Компьютерная фотовыводка: технология CTP в современной печати

Imaging technology used in printing
Computer to plate (CTP) is an imaging technology used in modern printing processes. In this technology, an image created in a desktop publishing (DTP) application is output directly to a printing plate. This compares with the older technology, computer to film (CTF), where the computer file is output onto a photographic film. This film is then used to make a printing plate, in a similar manner to a contact proof in darkroom photography.

CTP methods differ according to the construction type of the imagesetter and the light source used to expose the plate surface. In terms of construction type there are three different kinds: internal drum, external drum and flat bed imagesetters. The two main light sources used to expose the plate are ultraviolet light lamps and laser diodes. The wavelength and energy of the laser diodes depend on the type of plate that is used. This technology is mainly used in newspaper and magazine printing.

In an internal drum image setter the plate is put into a cylinder, while the imaging head moves along the axis of the cylinder to expose the plate. The rotating mirror at the end of the imaging head rotates around its axis to beam the laser on the desired part of the fixed plate. The construction type makes it more difficult to use several lasers at the same time, but just using one laser also has advantages such as uniform beam intensity over the whole plate imaging period. Plates are produced in less time, are more consistent, and at a lower cost. CTP can also improve registration and image to edge repeatability over traditional methods. More recent advancements in CTP plate technology made by companies such as DuPont and PlateCrafters have further widened the gap with analog plates, allowing for very fine vignettes and minimal dot gain on press. In CTP, the media is registered (held in precise position) in the platesetter during imaging, and does not rely on a separately aligned pin grid, as is the case with film. Defects due to dust, scratches or other artifacts are minimized. CTP systems can significantly increase plate production outputs. Platesetters for newspaper production can output up to 300 plates per hour at 1,270 dpi (dots per inch), whilst for commercial applications a CTP system could output 60 B1 plates at 2,400 dpi for higher screen rulings. Small portrait presses that typically create one or two color output can use anything from a standard laser printer, for low quality/low volume, up to a higher end dedicated platesetter for higher quality and volume.

CTP is restricted to digital format. CTP productions require that the basis for the printed matter as well as the imposition be digital. In CTP, if for some reason a plate is damaged, if an error occurs when it is ripped or if something has to be corrected after the plate is exposed, a completely new, imposed plate must be created.