LIDAR speed gun

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Police officer using the 'LTI-20/20 Ultra Lyte Laser' hand-held LiDAR speed gun.

A LiDAR speed gun is a device that police use to measure vehicle speed, to see if the target vehicle is exceeding the speed limit. It uses LiDAR to detect the speed of a vehicle. Unlike Radar speed guns, which rely on doppler shifts to measure speed, these devices let a police officer measure the speed of an individual vehicle within a stream of traffic.

How police LiDAR guns work[edit]

Note: This article uses some Imperial units—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 Per Hour.

LiDAR relies on the principle of time-of-flight of two or more short 905 nm wavelength (near infrared - NIR) LASER pulses. The police officer aims the LiDAR through a telescopic monocular (2X - 8X, depending on model) built into the LiDAR gun. The scope helps the police officer see the target vehicle before the driver sees the police officer—generally at a distance of 1000 ft and up to 4000 ft. The police officer aims the pulsed 4 milliradian laser at the license plate. License plates are coated with a retro-reflective coating that reflects the laser pulses back to the LIDAR gun receiver aperture. Range varies by LiDAR gun manufacturer, target vehicle aim-point reflectivity, and weather conditions (temperature, humidity, precipitation). The LiDAR can record vehicle speed anywhere from ~5 feet to ~4,000 feet away. Most police Lidar units use a magnification of 2X. An 8X magnification scope makes acquiring and tracking a quickly moving vehicle more difficult. LiDAR gun manufactures have begun to concentrate on extending the speed-detection range of the devices (2014).

Some LiDAR units produce a tone to indicate they are receiving a good return signal. The tone may vary from target to target, so the operator can sample multiple vehicles and select a particular one. The 3-4 milliradian cone presents an area of illumination of about 1 square meter at 300 meters distance. Therefore, the police officer can select a single vehicle out of a group. A vehicle in the "shadow" of another vehicle cannot be measured.

To operate the device, the police officer presses the LiDAR gun trigger. The gun emits short LiDAR laser pulses, with a pulse width (duration of pulse) of 30 nanoseconds or less. Depending on the LiDAR gun, the number of pulses per second (pps) ranges 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 that rejects inaccuracies. All manufacturers use proprietary error rejection methods.

LiDAR speed measurement takes place in these steps:

  1. On a trigger pull, the gun sends a series of short (typically 30 nanosecond) laser pulses (100-600 per second) and starts a timer.
  2. The gun stores the time that each pulse's reflection reached the gun's detector.
  3. The gun uses elapsed “time-of-flight” to determine the distance each pulse traveled, and uses the difference between pulse distances to calculate speed.[1]

Distance_traveled = Distance_2 - Distance_1 Speed = Distance_traveled / Elapsed_time

Current LIDAR guns typically acquire and validate target vehicle speed in under half a second (250 to 400 ms). LiDAR guns must meet a law enforcement accuracy requirement of +1 MPH or -2 MPH. All attain +/-1 MPH (same as RADAR).[2][3]

Since the LiDAR gun may be off, and is only triggered when the police officer targets a vehicle, there are no "stray" LiDAR signals to detect, so driver-operated RADAR/LiDAR detectors are not very effective. A police officer targeting many vehicles at long range may create some stray laser reflections that a LiDAR detector may detect, but this isn't likely. If a RADAR/LiDAR detector in a targeted vehicle sounds an alarm, it does no good because the gun has completed the speed measurement long before the driver can react.

In addition to fast target acquisition, some LiDAR guns produce pulse patterns that are not evenly spaced in time. Many LiDAR detectors use photo-diodes to detect 904-905 nm laser through a narrow band-width filter (to reject irrelevant wavelengths) and detect only even pulsing. These "stealth" LiDAR signals are invisible to most LiDAR detectors.

Other than distance, other factors that degrade vehicle speed measurement include:

  • Refraction by differences in air density due to "heat waves" off a hot road surface, etc.
  • Weather conditions, such as rain, heavy fog, snow—which have a negligible impact on the laser pulse but may impair the police officer' ability to target.
  • Windshield glass tends to scatter the IR pulses. This is usually negligible, but if the windshield is wet with rain, fogged, or splattered with snow, that can reduce LiDAR range.
  • Front or rear vehicle targeting makes a difference because, generally, automobile rears present a stronger reflection. In states that don't require front plates, or in the case of motorcycles, the rear is surely the strongest reflection point However, police set up to detect vehicles from the front around 90% of the time so they can wave offending vehicles over without having to chase them.
  • Darkness can impede the ability of police officer to identify and target and a vehicle. LiDAR units do not yet include light amplification or "night vision."
  • Occlusion by the sun, where the sun is behind the target, can prevent the LiDAR unit from reading. The Sun may "wash out" incoming LiDAR pulses. The sun also impedes targeting. 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) at the LiDAR gun receiver aperture rejects light outside the Laser's operating range (sun, HID Headlights, etc.)[4]
  1. Cosine error is an issue with LiDAR, just as it is with RADAR speed measurement. The closer the LiDAR gun is to directly in-front or behind the target vehicle, the more accurate the reading. Police often use off-center angles of as much as 15 degrees, but they know this cosine angle works against them. The gun calculates a lower-than-actual speed because the calculated distance between pulses is less because the car is moving at an angle to the gun, instead of directly towards or away. This is why LiDAR units often operate close to the edge of the road.
  2. Police misuse (sweeping) refers to the act of sweeping the LiDAR gun while pressing the trigger, so that—particularly at long range where angular separation between targets is slight—pulses from more than one target create a false reading.
  3. Police in motion, in a moving police vehicle, must stop the vehicle to get an accurate speed measurement.

For example: A police officer is trying to read a group of vehicles over a mile away (near the range limit). The officer is operating the unit "hand held" (without a tripod). "Camera shake" makes the LiDAR pulse beam 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 make the LiDAR "think" it read a single vehicle. The pulse time-in-flight between the initial and final vehicle being much shorter, the LiDAR unit provides a reading much higher than the target vehicle speed.

A specific "sweep error" scenario: A car is traveling at the 45 MPH posted speed limit alongside another vehicle that's further forward, also traveling at the posted speed. If the officer's unsteady hand makes the LiDAR beam sweep from the first vehicle to the second, forward vehicle in a single read, the LiDAR gun might produce a reading as high as 80 MPH, depending on the distance between the vehicles. The officer may think this is the target vehicle's actual speed and ticket the driver. The probability of this is high because "pulling" the trigger instead of "squeezing" it can make the gun sweep. Police officers are trained in 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 a violation occurred.

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].

Some LiDAR guns have logic that tries to detect when a vehicle is operating some form of LiDAR "Jamming" signal and may report suspected jamming. There are many instances of false positives, however, so police are never sure if a vehicle is operating a jamming device. Also, some LiDAR guns are more susceptible to LiDAR jamming than others.

LiDAR jammers assume the Police Officer is within 30 degrees of center front (in most cases) or 30 degrees of center rear (in lesser cases). The Police Officer must operate within these angles to limit cosine error, which always favors the speeder. In fact, many police officers try to get as close to 0 degrees as possible to produce the most accurate reading. They may stand near the edge of the road, or even leaning out into it. Jammers generate a large number of short 905 nm laser pulses in a 30 degrees wide (or less) beam. This makes the LiDAR gun detect so many returning pulses that it becomes "confused." The error-correction algorithm ends up rejecting the readings and prevents the LiDAR gun from determining vehicle speed . LiDAR units are becoming more sophisticated, and some can detect jamming attempts. LiDAR gun manufactures buy every new LiDAR detector and jammer and analyse them to determine how to improve LiDAR gun software. Some may offer police-user upgradable firmware.

Capabilities of police LiDAR speed guns[edit]

Differences in range for target vehicles moving away vs toward LiDAR operator
Chart shows there is negligible degradation in performance due to inclement weather.

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

1. The best reflector is a license plate. It's highly reflective retro-reflective coating matches the 4 milliradian cone angle of the police LiDAR units, and returns that beam effeciently. 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.[5]

2. Very good reflectors include headlights—often the backing within a headlight is a semi-parabolic reflecting first-surface mirror. Police use these on oncoming motorcycles and in states that don't require vehicles to have front plates.

3. Nice reflectors include other lighting, such as turn signals with retro-reflective trihedral corner reflectors. Tail lights generally have the largest area of these retro-reflectors and make good targets even for long range detection. Different vehicle designs present different degrees of bezel retro-reflection.

4. Good reflectors are chrome trim such as bumpers and grills, which present a great first-surface mirror reflector, but are less good for long-range speed measuring.

