GRAPHIC ARTS - Thermal printing benefits from better lasers
Higher output and better reliability make new diode-laser-based digital prepress systems cost-effective.
The printing industry is continuing to change rapidly, driven by the unprecedented availability and cost-effectiveness of new digital technologies. All-digital prepress workflows provide improved quality, reduced turnaround time, and lower costs. Now a key focus is developing all-digital prepress output devices and creating a seamless, streamlined, all-digital workflow right up to the printing press.
The latest advances benefit from new high-power, high-brightness, high-reliability diode and diode-based lasers that enable thermal imaging processes. Thermal technologies have the potential to increase total throughput, improve printed-product quality, and reduce total costs. As we approach the printing industry's major trade show, DRUPA 2000, held every five years in Dusseldorf, Germany, industry experts expect these new thermal-based output systems to dominate.
Lasers in printing
Laser technology has always played a critical role in the development of digital prepress output systems. The first steps to all-digital systems were taken in the early 1970s, with the development of helium-neon-based imagesetters. These devices imaged conventional silver-halide (AgX) film to create the four color separations required by offset printing presses. These film images were then used to create printing plates for the press, using a broad-area, analog UV exposure system. Since that time, imagesetters have evolved to use primarily visible and infrared (IR) single-mode diode lasers as the imaging light source. According to the April 19, 1999, Seybold Report, these systems dominate the installed base for prepress output, with an estimated 90,000 units installed. The analog processing steps required with this approach, however, make it far from ideal in an otherwise all-digital environment.
Arrays of fiber-coupled diodes, used in several new thermal computer-to-plate systems and direct digital color-proofing systems, require very high device reliability in demanding environments.
The situation began to improve in the early 1990s, when a variety of systems for directly imaging printing plates with lasers were developed. The most successful of these early computer-to-plate (CTP) systems used conventional AgX film technology adapted to the printing plate, imaged with low-power visible laser sources (HeNe, air-cooled argon, diode). This approach has the advantages of high optical sensitivity and familiar technology, but suffers from the same drawbacks that plague film-based systems, namely ambient-light sensitivity, limited shelf life, and costly, time-consuming, and environmentally unfriendly processing. Other CTP systems were introduced that used photopolymer imaging media sensitive in the blue and green. Like the AgX plates, these materials require post-image processing, but do not have the advantage of high sensitivity. During this period, the earliest systems for imaging thermal films and plates were also introduced.
At DRUPA 1995, a wide range of CTP products and technologies was introduced. Since then, significant advances have been made in lasers, materials, and output systems. The industry's main participants have now embraced the thermal approach and are working to develop thermal products in time for strong new-product presentations at DRUPA 2000.
There are several reasons for this convergence on thermal technology. Most important, thermal is the only technology that does not require chemical processing. This reduces cost by eliminating the need for processor hardware and chemicals, improves quality by eliminating an unstable, difficult-to-control process, and improves throughput and total productivity. Also, thermal plates have a long shelf life and are daylight-safe, eliminating the need for special plate cassettes loaded in darkrooms. Finally, thermal technology does not generate hazardous waste that requires special treatment and disposal procedures.
FIGURE 1. A fiber laser is the light source in a direct digital color-proofing system from Polaroid Corp. (Cambridge, MA).
Thermal technology also enables a much broader range of all-digital printing systems than any other direct-digital approach. Eliminating complex chemical processing means that plates can be imaged directly on the printing press. This digital offset press approach was pioneered in the early 1990s. Today, every major press manufacturer is pursuing some type of digital offset press. Also, thermal technology is used in today's dominant direct digital color-proofing (DDCP) systems that provide sample images for customer approval before the press is run (see Fig. 1). Development and acceptance of DDCP technology is necessary for the continued growth of CTP. Thermal technology is also starting to allow practical all-digital systems for imaging rubber or polymer cylinders for flexographic printing and copper cylinders for gravure printing.
Thermal materials, however, are 10,000 to 100,000 times less sensitive to light than AgX materials, placing significant demands on laser sources. Since DRUPA 1995, the industry has responded with a new generation of diode and diode-based lasers that offer higher power, brightness, and reliability.
Two prepress imaging architectures
The dominant architectures for prepress imaging systems-internal drum and external drum-have radically different laser requirements. In the internal drum approach, the surface to be imaged, either a sheet of film or a printing plate, lies on the inside of a stationary cylinder. The laser beam is directed down the central axis of the cylinder to a spinning mirror that directs the laser beam to the image surface. The spinning mirror is then moved along the cylinder axis to expose the entire image.
