WHY IS BLACK INK NECESSARY FOR CMYK PRINTING?
On a computer monitor, equal amounts of red, green, and blue light combine to produce white light. Expressed differently, the presence of all colors of light creates the color white. In contrast, when you combine cyan, magenta, and yellow inks, you get black ink (the presence of equal amounts of all colors of ink creates the color black).
Unfortunately, while this works in theory, in real life this is not exactly true. Theoretically, cyan, magenta, and yellow inks form black, but in practice they form a muddy brown. Printers add the black ink (the K, in CMYK printing) in order to deepen the shadows, add contrast, and provide a true black to the color palette.
INK REPLACEMENT TRICK
The usual assumption would be that all CMYK inks are always the same at every print provider on every job. While this is usually the case, to create dramatic effects some printers replace one or more of the traditional process inks with fluorescent ink. While the overall look of the photographs may not be 100 percent true to life, the images can nevertheless be quite vibrant. This can provide a dramatic way to make colors “pop,” particularly on an uncoated press sheet that might otherwise dull down the color.
As with any other design challenge, it’s best to provide your printer with a sample of what you expect (if possible both a printed sample and an unprinted paper swatch). Also, discuss with your printer exactly how you want the photos to look to ensure that problems don’t arise (such as odd-looking skin tones).
Have you ever seen a photo of a sunset in a magazine with a brilliant sun that seems so bright it could hurt your eyes? It seems to be brighter than anything else on the page, yet, clearly, nothing can be whiter or brighter than the color of the press sheet (unless, of course, you’re adding a separate color like opaque white). This is an optical illusion, a rather dramatic one.
Along these lines, it is interesting to note that colors in general look different based on what other colors they are adjacent to. For example, if you printed a square of a neutral grey ink build on a yellow background, the grey would actually appear darker to your eye than the exact same grey printed on a square of black ink.
The rule of thumb is that dark colors appear darker on a light background than on a dark background. But an even more important thing to remember is that color in general appears different depending on the color surrounding it. If you understand this aspect of color theory, you can use it to your benefit (as with the brilliant sun that seems brighter than the white sheet on which it’s printed—but only because of the darker hues surrounding it).
NEUTRAL GREY PAPER
Not every job is printed on a white press sheet. The choice of paper color actually opens up whole new creative options—and potentially economical benefits as well, since this design choice doesn’t add extra inks to the press run.
To put this in concrete terms, you may choose a grey press sheet to give a more subtle and subdued look to a promotional piece. Perhaps you want the brochure to look sophisticated or understated, or perhaps more environmentally minded.
Beyond appreciating the aesthetics of the unprinted sheet, it behooves you to realize that colored paper usually changes the color of inks printed on it. This is because many printing inks are transparent; hence, they act like filters, changing the perceived hue of the substrate on which they are printed.
With this in mind, consider the following. Neutral grey paper (grey without a color cast of any sort) absorbs “quantity” of light but not “quality” of light. That is, it absorbs all colors equally, so the inks printed on such a sheet do not change in hue (their quality of redness or blueness). However, a neutral grey sheet will darken (or lower in value) any color printed on it.
TRIM TOLERANCES IN DIGITAL PRINTING
Offset and digital presses aren’t perfect. Paper moves slightly in the printing process, and post-press processes such as folding and trimming amplify this problem. In fact, each successive operation in the printing process can make flaws a little more obvious.
Printers speak of folding and trimming tolerances, and a good rule of thumb is that within the arena of traditional offset printing and finishing, movement of up to 1/16-inch is considered acceptable (1/32-inch is ideal, but 1/16-inch is not uncommon). That is, such an error is acceptable—or within tolerance.
This is useful information for you if you’re designing precise folds within a brochure, expecting everything to line up exactly from panel to panel when the brochure is trimmed and folded. This would be nice, but it doesn’t always happen, and a prudent designer will avoid the “need” for perfect alignment whenever possible.
That said, digital presses complicate matters. A digital press is essentially a high-end photocopy machine (better, but the same technology). Paper does not move through these presses with even the same precision as it moves through an offset press (and, as noted above, even that will produce slight misalignments).
Printers I have spoken with say that it’s good to expect the play from side to side within a digital press to cause the paper to move up to 1/32-inch. This must then be added to the customary 1/16-inch tolerance within an offset press workflow. In essence, the post-press operations of folding and trimming that are common to both offset and digital printing will add to problems, especially when you’re already starting with more movement of the paper within the digital press than on an offset press. If you start with a variance of 1/32-inch in the digital press, and then fold and trim the sheets, you will make the flaws more obvious—unless you keep this in mind when designing for digital printing.
In short, when designing for digital printing, try to avoid the need for precise folds and trims. Avoid crossovers from page to page, for instance. Don’t put a box rule around a page, assuming that the rule will have equal clearance on all sides once the job has been printed and trimmed. Involve your printer early in the design process, and ask for suggestions regarding how to make your design compensate as much as possible for the limitations of digital presses.
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]