G7 Calibration for Effective Process Control - Part 2

Calibrating with G7
G7®, sometimes referred to as the G7 Method, is a registered trademark of IDEAlliance. The original specification was developed in 2006 by Don Hutcheson, then chairman of the IDEAlliance GRACoL committee. IDEAlliance, also known as the International Digital Enterprise Alliance, is a non-profit organization founded in 1966 as the Graphic Communications Association. Its purpose is to serve the commercial print and publishing industries; members include print buyers, agencies, publishers and print providers. It promotes standards and best practices for the production of print and digital content.

GRACoL® and SWOP® are two of the more commonly known print specifications developed and promoted by IDEAlliance. Both GRACoL (General Requirements for Applications in Commercial Offset Lithography) and SWOP (Specifications for Web Offset Publications) are registered trademarks of IDEAlliance. G7 was developed as an outgrowth of the GRACoL 6 specification, but has become a separate specification in its own right, and can be implemented independent of GRACoL or SWOP.

The G7 specification is also an official ANSI standard; the formal specification, including aim points, viewing and print requirements, is described in ANSI/CGATS Technical Report TR015-2013, published by the American National Standards Institute’s Committee for Graphic Arts Technologies Standards. The specification is available free of charge from the NPES. The G7 Expert Certification and G7 Master Qualification programs are maintained by the Print Properties and Colorimetric Council of IDEAlliance. Also, TR015 is referenced in the proposed ISO 15339-1 document, which is currently under review by the International Organization for Standardization (ISO) based in Geneva.

Although G7 is a formal ANSI standard, only IDEAlliance may certify a product, print system or provider as being G7 compliant; and, only products and providers certified by IDEAlliance may use the G7 logo for promotional purposes. Software, print systems, print providers and consultants may submit to IDEAlliance for G7 certification. Certification is typically carried out through an independent public 
institution, which currently includes the Rochester Institute of Technology (RIT) in Rochester, New York, and the California Polytechnic State University (Cal Poly) in San Luis Obispo, California.

The Science of G7
The science behind G7 goes back nearly 100 years, to some of the earliest experiments into how a person with normal vision interprets color (referred to as the Standard Observer). Experiments with numerous subjects lead to the development of several common color models still used today. One important discovery was that the human vision system is particularly sensitive to gray balance. Color can be understood as the perception of chroma (color intensity) for a given hue relative to the achromatic (gray) equivalent of the same lightness. We perceive color relative to a neutral stimulus; we may not know if a particular color is correct without a visual point of reference (a proof) but we can recognize if an image possesses a color cast, even if subtle.
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From the earliest days of color film photography, calibrating the exposure of the red, green and blue plates was accomplished by calibrating to a neutral density (a common grayscale); the G7 method is based on these same color photography processes. Later, methods of calibrating to a common neutral density similar to G7 were used to calibrate high-end color scanners and automatic film processing equipment. So, while G7 as a name and a print specification may appear new, in practice the fundamentals that underpin G7 have been used to calibrate color output since the first practical color imaging devices appeared in the early 20th century.

In most cases, at least for inkjet applications, G7 calibration can simply replace conventional linearization, avoiding the need to add an additional step in the color workflow. Rather than linearizing the printer in the conventional way (which has the effect of calibrating the printer only to itself), the printer is calibrated to a set of system-independent aim points derived from the native white and black points unique to the printer, its ink and media. The number of patches necessary to calibrate the printer need be no more than the typical number used for linearization; it requires no more time nor expertise, and the resulting curves can be applied in the same way as conventional linearization curves.

Primary ink limits may still be applied, depending on the printer and the substrate to be printed, but secondary ink limits are generally unnecessary. In some cases, depending on the RIP configuration, the calibration curves can be imported directly into the RIP software in lieu of linearization curves; in other cases, the linearization curves are kept null and the calibration curves are applied as 1D lookup tables inside the ICC profile. Either way, should color drift begin to occur, the calibration curves may simply be iterated and reapplied; in most situations this avoids the need to recreate the ICC profile.

G7 Calibration vs. Linearization
The key difference between conventional linearization and G7 calibration is that G7 establishes a known good condition against which any printer may be calibrated and tested for conformity. Regardless of what happens on the printer – ink changes, media changes, maintenance, wear-and-tear, environmental variation – the known good condition is always available to calibrate back to. Linearization, on the other hand, calibrates only to a transient condition that can change over time. Without the ability to calibrate to a defined known good condition independent of any one printer, with traditional linearization there is no good way to ensure consistent color reproduction day-after-day.

Coming up, Part 3...