Raster scanning system utilizing overfilled polygon facet design with non-reflective facet edges

Abstract: An overfilled polygon scanning system is implemented by first determining which portions of the polygon facet can be formed with a degree of flatness within specification. Typically, the flatness goes increasingly out of spec adjacent to the facet edges, causing stray light and subsequent reduction in contrast in a subsequent exposure at a photoreceptor image plane. Once the areas of the non-spec portions are determined, these areas which constitute the oversized portion of the polygon are coated with a non-reflective material. This coating eliminates light reflected from these areas and assures that the light is reflected only along the non-coated (in-spec) portion of each facet.

Description: BACKGROUND AND INFORMATION DISCLOSURE STATEMENT This invention relates to a raster scanning system utilizing a plurality of overfilled polygon facets for forming latent images at a photosensitive medium according to the preamble of claim 1. Many prior art raster output scanners (ROS) utilize a rotating polygon having flat reflective surfaces, or facets, in parallel with the axis of rotation of the polygon. In a typical system, a beam is emitted from a light source such as a helium-neon laser. The light is directed through a pre-polygon conditioning optics, modulated according to an input electrical signal, onto the rotating polygon surfaces. The high speed rotation of the polygon, typically in a 3 to 15 krpm range, then scans the beam through a post-polygon conditioning lens and images the laser spot across the full process width of a photosensitive image plane. In these prior art ROS systems, the pre-polygon conditioning optics typically are incorporated in an underfilled facet design; e.g., the light beam directed against the rotating polygon illuminates only a portion of each rotating surface (facet). Overfilled facet designs, where the light beam completely illuminates each facet and a small portion of adjacent facets, have been used to some degree, but have not gained wide acceptance. Comparing the two designs, in an overfilled design the facet size required to produce a given spot size at the image plane is greatly reduced allowing many more facets to be accommodated on the same diameter polygon. This, in turn, permits the scan system to operate it at a relatively low rotation rate permitting the use of less powerful (and less expensive) polygon motor drives. This advantage has, heretofore, been more than offset, by two factors: low throughput efficiency and non-uniform illumination at the polygon facets. In order to tolerate the low efficiency (typically 10 to 15%), a higher powered laser diode is required. The non-uniformity problem occurs because the light beam directed across the rotating polygon has a Gaussian spot shape which has been expanded so that more than one facet of the polygon is illuminated. As the polygon rotates to scan the spot across an output medium, the amount of light reflected to the medium varies because the facets are sampling different parts of the Gaussian illumination profile and the effective area of the polygon is changing. Attempts to compensate for this illumination uniformity have taken several forms. One method is to change the spot intensity with scan angle by using a smile correction algorithm which changes the output of the laser diode to maintain uniform exposure at the photoreceptor image plane. Another method, disclosed in document US-A-4 941 271 is to introduce an aspherical lens system between the laser source and the rotating polygon. The aspheric lens transforms the non-uniform Gausian light beam profile into a colliminated flat intensity beam directed across the width of the polygon facets, the beam overfilling each facet. With either of these two methods, however, there still remains a residual non-uniformity problem which is inherent in the basic design of any polygon scanner. The mirrored surfaces (facets) are manufactured with a high degree of flatness precision. Typically ¼ wavelength of light is the tolerance, but the facets cannot be held to this tolerance at the facet edges. Any deviation from the flatness specification near the corner edges of each facet will result in a degradation of the spot imaged at the photoreceptor. The amount of degradation of the spot depends upon the magnitude of departure from flatness and the areas of the facet over which the departure exists. From GB-A-2191303, a device for providing an alternating scan of light beam in a plane is known which device uses a polgonal prism having odd numbers of sides. An incident light beam is deflected for scanning by the rotating prism. The sides of the prism includes stripes at its edges parallel to the axis of rotation. These absorbent or reflecting stripes prevent the beam whose cross-section has a small surface which is always greater than "zero" from dividing into two parts when it illuminates simulatenously to contigious surfaces of the side of the prism. US-A-4277141 discloses a multi-faceted mirror and a method of making such a mirror. The multi-faceted mirror is formed of individual plain mirrors bonded to a polygonal support element by cement in positions that are accurately located independently of the support element. Gaps between the mirrors and the support element are filled with cement and trim strips can be added to extend between and over the edges of the mirrors. It is the object of the present invention provide a raster scanning system as mentioned at the beginning of the description which provides a scanning beam having a uniform intensity. The raster scanning system according to the present invention comprises a laser light source