Category: Inkjet Quality

Posted on

Compensating for blocked or deviated nozzles

As digital presses become wider and higher resolution, supporting additional inks beyond CMYK, the total number of nozzles required per press increases, which in turn raises the likelihood of at least one of those nozzles failing to jet or becoming deviated (and needing to be disabled). Each nozzle that is out will result in a lack of ink where it should have been in a straight line along the direction of print.

One obvious response would be to fix the problematic nozzles, when they have been detected, but this kind of physical intervention interrupts production and can be impractical. Replacing the affected printhead altogether is another possibility, but again this will have an unwelcome financial impact.

Our sister company, Meteor Inkjet, offers NozzleFix™, an embedded software/hardware solution to compensate for missing nozzles.  But for digital printing equipment which does not incorporate Meteor printhead electronics, software compensation may also be beneficial.  In this post, we will look at a software-based approach where good working nozzles can compensate for any misbehaving neighbors as required.

Nozzle-Out compensation in software

As the word “compensation” suggests, this solution works by telling certain nozzles to jet more ink than they otherwise would have to make up for a deficit caused by nozzles that were not jetting or had to be turned off because they had deviated.

For one-drop screening, this could mean that whenever a drop should have been output by a nozzle that’s out, a nozzle on either side could be asked to jet that drop instead. An alternative to this, which can accommodate any drop size, is to apply compensation before screening, working with intensities rather than drops.

Here’s how it works: when a nozzle is off, the intensity for each pixel associated with that nozzle is used to boost the intensity of neighboring pixels, which are then screened as normal.

This is best understood by way of an example. Consider an 8-bit continuous tone input TIFF™ which has the value 160 for every pixel. If we are using one neighbor on either side of a pixel to compensate, we will add 80 to their intensity before screening. If we are using two neighbors on each side, we would add 40.

The greatly zoomed-in images above illustrate the results after 3-drop screening for this example: the left image has one nozzle out and no compensation; the middle image shows compensation with one neighbor either side; the right image shows the use of two neighbors on each side.

Of course, the impact is best judged from the actual prints, where the combination of high resolution and ink results in an impressive optical illusion!

Image shows nozzles out and then a compensated version

 

The scans above were printed at 1200dpi. The image on the left simulates a number of missing nozzles, which can be seen by the presence of thin horizontal white lines (you may need to enlarge it for the full effect). The image on the right shows the impact of applying compensation to each nozzle neighboring a missing nozzle.

Rapid response and adjustment

A key benefit of this software-based approach is that a rapid response and adjustment can happen as soon as a problematic nozzle is detected. There is no need to wait for engineer and parts availability. Additionally, there is no trade-off to make between taking costly action each time a problematic nozzle presents itself or having a tolerance or threshold where nothing is done until a certain number of quality issues have accumulated.

Learn more about the technologies available to improve the quality of your inkjet output by visiting our website.

About the author

Ian Bolton, Product Manager, Direct
Ian Bolton, Product Manager – Direct

Ian has over 15 years’ experience in industry as a software engineer focusing on high performance. With a passion for problem-solving, Ian’s role as product manager for the Direct range gives him the opportunity to work with printer OEMs and break down any new technology barriers that may be preventing them from reaching their digital printer’s full potential.

Further reading:

  1. What causes banding in inkjet? (And the smart software solution to fix it.)
  2. Streaks and Banding: Measuring macro uniformity in the context of optimization processes for inkjet printing

To be the first to receive our blog posts, news updates and product news why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn, Twitter, and YouTube

 

Posted on

Shade and color variation in textile printing – why it’s important and what you can do about it

Printing soft furnishings

With fewer design limitations, a faster turnaround, no minimum run length and higher margins (not to mention reduced use of power and water, and of pollution), it’s not surprising that the digitally printed textile market is growing.1 Inkjet has certainly made textile design and printing much more flexible than screen printing – and that goes for everybody involved, from the designer through the printing company, to the buyer.

But printing textiles on inkjet doesn’t come without its challenges: as a software provider focusing on print quality issues, we often hear from print service providers who can only digitally print two thirds of the jobs they receive because they would not be paid for the quality they could achieve on the others.

Shade or color variation is a common problem. It’s not new in digital printing (it’s always been an issue for screen-printed and dyed textiles as well) and is usually managed by providing a shade band, which printer operators refer to, to check allowable color variations between pieces.

