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Still used today to reproduce full colour output, halftone colour printing was introduced in the 1890s, although many years passed before its full potential was realised. Although colour reproduction theory was fairly well understood, the lack of colour film restricted colour work to studios where the necessary separation negatives had to be made directly from the subject, under the most exacting conditions. As reliable colour film became available in the 1930s, colour reproduction became both more common and more accurate. The offset plate is made of a base material - such as aluminium, stainless steel, or, for very short runs, paper - coated with a photoreactive substance. After exposure, the plate is developed and then treated to enhance its ink-attracting or water-repelInking rollers Dampening Plate rollers cylinder rapt J Ic4--+ PaDer out Impression cylinder The offsetlithographicprinfingprocessling properties. For very long print runs, bimetal plates are sometimes used; typically, copper forms the image area, while aluminium or chromium is used for non-image areas. Recent developments which have seen dramatic increases in the light sensitivity of photopolymer coatings offer the possibility of producing plates in future which will no longer require film exposure, but will instead be digitally imaged by a scanning laser. When printing process colours, two factors must be controlled to ensure the quality of the finished work, namely the number of halftone dots which print per inch (called the screen frequency or screen ruling) measured in lines per inch (lpi) and the angle at which they print (called the screen angle). If these factors are not correctly specified, the process inks may not print correctly in relation to one another, and distracting moire patterns may appear in the final printed colours. The default screen settings in the selected printer’s PPD are based on specifications from the printer manufacturer and are optimised for the printer. A prepress service provider may, however, suggest different settings in some circumstances.