LOW PRESSURE SODIUM VAPOUR LAMP
These
lamps were invented first in 1920 by ARTHUR H. COMPTON at Westinghouse, USA. It
is a gas-discharge lamp that uses sodium in an excited state to produce light.
WORKING PRINCIPLE
It
works by electric discharge (passage of electricity through sodium vapours at low
and high pressure) filaments of the lamp sputter (sputter means spit up in an
explosive manner) fast moving electrons, which hit the sodium atoms (vapour)
causing the valence electrons of the sodium atoms to excite to higher energy
levels and the electrons thus excited and are emitting the characteristic
monochromatic bright yellow light (589 nano-metre).
CONSTRUCTION
1.
A Low-Pressure Sodium (LPS) vapour lamp contains an inner
discharge tube made of borosilicate glass
that is fitted with
metal electrodes and filled with neon and
argon gas and a
little metallic sodium.
2.
Neon gas serves to start the discharge and to develop enough
heat to vaporize the sodium.
3.
Argon has a lower glow voltage, argon helps the smaller
lamps start at a lower voltage.
4.
The sodium vapour lamp is only suitable for alternating
current, and therefore requires choke
control.
5.
A voltage of the order of 380- 450 volts (depending on the
wattage) is necessary to start the
discharge, which is
obtained from a high reactance transformer
or an
auto-transformer.
4.
Its operating power factor is low (0.3) hence suitable
capacitor must be used to improve the power
factor.
OPERATION
1.
When the lamp is not in operation, the sodium is usually in
the form of solid deposited on the side
walls of the tube.
2.
When it is connected across the supply mains, current passes
between the electrodes, it ionizes the neon
and argon,
giving a red glow until the hot gas
vaporizes the sodium.
3.
The discharge tube is U-shaped. When the lamp is turned on
it emits a dim red/pink light to warm the
sodium metal and
within a few minutes it turns into the
common bright yellow
as the sodium metal vaporizes.
4.
LPS lamps have an outer glass vacuum envelope around the
inner discharge tube for thermal insulation,
which improves
their efficiency.
5.
At starting it creates a red glow due to the neon gas and the
neon gas lights at a lower temperature.
6.
As the temperature increases the sodium begins to vaporize
and the lamp turns to a pure yellow, which
makes objects
appear as grey.
ADVANTAGES
1.
Most energy efficient light source commercially available,
with an efficacy of 100 to 185 lumens per
watt.
2.
Lamps have average life in the 14,000 to 18,000 hour range
and have excellent lumen maintenance (very
little reduction
in lumen output over life of lamp).
3.
Most lamps will restart immediately after interruption of
power supply, but require some time to come
up to full
brightness.
4.
Provides superior uniformity of light distribution over all HID
lamps.
5.
Lumen output does not drop with age.
6.
High lamp efficacy leads to low energy consumption.
DISADVANTAGES
1.
These lamps have poor color rendering characteristics.
2.
It is almost impossible to distinguish colors under an LPS lamp
because light produced by this source is
monochromatic (a
single color).
3.
Most expensive lamp to install.
4.
Run time to full light output is the longest (7 to 10 minutes).
5.
Require special disposal considerations.
[Care should be taken in handling these
lamps, particularly,
when replacing inner U tube. If it is
broken, then sodium
comes in contact with moisture, fire will
result]
APPLICATIONS
1.
Road lighting and railway marshalling yards
2.
Airports, harbors and ports and, quarries, foundries and
rolling mills.
3.
Security and orientation lighting.
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