A special phosphor was designed for the Imaging Plate.   A certain substance has been known to emit light when irradiated with radiation, UV rays or an electron beam, or when heated, or mechanically hit or stimulated by chemical reaction in some cases.   Materials of this kind are generally called fluorescent substances.   In particular, the substances which are powders with practical applicabilities are often called phosphors.

      A phosphor emits light when stimulated by, for example, radiation.   The light disappears instantaneously when the stimulation ceases.   This phenomenon is called "fluorescence."   Some of the phosphors, however, continue emitting lights for a while after the stimulation stops, which is called "phosphorescence."   "Luminescence" incorporates both of these light emission phenomena.   The luminescence characteristics, specifically those of fluorescence and phosphorescence, should be taken into account when developing phosphors for practical usage.   And depending on the application, these characteristics are accurately adjusted by varying the phosphor composition or manufacturing conditions.

      The phosphor used for the Imaging Plate has special properties differing from those previously known, but which have not yet been put to practical use.   It utilizes the "photostimulated luminescence" (PSL) phenomenon which is neither "fluorescent" nor "phosphorescent."

      The PSL phenomenon is said to have been discovered by the world-famous Becquerel of France in the mid-19th century.   This phenomenon involves a substance that emits light again upon the second stimulation by light having a longer wavelength than the luminescence wavelength of the first stimulation by, for example, radiation.

      The PSL phenomenon, however, did not attract much interest until recently.   In Japan, it was studied by military researchers until the end of World War II to develop an infrared -ray detection system.   In the U.S., research was carried out in 1947 in which PSL was taken using photo-film for detecting the radiation image.   No other studies were reported for some time afterwards.

      This PSL phenomenon satisfies our basic concept of the Imaging Plate as an X-ray image sensor, which stores the first radiation information and releases that information as light.