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A Review Of Laser Speed Meters, A critical look at Laser Speed Meters
Paul D. Lee
post Thu, 10 Jul 2008 - 21:25
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For ease of reference, here is the latest copy of my Laser Speed Meters Report.

Attached File  report_v1_2_rqp.pdf ( 2.18MB ) Number of downloads: 2134


Attached File  beam_spread_wrong_clip_1_small.mpg ( 3.48MB ) Number of downloads: 1555

Attached File  beam_spread_wrong_clip_2_small.mpg ( 3.43MB ) Number of downloads: 729

Attached File  beam_spread_correct_clip_1_small.mpg ( 2.91MB ) Number of downloads: 729


(Edited 15 July 2008, with latest version of report, and example "Beam Spread" Clips)

This post has been edited by Paul D. Lee: Wed, 30 Jul 2008 - 22:06
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Mortimer
post Thu, 15 Oct 2009 - 12:09
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Which is why an up-to-date model of speed measuring device is unlikely to be allowed to be scrutinised closely in the way the device in the report was. Because the porcine DNA will be revealed perhaps?


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nomadros
post Thu, 15 Oct 2009 - 12:15
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QUOTE (Laura Bidding @ Thu, 15 Oct 2009 - 05:57) *
snip

In the years I was involved in speed enforcement (I am no longer), my unit had four models of LTI, each with a different software version. Every model was designed to give a speed reading but each progressive model also brought with it other ‘benefits’. These included less weight due to smaller components, different power options, digital compatibility which means far less storage space needed as video tapes not used, plus reduced viewing time when searching the resulting DVD. There were also software developments which dramatically improved the system’s abilities in poor weather conditions such as rain, something not available on early models/software versions.

snip


My FOI request to the HO asked if there had been any changes to any LTi model since their introduction and the reply was "none." There also had been no updates to the original TA for any model as there had been no changes. Given a TA covers hardware and software, I would have thought that the above suggests that there have been changes.
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bama
post Thu, 15 Oct 2009 - 12:26
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The requirements have not changed.

'up to date' is spurious. IF the original solution actually worked.

In fact the updates themselves testify that the device was not 'fully functional' when released.

Software maintenance comes in three flavours - corrective, adaptive and perfective.

Read about them here http://books.google.co.uk/books?id=FXiRmlJ...ive&f=false

and see if there could be any reason for any software maintenance to the LTI IF it worked out of the trap.

the answer is no IMO.


--------------------
Which facts in any situation or problem are “essential” and what makes them “essential”? If the “essential” facts are said to depend on the principles involved, then the whole business, all too obviously, goes right around in a circle. In the light of one principle or set of principles, one bunch of facts will be the “essential” ones; in the light of another principle or set of principles, a different bunch of facts will be “essential.” In order to settle on the right facts you first have to pick your principles, although the whole point of finding the facts was to indicate which principles apply.

Note that I am not legally qualified and any and all statements made are "Reserved". Liability for application lies with the reader.
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nomadros
post Thu, 15 Oct 2009 - 15:39
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All I know (as a full time, seriously well paid, software developer) is that any update, for whatever reason, has to be tested and then achieve client approval before release.

In the case of a product whose software testing is done prior to sale, the client (purchaser) would in all cases test the product prior to purchase and the results of those tests would form part of that approval to purchase.

In this case, testing for statutory approval for use after purchase would mean that software version x.y has been proven for use on hardware version G.

However the "same" product with version Y.z even on hardware version G is a totally different can of worms and you cannot rely on the manufacturer's word that everything is still kosher, whether the previous version worked lilke a charm or was a bug ridden POS.

If those guns don't have the same software and/or hardware that was originally tested for approval, then they are not the same guns and don't have type approval.....IMVHO.

Just got to convince a court of that though!
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bama
post Thu, 15 Oct 2009 - 15:52
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agreed.


--------------------
Which facts in any situation or problem are “essential” and what makes them “essential”? If the “essential” facts are said to depend on the principles involved, then the whole business, all too obviously, goes right around in a circle. In the light of one principle or set of principles, one bunch of facts will be the “essential” ones; in the light of another principle or set of principles, a different bunch of facts will be “essential.” In order to settle on the right facts you first have to pick your principles, although the whole point of finding the facts was to indicate which principles apply.

