Background of the Invention

Technical Field This invention relates generally to input and output scanners, and more particularly to such a scanner having a multifaceted rotating polygon that directs a beam of light through a post-facet lens system toward a surface to be scanned.

Background Art A multifaceted rotating polygon can appear to wobble. That is because not all facets are exactly parallel to the axis of rotation and that bearing free-play causes the axis to tilt. So the facets appear to wobble as the polygon rotates and that condition can cause scan-to-scan spot position errors at the surface to be scanned (subsequently referred to hereinbelow by way of example as a photoreceptor) . In order to compensate for wobble, some early scanners included a wobble-correcting cylinder lens in a post-facet position between the polygon and the photoreceptor. Sometimes referred to as a positive pyramid error compensating cylinder lens, it helped focus the facet along a desired scan line at the photoreceptor despite alignment errors in the cross-scan plane between adjacent facets. In that regard, the plane containing both the light beam and the scan line is referred to as the scan plane while a perpendicular plane containing the central position of the light beam (i.e., the position occupied by the light beam when it is directed toward the center of the scan line) is referred to as the cross scan plane. The cylinder lens had little or no power in the scan direction so that it had essentially no effect in the scan plane, but it had power in the cross-scan direction ^' and so sagittal field curvature was objectionable, especially as the scan angle increased

with a decrease in facet-to-photoreceptor distance.

Some scanners had no optics between the facet and the photoreceptor to correct for field curvature (i.e., to flatten the field) . Others simply adapted known optical designs, such as those referred to as the Cooke Triplet and the Double Gauss. But then the two-element "f-theta" lens appeared ("f" representing the focal length and "theta" the scan angle) . It helped flatten the field as described in U.S. Patent Nos. 4,108,532 (Minoura) and 4,179,183 (Tateoka and Minoura). In addition, using two elements to flatten the field left a free design parameter available for use in correcting another design problem. It was used to compensate for scanner non- linearity. Scanner non-linearity refers to the change in spot velocity occurring as the light beam scans across the photoreceptor, the velocity being greater at the beginning and end of the scan. That change occurs for a constant polygon rotational rate because the spot on the photoreceptor is farther from the facet at the ends of the scan line than it is at the central position. The Minoura patents taught that linearity can be treated as distortion, a known aberration. By introducing third order barrel distortion of the proper amount, the third order term of spot velocity can be cancelled. Then, to third order accuracy, the spot velocity is constant with constant polygon angular velocity.

Although the f-theta lens flattened the field and compensated for scanner non-linearity, compensation for wobble was left to other means. As a result, some existing scanners include a two-element f-theta lens and a wobble correcting element, for a total component count of three. In addition to the drawback of increased component count, forcing distortion onto the f-theta lens design can be a significant penalty. It can complicate the design, increase cost, and produce unwanted aberrations such as third and fifth order field

curvatures in the scan and cross-scan planes. The field curvatures limit the margin for focusing of the spot within the depth of focus acceptable to the photoreceptor. Manufacturing and assembly tolerances have to be reduced, which are not easy to do, in order to be able to preserve an acceptable margin in view of the field curvatures. Thus, it is desirable to have some way to simplify and improve the scanners in that respect.

Further, it is desirable to use plastic lens elements as it further reduces fabrication costs. Heretofore, existing post-facet lens systems do not use plastic lens elements because plastic exhibits greater changes in refractive index with temperature than does glass and that can change the location of the focus beyond acceptable limits. In that regard, the wobble- compensating element in many existing scanner designs causes the cross-scan plane to be several times more sensitive to temperature defocusing than the scan plane. That is because the wobble compensating element magnifies the image of the facet at the photoreceptor and thus magnifies the effect of defocusing. Also, using plastic involves problems such as the indeterminate surface tension of hot plastic. It can cause flat surfaces to deform during cooling and thereby complicate the manufacturing process.

Summary of the Invention

This invention solves the problems outlined above by providing a scanner with a post-facet lens system that flattens the field and compensates for wobble without introducing aberrations by not optically compensating for scanner non-linearity. Thus, the post-facet lens system is less complicated. It is less costly. It avoids the introduction of unwanted aberrations. It removes a major constrain on the design. It can be configured with just two elements, and it still allows non-optical compensation for scanner non-linearity if desired.

