Strange Rainbows in Your Dash Cam Videos?

Dashmellow

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Strange Rainbows in Your Videos?
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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.

spoon.jpg

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.

crossed_polarizers.jpg

Typical glass prism refraction rainbow.
light-prism-color-angle-colors.jpg

Stress birefringence seen in a DVD Jewel Box photographed with two crossed polarizers.
CDbirefringence.jpeg

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.

rainbow windshield.jpeg

stress_bifringence.jpg

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..
patterns.jpg

Looking through side window tempered glass with a polarized lens.
side window_tempered.jpg

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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Dashmellow

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Here are some of the many images posted by other members who have come to DCT looking for answers about the weird rainbow effects they were seeing in their dash cam videos.

If you too have come here looking to find out what they are and any of your video images look like these, you've come to the right thread! :)

All of these images show bifringence caused by the combination of tinted windshields imaged with dash cams that have CPLs installed.

CPL-Halo.jpg

rainbows1.jpg

refractions3.jpg

refractions2.jpgrefractions.jpg

PastedGraphic.jpg

rainbows3.jpg
 
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Dashmellow

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The solution for windshield reflections without using a polarizer is to install a black dash mat on your dash board. In fact, a dash mat alone can actually exceed the performance of a CPL with the added benefit of not increasing the exposure factor for your camera because a dark glass CPL filter will otherwise cause increased motion blur and less effective night time performance.

Below is an example of a black dash mat alone with no CPL installed on the camera.

Notice the area where the custom dash mat has a pre-cut hole for my defroster vent. You can see the vent, some of the gray textured plastic surface of my dashboard, a bit of the dash mat and the pronounced reflection they create in my windshield, except that the reflection happens to be confined to the hood area of my truck where it is not causing any particular problems for my dash cam video coverage. The rest of the windshield however, is completely free of any reflections due to how amazingly well the black dash mat works! Even a CPL will not generally eliminate reflections to this degree!

lake_reflection_a.jpg

Here, as a demonstration, I've placed a couple of objects on my black dash mat covered dashboard. Obviously, the reflections look pretty bad, but of course, that's the point. It's all about showing just how well a dash mat alone works to eliminate reflections in your windshield . If you can't use your CPL because of strange rainbow effects caused by your window tint and you want to eliminate your troublesome windshield reflections, get yourself a matte black dash mat! You'll get to enjoy the benefits of your fancy window tint job but not have to make any visual compromises with your dash cam. In fact, a dash mat will improve your camera's performance.

lake_reflection_b.jpg

Some people absolutely refuse to install dash mats because they don't want to spoil the look of their original car interior and they believe they look ugly. That's understandable, and some dash mats are indeed pretty ugly. When I first installed a dash mat in my truck five years ago, I thought it would look hideous but I was pleasantly surprised by how nice it looked. Before long I hardly noticed it. I sometimes get questions about it and even some compliments. The real surprise was how much it exceeded my expectations for how well it functioned.

What it comes down to is that if you have a serious problem with reflections in your dash cam videos and you've got a windshield tint, a dash mat is really your only option if you can't use your CPL and it is well worth it.

If you decide to buy a dash mat for your vehicle, you want one that is as smooth and flat matte black as possible. I recommend the matte black “brushed suede” type. They're not real suede of course, but they have a short, non-reflective napped finish and that is what you want. Avoid anything with holes in it and definitely avoid anything made of floor carpeting which are not only ugly but usually are made with shiny fibers that will reflect in your car's windshield glass.

Here are some photos of dash mat installations. Custom dash mats are available for most makes and models of cars and trucks. Rear deck mats are also available. Right hand drive versions can be hard to find but they too are indeed available and I've recently found more vendors that carry them such as on AliExpress.

For-Toyota.jpg


dashmat_suede_dashboard_cover_aa_black.jpg


brushed-suede.jpg

Right hand drive
right_hand-drive.jpg
 
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kamkar

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You did not make a wrong turn and somehow ended up on the Bifrost,,,,, thrust me drunk Danes have been thinking that since the first Ox and cart. :LOL:
 
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Dashmellow

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You did not make a wrong turn and somehow ended up on the Bifrost,,,,, thrust me drunk Danes have been thinking that since the first Ox and cart. :LOL:

The Bifrost?! That must be it! And here I was thinking it was a flashback from the acid I dropped back in the day! :smuggrin:

bifrost.jpg
 

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Dashmellow

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This really isn't a thread about dash mats and glare. We have other threads for discussing DIY dash mats and such.