Note that I am not legally qualified and any and all statements made are "Reserved". Liability for application lies with the reader.
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SMURF POWER
post Thu, 15 Oct 2009 - 18:25
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QUOTE (Laura Bidding @ Thu, 15 Oct 2009 - 05:57) *
......my unit had four models of LTI, each with a different software version. ...... .[/size][/font]

To clarify would it be the 4 below
1.Marksman
2.TSM
3.100
4.1000
Or was it 4 TSM with diff' software, I suspect they were four LTI models which can all have different software which relate to the UK version of the device.


QUOTE (andy_foster @ Thu, 15 Oct 2009 - 06:33) *
The problem highlighted by Paul's report is a direct result of a fundamental part of how the device was designed to work - it 'locks on' to the first signal received above a pre-set threshold. Unless this has been changed, or the beam divergence substantially reduced, the device would almost certainly produce the same results even with the new super-wizzy software.

As Toad Hall, who is apparently in charge of selecting devices for HOTA testing, was at the Elvington tests, one would assume that if they had ironed out this 'undocumented feature', he'd have made some noise about it.
However, he and his boss Med "90 in a 60 limit" Hughes seem to believe that the integrity of devices relied on in prosecutions can best be assured by threatening to slap anyone who dares to challenge them with substantial and disproportionate costs.

If you know how we can get borrow a current HOTA UK police spec device without having to get a Crown Court judge to order it, I'm sure that we could arrange another test.


You are spot on with much of what you say… how ever when you do it again perhaps start at the distance the bike was caught and not at the 500 - 600 + metres the engineered test were conducted at. Engineering failure is very much different to reality …..

QUOTE
Out of many hundreds of thousands of trigger pulls and umpteen detected offences, I never had cause to doubt what was being displayed on the back of the speedscope or being recorded. If the system had given me cause to question its results, I would have aborted the check and reported the symptoms. I never had cause to do that. I have no reason to lie. I have no vested interest. I don’t owe anyone anything and have nothing to gain by fibbing.
So do I have any basis for saying that newer equipment would not fail under the same circumstances?
LTI officer
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smeggy
post Wed, 5 Jan 2011 - 23:46
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I've given some thought to the issue of the speculated 'algorithm error' described in page 61 of the report.

This failure mode may seem plausible, but my own testing (with an actual TS/M, one that was still used 'in anger' well after those Elvington tests) indicated no such mechanism.

An alternative explanation may be a consequence of the well-known 'gaps' of the emitted laser beam pattern.
These gaps are expected; however, the actual gaps are variable. They are difficult to measure, so this usually remains unknown for individual LTIs.
The beam is emitted from three apertures; all are vertical, narrow strips, horizontally adjacent to each other. The resulting emitted beam is merely a projection of this arrangement, be it somewhat out of focus (as well as being proportionally larger with distance).

The attachment is an animated gif. It shows how the alternative explanation is possible.
This method of presentation was chosen to enable easy comparison and confirmation of appropriate placements and scalings; I've tried to be diligent with these. I don't claim pixel-perfect accuracy, but it is good enough for the simple explanation.
The images contained within are from several different runs, all extracted from the report.
All the images have the same (actual) LTI beam pattern superimposed onto them, showing what surfaces could be struck and missed by the laser. The vertical turquoise lines are to help indicate the boundaries of the lobes.
The alignments of the laser/sighting scope/video camera are reasonably assumed as fixed throughout.



As per the guidelines: operators and processing staff should not initiate or process cases where there are multiple vehicles within the active area; however, they're not perfect. Evidence showing multiple vehicles in close ‘line-of-sight’ proximity must be carefully scrutinised as there is evidently a significant risk of innocent drivers being prosecuted.

This is still one of the failings of the LTI, even if it is for a different reason than expected.

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smeggy
post Thu, 27 Jan 2011 - 22:31
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I've had some questions posed to me by PM.

I will reproduce the relevant content in this thread so that everyone can digest them.

QUOTE
If your postulation is true then how do you account for this effect not showing up all the time, firstly at Elvington (it only happened once yet the tests were repeated multiple times),

It happened twice at Elvington.
My explanation shows both these events:
1) 12:21:57NR19
2) 12:27:33NR36

QUOTE
secondly during pre-post session alignment checks,

- Was it checked for?
- Do people know what to look out for?

