Internal construction looks a bit like a linear sodium lamp, but that's all I have

Nope, that's not it. It's a much smaller lamp. Different technology too.

Oh, that’s a tricky one. Trying to workout the size. It looks like it might be about the size of a car halogen H4 lamp. Was wondering if it’s some kind of special purpose UV lamp but I can’t work out what looks like two holes in the envelopes.

You're getting there, but that's no UV lamp.

Your file name may have given a clue or two. Based on that, the automotive connection seems right, suggesting it is one of these ‘so-called’ xenon headlamps. However, your picture shows the lead-in section of the lamp. This leads me to think there has been a failure and the lamp envelope has been punctured. Is this possible! Wild guess: The positioning looks like dielectric breakdown of the tube followed by sustained arcing. I know the strike voltage, and in particular, the restrike voltage can be 30kV for these but there wouldn’t be enough current to melt glass. Perhaps the arc was sustained.

What a fantastic close up shot.
I've got no chance of getting a shot like this, even though my camera is capable of doing it.
With my vision problems, it's me that can't focus sharply enough.

As usual, I have absolutely no idea

Sammi - Thank you. I'm sure you can achieve similar results. Let the camera do the focus work on your subject and make sure you have some good lighting.

Tuopeek - The file name... forgot to clear it. OK, so, it's a xenon metal halide lamp for automotive headlights indeed (a Philips D4s, if that wasn't clear by now). The lamp is brand new and hasn't suffered any catastrophic failure. So, what are those features visible in the picture

Ah, ok. I'll have another go. Could the mark on the outer envelope, middle of the image, actually be a type of seal-off point with a flat profile to protect the optical performance. Similarly, at the top, looks like graphite painted onto the outside of the arc tube at the electrode end. Could this be part of the optical control for beam pattern when in use?

Well my idea got blown out of the water.
I do know of one type of compact fluorescent lamp, with the tube in a sealed outer envelope; the tube has a hole in it to allow gas into the outer envelope, I think it was to equalise pressure. Is this a similar construction idea?

I see what you mean (the Yorkville-Hitachi CF20, and also the Osram Centralux had that design). It's about a discharge indeed, but not like that. The burner is definitely not open to the outer bulb.

Tuopeek - Yes, you are correct, that's a seal-off point, one closed by a laser. That's just a method used by Philips (no impact on the lamp's optical properties) to fill in the outer jacket with a specific gaseous atmosphere. Now, the latter bit of info is related to the second feature, that coating on the burner's leg. So, what's its purpose exactly?

Cleaver sealing technique. I was always impressed previously by the seal off technique for XBO lamps too. So if the second feature isn't for optical control, could it be for temperature control of the burner at the electrode end. Perhaps it being dark can conduct the heat into the specific atmosphere in the outer jacket

Max & Mark - Always fascinating to read the detailed technical discussions between you two, I don't always understand a lot of it, but it's still interesting, keep it up

Ria, I enjoy these discussions too, but don't always get it right so the learning is fun.

Max, noticed one of your well detailed explanations on a similar lamp mentioned an outer gas being used as strike assist. So, I'm now thinking the coating may be to encourage an initial outer discharge creating a temporary conductive jacket of ionised gas. This would also emitting some UV at the same time to assist the main arc striking. Think I've covered all bases with my answers. If that's not it I'll submit a patent.

Your memory serves you well (and sorry, no patent possible here, it's been done already). The purpose of the gas-filled outer bulb and of the metallic coating is to facilitate the ignition of the main discharge in the burner indeed The metallic coating is an electrode which is capacitively coupled to the burner's moly seal, and its nature helps with the ignition of the low-pressure discharge in the outer bulb via the emission of secondary electrons upon impact with impinging ions. The all-quartz construction of this part of the lamp does not help with ignition because of the low secondary emission coefficient of silica, hence the need for a metal part. That's a very clever design, I'd say.

Ria - I'm happy that you are stimulated by our discussions. There will be more of those for sure