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 ceramic, or metal, deposited onto the film.
A common misconception is that window tint is dark and nighttime driving is impossible when a car is tinted. The truth is that there are films of any darkness that suits your preference. Also, unlike sunglasses that do impair your ability to drive at night, tinting film is designed to reduce glare and not impede nighttime driving.
Once tinting film is applied to a window, the characteristics of how visible light comes into the interior 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). Almost all the official names of films include the VLT%. The VLT% tells you how much visible light is allowed to shine into the car and also indirectly how dark the tint looks.
For maximum heat rejection, look for films with a high solar energy reflectance rating.