More and more digital printers support the use of white ink, which is a welcome feature in many applications. But to be really useful the “covering power” – the opacity of the white ink – needs to be very high to match that of analogue printing technologies like for example screen printing. We have used the upcoming ISO standard for this to put actual numbers on the opacity of the white ink used on three large digital format printers from Agfa.
Anyone interested in or performing tests of printing devices should follow the development of ISO standard 15311-1. Its full name is “Graphic Technology – Requirements for printed matter for commercial and industrial production – Part 1: Measurement methods and reporting schema”. In chapter four you find a whole range of tests that describe how to measure and assess print quality, for example colour accuracy, gloss, tone smoothness, colour gamut, uniformity, banding, permanence (including indoor light stability, weathering and thermal stability), water-, scratch- and abrasion-resistance – the list goes on. New test methods are added when they have been fully evaluated and deemed to give a repeatable and relevant result. How to measure ink opacity is one of the test methods currently under evaluation.
You might think that there should be an established and agreed upon method for how to measure this by now, but in fact there isn’t. There are some different reasons for why not, for example the lack of defined reference materials, and the complications that quickly builds if you were to combine several inks printed on top of each other. But for the fairly straightforward scenario of printing white ink on a dark substrate the experts in the ISO technical committee responsible (ISO TC 130 – Graphics technology) are now very close to agreeing on a method and formula that gives a consistent result and where the measured result also matches well with how the prints look when visually assessed.