Im not completely sure so take this with a pinch of salt but I think it may be cellulose acetate
https://en.wikipedia.org/wiki/Cellulose_acetate
it was used for film and for things like reel to reel tape until other plastics proved more reliable. I think it ages and you are seeing some breakdown of the material.
for the replacement , you could perhaps change the bulbs if heat is an issue maybe LED’s but I think I’d take your sample to a sign shop. they have CNC machines that can cut various kinds of plastic film and I think it’s likely they would just be able to give you some blue/green scraps or maybe if you wanted they could also transfer or help recreate the image. Its basically like photographic film and I guess it would be possible to take a picture and reproduce film using old darkroom equipment.
if the blue film is not thick enough you could sandwich layers if you need to but I am not completely sure how well it will stand up to the heat from the dial lamps. I think that old acetate stuff just kind of decomposes and gets sort of brittle through time.
you could check a place like staples, they have those clear folders for presenting documents, you may also find they have some kinds of film like overhead projector sheets. I think you can photocopy onto those so maybe that would be another way to transfer an image that you could perhaps create on the computer in a graphics program, if you can get the mylar material in green you could use that , otherwise maybe you can sandwich mylar with colored acetate sheets. I think they use it for things like kids art projects but that stuff is very thin. they might call it acetate film but it is indeed more modern plastic.
thats about all I know and I am not even sure if that is what it is but if you look at the link it kind of fits that plastics and photographic film from that era were that. you can transfer from one type to another through making simple contact prints (not requiring an enlarger or camera) similar processes may be used to make the screens for silk screen printing.
in printing there is a process called “spectrum proofing” where sheets of different colors are loaded onto a drum and colors are imaged onto the paper. the light hits the film and that causes the color to transfer. those machines use mylar sheets with colors attached and the process removes the color and transfers it to paper. the spent sheets go in the garbage and those ones have most of the color on them except what was used in the process.
there is an older system called matchprints. both of these processes use colored mylar sheets.. Id try staples and see what you can find but if you happen to have an in with a local printshop they may have types of film that would be similar.
anyway to be simple some print shops may have access to colored films but I think approaching a sign shop would be my first choice. I used to work in priniting so I am pretty familiar with the varuious systems for press proofiing and some of those use colored mylar film materials. most sign shops are set up to deal with the public and they may be more helpful wit this.
Phil
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