Streaks and Banding: Measuring macro uniformity in the context of optimization processes for inkjet printing

Dr Danny Hall, Chief Screening Scientist, Global Graphics Software

Global Graphics Software’s chief screening scientist, Dr Danny Hall, discusses the emerging standards designed to objectively characterize directional print variations with particular reference to the ISO TS 18621.21 standard:

Directional printing artifacts like streaks and banding are commonly encountered problems in digital printing systems. For example, inkjet systems may produce characteristic density variations due to inconsistencies between printheads or intra-printhead variations between nozzles. When these variations have a high spatial frequency they can be characterized as causing ‘streaking’ in the direction of print, where the variations have a low spatial frequency this can cause the appearance of ‘banding’ in the direction of print.

Other causes of directional streaking and banding effects may be due, for example, to variations in the speed of printhead or substrate velocities resulting in density variations across the direction of printing. The ‘wow’ and ‘flutter’ of the digital printing age.

In the décor market there is a visual perceptual test sometimes referred to as a ‘porthole test’. In this test a human subject is presented with a print (e.g. wallpaper or floor covering) rotating slowly behind a round window under controlled viewing conditions. If they can determine the direction of printing then it test is a ‘fail’. One aspect of the porthole test is that it allows for the perceptual response differences between different printed images, for example the same press and conditions may be able to print one job containing a lot of graphical detail, but still fail on another job requiring flat tints.

There are currently emerging standards designed to objectively characterize this type of directional print variation.  For example, the proposed ISO TS 18621-21 standard defines a measurement method for the evaluation of distortions in the macroscopic uniformity of printed areas that are oriented in the horizontal and/or vertical direction, like streaks and bands.

Such recognized standards could be very useful for the development and maintenance of printing systems; as well as potentially allowing for the quantitative comparison of directional quality between different printing systems.

Having an objective ISO measurement of directional uniformity would therefore be a very useful step forward and something we at Global Graphics would like to encourage.

As a first step the current ISO TS 18621-21 proposal looks good and useable and provides for a robust and simple metric that can be calculated using standard equipment.

However, in exploring the potential use of this standard we also note a few limitations which may constrain the widest possible utility for a general directional measure in printing. For example, the frequency response of the proposed measurement technique may limit the response of the measure to higher frequency ‘streaking’ artifacts, this may be inevitable with the measurement devices available but this potential spatial frequency bias needs to be clearly understood and accepted.

Another challenge in standardizing such a metric across different printing platforms is the difficulty in selecting some kind of objective color tint to measure. The ‘goodness’ of the proposed ISO TS 18621-21 metric will depend on the color tint chosen for measurement; therefore making such measurements standard between systems with different color gamuts is a difficult and perhaps impossible task. Nonetheless we would like to propose a color tint selection strategy which at least a priori could have the potential to provide a selection of standardized color tints that could be used meaningfully with ISO TS 18621-21 across a range of different printers.

Frequency response

The frequency response is discussed in the ISO proposal.  There is a potential bias in the measuring methodology towards lower frequencies due to the suggested 6mm sampling cut-off. For example, in our experience the main frequency elements of ‘streakiness’ may not be captured by this methodology potentially resulting in a bias towards lower-frequency ‘banding’ effects. That’s not necessarily a problem, it just needs to be understood that this metric may be biased towards ‘banding’ over ‘streakiness’ determination.

Where any streakiness is random and un-correlated with lower frequency banding: changes in high frequency streakiness can be expected to show up statistically as variations at lower frequencies (white noise). However, there are currently printing compensation systems available (such as PrintFlat ™) which can correct for directional variations so that high and low frequency variations are no longer correlated in a gaussian way. In such a case the proposed metric could in the worst case be blind to any underlying changes in high frequency streakiness variation above the band-pass of the sampling system.

Color selection

The proposed standard does not specify the printed color to use, which may make objective comparisons between systems based on this metric difficult and the metric itself is correlated with the underlying contrast of the tint selected. For example, one can expect an apparently better metric to result from printing a 5% tint compared to a 70% tint of the same ink. Therefore, an objective method for selecting color tints could be helpful and this is something we would like to explore.

This is an abstract from Danny’s forthcoming talk at the TAGA Annual Technical Conference, March 17 – 20, 2019 in Minneapolis, MN.

Register here: https://www.taga.org/register/

New to inkjet? Come and see us at Hunkeler Innovationdays

Martin Bailey, CTO, Global Graphics Software
Martin Bailey, CTO, Global Graphics Software

If you are new to inkjet and are building your first press no doubt you’ll have many questions about the workflow and the Digital Front End.

In fact, you’re probably wondering how to scope out the functionality you need to create a DFE that is customised to exactly what your customers require. Among your concerns will be how you’re going to achieve the throughput you need to keep the press running at rated speed, especially when handling variable data. Or it might be handling special colours or achieving acceptable image quality that is keeping you awake at night.  And how to achieve this without increasing the bill of materials for your press?

