Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-04-05
2003-11-18
Tran, Huan (Department: 2863)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S255000, C702S001000, C702S085000, C702S086000
Reexamination Certificate
active
06650353
ABSTRACT:
BACKGROUND OF THE INVENTION
Imagesetters and platesetters are used to expose the media that are used in many conventional offset printing systems. Imagesetters are typically used to expose film that is then used to make the plates for the printing system. Platesetters are used to directly expose the plates.
In imagesetters and platesetters, throughput and uptime are critical metrics. These systems typically operate in commercial environments. Their throughput is often used as the criteria for selecting between the various commercially available systems.
The cycle time, and consequently throughput, for a platesetter or imagesetter is largely dictated by the time that the imaging engine requires to expose the media. Most conventional systems expose the media by scanning. In a common implementation, the plate or film media is fixed to the outside or inside of a drum and then scanned with a laser source in a raster fashion. The laser's dot is moved longitudinally along the drum's axis, while the drum is rotated under the dot. As a result, by modulating the laser, the media is selectively exposed in a continuous helical scan.
In these drum-scanning systems, a number of criticalities can dictate the cycle time. One limitation can be the speed at which the laser is modulated. This is related to the resolution that is required on the media. Another limitation is laser power. As the scan rates increase, the power that the laser generates must also be increased since the time to expose each pixel on the media decreases.
To overcome some of these inherent limitations, systems are being proposed that use a combination of a light source and a spatial light modulator (SLM). Such modulators are usually based on liquid crystal technology. In one example, the light source is pulsed with a fixed periodicity. The data determining the plate exposure is then used to drive the spatial light modulator. This results in the media being exposed in a series of separate sub-images in the fashion of a stepper. As a result, the speed of operation is no longer limited by the rate at which the laser can be modulated or the power that can be extracted from that single laser.
SUMMARY OF THE INVENTION
One issue that arises in these SLM-based systems concerns the focus setting for optics that projects the light through the SLM. The process is more complex than conventional systems that merely focus the laser spot onto the drum. As a result, even if the focus setting is accurately determined in the factory, over time as components age and with thermal cycling, the imagesetter or platesetter can drift out of its best focus.
In general, according to one aspect, the invention features a calibration system for a platesetter or imagesetter. It comprises a media drum and a carriage that includes a light source and a spatial light modulator for selectively exposing media that is held against the drum.
This calibration system comprises a calibration sensor. The spatial light modulator is scanned relative to this calibration sensor. The controller analyzes the response of the calibration sensor to develop focus information that is used to control how light is projected through the spatial light modulator and onto the drum.
In the preferred embodiment, the calibration sensor comprises a photodiode and a slit aperture that enable the detection of responses of individual elements of the spatial light modulator.
In operation, the controller compares exposure levels provided by the spatial light modulator for different focus settings. In the preferred embodiment, the controller also compiles the dark levels provided by the spatial light modulator. Preferably, the controller generates a focus setting based on the best contrast ratio between the exposure levels and the dark levels provided by the spatial light modulator. In this way, the system optimizes focus for the contrast ratio, which is a figure of merit for the system's performance.
REFERENCES:
patent: 5072239 (1991-12-01), Mitcham et al.
patent: 5323002 (1994-06-01), Sampsell et al.
patent: 437766 (1991-07-01), None
patent: 0791863 (1995-10-01), None
patent: WO 91/15843 (1991-10-01), None
patent: 0069631 (2000-11-01), None
Allen Roy D
Bailey Dwayne
Comeau Bryan M.
Romano David Joel
AGFA Corporation
Houston Grant
Sabourin Robert A.
Tran Huan
LandOfFree
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