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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


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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
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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.
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Mercury-Halide

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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.

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Low Pressure Sodium

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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.
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Super High Pressure

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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.
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