Irradiation, exposure to radiation, such as X rays or ultraviolet rays, usually for a specific purpose. Irradiation has many uses. Vitamins D2 and D3 are formed by irradiating provitamins D (compounds from which D vitamins are formed) with ultraviolet rays. Drugs, enriched products, and milk and other foods are often subjected to ultraviolet irradiation to make them germ-free. Irradiation with gamma rays is sometimes used to prevent potatoes from sprouting, slow the ripening of fruit, and sterilize uncanned foods so that they may be safely stored. A similar treatment is used to kill insect larvae and eggs that infest seed.

Cancer, leukemia, and certain skin diseases are commonly treated by irradiating the affected areas with X rays or gamma rays. Industrially, the irradiation of automobile piston rings is an effective means of measuring friction and wear. Plastics are often irradiated to give them certain desirable characteristics, such as increased strength or higher melting points.