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Apr 1, 2005 12:00 AM
Today’s screening technology is enabling printers to
achieve truly dazzling results. But before reviewing the latest AM,
FM and hybrid offerings, printers should consider what
they’re trying to do, rather than how it can be done. In
other words, they should think outside the dot.
"Printing has always been about putting little dots of ink on paper; about reproducing a customer’s original, which has no dots," says Gordon Pritchard, Creo’s (Vancouver, Canada) Value in Print marketing manager. "All printers are trying to meet customer requirements while reducing manufacturing costs. Screening impacts both of those areas."
What will work best for your customers? Conventional AM remains as popular as ever, while second-order FM has solved most of the problems that plagued the first generation. And then there’s hybrid, which reportedly combines the best AM/FM attributes. (For an excellent explanation of all things screening, see "BRIDG’s Guide to Halftone Screening," available at www.ipa.org.)
AM: size matters
Amplitude modulated (AM) or conventional screening has been around in one form or another since the 1880s. With AM, the size ("amplitude") of dots is modulated on a regularly spaced grid to create the illusion of different tones. The dots are arranged along lines in both directions, forming a grid. When combining the color separations in four-color printing, these grids must be rotated at predetermined angles. Moiré appears in the image if the angles are incorrect or slightly out of place; the problem increases as you add more colors. Conventional screens also can create a jump in color tone in what should be a smooth gradient blend. This occurs when, as the tone darkens, the dots get larger and suddenly touch each other.
My AM is true
Despite these potential challenges, AM has many advantages. Ray Cassino, Heidelberg USA’s (Kennesaw, GA) director of prepress product management, maintains that one kind of traditional AM, irrational screening (IS), is the best.
"IS can be used for any type of printing," says Cassino.
"It’ll give you the best fleshtone and vignette
reproduction—and for general commercial printing, it’s
the easiest set of angles to print with."
Heidelberg’s IS Classic hasn’t changed since its 1982 introduction. "The angle is the angle, same as it was 100 years ago," says Cassino. "But those conventional camera angles (0°, 15°, 45° and 75°) were never able to be reproduced electronically until the Hell company made them available in 1982."
Web printing is where AM screening really shines, according to Cassino. "Its advantage is that it’s very forgiving on press," he says, "especially on web presses." Misregistration can cause artifacts and doubling to show up on web printing. "IS screening doesn’t let that happen," says Cassino. "For instance, we’ve got screening systems particularly for web presses that have no color in the 0° angle, because that’s the way the job is fed through the press to prevent doubling issues."
Heidelberg’s Prinect MetaDimension RIP has a series of queues for the 12 standard screening products shipped with the basic RIP. Cassino explains: "The user takes Job A to queue 5 for IS screening, and the parameters are built in to make the plate with the right calibration curve, the right dot gain curve and so on. A different output queue for stochastic (FM) screening can be set up with its necessary parameters."
A new breed of AM
Esko-Graphics’ (Ghent, Belgium) new "high-definition AM" screening technology, HighLine, incorporates digital dot gain compensation curves to produce small dots without the use of FM screening. It can produce high screen rulings at lower output resolutions than traditionally are required, while a greater set of angle combinations helps avoid moiré issues.
Andy Redman, Esko-Graphics’ business development manager, automated workflows, explains: "It applies a ‘dithering algorithm’ to both ends of the spectrum, to keep those small dots while making a full gradient that is very smooth all the way across."
Esko-Graphics introduced HighLine in July 2002. It’s
available in different bundling packages or as an add-on for
"The whole idea of HighLine screening," says Redman, "is that it enables you to output a higher line screen at the typical output resolution you’re using right now. 308 lpi at 2,400 dpi is typical, even for web presses. The quality level is as good as or better than stochastic."
HighLine reportedly offers more on-press flexibility than stochastic. Redman says, "If you want to make a color move on press, you sometimes have to pull an FM job off and make a new set of plates with a different curve applied, while the press sits idle." As with conventional AM screening, HighLine allows press operators to manipulate colors on press via the ink console.
