Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2003-10-29
2004-09-28
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S216100, C359S217200, C347S235000, C347S243000, C250S235000
Reexamination Certificate
active
06798549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using it and, more particularly, the invention is suitably applicable to image forming apparatus, for example, such as laser beam printers, digital copiers, and so on, capable of implementing high-quality printing in relatively simple structure and at high speed.
2. Related Background Art
Scanning optical systems used heretofore in the image forming apparatus such as the laser beam printers, the digital copiers, and so on are constructed in such structure that light emitted from a light source is guided to a deflecting means by an incidence optical means, that the light deflected by the deflecting means is focused in a spot shape on a surface of a photosensitive drum, which is a surface to be scanned, by a scanning optical means, and that the surface of the photosensitive drum is optically scanned by the light.
With a recent trend toward higher performance and advanced functions of the image forming apparatus, there is the growing need for higher speed and use of plural light sources is under study in order to meet the need. For example, Japanese Patent Application Laid-Open No. 09-54263 suggests the multi-beam scanning optical system using a multi-beam laser chip as a light source, which is a light source of a single chip for emitting a plurality of laser beams aligned on a straight line.
In the case of such multi-beam scanning optical systems, it is common practice to provide an optical means for detection of synchronism (BD optical system) immediately before writing of image signals in order to accurately control start positions of images.
FIG. 22
is a principal, cross-sectional view in the main scanning direction of a conventional multi-beam scanning optical system (which is a main scanning section view). In the same figure, numeral
51
designates a light source unit, for example, having two light-emitting regions (light sources) of semiconductor laser. The two light-emitting regions are spaced from each other in the main scanning direction and in the sub-scanning direction. Numeral
52
denotes an aperture stop, which shapes each of beams emitted from the respective light-emitting regions, into a desired optimal beam shape. Numeral
53
indicates a collimator lens, which converts the beams having passed through the aperture stop
52
, into nearly parallel beams. Numeral
54
represents a cylindrical lens, which has a predetermined refractive power only in the sub-scanning direction. Each of such elements as the aperture stop
52
, the collimator lens
53
, and the cylindrical lens
54
composes an element of the incidence optical means
62
.
Numeral
55
designates a deflecting means, which is comprised, for example, of a rotary polygon mirror and which is rotated at a constant speed in a direction of an arrow A in the drawing by a driving means such as a motor or the like (not illustrated). Numeral
56
denotes a scanning optical means having the f-&thgr; characteristic, which has two f-&thgr; lenses as first and second f-&thgr; lenses. The scanning optical means
56
establishes a conjugate relation between the vicinity of a deflecting facet
55
a
of the optical deflector
55
and the vicinity of the photosensitive drum surface
57
as a surface to be scanned, in the sub-scanning section, thus having an inclination correction function.
Numeral
58
indicates a return mirror (which will be referred to hereinafter as a “BD mirror”), which reflects a plurality of beams (BD beams) for detection of synchronous signals for adjusting the timing of scan start positions on the photosensitive drum surface
57
, toward a synchronism detector
61
described hereinafter. Numeral
59
represents a slit plate (hereinafter referred to as a “BD slit plate”), which is located at a position equivalent to the photosensitive drum surface
57
. Numeral
60
denotes an imaging lens (hereinafter referred to as a “BD lens”), which is mounted for establishing a conjugate relation between the BD mirror
58
and the synchronism detector
61
and which corrects surface inclination of the BD mirror
58
. Numeral
61
designates a photosensor as a synchronism detector (which will be referred to hereinafter as a “BD sensor”). Each of such elements as the return mirror
58
, the BD slit plate
59
, the BD lens
60
, and the BD sensor
61
constitutes an element of a synchronism-detecting optical means (or BD optical system).
In the same figure, BD detection is carried out for each of the BD beams and the timing of the scan start position for image recording onto the photosensitive drum surface
57
is adjusted for each of the BD, beams by use of output from the BD sensor
61
.
Incidentally, in the case of the multi-beam scanning optical systems with a plurality of light-emitting regions (light sources), if the spacing in the main scanning direction between the light sources varies with progress in scanning for various reasons, it will result in deteriorating a printed image. The printed image will also deteriorate if there is deviation between writing start positions of the respective light-emitting regions, even without the variation in the spacing in the main scanning direction between the light-emitting regions during scanning.
A cause to induce the above phenomenon is conceivably existence of difference between defocus amount of the BD beams on the BD slit surface and defocus amount of the scanning beams on the surface to be scanned.
This will be explained below with reference to
FIGS. 16A and 16B
to
FIGS. 21A
,
21
B, and
21
C. It is noted that marginal rays are omitted in
FIG. 17A
,
FIG. 18A
,
FIG. 20A
, and
FIG. 21A
in order to avoid complication of illustrations.
FIG. 16A
shows a state in which each of beams (A- and B-beams in this case) is focused just at one edge on the BD slit plate in the main scanning direction. The A-beam scans the slit plate from left to right in the drawing and first enters the BD sensor just at the left edge of a slit in the BD slit plate, whereupon the BD sensor outputs a signal to indicate the entrance of the A-beam. The B-beam also scans the slit plate from left to right and, just as the A-beam, it first enters the BD sensor just at the left edge of a slit in the BD slit plate, whereupon the BD sensor outputs a signal to indicate the entrance of the B-beam. The timing of the writing start positions of the A- and B-beams is adjusted by detecting the timing of these two signals.
However, if the focus position in the main scanning section of the A- and B-beams having passed through the BD optical system is shifted by &dgr;M to this side, i.e., toward the deflecting means as illustrated in
FIG. 17A
, there will occur the phenomenon as described below, so as to cause the difference between start positions of the A- and B-beams. The A-beam without defocus (actual A-beam) is converged at the left edge of the slit in the BD slit plate and is about to enter the BD sensor at this point. In contrast, the A-beam with defocus (original A-beam) has already entered the surface of the BD sensor (the right dashed line in the figure). The A-beam actually starts entering the BD sensor when arriving at the position of the left solid line in the figure. Therefore, the start timing of the A-beam is earlier by the degree of deviation between the dashed line and the solid line. On the other hand, the B-beam (original B-beam) fails to enter the sensor because of the defocus though it should start entering the BD sensor at the left dashed line. Actually, the B-beam can first enter the BD sensor at the position of the right solid line (actual B-beam) and thus the start timing of the B-beam becomes later by the deviation between the left dashed line and the right solid line. As a result, the start positions of the A- and B-beams will have a difference equal to the distance between the two dashed lines, on the BD slit surface.
The difference &dgr;Y between the start positions of the A- and B-beams is determined by the defocus a
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