The mystery of the "invisible" license plate on my LPR cameras

[wtimothyholman]

I have run across a very interesting phenomenon with my LPR system, and I'm wondering if anyone else has encountered this, or can suggest a change to my camera settings to resolve it. Quite by accident, I realized that the specialty plates on my wife's RAV4 cannot be viewed while my Dahua IPC-HFW5231E-Z12 cameras are in nighttime mode. The plate is just a blur, although other license plates show up fine. Attached is an image of the RAV4 plate in the daytime and the nighttime. I've also attached an image of a standard license plate, which is quite clear and readable, taken just a minute before the RAV4 plate.

So in effect my wife's vehicle is "invisible" both to LPR and to standard recording, at least with the current LPR settings on my cameras. I've included snapshots of my camera settings, which are pretty standard for LPR usage. Has anyone seen this before? Any thoughts to what I can do to make these types of plates readable?

 

[mat200]

I have run across a very interesting phenomenon with my LPR system, and I'm wondering if anyone else has encountered this, or can suggest a change to my camera settings to resolve it. Quite by accident, I realized that the specialty plates on my wife's RAV4 cannot be viewed while my Dahua IPC-HFW5231E-Z12 cameras are in nighttime mode. The plate is just a blur, although other license plates show up fine. Attached is an image of the RAV4 plate in the daytime and the nighttime. I've also attached an image of a standard license plate, which is quite clear and readable, taken just a minute before the RAV4 plate.

So in effect my wife's vehicle is "invisible" both to LPR and to standard recording, at least with the current LPR settings on my cameras. I've included snapshots of my camera settings, which are pretty standard for LPR usage. Has anyone seen this before? Any thoughts to what I can do to make these types of plates readable?

HI @wtimothyholman

Wondering if they're now using different paint to allow the state's cameras to get better captures?

How does the plate look at night with a non-lpr custom tuned camera?

 

[bigredfish]

Try a lot more Contrast, less DNR..

Looks like a tough angle? What distance?

I see plates like that, some are just old and dirty and hard to read... and local LE now uses some type of spray that doesnt reflect IR..

 

[wtimothyholman]

Try a lot more Contrast, less DNR..

Looks like a tough angle? What distance?

I see plates like that, some are just old and dirty and hard to read... and local LE now uses some type of spray that doesnt reflect IR..

The distance is only about 70 feet. The angle is the best that I can do with the cameras mounted under the eaves of my roof, given the surrounding obstacles. But this isn't a matter of angle or distance, as the other plate image shows. It's a matter of contrast, and only for that particular type of plate.

When I realized that this was happening, the very first thing I did was to clean the plate on my wife's RAV4, with no effect. This weird lack of contrast has something to do with this particular type of specialty plate.

Overall, my LPR system works great, despite the angle. I capture most plates running OpenALPR on an Nvidia graphics card and operating both cameras at 2MP resolution @ 50 fps. But having realized what is happening, I am now wondering if there are other specialty plates with graphical backgrounds that I've been missing.

I'll experiment with the contrast and see if I can get better results.

 

[bigredfish]

the daytime pic isnt great. I asked about distance and and angle because the day image was mediocre at best.

I run contrast at 70-80

 

[wtimothyholman]

How does the plate look at night with a non-lpr custom tuned camera?

I'll try viewing the plate with my PTZ camera in its normal nighttime mode and see how readable it is.

 

[wtimothyholman]

the daytime pic isnt great. I asked about distance and and angle because the day image was mediocre at best.

I run contrast at 70-80

What is posted is actually a crop of a much larger image. It's a compromise between capturing a sufficiently wide view versus capturing enough pixels to identify the plate.

I'll crank up my contrast and see if that helps.

 

[richardgohth]

Take a photo of the plate at night with normal camera with flash

 

[Laserray]

Try a lot more Contrast, less DNR..

Looks like a tough angle? What distance?

I see plates like that, some are just old and dirty and hard to read... and local LE now uses some type of spray that doesnt reflect IR..

My question would be why does local PD spray their plates?

Op,
Question how many feet away? I see this on some plates. Dirty and temp plates in particular. It got better when I moved the target closer by changing angles, something you can't do.
More IR light? but if what bigredfish says is true well........

