The Beginning of Computer to Plate (CTP)

CTP technology evolved from computer-to-film (CTF) technology which we call imagesetters. There are two types of imagesetters, drum and capstan. With drum imagesetters the media is vacuumed to the internal surface of the drum and exposed by a laser beam reflected by a mirror (or mirrors) mounted on a high speed spinner motor.

The laser is positioned a considerable distance from the media and moves across it to expose the image. Capstan imagesetters also reflect the image onto a mirror with a high speed spinner motor, however the spinner motor remains stationary and the film moves through a set of rollers. When the first imagesetters entered the marketplace drum imagesetter were the imagesetter of choice because they registered film better than capstan imagesetters. The reason was because the the film was stationary on the drum, where it was constantly moving in a capstan imagesetter. But as time went on capstan technology improved greatly and capstan imagesetter registration became as good as drum registration. Most companies today that continue to use imagesetters (screen printers, flexo printers, and even some commercial printers) actually prefer capstan imagesetters over drum imagesetters.

As imagesetter technology integrated itself into CTP equipment, the most practical laser was a 532 nanometer (nm) green laser, referred to as a YAG. Other available lasers were the 633 nm red and the 488 nm blue gas lasers. The most workable media was a silver-based plate, which could be exposed by all three laser types. Internal drums, using a single laser diode, were the technology of choice with the first CTP systems in the early 1990’s. The first CTP manufacturers Agfa, Autologic, Cymbolic Sciences, Purup, and Western Litho, chose the 532 nm YAG laser, following the lead of Creo, who pioneered CTP technology with their 3244 Trendsetter. Competitors such as Barco and ECRM chose the 488 nm blue gas laser. History proved the choice of the blue laser to be a mistake, primarily because of its high failure rate, often in less than 1,000 hours. A few manufacturers also offered equipment with the 633 nm red laser, since this laser was well accepted and understood in imagesetter technology. From these laser technologies evolved violet laser diode technology, which is used in many platesetters today.
The violet diodes cost less than the blue and green lasers, and can also be used in a more user-friendly yellow safe-light environment.