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Fluorescence is a property of many substances. It consists of an emission of light under excitation by radiation whose wave length falls within a critical absorption band. When the emission persists for an appreciable period after the existing radiation has been extinguished, it is known as phosphorescence. The fundamental principles of fluorescence are described in detail.

At low contents of the manganese activator, the decay of phosphorescence takes place in two stages. The initial stage is exponential. It is complete in less than 0.1 second and is not affected by temperature. The second stage is at a much lower intensity level. It becomes noticeable after 0.03 second and persists for more than an hour. Its speed of decay increases with temperature. At − 196°C it is completely latent. A change in the intensity of the exciting light alters proportionally the luminescence caused by each stage of decay and has no effect upon the shape of the decay curves.About fifty times as many electrons are involved in the second stage of decay as in the first. It is assumed that the second stage is caused by the retention of electrons in a metastable state, probably one associated with the presence of lattice imperfections. The exponential decay of the first stage would result from the recombination of manganese ions, relatively in excess, with those free electrons which had not been capt...

In the presence of a relatively high percentage of lead or certain other agents and after the usual excitation, zinc sulfide phosphors become capable of storing luminescence energy at such a high potential that, unlike phosphorescence, it can be released only by radiation with the near infra-red. There are two stimulating bands—a strong one at 1.25–1.60μ and a weaker one adjoining the visible. The resulting luminescence is increased by a small content of copper. Its decay, under continued stimulation, is monomolecular in contrast to the decay of phosphorescence, which is bimolecular. In the case of lead, the color of the stimulated luminescence is green, that of the shorter of its two fluorescence bands. These fluorescence bands, in the green and yellow, are excited by different portions of the spectrum, and their phosphorescence decay is at different rates. After excitation at 77°K, there is also a bright emission observed on radiating with infra-red of 1–3μ, but this is due to the release of stored luminescence energy which is normally latent at 77°K, appearing at higher temperatures as phosphorescence. It does not affect the magnitude of the luminescence which is subsequently obtainable, after warming to 25°C, by stimulation with infra-red of the same range.

Calcium silicate phosphors activated with varying amounts of manganese luminesce yellow to orange under cathode-ray excitation, but are not excited by ultraviolet λ2537. If a small amount of lead is added with the manganese, the resulting material responds to λ2537 to give the same color as that produced by cathode rays, and in addition to the visible luminescence, there is an ultraviolet band at λ3400. The spectral distribution in the visible is dependent upon manganese concentration and on temperature.If lead alone is used as an activator, the product gives emission entirely in the ultraviolet. Three emission bands are evident at approximately 3000A, 3340A, and 3500A, any one of which may be predominant, depending upon the conditions of preparation. The excitation spectrum also varies widely, having several maxima between 2000A and 3000A. When the phosphor is excited by 2288A line from a low pressure cadmium arc, a fourth broad band appears in the emission, which peaks about 3850A and extends far enough to the longer wave-lengths to be easily observed visually.X-ray analysis with a Philips Geiger counter spectrometer demonstrates that the emission band at 3000A is due to the α-metasilicate, the one at 3500A to the β-metasilicate, while the 3340A band is due to the β-orthosilicate. During the firing process the orthosilicate is formed first by the reaction between the calcium oxide and silica in equi-molar proportions, and it reacts thereafter with additional silica to form metasilicate.

Unexpected emission bands have been observed for ZnCdS: Ag high in cadmium. With high Ag, the main emission band under cathode-ray excitation at 10 kv appeared to be shifted to shorter wavelength on increase in current density. Under ultraviolet excitation at −196°C, the apparent shift was resolved into two emission bands, one of which corresponded to the new band that appeared under cathode rays; the other, to the normal band of the unactivated phosphor. With low Ag, edge emission bands appeared under high current density cathode rays and sodium bands under ultraviolet at −196°C.

Two groups of oxygen-containing phosphors are examined for the effect of activator concentration and temperature upon the location of their peak emission. The first group includes five phosphors whose emission is distributed between two main spectral bands to an extent dependent on activator concentration. Sufficient decrease in activator content causes all of the emission to appear in the shorter wave-length band. An increase in temperature has the same effect. In the second group, represented by three phosphors, the emission cannot be resolved into component bands by change in activator content. An increase in temperature shifts the emission to shorter wave-length.

The spectral distribution for the excitation of phosphors is so erratic that it seems at present impossible to predict the location of strong excitation bands. Out of many phosphors examined in rare gas discharges, none was found to exceed in brightness the calcium tungstate and zinc silicate already in commercial use. The light output of the latter, however, was considerably improved by the elimination of the free silica usually present, which evidently has an appreciable absorption to radiation in the Schumann region. On the reasonable assumption that the resonance radiation is the source of excitation, the efficiency of fluorescence emitted by the zinc orthosilicate phosphor in the neon discharge denotes a quantum conversion near unity, just as in the low pressure mercury discharge.