Dashmellow
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Strange Rainbows in Your Videos?
With a steady stream of posts to the forum from members trying to understand why they are suddenly seeing unusual rainbows appearing in their dash cam footage I decided to create a sticky where we can learn about and discuss the problem and where visitors searching for some answers can find them.
Quick Start Spoiler: Ultimately, the problem of unusual, internally generated rainbows appearing in your dash cam videos will require removing the CPL from your camera if you want to eliminate them but I will talk more about that below, along with a workaround for eliminating bothersome windscreen reflections that does not require the use of a CPL.
In the meantime, we can learn about why the problem is occurring in the first place.
In virtually all situations, rainbows like we are talking about here will only manifest if you have or have recently installed tint material to your front or rear windshield in combination with having a CPL installed on your dash cam.
The cause of these odd windshield rainbows appearing in your dash cam videos is a phenomenon called BIREFRINGENCE, (bī′rĭ-frĭn′jəns) - (bye-ree-frin-jense) also known as a double refraction.
While birefringence can be a complex phenomenon in physics, for our purposes the problem is simply the resulting double refraction, which is the splitting of a ray of light into two parallel rays polarized perpendicularly and this is caused by the interaction of the CPL and the window tint which like the CPL is also acting as a polarizer. When your polarized camera lens looks through your polarized window tint, the resulting double refraction will manifest as rainbows in your videos.
Birefringence - the splitting of a light wave into two unequally reflected or transmitted waves by an optically anisotropic medium.
Anisotropic - having physical properties that are different in measurement along different axes or directions.
Refraction - the change in direction of a wave passing from one medium to another, like when a spoon suddenly appears bent or distorted when placed in a clear glass of water.
Another term for this phenomenon is CROSS-POLARIZATION. One polarized surface traps light traveling vertically, the other traps light traveling horizontally. With the CPL and the window tint both polarizing incident light entering your car, the result is the splitting of the light rays into various frequencies (colors) of the electromagnetic spectrum creating rainbows, not unlike you might see from a glass prism or in a raindrop.
Typical glass prism refraction rainbow.
Stress birefringence seen in a DVD Jewel Box photographed with two crossed polarizers.
You will notice similar birefringent rainbows if you look through a tinted windscreen from the inside (or outside) of your vehicle while wearing polarized sunglasses. Here, you are witnessing the same phenomenon seen by your polarized dash camera lens.
Wearing polarized sunglasses, you might also see some strange optical effects such as dots, lines or square shaped patterns depending upon which particular window of your car you may be looking through. You might also see a rainbow effect. This is because your front windshield, your rear window and your side windows are made from different types of glass.
Your vehicle's front windshield (and usually rear) are made from laminated safety glass. Without any tinting applied to your front windshield you won't see any rainbows in your images even with a CPL because laminated glass is usually clear and structurally aligned (on a molecular level). Your side windows (and sometimes rear) are made of tempered glass which is heated and cooled during manufacture and this process creates stress patterns that cause the aformentioned dots, lines and square shapes you may see along with rainbow colors that may also be visible when wearing polarized sunglasses.
Rear window tempered glass stress patterns seen through a polarized lens..
Looking through side window tempered glass with a polarized lens.
OK, so now you know the explanation for what's causing your weird rainbow problem. The question is what to do about it. To reiterate what I said earlier, if you want to eliminate these unwanted rainbows from your videos, the ONLY solution is to either remove your CPL from your camera or to remove your window tint.
Of course, removing the CPL still leaves us with the problem of reflections of your dashboard in your windshield obscuring your video captures. Some folks find they can simply live with intrusive windshield reflections and get used to them, but most dash cam users really want the clearest possible image they can achieve and fortunately there is a “fix-it” for those willing to give it a go. I'll talk about that below, but first we'll have a look at some further examples of dash cam birefringence.
With a steady stream of posts to the forum from members trying to understand why they are suddenly seeing unusual rainbows appearing in their dash cam footage I decided to create a sticky where we can learn about and discuss the problem and where visitors searching for some answers can find them.
Quick Start Spoiler: Ultimately, the problem of unusual, internally generated rainbows appearing in your dash cam videos will require removing the CPL from your camera if you want to eliminate them but I will talk more about that below, along with a workaround for eliminating bothersome windscreen reflections that does not require the use of a CPL.
In the meantime, we can learn about why the problem is occurring in the first place.
In virtually all situations, rainbows like we are talking about here will only manifest if you have or have recently installed tint material to your front or rear windshield in combination with having a CPL installed on your dash cam.
The cause of these odd windshield rainbows appearing in your dash cam videos is a phenomenon called BIREFRINGENCE, (bī′rĭ-frĭn′jəns) - (bye-ree-frin-jense) also known as a double refraction.
While birefringence can be a complex phenomenon in physics, for our purposes the problem is simply the resulting double refraction, which is the splitting of a ray of light into two parallel rays polarized perpendicularly and this is caused by the interaction of the CPL and the window tint which like the CPL is also acting as a polarizer. When your polarized camera lens looks through your polarized window tint, the resulting double refraction will manifest as rainbows in your videos.
Birefringence - the splitting of a light wave into two unequally reflected or transmitted waves by an optically anisotropic medium.
Anisotropic - having physical properties that are different in measurement along different axes or directions.
Refraction - the change in direction of a wave passing from one medium to another, like when a spoon suddenly appears bent or distorted when placed in a clear glass of water.
Another term for this phenomenon is CROSS-POLARIZATION. One polarized surface traps light traveling vertically, the other traps light traveling horizontally. With the CPL and the window tint both polarizing incident light entering your car, the result is the splitting of the light rays into various frequencies (colors) of the electromagnetic spectrum creating rainbows, not unlike you might see from a glass prism or in a raindrop.
Typical glass prism refraction rainbow.
Stress birefringence seen in a DVD Jewel Box photographed with two crossed polarizers.
You will notice similar birefringent rainbows if you look through a tinted windscreen from the inside (or outside) of your vehicle while wearing polarized sunglasses. Here, you are witnessing the same phenomenon seen by your polarized dash camera lens.
Wearing polarized sunglasses, you might also see some strange optical effects such as dots, lines or square shaped patterns depending upon which particular window of your car you may be looking through. You might also see a rainbow effect. This is because your front windshield, your rear window and your side windows are made from different types of glass.
Your vehicle's front windshield (and usually rear) are made from laminated safety glass. Without any tinting applied to your front windshield you won't see any rainbows in your images even with a CPL because laminated glass is usually clear and structurally aligned (on a molecular level). Your side windows (and sometimes rear) are made of tempered glass which is heated and cooled during manufacture and this process creates stress patterns that cause the aformentioned dots, lines and square shapes you may see along with rainbow colors that may also be visible when wearing polarized sunglasses.
Rear window tempered glass stress patterns seen through a polarized lens..
Looking through side window tempered glass with a polarized lens.
OK, so now you know the explanation for what's causing your weird rainbow problem. The question is what to do about it. To reiterate what I said earlier, if you want to eliminate these unwanted rainbows from your videos, the ONLY solution is to either remove your CPL from your camera or to remove your window tint.
Of course, removing the CPL still leaves us with the problem of reflections of your dashboard in your windshield obscuring your video captures. Some folks find they can simply live with intrusive windshield reflections and get used to them, but most dash cam users really want the clearest possible image they can achieve and fortunately there is a “fix-it” for those willing to give it a go. I'll talk about that below, but first we'll have a look at some further examples of dash cam birefringence.
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