ink trapping
TRANSCRIPT
INK TRAPPING
Trapping is a term most commonly used in the prepress industry to describe the compensation for misregistration between printing units on a multicolor press. This misregistration causes unsightly gaps or white-space on the final printed work.
Trapping describes the measures and methods that can be used to eliminate or conceal the occurrence of flashes.
FLASH: white gap that appears between two relatively dark areas which are supposed to meet.
This happens when the colors are made up of
different separations and the separations do not register exactly on the border between the colors.
A small area of paper remains unprinted, which, in contrast to the two darker colors, appears as a bright, very noticeable stripe.
One of the most conspicuous irregularities are areas of unprinted paper called flashes or halos
The unprinted area is sometimes called a "flash"
Inaccuracies in the image setter
Instability of the image carrier, e.g., Stretch in film or plate
Inaccuracy in the film-to-plate or film-to-film copying steps
Instability of the press
Instability of the final media
Human error
MIS REGISTRATIONS ARE DUE TO
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 modifications: Overlapping colored objects to prevent unwanted areas of
unprinted paper from appearing.
Deliberately creating gaps to prevent inks from being mixed and avoiding unacceptable new colors.
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. If the design includes fine linework or small serif type. If two or more adjoining colors share a common color. 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.
OVERPRINT
Objects printed over other objects are overprinted. This mix can result in unexpected colors
KNOCKOUTIn 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 plate:
CHOKEThe 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.
SPREADThe 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.
TRAPPING TYPES
Photographic or Conventional TrappingPhotographic 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.
Disadvantages: Photographic trapping requires a great deal of experience and
craftsmanship and is time consuming.
It can only be used for fully filled areas, but not when half tone screening is involved.
Only certain objects or colors can be trapped selectively.
The results can be very difficult to reproduce, especially when overexposure is used.
Photographic trapping cannot be used when printing to plate. There is no means to photographically copy plates
Electronic trappingBy comparison, electronic trapping has many advantages:
Electronic trapping can be applied to tinted objects.
Traps can be generated from tinted objects or can have varying widths.
Electronic trapping can always be faithfully repeated.
Electronic trapping can be controlled more easily. It can be restricted to certain objects or colors.
Electronic trapping 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:
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)
Displays the colors at their 100% values Converted to grayscale
NOTES: 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.
Black frames are used in the borders of colored areas in comic books.
Opaque inks are almost always printed last.
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.
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