Math not possible?

Kruppa · November 20, 2018

Hey guys, I'm a customer, not an installer. I've had many vehicles tinted over the years, always wanting to get an even appearance front/rear on my SUVs with factory privacy glass in rears which adds some complexity. I have always used math to figure it out and it's always worked nearly perfectly for me yet every time I use a new place they say "it doesn't work like that". Can somebody tell me if I'm wrong please? I'm well aware below makes it sound like I know the answer already, but I am far from a window tint expert and am interested to get installer opinion.

Ignore all legality, product offerings etc. as it relates to %'s here I'm just doing it as an example. I also understand with all windows the same % the front will still look lighter than the rears due to light entering from the windshield. Also I know just because a product says it's 20% VLT, one batch could possibly be 19% and another 21% etc. but let's just assume that is constant. Also of course privacy glass and aftermarket tint can have varying hues etc.

Let's say my front windows meter 20% after film is applied and I want the rears to read the same VLT. If my factory rear privacy glass reads 40% and the aftermarket tint applied is 50% VLT, math (0.4 * 0.5 = 0.2) says it will yield 20% and therefore match... right?

I've always tried to get the front 5%-10% darker to compensate for the windshield light and it's worked pretty well.

Edited November 20, 2018 by Kruppa

jh812 · November 20, 2018

What exactly are you driving? Only reason I ask most factory windows are 20ish with some being 30. BMW I think has some that light.

He's right though that math is not right. I've proven it wrong several times with a meter, and the lighter you go the worse it gets.

Kruppa · November 20, 2018

2 minutes ago, jh812 said:

What exactly are you driving? Only reason I ask most factory windows are 20ish with some being 30. BMW I think has some that light.

He's right though that math is not right. I've proven it wrong several times with a meter, and the lighter you go the worse it gets.

Sorry should have specified, that is not my actual factory privacy glass VLT. It was just easy math with 40% for an example

Very interesting, so what is the preferred process you go through with a customer who would say he wants even appearance of darkness all around?

jh812 · November 20, 2018

9 minutes ago, Kruppa said:

Sorry should have specified, that is not my actual factory privacy glass VLT. It was just easy math with 40% for an example

Very interesting, so what is the preferred process you go through with a customer who would say he wants even appearance of darkness all around?

I meter everything, that's the only way to know exactly. You do have a good knowledge of it though. I explain just like you said the fronts can be just a tad darker and still match up. 20 imo on most vehicles does that just well. With the film I carry anyways. Also you can always get a small sample installed and meter that to see where things land.

Kruppa · November 20, 2018

14 minutes ago, jh812 said:

I meter everything, that's the only way to know exactly. You do have a good knowledge of it though. I explain just like you said the fronts can be just a tad darker and still match up. 20 imo on most vehicles does that just well. With the film I carry anyways. Also you can always get a small sample installed and meter that to see where things land.

Thank you very much for the quick replies. I’ve never asked for a small sample installed and then meter as a test. Do you think that’s a reasonable request that most shops would say ok to doing?

Bham · November 20, 2018

I have a piece of plate glass that meters 35%. I just put a piece of 40% film on it and it now meters bouncing 13%-14%.

By the formula (VLT#1 x VLT#2) x 100 = ?? it works out to be correct.

Now that being said,,, I also used 70% ceramic on a 20% factory glass on an Acadia (( all metered before hand to be sure)) and after install it metered 15%. Which again, by the formula comes out to be correct.

I also know that 20% on top of 20% usually comes out around 4%, which again works out by formula.

So i'm not sure what the problem is here? Maybe I'm missing something :dunno

Edited November 20, 2018 by Bham

jh812 · November 20, 2018

19 minutes ago, Bham said:

I have a piece of plate glass that meters 35%. I just put a piece of 40% film on it and it now meters bouncing 13%-14%.

By the formula (VLT#1 x VLT#2) x 100 = ?? it works out to be correct.

Now that being said,,, I also used 70% ceramic on a 20% factory glass on an Acadia (( all metered before hand to be sure)) and after install it metered 15%. Which again, by the formula comes out to be correct.

I also know that 20% on top of 20% usually comes out around 4%, which again works out by formula.

So i'm not sure what the problem is here? Maybe I'm missing something

Try it on standard automotive glass and anything in the 35 to 45% range. It's usually a few points off. Now if you're wanting something to say yeah round about this it's fine. If you're sitting at an inspection station though try arguing math with them :lol

No asking for a test piece isn't unreasonable imo. I do it all the time for customers.

Bham · November 20, 2018

16 minutes ago, jh812 said:

Try it on standard automotive glass and anything in the 35 to 45% range. It's usually a few points off. Now if you're wanting something to say yeah round about this it's fine. If you're sitting at an inspection station though try arguing math with them

No asking for a test piece isn't unreasonable imo. I do it all the time for customers.

Varying degrees of blue-green-yellow factory “”clear glass”” makes it very hard to get right on the number. Not to mention quality of meters. Cheap meters and meters with bad batteries will always read low. And there is no way to calibrate them, only to check if they are calibrated. :crazy

and I agree, a test piece is not out of the question. Grief about having to do it would be a flag for me as a consumer.

DynamicATL · November 20, 2018

Personally, I think the math is correct...that is the best formula you can use to see the total VLT. However, there are too many variables involved which means you will not always hit that number. Your stock glass % varies car to car, window film % varies not only between batches but many films are darker/lighter than stated on the box, and even the meter you are using can vary in readouts. Some films metering the same out the box can drop 3%-5% while other 5%-7% once again depending on the stock glass. For example, 3M Crystalline 20 meters exactly 20%, but on a BMW X5 front doors metered 13%. So since the visible light was reduced down to 1/5, if you multiply the 13% it shows the stock glass would be 65% which would be much lower than it should be stock. So use the math as a guide to give you a roundabout answer but know final results can be different.

Edited November 20, 2018 by DynamicATL

Bham · November 20, 2018

All of this only gets WORSE when they start wanting to match the color of the rear factory glass. Outside in is one thing, but inside out always makes them complain. (Toyota yellow glass on the front doors is my biggest headache in this department )

juss sayin :heh