- #1
RedneckPhysics
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Hi all, I'm new to the forums (in posting at least!). I recently graduated with a B.S. in Applied Physics, and have viewed topics here on and off throughout my educational experience. I definitely love this forum!
I've been fortunate enough to have the opportunity to buy a new car, and recently had the windows tinted. The tint job itself is another story altogether... not the best quality tinting material and workmanship in the world. Now, I know some of you have probably seen this effect in person before: when wearing polarized glasses, the tinted windows seem to impart coloration to scenery viewed through the window.
I'm a bit weak in the optics discipline (I focused on E&M, Engineering Physics and Laboratory Electronics and Automation), but I have had some radiometry, and have extensively studied the causes of induced birefringence via the Faraday Effect. I was hoping to run the issue by everyone here on the forums, to see if anyone knew more about what exactly is happening with the effect.
So, first, some background on the problem:
When I first got the car, I immediately started using a pair of dark amber (almost a bronze) tinted polarized glasses for daytime driving. I could immediately tell that the front side windows were nicely tempered, as they distinctly showed a pattern of darkened areas. The pattern resembled a matrix of 2cm to 3cm circles, with no obvious hue at all (just a black "coloration", causing a darkening of the color transmitting through the window, through the glasses, and to my eyes). The windows are PGW (formerly PPG) Solargreen glass. I found a super handy transmittance and reflectance chart on their website:
Like many manufacturers, they didn't provide hard data points, but we shouldn't need them to solve this issue. Look at that sharp UV cutoff! Also, the curve peaks nicely around the 500's, which I assume is great for driver visibility from the photometry/safety standpoint. Still, I'm not at all sure how the glass achieves these optical characteristics. Perhaps a patent search is in store?
After tinting, the film still had a good amount of water (or perhaps water and soap... whatever they use these days) trapped between the film-window interface. I put the glasses on for a drive home, and lo' and behold, it looked like I was swept back to the 60's. Pinching myself a few times, and making sure the new car scent wasn't actually something else, I noticed that the film was definitely the culprit of a newfound "rainbow road" effect. It really looked like I was driving a car protected by soap bubbles.
After some time in the sun, the tints apparently absorb enough radiation to self heat and smooth out... a process many refer to as "curing". So, the rainbow gradually went away day by day... but plateaued at the level it is now after about 4 sunny days outside. The color shift effect is there, but it only gets noticeable when viewing a reflection off of a smooth surface (more on that in a minute).
One thing that I did notice is that the effect initially displayed the full spectrum of colors, but now the "rainbow tint" seems to be dichromatic, with only varying intensities of magenta and cyan being noticeable. Here are two pictures, taken through a photographic quality linear polarizer, rotated 90 degrees after the first shot:
My sunglasses also showed a very similar colored effect as the polarizing filter, so I won't show a comparison shot with the glasses (although, as I should know, our eyes are very selective instruments, and the digital camera may in fact reveal that there is actually more than meets the eye (pun totally intended))
Naturally, being the curious kid that I am, I ended up wanting to get to the cause behind the effect itself. Like many topics in physics, the more I thought about it and the more I researched, the more uncertain I became about what's really happening with the windows. Clearly, this phenomenon involves polarization at several points in the optical chain. The real question is, what exactly is occurring at each point along the way?
Given that the formulation from first principles was going to take a while, and the fact that I go back to have the windows re-tinted in a few areas on Monday, I wanted to get to the bottom of this a bit quicker. It is still at the point of being annoying when driving around, so it will either be new tints, or new glasses if it continues. So, I decided to do a few "cheap and dirty" experiments (I mean after all, I did just call myself "redneckphysics", right?) in order to deductively eliminate certain theories.
First, I repeated the above two shots with a "party-cloudy sky" background, for comparison:
This first shot shows a similar pattern that I saw with the untinted window darkening, of "holes" and/or dots, with the now magenta coloration.
I don't have the pictures for some reason (will try to re-create tomorrow), but I then took pictures near the top edge of the window. This revealed a more consistent pattern (perhaps due to the stress, or perhaps due to the tints, it's unclear which), and the colors flip-flopped, with magenta and cyan hues switching places upon a 90 degree rotation.
Then, I decided to make a simple polarizer network with the photographic filter and my sunglasses. As I'm sure may of you have seen demonstrated before, linear polarizers arranged with their direction of polarization will cause an almost complete extinction of transmittance, due to the selective sequential filtration process. I laid the filter on top of an led flashlight, then took pictures through the sunglasses positioned at around 0 or 180 degrees separation between polarizer directions (the two are basically indistinguishable), and at 90 or 270 degrees separation. The glasses were actually nicely polarized, as you can see here in the following shots:
(And yes, that did cover up the writing underneath!)
Now we can analyze the window, soapy film and tint for their effects on the overall polarization of transmitted light in the system. Photographing the light passing through the photographic polarizer first, then window soapy film and tint, and then finally the glasses, gave some interesting results!
