What you're looking at here is the inside of one end of a 15-watt compact fluorescent bulb (CFL).
I was motivated to explore this thing because of an interesting failure. The bulb stopped working. That's not unusual, particularly for early models. However, when I replaced it, I happened to notice something that was very unusual: a small hole melted through the glass envelope very near one end of the tube.
Here's the overview:
On closer examination, the hole looks like this. It's a little hard to tell, even in stereo, but the overall shape is a funnel-shaped crater, apparently caused by melting a small area of glass and sucking it in courtesy the near vacuum inside the tube. There's also some cracking around the edges of the crater. This is true stereo, two short stacks shot from different viewpoints.
A while later, I had figured out how to cut through the glass tube without shattering it, and had removed most of the tube.
At the end without the hole, I removed enough tube to fully expose the filament, so I could get a good shot at it. That's what shows in the picture we started with.
At the end with the hole, I left a longer chunk of tube, about 2 cm, hoping that I could peer in and get a good shot of the crater from the inside. That proved more challenging than I had expected, because to my surprise it's dark in there! Very dark. So dark, in fact, that the only way I ever succeeded in illuminating some areas was with axial lighting bounced off a big square microscope slide that I also shot through. (There's a separate thread about that, HERE.)
Here's the overview of the end with the hole. Bright spots back around the broken filament are light shining through from behind the glass.
Looking closer, here's a stereo rendering of the critical area. This is synthetic stereo, cropped tightly from the same stack as the overview. Again, the bright spots are light shining through from the back.
I'm not sure exactly how the failure proceeded. Best I can guess is the other filament gave out first, so the arc wouldn't start, and then the bulb left this filament heating for so long that it softened, warped into contact with the glass, melted the glass, and finally blew itself to smithereens when the air rushed in. Notice that there's a short chunk of filament sitting all by itself over near the right edge of the picture, and neither end of the long chunk is connected to anything.
It's an interesting failure, and presumably one that the manufacturer really tries to avoid because after the air rushes in, the mercury vapor seeps out. Kind of a slow motion self-breaking bulb. Not a good reputation to have.
By the way, in that very first image, you might have noticed an odd structure to the filament. Instead of being just a coiled coil like incandescents use, the filament is actually a coil of fine wire wrapped around a coil of coarse wire. Apparently this structure is something specific to fluorescent bulbs and has to do with its function as a source of electrons rather than light. There's a brief discussion and an SEM picture HERE.
However, it would be a mistake to think that coil overwrap construction is found in all fluorescents. Here's what I found inside a different model of bulb, when I took it apart to perfect my skills in glass cutting. It's a conventional coil, just one strand of wire.
I hope you find this interesting. If anybody knows more about that coil overwrap filament structure, I'd like to hear about it.
--Rik
Notes:
1. Most images were shot with a Canon 100 mm f/2.8 L IS USM, stepping focus manually with EOS Utility (8 ">" per step at f/5.6). High mag views of the coils were shot with Olympus 38 mm f/2.8 bellows macro lens at f/4, stepped with a StackShot.
2. Glass tubes were cut outdoors, using a small abrasive wheel on a Dremel Moto Tool. No living areas were damaged in the making of these photographs.
Edit: added link to the illumination thread
