There are a number of facts that make discharge lights interesting. Firstly, unlike an ordinary filament lamp the light does not come from a white hot glowing wire. The light is emitted entirely from the gas atoms excited from conducting electricity. As the gas atoms emit the light as a result of electron transitions within the atom, the light is unique to the type of gas. Atoms in this state are known as ionised which is often referred to as 'Plasma' - the 'fourth state of matter'. The generic neon sign has exploited the unique colours produced by electrical discharge of gases at low pressure. Neon actually produces a red light but many other gases are used to produce the colours seen in neon lighting such as argon, xenon and even mercury vapour. Some gases produce more visible light than others and have found their way into high intensity discharge lamps for industrial lighting.
The images here show most lamps running and go off as you mouse over them. Links on the images will take you to the line spectrum for the lamp. There are also 3 other links on the page; more information on ballasts, electrodes, vintage lamps, and Deuterim lamp circuits.
This image shows a Neon indicator lamp running on 240v AC. The neon gas around the metal spiral and disk in the centre is glowing due to the electrical discharge.
The lamp shown here is a 400W mercury vapour lamp.
Mercury vapour is probably the most commonly used discharge medium. It is found in various lamps from fluorescent tubes to high pressure sodium lamps. The visible light from the mercury discharge is not particularly good. It is not as efficient as low pressure sodium and the colour is blue-green which has poor colour rendering properties. Where the mercury lamp succeeds is in it's ultra-violet (UV) emission. This can be converted into useful light which complements the colour already available from the discharge. If a fluorescent coating which produces red or yellow light from absorbing UV, a colour balance close to white light can be produced. This is what happens in a fluorescent tube or as in the high pressure mercury lamp above. The actual lamp or arc tube can't be seen but the coating on the elliptical outer bulb is converting the UV light into red light which produces an overall white light when mixed with the visible mercury discharge. With only filtered UV light on the lamp the fluorescing colour can be seen in the right hand image above.
An early high pressure mercury lamp with no fluorescent coating is shown below. The light is a cold bluish colour. These lamps were replaced with the type above which improved colour quality and increased light output.
The arc can be seen in the quartz tube on the right. This is housed inside the outer lamp envelope. This lamp actually has a filter outer envelope known as 'Woods' glass. This removes most of the visible light but some blue does get transmitted. The long-wave UV is also transmitted but not the short-wave which is very dangerous to the eyes and skin. These lamps are also know as 'Black lights' and are used for various UV lighting effects.
Other elements can be added to the mercury discharge to change or improve the light quality. This is best seen in Mercury-Halide lamps. Elements from the halogen group can be added to the discharge. The lamp on the right produces a bright green light as a result of this technique.
Another commonly seen blue halide is shown here.
The Mercury-Halide lamp lower right produces a cool white light which has good colour rendering properties. These can be good enough to use in stage lighting.
One of the most remarkable discharge lamps is the low pressure sodium lamp, well known for its use in street lighting and it's yellow light. Two interesting facts about sodium light are that it emits almost only yellow light from just two close wavelengths in the visible spectrum. These wavelengths are also very close to the maximum sensitivity of our eyes. This means that the sodium light is very efficient at doing its job, producing light. With efficiencies up to 200lm/watt these are still the most efficient, mass produced, light source in the world.
The low pressure sodium lamp shown upper left produces a monochromatic yellow light. The inner arc tube is housed in an evacuated outer jacket. Preventing heat loss from the arc increases the efficiency of the lamp.
The lower right picture shows the lamp switched off but the sodium vapour is still at operating temperature and pressure. The lamp is being illuminated from another sodium source. The discharge tube appears to be cloudy. This is actually due to the absorption of the light by the sodium vapour and it's stimulated re-emission. The light re-emitted is less than the illumination so it appears cloudy.
The so called high pressure sodium lamp is actually a blend of sodium and mercury operating at high pressure. It was difficult to develop the arc tube for this lamp due to the intense chemical activity of sodium and high running temperatures. The arc tube is made from sintered aluminium oxide which is able to withstand the heat and intense chemical attack but still transmit light.
The discharge lamp on the right is now replacing tungsten filament lamps in car head lamps. They produce more light for a given wattage and the lamps last significantly longer than the filament type. Although these lamps are known as Xenon they only start on Xenon. They also contain mercury/halides which predominate the arc output after a few seconds of operation. p>
A more typical example of a short arc Xenon lamp is shown on the left. This type of lamp is found in large projectors and search lights. This 2kW version is quite large and the xenon gas pressure is many times atmospheric. Due to the gas pressure these lamps are hazardous. Any stress on the quartz envelope can result in the lamp exploding with a high risk of injury. The lamps must be stored and handled while enclosed in their safety cover. The second picture below shows the lamp in it's protective cover. It is shown here being pulsed but not running. The casing would melt quickly if the lamp struck. The arc running voltage is as low as 24V but the striking voltage is in excess of 30kV. A special ignitor circuit based on a Tesla coil is required to create the ignition pulses.
A number of examples of special purpose lamps are shown here. The super high pressure mercury lamp on the right is a high intensity lamp operating at several atmospheres. This is a very intense source of UV light. Like the XBO it is often used in projection systems.
An example of a Cadmium lamp is shown on the right. This lamp has no commercial use but the light from lamps like these can be used for spectroscopy.
The lamp on the left is a Deuterium lamp used in spectroscopy and fluorescence measurement. 'Click' on the 'more' link for further information.
A Low pressure Mercury UV lamp, shown below, is made with a quartz tube. It uses hollow cold cathodes and requires 5kV to strike the starter gas (probably Argon). The lamp is old and has a 4-pin valve style base for connection.
This lamp is primarily designed for use in spectroscopy and has no commercial application. The tube strikes at just over 2.5 kV and produces a dim light. An interesting concept is that hydrogen is the simplest atom with just one electron and one proton. When ionised in the discharge, it can exist as hydrogen without an electron; an H+ ion (Proton).
Like the hydrogen lamp above, this lamp is primarily designed for use in education and spectroscopy. It has no commercial application. The tube produces a dim light. Not dissimilar to hydrogen but the spectrum is very different.