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How Does A Laser Detector Work?  
User currently offlineStartvalve From , joined Dec 1969, posts, RR:
Posted (10 years 6 months 2 weeks 1 day 20 hours ago) and read 4635 times:

We are all familiar with radar detectors and how they detect radar emissions is pretty obvious. But now with more and more departments going to lasers for speed enforcement there are laser detectors popping up everywhere. How can a laser detector possibly work?

7 replies: All unread, jump to last
 
User currently offlineYbacpa From United States of America, joined Feb 2001, 1108 posts, RR: 1
Reply 1, posted (10 years 6 months 2 weeks 1 day 20 hours ago) and read 4627 times:

A lidar detector works by looking for the laser pulse (its actually infrared, btw), similar to how a television looks for pulses from a remote control. Compared to a conventional radar detector, which listens for radar noise, this is highly inefficient, since the beam used for laser guns is only a degree or so wide.

Lidar detectors are really not that reliable, especially considering that generally speaking when lidar is being used, it isn't used in an always-on state, making detection extremely difficult. If you're thinking about buying one, don't, save your money for the ticket you're going to get regardless.



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User currently offlineSccutler From United States of America, joined Jan 2000, 5520 posts, RR: 28
Reply 2, posted (10 years 6 months 2 weeks 1 day 10 hours ago) and read 4599 times:

Ybacpa is right on.

I still recommend the Valentine One radar and laser detector as the best on the market. Its radar functionality- especially, the discrimination between front and rear signals- is amazing.

It also has laser detection which, when I bought it, I assumed would be useless since, I have always reasoned, the laser detector only fires off when you are being clocked and (so I assumed) that was too late to take any action.

Since getting the V1, though, I have discovered that the laser detection sometimes picks up "scatter" when others are being clocked ahead, so I now no longer call it "useless." Just "almost useless."



...three miles from BRONS, clear for the ILS one five approach...
User currently offlineStartvalve From , joined Dec 1969, posts, RR:
Reply 3, posted (10 years 6 months 2 weeks 1 day 1 hour ago) and read 4581 times:

I had no intention of getting one. I was just wondering since someone was talking about how fabulous they were in another thread which brought back memories of when me and about 5 other science geeks tried like hell to figure out a scientific explanation as to how they worked. Our theory ended up being that they didn't work at all. We sort of figured the only thing it was good for was letting you know the cop had you painted.

User currently offlineYbacpa From United States of America, joined Feb 2001, 1108 posts, RR: 1
Reply 4, posted (10 years 6 months 2 weeks 1 day ago) and read 4571 times:

...tried like hell to figure out a scientific explanation as to how they worked

They work by calculating the change in the amount of time it takes for the pulse of light to return to the gun. Knowing the speed of light and the delta of the amount of time it takes for it to return allows the gun to calculate speed (extremely accurately, I might add).

I used to have a lidar detector, and other than in a few isolated circumstances, it never alerted me to their use (although, at the time, lidar use was very isolated.. this was about 10 years ago). I don't even bother using a radar detector anymore either, as it never really seemed to accurately tell me when a trap was set up.

-yb



SkyTeam: The alliance for third rate airlines finally getting their act together!
User currently offlineStartvalve From , joined Dec 1969, posts, RR:
Reply 5, posted (10 years 6 months 2 weeks 1 day ago) and read 4567 times:

we knew how the lidar gun worked to figure out speed. We just couldn't figure out how something was going to detect someone else getting lased. Our research showed that most of the energy was going to go pretty much straight back to the cop and any scatter would be hard to discern from sunlight to a light detector.

User currently offlineYbacpa From United States of America, joined Feb 2001, 1108 posts, RR: 1
Reply 6, posted (10 years 6 months 1 week 6 days 16 hours ago) and read 4548 times:

couldn't figure out how something was going to detect someone else getting lased

Ah, I see what you're getting at now. For exactly the reasons you listed, they do not work well.

Our research showed that most of the energy was going to go pretty much straight back to the cop
This isn't necessarily accurate. The beam produced is slightly cone-shaped (typically 1-2 degrees, producing a 'spot' about 3' in diameter at its usual useable range), and a typical car isn't flat, so there will be some scatter. Generally speaking, in practice, when in use the guns are aimed at the car's license plate, as it is typically the flattest metallic surface.

It is true that it is difficult to separate lidar from sunlight, but, going back to my low-tech analogy: your television, even if the sun is shining on it, can usually still receive the pulses from a remote, although typically at reduced range. Its all a matter of the electronics being sensative enough to work at a poor signal-to-noise ratio.

In other words, while in theory lidar detectors work, in practice they just aren't functional.

By the way, I'm by no means an expert in the field, my experience comes from some of the engineering companies I've done business with in the past.



SkyTeam: The alliance for third rate airlines finally getting their act together!
User currently offlineJwenting From Netherlands, joined Apr 2001, 10213 posts, RR: 18
Reply 7, posted (10 years 6 months 1 week 6 days 9 hours ago) and read 4534 times:

not just signal to noise ratio.
The signal also has a known modulation and frequency range which makes filtering out extraneous sources easy.

A lidar has that as well, but of course the manufacturers don't publish that data (except I guess to customers) and units can more likely than not be tuned to different settings in order to make it possible to use several close together (say one on either side of a road).

Detecting whether you're being hit by one is possible in theory, but you'd need an extremely sensitive detector over the entire body of the vehicle.
The simple sensor behind the windscreen most likely won't get a whiff of a narrowbeam transmission aimed at the license plate (at most it might get something for a split second while the police officer is aiming if he moves the lidar unit up and down).



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