arranged to provide a modulated output light beam, said output light beam being directed along an optical path which extends from the light source to said photosensitive medium; means for expanding the modulated beams so that the beam, when incident on the facets is wider than each facet; means for modifying the expanded modulated beam to a controlled energy distribution beam; a polygon having at least one reflective facet positioned in the optical path of said modulated beam and adapted to scan said beam across said medium, each of said facets has two strips of non-reflective material coated on the respective facet adjacent each facet edge, strip extending along the full height of the facet; and post polygon lenses to provide a focused scan beam across the medium. Some relevant prior art disclosures are found in the following patents: U.S. Patent No. 3,944,323 to Starkweather, assigned to Xerox Corporation, discloses a variable spot size scanning system. A means is provided for covering a portion of the lower half of the facets. This system is intended to provide a more continuous gray scale for a scanner. U.S. Patent No. 4,357,071 to Mankel et al. discloses an optical fault seeking apparatus. A polygon mirror is shaped with flat facets and concave corners. The apparatus is used for scanning fabrics for faults in workmanship. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic bottom plan view of a polygon ROS system operating in an overfilled facet design, and including a preferred embodiment of the polygon of the present invention. Fig. 2 shows a front edge view of a portion of the polygon of Fig. 1 showing edge corner strips coated with a non-reflective coating. Fig. 3 shows an illumination profile for an overfilled polygon facet used to determine facet areas to be coated. DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1 of the drawings, there is shown a ROS system 10 of the type having an overfilled polygon facet design. As shown, a laser diode 12 serves as the source of high intensity radiation. The laser diode 12 is self-modulating and the output beam of light is modulated in conformance with the information contained in a video signal. The output of diode 12, in a preferred embodiment, varies in response to a smile correction algorithm in the input video signal. The modulated beam is expanded by the two lens beam expanders 14, 16 following which the expanded beam of light 18 passes through a line-like aperture or slit 20′ formed by aperture plate 20. The beam 18 passes through pre-polygon system 22. Lens system 22 focuses the beam 18 to a controlled energy distribution beam 24 which is reflected from mirrored facets 30 of rotating polygon scanner 32. Facets 30 of polygon 32 are mirrored surfaces having a flatness within ¼ wavelength of light tolerance save for a narrow strip adjacent each edge. As shown in Figs. 2 and 3, strips 30A through 30N at the corner edges of each facet are coated with a non-reflective coating such as Sikken's™ epoxy top coat, black #463-3-8. The width of coatings 30A - 30N is determined during polygon design by directing an incident beam 24, as shown in Fig. 3, to the polygon surface. Each facet 30 has an area AR which has a flatness within the required specification, while areas 30A, 30B, 30N are outside the required flatness. Each facet is thus designed to be oversized by the areas 30A, 30B, 30N. It is these areas which are rotated into the brightest part of the scan. A typical width for each strip 30A, 30N is between 0.1mm and 0.5mm. The polygon design is them completed with facets 30 oversized by the sum of the two edge strip widths. Area 30A through 30N are coated, prior to polygon balancing, with the above-identified non-reflective coating to eliminate light which would be reflected from this area. The coating was applied using a catalyst solution or converter and a thinner (three parts of paint to one of converter and thinner). The area was masked to define the areas to be painted, and an air brush was used to apply the paint. With the rotation of polygon 32 in the direction indicated by the arrow (Figure 1), light beam 36 is reflected from each illuminated facet 30 and passes through a series of post-polygon lenses (not shown, but conventional in the art) which image the beam 36 across the full process width at a photosensitive image plane 46. The beam 36 strikes the image plane with a uniform intensity. While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth, but is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: A raster scanning system (10) utilizing a plurality of overfilled polygon facets for forming latent images at a photosensitive medium, comprising a laser light source (12) arranged to provide a modulated output light beam, said output light beam being directed along an optical path which extends from the light source to said photosensitive medium; means (14,16) for expanding the modulated beam so that the beam, when incident on the facets, is wider than each facet; means (22) for modifying the expanded modulated beam (18) to a controlled energy distribution beam; a polygon (32) having at least one reflective facet (30) positioned in the optical path of said modulated beam and adapted to scan said beam across said medium; post polygon lenses to provide a focused scan beam across the medium characterised in that each of said facets has two strips of non-reflective material coated on the respective facet adjacent each facet edge, each strip extending along the full height of the facet. The scanning system of claim 1, wherein said strips have a width between 0.1 and 0.5 mm.

Inventors: APPEL JAMES J, APPEL, JAMES J.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: G02B26
IPC codes: G02B26, G02B5, H04N1