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Discussion Starter · #1 · (Edited)
I continue to see misunderstandings about window glass, window tint, the specs, and the math involved. I have made little 1 or 2 sentence suggestions in various posts, but finally wrote something up to try to help.


Glass markings

ASX is a glass safety classification - not a reading of or specification for this piece of glass's actual tint.
"AS1 - Glass with this designation is typically used on the vehicle's windshield, as AS1 glass must be laminated and allow no less than 70% of incoming light to pass through it.
AS2 - This glass designation describes tempered glass with 70% light passage and may be used in any non-windshield application.
AS3 - Glass with an AS3 designation can either be laminated or tempered, but it does not permit more than 70% of incoming light to pass. This glass cannot be used on the windshields nor the driver or passenger side front windows."

I happen to own a 99 Durango with 'double dark rears' that a Trooper had metered for me at 89% front seat, 11% rear seat VLTs. (The Trooper wanted to call me out on the rears, but I reminded him that was OE/DOT glass - not tint film.)
I went and looked through the markings (too small for pictures) and the glass lined up exactly with class specifications.
Front windshield, light tint - marked AS-1 - which is front laminate, greater than 70% VLT.
Front seat, light tint - marked AS-2 - which is front tempered, greater than 70% VLT.
Rear seats, dark tint - marked AS-3 - which is rear tempered, less than 70% VLT.

The scheme works - the marking may include > or < 70 - but that is a class marking, not a spec for this piece of glass. It indicates this is safety glass, what tint group for that safety group, and thus what positions are legal uses.
If it helps, remember that if the glass was marked 70% for exact specifications, someone would be suing somebody when they found 68% or 72% glass.
I bet they always chose their tint points well away from Class divisions to stay clear of problems from factory variances.
Also glass with no tint at all could properly be marked AS1 or AS2, and could have tint group markings of >70.


Percentages of VLT for adding tint.

VLT is Visible Light Transmission - the light that passes through. (Tint 'used to be' identified by blockage - 'blocks 80% blah, blah, blah' - but checking today I can find no Pros referring to tint as other than by transmission value. They thus sell what sounds like smaller numbers, but consumers have mostly come to understand that 20% is cooler - both literally and aesthetically - than 80%.)

So, sunshine shines on a piece of glass, if it is tinted some of the light is blocked, some is passed through.
The light continuing on through is the VLT - transmitted on through.
(You have to have this number to plan a legal tint job.)

On my 16 SXT, the front sides read 82% - 82% of the light passes through, the VLT of the glass alone is 82%.
The other 18% is absorbed, rejected, managed by the factory tint - it is gone, and the number will not be used in any calculation.

I added 35% tint film - the tint film alone is 35% VLT as the manufacturer does not know what my factory glass is, and cannot predict - see ASX above. They expect us to be able to do the math for a legal job.
The other 65% is absorbed, rejected, managed by the tint film - it is gone, and the number will not be used in any calculation.

So, in a 'film tinted over factory tinted window' job the light will lose VLT in the glass, and then lose VLT in the film.
The light loses the complement of the VLT for the glass, and that light then loses the complement of the VLT in the tint.

The light keeps the VLT for the glass, and that light then keeps the VLT for the tint.

The VLTs do not add, subtract, or anything but - they only multiply as percentages reduced to ratios to 1.
The 'effects' of the 2 tints do add to each other, but the math for it is only correct when multiplying 2 ratios to 1.

In my case, the glass passes through 82%, and the film passes through 35% of that remaining light.

The math would be:

.82 (remaining light exposed to film) reduced to .35 of that (light that gets through film) = .287, or 28.7% VLT for the combo.

.82 X .35 = .287, X 100 = 28.7% VLT

28.7% is then compared to the exact wording your legislators used.
Mine (VA) said 'total... transmittance... no less than 35%' with a measurement tolerance of 'minus seven' - so 28% is OK - 27.9 is not (I have Optho letter).

Mine have tested perfect by getting checked before and then again after by applying this method.


Notes:

Film typically has a tolerance - it would be wise for close tolerances to test measured through the glass and (peeled) tint before purchase.

I have only encountered Troopers happy to help someone comply with law - they did before and afters for me on 2 cars.

In 6 different instances I have seen Troopers only with a fork type meter - slips over glass edge - I have never seen a 2 piece that could read a rear window.

In 2 stops by Troopers - tested and passed cars were never stopped again.
(I suspect they keep a peer-to-peer database of checked cars. Fine with me, no sense in wasting everyone's time again!)

I have also only ever seen 2 digits - no decimal. Device probably rounds up from .5/.6 to whole percent.

In looking at other states... I see lot of states clustered around 35% and a lot clustered around 70% - regardless of that states' sunshine or weather. I wonder if legislators have also been confused by % blocked vs VLT% in the past...

Based solely on my example:
VLT Glass"Tint"VLT TintVLT NetNet % (Trooper!)
0.8250%0.50.4141%
0.8240%0.40.32833%
0.8235%0.350.28729%
0.8230%0.30.24625%
0.8225%0.250.20520%
0.8220%0.20.16416%
0.8215%0.150.12312%
0.8210%0.10.08208%


Hope that helps!
 
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