G7™ Color Management and Flexo
G7™ is a hot new buzzword in the industry, referring to a new calibration method developed by IDEAlliance for its latest SWOP® (Specifications Web Offset Publications) and GRACoL® (General Requirements for Applications in Commercial Offset Lithography) specifications. Since its initial publication two years ago, G7 has raised a lot of interest internationally among groups involved in setting printing standards or specifications. Among those groups is Flexographic Technical Association (FTA), which is well on its way to adopting G7 for use in flexographic printing, and will include initial test results in the upcoming fourth edition of Flexographic Image Reproduction Specifications and Tolerances.
What is G7™?
G7 is a new way of calibrating and controlling any CMYK imaging process. Instead of measuring traditional CMYK scales for the four inks, G7 measures just two grayscales, one printed in black ink alone and another printed in pre-determined ratios of CMY. These scales are contained in the P2P (proof-to-press) target (see Figure 1) which is available free at www.gracol.org.
Instead of measuring dot area or dot gain values (a.k.a. tone value increase, or TVI), the scales are measured in simple neutral density values, which are plotted as NPDC curves either manually on graph paper (free graph paper method) or using software. Gray balance is either adjusted on press, or corrected with different CMY RIP curves calculated automatically in software, or manually with the aid of the "grayfinder" target.
The four main functions for which a printer or prepress shop might use G7 are:
- Calibrating a press to a constant visual grayscale appearance
- Calibrating a proofing system to a constant visual grayscale appearance
- Controlling a press to maintain the same grayscale appearance from run to run
- Controlling a proofer to maintain the same grayscale appearance from proof to proof
The main benefit of G7 is that it produces a more visually-constant printed appearance, with calibration curves alone, than can be achieved with traditional dot-based calibration methods. The simple explanation for this is that dot size alone is not a reliable indicator of visual appearance, because it ignores other factors like ink hue, ink film thickness and trapping.
Excerpts from "Applying G7 To Flexo" by: Don Hutcheson.
This article appeared in the June, 2007 issue of FLEXO® Magazine
