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

[mezger]

Looks like caliche dust makes for a good IR block. Couldn't get the dirty plate with more contrast and less DNR. Tried 72W of not very well focused IR placed at the road. I can get it then, but it's still pretty weak. I imagine something focused and 50W would make a pretty noticeable difference.

Also, looks like my IPC-HFW5241E-Z12E has the same issue with losing its focus at day/night changeover as did the 5231. Will have to implement some form of remedy for that.

 

[erkme73]

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

I guess we'd have to know for certain that cop plates were deliberately/intentionally IR obscured. If so, I cannot think of any legitimate reason that doesn't involve some shielding from photo enforcement (tolls, red-light cameras, etc). There was a big story in the Sentinel a few years ago about police officers routinely using the Orlando tollways in their personal vehicles without transponders. There was no fear of 'pay-by-plate' since all officers' personal vehicle tags are scrubbed from all enforcement/billing databases. If you run a cop's personal tag, it comes back as blocked.

I suppose there is some legitimacy to this, as having access to home addresses of police officers is probably a security concern. The problem, as the article pointed out, was that more municipalities were adding non-LEO government workers to the same shielding (think city council members, maintenance department heads, firefighters, etc). And given the numbers of 'shielded' tags that routinely used the tollways (at 90+ MPH averages) without any recourse to collect fees - it seemed a bit abusive.

But, you'd think a marked (or even unmarked) patrol car could easily be identified via the numerous vehicle numbers on the rear, sides, and roof. It is a head scratcher.

 

[Purduephotog]

License plates include matte beads which are a 'retroreflector'. Obviously something is different with that tag- you should notice it immediately if you park it down the road a bit and shine a flashlight at it. Compared to your other vehicle, it doesn't light up.
My guess is that it's just regular paint, and that that paint doesn't include the overcoat of reflective beads (actually, internally reflective). So you're basically trying to image at night with no help. You'll need either more IR light or a flash or something to freeze the motion.

LE can probably just paint their plates the same color. Don't need to scatter IR light if you don't reflect it back.

 

[Purduephotog]

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?

No, they just don't believe people should know where they are... funny about that. All that surveillance aimed at the general population (read why you should block your toll-tag when not in use) and yet when it might be used on them they balk.

 

[Laserray]

Hopefully not off subject but does anybody have an IR reflective spaint pait I can use for targets? Say spraying an "X" on the road. Marks on posts etc.

 

[richardgohth]

can get single side adhesive retro reflective tape online

 

[Purduephotog]

Hopefully not off subject but does anybody have an IR reflective spraypaint I can use for targets? Say spraying an "X" on the road. Marks on posts etc.

"IR" paint is regular paint with tiny beads embedded in it. So... some adhesive targets is best. Look for 'retroreflective' tape. Depending on weatherproofing you can get away with cheap stuff.

 

[mech]

DOT retroreflective tape like Reflexite V82 is very reflective if that suits your purpose. You should be able to get it at auto-parts stores alongside reflectors and taillights intended for trailers. If you want something stealthy, there's also black retroreflective tape (search Ebay for "black reflective tape").

You can also get clear reflective spray paint, I have a can here of RustOleum 214944, UPC code 020066133221.

 

[Purduephotog]

DOT retroreflective tape like Reflexite V82 is very reflective if that suits your purpose. You should be able to get it at auto-parts stores alongside reflectors and taillights intended for trailers. If you want something stealthy, there's also black retroreflective tape (search Ebay for "black reflective tape").

You can also get clear reflective spray paint, I have a can here of RustOleum 214944, UPC code 020066133221.

That's cool, I thought it would be outlawed by now for microplastic beads. Interesting.

 

[th182]

License plates include matte beads which are a 'retroreflector'. Obviously something is different with that tag- you should notice it immediately if you park it down the road a bit and shine a flashlight at it. Compared to your other vehicle, it doesn't light up.
My guess is that it's just regular paint, and that that paint doesn't include the overcoat of reflective beads (actually, internally reflective). So you're basically trying to image at night with no help. You'll need either more IR light or a flash or something to freeze the motion.

LE can probably just paint their plates the same color. Don't need to scatter IR light if you don't reflect it back.

Is it possible the LE plates are not made on the same line/process as regular plates? Maybe they have a stock of blanks printed with the logo on them and then the city sign shop or someone adds the numbers on an as-needed based on what car it goes on? Therefore the lettering added doesn't have the reflective material a true plate would have. Probably out of convenience/cost savings rather than actually trying to be stealthy.

Here in MN we don't have actual license plates on squads but rather just plates that say POLICE. As far as I know the cars are registered with the DMV by VIN, just not assigned an actual plate number.

 

[mech]

Is it possible the LE plates are not made on the same line/process as regular plates? Maybe they have a stock of blanks printed with the logo on them and then the city sign shop or someone adds the numbers on an as-needed based on what car it goes on? Therefore the lettering added doesn't have the reflective material a true plate would have. Probably out of convenience/cost savings rather than actually trying to be stealthy.

Here in MN we don't have actual license plates on squads but rather just plates that say POLICE. As far as I know the cars are registered with the DMV by VIN, just not assigned an actual plate number.

In my state, the base layer of the plate is reflective, and the letters are NOT reflective, which is why they have good contrast in reflective situations. I suspect the odd plates are made differently, with a tinted overlay which either is functionally reflective in nature, or which is not opaque enough to block the reflectivity of the background. Hence the very low contrast. I've noticed that effect on a nearby state's specialty plates on civilian vehicles, which also lends support to the idea that they use a different process for low-volume special plates.