Generally a scanner system constructed according to the invention includes a light source for producing a light beam and scanning means for directing the light beam toward a surface to be scanned, such as that of a photoreceptor, in order to scan the light beam along a scan line across the photoreceptor. Those components may be similar in many respects to the components used in existing scanning systems, the scanning means including a rotatable element with a plurality of light reflecting facets. In addition, the scanner system includes a post- facet lens system interposed between the scanning means and the photoreceptor. But the post-facet lens system differs from existing designs. According to a major aspect of the invention, it includes first and second elements configured to compensate for field curvature and wobble without compensating for scanner non-linearity.

Preferably, just two elements are used and they are so disposed that the light beam passes first through the first element and then through the second element. The first element preferably includes first and second surfaces such that the light beam passes from the first surface of the first element to the second surface of the first element, while the second element includes first and second surfaces such that the light beam passes from the first surface of the second element to the second surface of the first element. The first and second surfaces of each of the first and second elements are configured to compensate for field curvature and wobble. In one embodiment of the invention, the lens elements are made of glass. The first surface of the first element is spherical and the second surface is cylindrical, having curvature in the scan plane and essentially no curvature in the cross-scan plane. In addition, the first surface of the second element is spherical while the second surface is toroidal and so configured according to known optical design techniques that it has primary effect in compensating for wobble.

The separation between the first and second surfaces of the second element is substantially uniform in the scan plane so as to minimize aberrations. From the foregoing and subsequent descriptions, it is apparent that all surfaces could be toroidal, but the foregoing arrangement simplifies certain aspects of the design without significantly impairing performance.

In another embodiment of the present invention, plastic lens elements are used. The use of plastic lens elements in the scanner is made possible by the concept of providing lens elements that compensate for field curvature and wobble without introducing aberrations by not optically compensating for scanner non-linearity. Among other things, by not compensating for scanner non- linearity, that removes a severe restraint and permits much better performance of the optical design compared to existing post-facet lens systems that are configured to compensate for scanner non-linearity as well. As a result, the elements can be configured in the cross-scan plane (the wobble correction plane) to have less magnification than elements of existing post-facet lens systems. That is important because less magnification means less change in refractive index and thus less change of focus with changes in temperature. In addition, more of the depth-of-focus budget can be allocated to temperature changes in focus because none is required for field curvature. In other words, the depth- of-focus for any particular design is often allocated to manufacturing tolerances, field curvature, and changes in focus. But with a flat field, none is required for field curvature so that more is available for changes in focus and, therefore, the design can tolerate greater changes in focus. The post-facet plastic elements are configured with curved surfaces. Curving the surfaces overcomes the indeterminate surface tension problem while less magnification reduces the change of focus occurring with changes in temperature. Thus, the scanner enjoys the

benefit of using less costly plastic lens elements without being unacceptably temperature sensitive.

Preferably, the surfaces of the first and second elements are curved in both the scan plane and the cross- scan plane, one embodiment having a spherical first surface of the first element and toroidal surfaces thereafter. Preferably, the post-facet lens system is configured to have less magnification (e.g., less than four) than existing post-facet lens systems (e.g., a system using an f-theta lens) . In addition, the first and second elements of the post-facet lens system are configured to compensate for field curvature and wobble without compensating for scanner non-linearity and they are diffraction limited at one of 300, 400, 500, or 600 dots-per-inch (dpi) (dots-per-2.54cm) .

According to another aspect of the invention, the post-facet lens system may include a base that serves to mechanically link the first and second elements to the predetermined location at which the surface to be scanned is located. The base is at least partially composed of a material that exhibits a thermal coefficient of expansion such that dimensional changes in the structure occurring with changes in temperature at least partially compensates for changes in focus occurring with changes in temperature.

Brief Description of Drawings

FIGURE 1 of the drawings is a diagrammatic representation of a scanner constructed according to the invention;

FIGURE 2 is a diagrammatic representation of the scanner in the scan plane;

FIGURE 3 is an enlarged diagrammatic representation of the post-facet lens system in the scan plane; and FIGURE 4 is a diagrammatic representation of the post-facet lens system in the cross-scan plane.

