Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-06-29
2003-02-04
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S246000
Reexamination Certificate
active
06515693
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to a printhead in an image producing apparatus and more particularly to a calibration station for adjusting output power of a printhead that writes onto an exposure-sensitive medium and to a method for calibrating output power of such a printhead.
BACKGROUND OF THE INVENTION
Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material. This procedure avoids the high cost and time required to actually produce printing plates and also avoids setting-up a high-speed, high-volume, printing press to produce a single example of an intended image. In the absence of pre-press proofing, the intended image may require several corrections and be reproduced several times to satisfy customer requirements which result in reduced profits. By utilizing pre-press color proofing, time and money are saved.
A laser thermal printer having half-tone color proofing capabilities is disclosed in commonly assigned U.S. Pat. No. 5,268,708 titled “Laser Thermal Printer With An Automatic Material Supply” issued Dec. 7, 1993, in the name of R. Jack Harshbarger et al. The Harshbarger et al., device is capable of forming an image on a sheet of thermal print receiver by transferring colorant from a roll (i.e., web) of colorant donor material to the thermal print receiver. This is achieved by applying a sufficient amount of thermal energy to the colorant donor material to form the image on the thermal print receiver. This apparatus generally comprises a material supply assembly; a lathe bed scanning subsystem, which includes a lathe bed scanning frame, a translation drive, a translation stage member, a laser printhead, and a vacuum imaging drum; and exit transports for exit of thermal print receiver and colorant donor material from the printer.
The Harshbarger et al. device writes an image using a plurality of laser disposed in an array at the laser printhead. In order to write the image, individual lasers are energized in coordination with imaging and timing signals to write the output image onto the donor material in a continuous swath. As is well known in the laser thermal printing art, there can be differences in output power from one laser to the next. A printer of this type can employ 20 or more lasers, each of which may vary from its neighbors in terms of the dependence of its output power upon wavelength. Because the achieved output density is dependent upon the applied power absorbed by the image-recording medium, imaging anomalies such as banding can result when lasers in the array emit different power levels, causing a print to be unacceptable for its intended purpose.
For printers of the type disclosed in the Harshbarger et al. patent, calibration procedures are used to compensate for laser-to-laser output power differences. Laser calibration procedures are also employed in the data recording art, such as for writing digital data onto optical disks. As some examples, U.S. Pat. No. 5,687,156 (Hurst, Fr.), U.S. Pat. No. 5,185,733 (Finkelstein, et al.), and U.S. Pat. No. 5,216,659 (Call et al.) disclose techniques used to calibrate lasers in optical disk writing. However, for purposes of recording digital data, represented in sequences of binary 1/0 data, only two discrete levels of laser power are needed. In contrast, when writing image data using a device such as is disclosed in the Harshbarger et al. patent, output laser power is related to achievable density, so that power must be accurately adjustable over a range, wherein each discrete value within the range can be correlated to corresponding density of donor colorant transferred to the receiver. Even when applying or withholding only one level of laser power to expose a halftone image on an image-recording material whose image density varies with exposure, that applied power level must be set accurately to the intended value in order to render the image with fidelity.
There are detailed calibration procedures developed to systematically adjust the power output of each laser in order to minimize banding and related anomalies. U.S. Pat. Nos. 5,921,221 and 5,323,179, Sanger et al., disclose use of a calibration station and sensor for laser calibration in a multichannel printer. The method disclosed in the Sanger et al. patents involves both direct measurement of laser power and measurement of densities for colorant output on a receiver medium. From the detailed description of the laser calibration process, it is clear that it would be advantageous to eliminate steps in the overall calibration procedure to simplify this procedure where possible.
While such methods developed for power calibration compensate for differences in laser output power, there is room for improvement. It has been observed that even if two writing lasers are very closely matched in terms of measured output power, the lasers may yet achieve different efficiencies in donor colorant transfer. It is known that the donor colorant exhibits more efficient transfer for some wavelengths of the light source than for others. Moreover, while each writing laser in the array is manufactured to emit wavelengths within a narrow range, there are differences in laser fabrication that result in diode lasers having slightly different wavelengths. For example, while the specified wavelength of each laser in an array may be 840 nm, nominal, the actual wavelengths obtained may range from 832 nm to 846 nm. It is known that each diode laser provides the substantial portion of its output within a narrow 1 nm band. Alternatives to compensate for wavelength effects, such as manufacturing diode lasers to within tighter wavelength tolerances or manufacturing a donor colorant medium that is less wavelength-dependent are very costly.
There are methods for tuning some types of lasers to adjust frequency, thereby adjusting laser output wavelength. As one example, U.S. Pat. No. 5,033,114, Jayaraman et al., discloses a tunable laser used in data communications. A feedback control loop for an optogalvanic glow-discharge modulator comprises beamsplitters and detectors used to control modulation of the output laser to achieve a desired wavelength. As part of the feedback loop, an interference filter is used to select that portion of the sensed feedback signal that is needed to achieve output frequency and wavelength tuning. These tuning procedures are not applicable to diode lasers, however, and maintenance of all emission at a specific wavelength is not required for the type of image-recording material used in imaging applications.
Interference filters have been used as part of a calibration feedback loop for laser frequency tuning control, as disclosed in the Jayaraman et al. patent noted above. Interference filters have also been used to isolate specific wavelength components of a sensor signal, such as is disclosed in U.S. Pat. No. 5,275,327, Watkins et al., for laser-based sensing during an arc welding operation. Interference filter transmission profiles have been adapted to isolate specific wavelengths for measurement by a sensor, but without adaptation to a response profile of an imaging medium.
As a result of wavelength dependence, an operator calibrating a printhead may be required to measure laser wavelength for each diode laser in an array and to compensate by making power adjustments corresponding to each wavelength. Alternatively, an operator may be forced to perform additional cycles of calibration, preparation, and measurement procedures, such as manually adjusting power output to achieve uniform density response. A radiation source may also change the wavelength distribution of its emitted power during exposure of an image, so that a suitably prepared feedback-power-controller would be desirable to maintain constant deposited energy in the image-recording medium. Thus, there is a need for a simple and inexpensive solution that allows a calibration procedure to adjust lasers for output power for a wavelength-sensitive image recording material.
There is a need for a printer
Haas Daniel D.
Tredwell Timothy J.
Eastman Kodak Company
Nelson Adrian Blish
Pham Hai
LandOfFree
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