The polarization test is a simple, non-destructive method to identify the types of film in your collection. This test involves the use of a simple tool which relies on the properties of light as it travels through two pieces of polarized film.
This tool can be used to help you identify a piece of film as polyester or cellulosic, but it cannot be used to differentiate between the cellulose acetate and cellulose nitrate films. This identification is important because all cellulosic film requires cold storage, and most polyester films are stable in a normal archival environment. Here's a brief demonstration on the use of this tool.
This is a piece of polyester film, and as most films, the center of the film is very dark, so it's hard to get a good read on the film through the polarizing lens. however, if you take a part of the film that is light, like along the edges generally, put it between the viewer, hold the viewer up to the light and look through it, and if it's a polyester, you'll see what are called interference colors, or the reds and green colors. If you take a piece of cellulose acetate film, do the same thing, find a spot that's light, put it between the viewer, the two polarized films, hold it up to a light source, it will be dark. It will be dark black or blue and there will be no interference colors. This identification technique comes in very handy if the film is in relatively good condition and also if it's not edge marked and you can't readily tell if it says nitrate or safety.