Neon sign

  Neon signs are luminous-tube signs that contain neon or other inert gases at a low pressure. Applying a high voltage (usually a few thousand volts) makes the gas glow brightly. They are produced by the craft of bending glass tubing into shapes. A worker skilled in this craft is known as a glass bender, neon or tube bender.

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History

The neon sign is an evolution of the earlier Geissler tube (also called a Crookes tube), which is a glass tube for demonstrating the principles of electrical discharge. Neon signs are used for many purposes.

At the 1893 World's Fair, the World Columbian Exposition in Chicago, Illinois, Nikola Tesla's neon lamp signs were displayed. The development of neon signs is credited to Georges Claude and the first public display of a neon sign was of two 38-foot long tubes in December of 1910 at the Paris Expo. The first commercial sign was sold by Jaques Fonseque, Claude’s associate, in 1912 to a Paris barber.

Manufacturing process

  Lead glass tubing in external diameters ranging from about 8 to 15 mm is most commonly used. The tube is heated in sections using several types of burners that are selected according to the amount of glass to be heated for each bend. These burners include ribbon, cannon, or crossfires, as well as a variety of torches that run on a simple combination of natural gas (butane or propane work better, however natural gas is cheapest) and air.

A section of the glass is heated until it is malleable; then it is bent into shape and aligned to a pattern containing the graphics or lettering that the final product will ultimately conform to.

An electrode is melted (or welded) to each end of the tube as it is finished. The electrodes are also lead glass and contain a small metal shell with two wires protruding through the glass to which the sign wiring will later be attached. All welds and seals must be perfectly leak-proof before proceeding further.

The tube is attached to a manifold which is itself attached to a high-quality vacuum pump. The tube is then evacuated of air until it reaches near-vacuum. During evacuation, a high current is forced through the tube via the wires protruding from each electrode (in a process known as "bombarding"). The current depends on the specific electrodes used and the diameter of the tube, but is typically in the 500mA to 1000mA range, at an applied voltage usually between 15,000 to 25,000V. The bombarding transformer acts as an adjustable constant current source, and the voltage produced depends on the length and pressure of the tube. Typically the operator will maintain the pressure as high as the bombarder will allow to ensure maximum power dissipation and heating. This very high power dissipation in the tube heats the glass to a temperature of several hundred degrees Celsius, and any dirt and impurities within are drawn off in the gasified form by the vacuum pump. The current also heats the electrode metal to over 600 degrees Celsius, which activates a special coating that scavenges unwanted contaminants in the tube and reduces the work function of the electrode for cathodic emission. When completed properly, this process results in a very clean interior at a high vacuum.

While still attached to the manifold, the tube is allowed to cool while pumping down to the lowest pressure the system can achieve. It is then filled to a pressure of a few torr with one of the noble gases, or a mixture of them, and sometimes a small amount of mercury. The required pressure depends on the gas used and the diameter of the tube, with optimal values ranging from 6 torr (for a long 20mm tube filled with argon/mercury) to 27 torr (for a short 8mm diameter tube filled with pure neon). Neon or argon are the most common gases used; krypton, xenon, and helium are used by artists for special purposes but are not used alone in normal signs. A premixed combination of argon and helium is often used in lieu of pure argon when a tube is to be installed in a cold climate, since the helium increases voltage drop (and thus power dissipation), warming the tube to operating temperature faster. Neon glows bright red or reddish orange when lit. When argon or argon/helium is used, a tiny droplet of mercury is added. Argon by itself is very dim pale lavender when lit, but the droplet of mercury fills the tube with mercury vapor when sealed, which then emits ultraviolet light upon electrification. This ultraviolet emission allows finished argon/mercury tubes to glow with a variety of bright colors when the tube has been coated on the interior with ultraviolet-sensitive phosphors after being bent into shape. Plain argon/mercury fill in clear glass is used for a bright but pale blue (or bluish white) color often seen in signs where cost is a major factor.

The finished glass pieces are illuminated by either a transformer or a switching power supply running at voltages ranging between 3,000 and 15,000 volts and currents between 20 and 60 mA. These power supplies operate as constant-current sources (a high voltage supply with a very high internal impedance), since the tube has a negative characteristic electrical impedance. The most common current rating is 30mA for general use, with 60mA used for high-brightness applications like channel letters or architectural lighting. 120mA sources are occasionally seen in illuminating applications, but are uncommon since special electrodes are required to withstand the current, and an accidental shock from a 120mA transformer is much more likely to be fatal than from the lower current supplies. Neon signs are a type of cold cathode lighting.

Applications

The light-emitting tubes form colored lines with which a text can be written or a picture drawn, including various decorations, especially in advertising and commercial signage. By programming sequences of switching parts on and off, there are many possibilities for dynamic light patterns that form animated images.

Images

See also

• 1911 in science
• Neon
• Neon lamp
• Neon message board
• Timeline of lighting technology
• noble gases