G7 is a color calibration process that helps commercial printers print consistent color with less preparation and less waste. This methodology allows printers who maintain a G7 certification to reliably produce high quality commercial printing that matches from proof to press, and from one G7 press to other G7 presses.
Developed by the IDEAlliance and the GRACoL Committee, G7 is named for its gray scale calibration technique and the 7 ISO ink colors it requires. The ?G? refers to calibrating Gray values, while the ?7? refers to the seven primary color values: Cyan, Magenta, Yellow, Black (K), Red (M&Y), Green (C&Y) and Blue (C&M). It describes how to use the principles of digital imaging, spectrophotometry, and computer-to-plate (CtP) technologies to achieve these color matches using quality inks and papers.
Essentially, G7 is a set of instructions on the methods for calibrating a press or proofing system based on spectrophotometry (colorimetric data) for gray balance instead of traditional densitometric (dot gain/TVI) methods.
Why gray balance?
The concept of gray balance is essential for consistent color reproduction. Images that are not in gray balance are considered ?casted.? Images that are casted show a noticeable magenta, cyan, or yellow color appearance that requires color correction to remove. If the original file or the proofing system introduces a cast, then all the color is shifted away from gray balance. A proof that is casted will require the press to print away from neutral gray to match the proof. If you?re casted, color reproduction suffers.
While a job is on-press, press operators might look at a printed press sheet and notice casts of too much magenta, cyan, or yellow and reduce whichever color is creating the cast. At this stage, the only control for the press operator is more or less ink; however, the TVI (tone value increase), or dot area, is equally important to color fidelity. The press operator can?t change the size of the dots on the plate, but he or she can change the gain by adding or subtracting ink.
It is impossible for the pressroom to have absolute control over gray balance and color with the wrong size dots on the plates. The old procedure was to print a test form at the required density, then compare the scales against a standard with a reflection densitometer and adjust the plate values accordingly for each color. This was a time-consuming and inefficient, but necessary, process before the introduction of the G7 standard.
With the G7 process, many of these difficulties are overcome. The human eye ?measures? light in colorimetric terms in much the same way that the spectrophotometer does. It measures color in Red, Green, and Blue values, which is also known as additive color. (All the colors combined add up to white.) However, printing inks on paper uses subtractive color. (All the color removed leaves you with white.) When you look at printed colors, your eye is actually seeing white light bouncing off of a color, and subtracting its complementary color from the lightsource. The human eye is a very good judge of when printed material is off-color balance.
In order to achieve consistent color, a different way is needed to describe precisely what colors should look like, rather than specifying different percentages of RGB or CMYK. With G7, that?s where L*a*b* color comes in.
What is L*a*b* color?
?Lab? is an advanced color space description that allows precise and accurate color specifications. Its color gamut (or color range) is similar to RGB and it has three components to its makeup:
L = Luminance, the brightness or darkness of a color, (i.e. light blue vs. dark blue)
a and b are value scales for complementary colors:
a = a scale from red to its complementary color, green. (-a = green, +a = red)
b = a scale from yellow to its complementary color, blue. (-b=blue, +b=yellow)
Using Lab color, it?s easy to determine gray values: Where the 2 lines cross ( zero a and zero b) is neutral gray. And to measure Lab color, you need a spectrophotometer, which is why G7 methodologies require it ? instead of a densitometer.
Being able to precisely and accurately measure color and gray balance comes in handy on a properly-calibrated press, like when you want a device to be able to predict a visual match from proof to press by statistically matching a large sampling of the colors. That?s what the G7 methodologies do - they provide an accepted common standard that can measure gray values on press to simply and efficiently ensure that colors match what they are supposed to. This makes your color proofs more accurate and simplifies press makereadies.
Once a press has been calibrated using the G7 standards, an ICC Profile can be generated for that press that allows easy color consistency from the original electronic design to when it comes off-press.