LIDAR speed gun

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Police officer using the 'LTI-20/20 Ultra Lyte Laser' hand-held LiDAR speed gun.

A LiDAR speed gun is a device used by the police for speed limit enforcement which uses LiDAR to detect the speed of a vehicle. Unlike Radar speed guns, which rely on doppler shifts to measure the speed of a vehicle, these devices allow a police officer to measure the speed of an individual vehicle within a stream of traffic.

How police LiDAR guns work[edit]

Note: Some US measures are used in this article—for reference: 1 mile = 1.6 kilometers; 1 yard = 0.91 meters; 1 foot = 30.5 cm; 1 Mile Per Hour (MPH) = 1.6 kilometers.

LiDAR relies on the principle of time-of-flight of two (or more) short 905 nm wavelength (near infrared - NIR) LASER pulses.

Here is the sequence of events in detail—these events happen over the course of a fraction of second actuated by a single press of the trigger of LiDAR gun:

1. Aiming: The police officer aims the LiDAR "cross-hairs" through a telescopic monocular (usually a 2X - 8X monocular scope depending on model) built into the LiDAR gun. The scope allows the police officer to see the target vehicle before the target vehicle operator sees the police officer generally at a distance of 1000 ft and up to 4000 ft. The police officer is trained to aim the pulsed 4 milliradian cone of the laser at the license plate. Licence plates have been coated with a retroreflective coating designed to create a 4 milliradian return reflection (back to the police officer's LIDAR gun receiver aperture). Range varies by LiDAR gun manufacturer, target vehicle aim-point reflectivity, and weather conditions (temperature and humidity, precipitation). LiDAR speed measurement can be recorded by the LiDAR unit anywhere from ~5 feet to ~4,000 feet. Most police Lidar units actually use a magnification of 2X. The use of a laser with an 8X magnification scope would make acquiring and tracking a quickly moving vehicle problematic at best. Manufactures of LiDAR guns have begun to concentrate on extending the speed-detection range of the devices (2014).

2. Firing laser: The police officer takes aim (usually at the licence plate or secondary reflector such as headlight, chrome grill-work, etc.) Some LiDAR units have a tone to tell the operator if they are getting a good return signal and the tone varies from target to target so the operator can sample a number of vehicles and select the vehicle they think is the fastest moving vehicle if they choose. The 3-4 milliradian cone presents an area of illumination of about 1 square meter at 300 meters distance. Therefore a single vehicle can be selected out of a group for speed measurement. Vehicles in the "shadow" of other vehicles cannot be speed measured.

To operate the device, the police officer presses the trigger of the LiDAR gun and it emits very short LiDAR laser pulses with a pulse width (duration of pulse) of 30 nanoseconds or less. Depending on the LiDAR gun in use the number of pulses per second (pps) range from 100 to 380 in the USA or up to 600 pps for countries outside the USA. The LiDAR gun's internal software uses an algorithm to reject inaccuracies and all manufacturers have their own proprietary method of doing error rejection.

To sum up the full function of the LiDAR speed measurement system in simple steps:

   1. The pulsed laser sends a short pulse of light starting the clock. 
   2. The detector detects the reflected pulse and stops the clock. 
   3. The elapsed time which is referred to the “time-of-flight” is used to 
      determine the distance.[1]

A Detailed Scenario: Laser pulse [Pulse A] "flies" towards the target and hits the target vehicle license plate. At the same time the pulse is released into flight [Release Time A], a timer is started by a high resolution high-speed timer built into the LiDAR gun. Additional laser pulses are also sent at a rate of 100-600 pulses per second. The advent of high-speed timers was a critical technology, making "time-in-flight" measurement of light possible. So the LiDAR measures time-in-flight of each pulse and requires only 2 pulses over a period of time of as little as 3 ms (theoretically) to determine the velocity of a vehicle. The LiDAR unit, however, takes additional measures in order to provide some additional validation that the measure is correct. Generally the LIDAR guns currently on the market acquire the target vehicle speed and validate that speed in 300 to 400 ms. Accuracy required by manufactures of LiDAR guns is +1 MPH or -2 MPH and all attain +/-1 MPH (same as RADAR).