Given the 'algorithm error' was first postulated subsequent to the Elvington testing, I would say the answer to both my questions are "no".

QUOTE
and thirdly during enforcement sessions on motorbikes?

I have repeatedly given my answer to this on the open forums. There are procedures and processes in place to prevent this type of error resulting with a prosecution. Operators should not target when multiple vehicles are in the active area; the processing staff also check for this.

So it shouldn't happen anyway. Even if it does, the case should be dropped due to the risk of error, regardless of whether a speed error is actually apparent.

Further to that are the circumstances needed to induce this error. Many other things have to come together:
- The optics must be set to have the gaps in the beams
- Targets must be distant enough for size of gaps to be significant enough
- The vehicle further behind must have highly reflective surfaces within the lobes; this is not a given.

QUOTE
At Elvington a Leica plate 10cm x 15cm was used vertically at 200m distance. Beam spread was measured at 4.3mR so we have a 10cm plate and an 86cm beam. If the beam has gaps that matter then the result should have been wee-waa-wee-waa-wee during alignment, but it was not.

How do you know it wasn't? Was it tested for? There is no mention of such testing in PDL’s report.

Don't forget, the gaps get larger the further the further away the target surface is; ideally, this would be directly proportional.
Then there is the further issue of focus, meaning the gaps will (and does) become disproportionately larger with distance – like I said in the open forums: the pattern becomes more defined with distance.

Further to that is the issue of implied intensity that I recently postulated and confirmed.
At 200m, gun could be getting enough of a signal from other parts of the bike, meaning a staggered (stretched) return, enough to fool the gun into deducing a strong signal, even when the return has actually become substantially weaker because a small part of the beam caught the edge of the plate. You will need to read my post on the matter to understand this.

I speak from first-hand, expert experience: moving the gun by hand to get the waa-wee-waa-wee-waa-wee-waa is difficult to do. Don’t forget we’re talking about pans of only 1/20th of a degree (from gap to lobe).
I have a motorised, star-tracking tripod that has a full manual mode via remote control, to allow very slow panning; this is the way to do it.

QUOTE
If there were such a figurative and literal hole in the operation, I'm pretty sure someone at LTI would have noticed this during development.

That literal hole exists, there is no (genuine) question of it.
It has been successfully explained, as well as confirmed by actual testing.

Do you think it would have made any different to LTI if they had or hadn't been aware of these lobes?

I have LTI documentation (1993) that unequivocally shows that Jeremy Dunne knows about how the beams are projected – yes through air! I’ve just now checked it again. He indeed has taken measurements of them, and his results are in total agreement to mine – a simple projection, resulting with thin strips of illumination on the target surface (assuming correct focus); there is no multi-modal 'smearing'.
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Hoof_Hearted
post Sat, 2 Apr 2011 - 13:03
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QUOTE (smeggy @ Thu, 27 Jan 2011 - 23:31) *
I've had some questions posed to me by PM.

I will reproduce the relevant content in this thread so that everyone can digest them.

QUOTE
and thirdly during enforcement sessions on motorbikes?

I have repeatedly given my answer to this on the open forums. There are procedures and processes in place to prevent this type of error resulting with a prosecution. Operators should not target when multiple vehicles are in the active area; the processing staff also check for this.

So it shouldn't happen anyway. Even if it does, the case should be dropped due to the risk of error, regardless of whether a speed error is actually apparent
.


In principle, you are 100% correct. And when the alleged speed is recorded on video (by either a proper BiB, or a civvy SCP operator) - those checks and balances should indeed be carried out by the processing staff - and those cases be dropped. But this isn't always the case - there are many examples, on many differing forums, where drivers have been summonsed when two vehicles are clearly within the defined target area.

But the real crux of concern is when the laser readings are obtained by a Police officer, and where the laser device is NOT linked to any kind of video recording device - I'm 100% confident in my opinion that in these instances, these 'checks and balances' are NOT complied with.


QUOTE (smeggy @ Thu, 27 Jan 2011 - 23:31) *
Don't forget, the gaps get larger the further the further away the target surface is; ideally, this would be directly proportional.


You are correct that the gaps get larger as the distance increases - but you fail to mention that the 'edge defintion' of the beams becomes LESS defined as distance increases. See my next sentence for the explanation.


