Inter-Instrument Agreement in Colour and Density Measurement

Summary

“…metrology, standardisation and conformity assessment are the pillars of knowledge for developing a technical infrastructure, and thereby enabling sustainable development and full participation in international trade. And they are firmly linked together.” — from ISO’s promotional brochure “Metrology, Standardisation and Conformity Assessment”. Colour measurement has come to the fore in the graphic arts industry as it has finally acquired instruments and ISO Standards. Ironically, colour measurement and colour management now offer better control of standardised procedures between proof and print than density measurement, though the printing industry has been using density measurement for far longer to control film processing, plate making and printing – film and plate processing now being almost completely replaced by a CTP process. Ironical, too, in this area of CTP plate processing control, is that while densitometry may remain as a hidden part of a closed loop control system, a new generations of CCD-based measurement devices has emerged using image analysis for plate halftone dot size measurement and registration control. Densitometry for plate quality control is now largely obsolete. One of the reasons why densitometry has failed the user in printing is the multitude of standards and the lack of a common agreement about how density should be measured. Differences are found between absolute and relative density measurement, differences in wide-band blue filter responses, in wideband versus narrow band filters, the inclusion or exclusion of fluorescence, differences between spectral density measurement versus colour filter measurement, with polarisation filter or not, and differences in how the black ink is measured. In the last year, there has been a realisation that such differences are obstacles to a global standard for print process control. Generally, it would be much better to have just one standard configuration and method for density measurement. The individual fourcolour inks have almost identical spectral properties, regardless of the printing process – only the paper colour on which the inks are measured brings some discontinuity to the process ink standards. Apart from these, other measurement differences occur due to the instrument design and measurement process errors. Instruments from different manufacturers measure differently. Often, it is even difficult to compare measurement results between print sites as well as between printer and customer. Sometimes even one and the same instrument measures differently depending on its orientation in relation to the measuring target. This problem is true for both, densitometers and spectrophotometers. All the factors which contribute to these measurement differences apply regardless of which of the major printing processes is used. Consequently, there is a strong case for agreement on having instruments designed for the graphic arts generally and to separate the instruments for print density measurement from those for colour measurement. Density measurement is still the preferred method for press inking control – manual or closed-loop full colorimetric print process control is still something for the future. On the colorimetric side, additional refinements to ISO standards, colour measurement instruments, colour difference formulae and viewing booths are in preparation to improve the correlation of colour measurement data with appearance and with the matching of an approved proof to the printed sheet. Overall, the objectives of this report are to improve measurement accuracy and precision for colour and density, to improve interinstrument agreement and to maintain repeatability for hand-held spectrophoto meters and densitometers. It is a conclusion of this Ifra Special Report on Colour and Density Measurement that the main improvements in inter-instrument agreement can only be reached first through rationalising and improving certain ISO standards and then by using instruments that conform. Differences caused by the measurement conditions and method are more easily overcome.