Understanding Prepress Techniques and Desktop Printing
written by: Gwen Hagaman•edited by: Wendy Finn•updated: 8/31/2011
Why does artwork look great on the computer AND print perfectly to an inkjet, but the colors look really different when the local print shop makes thousands of them? The differences between desktop printing and prepress techniques may surprise you. Here are some ways to get more predictable results.
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The RGB Color Gamut
RGB stands for Red, Green and Blue colors of light, that blend to create a color spectrum similar to our own vision. A value between 0 and 255 for each of the three colors determines RGB color mixes.
This color standard is used primarily by light-emitting devices including televisions, computer monitors, cell phones, digital cameras and other electronics. Since the color emissions and display features of each device is different, a specific RGB color's appearance will vary somewhat when viewed on each piece of equipment.
This color standard grew out of James Clark Maxwell's work in development of the color triangle in the mid-1800s. He was able to produce color photographs using three different color filters in a darkroom process. Maxwell is credited with the first permanent color photograph in 1861.
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The CMYK Color Gamut
CMYK color is all about ink on paper: Cyan, Magenta, Yellow and black (the Key plate to which the other colors register). CMYK color mixes are shown using percentages from 0 to 100 for each color.
Ink on paper is a reflective color environment. Instead of emitting color lights, paper surfaces absorb incoming light, acting as a filter to the color of light reflected back. Cyan, magenta and yellow inks are similar to the basic three color mixture of RGB. And black is often called the "punch" plate by printers because it adds contrast to the mix.
In order to print color images using inks, it was necessary to convert artwork (initially mostly photographs) into mechanical separations which would each carry one color of ink. To reproduce gradations of color, a series of dots in varying sizes, called half-tones, are used to deposit very small amounts of ink. The mixing of halftone screens at compatible angles allows the visual mixing of four ink colors, providing a large spectral range. Printers use CMYK inks with a variety of dot shapes for special effects.
Almost every mass production method that uses ink requires use of color separated artwork including offset printing, screen printing, and flexography.
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So convenient, desktop printers allow us to print our own color documents as we need them. There is always some variation in color from the monitor to the paper print. ICC profiles (International Color Consortium) can be helpful in minimizing color variations. They act as color interpretation translators between various devices. See the reference Adobe Computer Darkroom to learn more about using ICC profiles.
Whether inkjet or laser systems, desktop printers are normally used to print "composite" images. From the computer, digital camera, smart phone or other sending device, the desktop printer maintains an RGB color environment. By staying inside the same color gamut, less variation is seen from the screen image to the paper image even though the paper is reflective.
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The biggest difference in desktop printing and prepress techniques is the need to make color separations. A big color shift happens when color separations are required and the artwork is converted to CMYK colors.
The best way to see what happens in CMYK conversion is to do it yourself. Most desktop publishing software allows you to convert your color mode (Adobe) or color model (Quark). Microsoft products are used for desktop printing purposes and require non-standard installation of a third-party CMYK color profile.
Placed images should be converted in their native applications. So, open an image in Photoshop. You'll see "RGB" at the top of the window. Pull down Image/Mode/CMYK and see what happens. In general you will see a flatter, less bright image. Vibrant turquoise, lavender and gold hues will experience the most drastic color shifts. Some color corrections may be done before sending your project to production. See the tutorials for your software to make the best use of its color management tools.
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Using Spot Colors
Digital artwork preparation has us all trained to use composite colors. But sometimes the project requires using spot colors other than CMYK. The Pantone Matching System (PMS) is most popular with offset printers and provides standardized ink mixing formulas for hundreds of colors.
Let's say, for example, that your company wants to print a flyer announcing a holiday event using PMS 350 Green and PMS 032 Red. Since it's a casual event, it will save money to use only two ink colors. You would load the spot colors into your color palette and be sure to use ONLY them in your document.
For an in-office proof, you will still print a composite from your desktop printer. Once everyone has approved the flyer, you are ready to send it to the print shop for mass printing. But if you want to make sure the document separates properly - and that no color errors have been made - you can test it first.
Print to "separations" using your desktop printer. You should have a grayscale print for each color plate. In this example you should only have two prints come out. If an element of artwork separates into CMYK colors (giving you six prints in this example), you will be able to see where it is and correct it before the file leaves your office. If you can print accurate separations from your file, your print shop will be able to provide you with the results you are expecting.