When trying to evaluate colour and achieve some form of process control, we often neglect grey and its power. Grey is more than just a neutral, achromatic colour midway between white and black. Instead, think of grey as the foundation of colour. It is the first element you should look at when evaluating colour, and it is the most telling indication when things go wrong on your proofing device or press.
Grey balance and process control
Using grey balance for process control is easy, practical, and effective. One method of evaluating grey balance on press is to place two different grey patches side by side, as seen in the image to the right. One patch is a chromatic grey (CMY) and the other is a true grey (K). The goal is to get the two patches to look the same. Good grey balance is dependent on printing parameters that are influenced by at least four factors: the dot gain, the solid colour inking of the primary colours, overprinting, and paper colour. A pressman can make moves on press to correct for grey balance, but a full evaluation of the CTP curves may be required to ensure plates are created properly.
The human eye is very good at metering or evaluating colours that are directly adjacent to each other, and is particularly sensitive to colour differences within the greys. The example below shows how much easier it is to see variations in grey than in other colours.
Improper grey balance will quickly identify if the “spread” or deviation between colours is too great. When using process inks in offset printing, inking and damping fluctuations show up more distinctly in chromatic grey image areas than in coloured image areas, which makes grey balance patches in colour control bars instrumental in indicating printing process disorders.
G7™ and its role in grey balance
As a new calibration process that specifies a colour printing condition using tone reproduction and grey balance, G7 is a major departure from the previous implementations of GRACoL where only densities and dot gains or tone value increase (TVI) were specified for various substrates.
G7 is a new calibration method developed to support the GRACoL 7 specification. The “G” refers to calibrating grey values and the “7” refers to the seven primary colour values defined in the ISO 12647–2 printing standard: Cyan, Magenta, Yellow, Black (K), Red (M Y), Green (C Y) and Blue (C M). Although originally intended for commercial offset printing, the G7 method is applicable to virtually any CMYK imaging process, and has been successfully tested on a wide range of processes, including coated and uncoated offset, newsprint, gravure, flexography, dye–sublimation, ink–jet, and electrophotography, as well as a wide range of AM and FM screening methods.
Resources
The ECI/bvdm Grey Control Strip is freeware available for download. You may use the ECI/bvdm Grey Control Strip at your own discretion for as long as you want, on as many computer workstations as you need, and in whichever country you wish. However, you are not permitted to distribute the ECI/bvdm Grey Control Strip or to change or modify it, in whole or in part, without written permission from European Colour Initiative (ECI) or the Bundesverband Druck und Medien (bvdm). You use the ECI/bvdm Grey Control Strip at your own risk.
Angus Pady is the president of Digital Solutions. Complete colour control from desktop to press.
T: 905.764.6003 • Angus@ColourManagement.ca • www.colourmanagement.ca
