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Zooming in on Digital Printing Technologies



Zooming in on Digital Printing Technologies

By Harald Johnson

The process of creating high-quality digital prints has, for the majority of photographers and digital imagers working today, coalesced around four major output methods or technologies: digital photoprint, dye sublimation, electrophotography, and inkjet. To be sure, there are plenty of other digital processes that a digital imager can use, but these are either more obscure, more expensive, or too low on the quality scale, so I won't be including them here.

The Durst Lambda 130 digital laser imager. Courtesy of Durst U.S.

Digital Photoprint This is top-of-the-line, continuous-tone photo output, and you'll only find the pricey devices for doing this in photo labs, repro shops, service bureaus, and 'imaging centers.' (I like the term 'digital photoprint'; others use words like laser photo printing, although not all devices use lasers.) Using either three-color lasers (red, green, blue) or light-emitting diodes (LEDs), these printers produce extremely high-resolution prints on conventional, light-sensitive, color photo paper that's typically processed in the normal 'wet' photographic manner using RA-4 chemistry (although other processing back ends can be used). For smaller prints, digital minilabs made by Agfa, Noritsu, and Fuji are the standard at many photofinishing labs and new online processors such as Shutterfly.com. Examples of digital photoprint printers include the Durst Lambda 130 and 131, Océ LightJet 430 and 500XL, and ZBE Chromira (wide format), and the Fuji Frontier and Noritsu QSS (narrow format). Digital Photoprint: For What and for Whom? Photographers like the output from digital photoprint because it looks like a real photograph. In fact, it is a real photograph! Designers, art directors, and fine artists also use this kind of printing for large displays and exhibition prints. The primary negatives are high print cost (for the wide-format variety only), limited paper choices (photographic only), and the fact that you definitely can't do this yourself because the devices are much too expensive for self-printers to own.

The Olympus P-400 dye-sub printer (a newer model, the P-440, is even faster. Courtesy of Olympus America, Inc.

Dye Sublimation With dye sublimation, a single-color ribbon containing dye is heated by a special heating head that runs the width of the paper. This head has thousands of tiny elements that, when they heat up, vaporize ('sublimate') the dye at that location. The gaseous dye spot is then absorbed into the surface of the paper. Since the paper receives separate cyan, magenta, yellow, and sometimes black passes of the dye ribbons to make up the final image, the resulting layering of color provides a smooth, seamless image. Photo dye-sub printers only have 300 or so dpi resolution, but they can deliver continuous-tone images because of this layering and the way the dyes diffuse, or cloud into the paper. Some dye subs add a protective layer (a clear UV laminate) as a fourth and final step after the single-color passes. Examples of dye-sub printers include the Olympus P-400 and P-440, Kodak Pro 8500, and Mitsubishi CP-8000 and CP-3020.

Dye Sub: For What and for Whom? Dye sub is popular with pro and advanced-amateur photographers who want continuous-tone, photographic image quality. Print speeds can be very fast (75 seconds per 8x10 on the Kodak 8500 or the Olympus P-440), and some prints are more scratch-resistant than with those created via inkjet. Design and retouch studios often use the cousin of dye sub for proofing: the Fuji Pictrography. While dye sub yields photographic quality to the naked eye, disadvantages are the high cost of the larger-format printers, the high cost of the consumable supplies, and the limited choice of special papers (glossy and matte only).

The Xerox Phaser 7700 color printer. Courtesy of Xerox Network Printers

Electrophotography Electrophotography involves the use of laser printers and printers/copiers. We're not talking about run-of-the-mill office copiers-the technology has taken huge leaps in recent years. Many use hair-thin lasers to etch a latent image onto four rotating drums, one each for the four printing colors. The drums attract electrically charged, dry, plastic-based pigment toner and then transfer the image to an intermediate transfer belt and then onto the paper, where it's fused. Other laser printers transfer the toner directly to the paper without the intermediate step. Electrophotographic printing is fast and reasonable, with 8x10 prints well under $1 at retail, and images can be printed on a small range of substrates, including matte paper and commercial printing stocks. Examples of dye-sub printers include the Xerox Phaser line (Phaser 7700), the Canon CLC line, and the Minolta Magicolor line of color laser printers.

Electrophotography: For What and for Whom? Traditionally used as proof printers by prepress departments and production printing operations, color laser printers sometimes double as color copiers but less so all the time with more direct-data input options. They're also used as primary color output devices in graphic arts departments, design studios, and smaller print shops, and now by artists, especially photographers. Indiana photographer Seth Rossman loves this type of output. "For photographers, it's an almost perfect medium," he says. "I use it in continuous-tone mode, which gives it more of a dithered effect, so [there are] no dots." The main disadvantages of electrophotography are the limited maximum output size (usually 12x18 inches) and the high initial cost of the machines.

Inkjet Inkjet is the current technology of choice for many digital imagers, especially those doing their own printing ('self-printers'). Inkjets use nozzles to spray millions of tiny droplets of ink onto a surface, typically paper. While earlier devices had an obvious digital signature, the newer printers are so much further along that inkjet images can now be considered continuous tone for all practical purposes.

