This invention relates to a wobble correction lens for a raster optical scanner, and, more particularly, to a wobble correction lens with a binary diffractive optic surface and a refractive cylindrical surface.
Many conventional raster optical scanners utilize a multi-faceted rotating polygon mirror as the scanning element. A collimated beam of light, as, for example, from a laser, strikes the facets of the rotating polygon which causes the reflected light to revolve about an axis near the center of rotation of the rotating polygon and scan a straight line. This reflected light can be utilized to scan a document as the input of an imaging system or can be used to impinge upon a photosensitive medium, such as a xerographic drum, as the output of an imaging system.
The post-polygon optics of the raster optical scanner are the optical elements in the optical path between the facets of the rotating polygon mirror and the scan line of the raster optical scanner.
Wobble is defined as an error in the post-polygon optics of the optical scanning system caused by the rotating facet not being exactly parallel to the vertical axis. The beam reflected from the facet is thereby angled up or down a small amount resulting in scan line displacement errors in the cross-scan direction.
The first order effect of this wobble can be removed by using a wobble correction mirror. Since, in the cross-scan plane or vertical direction, the beam is focused at the facet, and since the mirror will again focus the beam at the scan line, the angle, either up or down, will be removed. That is, if the beam is going upward a small amount, the mirror, by refocusing the beam, will bend it down an equal amount to strike the scan line at the same spot.
What slight wobble remains in the optical scanner is a second order effect caused by focusing problems. The beam, vertically, can not be focused at the facet along the entire scan line since the field is curved by the difference in glass thickness as the beam scans through thinner and thicker portions of the scan lens. To the extent that the beam is out of focus as it hits the scan line, so also will it have a small wobble at the ends of the scan line.
Angular 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 be irregular surfaces which do not lie parallel to each other.
Extreme precision in the manufacture of the motor, motor shaft and 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 prohibitive in cost.
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.
High resolution laser optical scanner systems may have low cross-scan scan line displacement errors due to a combination of motor shaft wobble, polygon facet tilt errors, and motor vibration. These motor/polygon wobble errors produce high frequency banding which degrades the scanning quality to an unacceptable level. Wobble correction is performed optically by imaging the polygon mirror facet onto the scan plane. The facet and scan plane are geometric object-image optical conjugates formed by optical elements with power in the cross-scan direction.
Currently there are three basic types of of optical elements that are used to perform wobble correction in the post-polygon optics of the optical scanning systems: (1) a wobble correction cylindrical mirror, (2) a toroidal lens, and (3) a cylindrical lens. Each type has its own unique advantages and disadvantages. The wobble correction mirror contributes no cross-scan field curvature but imposes some mechanical constraints on the size of the optical scanning system and how the beam is folded onto the scan plane. The toroidal lens also has a flat cross-scan field and also allows greater freedom and flexibility in the optical scanning system design, and, if an external fold mirror is used, the critical optical components can be confined to a small space. However, the toroidal lens is difficult to manufacture and is therefore expensive. The cylinder lens is much easier to produce and also reduces the mechanical packaging constraints, but has a large cross-scan field curvature producing a curved scan line.
All three types of wobble correction elements rely on either reflective or refractive surfaces to perform the desired optical transformations. These surfaces may have shapes that are difficult and expensive to fabricate, and may not have all of the desired optical correction characteristics. For laser diode light sources for the optical scanning system, the waveband of light emitted is small enough so that chromatic correction is typically not required for refractive optics used with laser diode sources.
Another type of surface for an optical element uses the process of diffraction to obtain the desired optical transformation characteristics. These diffractive surfaces have surface profiles that can focus and redirect light, and can be designed to have optical correction properties that are not available with easily manufacturable refractive and reflective surface shapes. In addition, many of these diffractive surface profiles can be fabricated using a multi-level profile structure (binary diffractive optics technology) on a flat substrate. Optical elements that use diffractive surfaces are highly dispersive, more dispersive than refractive elements. In fact, they are so dispersive they cannot be used as the primary source of optical power in systems that use laser diodes for optical scanning systems.
Binary diffractive optic lenses are formed by etching or molding very shallow and precise steps or grooves into the surface of a transparent optical element. Binary diffractive optic lenses present substantial cost savings over conventional precision glass or plastic optical lenses.
It is an object of this invention to provide a type of optical element that will enable reasonable levels of wobble correction and correct cross-scan field curvature to produce a straight scan line.
It is another object of this invention to provide a more flexible and more compact optical scanning system without the high manufacturing cost associated with a toroidal lens.
It is yet another object of this invention to provide a wobble correction lens that combines a refractive cylindrical lens with a binary diffractive optic lens to correct wobble in the scan line and to correct cross-scan field curvature to produce a straight scan line.