Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2003-03-20
2004-09-28
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S248000
Reexamination Certificate
active
06798437
ABSTRACT:
Priority to German Patent Application No. 102 12 532.5, filed Mar. 21, 2002 and hereby incorporated by reference herein, is claimed.
BACKGROUND INFORMATION
The present invention relates to a method for imaging a printing form, using an imaging device containing a laser diode bar which has a number n of individually controllable laser diodes, the imaging spots of the activated laser diodes lying essentially in a row on the printing form, for the case that at least one laser diode on the laser diode bar cannot be activated and that the laser diode bar has a maximum number m of activatable laser diodes whose neighboring imaging spots on the printing form have a distance a. The present invention further relates to a method for imaging a printing form using a number b of imaging devices which each contain a laser diode bar having a number n of individually controllable laser diodes, the imaging spots of the activated laser diodes of the number b of imaging devices lying essentially in a row on the printing form, for the case that at least one laser diode on one of the b laser diode bars cannot be activated and that all laser diode bars have a number m of activatable laser diodes whose neighboring imaging spots on the printing form have a distance a.
In printing form imaging units or printing units of printing presses featuring imaging devices (so-called “direct imaging printing units”), frequently, several imaging beams, in particular generated by laser diodes, are used concurrently to efficiently reduce the imaging time for the imaging of the two-dimensional surface of the printing form. If a non-redundant imaging method is used, that is, if the imaging beams are moved over the two-dimensional surface of the printing form in such a manner that the location of each printing dot to be placed is passed over by an imaging beam exactly once, the imaging time for the total surface to be imaged using an imaging device having n imaging beams is reduced to (1
) of the time. Further shortening can be achieved in an equally efficient manner by parallel use of b imaging devices which each image sections of the printing form in a non-redundant manner, analogously to the above-described procedure. Then, the imaging time for the total surface to be imaged is reduced to (1/b) of the time; to be more precise, using b imaging devices having n imaging beams to (1/(bn)) of the time.
Thus, the substantial shortening of the imaging time by non-redundant parallelization strongly depends on the number of available (activatable) or used imaging beams, since, without parallelization, the imaging time for the total surface to be imaged is equal to the number of all printing dots to be placed multiplied by the exposure time thereof, to be more precise, multiplied by the maximum available time for imaging one printing dot when passing over the locations of the surface in a non-redundant manner. Non-redundancy may be desirable not only because of these time considerations but also for reasons of mounting space or cost.
In order to pass a number of imaging beams (independently of whether they are arranged on one or on several imaging devices) over the locations of a two-dimensional surface of a printing form on which printing dots are to be placed, it is required to observe certain feed rules for the passage of locations that are imaged in a preceding step with respect to locations that are imaged in a subsequent step. These feed rules must be strictly complied with, especially if in an imaging step, n imaging beams place n printing dots at locations which do not lie densely together on the printing form, i.e., whose distance is not the minimum printing dot spacing p (typically 10 micrometers). In order to achieve a dense imaging, printing dots are placed between already imaged printing dots in a subsequent imaging step. This procedure is also known by the term “interleaving method” (interleaving). An interleaving method for imaging printing forms is characterized, for example, in German Patent Application No. DE 100 31 915 A1: Given a minimum printing dot spacing p, for a number of n imaging channels on a setting line, which are equally spaced from each other and whose neighboring imaging spots on the printing form have a distance a which is a multiple of the minimum printing dot spacing p, a non-redundant feed by the distance (np) in the direction of the setting line is guaranteed if the natural numbers n and (a/p) are relatively prime.
The implementation of a non-redundant interleaving method according to DE 100 31 915 A1 critically depends on that n equally spaced imaging beams are available, i.e., activatable, on a setting line. The strategy proposed in this document in case of failure or inoperability of an imaging beam is to use the largest, still connected section of the equally spaced imaging channels if the intention is to avoid non-imaged strips on the printing form and to ensure a constant imaging quality. It is clear that, for implementing a non-redundant interleaving method according to this document, it is required to choose a number of imaging beams of the still connected section that is relatively prime to the multiple distance (a/p). Pursuant to this strategy, failures or inoperabilities of further imaging channels result in only very short sections of the originally n parallel imaging beams. Consequently, the imaging time considerably increases with the decrease of parallelization still available. For example, in the worst case that in each case one imaging beam in the middle of the largest connected section on the setting line fails, the imaging time is in each case increased to the double, that is, for several failures, to a multiple of the original parallelized imaging time. In practice, this is completely unacceptable.
When using laser diode bars in imaging devices, failure or inoperability of a laser diode is generally particularly critical if each imaging beam is generated by exactly one laser diode, because in order to restore the original performance, it is necessary to replace the whole laser diode bar. For economic reasons alone, this is not sensible since the other laser diodes on the bar are generally still functional, that is, the laser diode bar has not become completely inoperable.
In U.S. Pat. No. 6,181,362 B1, it is proposed to assign each imaging beam two laser diodes on a laser diode bar. To image a printing form, one laser diode is used per imaging beam. If the first laser diode fails, the second laser diode is used instead. However, the document leaves open how to proceed if the redundant laser diodes for generating the imaging beam fail at the same time.
As an alternative to this, U.S. Pat. No. 6,252,622 B1 proposes to assign each imaging beam a first laser diode on a first laser diode bar and a second laser diode on a second laser diode bar. To image a printing form, one laser diode of one of the two laser diode bars is used per imaging beam. If the first laser diode on the first laser diode bar in one imaging channel fails, the second laser diode on the second laser diode bar is used instead. However, the document leaves open how to proceed if the redundant laser diodes for generating the imaging beam fail at the same time.
The design approaches of U.S. Pat. No. 6,181,362 B1 and U.S. Pat. No. 6,252,622 B1 have in common that, roughly speaking, for the generation of each imaging beam, simply a replacement laser diode is kept available in case of inoperability. In consequence, this is cost-intensive. To guarantee a reliable strategy, double the number of laser diodes are required right from the beginning. In practice, many of the replacement diodes are a priori not needed at all. Both documents fail to provide a fundamental solution to the problem of how to proceed in case of a failure of one imaging beam or of several imaging beams.
SUMMARY OF THE INVENTION
An object of the present invention is to carry out a fast imaging of a printing form using an imaging device which contains a laser diode bar having n laser diodes of which one or more laser diodes have failed.
The
Davidson Davidson & Kappel LLC
Heidelberger Druckmaschinen AG
Pham Hai
LandOfFree
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