PRE PRESS SECTION (UNIT 4)

A imagesetter.

Prepress is the term used in the printing and publishing industries for the processes and procedures that occur between the creation of a print layout and the final printing. The prepress procedure includes the manufacture of a printing plate, image carrier or form, ready for mounting on a printing press, as well as the adjustment of images and texts or the creation of a high-quality print file. In today's prepress shop, the form of delivery from the customer is usually electronic, either a PDF or application files created from such programs as Adobe InDesign or QuarkXPress.

The following items have each been considered part of pre press at one time or another:
1. Screening and adjustment of a continuous-tone of images such as photographs

2. Imposition, or the combination of many pages into a single signature form.

3. Separation, or specifying images or text to be put on plates applying individual printing media (inks, varnishes, etc.) to a common print.

4. Making of plates - The usage of different materials of plates should meet the needs of printing  method. Usually rubber, plastic, aluminum are used for plates as well as film which is the photomechanical exposure and processing of light-sensitive emulsion on a printing plate. Manufacturing of plates should be well planned and delivered beforehand. Also, the cost should be calculated as well.
Manufacturing of a high-quality print (PDF) file, which is used for the final printing.
Paper select, choosing a proper paper is also a very important step in prepress.


Image setter 

An imagesetter is an ultra-high resolution large-format computer output device. It exposes rolls or sheets of either photographic film or bromide paper to a laser light source. Once the film or paper is developed, a very high quality black and white image is revealed. Development ( processing) usually occurs in a unit separate to the image setter, as does raster image processing.
Image setter output ranges in width; usually between 12 and 44 inches. The resolution of an image setter is typically between 1200 and 4800 dpi.
The image setter has been largely superseded by the platesetter.

Image setter film is a silver halide-coated plastic film very similar to normal black & white photographic film, except the spectral sensitivity is reduced to a much narrower band around the output of the laser of the individual image setter. This allows the film to be handled under a (usually red) safelight, instead of in total darkness like most photographic film.

Output on bromide paper
The use of bromide paper instead of film in an image setter allows it to produce paper output called bromides. Bromides are similar to ordinary laser prints but with four major differences.
The output can be much bigger
The output is of much higher resolution
The black is darker, and is of much more consistent color
The cost is approximately 100 times that of an equivalent laser print
The main use of bromides is for proofing documents before expensive lithography plates are made. Proofing on bromide paper is much easier than film, since it doesn't require a light table or other backlighting in order to read. Because of the uptake of plate setters, this function of image setters in modern printing houses has been reduced, although not completely eliminated.
Bromide paper is very similar to black & white photographic paper, except the backing is much thinner, and like image setter film is of diminished spectral sensitivity.

Computer to Plate (CTP) 

Computer to Plate (CTP) is a newer technology that allows the imaging of metal or polyester plates without the use of film. Eliminating the stripping, compositing, and traditional plate making processes, CTP revolutionized the printing industry and led to reduced prepress times, lower costs of labor, and improved print quality.
Most CTP systems used thermal CTP as opposed to violet CTP, though both systems are effective, depending on the needs of the printing job. Thermal CTP does have the advantage of extremely high quality, but Violet CTP does cost significantly less. Thermal plates are generally used for longer runs, while Violet CTP is employed for shorter runs, and popular with 2-up and 4-up applications (Bruno, Romano, and Riordan 126).

Thermal CTP has the added bonus of utilizing binary exposure, which limits the risk of under or overexposure, and makes it possible to work under yellow light (Bruno, Romano, and Riordan 126).
Thermal CTP involves the use of thermal lasers to expose and/or remove areas of coating while the plate is being imaged. This depends on whether the plate is negative, or positive working. These lasers are generally at a wavelength of 830 nanometers, but vary in their energy usage depending on whether they are used to expose or ablate material. Violet CTP lasers have a much lower wavelength, 405–410 nanometers. Violet CTP is “based on emulsion tuned to visible light exposure,” (Bruno, Romano and Riordan 126). The general trend of platesetters has been to move toward coatings whose success on press is independent of post imaging chemical bath processing.