A printed colour picture is made up of tiny dots of three primary colours — cyan, magenta, yellow and black. For printing, therefore, the colour original is first separated by scanning into negatives for cyan, yellow, magenta and black.
During scanning, a screen of dots is also used so that the images on the four negatives are in the form of fine dots which is necessary for printing.
The most popular colour printing process used these days is offset printing. Offset printing employs specially treated aluminium plates on which the printing ink sticks only to selected areas. When the plate, made sensitive to light by chemical coating, is exposed to light through the film negative and washed, coated areas that repel water but accept oil based ink remain. The plate is mounted on a cylinder in printing machine and wetted so that only the printed areas are inked. The impression from the printing plate is first transferred to a cylinder covered with a rubber blanket which finally transfers the impression onto paper.
