Net VLT Definition and Calculation
Most factory glass isn't clear.
Net VLT (Visible Light Transmission) is a very common term in automotive tint laws. The "NET" portion refers to the combination of both glass and film, and not just the film alone.
Automotive glass usually contains some level of factory tinting. Most automotive glass is in the range of 70% - 85% light transmission. Only the clearest glass is above 90% and is not very common in most markets. When the glass is coated with window film the resulting light transmission is lower than the film alone and is called NET VLT. If the glass is 80% VLT it will darken the film by 20%. A simple rule of thumb is NET VLT equals the transmission of the glass multiplied by the transmission of the film. This method is not perfect but it is close enough to keep you and your customers out of trouble.
Here is an example using 80% glass and a 35% film:
VLT of Glass times VLT of Film = NET VLT
80% X 35% = 28%
The NET VLT of this combination is 28%. Obviously you cannot use 35% film in an area that has a NET 35% law. Installers in these areas should keep a VLT Meter on hand to test the glass prior to an installation. Perform the math as outlined above and recommend the correct film for your customer. You will earn repeat business through your knowledge and honesty while protecting your customer from unwanted encounters with the local police.