Spot-defined exposure system for a laser printer

Abstract: An exposure system for electrophotographic apparatus having an aperture plate for masking one or more laser beams. The plate (30) contains one or more apertures (52, 54, 56) which have irregular edges to form a gaussian light distribution for the light beams passing through the apertures. A saw-toothed edge is used with a predetermined amplitude ratio to predictably attenuate the light at the edges which correspond to the process or in-track direction in the exposed image. The gaussian distribution is more tolerable of spacing changes in scan lines and improves the continuity of process direction lines in the finished image.

Description: This invention relates to an exposure system for selectively irradiating a photosensitive member (28) simultaneously with a plurality of laser light beams (22, 24), having a laser light source (20) for producing said laser light beams (22, 24), and deflecting means (32) for simultaneously scanning said laser light beams across the photosensitive member in a cross-track direction (36) that means in the direction of the scan lines (40, 42, 44), and having an aperture plate (30) positioned between the deflecting means and the light source and having a plurality of apertures (60) each of which in use passes at least a portion of a particular laser beam (58) before being scanned by the deflecting means and having a shape which provides a substantially gaussian light distribution in at least one dimension after passing through the aperture. US-A-4 722 581 describes such an exposure system. Many traditional laser printers use a single laser beam which is modulated and scanned across the surface of a photosensitive member. The selective exposure of a photosensitive member provided by this system creates a latent image which can be developed and transferred to the hard copy output medium, such as a sheet of paper. Since the beam must scan the entire page area to create the image, the throughput or speed of the printer is dependent upon the scanning time. In order to produce faster printers with comparable quality outputs, some printers use more than one modulated laser beam in the scanning and exposure process. For example, a system which uses three simultaneously modulated laser beams which are scanned together across the photosensitive surface or member can expose the member in one third the time required by the single laser scanning system. Each of the three beams can scan adjacent lines in the image being created or they can be interlaced with other lines to be scanned in subsequent scan passes. The individual laser beams can be derived from individual laser devices, such as laser diodes, or from a single laser device whose beam has been split, before modulation, into two or more beams. U.S. Patent 4,884,857, issued on December 5, 1989 to the same assignee as the present invention discloses a multiple-beam laser scanning system. One of the difficulties of multiple-beam laser scanning systems is that it is difficult to maintain exact placement of the beams in the process or in-track direction of the scan. This is needed to prevent lines or characters which have continuity in the process direction from appearing irregular. This is especially true in the case where laser diodes are used as the light producing device and the optical components of the system effectively enlarge the spacing between the diodes when imaged onto the photosensitive member. One way to reduce the tight tolerance requirements needed for the diode spacing, and hence the laser beam spacing, is disclosed in the referenced patent. As taught therein, an aperture plate is positioned between the stationary beams and the optical system which provides the deflection necessary to scan the beams across the photosensitive member. The aperture plate contains apertures or openings through which the beams pass. The beam impinging upon an aperture is larger than the aperture, thus causing only the portion of the beam passed by the aperture to reach the photosensitive member. This effectively causes the separation between the apertures in the plate to govern the scan line spacing on the photosensitive member. The plate maintains that spacing even with some variation in the laser beam position due to physical or electrical changes in manufacturing or operation of the laser diode array. Even though the arrangement disclosed in the referenced patent is advantageous in certain applications, it can experience the difficulty of irregular lines when the spacing provided by the aperture is not exact or when process movement of interlaced scans causes some in-track or process direction deviation from the exact position of the scanned line. The process or in-track scan direction refers to the direction in which the photosensitive member travels past the imaged laser beam. The scan or cross-track direction refers to the direction in which the beam moves across the photosensitive member as a result of the scanning action of the beam deflecting system. The terms process and scan are also defined in the referenced patent. In order to overcome the irregularity aspect of the prior art, the present invention uses an aperture plate which has apertures designed to give other than a sharp edge to the beam which passes through the aperture. This is contrary to conventional practice for apertures. Normally, an aperture is used for sharply cutting off the edges of the beam passing through the aperture. Several prior art patents show