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 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.
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
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.
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.
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.
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.
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.
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.
- 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.
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