Colour measurement and viewing conditions for non-paperbased substrates

ICC-based colour management is an important part of modern prepress workflows to ensure that colour information is transported correctly and unequivocally from input devices to workstations and output devices. For instance, an ICC-based colour management process provides the possibility to reproduce original colour information of a scene on a different medium, such as a sheet of paper, as accurately as possible. Other possibilities offered include accurate simulation of colour appearance of a print on a specific paper; a specific printing device on a different paper; and using a different printing device. To achieve this aim, ICC colour management links device dependent colour information, such as the RGB values provided by a scanner, to the CYMK values of a printing process and to device independent colour information such as CIELAB. For printing processes, such information is generated through the colour measurement of a set of colour swatches using a spectrophotometer. Such measurements are uncomplicated for most common paper-based substrates and inks used in the printing industry and the results are correspondingly close to the colour perception of the average human observer. This colour management process is widely accepted in the printing industry and is effective for paper based substrates but there are significant limitations with some other substrates. The important group of non-paperbased substrates, such as metal foils and translucent materials used widely in the packaging industry, give spurious results which mismatch when different spectrophotometers are compared. Most importantly, the results obtained do not correlate closely enough with the colours perceived by the human observer, rendering the colour management of such materials unreliable. The aim of this research project was to identify a method giving better correlation between measurements of coloured samples and prints on difficult substrates typically used in packaging and those observed by the human eye in order to provide a more reliable colour management system. To achieve this goal, the ICC-based colour management processes for different typical packaging substrates was reviewed with regard to its ability to give a good visual match between print and proof. From this group of materials a problematic substrate was identified. The different steps in ICC-based colour management required to give a good visual proof to print match were analysed and reasons for their failure established. A solution to overcome every failing step in the process and improve the relationship between proof and print was developed based on consistency between illumination and pick-up geometries during sample measurement and proof or print evaluation in a viewing booth