various systems which use apertures of various configurations to perform different functions, none of which is similar to the present invention. U.S. Patents 4,321,630, issued on March 23, 1982, and 4,057,342, issued on November 8, 1977, are representative of references which use specially shaped slits or apertures across the width of the image to regulate or shape the image-wide light which is irradiated onto the photosensitive member or received from the original document being copied. See Figures 9 through 13 of the 4,057,342 patent and Figure 8 of the 4,321,630 patent. U.S. Patents 3,813,140, issued on May 28, 1974, and 4,725,729, issued on February 16, 1988, both represent references which use apertures or openings having irregular shapes to compensate for other nonlinear properties of the scanning system which results in light changes being dependent upon the scan angle. See Figures 4 and 5 of the 4,725,729 patent and Figure 6 of the 3,813,140 patent. In both representative cases, the beam of light passes through the apertures at different positions, depending upon the angle of the light ray or beam which enters the aperture. U. S. Patents 3,055,263, issued on September 25, 1962, and 4,433,991, issued on February 28, 1984, are representative of references which use specially shaped apertures or openings to determine distances or locations of object images passing through the aperture. See Figure 7 of the 3,055,263 patent and Figures 1 and 2 of the 4,433,911 patent. It is an object of this invention, to provide an exposure system which improves the process direction continuity of adjacent scan lines. This is achieved in that said apertures are defined by at least two edges which have predetermined irregularities spaced along said edges to partially attenuate some of the light passing through the aperture in the vicinity of said edges, such that the attenuated laser light beam, being oriented in a direction after being scanned by the deflection means (32), has a gaussian distribution of the light in the process direction (38) that means perpendicular to said scan lines on the photosensitive member. There is disclosed herein a new and useful exposure system for irradiating a photosensitive member in an electrophotographic device, such as a printer, copier, or duplicator. The exposure system conditions the light beams impinging upon the photosensitive member to provide a gaussian light intensity distribution or profile in the process direction that means perpendicular to said scan lines on the photosensitive member. This affords higher quality adjacent scan line continuity even with variations in scan line position caused by process deviation or laser beam misalignment. The invention is particularly useful in systems employing multiple-spot beam scanning. According to a specific embodiment of the invention, a multiple-spot laser beam passes through an aperture plate containing an aperture for each beam. After passing through the apertures, the beams are deflected by a rotating polygon to simultaneously scan the lines across the photosensitive member. Each aperture is defined by a generally rectangular opening which is slightly larger than the laser beam in a dimension ultimately corresponding to the cross-track or scan direction of the beam. The opening is slightly smaller than the laser beam in a dimension which ultimately corresponds to the in-track or process direction of the beam. Two sides of the opening are defined by irregular, non-smooth edges having a saw-toothed shape. These edges regulate the cut-off of the light at the edges and determine the light intensity distribution, or profile, in the process direction of the scan line. By proper selection of the amplitude of the saw-toothed shape, the desired gaussian distribution can be achieved with an inexpensive device and without appreciable loss in light energy. This reduces the effect of discontinuities in the output image due to changes in scan line spacing from that produced by prior art aperture plates. A value between .175 and .225 for the ratio of amplitude to the repeating distance of the saw-toothed edges provides an acceptable compromise between intensity profile, intensity attenuation, and tolerance of scan line variations. Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawings, in which: Figure 1 is a diagramatic view of an exposure system constructed according to this invention; Figure 2 is a view illustrating the light intensity distribution after passing through an aperture constructed according to the prior art; Figure 3 is a plan view of an aperture plate constructed according to a specific embodiment of this invention; Figure 4 is a view illustrating the light intensity distribution after passing through an aperture constructed according to the embodiment shown in Figure 3; Figure 5 depicts the desired or ideal intensity profile for a three-line scan; Figure 6 depicts a three-line scan and the corresponding intensity profile for laser beams passing through the prior art aperture plate and having a space therebetween; Figure 7 depicts a three-line scan and the corresponding intensity profile for laser beams passing through the prior art aperture plate and being overlapped; Figure 8 illustrates a three-line scan intensity profile of overlapping beams which have passed through the aperture plate of this invention; Figure 9 illustrates a three-line scan intensity profile of spaced beams which have passed through the aperture plate of this invention; Figure 10 is a partial view of an