Window tinting for automobiles, trucks, and recreational vehicles all have visible light transmission laws that vary from state to state. Visible light transmission means the amount of light passing through any piece of glass to the interior of the vehicle. For example a clear piece of glass would have a 100% visible light transmission.
All of the window film manufacturers of today offer their products in different visible light transmissions or VLT's which they can be referred to as well. I will use SolarGard HP Smoke for an example. This particular window film comes in VLT's of fifty percent, thirty five percent, twenty percent, and five percent. They do not offer any different light transmissions in between the ones provided. It is important to note that these VLT's are all based on 100% clear glass. So for example a 35% visible light transmission installed on a factory supplied automotive glass with a 75% visible light transmission would give it approximately a 26% visible light transmission. So it is important to know the VLT of the glass before the installation of any window tinting. This can easily be checked with a meter which slides over the edge of the glass and reads the visible light transmission. Any reputable window tinting business will have one of these meters, and I am sure will check your glass for no cost. If one is not available in your area, and you have trouble attaining a meter they are available for purchase with a starting retail of eighty nine dollars.
The benefit of having window film that is too dark is very little. Staying along with our previous example of the SolarGard HP Smoke. The heat difference between the 50% light transmission, and the darkest available the 5% light transmission is a mere 10%. So to capture that example the window film is 45% darker, but only achieving an additional 10% in heat rejection! So darker does not necessarily mean cooler. It simply means more light is being taken out.
Most all glass installed in vehicles today have some sort of tinting to them. It varies from the automobile manufacturer, and the model of the vehicle. Generally they range from 75% on the front two windows, and as dark as 15% on the rear doors and back glass. The windshield is only allowed to have tinting on the top down to the AS1 line marked in the glass or six inches if the line is not marked.