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Factory Tinted Window Question
My tinting on the rear window has a sort of patterned look to it...only when viewed from an angle, but you can see small circles repeated in straight lines horizontally on the window.
Anyone have any idea what I'm talking about? Suggestions? |
I know what you're talking about, I've seen it on other cars before, but only when I was wearing polarized sunglasses. It has to do with the UV protection, I think.
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I know what you are saying but my factory tint doesn't have that
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Re: Factory Tinted Window Question
Originally posted by FighterPilot My tinting on the rear window has a sort of patterned look to it...only when viewed from an angle, but you can see small circles repeated in straight lines horizontally on the window. Anyone have any idea what I'm talking about? Suggestions? Update - here's one explanation - the tempering process http://bimmer.roadfly.org/3series/me...w05/16379.html |
It's called "spot-polarization" (seriously). If the glass were completely polarized, you'd receive light in through it that arrived only at a specific angle. What the spot polarization does is attempts to limit the amount of light that comes in through the window from all manner of random angles to cut down on glare. Think of it this way - if the entire back window was polarized glass and you were wearing polarized sunglasses as well, you'd see little or nothing through it because the two would be mutually exclusive polarized filters. The back seats of the Navy's S-3 Viking jet have windows that work on this principle (rotate the inner polarized "filter" to essentially go from clear to blacked out windows to allow monitoring tactical displays in bright daylight).
Make sense? |
funny. I always see this with polarized glasses as well. LOL thanks for clarifying
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Originally posted by Pure Tremble funny. I always see this with polarized glasses as well. LOL thanks for clarifying |
I read somewhere that those "Spots" you see (especially with polarized glasses) is a result of the manufacturing process for saftey glass to ensure the glass breaks in small pieces. I recall that it was from cooling the glass using water jets. Not sure if its true but it makes sense.
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If ya ever go to an amusement park where they have the 3D movies, take the glasses you get off your face. Now rotate them 90 degrees and look at the other people. Notice all the black looking lenses? Thats how things look through very specific polarization filters. Your vieweing a more subdued version of that.
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Yep - excellent example Genom.
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Originally posted by Golfer I read somewhere that those "Spots" you see (especially with polarized glasses) is a result of the manufacturing process for saftey glass to ensure the glass breaks in small pieces. I recall that it was from cooling the glass using water jets. Not sure if its true but it makes sense. What you are seeing is varying light polarization caused by the glass tempering process. The rear window glass is tempered both to toughen it and to prevent shards when the glass does shatter. Tempering is accomplished by prestressing the glass surface with differential cooling. This is done with air jets -- think of an air hockey table. The spacing of the jets determines the checkerboard pattern. The difference in the glass density and index of refraction is very tiny, but your eye is tuned to detect subtle patterns. It's not a problem, it won't get worse or better, and it cannot be polished away. Think of it as confirmation that your glass was properly tempered. |
Another reference:
When viewed under particular lighting conditions, especially when viewed at a grazing angle, faint shadowy spots caused by the air quench process can often be seen in heat treated glass. These spots can be seen in transmission and in reflection, and at most viewing angles, other than directly facing the glass. They become very visible when polarizing sun glasses are worn. They are easily seen on a sunny day when the light comes from the blue sky or is reflected from clouds. They probably would not be visible on a grey, cloudy day. These spots are a normal function of properly tempered glass and are mentioned in section 7.5 of ASTM C 1048 standard for heat treated glass. They are more visible when both lights of glass in an insulating glass (IG) unit are tempered. They are often seen in the sloping tempered back windows of cars, and in Asia they are very obvious in some small truck tempered front windshields. The attached illustrations show a simplified explanation of the physics involved in their formation: Illustration #1 shows how light coming from the sun is made up of oscillating waves in planes at different angles to each other. When sunlight interacts with molecules of air and very small dust particles in the sky it becomes polarized. The polarized waves vibrate mostly in one direction. Illustration #2 shows how the quench jets in a tempering furnace cannot cool the glass completely uniformly. As a result some areas are cooled faster than others resulting in differential shrinkage which creates areas of different compressive stress. When glass is stressed in the quenching operation, with its corresponding very slight change in density, it becomes birefringant or polarizing, that is it partially blocks or passes polarized light. The greater the stress the greater will be the polarizing effect. Illustration #3 shows schematically how vertically polarized light can easily pass through the vertically polarized sections of the glass to give bright spots but it is diminished when passing through other areas polarized at a different angle to give relatively darker spots. The spots are not an indication of absolute tempering level; they simply show areas of relatively more, or less, tempering stress. Given the nature of the quenching process it is physically impossible to quench a plate with absolute uniformity. The degree of tempering and the uniformity of tempering throughout the plate can be tested by other methods such as surface stress instruments or by examining the break pattern after fracture. |
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