Internal Process Controls - Control of Color Imaging Systems: Analysis and Design - page 504

measurement-process-actuation cycle, measurements can be carried out at different
area coverages leading to a higher bandwidth tone correction with control of the
entire tone curve after sampling all area coverage levels. Correcting one gray level at
a time using process actuators has the risk of affecting other gray levels if the
actuators are not simultaneously optimized for all levels. Also, complexity and the
cost of implementation may prevent the use of such control loops. Whereas, the use
of level 3 control can easily overcome such barriers and, hence, is the most widely
used control system for linearizing tone levels in many digital printers.

9.9.1 S TATIC TRC I NVERSION P ROCESS
To completely measure the TRC, every possible area coverage that can be printed
must be measured, but this results in too many patches and may be unnecessary
when the TRC has less structure. The printer tone curve is generally sensed at 7 to 10
different area coverages, depending on the measurement resources supplied by the
print engine. A curve fitting algorithm [27] can be used to reconstruct the TRC for all

area coverages between 0 and 255. In addition, noise in the measurements must be

final best estimate of the actual TRC. Also, the curve is
forced to pass through 0% and 100% area coverage, which is consistent with the fact
that these points always remain

filtered out to obtain the

fixed, thanks to levels 1 and 2 controls.
If the reconstructed TRC has values available at all area coverages and is mono-
tonic, then the inverse TRC is obtained using the

static inverse techniques illus-

'' ''

trated graphically in Figures 9.20 and 9.21. In these figures, the best estimate of the

reconstructed TRC that passes through all the measurements is shown schematically
by the continuous solid curve. The normalized DMA or D E from paper (referred

to as the output tone value) can have values lower or higher than the reference curve
as illustrated in Figure 9.19 (right). To demonstrate how the

works, let
us consider these cases separately as illustrated in Figures 9.20 and 9.21. Let the

static inverse

'' ''

Linearizing curve of DMA on photoconductor or
E from paper from which TRC curve for single
separation is determined

100%

E

*

DMA on Photoconductor
or E from paper normalized
to a maximum value of
100% or 255

The TRC of DMA on
photoconductor or
Δ E from paper

B

C

D

0 A 100%

E

Area coverage scaled to a maximum value
of 100% or 255

FIGURE 9.20 Schematic diagram illustrating graphically the static TRC inversion process
shown for one measurement point at area coverage A when the measured tone value is smaller
than the reference tone value.