The types of neons used in sketch and etch rely on the same principles as the typical helium-neon discharge tube. The difference between a sketch or etch neon sign, and a standard one is that there are additional parts in a regular sketch or etch sign to achieve the desired length, size, and brightness. These include:
The typical modern-day neon tube, whether purpose-made for an application such as a vacuum tube replacement in a radio receiver or made by recycling discarded fluorescent lamps (as was once common), consists of a glass envelope with at least one electrode sealed inside through which electrons flow freely. A large enough voltage applied between two electrodes will cause an electrical arc to strike across from one electrode to another. The glass tube is filled with approximately 80% neon and 20% other gases, creating an electrical glow discharge between the two electrodes. The key to the proper lighting of neon tubes is to control this initial “ignition” voltage. This is controlled by keeping it below the striking voltage necessary for arc formation, allowing the electrons sufficient time to flow into one electrode before they reach too high a level. This can be achieved by keeping power applied across the electrodes low enough that only small numbers of electrons are given sufficient energy to ionize the gas prematurely (about their distance between electrodes).
To achieve bright neons which continue working for extended periods, designers must make several tradeoffs against lifetime considerations; these include:
In a sketch or etch neon, the electrodes are brought very close together to form a fine wire or needle, and a single gas is used. There would be two long electrodes in a standard helium-neon tube instead of one short electrode. A voltage higher than usual for a standard tube can therefore be applied to get more light from the same power source. In addition, the larger volume of atoms increases brightness by allowing more electrons through before all of them have been ionized. This means that these types of neons need less work per unit length compared with standard neons to achieve similar brightness levels.
The most straightforward neon sign design uses two electrodes placed about an inch apart on opposite sides of a long glass tube that has been partially evacuated of air. A high voltage power supply charges the gas between these electrodes, causing them to emit ultraviolet light that then energizes phosphor materials found inside the glass tube, causing them to fluoresce with a bright visible light. The glass tubing must be designed for low-pressure operation, typically around 25 millibars (3 PSI). In addition, the neon sign requires a capacitor connected to the power supply to keep its electrodes from constantly discharging at high voltage through unintended paths.
Small diameter glass tubing is used in sketch and etch neons, usually 10mm or less in diameter. This means that it can be bent into tighter curves than standard neon tubes because there will not be as much glass to heat up and expand during use. It also means that very small-scale letters can be constructed with relative ease by using multiple tiny glass tubes stuck together in series.