COLOR SEPARATIONS
MOIRÉ
Any pattern which results from the combination of two or more ordered "periodic" patterns, is called a "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 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" 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 be avoided at the digital halftoning step by using Wasatch Precision Rosette Screens, a high precision digital screening method.
The traditional rosette structure can also fail due to misalignment of film, or of screens or plates during the printing process. It's a "weakest link" situation, and the production of perfect traditional rosette color is demanding.
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 "FinalResult 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.
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.
Much more on the topic of moiré, stochastic and hybrid halftoning methods, and dMax and other issues related to the production of graphic arts separations, can be found in Wasatch SoftRIP online help.
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