First, I rotated the photographic filter to what appeared to be the extinction point with respect to the sunglasses, and placed everything into position, so that half of the photo filter transmits light above the window pane, and half of the photo filter is aligned to transmit through the tinted glass. Note that there is a very small strip of untinted glass still visible as well:
First off, there are a lot more colors present than simply magenta and cyan! Note that the un tinted strip does appear to be monochromatic, but also shows transmitted light that was not blocked by the sunglasses. Does this indicate that the glass does indeed cause optical rotation?
Then, I tried to rotate the filter as close to 90 degrees away from the initial point as possible:
Note how the coloration again seems to shift in a complementary fashion! Interesting effect!
Just for good measure, have another set of photos:
And the second (albeit a bit under exposed!):
My initial theory is that the glass itself is causing varying degrees of optical activity (some polarization and possibly optical rotation of polarization through stress-induced birefringence). I do recall that rotating the sunglasses did cause a change in the observed pattern on the window, so I can say that the glass itself is somehow selectively polarizing. It's perplexing, because as we know, optical rotation in many optically active materials shows a spectral dependence (or rather, the angle of rotation increases as wavelength decreases, due to the nature of refraction, including the speed of light in various media, etc.).
Based on the very first two shots I showed you all, I would say that perhaps the tint is causing an additive color bias by shifting the overall transmitted hue to blue (or reducing the visible red and green). Then I think the soap film or the tint is somehow dichroic, causing red and green wavelengths to be absorbed at different angles of orientation. Perhaps those different angles of rotation just so happen to be perpendicular to one another? Thus, you would get the magenta and cyan switching places. However, this sounds like an incomplete explanation of the phenomenon. The tint film is likely of low quality (but still pro grade), and thus uses a dye (or several) to filter light. I do find it interesting that polarized rays cause the effect to increase (that light reflecting off of the satin concrete finish and car body is likely linearly polarized due to the reflection).
Something that keeps bugging me is the change in effect intensity and diminishing colors with the evaporation of the water/soap mixture over time. Perhaps there is some thin film interference involved as well? If the film left behind is indeed causing some constructive and deconstructive interference, how would this appear? I wonder if some of the coloration apparent when viewing the glass edge through the polarizer network is caused by the variation of thickness in the underlying soap film, vs. stresses in the window or tint alone? Is birefringence the only cause, or is it even a factor at all?
So, I would love to hear your thoughts on this! Does the initial theory sound correct? Again, it seems like I'm not tying everything together, but that's likely due to my limited knowledge of optics. Anything else that you can think of to test things further? I imagine a filter network test repositioned amongst the matrix of dots is in order... to perhaps see if the window's polarization properties influence the color variation, vs. the tint or soapy film. Without a control window to work with, it is difficult to deduce things experimentally!
Thanks for your help on this!
Mike
I've been fortunate enough to have the opportunity to buy a new car, and recently had the windows tinted. The tint job itself is another story altogether... not the best quality tinting material and workmanship in the world. Now, I know some of you have probably seen this effect in person before: when wearing polarized glasses, the tinted windows seem to impart coloration to scenery viewed through the window.
I'm a bit weak in the optics discipline (I focused on E&M, Engineering Physics and Laboratory Electronics and Automation), but I have had some radiometry, and have extensively studied the causes of induced birefringence via the Faraday Effect. I was hoping to run the issue by everyone here on the forums, to see if anyone knew more about what exactly is happening with the effect.
So, first, some background on the problem:
When I first got the car, I immediately started using a pair of dark amber (almost a bronze) tinted polarized glasses for daytime driving. I could immediately tell that the front side windows were nicely tempered, as they distinctly showed a pattern of darkened areas. The pattern resembled a matrix of 2cm to 3cm circles, with no obvious hue at all (just a black "coloration", causing a darkening of the color transmitting through the window, through the glasses, and to my eyes). The windows are PGW (formerly PPG) Solargreen glass. I found a super handy transmittance and reflectance chart on their website:
Like many manufacturers, they didn't provide hard data points, but we shouldn't need them to solve this issue. Look at that sharp UV cutoff! Also, the curve peaks nicely around the 500's, which I assume is great for driver visibility from the photometry/safety standpoint. Still, I'm not at all sure how the glass achieves these optical characteristics. Perhaps a patent search is in store?
After tinting, the film still had a good amount of water (or perhaps water and soap... whatever they use these days) trapped between the film-window interface. I put the glasses on for a drive home, and lo' and behold, it looked like I was swept back to the 60's. Pinching myself a few times, and making sure the new car scent wasn't actually something else, I noticed that the film was definitely the culprit of a newfound "rainbow road" effect. It really looked like I was driving a car protected by soap bubbles.
After some time in the sun, the tints apparently absorb enough radiation to self heat and smooth out... a process many refer to as "curing". So, the rainbow gradually went away day by day... but plateaued at the level it is now after about 4 sunny days outside. The color shift effect is there, but it only gets noticeable when viewing a reflection off of a smooth surface (more on that in a minute).