aperture illustrating dimensional quantities; Figure 11 depicts an actual one-beam intensity profile produced by the present invention for a "T" value of 1.0; Figure 12 depicts an actual one-beam intensity profile produced by the present invention for a "T" value of 1.4; and Figure 13 depicts an actual one-beam intensity profile produced by the present invention for a "T" value of 1.8. Throughout the following description, similar reference characters refer to similar elements or members in all of the figures of the drawings. Referring now to the drawings, and to Figure 1 in particular, there is shown an exposure system constructed according to this invention. A light source 20 is used to produce several parallel and stationary light beams, such as beams 22, 24 and 26. These beams may be produced by individual laser diodes or may be produced by dividing the beam from a single laser source. Although not shown in Figure 1, the individual laser or light beams would be modulated appropriately with the information content necessary to selectively expose the image area on the photosensitive member 28. The light beams 22, 24 and 26 pass through apertures in the aperture plate 30 and are deflected by the polygon 32 which rotates around the axis 34. Such rotation causes the light beams to scan across the photosensitive member 28 in the direction indicated by the double-ended arrow 36. This is referred to as the cross-track or scan direction. Although the three scan lines depicted in Figure 1 have a gap therebetween in the process or in-track direction, indicated by arrow 38, the actual spacing desired in an actual system would be much less than indicated. For a high quality output, the spacing between the scan lines 40, 42 and 44 should be essentially equal to zero. For clarity reasons, some of the conventional pre-scan and post-scan optical elements normally required with such systems are not shown in Figure 1. Also not shown in Figure 1 are some of the other process stations which would work in conjunction with the exposure system. These would include, normally, the toning or developing station, the transfer station which transfers the developed image on the photosensitive member to a hard copy medium such as a sheet of paper, and a fixing or fusing station for permanently fusing the transferred image to the paper. The aperture plate 30 used in the exposure system of Figure 1 contains three apertures or openings which are aligned to prevent the passage of a portion of the three light beams. By using the aperture plate 30, the shape of the beams impinging upon the photosensitive member 28 can be controlled. The aperture plate 30 also allows the spacing between the scanned lines to be maintained more accurately even with differences or changes in the direction and spacing of the beams emitted from the light source 20. However, conventional aperture plates with smooth rectangular openings or apertures exhibit certain properties which can be detrimental to the overall image quality of the produced image, especially in the process direction. Figure 2 illustrates a prior art aperture geometry and the resulting light intensity distribution of a laser or light beam after passing through the aperture. According to Figure 2, the aperture 46 has a rectangular shape which is smaller in the vertical direction than the size of the laser beam 48 which impinges upon the aperture. The distribution of the light intensity "I" as a function of the vertical direction or dimension "y" is shown by the curve 50 in Figure 2. As can be seen, the distribution of the light intensity has a substantially square wave characteristic in that all of the light intensity is contained within the region corresponding to the opening or aperture 46. Substantially no light exists in the regions above and below the aperture opening. It is to be understood that, in an actual system, there may be some magnification or demagnification involved in the light beam before it impinges upon the photosensitive member. Consequently, the size of the spot does not necessarily correspond one-for-one with the size of the aperture. The important thing to note here is that the distribution of the light intensity drops off sharply because of the attenuating or masking effect of the aperture, as will be described in more detail hereinafter. Difficulties with this type of distribution can occur in producing smooth and continuous image lines in the process direction. Figure 3 illustrates, in greater detail, the aperture plate 30 shown in Figure 1. According to Figure 3, the apertures or openings 52, 54 and 56 are impinged by the laser beams 22, 24 and 26, respectively, shown in Figure 1. Although not drawn to scale in Figure 3, the apertures in the plate 30 are no larger than is necessary to mask one single laser beam with some additional area on each side of the laser beam which is not masked. The non-masked area effectively leaves the laser beam unaltered in the cross-track or scan direction. As shown in Figure 3, the apertures 52, 54 and 56 are generally rectangular in shape but the two opposite sides thereof, which are generally parallel, have irregular or non-smooth surfaces as compared to the prior art aperture geometry. The saw-toothed shape of the two sides defining each aperture is used to define and develop a specific light intensity distribution or profile of the laser beam passing through the aperture. It is the enhanced distribution of the light intensity which provides the advantages of the invention over the