Since the thread is a sticky dedicated to discussing BIFRINGENCE rainbows caused by window tinting, I'm hoping we can more or less "stick" to the topic at hand even if dash mats might be part of the conversation.
 
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kamkar

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It is nice you put this up, as you know the problem come up ever so often, so nice to be able to drop a link to this thread.
 
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Dashmellow

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It is nice you put this up, as you know the problem come up ever so often, so nice to be able to drop a link to this thread.

Thanks!

Yeah, I'm often the guy who tries to explain what's happening when puzzled members show up to ask what the heck is going on, so it seemed like a sticky would be a good idea, especially after noticing how many new posts there have been about it. Aftermarket window tinting seems to be gaining popularity so I expect we'll be seeing more new dash cam users or new window tint adopters asking about weird rainbows.
 
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jokiin

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Thanks!

Yeah, I'm often the guy who tries to explain what's happening when puzzled members show up to ask what the heck is going on
much easier than trying to find all the different posts you have already done about this, nice to have it all in one thread and easy to find
 

Dash cams Serbia

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Regarding reflections, I haven't tried it myself, but if you block/cover the underside of the camera/lens with a piece of plastic/cardboard/whatever (without it entering the FOV of the camera), it should prevent the light bouncing of the dashboard entering the lens.
 
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Dashmellow

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Regarding reflections, I haven't tried it myself, but if you block/cover the underside of the camera/lens with a piece of plastic/cardboard/whatever (without it entering the FOV of the camera), it should prevent the light bouncing of the dashboard entering the lens.

You are not the first person to suggest this idea but like everyone else, if you did try this yourself you would find that it does not work. The wide angle lens of your camera captures an image of your entire windshield and your dashboard is reflecting across your entire windshield. A small piece of plastic or cardboard under your camera lens only covers a tiny portion of this area and would never be able to prevent reflections across the entire angle of view seen by the camera.
 
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Dashmellow

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Check this video:

Firstly, the thing this guy made is enormous (12 inches wide!) and ugly. It draws a lot of attention to itself and the camera. In many jurisdictions sticking a 12 inch wide object on your windshield is illegal. Secondly, this only works at all because he has the camera tilted WAY too high to be practical for optimal dash cam use. For optimal dash cam imaging and exposure you want about 40 percent sky above the horizon and 60 percent road below in the horizon in the image. He has the opposite or more with as much as 65 or 70 percent sky and only about 30 or 35 percent below the horizon. You'll notice that when he shows his first footage before he installed his black shield he had the camera pointed much lower. If he had the camera aimed properly then this would not work at all and /or the shield he made would be halfway down his windshield and would need to be even larger.

As I've said, a number of DCT members have suggested and tried this technique but zero have found the idea to be viable enough to continue. It simply doesn't take into account how AOV (Angle of View) works optically. Certainly, if you want to try this yourself, go right ahead. Afterwards, please report back with your results.
 
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SawMaster

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For a short time Akaso offered a 1080p 'wedge' cam which mounted to the windshield with a similar 'flared' collar blocking all directions. The videos I saw with it weren't great and it had a rather narrow FOV. My guess is that they stopped making it because of low sales. The concept has seen some experimental work and it's just not practical in most cases. Fitting a CPL can be effective for this although you may need to brighten the exposure setting to keep night-time vids from being too dark. A non-reflective black dash mat can work too.

My old van had some dash reflections, but not too bad. Using a CPL helped considerably; the reflections are much less and it also helps cut down on glare when driving into a low sun. They are a good solution which lots and lots of people use to very good effect.

Phil
 
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Dashmellow

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Well, the whole point of this thread is that you can't use a CPL with tinted windshields or you will get "strange rainbows".

A "flared collar blocking all directions" is called a lens hood which is a different animal than a piece of cardboard or plastic mounted below the camera. Usually cameras that are factory installled with lens hoods like that are fitted directly to the glass so that any reflections of the windshield are eliminated entirely. These lens hoods tend to require a narrower AOV lens. The 2016 Corvette Stingray came with an onboard dash cam built like that and it was integral to the windshield.
 
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Dash cams Serbia

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Like I suggested in his video, I though something like this would be better?


But not as exaggerated, or mounted a bit lower, still on the dash cam itself.


When I get a better car next year, I will get a new dash cam as well. And I will try something like this.
 
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