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Feb 1, 2010 12:00 AM
Easier color space transformations
Ink optimization software uses gray component replacement (GCR) to change the separation of an image. The software will take color from the file and increase the black channel. It can be set to take more or less color out depending on the user's needs. Despite GCR's function — the removal of color and replacement of black — a nearly identical separation results, but it uses less ink. Ink use and cost reductions typically range from 10% to 30%. Reduced ink usage is the most obvious benefit, but there are many others.
Scanner operators often apply GCR to an image to make the image more neutral. Ink optimization software does the same thing, but rather than just applying GCR to images, the software applies GCR to all elements, vectors and images. Not only does the GCR make the image appear more neutral, but the color removal and black insertion makes the printed result more stable. This can be seen on press when running a job with GCR by adjusting an ink key. On the page without GCR, the ink key adjustment is quick and obvious. On the page with GCR, the adjustment has less of an effect and, in the case of high GCR, almost no effect. This results in faster makeready and improved consistency across a press run.
GCR software also typically adjusts the total ink coverage (TAC) — in some cases, globally appling the desired TAC to the entire document. For example, if a web printer's customer created a document in Adobe Creative Suite with a TAC of 400 percent, the software would automatically adjust the file and all elements down to the web printer's desired TAC of 260. Less ink is put on the page, resulting in easier printing and control for the press operators.
Systems applying GCR also can perform gamut mapping. Many GCR systems use device links (two ICC profiles used to make a direct color conversion), which enable the conversion of files from one color space to another. Users can make the press print to a standard print condition such as GRACoL or SWOP, as well as take advantage of ink savings. While many printers still use press curves and a GCR system, with gamut mapping, press curves aren't always necessary. There are many parts of the print space (such as the three-quarter tones) where it is physically impossible to perfectly match the print specifications. Gamut mapping also facilitates moving from one color space to another. Suppose you are using a print process that requires a non-traditional rotation, such as moving the Y to another position. The GCR software can map color so that the color of traps and other elements appears correct. In this scenario, it would be physically impossible to match GRACoL or SWOP. But the color mapping will correct the values in the file so the press output will appear correct. (It also can be used to match a press to a client's custom proof condition.)
Danger areas for GCR range from the simple to the complex. If you have less ink on the page, it follows that you will have less control. On a web press without closed-loop color control, less ink is helpful, as an operator has limited control over the color. On a sheetfed press, where an operator might be making precise color adjustments during the press OK, you might actually need the color on the sheet. For those employing custom GCR based on press conditions (rather than a more generic GCR), watch and adjust as your press conditions change.
Ron Ellis is a Boston-based consultant specializing in color management, worflow integration, and press calibration. He has provided installation and training services to dealers, manufacturers and content creators since 1986. An IDEAlliance G7 Expert and chair of the GRACoL Committee, he has performed more than 100 G7 calibrations. Contact him via www.ronellisconsulting.com.
Gray component replacement (GCR) is total undercolor removal. GCR reduces cyan, magenta and yellow in the neutral and trichromatic color throughout an image. For a concise overview, see Joe Marin's January 2006 article, “Match game,” at www.americanprinter.com.