The short answer
VLT means visible-light transmission: the percentage of visible light that passes through a window. Arizona's statutory definition measures the product or material together with the vehicle glazing. That distinction matters because a window is rarely a perfectly clear starting surface. Factory glass may have a light tint, solar treatment or privacy coloring before aftermarket film is added.
Film ratings describe a product under specified test conditions, while the finished vehicle reading reflects the combined assembly. For example, multiplying a hypothetical 80-percent glass transmission by a hypothetical 40-percent film transmission produces about 32 percent before accounting for testing tolerances; the labels should not simply be added or treated as the final result. Manufacturer performance sheets may also distinguish film-only values from film-on-glass values and may use different glass in their tests. A knowledgeable installer should inspect the existing glazing, choose a product with a reasonable compliance margin and avoid promising an exact completed reading from the film name alone. Arizona also regulates luminous reflectance, so VLT is not the only legal measurement to consider.
