Text 2 Reproduction techniques for offset lithography

VI. Compose a plan and write a short summary of the text.

V. Compare different types of originals.

These can be broadly grouped under three headings: line origination, halftone origination, and colour separation.

Line origination For this the original, normally black-and-white, is placed on the copy board of the process camera (a very large version of an ordinary camera). Film coated with a light-sensitive emulsion is placed in the back of the camera, behind the lens. On exposure, light reflected from the white areas of the original passes through the lens to fall on the film emulsion, which contains grains of silver halide that respond to light and blacken during the development process. After development, unexposed silver, which has not developed black, is dissolved in a fixing solution of sodium thiosulphate (Hypo). The developed negative can then be used for platemaking or sometimes, where the process requires it, contact-printed to make a positive.

Halftone origination This process is used to produce a halftone negative or positive from continuous-tone originals, such as black-and-white photographs.

This halftone effect is obtained by placing a 'screen' in the back of the camera, between the lens and the film. This screen (normally made of film) carries a fine grid of lines and thereby breaks the image up into dots, so producing a halftone negative. Halftones can also be produced as PMTs (photomechanical transfers). PMTs are screened halftone prints on photographic paper. This procedure can give reasonable quality as long as the screen used is no finer than 120 lines per inch.

The key to good halftone reproduction is the control of the density range — that is, the range between the highlight (nearly white) areas and the shadow (black) areas. A typical range will have highlight dots printing only 5 per cent of their screened area and shadow dots printing 95 per cent, but this can be adjusted on the camera to give different results where the original requires it, What is needed is a compromise between contrast and detail.

Colour separation Colour originals have to be 'separated' — that is, the effect of full colour has to be achieved in printing by breaking the original picture down into four components, corresponding to the three basic colours — magenta, cyan and yellow — plus black, added to give finer detail and great density in dark areas. This separation process, known as the 'four-colour process', results in four pieces of film, one for each colour. Virtually any colour can be reproduced from combinations of these four colours; for example, greens are produced by combining a certain amount of yellow with a certain amount of cyan, the proportions varying according to the particular shade of green desired. The combination of red and blue together produce magenta; and green and blue give cyan. Magenta, yellow and cyan are therefore known as 'subtractive' primaries — or, more formally, as 'secondary' colours.

To correct colour casts or to compensate for the limitations of the process, the camera operator makes masks, which separated negatives are made with special filters and combined with the original negatives.

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