G7® and Paper Color: The Challenge of Achieving an Acceptable Color Match

By Ron Ellis
 G7® has proven to be a valuable method for calibrating printing devices, as well as specifying color. One of the limitations of G7, as well as other calibration methods, has been the problem of paper color.  The G7 method is based upon ISO 10128. This means that the LAB values for inks, the traps, and the gray curve are all based on the global print standard, ISO 12647-2. When performed on the stock specified, the process works spectacularly well. Because of this G7 has been very successful. Part of the consistent results we see from G7 are due to the fact that G7 sits on top of ISO 12647-2. In the world of standard papers and inks everything works well.

When we use papers that are out of spec then we begin to see color matching problems. The more out of spec the paper is the worse the color match will be. (By out of spec we mean the paper color is shifting away from the neutral paper specified by ISO 12647-2.) Paper color is a big deal and can cause all sorts of color management issues. The more colorful (and away from the standard) the paper is, typically towards blue or yellow, the more the printed color shifts away from the desired color. Deviating from the standard sounds like an easy problem to fix. The easy solution is just to make sure that the paper you are using meets the standard. This however, is not easy. First, the paper described in ISO 12647-2 (verify) is not easily obtainable. In commercial markets paper grades have brightened dramatically in the past few years. For packaging the typical SBS paper is more yellow than the standard describes. Very few papers match the specifications of ISO 12647-2.  Second, print buyers are selecting papers that are much whiter and brighter than the standard ISO paper. In some industries like packaging the customer is supplying them with a GRACoL C1 proof (which is based on a more neutral paper) even though the packaging substrate is more yellow.

The scenario tends to go as follows: The customer submits a print job along with a GRACoL proof. The proof is to GRACoL specifications and scanning the control strip indicates that the proof is GRACoL compliant. The paper selected for the job is a new and popular paper, which is both much more blue and even more red than the ISO specified paper that GRACoL is specified on. On press the operator quickly comes up to color, pulling to within 5 points of the target densities. The pressman looks at the proof and press sheet and realizes he has a problem. His normal procedure ,he begins to adjust the solid densities to visually match the proof. Though he can get some parts of the sheet in the mid-tone to match the proof, other objects such as the light tan process builds and the dark purple builds are way off. The plant manager comes out and asks if the press operator has run to the numbers – which he has.

It is important to note that this isn’t just a problem with GRACoL, but that this same problem applies to all proof to press matches because they are based on an assumed paper color. In the case above, because of the paper color change it is impossible to get a reasonable match to the proof. While the buyer is using the proof and the ISO values in the control strip religiously, the painful truth is that because of the paper shift these values are no longer valid  — on the paper stock specified by the print buyer, the printer cannot achieve the specified overprints. The paper is so blue that all of the solids and overprints are shifted, and in the case of the beige quartertones everything looks slightly green and no amount of curve changing will make those light highlight areas match the proof. The scenario above is dramatic. Many stocks are close enough that the press operator can often hit the proof. Brand owners and those seeking precision expect perfect proof matches from their printer – and when the match isn’t there on an odd stock, they don’t understand why. Paper is as we have all heard over and over, the fifth color.

IDEAlliance, the organization that owns G7, GRACoL and SWOP has been aware of this disconnect.  Standards are like ISO are standards for a reason, and when you stop using standard material (like using an extremely different shade of paper) then the standard becomes broken and does not work as intended.  The problem is bigger than just sheetfed printing, where we have more control over the sheets we print on. In other print processes like flexo, or board printing, the substrates are even more out of spec. In order to have a brand render correctly across a variety of substrates, and to give the printer a fair chance at matching color, G7 and additional color management may be required. One member of the IDEAlliance Print Properties & Colorimetric Committee who has thought long and hard about this is David McDowell. David is a color scientist and US ISO delegate. He has developed a formula for paper scaling which the GRACoL Tolerance Committee is currently testing. The formula has been integrated into a spreadsheet, which contains the GRACoL and SWOP datasets. By typing the paper color into the spreadsheet, the spreadsheet will do the math and recalculate the dataset with new aim points based on using that specific paper color. It’s a very interesting concept and in near future IDEAlliance may release it as a tool with a set of guidelines for printers to test.

How would a paper scaling tool work? The idea is that by typing in paper color you would be presented with a new dataset that is more realistic for that paper. This includes aim points that are more appropriate for the paper. Great news – but we’re not done yet. This doesn’t mean that the press sheet will now magically match the proof. There is just no way  that the  specified super blue substrate is going to render color the way the neutral paper color of ISO would. To make it work the proof should be created using this new dataset as well. That means making a new proof profile based on these numbers. It is not something you would want to do for every job but something that could be very important if you are printing on non-standard substrates, custom colored paper or other situations where you will be expected to match a proof precisely. In practical terms, this could result in a printer having a traditional GRACoL C1 proof as well as one or two other for non-standard substrates such as blue/brightened or SBS.
Intriguing? Once tested, if it works the method and procedure will be available for download by IDEAlliance members. (To learn more watch the GRACoL web site at www.gracol.org where there are updates on the progress as well as updated downloads.) Until then keep in mind the challenge paper brings to our industry. Not everything will match a supplied proof, and more often than not, the problem could be the paper. Paper is the fifth color, and just ignoring it doesn’t help.

 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, Ron has performed over 100 G7 calibrations. In addition to calibrating pressrooms for customers such as Pantone, Ron also specializes in creating internal working spaces for brands and agencies that allow them to work more efficiently with vendors, saving both time and money. Ron is published frequently in industry magazines, and has produced training materials for numerous printing industry vendors and publishers. He can be contacted at 603-498-4553 or through his web site at www.ronellisconsulting.com.

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