A type of printing plate used in offset lithography based on the same principle as diazo plates, in which a light-sensitive coating is applied to the surface of an aluminum plate which is then exposed to light via a photographic negative or positive, which causes the coating to harden in image areas and remain soluble in non-image areas (when exposed via a photographic negative; see also Positive-Working Plate). A chemical treatment after exposure removes the coating in the non-image areas, making those portions of the plate water-receptive, while the hard, durable image areas remain ink-receptive. Photopolymer plates utilize coatings that provide more durable and abrasion-resistant image areas than do diazo coatings, and consequently photopolymer plates can be used for longer print runs (up to 250,000 impressions). Upon exposure to light, the molecules in the coating undergo or the joining together of smaller molecules to form long chains of higher-molecular-weight molecules, which are responsible for the high degree of durability. Special thermal curing processes after exposure and developing make the image areas even more durable, and have been known to be able to last for print runs in excess of 1,000,000 impressions. New developments in photopolymer plates include dye-sensitized photopolymers that can be etched by lasers, making them well-suited for use in computer-to-plate platemaking.
A photopolymer plate also refers to one of two types of plates used in flexography.