Two or more readings are used to plot velocity of the vehicle and the system looks at variance to indicate errors and rejects possible errors from its internal dataset. It is possible that police LiDAR can attain a single accurate speed reading in as little as 250 ms.[2][3]

Since the LiDAR gun may be off and only triggered by the Police Officer when your vehicle is targeted (by pressing the gun's trigger) there may be no "stray" LiDAR signals to detect and so the effectiveness of any RADAR/LiDAR detector is questionable when LiDAR is operated in this way. The best hope for a speeding vehicle is that the Police Officer is targeting many vehicles at long range and so creating some "scatter" (stray laser reflections) that a LiDAR detector may detect and so forewarn the speeder. Generally speaking if your vehicle is in range (has a strong enough return reflection) and this sets off your LiDAR detector it is humanly impossible to slow down fast enough to correct your speed before it can be measured. Considering that it takes we humans about the same time to blink our eye (about 400 ms) I think you will agree.

In addition to the fast target acquisition speed some LiDAR guns are incorporating pulse patterns where pulses are not evenly spaced in time. Since many LiDAR detectors are using Photodiodes to detect 904-905 nm laser through a narrow band-width filter (to reject other irrelevant wavelengths of light) and looking only for even pulsing these "Stealth" LiDAR signals may go un-noticed by LiDAR detectors that have not kept up with the state-of-the-art.

3. Reflection: The laser pulse is reflected from the target vehicle (preferably the license plate). Testing protocol for LiDAR units actually use a piece of retro-reflective material the size of a standard automobile license plate on a black background for testing. LiDAR guns use 50 µW (microwatt) laser diodes providing 30 ns (or faster) pulses at a rate of 100-380 pps (USA) and upwards of 600 for non-USA countries. Reflection is enhanced by "scatter" caused by strong reflective surfaces on modern automobiles (e.g. chrome bumpers, chrome grill work, headlight reflectors). "Scatter" is also what modern LiDAR detectors detect in order to give users advanced notice that LiDAR is being used ahead.

4. Detection: This is merely an internal calculation based on the speed-of-light and time-in-flight of the laser pulse, divided by 2 (halved, to produce a one-way trip time). After 2 pulses have been measured the LiDAR unit "know" the distance to the vehicle at the first pulse and the distance at the second pulse and the time that has elapsed between the first pulse and the second. With that information the LiDAR gun can easily calculate the speed of the vehicle:

A. First the total distance traveled is calculated: Distance_Traveled = D1 - D2 B. Then the speed: Distance_Traveled / ET C. Speed is then converted to the standard MPH or KPH

D1 = Distance at Pulse 1 D2 = Distance at Pulse 2 ET = Elapsed time between pulse 1 and 2

Other than the distance, the ability for police to measure vehicle speed is degraded by the following factors:

  1. Refraction. Refraction by differences in air density due to "heat waves" coming off a hot road surface, etc.
  2. Weather. Rain, heavy fog, snow or other precipitation (negligible impact on the laser pulse, however, the police officer must be able to see the target and so visibility impacts the ability of the police officer to properly target and identify the vehicle).
  3. Through the windshield. Police windshield (if firing it through glass tends to scatter IR to a negligible degree). If the windshield has the added challenge of rain, fogging, or snow, etc. that would reduce the range of the LiDAR.
  4. Front or rear vehicle targeting. Direction of vehicle travel (away or toward LiDAR operator because generally the rear of automobiles present a stronger reflection. In states where only rear license plates or in the case of motorcycles (which do not have front plates) the rear is surely the strongest reflection point However, the police generally set up to detect vehicles from the front in about 90% or more of the time. This is so they can wave the offending vehicle over to ticket without having to take chase.
  5. Darkness. Darkness can impede the ability of police officer to target and identify a vehicle. LiDAR units do not yet include any type of light amplification or "night vision."
  6. Occlusion by the sun. Shooting at targets that have the sun in the background can prevent the LiDAR unit from getting a reading. The Sun may be able to "wash out" any incoming LiDAR pulses. The sun also prevents the police officer from seeing their target and having sun light come in through an 8X monocular can be very damaging to the eyes, so good sense dictates that police do not operate LiDAR in the direction of the sun. Aiming the LiDAR into the sun may cause LiDAR gun faults (rejected readings) even though a narrow bandpass filter (+/-5 nm; 899 nm-909 nm) is used at the LiDAR gun receiver aperture to reject light outside of the Laser's operating range (sun, HID Headlights, etc.)[4]
  7. Cosine Error. Just as it is with the use of RADAR to measure the speed of a vehicle the close the police officer gets to being directly in-front or behind the target vehicle the more accurate the reading. Off-center angles as much as 15 degrees are often used but most police know that this cosine angle works against them (they are getting a lower speed reading than your actual speed). This is why you may often see LiDAR units operated close to the edge of the road.
  8. Police misuse (sweeping). It is possible, particularly at long range where angular separation between targets are slight, to "sweep" the LiDAR gun across more than one target while pulses are being returned and create a false reading — sometimes in favor of the target vehicle.
  9. Police in Motion. LiDAR cannot be operated from a moving Police vehicle. The Police Officer must stop his vehicle to get a speed mesurement.

