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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.

Neon sign tubes are distinguished from neon lamp bulbs by their length, customized shapes, higher operating voltages, and range of colors.

Contents

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.

There are conflicting stories for the origin of neon signs. At the 1893 World's Fair, the World Columbian Exposition in Chicago, Illinois, Nikola Tesla's signs were displayed. Supposedly, a sign created by Perley G. Nutting and displaying the word “neon” was shown at the Louisiana Purchase Exposition of 1904; however, this claim has been disputed.[1] The development of neon signs has also been credited to Georges Claude; another early public display of a neon sign was of two 38-foot (12 m) long tubes in December 1910 at the Paris Expo. The first commercial sign was sold by Jaques Fonseque, Claude’s associate, in 1912 to a Paris barber.

In 1923, Georges Claude and his French company Claude Neon, introduced neon gas signs to the United States, by selling two to a Packard car dealership in Los Angeles. Earle C. Anthony purchased the two signs reading "Packard" for $1,250 apiece. Neon lighting quickly became a popular fixture in outdoor advertising. Visible even in daylight, people would stop and stare at the first neon signs for hours, dubbed "liquid fire."[2]

While neon lighting was used around 1930 in France for general illumination, it was no more energy-efficient than conventional incandescent lighting and neon lighting came to be used primarily for eye-catching signs and advertisements. Phosphor coating manufacturers have developed high efficiency triphosphor coatings for neon lamps that rival the best fluorescent tubes for efficiency, especially when used at small diameters.

Construction

While many people see neon tubes every day, what makes them work is a complete mystery to most. The neon tube is made out of 3-4' straight sticks of hollow glass sold by sign suppliers to neon shops worldwide who hand produce them into individual custom designed and manufactured lamps. Manufacturing is a cottage industry and an eclectic art, in most cases, often a small family business. All neon tubes are hand made and labor intensive, even today, and the shop equipment is normally custom assembled from scratch by the craftsmen themselves from parts. There are many dozens of colors available. The color is chosen by the type of tubing used, and the gas filling.

The interior of the tubes may, or may not, be coated with a thin phosphorescent powder coating, affixed to the interior wall of the tube by a binding material. When processed, the tube is filled with a purified gas mixture, and the gas ionized by a high voltage applied between the ends of the sealed tube through cold cathodes welded onto the ends. What you see when you observe an illuminated neon tube is only the glowing interior gas, which may or may not depending on the tube's intended color, be used to illuminate the phosphor on the interior of the tube. Different phosphor coated tubing sections may be butt welded together using glass working torches to form a single tube of varying colors in the span of the same filled tube, for effects such as a sign where each letter displays a different color letter within a single word, such as shown in the OPEN sign in the photo above right.

Even though the term "neon" is used to denote the general field, this is a misnomer; inert neon gas, which glows reddish orange by itself, is only one of the two types of tube gases principally used in commercial application and in fact pure elemental neon gas is used to produce only about a third of the colors. The greatest number of colors is produced by filling with another inert gas, argon, which itself produces very little light. What makes these tubes work is a tiny droplet of mercury (Hg) which is added to the tube immediately after purification. When the tube is ionized by electrification, the pure mercury droplets disperse into tiny drops, migrate evenly throughout the tube, coat the phosphors, and evaporate into mercury vapor, which fills the tube and produces strong ultraviolet light. The ultraviolet light thus produced excites the various phosphor coatings that come coated inside the tubing which contain exotic rare earth phosphors designed to produce different shades of color. Even though this class of neon tubes have absolutely no neon in them whatever, they are still denoted as "neon." In this sense there is little difference between the physical ionization and light production process of a standard fluorescent tube and a neon tube, except for the cathodes, the diameter of the tubes, the bending, and the gas interior chosen.

Each type of neon tubing produces two completely different possible colors, one with neon gas and the other with argon/mercury. Some "neon" tubes are made without phosphor coatings for some of the colors. Clear tubing filled with neon gas produces the ubiquitous yellowish orange color with the interior plasma column clearly visible, and is the cheapest and simplest tube to make. Traditional neon glasses in America over 20 years old are lead glass that are easy to soften in gas fires, but recent environmental and health concerns of the workers has prompted manufacturers to seek more environmentally safe special soft glass formulas.[3] One of the vexing problems avoided this way is lead glass' tendency to burn into a black spot emitting lead fumes in a bending flame too rich in the fuel/oxygen mixture. Another traditional line of glasses was colored soda lime glasses coming in a myriad of glass color choices, which produce the highest quality, most hypnotically vibrant and saturated hues. Still more color choices are afforded in either coating, or not coating, these colored glasses with the various available exotic phosphors.

