Lasers - How can they prove it was you?
Discussion
if it's a scamera van, it had a video camera recording everything on SVHS tape, and usually a printer that prints a picture of your car, complete with 'burned in' crosshairs and other details such as speed, 'TIMEOUT' - (shouldn't say that actually) etc.
If it's a plod on his own, then the laser only confirms his opinion. The one I used had sights on it, and divergence is about 3mRad. Or 30cm every 100 meters, so reasonably accurate.
If it's a plod on his own, then the laser only confirms his opinion. The one I used had sights on it, and divergence is about 3mRad. Or 30cm every 100 meters, so reasonably accurate.
bongotvr said:I believe they can't. The low level laser output (combined with the typical spread over distance) will be completely swamped by ambient light. They can only prove that the laser should be on target (in the crosshairs) from calibration documents.
If you a caught speeding by a policeman pointing a laser beam at you, how can they prove they were pointing it at you?!
(reposting because first attempt doesn't seem to be working)
I stand to be corrected.
Supraman,
They don't get swamped by ambient light I'm afraid.
The laser is more or less pure 905nM Infra Red coherent light, modulated at about 33kHz.
As the reciever has filters, both physical and electronic to detect this light, you would need a heck of a lot of ambient to swamped it, or something very similar (such as posh discharge bulbs that you see on BMW's, which flicker at, or at a harmonic to 33kHz)
30mW O/P gives you a theoretical range of about 20 miles if bounced off a retro-reflective object such as balatino (number plate backing)
They don't get swamped by ambient light I'm afraid.
The laser is more or less pure 905nM Infra Red coherent light, modulated at about 33kHz.
As the reciever has filters, both physical and electronic to detect this light, you would need a heck of a lot of ambient to swamped it, or something very similar (such as posh discharge bulbs that you see on BMW's, which flicker at, or at a harmonic to 33kHz)
30mW O/P gives you a theoretical range of about 20 miles if bounced off a retro-reflective object such as balatino (number plate backing)
I'm a bit confused!!
Scenario:
Driving down the motorway and a policeman in front of you hiding behind a bridge pointing a laser beam at you. You see him, but too late before he gets you with a laser, clocking you at approx 90mph. As you pass him, having slowed to 70 he sets off, and duly pulls you over.
Question:
If this happens, why can't I say I was doing 70, and he must have been pointing the laser at someone else?
His mistake, my word, his word?
Sorry if I am being stupid!
Scenario:
Driving down the motorway and a policeman in front of you hiding behind a bridge pointing a laser beam at you. You see him, but too late before he gets you with a laser, clocking you at approx 90mph. As you pass him, having slowed to 70 he sets off, and duly pulls you over.
Question:
If this happens, why can't I say I was doing 70, and he must have been pointing the laser at someone else?
His mistake, my word, his word?
Sorry if I am being stupid!
I *think* the legal position is that the bib forms the opinion you are speeding, points the gun at you, and the reading confirms or refutes that opinion. So in court, the gun is like a witness to bib opinion of speeding.
In the same way that the opinion of two bib is sufficient for a speeding conviction with no measurements at all, I believe.
Waiting for someone who knows the facts to confirm / deny...
In the same way that the opinion of two bib is sufficient for a speeding conviction with no measurements at all, I believe.
Waiting for someone who knows the facts to confirm / deny...
bryan35 said:
Supraman,
They don't get swamped by ambient light I'm afraid.
The laser is more or less pure 905nM Infra Red coherent light, modulated at about 33kHz.
As the reciever has filters, both physical and electronic to detect this light, you would need a heck of a lot of ambient to swamped it, or something very similar (such as posh discharge bulbs that you see on BMW's, which flicker at, or at a harmonic to 33kHz)
30mW O/P gives you a theoretical range of about 20 miles if bounced off a retro-reflective object such as balatino (number plate backing)
Thanks Bryan.
I wish to continue as I would like the answer to this.
What you say is correct with regard to the speed measuring module/sensor.
However, surely the video camera system (the only possible method for providing real evidence of laser position relative to target) won’t have ambient light rejection filters? otherwise the car/driver won’t be seen on the video, (it’s usual for video optics to have IR rejection filters). The video imager can’t distinguish the 33khz modulation (I thought it was lower, doesn’t really matter) as it is running at 50 fps.
edited for noobie use of English
>> Edited by supraman2954 on Monday 9th May 14:09
Size Nine Elm said:
Type approval and a calibration certificate prove that it was pointing at exactly what it shows on the video.
The S172 will show (eventually) who was in the target vehicle.
Policing by numbers...
There is nothing to say the laser beam from the gun is pointing at the same spot as the crosshairs in the video unless as aiming test was carried out at initiation of setting up. Hence why you should always insist on seeing this evidence. (rarely available may I add) and an issue coming to light in a few court cases.
The 'TIMEOUT' appears in the display shortly after the reading is given.
This prevents the scam man then pointing the laser at someone else.
If 'TIMEOUT' is on the evidential image, it aint evidence, as the speed reading could have been from anywhere.
It goes like this.........
Point gun,
press trigger,
44 light pulses over 0.3 seconds,
recieved pulses go through the various error trapping algorithms etc.
A speed reading (or error reading) is displayed,
VERY shortly afterwards 'TIMEOUT' appears.
The video therefore has a few frames of video with image of a car with a speed reading, then 'TIMEOUT'
If not, you could use one fast travelling car as the speed for everyone.
I'd have thought that you'd know that Destroyer?
This prevents the scam man then pointing the laser at someone else.
If 'TIMEOUT' is on the evidential image, it aint evidence, as the speed reading could have been from anywhere.
It goes like this.........
Point gun,
press trigger,
44 light pulses over 0.3 seconds,
recieved pulses go through the various error trapping algorithms etc.
A speed reading (or error reading) is displayed,
VERY shortly afterwards 'TIMEOUT' appears.
The video therefore has a few frames of video with image of a car with a speed reading, then 'TIMEOUT'
If not, you could use one fast travelling car as the speed for everyone.
I'd have thought that you'd know that Destroyer?
jonnyb said:
In law the bibs opinion is worth more than yours. So they will always err on the side of the bib if it comes down to your word against theirs.
Totally wrong I'm afraid.
In practice in the courts that happens, but it is completely untrue that a police officer's word is in some way more valuable, believable or truthful than any other credible witness.
The onus of proof is always on the Crown, NOT the accused; you have to really stick to your guns in court and often remind them of that.
cptsideways said:
Type approval and a calibration certificate prove that it was pointing at exactly what it shows on the video.
The S172 will show (eventually) who was in the target vehicle.
Policing by numbers...
There is nothing to say the laser beam from the gun is pointing at the same spot as the crosshairs in the video unless as aiming test was carried out at initiation of setting up. Hence why you should always insist on seeing this evidence. (rarely available may I add) and an issue coming to light in a few court cases.
That's what I meant by calibration...
It doesn't prove scientifically that the laser is functioning, but it is sufficient to 'prove' it in a court.
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