QUOTE (smeggy @ Thu, 27 Jan 2011 - 23:31) *
Then there is the further issue of focus, meaning the gaps will (and does) become disproportionately larger with distance – like I said in the open forums: the pattern becomes more defined with distance.


That is just plain wrong. Unless you are operating the laser in a VACUUM !!! In 'real life', air is contaminated with airbourne dust. And just as the number of oxygen (and nitrogen, and other 'gases' which make up the atmosphere) molecules will be exponentially different from say one cubic metre of air compared to 100 cubic metres - so does the amount of 'dust particles' increase. Each one of these dust particle contaminents can distort and/or deflect the laser beam - and is therefore fundamental 'science' which would dictate that the actually accuracy of the 'focus' will deteriorate with increasing distance.


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Thanks, H_H
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justforthepictur...
post Sat, 2 Apr 2011 - 15:06
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QUOTE (Hoof_Hearted @ Sat, 2 Apr 2011 - 14:03) *
But the real crux of concern is when the laser readings are obtained by a Police officer, and where the laser device is NOT linked to any kind of video recording device - I'm 100% confident in my opinion that in these instances, these 'checks and balances' are NOT complied with.

Just to confirm what you are saying?

You much prefer the actions of the operator (whoever that may be), to be recorded to some form of media (tape or DVD), so there is a way of verifying exactly what was done in the way of checks?

Can you list the ‘checks and balances’ you refer to that should be carried out by the processing staff?

You also say you are 100% confident that there are checks and balances that aren’t done by police officers. What are they and how you have arrived at such a stoic conclusion?

Incidentally, what is the relevance of your ‘That is just plain wrong. Unless you are operating the laser in a VACUUM !!!’

Isn’t the vacuum of space packed with floating particles? I was taught that the Earth collects about 40,000 tons (35,000 metric tons) of dust in one annual trip around the Sun.

Also, you state that ‘Each one of these dust particle contaminants can distort and/or deflect the laser beam - and is therefore fundamental 'science' which would dictate that the actually accuracy of the 'focus' will deteriorate with increasing distance.’

What particle density would it take to deflect the laser beam? Over what distance? Do you mean that the particles would form enough of a barrier to deflect it so it would actually no longer travel in a straight line?

This post has been edited by justforthepictures: Sat, 2 Apr 2011 - 18:42


--------------------
'Speed Kills' simply means, hit something or someone fast enough and it will result in a fatality.
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smeggy
post Sat, 2 Apr 2011 - 19:15
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QUOTE (Hoof_Hearted @ Sat, 2 Apr 2011 - 13:03) *
QUOTE (smeggy @ Thu, 27 Jan 2011 - 23:31) *
Then there is the further issue of focus, meaning the gaps will (and does) become disproportionately larger with distance – like I said in the open forums: the pattern becomes more defined with distance.


That is just plain wrong. Unless you are operating the laser in a VACUUM !!! In 'real life', air is contaminated with airbourne dust. And just as the number of oxygen (and nitrogen, and other 'gases' which make up the atmosphere) molecules will be exponentially different from say one cubic metre of air compared to 100 cubic metres - so does the amount of 'dust particles' increase. Each one of these dust particle contaminents can distort and/or deflect the laser beam - and is therefore fundamental 'science' which would dictate that the actually accuracy of the 'focus' will deteriorate with increasing distance.

What I said was actually correct. The effect you mention is a technically valid one to raise, but it is merely additional and doesn’t invalidate what I said.

The issue you raise is more of an issue for shorter wavelengths (Rayleigh scattering: is why the sky is blue). The much longer 904nm is far less affected, although that is not to say there is no effect as that wavelength. However, for the effect at 904nm to be significant, that would inescapably result with the effect being much more obvious at wavelengths we can see - especially green colours, but there is no such effect there.

I have photos of various LTI beams at various distances. There isn’t significant edge spread (that could be detected) beyond what could be attributed to diffraction or the lack of focus.

