAL CENTER
NEW YORK
NY
10281-2101
US
|
Family ID: |
34857933 |
Appl. No.: |
11/059613 |
Filed: |
February 15, 2005 |
Current U.S.
Class: |
359/205.1 |
Current CPC
Class: |
G02B 26/127 20130101;
G02B 26/123 20130101; H04N 1/1135 20130101 |
Class at
Publication: |
359/205 |
International
Class: |
G02B 026/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2004 |
JP |
2004-041483 |
Claims
1. An optical scanning apparatus comprising: light source means
having a light emitting portion emitting light beam; deflecting
means; an imaging optical system for causing the light beam
deflected by the deflecting means to be imaged into a spot shape on
a surface to be scanned; and a plurality of mirrors provided in an
optical path between the deflecting means and the surface to be
scanned, wherein at least two of the plurality of mirrors differ in
reflectance for an on-axial ray from each other, and reflectance
for the on-axial ray and reflectance for an off-axial ray of each
of the at least two of the plurality of mirrors are different from
each other.
2. An optical scanning apparatus according to claim 1, wherein the
reflectance for the on-axial ray and the reflectance for the
off-axial ray of each of said at least two of said plurality of
mirrors are made different from each other in order to compensate
for unevenness in image plane illuminance on said surface to be
scanned.
3. An optical scanning apparatus comprising: light source means
having a light emitting portion emitting light beam; deflecting
means; an imaging optical system for causing the light beam
deflected by the deflecting means to be imaged into a spot shape on
a surface to be scanned; and a plurality of mirrors provided in an
optical path between the deflecting means and the surface to be
scanned, wherein at least two of the plurality of mirrors differ in
reflectance for an on-axial ray from each other, and reflectance
for the on-axial ray and reflectance for an off-axial ray of a
mirror having the lowest reflectance among the plurality of mirrors
are different from each other.
4. An optical scanning apparatus according to claim 3, wherein
reflectance for the on-axial ray and reflectance for the off-axial
ray of said mirror having the lowest reflectance among said
plurality of mirrors are made different from each other in order to
compensate for unevenness in image plane illuminance on said
surface to be scanned.
5. An optical scanning apparatus according to claim 3, wherein
light beam emitted from said light source means is P-polarized with
respect to and incident on an incidence surface of an f.theta. lens
constituting said imaging optical system; and wherein reflectance
for the on-axial ray of said mirror of the lowest reflectance among
said plurality of mirrors than that for the off-axial ray
thereof.
6. An optical scanning apparatus comprising: light source means
having a light emitting portion emitting a beam; deflecting means,
an imaging optical system for causing the beam deflected by the
deflecting means to be imaged into a spot shape on a surface to be
scanned; and a plurality of mirrors provided in an optical path
between the deflecting means and the surface to be scanned, wherein
a width of the light beam incident on a deflecting surface of the
deflecting means in a main scanning direction is greater than the
width of the deflecting surface in the main scanning direction;
wherein at least two of the plurality of mirrors differ in
reflectance for an on-axial ray from each other; and wherein
reflectance for the on-axial ray of a mirror having the lowest
reflectance among the plurality of mirrors is smaller than
reflectance for an off-axial ray thereof.
7. An optical scanning apparatus according to claim 6, wherein
reflectance for the on-axial ray of said mirror of the lowest
reflectance among said plurality of mirrors is made smaller than
that for the off-axial ray thereof in order to compensate for
unevenness in image plane illuminance on said surface to be
scanned.
8. An optical scanning apparatus comprising: light source means
having a light emitting portion emitting a beam; deflecting means;
an imaging optical system for causing the beam deflected by the
deflecting means to be imaged into a spot shape on a surface to be
scanned; and a plurality of mirrors provided in an optical path
between the deflecting means and the surface to be scanned, wherein
reflectance for an on-axial ray of each of at least two of the
plurality of mirrors is 90% or less, and reflectance for the
on-axial ray thereof and reflectance for an off-axial ray thereof
are different from each other.
9. An optical scanning apparatus according to claim 8, wherein said
reflectance for the on-axial ray of each of said at least two of
said plurality of mirrors and said reflectance for the off-axial
ray thereof are made different from each other in order to
compensate for unevenness in image plane illuminance on said
surface to be scanned.
10. An optical scanning apparatus according to claim 1, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
11. An image forming apparatus comprising: an optical scanning
apparatus according to claim 10; a photosensitive member disposed
on said surface to be scanned; a developing device for developing
an electrostatic latent image formed on said photosensitive member
by the beam scanned by said optical scanning apparatus as a toner
image; a transferring device for transferring said developed toner
image to a transfer material; and a fixing device for fixing the
transferred toner image on the transfer material.
12. An image forming apparatus comprising: the optical scanning
apparatus set out in claim 11; and a printer controller for
converting code data inputted from an external device into an image
signal and inputting it to the optical scanning apparatus.
13. A color image forming apparatus comprising: a plurality of
optical scanning apparatuses comprises the optical scanning
apparatus set out in claim 10; and a plurality of image bearing
members disposed on the surfaces to be scanned of the respective
optical scanning apparatuses for forming images of different colors
thereon.
14. A color image forming apparatus according to claim 13, further
comprising a printer controller for converting a color signal
inputted from an external device into image data of different
colors and inputting them to the respective optical scanning
apparatuses.
15. An optical scanning apparatus according to claim 2, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
16. An optical scanning apparatus according to claim 3, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
17. An optical scanning apparatus according to claim 4, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
18. An optical scanning apparatus according to claim 5, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
19. An optical scanning apparatus according to claim 6, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
20. An optical scanning apparatus according to claim 7, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
21. An optical scanning apparatus according to claim 8, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
22. An optical scanning apparatus according to claim 9, wherein an
f.theta. lens constituting said imaging optical system is a plastic
lens.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an optical scanning apparatus for
use in an image forming apparatus such as a laser beam printer or a
digital copying machine.
[0003] 2. Related Background Art
[0004] An optical scanning apparatus heretofore used in an image
forming apparatus such as a laser beam printer or a digital copying
machine directs a beam emitted from a light source to a deflecting
element by incidence optical means, causes the beam deflected by
the deflecting element to be imaged into a spot shape on a
photosensitive drum surface which is a surface to be scanned by
scanning optical means, and optically scans on the photosensitive
drum surface with the beam.
[0005] In such an optical scanning apparatus, the beam emitted from
the light source is converted into substantially parallel light
beam by a collimator lens or the like, and in order to effect an
optical face tangle error correction, the beam converted in
substantially parallel light is formed into a linear image near a
deflecting surface by a cylindrical lens. The photosensitive drum
surface is scanned with the beam deflected by the deflecting
surface of the deflector at a substantially constant speed by a
scanning lens, and forms a spot.
