Subscriber Discussion

Preventing LPR Camera Lens Reflection Of Headlights At Night

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Carter Maslan
Nov 19, 2016
Camio

Any ideas to prevent the reflection of headlights in an LPR lens?

Notice the 2 white circles that start in the lower left of the scene as this police cruiser passes at night in this video:

https://youtu.be/oEgeTNSmjIw

This Zavio does a great job of capturing the license plate, but the reflected headlights complicate Camio's video processing. 

Zavio LPR lens

JH
John Honovich
Nov 19, 2016
IPVM

Carter, see: How to Handle Headlight Surveillance Problems

Best bet is to add an external IR to overcome / balance out lighting. Headlights are a common problem.

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Carter Maslan
Nov 19, 2016
Camio

John - that article was helpful but didn't discuss the reflection problem that I've circled in the lower left that moves across the scene. Since that's clearly bouncing off the camera's own glass, are there any lens covers that inhibit that bounce? (instead of having to make the whole scene brighter)

reflection in lower left circled

U
Undisclosed #1
Nov 19, 2016
IPVMU Certified

are there any lens covers that inhibit that bounce?

A polarizing filter might do it.

Testing Polarizing Filters on HD Cameras

MM
Michael Miller
Nov 19, 2016

The plate is overexposed till the last couple of frames. You need to have some type of image adjustment on the White Levels in live/recorded video or you need to add more light while upping your exposure settings in the camera(1/1000+). White Level adjustment will be more forgiving.

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Jon Dillabaugh
Nov 19, 2016
Pro Focus LLC

As Michael said, your scene is overexposed. You need to increase your minimum shutter speed. This will negate both the overexposure and the reflections. Give that a shot and see if it corrects both issues.

U
Undisclosed #1
Nov 19, 2016
IPVMU Certified

Agree with all that a slightly faster shutter should reduce the reflections without making the plate too dark.

A straighter shot would have less reflections as well.

Does the enclosure add a piece of glass in front of the lens? How much space is there between them?

JH
John Honovich
Nov 19, 2016
IPVM

There's a reason almost every real LPC / LPR comes with IR illuminators. You can shorten the shutter speed but that will not change the ratio of the headlights illumination to the rest of the scene. And, in particular, it will make reading the license plate even more difficult since there is much less illumination there.

Even adding a cheap IR illuminator is likely to help significantly, e.g., 5 Low Cost IR Illuminators Tested

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Jon Dillabaugh
Nov 19, 2016
Pro Focus LLC

John, normally that is true, but here he has plenty of illumination. That's why he needs to shorten the exposure.

U
Undisclosed #1
Nov 19, 2016
IPVMU Certified

I'd say it depends on what Carter means by

This Zavio does a great job of capturing the license plate, but the reflected headlights complicate Camio's video processing.

If he needs to eliminate or drastically reduce the reflected light because of some technical reason (false motion?), then you might not get there by upping the shutter speed.

In that case you would need more contrast between the visible and IR light, one way to get that would be by external illuminator or possibly increasing the IR power, if configurable.

If the camera is used only for LPR, you could also use an ir-pass filter, though that would be pricey as well.

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Carter Maslan
Nov 19, 2016
Camio

To clarify, I mean that the two "phantom headlights" (from the reflection that starts in the lower left and then moves up and to the right in the inverse direction as the car passes in the video) are currently interpreted as moving headlights too rather than the single car's actual headlights. So if we can eliminate those phantom reflections, we don't have to remove them with algorithms.

I've asked the customer to try lowering the exposure to see if that eliminates the lens reflection without making things too dark. Then we'll try the lens filters and IR illuminator suggestions if the lower exposure doesn't work.

JH
John Honovich
Nov 20, 2016
IPVM

but here he has plenty of illumination

No, he doesn't, look at the scene:

Overall, it is fairly dark and (notice how dark the grill, sides of the car, background is, etc.) once you shorten the exposure it is going to get much worse. Again, ergo why IR illumination is standard in professional LPC setups.

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Jon Dillabaugh
Nov 20, 2016
Pro Focus LLC

John, in LPR/ANPR setups, you generally don't care about the grill or background. You only care about the plate. If you also need context of make, model, color, etc, you would supplement the scene with a low light camera for that purpose. He absolutely needs to shorten the exposure, which will compensate for both the headlights and the overexposed plate shown in his image.

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JH
John Honovich
Nov 20, 2016
IPVM

John, in LPR/ANPR setups, you generally don't care about the grill or background.

Obviously the grill is not important. The point is that given much of the scene is already dark (like the grill), once you shorten the exposure to reduce the impact of the headlights, the license plate will become difficult to read since the headlights are the brightest part of the image. Ergo, adding IR illumination will balance out the lighting of the scene and provide better capture of the plate.

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Jon Dillabaugh
Nov 20, 2016
Pro Focus LLC

John, the plate is overexposed, no?

JH
John Honovich
Nov 20, 2016
IPVM

Yes, that plate is overexposed but it's one car and even with it overexposed, the headlights are still stronger.

If all you do is reduce the exposure, the whole scene will get darker and there will almost certainly be many cases where the plate is too dark. One thing we found in our testing is that the strength of headlights vary significantly and with cars with headlights are particularly strong, the plate will be particularly dark.

Again, this is why the standard for LPC cameras is to include IR illumination to overcome this.

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Jon Dillabaugh
Nov 20, 2016
Pro Focus LLC

And what is the source of the light being reflected off the plate?

JH
John Honovich
Nov 20, 2016
IPVM

I stand corrected. The pictures shows that the Zavio has integrated IR.

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Carter Maslan
Nov 22, 2016
Camio

To close the loop on this question, reducing the exposure using 1/1000 shutter speed reduced the reflection somewhat (because there was less light) but not sufficiently to avoid having the phantom headlights in the scene.

The manufacturer Zavio replied with this answer:

"This is an physical phenomena. The strong head light cause reflection inside the camera lens. Adjust the camera angle might help but the clearance (sic) of license plate will be influenced too. Sorry there is no way to reduce the reflection by adjusting the configuration."

U
Undisclosed #1
Nov 23, 2016
IPVMU Certified

I'm not sure if you are trying to solve the problem of this particular camera getting reliable LPR when mounted in this particular location, but if you are, I believe you may be able to enable privacy masking over the parts of the display in which the phantom travel, like so:

Obviously, this is not ideal, but might buy you some time.

Also, I don't think its so much the glass of the lens as the glass of the enclosure causing the phantoms. Can you confirm that there is a seperate piece of flat glass in addition to the lens?

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Carter Maslan
Nov 23, 2016
Camio

Thanks but we need to recognize rear license plates traveling in the opposite direction through that mask area. Yes, there's outer flat glass as pictured at the top of this thread. It looks like algorithms are needed to solve for this problem generally (was hoping for some kind of anti-reflection lens filter).

U
Undisclosed #1
Nov 23, 2016
IPVMU Certified

(was hoping for some kind of anti-reflection lens filter).

Both these filters from Banner look promising for your application, (assuming CS mount lens):

There may be cheaper equivalents available elsewhere.

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U
Undisclosed #1
Nov 24, 2016
IPVMU Certified

Hey, I just saw that Edmunds Optics sells a polarizing laminate film, for $22 bucks its worth a shot...

JE
Jim Elder
Nov 23, 2016
IPVMU Certified

Possibly try a polorizing filter? Cheap, but it could impact the image.

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