Gas-discharge lamp

Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, such as mercury, sodium, and metal halides, which are vaporized during start-up to become part of the gas mixture.

Single-ended self-starting lamps are insulated with a mica disc and contained in a borosilicate glass gas discharge tube (arc tube) and a metal cap. They include the sodium-vapor lamp that is used in gas-discharge lamps in some street lighting.

In operation, some of the electrons are forced to leave the atoms of the gas near the anode by the electric field applied between the two electrodes, leaving these atoms positively ionized. The free electrons thus released flow to the anode, while the cations thus formed are accelerated by the electric field and flow towards the cathode.

The ions typically cover only a very short distance before colliding with neutral gas atoms, which give the ions their electrons. The atoms which lost an electron during the collisions ionize and speed toward the cathode while the ions which gained an electron during the collisions return to a lower energy state, releasing energy in the form of photons. Light of a characteristic frequency is thus emitted. In this way, electrons are relayed through the gas from the cathode to the anode.

The color of the light produced depends on the emission spectra of the atoms making up the gas, as well as the pressure of the gas, current density, and other variables. Gas discharge lamps can produce a wide range of colors. Some lamps produce ultraviolet radiation which is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface. The fluorescent lamp is perhaps the best known gas-discharge lamp.

Compared to incandescent lamps, gas-discharge lamps offer higher efficiency, but are more complicated to manufacture and most exhibit negative resistance, causing the resistance in the plasma to decrease as the current flow increases. Therefore, they usually require auxiliary electronic equipment such as ballasts to control current flow through the gas, preventing current runaway (arc flash).

Some gas-discharge lamps also have a perceivable start-up time to achieve their full light output. Still, owing to their greater efficiency, gas-discharge lamps were preferred over incandescent lights in many lighting applications, until recent improvements in LED lamp technology.

History

The history of gas-discharge lamps began in 1675 when the French astronomer Jean Picard observed that the empty space in his mercury barometer glowed as the mercury jiggled while he was carrying the barometer. Investigators, including Francis Hauksbee, tried to determine the cause of the phenomenon. Hauksbee first demonstrated a gas-discharge lamp in 1705. He showed that an evacuated or partially evacuated glass globe, in which he placed a small amount of mercury, while charged by static electricity could produce a light bright enough to read by. The phenomenon of electric arc was first described by Vasily V. Petrov in 1802. In 1809, Sir Humphry Davy demonstrated the electric arc at the Royal Institution of Great Britain. Since then, discharge light sources have been researched because they create light from electricity considerably more efficiently than incandescent light bulbs.

The father of the low-pressure gas discharge tube was German glassblower Heinrich Geissler, who beginning in 1857 constructed colorful artistic cold cathode tubes with different gases in them which glowed with many different colors, called Geissler tubes. It was found that inert gases such as the noble gases neon, argon, krypton or xenon, as well as carbon dioxide worked well in tubes. This technology was commercialized by the French engineer Georges Claude in 1910 and became neon lighting, used in neon signs.

The introduction of the metal vapor lamp, including various metals within the discharge tube, was a later advance. The heat of the gas discharge vaporizes some of the metal and the discharge is then produced almost exclusively by the metal vapor. The usual metals are sodium and mercury owing to their visible spectrum emission.

One hundred years of research later led to lamps without electrodes which are instead energized by microwave or radio-frequency sources. In addition, light sources of much lower output have been created, extending the applications of discharge lighting to home or indoor use.

The "Ruhmkorff" lamp

Ruhmkorff lamps were an early form of portable electric lamp, named after Heinrich Daniel Ruhmkorff and first used in the 1860s. The lamp consisted of a Geissler tube that was excited by a battery-powered Ruhmkorff induction coil; an early transformer capable of converting DC currents of low voltage into rapid high-voltage pulses. Initially the lamp generated white light by using a Geissler tube filled with carbon dioxide. However, the carbon dioxide tended to break down. Hence in later lamps, the Geissler tube was filled with nitrogen (which generated red light), and the clear glass was replaced with uranium glass (which fluoresced with a green light).

Intended for use in the potentially explosive environment of mining, as well as oxygen-free environments like diving or for a heatless lamp for possible use in surgery, the lamp was actually developed both by Alphonse Dumas, an engineer at the iron mines of Saint-Priest and of Lac, near Privas, in the department of Ardèche, France, and by Dr Camille Benoît, a medical doctor in Privas. In 1864, the French Academy of Sciences awarded Dumas and Benoît a prize of 1,000 francs for their invention. The lamps, cutting-edge technology in their time, gained fame after being described in several of Jules Verne's science-fiction novels.