FIGURE 5 of the drawings is a diagrammatic

representation of a scanner constructed according to another embodiment of the invention;

FIGURE 6 is a diagrammatic representation of the scanner in FIGURE 5 in the scan plane; FIGURE 7 is an enlarged diagrammatic representation of the post-facet lens system in FIGURE 5 in the scan plane; and

FIGURE 8 is a diagrammatic representation of the post-facet lens system in FIGURE 5 in the cross-scan plane.

Description of Invention

Fig. 1 illustrates a scanner system 10 constructed according to the invention. Generally, it includes a light source 11 for generating a light beam 12, a photoreceptor 13, and scanning means 14 for scanning the light beam 12 across the photoreceptor 13. The scanning means 14 includes a rotatable element or polygon 15 with a plurality of light reflecting facets 16 (eight facets being illustrated) . The scanning means 14 may include other known mechanical components that are depicted in Fig. 1 by the polygon 15 rotating about a rotational axis 17 in the direction of an arrow 18.

As the polygon 15 rotates, the light beam 12 is directed by the facets 16 toward an image plane at the photoreceptor 13, scanning across the photoreceptor 13 in a known manner along a scan line 19 from a first end 20 of the scan line 19 past a center 21 of the scan line and on to a second end 22. Thus, the light beam 12 scans in a scan plane defined as a plane containing both the scan line 19 and a central light beam position 23 that is the position occupied by the light beam when it is directed toward the center 21 of the scan line 19 (i.e., the position of the light beam 12 that is illustrated in Fig. l) . Wobble results in the light beam 12 being directed above or below the scan line 19 in a direction perpendicular to the scan plane. In that regard, a plane

containing both the central light beam position 23 and a line perpendicular to the scan plane is referred to as the cross-scan plane (X-scan plane) .

The foregoing components may be similar in many respects to corresponding components in existing scanner systems. The light source 11 may include an infrared laser diode and known conditions optics, for example, it forms a beam at the facets 16 that is collimated in the scan plane and focused in the cross-scan plane. That is done so that the beam has a proper diameter in the scan plane and a proper Numeric Aperture (NA) in the cross- scan plane. In the scan plane, the conditioning optical can be a single aspheric lens of short focal length, for example, or a laser diode collector which is similar to a microscope objective but designed for infrared light and for infinite conjugates. In the cross-scan plane. The optic has the same diode collector, with the addition of a cylinder lens to focus the collimated beam at the facets 16. Many of those things are well known in the art and reference is made to Brueggemann U.S. Patent Nos. 4,512,625, 4,247,160, 4,230,394, 4,662,709, 4,805,974, 4,682,842, 4,624,528 and 4,595,947 for the various details of scanner system construction provided. For that purpose, reference is also made to Starkweather U.S. Patent Nos. 4,475,787, 3,995,110, 3,867,571, 4,040,096, and 4,034,408.

Best Mode for Carrying Out the Invention - First Embodiment with Glass Lenses A major way the scanner system 10 differs from existing designs is in having a post-facet lens system 30 as subsequently described with this reference to Figs. 1- 4. The post-facet lens system 30 is interposed between the facets 16 and the photoreceptor 13, in the optical path of the light beam 12, and it includes a first element 31, and a second element 32. According to a major aspect of the present invention, the first and

second elements 31 and 32 are configured to compensate for field curvature and wobble without compensating for scanner non-linearity, thus without introducing aberrations otherwise experienced with the f-theta lenses. Based on the foregoing and subsequent descriptions, that can be done according to known optical design techniques.

Preferably, the first and second elements 31 and 32 are disposed as illustrated in Figs. 1-4 so that the light beam 12 passes first through the first element 31 and then through the second element 32. Both elements are made of glass of the type commonly designated as BK7. In addition, the first element includes a first surface 33 and a second surface 34 such that the light beam 12 passes from first surface 33 to the second surface 34. Furthermore, the second element includes a first surface 35 and a second surface 36 such that the light beam 12 passes from the first surface 35 to the second surface 36. And, the surfaces 33-36 are so configured that they compensate for field curvature and wobble without introducing aberrations by not optically compensating for scanner non-linearity.