But, unlike screen-printing or dyeing, the color variation on an inkjet press can be visible over a small distance, just a few centimeters, and this results in visible bands across the output. Banding describes features that tend to be 1 – 10 cm across and they’re often caused by variation of inkjet pressure or voltage differences within the head, which typically results in a frown or smile shape. We also see a certain amount of manufacturing variation between heads so that one may print lighter or darker than the head next to it in a print bar. Some types of heads can also wear in use, which can result in less regular banding that can change over time. This means that large areas which should be flat color may not be.

When such a variation occurs it can greatly complicate a lot of post-print steps, especially if you need to put more than one piece of textile together, either in sewing or use (such as a pair of curtains). If that’s the case, then a significant difference may be unacceptable and your printing rejected by your buyer. Ultimately this leads to print service providers rejecting jobs, because they know their digital press can’t handle printing those tricky flat tints or smooth tones.

What can you do about it?

The first thing many companies do to try to overcome this banding is to adjust the voltage to the inkjet head, but this is often time-consuming and expensive because it requires an expert technician. A better alternative is to make the correction in software, which is a more cost-effective and faster solution. It means it can be automated and can act at a much finer granularity, so printing is more accurate. There’s no need to mess with controls that could damage the press, and printing companies themselves can make corrections without the vendor sending a technician on-site.

Our solution at Global Graphics Software for improving banding is PrintFlat™. It corrects tonality to hide banding based on measurements from the press. It adjusts every nozzle separately and doesn’t need a specialist engineer to make press adjustments. PrintFlat can be integrated into different digital front ends, using a variety of RIPs, including Caldera and Colorgate and, not to mention, our own Harlequin RIP®.

Over the years of working with many press manufacturers we’ve discovered that many technical issues and solutions are common across different sectors, including transactional, wide-format, commercial, labels and packaging, and industrial, including ceramics, wall coverings, flooring and of course textiles. That means that we already have years of experience in correcting for banding. Using PrintFlat in your press means print service providers can now take on those jobs they would normally reject.

To learn more about how to eliminate shade and color variation when printing on an inkjet press, listen to Global Graphics Software’s CTO Martin Bailey’s talk for FESPA 2020:

“New techniques to eliminate in-lot shade variation when printing textiles with inkjet.”

Or visit the PrintFlat website: https://www.printflat.com/

Further reading:

  1. What causes banding in inkjet? (And the smart software solution to fix it.)
  2. Streaks and Banding: Measuring macro uniformity in the context of optimization processes for inkjet printing

To be the first to receive our blog posts, news updates and product news why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn and Twitter

_______________________________

  1. Digitally printed textiles are estimated to be between 2% – 5% of the total printed textiles market. Estimated at $146.5 billion in 2018 by Grand View Research
Posted on

What causes banding in inkjet? (And the smart software solution to fix it.)

Banding, or non-uniformity, is a common problem in inkjet printing that can often result in print production downtime and loss of revenue. In this post, I’ll discuss the challenges printer OEMs and print service providers face when trying to reduce banding and provide an insight into the work we’ve been doing at Global Graphics Software to remove banding and streaking artifacts from the print output, enhancing print quality and raising productivity.

What causes banding in inkjet?

Inkjet printheads produce variable density output both across an individual printhead (known as the inkjet ‘smile’) and when comparing output from one printhead with another. The output from a printhead can also change with time, as the printhead wears or ages. Additionally, the overlapping stitch area between printheads in a single-pass printer, or between overlapping passes in a multi-pass printer, can also cause density variations. Such variable density becomes visible in the printed output as ‘banding’ and ‘stripes’, and means that it is not possible for print providers to digitally print jobs with certain image features (such as flat areas or gradients), or that they must sell the lower quality output produced at a significant discount.

Why is uniformity in inkjet a challenge?

Fixing banding or streaking in inkjet is not without its challenges:

  1. In the printer design phase, the use of interlacing in the printing process can be effective at reducing banding and improving uniformity, but significantly impacts the speed and/or cost of the printer. This approach is especially undesirable in single-pass systems, where the only option to interlace is by doubling the quantity, and hence cost, of printheads in the printer.
  2. Currently most OEMs attempt to correct uniformity issues with hardware solutions such as drive voltage tuning, but these give only limited improvement and are slow, complex and costly to implement. Most printheads have only limited voltage adjustment for banks of many nozzles together, or even the entire printhead as a whole, and do not allow adjustment of drive voltage for individual nozzles – such adjustment does not have the granularity necessary to really eliminate banding. Additionally, adjusting drive voltage to balance output density (drop volume), is undesirable as this is likely to negatively impact drop velocity, printing reliability (jetting stability) and even printhead lifetime. As the printer performance changes over time, and when printheads are replaced, service and support engineers must spend a significant amount of time onsite re-making these complex adjustments to achieve quality that is, at best, a compromise.

