“Toto, we’re not in RGB anymore…”

Posted on July 12th, 2008 | Permalink

Most new designers I meet nowadays started their career working on the computer, and what they know about the printing world has come from slow osmosis as they have sent work to the press. They know how RGB colors work since they use them on the computer every day, but when it comes to understanding the CMYK colors that the press uses, they don’t even know what questions to ask. The brave few who attempt to delve into the workings of the press usually end up with lost in a sea of terms like coverage, trapping, and dampening system.

So let’s start with the basics. Thinking in terms of ink and paper is critical to understanding the printing process, and so also to properly preparing things for the press. Ink behaves differently than the sharp glowing pixels on your LCD monitor, and the type of paper it’s going onto and the press that’s putting the two together each have their own quirks.

That being said, the first thing I usually consider is dot gain. When dots of ink get printed onto paper, they spread out as they soak in, causing the dots to gain size, what the industry calls dot gain. The darker the color, the more ink is used, so the bigger the dots get as they soak in. Dot gain is usually measured in percentages; for example, with a 20% dot gain what started out as a ten percent gray would now be twelve percent, and a seventy-five percent gray would jump up to a whopping ninety percent. Dot gain is one of the reasons why printed jobs often seem darker on paper than they look on the computer screen.

Just how much dot gain you get on a particular job depends in part on the quality of the press. Web presses, ones that print on one continuous sheet of paper that comes off a roll, tend to get more dot gain because they usually run faster and use more fluid, less expensive inks. Sheet-fed presses, on the other hand, pull one sheet of paper at a time through the press, holding it more precisely and causing less smearing. Paper affects dot gain as well, with shiny coated stocks (like the stuff used in magazines) usually minimizing dot gain, in contrast to the dull uncoated paper (similar to copy paper) that lets the ink soak in much more broadly. Dot gain will never really go away, so your job is to make choices that prevent it as much as possible, then compensate for what’s left.

The type of paper you choose is usually the deciding factor in the next consideration, the total ink limit. Paper can only soak up so much ink before it starts to distort as it turns back into the mush it was created from, so the amount of ink you use must be limited to prevent this. The total ink amount, or coverage, is measured by adding up the percentage of each ink being put down in a particular spot. For example, printing 80% of all four CMYK colors (cyan, magenta, yellow, and black) would give you a total ink coverage of 320%.

If you tried to print something with a total ink coverage of 400%, it would be too much for just about any paper to handle, whereas a stingy 60% of each ink, totaling 240%, is safe in just about any case, so you will usually end up somewhere in between. Choosing a heavier weight paper, such as 80 lb., will allow you to use more ink than 60 lb. paper, since it has more substance for the ink to soak into before the paper starts to distort. Coated paper is designed to let less ink soak into the paper itself, so a higher amount of total ink is possible before the paper starts to be affected.

Ink standards will affect the look of your job as well. Ink manufacturers use differing processes and materials to create their inks, and how the ink behaves on the press depends on who you got it from. Organizations such as SWOP have defined standards for things like exactly what color “cyan” ink should be, giving you the confidence of knowing that if you’re using SWOP standard inks, 100% cyan will always look pretty much the same. Using a consistent ink standard is especially for color-sensitive things like logos and letterheads.

Using a program like Photoshop, you can create or choose a CMYK color profile with this information in it, and the program will give you a better preview of what your job will really look like on the press. Photoshop comes with quite a few generic CMYK color profiles that are set up with typical dot gains, total ink limits, and ink standards to get you started. For instance, the default CMYK profile in Photoshop CS2 is “U.S. Web Coated (SWOP) v2″, designed for U.S. printing processes on a web press, using coated paper with SWOP standard ink colors. If you choose that profile, Photoshop will estimate a dot gain of 20% and a total ink limit of 300%.

However, there are varying degrees of quality for both presses and paper, and as such the generic profiles will never quite be accurate. For instance, I once sent out a job where the dot gain was 17% and the total ink limit was 275% on 80 lb. paper, then turned around and watched my dad print an almost identical job with a 4% dot gain and a total ink of 300% because of the superior quality of his press and printing materials. That said, your best option is to tell the printer what paper you will be using for the job, then ask him what his ink standard, dot gain, total ink limit (or maximum ink coverage, which is the same thing) will be and create a custom CMYK profile using these settings. If he doesn’t know this time around, play it safe on your total ink (~280%) and ask him to check those settings as he runs the job this time so you will know for next time.

While there are many more complexities to consider when printing a job, these are some of the biggest variables for a standard four-color job, and will give you a significantly better preview of how your job will change when you go from RGB to CMYK.


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