 

[K175un3]

You could try playing around with manual IR and shutter speed, as it kind of looks like glare and too high a shutter speed at night would loose detail.

Has anyone got the LPR tweaking or tuning link available please for wtimothyholman?

 

[bigredfish]

My question would be why does local PD spray their plates?

Op,
Question how many feet away? I see this on some plates. Dirty and temp plates in particular. It got better when I moved the target closer by changing angles, something you can't do.
More IR light? but if what bigredfish says is true well........

No idea... Im trying to find those caps but may have deleted them. I'll watch for them and get another.. I asked one of the deputies one night I was helping them with v ideo of a burglar we ended up helping put behind bars and he just shrugged, grinned and said he didnt know what i was talking about

 

[wtimothyholman]

I've tried increasing the contrast and turning off noise reduction. The RAV4's license plate is still washed out. But standard plates look just fine, as can be seen from the latest two images saved by my system (attached).

Her specialty plate is a women's veteran plate with a blue pattern on a white background. There is something about the plate that seems to scatter the IR and prevents the numbers from showing up.

I'm going to do some experiments this weekend using a -Z12E camera monitoring my alleyway, and see if I can find a combination of settings that makes the plate readable.

 

[bigredfish]

I’d say those caps are pretty good, congrats

 

[wtimothyholman]

I’d say those caps are pretty good, congrats

Thanks. Your suggestion to increase the contrast was a very good one. I just got a "hit" on a D* temporary paper tag, which I've never been able to read in the nighttime before. I always assumed that a paper tag wouldn't be readable. The image isn't great, but it can be seen.

My cameras seem quite capable of reading other types of specialty tags, e.g. the 9TG940 tag. It's the women's veteran tag that is the exception. I'm going to try different angles and settings this weekend and see if that makes a difference.

 

[bigredfish]

No idea... Im trying to find those caps but may have deleted them. I'll watch for them and get another.. I asked one of the deputies one night I was helping them with v ideo of a burglar we ended up helping put behind bars and he just shrugged, grinned and said he didnt know what i was talking about

Found 'em

Two different OCSO cars. I had called them about a burglary and happened to catch their tags..




Ad here's a normal capture from the same night

 

[wtimothyholman]

Found 'em

Two different OCSO cars. I had called them about a burglary and happened to catch their tags..

Ad here's a normal capture from the same night

That's very interesting. Whatever they're using is absorbing the UV light very effectively. If this stuff becomes widespread, it could put a real crimp in the utility of custom LPR systems, at least at night.

But the more interesting question is "why"? Are they anticipating the use of LPR by the criminal element, and trying to make their vehicles stealthy?

 

[bigredfish]

I think it may be less nefarious

We have a LOT of toll roads around Mickey Central. ...Stick it to the tourists..

 

[wtimothyholman]

I think it may be less nefarious

We have a LOT of toll roads around Mickey Central. ...Stick it to the tourists..

I was always under the impression that toll road cameras didn't use IR, but instead photographed plates using normal (bright) roadside lights for illumination. Regardless, this trick would still be useless in the daytime.

And perhaps that also explains why this trick isn't used more often; professional LPR systems installed on roads will have sufficient illumination to avoid the use of IR. The spray doesn't help.

 

[bigredfish]

The toll road cameras around here dont appear to use white light... but yes you're right wouldnt help in daytime.

We do have a backup in case thew two LPR cams dont catch your tag, we use an older 2MP Axis with really good light pickup and no IR and catch as they exit. We can only do thi because of the bright streetlight at the entrance..

 

[mezger]

I've been at it for 2 days with a 5241 Z12 I picked up from Andy. I'm at about 110' with a 50° angle. I have almost no artificial light, working ~100% from the camera's IR with maybe a little spillover I have from some banks pointed elsewhere. Did some rough dialing in by putting two plates on a stand on the side of the road, one dirty, one clean. Our plates don't have raised lettering. I found that a dirty plate is very difficult to read at night, while a clean plate is trivially easy to read. Will try the 'more contrast, less DNR' suggestion. If that fails, both plates have a detectable response to the camera's IR, so I may end up picking up >40W of <=10° focused IR to throw downrange.