An Overview of Digital Printing Systems - Control of Color Imaging Systems: Analysis and Design - page 12

adjustment is required during runtime. New density sensors were developed for
measuring various tone densities for monochrome and color toners. A hierarchical
and multilevel control loop architecture was simultaneously developed to address
these problems, which required the use of modern control theory and methods [9 12].

For production quality color EP printers, the control challenges are several orders
of magnitude higher since they compete in the traditional offset market that has a
reputation for high quality. Materials affect the print quality stability and stability of
color balance in prints. The control loops should not only maintain process stability
for individual color separations, but also adjust the color for varying media condi-
tions and wide array of media stocks (e.g., coated, uncoated, textured, smooth, and
specialty) in order to compensate for overlay colors, sheet-to-sheet differences,
temperature, humidity, PR aging and wear in drives, etc. They should maintain
much tighter control on image registration between separations (for simplex and
duplex functions) and paper motion at various regions in the paper path. To make
them competitive with offset printers in terms of operational cost, many of the press
makeready costs should be eliminated using automated setups. Thus, due to many
new challenges, to deliver quality and high productivity at a low run cost, techno-
logical advances are required in different areas including sensing, algorithms, and
processes. To compete, color EP printers also need color accuracy improvements
against industry standards (e.g., General Requirements for Applications in Commer-
cial Offset Lithography [13] [GRACoL], International Organization for Standards
[ISO]). In addition to these, they should also closely match offset printing standards
in time, that is, not require too much time on the press to adjust the color manually
while following the international color consortium (ICC) [14] work

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ow for produ-
cing higher performance photo quality prints. Neutral grays and highlights should
be reproduced at offset quality; photo smoothness in faces should be retained while
maintaining sharp background and shadow details. Color EP printers should also
retain highlight, midtone color balance without injecting any contours or blocking
and offer high de nition image quality at high speed.

Because measurement devices have different response rates, accurate color
control methods require accurate and repeatable color measurements referenced to
some device or sensor in the master printer. Each

color standard may specify a particular device, such as an X-Rite iSis or DTP70
Autoscan spectrophotometer, for generating the reference target (aim) measure-
ments. For example, in the Pantone 1 matching system, the device-independent
targets supplied by Pantone are measured by their standard instrument, an iSis
spectrophotometer. A correction that adjusts the sensor output to correlate
between color sensors in different printers may be required to maintain instrument-
to-instrument variability to be within tightly speci

golden measurement standard

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ed limits.

media-based contactless, noninvasive measurements at high-
speed, in-line embedded spectrophotometers are used in the paper path to measure
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To achieve paper =

toner patches. There are many sensor-related issues to consider when
the embedded spectrophotometers are located inside the print engine. In-line spec-
trophotometers sensors have been found to give different measurements on just-
fused sheets as compared to those when the printed sheets have been cooled. A
change in temperature can cause a chromatic shift in color pigments. Similarly, the

just-fused

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