PHOSPHORESCENCE, a name given to a variety of physical phenomena due to different causes, but all consisting in the emission of a pale, more or less ill-defined light, not obviously due to combustion. The word was first used by physicists to describe the property possessed by many substances of themselves becoming luminous after exposure to light. This property has been noticed from early times. Pliny speaks of various gems which shine with a light of their own, and Albertus Magnus knew that the diamond becomes phosphorescent when moderately heated. But the first discovery of this property which apparently attracted scientific attention seems to have been that of the Bologna stone (barium sulphide), which was discovered by Vincenzo Cascariolo, a cobbler of Bologna, in about 1602. This was followed by the discovery of a number of other substances which become luminous either after exposure to light or on heating, or by attrition, and to which the general name of " phosphori " (from 4s)r and 4 Epos, bringing light) was given. Among these may be mentioned Homberg's phosphorus (calcium chloride), John Canton's phosphorus (calcium sulphide) and Balduin's phosphorus (calcium nitrate). Of late years it has been found convenient to limit the strict meaning of the word " phosphorescence " to the case of bodies which, after exposure to light, become self-luminous (even if only for a fraction of a second). The general term "luminescence" has been proposed by E. Wiedemann to include all cases in which bodies give off light not due to ignition. This general term embraces several subdivisions. Thus, fluorescence and phosphorescence are included under the same heading, " photoluminescence," being distinguished from each other only by the fact that fluorescent bodies emit their characteristic light only while under the influence of the exciting illumination, while phosphorescent bodies are luminous for an appreciable time after the exciting light is cut off.
Phosphorescence, in its restricted meaning as above explained, is most strikingly exhibited by the artificial sulphides of calcium, strontium and barium. If any of these substances is exposed for some time to daylight, or, better, to direct sunlight, or to the light of the electric arc, it will shine for hours in the dark with a soft coloured light. The colour depends not only on the nature of the substance, but also on its physical condition, and on its temperature during insolation, that is, exposure to the sun's rays. Thus the phosphorescent light emitted by calcium sulphide may be orangeyellow, yellow, green or violet, according to the method of preparation and the materials used. Balmain's luminous paint, a preparation of calcium sulphide, shines with a white light. The colour also depends on the temperature during exposure to light. Thus A. E. Becquerel found that the light given by a specimen of strontium sulphide changed from violet to blue, green, yellow and orange, as the temperature during the corresponding previous insolation was 20°, 40 0, 70°, 100° or 200° C. The duration of phosphorescence varies greatly with different substances. It may last for days or for only a fraction of a second.
As in the case of fluorescent bodies, the light produced by phosphorescent substances consists commonly of rays less refrangible than those of the exciting light. Thus the ultra-violet portion of the spectrum is usually the most efficient in exciting rays belonging to the visible part of the spectrum. V. Klatt and Ph. Lenard (Wied. Ann., 1889, xxxviii. 90), have shown that the phosphorescence of calcium sulphide and other phosphori depends on the presence of minute quantities of other substances, such as copper, bismuth and manganese. The maximum intensity of phosphorescent light is obtained when a certain definite proportion of the impurity is present, and the intensity is diminished if this proportion is increased.
It appears likely that when a phosphorescent body is exposed to light, the energy of the light is stored up in some kind of strain energy, and that the phosphorescent light is given out during a more or less slow recovery from this state of strain. Klatt and Lenard have shown that the sulphides of the alkaline earths lose the property of phosphorescing when subjected to heavy pressure. Many fluorescent solutions become briefly phosphorescent when rendered solid by gelatin.
When the duration of phosphorescence is brief, some mechanical device becomes necessary to detect it. The earliest and bestknown instrument for this purpose is Becquerel's phosphoroscope. It consists essentially of a shallow drum, in whose ends two eccentric holes, exactly opposite one another, are cut. Inside it are fixed two equal metal disks, attached perpendicularly to an axis, and divided into the same number of sectors, the alternate sectors of each being cut out. One of these disks is close to one end of the drum, the other to the opposite end, and the sectors are so arranged that, when the disks are made to rotate, the hole in one end is open while that in the other is closed, and vice versa. If the eye be placed near one hole, and a ray of sunlight be admitted by the other, it is obvious that while the sun shines on an object inside the drum the aperture next the eye is closed, and vice versa. If the disks be made to revolve with great velocity by means of a train of toothed wheels the object will be presented to the eye almost instantly after it has been exposed to sunlight, and these presentations succeed one another so rapidly as to produce a sense of continuous vision. By means of this apparatus we can test with considerable accuracy the duration of the phenomenon after the light has been cut off. For this purpose we require to know merely the number of sectors in the disks and the rate at which they are turned.
The term luminescence is used to describe a process by which light is produced other than by heating. The production of light from heat, or incandescence, is familiar to everyone. The Sun gives off both heat and light as a result of nuclear reactions in its core. An incandescent lightbulb gives off light when a wire filament inside the bulb is heated to white heat. One can read by the light of a candle flame because burning wax gives off both heat and light.
But light can also be produced by other processes in which heat is not involved. For example, fireflies produce light by means of chemical reactions that take place within their bodies. They convert a compound known as luciferin from one form into another. As that process occurs, light is given off.
Fluorescence and phosphorescence
Two forms of luminescence can be identified, depending on the amount of time emitted light continues to glow. By definition, fluorescence refers to the release of light that lasts no more than about 10 nanoseconds (10 billionths of a second) after it begins. Phosphorescence refers to the release of light that lasts longer than 10 nanoseconds.
Substances that glow in the dark have many practical applications today. Clocks and watches, for example, often have their numbers and hands coated with phosphorescent paints so we can see what time it is in the dark. Emergency doors and stairways are also highlighted with these paints so that people can find their way out in case of a power failure.
Probably the most familiar form of fluorescence is a fluorescent lightbulb. Fluorescent light is produced when an electrical current passes through mercury vapor in the lightbulb. Electrons produced from the mercury vapor collide with a chemical painted on the inside of the bulb, causing fluorescence. The moment the bulb is turned off, however, the chemical stops glowing. The light produced by this process, therefore, is an example of fluorescence