OSRAM DULUX F - Manual (Page 71)

Economical long-life light sources with plug-in bases
Compact Fluorescent Lamps OSRAM DULUX ®
Technical Guide
69
Picture of an electrode for low pressure discharge lamps, consisting of 2 lead wires and a tungsten electrode
coil which is covered by (white) emitter.
The problem with these oxides is that they are highly hygroscopic. This means, if they come into
contact with air, they will suck up a lot of moisture which then is trapped inside of the lamp, leading
to low light output, high lamp voltage and short lamp life. The trick, lamp manufacturers use, is to
put Barium-, Strontium- and Calcium-Carbonate on the electrodes instead of the oxides. The
carbonates are stable when they are in contact with air. During the exhaust process of the lamp,
when the air is pumped out of the lamp and the lamp is then filled with the designed fill gas, the
electrodes which are covered with the carbonates are heated up to a temperature of 600°C and
above. At that high temperature the carbonate changes into oxide by releasing CO
2
as shown in the
equation.
After this reaction is finished, another chemical reaction is necessary to reduce the electron work
function of the electrode-emitter system, where atomic Barium is released and transported to the
surface of the emitter. The reaction takes place at the surface of the tungsten wire, which the
electrode is made of below the emitter coating and follows the equation
Once enough atomic Ba has reached the outer surface of the emitter, the electrode is ready for its
work. This last reaction then continues over the whole life of the lamp and electrode.
When the reaction is started for the first time in a new lamp, it will take a significant amount of time
until the first coverage of Ba on the emitter surface is ready. This process needs a high emitter and
coil temperature, which usually is only given when the lamp operates at the rated discharge current
with a hot spot on the electrode where a temperature of about 1900K is reached. If the lamp is
operated in a dimmed mode, the temperature on the coil is lower and got a wider distribution.
Therefore the described process does not happen as effectively as in the operation at rated current.
The consequence then is that the electrode is not in a proper shape and the electron work function
is higher than with a properly conditioned electrode. This makes an increased temperature on the
electrode necessary which is realized by an increased cathode fall voltage in front of the electrode.
By this cathode fall voltage, ions from the plasma are accelerated to the electrode and yield an extra