Download - Industrial and Bearing Supplies

129
TECHNICAL
Operating principle of an
Incandescent Lamp
The higher the filament temperature, the
higher the luminous efficacy, but the shorter
the lamp life.
A reduction in lamp life is a consequence of
the rapid increase in the rate of vaporization
of the Tungsten atoms as the temperature
rises; this process not only produces a black
coating on the bulb but also ultimately causes
the filament to break.
Blackening of the bulb can be effectively
countered by using as heavy an inert gas
as possible (Argon, Krypton or Xenon) and
ensuring that this gas is at high pressure
to reduce the rate at which the Tungsten
vaporises.
The principal types of incandescent lamp
are: general-purpose lamps, special-purpose
lamps, decorative lamps and incandescent
reflector lamps. The luminous efficacy of
incandescent lamps in the 25 to 1000W range
is between about 9 and 19 lm/W for lamps
with an average life of 1000 hours.
%
250
200
65% = 40 000% life
75% = 10 000% life
75% = 3600% life
80% = 1700% life
85% = 1000% life
90% = 600% life
Tungsten-Halogen Lamps
Tungsten-Halogen lamps operate in the
same way as incandescent lamps and have
a similar design. The small quantities of
Halogens (Bromine, Chlorine, Fluorine and
Iodine) and their compounds added to the
filler gas, almost entirely prevent the bulb
from blackening due to vaporizing Tungsten
atoms within a particular temperature range,
so there is no associated drop in luminous
flux. The bulbs in Tungsten-Halogen lamps
can therefore be made much smaller, which
means the pressure of the filler gas can be
increased and more economical use can
therefore be made of expensive inert gases
like Krypton and Xenon. The life of Tungsten
lamps varies from 50 hours for photographic
studio types, to over 2000 hours for
commercial display lighting types. Automotive
Halogen lamps are generally rated in the 300
hour range.
95% = 190% life
Halogen Cycle
As already mentioned, the main
characteristics of an incandescent lamp,
namely its luminous efficacy and service
life, are determined to a large extent by the
filament temperature. The higher the filament
temperature, the higher the luminous efficacy
but the shorter the life of the lamp.
This reduction in the life of the lamp is a
consequence of the rate at which Tungsten
vaporises from the filament. This rate
increases rapidly with temperature. Not only
does it blacken the inside of the bulb, it
causes the filament ultimately to burn right
through.
This blackening of the bulb can be effectively
countered by adding halogens to the filler
gas to keep the vaporized Tungsten away
from the bulb wall in a cycle process . The
Tungsten that vaporises from the filament
during normal operation drifts towards
the bulb wall by diffusion or convection
in thermal regions (•1 1400°C) where
they disintegrate. Some of the Tungsten is
transported back to the filament, but not
to its original location. The normal Halogen
cycle therefore merely prevents the bulb from
blackening, it does not extend the life of the
lamp. The lamp comes to the end of its life
when the filament breaks at one of its
hot spots.
150
Lamp life
Light output
Tungsten
Halogenide
Lamp power
Halogen cycle
100
Tungsten
Halogenide
Lamp power
Halogen cycle
50
Light output
Tungsten
Lamp life
Halogenide
0
70 80 90 100 110 120 130 %
Halogen cycle
Halogen Cycle
A regenerative cycle would be feasible with Fluorine
but because this Halogen is so corrosive towards
the quartz and hard glass used for the bulb, and
resistant to the Halogen currently used, a satisfactory
solution has not yet been found.