The purpose of the present investigation was to obtain data concerning the duration of phosphorescence of minerals. Some minerals with protracted emissions have been discovered and the features of their phosphorescence determined with dark-adapted eyes, and in some cases, with photographic film.
Studies have shown that the phosphorescence caused by a short exposure to 2437A ultraviolet rays will continue for a long period of time. Dark-adapted eyes were used to determine the duration of colored and colorless phosphorescence and photographic film was used to obtain phosphorographs and thereby record the emission of light long after it became imperceptible to the eye. Protracted phosphorescence has been discovered in calcite, fluorite, spodumene, wernerite, willemite, and synthetic phosphors. After an initial exposure of one minute, Texas calcite emitted an afterglow which was distinguishable to dark-adapted eyes for 4600 hours and was capable of producing a photographic image after more than 5½ years. Fluorite from Trumbull, Connecticut, similarly exposed, continued to emit a phosphorescence perceptible to the eye for more than four years. The Texas calcite, fluorite, and spodumene are still phosphorescing.
Data on color and duration of visible phosphorescence of natural minerals are tabulated.
HENRY E. MILLSON and HENRY E. MILLSON, JR., "Observations on Exceptional Duration of Mineral Phosphorescence," J. Opt. Soc. Am. 40, 430-434 (1950)