By adjusting the ink neutral density (ND) values that the selected trapping engine uses, you can determine the precise placement of traps. The default ND values for process inks are based on the neutral density readings of process ink swatches that conform to industry standards in different parts of the world. The language version determines which standard it conforms to. For example, the ND values for the U.S. English and Canadian versions conform to the Specifications for Web Offset Publications (SWOP) solid ink density values published by the Graphic Arts Technical Foundation of North America. You can adjust process ink neutral densities to match printing industry standards in other parts of the world.
The trapping engine derives the ND values for a spot color from its CMYK equivalent. For most spot colors, the ND values of their CMYK equivalents are accurate enough for proper trap creation. Spot inks that aren’t easily simulated using process inks, such as metallic inks and varnishes, may need their ND values adjusted so that the trapping engine can trap them correctly. By typing new values, you can ensure that an ink that is observably darker or lighter is recognized that way by the trapping engine; the appropriate trap placement is then applied automatically.
You can get the appropriate neutral density value for a given ink by asking your commercial printer. The most accurate method of determining an ink’s ND value is by measuring a swatch of the ink with a commercial densitometer. Read the “V” or visual density of the ink (don’t use process filters). If the value differs from the default setting, type the new value in the ND text box.
Changing the neutral density for a spot color affects only how that color will trap. It doesn’t change the appearance of that color in your document.
Metallic and opaque inks
Metallic inks are usually darker than their CMYK equivalents, while opaque inks obscure any ink beneath them. In general, you should set the ND values for both metallic and opaque spot colors much higher than their default values to ensure that these spot colors won’t spread.
Setting an ink to Opaque or Opaque Ignore in the Type menu of the Ink Manager prevents an opaque ink from spreading into other colors, unless another opaque ink has a higher ND value.
These inks are normally lighter than their process equivalents. You may want to set the ND value for these inks lower than their default values to ensure that they spread into adjacent darker colors.
Other spot inks
Some spot colors, such as turquoise or neon orange, are significantly darker or lighter than their CMYK equivalents. You can determine whether this is the case by comparing printed swatches of the actual spot inks to printed swatches of their CMYK equivalents. You can adjust the spot ink’s ND value higher or lower as necessary.
Using certain inks involves special trapping considerations. For example, if you are using a varnish on your document, you don’t want the varnish to affect trapping. However, if you’re overprinting certain areas with a completely opaque ink, you don’t need to create traps for items underneath. Ink options are available for these situations. It’s usually best not to change the default settings, unless your prepress service provider recommends changing them.
The speciality inks and varnishes used in the document may have been created by mixing two spot inks or by mixing a spot ink with one or more process inks.
The trapping sequence (also called the trapping order) matches the order in which inks are printed at the press, but it doesn’t match the order in which separations are produced at the output device.
The trapping sequence is particularly important when you’re printing with multiple opaque colors, such as metallic inks. Opaque inks with lower sequence numbers are spread under opaque inks with higher sequence numbers. This process prevents the last applied ink from being spread, and it still creates good traps.
Don’t alter the default trapping sequence without first consulting with your prepress service provider.