Long Lifetime

It is the wide range of colors and the ability to make a tube that can last for years if not decades without replacement, that makes this an art. If it were not possible to make a tube that when well processed had the longevity that neon has, lasting up to a decade or more, there would be very little economic viability in a tube that requires so much custom labor. The intensity of neon light produced increases as the tube diameter grows smaller, by the inverse square root of the interior diameter of the tubing, and the resistance of the tube increases as the tubing diameter decreases accordingly, because tube ionization is greatest at the center of the tube, and the ions migrate to and are recaptured and neutralized at the tube walls. The greatest cause of neon tube failure is the gradual absorption of neon gas by high voltage ion implantation into the interior glass walls of the tubes which depletes the gas, and eventually causes the tube resistance to rise to a level that it can no longer light at the rated voltage, but this may take 7-10 years.

The actual cause of 80% of neon sign failures in strip malls on buildings conspicuous from the street has nothing to do with the tube life, but unbeknownst to the public is caused by the burnout of the high voltage electrical wires connecting the tubes inside of metal conduits, a traditional way of wiring neon that was formulated in a low voltage electrician's world. A very common type of neon sign is made from a formed metal box having a colored translucent face, called "channel lettering." Newer channel letter signs are being replaced by high brightness LEDs.

Tubing in external diameters ranging from about 8-15 mm with a 1 mm wall thickness is most commonly used, although 6mm tubing is now commercially available in colored glass tubes. 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. Ribbon burners are strips of fire that make the gradual bends while crossfires, when used, make the sharp bends.

This long lifetime has created a practical market for neon use for interior architectural cove lighting in a wide variety of uses including homes, where the tube can be bent to any shape, fitted in a small space, and can do so without requiring tube replacement for a decade or more.

Hand fabrication of neon tubes

Tube Bending

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. This is where the real art of neon comes in that takes some artisans from a year up to several years of practice to master. A tube bender corks off the hollow tube before heating and holds a latex rubber blow hose at the other end, through which he gently presses a small amount of air in order to keep the tube diameter constant as she is bending. The technique is similar to but much simpler than glass bending. The trick of bending is to bend one small section or bend at a time, heating one part of the tubing so that it is soft, without heating some other part of the tube as well, which would make the bend uncontrollable. A bend, once the glass is heated, must be brought to the pattern and fitted rapidly with vigilant forethought before the glass hardens again because it is difficult to reheat once completely cooled without risking breakage. To do this, it is frequently necessary to skip one or more bends and come back to it later, by measuring carefully along the length of the tube. One tube letter may contain 7-10 small bends, and mistakes are not easily corrected without going back and starting all over again. Once one stick of neon is bent, if more neon glass is needed to lengthen, another one is cut off and welded onto it, or the parts can be all welded onto each other at the final step. The finished tube must be absolutely vacuum tight in order to operate, and it must be vacuum clean inside. Once the tube is filled with mercury, if any mistake is made after that, the entire tube had, or should, be started over again, because breathing heated mercury impregnated glass and phosphor causes long term heavy metal poisoning in neon workers. Sticks of tubing are joined until the tube reaches an impractical size, and several tubes are joined in series with the high voltage neon transformer. In large installations this is done through by point electrical wiring, where extreme ends of the electrical circuit must be isolated from each other to prevent tube puncture and buzzing from corona effect.

"Bombardment"

