Fifty shades of greyscale
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By Sophie Matthews-Paul
6 August 2012
The Xaar 1001 greyscale print-head removes the need for additional colours, so reducing production costs
In monochromatic digital image terms, there are actually many levels of greyscale but, in print-head terminology, we are referring to variable droplet sizes compared with only two levels in binary. So what does this mean when it comes to discussing the capabilities of print-heads and their respective performance? In simple terms, a drop-on-demand greyscale print-head is able to eject droplets of different sizes intended to improve print quality, whereas the binary option means that all drops ejected by the print-head are the same volume.
An improvement in wide-format ink-jet quality might not seem to be particularly important for large displays and banners only seen from a distance, yet increasingly printer users demand better quality for application versatility. Fine standards have always been important with single-pass print devices where close viewing demands greater accuracy.
Whether or not the print-heads are greyscale and/or binary is a decision made at the time the technology is being incorporated into the engine design. Typically in the display market we see platforms where the print-head moves in a process called multiple-pass printing, and this is known as scanning. Where the heads are fixed and only the material moves the functionality is called single-pass printing, bringing with it the ability for higher speeds and more productive throughput.
The relationship between print-head and final result is probably closer than many people realise. In terms of resolution, binary options print at a resolution that is defined clearly by droplet size, assisted by the number of passes. Although the drop placement can be such that it gives the illusion of higher quality, in truth this is merely a trick of the eye, particularly when output is for distance viewing and not for very close attention.
The binary model is based on each individual pixel either containing no colour or a fixed amount of colour. It is coded so that the drops produced are standardised in the picolitre range, normally between 28 and 100 or more, proving to be reliable and stable in multi-pass technology. In essence, using a relatively simple algorithm either a drop is printed or it's not printed, so the perceived limitations in resulting output quality don't take rocket science to establish. Nonetheless, print-heads incorporating this technology have served their purpose well, particularly in multiple pass applications, and for general interior and exterior wide-format jobs.
Greyscale benefits from allowing greater flexibility in how the ink is put down to form the image. By enabling variable drop sizes to be used, the overall perception can be that the printed graphic is far higher quality than the resolution which is attributed to it. This effect is often quantified by the terms 'visual' or 'apparent' versus 'native' resolution but, with the right algorithms, this print-head technology can be used to create output that the eye sees as smooth and precise.
With greyscale, much smaller drops can be generated that join together prior to meeting the material surface, resulting in an accurate and clean image. These drops are of variable sizes with the intention being that overall images appear to be of superior quality; the human eye perceives output as being of a much higher resolution, even when it isn't. This effect can't be reproduced simply by using smaller uniform drop sizes because it is the variation which gives the impression of greater quality.
With binary printing it is easy to see how extensions to the traditional CMYK ink-set have been needed. These heads are restricted to four plain colours so light cyan and light magenta have become essential where fine gradients and skin tones are required. Because greyscale technology uses variable dot sizes it is possible to simulate a much broader colour range and a higher resolution than the firing frequency and nozzle spacing would suggest. In reality, and in many application areas, the need for additional colours is removed and good quality can be maintained without requiring any additional ink options. This is becoming particularly pertinent for many specialist markets, such as ceramics, where fine gradients and crisp text are both essential prerequisites.
Within the wide-format print sector there is a danger that we can become obsessed with output resolution, even for jobs which are only likely to be viewed at a considerable distance. Nonetheless, manufacturers are now increasingly looking to incorporate greyscale technology into their display platforms to generate better results and lower ink consumption. As printing machines become more sophisticated, we can expect more satisfactory results across all sizes.
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