Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-02-28
2003-10-28
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S209100, C359S216100, C359S212100, C347S259000, C347S260000
Reexamination Certificate
active
06639704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical device and an image forming apparatus using the same and, more particularly, to a device suitable for an apparatus such as a laser beam printer, digital copying machine, or the like, which has, e.g., an electrophotography process for recording image information by reflecting and deflecting (deflecting and scanning) at least one light beam, which has been optically modulated and emitted by light source means, by deflection means comprising a rotary polygonal mirror or the like, and scanning a surface to be scanned with the light beam via image forming means having at least one diffraction surface.
2. Related Background Art
Conventionally, in a scanning optical device of, e.g., a laser beam printer (LBP) or the like, a light beam, which has been optically modulated and emitted by light source means in accordance with an image signal, is periodically deflected by a beam deflector comprising, e.g., a rotary polygonal mirror, and is focused by image forming means having f-&thgr; characteristics to form a spot on the surface of a photosensitive recording medium (photosensitive drum), and to scan that surface with the beam spot, thus recording an image.
Furthermore, various scanning optical devices having diffraction surfaces in a portion of image forming means (scanning optical means) have been proposed in, e.g., Japanese Patent Application Laid-Open No. 10-68903 and the like. In Japanese Patent Application Laid-Open No. 10-68903, image forming means uses an optical element having a refraction portion (refraction surface) and diffraction portion (diffraction surface). When the powers of the refraction and diffraction portions are set to satisfy a desired condition, changes in magnification and focus in the main scanning direction due to the temperature drift of the scanning optical device are corrected by a change in power of the refraction and diffraction portions of the image forming means, and the wavelength drift of a semiconductor laser as light source means. With this arrangement, even when the temperature drifts, a high-quality image can be obtained.
The diffraction surface of the optical element which serves as the image forming means and has the refraction and diffraction surfaces is normally formed to have a grating pattern so that first-order diffraction light as diffraction light of an order to be used (use diffraction light) has a maximum intensity. At this time, of the diffraction light components diffracted by the diffraction surface, the amount of unnecessary (high-order) diffraction light components is smaller than the diffraction light of the order used to form a spot on the surface to be scanned. However, in a scanning optical device in which the angle of incidence onto the diffraction surface changes depending on the image height, the unnecessary (high-order) diffraction light components increase/decrease in correspondence with the image height. In the actual manufacturing process, since a manufacturing error is produced with respect to an ideal diffraction grating pattern, unnecessary (high-order) diffraction light components often increase.
Such unnecessary (high-order) diffraction light becomes flare as stray light, and adversely influences image quality.
Furthermore, a scanning lens system (image forming means) of a scanning optical device including such diffraction optical element is normally made up of a plastic lens, and an anti-reflection coat to be formed on the refraction surface is often omitted since it suffers many technical and cost problems. For this reason, reflected diffraction light produced by the diffraction surface is reflected by the refraction surface of the plastic lens without any anti-reflection coat, and becomes ghost as stray light.
This state will be explained below using
FIGS. 10 and 11
.
FIG. 10
is a sectional view showing principal part of the conventional scanning optical device in the main scanning direction.
In
FIG. 10
, a divergent light beam emitted by a light source means
91
is converted into a nearly collimated light beam by a collimator lens
92
, and is limited by a stop
93
. The light beam then enters a cylindrical lens
94
having a predetermined refracting power in only the sub scanning direction. Of the nearly collimated light beam that has entered the cylindrical lens
94
, light components in the main scanning section leave the lens as they are. On the other hand, light components in the sub scanning section are focused, and form a nearly linear image on a deflection surface (reflection surface)
95
a
of a beam deflector
95
comprising a polygonal mirror.
A light beam
15
(
15
P,
15
U,
15
L) reflected and deflected by the beam deflector enters an image forming means (scanning lens system)
85
which comprises a refraction optical element
81
and diffraction optical element
82
. In
FIG. 10
, a plastic toric lens
81
and long diffraction optical element
82
are inserted in turn from the side of the beam deflector
95
. The long diffraction optical element
82
has different powers in the main scanning direction and sub scanning direction, forms image of the light beam coming from the beam deflector
95
on a surface
96
to be scanned, and corrects any inclination of the deflection surface (mirror surface) of the beam deflector
95
. The light beam which has left the image forming means
85
forms an image on the surface
96
to be scanned, and optically scans the surface
96
to be scanned in the direction of an arrow B (main scanning direction) upon rotating the beam deflector
95
in the direction of an arrow A, thus recording image information.
In
FIG. 10
, the long diffraction optical element
82
has an entrance surface
83
serving as a refraction surface, and an exit surface
84
serving as a diffraction surface (diffraction grating surface). Most light components of the light beam
15
(
15
P,
15
U,
15
L) reflected and deflected by the beam deflector
95
are imaged on the surface
96
to be scanned as use diffraction light (normally, +1st-order diffraction light), thus forming a beam spot (not shown).
However, some light components of the light beam
15
(
15
P,
15
U,
15
L) reflected and deflected by the beam deflector
95
become unnecessary high-order diffraction light. Of these light components, sixth-order reflected diffraction light (reflected sixth-order diffraction light) diffracted by the diffraction surface
84
will be examined below.
In
FIG. 10
, of the reflected sixth-order diffraction light, a light beam
16
(
16
P,
16
U,
16
L) is surface-reflected by the refraction surface
83
, is also diffracted by the diffraction surface
84
, and travels toward the surface
96
to be scanned as use diffraction light (normally, +1st-order diffraction light). As can be seen from
FIG. 10
, such reflected sixth-order diffraction light hits the surface
96
to be scanned as stray light although it does not form any image.
The behavior of stray light of the reflected sixth-order diffraction light that scans the surface to be scanned will be explained below using FIG.
11
. In
FIG. 11
, the abscissa plots the image height of a primary beam spot which reaches the surface
96
to be scanned, and the ordinate plots the position of stray light of the reflected sixth-order diffraction light on the surface
96
to be scanned. When the primary beam spot scans the surface
96
to be scanned, the stray light of the reflected sixth-order diffraction light scans the surface
96
to be scanned accordingly, and the scan speed lowers at image height positions near ±80 mm. As a result, many stray light components gather around the image height positions of ±80 mm, thus considerably deteriorating image quality.
Stray light such as flare, ghost, or the like blurs an image on the surface to be scanned. For example, in a laser beam printer (LBP), a blurred image is printed. Furthermore, in recent years, since the sensitivity of a photosensitive drum is increasing to express a ha
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