an apparatus for measuring the duration of phosphorescence and determining the laws governing the decay of phosphorescence in the range of times from 10^–1 to 10^–7 sec. For durations >10^–5 sec, the decay curve may be obtained mechanically.

In single-disk phosphoroscopes, the material being studied is applied to the edge of the disk and a given narrow segment of the disk is excited. Upon rotation of the disk, this segment is removed from the excitation zone, and the luminescence undergoes decay. By measuring the intensity of the afterglow at various angular displacements, the law governing the decay can be determined. Single-disk phosphoroscopes are unsuitable for studying the luminescence of liquid phosphors.

In double-disk phosphoroscopes, the luminescing material is placed between two disks. The disks have slots and are mounted on the same axis. The slots of one disk are shifted at a certain angle relative to the slots of the other. The phosphor is placed opposite one of the slots of the first disk, and the afterglow is observed through the slots of the second disk. By changing the angle between the disk slots and the angular velocity of the disks, the intensity of the afterglow at various intervals of time after the cessation of excitation may be measured. With this type of phosphoroscope, afterglow lasting as little as ~10^–4 sec may be detected. Phosphoroscopes in which the time scan is accomplished by a rotating mirror and the excitation by a short electrical discharge permit measurements of afterglow durations of ~10^–5 sec.

For the measurement of durations of ~10^–5 sec and less, photoelectric scanning methods with pulsed excitation are used. Here, a photomultiplier is used to detect the afterglow. The out put of the photomultiplier can be displayed on a cathode-ray oscillograph. Measurements in the range 10^–8–10^–9 sec are accomplished by fluorometers.