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Index Color: The Fastest Proof-to-Print System?

(February 2008) posted on Wed Feb 06, 2008

Index separations are often avoided by those who fear being stuck with too many colors to print. This month, Trimingham debunks some myths about index color and explains how to use it as an effective tool in prepress.

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By Thomas Trimingham

Following three simple steps will enable you to properly use an index proofing and separating system and get everything running smoothly. The first step is artwork modification to minimize the number of colors in the design. My previous column about color crunching (Dec. ’07, pg. 28) explained how to do this in greater detail. The technique is worthy of review in this context because of the nature of how index color splits a design. The second part is the actual separation method using index color, which is deceptively simple at first glance but still requires some serious finesse to do well. Last is the integration of the separated index file into a vector program like CorelDRAW, where it can be customized, proofed, and then separated.


Artwork modification for index separation

Artwork modification is the most overlooked area of the square-dot-based separation method and typically one of the reasons that it has obtained such a poor reputation for needing too many colors to reproduce designs. A design on a screen-printed shirt has the primary job of looking good and hopefully making the person wearing it look good as well. Unless there is a necessity for specific color matches on multiple areas of a design, you should simplify any colors that are similar in hue and try to combine these colors into a smaller palette before you start separating. The easiest way to accomplish this is by crunching colors.

Let’s review the crunching method in the context of prepping an image for index, rather than simulated color, separations. The wolf photo in Figure 1 needed some adjustments before it could be used for an index separation. Animal photos like this one commonly have good areas and bad ones, so the key is to find pieces that will work and build from there. In this case I used the right eye and duplicated it and flipped it to work on the other side of the face. I followed the same idea with the ears by copying the left ear and using it on the right side of the face. The next step was prepping the overall image by bumping up the contrast 15 points and then changing the image mode to L*a*b* color and using the Unsharp Mask filter on the lightness channel to help define the edge quality of the image.


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