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.

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 licence 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.

Table of Contents

1. 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

   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 licence 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 cross section. This is a very weak function, and so the range for your state probably falls between these extremes of measurement range to the best and worst licence plates measured:

   • Georgia licence plate, by itself = 2577 ft.
   • Colorado licence plate, by itself = 1339 ft.
   • With no plate- detection range of a Mazda RX7 with headlights retracted is around 800 ft.
   • No plate, and the lights covered - detection range < 500 ft.

   The Uniden detector ad claims that:

   • Uniden detector is good to about 8000 feet.
   • Lidar guns are effective to about 2000 feet.
   • Average operating range of police lidar is about 800 feet.

   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 licence 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 licence plate is the strongest target on the front of the car. Licence plates have retroreflective material that returns light in a 4 milliradian cone angle (Colorado plate measurement)

   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 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 reflector. 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 licence 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 licence plate is the most important on the front of the car, unless you are blessed with a plate without retroreflectors 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 colour. 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 cross section reducing licence plate cover

   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 licence 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 licence plate cover over your headlights. Remove the fog lights, glue licence 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 cross section 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 licence 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

   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 sophisticated jammer might work.

   Is it legal? Jamming lidar is not illegal under FCC rules or UK 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 Shot Noise = 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 licence plate.

   Pulsed Jamming sources

   LEDs:

   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. 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

   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.

   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 set-up 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)

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