[sputnik]

Recently I've spent a whole lot of time staring at my Insight's windows, working through window tinting and vent visor questions. ( we really need to plan better weekends!)

There is an odd optical effect on the hatchback glass, noticable when viewing the glass from outdoors, and wearing polarized sunglasses....the entire hatchback window surface appears covered in whitish thumb-size spots! The pattern is very regular and covers the whole surface.

I wondered if the glass was already tinted (is there a way to tell?) and if this could be an effect from a tinting film. My husband thinks that the glass itelf has become polarized due to the heat from the defrosting wires. We checked the hatchback glass on two other cars w/ defrosting wires, but the pattern was no where near as evident as it is on the Insight.

8)

 

[james]

I see this spotted polarization pattern on most auto glass, especially hatchback glass, and sometimes on things like patio doors. I'm not sure exactly what causes it, but I'd bet it's a side-effect of the tempering process that makes it shatter into little bits when it breaks.

 

[b1shmu63]

It may infact be a triple layer glass with a plastic layer in the middle. reflected light can be polarised. That is why polarised glasses can reduce glare.

 

[james]

I don't think so. Windshields have the triple layering, I think, so that when they shatter all the pieces are held together by the plastic. But the rear glass doesn't have the plastic layer. When it breaks, it crumbles into millions of pea-sized chunks. After two months, I still was vacuuming up a few of them

 

[james]

OK, you went and made me curious, so I looked it up. Tempered glass is made by cooling the surfaces of the glass faster than the inside. This creates surface stress that in effect holds the piece together. One way this is done is by jets of air blowing on it. So each of those little spots is where the air jet blew on the window as it was made. The glass there cooled a bit faster, so has different stresses, and is maybe a bit thinner or thicker, than the area between spots.

The curious thing about tempered glass is that it has to be made all in one piece: if you try to cut or shape it, it shatters.

 

[b1shmu63]

Hey, I saw the spots on it today even without plarized glasses.

Congradulations on finding that info James.

 

[sputnik]

Thanks b1shmu63 and James for the replies and the useful information.

Yesterday afternoon, wearing polarized shades 8) , I clearly saw the spot pattern on the rear window of a car in front of me (not an Insight). It's a relief to know that I haven't acquired insect vision (can you imagine the wierdness of seeing like a bumble bee?!) and that the pattern is a result of the glass tempering process.

I wonder if this same glass tempering process makes it difficult for a spray-on tint to adhere (glass surface characteristics vary at each spot) and maybe that's the reason window tinting is done w/ stickdown films?