[0006] In FIG. 9 of the accompanying drawings, the reference
characters 1a, 1b, 1c and 1d designate light source means, each of
which comprises, for example, a semiconductor laser. The reference
characters 2a, 2b, 2c and 2d denote cylindrical lenses, each of
which has predetermined refractive power in only a sub-scanning
direction. Each of the light source means 1 and the cylindrical
lenses 2 constitutes an element of incidence optical means.
[0007] The reference numeral 3 designates a light deflector as a
deflecting element, and it comprises, for example, a rotating
polygon mirror, and is rotated at a constant speed in the direction
of arrow A by driving means (not shown) such as a motor. The
reference numeral 11 denotes three f.theta. lenses having an
f.theta. characteristic, and in a sub-scanning cross section, they
bring the vicinity of the deflecting surfaces 3a and 3b of the
light deflector 3 and the vicinity of photosensitive drum surfaces
100a, 100b, 100c and 100d as surfaces to be scanned into a
conjugate relation to thereby have an optical face tangle error
correcting function.
[0008] Also, in order to make the image forming apparatus compact,
a return mirror for folding an optical path is disposed downstream
of the deflector to thereby achieve compactness.
[0009] In the above-described conventional optical scanning
apparatus, since a scanning lens is constituted by glass lens,
usually an antireflection coat is provided on the surface of the
lens and surface reflection does not occur, that is, it is
difficult for an unevenness in light quantity on the surface to be
scanned to occur.
[0010] The scanning lens is recent years, however, is constituted
by a plastic lens and is designed to achieve a lower price and
higher performance. In an optical scanning apparatus comprised of
such a plastic lens, it is generally difficult to provide an
antireflection coat on the surface of the lens and therefore, there
occurs unevenness in light quantity on the surface to be scanned
caused by Fresnel reflection on the surface of the lens being
changed by a scanning angle.
[0011] In order to solve these problems, as disclosed in Japanese
Patent Application Laid-open No. H2-35413, it is known to set the
reflectance of a return mirror so that reflectance near an optical
axis and off-axis reflectance may differ from each other, and
correct the unevenness in light quantity on the surface to be
scanned. Also, as disclosed in Japanese Patent Application
Laid-open No. H7-294837, there is known a technique of converting
linearly polarized light into elliptically polarized light on this
side of deflecting means, and correcting the unevenness in light
quantity on the surface to be scanned, and as disclosed in U.S.
Pat. No. 6,084,696, there is known a technique of changing the
transmittance of an optical material to thereby correct the
unevenness in light quantity on the surface to be scanned.
[0012] In an optical scanning apparatus described in Japanese
Patent Application Laid-open No. H2-35413, however,
[0013] (1) consideration is given only to the reflecting
characteristic depending on a polarizing direction,
[0014] (2) consideration is given only to the unevenness in light
quantity due to the transmittance of the interior of the lens,
and
[0015] (3) there is disclosed only a technique when a single return
mirror is used in an optical path, and therefore, nothing is stated
clearly about an unevenness in light quantity caused by surface
reflection when the scanning lens is formed of plastics, and the
correction of an unevenness in light quantity when there are two or
more return mirrors in the optical path. Also, the technique
disclosed in Japanese Patent Application Laid-open No. H7-294837
further requires optical members, and in the technique disclosed in
U.S. Pat. No. 6,084,696, it is foreseen that the optical material
is expensive, and by any of these techniques, it is difficult to
construct an inexpensive optical scanning apparatus.
SUMMARY OF THE INVENTION
[0016] The present invention can provide an image forming apparatus
of a low price and high quality having an optical scanning
apparatus in which a scanning lens is formed of plastics and which
has two or more return mirrors in an optical path, wherein at least
one of the return mirrors is constituted by such reflecting coating
as can obtain desired reflectance in conformity with an incidence
angle and a polarizing direction, and an unevenness in light
quantity on a surface to be scanned by reflection on the surface of
the lens is corrected. Also, the present invention can provide at a
low price an image forming apparatus of a high speed which can also
correct unevenness in image plane illuminance due to the fall of
the light amount at the end portion of an image in an overfilled
scanning apparatus.
[0017] According to one aspect of the invention, an optical
scanning apparatus comprises light source means having a light
emitting portion emitting a beam, deflecting means, an imaging
optical system for causing the beam deflected by the deflecting
means to be imaged into a spot shape on a surface to be scanned,
and a plurality of mirrors provided in an optical path between the
deflecting means and the surface to be scanned, wherein at least
two of the plurality of mirrors differ in reflectance for an
on-axial ray from each other, and reflectance for the on-axial ray
and reflectance for an off-axial ray of each of the at least two of
the plurality of mirrors are different from each other.
[0018] According to a further aspect of the invention, in the
optical scanning apparatus, the reflectance for the on-axial ray
and the reflectance for the off-axial ray of each of said at least
two of said plurality of mirrors are made different from each other
in order to compensate for unevenness in image plane illuminance on
said surface to be scanned.
[0019] According to another aspect of the invention, an optical
scanning apparatus comprises light source means having a light
emitting portion emitting a beam, deflecting means, an imaging
optical system for causing the beam deflected by the deflecting
means to be imaged into a spot shape on a surface to be scanned,
and a plurality of mirrors provided in an optical path between the
deflecting means and the surface to be scanned, wherein at least
two of the plurality of mirrors differ in reflectance for an
on-axial ray from each other, and reflectance for the on-axial ray
and reflectance for an off-axial ray of a mirror having the lowest
reflectance among the plurality of mirrors are different from each
other.
[0020] According to a further aspect of the invention, in the
optical scanning apparatus, reflectance for the on-axial ray and
reflectance for the off-axial ray of said mirror having the lowest
reflectance among said plurality of mirrors are made different from
each other in order to compensate for unevenness in image plane
illuminance on said surface to be scanned.
[0021] According to a further aspect of the invention, in the
optical scanning apparatus, light beam emitted from said light
source means is P-polarized with respect to and incident on an
incidence surface of an f.theta. lens constituting said imaging
optical system; and wherein reflectance for the on-axial ray of
said mirror of the lowest reflectance among said plurality of
mirrors than that for the off-axial ray thereof.
