luminescence

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The emission of light that does not derive energy from the temperature of the emitting body, as in phosphorescence, fluorescence, and bioluminescence. Luminescence is caused by chemical, biochemical, or crystallographic changes, the motions of subatomic particles, or radiation-induced excitation of an atomic system.
2. n. The light so emitted.

Century Dictionary and Cyclopedia

1. n. See the quotation.
2. n. The emission of light from causes other than that which produces incandescence. Radiation is emitted by all bodies at all temperatures; but below a certain temperature, that of the red heat, the wave-lengths emitted do not affect the eye. At that temperature (about 450° C) wave-lengths of the visible spectrum begin to have sufficient intensity to produce luminous effects. The body is then said to be incandescent. The luminous intensity of incandescent bodies increases rapidly with further rise of temperature, the total radiation being proportional to the fourth power of the temperature, and the luminous intensity increasing at an even greater rate. See radiation, 1. There are numerous cases in which bodies emit light at temperatures below that of incandescence, and in which at higher temperatures the intensity and character of the light, emitted differs from that which we should expect from the law of ordinary radiation. All such cases of extraordinary light-emission are included under the term luminescence, and the phenomenon is variably described as photoluminescence, thermolnminescence, chemiluminescence, triboluminescence, piezoluminescence, lyoluminescence, eathodoluminescence, X-luminescence, or autoluminescence, according to the exciting cause to which the phenomenon is ascribed. Luminescence produced by the exposure of the substance to light or to ultra-violet rays is called photoluminescence. When observed while the substance is still under exposure to light, it is known as fluorescence, a name proposed by Stokes (1857), who first systematically studied the luminescence of fluor-spar, whence the name. However, the phenomenon had previously been observed by Hersehel and others. When, as is sometimes the case, photoluminescence persists after the removal of the exciting light, the phenomenon is termed phosphorescence, on account of an imagined analogy to the power of phosphorus to shine in the dark. The glow of phosphorus is, however, a form of chemiluminescence. Since the luminous energy emitted by a photoluminescent body has its source in the exciting light, it follows that a portion of this light must have been absorbed by the body; and, in fact, it is found that photoluminescence is associated with the presence of an absorption-band in the transmission-spectrum of the substance, and that light of wave-lengths corresponding to this band is chiefly active in producing the luminescence. Luminescent light is not monochromatic, but forms an emission-band in the spectrum having a well-defined maximum the wave-length of which is always greater than the wave-length corresponding to the minimum of the absorption-band to which the luminescence is due. It was thought by Stokes that the shortest wave-length of the fiuorescent light always exceeded the longest wavelength of the exciting light (Stokes's law), but subsequent measurements have shown that the absorption-band and the luminescence-band frequently overlap. Photoluminescence is exhibited not only by fluor-spar, but by numerous other solids, of which zinc sulphid and calcium sulphid are perhaps the best-known examples. The phenomenon of fluorescence is more readily observed, however, in the case of the solutions of certain organic dyestnffs, such as eosin, resorcin blue, and naphthalene red. One of these substances has received the name fluorescein on account of the extraordinary green fluorescence exhibited by it. The color of the fluorescent light depends upon the position of the absorption-band to which it is due; but it is noteworthy that not all substances the spectra of which show absorption-bands are fluorescent, and that a substance may have several absorption-bands only one of which has the corresponding fluorescence-band. The cause of fluorescence in organic solutions has not been satisfactorily determined, but it is known that the photoluminescence of most, if not all, inorganic solids is due to the admixture in minute quantities of certain impurities, such as the salts of copper, manganese, bismuth, lead, nickel, antimony, zinc, etc., and that each of these metallic salts produces its own characteristic photoluminescence. Many inorganic compounds, when exposed to light or to the action of cathode rays and then heated, emit light far below the temperature of incandescence, or, when red-hot, radiate light other than that due to ordinary incandescence. Such emission of light, in which previously stored energy is set free, is termed thermoluminescence. Many specimens of fluorite and of flint and crown-glass exhibit the phenomenon, as do the haloid salts of the alkalis and the sulphates of ziuc, calcium, barium, magnesium, etc. Frequently a substance which shows thermoluminescence will emit light when, instead of being heated, it is rubbed (triboluminescence), or is subjected to pressure (piezoluminescence), or is dissolved in water (lyoluminescence). Thus Wiedemann and Schmidt observed all three types in the case of sodium chlorid and potassium chlorid: thermolumiescence and lyoluminescence in the case of lithium chlorid; and thermoluminescence and triboluminescence in the case of potassium bromide. The salts had in all cases been previously exposed to the cathode rays. Many substances, when exposed to the electric discharge within a vacuum-tube, emit light. Such luminescence, which is sometimes spoken of as electroluminescence, is, however, due either to the ultra-violet or visible rays from the discharge (in which case it is to be classed as photoluminescence) or to the action of cathode rays (in which case it is cathodolnminescence). Röntgen rays are also capable of exciting luminescence (X-luminescence) in many substances, such as calcium tnngstate, platinocyanide of barium, and Sidot-blende, from which the screens of fiuoroseopes are made. The glow of radium, and other radioactive substances, which appears to be independent of excitation from without, is termed autoluminescance. The term chemiluminescence is applied to all cases in which chemical changes accompany the emission of light by a luminescent body. When, for example, sodium chlorid is exposed to cathode rays, unstable subchlorids, giving the surface a brown or blue color, are formed. Heating or friction restores the substance to its original color and composition, and the thermoluminescence or triboluminescence, respectively, which accompanies the reaction is classed as chemiluminescence. Whether all luminescence is of the nature of chemiluminescence has not, as yet, been definitely determined.

Wiktionary

1. n. physics Any emission of light that cannot be attributed merely to the temperature of the emitting body.

GNU Webster's 1913

1. n. (Physics) Any emission of light not ascribable directly to incandescence, and therefore occurring at low temperatures, as in phosphorescence and fluorescence or other luminous radiation resulting from vital processes, chemical action, friction, solution, or the influence of light or of ultraviolet or cathode rays, etc.
2. n. The faculty or power of producing light by biological processes, as in the firefly and glowworm. Also called bioluminescence.
3. n. The light produced by biological or biochemical processes. Also called bioluminescence.

WordNet 3.0

1. n. light not due to incandescence; occurs at low temperatures
2. n. light from nonthermal sources