The Reason Polarized Lenses Cause Crazy Patterns in Car Windows
If you put on your sunglasses and notice that your car windows have suddenly become covered in wavering colors, you’re not going crazy. Nor can it be chalked up to a bad window tinting job. And it doesn’t mean that your lenses are defective.
What you’re experiencing is just a simple matter of physics.
Many rear and side car windows are tempered — designed to shatter without causing sharp pieces. During the tempering process, the glass is heated and then rapidly cooled to room temperature. The surface of the glass cools much faster than the center of the glass and contracts, causing compressive stresses, while the center of the glass expands because of its temperature, producing tensile stresses.
What you’re seeing when you notice a checkerboard or rainbow pattern in a car window is an effect called stressed birefringence. Stress on optically clear materials often produces birefringence, which basically means that the material changes the polarization of the light.
The pattern you may notice in rear car windows is the strain pattern from the tempering process.
Those little dots or lines you see are sections of the glass that partially polarize light on a horizontal axis. Since your polarized lenses have a vertical axis, that light is blocked, which is why it appears as dark dots or lines.
It’s essentially like having two polarized filters, each aligned perpendicular to one another. One traps light traveling vertically, the other traps light traveling horizontally, and so nothing gets through the combined filters.
You can sometimes see this pattern without polarized lenses because there are other factors in the environment that can cause light to become polarized. So, if light is already polarized, and then it encounters the window, it’s essentially passing through a second filter. The dots are more defined while wearing polarized lenses because your lenses are a true filter designed specifically to polarize light. The car window only partially polarizes light.
The pattern isn’t consistent across all cars because manufacturers use different tempering processes. In some vehicles, the pattern may look like diamonds, on others it’s dots, and others are more square in appearance. It depends on the heating pattern that was employed during the tempering process.
As for the rainbows in car windows, this is also an effect of stressed birefringence.
Incident light from outside the car passes through the rear or side windows. The stresses caused by the tempering rotate the polarization. The amount of rotation depends on the wavelengths, thus resulting in the varied colors. Again, what you’re seeing is the stresses in the car window caused by the tempering process.
This phenomenon is somewhat similar to a prism or a raindrop that causes light to bend, or refract, thereby showing the different colors that make up light. You only notice the rainbows in the car windows while wearing sunglasses because your polarized lenses filter out particular wavelengths, allowing you to see the patterns.
You can also observe stressed birefringence in objects such as plastic utensils or cups. If you put on polarized lenses and hold a plastic cup in front of a computer screen, you should be able to see the same rainbow pattern you observe in a car. This method is known as photoelasticity, and scientists and structural engineers use it to determine the stress distribution in a material.
One theory as to why the rainbow effect is so much stronger in windows with window tinting is that the film used on window tinting is most often made up of polyester, which is a form of plastic. In the same way that you can see the stresses in a plastic cup or spoon, the rainbow pattern you’re viewing in your tinted car windows is a result of the stresses within that plastic film.
The reason you don’t see any rainbows or patterns in a windshield is that windshields are made of laminated safety glass, not tempered glass.
If the rainbow effect is distracting and driving you a little nuts, then you may want to consider switching out to non-polarized lenses — at least while you’re driving. And if anyone else comments on the rainbows, now you’ll have an answer for them: