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Frequently Asked Questions on Police Lidar

With the Lidar detectors on the market, you can detect lidar pointed at your car, but lidar measures your speed so fast you don't have time to react.

What can you do? Details are in the FAQ, here's the summary.


  • A detector will save you sometimes If they target a car ahead of you, and you are ideally situated to detect the lidar, your detector might go off.
  • Stealth in combination with a Lidar detector will save you at long range. If they are targeting cars at 1000 feet, and your car has reduced reflectivity, they don't get a speed reading until you come into range. Your detector goes off and you have time to slow down. At short range, if they point the gun at your license plate and you have a stealth plate cover, your detector goes off, and you have time to slow before they retarget another part of your car.
  • Archive-name: PoliceLidar-faq

    Version: 1.0

    Last-modified: September 1, 1995

    Author's Home Page

    Frequently Asked Questions about Police Lidar

    Table of Contents

    1. Lidar Gun Description
    2. Lidar power estimates
    3. Lidar Range
    4. Stealth
      1. Lidar targets on your car. (what is most important)
      2. Stealth Techniques. (what to do about these Lidar targets)
    5. Jamming Police Lidar
      1. CW Jamming sources
      2. Pulsed Jamming sources
    6. Fighting Tickets
    7. Caveats
    8. References
      1. Technical References
      2. Popular Magazine References
    9. Product Reviews

    1. Lidar Gun Description

    Police lidar is 904 nanometers, 5 nanosecond pulses of 25 Watt instantaneous power delivered into a 4 milliradian cone angle at 1 KHz repetition rate. The long wavelength and low average power aids eye safety. The divergence angle on transmit allows the units to be used without a tripod. The time of flight of the pulses are multiplied by speed of light and the resulting distances are plotted as a function of time. A least square fit is used -the slope gives the car speed, and the variance gives a validity test [1] .

    2. Lidar power estimates

    You need to know this power if you want to build a jammer.

    The lidar beam width at 250 meters is about 1 meter^2. The license plate (Colorado) has about .001 meter^2 of retroreflective paint. - 30 dB loss. They are illuminating the retroreflective paint with 25 milliWatts

    The return beam (from the retroreflective paint) is also about 1 meter^2 and the receiving aperture is about .001 meter^2 - 30 dB loss.

    The return (Colorado) is thus 25 microWatts instantaneous power. The effective measurement bandwidth is 30 GHz. Other states have stronger returns, with Georgia 14 times as strong.

    3. Lidar Range

    Lidar range goes as the fourth root of target crossection. This is a very weak function, and so the range for your state probably falls between these extremes of measurment range to the best and worst liscense plates measured by Tom Bell:

    The Uniden detector ad (Aug 95 Road&Track p31) claims that:

    Craig Peterson has written alot on police radar and lidar, and he makes the argument in AUTOtronics, March 95 that typical radar ambushes are 600-800 feet because the police must identify the car and make a visual estimate of speed as well. This argument may not apply to lidar with its narrower beam, since the beam itself identifies the car.

    4. Stealth

    Stealth is not invisibility, it is just reduction of the range. This may help in combination with a detector, if the police target cars at the far half of their range as defined by the reflectivity of the average car. You get the warning before they get the speed measurement. Without stealth, in most circumstances, lidar detectors only go off when you have been targeted, and you don't have the time to slow down. With some range reduction due to stealth, they might target you before you come into range. They are trained to point at your license plate. If you have a stealth plate cover, your detector goes off, and you have time to slow before they retarget another part of your car.

    A. Lidar targets on your car. (what is most important)

    To find out what is most important on your car, stand with the light source behind you (park in the sun at dawn or dusk), and look at the region of the car right next to the shadow of your ear. You are looking at light reflected back towards the source. Alternately, you can at night use a flashlight held against your ear and pointed at the car. Stand back by at least 30 feet to see light returned at an angle close to straight back at the source. Another technique is to park your car on a dark street, stand >30 feet away and take a flash photo. Most car parts have similar properties in the visible and near IR. Some special materials absorb in the near IR and transmit in the visible and hence this test using visible light is not indicative that a special police-lidar countermeasure is working.

    The license plate is the strongest target on the front of the car. License plates have retroreflective material that returns light in a 4 milliradian cone angle (Colorado plate measurement) [2] . Different States have a very large difference in the lidar return from the plates. Colorado is one of the lowest, using retroreflective paint on only the letters and a thin border. Most large states [3] have better retroreflective material covering a larger area of the plate.

    The strongest reflection of the rear of the car is an array of little corner cubes in the red tail light cover. All cars have a hexagonal array in the plastic tail light covers. These are just like a bicycle reflectors. You can distinguish these retroreflectors from the tail light lens by the array pattern. The tail light lenses are in a rectangular array, and this corner cube array is in a hexagonal pattern. The second strongest target on the rear is the rear liscense plate.

    There is also a corner cube array on the side-front turn signal indicators. These do not present much of a target unless the police target you from the side in which case the cosine factor in speed measurement will give them too low a speed to worry about.

    Next in strength are head lights, brake lights, turn signal indicators, and fog lights. Of these, headlights are highest, followed by turn signals and fog lights. The retroreflection mechanism is interesting. Light enters one-half of the light and is returned from the other half. I suspect that the light enters one half, hits the back reflector, is focused near the filament, expands to the other side back reflector, and is recollimated returning towards the original source. This phenomena happens over a moderately narrow range of angles directly in front of the car.

    Grills, forward facing chrome, and any rounded specular material are the next strongest targets. Look for a bright glint from any rounded surface which always presents one small region facing the source. Flat regions are almost always pointed away from the source and hence do not contribute to the return.

    B. Stealth Techniques. (what to do about these Lidar targets)

    The license plate is the most important on the front of the car, unless you are blessed with a plate without retroreflectors (The only example I know of this is the old California yellow on blue plates) In some states you can legally remove the front plate. In some states it may be sufficient to paint over the retroreflective paint with glossy house paint to match the color. Check your local laws first. Many states use 3M retroreflective material, which would be difficult to paint over inconspicuously. For these states it is preferable to use a lidar crossection reducing license plate cover. One such cover , is reviewed at the end of this document.

    The array of corner cubes in the tail light cover. These are on all cars, and are probably required by law. If you paint them black, a car driving by would not see your car on the side of the road as easily. The only thing I can figure is to get one of the liscense plate covers that block IR, and cut out a piece and glue it over the corner-cube array. (expect new products for this application to come out soon)

    Head lamps, turn signal lamps, and fog lights. Get retractable headlights or glue a section of IR absorbing liscense plate cover over your headlights. Remove the fog lights, glue liscense plate cover over your turn signal indicators. The "smoke" headlight covers that are on the market do a fine job of reducing retroreflection from headlights. The down side to these things is that they really aren't legal in most states.

    The most extreme stealth includes taking care of all other possible glints. Get a flat black car bra, paint over flat surfaces with glossy paint, paint over rounded surfaces with flat paint. These returns are small compared to the Plates, cornercube array, and lamps.

    Lidar (and radar) range goes as the fourth root of return signal power. By eliminating the single strongest return, the best you can hope for is to reduce the range by about a factor of 2 to 4, from reducing your crossection by a factor of 16 to 256. This is because even if you reduce the strongest return by a larger factor than 256, you probably have something else which returns the lidar signal by at least 1/256 as much as this strongest factor. When you eliminate the strong return you still have the other source of returns. To get a range reduction of more than a factor of 4, you probably have to apply stealth measures to multiple reflectors on your car. If you get the license plate, the corner-cube array (back of car only) and the lamps I think you'll get your visibility under the average range at which police target cars.

    5. Jamming Police Lidar

    Craig Peterson wrote reviews of radar and lidar jammers in which he tested products that use these jammer techniques and found they didn't work.

    Is Jamming feasible? The two techniques outlined here do not appear to be viable on technical grounds. Assumptions about these techniques are built into the descriptions below, and a more $ophisticated jammer might work.

    Is it legal? Jamming lidar is not illegal under FCC rules since they don't regulate this part of the spectrum, but most jurisdictions have a law which makes it illegal to "interfere with the duties of a police officer." I am not a lawyer and the above should not be considered legal advice.

    There are two kinds of jamming proposed and on the market -

    pulsed LEDs and CW Headlights.

    My calculations indicate that neither of these work without combining them with stealth measures. These calculations are specific to the range of 250 meters.

    CW Jamming sources.

    Headlights aimed into .5 by .2 radian distribute their power over 0.1 steradians, at 250 meters range, this illuminates 6000 square meters or 10^(-6) of the police receiving aperture. 200 Watt lights put 200 microWatts into the lidar gun. Presumably the lidar gun has a narrow band filter passing about 10 nanometer of the spectrum, reducing this CW jammer by a factor of about 40, meaning that the light is now 5 microWatts. The detector is AC coupled so we calculate the shot noise due to this background ShotNoise = SQRT[RecievedPower * PhotonEnergy * MeasurementBandwidth] Sqrt[5. Micro Watt PlanckConstant SpeedOfLight/(900 Nano Meter)*30 Giga Hertz] = 0.200 microWatt equivalent optical power. This is small compared to the 25 microWatts return from a license plate. The Car and Driver article indicates that this jamming technique works, contrary to this calculation, and to Craig Peterson's article.

    Pulsed Jamming sources


    At a 250 meter range, LEDs broadcast into .005 steradians (.5 radian horizontal times .01 radian vertical) would have to be 500 times brighter than the 25 milliWatts they hit you with to beat the retroreflective paint which broadcasts into only 10^(-5) = ( 4 milliradian times 4 milliradian) return.

    This is 12 Watts, well beyond the power of an LED. At shorter range, the problem of jamming is worse. The police lidar power grows as 1/(Range^4) power as the range decreases, and your jammer power grows only as 1/(Range^2). The reason jamming is not feasible is that you have to broadcast into all directions, reducing the power aimed at the lidar gun.

    Laser diodes:

    The strongest laser diode you can buy is the one they put in the lidar gun (see product data sheet below). Unless you actively steer this jamming signal towards the police lidar gun, it will only be an effective jammer at ranges farther than about 200 meters for a jamming signal broadcast into .005 steradians.