This post has been edited by smeggy: Sat, 2 Apr 2011 - 20:45
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The Rookie
post Sun, 3 Apr 2011 - 20:33
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QUOTE (Hoof_Hearted @ Sat, 2 Apr 2011 - 14:03) *
and thirdly during enforcement sessions on motorbikes? I have repeatedly given my answer to this on the open forums. There are procedures and processes in place to prevent this type of error resulting with a prosecution. Operators should not target when multiple vehicles are in the active area; the processing staff also check for this.

So it shouldn't happen anyway. Even if it does, the case should be dropped due to the risk of error, regardless of whether a speed error is actually apparent
.
In principle, you are 100% correct. And when the alleged speed is recorded on video (by either a proper BiB, or a civvy SCP operator) - those checks and balances should indeed be carried out by the processing staff - and those cases be dropped. But this isn't always the case - there are many examples, on many differing forums, where drivers have been summonsed when two vehicles are clearly within the defined target area.

Disagree, its the operators responsibility to be sure there is suitable targetting, and we rarely see video or still were another car is in the 'target area (by which I read as the area covered by the laser, not jsut the area in video shot.

It may be the case with unmanned radar devices, but not with laser.

Simon


--------------------
There is no such thing as a law abiding motorist, just those who have been scammed and those yet to be scammed!

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justforthepictur...
post Mon, 4 Apr 2011 - 16:28
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QUOTE (The Rookie @ Sun, 3 Apr 2011 - 21:33) *
QUOTE (Hoof_Hearted @ Sat, 2 Apr 2011 - 14:03) *
and thirdly during enforcement sessions on motorbikes? I have repeatedly given my answer to this on the open forums. There are procedures and processes in place to prevent this type of error resulting with a prosecution. Operators should not target when multiple vehicles are in the active area; the processing staff also check for this.

So it shouldn't happen anyway. Even if it does, the case should be dropped due to the risk of error, regardless of whether a speed error is actually apparent
.
In principle, you are 100% correct. And when the alleged speed is recorded on video (by either a proper BiB, or a civvy SCP operator) - those checks and balances should indeed be carried out by the processing staff - and those cases be dropped. But this isn't always the case - there are many examples, on many differing forums, where drivers have been summonsed when two vehicles are clearly within the defined target area.

Disagree, its the operators responsibility to be sure there is suitable targetting, and we rarely see video or still were another car is in the 'target area (by which I read as the area covered by the laser, not jsut the area in video shot.

It may be the case with unmanned radar devices, but not with laser.

Simon

I will add to that;

There will be instances where although the target vehicle was isolated at the point of pulling the trigger, the chronological lag before the asterix appears can often mean that another vehicle may then encroach into the 'offence shot', i.e. the moment when the speed reading has appeared.

This is relatively common with 'crossover', when detecting across opposing lines or multi-carriageways of uni-directional traffic. Whilst it is the person behind the LTI who dictates what is recorded, it isn't always possible to obtain 100% clean evidence due to unanticipated vehicular interaction.

Although processing unit viewing teams are usually very strict in what they allow through, it isn't unknown for the resulting witness statements that need your signature to include a copy of the offence shot where another vehicle has crept into the field of view.

Now of course, this would have had no bearing on the speed of target vehicle when measured, but it does encourage debate as to the uncertainty of the accuracy of the ping. Therefore. no signature would be added, a note attached saying you aren't prepared to defend this and the case would be dropped.

Obviously, statement signing is some way down the line, but very few questionable pings make it as far as court, because they are filtered out at some stage long before making it that far.

This post has been edited by justforthepictures: Wed, 6 Apr 2011 - 07:13


--------------------
'Speed Kills' simply means, hit something or someone fast enough and it will result in a fatality.
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smeggy
post Wed, 6 Apr 2011 - 16:26
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QUOTE (justforthepictures @ Mon, 4 Apr 2011 - 16:28) *
QUOTE (The Rookie @ Sun, 3 Apr 2011 - 21:33) *

QUOTE (Hoof_Hearted @ Sat, 2 Apr 2011 - 14:03) *

... And when the alleged speed is recorded on video (by either a proper BiB, or a civvy SCP operator) - those checks and balances should indeed be carried out by the processing staff - and those cases be dropped. But this isn't always the case - there are many examples, on many differing forums, where drivers have been summonsed when two vehicles are clearly within the defined target area.

Disagree, its the operators responsibility to be sure there is suitable targetting, and we rarely see video or still were another car is in the 'target area (by which I read as the area covered by the laser, not jsut the area in video shot.