For example: The police officer is trying to get a reading on a group of vehicles at over a mile away (near limit of range). The police officer is operating the unit "hand held" (without a tripod), due to the very unstable nature of holding the LiDAR there is "Camera Shake" and this causes the LiDAR pulse beam to sweep across two or more targets during a single read. If the initial pulses reflect off a vehicle that is further away and final pulses bounce off a vehicle that is much closer, this may cause the LiDAR to "believe" that a single vehicle was read. The pulse time-in-flight between the initial and final vehicle being much shorter, the LiDAR unit may provide an erroneous reading that is much higher than the velocity at which target vehicle was traveling.

A specific "sweep error" scenario: You could be traveling at the 45 MPH posted speed limit alongside another vehicle forward of your position, also traveling at the posted speed limit. If the officer's unsteady hand causes the LiDAR beam to sweep from your vehicle to the forward-running vehicle during a single read, the police officer could get a reading from the LiDAR measuring your speed to be erroneously 80 MPH (value depending on the lead distance that the other vehicle has on you). The officer may think this is your actual measured speed and you could be charged with "Reckless Driving." The probability of this is high because the "pulling" of the trigger instead of "squeezing" the trigger can cause the gun to sweep. It is thought that because police officers are trained in the handgun marksmanship and learn to "squeeze" the trigger correctly, but at ranges of a mile this effect can be amplified and go unnoticed by the police officer who, with good intentions, may still believe you violated the law when you did not.

Note: See capabilities section for more detailed info on effects of a., c., d. above. Also note that any one factor is negligible, however, the cumulative effect of more than one of these factors can have a negative impact on range (range of speed detection).

4. Return flight: The pulse [Pulse A], having been reflected is returned to the LiDAR gun and detected by a photodiode filtered to look specifically for 905 nm laser. The time of arrival is captured [Arrival Time A].

5. Calculating target distance: The [Release Time A] is subtracted from [Arrival Time A] to determine the [Time In Flight A]. Due to the fact that the pulse made a two way trip (to the target and back again) that [Time In Flight A] is divided by half to determine [Flight Time A to Target]. Now, knowing the Speed of Light (C) the distance to the target vehicle can be calculated by the LiDAR gun microprocessor as follows: [Flight Time A to Target] x C = [Distance to Target A]. (Distance = Time X Speed].

Now we have the distance to the target and that is all. We still don't have the speed. So an additional pulse is required to determine the speed as follows:

6. New target distance acquired: The police officer has not yet removed his finger from the LiDAR trigger. The LiDAR gun emits another pulse [Pulse B] and also captures its release time [Release Time B] (exactly the same way as it was done for pulse A -- see #2 above). The same sequence of events occur in #3 through #5 above and [Flight Time B to Target] is used to calculate [Distance to Target B] (the same target actually). We also have the [Arrival Time B].

7. Calculating distance traveled by target: Now having the two distance of the vehicle when pulse A hit it and pulse B hit it the LiDAR can calculate the distance traveled by the target vehicle between pulses as follows: [Distance to Target B] - [Distance to Target A] = [Distance Traveled by Target].

8. Calculate the speed of vehicle: Now we have the [Distance Traveled by the Target] between pulse A and pulse B and we have the [Release Time A] and [Arrival Time B]. By taking [Release Time B] and subtracting [Release Time A] we get the [Total Time of Target Travel]. This is all the LiDAR gun now needs to calculate the speed of the vehicle as: [Distance Traveled by the Target] / [Total Time of Target Travel] = [Speed of Vehicle] or Speed = Distance/ Time.

