Facsimile and static presentation processing – Facsimile – Recording apparatus
Reexamination Certificate
2000-04-21
2004-07-27
Coles, Edward (Department: 2622)
Facsimile and static presentation processing
Facsimile
Recording apparatus
C358S493000, C358S451000, C369S044110, C369S044120, C359S204200
Reexamination Certificate
active
06768562
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus for scanning a plurality of combined light beams by using a common scanning optical system and its correcting method.
BACKGROUND OF THE INVENTION
There has been known a light beam scanning apparatus for scanning a plurality of light beams such as laser beams on a recording sheet put on an exposure plane by using a common scanning optical system to record an image on the recording sheet. By using a plurality of light beams, an image can be recorded at a high speed (for example, U.S. Pat. No. 5,502,709).
All of the plurality of light beams or at least light beams other than a basic or reference beam are deflected by individual optical deflecting devices independently to be controlled so that scanning lines formed by respective light beams are put in straight lines at regular intervals on a recording sheet. Since the light beams pass through a common scanning optical system simultaneously, an effect of deterioration with age of the scanning optical system is on a level for all the light beams and therefore it does not matter in general.
Prior to combining respective light beams in combining optical systems, however, the optical systems emitting the light beams are discrete and therefore deterioration with age of these optical systems directly affects positions between the light beams. Light beam intervals or phases on scanning lines fluctuate according to, for example, a temperature change or deterioration with age of the optical systems of respective light beams. It causes a problem that a quality of a recorded image is lowered.
Therefore, light beams are split by a beam splitter and the split light beams are guided to a beam position detecting device to detect respective positions of the light beams. For example, the positions of the light beams are detected, before they enter the scanning optical system, by a 4-split light beam detecting device, a PSD (position sensing device, two-dimensional position sensor) or the like which is positioned on a conjugate plane optically conjugate with an exposure plane or in a position slightly displaced from the conjugate plane. On the basis of a result of the detection, the deflections by the optical deflecting devices are corrected so that the relative positions of the light beams are appropriate.
The light beams, however, frequently show an optical power distribution which is not uniform in a circumferential direction within a range of its diameter. Accordingly it has been found that a recording position on the exposure plane after passing the scanning optical system sometimes does not precisely match the beam position before impinging on the scanning optical system detected by the beam position detecting device.
FIGS. 9A. 9B
are diagrams of assistance in explaining the reason why there is caused the disagreement between the recording position on the exposure plane and the beam position detected by the beam position detecting device.
FIG. 9A
shows an optical power distribution of the light beam, with an abscissa axis (x) indicating a position on a plane perpendicular to the light beam and an ordinate axis (p) indicating an optical power (or radiant power). As shown in this diagram the optical power (p) is unsymmetric around the maximum optical power position (x
0
).
On the other hand, a 4-split beam detecting device or a PSD is generally used as a beam position detecting device and they are used to detect a center of gravity of an optical power. In other words, in
FIG. 9A
, a position (x
1
) between horizontally equal areas enclosed by the distribution curve of the optical power (p) and the abscissa axis is detected as a beam position. A recording density (D) of the light beam on a film on the exposure plane is as shown in FIG.
9
B. In this diagram the abscissa axis (x) indicates a position on the exposure plane and the ordinate axis (D) indicates a recording density. This recording density (D) is the highest in the position x
0
where the optical power (p) is the maximum.
Therefore, unless the optical power distribution of the light beam is completely symmetric in a radial direction, an error &Dgr;x occurs between the beam position (x
1
) detected by the beam position detecting device and the recording position (x
0
) on the exposure plane (recording surface). This positional error &Dgr;x always occurs between a plurality of light beams and therefore it causes deterioration of an output image quality.
To output an image having a high resolution, it is required to expand a beam diameter of a light beam before narrowing the beam to be focused on the exposure plane. To expand the beam diameter, a lens having a large aperture or a mirror is required, while these members easily cause significant astigmatism due to a restriction on manufacturing. On the other hand, to detect the beam position of the light beam, the light beam is split before impinging on an beam expander and is introduced to the beam position detecting device. The split light beam has a small diameter and therefore astigmatism of this light beam is small. It will be described in detail below by using
FIGS. 10A-10C
.
FIG. 10A
shows an example of a constitution of a light beam scanning apparatus of a drum inner surface scanning type. In this figure, two light beams LB
1
and LB
2
composed of lasers are combined with each other. These two light beams LB
1
, and LB
2
are emitted from laser diodes LD
1
and LD
2
and then deflected by acousto-optic deflecting devices AOD
1
, and AOD
2
, respectively. These light beams LB
1
, and LB
2
have sufficiently small diameters. These light beams LB
1
, and LB
2
having small diameters are combined by a combining mirror M
1
and expanded in their diameters by a beam expander EX formed by lenses L
1
and L
2
.
The expanded light beams pass through an aperture AP, guided to a condenser or focusing lens L
3
, and then guided to an exposure plane S by a spinner SP. The spinner SP has a mirror rotating at a high speed coaxially with the expander EX and the condenser lens L
3
. The exposure plane S is formed on an inner surface of the drum coaxial with the spinner SP and relatively moves in a rotary axis extending direction of the spinner SP synchronously with a rotation of the spinner SP.
In such a scanning apparatus, the light beams LB
1
and LB
2
having small diameters before impinging on the expander EX are split by the splitting mirror M
2
and then pass through the condenser lens or image focusing lens L
4
so as to be guided to the beam position detecting device PS. Therefore, a lens having a small diameter can be sufficiently used as the condenser lens L
4
and its astigmatism can be also small.
FIG. 10B
shows an enlarged diagram of an image formation in this beam position detecting device PS. As apparent from
FIG. 10B
, a difference between focal positions (astigmatism) is almost zero at a view in two directions perpendicular to each other (x and y directions) on the image focusing plane. An axial z indicates a light beam traveling direction in FIG.
10
B.
On the other hand,
FIG. 10C
shows an enlarged diagram of the image formation on the exposure plane S. As shown in
FIG. 10C
, there is increased a difference (astigmatism) &agr; between a focal position on a z-y plane viewed in the x direction and a focal position on a z-x plane viewed in the y direction. This is because the aperture of the condenser lens L
3
is large as mentioned above.
In this manner if there is an error &Dgr;x caused by an unsymmetric optical power distribution of the light beam described by referring to
FIG. 9
or an error caused by astigmatism &mgr; described by referring to
FIG. 10
, an image quality for recording is reduced. Particularly in an image setter for printing requiring a high-precision image recording, there sometimes appear moire stripes generated by a slight density change which may periodically occur on the image combined with dots. Therefore, it has a problem that an image quality may be deteriorated.
SUMMARY OF THE INVENTION
The present invention has been accomp
Fujii Takeshi
Takada Norihisa
Coles Edward
Fuji Photo Film Co. , Ltd.
Sherrill Jason
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