Everybody has seen neon lighting in some form or other. Its main application is in advertising signage, from small and simple 'open' signs in shops and restaurants to enormous signs pouring out their commercial message to the masses, screaming for your attention. This entry tries to shed some light of its own on the history, working principles, and manufacturing techniques of neon lighting.
The phenomenon of luminous tubes has been known since as early as 1675, when the French astronomer Jean Picard noticed that a mercury-filled barometer tube emitted a bluish glow when shaken. The scientific principle that was responsible was not understood at that time, however. It was not until 1910, when the Frenchman Georges Claude was the first to present a commercial luminous tube using neon gas. Since then, neon lighting has become one of the most prominent forms of signage.
How It Works
Simply put, a vessel (often a glass tube), with a metal electrode at each side, is filled with gas at a low pressure. Then a high voltage is applied across the electrodes. The gas molecules1 inside the tube are accelerated and travel in all directions, colliding with one another constantly. Actually, the positive molecule parts (ions) travel towards the negative electrode, while the negative parts (electrons) travel towards the positive electrode. Each time a collision takes place, the molecules involved will emit light of a certain frequency of the spectrum. Since an enormous number of collisions occur each moment, we view this simply as 'light'.
Only two basic colours are used in modern neon signage:
Red, where the tube is filled with neon gas, which is responsible for the light emission.
Blue, where the tube is filled with argon gas and a drop of mercury. In this case, the evaporated mercury molecules inside are responsible for the light emission, whereas the argon gas only serves as a carrier.
So you may be wondering - how come we see so many other colours in neon2 signs?
This is done by adding other colours to the picture, by:
Using coloured glass - for example, filling a yellow tube with neon (red) gas will result in an orange light.
Coating the inside of the glass with a fluorescent powder - these are available in many variations, allowing for different colours.
Using a combination of these techniques.
By these methods almost any colour can be made. It's even possible to make a tube that glows red using a mercury/argon filling which by itself would emit blue light.
Neon lighting is available in many different forms: straight tubes, curved tubes, and also highly complicated bent shapes. These are made by someone quite appropriately called a 'neon bender' or glass-blower. These people heat the glass using burners that run on a combination of gas, air and sometimes oxygen. When the glass is hot they are able to shape it as desired, by bending it and blowing into the tube. This is a difficult skill which takes many years to learn and many more to master.
Then the electrodes are fused to the glass tube. Electrodes are small metal shells sealed inside a glass tube that's open at one end. Two metal strands run from the shell through the closed end of the glass tube. Through these the voltage can be applied. One of the electrodes also has a small-diameter glass tube sealed to it, called a 'tubulation'.
Now comes the most important part in neon manufacturing: evacuating the air from the unit and filling it with gas. The glass part is connected with the tubulated electrode to a special vacuum system. Then a high voltage (between 10,000 and 15,000 volts) is applied across the electrodes. This causes the glass to heat up very quickly, releasing any impurities that are present in the glass itself. Removing impurities is very important, since any that remain will considerably shorten the life-span of the tube. They are removed by the vacuum system.
When a sufficient temperature has been reached (230° - 250°C), the voltage is switched off and the vacuum system starts pulling the tube vacuum - ie, sucking out the gas inside - while it cools down. When the tube has cooled down sufficiently, the rare gas (neon, argon, or a mixture of these) is added to the tube. In the case of an argon filling, a minute quantity of mercury is added. The tube is now melted off at the tubulation, and presto! a new neon tube is born.
The tube can now be installed in the sign installation. In order to light it, a high voltage transformer (typically 1,000 - 15,000 volts) is connected.