Raster output scanner employing rotating prism facet tracking

Abstract: A raster scanning system is disclosed which utilizes a rotating prism (18) for facet tracking. The rotating prism has a plurality of facets (20a-20d, 20a'-20d') and each facet is parallel to an opposite facet. A light beam (14) enters through one facet (20c) of the rotating prism and exits the corresponding parallel facet (20c'). The light beam passing through the rotating prism (18) deflects and since the the prism rotates the light beam also shifts. The prism shifts the light beam in such a manner to keep the beam at the center of a mirror facet (24) of a rotating polygon mirror (22).

Description: This invention relates to a raster scanning system and, more particularly, to a raster scanning system which uses a rotating multifaceted member. A conventional raster scanning system utilizes a light source, a modulator and a multi-faceted rotating polygon mirror as a scanning element. The light source, which can be a laser source, emits a light beam and sends it to a modulator. The modulator receives pixel information for modulating the light beam. The modulated light beam will be directed onto a rotating polygon mirror. The rotating polygon mirror reflects the light beam and also 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 beam can be utilized to scan a document at the input of an imaging system or can be used to impinge upon a photographic film or a photosensitive medium, such as a xerographic drum at the output of the imaging system. Many of the conventional raster scanning systems exhibit a modulated light beam which has a varying intensity over one scan line. For instance, the modulated light beam at the start of the scan line and at the end of the scan line has less intensity than at the center of the scan line. If the intensity variation (difference between the highest intensity and the lowest intensity) becomes more than a certain specified limit, it can be observed on the printed document as lighter print at the edges of the document and as darker print at the center of the document. The problem comes mainly from a facet tracking feature used in the modulators. Facet tracking is a feature of an underfilled scanning system that shifts the beam while the mirror facet is rotating in order to keep the beam at the center of the mirror facet. A piezoelectric transducer in the modulator resonates, thus shifting the beam to track the center of the mirror facet. The transducer utilizes a variable carrier frequency which gradually increases as the beam moves from the start of the scan line to the end of the scan line. This causes the transducer to oscillate with increasing frequency. The increasing frequency of the transducer's oscillation causes the beam to shift while it is deflected by a Bragg cell. However, from basic mechanics, a transducer performs best at its resonant frequency. As the frequency changes and moves away from the resonant frequency, the performance of the transducer degrades and therefore the intensity of the reflected beam decreases. Typically, the transducers are fabricated to have a resonant frequency at the mid range of the variable frequency which is at the center of the scan line. Therefore, at the start and at the end of the scan line, the performance of the transducer is less than the peak performance of the transducer which is at the center of the scan line. This is the main cause of the nonuniformity of the intensity of the beam since the light is being deflected more efficiently where the transducer is at its peak performance. Also, conventional raster scanning systems such as tri-level, highlight color imaging, which use a high frequency for modulation, have an inherent problem called "beating". Beating occurs since the frequency used for facet tracking is close to the frequency of modulation. Beating is a phenomenon happening between two close frequencies. When beating happens at some points the amplitudes of the two waves add up to each other and at some points the amplitudes cancel each other. A beating problem on a printed document can be observed as periodic dark and light stripes. It is an object of the present invention to provide a raster scanning apparatus in which these problems are overcome. According to the invention there is provided a raster scanning apparatus comprising a light source for emitting a light beam, and a scanning means mounted for rotation about a first axis, the scanning means comprising a plurality of facets arranged to scan the light beam in succession as the scanning means rotates, characterised by a light beam shifting means positioned in the optical path between the light source and the scanning means and mounted for rotation about a second axis parallel to the first axis, the beam shifting means being arranged to shift the light beam to track the centre of each facet as that facet scans the beam. The apparatus of the invention reduces the nonuniformity of the intensity of the light beam by utilizing a rotating prism for facet tracking. A raster scanning apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of