Preferably, compensation for field curvature and wobble without compensating for scanner non-linearity is accomplished by configuring the first and second elements 31 and 32 of the post-facet lens system 30 according to known optical design techniques so that the first surface 33 of the first element 31 is spherical, the second surface 34 of the first element 31 is cylindrical, the first surface 35 of the second element 32 is spherical, and the second surface 36 of the second element is toroidal. Also, the first surface 33 of the first element 31 is concave, the second surface 34 of the first element 31 is convex in the scan plane and flat in the cross-scan plane, the first surface 35 of the second element 32 is concave, and the second surface 36 of the second element 32 is convex. The separation between the

first and second surfaces 35 and 36 of the second element 32 is substantially uniform in the scan plane. Moreover, the second surface 36 of the second element 32 is so configured that it has primary effect in correcting for wobble.

Stated another way, the first surface 33 of the first element 31 is spherical, the second surface 34 of the first element 31 has curvature in the scan plane and essentially no curvature in the cross-scan plane, the first surface 35 of the second element 32 is spherical, and the second surface 36 of the second element 32 has a first curvature in the scan plane and a second different curvature in the cross-scan plane. But from the foregoing and subsequent descriptions, it becomes apparent that the first and second elements 31 and 32 and their surfaces 33-36 can be configured in any of various ways according to known optical design techniques to compensate for curvature and wobble without compensating for scanner non-linearity. All the surfaces 33-36 could be toroidal.

Table A shows a prescription for the post-facet lens system 30 while Table B show a prescription for the scanner system 10 (all dimensions in inches (x 2.54cm) unless indicated otherwise) . Surface 1 refers to surface 33 of the first element 31, surface 2 refers to surface

34 of the first element 31, surface 3 refers to surface

35 of the second element 32, and surface 4 refers to surface 36 of the second element 32.

Table A

Radius of Thickness (distance Intervening Surface Curvature between surfaces) material Remark

B 7 Spherical

Air Cylinder

BK7 Spherical 4 Air Toroidal

Table B

Radius of Thickness (distance Intervening

Surface Curvature between surfaces) material Remark

Wavelength: 632.8 nanometer This design is diffraction limited.

Table B specifies that the design is diffraction limited. That refers to the physical size (FWHM) of the scanning spot produced by the light beam 12 on the photoreceptor 13. Spot size is commonly referred to in terms of dots-per-inch (dpi) (or dots-per-2.54cm) , the reciprocal of the actual spot size. For example, at 300 dots-per-inch, the actual spot size is 1/300 inches measured at what is commonly referred to as Full width Half Max (FWHM) . Table A could also specify that the design is diffraction limited without departing from the broader inventive concepts disclosed. That may be done for a selected resolution of 300 dots-per-inch, 400 dots- per-inch, 500 dots-per-inch, or 600 dots-per-inch, for example.

It is noted that the separation of the surfaces of the second element is substantially uniform in the scan plane (curvatures of -3.132 inches (x2.54cm) and -3.037 inches (x2.54cm) respectively.)

According to another aspect of the invention, the light source 11 is configured to electronically compensate for scanner non-linearity. The light source 11 may be configured, for example, to include a scanning clock generating device for that purpose as described in Shi ada et al. U.S. Patent No. 4,729,617. That patent

is incorporated by reference for the details provided.

Thus, the above described embodiment of the invention provides a scanner with a post-facet lens system that flattens the field and compensates for wobble without compensating for scanner non-linearity. The post-facet lens system is less complicated. It is less costly. It avoids the introduction of unwanted aberrations. It removes many design constraints that otherwise exist. It can be configured with just two elements, and it still allows non-optical compensation for scanner non-linearity if desired.

- Second Embodiment with Plastic Lenses

The second embodiment of the present invention provides a scanner with a post-facet lens system having plastic lens elements. They are also configured to compensate for field curvature and wobble without introducing aberrations by not optically compensating for scanner non-linearity. That relaxes the design so that the element surfaces can be configured with less magnification to reduce the change in focus occurring with changes in temperature. So the scanner is less costly and yet it avoids the problems associated with plastic lens elements. Where desired, a base is added as a mechanical linkage that helps reduce changes in focus otherwise occurring with temperature changes.