In actuality—many pulses: The above example shows just two pulses but in actual practice the LiDAR gun sends out a burst of 100 to 380 pulses per second (in USA and up to 600 pps outside USA). The speed calculation can be performed and error-checked likely 4X in a single second! [5] This way any pulses that don't return or other interference can be negated in a form of signal quality assurance or "Error Rejection." This is the most accurate form of police speed measuring devices in use today. The multiple pulses also enables the LiDAR to distinguish whether the target vehicle is accelerating or decelerating and by how much. If accelerating the police officer may re-measure the target vehicle through additional triggering to determine its fastest speed.

Some LiDAR guns also have algorithms that try to determine when a vehicle is operating some form of LiDAR "Jamming" signal and may report on suspected jamming. There are many instances of "false positives" however so the Police Officer is never sure if a vehicle is operating a jamming device. Also, some LiDAR guns are more susceptible to LiDAR jamming techniques than others.

LiDAR Jammers are effective because they assume the Police Officer is within 30 degrees of center front (in most cases) or 30 degrees of canter rear (in lessor cases). The Police Officer must operate in these angles in order to control cosine error which always favors the speeder—that is known. In fact, many Police Officers try to get as close to 0 degrees as possible in order to get the most accurate reading. That is why you may see then standing near the edge of the road or even leaning out into it. Jammers just simply generate a large number of short 905 nm laser pulses and project them into an area that is about 30 degrees wide (or less). The concept is that if the jamming is operating when the Police Officer first illuminates the target vehicle with LiDAR then the LiDAR detector will "see" so many returning pulses that it would become "confused." Basically, the error-correction algorithm would end up rejecting all the readings and getting the speed of the vehicle would be impossible. This is a war of software however, and LiDAR units are becoming smarter all the time and should be able to detect some of the jamming techniques that have been on the market. LiDAR gun manufactures buy every LiDAR detector and Jammer on the market and analyse them to determine how the function and diligently work to improve their software. Quite possibly there will be a few who will offer Police-user upgradable firmware. The Police wouldn't need to purchase a new gun to stay on top of things but simply download the new software from the LiDAR manufacture's web site and plug the gun up to a USB port on their computer and upgrade the firmware with new software immediately overcoming whatever new detection or jamming technique that is on the RADAR-LiDAR Detector market.

Capabilities of police LiDAR speed guns[edit]

Capabilities: LiDAR can be used to measure vehicles coming toward the LiDAR gun or moving away from the LiDAR gun. More often it is used to measure the target vehicle coming toward the LiDAR gun because this allows the police officer, who may be operating alone, to wave the driver down for ticketing/arrest.

Differences in range for target vehicles moving away vs toward LiDAR operator

The LiDAR is generally used as a stationary device and fired in clear air. There is a low probability that a police officer will try to operate it in heavy rain or through a windshield from inside his vehicle. Unlike police radar, it is able to pick a single vehicle out of a group.

Chart shows there is negligible degradation in performance due to inclement weather.

Trained aim points: Police may want to measure the speed of a target vehicle when they are far enough away, so they can avoid detection. To optimize their range, they are trained to use the following aim points, arranged in order of greatest reflectivity to 904 nm laser pulses:

1. Best reflector: Front/rear license plate. This has a highly reflective retro-reflective coating designed to match the 4 milliradian cone angle of the police LiDAR units and return that beam back to the point of origin (the police LiDAR receiving aperture). For motorcycles or states where front license plates are not required police are trained to aim at the headlight (see #2 below) or chrome grill work. Most states use 3M(tm) retroreflective surfaces on their license plates that are specifically designed to ease LiDAR speed detection of your vehicle.[6]

2. Very good reflector: Headlight reflectors—often the backing within a headlight is a semi-parabolic reflecting first-surface mirror.

3. Nice reflector: Other lighting such as turn signals may have retro-reflective "trihedral corner reflectors" in them that are designed to make the vehicle more visible when it is parked at night and not being operated (lights are off). Tail lights generally have the largest area of these retro-reflectors in them and make good target even for long range detection. Different vehicle designs present differing degrees of bezel retro-reflection.

4. Good reflector: Chrome trim such as bumpers and grills present a great first-surface mirror reflector. Probably not good for long-range speed measuring.

5. Poor reflector: Windshields and car body panels make poor reflectors but still useful at closer ranges.

Cosine angle error: Just like radar this technology is subject to cosine angle error. This means that the police officer (as trained) will try to take a position at 0 degrees directly ahead of you or behind you when the measure is taken. This is generally not the case as the police officer's position must be safely off the road so there is always some cosine error which works to the benefit of the target vehicle and isn't considered (other than trying to reduce it to a minimum) during punishment (i.e. you may have been measured speeding at 83 MPH but due to cosine error your actual speed was 85 MPH). Sometimes police officers step up to the edge of the road (or even in the road) to eliminate this error to the greatest degree possible.

