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]

LiDAR relies on the principle of time-of-flight of two or more short 905 nanometres (3.56×10−5 in) 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 and select the best aim point often before the driver sees the police officer—generally at a distance of 1,000 feet (300 m) and up to 4,000 feet (1,200 m) The police officer aims the pulsed 4 milliradian laser at the license plate as most often the retro-reflective film on US plates provide a very strong return signal. License plates are coated with a retro-reflective coating that reflects the laser pulses back to the LIDAR gun receiver aperture. In the case of vehicles without front plates the officer will choose a headlight or chrome bumper to bounce the LiDAR signal off of. Headlights generally have a back reflector that can provide a strong return signal. 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 to 4,000 feet (1.5 to 1,219.2 m) 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 metre (11 sq ft) at 300 metres (980 ft) 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]

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. has some impact though minor.
  • 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. The Police Officer would need to operate LiDAR from outside the vehicle—possibly from under a bridge overpass, etc. Weather has little impact on the LiDAR signal.
  • Windshield glass can scatter the IR pulses if the windshield is wet with rain, fogged, or splattered with snow. This makes it very hard to operate a LiDAR unit from inside a Police vehicle in inclement weather.
  • Front or rear vehicle targeting makes a difference because, generally, automobile rears present a stronger overall reflection. In states that don't require front plates, or in the case of motorcycles, the rear is surely presents the strongest reflection. However, police often prefer to measure the speed of vehicles from the front so they can wave offending vehicles over without having to chase after them. Consider that in about 95% of the time that LiDAR is in use you will be targeted from the front.
  • Darkness can impede the ability of police officer to identify and target the license plate on a vehicle at a distance. LiDAR units do not yet include light amplification or "night vision" scopes. This may be a future improvement.
  • 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]
  • Cosine error is an issue with LiDAR. The closer the LiDAR gun is to directly in-front or behind the target vehicle, the more accurate the reading. When police use off-center angles, 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. To prevent cosine error from lowering measured speed of the target vehicle you will often see the Police Officer operating the LiDAR right up to the edge of the road or even stepping into the road slightly to get a higher reading. Cosine error works against the Police Officer and so they try to minimize its effect.
  • 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.
  • Police in motion: When in a police vehicle, the police must stop to get an accurate speed measurement. Presently LiDAR cannot be operated from a moving Police vehicle (as RADAR can).

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. 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 error. 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 vendors offer police user-upgradable firmware.

Police strategies[edit]

Front vs. rear targeting: Rear targeting often extends detection range by about 5.3% average.[5]

Police LiDAR countermeasures[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." Some devices try to measure the pulse per second (PPS) rate of Police LiDAR unit to identify what type of unit is in use.
  2. LiDAR Scatter/Background Noise: Another technique that may legally be considered "Jamming" is to use a number of 905 nm 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 μW 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 905 nm (+/-10 nm) and with the ability to absorb 100% of incident LiDAR rays. Note: 905 nm is the center wavelength (CWL) of many LiDAR units—some advertise a CWL of 904 nm. Generally the operate in the range of 895 nm to 915 nm (10 nm of +/- accuracy).
  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 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. 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. Cold Mirrors Do Not Reflect LiDAR The mirror of a motorcycle (when illuminated by LiDAR from the back can present a strong return signal. A technology called a "Cold Mirror" is a mirror with a specially tuned surface that reflects light except for heat/light over a particular wavelength. Using a mirror with a "cut off" frequency of 800 nm (and above—for example) would prohibit the return reflection of LiDAR laser.
  9. 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.
  10. 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).
  11. 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. A mention here even though the technology specifically applies to RADAR only and not LiDAR. 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 reacts 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 and will have its place in moving Police vehicles. X-band is now rarely used but K and Ka band are used all over the USA (Ku band used in Europe).
  12. 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.
  13. 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."
  14. 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 note 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.
  15. 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.[6]

Use in court[edit]

United States[edit]

In 2008, the D.C. Superior Court upheld the admissibility of LiDAR evidence in its jurisdiction. In addition 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.[7]

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.[8]

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 905 nm 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 900 nm 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. ^ "Range Testing by". 
  6. ^ "Mobile Speed Cameras". 
  7. ^ [1] p.866
  8. ^ "How Laser Jammers Work".