As far as sox plus 180w goes – the sox plus range was a marketing and sales tool to offer a 2 year guarantee against failure for an additional price – I believe 180w did not have any significant competition in the market, so was not required.

I’m kind of wondering why the E36w lamps are being run on 55w gear? I personally have not run a test for this and we have never/ do not recommend this. Like you say the operating currents are different (although technically the same Discharge Tube) the coatings on the outer bulb are different to optimise the SOX E lamps at maximum efficacy in combination with the correct gear. Our life testing shows the 55W and E36w lamps are performing well for life.

Sodium at the electrodes does not cause short life – we have sodium resistant electrodes.

The correct lamp type should be run on the correct gear.

Blown electrodes can be due to a number of factors – usually a low supply voltage, causing emitter exhaustion on the electrodes and then the tungsten of the electrode emits and falls off. If the emitter (white coating on the electrode) is still present and the electrodes are half way up the tubes then it can be a sign of a lighting strike.

We no longer manufacture the srs range of lanterns – the luminaires division closed here at the end of march (don’t read this as the end of SOX though, there are other manufacturers producing their own designs and we still have an install base of over 4 million lighting points using sox.

Blackening of the DT at the cathodes is not a defect
The extent of the blackening is variable, but it has only a temporary effect on lumens output
If the DTs were not blackened, the lamps would exhibit short life
 
·         Why blackening occurs;
The blackening of the DT at the cathodes is metallic barium deposited on the tube wall
This is done just before the DT is sealed and it acts as a getter (vacuum pump) removing any residual contaminants from the DT
When any contaminant gasses (eg oxygen, water etc) contact the barium, they react with it – forming barium oxide or barium hydroxide etc
By keeping the DT clean (uncontaminated), the ignition performance of the lamp is more reliable and it lives longer
To make the blackening, we pass a large electrical current through the cathodes to heat it white hot
The tungsten cathode is coated with an emitter compound (calcium-barium-strontium oxide)
Some of the emitter barium oxide reduces to barium metal evaporates and then condenses on the colder DT wall
 (The blackening of the outer bulb at the cap end is exactly the same – metallic barium)
 
·         Why blackening only appears on some lamps and not others;
The temperature/time caused by the electrical current we apply during DT manufacture is what controls the extent of the blackening
This cannot be directly controlled, so the visual appearance varies
All DT are subject to this process to ensure good lamp life
 
·         If the blackened DT reduces lumen output
The blackening is metallic barium and this is present to ‘fix’ any residual oxygen etc
Metallic barium is not transparent, but barium oxide and barium hydroxide are transparent
So when the lamp is burned, this black (metallic) coating disappears as it is converted into a transparent (oxide or hydroxide) coating
Most of the blackening has disappeared after a couple of hundred hours burning
 
Please note, 
as the lamp gets near to end of life, tungsten from the cathode will start to evaporate and condense on the DT
Typically this starts to occur after about 12,000 hrs
This can look a lot like the blackening we apply in the factory – but it is not the same
 
·         Does the blackened DT affect lifetime or lumen maintenance;
Yes
Without the blackening, lamp life and lumens maintenance would be worse