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| Incandescent Filament Lamps
In conventional filament lamps or light bulbs, electric current
is
passed
through
a coiled tungsten filament, contained in a
glass envelope that
is filled with an
inert gas. When heated
by an
electrical current, the
filament emits electromagnetic
radiation. At lower temperatures, radiation
is mainly emitted
in the infrared
part of
the spectrum as heat. At higher
temperatures,
the proportion
of radiation at wavelengths
ranging
from
380 to 780 nm increases
and visible light is
produced.
In a conventional
lamp, the filament temperatures
are limited to
about
2700 Kelvin.
The
tungsten filament starts
to evaporate and
as a
result it leads to the
blackening
of the
inside of the lamp envelope.
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| Halogen Lamps
Halogen lamps are also filament lamps. However, halogen is
added
to
the
fill gas to prevent evaporated tungsten from condensing on the
inside
of
the
lamp envelope. This feature
is used to exploit higher
filament
temperatures
of 3000K and beyond and allows the size of the
lamp envelope
to be
significantly
reduced. |
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Gas Discharge Lamps
In a gas discharge lamp, once a sufficient voltage is applied, electrons
are
emitted from a heated electrode, creating a
plasma or a gas capable
of
conducting electricity. In the
plasma mobile electrons collide with atoms
(predominantly mercury), transfer energy to the atoms and elevate them
to
an excited state. When these atoms fall back to their original status they
emit photons (packages of energy). In many low pressure gas discharge
lamps the wavelength of the emitted photon is not in the range of visible
light. Mercury, for example,
has its major emission in the ultraviolet at
254 nm. |
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Principle of low-pressure mercury gas discharge fluorescent lamp |
Ultraviolet photons have
the capability to excite fluorescent powders, which are coated on
the inside of
the tube, with a high degree
of efficiency. As a result, these powders emit visible
radiation in a range of colours. |
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Lamps based on these principles and operating at low
internal
gas pressure are called “fluorescent lamps”. |
Principle of a high intensity discharge lamp (e.g. metal halide lamp) |
| High pressure gas discharge lamps emit radiation directly as visible light. In this type of lamp
the combination of different element atoms in the hot
gas plasma, each emitting
at specific
wavelengths, determines the colour characteristics of the lamp
as a whole, as well as the
quality of colour rendition properties. |
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Most gas discharge lamps need at least
one free electron combined with a high pulse to start
the lamp operation and to produce light. Usually
minute quantities of materials like tritium
or
krypton-85 are applied either in the lamps themselves
or in starter devices as a source
for electrons. |
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