5. Poor reflector include windshields and car body panels, which make poor reflectors but may work at close range.

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.[6] 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, police are more likely to target cars from the front so they can wave you off for a ticket. They often take position over a hill or around a curve or other obstacle so they can operate LiDAR at close range. They often operate a chase car, to which the LiDAR operator radios 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 give the offending driver enough time to change lane positions to complicate identification.

Ambush technique: This is the most effective LiDAR technique. 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 on the vehicle 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 may 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: Somewhat confusing legal issues surround the use and possession of Radar/LiDAR detectors under circumstances where they are illegal in the USA. Throughout the US commercial truckers are disallowed the use of Radar/LiDAR detectors. Also, the state of Virginia and Washington, DC have laws making their use illegal to all. There are no such devices for sale in Virginia but they can be mail-ordered into the state. They are legal to possess ostensibly for use outside Virginia. A person may legally possess a Radar/LiDAR detector in Virginia but if they have it in their vehicle and it is within their reach the police may assume it was in use and charge the driver with its use whether it was actually in use or not. Spetre IV/Elite and other such Radar-Detector-Detectors (RDD) are in common use throughout the USA but very often in use in localities where Radar/LiDAR detector are illegal for all to use. In Virginia (particularly), the police use RDD technology to detect the presence of a Radar/LiDAR detector in the vehicles around them.

Spectre Elite is extremely sensitive and there are NO Radar Detectors on the market that can avoid detection by the device. The device is effective because it leverages a weakness in the design of consumer Radar detectors that makes those Radar detectors affordable to the public. Electronic component that can operate at the high frequencies that police Radar operates (10-36GHz) would be very expensive to use to build a radar detector so radar detector manufactures use a technique called superheterodyning. In superheterdyning the police Radar that is coming into the Radar antenna is mixed with a lower frequency RF generated by a local-oscillator. That mixing causes hetrodyning and a lower frequency is created that is well within range of affordable electronic components within the Radar detector. It is this local-oscillator and the mixing circuit within the radar detector that "leaks" a 10GHz to 25GHz RF signal that the Spectre and other RDDs sense. Though all of the detector manufactures provide shielding of the local-oscillator and mixer portions on their circuit boards few provide any additional shielding or radar absorbing materials to attenuate the RF and the small amounts if RF that leaks can be detected from distance of likely 250 feet or more. To sense weak signals, Spectre amplifies radar detector singnals to such a degree that it is also a cause for false positives. Other RF sources and transient conditions can cause a false positive that may cause the police officer to pull someone over. When the police do get a indication there is a radar detector in use, they have probable cause to make a traffic stop and can search your vehicle.

The device can be hand carried (on a police belt in a holster) however it is often positioned at the center ceiling inside the police vehicle on a pivot so that it can be turned 360 degrees. When the device triggers it shows the police officer the signal strength of the device it is sensing. The officer can then rotate the Spectre to point directionally at various possible alternative targets while watching the signal strength (shown as an LED bar graph) to provide some isolation of the signal and determine the likelihood that it is being radiated from your vehicle. All consumer Radar detectors known leak RF in the 10 GHz to 25 GHz range. This RF leakage come from the local-oscillator within the Radar detector. Some Radar detectors can switch off the local-ocillator to stealth themselves from RDD but if you are coming around a blind-curve or hill when they hit you with Radar/LiDAR -- at that close range they may already have detected a likely Radar detector in use.[7]

The best way to improve on the stealthing capabilities of a Radar detector to make it less prone to RDD is by application one or more of the following techniques to reduce local-oscillator leakage:

