Very very cool I've never heard of these! From the looks of it it's pretty much just a neon sign tube, though I can't tell the diameter. What is that dark stuff surrounding the area where the phosphor starts? I wonder what it's efficiency is

The HTL-type of fluorescent tube is indeed not well known as its use in general lighting was very brief - spanning typically from the mid 1920s to the late 1930s. This is indeed related to the neon sign tube, the same kind of hollow cathode is used. As a matter of fact, Philips's HTL200 was derived from George Claude's neon and mercury sign tubes (France), whose technology was licensed to Philips when the Dutch began researching and developing their first positive-column gas discharge tubes in November 1924.

As for the efficacy of this tube, the data published by Philips in 1939 relative to its HTL200 indicate a power consumption of 58.5 W for a 2000 lm output for each tube, which corresponds to a 34 lm/W efficacy. Now, I haven't measured the exact luminous characteristics of the present #10-76133 tube, but I think its efficacy should be in the same ballpark given the more efficient operation of the mercury discharge at low current, compensated by the less efficacious red-emitting phosphor. Precise measurements will have to be done in order to answer that question properly though.

The "dark stuff" near where the phosphor coating starts is just black paint that was applied to block off the bluish (uncorrected) light from the mercury discharge. If you're referring to the darker trace visible along the phosphor-coated tube, that's just some excess fluorescent material that accumulated there when the tube was coated (which was done horizontally). Interestingly, this feature is intentional as it acts as a light reflector that increases the source luminance in the direction opposite to the electrode locations, a characteristic and a design which was patented. In its application the electrodes of the HTL200 where in the upward position while light was directed mostly downward. In essence, the first Dutch fluorescent tube was of a reflective kind!

Another wonderful picture.!

Thank you, Sammi. I hope you like the description as well, pictures are only half the story.

Ahh okay interesting that does make sense, I suppose it's a similar concept to the black paint used on neon signs. I didn't even notice the uneven phosphor but that is so interesting that it was intentional. 34 lm/W is decent for the time I suppose, I think that's decently better than some low wattage HPMV lamps but I forget. Did these have dedicated ballasts or were they just meant to run in series on a neon sign transformer?

Again, another amazing picture (and description..!) from our Max. I really must get around to posting more, it's been rather hectic here over the past few weeks and things show no sign of letting up

Interesting lamp there! I like how the gas discharge peeks out by the cathodes. Does the black paint hide a rough edge of the phosphor coating, or just to hide most of the uncoated ends?

That's definitely not your usual fluorescent tube. As far as I can tell, the black paint covers the transition between two tubes, which where initially two separate parts: the phosphored tube and the clear one containing the electrode. The edge between the two parts is quite clean.

Ria - Thanks a lot! Please find/make some time to post more, you have a nice collection and I'd certainly like to see more of it here.

Drew - Those lamps (i.e., the HTL200) were meant to be run with several units in series, connected to a dedicated high-voltage transformer. In their December 1939 publication (Schouwstra and Zecher, Philips Technical Review) they point to installations where 9 lamps are connected end-to-end for a total luminous tube length of 18 meters for each transformer. This approach to the HTL system operation was meant to mitigate the relatively high losses from the transformer ballast. The overall system efficacy was 30 lm/W, which compared favorably to incandescent lamps (13.5 lm/W), the reference for the intended application, i.e., interior lighting.

Max, I love the descriptions as well.
very detailed, and a history lesson as well.

Glad you like this, it's always good to know that it benefits a readership who still has a sense of curiosity and is not there just for the pretty pictures or because of some shared personality/mental disorders ;P

What do you mean by mental disorder.?

You've never been to LG? That's actually a recurring topic there...

All right Fairy Nuff, we like to think we're here for a shared love of quality lighting.
The less said about LG the better.