Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2003-05-06
2004-09-14
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S216100, C347S243000
Reexamination Certificate
active
06791728
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical device and an image forming device using the scanning optical apparatus. More particularly, the present invention relates to a scanning optical device suitable to an apparatus, such as a laser beam printer or a digital copying machine, in which image formation is conducted using a laser beam produced from light emitting points of a plurality of light sources in order to achieve high speed operation and an increase in recording density, in particular, which uses an over field scanning optical system (hereinafter referred to as an OFS scanning optical system) for receiving a light beam emitted from a light source unit on a wider region than a deflection surface in a main scanning direction and scanning a surface of a photosensitive member using light.
2. Related Background Art
In recent years, high speed operation has been demanded in a digital copying machine and a laser beam printer. In order to meet the demand, such inventions have been made, which include an OFS optical system capable of scanning using the increased number of deflection surfaces of a polygon mirror as a deflector and a multi-beam scanning optical device that can simultaneously scan a plurality of scanning lines by using the increased number of light sources.
In general, an element used for a light source is a semiconductor laser and a light beam having a Gaussian distribution is emitted from the light emitting point so as to have a given spreading width.
After the emitted light beam is formed into a light beam at a spreading angle, which is regulated by an incident optical system, a part of the incident light beam is cut away by the deflection surfaces of the polygon mirror to form a scanning light beam. Because the light beam is fixed against the rotated deflection surfaces, a usage region of the light beam to be used is changed according to the rotation.
When a cutaway manner of the light beam having the Gaussian distribution is momently changed, a distribution of intensity of the scanning light beam is produced. The distribution becomes an illumination distribution on a surface to be scanned, thereby causing unevenness in density of an image.
In order to solve the above problem, various correction techniques have been created, such as a technique using a gradation ND filter disclosed in Japanese Patent Application Laid-Open No. 11-014923 and a technique for conducting electrical correction disclosed in Japanese Patent Application Laid-Open No. 04-255874. Further, an idea is devised in Japanese Patent Application Laid-Open No. 11-218702, in which an opening portion is provided in an incident side to simplify a structure.
According to the above-mentioned techniques, the illumination distribution can be improved. However, there is a demerit in that structures of parts are complicated and a cost is increased.
In the technique in which the ND filter or the opening portion is provided, there is also a problem in that a sufficient effect cannot be obtained depending on processing precision of parts, an assembly tolerance, or a variation in parts.
However, in the above-mentioned various techniques, it is assumed that the light source is disposed in a normal state. When a chip of the semiconductor laser as the light source is assembled so as to be tilted toward a main scanning direction with respect to an optical axis, the center of intensity of a substantially parallel light beam, which has emerged from a collimator lens, is shifted from the optical axis. Thus, an intensity distribution of the incident light beam, which is cut away by the polygon mirror, becomes asymmetrical.
In an extreme case, an intensity peak of a scanning line formed on a surface to be scanned is located outside a scanning effective region and the intensity distribution is tilted, so that the intensity at the peak becomes two or more times larger than an initial intensity.
When an OFS scanning optical device using a plurality of light sources is considered, it is expected that distributions of light emitted from respective semiconductor lasers are different from one another. Thus, even if all light beams are aligned by the polygon mirror deflection surfaces as deflection surfaces, when scanning is conducted using such a light beam, an illumination distribution is changed for each scanning line. Therefore, there occurs a situation where the entire image has an uneven density.
It is difficult to prevent the above problem by measures according to the above-mentioned techniques.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problems. Therefore, an object of the present invention is to provide a scanning optical device in which distributions and exit axes of light beams emitted from light sources are specified when scanning is conducted by an OFS scanning optical device using a plurality of light sources, so that an illumination distribution on a surface to be scanned is made substantially symmetric, thereby obtaining a high image quality and high speed operation simultaneously.
Also, another object of the present invention is to provide a scanning optical device capable of making the illumination distribution substantially symmetric with respect to a scanning center axis while the extension of a structure according to the improvement of an image quality is minimized to suppress an increase in cost.
In order to attain the above-mentioned objects, according to a first aspect of the present invention, there is provided a scanning optical device including: a first optical system for guiding at least two light beams emitted from at least two light emitting points to deflection means; and a second optical system for imaging as light spots the plurality of light beams deflected-by the deflection means on a surface to be scanned, characterized in that:
the light beams incident to a deflection surface of the deflection means are set to light beams wider than the deflection surface in a main scanning direction; and
in a main scanning section, when respective angles formed between the center axes of emitted distributions of the light beams from the at least two light emitting points and a corresponding optical axis of the first optical system are given by &agr;i, the angles among &agr;i with respect to two light beams located at the longest distance from the optical axis are each given by &agr;1 and &agr;2, a maximum difference formed between the angles of &agr;i is given by &agr;&egr;, respective total angular widths of radiation angles of the light beams from the at least two light emitting points within the main scanning section are given by &thgr;i, two beam radiation angles in which a difference of the total angular widths is maximum are given by &thgr;1 and &thgr;2, and a difference of the beam radiation angles is given by &thgr;&egr;=|&thgr;1−&thgr;2|,
2≧|&agr;1−&agr;2|≧0
4≧|&agr;1+&agr;2|≧0, and
2≧&agr;&egr;/(1+&thgr;&egr;)≧0
are satisfied, where &thgr;i is a range up to 1/e
2
of a peak of an intensity distribution of each of the light beams.
According to a second aspect of the present invention, in the first aspect of the invention, the scanning optical device is characterized in that a difference of intensities of illumination distributions of the light beams in both end parts of an effective scanning region on the surface to be scanned is within 4%.
According to a third aspect of the present invention, in the first or second aspects of the invention, the scanning optical device is characterized in that the first optical system includes a third optical system and a fourth optical system, and an optical axis of the third optical system which guides a light beam emitted from one of the at least two light emitting points to the deflection means and an optical axis of the fourth optical system which guides a light beam emitted from the other light emitting point to the deflection means are incident to the def
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