Calibrating, Printing and Proofing by the G7™ Method

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Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006 7 Adjusting NPDC Curves This section describes how to determine the RIP correction values from the P2P target that will force the printing device to match the specified G7 NPDC curves. 7.1 Allow sample sheets to dry Set aside several good sheets to dry prior to measuring. 7.2 Measure the P2P target From one or more good calibration sheets (as close to target goals as possible after drying), measure the K-only gray scale (column 4) and CMYonly (column 5) of the P2P target. The latest target (P2P23x) lets you measure manually or with an automated device such as the GretagMacbeth EyeOne Pro or X-Rite DTP70. For the graph-paper method, automated measurement files must be converted to Neutral Density values. Measure at least two sets of scales from different parts of each sheet and average the readings. NOTE: If taking manual readings with a hand-held densitometer, set the display to 'visual density' and zero on the substrate or paper base. If you cannot zero the densitometer on the substrate, subtract the substrate density (measured in a clear white area) from all density readings. 7.3 Plot the curves NOTE: If using IDEAlink Curve proceed to step 7.7 P2P23x Target Plot the graph of column 5 as neutral density vs-dot% on the CMY graph paper. For convenience, the red vertical lines on the graph paper correspond to each percentage step in the P2P target. Typical sample graph compared to the G7 CMY Target graph Repeat for K on a separate sheet of graph paper. 24 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.
Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006 7.4 Find nearest target graph with same D-max Find the target graph nearest to the D-max (Maximum Neutral density) of your graph. If your device is not close to any of the pre-printed graphs, use the two nearest graphs (higher and lower D-max) to assist in re-drawing a new reference graph that parallels its neighbors and precisely meets your device curve at 100%. Finding the target graph that meets your sample graph at 100% 7.5 Choose the optimum curve points Examine the graphs and decide how many 'curve points' are needed - typically wherever your curve bends significantly. For example, the curve at right requires points at 2% and 15%, and ideally at 10% and 20%, for maximum accuracy. The green line shows how profiling software falsely interprets real performance (red) from 10%-spaced data (blue) unless extra curve points (circled) are added. NOTE: Some RIPs require specific curve points. Fewer points may reduce the chance of banding, but extra precision is often needed in lighter tones. 7.6 Calculate the RIP curve correction values For each curve point on the graph paper (or curve points for your RIP) do the following: • Draw a vertical line UP to meet the target graph. • From there draw a horizontal line ACROSS (left or right) to the device graph. • From there draw a vertical line DOWN to the bottom axis to get the 'new aim value.' Record that in the 'new aims value' column on the graph paper. • Repeat for each curve point except 0% and 100%, which never change. 25 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.
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Calibrating, Printing and Proofing by the G7™ Method

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