Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-06-26
2001-09-11
Brase, Sandra (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S241000, C347S256000
Reexamination Certificate
active
06288734
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a scanning optical device and an image forming apparatus realized by using such a device. A scanning optical device according to the invention can suitably be used for an image forming apparatus such as a laser beam printer (LBP) or a digital copying machine.
2. Related Background Art
Scanning optical devices to be used for imageforming apparatus including laser beam printers and digital copying machines are adapted to cyclically deflect a light beam such as a laser beam that is optically modulated according to an image signal and emitted from a light source by means of an optical deflector such as a rotary polygon mirror, converge the deflected light beam to a spot of light on a surface of a photosensitive recording medium (photosensitive drum) by means of an imaging optical system having a so-called f&thgr; feature and cause the light beam to scan the surface in order to record an image.
The optical requirements to be met by such a scanning optical system include that it can excellently focus the light beam on the photosensitive drum and it shows an f&thgr; feature in the main-scanning direction. Japanese Patent Application Laid-Open No. 9-230274 proposes a scanning optical system satisfying those requirements.
FIG. 1
of the accompanying drawings is a schematic perspective view of a principal portion of such a known typical optical scanner as applied to an image forming apparatus such as a laser beam printer or a digital copying machine.
Referring to
FIG. 1
, the light beam emitted from a semiconductor laser
21
is substantially collimated by a collimator lens
22
and then transformed into a divergent light beam by a spherical lens
46
having negative power. Then, the divergent light beam is made to enter a cylindrical lens
24
by way of a first fold mirror
18
to be converged in the sub-scanning section, i.e. a plane intersecting the optical axis along the sub-scanning direction. The converged light beam is reflected by a second fold mirror
25
and transmitted through an f&thgr; lens system
27
having a spherical lens
27
a
and a toric lens
27
b
before it strikes the deflection surface (reflection surface)
26
A of an optical deflector
26
and becomes focussed to form a substantially linear image (extending in the main-scanning direction) near the deflection surface. Note that the light beam striking the deflection surface
26
A is made to show a predetermined angle relative to a plane (in which the optical deflector rotates and which is) perpendicular to the axis of rotation of the optical deflector in the sub-scanning section containing the axis of rotation of the optical deflector
26
and the optical axis of the f&thgr; lens system
27
.
On the other hand, the light beam entering the cylindrical lens
24
is not modified in the main-scanning section, i.e. a plane intersecting the optical axis along the main-scanning direction, and is then reflected by the second fold mirror
25
and transmitted through the f&thgr; lens system
27
before it strikes the deflection surface
26
A of the optical deflector
26
substantially along the center line of the deflection angle of the optical deflector
26
(front incidence). At this time, the divergent light beam is made to show a sufficiently large width relative to the facet width of the deflection surface
26
A of the optical deflector
26
in the main-scanning direction by the collimator lens
22
and the spherical lens
46
. Such an optical system is referred to as overfilled optical system.
The light beam deflected/reflected by the deflection surface
26
A of the optical deflector
26
is led to the surface
31
of the photosensitive drum by way of the f&thgr; lens system
27
, a plane mirror
28
and a cylindrical mirror
34
having predetermined power only in the sub-scanning direction. Then, the light beam optically scans the surface
31
of the photosensitive drum in the direction of arrow B (main-scanning direction) as the optical deflector
26
is driven to rotate in the direction of arrow A. As a result of this scanning operation, an image is recorded on the surface
31
of the photosensitive drum operating as recording medium.
Referring to
FIG. 1
, an anti-dust glass panel
30
is arranged between the cylindrical mirror
34
and the surface
31
of the photosensitive drum to prevent fine particles of toner and paper floating in air near the surface
31
of the photosensitive drum from colliding with and adhering to the optical elements (on the rotary polygon mirror side).
A scanning optical device comprising an overfilled optical system and having a configuration as described above shows an excellent deflection efficiency due to its optical deflector and hence is adapted to high speed scanning operation. On the other hand, however, the quantity of light getting to a unit area of the surface to be scanned (surface of the photosensitive drum) varies between on axis and off axis in the main-scanning direction to make it impossible to realize a uniform distribution of quantity of light in the main-scanning direction. This is because the deflection surface having a width smaller than that of the light beam moves in the light beam with a changing deflection angle along the main-scanning direction. More specifically, the deflection surface is located at a position squarely facing the light beam when the latter is scanning an area on and near the optical axis, whereas it is located at a position inclined relative to the light beam when the latter is scanning an off-axis area. Thus, the ratio of the quantity of light striking the deflection surface to the total quantity of light emitted from the optical system for incident light of the device and hence the quantity of light getting to a unit area of the surface to be scanned vary as a function of the angle of the deflection surface.
Meanwhile, the intensity distribution of the light beam from a semiconductor laser is normally a Gaussian distribution, where the intensity of light is higher at the center than at the periphery of the light beam. Therefore, the quantity of light getting to a unit area of the surface to be scanned (surface of the photosensitive drum) is greater on axis than off axis in the main-scanning direction to produce an uneven distribution of quantity of light.
SUMMARY OF THE INVENTION
Therefore, it is the object of the present invention to provide a scanning optical device that is free from the above identified problems of the prior art and provides a substantially uniform distribution of quantity of light and an image forming apparatus comprising such a scanning optical device.
According to the invention, the above object is achieved by providing a scanning optical device comprising:
a light source;
an optical deflector having a deflection surface adapted to deflect the light beam emitted from the light source in the main-scanning direction;
a first optical system for leading the light beam emitted from the light source to the optical deflector, the first optical system being adapted to make the light beam strike the deflection surface of the optical deflector with a width greater than that of the deflection surface in the main-scanning direction; and
a second optical system for focussing the light beam deflected by the optical deflector onto a surface to be scanned;
the second optical system having an optical element provided on at least an optical surface thereof with an anti-reflection film having a film thickness so adapted as to maximize its light transmittance for the angle of incidence of the most off-axis ray of the light beam deflected by the optical deflector and striking the optical surface.
In another aspect of the invention, there is provided an image forming apparatus comprising:
scanning optical device having the above constitution;
a photosensitive member arranged at the surface to be scanned;
a developing unit for developing an electrostatic latent image formed on a surface of the photosensitive member by the light beam made to scan the surface
Brase Sandra
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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