In order to overcome the indeterminate surface tension problem of hot plastic, the lenses are configured with curved surfaces. In order to overcome the change in focus accompanying changes in refractive index that plastic exhibits with changes in temperature, they are configured with less magnification (e.g. less than four in the cross-scan plane) than the elements of existing post-facet lens system. And, with a flat field, more of the depth-of-focus budget can be allocated to changes in focus. Based on the foregoing and subsequent descriptions, those things can be done according to known

optical design techniques.

The present embodiment is in many ways similar to the previous embodiment except for the specific lens configuration. Reference is made to Figs. 5-8 wherein like parts are given the same reference numeral as in Figs. 1-4. The first and second elements 31 and 32 are disposed as illustrated in Figs. 5-8 so that the light beam 12 passes first through the first element 131 and then through the second element 132. In addition, the first element includes a first surface 133 and a second surface 134 such that the light beam 12 passes from the first surface 133 to the second surface 134. Furthermore, the second element includes a first surface 135 and a second surface 136 such that the light beam 12 passes from the first surface 135 to the second surface 136. And, the surfaces 133-136 are so configured that they compensate for field curvature and wobble without introducing aberrations by not optically compensating for scanner non-linearity. In that regard, the term "compensate" is not limited to completely correcting for the condition. It includes partial correction as well.

Preferably, compensation for field curvature and wobble without introducing aberrations by not optically compensating for scanner non-linearity is accomplished by configuring the first and second elements 131 and 132 of the post-facet lens system 130 according to known optical design techniques so that the first surface 133 of the first element 131 is spherical, while the second surface 134 of the first element 131 and the first and second surfaces 135 and 136 of the second element 32 are toroidal. Also, the first surface 133 of the first element 131 is concave, and the second surface 134 of the first element 131 is convex, the first surface 135 of the second element 132 is concave, and the second surface of the second element 132 is convex. Moreover, the second surface 136 of the second element 132 is so configured that it has primary effect in correcting for wobble.

Stated another way, the first surface 133 of the first element 131 is spherical, the second surface 134 of the first element 131 has a first curvature in the scan plane and a second different curvature in the cross-scan plane, the first surface 135 of the second element 132 has a third curvature in the scan plane and a fourth different curvature in the cross-scan plane, and the second surface 136 of the second element 131 has a fifth curvature in the scan plane and a sixth different curvature in the cross-scan plane. Unlike the post-facet lens system described in the previous embodiment, the second surface 134 of the first element 131 has curvature in both the scan plane and the cross-scan plane in order to overcome the problem of indeterminate surface tension. From the foregoing and subsequent descriptions, it becomes apparent that the first and second elements 131 and 132 and their surfaces 133-136 can be configured in any of various other ways according to known optical design techniques to compensate for field curvature and wobble without compensating for scanner non-linearity. All the surfaces 133-136 could be toroidal, for example. Table C shows a prescription for the post-facet lens system 130 while Table D shows a prescription for the scanner system 110 for a 780 nm wavelength light beam (dimension in inches (x2.54cm) unless otherwise stated).

Table C

Radius of Thickness (distance Intervening Surface Curvature between surfaces) material Remark

0.250

2.300 3 0.250

PolyC = .a polycarbonate material such as that available from General Electric Co. under the trademark LEXAN.

Table D

Radius of Thickness (distance Intervening Surface Curvature between surfaces) material Remark

PolyC = a polycarbonate material such as that available from General Electric Co. under the trademark LEXAN. This design is diffraction limited.

Tables C and D specify prescriptions in a commonly employed manner. For example, the first line of the prescription in Table D specifies that the facet is flat, that there are 0.800 inches (x2.54cm) from the facet to the next surface (designated 1) , and that there is air between the facet and the surface designated 1. The second line of the prescription specifies that the surface designated 1 (i.e., the first surface 133 of the first element 131) has a curvature in the scan plane of -2.3797 inches (x2.54 cm), that there are 0.250 inches (x2.54cm) to the next surface (designated 2), that there is polycarbonate material between the surface designated 1 and the surface designated 2 (i.e., the first element 131 is composed of polycarbonate) , and that the first surface 133 is spherical. The third line of the prescription specifies that the surface designated 1 has a curvature of -2.3797 inches (x2.54cm) in the X-scan plane (i.e., the cross-scan plane), the curvature being the same because the surface is spherical.