Police strategies[edit]

Police working alone generally target oncoming vehicles because they can wave violators off the road for ticketing/arrest. The LiDAR gun can capture the speeds of many vehicles and save them. Some LiDAR guns also record a video image of the vehicle, license plate, and possibly LiDAR aim-point. Detection ranges in clear air range from an average of 4,679 feet for oncoming targets and 4,887 ft for egressing targets (moving away from the LiDAR gun). Note: 1 mile is 5280 feet so this is less than 1 mile at best!

Front vs. rear targeting: Rear targeting often extends detection range by about 5.3% average.[7] However, that isn't easy to accomplish without a police chase car the police officer radios to take chase far ahead. Therefore, on Interstate highways front and rear measures are often taken by a single police officer who calls in a description of your vehicle, lane you are in, and the last few plate numerals so one of a few chase vehicles a half a mile head can pull you over. Long-range measures using a tripod mounted LiDAR gun are useful in this case. Motorcycles do not present a front license plate and so may present a harder target to get a speed reading on—they must be closer to the police LiDAR to get a reading. Motorcycles also present a smaller return signal from the back as the retro-reflective license plates present about 25% of the surface area of automobile license plates.

On non-Interstate highways you are more likely to be targeted from the front and waved off the road for a ticket/arrest. Often over a hill or around a curve or other obstacle that allows them to operate LiDAR at fairly close range. A chase car is often operated in which the LiDAR operator calls out a description of the offending vehicle (lane position, color of car, last few numbers of license plate). Sometimes the detection point and chase point are separated by 1/4 mile. This can allow you enough time to change lane positions to complicate identifying you in heavy traffic.

Ambush technique: This is the most effective of LiDAR techniques. This strategy helps them to overcome the use of consumer LiDAR detectors. LiDAR detectors are only effective when they provide enough advance notice to operators by detecting stray LiDAR pulses, misses (LiDAR traveling under a vehicle), or out-of-range attempts. Other than "out of range" attempts, when LiDAR detectors detect a "direct hit" it is already too late to do anything about it (detection of your vehicle speed can theoretically occur in 1/250th second). Police use this technique at close range when you are coming over a blind hill or around a blind curve. Generally they will operate "hand held" and stand very close to the edge of the road to counter as much cosine error as possible (police are trained to stay within 15 degrees of the center of the road to reduce cosine error — often they will stand on the edge of the road to optimize their readings). They will trigger the gun after precisely aiming at your front license plate as it come into view over a blind hill or around a blind curve, where you are likely unable to react fast enough to slow down to posted speed limit. However, hitting your brake to bring your car to posted speed limit is a good practice (after you check to ensure following vehicles will not hit you). These ambush techniques negate the ability of LiDAR sensing "RADAR Detectors" as no LiDAR is detected until after your vehicle has been "hit" and the police officer has measured your vehicle speed. Heavy traffic and multiple lanes may require the LiDAR operator to hit vehicles near you and your detector may pick up the "scatter" and allow you to reduce your speed to posted speed limit therefore avoiding a ticket.

Coming over a blind hill, you may attain a slight benefit by mounting a single LiDAR sensor high in order to capture some scatter or overshoot while your license plate is still obscured to the police officer. Generally these sensors are mounted close the license plate (the prime target). Note: This is in addition to having a sensor very close to your license plate (the prime target). Otherwise, it is possible that your LiDAR detector will detect nothing because the angular size of the beam can be very small at close range.

Coming around a blind curve, you may attain a slight benefit by mounting LiDAR sensors to the outer most left and right margins of your vehicle (e.g.on your side view mirrors), so that the sensor is able to get a glimpse around a curve and possibly capture some LiDAR scatter from other vehicles being targeted before your license plate comes into view of the police officer. Note: This is in addition to having a sensor very close to your license plate (the prime target). Otherwise, it is possible that your LiDAR detector will detect nothing because the angular size of the beam can be very small at close range. (The inherent complexity of installing such an array of sensors raises serious questions about whether it is worthwhile for the driver to pursue such an installation—as opposed to simply adhering to the speed limit.)

