Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-09-27
2002-06-04
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S216100
Reexamination Certificate
active
06400488
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a light beam scanner that uses a laser diode and the like as the light source.
(2) Related Art
In the electrophotographic method printer or the digital copying machine, for instance, an image is formed in the following manner. The surface of the photoconductive drum is exposed by the light beam scanner to form an electrostatic latent image. Then, the electrostatic latent image is developed with toner and the toner image is transferred on a recording sheet.
In the light beam scanner, a laser beam from a semiconductor laser is applied onto a deflecting facet of a rotating polygon mirror via an incident optical system (the first optical system) such as a collimator lens. Next, the laser beam is deflected at a scanning angle within a certain range. The deflected beam is then applied onto the surface of the photoconductive drum via an optical scanning system (the second optical system) that includes a scanning lens so as to expose the surface of the photoconductive drum in the main scanning direction at a constant scanning speed. Note that a laser beam that is applied to the polygon mirror is referred to as an “incident beam” and a laser beam that is deflected by the polygon mirror is a “deflected beam” in this specification.
The light beam scanner is classified into two types, i.e., the underfilled optical system and the overfilled optical system, according to the relationship between the width of each of the mirrors (deflecting facets) of the polygon mirror in the main scanning direction (referred to as a “mirror width” in this specification) and the luminous flux width of the incident beam in the main scanning direction (referred to as an “incident beam width” in this specification). More specifically, the incident beam width is set to be smaller than the mirror width for the underfilled optical system. On the other hand, the incident beam width is set to be larger than the mirror width for the overfilled optical system.
For this structure of the overfilled optical system, the luminous flux width of the deflected beam can be set to be same as the mirror width, so that the number of deflecting facets of the polygon mirror can be increased without increasing the polygon mirror diameter. As a result, the scanning speed can be easily increased without upsizing the light beam scanner or increasing the driving force of the polygon motor for drive. For this reason, the overfilled optical system has recently received increasing attention as the demand for high density and high speed has grown.
On the other hand, however, the overfilled optical system has a problematic point. According to the angle that the incident beam forms with one deflecting facet, the width and the location of the incident beam cut by the deflecting facet vary and eventually the exposure quantity and the beam diameter in the main scanning direction vary. As a result, the light quantity is uneven in one scanning line and the image quality deteriorates.
One solution that has been proposed to this problem of the overfilled optical system is to set the principal ray of the incident beam, the scanning center axis of the optical scanning system, and the rotating axis of the polygon mirror to be approximately included in one plain, and to set the angle that the principal ray of the incident beam forms with the scanning center axis in the main scanning direction to be approximately 0°. (The optical system of this kind will be referred to as the “front incident optical system” in this specification, and suppose that the angle between the positions of the deflected beam at the start and finish of a scan of an object to be scanned is a “scanning angle”, the principal ray of the deflected beam that bisects the scanning angle is defined as the “scanning center axis”.)
In the front incident optical system, the incident beam is deflected so that deflected beams are approximately symmetric with respect to the scanning center axis. As a result, a required scanning angle can be obtained without drastically changing the angle that the incident beam forms with the deflecting facet and eventually the luminous flux width and the quantity of light of the deflected beam are relatively stable.
Note that the incident beam is reflected by the same deflecting facet to be applied to the scanned surface in the underfilled optical system, so that no problem arises as to the light quantity change. However, since the relationship between the luminous flux width of the deflected beam and the mirror width depends on the incident angle, it is also desirable for the underfilled optical system to have the structure of the front incident optical system in order to set the mirror width as small.
At the same time, the scanning speed needs to be raised due to the recent demand for high speed processing. However, when the rotation speed of the polygon mirror is raised for this purpose, the friction between the polygon mirror and the surrounding air is increased and eventually the power consumption and the wind noise caused by the friction become too large to be ignored.
A conventional solution to this problem is to enclose the polygon mirror in a housing. By doing so, disturbance by outside air is prevented, so that an air resistance is reduced and the wind noise is prevented.
One part of the housing is formed by a window (referred to as the “polygon window” in this specification) that is made of a light-transparent material such as glass and the incident beam passes through the polygon window to be applied to the polygon mirror. In the front incident optical system, both of the incident beam and the deflected beam pass through the same polygon window. The polygon window is set so that the surface of the polygon window is in parallel with the main scanning direction and the sub scanning direction in order to keep the symmetry of the deflected beam in the main scanning direction with respect to the scanning center axis.
However, the polygon window may reflect a part of the incident beam. When reaching the photoconductive drum, the reflected light (referred to as the “ghost beam” in this specification) may expose an undesired part of the surface of the photoconductive drum to deteriorate the image quality.
Especially, in the case of the front incident optical system, the optical path of the incident beam tends to be close to the optical path of the deflected beam in the sub scanning direction. Also, the polygon window is set so that the surface of the polygon window is in parallel with the main and sub scanning directions. As a result, the ghost beam travels close to the scanning center axis of the optical scanning system and the ghost beam is highly possibly applied to the photoconductive drum to deteriorate the image quality. This is highly problematic.
One solution to this problem is the multi-coating of the surface of the polygon window to prevent the ghost beam from arising as less frequently as possible. It is difficult, however, to completely eliminate the ghost beam. In addition, such coating raises the cost.
Another solution to this problem is to curve the surface of the polygon window to diffuse the reflected light. Even in this solution, however, it is difficult to eliminate the ghost beam. In addition, unnecessary optical power is added to the polygon window by the curve, and the precision of the drawing by the deflected beam that passes through the polygon window is degraded.
SUMMARY OF THE INVENTION
The present invention to provide a light beam scanner according to which the image is hardly deteriorated by the reflected light on the surface of the polygon window.
The includes a light beam scanner that includes: a light source unit; a polygon mirror for deflecting an incident beam from the light source unit, the polygon mirror being rotated for having a deflected beam scan a scanned surface in a main scanning direction, a rotational axis of the polygon mirror, a principal ray of the incident beam, and a scanning center axis being substantially included in a f
Kohsaka Jun
Nagasaka Yasushi
Takeshita Kenji
Cherry Euncha
Minolta Co. , Ltd.
Morrison & Foerster / LLP
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