Line Screens

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Line Screens

Postby Masters » Fri Dec 11, 2009 5:54 pm

Is there any reason one would have different line screen frequencies for each color separations? From my experience they should all be the same. Anythoughts ??
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Re: Line Screens

Postby cheapthrills » Fri Dec 11, 2009 7:08 pm

Makes no sense to me. Play havoc in prepress, inks, platemaking, anilox's and a different dot compensation for each color. I like standardization. If there are advantages and they make sense, I'd try anything once.
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Re: Line Screens

Postby Luc St-Pierre » Fri Dec 18, 2009 1:14 pm

When needed on some specific images, a different frequency on yellow will avoid moiré with black.
Luc St-Pierre
Flexographic 7 Inc.
Luc St-Pierre
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Location: St-Jacques Canada

Re: Line Screens

Postby ljgoldberg » Wed Jan 06, 2010 11:04 pm

Masters wrote:Is there any reason one would have different line screen frequencies for each color separations? From my experience they should all be the same. Anythoughts ??

Before computerized screening became the norm, camera halftones were created from contact screens, and from glass screens before that. Glass halftone screens were often supplied as round screens, rotated to the correct angle on the camera back. One screen could produce any desired angle, all naturally, of exactly the same screen ruling.

Providing four harmonious screen angles with uniform screen rulings is an unsolvable problem, although many compromises produce commercially acceptable results.

Modern screening and RIPs are enormously clever systems, able to solve moire, rosette, and other problems. Now that computing power, RAM, and disk space are trivially cheap, allowing the RIP to optimize the screening makes sense. Limitations of laser resolution (addressability) and other variables can be elegantly "worked around" by the RIP. The limits of these solutions are bounded by interactions with anilox rulings and angles, so the prepress operator has to communicate with the pressmen.

Small variations in screen ruling and screen angles can sometimes greatly improve halftone results, the effect of these changes should be tested against the standard anilox rolls.

There is nothing magical about the old standard screen rulings; 85, 100, 120, 133, 150, etc. were what was available on glass and contact screens. The original Betaflex software came out while the CTP revolution was just getting up to speed. We measured the true screen ruling and displayed the nearest standard value. It had a calming effect on the prepress guys. :)

As digital screening eclipsed camera halftones, prepress operators began to notice and eventually understand the strange rulings and angles. TIFF viewers made it very clear what was happening. Knowledgeable operators could solve most any subject or screen moire problem with help from the RIP vendors.

Standardization is a good thing. Many pressrooms are like the Wild West where anything goes. Anilox volumes varying wildly by color giving fits to the ink guys, etc.

Screening is another tool in the toolbox. Its the mechanic that fixes the problem.


Larry Goldberg
Beta Industries
Carlstadt, NJ
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