A solution in software

Global Graphics Software has been working with printer OEMs and print service providers to significantly enhance the quality of their inkjet output, one such company being Ellerhold AG, a leading poster printing house and press manufacturer in Germany.

Ellerhold wanted to enhance the printing quality of it’s large-format posters. Specifically, the printheads on its digital printing machine showed variation in printed density both between the heads and across each head, which produced clearly visible bands within some types of printed output.

Together with Ellerhold we were able to enhance the quality of the printed output using our ScreenPro™ screening engine with PrintFlat™ technology. ScreenPro is a very fast and efficient multi-level screening engine that mitigates artifacts such as banding or streaking and mottling from the inkjet print process and can be used in any print workflow, including Adobe®, Caldera, Esko, EFI and Sofha, with any combination of inks, substrates, printheads and electronics. In ScreenPro every nozzle can be addressed separately on any head/electronics to achieve very fine granularity. The PrintFlat technology adjusts the density within ScreenPro to produce uniform density across a print bar, thereby optimizing print quality.

The project brought many technical challenges: As it was a multi-pass process we needed to efficiently capture repeating density variations across the entire print area in an unbiased way. We carried out tests, analyzed the scanned prints and created a PrintFlat calibration workflow for the press designed to compensate for the non-uniformity in output across the print bar. The team also used a variant of Global Graphics Software’s Advanced Inkjet Screens™, available with ScreenPro and the Harlequin RIP®, which they adapted specially for scanning-head systems. These proved very effective.

You can watch the short case study film here:

PrintFlat technology provided the ideal solution, giving smooth, uniform tints and accurate tone reproduction via a simple ‘fingerprint’ calibration of the screening process, where the density compensation is then built into the screen halftone definition. This means that the PrintFlat calibration is applied during the screening process at runtime and enhances the quality of your output without any compromise on speed. The PrintFlat approach addresses every individual nozzle, has no negative effect on other printing parameters, and allows drive voltage to be used to maximize printing stability and reliability instead.

A valuable additional benefit is in increasing overall productivity. Achieving higher quality with fewer print passes allows for greater use of faster print modes. Jobs that require 4-pass quality can be printed in 2-pass mode with PrintFlat.

The process can be automated for closed-loop correction and, unlike correction by adjustment of voltages, there is no effect on jetting stability or head lifetime, nor ink pressure and timing/drop speed variation.

PrintFlat can increase the added value of your service engineers’ visits, producing a much higher quality result in less time. Alternatively, the print service provider can operate the PrintFlat calibration process to maximize their output quality themselves.

Sunflower web image before PrintFlat is applied Sunflower web image after PrintFlat is applied.Before and after images illustrating how effective PrintFlat technology is at improving print uniformity.

 

For more information about PrintFlat technology visit: https://www.globalgraphics.com/technologies/printflat

For further reading about the causes of banding and streaking in inkjet output see our related blog posts:

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

  2. Where is screening performed in the workflow

About the author

Jimmy Fox, Inkjet Printing Technologist, Global Graphics Software
Jimmy Fox, Inkjet Printing Technologist, Global Graphics Software

Jimmy Fox is an inkjet printing technologist with 25 years’ experience of developing inkjet printers, inks and applications.

Be the first to receive our news updates and product news. Why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn and Twitter

Posted on

Why does optimization of VDP jobs matter?

Would you fill your brand-new Ferrari with cheap and inferior fuel? It’s a question posed by Martin Bailey in his new guide: ‘Full Speed Ahead – how to make variable data PDF files that won’t slow your digital press’. It’s an analogy he uses to explain the importance of putting well-constructed PDF files through your DFE so that they don’t disrupt the printing process and the DFE runs as efficiently as possible. 

Here are Martin’s recommendations to help you avoid making jobs that delay the printing process, so you can be assured that you’ll meet your print deadline reliably and achieve your printing goals effectively:

If you’re printing work that doesn’t make use of variable data on a digital press, you’re probably producing short runs. If you weren’t, you’d be more likely to choose an offset or flexo press instead. But “short runs” very rarely means a single copy.

Let’s assume that you’re printing, for example, 50 copies of a series of booklets, or of an imposed form of labels. In this case the DFE on your digital press only needs to RIP each PDF page once.

