G7® is a Specification defined by the Print Properties and Colorimetrics Working Group of IDEAlliance.
G7 is a method for matching color across multiple printing devices, such as printing presses and proofing systems. It can be applied to any printer that has a calibration process (usually involving adjusting color "curves" in a RIP) that allows control over CMYK inks. These would include web presses, sheet-fed presses, proofing systems, inkjet printers being driven by RIPs. The goal of G7 is to provide visually consistent, repeatable color between different devices (like proofers and printers) as well as different presses, and even different facilities. The emphasis in this specification is on gray balance. Since human vision sees differences in gray more readily than more saturated colors, this specification is more likely to result in successful visual "matching" than merely matching solid ink densities which is typically done for more traditional processes.
Note that G7 is a specification - not a standard. It can be used in addition to other industry printing specifications such as SWOP, or GRACoL.
The G in G7 stands for Grayscale, and the 7 stands for seven colors: cyan, magenta, yellow and black - the subtractive colors used in printing, and red, green, and blue - the additive colors used in proofing.
From the IDEAlliance website
http://www.idealliance.org/videos/Just_Enough_Publish/JustEnough_G7/index.html G7® specifies the components of an image that define a similar "visual appearance" to the human eye. To do this, the G7 Specification:
- defines a colormetric definition for gray balance
- specifies gray balance in the midtones, image weight and image contrast from the highlights to the shadows are the factors that determine likeness of the visual appearance of an image.
Initially G7 was developed by the IDEAlliance GRACoL Working Group. But as adoption of G7 grew, it became clear that the application of this specification that enables printers to reproduce a similar visual appearance across printing types and substrates should be addressed by a group with reach beyond the focus on sheetfed offset printing that is unique to the GRACoL working group. Today, through the PPC Working Group, experts from across the spectrum of printing disciplines contribute to this important IDEAlliance Specification.
How to get it
The G7 specification is managed by IDEAlliance. Documentation describing the process is freely available at the IDEAlliance website which describes the specification and how to achieve it.
- "How to" documents are available here:
A list of downloadable G7 documents is here: http://www.idealliance.org/specifications/gracol/resources/9
IDEAlink Curve was the first software that greatly simplified the process of G7 calibration. Doing it "by hand" involved manually drawing out curves on graph paper. This software made the process automatic, consistent, allowed for automatic averaging of multiple press sheets, compared the press run to SWOP and GRACoL standards, automatically created correction values for the most popular RIPs, provided several visual graphs for feedback on your press condition, and more.
As of 2010, IDEALink Curve is no longer available for purchase new. It was replaced by Curve2, which was in turn replaced by Curve3. https://www2.chromix.com/idealink
In 2009 CHROMiX and Hutchcolor released Curve2. This was an upgrade to the original IDEAlink Curve software. Curve2 is not an IDEAlliance product as was the original Curve software.
Links to download the software (requires username and password to run), links to beta software, and the registration page are at the CHROMiX website:
In April of 2013, CHROMiX and Hutchcolor released a new version called Curve3.
Curve3 allows calibration of special (non-process) inks, TVI calibration, introduces a high quality smoothing function, and more options for inkjet calibration.
In September of 2016, the Verify-only version of Curve4 was released.
In May of 2017, the full versions of Curve4 were released which added the Calibrate and Blend tools.
While retaining the functions of Curve3, Curve4 added many new features and entirely new tools. Some of the main differences were:
- Ability to drive most of the common spectrophotometers directly in the Curve4 software.
- Verifying measurements according to G7 Grayscale, G7 Targeted, or G7 ColorSpace compliance.
- Adding the Blend tool to aid in combining, averaging and saving out specific target patch collections.
- Adding a white point and black point adaptation to scale data to new white point / black point.
- Adding the ability to calibrate using targets other than the usual P2P targets.
- Adding the ability to fine-tune a calibration using smaller targets, some as small as 6 patches.
- Introducing a Ink Limiting feature with diagnostic tools for making calculated ink limiting decisions in a RIP or outside of a RIP.
- Adding Spot Color Tone Value (SCTV) support, a method of calibrating Spot inks that has been embraced by the industry.
- Adding support for importing and exporting i1Profiler .cxf files, and other common file types.
- Integration with Maxwell, the CHROMiX cloud-based color tracking system.
The software package can be downloaded at no charge from the CHROMiX website. It can be operated in Demo mode. A license needs to be purchased before the user can run it using his own files. Curve 4 information page at CHROMiX.
- Article: The Ins and Outs of GRACoL 7 and G7 This is an article by Steve Upton which originally appeared in CHROMiX ColorNews. He discusses some of the history of why we have come to the point of using gray balance to calibrate presses, the differences between GRACoL 7 and G7, and how CHROMiX has become involved.
- Article: "What is G7 and Why Should You Care?"
- Article: "What defines accuracy?"
- Hutchcolor G7 page: