When the sun’s rays hit a surface coated with dyed film, the dark color absorbs the sun’s light and heat. The natural flow of air around a car then whisks it away from the glass, reducing how much penetrates to the vehicle’s interior. Dyed films tend to be less expensive than other options, but some can discolor over time.

In metallic films, extremely thin layers of reflective metals rest between layers of polyester—you might be able to identify a car with this type of film by its slightly mirrored look. These metals do an excellent job of blocking harmful rays and reducing interior heat, but there’s one potential drawback: the metal in some of these films can slightly distort radio, phone, or GPS signals.

Made using a combination of dyed and metallic materials, hybrid films boast many of the benefits that these separate techniques offer. For instance, combining a gray dye with a thin layer of titanium produces a window film that’s neither too dark nor too reflective.

This durable, scratch-resistant material was developed for the thermal-protection systems of NASA’s space shuttles, and it works exceptionally well on earth. Ceramic films are among the thinnest on the market and are even available for windshields, since they can be made completely transparent. Beyond their automotive use, ceramic films are a popular choice for home windows, blocking out heat in the summer and holding it in during the winter.