This application relates to U.S. application Ser. No. 07/862,154 now U.S. Pat. No. 5,319,393, "A Sensor for Detecting Beam Position and Start of Scan Position" Attorney Docket No. D/93019Q (Common Assignee) Filed concurrently herewith.
This invention relates to a raster scanning system, and more particularly, to a raster scanning system which utilizes a dual light source for wobble correction caused by different factors such as an imperfect facet of a scanning polygon.
Referring to FIG. 1, a conventional raster scanning system utilizes a light source 12, a collimator 14, a pre-polygon optics 16, a multi-faceted rotating polygon mirror 18 as the scanning element, a post polygon optics 20 and a photosensitive medium 22. The light source 12, which can be a laser source, produces a light beam 24 and sends it to the polygon 18 through the collimator 14 and the pre-polygon optics 16. The rotating polygon has a plurality of facets 26, each of which is a plane mirror. The facets 26 of the rotating polygon mirror 18 reflect the light beam 24 and also cause the reflected light 24 to revolve about an axis near the center of rotation of the rotating polygon mirror 18 scanning a line. This reflected light beam can be utilized to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photographic film or a photosensitive medium 22, such as a xerographic drum at the output of the imaging system.
In real world situations, a raster scanner typically includes a number of optical elements to accommodate a specific design. Unavoidable imprecision in the shape and/or mounting of these optical elements will inevitably introduce certain anomalies in the quality of the scan line on the photoreceptor such as wobble or bow.
Wobble is defined as the displacement of a scan line in the cross-scan or sagittal plane from its intended position. Typically, a wobble error in an optical scanning system is caused by different factors such as the rotating facet not being exactly parallel to the vertical axis. In this case, the beam reflected from the facet is thereby angled up or down for a small amount resulting in scan line displacement errors in the sagittal plane.
Wobble errors can be caused by several factors. The motor driving the rotating polygon mirror can vibrate during operation. The motor shaft can deviate from its rotational axis. The facets of the polygon mirror can have irregular surfaces which do not lie parallel to each other.
Extreme precision in the manufacturing of the motor, the motor shaft and the polygon mirror can reduce wobble, but not eliminate it. Such precision also increases the steps of production and makes mass production of the scanning system virtually impractical and commercially cost prohibitive.
Many systems have been disclosed in the art to overcome a wobble caused by different factors. Typically, they involve an intricate series of anamorphic lenses or non cylindrical and non-spherical lenses such as toroidal-shaped ones or mirrors to reflect the beam off the facet several times or even deviation feedback by optical sensors to provide electronic synchronization of the beam. Also, proposed has been a series of f-theta lenses which can be adjusted relative to each other based on complex formulae to correct for wobble and other optical distortions.
A problem with the prior art systems to correct wobble is that some of the lens elements are very expensive. Another factor in the high cost of correcting wobble is the high fabrication and assembly tolerances required for the optical scanning system to work efficiently and properly.
The next anomaly in a raster scanner is a bow. A bow is an undesirable character of a scan line which does not form a straight line and bows about a central midpoint. Depending on the types of imprecision in the construction of the apparatus, the bow may bend in either sagittal direction relative to a bow free line. A bow situated downward is called a frown and a bow situated upward is called a smile.
Typically a bow happens when the center ray of a light beam scanning a lens does not scan along the optical axis of the lens. The farther the center ray of a beam is from the optical axis of the lens, the more the curvature of the bow.
It is an object of this invention to provide a simple and inexpensive means which can be used in a raster scanning system to correct wobble or can be used in a raster scanning system to correct a bow.