One thing that I did notice is that the effect initially displayed the full spectrum of colors, but now the "rainbow tint" seems to be dichromatic, with only varying intensities of magenta and cyan being noticeable. Here are two pictures, taken through a photographic quality linear polarizer, rotated 90 degrees after the first shot:
My sunglasses also showed a very similar colored effect as the polarizing filter, so I won't show a comparison shot with the glasses (although, as I should know, our eyes are very selective instruments, and the digital camera may in fact reveal that there is actually more than meets the eye (pun totally intended))
Naturally, being the curious kid that I am, I ended up wanting to get to the cause behind the effect itself. Like many topics in physics, the more I thought about it and the more I researched, the more uncertain I became about what's really happening with the windows. Clearly, this phenomenon involves polarization at several points in the optical chain. The real question is, what exactly is occurring at each point along the way?
Given that the formulation from first principles was going to take a while, and the fact that I go back to have the windows re-tinted in a few areas on Monday, I wanted to get to the bottom of this a bit quicker. It is still at the point of being annoying when driving around, so it will either be new tints, or new glasses if it continues. So, I decided to do a few "cheap and dirty" experiments (I mean after all, I did just call myself "redneckphysics", right?) in order to deductively eliminate certain theories.
First, I repeated the above two shots with a "party-cloudy sky" background, for comparison:
This first shot shows a similar pattern that I saw with the untinted window darkening, of "holes" and/or dots, with the now magenta coloration.
I don't have the pictures for some reason (will try to re-create tomorrow), but I then took pictures near the top edge of the window. This revealed a more consistent pattern (perhaps due to the stress, or perhaps due to the tints, it's unclear which), and the colors flip-flopped, with magenta and cyan hues switching places upon a 90 degree rotation.
Then, I decided to make a simple polarizer network with the photographic filter and my sunglasses. As I'm sure may of you have seen demonstrated before, linear polarizers arranged with their direction of polarization will cause an almost complete extinction of transmittance, due to the selective sequential filtration process. I laid the filter on top of an led flashlight, then took pictures through the sunglasses positioned at around 0 or 180 degrees separation between polarizer directions (the two are basically indistinguishable), and at 90 or 270 degrees separation. The glasses were actually nicely polarized, as you can see here in the following shots:
(And yes, that did cover up the writing underneath!)
Now we can analyze the window, soapy film and tint for their effects on the overall polarization of transmitted light in the system. Photographing the light passing through the photographic polarizer first, then window soapy film and tint, and then finally the glasses, gave some interesting results!
First, I rotated the photographic filter to what appeared to be the extinction point with respect to the sunglasses, and placed everything into position, so that half of the photo filter transmits light above the window pane, and half of the photo filter is aligned to transmit through the tinted glass. Note that there is a very small strip of untinted glass still visible as well:
First off, there are a lot more colors present than simply magenta and cyan! Note that the un tinted strip does appear to be monochromatic, but also shows transmitted light that was not blocked by the sunglasses. Does this indicate that the glass does indeed cause optical rotation?
Then, I tried to rotate the filter as close to 90 degrees away from the initial point as possible:
Note how the coloration again seems to shift in a complementary fashion! Interesting effect!
Just for good measure, have another set of photos:
And the second (albeit a bit under exposed!):
My initial theory is that the glass itself is causing varying degrees of optical activity (some polarization and possibly optical rotation of polarization through stress-induced birefringence). I do recall that rotating the sunglasses did cause a change in the observed pattern on the window, so I can say that the glass itself is somehow selectively polarizing. It's perplexing, because as we know, optical rotation in many optically active materials shows a spectral dependence (or rather, the angle of rotation increases as wavelength decreases, due to the nature of refraction, including the speed of light in various media, etc.).
Based on the very first two shots I showed you all, I would say that perhaps the tint is causing an additive color bias by shifting the overall transmitted hue to blue (or reducing the visible red and green). Then I think the soap film or the tint is somehow dichroic, causing red and green wavelengths to be absorbed at different angles of orientation. Perhaps those different angles of rotation just so happen to be perpendicular to one another? Thus, you would get the magenta and cyan switching places. However, this sounds like an incomplete explanation of the phenomenon. The tint film is likely of low quality (but still pro grade), and thus uses a dye (or several) to filter light. I do find it interesting that polarized rays cause the effect to increase (that light reflecting off of the satin concrete finish and car body is likely linearly polarized due to the reflection).
Something that keeps bugging me is the change in effect intensity and diminishing colors with the evaporation of the water/soap mixture over time. Perhaps there is some thin film interference involved as well? If the film left behind is indeed causing some constructive and deconstructive interference, how would this appear? I wonder if some of the coloration apparent when viewing the glass edge through the polarizer network is caused by the variation of thickness in the underlying soap film, vs. stresses in the window or tint alone? Is birefringence the only cause, or is it even a factor at all?
So, I would love to hear your thoughts on this! Does the initial theory sound correct? Again, it seems like I'm not tying everything together, but that's likely due to my limited knowledge of optics. Anything else that you can think of to test things further? I imagine a filter network test repositioned amongst the matrix of dots is in order... to perhaps see if the window's polarization properties influence the color variation, vs. the tint or soapy film. Without a control window to work with, it is difficult to deduce things experimentally!
Thanks for your help on this!
Mike
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