It’s not often that we give much thought to the glass windows in our cars – at least not if they’re intact and clean.
If you have a car long enough, however, you eventually become familiar with every imperfection, scratch and stone chip.
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Light spots on the rear window
A phenomenon seen on the rear window of many cars is that a pattern with bright spots often becomes visible in direct sunlight.
The spots become particularly obvious if you look through polarized sunglasses, and as a car owner you may wonder what they are due to.
If the car is reasonably new, one may also wonder if the spots are a defect that should be repaired under warranty.
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The spots are not a defect
The bright spots on the windshield are not actually a defect, but instead a byproduct of how a rear window is typically manufactured and how light is reflected through the glass.
The majority of all cars have rear and side windows made of tempered glass, which is stronger and more impact resistant than regular glass.
As a rule, the stains do not occur on the windshield, as this is made of laminated glass where two layers of glass are glued together with a plastic film in between.
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This is how the spots appear
During the manufacturing process, the glass panels are heated up to over 500 degrees and then quickly cooled down with a number of small nozzles.
However, the glass cannot be cooled completely evenly, and this leads to different parts of the glass surface shrinking at different rates. This leads to a shifting structure that reflects the light in different ways.
The spots are therefore nothing to worry about, nor something to complain about.
Despite this, it happens that car manufacturers have to send out information about the phenomenon. An example of this is General Motors, which in 2021 sent out information to its workshops after a number of customers heard complaints about stained rear windows.
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