Philips Entertainment Lamps catalogue

The halogen lamp
The difference between a halogen lamp and a conventional lamp is the
adding of a halogen to the inert-gas filling of the lamp.
The halogens are I(Iodine), Br(Bromine), F(Fluorine) and Cl(Chlorine).
Non-halogen lamps have large bulbs to spread the evaporated tungsten
over a large area. With the halogen cycle the lamp no longer blackens
during its life-cycle, therefore the bulb can be made much smaller.
An additional advantage of these small bulbs is that they can withstand
high filling pressures. A high filling pressure will reduce the dissipation
of tungsten from the filament and can be used to increase the life of
the lamp or the output (Im/W).
In the lamp a complex chemical process takes place where the basic
reaction can be described as follows:
W
(solid)
Nx
(gas)
in which W = Tungsten
See also fig. 1
X = Halogen
low temperature
high temperature
WXn
(gas)
In the vicinity of the filament the temperature is so high that only
atoms of tungsten and halogens are present. Near the bulb wall will
be found a compound of WXn. The transition between these two
situations depends on the kind of halogen which is used in the lamp.
Generally this will be at 500 to 1600 °C for the gas phase and at 1200
to 1600 °C for the solid (dissociated) phase. In the lamp one
finds several areas in which these processes are taking place. See fig. 2.
The halogen cycle
So what happens in the lamp?
The evaporated tungsten of the filament associates near the bulb wall
(area A) with the free halogens. Due to partial pressure this gas is
transported back to area B where it dissociates and the tungsten is
deposited back on the filament. The halogen can then be used for the
next identical process.
Unfortunately the tungsten particles are not deposited on exactly the
same spot from which they evaporated so that, in due time, certain
parts of the filament get thinner; this causes so-called “hot-spots”.
At a hot spot the evaporation rate is higher than average due to the
higher temperature. This phenomenon results in the failure of the lamp.
W WXn
Fig. 1 The tungsten regenerative cycle
WXn W + nX (high temperature)
W nX WXn (low temperature)
Nx
Philips Entertainment Lamps catalogue 113
Appendix