Ed #3 Stochastic and Conventional

How would you like deeper reds, brighter blues and more vibrant yellows?

How would you like greater fidelity, more open shadows and finer detail?

Still want more? How about shorter press times, sharper textures and printed images that look more like photography than ever before?

Those are some of the benefits promised by stochastic printing. First launched about a decade ago, only to be rejected by the market, stochastic printing (or, more accurately, stochastic screening) has recently made a comeback with the help of advances in technology. In fact, a rapidly growing number of printers have adopted some variation of the technique, to solve some of the long-standing problems associated with conventional printing and gain a number of other advantages.

The questions are, what sets stochastic apart from conventional printing? How does it work? Are the benefits it promises for real, or are they just hype? And is it right for you?

The answers begin with a comparison: Conventional printing is AM (amplitude modulation). Stochastic printing is FM (frequency modulation).

The vast majority of the commercially printed images that appear today are halftone images, much like the first one that appeared in 1880. Images are printed from plates made using one or more grid-like screens that separate the image into evenly spaced dots that are larger in size, or amplitude, in the dark areas of the image, and smaller in the light areas. In color printing, separate screens are used to reproduce each of the subtractive primaries—cyan, magenta and yellow—and, to add depth and sharpness, black.

Stochastic printing (also known as Staccato,^® Diamond^™ and other trade names or, more generically, FM screening) takes an entirely different approach. Instead of changing, or modulating, the amplitude of the dot as in conventional AM printing, the second order stochastic printing that is being adopted today modulates both the size of the dots and how many dots appear, or their frequency (hence, FM screening). And rather than being equally spaced in a rigid pattern, the new dots, which are smaller than even the smallest traditional halftone dot, vary according to the tone value to be reproduced. Lighter portions of the image have relatively few dots, and darker areas have more dots that can begin to form into tiny wormlike shapes or clumps of color.

How small are the dots? Almost as small as the odds of having your boss give you an extra two weeks of vacation. Conventional screens are measured in dots (or lines) per inch, with a 200-line screen generally considered to be the maximum that is commercially feasible. But with FM screens, dot size is no longer limited by the halftone screen—because there isn’t one. Stochastic dots are measured in microns—or millionths of a meter. The period at the end of a sentence, typically about 1/64 of an inch, equals 615 microns. Stochastic dots, which are sometimes square in shape, can be as small as 10 microns. The 20-micron dots that are used most often are barely visible to the naked eye—roughly the same size as a mold spore.

The dots are not only small—they are virtually random. Highly developed software programs use complex algorithms to place the dots more or less often as required in order to reproduce the image with no determinable pattern—and no rigidly patterned screens. It’s this appearance of randomness that gives stochastic printing its name. (“Stochastic” is the mathematical term for having to do with random variables patterned by probability or chance.)

Small—and random—is good. As anyone who has used fine screens knows, the smaller the dot, the sharper the resolution. More dots capture more of the data found in the original image, which in turn equals more apparent detail.

Stochastic screens carry that idea to the extreme. Done right, the small dots of stochastic screens can create a look that comes close to the continuous tones of a photograph. That makes them particularly well suited for handling fine lines, small sizes of type—especially when reversed out of a dark background—and high resolution images. This ability to capture more detail, especially when compared to conventional 150-line, or even 175-line, screens, is often considered to be the greatest advantage of stochastic techniques.

But small dots bring other advantages too. Small dots require less ink than the larger dots used in conventional halftone printing, which means that the film of ink applied by the press is more even. And because the ink film is thinner, more light is reflected back to the eye, especially when a bright, reflective, white paper, our premium-grade is used. Printed images can begin to gain the luminous depth of a transparency. In theory at least, small dots also mean that the ink dries faster too, which can save time on-press, get the job into the bindery more quickly, and reduce the need to use spray powder to prevent pages from sticking together. In reality, however, most printers notice little if any improvement.

If small is good, random may be even better, because it helps to eliminate several types of troublesome moirés—the distracting, wavy patterns that sometimes appear in printed images. While they can’t be seen on monitors, moirés can cause big problems on press. Subject moirés appear when the image that is reproduced includes grid or line patterns—such as those found in fabrics or fences—that clash with the pattern of halftone dots on conventional ruled screens. Screen angle moirés appear when halftone screens are placed at too shallow an angle to avoid interfering with other screens, one of the biggest reasons why printing jobs in more than four colors using conventional techniques can be a challenge. Scan moirés occur when the size of the image is changed and a pattern in the original image interferes with the rigid grid of pixels in the scan.

By eliminating conventional halftone screens, FM screens also eliminate screen lines and angles, which eliminates both subject and screen moirés. By eliminating these concerns, stochastic screens can make it easier to print in more than four colors. Additional colors and numerous “touch” or “bump” plates, which increase color saturation and contrast, are easier to use, so colors can be matched more precisely. (In fact, one catalog retailer reports that the use of stochastic printing has reduced its color-related returns.)

Yet stochastic printers are not entirely free to ask, like Alfred E. Newman, “What, we moiré?” Although they occur far less frequently than the other two, scan moirés can still be a problem.

Stochastic screening can help eliminate other problems as well, including the need to precisely register color rosettes. There also is less risk of finding the rough zipper or sawtooth edges that sometimes appear with conventional halftones.

With all the benefits it offers, it’s no wonder that the use of stochastic printing is growing rapidly—in fact, printers you work with may be using it already. So why isn’t every printer rushing to go stochastic? Some remember that when first order stochastic technology was introduced in the early 1990s, it wasn’t ready for prime time. Now, however, with the widespread use of direct to plate technology it’s much more viable.