Radar/LiDAR detector detectors: Legal issues must also be taken into account. For example, though Virginia doesn't disallow the purchase and possession of a radar/LiDAR detector, they do disallow its use. If such a device is within your vehicle, even if it is disconnected or not operating, it could be considered evidence of its use and it would be confiscated and the driver penalized. It may also be illegal for commercial truckers/drivers to operate such devices in some states. In Virginia (particularly), the police use Spectre or VG Radar detector detector (RDD) to detect the harmonic RF leakage from these devices and then go after the violators. Often LiDAR detection and radar detection is present within the same device.

Here is how it is used:

  1. The operating police officer has a radar detector detector attached to a rotatable mount on the ceiling of his or her vehicle.
  2. A sound and/or light indicates detection of "local oscillator leakage" from a radar detector.
  3. The unit is directional so it is aimed toward vehicles as the officer is passing them or as they pass by the officer, which helps isolate the exact vehicle emitting the tell-tale RF oscillator leakage.

Police LiDAR countermeasures[citation needed][edit]

  1. LiDAR jamming devices (Blinder(TM), etc.). These units create slurry of 905 nm pulses to try to confuse the Police LiDAR unit. Some units can indicate if an attempt at jamming has occurred and in some states it is illegal to jam LiDAR (such as Virginia laws state, in summary, "Any attempt to thwart or negate police attempts to measure traffic speed is illegal."
  2. LiDAR 905 nm specific absorbing pigments and dyes: Since 905 nm LiDAR is simply a 50 uW pulsed laser there are pigments and dyes on the market that can be made into paints and nearly clear-coats that will absorb most of the LiDAR that strikes the body of a vehicle (or simply the license plates, headlights, and retro-reflectors in tail-lights/reflectors. Absorbing dyes and pigments are often dark green, black, or rust-brown in visual appearance. Veil(tm) is one product reported to absorb some of the LiDAR signal.
  3. LiDAR 905 nm specific deflectors [reflecting LiDAR away from police point-of-origin]: On top of #2 above it is possible to design a vehicle with angled surfaces that further prevent reflections from going back to the point of origin (the police LiDAR unit).
  4. License Plate Shaping / Angling Technique: One simple method of reducing a vehicle reflectivity to LIDAR is to bend the license plate so that LIDAR beams hitting it will be deflected at a slightly upward angle (into the sky). To determine if this will work for you you first need to determine if you have a "Directionally Imaged Retroreflective" coating on your plate. Do this by going outside at night and hitting your plate with a low powered laser pointer. A strong return when the laser is positioned close to your eye will reveal that you have a such a coating on your plate. Standing about 1000 ft away bend the plate upward in small increments until you get little or no return. Also bending the plate edge to edge so that it is no longer flat but has a slight radius may reduce the amount of return.
  5. License Plate Painting Technique: This is specifically illegal in many states: Painting over the retro-reflective (white portions) of the license plate with a white acrylic primer paint (usually Titanium Dioxide as a pigment) will negate the retro-reflectivity of the surface. Painting this over a coat of carbon-black paint (Kylon(tm) Ultra-Flat Black) will further increase the effectiveness of this coating. The plate will not be reflective to headlights at night and probably draw the attention of police however. A light fogging of white paint may reduce retro-reflectivity without completely negating it and used in conjunction with Shaping/Angling (above) may drastically reduce the reflection. Also powdering the plate with talcum powder will reduce the retro-reflectivity of the plate to some unknown degree (wipe talc from lettering!).
  6. Motorcycle Plates Present a Weaker Reflection Standard automobile license plates are about 6" high X 12" wide. A motorcycle, having a plate size of 4" high x 7" wide has 38.9% less reflective surface area and so naturally presents a lessor amount of LIDAR return (given the same distance). Manipulating, over-coating, or otherwise modifying the surface of a license plate to reduce its reflectivity is against the law in many states.
  7. High Technology AR Coatings LiDAR 904 nm specific (AR) anti-reflective coatings (for clear glass/plastics). There are companies that produce tuned coatings that are narrow-band specific and can be made to peak at 904 nm there by negating all but a small 3% or less reflection. These coatings can be used on clear surfaces such as windshields, headlights, or even bexel retro-reflectors in various reflectors, turn signal indicators, tail lights, etc. on most modern vehicles.
  8. LiDAR obfuscation measures and theories — Light travels at differing velocities through differing materials. It may be possible to use that characteristic to create a passive system that provides a strong reflection back to the LiDAR unit with a number of overlapping pulses that have been delayed by minute but varying amounts. The theory is that this would confuse the LiDAR unit into trying again until it gets a clean reading (which conceivably — it may never get).
