kbuskill
***CAUTION*** I do my own stunts!
I'm actually a journeyman glazier. Glass is what I do and I understand tints, colors, light transmittance, etc although I deal with mainly commercial glazing especially towers (shyscrapers). I did do Auto Glass though for years a long time ago. Nearly all of today's Auto Glass is tinted to some extent especially windshields. Only the absolute cheapest cars have any " clear" glass and that would be the side windows. And even that doesn't have 100% light transmittance. Most windshields are tinted at 70-80% factory. If you add a 70% film on top of that, it doesn't stay 70% total, the film blocks 70% of the 70% the windshield was letting in. Here is a good explanation from the window film industry:
"Car window tinting films are measured in visible light transmission levels (also known as VLT), and these levels are represented as a percentage of the visible light transmitted through the windows. So, when you see a window film being referred to as a percentage, this is the VLT. “Let’s do 35% on the back windows,” means 35% of the visible light will pass through the window film.
In the simplest terms, the lower the percentage, the darker the window film will be. A 5% VLT film is very dark as it only lets though 5% of visible light and a 70% film is very light as it lets through 70%. There is, however, one little complication, and that is that windows as installed by the factory do not allow 100% of light to pass through. Most auto makers very slightly tint their glass, usually with a VLT of about 80%.
So how do you calculate the true VLT of your windows?
To calculate the actual VLT of the glass with film applied to it, you need to multiply the VLT of the window tint applied by the VLT of the glass. For example, applying a 5% film to glass with an 80% tint to it, you would multiply 5% x 80% = (0.05×0.80)x100 = 4%. So the glass and window tint would have a combined VLT of 4%."
BTW all glass, even clear, blocks a small amount of light depending on it's composition and thickness among other things. 1/4" clear has a higher % of vlt than 3/4" clear; tempering changes the vlt; any laminated safety glass has a lower vlt than a solid glass, etc.
Edit: so 70% film on a stock 80% windshield is .70 x .80 x100= 56%..
Well I guess if you want to get all technical and shit... lol
I was just saying it's not like guys running 35% on there windshield and telling you it's no big deal. I did it for the heat rejection and to try to keep the already crack prone dash in better shape from the UV rejection.
Thanks for the lesson.