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Computer-to-plate (CTP) is an
imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
technology used in modern printing processes. In this technology, an image created in a
Desktop Publishing Desktop publishing (DTP) is the creation of documents using page layout software on a personal ("desktop") computer. It was first used almost exclusively for print publications, but now it also assists in the creation of various forms of online ...
(DTP) application is output directly to a
printing plate Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ea ...
. This compares with the older technology,
computer-to-film Computer to film (CTF) is a print workflow involving printing from a computer straight to film through an imagesetter. Designs are typically created in Adobe Illustrator or CorelDRAW, however they can also be produced in AutoCAD, Inkscape and ma ...
(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.


Technology

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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
light lamps and
laser diodes The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
. 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.


Internal drum imagesetters

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.


External drum image setters

The plate is wrapped around a drum that can rotate around its axis, while the imaging head that projects the image on the plate can move along this axis to focus the laser beam on the surface. The simple construction type makes it possible to arrange multiple image heads next to each other and use them simultaneously. This leads to a decrease of the imaging time needed to expose a whole printing plate.


Flat-bed image setters

The image is reproduced line by line on a fixed flat printing plate by a laser beam that is deflected by a rotating polygon mirror and then onto the printing plate. Because of the setup the laser beam becomes less accurate at the edges of the printing plate and is therefore mainly smaller formats or production with lower quality expectations.


Types of plates


Photopolymer plates

* Photosensitivity between 380 nm and 550 nm; * low laser energy of 30 to 100 µJ/cm2 needed for exposure; * run lengths of up to 300,000 prints; * negative plate.


Silverhalogen plates

* Silverhalogen aluminium printing plates; * photosensitivity between 400 nm and 700 nm; * low laser energy of 1 to 2 µJ/cm2 needed for exposure; * run lengths of up to 150,000 prints; * resolution to 450 LPI possible; * dot reproduction 1–99%; * positive plate.


Thermal plates

* Photosensitivity between 830 nm and 1.070 nm; * laser energy between 70 and 200 mJ/cm2 needed for exposure; * run lengths from 100,000 up to 500,000 prints; * resolution to 200 LPI–400 LPI possible; * dot reproduction 1–99%; * high process stability; * many providers; * processing in the daylight possible.


Comparison with CTP


Advantages

CTP has several advantages over conventional platemaking. In CTP, one generation (transfer of film image to the printing plate) is removed from the printing process (eliminating the need for film and related developer chemicals), increasing sharpness and detail. CTP avoids potential losses in quality that may occur during film processing, including scratches in the film, and variations in the exposure. An imagesetter usually has an accuracy rate of ±2%. Plates are produced in less time, are more consistent, and at a lower cost. CTP can also improve
registration Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), th ...
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 Dots per inch (DPI, or dpiThe acronym appears in sources as either "DPI" or lowercase "dpi". See "Print Resolution Understanding 4-bit depth – Xerox" (PDF). Xerox.com. September 2012.) is a measure of spatial printing, video or image scanner ...
), 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 1 or 2 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.


Disadvantages

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's ripped or if something has to be corrected after the plate is exposed, a completely new, imposed plate must be created.


See also

*
Computer-to-film Computer to film (CTF) is a print workflow involving printing from a computer straight to film through an imagesetter. Designs are typically created in Adobe Illustrator or CorelDRAW, however they can also be produced in AutoCAD, Inkscape and ma ...
* Color printing * Digital press


References


External links

{{commons category, Computer to Plate
CTP market in India

Michael Limburg: ''Der digitale Gutenberg''
in Google Books Germany Print production Printing terminology