Tektronix 4051 screen filter / protector cracked

[Xenon]

Hello all,
Currently I am repairing a Tektronix 4051 for the Dutch Homecomputermuseum. It is in terrible condition and I hope I can get it working again.
After powering it up (yes, checked the voltages first) I heard a klicking noise and smelled something burning. It appeared that a 5kV capacitor was arcing short (cracked). As soon as that is replaced I can test the rest.

The green-ish filter in front of the tube is cracked. It seems like ordinary layered glass, but there is something special about it.
On the back it has a metal coating on the top and bottom edge, and aluminum/aluminium tape at the corners. The pieces of tape push against springs connected to chassis ground.
When held against the light you see a blue/red oily kind of glow on the back of the glass, like there is a special coating on it.
Does anybody know if it can be replaced by ordinary green-ish glass, or does this coating have a special function for the tube to function properly? Or is it just shielding to protect the user from tube emissions?

[David Hess]

My guess is that is EMI shielding.  On many Tektronix instruments EMI shielding, which included a metal screen over the CRT, was an option.

It it is not necessary for proper functioning so the CRT shield can be replaced with one that lacks the shielding.

[YetAnotherTechie]

Have a look under a magnifier to see if there is a mesh inside.

[james_s]

I'm only speculating but I would guess it's an anti-reflective coating to prevent a "hall of mirrors" effect from light reflecting off the glass and back off the face of the CRT. The EMI shielding is another good guess, it could even be both.

I think plain glass or even acrylic would probably work fine, especially for museum duty. Certainly it would be better than the smashed glass.