Co-Res mi fa so la ti do
Another AM alternative, Co-Res, also enables users to print high-screen rulings while using standard output. The FujiFilm product is distrbuted by Enovation Graphic Systems (Hanover Park, IL). "The goal of any screening technology is to get the printed piece as close to continuous tone as possible," explains Bryan Hughes, product manager, electronic products. "Unlike most stochastic or hybrid line screen technologies, Co-Res doesn’t compress your process control window as much as FM or increase the time it will take to produce the job. It delivers higher line screens but handles more like an AM job."
With conventional technology, screen rulings of 175 lpi and 300 lpi require output resolution ranges of 2,800 dpi and 4,800 dpi, respectively. With Fujifilm’s Co-Res, 175-lpi screen rulings can be output with a 1,200-dpi resolution, and 300 lpi with a 2,400-dpi resolution.
"Co-Res comes from Fuji’s years of experience with color science," explains Hughes. "It keys on the the human eye and brain—why we perceive some things but not others. Fuji has created an algorithm that compensates for noise, placing it where our eyes don’t perceive it."
Fuji introduced Co-Res in 2003 and currently has about 70 installations in the United States. Taffetta, its second-order FM product, will be released this month.
Little bit country, little bit rock-and-roll
Screening that doesn’t follow the classic AM or FM methodology is generally referred to as "hybrid." Hybrid screens typically use AM technology in the midtones (about 10 percent to 90 percent) and stochastic in the highlights and shadow areas. The idea is to achieve rich detail in the highlights and shadows while avoiding graininess in the midtones. From an implementation perspective, going from AM to hybrid screening is less radical in production terms than a move to FM.
Says Michele Zajac, Screen USA’s (Rolling Meadows, IL) workflow product manager, "Hybrid takes the best of both AM and FM technology—working with an optimized, randomized FM dot pattern in the highlights and shadows, then combining it with an optimized, randomized white space pattern of AM in the midtones. So there are no angles, no traditional AM rosettes."
Screen’s Spekta hybrid screening product was launched at
Print 01 and currently has about 150 users in the United States. It
is a closed system product, requiring Screen software and
equipment. Says Zajac, "If you can run a 175 line screen, you can
run Spekta." Spekta also comes in 300-lpi/2,400-dpi and
400-lpi/2,400-dpi versions. Screen recommends thermal plates for
their ability to hold a dot, and offers a set of certified plates
for use with the Spekta product.
Zajac explains: "AM screening is a known animal—people know how to run it. FM has an advantage in quality but requires micromanagement in the press environment. Hybrid screening has curves already inherent and doesn’t require any prepress or press changes."
Additionally, Screen’s workflow has a multiscreening option that enables a prepress operator to apply different screening types to the same plate. "For example," says Zajac, "You might not want to use stochastic for a large, flat tint background. Hybrid does much better with them, but hybrid’s strength is in the details. So for a large wash, multiscreening allows you to assign a background as AM 12-bit and assign images and illustrations as hybrid."
XM marks the spot
Agfa (Ridgefield Park, NJ) categorizes its hybrid :Sublima product as cross modulated (XM) screening. (Similar products include Creo’s Maxtone and Esko Graphics’ Samba.) "Printers find that XM is easier to work with on press vs. first- or second-generation stochastic screening," says Steve Musselman, Agfa Graphic Systems worldwide business development. "We’re all trying to improve on first generation stochastic’s noise and graininess."
Sublima combines AM halftone screening in the midtones with FM
in the highlights and shadows. Both screens are maintained at the
same halftone angles to ensure a smooth transition from AM-screened
areas to FM. A tiling algorithm reportedly makes screening
Printers that can print conventionally at 175 lpi and hold a two-percent dot will find :Sublima smooth sailing. "For 240 to 340 line screening with :Sublima, the smallest dot we use is 21 microns," says Musselman. "If you take a 2 x 2 pixel on a 2,400-dpi platesetter, side by side and up and down, that’s a four-spot dot. That equates to a two percent dot at 175 lpi."
Some :Sublima users position it as "spot-free" CTP.
