Fluorescence

Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colored visible light. The color of the light emitted depends on the chemical composition of the substance. Fluorescent materials generally cease to glow nearly immediately when the radiation source stops. This distinguishes them from the other type of light emission, phosphorescence. Phosphorescent materials continue to emit light for some time after the radiation stops. This difference in duration is a result of quantum spin effects.

Fluorescence occurs when a photon from incoming radiation is absorbed by a molecule, exciting it to a higher energy level, followed by the emission of light as the molecule returns to a lower energy state. The emitted light may have a longer wavelength and, therefore, a lower photon energy than the absorbed radiation. For example, the absorbed radiation could be in the ultraviolet region of the electromagnetic spectrum (invisible to the human eye), while the emitted light is in the visible region. This gives the fluorescent substance a distinct color, best seen when exposed to UV light, making it appear to glow in the dark. However, any light with a shorter wavelength may cause a material to fluoresce at a longer wavelength. Fluorescent materials may also be excited by certain wavelengths of visible light, which can mask the glow, yet their colors may appear bright and intensified. Other fluorescent materials emit their light in the infrared or even the ultraviolet regions of the spectrum.

Fluorescence has many practical applications, including mineralogy, gemology, medicine, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, cosmic-ray detection, vacuum fluorescent displays, and cathode-ray tubes. Its most common everyday application is in (gas-discharge) fluorescent lamps and LED lamps, where fluorescent coatings convert UV or blue light into longer wavelengths, resulting in white light, which can appear indistinguishable from that of the traditional but energy-inefficient incandescent lamp.

Fluorescence also occurs frequently in nature, appearing in some minerals and many biological forms across all kingdoms of life. The latter is often referred to as biofluorescence, indicating that the fluorophore is part of or derived from a living organism (rather than an inorganic dye or stain). However, since fluorescence results from a specific chemical property that can often be synthesized artificially, it is generally sufficient to describe the substance itself as fluorescent.

Fluorescence was observed long before it was named and understood. An early observation of fluorescence was known to the Aztecs and described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as lignum nephriticum (Latin for "kidney wood"). It was derived from the wood of two tree species, Pterocarpus indicus and Eysenhardtia polystachya. The chemical compound responsible for this fluorescence is matlaline, which is the oxidation product of one of the flavonoids found in this wood.

In 1819, E.D. Clarke and in 1822 René Just Haüy described some varieties of fluorites that had a different color depending on whether the light was reflected or (apparently) transmitted. Haüy incorrectly viewed the effect as light scattering similar to opalescence. In 1833 Sir David Brewster described a similar effect in chlorophyll which he also considered a form of opalescence. Sir John Herschel studied quinine in 1845 and came to a different incorrect conclusion.

In 1842, A.E. Becquerel observed that calcium sulfide emits light after being exposed to solar ultraviolet, making him the first to state that the emitted light is of longer wavelength than the incident light. While his observation of photoluminescence was similar to that described 10 years later by Stokes, who observed a fluorescence of a solution of quinine, the phenomenon that Becquerel described with calcium sulfide is now called phosphorescence.

In his 1852 paper on the "Refrangibility" (wavelength change) of light, George Gabriel Stokes described the ability of fluorspar, uranium glass and many other substances to change invisible light beyond the violet end of the visible spectrum into visible light. He named this phenomenon fluorescence