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PostPosted: Fri Aug 06, 2010 14:52 
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If you deliberately take measures to avoid detection for an offence that would come under conspiring to pervert the course of justice, precisely what they use to prosecute those that lie about who was in the car at the time of a speeding offence detection.
The anpr cameras also use an inverted image, the "negative".
They also use polarised filters.

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56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing


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PostPosted: Fri Aug 06, 2010 15:15 
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jomukuk wrote:
The anpr cameras ... also use polarised filters.

Really? (I'm not disputing this)
Do you know why, and which direction/type the filter is?

This seems odd for a reflection straight from a retroreflective surface - unless the emitted light from the array is polarised?

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PostPosted: Fri Aug 06, 2010 16:34 
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Yeah, it's true that anyone who tries to evade prosecution for diddling the congestion charge could be accused of perverting the course of justice. And any tampering with a number plate is strictly illegal too. But it's not heinous, is it? I broke the law last Friday and I plan to do the same tonight - drunk on the way back from the pub in a public place. But I didn't nip over next door's hedge, put their window out, punch them in the eye and nick their telly. Shades of grey in my humble opinion.

I'd guess an inverted image helps with the automatic character recognition algorithms. But a polarising filter? Yeah, perhaps if the IR LED illumination is polarised, but otherwise all I can see it doing is reducing intensity. I'd VERY interested though if you know more...

Ash


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PostPosted: Fri Aug 06, 2010 17:22 
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Why not just conform to having the correct clean number plates instead of running the risk of being arrested and charged with attempting to or perverting the course of justice. C'mon Lads! get back to the real world. I have insider dealings with the ANPR system, It has it's faults but its not stupid!


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PostPosted: Fri Aug 06, 2010 17:55 
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Its very far from stupid, you're right. But I've happily saved a fair few bob when driving in and out of central London by working out it's foibles and getting round them. Maybe I'd change my tune if I was in court charged with perverting the course of justice, but my assessment of the chance of that happening is pretty small. Next to nothing, actually. Not without some sort of lower level warning at least, like being pinched by a mobile unit.

Nah - I don't like the idea that someone, somewhere can track me from one end of the country to the other. I didn't vote for that. So it's two fingers and 'bol*ocks' from me.

Am I alone?


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PostPosted: Fri Aug 06, 2010 18:55 
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jomukuk wrote:
If you deliberately take measures to avoid detection for an offence that would come under conspiring to pervert the course of justice

Obstructing a police officer in the execution of his duty, surely. Perverting the course of justice can only only occur after the miscreant has been charged.[/quote]

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PostPosted: Fri Aug 06, 2010 20:01 
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In English, Canadian or Irish, perversion of the course of justice is a criminal offence in which someone acts in a manner that in some way prevents justice being served on either themselves or on a third party. Perverting the course of justice is an offence in common law. It carries a theoretical maximum sentence of life imprisonment, although no sentence of more than 10 years has been handed down in the past one hundred years


https://encrypted.google.com/search?hl=en&q=polarising+filters+in+anpr+cameras&aq=f&aqi=&aql=&oq=&gs_rfai=

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56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing


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PostPosted: Fri Aug 06, 2010 20:47 
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Ah - thanks for that!

Polarising filters being used to reduce windscreen reflections in an imaged scene. In order to better view a vehicle occupant. Magic! How long before ANPR moves on from reading number plates to recognising faces? Don't scoff - facial recognition software is already on the market.

Yeah, but they're not interested in you unless you have something to hide...

Come on people - wake up!!!!!


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PostPosted: Fri Aug 06, 2010 21:27 
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Ash wrote:
Polarising filters being used to reduce windscreen reflections in an imaged scene. In order to better view a vehicle occupant.

I already knew that (forum link) ;)

My question was it relating to the ANPR part of the system.

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PostPosted: Fri Aug 06, 2010 22:33 
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Steve wrote:
Ash wrote:
Polarising filters being used to reduce windscreen reflections in an imaged scene. In order to better view a vehicle occupant.

I already knew that (forum link) ;)

Did you use a sheet of polarising material? If so, it's possible it interfered with the focussing. Use a Hoya PL filter instead of a Cokin one and you will see what a difference a quality filter makes!

There is good answer to the tactic of using polarising filters to see through the glare - use a 90° screen sized polarising filter, and YOU can see through it while driving* - but anything with a polarising filter which would normally eliminate glare, would render the screen as BLACK! :lol:

The only difficulty is the availability of such a large piece of material, it's density, and the fact you *could not wear polarising shades in the car while driving! :(

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PostPosted: Fri Aug 06, 2010 22:36 
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Ernest Marsh wrote:
Did you use a sheet of polarising material? If so, it's possible it interfered with the focussing. Use a Hoya PL filter instead of a Cokin one and you will see what a difference a quality filter makes!

