Luminescence occurs in nature in the form of bioluminescence (glow-worms, fire-flies and lightning-bugs), chemiluminescence (decaying vegetation and marsh gas) and triboluminescence (as a result of friction). Since the beginning of the twentieth century, many forms of artificial electroluminescence have become available (to include discharge tubes, neon signs, television and the video display tube).

In the common discharge tube, a small quantity of gas or metal-vapour is sealed within a cylindrical glass or quartz tube. When an electrical current is passed or 'discharged' between the electrode ends, a flow of electrons is released which excites the element (which is most usually mercury vapour) to emit its characteristic spectral wavelengths. (In the high-pressure mercury-vapour lamp these combine optically to produce a bluish-white emission, commonly used in street lighting since 1933.)

Other familiar discharge lamps are the yellow low-pressure sodium-vapour and pink high-pressure sodium street-lamp. (Increasing the pressure within the tube will tend to broaden the emission spectra to wavelengths beyond the spectral lines normally associated with each element.) The xenon lamp emits a combination of spectral lines which fuse in the human eye to yield a white closely similar to daylight.

Almost all solid materials capable of luminescence consist of a so-called 'host crystal' activated by an impurity, to which it usually owes its colour appearance. For example the host crystal zinc sulphide appears yellow if its impurity is manganese, blue is silver, or green if bismuth or copper. Such crystals are stimulated to emit light by forcing an electric current through them. One arrangement utilises luminescent powder 'sandwiched' between glass and a reflective metal plate; an electrical circuit is made complete by placing a sheet of conducting glass over the metal and sealing the edges. Relatively little current is drawn, and the whole flat panel is illuminated. The intensity of the light emitted depends on the magnitude of the current and the colour on the selection of crystals used. A similar construction is used for lightweight television and computer display systems.

A particular limitation in the use of the mercury vapour street-lamp is its absence of emission in the red region of the spectrum; in an interior setting, this would cause red objects to appear dark and the human complexion unflattering. A light source which imitates daylight more closely can be obtained by coating the inside of the mercury lamp with a luminescent powder (commonly an impure calcium halophosphate). The energy emitted by the mercury arc is rich in ultraviolet energy (at 185 and 253 nanometres) and the luminescence produced in the coating converts the invisible ultraviolet wavelengths into shorter wavelengths within the visible range; this extends the lamp's bandwidth into the red, thereby offering a more 'balanced' and useful source of white, interior illumination.

Light emitted artificially from the discharge of static electricity was demonstrated by Otto von Guericke as early as 1683. In 1744 Johann Winkler conducted an experiment in which he produced light by shaking up a vacuum tube filled with mercury. The first modern discharge tube did not appear however until 1856, when Heinrich Geissler experimented with a sealed, low-pressure tube powered by a high-voltage alternating current. Important developments followed from advances made in New York City by Nikola Tesla, competing for an alternative to Edison's new incandescent lamp. The earliest luminescent advertising signs, by McFarland Moore (Newark, New Jersey, 1904) were ultimately developed by Georges Claude in Paris, who manufactured and exhbited his first red neon sign at the Grand Palais in 1910. (The device was patented in 1915.) The first of the line of modern mercury-vapour lamps was exhibited in Cincinnati, Ohio, in 1935.

N.B. Luminescence may occur either during or after the absorption of light energy at another wavelength. Emission which occurs only as long as the exciting input is being received is specified by the term fluorescence; emission which continues for some time after the energy input has ceased (as on the dial of an alarm clock) is said to exhibit 'afterglow' or the attribute of so-called phosphorescence.

R.O.

References:

Fred Billmeyer & Max Satzman (1966), Principles of Color Technology. New York: John Wiley. Second edition 1981.

M.A. Cayless & A.M. Marsden, Lamps and Lighting. London: Edward Arnold. Third edition 1983.

Adrian Bernard Klein (1937), Coloured Light: An Art Medium. London: Technical Press.

William Sproson (1983), Colour Science in Television and Display Systems. Bristol: Adam Hilger.