The fourth line of the prescription in Table D specifies that the surface designated 2 (i.e., the second surface 134 of the first element 31) has a first curvature in the scan plane of -1.8583 inches (x2.54cm) . It specifies that there is air between the surface

designated 2 and the surface designate 3, and that the second surface 134 is toroidal. The fifth line of the prescription specifies that the surface designated 2 has a second curvature (different from the first curvature) of -2.4834 inches (x2.54cm) in the X-scan plane. The sixth line of the prescription in Table D specifies that the surface designated 3 (i.e., the first surface 135 of the second element 132) has a third curvature in the scan plane of -4.2680 inches (x2.54cm). It specifies that there are 0.250 inches (x2.54cm) to the next surface (designated 4) , that there is polycarbonate material between the surface designated 3 and the surface designated 4 (i.e., the second element 132 is composed of polycarbonate) , and that the surface designated 3 is toroidal. The seventh line of the prescription specifies that the surface designated 3 has a fourth curvature (different from the third curvature) of -2.0902 inches (x2.54cm) in the X-scan plane.

The eighth line of the prescription in Table D specifies that the surface designated 4 (i.e., the second surface 136 of the second element 132) has a fifth curvature in the scan plane of -3.4064 inches (x2.54cm). It specifies that there are 7.891 inches (x2.54cm) to the next surface (the image at the photoreceptor 13) , that there is air between the surface designated 4 and the photoreceptor 13. The ninth line of the prescription specifies that the surface designated 4 has a sixth curvature (different from the sixth curvature) of -0.8756 inches (x2.54cm) in the X-scan plane. The tenth line of the prescription specifies that the image is flat. The eleventh and twelfth lines provide information about the polycarbonate material used for the elements, and the last line specifies that the design is diffraction limited. It is noted that the separation of the surfaces of the first ^" element 131 is substantially uniform in the cross-scan plane (curvatures of -2.3797 inches (x2.54cm)

and -2.4834 inches (x2.54cm) for the surfaces 133 and 134 respectively) .

As in the previous embodiment, the light source 11 may be configured to electronically compensate for scanner non-linearity. The light source 11 may be configured, for example, to include a scanning clock generating device for that purpose as described in Shimada et al. U.S. Patent No. 4,729,617. That patent is incorporated by reference for the details provided. According to yet another aspect of the invention, the post-facet lens system 110 includes means defining a base or other suitable structure for linking the first and second lens elements to the predetermined location at which the surface to be scanned is located (i.e., the photoreceptor 13) . Such a structure is depicted diagrammatically in Fig. 8 by a structure 40. It is configured according to known techniques and mechanically connected by suitable known means to the first and second lens elements 131 and 132 as depicted in Fig. 8 by broken lines extending from the structure 40 to the first and second elements 131 and 132. The structure 40 is also mechanically connected to the photoreceptor 13 by suitable known means, such as a yoke that connects it at the rotational axis 13a. That is depicted in Fig. 8 by a broken line extending from the structure 40 to the rotational axis 13a.

The structure 40 is at least partially composed of a material having a thermal coefficient of expansion such that dimensional changes in the structure occurring with changes in temperature at least partially compensates for changes in focus occurring with changes in temperature. In other words, a dimensional change in the structure 40 caused by a change in temperature causes a change in the positions of the first and second elements 131 and 132 relative to the photoreceptor 13. That in turn causes an offsetting change in the focal position of the post-facet lens system that at least partially compensates for a

change in focus caused by changes in the refractive index of the elements 131 and 132 accompanying the temperature change. Aluminum exhibits the desired characteristic, but existing fiber reinforced plastics do not. Preferably, the structure 40 is composed of a polycarbonate material having minimum fiber filling for strength. Of course, the structure 40 can be omitted without departing from the broader inventive concepts disclosed whereby the post-facet lens system includes plastic elements.

From the foregoing, it is apparent that the invention can be used for any of various input and output scanner configurations, including a input scanner used for reading a document or an output scanner used for printing a document. Thus, it is intended that the claims cover both input and output scanners.