Even so, it’s not easy to go stochastic.

Small dots are both a blessing and a curse. Stochastic screens can bring out all of the details in the image, including those that may not always be wanted. Sometimes, for example, you can see the grain of the film in flesh tones.

There are production hurdles too. Capturing—and holding—a tiny dot of ink on a rapidly moving piece of paper is the greatest challenge in printing. And the smaller the dot, the smaller the margin for error, every step of the way. That makes stochastic screens very unforgiving when they are on the press.

Virtually everyone who has used it says that doing stochastic printing right requires tighter controls than conventional screening.

Dot gain—the tendency for the size of the dot to become larger when it is printed—is another concern. While more dots equal more detail, more dots also equal less space between the dots and thus, less room to spare. Stochastic screens can experience as much as 20% more dot gain than traditional screens. To help control dot gain, it may be necessary to use a different reproduction curve than the one used with conventional screening techniques when the printer makes the plate. Different blankets and higher tack inks also may be required to control dot gain when the job is on-press.

Proofing remains something of a problem too. It’s important to start with good scans and calibrate the imager to ensure that the scan reproduces properly on press. Then the calibration must be changed for different press conditions or paper stocks. And remember that because the dots are so small, corrections can’t be made while the job is on-press. As some printers say, with stochastic, “the proof is the press check.” If any changes are required, so is a new plate.

Although some people think that stochastic is more forgiving when it comes to registration, that may not be entirely true. Stochastic printing can help to eliminate concerns about the misregistration of rosettes as well as lines hang-ing out, but maintaining good register is still important. Stochastic or not, misregistration will still blur the edges of fine lines and shift the color.

Printers also note that the quality of the paper stock is especially important. Ed thinks that is true for any project, but it may be especially important with stochastic screens. Smoothness counts more than ever, because even small imperfections in the surface can swallow the tiny dots. For those reasons some printers counsel against using stochastic methods with uncoated, or even matte coated, paper. A gloss coated paper will yield the best results.

You should certainly use conventional AM printing for legacy assignments—reprints or one of a continuing series of brochures—where it is important to match previous AM work. Conventional printing techniques also are preferred if you want the greatest latitude in modifying the color on-press. The relatively large dots of conventional halftones tend to be more forgiving than their stochastic counterparts.

Most printers agree that conventional techniques are also better if you intend to put a large amount of ink on the paper. When you are using metallic or opaque inks in halftone applications, conventional techniques are likely to produce a bigger impact because you can push the ink more, especially when using screens of 175 lines or less. By the same token, some printers say conventional techniques are likely to work better for one or two-color projects, where stochastic screens can sometimes make images appear grainy or rough. Conventional techniques also may be better for skin tones, because high resolution stochastic dots can tend to bring out imperfections and too much detail.

In many applications, however, stochastic screens shine. They are typically the best way to print textiles and other materials that are plagued by moirés. FM screening may also be best for handling projects with more than four colors or a number of touchplates, and those that feature charts, fine rules and screened type. Tritones, quadtones (see Ed #2) and conventional duotones also can show to advantage because shadow areas tend to be more open and details more crisp. And once printing and prepress systems are brought up to the required standards, stochastic screens can produce the most consistent results, ensuring that the last piece in the run—or a reprint—looks exactly like the first piece off the press.

But while stochastic screens offer a number of advantages over conventional screens, at the moment at least, the differences can sometimes be difficult to see, especially when compared to 175 or 200-line screens. Most printers agree that stochastic stacks up best against 150-line screens, where there’s often a perceptible increase in quality. Beyond that point, top quality conventional printing can often look every bit as good as the newer technology, even when it comes to capturing fine lines and other high-resolution details.

There also are efforts to marry the two. Sometimes the yellow ink is printed using stochastic screens, which helps to reduce the risk of moirés and produce smoother green tones. Sometimes midtones may be printed using conventional plates, while highlights and shadow tones are printed using stochastic processes. In other cases, the line ruling of conventional screens is varied. The goal is to gain the benefits of stochastic screens without the time and expense associated with implementing fully stochastic processes or losing the advantage of conventional printing. Because the two types of screens behave differently on-press, however, controlling the color can be difficult, and changes in one area can have unintended consequences in another.

In the near future, there’s likely to be room for all three approaches— conventional, stochastic, and hybrid. There’s not a single, undisputed champion. Instead of looking for a winner, it’s probably better to think in terms of having more tools to use, and new ways to achieve the results you’re after. We all win. And no matter what the future brings, it is certain to be more colorful and true-to-life than ever before.

No matter what screen you use, the results will look best on coated paper, such as the gloss, dull and matte papers show­cased in this publication. That’s because the hard, nonporous surface of coated paper holds each halftone or stochastic dot precisely, without allowing it to run into other dots or be absorbed into the capillaries of the paper. This superior dot holdout—and reduced dot gain—means that each layer of ink can precisely filter the light that strikes the surface of the paper, and reflect back clean, unmuddied colors and tones. The smoother the surface, the better the print quality. Simple.

You can find the right kind of coated paper for practically every project, and different coated paper finishes help you achieve different things. Gloss allows you to print highly reflective art, such as photography, with wonderful clarity and sharpness of detail. Dull combines lower light reflection with better readability and uniform print smoothness. Glare-free, easy-to-read matte has a rich, tactile feel as well as increased bulk to help add more substance to the project.