Substances that glow under ultraviolet light, the phenomenon known as fluorescence, are illustrated in the Thomas S. Warren Museum of Fluorescence in various ways — by minerals, rocks, household objects, decorative objects, even liquids. Below we offer some general information on this phenomenon, starting with some of the terms used to describe it.
Fluorescence in common parlance refers to the emission of visible light from a substance being irradiated by ultraviolet light, which the human eye cannot see. Fluorescent art posters viewed under “black light” are familiar examples of this phenomenon in everyday life. In reality, however, this definition is too narrow, for the light given off by many substances falls outside the visible range. Some minerals, for example, fluoresce in the infrared, but we cannot see the emitted light. Moreover, the means by which we cause a given substance to fluoresce need not be ultraviolet lightóit could just as well be an electron beam, or X-rays, or even visible light of a different wavelength from that emitted. All fluorescence, however, involves the addition of energy by some means to a substance, and the reemission of part of that energy as electromagnetic radiation.
Fluorescence is a process distinct from incandescence, which refers to emission of light from an object heated to such high temperatures that it glows. The glowing filament in a conventional light bulb, and glowing embers in a fireplace, are examples of incandescence. Heating, however, is generally detrimental to the process of fluorescence. Moreover, most substances, when fluorescing, produce very little heat. For this reason fluorescence has commonly been referred to as “cold light.”
To most people phosphorescence refers to the continued emission of light from a substance after being exposed to ultraviolet light. In this sense it is equivalent to an afterglow, as when a diamond is seen to continue glowing after the ultraviolet lamp used to excite its fluorescence has been turned off. Items that commonly utilize phosphorescent materials include the hands on dial watches and the wide variety of plastic “Glow-in-the-Dark” stars, Halloween ornaments, insects, and the like.
This simple view of phosphorescence as a visible afterglow is different from the definition accepted by physicists, wherein the difference between fluorescence and phosphorescence depends on the atomic mechanism whereby the light is emitted. In most of the hobbyist literature, however, fluorescence refers to what one sees when the ultraviolet lamp is on, and phosphorescence to what one sees after it is off.
Luminescence is a general term that embraces both fluorescence and phosphorescence. It comes from the same root word as the terms “luminous” and “lumen” (a measure of light intensity), all of which refer to a glowing object. The related word “luminary” refers to a person who is figuratively glowingóthat is, a person who occupies an important or exalted position.
The term “fluorescence” is commonly used in everyday parlance and in the hobbyist literature where the more general term “luminescence” would be more appropriate.
Types of Luminescence
A potentially confusing array of terms greets those who delve into the literature on this subject. Most of those terms, however, simply refer to the means by which energy is added to a substance to excite it into luminescence. Listed below are some of them; there are numerous others:
Cathodoluminescence – luminescence excited by an electron beam (as opposed to a beam of photons). The light produced by conventional (not flat-screen) televisions and computer monitors is cathodoluminescent light.
Chemiluminescence – luminescence that results from energy released during a chemical reaction. The “Glow Sticks” widely found on the market today glow for hours after the stick is "broken" to allow the two chemicals inside to mix. The mechanism is similar to that used by fireflies on summer evenings.
Crystalloluminescence- light emitted from a substance as it crystallizes from a melt or solution.
Photoluminescence – luminescence caused by a beam of photons (light). When the photons fall within the ultraviolet range, as with the hobbyist lamps used to excite fluorescence in minerals, the appropriate term is “ultraviolet photoluminescence.” When hobbyists use the term “fluorescence,” this is generally what they mean.
Thermoluminescence – luminescence (actually a phosphorescence) stimulated by the application of heat, to temperatures below those that result in incandescence.
Triboluminescence – luminescence that results from hitting, crushing, scratching, or abrading a substance, as when two pieces of quartz are rubbed together to produce light.
To this list we should add bioluminescence, light emitted from an organism as a result of a biological process. The actual process involved in generating this light is generally chemiluminescence, as in glowing fireflies, bacteria, and some types of plankton.
Mechanism of Fluorescence
In the standard conceptual model of an atom the electrons are envisioned as revolving in orbits ("shells") around a central nucleus. An atom in this sense resembles a miniature Solar System, with the nucleus taking the place of the Sun and the electrons the planets at various distances from it. This simple view of an atom is not quite correct, especially as regards orbital shapes, but it does serve to outline the general process of fluorescence.