"Essentially, you don’t have to use spot colors" explains
Musselman. "At these fine frequencies, you don’t see the
rosettes with the naked eye—it looks like a solid color. You
could even build your text out of process builds."
Because CMYK can be used for spot color effects, customers have more options. "They can put the job on a six-color press rather than an eight-color one or use the extra units for spot varnish or metallics," says Musselman. "You get more bang for the buck."
Agfa still offers Cristal Raster, an FM screening product that debuted in 1993, but according to Musselman, most customers prefer :Sublima. "Customers don’t want stochastic so much as they want increased print fidelity and consistency as well as removal of screening artifacts."
Come on feel the noise
With frequency modulated (FM) screening, the size of the dot remains the same, but the number ("frequency") of dots varies. Since dots are placed pseudo-randomly, there is no direction to the screen, screening angle or rosette structure. First-order stochastic products used fixed-size dots and variable spacing, while second-order borrows the idea of variable dot size from halftone screening but also employs variable spacing.
Second-order FM screening is a new generation of stochastic developed to address quality issues the early FM adopters encountered, particularly in reproducing midtones. Whereas FM screening at first clustered dots to achieve dark tonal values, the second-order variety applies algorithms to cluster the dots into more orderly patterns and to modify the size and shape of the dots. The resulting output is said to be smoother and to have less dot gain.
FM screening solves the problem of moiré patterns on press because screen angle interference is eliminated from the process. This is particularly important for presses running full-color jobs. Cassino notes, "The more screen angles you have, the more chance of a moiré there is between screen angles. Today, with more jobs going to five, six or seven colors, there’s a higher applicability for FM screening." For multicolor jobs, he explains, the press run stability FM screening provides is an advantage. However, FM’s stability on press can make color adjustments difficult to near impossible. "You shouldn’t use a printing press like a Photoshop extension," Cassino says. "That’s the wrong place to start color corrections." FM relies on tight process controls in prepress and in the pressroom.
With FM screening, there is no screen ruling per se. Cassino explains: "It prints about equivalent to 280 lpi, which is why you need more process controls. If it were just as easy to print a 300 line screen as a 175, everyone would be doing it because 300 lpi looks really good! Most people don’t print it because it’s a lot harder to keep consistent."
CTP provides consistency
"CTP really paved the way for screening technology to flourish," says Cassino. "When stochastic screening came out in 1993, all we had were imagesetters." He cites problems associated with processing film such as imagesetter calibration, contacting the film and creating analog proofs. "Frankly, those problems outweighed the benefits of FM screening for most printers," he explains.
Cassino notes that inkjet devices, which produce proofs without dots, are ideally suited for use with FM work. "Going to FM enhances the acceptability of inkjet proofs—they’re certainly two peas in a pod," he says.
Jack Ryan, prepress manager for Perfect Printing (Moorestown, NJ), concurs: "We use an Epson 10600 with a DuPont CromaPro front end for color management. FM helps our customers, who have pointed out on many occcasions that the printing looks more like the proof because neither has an actual dot."
Heidelberg’s Satin Screening product currently has about 50 users. Some run a Heidelberg shop; others, by outputting a TIFF-B file from Heidelberg’s MetaDimension RIP, combine equipment from numerous vendors to run a stochastic job.
Creo’s second-order Staccato stochastic screening is available in multiple dot sizes, including Staccato 10, which uses a 10-micron dot. Pritchard cites some key differences between Staccato and the orginal first-order FM screen. "It could also be called a hybrid FM. A strict FM screen uses small dots that are all the same size, and more of them when you want a darker tone. But with Staccato, similar to an AM screen, dots actually grow on the tone scale. Through the midtone, the dots will actually grow in one dimension, forming these little ‘worms’ that smooth out the graininess associated with first-generation FM."
In addition to maintaining a higher level of fidelity, FM can provide some attractive manufacturing benefits. "You’re printing more consistently with less ink," says Pritchard. "Even if a customer doesn’t care about dots, the printer is helping reduce manufacturing costs because the job comes up to color faster and you can get more vibrant color on a poorer grade of paper."
