Diffractive deflection beam scanning system

Details Diffractive deflection beam scanning system Diffractive deflection beam scanning system

1999-11-09

2001-02-06

Schuberg, Darren (Department: 2872)

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

C359S204200, C359S209100, C347S241000, C347S243000

Reexamination Certificate

active

06185017

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to beam scanning systems for use in an electrophotographic type image forming apparatus and, more particularly, to a beam scanning system for diffracting and deflecting beams emitted from light sources using a disc, in which the arrangement of reflecting mirrors that reflect the diffracted and deflected beam toward a photosensitive medium is improved.
2. Description of the Related Art
In general, beam scanning systems are employed by electrophotographic image forming apparatuses for use in forming an electrostatic latent image on a photosensitive medium such as a photoreceptor web by, for example, scanning beams emitted from a laser scanning unit and a light source. Recently, a multi-beam scanning system which diffracts and deflects beams emitted from light sources by adopting a rotary deflection disc, instead of by adopting a rotary polygon used in a conventional beam scanning system, has been introduced.
FIG. 1
shows a schematic configuration thereof.
Referring to
FIG. 1
, the beam scanning system includes a light source
10
and a deflection disc
11
rotatably mounted over the light source
10
. The deflection disc
11
is coupled to a driving motor
12
which rapidly rotates the deflection disc
11
. The deflection disc
11
includes a plurality of sectors having diffraction patterns formed on the surface thereof.
A beam emitted from the light source
10
is diffracted by the diffraction patterns while passing through a rotating deflection disc
11
. Since the diffraction patterns are formed to have different diffraction angles according to the rotation angle of the deflection disc
11
, beams that are emitted from the same light source
10
, are diffracted at different angles with the rotation of the deflection disc
11
, to create a single scanline of beams. The beams diffracted by the deflection disc
11
are reflected by a plurality of reflecting mirrors
13
and
14
, so that the traveling direction is changed.
The reflected beams come to pass through a beam correction means. In general, the beam correction means includes a condensing mirror
15
for condensing and reflecting the beam, and an hologram element
16
for diffracting and transmitting the beam to direct the beam toward a photosensitive medium (not shown) such as a photoreceptor web. Alternatively, the beam correction means may be replaced with an F-&thgr; lens (not shown) that corrects the focal position and scanwidth of the beam. The F-&thgr; lens corrects aberrations of the beam scanned in a primary scan direction and sets the form of the beam as the deflection disc
11
rotates.
Through the above operations, beams emitted from the light source
10
can form a scanline on the photoreceptor web in the primary scan direction, that is, in a direction perpendicular to the traveling direction of the photoreceptor web.
Only one light source
10
is illustrated in FIG.
1
. However, a color printer needs a plurality light sources for the colors of yellow A), magenta (M), cyan (C) and black (B). A deflection disc
20
and a plurality of light sources
21
,
22
,
23
and
24
, of a multi-beam scanning system that requires a plurality of light sources, are illustrated in FIG.
2
. As the diffraction disc
20
rotates, beams emitted from each of the light sources
21
,
22
,
23
and
24
diffract and transmit the diffraction patterns formed on each different sector of the deflection disc
20
to create scanlines L
1
, L
2
, L
3
and L
4
, respectively. The scan directions of the scan lines L
1
, L
2
, L
3
and L
4
are tangential with respect to the deflection disc
20
.
In the multi-beam scanning system, after the beams emitted from the light sources
21
,
22
,
23
and
24
are diffracted and deflected by the deflection disc
20
, the traveling paths of the beams are changed toward the same direction, that is, the X-axis direction, to scan beams parallel onto a photoreceptor web (not shown). For the parallel scanning of the beams, as shown in
FIG. 3
, there are disposed a plurality of first reflecting mirrors
31
,
32
and
33
and a plurality of second reflecting mirrors
41
,
42
,
43
and
44
over the deflection disc
20
. That is, beams emitted from the light sources
21
,
22
and
23
are diffracted and deflected while passing through each different sector of the deflection disc
20
, are reflected by the first reflecting mirrors
31
,
32
and
33
, and are then reflected by the second reflecting mirrors
41
,
42
and
43
, thereby heading in the X-axis direction. Also, the beam emitted from the light source
24
(see FIG.
2
), which is diffracted and deflected by the deflection disc
20
, is reflected by another first reflecting mirror (not shown) and the second reflecting mirror
44
in sequence, thus heading in the X-axis direction.
Preferably, the second reflecting mirrors
41
,
42
,
43
and
44
are arranged over the center of the deflection disc
20
at different heights, as shown in
FIG. 3
, for easy arrangement and scanline stability. However, in the case where the light sources
21
,
22
,
23
and
24
are symmetrically disposed with respect to the center of the deflection disc
20
, as shown in
FIGS. 2 and 3
, directions of each scanline do not coincide with each other. The problem associated with the symmetrical arrangement of the light sources will be described in greater detail with reference to
FIGS. 4A through 4D
.
FIG. 4A
illustrates the path of beams emitted from the light source
24
. That is, a beam emitted from the light source
24
is reflected by the first reflecting mirror
34
, and the scanline thereof heads in the X-axis direction. Then, the beam is reflected again by the second reflecting mirror
44
disposed over the center of the deflection disc
20
, so that the scanline heads in the −Y-axis direction as indicated by an arrow D
1
. Similarly, as shown in
FIGS. 4B and 4C
, scanlines of beams reflected by the first mirrors
32
and
33
and then reflected by the second reflecting mirrors
42
and
43
, respectively, which have been emitted from the light sources
22
and
23
, also head in the −Y-axis direction as indicated by each arrow D
1
.
However, referring now to
FIG. 4D
, the scanline of the beam, which is emitted from the light source
21
and then reflected by the first and second reflecting mirrors
31
and
42
in sequence, heads in the Y-axis direction as indicated by an arrow D
2
, which is opposite to the scanline directions D
1
of the beams emitted from the light sources
22
,
23
and
24
. Such noncoincidence of the scanline directions must be corrected by an additional circuit or mechanical device prior to scanning it onto a photoreceptor web.
To avoid noncoincidence of the scanline directions, which occurs where the light sources
21
)
22
,
23
and
24
are symmetrically arranged with respect to the center of the deflection disc
20
, a configuration shown in
FIG. 5
has been suggested, where all light sources
51
,
52
,
53
and
54
are arranged within one section divided by a bisecting line S, which passes through the center of a deflection disc
50
and is parallel to the Y-axis.
The light sources
51
,
52
,
53
and
54
are disposed at an intermediate angle that measures 60°. The arrangement of the light sources
51
,
52
,
53
and
54
, which is illustrated in
FIG. 5
, provides an advantage of providing the same scanline directions. However, there is a problem associated with asymmetry of scanlines from a light source, which will be described below with reference to FIG.
6
.
FIG. 6
shows the path of the beam emitted from the light source
52
. The beam emitted from the light source
52
is diffracted and deflected by a predetermined pattern of the rotating deflection disc
50
, and is then reflected by a reflecting mirror
55
disposed over the center of the deflection disc
50
, so that it heads in the X-direction. Here, a central beam B
1
of scanlines travels along a central line C, which is parallel to the X-axis, after bei

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