It may be the case with unmanned radar devices, but not with laser.

Simon

I will add to that;

There will be instances where although the target vehicle was isolated at the point of pulling the trigger, the chronological lag before the asterix appears can often mean that another vehicle may then encroach into the 'offence shot', i.e. the moment when the speed reading has appeared.

This is relatively common with 'crossover', when detecting across opposing lines or multi-carriageways of uni-directional traffic. Whilst it is the person behind the LTI who dictates what is recorded, it isn't always possible to obtain 100% clean evidence due to unanticipated vehicular interaction.

Although processing unit viewing teams are usually very strict in what they allow through, it isn't unknown for the resulting witness statements that need your signature to include a copy of the offence shot where another vehicle has crept into the field of view.

Now of course, this would have had no bearing on the speed of target vehicle when measured, but it does encourage debate as to the uncertainty of the accuracy of the ping. Therefore. no signature would be added, a note attached saying you aren't prepared to defend this and the case would be dropped.

Obviously, statement signing is some way down the line, but very few questionable pings make it as far as court, because they are filtered out at some stage long before making it that far.

An intelligent response.

If I may add to it:

At close ranges: there isn’t much of an issue because the size of the laser dot (at the target distance) is small, so it likely won't hit multiple vehicles (excluding secondary bounces).

When enforcing over large distances: the camera set up will be very telephoto (400mm sitting on a 6x crop sensor = 2400mm). It will be difficult to align the camera to set in perfect alignment with the laser i.e. with the centres overlapping.
Therefore, seeing two vehicles within the crosshairs doesn’t mean the laser actually hit both vehicles; that apparent error could merely be a result of the camera being only just within alignment.

The issue of camera alignment has always been a contentious one, and has previously cast reasonably grave doubts on speculations of various “false readings”.
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norahl
post Wed, 6 Apr 2011 - 17:48
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QUOTE (justforthepictures @ Mon, 4 Apr 2011 - 17:28) *
... the chronological lag before the asterix appears can often mean that another vehicle may then encroach into the 'offence shot', i.e. the moment when the speed reading has appeared...

...and what would that lag be?
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Jimzzr
post Wed, 6 Apr 2011 - 17:51
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QUOTE (smeggy @ Wed, 6 Apr 2011 - 16:26) *
QUOTE (justforthepictures @ Mon, 4 Apr 2011 - 16:28) *
QUOTE (The Rookie @ Sun, 3 Apr 2011 - 21:33) *

QUOTE (Hoof_Hearted @ Sat, 2 Apr 2011 - 14:03) *

... And when the alleged speed is recorded on video (by either a proper BiB, or a civvy SCP operator) - those checks and balances should indeed be carried out by the processing staff - and those cases be dropped. But this isn't always the case - there are many examples, on many differing forums, where drivers have been summonsed when two vehicles are clearly within the defined target area.

Disagree, its the operators responsibility to be sure there is suitable targetting, and we rarely see video or still were another car is in the 'target area (by which I read as the area covered by the laser, not jsut the area in video shot.

It may be the case with unmanned radar devices, but not with laser.

Simon

I will add to that;

There will be instances where although the target vehicle was isolated at the point of pulling the trigger, the chronological lag before the asterix appears can often mean that another vehicle may then encroach into the 'offence shot', i.e. the moment when the speed reading has appeared.

This is relatively common with 'crossover', when detecting across opposing lines or multi-carriageways of uni-directional traffic. Whilst it is the person behind the LTI who dictates what is recorded, it isn't always possible to obtain 100% clean evidence due to unanticipated vehicular interaction.

Although processing unit viewing teams are usually very strict in what they allow through, it isn't unknown for the resulting witness statements that need your signature to include a copy of the offence shot where another vehicle has crept into the field of view.

Now of course, this would have had no bearing on the speed of target vehicle when measured, but it does encourage debate as to the uncertainty of the accuracy of the ping. Therefore. no signature would be added, a note attached saying you aren't prepared to defend this and the case would be dropped.

Obviously, statement signing is some way down the line, but very few questionable pings make it as far as court, because they are filtered out at some stage long before making it that far.

An intelligent response.

If I may add to it:

At close ranges: there isn’t much of an issue because the size of the laser dot (at the target distance) is small, so it likely won't hit multiple vehicles (excluding secondary bounces).