  9. Beating the RDD Radar detector detector: Radar detector detectors are a police tool and detect the harmonic RF leakage that emanates from the oscillating circuit in most Radar detectors. You can beat this tool by carefully choosing a RADAR-LiDAR detector that has improved circuitry preventing harmonics leakage in the ranges that the RDD units are looking for. However, every time the radar-LiDAR detector manufactures react to a new Law Enforcement capability with a safeguard so too the Law Enforcement suppliers react by re-designing their RDD to detect even formerly undetectable radar-LiDAR Detectors. Radar has not died out. X-band is now rarely used but K and Ka band are used all over the USA (Ku band used in Europe).
  10. Optical Filters/Deflectors. Optical windows that are wavelength specific filters (absorbing 905 nm) or "Hot Mirrors" that reflect specific wavelengths (reflect the in-bound LiDAR beam away from its point-of-origin detection point) can be used creatively to reduce the effectiveness of Police LiDAR. For instance, a Hot Mirror set at a 45 degree angle to deflect 905 nm laser up into the sky could be place over headlights to negate their effectiveness as a LiDAR reflector.
  11. Multiplicative Passive Reflection. It is a fact of physical science that light (including LiDAR Laser) travels at a different speeds through different "optically transparent" materials. It is possible to design an array of corner-cube-reflectors (retro reflectors) that all reflect the in-bound LiDAR beam back to the Police detection point-of-origin so that each reflector is made of differing materials to create a variance in multiple reflections all happening simultaneously in a single pulse return. This would indicting different "time-in-flight" measurements in a single pulse. This can be done also by creating multiple length (multiple distance) reflection paths for a single LiDAR pulse. It may also be possible to use both of these techniques in cooperation to "confuse" the LiDAR unit into identifying the reflection as an error and then repeatedly rejecting it. So another trick may be to create reflections that "don't make logical sense."
  12. Testing and Negating Retro-reflection. If you go outside at night and while holding a flashlight close to your head (close to an eye) project it at your car from a distance of 20 feet or so. Do this while standing at the center-line (0 degrees of incidence) front and back and not the strong reflections you receive directly back to the light point-of-origin (and your eye). These are likely retro-reflectors that are built into the license plate surface, the amber and red tail light, brake light, and turn signal bezels and more. All of these things are designed to betray you by reflecting Police LiDAR back to its detector so that your speed can be easily measured at greater distances. Some people have negated these by grinding out the retro-reflectors/replacing their bezels with aftermarket ones that don't include retro-reflectors. Some of these signal bezels may also include a back reflector that has a mirror-like finish. If the signal lights are replaced with LED units the back-reflectors can be removed, ground-away, or painted with a black-carbon based paint. Black Iron-Oxide based pigments are very effective light absorbers in the 905 nm wavelength that LiDAR uses as well so finding a black pigment (PBk) based on iron-oxide that is a heavily-loaded pigment will dramatically reduce your LiDAR return. For the "shock of your life" take a simple laser pointer (red/green) outside and standing about 100 yards away hold it to the side of your head (next to your eye) and project it onto your license plate. Just like this laser the LiDAR is also reflected very strongly as well.
  13. Crowd-Sourced Intelligence: Applications such as (and others) provide a method for traffic to report on the GPS location of police speed traps and traffic cameras, etc. and users can gain real-time intelligence. Users get a warning on their smart phone in advance of moving into the trap zone. As of October 2013 Waze had 17M users.

Erroneous readings are possible[edit]

In 2005 a BBC program Inside Out demonstrated how the LiDAR speed gun most commonly used in the UK, the LTI 20.20 could create exaggerated reading. Errors came from two sources. 'Sweep errors' were as a result of the laser beam not measuring the distance to a fixed point on the vehicle but instead being 'swept' along the side of the vehicle.

In the step-by-step example above this would be a case where the LiDAR gun aim-point is shifted so initial target calculated distance comes from a vehicle further away and the Laser is swept to a closer target for the second reading causing an exaggerated speed calculation. This can happen particularly when a vehicle behind the intended target is read first. Take for instance when attempting to get a speed reading on a motorcycle which has a very low LiDAR frontal cross-section. A large automobile presenting a retro-reflective license plate could product the initial distance calculation and then the motorcycle the second distance reading causing the LiDAR unit to calculate the speed of the motorcycle incorrectly [Distance Traveled = Distance of Car - Distance of Motorcycle]. This could easily happen if the police officer was operating the unit as a "hand held" gun and trying to get readings at long ranges.

