COLOR SEPARATIONS

MOIRÉ

Moiré with Traditional Rosette Screens

CMK Rosette
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CMK Rosette

Failed CMK Rosette
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Failed CMK Rosette

Any pattern which results from the combination of two or more ordered "periodic" patterns is called moiré. Moiré can be produced any time two or more color separations are combined. With traditional screening, moiré in the Cyan, Magenta, and Black separations is minimized by placing these three screens at angles that are exact multiples of 30 degrees. This produces the "CMK rosette" pattern shown in the image on the left above.

This traditional pattern is an "unstable" moiré–free arrangement. Any slight deviation in the angles or lpi of the separations can produce a pattern like that shown in the "Failed CMK Rosette" image on the right above (you may find it helpful to sit a few feet back from your computer screen to see this).

This failure of the CMK rosette structure can result from ordinary digital screening. It can also result from misalignment of film, screens, or plates during the printing process.

The production of perfect traditional rosette color is demanding, but by using Wasatch Precision Rosette Screens, a high-precision digital screening method, failure of the CMK rosette structure can be avoided.

15 Degree Yellow Moiré
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15 Degree Yellow Moiré

Final Result in CMYK Color
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Final Result in CMYK Color

Only three screens can be placed at 30-degree angles, so one of your CMYK separations must be placed in a way that does allow moiré to occur. It is traditional to make this the yellow separation, and to place it at 15 degrees from the cyan and magenta separations. Simple mathematics shows that this will produce a moiré with "squares" that are 3.83 times the size of the halftone dots. This yellow moiré, which is shown above, will look familiar to all prepress workers who work with films.

Yellow is chosen because it is the faintest of the four colors in a CMYK separation. The above "Final Result in CMYK Color" shows the same separations as the "15 Degree Yellow Moiré", but the rosette structure is much more pleasing because the CMK separations are visually stronger than the yellow separation.

Looking for creative ways to eliminate yellow moiré?

Wasatch's innovative Hybrid Rosette/Stochastic Screens produce separations with the rosette structure that print buyers expect while using stochastic screening to completely eliminate yellow or "fourth screen" moiré.

You can find more information about moiré, stochastic and hybrid halftoning methods, and dMax and other issues related to the production of graphic arts separations in Wasatch SoftRIP's online help.

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