If you print on an inkjet press you’ll know that the problem of non-uniformity or banding is a particularly difficult one to resolve. It’s especially acute on areas of flat tints with the result that printed output is unacceptable to you and to your customers. This means you either don’t run certain jobs on your inkjet press or, in some sectors of the market, are forced to sell your output at a discount.
The good news is that with PrintFlat you have a solution that is quick to deploy and cost-effective, and it can be applied to any workflow with or without a RIP. With more press vendors adopting this technology, watch our new explainer video to see how you might benefit.
Halftone screening, also sometimes called halftoning, screening or dithering, is a technique to reliably produce optical illusions that fool the eye into seeing tones and colors that are not actually present on the printed matter.
Most printing technologies are not capable of printing a significant number of different levels for any single color. Offset and flexo presses and some inkjet presses can only place ink or no ink. Halftone screening is a method to make it look as if many more levels of gray are visible in the print by laying down ink in some areas and not in others, and using such a small pattern of dots that the individual dots cannot be seen at normal viewing distance.
Conventional screening, for offset and flexo presses, breaks a continuous tone black and white image into a series of dots of varying sizes and places these dots in a rigid grid pattern. Smaller dots give lighter tones and the dot sizes within the grid are increased in size to give progressively darker shades until the dots grow so large that they tile with adjacent dots to form a solid of maximum density (100%). But this approach is mainly because those presses cannot print single pixels or very small groups, and it introduces other challenges, such as moiré between colorants and reduces the amount of detail that can be reproduced.
Most inkjet presses can print even single dots on their own and produce a fairly uniform tone from them. They can therefore use dispersed screens, sometimes called FM or stochastic halftones.
A dispersed screen uses dots that are all (more or less) the same size, but the distance between them is varied to give lighter or darker tones. There is no regular grid placement, in fact the placement is more or less randomized (which is what the word ‘stochastic’ means), but truly random placement leads to a very ‘noisy’ result with uneven tonality, so the placement algorithms are carefully set to avoid this.
Inkjet is being used more and more in labels, packaging, photo finishing and industrial print, all of which often use more than four inks, so the fact that a dispersed screen avoids moiré problems is also very helpful.
Dispersed screening can retain more detail and tonal subtlety than conventional screening can at the same resolution. This makes such screens particularly relevant to single-pass inkjet presses, which tend to have lower resolutions than the imaging methods used on, say, offset lithography. An AM screen at 600 dots per inch (dpi) would be very visible from a reading distance of less than a meter or so, while an FM screen can use dots that are sufficiently small that they produce the optical illusion that there are no dots at all, just smooth tones. Many inkjet presses are now stepping up to 1200dpi, but that’s still lower resolution than a lot of offset and flexo printing.
This blog post has concentrated on binary screening for simplicity. Many inkjet presses can place different amounts of ink at a single location (often described as using different drop sizes or more than one bit per pixel), and therefore require multi-level screening. And inkjet presses often also benefit from halftone patterns that are more structured than FM screens, but that don’t cluster into discrete dots in the same way as AM screens.
The above is an excerpt from our latest white paper: How to mitigate artifacts in high-speed inkjet printing. Download the white paper here.
The Inkjet Conference Düsseldorf has been and gone for another year and we’re already looking ahead to the 2019 events that will be organised by ESMA.
This year delegates in the audience were able to submit questions via an app for the first time. I’m grateful to the IJC for sending me the questions that we either didn’t have time to cover after my presentation, or that occurred subsequently. So here they are with my responses:
Is it possible to increase the paper diversity with software by e.g. eliminating paper related mottling?
Yes, we have yet to come across a media/ink combination ScreenPro™ will not work well with. The major artefact we correct for is mottle. This may mean you can print satisfactory results with ScreenPro on papers where the mottle was unacceptable previously, so increasing the diversity of papers that can be used.
It sounds like ScreenPro is very good at tuning a single machine. How do you also then match that output quality among several machines?
There are two technologies in ScreenPro, the screening core itself with the Advanced Inkjet Screens (AIS), and PrintFlat™ to correct for cross web banding. ScreenPro generally improves print quality and Mirror and Pearl screens (AIS) work in the majority of screening situations. PrintFlat, however, needs to be tuned to every press and if the press changes significantly over time, if a head is changed for example, it will have to be recalibrated. This calibration actually makes subsequent ink linearization and colour profiling more consistent between machines as you have removed the cross-web density fluctuations (which are machine specific) from the test charts used to generate these profiles.
“We haven’t found ink or substrate that we couldn’t print with.” Does this include functional materials, such as metals, wood, rubber? or is it limited to cmyk-like processes?
No – with ScreenPro we have only worked with CMYK-like process colours, i.e. print that is designed to be viewed with colour matching etc. ScreenPro is designed to improve image quality and appearance. I see no reason why ScreenPro would not work with functional materials but I would like to understand what problems it is trying to solve.
What is the main innovation of the screening software in terms of how it works as opposed to what it can do?
“How it works” encompasses placing the drops differently on the substrate in order to work around common inkjet artefacts. The innovation is therefore in the algorithms used to generate the screens.
By Tom Mooney, product manager for Global Graphics Software
I’ve just returned from a road trip in the US to inkjet press manufacturers who are all interested in using ScreenPro.
The meetings have gone in a very similar manner with the opening line: “We have a print shop that wants to print this job, but take a look at this area.” They point to an area of the image, usually in the shadows, and it is either a muddy brown mess or crusty and flaky, the typical ‘orange peel’ effect. We all agree the print is unacceptable and cannot be sold, so we discuss what can be done.
Firstly, we look at the ink limitation, but this kills the color saturation in the rest of the print. We look at color management and under color removal, but this only moves the problem to a different area on the image.
Then we see what ScreenPro can do.
We try our Advanced Inkjet Screens™ and use Pearl screen on the muddy mess and Mirror on the orange peel. This does the trick and makes the prints acceptable, so the print shop can sell that print job.
As long as this quality threshold is met the customer is happy. This quality is achieved by a combination of hardware, media and ink and software. Color management is only part of the story with the software – ScreenPro makes a real impact on those hard-to-solve killer jobs.
They say a problem shared is a problem halved. Well, two weeks on from our launch of our Advanced Inkjet Screens it’s been gratifying to see how much the discussion of inkjet output quality has resonated among the press vendor community.
Just in case you missed it, we’ve introduced a set of screens that mitigate the most common artifacts that occur in inkjet printing, particularly in single-pass inkjet but also in scanning heads. Those of you who’ve attended Martin Bailey’s presentations at the InkJet Conference ( The IJC) will know that we’ve been building up to making these screens available for some time. And we’ve worked with a range of industry partners who’ve approached us for help because they’ve struggled to resolve problems with streaking and orange peel effect on their own.
Well, now Advanced Inkjet Screens are available as standard screens that are applied by our ScreenPro screening engine. They can be used in any workflow with any RIP that allows access to unscreened raster data, so that’s any Adobe PDF RIP including Esko. Vendors can replace their existing screening engine with ScreenPro to immediately benefit from improved quality, not to mention the high data rates achievable. We’ve seen huge improvements in labels and packaging workflows. Advanced Inkjet Screens are effective with all the major inkjet printheads and combinations of electronics. They work at any device resolution with any ink technology.
Why does a halftone in software work so well? Halftones create an optical illusion depending on how you place the dots. Streaking or graining on both wettable and non-absorbent substrates can be corrected. Why does this work in software so well? Halftoning controls precisely where you place the dots. It just goes to show that the assumption that everything needs to be fixed in hardware is false. We’ve published a white paper if you’re interested in finding out more.