In offset lithography the image is printed from a rubber coated roller (blanket) to which the ink is transferred from the plate. The image is firstly created on a printing plate, either photographically or by using a CTP (computer-to-plate) digital system. CTP systems are not yet the standard, but this method is set to dominate within the next few years. When the photopolymer resin coating of the plate is exposed to ultraviolet light under a process camera, exposed areas are hardened. The unexposed areas are subsequently washed off during processing. Gum Arabic is then applied to the surface to make the non-image areas water-receptive and grease-repellent (ink-repellent). Plates can either be negative or positive working. Negative-working plates are much less expensive and are generally used for single-colour work. The same chemistry applies to negative-working plates but in reverse : it is the exposed areas that become unstable leaving the unexposed areas to print.