    6. Fighting Tickets

    This section contributed by

    National Motorists Association
    Join the NMA! $29 a year.

    Web Page For the NMA Email nma@genie.geis.com

    For anyone who gets a laser ticket, we encourage you to fight it. That in itself isn't news, (we encourage everyone to fight all speeding tickets) but lasers are not on judicial notice in New Jersey. In fact, they're not on notice in 95% or more of the country's courts. The prosecution teams around the country are glad you don't know that.

    What's "judicial notice" mean? When something has been given judicial notice, it means that the theory of operation has been proven to the court and expert testimony to prove that is no longer needed. Radar is one such example. Aircraft speed traps are another.

    Laser, however, has not been proven to the courts, so the prosecution will have to fly in expert testimony to prop up their case and new toy) to prove that their latest gadgetry actually works as advertised.

    As it so happens, we at NMA keep a list of experts for our members, and one of those categories just happens to be radar and speed measurement devices. ;-) (Sorry, access to the list is for members only)

    Laser -cannot- be used while the cop car is rolling. The vibrations would scatter the beam everywhere. I've only been ambushed by a laser trap once, and the setup had one officer doing the laser, another next to him radioing the to the chase vehicles, and 4 or 5 chasers. So, this is hardly a valuable use of scarce police resources!

    In cases of aircraft tickets, if someone contests one, both the arresting officer and airplane spotter (or pilot, if the same) have to be hauled into court to say that yes, that driver was the one we stopped. Theoretically the same should happen with laser traps, but don't count on it until someone says something. ("Shhh. It's our secret. Don't let the public know" kind of stuff) One of our members in Texas has been fighting a laser ticket for 2 years now. He's only asked for one or two continuances (extensions) the rest of the time has been the prosecutions fault. Still no outcome. This works to his advantage. Since laser isn't on judicial notice there, they had to fly in experts.

  • [Also from nma@genie.geis.com, some info on lidar in New Jersey, and who is paying for lidar guns]
  • State Police based in Morris County have begun using 23 of the $3,800 LTI 20-20 Marksman laser guns. Newspaper accounts say that the new units will be distributed to other parts of the state when a municipal judge convicts a driver of speeding with a laser (i.e., no judicial notice yet). The initial group of units was purchased with a grant from the National Highway Traffic Safety Administration. New Jersey is the 15th state to adopt lasers.

    7. Caveats

    Some of the stuff I put in the FAQ is speculative.

    The numbers I assigned to the lidar are estimates.

    The calculations are 1 significant digit.

    Exact numbers are not important because the lidar problem is one where the signal power falls as 1/(Range^4), and consequently a factor of 2 error in estimate of signal power leads to a small error in estimate of range.

    The jamming noise goes as the square root of measurement bandwidth, making the exact number not that important. The use of 30GHz gives a 10 millimeter range resolution.

    8. References

    1. These numbers are best guess averages from the technical references (below). The data processing is described in Samuels and in Fischer in the technical references section. [Return To Text]
    2. I measured the cone angle returned from a HeNe laser incident on the white retroreflective paint. [Return To Text]
    3. I believe the material is 3M material, and it appears to be a much better retroreflector than the retroreflective paint. I have found highly retroreflective material on new plates from California, Kansas, Oklahoma, Texas, Virginia, New Mexico, Wisconsin, Texas, Hawaii, Illinois. This is by no means an exhaustive list, as it is just some states that you find on a stroll through the U. Colorado student parking lot. [Return To Text]

    Technical References:

    This reference is highly technical, but it describes the design of the actual lidar, and hence gives the best data.

    I suspect that this is the actual diode used in police lidar.

    Includes some lidar information as well as radar. IEEE Spectrum is in between a "technical" source and a "popular" source. It is a popular rag for all electronics engineers, and it is easy reading.

    Popular Magazine References:

    Craig Peterson Articles:

    9. Product Reviews

    - ----Review of the Taylor-Bell Laser-Guard(TM) license plate cover 8/2/95-----

    reviewed by Robert T. Weverka [Return To Stealth Techniques]

    Company Contact: tombell@ix.netcom.com
    Taylor-Bell Technologies, Inc.
    P.O. Box 420002
    Atlanta, GA 30342
    Order line: 800-945-1141
    Business phone: 404-751-5787

    I got this plate cover and I am impressed.

    Visual inspection:

    In transmission in the visible it is indistinguishable to the eye from clear plastic. To see that it is not just clear plastic you must hold it to directly reflect a light source and looking at the reflection you see a very slight color change.

    Optical measurement:

    I measured 2.8% one-way transmission at 900 nanometers, and the transmission spectrogram shows the transmission staying within a factor of two of this value from about 830 to 960 nanometers.

    Effective Stealth protection analysis:

    The one-way transmission is 0.028. The two way reduction in lidar return from the plate should be (0.028)^2 = .00078. If the plate were the only target on the car, this would give an 84% reduction in detection range, since lidar range goes as the fourth root of the signal strength and 1 - .00078^(1/4) = 84%.

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