Performance metrics & physics
Lumens
Lumens is the standard unit of luminous flux, measuring how much visible light is emitted by a source; it is not directly used in window film specs but provides intuition about light levels.
For context: direct noon sun delivers about 100,000 lux on a flat surface. A bright office at 500 lux. A street lamp at night ~10 lux. The dynamic range matters for understanding why VLT regulations exist.
Window films don't quote performance in lumens, but the underlying physics is about controlling how much luminous flux passes through.
For Singapore drivers facing direct sun, even a "clear" 70% VLT film reduces transmitted lumens by 30% — meaningful for glare reduction without compromising visibility.