Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
2 New Variables and Definitions
This section explains some new variables introduced with the G7 method, including 'NPDC', 'HR', 'SC',
and 'HC', and some new definitions or uses for old variables such as gray balance and TVI.
2.1 Neutral Print Density Curve (NPDC)
A new concept introduced with G7 is the definition of a 'Neutral Print Density Curve' (NPDC), which is
the relationship between measured neutral density and original halftone percentages on a printed gray
scale. Because neutral density is an absolute value, while TVI is a relative function, NPDC ensures a
better contrast and density match between multiple devices.
Two NPDC curves are specified, one for a combined CMY gray scale and one for a black-ink gray
scale. NPDC calibration compares a printed gray scale to a reference scale and calculates RIP
correction values in dot percentage terms that force the press (or other imaging device) to the desired
NPDC shape. Curve correction values are calculated either manually by plotting graphs on special
graph paper (see Appendix B), or automatically in the IDEAlink Curve software available from
www.printtools.org.
2.1.1 Determining 'natural' CtP curves
To determine the 'natural' print curves of commercial CtP-based printing, the GRACoL technical
committee analyzed multiple ISO-standard press runs made with a variety of plates imaged on uncalibrated
CtP systems, set up to manufacturer specifications.
While every press run is unique, we believe the G7 NPDC formula represents what an average press
running to ISO 12647-2 specifications would produce with typical un-calibrated CtP plates, on high
quality commercial stock.
For an explanation of how the NPDC curves were mathematically smoothed, see Appendix E.4
REMEMBER: G7 NPDC curves are NOT an official ISO standard. Other valid interpretations of ISO
12647-2 may produce slightly different NPDC curves.
2.2 Highlight Range (HR)
HR is a single measurement that quickly tests the mid-tone density of a previously-calibrated device
during production, for example while a press is running or after each proof is made. HR replaces
individual TVI readings as the primary measure of overall print darkness and gray balance, however TVI
is still a useful press control metric - (see 2.8 When to Use TVI).
HR is computed twice, once for CMY ('HR_cmy') and again for black ('HR_k'). Just two measurements
are required for each, or just one each (total 2) in absolute mode (see2.2.3), compared to three
measurements each for C, M, Y and K TVI (total 12), yet HR gives a more reliable indication of pictorial
darkness and contrast because HR is absolute while TVI is only a relative measurement.
For convenience, HR is expressed in neutral density (ND), which can be measured with a densitometer
set to the black channel, or 'Visual density'. ND can also be computed from CIEXYZ_Y, in fact GRACoL
officially defines Neutral Density as ;
ND = Log10(100/Y); (where Y > 0 < 100)
2.2.1 Measuring HR_cmy
HR_cmy is computed by measuring the neutral density (ND) of a combined CMY gray patch (50c, 40m,
40y) and subtracting the neutral density of paper.
HR_cmy = ND(50c,40m,40y) - ND(paper) = 0.54 (typical)
For devices with a dynamic range of about 1.3 ND or higher, HR_cmy is effectively a constant of 0.54.
For low dynamic range devices, such as newsprint, HR_cmy may be lower. (See Appendix A.5 for a
graph of HR_cmy vs. dynamic range.) HR_cmy is calculated automatically by IDEAlink Curve.
8 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.