Problem with crosshatched/quilt pattern in seperations

[gobostogo]

Hello,
We have a Harlequin RIP Version 5.5 Revision 1c and SelectSet Avantra 25XT/Avantra 20/25 OLP. We see these patterns in the cyan & magenta films. We can also see the same pattern when we roam. I was wondering if the lpi versus dpi has to do with this; 425lpi & 2540dpi. When we changed the lpi to 300 it alleviates these patterns. We need the lpi to be 425 for the type of work we do.
Any help would be much appreciated.

[meddington]

You could try altering the default HPS options. If you alter the frequency deviation in the HPS settings, from 7 to 15 or so, the pattern may diminish (or if you use a different dot shape or lpi, ect). Achieving a certain freq/angle is tricky, and this is where the frequency deviation comes in. The more you allow the RIP to deviate, the better chance it has of finding a good screen set with less patterning/memory.

Disabling hps all together might actually remove this pattern issue from your film, BUT... one things for sure...you do NOT want to image without HPS enabled unless you like moire patterns...trust me

[gordo]

Hello,
We have a Harlequin RIP Version 5.5 Revision 1c and SelectSet Avantra 25XT/Avantra 20/25 OLP. We see these patterns in the cyan & magenta films. We can also see the same pattern when we roam. I was wondering if the lpi versus dpi has to do with this; 425lpi & 2540dpi. When we changed the lpi to 300 it alleviates these patterns. We need the lpi to be 425 for the type of work we do.
Any help would be much appreciated.

I'm not that familiar with your RIP, but it sounds like single channel moiré.
When you ask for 425 lpi you are likely actually getting something like 414.792 lpi with a halftone cell size of 4.33 pixels. Since your device can't image a partial pixel, it is the .33 pixels that, depending upon how the RIP creates the screen will cause single channel moiré - the crosshatched herringbone effect you're seeing.
When you ask for 300 lpi you are likely actually getting something like 299.342 lpi with a halftone cell size of 6 pixels – hence the problem disappears.
You could try asking for 450 lpi where you will likely actually getting something like 449.013 lpi with a halftone cell size of 4 pixels, which should get rid of the issue - but then it won't be 425 lpi.

You could also try a round dot (round dot that does not go through a square shape at the 50% point)
You could also try offsetting all the angles by 7.5 degrees - this has somewhat the effect of adding noise which might mask the problem.

BTW, viewing film will exaggerate the visibility of the problem. It's quite possible that it won't actually be visible in the presswork.

best, gordon p

[meddington]

BTW, viewing film will exaggerate the visibility of the problem. It's quite possible that it won't actually be visible in the presswork.

Back in our film days, I at one point was hell-bent on getting rid of these patterns on the film itself, which is when I altered the hps settings until it diminished (and learned why you don't turn hps off!). But its true that these patterns never actually were incarnated visually on press.

[gordo]

Back in our film days, I at one point was hell-bent on getting rid of these patterns on the film itself, which is when I altered the hps settings until it diminished (and learned why you don't turn hps off!). But its true that these patterns never actually were incarnated visually on press.

I also had the same "problem" with my Agfa Selectset 7000 - back in the day. Interestingly there were all kinds of image detail in the film that also never ended up in the presswork either! LOL

best, gordon p

[Jim-CompuDoc]

425 lpi is about double what I would ever run on an XT.
you're talking a newspaper class avantra here, it was never intended to screen that fine.
You need a standard Avantra, or an S, and you need to be running it at 3600 dpi.

[gobostogo]

Thanks for the the input; we are experimenting with your ideas. We'll let you know how things work out.