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Window tinting films are measured in visible light transmission levels (or VLT). This means that when we discuss a particular film, be it for fitting to a car or any other application, we normally refer to it with it’s VLT value. VLT is measured in percentage ( % ), so if you hear about a tint product being referred to as a percentage, it is the VLT that defines that percentage value.
For example, a tinting film referred to as Charcoal 5% is a charcoal coloured tint with a VLT of 5% and likewise a film referred to as green 50% is a green coloured tint film with a VLT of 50%. But what does the number actually mean?
Well, in simple terms the VLT value is the percentage of visible light that will be allowed to travel through the window tinting film from the exterior face side of the film to the interior side. This means that a 5% film will only allow 5% light travel through and a 70% film will allow 70% light to travel. In effect, this means that lower VLT films will appear darker. For instance, it is normally 5% tints that we will see on limousines for privacy.
So, fitting a 5% tint to a window will allow 5% light to travel through the glass from outside to inside, right? NO! Because we need to take into consideration the actual VLT of the window before the tint is even installed. There is no such thing as a piece of glass, no matter how clear it appears, with a VLT of 100%. This is because glass naturally filters out a little bit of visible light.
Lets look at car window tinting as this is one area where we speak of VLT often due to the fact that many countries have laws in place limiting how dark car windows should be tinted. Most modern cars come from factory with windows reading a VLT somewhere between 72% and 78%, depending on manufacturer, model and country. Say, our example car’s windows read at 72% and we add a 50% window tinting film, what is the new and final VLT of our car’s windows after installation?
The sum is very simple: V1 x V2 = V3 (Where V1 is the original VLT of the glass before tinting, V2 is the VLT of the window tinting film and V3 is the final VLT value of the glass with tint film applied).
Our car’s windows original VLT = 72% and the tint = 50 %, thus V1 = 72 and V2 = 50
The sum is 72 x 0.50 giving us 36, which we will express as a percentage. So a window with an original VLT of 72% will then have a VLT of 36% after application of a 50% film.
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Source: http://ezinearticles.com/?How-to-Calculate-Window-Tint-Visible-Light-Transmission-(VLT)&id=5455994
