During restoration of an old oscilloscope, I found a blue/green plastic sheet in front of the CRT. This piece of plastic was heavily damaged and scratched, and clearly discolored due to exposure to light.
I realized then that most CR oscilloscopes I have used have such a blue/green cover, instead of the white background of the actual phosphor coating. What is the purpose of this? Is it just protection? Is there any harm in removing it (since I have no replacement).
The picture below shows the plastic with the bezel. The plastic can be removed and just the bezel placed back, giving a white background instead of the blue/green of the plastic in front of it.