Tint is created when tinting film is bonded onto a piece of window glass. Tinting film is usually made out of clear polyester film with a very thin and even layer of tinting agents such as dyes and/or metals deposited onto the film.
A common misconception is that window tint is dark, and night-time driving is impossible when a car is tinted. The truth is that there are films of ANY darkness that suits your preferences. Also, unlike sunglasses that do impair your ability to drive at night, tinting film is designed to reduce glare and not impede night-time driving. Another misconception is that window tint is bonded onto the outer surface of auto glass. Window tint is applied on the inner surface which also protects the film itself from flying debris outside the car.
Once tinting film is applied to a window, the characteristics of how visible light comes into the car changes. Normal auto glass without tint reflects around 5% of visible light (known as VLR%), absorbs another 5% (known as VLA%), and transmits 90% of visible light (known as VLT%) into the car.
Depending on the type and quality of tinting film applied to a window, these percentages change dramatically. Some tints are more reflective, and others absorb more light. The number you will most commonly see is the VLT% (Visible Light Transmittance). VLT tells you how much visible light is allowed to shine into the car, and also indirectly how dark the tint looks.
Tinting film doesn't just block visible light. It also blocks harmful cancer-causing ultra-violet (UV-A and UV-B) rays as well as infra-red (IR) rays that cause heat to build up in your car; however, how effective a film is in blocking these rays depends on the type of film, the manufacturer, and the quality of installation.