To continue the example, let’s assume that you’re printing on a press that can produce 100 pages per minute (or the equivalent area for labels etc.). If all your jobs are 50 copies long, you therefore need to RIP jobs at only two pages per minute (100ppm/50 copies). Once a job is fully RIPped and the copies are running on press you have plenty of time to get the next job prepared before the current one clears the press.

But VDP jobs place additional demands on the processing power available in a DFE because most pages are different to every other page and must therefore each be RIPped separately. If you’re printing at 100 pages per minute the DFE must RIP at 100 pages per minute; fifty times faster than it needed to process for fifty copies of a static job.

Each minor inefficiency in a VDP job will often only add between a few milliseconds and a second or two to the processing of each page, but those times need to be multiplied up by the number of pages in the job. An individual delay of half a second on every page of a 10,000-page job adds up to around an hour and a half for the whole job. For a really big job of a million pages it only takes an extra tenth of a second per page to add 24 hours to the total processing time.

If you’re printing at 120ppm the DFE must process each page in an average of half a second or less to keep up with the press. The fastest continuous feed inkjet presses at the time of writing are capable of printing an area equivalent to over 13,000 pages per minute, which means each page must be processed in just over 4ms. It doesn’t take much of a slow-down to start impacting throughput.

If you’re involved in this kind of calculation you may find the digital press data rate calculator useful: Download the data rate calculator

Global Graphics Software’s digital press data rate calculator.
Global Graphics Software’s digital press data rate calculator.

This extra load has led DFE builders to develop a variety of optimizations. Most of these work by reducing the amount of data that must be RIPped. But even with those optimizations a complex VDP job typically requires significantly more processing power than a ‘static’ job where every copy is the same.

The amount of processing required to prepare a PDF file for print in a DFE can vary hugely without affecting the visual appearance of the printed result, depending on how it is constructed.

Poorly constructed PDF files can therefore impact a print service provider in one or both of two ways:

  • Output is not achieved at engine speed, reducing return on investment (ROI) because fewer jobs can be produced per shift. In extreme cases when printing on a continuous feed (web-fed) press a failure to deliver rasters for printing fast enough can also lead to media wastage and may confuse in-line or near-line finishing.
  • In order to compensate for jobs that take longer to process in the DFE, press vendors often provide more hardware to expand the processing capability, increasing the bill of materials, and therefore the capital cost of the DFE.

Once the press is installed and running the production manager will usually calculate and tune their understanding of how many jobs of what type can be printed in a shift. Customer services representatives work to ensure that customer expectations are set appropriately, and the company falls into a regular pattern. Most jobs are quoted on an acceptable turn-round time and delivered on schedule.

Depending on how many presses the print site has, and how they are connected to one or more DFEs this may lead to a press sitting idle, waiting for pages to print. It may also delay other jobs in the queue or mean that they must be moved to a different press. Moving jobs at the last minute may not be easy if the presses available are not identical. Different presses may require different print streams or imposition and there may be limitations on stock availability, etc.

Many jobs have tight deadlines on delivery schedules; they may need to be ready for a specific time, with penalties for late delivery, or the potential for reduced return for the marketing department behind a direct mail campaign. Brand owners may be ordering labels or cartons on a just in time (JIT) plan, and there may be consequences for late delivery ranging from an annoyed customer to penalty clauses being invoked.

Those problems for the print service provider percolate upstream to brand owners and other groups commissioning digital print. Producing an inefficiently constructed PDF file will increase the risk that your job will not be delivered by the expected time.

You shouldn’t take these recommendations as suggesting that the DFE on any press is inadequate. Think of it as the equivalent of a suggestion that you should not fill your brand-new Ferrari with cheap and inferior fuel!

Full Speed Ahead: how to make variable data PDF files that won't slow your digital press edited by Global Graphics Software

The above is an excerpt from Full Speed Ahead: how to make variable data PDF files that won’t slow your digital press. The guide is designed to help you avoid making jobs that disrupt and delay the printing process, increasing the probability of everyone involved in delivering the printed piece; hitting their deadlines reliably and achieving their goals effectively.

DOWNLOAD THE FREE FULL GUIDE

To be the first to receive our blog posts, news updates and product news why not subscribe to our monthly newsletter? Subscribe here

About the author:

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

Martin Bailey first joined what has now become Global Graphics Software in the early nineties, and has worked in customer support, development and product management for the Harlequin RIP as well as becoming the company’s Chief Technology Officer. During that time he’s also been actively involved in a number of print-related standards activities, including chairing CIP4, CGATS and the ISO PDF/X committee. He’s currently the primary UK expert to the ISO committees maintaining and developing PDF and PDF/VT.

