A neon lamp is a gas discharge lamp containing primarily neon gas at low pressure. The term is sometimes used for similar devices filled with other noble gases, usually to produce different colors.
Description
A small electric current, which may be AC or DC, is allowed through the tube, causing it to glow orange-red. The exact formulation of the gas is typically the classic Penning mixture, 99.5% neon and 0.5% argon, which has lower striking voltage than pure neon. The applied voltage must initially reach the striking voltage before the lamp can light. Once lit, the voltage required to sustain operation is significantly (~30%) lower. When driven from a DC source, only the negatively charged electrode (cathode) will glow. When driven from an AC source, both electrodes will glow (each during alternate half cycles). Neon lamps operate using a low current glow discharge. Higher power devices, such as mercury-vapor lamps or metal halide lamps use a higher current arc discharge.
Graph showing the relationship between current and voltage across a neon lamp.Once lit, a neon lamp has a negative resistance characteristic: increasing the current through the device increases the number of ions, thereby decreasing the resistance of the lamp and allowing even more current. (This behavior occurs between the points labeled A and B on the lamp's current vs. voltage graph.) Because of this characteristic, electrical circuitry external to the neon lamp must provide a means to limit current through the circuit or else the current will rapidly increase until the lamp is destroyed. For indicator-sized lamps, a resistor is conventionally used to limit the current. Larger neon sign sized lamps often use a specially constructed high voltage transformer or ballast to limit the available current, usually by introducing a large amount of leakage inductance in the secondary winding.
When the current through the lamp is lower than the current for the highest-current discharge path, the glow discharge may become unstable and not cover the entire surface of the electrodes. This may be a sign of aging of the indicator bulb, and is exploited in the decorative "flicker flame" neon lamps. However, while too low a current causes flickering, too high a current increases the wear of the electrodes by stimulating sputtering, which coats the internal surface of the lamp with metal and causes it to darken.
The flickering effect is caused by the differences of the ionization potential of the gas, which depends on spacing of the electrodes, temperature, ambient radiation, and the pressure of the gas. The potential needed to strike the discharge is higher than what is needed to sustain the discharge. When there is not enough current to ionize the entire volume of the gas around the electrodes, only partial ionization occurs and the glow forms around only part of the electrode surface. Convective currents make the glowing areas flow upwards, not unlike the discharge in a Jacob's ladder. A photoionization effect can also be observed here, as the electrode area covered with the discharge can be increased by shining light at the lamp.
In comparison with incandescent light bulbs, neon lamps have much higher luminous efficacy. This is due to the fact that incandescence is heat-driven light emission, thus a large portion of the electric energy put into an incandescent bulb is converted into heat. Non-incandescent light sources such as neon light bulbs, fluorescent light bulbs, and light emitting diodes are therefore much more energy efficient than normal incandescent light bulbs. Green neon bulbs can produce up to 65 lumens per watt of power input, while white neon bulbs have an efficiency of around 50 lumens per watt. In contrast, a standard incandescent light bulb only produces around 13.5 lumens per watt.
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