  1. ATTENUATE LOCAL-OSCILLATOR AND MIXER RF ON COMPONENTS DIRECTLY: Attenuate RF from local-oscillator component and all ICs with the Local-Oscillaor and Mixer Stages: Generally these are the areas of the circuit board that are covered by an RF shield.
  2. ATTENUATE RESONATING RF INSIDE EXISTING SHIELDING: Attenuate RF within the shield chamber by open the Radar detector and open the metal shielding around the local-oscillaor circuitry and install a type of Radar Absorbing Materials (RAM) called a resonate absorber. This absorber should be tuned to absorb specifically 10-25GHz. Use a thin insulator between the circuit and this conductive resonator material to prevent it from shorting components on the circuit board. Re-install the shield soldering it down or ensuring a good tight metal to metal contact devoid of gaps or insulators such a oils from your hands.
  3. ATTENUATE SURFACE CURRENTS AND RF ON OUTSIDE OF SHIELD: Attenuate Surface E-waves on the Shield Chamber by applying resonator-type RAM specific to 10-25GHz to the outside of the metal shield chamber. Complete coverage is not necessary -- a small square in the center of each side or surface of the chamber provides some absorption of RF that could be regenerated on the outside of the shield.
  4. ATTENUATE RF THROUGH FULL SHIELDING OF REMOTE ANTENNA: Add-on a solid copper shield (box)you can build for the remote antenna of some Radar detectors. Keep this copper shield electrically isolated from the antenna ground using insulating standoffs or GE-Silicone II RTV. The RTV and copper can be purchased at a hardware store or hobby shop. Don't get foil-thin material but as thick as you cut with metal shears easily and build a box to enclose the antenna completely soldering the edges. You can also opt to extend out the wave-form guide to capture more police radar and improve the sensitivity of your radar detector if you like to experiment. Block the exit of wire/cable from the box with additional copper plating so that the RF within cannot follow the insulation out of the box and get a clear-shot into the air -- create a kind of "light trap" effect so any escaping RF will need to make one or more turns. Cover use additional resonator type RAM or lossy carbon RAM absorber material also specific to 10-25GHz. Additionally the wire/cable leading to the antenna can be enclosed in a copper tubing used as a conduit and soldered to the copper shield. The opening for the Antenna wave-form guide (horn) should be left open but either soldered or gasketed using stainless-steel fabric gasketing material. Leave an extra inch of space on the side of your shield facing the local-oscillator and apply a wide-band lossy carbonized foam RAM to the inside panel of the shield. Ground the shield to the chassis ground of your vehicle through a 377 ohm 1/2 watt resistor.

Police protocol for use of RDD (Spectre, VG2, etc.):

  • MOUNTING: The police officer has an RDD device, generally attached to a rotatable mount on the ceiling of the police vehicle. The device may also be mobile—attached to the officer's belt.
  • DETECTION & ISOLATION: A sound and/or light indicates detection of "local oscillator leakage" from a radar detector and the signal strength displays in a bar graph line of LEDs. The unit is directional so an officer aims toward vehicles while passing them or as they pass by, which helps isolate the exact vehicle emitting the tell-tale RF oscillator leakage.

The police may operate the RDD in a stationary position with radar or LiDAR. If a single vehicle comes around the curve and the RDD indicates a signal, they already have the car isolated. It is instantly likely that the signal is from that vehicle and the driver is operating a Radar detector. The police may operate the RDD from within their vehicle while following you in traffic. If they gets a signal, they may rotate the unit to be sure the signal is from your vehicle, and then follow you through a number of turns to isolate you from other traffic that may be radiating the signal. Once they're sure where the signal is from, they pull over the suspect vehicle and search for the radar detector.

  1. SIGNAL-DOWN VALIDATION: Protocol calls for police officers to sit in their car and wait for the suspect driver to shut off their engine. Then they look at the RDD signal, which should stabilize when the two vehicles aren't moving. The officer also looks at the RDD to see its reaction when the suspect vehicle's engine turns off. Since many Radar detectors are wired to ignition power, the officers are looking for the RDD signal to drop off "coincidently" when the suspect vehicle's engine turns off. That strongly indicates that the suspect driver is operating a radar detector. If the signal drops before the engine goes off, the police may conclude the vehicle has a radar detector on/off switch somewhere on the dash. If the signal continues but reduces on engine turn off, it may indicate a stealth radar detector (hidden remote antenna in the suspect vehicle grille or elsewhere).
  2. OPERATOR MOVEMENTS/BEHAVIOR: If you are in a vehicle where the officers can see you through the rear window, they look for movements that might indicate that you are trying to hide your radar detector. That may also indicate that you have a portable device. If you don't move and the signal drops when the ignition is turned off that may indicate that you have remote radar antennas and likely a somewhat stealth device with a hidden switch to turn it on and off. That indicates that they should look for add-on switches and indicator LEDs.
  3. INTERVIEW: The police officer first asks if you have a radar detector. If you say you do, they confiscated it and write a summons, and later you pay a fine. If you deny you have one, they search your vehicle with probable cause. They may also search the front and rear of the vehicle for a radar antenna if they think you are operating a remote antenna.
  4. ENGINE START TEST: If the police officers get an indication from the RDD that you are operating a Radar detector, but can't find it, they may ask you to re-start your engine. They either go back to the police car to view the RDD to see if it again indicates a radar detector, or they may use a hand-held RDD to try to isolate the signal.
  5. INVESTIGATE ALTERNATIVE SOURCES OF RF: The police officers may ask if you have other electronic devices in the vehicle or on you. In asking these questions they are considering the possibility that something else might be causing the signal. Under certain conditions, some devices may create a transient false positive indication on the RDD. HAM radio, walkie-talkie, cell phone, any WiFi devices, or transponder such as Flex Pass may be the source of the RF. False positives happen all the time, and police must consider that a passing vehicle, a number of passing vehicles, or even a shopping center automatic door opener (10GHz) may have generated the signal.

