by Mark Ritchie, Product Manager at Xaar
The subject of print quality is very complex, subjective and difficult to quantify, particularly when printing four colour images. Some recent articles have been playing the numbers game, using the number of dots or lines per inch as the key factor for print quality. This is a simplistic approach which does not consider the other important factors, many of which are systems related rather than specific to the printhead.
So, what are the other contributing factors for print quality?
It's all about the dots
Although dots or lines per inch are important, we also need to look at the addressability of the whole system, not just the printhead. Addressability can be increased by interleaving multiple printheads to double the effective npi.
The use of variable dot greyscale can also increase the resolution visible to the human eye compared to binary print.
Does size matter?
Size really does matter when looking at print quality. A small drop size produces a small dot with a low colour density which can produce smooth edges on text when used to fill in spaces around larger dots. However, large drop sizes ensure solid coverage and stronger or higher opacity colours.
Hitting the sweet spot
To ensure precise imaging, each dot must be precise in its positioning on the substrate. Misplaced dots will affect the quality of text and look ragged around the edges and result in white lines in images and solid areas.
Consistency is critical
Consistency of drop size is critical to visual print quality. Bands of different colour density visible in an image are highly undesirable, but can result from inconsistent drop volumes across a single printhead and the entire print width. This is influenced both by the physical capabilities of the printhead and the variation in ink temperatures.
How close can you get?
Ultimately it is the capability of the human eye which is the final test of perceived print quality. The power of human eye with 20/20 vision translates to a dot size of 29 microns at the eye's closest focusing distance of 10 cm. This in turn equates to an effective resolution of 876 dpi which is well within the capabilities of a 360 npi 8 grey level printhead. Higher resolutions are therefore of little benefit when it is beyond the capacity of the human eye.
All the factors outlined here contribute to the final print quality on the substrate and therefore all need to be considered. Our recent white paper "Print quality requirements for single-pass inkjet printing – the whole picture", gives a more detailed understanding of the determinants of print quality and how it is perceived.