A cold cathode electrode is melted (or welded) to each end of the tube as it is finished. The electrodes are also traditionally 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 to high vacuum 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"). This current and voltage is far above the level that occurs in final operation of the tube. The current depends on the specific electrodes used and the diameter of the tube, but is typically in the 450 mA to 800 mA range, at an applied voltage usually between 22,000-26,000 V. 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 walls 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 greatest impurities that are driven off this way are the gases that coat the inside wall of the tubing by adsorption, mainly oxygen, carbon dioxide, and especially water vapor. The current also heats the electrode metal to over 600oC, an bright orange incandescent color. The cathodes are prefabricated hollow metal shells with a small opening (sometimes a ceramic donut aperture) which contains in the interior surface of the shell a light dusting of a cold cathode low work function powder (usually a powder ceramic molar eutectic point mixture including BaCO2, combined with other alkaline earth oxides, which reduces to BaO2 when heated to about 500 degrees F, and reduces the work function of the electrode for cathodic emission. Barium Oxide has a work function of roughly 2 wherease tungsten at room temperature has a work function exponentially 100 times more, or 4.0. This represents the cathode drop or electron energy required to remove electroons from the surface of the cathode. This avoids the necessity of using a hot wire thermoelectric cathode such as is used in conventional fluorescent lamps. And for that reason, neon tubes are extremely long lived when properly processed, in contrast to fluorescent tubing, because there is no wire filament as there is in a fluorsecent tube to burn out like a common light bulb. The principal purpose of doing this is to purify the interior of the tube before the tube is sealed off so that when it is operated, these gases and impurities are not driven off and relesed by the plasma and the heat generated into the sealed tube, which would quickly burn the metal cathodes and mercury droplets (if pumped with argon/mercury) and oxidize the interior gases and cause immediate tube failure. The more thorough the purification of the tube is, the longer lasting and stable the tube will be in actual operation. Once these gases and impurities are liberated under pre-filling bombardment into the tube interior they are quickly evacuated by the pump.

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 low pressure of a few torr with one of the noble gases, or a mixture of them, and sometimes a small amount of mercury. This gas fill pressure represents roughly 1/100th of the pressure of the atmosphere. The required pressure depends on the gas used and the diameter of the tube, with optimal values ranging from 6 torr (mm Hg pressure) (for a long 20 mm tube filled with argon/mercury) to 27 torr (for a short 8 mm 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.

Heat processed neon tubes

An alternate way of processing finished neon tubes has also been used. Because the only purpose of bombardment by electrical means is to purify the interior of tubes, it is also possible to produce a tube by heating the tube externally either with a torch or with an oven, while heating the electrode with a Radio Frequency Induction Heating coil (RFIH). While this is less productive, it creates a cleaner custom tube with significantly less cathode damage, longer life and brilliance, and can produce tubes of very small sizes and diameters, down to 6mm OD. The tube is heated thoroughly under high vacuum without external electrical application, until the outgassed gases can be seen to have been totally depleted and the pressure drops to a high vacuum again. Then the tube is filled, sealed and the mercury dropped and shaken.

Electrical wiring

The finished glass pieces are illuminated by either a neon sign transformer or a switching power supply running at voltages ranging between 3-15 kV and currents between 20 and 120 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. Standard tube tables established in the early days of neon are still used that specify the gas fill pressures, in either Ne or Hg/Ar, as a function of tube length in feet, tube diameter and transformer voltage.

The standard traditional neon transformer, a magnetic shunt transformer, is a special non-linear type designed to keep the voltage across the tube raised to whatever level is necessary to produce the fixed current needed, up to the maximum limit of the neon footage possible.

Newer, compact high frequency inverter-converter transformers developed in the early '90s are used as well, especially when low Radio Frequency Interference (RFI) is needed, such as in locations near high-fidelity sound equipment. The reason for this is that at 20kHz, the typical frequency of these solid state transformers, the plasma electron-ion recombination time is too short to extinguish and reignite the plasma in the tube at every 1/120th second as in 50/60Hz transformers and so the plasma does not broadcast high frequency switching noise. The plasma simply remains lit at all times, becoming radio noise free.

The most common current rating is 30 mA for general use, with 60 mA used for high-brightness applications like channel letters or architectural lighting. 120 mA 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 120 mA transformer is much more likely to be fatal than from the lower current supplies. Neon signs are a type of cold cathode lighting.

Blocking out and coating

The trick of the eye neon plays is produced by blocking out parts of the tube with blockout paint, which is either painted on or dipped. One complete tube is actually composed of contiguous tube elements joined by glass welding to one another so that the entire voltage passes through, say, several letters end to end from cathode to cathode. To the untrained eye, it looks like separate tubes, but the electrical splice is the plasma inside the crossover glass itself. The entire tube lights up, unless the parts that the viewer is not supposed to see are covered with highly opaque special black or gray glass paint. Without blockout paint the display would appear confusing. In most mass produced signs today, a glass paint is often dipped to make ordinary tubing look like high quality colored glass tubing.

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 of neon signs

See also

References

  1. ^ John K. Howard (February 2009). "OSA’s First Four Presidents". Optics & Photonics News. http://www.opnmagazine-digital.com/opn/200902/?pg=14. Retrieved 2009-02-21.  
  2. ^ The history of neon signs
  3. ^ [1]

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