Yenya's World

Wed, 21 Sep 2005

Car windows polarized

Do you have a polarizing filter or polarizing sunglasses? It is interesting how many unusual things can be seen through the polarizing filter.

car window polarized

I have found that when you look at car windows through the polarizing filter, strips, spots, or chequered pattern can be seen. On most cars only the rear window shows this behaviour, altough I have seen an old Wartburg, which had this pattern on all windows, including the windscreen.

Does anybody know how this pattern is created, and why only the rear windows have this? Does it have something to do with rear window heating/defrosting?

Section: /world (RSS feed) | Permanent link | 9 writebacks

9 replies for this story:

Petr Holub (hopet) wrote:

Hi Yenya, it think it might be due to tensions in the glass (may be bad tempered glass with lots of tension in it) and maybe some other inhomogenities. The analysis using polarizing filter is often used for tension analysis of transparent material. - Petr

Yenya wrote:

But this does not explain why rear window only and why such (semi-)regular pattern.

Keith wrote:

Check out the article in the following link: Scroll all the way to the bottom to the section 'A phenomenon' In a nutshell, it's the tempered glass that makes the lines/spots/hashes and the windshield/windscreen is laminated, not tempered.

mansoor wrote:

ull only see this in rare windows which have sunflim sheet coated on them. if u ever see how they put sunflim (dark paper) on a cars rare window ur question will be answered.

GodFather wrote:

Those brownish stripes you see on the rear window of many cars today is the defog/defrosting mechanism. When the rear window is fogged, activating this device warms the rear window slightly to remove condensation on the glass, thereby defogging it. I can't explain the other two things you are seeing in that rear window but it is not due to stress.

Yenya wrote: Re: GodFather

I of cource know that the brown stripes are from defrosting wires. They are visible even w/o polarized sunglasses. I am talking about the white spots (in my picture), sometimes seen as light stripes or checquered pattern. Other have suggested that it is caused by a manufacturing process.

Roman wrote:

it has to do with tinting that the companies use on the window.

Kurt wrote:

The patterns you see are caused by a matrix of heating jets (like 1000 little blow torches) used when tempering and shaping the glass. All auto safety glass is tempered and then quick-cooled to create tension by design. This tension is what causes the glass to break up into small pieces when broken instead of razor-sharp shards of jugular slicing death.

Jim wrote: Will the Real Answer Please Stand UP

I worked for the company that made the equipment that produces the glass that gives the pattern that you see. There are no "heating jets" in the systems that are am familiar with. The tempering equipment is typically a highly insulated box with electrical heating elements that raise glass to about 650 C. Perhaps a little hotter in bending systems. The patterns are stress patterns caused by "air jets" that rapidly cool the surface of the glass. Rear windows typically are in a "ring mold" and stationary during the tempering process (cooling portion). Side lites (side windows) have typically only a single curve and can be tempered continuously moving and thus not have the same noticeable pattern of stress. The pattern seen is light being scattered differently do to the tempering process. The pattern is caused by some sort of random polarization that changes from one area to another. Your sunglasses fully block light that is polarized perpendicular to the way your glasses are made. Some light gets through, some doesn't - this causes the pattern. I just inspected a building last night looking for stress patterns in the tempered glass. This is what I do for a living. I'm just not making it up to give you an answer. I could be wrong - but I'm probably pretty close to being right! Enjoy. PS Some systems for bending glass use molds that press the glass, some are gas fired. The stress is caused during the cooling process since the stress is nearly fully relieved by heating glass to 650 C.

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