Landmarks in printing; from movable type to the microchip

UNESCO Courier, July, 1988 by Werner Merkli

The printed image | Throughout the Middle Ages, beautiful copperplate engravings, etchings, woodcuts (26) and lithographic prints were produced by draughtsmen and painters by carving or drawing on printing blocks of wood or stone, or by engraving on metal plates. Discoveries regarding light and the theory of colour, together with the invention of photography in the nineteenth century, brought the potential of photographic processes to the printing industry (27).

Although solid lines can be printed directly using the letterpress and offset processes, the intermediate tones in photographs cannot. In 1881, the Munich copperplate engraver Georg Meisenbach succeeded, by photographing an image through a screen of crossed lines, in breaking down the image into tiny dots. In the resulting positive, the closely-spaced dots combine to form the darker tones of the original image, and the less dense dots yield the light tones. The screened image can then be recorded on to a printing plate in a process known as halftone etching. Screening is often carried out by electronic scanning today (28).

For the reproduction of tonal images in colour, however, three plates must be made--one each to print red, blue and yellow ink. Usually, a black plate is also made, because black ink adds sharpness to the printed copy. The first step is to separate the colours photographically in the original image. The colour separations are made by taking four separate photographs through different filters which block out all colours except the desired colour, and also through a halftone screen to produce the dot pattern required for printing. In colour printing, some of the different coloured dots fall close together and some are superimposed. The eye mixes the colours of the dots on the printed page into all the tones of the original. For example, what the eye sees as green is really an area of tiny blue and yellow dots. Experiments carried out in the USA between 1946 and 1950 led to the development of a process for making colour separations by electronic scanning, and by the late 1970s, following developments in electronics technology, the impulses from the scanner beam were digitized. The tiny dots which produce the tonal values of the reproduction are then reconstituted by laser beam, either on film or directly onto the printing plate.

The electronic future | Developments in the printing industry during the coming decades will be primarily determined by advances in electronics. Desktop publishing technology (see article page 16) emerged in the 1980s for the processing of both text and image. Using electronic pencils and brushes, computer graphics can be created directly on the screen and integrated into the stored text. Furthermore, magnetic discs are being developed which can store over 1,000 million typographical signs (500,000 typewritten pages) and integrated databases will improve access to information.

Metal typesetting and letterpress printing processes have virtually disappeared within a short period. Thanks to the simplification of platemaking, however, the offset and photogravure processes have survived. Electrostatic printing, whereby an electrostatically-charged plate transmits a dry powder or liquid ink toner to plain paper, and inkjet printing, in which computer-controlled jets are used to spray hundreds of thousands of electrostatically-changed drops of colour per second onto the paper to produce text and images, are just two of the techniques which are leading to contactless printing, instead of printing from an inked plate.