Selection of the optimum offset ink set for colour digital image reproduction by gamut matching

A number of researchers have carried out the research on matching the colour of the original to the printed output for many years in various aspects. A quantity of original painted with artistic colours, and several photos with colour produced by dye, cannot be matched with only four-colour process ink. This difficulty has been solved to a certain extent by using High-Fidelity colour printing. Nevertheless, not many coloured inks are added for halftone printing at present. This research was concerned about the introduction of a method of selection of custom inks in any combination and number provided the shape and size of the colour gamut that was similar to the original. Calibration prints of three sets of four-colour process inks and twenty-one special inks were prepared on glossy paper so as to determine their optical properties using the two-constant method of Kubelka-Munk theory. The spreadsheet tools were used to calculate the optical properties and to obtain colour gamuts for any combination of inks. The numbers of inks in the formulations used to determine the colour gamut were five, six, seven, nine and ten. The digital originals employed were the SHIPP standard digital images. The RGB colour values of all pixels of the digital orginal were read and recorded using the MATLAB code. They were converted to XYZ tristimulus values and CIEL*a*b* coordinates using an appropriate conversion method for SHIPP images. Two criteria for selecting the optimum ink in order to match the colour gamut of the colour digital image reproduction were the gamut volume as well as the size and the shape of the inks selected. By using the tool developed, it showed that the gamut of some of the SHIPP digital images can be covered using four-colour process ink. However, the colour gamut of the colourful SHIPP image needed more than four-colour inks to cover it. The ink-set that is suitable for an image can be investigated using the tools developed.