[0022] According to another aspect of the invention, an optical
scanning apparatus comprises light source means having a light
emitting portion emitting a beam, deflecting means, an imaging
optical system for causing the beam deflected by the deflecting
means to be imaged into a spot shape on a surface to be scanned,
and a plurality of mirrors provided in an optical path between the
deflecting means and the surface to be scanned, wherein a width of
the light beam incident on a deflecting surface of the deflecting
means in a main scanning direction is greater than the width of the
deflecting surface in the main scanning direction; wherein at least
two of the plurality of mirrors differ in reflectance for an
on-axial ray from each other; and wherein reflectance for the
on-axial ray of a mirror having the lowest reflectance among the
plurality of mirrors is smaller than reflectance for an off-axial
ray thereof.
[0023] According to a further aspect of the invention, in the
optical scanning apparatus, reflectance for the on-axial ray of
said mirror of the lowest reflectance among said plurality of
mirrors is made smaller than that for the off-axial ray thereof in
order to compensate for unevenness in image plane illuminance on
said surface to be scanned.
[0024] According to another aspect of the invention, an optical
scanning apparatus comprises light source means having a light
emitting portion emitting a beam, deflecting means, an imaging
optical system for causing the beam deflected by the deflecting
means to be imaged into a spot shape on a surface to be scanned,
and a plurality of mirrors provided in an optical path between the
deflecting means and the surface to be scanned, wherein reflectance
for an on-axial ray of each of at least two of the plurality of
mirrors is 90% or less, and reflectance for the on-axial ray
thereof and reflectance for an off-axial ray thereof are different
from each other.
[0025] According to a further aspect of the invention, in the
optical scanning apparatus, an f.theta. lens constituting the
imaging optical system is a plastic lens.
[0026] According to another aspect of the invention, an image
forming apparatus comprises the optical scanning apparatus set out
in the foregoing, a photosensitive member disposed on the surface
to be scanned, a developing device for developing an electrostatic
latent image formed on the photosensitive member by the beam
scanned by the optical scanning apparatus as a toner image, a
transferring device for transferring the developed toner image to a
transfer material, and a fixing device for fixing the transferred
toner image on the transfer material.
[0027] According to another aspect of the invention, an image
forming apparatus comprises the optical scanning apparatus set out
in the foregoing, and a printer controller for converting code data
inputted from an external device into an image signal and inputting
it to the optical scanning apparatus.
[0028] According to another aspect of the invention, a color image
forming apparatus comprises a plurality of optical scanning
apparatuses, each of which comprises the optical scanning apparatus
set out in the foregoing, and a plurality of image bearing members
disposed on the surfaces to be scanned of the respective optical
scanning apparatuses for forming images of different colors
thereon.
[0029] According to a further aspect of the invention, the color
image forming apparatus further comprises a printer controller for
converting a color signal inputted from an external device into
image data of different colors and inputting them to the respective
optical scanning apparatuses.
[0030] According to the present invention, as previously described,
in the optical scanning apparatus constituted by two or more
reflecting mirrors, the optical characteristics of the mirrors are
made different from one another, and the unevenness in light
quantity on the surface to be scanned is corrected, whereby it
becomes possible to provide a compact optical scanning apparatus of
high performance.
[0031] Also, according to the present invention, it is possible to
reduce unevenness of a formed image in density occurring in a color
image forming apparatus for superimposing different images on a
plurality of photosensitive members one upon another.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a sub-scanning cross-sectional view in an optical
scanning apparatus according to Embodiment 1.
[0033] FIG. 2 is a developed view in a main scanning cross section
of an optical system in Embodiment 1.
[0034] FIG. 3 is a developed view in a sub-scanning cross section
of the optical system in Embodiment 1.
[0035] FIG. 4 shows the incidence angle and the scanning angle of
the present embodiment.
[0036] FIG. 5 shows the uneven image plane illuminance correction
of Embodiment 1.
[0037] FIG. 6 is a sub-scanning cross-sectional view of Embodiment
2.
[0038] FIG. 7 shows the image plane illuminance unevenness
correction of Embodiment 2.
[0039] FIG. 8 shows the uneven image plane illuminance correction
of Embodiment 3.
[0040] FIG. 9 shows a conventional optical scanning apparatus.
[0041] FIG. 10 shows the image forming apparatus of the present
invention.
[0042] FIG. 11 is a main scanning cross-sectional view of
Embodiment 4.
[0043] FIG. 12 is a typical view of an overfilled scanning
system.
[0044] FIG. 13 shows the image plane illuminance unevenness
correction amount of Embodiment 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] Embodiment 1
[0046] FIG. 1 is a sub-scanning cross-sectional view of the optical
scanning apparatus of the present invention. FIG. 2 is a developed
view in a main scanning cross section of an optical system in the
optical scanning apparatus of the present invention.
[0047] The return mirror of the present invention is a plane
mirror, and has no optical power in a main scanning direction and a
sub-scanning direction.
[0048] In FIG. 2, the reference numeral 91 designates a light
source such as a semiconductor laser, and the reference numeral 92
denotes a collimator lens for converting divergent light from the
light source into a parallel beam. The beam emitted from the light
emitting portion of the light source 91 is single.
[0049] Also, the optical scanning apparatus adopts an underfilled
optical system (UFS) in which the width of the beam incident on a
polygon mirror 95 in the main scanning direction is smaller than
the width of a deflecting surface 95A in the main scanning
direction.
[0050] The present invention, however, can also be applied to a
light source having two or more light emitting portions. Further,
it can also be applied to a multi-beam light source having three or
more light emitting portions. For example, an end surface light
emitting type monolithic multi-semiconductor laser and a plane
light emitting type monolithic multi-semiconductor laser may be
mentioned.
[0051] The reference numeral 93 designates an aperture stop which
adjusts the diameter of a beam passing therethrough.
[0052] The reference numeral 94 denotes a cylindrical lens as a
second optical system, and it has predetermined refractive power
only in the sub-scanning direction, and causes the beam to be
imaged as a substantially linear image on the deflecting surface
95A of the deflector 95 in a sub-scanning cross section after the
beam has passed through the aperture stop 93.
[0053] The reference numeral 95 designates a polygon mirror as a
deflector, and it is rotated at a uniform speed in the direction of
arrow.
[0054] The reference numeral 96 denotes an imaging optical system
as a third optical system, and each of an f.theta. lens 96a and an
f.theta. lens 96b is constituted by an anamorphic lens of an
aspherical shape in a main scanning cross section, and it causes
the beam deflected by the deflector 95 to be imaged on a
photosensitive drum surface as a surface to be scanned, and
corrects the surface tangle error of the deflector (optical face
tangle error correcting system).
[0055] In the present embodiment, the shapes of the first and
second scanning lenses 96a and 96b constituting a scanning lens
system 6 are represented by the functions of the following
expressions.
