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I wanted to give VFDs a try so I ordered a couple of 8x14 segments display modules.

They arrived today, but I'm concerned because each has a big black stain in the corner. If it were on an LCD I'd suspect they're broken, but since I don't know this display technology at all, can someone tell me:

  • Are those displays broken or is the stain a normal thing?
  • If this thing has a name, what is it called?

Here is a picture of the modules (the stain is on the top right corner):

suspicious VFD display modules

Note that after checking on the seller website and a few pictures on other online resources, this kind of stain is visible on some of them, so it may be a normal thing.

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  • \$\begingroup\$ I wonder if it's some material injected to seal the case after sucking out the air. \$\endgroup\$ – Peter Green May 17 '16 at 7:51
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    \$\begingroup\$ @PeterGreen - It's not injected, it's evaporation deposited in-place. Basically, there's a loop of wire coated or plated in some manner with the getter metal. The tube is pumped down to vacuum, baked out, and the normal vacuum sealing process is employed. After the tube is at vacuum, an induction heater is used to heat the loop of wire until the getter coating literally evaporates off the surface of the loop, and deposits on nearby surfaces. This is done because the vacuum deposited surface is then completely fresh getter, and it increases the getter surface area dramatically. \$\endgroup\$ – Connor Wolf May 17 '16 at 19:26
  • \$\begingroup\$ Thanks everyone for the answers! TIL a new thing about VFD. \$\endgroup\$ – LeFauve May 18 '16 at 2:36
  • \$\begingroup\$ Just a coffee stain left over from when the guys in manufacturing accidentally spilled a pot. Nothing to worry about. \$\endgroup\$ – Olin Lathrop May 18 '16 at 18:31
  • \$\begingroup\$ Well, if it was a coffee stain, why would it appear on most of the VFDs manufactured? \$\endgroup\$ – Brick May 19 '16 at 9:56
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It's getter, it should absorb any residual gases in the display. Vacuum tubes have getters too (the black part on top).

If that spot turns white (though in some cases I have seen it become transparent/invisible or just fall off) it means that the glass is broken and the VFD or tube is full of air and is no longer functional.

vacuum tubes

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    \$\begingroup\$ Note that for most applications like the mentioned VFD magnesium getters are used that will only absorb oxygen, and the remaining gasses is not cared about. For more sophisticated applications you will find barium or rare earth getters that are able to absorb nitrogen too. \$\endgroup\$ – PlasmaHH May 17 '16 at 8:44
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It is called a getter. It's design to be a reactive deposit to absorb the remaining air in the vacuum. The ones in the linked photo are probably fine, the Wikipedia article has photos of examples where the vacuum has been lost (they become completely whitened).

Image from Wikipedia showing a white getter dot on a VFD

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It is as designed. No worries! It's called a getter. It'll get the remaining O2 that was left in there.

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