Graphic Design
Moire:
Cause
The film service
bureau made film without paying close attention to the screen
angles of the film seperations.
Solution
To avoid moire
patterns on duo tones and tri tones use the following angulation:
45 / 75 degrees.
You may use other
rulings but a difference of 30° or 60° between films is
necessary.
For CMYK films
follow these angles:
Cyan
15 |
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/
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