When choosing the shade of your tints, it’s important to follow the law. Different states have different regulations about how dark your car windows can be. The shade of window film is measured by the percentage of light that can pass through. This is called visible light transmission, or VLT. The lower the VLT is, the darker your windows will be. In California, the law requires that your front windshield and front side windows have a 70% VLT. However, you can tint your rear and backside windows as dark as you desire.