Offset Lithography - In this printing process the image is carried flat on the surface of the printing plate. During the preparation stage, colour separations are made and each is screened from a different angle, the same as all four colour printing methods. When the image is combined at the printing stage, the dots will be superimposed to differing degrees, although the inks do not actually merge. The eye reads the size and overlapping colour of each dot against the plain background of the paper, which also appears visable through the transparent ink. The more ink, the less light is reflected through, and vice versa, and from this the brain perceives the intensity of colour and tone.

An aluminium or zinc plate is made chemically sensitive to light and a film positive of the colour separation is photographically imposed. The chemical composition of the image created on the surface of the plate is such that it will attract ink but repel the water which is used to moisten the plate. The plate is wrapped around a cylinder, and after being wetted and inked it is rotated (offset) against a rubber blanket cylinder to which the image is transferred. From this, the image is then transferred to the paper, with the intermediate blanket cylinder prevents water from the plate reaching the paper. As with all these methods each colour is dealt with separately and the image is built up from consecutive passes through the press.

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