a raster scanning system incorporating a rotating prism for facet tracking; Figure 2 is a magnified view of a portion of the raster scanning system of Figure 1; and Figure 3 is another magnified view of a portion of the raster scanning system of Figure 1 showing the rotation of the rotating prism for shifting a light beam to track the center of a facet of a rotating polygon. Referring to Figures 1 and 2, there is shown a raster scanning system 10 of this invention. A helium neon laser is used as a light source 12. The light source 12 emits a light beam 14 onto a mirror 16 which is used to reflect the light beam 14 onto a rotating prism 18. The light beam passes through the rotating prism 18 which has a plurality of facets 20a, 20b, 20c, 20d, 20a', 20b', 20c' and 20d'. Then the light beam 14 is sent out onto a rotating polygon mirror 22 which has a plurality of mirror facets 24 for reflecting the light beam 14. The reflected light revolves about an axis near the center of rotation of the rotating polygon mirror 22. This reflected light 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 photosensitive medium, such as a photoreceptor 26, in the output mode as a raster output scanner. Referring to Figures 2 and 3, the rotating prism 18 is utilized to achieve facet tracking by shifting the beam 14 to track the center of the mirror facet 24 while the rotating polygon mirror 22 is rotating. The rotating prism 18 deflects the light beam 14. Since the prism 18 rotates, it causes the deflected light beam 14 to shift. Facets 20a, 20b, 20c and 20d are parallel to facets 20a', 20b', 20c', and 20d' respectively. The beam 14 always enters from one facet and exits from an exit facet which is parallel to the entrance facet. For example in Figure 2, the beam 14 enters the facet 20c and exits the facet 20c'. Also, the prism 18 is designed to deflect the light in such a manner that the light beam exiting the prism 18 is always parallel to the direction of the light entering the prism 18. The rotating prism 18 rotates in the opposite direction of the rotating polygon mirror 22 in order to deflect the light in such a manner to track the center of the mirror facet 24. As one mirror facet 24 reaches the end of the scan line, the light beam 14 deflected by the rotating prism 18 reaches the end of the exit facet i.e. 20c'. Then the light beam 14 moves onto the next exit facet. This causes the light beam 14 to jump to the center of the next mirror facet 24 which will be scanning the next scan line. The speed and the number of facets of the rotating prism 18 can be designed to match the requirements of the rotating polygon mirror 22. However, the number of the facets of the rotating prism 18 does not have to be equal to the number of the facets of the rotating polygon mirror 22. The raster scanning system of this invention eliminates the non-uniformity of the conventional raster scanning systems. Since this system is an optically passive system and does not use any frequency for shifting the beam 14, the intensity of the light beam 14 stays uniform. Since the facet tracking of this invention eliminates the conventional facet tracking systems and the ramp frequency used by the conventional facet tracking systems, the aforementioned problem of beating is also eliminated. In addition, this system eliminates the requirement for a modulator with a facet tracking feature. Therefore, an electronically modulated light source, such as a laser diode, can now be used in a scanning system with the facet tracking suggested by this invention without a need for a separate modulator.

Claims: A raster scanning apparatus comprising a light source (12) for emitting a light beam (14), and a scanning means (22) mounted for rotation about a first axis, the scanning means comprising a plurality of facets (24) arranged to scan the light beam in succession as the scanning means rotates, characterised by a light beam shifting means (18) positioned in the optical path between the light source (12) and the scanning means (22) and mounted for rotation about a second axis parallel to the first axis, the beam shifting means (18) being arranged to shift the light beam to track the centre of each facet (24) as that facet scans the beam. The apparatus of claim 1 wherein the beam shifting means (18) is mounted for rotation in the opposite rotational direction to the rotational direction of the scanning means (22). The apparatus of claim 1 or claim 2, wherein said rotating light beam shifting means (18) is a prism. The apparatus of claim 3, wherein said prism (18) is transparent and the light beam (14) passes through it. The apparatus of claim 4, wherein said prism (18) has an even number of facets, and each facet (20a-20d) of said prism has a corresponding parallel facet (20a'-20d') opposite thereto, said prism being arranged relative to said light source to receive said light beam by one of its facets and emit said light beam by a respective corresponding parallel facet.

Inventors: FEINBERG AMATZIA, FEINBERG, AMATZIA
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: G02B26
IPC codes: G02B26