[0056] Assuming, for example, each of the points of intersections
of the first and second scanning lenses 96a and 96b and an optical
axis be as respective origin, and the optical axis be the X-axis,
and a direction orthogonal to the optical axis in the main scanning
cross section be the Y-axis, and a direction orthogonal to the
optical axis in a sub-scanning cross section be the Z-axis, the
surface shape in the main scanning cross section on the scanning
start side with respect to the optical axis as shown in FIG. 1 is
represented by 1 x = y 2 R 1 + 1 - ( 1 + K ) ( y / R ) 2 + B 4 s y
4 + B 6 s y 6 + B 8 s y 8 + B 10 s y 10
[0057] and the surface shape in the main scanning cross section on
the scanning end side is represented by 2 x = y 2 R 1 + 1 - ( 1 + K
) ( y / R ) 2 + B 4 e y 4 + B 6 e y 6 + B 8 e y 8 + B 10 e y 10
[0058] where R represents a radius of curvature, and K, B4, B6, B8
and B10 represent aspherical surface coefficients.
[0059] In the present embodiment, the shapes of the first and
second scanning lenses 96a and 96b in the main scanning cross
section are formed substantially symmetrically with respect to the
optical axis, that is, the aspherical surface coefficients on the
scanning start side and the scanning end side are made coincident
with each other.
[0060] Also, in the sub-scanning cross section, on the scanning
starting side and the scanning end side with respect to the optical
axis, the radius of curvature of the emergence surface (the lens
surface most adjacent to the surface to be scanned) r4 of the
second scanning lens 96b is continuously changed in the effective
portion of the scanning lens 96b.
[0061] The emergence surface r4 is designed so as to be strongest
in refractive power (power, i.e. the reciprocal of a focal length)
among the plurality of lens surfaces of a plurality of lenses
constituting a scanning lens system. Also, the first scanning lens
96a includes a non-arcuate surface in the main scanning cross
section.
[0062] As shown in FIG. 2, assuming that the optical axis be the
X-axis, and a direction orthogonal to the optical axis in the main
scanning cross section be the Y-axis, and a direction orthogonal to
the optical axis in the sub-scanning cross section be the Z-axis,
the shapes of the first and second scanning lenses 96a and 96b in
the sub-scanning cross section can be represented by the following
continuous functions. The surface shape on the scanning start side
with respect to the optical axis is represented by 3 S = z 2 r ' 1
+ 1 - ( z / r ' ) 2 r ' = r ( 1 + D 2 s y 2 + D 4 s y 4 + D 6 s y 6
+ D 8 s y 8 + D 10 s y 10 )
[0063] and the surface shape on the scanning cross end side with
respect to the optical axis is represented by 4 S = z 2 r ' 1 + 1 -
( z / r ' ) 2 r ' = r ( 1 + D 2 e y 2 + D 4 e y 4 + D 6 e y 6 + D 8
e y 8 + D 10 e y 10 )
[0064] where r' represents the radius of curvature in the
sub-scanning direction, and D2, D4, D6, D8 and D10 represent
coefficients. And the suffix s of the coefficients represents the
scanning start side, and the suffix e of the coefficients
represents the scanning end side. The radius of curvature in the
sub-scanning direction refers to the radius of curvature within a
cross section orthogonal to the shape (meridian line) in the main
scanning direction.
[0065] The optical parameters of the present embodiment are shown
below.
1 Used Wavelength (mm) 7.90E-07 Refractive Index of f.theta. Lens
1.524 Incidence Angle in Main Scanning 90 Direction (deg.)
Incidence Angle in Sub-Scanning Direction 2.2 (deg.) Deflecting
Point -G1R1 (mm) 1.65E+01 Focal Length of f.theta. Lens (mm)
1.50E+02 Surface R1 Surface R2 Type Scanning Start Side Scanning
End Side Scanning Start Side Scanning End Side ST2 (s) (e) (s) (e)
Main d 6.00E+00 d 4.80E+01 Scanning R -3.62E+01 R -2.48E+01 K
-1.18E+00 K -1.18E+00 K -2.26E+00 K -2.26E+00 B4 5.67E-06 B4
5.67E-06 B4 -1.05E-05 B4 -1.05E-05 B6 2.76E-08 B6 2.76E-08 B6
2.55E-08 B6 2.55E-08 B8 -1.31E-10 B8 -1.31E-10 B8 -1.84E-11 B8
-1.84E-11 B10 1.13E-13 B10 1.13E-13 B10 -5.89E-14 B10 -5.89E-14
Sub- r -1.00E+0.3 r r -1.00E+0.3 r Scanning D2 0.00E+00 D2 0.00E+00
D2 0.00E+00 D2 0.00E+00 D4 0.00E+00 D4 0.00E+00 D4 0.00E+00 D4
0.00E+00 D6 0.00E+00 D6 0.00E+00 D6 0.00E+00 D6 0.00E+00 D8
0.00E+00 D8 0.00E+00 D8 0.00E+00 D8 0.00E+00 D10 0.00E+00 D10
0.00E+00 D10 0.00E+00 D10 0.00E+00 Surface R3 Surface R4 Type
Scanning Start Side Scanning End Side Scanning Start Side Scanning
End Side ST2 (s) (e) (s) (e) Main d 4.00E+00 d 9.95E+01 Scanning R
-4.61E+02 R 8.36E+02 K 0.00E+00 K 0.00E+00 K -3.58E+01 K -3.58E+01
B4 0.00E+00 B4 0.00E+00 B4 -1.02E-06 B4 -1.02E-06 B6 0.00E+00 B6
0.00E+00 B6 2.09E-10 B6 2.09E-10 B8 0.00E+00 B8 0.00E+00 B8
-3.39E-14 B8 -3.39E-14 B10 0.00E+00 B10 0.00E+00 B10 2.68E-18 B10
2.68E-18 Sub- r -1.00E+03 r r -2.14E+01 r Scanning D2 0.00E+00 D2
0.00E+00 D2 1.81E-04 D2 1.69E-04 D4 0.00E+00 D4 0.00E+00 D4
-8.03E-08 D4 -6.92E-08 D6 0.00E+00 D6 0.00E+00 D6 3.07E-11 D6
2.19E-11 D8 0.00E+00 D8 0.00E+00 D8 -7.61E-15 D8 -4.14E-15 D10
0.00E+00 D10 0.00E+00 D10 8.89E-19 D10 3.78E-19
[0066] FIG. 3 is a sub-scanning cross-sectional view of a scanning
optical system in the optical scanning apparatus of the present
invention.
