Re: pinhole window splotches

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Apr 28 2004 - 18:29:21 PDT 

Message-Id: <l03110713bcb60a16d2e8@[192.168.112.30]>
Date: Wed, 28 Apr 2004 18:29:21 -0700
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole window splotches

>Hi Eric

I didn't get the original question but here's one idea.

The windshields on most cars are complex layers of glass and plastic.
The windows are heated over flames.
The flames leave stresses in the plastic layers which then become birefringent.
When polarized light from the sky passes through the birefringent material
of the window and is reflected off the inside of the window to pass through
the layer a second time on the way out, and then at last is viewed through
polarizing sunglasses it allows you to see the pattern of polarization
created by the birefringence.
Different manufacturers use different flame patterns so the polarization
pattern is different.

I never heard about aluminizing.

Paul D

Hello again,
>
>Perhaps someone has already returned a response but I wanted to update my
>question a little based on yesterday's research.
>
>Upon entering various keywords on Google (windows, polarization, cars,
>etc.) I found what may be the best explanation so far. The description of
>a "polka dot pattern" caught my eye. The explanation was perhaps a little
>out of my normal realm of expertise and certainly not completely explained
>but to the best of my understanding . . . Thin layers of aluminum
>coatings are added to the window and a polarized filter reveals an
>interference pattern. A more expensive vehicle as a better application of
>the aluminum thus a smaller, more consistent pattern.
>
>Is this correct? Please help if you can. Thanks, Eric Plett.
>
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