An Overview of Digital Printing Systems - Control of Color Imaging Systems: Analysis and Design - page 13

Image Process

contro ls

DFE Image path PE Image path

CMYK

TRCs

3-4D LUT

CMYK
Recipe

TRCs

Electrostatics

ICC
profile

Gray
balance

Level
3

Level
2

Level
1

Spot
colors

Development
system

Color
controls

ESV

TC L oo p

Targets

Density sensors

Inline/offline spectrophotometer

FIGURE 1.8 Hierarchical time-based color and process control functions used for improving
consistency in a color EP print engine system.

cooling of glossy images can cause a shift in the lightness component, that is, L*, of
the color. These are some of the factors creating error in color measurements. Such
temperature related shifts have to be compensated for, so that the measurements are
closer to those of a golden measurement standard instrument. Sensor readings are very
dependent upon the displacement of the media from the focal point of the sense head.
As the medium moves through the point where the sensors are mounted in the paper
path, the effect of displacement of the medium on sensor output is signi cant and

should be compensated for, either through tight control of the media in the path, which
may be counterproductive for paper path control system, or through special displace-
ment insensitive (DI) optics [15]. Achieving accurate and repeatable readings from
the sensor also depends on the stability of the electronics, the light source and its
wavelength band, and the number of photons the system can integrate while the color
samples are present underneath the sensor on a passing medium.
As pointed out before (Figure 1.1), the processing of images can occur at various
levels inside and outside the printing and publishing system hierarchy. Many of the
processing techniques required for imaging and control functions occur at multiple
levels. A time-based separation is adopted with higher level functions occurring at a
slow timescale near the prepress and faster real-time control functions typically occur
in the print engine. The processing that goes on in the DFE will be at slower
timescale than in the print engine, but at a much faster timescale than the prepress.
There is also a time-based hierarchy being adopted local to the print engine. In
Figure 1.8, an example multilevel hierarchical structure is shown, which becomes the
foundation for navigating through the material of this topic.

1.6 PREPRESS-BASED PROCESSING
Prepress refers to the preparation of digital files for printing that begins after the design

decisions are made in the creation stage and ends when the document hits the press.
A whole segment of the printing industry is devoted to prepress, expanding on the
traditional prepress and removing limitations of printing methods. Digital prepress
work is largely aided by modern computing hardware, software tools, and imaging
instruments. Image capture and manipulation software, such as desktop publishing,