INDUSTRY STANDARDS — FOR THE COMMON GOOD
G7 and GRACoL 7
According to the “G7 How-to” primer released by IDEAlliance, this “calibrating, proofing and printing methodology” grew out of research and development efforts of the GRACoL Committee. The new methodology “defines gray balance and target neutral print density curves (NPDC) for three-color gray and black as the primary method for color control as opposed to previously developed methods that focus on ink density and TVI (Tone Value Increase, formerly known as dot gain).”
G7 originally was developed to support the GRACoL 7 specification. The “G” refers to calibrating gray values and the “7” stands for the seven primary color values defined in the ISO 12647-2 printing standard—Cyan, Magenta, Yellow, Black (K), Red (M+Y), Green (C+Y) and Blue (C+M).
The GRACoL Committee found the ISO standard’s “reliance on a small number of solid ink colors and TVI curves limits its value in ICC color managed workflows. The main problem is that ISO 12647-2 is ambiguous due to the use of multiple TVI curves, and the lack of a colorimetric definition for gray balance. Today’s users want a printing standard to define the ‘appearance’ of the final image more precisely than is guaranteed with ISO 12647-2.”
GRACoL 7/G7 eliminates the TVI problem entirely by replacing the ISO standard’s multiple TVI curves with the new concept of a single neutral print density curve. NPDC is described as a new concept based on 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,” explains the how-to guide.
According to its developers, “The primary advantage of the G7 calibration method is that the visual appearance of neutral tones and near-neutral colors is more effectively controlled than by traditional TVI-based methods. The main disadvantage is that gray balance can be a challenge to maintain in offset lithography.”