Purple windshields

[Stopper]

Has anyone noticed on certain GM vehicles that when you look at the windshield with polarized lenses, it has a purple sheen to it?Does any one have an explanation for this? Just curious.

[chip anderson]

Probably heat stress (which you see as grid patterns in Ford and many Japanese products) and U.V. filter in windshield.

Chip

[Stopper]

Chip,
It's not the grid pattern and it only occurs on the windshield. I noticed it mostly on GM minivans.

[edKENdance]

I posted about this before. I thought it was some kind of glare coating. I do notice it mostly on minivans though. Not sure of the manufacturer though. Heat stress would seem to be a bit of a stretch I think.

[Joann Raytar]

The Dr. Bill FAQ

What is that polka-dot or diamond-shaped pattern I see on the rear windows of cars? A caller asked this question, noting that polarized sunglasses seemed to create or enhance this pattern. (I have found this to be the case as well.)

Bill responded that the pattern was an interference pattern caused by the polarized sunglasses, although I don’t believe this is actually the case.

Having wondered about this myself, and noting some other aspects about the visibility of the pattern (why do I never see it on the windshield, rarely on the side windows, and almost always on the rear window (also called the backlight), I asked the same question of a fellow involved in automobile design. He believed that the pattern appeared in tempered glass, as a result of the manufacturing process. Tempered glass is designed to shatter into very small (smaller than pea-sized) chunks without creating sharp shards of glass. This is accomplished by rapidly cooling both sides of the hot sheet of glass with cold air streams. Because the outside cools much more quickly than the inside, a stress pattern is created in the glass that causes the glass to break in the desired manner (which you may have seen at the site of car accidents, and next to broken-into cars in the bad parts of town…).

Car windshields don’t use tempered glass, but rather laminated safety glass, in which multiple layers of glass are sandwiched with layers of plastic (or vice versa). This keeps the entire windshield in one piece (in most cases of impact), to keep the driver from having to deal with that amount of broken glass coming his way in an accident, or as a result of dings in the glass caused by pebbles.

While I believe side windows use tempered glass, they may be too thin to exhibit the polka-dot pattern in most lighting conditions.

http://www.pushback.com/Wattenburg/F...red-glass.html

A link with a good deal of information on Polarization (hey, that rhymes):
Polarization Concepts

[chip anderson]

Ed:

By heat-Stress, I ment stressin in the glass accomplished during the cooling process when the glass is moulded. (Sheet is placed on a grid of metal or wood and allowed to cool with or without air blast). You can see the patter of the grid.b You see this more on Fords and Japanese vehicles.

Chip

[edKENdance]

It's certainly a strange phenomenom. The sheen is really uniform. I would think the pattern would be more similar to looking at a glass through a polariscope. Some stressing here and there.

[Jim Schafer]

I have feeling that the product is the PPG Sungate window system. Yes, the same PPG Industries that is the parent company of Transitions Optical.
PPG Industries has three divisions, Chemical (which has a business unit that includes optical products like CR-39® and Transitions®Lenses.) Coatings and Resins (PPG Paints, Automotive Coatings etc) and the Glass Division that includes architectural, plate, aviation and auto glass). PPG is the number one global supplier of automotive windows and coatings (paints and clear coat systems).
Sungate is an architectural treatment that dramatically reduces glare and also reduces Infra Red Radiation. When applied to automotive glass the same benefits allow a parked or moving vehicle to be somewhat cooler in temp than a non Sungate fitted car. This allows AC systems to be more efficient, also smaller in size for better gas mileage.
GM jumped on this first with the intro Pontiac Aztec SUV and Olds Sillouette mini van. Most GM models have this feature now, I believe Nissan and Toyota along with some European manufacturers also use Sungate as OE glass.
I have heard feedback that folks driving these cars hardly ever pull out their sunwear for driving. The windows do all the work in cutting glare.
I never noticed the purple look on the windshield, I will now keep an eye out for that effect.
What originally caught my eye and got me to question the Glass folks about this product was how straight on looking at the side windows, the glass appears a light to medium tint titanium grade after market gray window film. Then, looking at an angle, the same windows have the G-15 Ray-Ban color.
It would be interesting to note how these features in new cars will affect future sales of driving lens type sales. Of course open sports car drivers will always need the polarized driving sunlens!
Jim

[Stopper]

Thanks for the imput from all. Again I wasn't referring to the checker board pattern just the uniform purple sheen that is seen and why is it only seen when wearing polarized lenses. I assumed it was some type of tint to block short wavelength light but I was just curious why it was only visible through polarized lenses. I can't explain that one.

[Pete Hanlin]

You can particularly notice the purple coloration to which Stopper is referring on a Chevy Lumina Van (if you can find one- they had a huge front window that looks pure purple through a polarized lens).

I am going to throw a guess in and say that it might have something to do with bi-refringence (that is, there is a difference of index of refraction among the layers of materials composing the windshield that is resulting in this effect). I don't know why polarization would enhance this effect. Also, I would imagine Jim would be the resident expert on matters like this, so I'm guessing his theory about the IR glass is probably pretty spot on.

Curiously, as a side note, isn't it interesting how internal stress in windshields doesn't seem to cause a problem for anyone- but internal stress in polycarbonate lenses is bally-hooed as "poor optical quality?" Particularly on the rear windshields of some Ford products, you will notice a "polkadot" effect when looking through a polarized filter.

[Blake]

We have an Olds Silhouette minivan, and I've noticed the purple hue even without polarized lenses, although it is more apparent with them.

[edKENdance]

Do you know of anyone that has a problem with the purple hue that drives one of these vehicles? You would assume that someone wearing polarised lenses on the inside of the van would notice this hue as well.

[Pete Hanlin]

My grandmother owns one of these vehicles, and I've driven it wearing polarized sunwear. As I recall, you don't notice any of the effect from inside the vehicle...

[Johns]

I have a 1977 Ford F-100 pick-up, and the only window that shows a patter through a polarized lens is the small vent (triangle) windows in the corner of the doors. (??)

[yzf-r1]

i thought the puple colour was due to an AR coating applied to the windshield because it was so uniform.

In england, is is Renault cars on which this phenomenon is most apparent. The purple sheen is clearly visible without a polariser. I am not sure whether it is enhanced py polarising sunglasses - will have to try sometime