Blue glass inside tubes

[morfic]

What was/is the purpose of the blue glass section in tubes, it's around the section with the plates, not above or below.
I am curious what it's for, blue and orange may near cancel each other out which may be a good military application at night? (yeah full on guess) But since my tube is brighter below that section anyway, i am not sure what the blue glass on the older versions was for (mine does not have this feature, it's a CV4014/EF91 tube)

Thanks for any info on this, googling mostly just finds ebay listings of such types (Amperex/Mullard) or stuff about blue glow, nothing about blue glass (same as search here it seems)

Daniel

[Colt45]

You mean the kind of graphite-ish coating inside?

Something like aquadag i guess. I believe it's supposed to have some shielding effect, iirc.


edit, here's a pic http://www.oddmix.com/tubes/px/6cb6a.jpg

[morfic]

Typically is more bright like this one.

[ilimzn]

Seen this on a couple of other tubes. In fact, I have several types of EF91 including 'civilian' ones, and all phillips/mullard have the blue coating. not sure what it is but it may well be blue in the visible spectrum, while absorbent in the IR spectrum, to facilitate cooling. Or, it could be slightly conductive and a form of shielding. The CEI/Thomson EF91 I also have, don't have it, but they do have a metallic coating reminiscent of a getter, but slightly different in color. The metalic nature would suggest a shield...