Avoiding Moire Patterns

Cause

The designer created an intricate design with fine lines and then the factory did not adjust the angle of the films to compensate for the design.  

Solution

Since CDman now prevents customers from supplying their own films, this is a rather obscure subject.  However, if you do receive your discs and you get a funky pattern showing up, your design contains angled vectors that conflict with our screen angles. Suffice to say; 

For CMYK films we follow these angles:

• Cyan15
• Magenta 75
• Yellow 60
• Black 45

Details

When two identical repetitive patterns of lines, circles, or arrays of dots are overlapped with imperfect alignment, the pattern of light and dark lines that we call a moire pattern appears. During the film developing stage, the image setter will align the dots on the page in a particular screen angle. If the angle for each color is too small or the same angle then a moire pattern results.

The moire pattern is not a pattern in the screens themselves, but rather a pattern in the image formed in your eye. In some places, black lines on the front screen hide the clear lines on the rear screen, creating a dark area. Where the black lines on the front screen align with black lines on the rear, the neighboring clear areas show through, leaving a light region. The patterns formed by the regions of dark and light are moire patterns.

In the case of the two sets of concentric circular lines, the dark lines are like the nodal lines of a two-source interference pattern. A typical two-source interference pattern is created when light passes through two slits. Along lines known as nodal lines, the peaks of the light waves from one slit and the valleys of the light waves from the other slit overlap and cancel each other. No light is detected along a nodal line.

In the black radiating lines of the moire pattern, the black lines of one moire pattern fill the transparent lines of the other. Note that as the patterns are moved apart, the dark, nodal lines move together. This is the same thing that happens when light passes through two slits and the slits are moved farther apart.

Moire patterns magnify differences between two repetitive patterns. If two patterns are exactly lined up, then no moire pattern appears. The slightest misalignment of two patterns will create a large-scale, easily visible moire pattern. As the misalignment increases, the lines of the moire pattern will appear thinner and closer together.

Want to see somthing cool?

http://www.mathematik.com/Moire/