[0067] The reference numeral 91 designates a light source such as a
semiconductor laser, and the reference numeral 92 denotes a
collimator lens for converting divergent light from the light
source into a parallel beam.
[0068] The reference numeral 93 designates an aperture stop which
adjusts the diameter of the beam passing therethrough.
[0069] The reference numeral 94 denotes a cylindrical lens as a
second optical system, and it has predetermined refractive power
only in the sub-scanning direction, and causes the beam to be
imaged as a substantially linear image on the deflecting surface of
a deflector 95 in the sub-scanning cross-section after the beam has
passed through the aperture stop 93.
[0070] The beam deflected by the deflecting surface 95A is imaged
on a photosensitive drum surface as a surface to be scanned by an
imaging optical system 96 as a third optical system.
[0071] The optical scanning apparatus according to the present
embodiment, as shown in FIG. 1, is constructed by the use of a
plurality of optical systems of FIGS. 2 and 3, and all of these
optical systems have the same optical characteristic.
[0072] Two return mirrors 99a and 99b in the present embodiment are
disposed in an optical path subsequent to the deflector 95 in order
to make an image forming apparatus compact, and are designed to
differ in reflectance from each other.
[0073] As shown in FIG. 4, the incidence angle .theta. onto the
return mirror is changed depending on a scanning angle of view
.alpha., and is given by the following expression by the use of the
incidence angle, .phi., of an on-axial beam onto the return
mirror.
.phi..sub.0=cos.sup.-1(cos .alpha. cos .phi.) (1)
[0074] When for example, the on-axis incidence angle .phi.=45 deg.
and the scanning angle of view .alpha.=40 deg., the incidence angle
.phi..sub.0 of the off-axial beam onto the return mirror is 57.2
deg.
[0075] Also, when the on-axial beam is incident on the return
mirror by S-polarization, the off-axial beam has the polarizing
direction with P-polarization intensity E.sub.p.sup.2 and
S-polarization intensity E.sub.s.sup.2 at the following rate. 5 E P
2 = tan 2 ( 1 tan ) 2 tan 2 + 1 + tan 2 ( 1 tan ) 2 ( 2 ) E S 2 =
tan 2 + 1 tan 2 + 1 + tan 2 ( 1 tan ) 2 ( 3 )
[0076] From the foregoing expressions, when the on-axis incidence
angle .phi.=45 deg. and the scanning angle of view .alpha.=40 deg.
and the on-axial beam is incident on the return mirror by
S-polarization, E.sub.p.sup.2:E.sub.s.sup.2=0.29:0.71, and from the
on-axis to the off-axis, the P-polarized component continuously
increases with the angle of view. That is, the incidence angle onto
the return mirror continuously changes and the polarizing direction
also continuously changes.
[0077] In the present embodiment, as shown in FIG. 4, the beam
emitted from the light source is linearly polarized light, and the
polarizing direction thereof is a direction 201. That is, a
P-polarized beam is incident on a deflector 102 and an f.theta.
lens 103. Also, a S-polarized beam is incident on a return mirror
104.
[0078] In the present embodiment, the light source means is
configured so that the beam emitted therefrom is incident on the
deflecting surface of the deflecting means 95 with being
P-polarized with respect to the deflecting surface. And the on-axis
incidence angle .phi. and the scanning angle .alpha. are set as
follows:
[0079] First return mirror 99a:.phi.=21 deg. .alpha.=33 deg.
[0080] Second return mirror 99b:.phi.=22 deg. .alpha.=29 deg.
[0081] In order to correct an unevenness in light quantity
(unevenness in image plane illuminance) on the drum surface, the
reflectance of the return mirror is set so as to be continuously
changed depending on the incidence angle and the polarized
direction, by the use of the fact that from the on-axis to the
off-axis, the incidence angle of a ray onto the return mirror and
the polarized direction continuously change.
[0082] FIG. 5 shows unevenness in image plane illuminance in the
present embodiment. By the surface reflection (Fresnel reflection)
of the f.theta. lenses 96a, 96b and dust-proof glass 98, the light
amount in the off-axis (the end portion of the image) is about 6%
greater than that in the on-axis (the center of the image) (before
correction) . The unevenness in light quantity caused by these
f.theta. lenses is corrected by continuously changing the incidence
angle of the aforedescribed deflected beam onto the return mirror
and the reflectance for the polarized direction. In the present
embodiment, as shown in FIG. 5, the reflectance of the first return
mirror for the on-axial beam is set to 95%. The reflecting coat of
the return mirror 99a is optimized so that a reflectance difference
between the on-axial beam and the off-axial beam may not occur,
that is, the reflectance for the off-axial beam may also be 95%,
with the angle and polarized direction of the incident beam taken
into account. Also, the reflectance of the second return mirror 99b
for the on-axial beam is set to 80%, and the reflecting coating is
optimized so that the reflectance for the off-axial beam may be 3%
lower than that for the on-axial beam, with the angle and polarized
direction of the incident beam taken into account. Consequently,
the unevenness in image plane illuminance is corrected to 3% (after
correction). Thereby, the unevenness in image plane illuminance is
made equal to that in an optical path wherein there is only one
return mirror (an optical path passing a mirror 100).
[0083] The inventor has found that when the reflectance of the
return mirror 99 (in the present embodiment, the second return
mirror 99b) is small, it is easy, in coating forming, to constitute
the reflecting coating by thin film in which reflectance for the
on-axial ray and reflectance for the off-axial ray are different
from each other.
[0084] So, in the present embodiment, the on-axis reflectance and
the off-axis reflectance of the second return mirror 99b are made
different from each other.
[0085] The inventor has found that reflectance of the
reflectingcoating, of which the on-axis reflectance and off-axis
reflectance can be easily made different from each other, is 90% or
less on the optical axis of the imaging optical system 96.
[0086] In the present embodiment, the return mirror, of which the
reflectance is 90% or less on the optical axis of the imaging
optical system 96, is defined as a mirror of small reflectance.
[0087] In the present embodiment, consideration is given only to
the Fresnel reflection component of the f.theta. lenses and the
dust-proof glass, but it is self-evident that it is also possible
to correct unevenness in image plane illuminance due to a
difference in the incidence angle dependency of the reflectance of
a polygon mirror or the diffraction efficiency of a diffracting
optical element, unevenness in image plane illuminance due to the
internal absorption by the f.theta. lens, the reduction of the
light amount of an overfilled optical system (OFS), etc.
[0088] The overfilled optical system (OFS) applied to the present
invention means an optical system in which the width of a beam
incident on the deflecting surface of deflecting means in the main
scanning direction is greater than the width of the deflecting
surface in the main scanning direction.