Pritchard notes that FM is no longer the niche application it was eight years ago. "We’re seeing Staccato used across all market segments from metal decorating to newspaper flyers for companies such as Wal-Mart and Home Depot. "Eighty percent of the Yellow Pages in North America are printed with Staccato," he says, "even though most people aren’t looking at the Yellow Pages with a loupe!"
FM screening saves the day
Two years ago, Perfect Printing (Moorestown, NJ) took a screening gamble. "We were working on a job that had light blends and a faded image from 100 percent to white stretched across a 24-inch-wide invitation," recalls Jack Ryan, Perfect Printing’s prepress manager. "We could not lose the banding." The 29-inch sheetfed shop uses Artwork Systems’ Nexus workflow, but had never tried Artwork’s 20-micron FM screening. "We really did not have any idea what was going to happen—and this was during a press check," relates Ryan. "It saved us. Our customer was ecstatic."
Photos look great
Using the same calibration file for Perfect Printing’s normal 175-line printing, Ryan tweaked the gray balance. "It was difficult to adjust for the 75 percent patch, but the 25 and 50 are very close to GRACOL standards," he says.
Ryan subsequently attended GATF’s Process Controls Boot Camp (www.gain.org) which he highly recommends: "It worked wonders for us."
Perfect Printing now uses FM for about 85 percent of its work. Challenges include holding jobs with a lot of highlight or shadow detail as well as printing large amounts of solid coverage, such as dark greens and blues.
On the positive side, FM reproduces photos cleanly "with no dots and obvious blends, even with PostScript Level 3," says Ryan. "It eliminates shark-tooth edges and prints small logos legibly—things that would be difficult to read with 150- or 175-line screens. It has been a fantastic addition to our business."
Finest level of quality and detail
Arandell Corp., (Menomonee Falls, WI) is the fourth largest cataloger in the U.S. The 660-employee, $250 million printer’s clients include Neiman Marcus, Macy’s, Coach, Godiva, Patagonia and others.
The web printer is a true pressroom pioneer: In 1994, Arandell installed the first Heidelberg Web (now Goss) single-web Sunday press and, more recently, helped develop its latest press, the Sunday 3000/32. A 72-inch web width enables the press to print 32 magazine-size pages per revolution at speeds up to 100,000 iph.
The company has been using Creo’s Staccato second-order FM screening technology since 2003.
"We want to be very productive, but we also want to give our clients the finest level of quality and detail that can be produced," explains Jim Giencke, president and COO.
"Staccato raises both without burdening our clients with additional costs."
Initial test results were excellent. "About 70 percent of the jobs had some discernible increase in detail," reports Giencke. "With some, we were able to achieve colors that we couldn’t with four-color process using a round dot. The color gamut is slightly larger."
Tight process control
Giencke agrees that FM screening is less forgiving than AM. "Stochastic requires tight process control. You have to like your proofs when you do your prepress, because Staccato is more solid in its movability on color—it won’t sway much throughout the run. Also, when you’re trying to hold open a 1.5 percent to 2 percent dot at 25 microns, your ink-and-water balance has to be very compatible."
Arandell tweaked its fountain solution and installed new filtration systems, but avoided major changes thanks to its disciplined manufacturing approach. "We’ve been told it’s unusual to find a printer that has its process and materials as well under control as we do," says Giencke.
Prior to implementing Staccato, Arandell spent 11 months working with a team of Creo consultants comparing conventional and Staccato forms and plates across a range of jobs. "It was testing, testing, testing. We tweaked the curves on everything from wood grains to food to hard goods to fashion until we had curves that gave us spectacular pieces," says Giencke.
Arandell then tailed in some Staccato sheets at the end of conventional runs to demonstrate the difference. Most customers gave immediate approvals. "It was a smooth transition," says Giencke. "Creo’s consultants did a fantastic job."
Like a 300-line screen
For now, Arandell is content with its 20-micron process. "It’s equivalent to a 300-line screen, which takes care of our full range," explains Giencke. "We’re not sure if a web press can get down to 15 micron—we’ll probably try it. But for now we’re settling in at 20 to 25 microns."
Denise Kapel and Katherine O’Brien can be contacted at email@example.com.