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Magazine quality screening at newspaper speeds?

Monday, 06 August 2007
By Print 21 Online Article

Gordon Pritchard, Author of BRIDG’s technical Guide to Halftone Screening gives the Kodak perspective.

As newspaper printers strive to find practical methods to deliver the higher reproduction fidelity that publishers and advertisers increasingly demand, attention has turned to screening technologies. Halftone screening is, after all, at the core of printing – the vehicle for delivering ink to paper.

The choice of halftone screening can have a significant impact on the visual appeal and reproduction fidelity of the presswork as well as its on-press printability, and hence impact the print manufacturing economics. A somewhat new class of halftone screening – the “hybrid AM” (a.k.a. “XM”) screen is increasingly touted as the preferred method for high-resolution printing and surpassing the benefits of second-order stochastic (FM) type screens. So, do the claims have any substance?

Hybrid screening basics

Hybrid AM/XM screens were initially developed to overcome resolution limitations in the plate imaging process. The lack of resolution, as in the rubber-like plates used in flexographic printing, can cause highlights (1%-4% dots) and/or shadows (96%-99% dots) to be “clipped” off when the plates are imaged.

A similar effect is seen in a film-to-plate workflow, where vacuum frames, draw down times, and chemistry variables resulted in a lack of imaging process control which in turn caused often caused the same highlight and shadow dots to be lost. Unfortunately, a similar lack of imaging integrity and process control can still be present in CtP imaging systems.

In fact, the inability to hold highlight and shadow dots on the plate, just as it is in film-to-plate exposure is an indicator of plate imaging issues. Unfortunately it is not just the highlight and shadow dots that are affected – dots throughout the tone scale will be affected, particularly as one goes to finer screens (AM, hybrid AM/X (pictured) or hybrid FM).

One work-around to this imaging issue – at least for highlight and shadow dots – is the hybrid AM/XM halftone screen. Rather than using the problematic smaller dots to render lighter tones, hybrid AM/XM screens use fewer, larger, dots in a given area to recover the otherwise lost values.

For example, with Kodak Maxtone (XM) screening the printer defines the minimum reproduceable dot size determined by the limitations of their imaging process (typically between 21 and 36 microns (2% – 3% dot at 175 lpi equivalent). Highlight and shadow dots can be set independently.

At that point, dots no longer get smaller. Instead those 2% or 3% dots are removed from the imaging grid in an FM-like manner – this is what makes it a hybrid screen, the dots are the same size while the number of dots varies. Because there are fewer dots in a given area it appears lighter.

The remaining dots are still placed on the AM grid, as are the dots in the rest of the tone scale, and process colors are still imaged at fixed screen angles. Indeed, in the majority of hybrid AM/XM (pictured) cases the underlying screen is the vendor’s conventional AM screening – there is no difference other than the dots at the extreme highlights and shadows.

On press with the different screening technologies

Assuming the plate imaging system can hold the small highlight and shadow dots, and the press is in reasonable mechanical condition, even newsprint can hold very fine screens – 175 lpi and finer. As a result, hybrid AM/XM screening are unlikely to provide any benefits over the equivalent halftone frequency conventional AM screen on press (e.g. 180 lpi AM vs 180 lpi hybrid AM/XM on newsprint).

This is because, the dots from 2%-96% are exactly the same between the two types of screen. While increasing the line frequency from the conventional 100-110 lpi certainly makes the screen less visible, one issue that can arise when using a hybrid AM/XM screen instead of an equivalent conventional AM screen is that the remaining dots in the highlight areas are still on the AM grid.

The result is often an appearancce of graininess in those areas especially when the minimum reproducible dot size is rather large. Also when using tile based screening, typical in modern halftone screen designs, the replication of the tile will become visible because of the non-homogeneity introduced by the FM part of the screen.

Hybrid AM/XM screens do not solve press ink chemistry issues – rather, they can mitigate some plate imaging issues (as in the case of flexography). From a press printability point of view, the organization of dots (AM, XM, or FM) is not the primary issue. Rather, it is the relative sizes of the dots through the tone scale that may reveal any deficiencies of the press/ink/water condition.

If on-press issues arise with a hybrid FM screen (second-order FM) they will also likely arise with an equivalent halftone hybrid AM/XM or AM screen under the same conditions as well.

On the other hand, a hybrid FM screen, such as 36 micron Kodak Staccato – equivalent to about a 240 lpi AM/XM screen – which has been tailored for newspaper application provides all the benefits of an FM screen without any of the issues associated with the equivalent AM or hybrid AM/XM screens.

With Staccato, the dots that form highlight areas are rendered at 30 microns – equivalent to a 2% dot at 100 lpi – well within the capabilities of a newspaper press and substrate to hold. Because the highlight dots are not constrained to an AM grid, dot placement is optimized for smoothness on highlight areas. There are no longer screen angles or frequencies – hence no subject or screening moirés and colour is more stable on press due to the smaller ink profile of the dots in the midtones compared to the equivalent hybrid AM/XM screen.

Indeed, in the U.S.A. over 80% of all Yellow Page phone directories have, for the past four years, been printed using second order FM screening, in the majority of cases Kodak Staccato screening. As well, over 40% of newspaper flyer inserts are using this class of screening.

The reasons are twofold. First, Kodak SQUAREspot thermal CtP technology enables accurate, consistent, and reliable imaging on plate of small highlight and shadow dots. Unlike a hybrid AM/XM screen, SQUAREspot imaging also maintains the integrity of halftone dots throughout the tone scale – whether those dots are organized as AM, hybrid AM/XM or hybrid FM screens. The result is that there is no need to compromise highlight or shadow dots with a hybrid AM/XM strategy.

Secondly, Staccato hybrid FM screening eliminates the moiré and screening issues associated with AM or hybrid AM/XM screening. These two elements – plate imaging integrity and Staccato screening have enabled Kodak to become the number one newspaper solution supplier in the U.S.A. market.

This article was brought to you by Kodak

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