Concentric Screening ?

[CKL]

Possible switch to concentric screening coming. Sheetfed. Should the press room be concerned ?

[gordo]

Both press room and prepress will need to be concerned.
Concentric screening is really an AM screen ruling multiplier and will have similar behaviour to any very fine AM or FM screen and carry many of the same benefits.
Patterning within each color channel can be an issue because it destroys visible uniformity. This patterning is partly influenced by imaging, plates and presswork and requires users to experiment till they find something suitable. Essentially, the screening math is hampered by geometric constraints (angle, resolution, lpi, dot shape + ring width).
Visible structure is kept to a minimum by specifying very narrow rings, BUT this pushes the frequency so high that process stability and imaging are compromised. Even when screens are free from visible artifacts, they are often so fine that it is difficult to support their use on plate let alone find a way to implement them in the press room. The problem is that some concentric screen settings drive rulings way over what plate imaging can support - on the order of 1-2 pixel widths for the rings, which is understandably problematic. For example, a 200 lpi screen with 2 pixel ringwidths = 600 lpi which is finer than, for example, Kodak Staccato 10.
Coarser ring widths are easier to implement but at that point it is probably more effective to use an AM screen of equivalent lpi. Since it is still an AM screen there is still the opportunity for screening and subject moiré - although the finer the screen the less likely that will be a problem.
From a print buyer point of view there will likely be no visible difference between a 2-300 lpi conventional AM screen and concentric screening - even under a loupe.

best gordo

[Sequoyah Graphics]

>Patterning within each color channel can be an issue because it destroys visible uniformity.

Hi Gordon,

I had not heard of this technology until I saw the first post in this thread. So I did a Google search and found some interesting links. This one below, page 6, seems to contradict your point directly. I realize it is by the vendor, but the disagreement seems to be one of fact.

http://print05.gasc.org/PDF/mse_individual.pdf

Al

Edited by: Al Ferrari on Feb 25, 2008 2:22 PM

[gordo]

I don't think there's a contradiction. We are speaking about two different issues. They are referring to the "graininess" sometimes associated with FM/stochastic screens (a separate discussion). I was referring to "patterning". Here's what the screen looks like:



I'll try to explain. In a conventional AM screen, the vertical and horizontal distance between successive dot centers is constant and is a function of the screen frequency. Unfortunately, the combination of screen frequency and angle results in halftone dot cells that, other than at 45 and 90 degrees, do not intersect the recorder grid in any consistent way. As a result, the halftone cells in a single color channel are not identically shaped and do not contain the same number of pixels. If the difference in dot shapes in a single color channel repeats itself , i.e. has a frequency, the result is a type of patterning within one screen called single channel moir�. (Unless the screening developer has taken steps to avoid it)
Dividing the AM dot into thin concentric rings adds complexity to the design of the screen while still being limited to the geometry of the AM screen. You can see this problem clearly in the illustration. This can result in the "patterning" that I referred to in my original post.

best, gordo

[Sequoyah Graphics]

Thanks for the illustrated response. I accept your explanation.

Learned several things today.

Al

[Green Printer]

The idea for this type of screening came from hollow dot printing. With improper chemistry solid dots will tend to print hollow in the middle.
To compensate for this ink water issue the screening was developed.

Pat Berger

Mercer Color

[Sequoyah Graphics]

Hi Pat,

Can you please say more about this "hollow dot printing", or post a link where we could follow up on it? I tried Googling for it, but didn't get too far.

Thanks,

Al

[gordo]

Re: Pat wrote:
"The idea for this type of screening came from hollow dot printing. With improper chemistry solid dots will tend to print hollow in the middle. To compensate for this ink water issue the screening was developed."

Pat, actually I believe this screening was born in the land of flexo, where Artworks was looking to texture the surfaces of the dots without using FM screening techniques which patents prevent them (and others) from doing. So, it's not to solve an ink/water issue at all.

best, gordo

[GinSu]

Yes, both prepress and pressroom need to be aware of what's going on.

From my testing, the key component to the ability to do concentric screening is your plate. While EAWS claims maintaining a 2x2 grid to plate may work, maintaining a 1x1 grid to plate (without patterning) works much better. I'm lucky in that our Javelin with Fuji LH-PJ plates were properly configured from the start and holding the 1x1 grid to plate happened by default. EAWS won't say it specifically but Fuji thermal plates are good, Kodak and Agfa not so much.

The pressman will also need a heads up. The thinner the ring widths, the less ability they'll have to move the color on press.

Are you using Paragon Screening? Concentric Screening is a unique dot shape for the Paragon Screens. Paragon Screens give the anti-moire with better rossettes. Pushing it to the Concentric dot will give you some ink savings and better consistency on press (ring width dependent) as well as slightly brighter/cleaner color.

[J_Lindgren]

GinSu gave you a pretty thorough explanation.

I'll just add some notes. Some of the pro's are:
- Reduced ink layer thickness - giving you far more chromaticity on your halftones (in comparison, regular AM screened will look as if there were some 10% dirtying colours with the main colour.)
- More stable in print due to inherent limit of ink layer thickness variation. Faster set-up time.
- High details as in FM, while avoiding the graininess of FM in flat tints.

Some of the con's:
- Demanding on plates and exposure. See GinSu's notes on Fuji thermal plates. When plates are in good shape, the printing should be no problem in modern press.
(Uh... Didn't come up with anything else...)

Gordon is wrong in what he says about the origin of Concentric Screening. Former AWS did not develop this for flexo. This is definitely for Offset. Maybe he is confusing this with PlateCell Patterning for adding pattern to solid areas to help ink transfer in flexo. Totally different thing.

Edited by: Jukka Lindgren on Mar 6, 2008 3:56 PM