Testing Polarizing Filters on HD Cameras

By: Ethan Ace, Published on May 30, 2014

Sun glare is particularly challenging for surveillance cameras. Even top cameras struggle with such conditions.

One technique suggested is using polarizing filters. Simply put, these filters pass certain polarizations of light, while blocking others, reducing sun glare at specific angles. 

To test how polarizers worked in real world scenarios, we tested 720p cameras in three configurations:

  • Auto-iris lens with polarizing filter
  • Auto-iris lens, no filter
  • P-iris lens, no filter

We tested these three cameras outdoors on a bright, sunny day, to see how the polarizing filter impacts performance.

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Comments (17)

It is not clear as to what would be the effect with the passage of time. If one fixed the polarizing filter in the morning to eliminate glare from a scene when the angle of incidence of the light would be say A, what would be the effect in the late afternoon, when the angle of incidence would be say B which may be as much as 90 deg. different from A. I suspect that the polarizing filter would need to be re-adjusted. This would grossly limit the usability of the filter in practical surveillance CCTV applications.

...in the morning to eliminate glare from a scene when the angle of incidence of the light would be say A, what would be the effect in the late afternoon...

Remember, this is not for doing a little back light compensation, IMO its best used for rather moderate to severe glare. Like the kind that causes such sensory sensor overload (AKA blooming), that WDR can't even fix it.

I don't doubt that you can come up with some scenarios where you have extreme morning glare and extreme late afternoon glare in the same static FOV, but typically its one or the other and then only for maybe a couple of hours once a day... What have you seen?

But, if the situation is that demanding, then maybe the camera can be angled down or out more, or maybe if Ethan has another filter lying around we could see what two partially out of phase ones do..(or if in NYC, I've heard certain Chinese takeout menus can be used as a glare hood :)

"I suspect that the polarizing filter would need to be re-adjusted."

Jayant, Good feedback.

I think the other thing that might be an issue, even if all you were interested in was dealing with just afternoon sun, e.g., is that the position of the setting and rising sun varies throughout the year. There's a map / calculator that shows that.

For example, here is what it is on January 31st for NYC:

And here is what it is today, May 31st:

I am not positive how this would impact it, and it's, by definition, not quick to test this, but theoretically if the angle of impact of the sun's glare shifted significantly, it would seem to undermine the glare reduction of the set polarized filter.

Just to be clear I wasn't saying Jayant couldn't be right, I just couldn't think of a typical situation where he would be... Do any come to mind? Maybe a lake scene where the sun is tracking, left to right across the sky?

Have you considered trying out one of these filters on your LPC shootout, talk about glare! It would have to do something, right?

The lake is certainly one possibility. Here is another more likely scenario. Just visualise an east-west row of cars facing north-south in a parking lot and the camera somewhere in the middle between 50 and 100 yards to the north or south. As the sun passes overhead, the windscreens of the cars would reflect the sunlight into the camera from different angles throughout the day.

As the sun passes overhead, the windscreens of the cars would reflect the sunlight into the camera from different angles throughout the day.

Again, I agree in theory, but I am not sure this disparity in angles would be enough to cause any concern, and even if it did somtimes occur, I don't think it would occur enough to

...grossly limit the usability of the filter in practical surveillance CCTV applications. (EA)

Which is a fairly strong knock against the filters, don't you think?

Here are the assumptions I'm using, let me know if you object:

1. Camera is a fixed box type with interchangable lenses/filters, with a horizontal view angle of less < 90.

2. Camera has a tilt angle of < 0

3. Glare, in this sense means unwanted light which causes excessive image degradation and severe over-exposure (so that 'even top cameras struggle')

Returning to your example of a camera pointed north-south, the difficulty lies in the fact that the sun itself is always more than 90 degrees off axis, therefore the total amount of reflected light itself, although the angle of the light is changing, never amounts to enough to warrant a filter. But, If you then point the camera to the east or west more, the glare will of course increase to the point of needing a filter, but only in that direction...

Glare, IMO, happens the most when the camera is slightly off axis to the sun, i.e. sunrise and sundown, and you can't have a camera pointed at both at the sametime without a wide angle lens (or a curved vertical mirror on the horizon, ski slope maybe?).

There's a lot of IPVM member controlled cameras out there, so its just a matter of time before somebody gives a convincing, real counter example. But the filters are probably effective in most cases, yes/no?

I have already posted a reply but on account of a power blink while submitting it, it may not have gone through. If it did go through, please ignore/delete this and accept my apologies for the inconvenience.

The reply:

I accept your three assumptions, but beyond that, both of us are making assumptions as to the actual impact. It would enlighten all of us if Ethan or some other IPVM member could give us feedback based on actual tests.

My reason for stating that the usability would be "grossly" limited is based on the assumption that if your surveillance is severely compromised for several hours a day for several days, then that surveillance cannot be relied upon and an alternate solution needs to be found.

...but beyond that, both of us are making assumptions as to the actual impact...

Here's the good news, we don't need no stinkin' filters!

Why not? Because I will concede to you that every scene, conformant to the agreed assumptions, which exhibits glare in the 'morning' and then again in the 'late afternoon' could very well be 'grossly limited', regardless of whether a filter is used or not...

So if you have access to any camera with a standard angle lens and the Sun is nearby, you might be be able come-up with a real-world candidate geometry that could be tested with filters.

But if we don't know of an actual double-time glare test case without filters, then what should we ask Ethan to test with filters? Surely we don't just want him to test a scene that actually has glare only once a day, do we?

P.S. I quickly tried the camera north-south east-west thing with only ten cars but I had a hard time getting any glare with the camera pointing 90 degrees off axis from the sun. I admit I would have tried a lot harder if I thought it was likely to get double glare in the first place, and so I am guilty of 1st degree confirmation bias. You might do a lot better, depending on your lattitude...(Reykjavic?)

You show the impact at night, but what is the impact on the image during the day when there isn't a situation that requires the filter?

Where can you buy this polarized filter?

Wow, I was just thinking about this post when Ericson replied to it.Thanks, Eric. I was able to see this first hand when my girlfriend commented on some fancy seats inside a car as we walked by. I told her I could not see the seats because of the glare. She could see them because she was wearing polarized Oakley sunglasses. Pretty neat.

Does anyone have information on where slip-ring mounts that will fit typical IP camera lenses can be purchased?

 any idea where I can buy those sliprings?

 

Let’s not forget Midwest Optical...

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