[0089] In the present invention, it is preferable that the image
plane illuminance ratio on the surface to be scanned be within 5%
in an effective scanning area with the on-axis image plane
illuminance as the reference.
[0090] Also, in the present embodiment, only two return mirrors are
disposed in the optical path, but three or more return mirrors may
be disposed, and further a reflecting optical element (curved
surface mirror) having optical power such as a cylindrical mirror
may be used as a return mirror.
[0091] As described above, in the present embodiment, design is
made such that the reflectance of the return mirror is continuously
changed depending on the incidence angle and the polarized
direction to thereby correct unevenness in image plane illuminance,
and unevenness in image plane illuminance between respective colors
in an in-line scanning system may become uniform, whereby there can
be provided a compact optical scanning apparatus of high
definition.
[0092] As shown in FIG. 5, in the present embodiment, variation in
the image plane illuminance ratio on the surface to be scanned is
compensated for within 5% in an effective scanning area with the
on-axis image plane illuminance as the reference.
[0093] In the present invention, a curved surface mirror having
optical power in the main scanning direction or/and the
sub-scanning direction may be used instead of the return mirror
(plane mirror). That is, in order to compensate for the unevenness
in image plane illuminance on the surface to be scanned, there may
be adopted a configuration in which the on-axis reflectance and
off-axis reflectance of the curved surface mirror are made
different from each other.
[0094] Embodiment 2
[0095] FIG. 6 shows a sub-scanning cross-sectional view of an
optical scanning apparatus according to this embodiment.
[0096] A beam from a light source is incident on the deflecting
surface of a deflector 85 by an incidence optical system, not
shown. The reference numeral 85 designates a polygon mirror as a
deflector, and it is rotated at a uniform speed. An imaging optical
system 86 is constituted by f.theta. lenses 86a and 86b. Each of
the f.theta. lenses 86a and 86b is constituted by an anamorphic
lens of an aspherical shape in the main scanning cross section, and
they cause the beam deflected by the deflector 85 to be imaged on a
photosensitive drum surface 87 as a surface to be scanned, and
correct the optical face tangle error of the deflector (optical
face tangle error correcting system).
[0097] Two return mirrors 89a and 89b in the present embodiment are
disposed in an optical path subsequent to the deflector 85 to make
the image forming apparatus compact, and are designed to differ in
reflectance from each other.
[0098] The differences of the present embodiment from Embodiment 1
are that the polarizing direction of a light source with respect to
the deflecting surface is S-polarized light (direction 202 in FIG.
4), and that the reflectance of a plurality of return mirrors in an
optical path is set low and the unevenness in image plane
illuminance is corrected to substantially 0%. In the other points,
the construction and effect of the present embodiment are similar
to those of Embodiment 1.
[0099] FIG. 7 shows the uneven image plane illuminance correction
in the present embodiment. Again in the present embodiment, as in
Embodiment 1, the light amount in the off-axis (the end portion of
an image) is about 6% more than that in the on-axis (the center of
the image) by the surface reflection (Fresnel reflection) of the
f.theta. lenses and the dust-proof glass. The unevenness in light
quantity occurring in the f.theta. lenses is corrected by
continuously changing the incidence angle of the aforedescribed
deflected beam onto the return mirror, and the reflectance for the
polarized direction.
[0100] As shown in FIG. 6, the optical scanning apparatus according
to the present embodiment has two return mirrors 89a and 89b in the
same optical path, and the reflectances for the on-axial beam of
the first and second return mirrors 89a and 89b, in order from the
deflecting means side, are set to 70% and 60%, respectively. Also,
as shown in FIG. 7, unevenness in light amount is corrected by 2%
by the first return mirror, and is corrected by 4% by the second
return mirror, whereby the unevenness in light quantity on the drum
surface is corrected to approximately 0% (substantially
uniformly).
[0101] The inventor has found that when the reflectance of the
return mirror 89 is small, it is easy in coating forming to realize
thin coating of which the on-axis reflectance and off-axis
reflectance differ from each other.
[0102] So, in the present embodiment, the reflectance of these
return mirrors 89a and 89b for the on-axial ray and the reflectance
thereof for the off-axial ray are made different from each
other.
[0103] The inventor has found that reflectance of the reflecting
coating, of which the reflectance for the on-axial ray and the
reflectance for the off-axial ray can be easily made different from
each other, is 90% or less on the optical axis of the imaging
optical system.
[0104] In the present embodiment, a return mirror of which the
reflectance is 90% or less on the optical axis of the imaging
optical system is defined as a mirror of small reflectance.
[0105] In the present embodiment, the polarized direction of the
laser diode as the light source means is set so as light beam to be
reflected with being S-polarized with respect to the deflector. For
this configuration, the laser diode is disposed so that a
direction, in which the emission angle of the laser diode is
narrower, is set to coincident with the main scanning direction,
and a direction, in which the emission angle of the laser diode is
wider, is set to coincident with the sub-scanning direction, to
thereby enable a necessary light quantity on the drum surface to be
secured even when a laser diode of low output is used. Moreover, as
compared with a configuration in which light beam emitted from the
light source with being P-polarized with respect to the deflecting
surface of the deflector is incident on the deflecting surface, the
focal length of the cylindrical lens can be shortened and
therefore, it also becomes possible to shorten the distance between
the laser diode and the deflecting surface.
[0106] In the present embodiment, the direction, in which the
emission angle of the laser diode as the light source means is
narrower, is made coincidence with the main scanning direction
(light beam is made incident on the deflecting surface with being
S-polarized with respect to the deflecting surface), whereby as
compared with a case where the light beam is made incident on the
deflecting surface with being P-polarized with respect to the
deflecting surface, the exposure light quantity is increased. This
leads to the merit that the reflectance of the return mirror can be
set low, whereby the reflectance incidence angle dependency and
polarizing characteristic of the return mirror can be secured
greatly. Also, correction is effected by two mirrors and therefore,
the correction amount can be made great.
[0107] In the present embodiment, consideration is given to only
the Fresnel reflection component of the f.theta. lenses and the
dust-proof glass, but it is self-evident that it is also possible
to correct unevenness in image plane illuminance due to the
difference in the incidence angle dependency of the reflectance of
the polygon mirror or the diffraction efficiency of the diffraction
optical element, unevenness in image plane illuminance due to the
internal absorption by the f.theta. lenses, the reduction of the
light quantity of the overfilled optical system (OFS), etc.
