It is not so
much the melting point of the tungsten (which, at 3653 K, is still a relatively
long way from the approx. 2800 K of the operating temperature of incandescents)
that hinders the construction of more efficient incandescent lamps, but rather
the increasing rate of evaporation of the filament that accompanies the increase
in temperature.
This
initially leads to lower performance due to the blackening of the surrounding
glass bulb until finally the filament burns through. The price to be paid for
an increase in luminous efficiency is therefore a shorter lamp life.
One
technical way of preventing the blackening of the glass is the adding of halogens
to the gas mixture inside the lamp. The evaporated tungsten combines with the
halogen to form a metal halide, which takes on the form of a gas at the
temperature in the outer section of the lamp and can therefore leave no
deposits on the glass bulb.
The metal
halide is split into tungsten and halogen once again at the considerably hotter
filament and the tungsten is then returned to the coil. The temperature of the
outer glass envelope has to be over 250° C to allow the development of the
halogen cycle to take place.
In order to
achieve this compact bulb of quartz glass is fitted tightly over the filament.
This compact form not only means an increase in temperature, but also an
increase in gas pressure, which in turn reduces the evaporation rate of the
tungsten.
Compared
with the conventional incandescent the halogen lamp gives a whiter light – a
result of its higher operating temperature of 3000 to 3300 K; its luminous colour
is still in the warm white range. The continuous spectrum produces excellent
colour rendering properties.
The compact
form of the halogen lamp makes it ideal as a point-source lamp; its light can be
handled easily and it can create attractive sparkling effects. The luminous efficacy
of halogen lamps is well above that of conventional incandescents – especially
in the low-voltage range.
Halogen
lamps may have a dichroic, heat reflecting coating inside the bulbs, which increases
the luminous efficacy of these lamps considerably. The lamp life of halogen
lamps is longer than that of conventional incandescents.
Halogen
lamps are dimmable. Like conventional incandescent lamps, they require no
additional control gear; low voltage halogen lamps do have to be run on a
transformer, however.
In the case of double-ended lamps, projector
lamps and special purpose lamps for studios the burning position is frequently
restricted.
Some
tungsten halogen lamps have to be operated with a protective glass cover.
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