The thought springs to mind that this might have been a way around the issues of rapid phosphor degradation which were big problems with the earlier compact lamps. Could the inner arms be uv transmissive with the phosphor on the outer bulb only?

Both inner and outer are phosphor coated from what we can see. Certainly a very clever design. May have been a way round the Philips SL patent?

It was a clever way of creating a cold spot in the lamp so as to keep mercury vapour pressure low and at the optimal level for peak efficacy. This was a problem in early CFLs prior to the use of amalgam fillings - if made too compact, they would overheat and lose efficacy. By allowing the discharge to pass into a large, relatively cool outer bulb where the mercuy condenses, its vapor pressure is reduced and efficacy is consequently higher than normal. The construction was fragile and expensive though - later designs superseded it by replacing the pure mercury with zinc, indium or bismuth amalgams which naturally have lower mercury vapour pressures at high ambient temperatures.

Very interesting, Thanks James!

Of course nothing to do with the phosphor...The shadow tells that. Teach me to engage brain before typing! Thanks for the clarification James. Definitely have one of these on the wish list simply for being such a wonderfully different bit of design.

LOL Zel, don't worry, I had no clue why this lamp was designed like this! It certainly is one to keep an eye out for! Very unique!