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Mar 1, 1997 12:00 AM

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Computer-to-plate (CTP) has been with us for many years. By the early 1990, for example, black-and-white CTP devices were being marketed and gaining acceptance among printers. Early units exposed paper and polyester plate materials. They filled a specific niche, but were not of much interest to those handling four-color work.

The picture changed in 1995, however, when systems addressing the color market burst onto the scene. At DRUPA 95 in May, experts estimated that 47 CTP systems were announced. But "announced" does not necessarily mean commercially available.

Fortunately, some of the 1995 announcements have turned into reality and color-capable CTP technology is starting to see more widespread application. By 1996, estimates indicated 400 color-capable computer-to-plate systems were installed worldwide, with the U.S. market accounting for roughly half of that number. Today, the installed base has increased to around 500 worldwide.

Future projections are optimistic. One market research firm, for example, estimates an installed base of 3,500 platesetters worldwide by 2000, expecting much of this growth to be fueled by two- and four-page units.

The worldwide metal plate market also is growing. Currently estimated at more than two billion sq. ft., CTP plates are projected to account for 20 percent to 30 percent of that number by the end of 2000.

Most of the installed base of CTP systems use visible laser light technology to digitally image metal plates. But, most recently, interest in thermal CTP technologies is heating up. Thermal computer-to-plate uses heat energy to image plates. Using this approach laser exposure is used to heat the surface of the plate material and change its composition. The plate is imaged as a result of heat applied to the surface.

To assist printers in analyzing the new thermal technologies for CTP, american printer, in partnership with Creo Products, has prepared the following Special Report.

So what's "hot" about thermal CTP?

Thermal imaging units are capable of exposing infrared-sensitive plates, however, they operate in a different manner compared to conventional or laser plates.

When handling light-sensitive plates, what matters is the total exposure time. For instance, a conventional plate can be exposed in one minute using one lamp or in 30 seconds using two lamps. It also is possible for the exposure to proceed for 30 seconds, stop the process and complete it later with another 30 seconds of exposure.

Thermal plates, on the other hand, respond to the peak temperature reached during exposure. At a specified temperature (usually a few hundred degrees centigrade), there is a physical or chemical change in the plate. Any temperature below the prescribed level has no effect. Likewise exposure beyond the threshold does not cause any change in the dot or overexposure.

This processing attribute gives thermal plates a sharpness advantage. Stray heat, or infrared energy, has no effect on the plate, preventing fogging or soft dot edges.

Thermal plates also tend to be insensitive to visible light and can operate in a daylight environment. While in the past visible light platesetters had to be light-tight or used in the dark, thermal plates make it possible to build simpler platesetters. Many platesetters are available with a fully automated plate loading apparatus. If, on the other hand, only a few dozen plates per day are being produced, semi-automatic or manual loading models also are offered.

Most recently thermal plates only were available to work at a wavelength (color of light) of 830 nm. Creo's external drum platesetters operate at 830 nm, for example, along with the Kodak Direct Imaging thermal plate. Both the Heidelberg GTO-DI and Quickmaster-DI presses also image at 830 nm. In comparison, ultraviolet light has a wavelength of 300 nm to 400 nm, while visible laser light is 400 nm to 700 nm.

Presstek's thermal plates, and several plates in development, operate over a broad range of wavelengths, typically 830 to 1100 nm, making them suitable for internal drum platesetters, that operate at 1064 nm when imaging thermal plates.

In any case, the key characteristics of thermal technologies is that a physical or chemical change in the plate (or plate system since some of the technologies use a transfer medium) occurs at a specific temperature. Using heat energy to write an image has implications for quality and workflow.

One way to look at the quality issue is to think of thermal plates as being "digital" in the basic sense of the word. Since a specific temperature must be reached to image the plate, spots are either there or not there--on or off. Theoretically, there should be no partial exposure or fogging of any areas of the plate because no change in the plate material occurs unless that temperature is reached. This allows for very precise imaging.

The workflow benefit is straightforward: thermal plates are sensitized for a fairly high temperature so they can be worked with in daylight conditions with no ill effects.

There has been some confusion regarding another possible benefit. In some cases, the term thermal has been used synonymously with "processorless." While thermal plates that require no processing are in development, the Kodak plate requires processing, albeit simplified, as do other announced products.

The Kodak plate consists of an aluminum substrate equivalent to conventional presensitized plates. The base is coated with a polymer sensitive to 830 nm for direct laser exposure. During imaging, the heat from the laser interacts with the polymer to begin the process of cross-linking the polymer to bind it to the aluminum substrate. This forms the image area.

Next, the plate is conveyed to a unit where it is preheated to strengthen the cross-linking started by the platesetter and to soften the background for removal by the processor.

The processor functions much like any automated plate finisher except that it uses an alkaline solution (potassium hydroxide) as a developer. The potassium hydroxide dissolves the non-image background, which then is scrubbed off with a roller and the plate rinsed with water. The plate then is gummed as it leaves the processor. Post-imaging processing is completed automatically.

The Presstek plate, however, is unique in that it does not require any processing or gumming steps. A wipe down with a clean cloth is required prior to mounting the plate on press.

Available in both "wet" and "dry" plates, the PEARLwet plate from Presstek can be mounted directly on the press where the fountain solution cleans off any residue during the makeready process. Additional gumming is not required as the original protective covering serves the same function as gumming in the conventional process.

