a long - afterglow phosphor is added to an electroluminescent lamp in order to continue to provide illumination after the electrical power has been removed from the lamp . when the electroluminescent lamp is powered , it emits light caused the stimulation of an electroluminescent phosphor in the electric field . the emitted light further stimulates a long - afterglow phosphor so that when the lamp is turned off , and the electroluminescent phosphor ceases to emit light , the afterglow phosphor continues to provide a visible illumination at a lower intensity for many additional minutes or hours .