A common result of beaming ultraviolet light on a material capable of absorbing it is that one or more of the electrons of an atom are “kicked” into a higher energy state, which here can be envisioned as moving to an orbit farther out from the nucleus. All such “excited” electronic states are unstable, and sooner or later the electron will lose its excess energy and fall back to its original orbit. This excess energy can be dissipated in several ways, the most common being simply to increase atomic vibrations in the material, but some materials also emit some of the energy as light. This is what we see as fluorescence.
Activators of Fluorescence
Some substances are intrinsically fluorescent. Scheelite, an ore mineral of tungsten with the chemical formula CaWO4, is an example. Here the cause of fluorescence is the tungstate polyatomic ion, (WO4)2-. Accordingly, even pure scheelite as synthesized in a laboratory will fluoresce.
some chemical impurity or a defect in its crystal structure. For example, pure zinc silicate (synthetic willemite, Zn2SiO4) prepared in a laboratory does not fluoresce under ultraviolet light, but if a bit of manganese (in the form of the divalent ion, Mn2+) is added the substance will glow bright yellow-green. The manganese in this instance is said to be the “activator” of fluorescence.
Numerous activators have been identified in minerals, but for most the suspected activators remain unproven, and for some they remain a genuine mystery. Chemical activators in minerals are diverse and include not only metallic elements such as manganese and lead, but also nonmetallic elements (e.g., nitrogen in diamond), polyatomic ions such as the uranyl and tungstate ions, and organic compounds.
Coactivators of Fluorescence
For any material to fluoresce it must be capable of absorbing ultraviolet light. The presence of an activator may or may not help; manganese, for example, is a potent activator of fluorescence, but a poor absorber of ultraviolet light. For some materials, then, an additional chemical impurity, one capable of absorbing ultraviolet light and then transmitting some of that added energy to the activator, is necessary for fluorescence. Impurities that operate in this way in conjunction with an activator are termed “coactivators.” Lead is perhaps the most common of these in minerals, both because it is a widespread element in the Earth's crust and because it is an effective coactivator even at low concentrations.
Poisoners of Fluorescence
Some chemical impurities deter fluorescence by promoting “radiationless transitions”—that is, by causing a given substance to lose energy by processes that do not involve the emission of light. Typically the added energy is dissipated as electronic vibrations instead. Among minerals the most notorious poisoners of fluorescence are iron, cobalt, nickel, and copper. For example, ruby corundum containing 1% chromium will probably fluoresce bright, deep red (chromium is the activator here), but if it also contains 1% iron it probably will not fluoresce at all.
Some materials that appear to fluoresce may not actually be doing so. The bright yellow fluorescence of some fluorite crystals from Illinois, for example, is due not to the fluorite itself, but to microscopic droplets of petroleum included in the mineral. Other minerals appear to fluoresce because of thin coatings of a second mineral whose presence may be difficult to detect in daylight, and still others because they are transmitting the light emitted by a mineral beneath. Such cases of spurious fluorescence are common and are the cause of many confusing reports in the hobbyist literature.
Uses of Fluorescence
Fluorescence is a part of our everyday lives, though many people seem not to realize it. Here are three examples.
Computer monitors and TV screens: The light that comes from these devices is emitted by tiny phosphor dots that coat the inside of the screen, and that are excited into fluorescence by a stream of electrons emitted by a cathode-ray tube. Three colors of dotsóblue, red, and greenóare used in most computer monitors. If you examine any on-screen image with a magnifying glass you should be able to see the dots.
Fluorescent office lights: The “fluorescent light tubes” commonly used for office and commercial lighting are well named, for the light they emit is indeed produced by the process of fluorescence. The insides of the tubes are coated with a phosphor that emits a bright white light when exposed to the ultraviolet light produced inside the tube.
Clothing: Nearly all laundry detergents contain a fluorescent dye that emits strongly in the blue when exposed to sunlight. The blue light counteracts the yellow tinge of old or incompletely cleaned clothing and thus makes clothes appear cleaner and brighter than they really are. The dye is designed to fluoresce in daylight.
The Fluorescent Mineral Society maintains a bibliography for those interested in learning more about fluorescence, particularly that of minerals. For a list of publications highly regarded by the Society click here. Those new to this subject are particularly referred to the section on Bibliography – Collecting.