Abstract:
Disclosed are methods and apparatus for homogenizing the exposure of different beams in a multi beam-plotter. The plotter typically operates on films or plates.

Description:
The present application claims benefit from U.S. Provisional Application No. 60/167,916 entitled “A Method for Homogenizing the Exposure of the Different Beams in a Multi Beam Plotter” and filed on Nov. 30, 1999, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of plotting on film or plate and more specifically to plotting by a multi-beam laser array. 
     BACKGROUND OF THE INVENTION 
     A multi-beam plotter is a device that uses an array of laser beams to produce an image on a light sensitive media like film or offset plate. In most such machines, the beams in the array are adjacent to one another. In order to receive a good image without artifacts, the intensity of the different laser beams on the exposed media should have a very similar value. Plotters usually have a light calibration system that uses a light detector to calibrate all the laser beams to work at the same intensity. In spite of the light calibration system, the exposed image looks, sometimes, not quite homogeneous. This non-homogeneous exposure can be caused, for example, by a slight difference in the wavelength of the different beams and a non-flat response of the media to the different wavelengths. 
     In order to solve the non-homogeneity problem, each beam has to be operated separately, its exposure result investigated and the intensity of each diode corrected accordingly. In a multi-beam plotter, exposing with a single beam takes, relatively, a very long time. Hence, This calibration procedure is very tedious and cumbersome. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for homogenizing the exposure of different beams in a multi-beam plotter, comprising the steps of: 
     creating digital data, wherein each row comprises data to be exposed by one of said beams; and 
     using said plotter to expose said digital data on a substrate, 
     wherein, following each exposure cycle by all of said beams moving in a first direction relative to said substrate, said beams move in a second direction, generally perpendicular to said first direction, relative to said substrate, 
     and wherein the distance traversed by said beams in said second direction is smaller than the sum of widths covered by all of said beams. 
     It is a further object of the present invention to provide a method for homogenizing the exposure of different beams in a multi-beam plotter, comprising the steps of creating 
     creating digital data, wherein each row comprises data to be exposed by one of said beams and wherein each column comprises a number of consecutive data elements, said number being equal to the number of said beams. 
     using said plotter to expose said digital data on a substrate, 
     wherein, following each exposure cycle by all of said beams moving in a first direction relative to said substrate, said beams move in a second direction, generally perpendicular to said first direction, relative to said substrate, 
     and wherein the distance traversed by said beams in said second direction is smaller than the sum of widths covered by all of said beams. 
     It is yet another object of the present invention to provide a method for homogenizing the exposure of different beams in a multi-beam plotter, comprising the steps of: 
     creating digital data, wherein each row comprises data to be exposed by one of said beams; and 
     using said plotter to expose said digital data on a substrate, 
     wherein, following each exposure cycle by all of said beams moving in a first direction relative to said substrate, said beams move in a second direction, generally perpendicular to said first direction, relative to said substrate, 
     and wherein the distance traversed by said beams in said second direction is equal to the width of one column covered by each one of said beams. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method to expose, in one pass of the machine, an image that is composed of a series of calibration patches, each one exposed by a different beam. The method combines exposing an especially designed file and a special mode of the carriage motion, carrying the exposure head. 
     It is an aim of the present invention to provide an exposed pattern that will facilitate the separate investigation of the light intensity of each laser beam, by exposing solid blocks, each exposed by one predetermined beam. 
     The digital file built for this purpose comprises, in each row, data to be exposed by a single beam. Each row is constructed of one-pixel width columns, comprising data-columns (ON) and non-data-columns (OFF). The distance between two adjacent data-columns in each row equals the number of working beams. The rows are similar to each other, but with a data-column shift of one pixel (in the direction of the carriage movement) between rows assigned to adjacent beams. 
     Normally, when exposing a pattern using a multi-beam array of N beams, N adjacent columns will be exposed on each revolution of the drum, followed by a carriage movement in the carriage movement direction, perpendicular to the exposure direction. The distance of the carriage movement equals the width of the array, resulting in a gap of the same width between consecutive columns exposed by each beam. 
     According to the present invention, while exposing the special file the carriage moves, after each revolution of the drum, a distance that is equal to the distance it would cover if the exposure had been done with less than the total number of beams, while the data distribution between the different beams does not change. In the extreme case, the carriage covers a distance corresponding to only one working beam. This reduced motion distance results in the shrinking of the exposed data in the direction of the carriage movement during exposure; i.e. the columns appear on the media closer than in the original file. If the distance is of one working beam, the columns exposed by each beam appear adjacent to one another, without the gaps that exist in the original file. Because of the one-pixel shifts in the design of the original file, a different beam exposes each row. The final result on the media is a slanted column of patches, each exposed by a different beam. This pattern gives the operator a tool for investigating the interaction between the media and each single beam, and for correcting the beam intensity accordingly. 
     Compared to the conventional method of exposing, creating these one-beam patches in the method of the present invention reduces the exposure time for creating the N patches by a factor equal to the number of working 
     Attention is drawn now to  FIG. 1 , showing a first example of a digital pattern to be exposed according to the present invention and to  FIG. 2 , showing the resulting exposed pattern of patches. In this example, the number of working beams is 4 and the carriage motion distance equals that of a one-beam system. 
     The digital pattern  20  comprises data patches. During the first revolution of the drum, patch  24  is exposed by beam B 1 , patch  26  is exposed by beam B 2 , patch  28  is exposed by beam B 3  and patch  30  is exposed by beam B 4 . Following this revolution, the carriage moves in the direction indicated by arrow  32 , a distance that equals one column width. On the other hand, the data system supplies each laser diode with the same data it would have been supplied in a normal working carriage motion, resulting in patch  34  being exposed by beam B 1 , etc. 
       FIG. 2  shows the resulting exposed pattern, where patch  36  was exposed by beam B 1 , patch  38  was exposed by beam B 2 , patch  40  was exposed by beam B 3  and patch  42  was exposed by beam B 4 . 
     Attention is drawn now to  FIG. 3 , showing a second example of a digital pattern to be exposed according to the present invention and to  FIG. 4 , showing the resulting exposed pattern of patches. Again, the number of working beams is  4  and the carriage motion distance equals that of a one-beam system. 
     The digital pattern  44  comprises data patches. During the first revolution of the drum, patches  46  and  48  are exposed by beam B 1 , patches  50  and  52  are exposed by beam B 2 , patches  54  and  56  are exposed by beam  83  and patches  58  and  60  are exposed by beam B 4 . Following this revolution, the carriage moves in the direction indicated by arrow  62 , a distance that equals one column width. On the other hand, the data system supplies each laser diode with the same data it would have been supplied in a normal working carriage motion, resulting in patches  64  and  66  being exposed by beam B 1 , etc. 
       FIG. 4  shows the resulting exposed pattern, where pattern  68  was exposed by beam B 1 , pattern  70  was exposed by beam B 2 , pattern  72  was exposed by beam B 3  aid pattern  74  was exposed by beam B 4 . 
     It will be understood by any person skilled in the art, that the present invention, described hereinabove in conjunction with a drum plotter, can also be applied to flat-bed plotters 
     It will also be understood that the platters that may benefit from the present invention may be film plotters (imagesetters) or platesetters, using multi-beam exposure systems. 
     It will further be understood that the present invention may also be applied to plateless printing systems, wherein the plate imaging is performed on the system.