prior art. Figure 4 illustrates the operation of the apertures constructed according to this invention and the resulting light intensity distribution or profile diagram. As shown in Figure 4, a laser beam 58 impinging upon the aperture 60 produces the light intensity illustrated by curve 62 after it passes through the aperture. As can be seen from curve 62, the light intensity distribution is substantially a gaussian distribution, which is more normal for light sources than the square wave type distribution, shown in Figure 2, provided by the prior art apertures. As will be indicated herein, gaussian distribution provides for a much more predictable and tolerable result in the output image, primarily for the vertical or process lines in the produced image which rely on accurate spacing and distribution between the scan lines. Figure 5 illustrates the ideal or desired condition for spacing and light distribution between scan lines on the output medium and, intermediately, on the photosensitive member. Ideally, the spacing between the scan lines 64, 66 and 68 should be zero as shown in Figure 5. With a sharp or step-type intensity profile, and neglecting any small intensity variations of the light between the sharp cut-off edges the vertical intensity profile of the three line combination, used here as an example, is represented by curve 70. This indicates that the intensity of the line in the vertical direction is constant over all three lines. While this is the desired condition, it should be appreciated that this is difficult to achieve with actual apparatus and some overlapping or gaps between the lines can occur which can detrimentally affect the continuity of the light intensity in the vertical direction. Figures 6 and 7 illustrate what happens to the light distribution in the vertical direction, when the scan lines have gaps or overlap, with apertures having the prior art geometry. As shown in Figure 6, a gap 63 between the scan lines 64, 66 and 68 causes gaps 65 in the output curve 72 because of the lack of light intensity existing in the three laser beams which form the scan lines outside of the area passing through the aperture. The distribution shown in curve 72 is detrimental to the overall quality of the produced image since lines in the process direction will not be uniform. A similar reduction in quality is experienced when the scan lines 64, 66 and 68 overlap each other as shown in Figure 7. With this condition, the output curve 74 includes spikes occurring at the area where the scan lines overlap, such as spikes 76 and 78. These are caused by the full intensity of the light adding at the region where the scan lines overlap. Thus, as can be seen in Figures 6 and 7, the difficult task of maintaining the scan lines in exact alignment to maintain high image quality is necessary. This is difficult to do in actual practice, especially with laser diode arrays. Alignment problems also exist when the scan lines are interlaced with other scan lines in the process direction. This type of deviation occurs mainly due to the mechanics of the interlacing system rather than to the spacing between the laser emitters. Whatever the cause of the spacing changes, the fact that the prior art aperture system produces a degraded image when the spacing is not ultimate is an important and difficult to achieve spacing requirement. Figure 8 represents the light intensity distribution realized when the scan lines overlap slightly when using the aperture plate of this invention. Since the distribution of each single laser beam is substantially gaussian, as shown by curve 62 in Figure 4, there is not the sharp cutoff of light intensity as the distance from the center of the beam is varied. In other words, some overlapping of the beams occur even when perfectly aligned and, when the spacing is decreased, the intensity is increased somewhat, but not by the amount of the step function dictated by the prior art light intensity distribution. In this regard, curve 80 of Figure 8 represents the composite three-line scan profile where there is some overlapping between the scan lines, with the curve peaks 82 and 84 being the only variation in the composite output from a desired smooth output. In Figure 9, the curve 86 corresponds to the situation where the scan lines have a gap therebetween, as in Figure 6. In this case, the dimples 88 and 90 correspond to the gap area between the scan lines. However, since the gaussian distribution includes some light intensity even within these gaps, the dimples 88 and 90 in curve 86 are not as pronounced as the irregularities or gaps 65 in the curve 72 of Figure 6. Therefore the overall quality and continuity of vertical lines produced by the distribution shown in Figure 9 is much better than that produced by prior art aperture plates. Although a saw-toothed shape is illustrated in Figures 3 and 4 for the irregular sides of the apertures, it is within the contemplation of this invention that other shapes may be used to produce the gaussian distribution of the light passing through the apertures, and thus the advantages in image quality afforded thereby. For example, the irregular side shape could be more sinusoidal than shown in the figures, or could be staggered so that the opposite side is not a mirror image of the other side. Whatever the configuration, the irregularity of the aperture openings causes a predictable amount of light to pass through the aperture in the vicinity of the defining edges. Various manufacturing processes can be used