In order for LiDAR to get an accurate reading the aim-point must be held on a single target point for the duration of the read. At long range this is accomplished through the use of a stationary tripod (to steady the aim). Errors can be demonstrated to police by sweeping the target along a wall (in demonstrations the LiDAR showed the stationary wall traveling at 58 mph). Another way of achieving a bogus reading is where the laser reflected off a wing mirror, hit a stationary reflective object and then returned reflecting off the mirror a second time adding additional time-in-flight to the initial distance calculation.[8]

Use in court[edit]

United States[edit]

In 2008, the D.C. Superior Court upheld the admissibility of LiDAR evidence in its jurisdiction. In additional to expert testimony, the court noted that it was factoring scientific publications into its decision:

The Court conducted an extensive four-day Frye [Daubert] hearing... [in which it] considered such issues as the basic science of laser technology, the technical methodology of, and theoretical challenges to, the reliability of radar guns... including the possibility of other “pulses” in the vicinity of use, difficulties in target identification, possible errors caused by vehicle license plates, windshield glass, shape, and color, and potential malfunction of the device. The Court also took judicial notice of at least six scientific publications on the subject in various journals of interest, together with two police-related studies in Florida, one New Jersey [study], and one independent study in Florida on this and similar radar devices, all of which met the standards set forth by [the] National Highway Safety Administration...

The court also noted that there was not one single court that had conducted full-blown hearings on the issue that had found LiDAR to be unreliable, while more than a dozen jurisdictions had decided that LiDAR is reliable.[9]

LiDAR speed gun jamming devices[edit]

Since the Federal Communications Commission (FCC) which promulgates regulations against Radio Jamming does not regulate LiDAR, there are jammers on the market. Virginia is the only state in the USA that outlaws the use of radar/LiDAR detectors and, in fact, vaguely outlaws any methods or techniques used to thwart the ability of Law Enforcement to measure the speeds of motorists. These devices cannot be legally used in Virginia.

LiDAR speed gun jamming devices, also known as laser jammers, are devices used by motorists to detect and block a LiDAR speed gun from registering a speed reading. The jamming devices work by detecting the gun's light and emitting light on the same 904 nm wavelength back at the gun. More recent versions of the devices will also emit light at the same rate (called pulse rate) that the gun uses to further confuse the gun.[10]

There are some problems with the theory of operation of these LiDAR jammers:

  1. How does the jammer know where the police LiDAR detector is so that it can paint it with a strong enough LASER pulse to disrupt it? Answer: To limit cosine error an assumption can be made that the LiDAR is within 15 degrees of the center-line of the vehicle front and back.
  1. If a LiDAR Jamming device promotes only 905 nm LEDs this may be a benefit at only close range? There are Laser Diodes on the market that when operated in pulsed modes at pulse widths of less than 30 ns (matching Police LiDAR) can attain extremely high power outputs and provide full coverage of the cosign angle that Police must operate within in order to gain accuracy. It is now very possible to "Jam" the narrow band-width filtered photodiode detector on the Police LiDAR gun from long ranges. Also, new array-style Avalanche NIR Detectors on the open market and used to in "smart bomb" technology can be used in conjunction with servos to aim the "Jamming" Laser at the Police point-of-origin completely "washing out" the LiDAR detector rendering it ineffective.


  1. ^ White Paper: "An Overview of avalanche photodiodes and pulsed lasers as they are used in 3D laser radar type applications" by Bruno Dion, CMC Electronics, Inc.
  2. ^ "Improving on Police RADAR". IEEE Spectrum: 38. July 1992. 
  3. ^ Anderson, Jay. "Stalker LIDAR LR Hand Held Specs". Stalker(tm). Retrieved 2013-11-11. 
  4. ^ (Article: They Have Lasers! | Road & Track Magazine, Nov. 1991, Page 106)
  5. ^ "How Police Laser Guns Work". 
  6. ^
  7. ^ "Range Testing by". 
  8. ^ "Mobile Speed Cameras". 
  9. ^ [1] p.866
  10. ^ "How Laser Jammers Work".