Stochastic, or FM, screening promised a lot: better quality, finer lines, finer type, no moirés. And it delivered on many of its promises. Images of fabrics and tweeds look clearer and sharper, without the moirés that plague such images with standard halftone screens. FM provides greater detail in highlights and shadows. Colors can leap off the page.
Stochastic screening works especially well for high-end catalogs and fine-art reproductions. And because it’s easier to add bump plates for special colors, stochastic is ideal for printing with more than four process colors.
“Using stochastic screening reduces color variations through the run, and it seems to use less ink — you get the same ink coverage on the paper running at lower ink densities,” says Gary Briney, vice president of color and technical sales at The Hennegan Co., a commercial and catalog printer in Florence, KY. Hennegan prints almost all its work using stochastic screening from Esko-Graphics, produced on its Creo platesetters. The company uses conventional halftone screens only when asked to by a customer.
Hardly perfect
“Stochastic” comes from a Greek word, which has been translated as “to aim” and “to predict.” It’s also interpreted as “random,” although that’s not technically correct. The process is sometimes referred to as FM, for “frequency modulated,” screening, distinct from traditional, amplitude modulated (AM) screening.
AM screens — used in the traditional halftone process — arrange color dots in a screen pattern, along straight lines. To get darker colors, the process uses larger dots — a larger amplitude. The screens for each of the four process colors (cyan, magenta, yellow, black) must be arranged carefully at angles relative to each other to avoid interference patterns from the dots — a.k.a. moirés.
Stochastic screening varies the placement of the dots: for darker color, it uses more dots per area, or a higher frequency of dots. Because there are no regular patterns of dots, there is no possibility of moiré. This also reduces the need for exact registration (moirés can also result from misaligned halftone screens).
Despite its advantages, though, stochastic screening has not captured more than a narrow niche of the market since its introduction to electronic prepress in the mid-1990s. One reason: its steep learning curve. “We recommend stochastic screening to our customers where it makes sense — but it requires a ton of education,” says Bill Peterson, process technical manager for catalog printer Banta Direct Marketing in Minneapolis.
Because the dot gain is much higher for FM screening than for AM, the images are often darker than with traditional screening. “You have to adjust the separation and the output curves together, or you get undesirable results,” explains Bill Flowers, prepress manager at Quebecor World Chicago Printing. “If you adjust only the separation curves, the proof won’t look as good, and then you have to explain to the customer that it will look all right on the press. If you adjust only the output curves, then you don’t control the color contamination.”
In fact, some first-order FM screens — those that vary only the frequency of the dots — produce grainy midtones and blends or vignettes with a “stair-stepping” effect rather than a smooth gradation in tone. Skin tones tend to be uneven, giving a “five-o’clock shadow” effect.
Happily, newer products address many of these problems. “Second-order” stochastic screens, which vary the size of the dots as well as their position, eliminate graininess and produce more-even blends and solids. “It’s the smoother tone transitions that our customers really appreciate,” says Flowers.
Some products combine AM screens in midtones and solids with FM in highlights and shadows. Agfa describes its Sublima screening technology as “XM,” or cross-modulated. In highlights and shadows, Sublima uses FM screening; in midtones, the system aligns the small FM dots along angles as AM screens. Fujifilm’s Co-Res (for “common resolution,” pronounced “koe-ray”) screening is an AM technology that Fuji says gives printers the same fine resolution as stochastic screens but without the drawbacks of grainy midtones. Because it uses halftone dots, it can be proofed on Fuji FinalProof halftone printers. Co-Res is currently available only for Fuji thermal and violet platesetters and proofing systems. Esko-Graphics’ Highline Screening is high-resolution, small-dot AM screening that offers many of the advantages of stochastic screening, such as finer detail, yet to the press operator reacts more like traditional AM screens. Esko Graphics has developed Intellicurve software to adjust for the higher dot gain.
Another drawback of stochastic screening is that it exacerbates “piling,” a buildup of ink or paper residue on the printing press blanket that sometimes occurs with the new, more environmentally friendly coatings on paper stocks. “If you’re going to have a problem with piling, it will show up faster when you’re printing stochastic,” Banta’s Peterson says. In fact, paper stock is even more important to the image when using stochastic screening — which means that printers must pay closer attention to this variable to ensure consistency and quality.
Some printers have also found they have to change plates and blankets more frequently when printing stochastic screens. “We don’t get the same plate life with stochastic screening,” notes John Gaspari, director of manufacturing at Quebecor World Chicago Printing. For catalogers with million-plus impression runs, this can add to costs, which they may not be able to pass on to the customer.
Why bother?
Stochastic screening is more expensive for the printer. It requires more process control, reliable color proofs are more expensive, and the press and the blankets need to be washed more frequently. “If you don’t have good process control, you won’t be successful with stochastic screening,” says Hennegan’s Briney.
But when you do have good process control, the results are stellar. “If you’re printing jobs with a lot of textures, fabrics, fine lines and screens, and small text, then stochastic is the answer,” Peterson says. “Problems solved.”
“For our customers, FM screening gives increased detail, smooth tone transitions, and better consistency,” Flowers adds. “For us, that means we can meet the customers’ needs, and that means happier customers.”
Briney agrees: “It’s a tremendous competitive edge.”
Terms of the Trade
For those who don’t work with printers on a daily basis, the terminology they use can be mystifying. Here, a few terms and definitions to make communications easier:
Blanket: The rubber mat on a printing press that transfers ink from the plate to paper.
CMYK: Shorthand for the four process colors of printing: cyan (blue), magenta, yellow, and key (black).
Color curves: Instructions in computer software that enable users to modify the components of a color (by adding more cyan and taking away some yellow, for instance).
Dot gain: When halftone or stochastic dots print larger on paper than they appear on films or plates, which can produce reduced contrast and detail in the final image; also called dot spread or press gain.
Hi-fi color: Use of six (or sometimes even more) colors, rather than the traditional four process colors, to produce brighter or more metallic results.
HLS table: A color-control option often available in production software that allows users to modify the hue, lightness, and saturation of colors.
Moiré: Caused by an undesirable pattern of halftone dots, it results in an image displaying an unwanted ripple effect.
Piling: A buildup of ink or paper residue on the blanket during printing.
Vignette: A background that gradually fades to white.