to create the irregular surfaces which define the apertures. A particularly useful method is to use a small wire in an EDM process to etch or form the edges of the aperture from a smooth surface. A wire with a radius of .1 millimeter can be used for this purpose. Figure 10 illustrates dimensions useful in defining the degree of amplitude or variance of the irregular surface from a smooth surface. The dimension 92 represents the amplitude of the irregular surface and changing this dimension can affect the overall distribution of the light passing through the aperture. The dimension 94 represents the repeating or cyclic dimension of the saw-toothed triangular shapes which, according to this specific embodiment, provide the gaussian distribution of the laser spot. An actual operating system constructed according to this invention used a dimension 94 value of .8 millimeters, and a dimension 92 value of .1 x "T", where T = 1.0. Figures 11, 12 and 13 are actual curves of spot intensity distributions produced on an optical bench from an aperture having the saw-toothed irregularities described herein. The amplitude of the teeth were varied to determine the response of the output beam with variations in the amplitude of the irregular surface. In the case where "T" = 1.0, curve 98 of Figure 11 represents the spot intensity profile. Although a gaussian distribution of the edges is apparent from Figure 11, the discontinuities at the top of the distribution curve 98 are not as smooth as desired. According to Figure 12, the curve 100 indicates the intensity profile in the case where the amplitude is governed by the value for "T" of 1.4, where the actual amplitude is equal to .1 x "T" millimeters. As can be seen in Figure 12, the overall response of the spot is approaching a true gaussian distribution. Figure 13 represents a case where "T" = 1.8. As shown by curve 102, a very well defined gaussian distribution is developed with the amplitude set at this value. A disadvantage of using higher "T" values is the fact that some of the amplitude of the laser light is lost in the resulting spot. Consequently, "T" values between the range of 1.4 and 1.8 provide the most economical spot profile consistent with maintaining adequate energy levels and efficiencies. The actual amplitude values used to develop the curves 100 and 102 in Figures 12 and 13 respectively, are .14 and .18 millimeters. In relation to Figure 10, the ratio of the amplitude of the saw-toothed shape, which distinguishes these edges from smooth edges, and the repeating distance of the saw-toothed shape, is between the values of .175 and .225, assuming that dimension 94 is .8 millimeters and dimension 92 is between .14 and .18 millimeters. The teachings of this invention provide for enhanced output quality under conditions where the scan lines are not exactly spaced as desired. It is emphasized that numerous changes may be made in the above-described system without departing from the teachings of the invention. It is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawings, shall be interpreted as illustrative rather than limiting.

Claims: Exposure system for selectively irradiating a photosensitive member (28) simultaneously with a plurality of laser light beams (22, 24), having a laser light source (20) for producing said laser light beams (22, 24), and deflecting means (32) for simultaneously scanning said laser light beams across the photosensitive member in a cross-track direction (36) that means in the direction of the scan lines (40, 42, 44), and having an aperture plate (30) positioned between the deflecting means and the light source and having a plurality of apertures (60) each of which in use passes at least a portion of a particular laser beam (58) before being scanned by the deflecting means and having a shape which provides a substantially gaussian light distribution in at least one dimension after passing through the aperture, characterized in that said apertures are defined by at least two edges which have predetermined irregularities spaced along said edges to partially attenuate some of the light passing through the aperture in the vicinity of said edges, such that the attenuated laser light beam, being oriented in a direction after being scanned by the deflection means (32), has a gaussian distribution of the light in the process direction (38) that means perpendicular to said scan lines on the photosensitive member. Exposure system of claim 1 wherein the apertures have at least two non-smooth edges which are generally parallel to each other. Exposure system of claim 2 wherein the non-smooth edges of the apertures have a saw-toothed shape. Exposure system of claim 3 wherein the amplitude of the saw-toothed shape is between .14 and .18 millimeters. Exposure system of claim 2 wherein the ratio of the amplitude of the saw-toothed shape, which distinguishes the edges from smooth edges, and the repeating distance of the saw-toothed shape, is between the values of .175 and .225. The exposure system of claim 1 wherein the light source includes a plurality of laser diodes to produce a plurality of stationary light beams, the deflecting means scans all of the produced light beams across the photosensitive member, and the aperture plate has a plurality of apertures each aligned with a different stationary light beam.

Inventors: PRAKASH RAVINDER, WOLFE LARRY LANCE, PRAKASH, RAVINDER, WOLFE, LARRY LANCE
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: G03G15
IPC codes: H04N1, G03G15, G02B26, B41J2