Product

Platform

Operating system

Error

Symptoms

Fleshtone or other moire in smooth tints for AM screen systems.

Cause

Solution

This is unavoidable with AM screening for any system, although we have tuned the AM screens in Prinergy to be the highest quality possible. An alternative screen set they can run with is the Creosettes Y60 screen system, which places the yellow screen between cyan and black. I have attached a small document which you can look at showing the screen angles in Prinergy. The way moire works with AM screens is that you will get more pronounced effects the closer two screens are to one another. If you take a look at the Creosettes screen system, it places yellow between cyan and magenta, so you end up with about equal amounts of midtone moire in both green (cyan-yellow) and red (magenta-yellow). The red, of course, is what you notice in fleshtones. If you go to the Y60 screen system, yellow is placed away from magenta, but you will still get green moire (most noticeable in synthetic vignettes of green). It's unavoidable with AM, as there's no good place to place the fourth, yellow screen.

If you swap magenta and black in Creosettes, which is a common strategy, magenta gets moved away from yellow, but now your black screens are going to be more noticeable when they're by themselves, as they'll not be at the least-noticeable position of 45 degrees.

The Creosettes screen system is the default one primarily because of comfort, as most people are used to having yellow at either 0 or 90 degrees. You can place 3 screens equally spaced around the circle, but not 4. This is a fundamental property of conventional AM screening. The most you can do is ensure that the rosettes formed by the CMYK screens are stable, so that if you have clear-centered rosettes at the centre of the press sheet, you'll get clear-centered rosettes at the corners, too, which is what we've done in Prinergy.

Running with FM screening such as Staccato, of course, avoids these moire issues altogether. Staccato screening with a Trendsetter is easily achievable for anyone who can print 150 to 175 lpi on press.

One of the reasons why the CMYK 75-15-90-45 wasn't included in the original list of screen sets released with Prinergy is that this particular set of angles has CMK only 30 degrees apart, instead of 60 degrees as with the original Creosettes screen set. This makes absolutely no difference for rotationally symmetric dot shapes like Round and Square. Dot shapes like Elliptical and Line, though, aren't so lucky. To see this, try drawing 3 ellipses intersecting at the same place, one set spaced 30 degrees apart and the other 60 degrees apart. What you'll see is that the set spaced at 60 degrees covers the paper much better than the set at 30 degrees, which end up overlapping with one another. Spacing dots like elliptical dots at 30 degrees makes the resulting screens a bit more susceptible to color shift as a result of press misregistration, because everything is so much more overlapped.

So, there are people out there who have been printing with the traditional angles for a long time, and been very successful with them. This would be because either they've got good control over press registration, or else they always print with Round, Euclidean, or other symmetric dot shapes. And, that's fine. It's only if someone really wants to print with elliptical or line dot shapes that the 30-60 degree angle issue becomes important. Of course, in changing the angle set, the look of the printing and the way it prints will change, which is exactly why we have customers who, having set up and comfortable with their existing angle sets, want those exact angle combinations. (Basically, the Customer is always right.)

Permanent fix:

See also

Diagrams of screen angles

Keywords

color, screening

Technical contact(s)

Rob Prior

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Date submitted