One of the most frequent production decisions
made in the printing industry is to
answer the question, “Do the print and
proof match?” Wherever it is made—in
prepress, in the press room, by the purchaser of a
proofing system, by a trade association certifying
proofing systems, etc.—it is a decision that today is
based on a visual comparison.
Many, particularly those trying to certify proofing
systems, would like to be able to use measurement
data to predict the quality of a match between a
proof and a print. The ultimate goal is to remove the
human element, the visual comparison, with all its
uncertainty and individual bias.
On a parallel note, recent discussions of digital
proofing at the February ICC (International Color Consortium)
meeting in Orlando have identified the need
to describe variables involved in both making proofs
from digital data and using measurement data to verify
the match of proofs and aim characterization data.
These two issues are more intimately related than
one would guess at first glance. As part of the validation
of any digital proofing system, it is important
to determine, and allocate, the variability involved in
making individual proofs. Many of these same variables
are involved in the analytical comparison of
proof and print.
Let’s assume that psychophysical testing can give
us some measure of the colorimetric variability that
can exist between images and still allow us to say
they match. We then must identify the sources of
variability between the proof and the aim characterization
data, or a print made according to that characterization
data, and apportion the allowed
variability among them. Of course, we must also
accept the possibility that there may be too much
variability to allow such a prediction to be made.
We will not know until we understand the
allowed tolerances based on psychophysical testing
and the variability associated with making and
measuring the proof.
The Scenario
Before we try to analyze the variables involved, we
need to put some boundaries on the problem. Let’s
assume we are starting with a set of CMYK digital
data that have been prepared using CGATS TR 001
as a reference. (For those not familiar with TR 001,
it is a characterization data set that provides the relationship
between CMYK data and the colors that are
measured on a sheet printed or proofed in accordance
with the SWOP aims—a digital definition of
SWOP.) The goal is to proof the images represented
by this data and also print the same data on press
and compare the results to each other and to the
reference data set. To provide the necessary colorimetric
data, we need to print a measurement target
at the same time we print the image data that is
used for visual comparison.