Introduction Polyethylene terephthalate (PET) film is widely used in automotive window films, architectural films, optical films, protective laminates, and other functional film applications because of its excellent mechanical strength, dimensional stability, transparency, and processability. However, when PET film is used in applications exposed to sunlight—particularly automotive glazing—controlling solar heat becomes an important technical challenge. A transparent film must allow sufficient visible light to pass through while reducing the amount of solar energy entering the vehicle or building. Simply making the film darker is not always an effective solution because visible-light absorption and infrared heat management are two different issues. One