I've seen these "spots" on windows every day, but I've never been able to explain how they got there, or is it an optical illusion? Even the camera has  hard time recording the image.

 

 

I have always wondered what caused that.

Take your polarizing sunglasses off.

wasn't wearing anything with polarizing lenses.

https://www.google.com/search?q=birefringence+in+auto+glass&ei=1BeLVeK-CMuqyAT2gaOwAg

I've seen people say it's from the aftermarket window tinting to the defroster on-off cycles creating abnormalities to your polarized sunglasses.  It all goes against the stuff I learned in physics class.  Some people insisted it was only for older cars, but I have seen the phenomena  on brand-new cars too.

 

The most reasonable explanation I found was:

 

These spots are caused during the tempering process. They are intentional stresses which are created in the glass to make it stronger and to control the way it breaks in an accident. The result of this is that it breaks into thousands of small pieces instead of as few large knife-like pieces..

Edited July 13, 201510 yr by TomoHawk