Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-13
2001-11-13
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S196100, C359S212100, C359S216100, C347S235000, C347S243000, C347S250000, C347S260000
Reexamination Certificate
active
06317244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a light-scanning optical system and also to an image forming apparatus comprising such a light-scanning optical system. More particularly, the present invention relates to a light-scanning optical system that is adapted to realize high definition printing and can effectively avoid any printing slippage in the main scanning direction by partly excluding the incident luminous flux entering the photodetector (BD sensor) for generating write-start position synchronizing signals. Such an optical system may suitably be used for a laser beam printer or digital copying machine.
2. Related Background Art
FIG. 1
of the accompanying drawings is a schematic illustration of a known light-scanning optical system, illustrating a principal area thereof. Referring to
FIG. 1
, the luminous flux emitted from a semiconductor laser
51
with optical modulation in response to the image information given to it is thinned in terms of its cross section by an aperture stop
52
and transformed into a substantially collimated or converged flux by a collimator lens
53
before entering a cylindrical lens
54
. The luminous flux that enters the cylindrical lens
54
is let out without any modification within the main scanning section but focused in the sub scanning section to produce a substantially linear image (running along the main scanning direction) on the deflection surface (reflection surface)
55
a
of light deflector
55
. The elements including the aperture stop
52
, the collimator lens
53
and the cylindrical lens
54
are those of the first optical system
62
. The luminous flux reflected and deflected by the deflection surface
55
a
of the light deflector
55
is then focused by an imaging optical system (f&thgr; lens)
56
operating as the second optical system onto the surface
57
of a photosensitive drum to produce a luminous spot, which is then made to optically scan the surface
57
of the photosensitive drum in the direction of arrow B (main scanning direction) at a uniform rate as the light deflector
55
is driven to rotate in the direction of arrow A. As a result, an image is recorded on the surface
57
of the photosensitive drum which is a recording medium.
In such a light-scanning optical system, generally, a photodetector is arranged for detecting a write-start synchronizing signal immediately before writing the image signal in order to accurately control the write-start position for writing the image signal.
In
FIG. 1
, reference numeral
58
denotes a bending mirror (to be referred to as “BD mirror” hereinafter) arranged to reflect the luminous flux for detecting the write-start position synchronizing signal to the BD sensor
61
in order to regulate the timing of spotting the scanning start point on the surface
57
of -the photosensitive drum and reference numeral
59
denotes a slit arranged at a position equivalent to the surface
57
of the photosensitive drum
57
. The slit
59
has a width of about 0.5 mm and a luminous flux having a diameter of about 0.1 mm passes therethrough. Reference numeral
60
denotes a BD lens operating as imaging means and arranged to take a role of establishing a conjugate relationship between the BD mirror
58
and the BD sensor
61
. It also takes a role of correcting the inclination of the BD mirror
58
. Reference numeral
61
denotes a photodetector (to be referred to as “BD sensor” hereinafter) operating as write-start position synchronizing signal detecting means.
Thus, the timing of spotting the scanning start point on the surface
57
of the photosensitive drum is regulated by means of the output signal of the BD sensor
61
in FIG.
1
.
Meanwhile, when arranging a light-scanning optical system in the image-forming apparatus main body, the write-start synchronizing signal (to be referred to as “BD signal” hereinafter) may have to be detected at the side opposite to the first optical system
62
relative to the optical axis of the second optical system (f&thgr; lens) as shown in
FIG. 2
depending on the positional restrictions due to the configuration of the main body and the arrangement of the electrical equipment. Then, the polygon mirror
55
has to be rotated in the direction opposite to that of FIG.
1
and the scanning luminous spot on the plane to be scanned
57
also has to be moved oppositely. Note that, in
FIG. 2
, the components same as those of
FIG. 1
are denoted respectively by the same reference symbols.
In light-scanning optical systems as shown in
FIGS. 1 and 2
, generally, the margin between the edge of the luminous flux getting to the opposite ends (point U and point L in
FIGS. 1 and 2
) of the image and the opposite ends in the longitudinal direction (main scanning direction) of the deflection surface
55
a
of the polygon mirror
55
is disregarded for ensuring good optical performance.
FIGS. 3A and 3B
are enlarged views of the deflection surface
55
a
of the polygon mirror
55
, illustrating the margin.
FIG. 3A
shows the luminous flux reflected by the polygon mirror
55
to get to the point U. The distance between the marginal end of the luminous flux and the corresponding longitudinal end of the deflection surface
55
a
of the polygon mirror
55
is defined as margin &Dgr;U. Similarly,
FIG. 3B
shows the luminous flux reflected by the polygon mirror
55
to get to the point L. The distance between the marginal end of the luminous flux and the corresponding longitudinal end of the deflection surface
55
a
of the polygon mirror
55
is defined as margin &Dgr;L.
In ordinary light-scanning optical systems, the following relationship is normally observed.
&Dgr;U>&Dgr;L
Therefore, if the scanning optical system has to be arranged in a manner as shown in
FIG. 2
, the BD signal has to be detected on the side where the margin of the deflection surface
55
a
of the polygon mirror
55
is scarce. This means that the scanning angle is limited or the diameter of the luminous flux is limited to minimize the scanning luminous spot to a great disadvantage of the performance of the system.
However, all the luminous flux coming from the first optical system
62
does not necessarily have to be reflected by the polygon mirror
55
so long as the luminous flux getting to the BD sensor
61
has a diameter small enough to pass through the slit
60
and provides a certain level of tolerance to the sensitivity of the BD sensor
61
.
Referring to
FIG. 4
, in known scanning optical systems, it is therefore typically so designed that the polygon mirror
55
is caused to intentionally vignette the luminous flux getting to the BD sensor (not shown) in order to provide a wide scanning luminous spot diameter without vignetting in the effective area of the image, while allowing a wide scanning angle.
However, such known light-scanning optical systems are more often than not accompanied by the problem of printing slippage in the main scanning direction because the quantity of light arriving to the BD sensor fluctuates depending on the deflection surfaces of the polygon mirror due to a possible eccentricity of the axis of rotation of the polygon mirror, uneven accuracy of machining the longitudinal edges of the deflection surfaces of the polygon mirror, the difference in the reflectivity of the films formed by evaporation on the deflection surfaces particularly in areas close to the edges and other factors.
Now, this phenomenon will be discussed by referring to
FIGS. 5 and 6
.
FIG. 5
is a timing chart of a BD signal (BD) and a laser drive signal (LD). Since the polygon mirror is rotating at a constant angular velocity, a BD signal is applied at regular temporal intervals and a laser drive signal is transmitted for a scanning line at predetermined time t
1
after the application of the BD signal for the scanning line. Thus, all the scanning lines are made to have an identical start point. The BD signal is output at time t
0
after the time when the output of the BD sensor gets to a predetermined slice level S as shown in FIG.
6
. Thus, th
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Schuberg Darren
LandOfFree
Light-scanning optical system and image-forming apparatus... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Light-scanning optical system and image-forming apparatus..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light-scanning optical system and image-forming apparatus... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2585291