Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2001-04-23
2002-10-15
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S315000
Reexamination Certificate
active
06466359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a multi-beam exposure apparatus that performs exposure by imaging a multi-beam light source onto recording materials such as photoreceptors, light-sensitive materials and heat-sensitive materials.
2. Prior Art
Lithographic platemaking using PS (presensitized) plates is quite common in the printing industry. To print a color image, reading with a scanner is done in three separated colors R (red), G (green) and B (blue), the image signals for these three colors are converted to color separated halftone signals for four colors C (cyan), M (magenta), Y (yellow) and Bk (black), light-sensitive materials called “lith films” are exposed for the respective colors by means of light beams modulated on the basis of the resulting color separated halftone signals so as to prepare lith plates for the respective colors, and halftone images for the respective colors are formed by exposure on PS plates using the prepared lith plates. As a result, lithographic printing plates of four colors C, M, Y and Bk are produced.
In recent years, direct platemaking and CTP (computer to plate) are drawing increasing attention since they contribute to simplifying the platlemaking process and shortening the time taken by it. These techniques eliminate the lith films and printing plates are made by drawing images directly on PS plates with light beams such as laser beams using the color separated halftone signals for four colors C, M, Y and Bk that have been acquired with the scanner system.
In order to produce print images of higher contrast and quality, the recording density mutst be increased up to 2400-2540 dpi so that the spot diameter of light beams that form halftone dots is reduced to about 10.0-10.6 &mgr;m. While it is necessary to form finer beam spots by increasing the density of printed images, a further reduction of the platemaking time is required and PS plates as large as 1100 mm×950 mm are desirably exposed in the shortest possible time, say, within a period of several minutes. This requirement for accomplishing high-density exposure of large areas exists not only in the printing field but also in many image recording areas.
In the case of the above-mentioned large-sized PS plates, high-density exposure with a single light beam requires that the drum (external drum) fitted with the PS plate should rotate for main scan at a speed of 10,000 rpm or more. However, from structural and control viewpoints, this need is almost impossible to meet at low cost.
Since high-density exposure with a single light beam cannot be achieved in a shorter time, it has been proposed that the exposure time be shortened by drawing several lines with a plurality of light beams. An apparatus operating on this principle is called a multi-beam exposure apparatus and relevant prior art examples are disclosed in U.S. Pat. No. 5,517,359, Japanese Patent Application (JPA) No. 1864901994 and International Publication (WO) No. 97/27065.
U.S. Pat. No. 5,517,359 teaches an apparatus for imaging the light from a laser diode on a multi-channel linear light valve; the light from
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emitters of a high-power (ca. 1 W) BALD (broad area laser diode) is imaged onto the linear light valve by means of a lens array in which the pitch between lenses is substantially the same as the pitch of the emitters; the images of the respective emitters are superposed and the small linear light valve is illuminated (coupled) with a high-power (20 W in total) LD (laser diode) array so that the desired image is formed on a heat-sensitive or light-sensitive material to realize effective CTP.
Since the small linear light valve array is illuminated with the 20 W high-power LD array, the apparatus requires fine adjustment of the relative positions of the two arrays. This poses two problems. First, if the LD light source fails, it must be replaced by a new LD array but the necessary adjustment is too complex to be performed by the user and the apparatus has to be brought to the factory or any appropriate service center where time-consuming repair and expensive parts replacement are performed. Second, in order to increase the reliability of the apparatus, the operating life of the high-power LD array has to be extended but this requires water cooling of the LD array, making the structure of the apparatus complex and increasing its cost.
The multi-beam recording apparatus disclosed in Japanese Patent Application (JPA) No. 186490/1994 comprises a plurality of light source portions each consisting of a discrete LD and collimating unit and which are arranged in a specified pattern to illuminate a perforated plate having a plurality of apertures formed in a pattern either identical or similar to the pattern of arrangement of the light source portions; light beams passing through the apertures are directed to imaging (reducing) optics so that they are imaged on a light-sensitive material (recording surface). With this recording apparatus, the individual light source portions need not be positioned in the specified pattern of arrangement in high precision and there is no need for prolonged adjustment but high-quality images can be recorded after simple adjustment.
If this apparatus is used to perform high-speed recording of large-sized PS plates, as many as several tens of light source portions must be used and in order to arrange them in a specified pattern, a light source unit of a comparatively large size must be employed.
The apparatus described in JPA No. 186490/1994 does not require as precise positioning as in the case where no perforated plate is used but, on the other hand, the apertures in the perforated plate must be aligned with the exit centers of the light beams from the respective LDs and replacement of a failing LD requires reasonably high positional precision and involves a complicated procedure. Second, due to the use of many expensive high-power LDs, the cost of the light source unit increases and the overall system reliability of the apparatus decreases. Thirdly, the light beams from all light source portions in the large-sized light source unit must be received by lenses, a parabolic mirror and other optical components of high precision and large size and, in addition, complex reducing (imaging) optics are required to reduce these light beams to a sufficiently small size on the recording surface of the light-sensitive material; these contribute to increasing the cost of the apparatus.
International Publication (WO) 97/27065 discloses an imaging apparatus for exposing platemaking materials and a platemaking apparatus using the same. In these apparatus, a plurality of 0.5-1.0 W optical fiber coupled LDs are arranged and a pattern of light beams emerging from the fibers are passed through telecentric optics so that they are imaged (exposed at smaller scale) on a platemaking material (heat-sensitive material or heat ablation material) fitted on an external drum so that the position and size of the exposing spot will have a specified precision in spite of changes in the distance from the exit end face of each fiber to the recording surface of the platemaking material.
If this apparatus is used in order to expose platemaking materials of the above-indicated large size within a duration on the order of several minutes, as many as several tens of LDs have to be used but then the cost of the apparatus increases and its overall system reliability decreases. If the number of LDs is reduced to, say, 24, the exposure time prolongs and the productivity decreases.
Ordinary laser printers use a polygonal mirror which deflects a single laser beam for main scan in a direction parallel to the rotating axis of a photoreceptor drum and they feature a much smaller size and a lower density than the platemaking apparatus. Japanese Utility Model Application (JMA) No. 137916/1986 proposes a laser printer which uses an acousto-optic light deflector (AOD) to deflect a laser beam in an auxiliary scanning direction (in which the photoreceptor drum rotates) so that a pluality of lines (raster) are
Fuji Photo Film Co. , Ltd.
Hindi Omar Z.
Mack Ricky
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