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There are two main types of inkjet technologies: continuous flow and drop-on-demand, which is further subdivided into thermal, piezoelectric, and solid ink categories. Continuous Flow: This is the original technology that started the entire high-quality, digital-printing boom. The most famous example is the Iris printer, which is no longer being manufactured. It's been revamped and is now called the Ixia, pronounced 'zia.' A single printhead moves along a rod above the paper, which is wrapped around a rotating drum. The printhead encloses four glass nozzles (one for each of the printing colors (cyan, magenta, yellow, and black), which are each connected to a bottle of translucent dye ink. In each head is a tiny, vibrating piezoelectric crystal that pushes out a million ink droplets per second. As the droplets exit the nozzle, some receive an electrostatic charge; most don't. The charged ink droplets are deflected away from the drum and recycled. But the uncharged ones - our heroes - pass through the deflector and end up hitting the paper to form the image.

Drop-on-Demand: This is the area where the technology advances are coming out at a dizzying pace. The reason it's called drop-on-demand is because only the ink droplets that are needed to form the image are produced, one at a time, in contrast to continuous flow, where most of the ink that's sprayed is not used. The three main types of drop-on-demand inkjet printing are: thermal, piezo, and solid ink.

Thermal: The thermal process is based on the heating of a resistor inside the printhead chamber. As the resistor heats up, a vapor bubble surrounded by ink is formed, and the increase in pressure pushes an ink droplet out of the printhead. After the bubble collapses, more ink is drawn in from the ink reservoir, and the cycle repeats. Both narrow-format (desktop) and wide-format, thermal plotter printers have printheads that move back and forth on a rail over the paper, which is pushed incrementally after each head pass. Most thermal printheads (one per color) are disposable with replacement periods varying according to the manufacturer; some have to be replaced with each ink change (if they're integratedwith the ink supply); and some can last for 10-12 changes (if the ink supply is separate). Examples of thermal inkjet printers include the HP Designjet 120 and 5500, ColorSpan DisplayMaker X-12, and Kodak/ENCAD NovaJet 880 (wide format), as well as the Canon i960 and i9000, HP PhotoSmart 7960, and Lexmark Z65 (desktop).

HP raises the inkjet bar with the PhotoSmart 7960 photo printer, which includes eight colors with three blacks. Courtesy of Hewlett-Packard Company

Piezoelectric: When certain kinds of crystals are subjected to an electric field, they undergo mechanical stress (i.e., they expand or contract). This is called the piezoelectric effect, and it's the key to this popular brand of digital printing, called 'piezo' for short. When the crystalline material deflects inside the confined chamber of the printhead, the pressure increases, and a tiny ink droplet shoots out toward the paper. The returning deflection refills the chamber with more ink. With both wide-format and desktop models of piezo printers, the printhead assembly goes back and forth over the paper to create the image. Piezo printheads are typically single units with all colors included; they are a permanent part of the machine and usually need no replacing. In the desktop category, there's only one piezo player, and that's Epson, with six- to seven-color inks in dye and pigment versions. Examples of piezoelectric inkjet printers include the Epson Stylus Pro 4000, 7600, and 9600; Roland Hi-Fi JET Pro II; and Mimaki JV4 (wide format), and the Epson Stylus C84, Stylus Photo R800 and 2200 (desktop).

The new Epson Stylus Pro 4000, for both photo and graphics printing, ships in January 2004. Courtesy of Epson America, Inc.

Solid Ink: Formerly called 'phase change,' solid ink technology is the inkjet oddball. The Xerox Phaser 8200 is a true piezoelectric inkjet, but there are several surprises. First, the pigmented colors come in the form of solid blocks of resin-based inks, although the ink still ends up as a liquid after heating (hence the term 'phase change').

The Xerox Phaser 8200 solid-ink printer; at left are the resin-based, solid ColorStix. Courtesy of Xerox Network Printers

And instead of a smaller, reciprocating printhead assembly, there is a single printhead that extends nearly the width of the paper with 88 nozzles in each of four rows. The same piezo substance we've already learned about shoots the ink droplets out as before, but in another twist, the ink doesn't go onto the paper; instead, the ink goes onto a turning offset drum that is kept warm so the ink doesn't solidify. The drum then transfers (in a single pass) the still-molten ink to the paper under pressure to form the image. The ink sits on top of the paper, creating a definite relief effect. The colors are brilliant and sharp since the ink drops don't spread or bleed. But because of that, the effect, even at 1200 dpi, is not always continuous tone. 'Near photographic' might describe it better. Solid ink inkjet is fast and it yields highly saturated images that many designers and illustrators love.

Inkjet: For What and For Whom? For the most flexibility in terms of choices of printer brands and types, inks, papers, sizes, and third-party hardware and software support, you can't go wrong with inkjet. There are photo printers, proof and comp printers-you name it. In addition, certain inkjet print combinations exceed all other standard, color photo print processes in terms of projected print longevity or permanence.

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