Trapping describes the measures and methods that can be used to
eliminate or conceal the occurrence of flashes.
||The unprinted area is sometimes
called a "flash".
Another problem caused by registration errors is an unwanted overlapping
of inks. This can result in a color that is conspicuously different
from the colors which are supposed to be
||The result of the Cyan and
Magenta inks mixing can be an unwanted purple color.
Causes of Registration Errors
Inaccuracy of the recording
devicesNot all recording devices can produce separations that
are consistent in all dimensions.
the separations belonging to one page are exposed in different positions
of the film, e.g. Cyan and Magenta exposed at the beginning of a
film roll and Black and Yellow in the middle of the film roll, the
separations produced may not match perfectly.
Distortion of the film
Exposing and developing
can stretch and distort the film.
Changes in the temperature
or humidity during the processing of a film can lead to registration
errors. Separations should be exposed on film from the same batch
Inaccuracies in film assembly
Errors in film assembly
and platemaking can cause mis-registration, even when greatest care is taken.
accuracy of the printing press.
or stripping material instability.
of the paper caused by pressure, temperature and moisture from atmospheric
humidity or dampening solution.
bounce in sheet-fed presses.
stretch in sheet-fed or web presses.
maintained or badly worn printing presses.
Improperly trained press
Preventing Flashes from Mis-registration
Preventing mis-registration and removing
all sources of error in each stage of production can be expensive.
There are also photographic and electronic methods of trapping which
can remedy more problems. These two methods offer the following
colored objects to prevent unwanted areas of unprinted paper from
creating gaps to prevent inks from being mixed and avoiding unacceptable
Adding objects such
as black frames along the borders of colored areas to hide any possible
gaps or overlapping.
When to Trap
If two colors touch or butt, trapping may be required. If the design
includes fine linework or small serif
type, the decision on how to trap is important. If two or more adjoining
colors share a common color, trapping is probably not necessary.
In simple trapping, where two colors are involved, the lighter
color is spread into the darker color. The darker, dominant color
defines the size and shape of the image.
Designing with the Printing Process in Mind
If two adjoining colors both share an
amount of the same process color, trapping is not usually required.
Transitions between pairs of colors that have most of their components
on different separations are a key target of trapping.
The following example is how the two colors are intended to appear:
No trapping is required between these two colors, if there
is a shift in a color the other colors will still be present.
This example exaggerates a shift to the right in the yellow
plate. Since there are substantial elements in the Cyan and
Magenta plates, trapping is not usually required. A
transitional Trap Color is found between the two intended colors.
If a purely cyan area borders on a purely magenta area, then a
mis-registration of the separations would
lead to a mis-registration of the objects. Either an overlapping or
a gap would appear between the objects. The gap between the objects
would appear as a visible flash of unprinted paper.
To avoid this, the following measures are possible:
Avoid adjacent areas
of strongly differing colors.
Add black frames to
print over the transitions between colors which would otherwise
have been trapped. This is often used with images.
Insert gaps or “white
frames” between colors that cannot overlap. These gaps should be
less visible than mixed colors that will result when a light color
such as yellow meets a dark color such as blue.
when using black text.
Choose suitable object
colors which have sufficient common shares of inks at critical transitions.
In trapping, the lighter color migrates or traps into the darker
color. The darker color is the dominant color. While the lighter
color traps, the darker color retains the integrity of the image.
Light areas are trapped
to dark areas.
Objects printed over other objects are overprinted. This mix can
result in unexpected colors.
In PostScript, every object that is placed on another object will
normally leave an unprinted image of itself on the lower object.
Trapping will be required for all cases because the “hole” is white
unprinted paper. The image below represents a knockout in the yellow
The background color is lighter than the foreground color. The
foreground or “island” color is darker. The lighter background color
is extended into the island color to provide a trap. No intermediate
color is used.
The island color is lighter than the background color. The lighter
island color is expanded into the background to provide a trap that
does not use an intermediate color.
Photographic or Conventional Trapping
Photographic trapping inserts layers of film between the original
and the copy during the copying process to allow the light to be
scattered around the objects for spread or around the background
for choke. Occasionally, this copy process is deliberately overexposed.
This method has several disadvantages:
requires a great deal of experience and craftsmanship and is time
It can only be used
for fully filled areas, but not when halftone screening is involved.
Only certain objects
or colors can be trapped selectively.
By using diffused light
or overexposure, the copy is usually blurred. This applies to the
entire image especially linework and fine
structures such as serifs and small sized text.
The results can be very
difficult to reproduce, especially when overexposure is used.
cannot be used when printing to plate. There is no means to photographically
By comparison, electronic trapping has many advantages:
can be applied to tinted objects.
Traps can be generated
from tinted objects or can have varying widths.
can always be faithfully repeated.
can be controlled more easily. It can be restricted to certain objects
can be performed automatically especially when standard tasks are
involved and manual intervention is not necessary.
Which Color is Darker
There are many ways of finding out which color is darker. The simplest
is to print the colors and then photocopy the proof. The colors
are transformed into grayscales at that point and can be visually
analyzed for relative darkness.
Another method to determine which color is darker is to determine
its Neutral Density. Neutral Density values are industry standards
for density readings of colors based on their C, M, Y and K components.
A third method to use the R, G and B components to determine the
relative darkness by applying the following formula:
Luminosity = .3 (Red
Value) + .59(Green Value) + .11 (Blue Value)
The following tables were created in Photoshop. The color table
on top was created first and displays the colors at their 100% values:
The grayscale table below provides an illustration as to which
colors are darker:
All color perception is subjective. If a light color abuts a darker
color, it may appear darker than its numeric value. For that reason,
trapping decision tables can be adjusted for specific situations.
When not to Trap
Trapping is not required in the four color process, when overprinting
is used or when either black or white margins are placed around
Trapping preserves the integrity
of the graphic design.
Trapping is used to preserve the
integrity or the image shape such as block serifs.
Trapping compensates for the limitations
of the printing process.
Designing with the printing process
in mind many problems can be avoided.
Black frames are used in the borders
of colored areas in comic books.
Opaque inks are almost always printed
Metallic inks are considered opaque.
Other opaque inks contain a high
content of white pigment so that the color of the underlying paper
cannot be seen.