[0108] Also, in the present embodiment, only two return mirrors are
disposed in the optical path, but three or more return mirrors may
be disposed, and a reflecting optical element having optical power
such as a cylindrical mirror can be used.
[0109] As described above, in the present embodiment, the polarized
direction of the incidence means is optimized and the reflectance
of the return mirrors is set low, whereby it becomes possible to
provide a compact optical scanning apparatus of high definition
which substantially uniformizes the unevenness in light quantity on
the surface to be scanned.
[0110] As shown in FIG. 7, in the present embodiment, variation in
the image plane illuminance ratio on the surface to be scanned is
compensated for within .+-.5% in the effective scanning area with
the on-axis image plane illuminance as the reference.
[0111] In the present invention, instead of the return mirror
(plane mirror), use may be made of a curved surface mirror having
optical power in the main scanning direction or/and the
sub-scanning direction. That is, in order to compensate for the
unevenness in image plane illuminance on the surface to be scanned,
there may be adopted a configuration in which the reflectance of
the curved surface mirror for the on-axial ray and the reflectance
thereof for the off-axial ray are made different from each
other.
[0112] Embodiment 3
[0113] The difference of this embodiment from Embodiment 1 is that
the correction amounts of the unevenness in image plane illuminance
by each of a plurality of mirrors are made substantially the same.
In the other points, the construction and effect of the present
embodiment are similar to those of Embodiment 1. The construction
of the optical scanning apparatus is similar to that shown in FIG.
1.
[0114] FIG. 8 shows the correction values of the unevenness in
image plane illuminance in the present embodiment. Again in the
present embodiment, as same as in Embodiment 1, due to the surface
reflection (Fresnel reflection) of the f.theta. lens and the
dust-proof glass, the light quantity in the off-axis (the end
portion of the image) is about 6% greater than that in the on-axis
(the center of the image). The unevenness in light quantity
occurring in this f.theta. lens is corrected by continuously
changing the incidence angle of the aforedescribed deflected beam
onto the return mirror, and the reflectance for the polarized
direction.
[0115] As described in FIG. 6, the optical scanning apparatus
according to the present embodiment has two return mirrors 87a and
87b in the same optical path, and the reflectance for the on-axial
beam is set to 90% in both of the first and second return mirrors
in order from the deflecting means side. Also, as shown in FIG. 8,
the unevenness in light quantity is corrected by 3% by the first
return mirror 87a and is corrected by 3% by the second return
mirror, whereby the unevenness in light quantity on the drum
surface is corrected to substantially 0% (substantially
uniformly).
[0116] The first return mirror and the second return mirror in the
present embodiment are the same in the reflectance for the on-axial
beam, but differ in the construction of the reflecting coating of
the mirrors 87a and 87b from each other. This is because the
incidence angle .phi. of the beam incident on each mirror differ
and scanning angle .alpha. differ.
[0117] In the present embodiment, the unevenness in image plane
illuminance is uniformly corrected by the two first and second
return mirrors, and even when use is made of a reflecting mirror of
80% or greater, the unevenness in image plane illuminance can be
corrected to substantially 0% (substantially uniformly).
Particularly in a system wherein the unevenness in image plane
illuminance is as great as 10% or greater, correction can be
effected by a plurality of mirrors without the reflectance of the
return mirrors being greatly lowered and therefore, a reduction in
the light quantity can be prevented and thus, a low output laser is
usable and also, there is the effect of reducing consumed
power.
[0118] In the present embodiment, consideration is given to only
the Fresnel reflection component of the f.theta. lenses and the
dust-proof glass, but it is self-evident that it is also possible
to correct the unevenness in image plane illuminance due to the
difference in the incidence angle dependency of the reflectance of
the polygon mirror or the difference in the diffraction efficiency
of the diffracting optical element, the unevenness in image plane
illuminance due to the internal absorption by the f.theta. lenses,
the reduction of the light quantity of the overfilled optical
system (OFS), etc.
[0119] Also, in the present embodiment, only two return mirrors are
disposed in the optical path, but three or more return mirrors may
be disposed, and a reflecting optical element having power such as
a cylindrical mirror can be used.
[0120] As described above, in the present embodiment, in an optical
scanning apparatus having a plurality of return mirrors of high
reflectance, reflecting coating is constructed so that the
correction amount of unevenness in image plane illuminance may be
substantially equal to one another between the respective mirrors,
whereby it becomes possible to provide a compact optical scanning
apparatus of high definition which substantially uniformizes the
unevenness in light quantity on the surface to be scanned.
[0121] As shown in FIG. 8, in the present embodiment, variation in
the image plane illuminance ratio on the surface to be scanned is
compensated for within .+-.5% in the effective scanning area with
the on-axis image plane illuminance as the reference.
[0122] Embodiment 4
[0123] FIG. 11 shows a main scanning cross-sectional view of an
optical system in this embodiment.
[0124] In Embodiment 4, an OFS optical system is applied instead of
the UFS optical system adopted in Embodiment 1. An optical scanning
apparatus according to Embodiment 4 is similar to that shown in
FIGS. 1, 2 and 3.
[0125] The reference numeral 71 designates a light source such as a
semiconductor laser which emits a P-polarized beam with respect to
a deflecting surface.
[0126] The reference numeral 72 denotes a collimator lens
constituted by two lenses 72a and 72b and converting divergent
light from the light source into a parallel beam.
[0127] The reference numeral 73 designates a cylindrical lens
having predetermined refractive power only in the sub-scanning
direction, and forming a linear an image in the vicinity of the
deflecting surface of a deflector 75 in the sub-scanning cross
section.
[0128] The reference numeral 74 denotes a plane reflecting mirror
which reflects the beam from the cylindrical lens toward the
deflector side.
[0129] The reference numeral 76 designates an imaging optical
system constituted by f.theta. lenses 76a, 76b and 76c, and
comprising spherical surfaces of curvatures shown in the table
below and a cylindrical lens. It causes the beam deflected by the
deflector 75 to be imaged on a photosensitive drum surface 77 as a
surface to be scanned, and corrects the optical surface tangle
error of the deflector (optical face tangle error correcting
system).
2 Used Wavelength (mm) 6.55E-07 Incidence Angle in Main Scanning 0
Direction (deg.) Incidence Angle in Sub-Scanning Direction 0.8
(deg.) Deflecting Point-G1R1 (mm) 2.50E+01 Focal Length of f.theta.
Lens (mm) 3.45E+02 Surface R1 Surface R2 N 1.77610E+00 N
1.00000E+00 d 4.00E+00 d 4.15E+01 R -3.56E+02 R .infin. Surface R3
Surface R4 N 1.69658E+00 N 1.00000E+00 d 1.50E+01 d 2.99E+02 Main R
0.00E+00 R -1.53E+02 Scanning Sub-Scanning r -1.00E+0.3 r .infin.
