This holds two camera lens filters parallel about 30mm apart. It is intended to hold two polarizing filters oriented perpendicular to each other so that the material under test can be placed between them to test for birefringence.
The object is currently configured to hold the 52 mm polarizing filters I have, but can be easily adjusted for whatever size filters you have by adjusting the values in the spreadsheet.
I have designed this because I have been digitally scanning some negatives and discovered that I had some from 1936, which means that they are definitely nitrate film. In researching nitrate film, I learned that if one has nitrate film, one should digitize it as soon as possible, and unless the film has historic value, it should be disposed of. When nitrate film gets old, it begins to decay, and out-gasses nitric acid, and other toxic substances. It is extremely flammable (same chemical compound as gun-cotton), and can spontaneously combust. Only keep this stuff around if you have the climate controlled fireproof environment in which to keep it. I also learned that the first safety film to replace nitrate film is acetate. While acetate film is much less flammable than nitrate film, it also decays, and out-gasses acetic acid (think vinegar). Neither nitrate nor acetate should be stored together with other film types as the acids they out-gas damage the color of color negatives. In the process of learning this I learned that one of the other types of safety film can be identified by checking for birefringence. hence this project.