Police LiDAR countermeasures[citation needed][edit]

  1. LiDAR Jamming Devices (Blinder(TM), etc.). These units create a 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 Scatter/Background Noise: Another technique that may also be consider legally to be "Jamming" is to use a number of 905nm LEDs facing forward and backward with as high intensity as you can find and with a protection angle to cover a full 30 degrees horizontally from the centerline of your vehicle and operate these. This will provide an increase in background-noise and will likely decrease the efficiency of the police LiDAR photo-diode signal-noise reduction circuit that attempts to adjust to ambient interference to allow the LiDAR unit to "pick out" its own reflected pulses.Varying the brightness of these LEDs and having a number of them and turning some on and off randomly may also prevent any logic within the police LiDAR noise-reduction system from being able to easily distinguish its own pulses from background noise. Police LiDAR can also read its pulses in LiDAR scatter at close range and so providing additional interfering "noise" by creating what appears to be a strong scatter (that isn't pulsing) will reduce the LiDAR gun's effective range even further.
  3. 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. There are pigments available specifically peaking in absorption at 905nm (+/-10nm) and with the ability to absorb 100% of incident LiDAR rays.
  4. 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).
  5. 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.
  6. License Plate Painting Technique: This is specifically illegal in all states: Painting over the retro-reflective (white portions) of the license plate with a white acrylic primer paint (look for Zinc oxide as a pigment -- not Titanium Dioxide) will help to 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!).
  7. 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 lesser 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.
  8. 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.
  9. 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).
  10. 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).
  11. 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.
  12. 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."
  13. 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.
  14. 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' resulted from the laser beam not measuring the distance to a fixed point on the vehicle but instead 'sweeping' along the side of the vehicle.

In the step-by-step example above this would be a case where the LiDAR gun aim-point shifts so initial calculated target distance comes from a vehicle further away and subsequent target distance is from a closer target, causing an exaggerated speed calculation. This can happen particularly when the LiDAR gun reads a vehicle behind the intended target first—for instance when attempting to get a speed reading on a motorcycle with a low LiDAR frontal cross-section. A large automobile with a retro-reflective license plate could product the initial distance calculation—and then the motorcycle returns the second distance reading, so the LiDAR unit calculates the motorcycle speed incorrectly [Distance Traveled = Distance of Car - Distance of Motorcycle]. This can easily happen if the police officer operates the unit as a "hand held" gun while trying to get long range readings.

For LiDAR to produce an accurate reading, the officer must hold the aim-point 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 kind of false reading is produced when the laser reflects off a wing mirror, hits a stationary reflective object and then returns to reflect 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 factored 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 not a single court had conducted full-blown hearings on the issue that found LiDAR 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 motorists use 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 to maintain 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.

At very close ranges police LiDAR can read LiDAR scatter and so it may be useful to provide a strong countering source of 905nm IR to provide a strong obfuscating scatter to help over-run the ability of the LiDAR photo-detector signal-to-noise reduction (digital signal processing) to isolate its own reflected pulses from the lock onto its own return pulse from the ambient light. This may be somewhat effective day or night as normal sunlight spectrum shows a dip in light intensity at about 900nm that may actually be providing a lower background noise even in the daytime thus improving Police LiDAR ability to operate in bright sunlight.


  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. ^
  6. ^ "Range Testing by". 
  7. ^ Spectre web-site and brosure:
  8. ^ "Mobile Speed Cameras". 
  9. ^ [1] p.866
  10. ^ "How Laser Jammers Work".