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Laser Scanners - Reprographics: Basics

Half-Tone Printing

Offset-lithographic printing is essentially a binary process where areas of colour are applied as a checkerboard pattern (Half-tone). Dots per unit area rather than ink density determines the final printed colour. Traditional half-tone screens, which were physical overlays used in a photographic process to create a grid of dots of a particular colour ("separation"), have long been replaced by equivalent output from reprographic scanners.

To produce a full colour image, four separations are needed, three primary colours; cyan, magenta, yellow plus black. The black separation is used to compensate for process errors and the non-ideal spectrum of the coloured inks. Screen ruling, typically 0.05mm to 0.5mm, and screen angle are two main parameters which characterise a half-tone separation. Each separation represents a spectral band of the original image, which when overprinted reproduces the original image in full colour.

The original image, which may be a transparency or photograph, is processed by the "analyse" part of the reprograhic system, point by point (analyse spot size is 0.01mm dia. typically). The analyser extracts colour and spatial information from the original, which after signal processing, is split into its spectral colours i.e. Separations. The colours are chosen to correspond to the printing ink . Images may also be made negative, reduced or enlarged as required.

Image information is transformed by the E.D.G. (Electronic Dot Generation) into a half-tone image. The E.D.G. replaces the overlay, and superimposes a two-dimensional function onto the image information.

The image resolution corresponds to the smallest pixel size that can be generated on the film or medium. Normally this is not limited by the E.D.G. but by the electro-optic system which switches the data. Screen ruling (equivalent to D.P.I. in desktop publishing) corresponds to the repeat pitch of the half-tone grid. Generally, high resolution is required only with finer rulings. Larger rulings give a much coarser image and are used for lower quality printing (e.g. Newspaper images).

Although the trend towards C.T.P. (Computer to plate) systems has begun, most expose engines still produce the final image on film. These images are transferred onto the photosensitive printing plate, by contact printing, as a 1 to 1 image. At some point within the whole process (e.g. by controls in the expose scanner) account is taken of any positive to negative (black to white) reversal which occurs after contact printing.

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