Many industrial lamps containing mercury have arc tubes made of quartz. These can transmit shortwave Ultra-Violet light. Ultra-Violet light is invisible but in the short wave form will cause severe sunburn and 'arc eyes' within minutes of exposure. The glass outer bulb does not transmit shortwave UV but if this becomes broken, skin or eyes must not be exposed to the light. 

Mercury Vapour 

  Discharge Lamps

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.

There are a number of facts that make discharge lights interesting.  Firstly, unlike an ordinary filament lamp the light does not come from heating a wire until it glows white hot. 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 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 gasses produce more visible light than others and have found their way into high intensity discharge lamps for industrial lighting.  

The lamp shown below 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 a 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 arc tube below. This is housed inside the outer lamp  envelope.  The small bit of plastic above is fluorescing due to the UV content.  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.

   UV Black light  

Follow this link for more information on lamp circuits.

-  on lamp electrodes and control circuits

- on early vintage lamps

Some of the lamps images have alternate pictures. To see more 'mouse-over' the images. 

'Clicking' on the image take you to the line spectrum emission for the lamp.

Mercury-Halide 

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 below produces a bright green light as a result of this technique.

Another commonly seen blue halide is shown below.

The Mercury-Halide lamp below produces a cool white light which has good colour rendering properties. These can be good enough to use in stage lighting. 

Low Pressure Sodium

One of the most remarkable discharge lamps is the low pressure sodium lamp, well known for its use in street lighting and its 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 below 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 picture on the right 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 its stimulated re-emission. The light re-emitted is less than the illumination so it appears cloudy.

High Pressure Sodium

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. 

Super High Pressure

Xenon HID

This discharge lamp 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.

A more typical example of a short arc Xenon lamp is shown on the right. 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 its 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.

Special application lamps

A number of examples of special purpose lamps are shown below. The super high pressure mercury lamp here is a high intensity lamp operating at several atmospheres.  This is a very intense source of UV light.  

Spectroscopy lamp

An example of a Cadmium lamp is shown below. This lamp has no commercial use but the light from lamps like these can be used for spectroscopy. 

Deuterium Lamp

The lamp below is a Deuterium lamp used in spectroscopy and fluorescence measurement. 'Click' on the link for more.

- on lamp control circuit