Oh don't worry about that; my toys have evolved well beyond that old crap :D :cloud9:

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PostPosted: Sat Aug 07, 2010 07:10 
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Quote:
Perverting the course of justice can be any of three acts:

* Fabricating or disposing of evidence
* Intimidating or threatening a witness or juror
* Intimidating or threatening a judge

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PostPosted: Sat Aug 07, 2010 08:05 
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Some degree of polarisation of reflected light occurs at any optical interface or surface, except at normal incidence. In the context of a camera viewing a number plate, the number plate is effectively a single surface. The reflective element is the white/yellow sheet material onto which the numbers are mounted. There will be polarisation when light reflects from it.

However, if there is a polarising filter over the CCD that captures that reflected light, the only thing it will do is reduce the intensity of the image to no advantage. The only conceivable way such a filter might help is if the plate was illuminated by considerable 'extraneous' polarised light, maybe from strong sunlight reflecting from a puddle underneath the front of the car or if the sun was illuminating the scene from a peculiar angle.

People who are using polarising filters over cameras are very specifically aiming to reduce reflections from windscreens in order to see through them. I didn't know that. Presumably forward facing speed cameras do that?

I like it on here - I'm beginning to learn stuff!


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PostPosted: Sat Aug 07, 2010 12:20 
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Ash wrote:
Some degree of polarisation of reflected light occurs at any optical interface or surface, except at normal incidence. In the context of a camera viewing a number plate, the number plate is effectively a single surface. The reflective element is the white/yellow sheet material onto which the numbers are mounted. There will be polarisation when light reflects from it.

What you say is correct, but it doesn't entirely apply in this case.
A registration plate does not act like a single surface. There is the top glossy layer, (followed by the characters) followed by the retro-reflective layer.
The registration plate is retro-reflective (ball type, not corner cube), so the reflected light (back to the camera from its emitter) is always at 'normal incidence'.

Ash wrote:
However, if there is a polarising filter over the CCD that captures that reflected light, the only thing it will do is reduce the intensity of the image to no advantage.

I've just a quick test with VRM plates, torches and polarising filters.
The plate preserves the polarisation angle of the emitted (and reflected) light. When setup properly, the plate appears comparatively brighter than everything else. So using polarised filters over the emitter and imager does give a slight, but not huge, advantage over non-polarised light sources.

Ash wrote:
The only conceivable way such a filter might help is if the plate was illuminated by considerable 'extraneous' polarised light, maybe from strong sunlight reflecting from a puddle underneath the front of the car or if the sun was illuminating the scene from a peculiar angle.

That makes sense too. I've just tried it and confirmed the effect :bighand: The loss of contrast is eliminated.

Something else I've noticed, it also removes the reflection of the light from the emitter array, specularly bouncing off the plate, specularly bouncing off the puddle, back to the camera, so preventing a ghost image of the plate on the ground.
Placing a filter over the emitter array again gives the slight advantage as described before.

Tell you what, the next time I pass SPECs (not driving) I will check it; I have lots of useful kit :D

Ash wrote:
People who are using polarising filters over cameras are very specifically aiming to reduce reflections from windscreens in order to see through them. I didn't know that. Presumably forward facing speed cameras do that?

I know for a fact that at least some speed cameras have exactly that, for exactly that reason.

Ash wrote:
I like it on here - I'm beginning to learn stuff!

I did say you might like it here :D

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PostPosted: Sat Aug 07, 2010 13:07 
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Thanks for that, Steve - very interesting.

Steve wrote:
I have lots of useful kit :D


Yeah? Me too. Mind, I'm not sure if this is the right place to discuss how to use it to 'best effect'.

Ash


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PostPosted: Sat Aug 07, 2010 14:21 
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Ash wrote:
....I don't like the idea that someone, somewhere can track me from one end of the country to the other. I didn't vote for that. So it's two fingers and 'bol*ocks' from me.

Am I alone?


:clap:

No you are not alone; I fully agree with you. Spying on us in that fashion is not acceptable.

If I had the ability to successfully evade such systems I'd definitely use it.

Best wishes all,
Dave.


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PostPosted: Sat Aug 07, 2010 15:19 
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:clap:

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.


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PostPosted: Sat Aug 07, 2010 15:27 
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You mean something like this?
Attachment:
Plate cover.JPG [21.98 KiB]
Downloaded 326 times


520x110mm sized IR dichroic. A device that splits a light spectrum into a transmitted porton and a reflected portion is called a dichroic. And there's only one way of doing it.

In this image it's offset to the right of the plate on my old Vette. This is viewed in a PIPS technology Sony 540TVL exwave CCD camera. Gives greater than 95% reflectivity from 750-1100nm at angles up to about 50 degrees, yet greater than 90% transmission from 420-680nm. So it's absolutely transparent to you, me and that policeman over there. But in the vast majority of ANPR cameras it looks like this.

There's a couple of disadvantages to it's use, not least of which it's illegality, but it works.

Ash


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PostPosted: Sat Aug 07, 2010 15:35 
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And apologies for the relatively poor quality of the pic, but it's been cut and pasted half a dozen times to be able to put it up here from the laptop I'm on at the moment.

If there's any interest, I guess I could do some new pics.

Ash


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PostPosted: Sat Aug 07, 2010 19:21 
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A patent application:-

http://www.ipo.gov.uk/p-find-publicatio ... umber=6285

Makes it sound alost legal! :roll:


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