Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2001-03-16
2003-07-08
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S144000
Reexamination Certificate
active
06590600
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for non-impact printing in general and more specifically to printing with scanning light beams on laser-thermal dye transfer medium using pulse width modulation.
BACKGROUND OF THE INVENTION
Pulse-width modulation, and pulse-number modulation, may be applied to various types of radiation sources used in non-impact printers: radiation sources incapable of emitting intermediate amounts of radiation, or radiation sources whose emissions do not attain consistent amplitudes; or radiation sources whose control is more easily or less expensively accomplished by pulse modulation rather than by amplitude modulation. Pulse-number modulation is described in Col. 2, Lines 9-19, of U.S. Pat. No. 4,375,064:
“the total optical energy applied to a picture element, i.e., the exposure H, is defined by the following expression:
H=N·&Dgr;h
where &Dgr;h is the optical energy which is applied to a photo-sensitive material by the semiconductor laser in response to one high frequency pulse and N is the number of high-frequency pulses (pulse number) which are provided according to the level of an input video signal for the picture element.”
Note that the original symbols E and &Dgr;e in this quotation have been changed to H and &Dgr;h, respectively, in order to conform to the nomenclature used in the remainder of this document. U.S. Pat. No. 4,375,064 modifies pulse-number modulation to produce pulse-width modulation in Col. 3, Lines 28-34, and its FIG. 3C by generating a single pulse whose duration of activation encompasses all of the pulses of an equivalent pulse-number modulated optical output. Even though the explanation of pulse-number modulation and FIG. 2 of U.S. Pat. No. 4,375,064 rely upon exposure in units of (energy per unit area) to attain a density level, the “Summary of the Invention” states in Col. 3, Lines 52-54, that its object is “to provide a signal with which a light beam is subjected to binary modulation to record an image having half-tones”, not continuous-tone images. The “Description of the Preferred Embodiments” specifies in Col. 4, Lines 25-29, that “It is desirable that the recording sheet . . . be a silver salt or electronic type which is capable of producing half-tones and is sensitive to the wavelength (red or infrared) of the semiconductor laser beam.” U.S. Pat. No. 4,375,064 never mentions the important roles of the optical output's irradiance profile on the image-recording medium and of the scanning speed in determining: the actual exposure deposited an any single location on that medium; and whether the resulting image is a halftone of binary density distribution or a continuous tone of many controlled density levels.
Pulse-width modulation is applied to laser-thermal imaging in commonly assigned U.S. Pat. No. 5,241,328 to improve “the linearity of the tone scale” in the Abstract. Printing of an intermediate density is accomplished in Col. 2, Lines 46-51, by an “LDCL circuit” which “immediately drives the laser from a threshold near-on value to an optimum ‘full-on’ condition, and then leaves the laser full-on, for a time corresponding to the weighted digital value of that respective binary word” and then reduces the laser to a threshold near-on value by Col. 6, Lines 8-9, and Claim 1, sufficiently low to not transfer further colorant as inferred from the zero density for both the pulse-width modulated and the amplitude modulated tonescales at the lower right corner of FIG. 5 and Col. 8, Lines 10-12 of U.S. Pat. No. 5,241,328. The fact that the pulse-width modulated tonescale curve attains the same image density as the horizontally sloped saturation regime of the amplitude-modulated tonescale curve at high exposure (the upper left corner of the graph of the image-recording medium's exposure response in FIG. 5 of U.S. Pat. No. 5,241,328) implies that the laser power during the pulsed exposures is great enough that an exposure lasting longer than the time for the writing spot to traverse its own full-width at half maximum attains the medium's saturated density level. In the Abstract, the apparatus scans “a finely focused spot of light from the laser along a line”, but the linkage between the size of that spot and its scanning speed in determining whether a halftone or a continuous-tone image results is not discussed
U.S. Pat. No. 6,060,208 uses pulse-width modulation to produce the visual semblance of intermediate image densities by superimposing a rudimentary line halftone upon an externally supplied halftone image at a spatial scale finer than the lines per inch at which that supplied halftone is encoded. “Pulsing a laser” exposes binary donors “creating tiny gap areas in the coverage of the colorant . . . much smaller than the screen dot . . . while operating in the saturation portion of the transfer function for the colorant” according to the Abstract and reiterated in Col. 2, Lines 46-51, in Col. 4, Lines 37-47, and again in Claim 1, to produce intermediate densities less than would be produced by uniformly deposited colorant in the image. The experiment reported in Col. 6, Lines 18-28, notes, “The spot size of the laser or other energy source used to transfer colorant from a donor to the substrate is typically a significant fraction of the area of each pixel. Therefore, one cannot simply turn off the laser (or other energy source) for 2% of the time to produce the desired apparent optical density of the deposited colorant. Varying duty cycle of a laser in a laser color proofer in the range of about 50% to about 80% can provide a useful range of apparent optical densities in some cases.” U.S. Pat. No. 6,060,208 does not teach a reason why the spot size of the laser precludes use of a 2% off-pulse to reduce image density, or how to predict the actual duty cycle that would be required to produce a desired exposure or density. The consequences of scanning speed for pulse duration and beam size are not discussed.
The first three embodiments of commonly assigned U.S. Pat. No. 5,874,981 are specifically stated in Col. 4, Lines 38-45, and references to T
traverse
, to complete exposure deposition by one pulse of the source in less time than its beam requires to traverse its own width on the donor. The second three embodiments encompass pulse durations longer than T
traverse
. U.S. Pat. No. 5,874,981 uses shaped pulses in addition to simple binary pulses of the exposing light to exploit intrinsic characteristics of laser-thermal dye transfer to obtain desirable tonescales in the consequent images while simplifying the electronic control of the light sources in Col. 2, Lines 34-38. Neither reproduction of desired exposure profiles nor adjustment of exposure deposited by multiple sources are stated as objects of this patent.
Multiple-source printers produce better image quality and reduced artifacts when all of the sources are matched to deposit identical exposure profiles generating identical image densities with identical spatial extent. This matching of exposure profiles deposited by all sources in a multiple-source printhead is sometimes called “printhead balance.” Imbalance of a printhead can produce undesirable streaking artifacts in its printed images because some scanlines are darker than others. The mechanism of creating some scanlines darker than others on a continuous-tone image-recording medium might be simply the difference in that medium's response to some sources emitting different powers than other sources in that printhead. Commonly assigned U.S. Pat. No. 5,266,973 measures the powers of individual sources in the printhead in response to a sequence of electrical currents applied during calibration, then rescales the currents encoding the image sent to each laser, eliciting the same power from each of the sources in order to impose balance during printing. Balance is attained when all of the lasers are adjusted to produce the same image densities over the same spatial extent in the image-recording medium within an acceptable tolerance. Commonly assigned U.S. Pat.
Blish Nelson Adrian
Eastman Kodak Company
Pham Hai
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