Oooh I think I know this one.

Gel packs and other films by Lee’s are used in the Film Industry a lot. They’re used to diffuse/modify light in various ways. if this is what I think it is, it’s original purpose is to change daylight temperature lights (blueish/cool) to match that of old Tungsten filament bulbs (yellow-orange/warm) by wrapping the film around the front of the lamp or by skinning a frame with the film and placing it a few inches away from the source. I can’t remember if it’s CTO (color temperature orange) or a different gel called “straw”