Modern Colour Difference Equations and Their Relationship to Printed Ink Densities

摘要

The purpose of this research is to investigate the correlation of printed ink densities and numerical colour differences and to find out whether the ISO standard colorimetric requirements can be translated into a set of densitometric values. Can densitometers be used in pressrooms as an effective tool for colour control? Does meeting the industry standards in terms of density values (i.e., GRACoL) yield to the colour conformity? How do paper characteristics such as surface structure, colour, brightness, etc., affect the reproduced colour? Also, which one of the colour difference equations should be used in the graphic arts industry? This paper attempts to answer these questions. The results show that none of the five Delta E equations is dominant in terms of better correlation with density variations. If one chooses to use the △E2000 which correlates better with the human eyes' perception of colour variations or DIN99 because of its more uniform colour space, different tolerances than those specified in ISO 12647-2:2004 must be used. Within the group of coated stocks, the target densities achieved as the best colour match, are very close to each other, therefore it is possible to establish a target density and variation tolerance for each primary colour, in order to be used as a starting point to achieve a colour with the lowest AE as quickly as possible. However, different ink coverage is required to achieve that target density depending on the surface structure of the paper. Also, different sets of inks have different hue errors and grayness which should be considered by the printers. Densitometers can be used effectively to control the density variations and therefore colour variations during a press run, but spectrophotometers are required-and becoming more crucial than before-to precisely control the conformity of the reproduced colours.