Surface R5 Surface R6 N 1.52757E+00 N 1.00000E+00 d 4.00E+00 d
1.68E+02 Main R -1.00E+0.3 R -1.00E+0.3 Scanning Sub-Scanning r
1.14E+02 r -1.08E+02 D2 6.63E-06 D2 8.05E-06
[0130] In the present embodiment, as shown in FIG. 12, since the
illuminance distribution of the beam emitted from the light source
means 71 comprising a semiconductor laser is Gaussian distribution,
the beam is a beam in which the light quantity is great at the
central portion of the beam and is small at the edge portion of the
beam. Consequently, when the beam to be incident on the deflecting
surface 75A is made incident on the deflecting surface 75A at a
predetermined angle with respect to the optical axis of the imaging
optical system 76 in the main scanning cross section, the beam
deflected and reflected by the deflecting surface 75A assumes a
light amount distribution asymmetrical with respect to the optical
axis of the imaging optical system, and there arises the problem of
compensating for the asymmetry of the image plane illuminance
distribution on the surface to be scanned. Therefore, it is
preferable to adopt, in the main scanning cross section, a front
incidence system, in which light beam is incident on the deflecting
surface 75A of the deflecting means 75 from the same direction as
the optical axis of the imaging optical system 76.
[0131] In that case, in the main scanning cross section, there is
adopted the front incidence system, in which light beam is incident
on the deflecting surface 75A of the deflecting means 75 from the
same direction as the optical axis of the imaging optical system
76. Therefore, in the case of OFS, the beam deflected and reflected
by the deflecting surface 75A becomes smaller in light amount
toward the off-axis than toward the on-axis.
[0132] In the OFS, the width of the beam incident on the deflecting
surface of the deflecting means in the main scanning direction is
greater than the width of the deflecting surface in the main
scanning direction and therefore, unlike the underfilled optical
system (UFS), the light quantity of the beam deflected and
reflected by the deflecting surface 75A becomes different depending
on the rotation angle of the deflecting means 75 which is a polygon
mirror. Because of front incidence, the beam traveling toward the
off-axis becomes smaller in light quantity than the beam traveling
toward the on-axis.
[0133] So, in the present invention, as shown in FIG. 13, in order
to increase the light quantity of the beam traveling toward the
off-axis, the reflectance of the second return mirror 79b for the
on-axial ray and the reflectance thereof for the off-axial ray are
made different from each other. That is, the reflectance of the
second return mirror 79b for the off-axial ray is made greater than
the reflectance thereof for the on-axial ray. In the present
embodiment, the reflectance of the first return mirror is set to
95%, and the reflecting coating of the return mirror 79a is
optimized so that a reflectance difference between the on-axial ray
and the off-axial ray may not occur, that is, the reflectance for
the off-axial ray may also be 95%, with the angle and polarized
direction of the incident beam taken into account. Also, the
reflectance of the second return mirror 79b for the on-axial ray is
80%, and the reflecting coating is optimized so that the
reflectance for the off-axial ray may be 5% higher than the
reflectance for the on-axial ray, with the angle and polarized
direction of the incident beam taken into account. Consequently,
the unevenness in image plane illuminance is corrected to 3% (after
correction).
[0134] As described above, in the present embodiment, the
reflectance of the return mirrors is continuously changed depending
on the incidence angle and the polarized direction to thereby
correct the unevenness in image plane illuminance, and design is
made such that the unevenness in image plane illuminance among
respective colors in an in-line scanning system becomes uniform,
whereby there can be provided a compact optical scanning apparatus
of high definition.
[0135] As shown in FIG. 13, in the present embodiment, variation in
the image plane illuminance ratio on the surface to be scanned is
compensated for within .+-.5% in the effective scanning area with
the on-axis image plane illuminance as the reference.
[0136] In the present embodiment, instead of return mirrors (plane
mirrors), use may be made of a curved surface mirror having optical
power in the main scanning direction or/and the sub-scanning
direction. That is, in order to compensate for the unevenness in
image plane illuminance on the surface to be scanned, there may be
adopted a configuration in which the on-axis reflectance of the
curved surface mirror and the off-axis reflectance thereof are made
different from each other.
[0137] (Image Forming Apparatus)
[0138] FIG. 10 is a cross-sectional view of essential portions in
the sub-scanning direction showing an embodiment of the image
forming apparatus of the present invention. In FIG. 10, the
reference numeral 60 designates a color image forming apparatus,
the reference numeral 11 denotes a scanning optical apparatus
having the construction shown in any one of Embodiments 1 to 3, the
reference numerals 21, 22, 23 and 24 designate photosensitive drums
as image bearing members, the reference numerals 31, 32, 33 and 34
denote developing devices, and the reference numeral 51 designates
a conveying belt.
[0139] In FIG. 10, red (R), green (G) and blue (B) color signals
are inputted from an external device 52 such as a personal computer
to the color image forming apparatus 60. These color signals are
converted into cyan (C), magenta (M), yellow (Y) and black (B)
image data (dot data) by a printer controller 53 in the color image
forming apparatus. These image data are inputted to the optical
scanning apparatus 11. Light beams 41, 42, 43 and 44 modulated in
conformity with the respective image data are emitted from this
scanning optical apparatus, and the photosensitive surfaces of the
photosensitive drums 21, 22, 23 and 24 are scanned in the main
scanning direction with these light beams.
[0140] The color image forming apparatus in the present embodiment
emits rays of light corresponding to respective colors, i.e., cyan
(C), magenta (M), yellow (Y) and black (B), from the single
scanning optical apparatus 11, records image signals (image
information) on the surfaces of the photosensitive drums 21, 22, 23
and 24, and prints a color image at a high speed.
[0141] The color image forming apparatus in the present embodiment,
as described above, forms latent images of the respective colors on
the surfaces of the photosensitive drums 21, 22, 23 and 24 by the
single optical scanning apparatus 11 by the use of light beams
based on the respective image data. Thereafter, it multiplexly
transfers those images to a recording material to thereby form a
sheet of full-color image.
[0142] As the external device 52, use may be made, for example, of
a color image reading apparatus provided with a CCD sensor. In this
case, this color image reading apparatus and the color image
forming apparatus 60 together constitute a color digital copying
machine.
[0143] This application claims priority from Japanese Patent
Application No. 2004-041483 filed Feb. 18, 2004, which is hereby
incorporated by reference herein.
* * * * *