The
incandescent lamp is a thermal radiator. The filament wire begins to glow when
it is heated to a sufficiently high temperature by an electric current. As the temperature
increases the spectrum of the radiated light shifts towards the shorter
wavelength range – the red heat of the filament shifts to the warm white light
of the incandescent lamp.
Depending on
lamp type and wattage the temperature of the filament can reach up to 3000 K,
in the case of halogen lamps over 3000 K. Maximum radiation at these temperatures
still lies in the infrared range, with the result that in comparison to the visible
spectrum there is a high degree of thermal radiation and very little UV
radiation.
Lack of a
suitable material for the filament means that it is not possible to increase
the temperature further, which would increase the luminous efficacy and produce
a cool white luminous colour. As is the case with all heated solid bodies – or
the highly compressed gas produced by the sun – the incandescent lamp radiates
a continuous spectrum.
The spectral
distribution curve is therefore continuous and does not consist of a set of
individual lines. The heating of the filament wire results from its high
electrical resistance – electrical energy is converted into radiant energy, of
which one part is visible light.
Although
this is basically a simple principle, there are a substantial number of
practical problems involved in the construction of an incandescent lamp. There
are only a few conducting materials, for example, that have a sufficiently high
melting point and at the same time a sufficiently low evaporation rate below
melting point that render them suitable for use as filament wires.
Nowadays
practically only tungsten is used for the manufacture of filament wires, because
it only melts at a temperature of 3653 K and has a low evaporation rate. The
tungsten is made into fine wires and is wound to make single or double coiled filaments.
In the case of the incandescent lamp the filament is located inside a soft
glass bulb, which is relatively large in order to keep light loss, due to
deposits of evaporated tungsten (blackening), to a minimum.
To prevent
the filament from oxidizing the outer envelope is evacuated for low wattages
and filled with nitrogen or a nitrogen-based inert gas mixture for higher
wattages. The thermal insulation properties of the gas used to fill the bulb increases
the temperature of the wire filament, but at the same time reduces the
evaporation rate of the tungsten, which in turn leads to increased luminous efficacy
and a longer lamp life.
The inert gases
predominantly used are argon and krypton. The krypton permits a higher operating
temperature – and greater luminous efficacy. Due to the fact that it is so
expensive, krypton is only used in special applications.
A
characteristic feature of incandescent lamps is their low colour temperature - the
light they produce is warm in comparison to daylight. The continuous colour spectrum
of the incandescent lamp provides excellent colour rendition.
As a point
source with a high luminance, sparkling effects can be produced on shiny
surfaces and the light easily controlled using optical equipment. Incandescent
lamps can therefore be applied for both narrow-beam accent lighting and for
wide-beam general lighting.
Incandescent
lamps can be easily dimmed. No additional control gear is required for their
operation and the lamps can be operated in any burning position. In spite of
these advantages, there are a number of disadvantages: low luminous efficacy,
for example, and a relatively short lamp life, while the lamp life relates significantly
to the operating voltage.
Special
incandescent lamps are available with a dichroic coating inside the bulb that
reflects the infrared component back to the wire filament, which increases the luminous
efficacy by up to 40 %
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