When enforcing over large distances: the camera set up will be very telephoto (400mm sitting on a 6x crop sensor = 2400mm). It will be difficult to align the camera to set in perfect alignment with the laser i.e. with the centres overlapping.
Therefore, seeing two vehicles within the crosshairs doesn’t mean the laser actually hit both vehicles; that apparent error could merely be a result of the camera being only just within alignment.

The issue of camera alignment has always been a contentious one, and has previously cast reasonably grave doubts on speculations of various “false readings”.


As an off the top of my head remark has anyone given any thought to the possibility of beam stearing due to changes in refractive index of 'air' (say crossing over a heated patch of black tarmac, when remainder of road is concrete) given that optical paths can be up to 1km?
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desktop_demon
post Wed, 6 Apr 2011 - 19:49
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In principle the 904nm infra red beam would refract differently to the visible wavelengths of light. So the resulting misalignment could introduce an error factor when targeting a given vehicle. I am not sure how one might measure such errors - the visible bit it easy - the infra red bit could be more difficult given the dynamic circumstances of the experiment.

This post has been edited by desktop_demon: Wed, 6 Apr 2011 - 19:51


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When your life finally flashes in front of you - let's hope there's something worth watching.
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justforthepictur...
post Wed, 6 Apr 2011 - 21:52
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It is also worth reiterating that the crosshairs are merely an indication of the laser targeting, that depends on the camera to subject distance in relation to where along the section of carriageway the laser/crosshair alignment has been carried out.

It is very common for a picture to be posted here for opinions on the ‘ping’ to be given, but the crosshairs aren’t used as the primary targeting method, this is done via the scope’s red-dot. So although a requested photo will show the crosshairs in relation to the target vehicle, parallax error in conjunction with the distance at which the crosshairs and laser have been aligned, will play a huge part in determining what value the crosshairs actually are. Then you must also accept that the apparent relationship between crosshairs and laser will alter depending on focal length set/used.

At the cessation of every period of enforcement, the LTI/camera is removed from the tripod and put back in its foam lined case, meaning the video camera lens is retracted. So every time it is used, the procedure of laser and crosshair alignment has to be carried out (the focal length will vary from site to site).

Many people wrongly interpret that the crosshairs are used to sight the target vehicle, but the crosshairs should only ever be regarded as a guide to laser positioning.


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'Speed Kills' simply means, hit something or someone fast enough and it will result in a fatality.
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smeggy
post Wed, 6 Apr 2011 - 23:43
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QUOTE (desktop_demon @ Wed, 6 Apr 2011 - 19:49) *
In principle the 904nm infra red beam would refract differently to the visible wavelengths of light. So the resulting misalignment could introduce an error factor when targeting a given vehicle. I am not sure how one might measure such errors - the visible bit it easy - the infra red bit could be more difficult given the dynamic circumstances of the experiment.

IR light refracts less (is not bent as much) than visible wavelengths. Do an Google image search for "prism".

Those experienced in photography biggrin.gif will confirm there is a 'twinkle' effect in such conditions. Such an effect would also show on the video as distant objects being blurry or jumping around (depending on the aperture of the lens). Assuming a uniform level of atmospheric turbulence between the camera and target, the amount of visible blurring/jumping of the targets vehicles is a good indication of how much the laser beam at the target is blurring/jumping (but that level will be slightly reduced for 904nm than for the visible wavelengths).


From my experience, I would say the effect is negligible (compared to the width of the laser at the target).
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smeggy
post Wed, 6 Apr 2011 - 23:56
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QUOTE (norahl @ Wed, 6 Apr 2011 - 17:48) *
QUOTE (justforthepictures @ Mon, 4 Apr 2011 - 17:28) *
... the chronological lag before the asterix appears can often mean that another vehicle may then encroach into the 'offence shot', i.e. the moment when the speed reading has appeared...

...and what would that lag be?

I don't know what the RS232 lag is, or what it could be.
Strictly speaking, the simple answer would account for the 'timeout' delay as any of the frames up to the timeout/blocks can be used as evidence. This is usually up to around 0.16 seconds.

More complicated answers could account for the samples that can be dropped, or even timing from the end of the 'acquisition' period.
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