home info renewables experiments museum
email page  
 

 

   

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. 

 

 

     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  Discharge Lamps

   Osram  neon 

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.

Thorn MBF/U      Lamp illuminated with 'Black Light'

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 its ultra-violet (UV)  emission. This could 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 over-all white light when mixed with the visible mercury discharge.  With just filtered UV light on the lamp the fluorescing colour can be seen in the right hand image above.

An early 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.

Thorn 250W MB

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.

EYE H400 BL   UV Black light  

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.

GEC 400W MBI   

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. 

 Philips HPI

 

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 wave lengths in the visible spectrum.  These wave lengths 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 light sources 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.

Philips SOX         scintillating Sodium

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.

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 at the high temperature the lamp runs at.  The arc tube is made from sintered aluminium oxide which was able to withstand the heat and transmit light. 

GEC 400W HPS/U (on),  GE 400W SON/T (off)

 

Special application lamps

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

Philips Cd vapour spectral source

The lamp below is a UV lamp made with a quartz tube. It uses hollow cold cathodes and requires 5kV to start. It runs with 2kV. The lamp is old and has a 4-pin valve style base for connection. 

Two examples of special purpose lamps are shown below. The super high pressure mercury lamp on the left is a high intensity lamp operating at several atmospheres.  This is a very intense source of U V light.  The lamp on the right is a Deuterium lamp used in spectroscopy and fluorescence measurement. 'Click' on the link for more.

Wotan HBO 200W                          Deuterium UV source    

workings of a Deuterium lamp - on lamp control circuit

Xenon HID

Discharge lamps are 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.

 

 

Follow this link for more information on lamp circuits.

 

 

electrodes and control circuits -  on lamp electrodes and control circuits

 

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.

 

 
 
  Copyright © 2010 Mark Klimek All Rights Reserved