Regardless of where you stand on the "processor" issue, thermal proponents and users cite the simplicity of the process and the environmentally friendly operation. Cohber Press (Rochester, NY) installed a Creo platesetter a year ago and uses Kodak developer in its digital plate processor. This developer can be casually disposed of at the end of the day.

It is true that environmental advantages do exist with thermal CTP. As we have already discussed, the Presstek process eliminates the chemistry and the gumming step, which can save both time and money.

On the other hand, users such as Marty Breslow, prepress manager at Continental Web Press (Itasca, IL), don't find the processing step a hardship. Turnaround time for creating a digital plate using the on-line process is as fast as conventional platemaking, assert users.

Once almost the exclusive province of large publication printers, today's thermal platesetters include models for medium-size commercial printing applications. High speed, high resolution and daylight operation are among the features prompting printers to consider thermal CTP options, along with improved quality and faster makereadies typical of all computer-to-plate installations.

Although still new to the marketplace, the prevailing wisdom holds that thermal CTP is the way of the future and will be quickly adopted. For in-depth information on early adopters of thermal CTP, see the three special case history articles in this supplement on pages 33 to 35. Also included is a Buyers' Guide to thermal CTP systems currently available or expected to be available by mid-summer of 1997. We also have included a breakdown of thermal plates on the market or about to be introduced. Please note that we expect more plate and system introductions during 1997, especially from Anitec, Imation and Polaroid.

For more information, please refer to the illustration on page 30 of the April 1997 American Printer.

Work in daylight conditions Environmentally safe; no chemistry disposal Increased dot sharpness Increased process latitude Decreased pressroom makeready Increased process reliability

Periodical printers have been the quickest to adopt computer-to-plate (CTP) technologies. This has undoubtedly been due to the CTP's pressroom benefits, including decreases in makeready times of as much as 25 percent. R.R. Donnelley has been a leader in implementing the technology, as well as Publishers Press and Quad/Graphics.

CTP vendors have seen the highest level of repeat buyers in the periodical printing marketplace, and these CTP devotees point out that the technology's benefits have been greater than anticipated.

In spite of quality improvements and decreased makeready that CTP provides, printers have to deal with existing film. They have devised various workflows to accommodate hybrid jobs--those requiring handling of both digital files and film--but are increasingly purchasing copydot scanners to deal with the issue.

What exactly do these specialized scanners do? Copydot scanners allow traditional film separations to be used in a digital prepress system. However, they are different from continuous-tone scanners, and the bitmap files generated also are unique.

In concept, the copydot scanning process is simple: halftone dots, linework and text on each film separation are captured in digital form so that this data can be reproduced on the plate.

Continuous-tone scanners capture shades of gray and colors. Copydot scanners, however, only see black-and-white; the separated halftone film has no shades of gray--it is either clear or black.

Copydot scanners can use either transmissive or reflective scanning techniques. Transmissive scanning uses a light source on one side of the film and a sensor on the other side to measure light that passes through the film. A reflective scanning system holds film tightly against a white surface, shines light on the film, and a scan head measures the light reflected back. This approach provides easy film handling and can scan reflective copy as well as film.

Because a copydot scanner creates a bitmap file, instructions must be given to produce the right resolution for the target imagesetter/platesetter or the RIP will resample the bitmap data to the output resolution of the imagesetter/platesetter. This resampling can create artifacts that result in poor image quality on the plate.

Additionally, it must be noted that RIPs do not change bitmap data. When working with copydot bitmap files, the RIP can't do color corrections, dot gain compensation or calibration for different plate types. These functions must be performed at the time the scan is done or by separate software.

It also is important to realize that bitmap files are large. For example, uncompressed film for an 81U2 x 11-inch separation at 2400 pixels per inch is about 70 Mb, and a full four-color set is close to 300 Mb.

Systems such as Creo's Renaissance can produce bitmap files in Tagged Image File Format (TIFF) or in Encapsulated PostScript (EPS). Both of these file formats contain the same bitmap data and will create the same image on the plate--they are just formatted differently.

Utilizing a program such as Adobe Photoshop, users can edit individual bitmap separation files to remove dust and scratch marks, edit linework, and perform simple image edits.

The final step in working with copydot data is to save or archive it for future use. No special steps are required, but since the files are very large, they should be compressed before storage. Even compressed, these files are large and will fill storage media quickly. A high-capacity, removeable-medium device with media management software is suitable for storing copydot files.

Here are a few considerations if you are planning the move to computer-to-plate. Suggestions are based on recommendations from the Graphic Arts Technical Foundation. 1 Implement color management systems throughout your scanning, color correction, proofing, output and press areas. 2 Become proficient at preflighting. 3 Do all trapping electronically. 4 Become comfortable with imposition software. 5 Become familiar with digital proofing. Decide on an approach to both internal color proofing and external color proofing in a digital workflow. Build customer confidence through discussions and education. 6 Become confident using a spectrophotometer and proof checking software to maintain quality and consistency of proofs. 7 Upgrade your network. 100BaseT is preferred. 8 Install and become comfortable with network servers that handle OPI functions and queue management. Consider one with color management capabilities. 9 Upgrade archiving systems and software. This means you must have the ability to store--and subsequently locate again--terabytes of information. 10 Implement computer-to-plate and reap the benefits of a digital workflow.