hi, they use "light meters" to test the VLT (visual light transmittance) if your car has 35% film on it, it lets in 35% of the light, 5%, 5% of the light etc. if your car like 90% of the cars today has a die in the glass that's added during manufacturing, 35% film applied to your car will be on average 27-32%, most company's like the one i work for now provide a 40% film, when once applied to said vehicles makes the VLT 35%. so whatever company you decide to go with please make sure they are aware of this. brands that are affected are all Toyota's, honda's, VW's, mitsubishi's, audi's, BMW's, merc's, pug's, citro's, some holdens, all fords and subaru's t, the list goes on, hope this helps.