One-way window film, also known as privacy window film or one-way mirror film, is a type of material applied to windows that allows one side of the glass to appear reflective and mirrored, while the other side maintains visibility. It works by using micro-thin layers of metal or reflective particles that control the passage of light.

During the day, when the exterior is brighter than the interior, the film creates a mirrored effect on the side facing the brighter environment. This enables people on the brighter side to see their reflection and prevents those on the darker side from seeing in. However, at night or when the interior is brighter, the effect reverses, and people on the darker side can see out, while those on the brighter side can't see in.

One-way window film is commonly used for privacy and security purposes in homes, offices, and vehicles. It can help reduce glare, block harmful UV rays, and improve energy efficiency by reducing solar heat gain. It's important to note that while this type of film provides privacy during the day, it's less effective at night when interior lighting can negate its reflective properties.