Imagesetter designs have been dominated by the internal drum approach because the optical and mechanical assemblies are relatively straightforward to design and cost-effective to manufacture, but they place stringent demands on the laser. The long optical path lengths require a single-spatial-mode laser beam with excellent pointing stability. Also, because the architecture is effectively limited to a single beam, the laser typically must deliver high output powers, the beam must be modulated very rapidly, and the mirror must rotate very quickly. These requirements become even more demanding at the high laser power levels required to image thermal materials.
The external drum architecture places the surface to be imaged on the outside of a rotating cylinder. This surface is typically imaged with multiple laser beams, with the lasers mounted close to the cylinder surface. The laser head is moved down the length of the cylinder to expose the entire image. Multiple beams mean that each beam can be lower power and modulated at a lower frequency. The external drum approach is inherently scalable, which allows the design of large, high-performance systems. The optical path lengths are typically short, so the system can use multiple-spatial-mode lasers, which are less expensive per watt of output power than single-mode devices. External drum architectures, however, are mechanically complex and demanding to manufacture. The large, rotating cylinder must maintain an excellent surface runout specification to accommodate the short depth of field of multimode laser beams. This limits maximum rotation rate, so achieving shorter imaging times requires more laser channels and potentially more total laser power. The use of multiple imaging channels also means that interleaving or stitching effects must be carefully considered and minimized in the design.
Lasers developed for thermal technologies now provide sufficient power, brightness, and reliability at reasonable cost for commercial systems. These are typically either diode or diode-based lasers and are used in five basic configurations. Multimode lasers for external drum systems are currently either free-space or fiber-coupled single-emitter diodes, or they are free-space multiple-emitter bars. Single-mode lasers for internal drum designs are either diode-pumped solid-state lasers or diode-pumped fiber lasers. Commercial systems are being developed with all five designs (see Fig. 2).
FIGURE 2. Diode lasers and diode-based lasers are reaching performance levels sufficient for new digital printing systems based on thermal technology.
A very popular configuration for new system designs uses multiple single-emitter diodes, up to 128 per system (see top photo). These are typically fiber-coupled, which improves beam uniformity and circularity and simplifies downstream optics design. Fiber coupling makes field service easier, as diodes are either supplied with connectors or fiber-spliced in the field. Fiber coupling, however, reduces delivered power and brightness, so some system designers prefer free-space lasers. This means the downstream optics must accommodate the typical asymmetry of laser diode output, which can increase costs significantly.
High channel counts demand high-reliability lasers, because each laser affects total system reliability. SDL offers, for example, high-brightness, fiber-coupled lasers that output 1 W from a 60-µm, 0.14-NA fiber and free-space lasers that emit 2 W from a 100-µm aperture and 3 W from a 200-µm aperture. The 1-W fiber-coupled device has a mean-time-before-failure of more than 500,000 hours at 25°C. Such devices are part of a new generation of high-performance, high-reliability thermal CTP and DDCP systems.
External drum systems based on multiple-emitter bars are also being successfully commercialized. These systems typically have optics that superimpose the outputs from the multiple emitters and create a uniform illumination source for a spatial light modulator. Bars for these systems must deliver high output powers at very low cost to compensate for the increased cost of the downstream optical design. Several manufacturers have recently increased the output power available from high-reliability bars from 20 to 40 W. This has dramatically decreased the cost per watt for laser power in this configuration and has enabled development of higher-performance thermal CTP and flexography systems.
Renewed interest in thermal-based internal drum designs has been sparked by the successful commercialization of fiber laser technology. SDL introduced the first commercial 9-W system to the printing market in 1997 and has since increased the commercial power level to 15 W and demonstrated 110 W in the laboratory. Fiber lasers offer the high beam quality and pointing stability required to enable new thermal CTP and DDCP products.
Thermal printing technology can provide graphic arts customers with reduced operating and facility cost and cost of ownership, increased productivity and operational flexibility, and improved product quality. These advantages are driving system development. The installed base of conventional AgX-film-based imagesetters is around 90,000 systems, while that of metal-plate CTP systems of all varieties is only around 3,000, with fewer than half of these systems using thermal technology. The industry expects that eventually the imagesetter installed base will be replaced by CTP systems and that the majority of these systems will employ thermal technology. New product introductions anticipated for DRUPA 2000 will set the stage for this transition.
GORDON MITCHARD is director of marketing in the materials processing and printing business unit at SDL Inc., 80 Rose Orchard Way, San Jose, CA 95134-1365; e-mail: firstname.lastname@example.org