Dot Gain: Friend and Foe?
by Ron Ellis
Very often when working at a printing plant I witness the following: A job arrives, accompanied by a printed sample. This sample was printed by a completely different printing company, and yet the customer expects this printer to match it. Usually the next step is to make a proof, and more often than not the proof looks nothing like the printed piece the printer is expected to match. If and when the printer asks the customer about the file and how is does not match, the customer usually says something like, “All our other printers were able to match this with no problems.” Which we all know is untrue. The customer may not be trying to lie to us – perhaps they are unaware of the behind the scenes work that often takes place when one printer is trying to match another printer’s printed sample. Of course this is not as simple as just dot gain. Inks, printing conditions, and many other variables factor into it. But dot gain is one of the main factors.
Dot gain, is the amount a dot grows when printed on press. For example a 50% dot when printed on press grows – maybe by 10% - maybe by 20%, depending on what type of printing conditions exist. Along with the ink solids, dot gain is one of the key factors we can use to control color on press. When using a procedure like the GRACoL and the G7 procedure dot gain is even used to control gray balance. With that being said you would think that everyone would be aware of what their dot gain actually as, and the implications of various dot gains. It is far from the truth though, while some printers know all their press metrics, many do not know the amount of dot gain they have and why. Often the dot gain they have was set up during a platesetter install as a generic dot gain curve. Depending on the dealer and the installer these dot gain results range from linear plates that show very low dot gain, to dot gain tuned to one of the old standards such as SWOP, GRACoL 6, or other recommended gains from the past such as an actual Kodak Approval or Matchprint. The trend with dot gain and CTP has been to use less gain on the newer systems, which results in problems with color systems such as the Pantone Process Tints that are based on a specific amount of dotgain. For example, the Pantone Process Tint book commonly used by many customers and printers are based on almost 30% dot gain. If you try to match some of those tints using a new CTP system or GRACoL 7 it can be very difficult to match those tints because the dot gain is just not there. The old Pantone Process Tints were created with high dot gain. The new Pantone Process Tint book, called Pantone ColorBridge reflects the new lower dot gains commonly found with CTP systems (as well as brighter paper, ISO inks, and other improvements.) ColorBridge uses completely different numbers to create the builds, so it is better for the CTP applications most of us use. The only potential problem is that Colorbridge was created when GRACoL 7 was still in development, so it has gains and printing conditions based on GRACoL 6. GRACoL 7 has significantly lower gain values when everything is done, and the gain values are adjusted by cylinder – so when using GRACoL 7 many users report problems matching the both the Pantone Process Tint values and the Colorbridge values. (Of course nobody has the ‘perfect’ gain and color values, so many printers have some problems matching the process tint builds is nothing new.)
Dot gain is simple, but not as simple as most people think. Even on a perfect press different cylinders gain differently. (Most of the basic CTP installs use one bump curve for all the cylinders). When the press has issues then it becomes even more of an issue and problem cylinders will show irregular gain. Just throwing a global gain curve for all plates may help add overall color gain but it doesn’t address color balance on press. The old school way to handle color balance has always been solid ink densities, but dot gain control can also be used to handle color balance. In addition, adjusting dot gain, when done correctly can allow you to run higher solid ink densities on press as long as you use the dot gain curves to ‘carve out’ some of the color in the midtones. By adjusting dot gain in the midtones, shadows, and highlights separately, dot gain can be used to give the image higher contrast and improve image quality dramatically. It can also be used as a starting point for matching one printing condition to another. But even though making changes to the dot gain curves are often done to ‘correct’ color on a job-by-job basis is very valuable, dot gain alone is not a valid way to control color.
Dot gain can also be used as a diagnostic tool to tell whether there are problems with a printing unit. By monitoring the gain on individual cylinders printers can tell if there is a problem with a specific unit, as well as verify why a proof no longer matches.
The variables that cause dot gain are numerous and include ink, paper, fountain solution, blanket and packing, plate, rollers, press speed, and coverage. Is dot gain good or bad? Dot gain is a fact of life in the printing industry, and most consider dot gain to be both good and bad. Without dot gain printing looks weak and powerless, and few would consider a job with no dot gain to be quality printing. On the other side, too much dot gain makes printing look dark, dirty, and makes the reproduction look poor. So dot gain is important – in moderation. We need some dot gain, but not too much.
How much dot gain is enough? Typically dot gain has is defined in print specifications such as SWOP, GRACoL6, and ISO specifications. While these specifications do not define color values they do define general dot gain values to achieve pleasing printing and reproduction. To take it one step further systems such as GRACoL 7, SWOP 11 use the G7 procedure, which develops custom dot gains for each press and cylinder based on color balance. Standards can help us define the right amount of dot gain, as well as give us something to compare and contrast to. Dot gain is important, and it is and key print metric both for troubleshooting and as a tool for managing print quality.
About the author: Ron Ellis is a prepress consultant specializing in workflow training and integration. He has consulted on numerous CTP installations and he provides color management, integration, training, workflow development, and troubleshooting solutions to the graphic arts community. He can be contacted at 603-498-4553 or through his web site at www.ronellisconsulting.com.