To be the first to receive our blog posts, news updates and product news why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn and Twitter

Posted on

Full Speed Ahead: How to make variable data PDF files that won’t slow your digital press

The use of variable data has increased exponentially over the past five years and is emerging in new applications such as industrial inkjet. Yet poorly designed variable data PDF files disrupt production and reduce ROI.

Watch the recent webinar with Global Graphics Software’s CTO Martin Bailey, the author of Full Speed Ahead, a new guide to offer advice to anyone with a stake in variable data printing, including graphic designers, print buyers, production managers, press operators, composition tool developers and users.

In the webinar Martin presents an overview of the guide and highlights some of the key tips and tricks for graphic designers, prepress and print service providers, showing how, when they all work together, VDP jobs can fly through digital presses.

Sponsored by Delphax Solutions, Digimarc, HP Indigo, HP PageWide Industrial, HYBRID Software, Kodak, Racami and WhatTheyThink, the guide is a practical format for easy reference and includes:

• Tips and tricks for making fast, efficient PDF files for variable data printing
• Helpful illustrations, photos and explanatory diagrams
• Real examples from industry

You can download your copy here: https://www.globalgraphics.com/full-speed-ahead

For further reading about PDF documents and standards:

  1. PDF Processing Steps – the next evolution in handling technical marks

To be the first to receive our blog posts, news updates and product news why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn and Twitter

Posted on

Improve inkjet output quality with PrintFlat™

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.

Find out more about PrintFlat here.

 

Posted on

Where is screening performed in the workflow?

In my last post I gave an introduction to halftone screening. Here, I explain where screening is performed in the workflow:

 

Halftone screening must always be performed after the page description language (such as PDF or PostScript) has been rendered into a raster by a RIP … at least conceptually.

In many cases it’s appropriate for the screening to be performed by that RIP, which may mean that in highly optimized systems it’s done in parallel with the final rendering of the pages, avoiding the overhead of generating an unscreened contone raster and then screening it. This usually delivers the highest throughput.

Global Graphics Software’s Harlequin RIP® is a world-leading RIP that’s used to drive some of the highest quality and highest speed digital presses today. The Harlequin RIP can apply a variety of different halftone types while rendering jobs, including Advanced Inkjet Screens™.

But an inkjet press vendor may also build their system to apply screening after the RIP, taking in an unscreened raster such as a TIFF file. This may be because:

  • An inkjet press vendor may already be using a RIP that doesn’t provide screening that’s high enough quality, or process fast enough, to drive their devices. In that situation it may be appropriate to use a stand-alone screening engine after that existing RIP.
  • To apply closed loop calibration to adjust for small variations in the tonality of the prints over time, and to do so while printing multiple copies of the same output, in other words, without the need for re-ripping that output.
  • When a variable data optimization technology such as Harlequin VariData™ is being used that requires multiple rasters to be recomposited after the RIP. It’s better to apply screening after that recomposition to avoid visible artifacts around some graphics caused by different halftone alignment.
  • To access sophisticated features that are only available in a stand-alone screening engine such as Global Graphics’ PrintFlat™ technology, which is applied in ScreenPro™.

Global Graphics Software has developed the ScreenPro stand-alone screening engine for these situations. It’s used in production to screen raster output produced using RIPs such as those from Esko, Caldera and ColorGate, as well as after Harlequin RIPs in order to access PrintFlat.

Achieve excellent quality at high speeds on your digital inkjet press: The ScreenPro engine from Global Graphics Software is available as a cross platform development component to integrate seamlessly into your workflow solution.
Achieve excellent quality at high speeds on your digital inkjet press: The ScreenPro engine from Global Graphics Software is available as a cross platform development component to integrate seamlessly into your workflow solution.

The above is an excerpt from our latest white paper: How to mitigate artifacts in high-speed inkjet printing. Download the white paper here.

For further reading about the causes of banding and streaking in inkjet output see our related blog posts:

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

  2. What causes banding in inkjet? (And the smart software solution to fix it.)

Be the first to receive our news updates and product news. Why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn and Twitter

Posted on

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/

For further reading about the causes of banding and streaking in inkjet output see our related blog posts:

  1. What causes banding? (And the smart software solution to fix it.)

  2. Where is screening performed in the workflow

Be the first to receive our news updates and product news. Why not subscribe to our monthly newsletter? Subscribe here

Follow us on LinkedIn and Twitter

Posted on

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.

Posted on

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