At Hunkeler Innovationdays we’ll have a range of resources available to address just such questions with some real case study examples of how our OEM customers have solved the problems that were causing them a headache using our technology and the skills of our Technical Services team.

For instance, how, on a personalised run, when every label or page might be different, can you stop the press from falling idle whilst the RIP catches up?  Our ScreenPro™ technology helps Mark Andy cut processing time by 50% on the Mark Andy Digital Series HD, enabling fully variable (every label is different) continuous printing at high-speed and at high-quality.

How can you avoid streaking on the image if your substrate is racing under your printheads at speeds of up to 300m/min for aqueous and maybe 90m/min for UV.  Or mottling? The Mirror and Pearl Advanced Inkjet Screens™ available with ScreenPro have been developed specifically to address these problems.

During the lifetime of the press, how can you avoid variations in quality that look like banding because your printheads have worn or been replaced?  Take a look at what Ellerhold AG has achieved by deploying PrintFlat™.

The ScreenPro screening engine is one of the building blocks you’ll need for your inkjet press. Our Fundamentals components provide other functions that are essential to the workflow such as job management, soft proofing, and colour management.

Using a variety of white papers, print samples, video footage and case studies , we’ll be sharing our experience.  So, come along and meet the team:  that’s me, Jeremy Spencer, Justin Bailey and our colleague Jonathan Wilson from Meteor Inkjet if you want to chat about their printhead driver electronics that are endorsed by the world’s leading industrial inkjet printhead manufacturers.

 

Join us at Hunkeler Innovationdays 2019

 

Screening for the next-generation high-definition devices

In days gone by, almost every job was more or less 600 dpi in both directions. Now there is a drive to higher definition, with higher resolutions and smaller drop sizes.

So we’ve introduced a new feature in ScreenPro™ that allows the resolution of a job to be “upscaled” meaning that a RIP can still render at 600 dpi through an existing workflow and then ScreenPro can upscale the job to the printer resolution. The benefit is that you don’t need to change your existing workflow, can cut down on RIP time by RIPping at 600 dpi, but print on a 1200 dpi machine for increased definition.

There are various ways of achieving higher resolutions: use the new generation of print heads running at 1200 dpi, use multiple print bars, or use scanning head printers for multiple passes. Sometimes it really is increased resolution that is required and other times it is higher addressability and, for example with textile printing, sometimes you just need to be able to put down more ink in any given location.

Once manufacturers have achieved 1200 x 1200 dpi there are other problems to solve. There is four times as much data generated that needs to be passed through the workflow pipeline to the press compared to a 600 dpi data path. There are some applications where the higher addressability isn’t needed, and 600 dpi is ok, in this case you could run the press twice as fast and get twice the production if you ran it at 1200 x 600 dpi, or three times as fast at 1200 x 400 dpi.

To solve the problem of too much data slowing down processing times we have implemented resolution upscaling in the latest release of ScreenPro. The typical example is that we have an existing press and workflow to go with it at 600dpi. The RIP delivers data at this resolution. We then have a choice – to send it to the 600 dpi printer, in which case we screen as normal, or we send to the 1200 dpi machine.

In this simple case we use ScreenPro to double the number of dots it produces in both directions. For non-square resolutions we multiply in one direction only. Also for non-square resolutions we have to change the shape of the screens, a circular screen will be distorted by the non-square printer resolution so we have to correct for that up front.

What this means is that you can continue to RIP at 600 dpi and keep the same workflow right up to the last process of screening. You keep the same PC processor requirements, same network data transfer speeds. Only at the last stage use ScreenPro to upscale to your desired resolution.

This will be a really useful feature for many customers developing the next generation of high definition digital printers.

Hunkeler Innovationdays 2019
Join us at Hunkeler Innovationdays 2019 to learn more about the new features in ScreenPro.

ScreenPro™ with PrintFlat™ removes banding on large format posters

Watch our latest video showcasing our award-winning technology, ScreenPro with PrintFlat.

Global Graphics Software’s Technical Services team worked with Ellerhold AG, the leading poster printing house in Germany, to enhance the printing quality of its large-format posters. The result was 100% customer satisfaction and an increased market share of outdoor advertising products in digital printing.

Questions from the Inkjet Conference, Düsseldorf

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.

Advanced Inkjet Screens are standard in the ScreenPro screening engine

 

ScreenPro™ makes a real impact on those hard-to-solve killer jobs

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.

ScreenPro’s Advanced Inkjet Screens solve two common problems: Mirror avoids orange peel mottle on non-absorbent, poorly wettable substrates and Pearl counteracts streaking on reasonably absorbent and/or wettable substrates
ScreenPro’s Advanced Inkjet Screens solve two common problems: Mirror avoids orange peel mottle on non-absorbent, poorly wettable substrates and Pearl counteracts streaking on reasonably absorbent and/or wettable substrates