Lidar (Laser)

Lidar technology is the newest method of measuring a driver’s speed. Lidar guns, or “Laser guns” are handheld devices that police officers point at cars to measure the car’s speed.

Lidar guns measure speed by using a “time-in-flight” method. The device shoots invisible, infrared laser beams one at a time at a rate of about 120-238 beams per second for a least a .3 seconds burst. The device then tracks the time it takes each of the beams to bounce back. The device simply measures how much time each beam spent “in flight” and multiplies the time by the speed of light (983,571,072 feet per second) and divides the product by two to calculate the distance between the gun and the target. Each pulse takes a distance measurement, so the Lidar gun measures the distance between it and the target 120-238 times per second.

The device then measures the change in distance during this distance measuring process and determines the speed of the target. For example, if during a .3 second burst, the target started out at 546 feet and ended at 510 feet away, the device knows the object traveled 36 feet in .3 seconds. This equals 120 feet per second or 81.8 miles per hour (mph).

There are many different types of Lidar units that may be used by Virginia law enforcement officers; however, any Lidar device used by law enforcement must be approved by the Virginia Division of Purchase and Supply (Va DPS). The most common Lidar units used in Virginia are the Pro Laser III and the Prol Lite +. Both devices are manufactured by Kustom Signals. Both devices are required to measure speed within +/- 1 mph in order to be used as evidence in court.

Lidar Range

The theoretical range of a handheld Lidar device is well over 2000 feet (aproximately1/3 mile). However, the effective range on Lidar is limited by several things including the beam width in relationship to the targets size, the reflective quality of the target, atmospheric conditions, and the steadiness of the operator’s hand. Range can also be limited when the officer is sending the beam through glass (such as the windshield) or when the lens of the Lidar device becomes dirty or scratched.

The width of Lidar’s infrared beam is usually about .003% of the distance from the gun. Therefore, if a police officer measures a car that is 1,000 feet away, the car is being struck by an infrared circle that is 36 inches in diameter. As the range becomes greater, the beam diameter gets bigger. As the diameter gets bigger, it becomes more and more difficult to measure only one target’s speed at a time and thus range suffers.

Most Lidar devices will not produce a measurement if a significant portion of the beam produces multiple distance readings simultaneously (i.e. half the beam is striking a car and half is over-shooting and striking an object behind the car).

Generally speaking, the range of a Lidar device is limited by the operator not the device. An officer will have trouble continuously holding the beam completely on a moving vehicle that is more than 900 feet away and will have a hard time even identifying the make and model of a vehicle in day light if it is more than 800 feet away. While it is possible to get a measurement beyond these distances, it is much harder for the officer to obtain a legitimate tracking history of the target vehicle in order to guarantee the accuracy of the readings. For this reason some jurisdictions outside of Virginia have created rules against using Lidar beyond 1000 feet.

In order for Lidar to work accurately, a certain percentage of the infrared beams emitted from the device must bounce back. If the beam is not pointed at something that reflects light well or if the object is not perpendicular to the device, then the portion of the beams that bounce back will be reduced and it will take longer for the device to acquire sufficient readings and the effective range will be reduced. Under ideal conditions it usually takes only .3 seconds of measurements for the Lidar device to produce a speed reading but the time it takes to acquire a reading gets longer as conditions worsen.

If not enough beams return to the gun, the Lidar device will not produce a result. Consequently, the police are trained to aim the beam at a car’s flat reflective surfaces, such as the license plate or headlights. Cars with hidden headlights or without a front license plate are harder to measure at distances beyond 700 feet in open air or more than 500 feet when through glass. Other conditions such as weather and the condition of the Lidar lens can affect the range and speed of target acquisition.

Lidar Sweep Error

Movement of the operator’s hand during the measurement process can cause erroneous speed measurements. If a police officer’s hand moves during the measurement process, and the Lidar beam moves from one object to another the difference in distance between the two objects may be read as if they were one object moving. This type of error is referred to as “sweep error.” The greater the distance between the operator and the target the higher the likelihood of sweep error.

A common form of sweep error is when an officer shoots at the windshield of a vehicle and then moves the beam to the front license plate in order to get a stronger signal. The change in distance between the windshield and the license plate may result in a 5-9 mph increase in speed. This phenomenon can be demonstrated by sweeping the Lidar beam rapidly from the windshield to the license plate on a parked vehicle. The stationary vehicle will produce a reading of 5-9 mphs.

This graphic explains the concept of “sweep error” in LiDAR devices.

Currently there are unsubstantiated reports of sweep error generated by officer’s who are measuring vehicles at a location where the road surface is somewhat perpendicular to the Lidar operator (i.e. shooting at hill or at a banking turn). Sweeping the Lidar beam along the road surfaces at these locations reportedly caused sweep errors of more than 90+ mph without any vehicle being present. The faster the officers tracked the beam along the road the faster the reading on the Lidar device.

Without a proper tracking history an officer cannot rule out the possibility sweep error. Sweep error is more likely where the officer is only taking a quick “snap shot” speed measurement (i.e. taking a single speed measurement instead of continuously measuring the vehicle’s speed over a period of several seconds). Sweep error is also hard to prevent at great distances where it is hard to visually estimate speed and where even slight tremors in the hands can result in rapid changes in the location of the beam.

Lidar Sight Misalignment

Law enforcement officers aim Lidar devices with a digital heads-up display (HUD) that contains cross hairs and a digital screen which can display speed, distance, and an estimated beam location. The HUD on any Lidar device should never be magnified (most are not). This is because magnification makes the device more difficult to use at close ranges and distorts the officer’s ability to visually estimate the vehicle’s speed, an essential component of verifying that a Lidar device is functioning correctly.

When an officer measures an object, the HUD generates a red circle over the general location of where the beam struck the target. The cross hairs and this red circle are the only indication of where the very narrow (and invisible) Lidar beam is being directed.

Like the site on a gun, the HUD can become misaligned. Dropping or mishandling a Lidar device can cause the HUD to aim inaccurately. If undetected or ignored, an error in the HUD can result in an officer attributing a speed measurement to the wrong driver (i.e pointing the gun at one car while measuring another car).

To prevent sight misalignment, an officer should perform a vertical and horizontal sight alignment test at the beginning and end of each shift. This will guarantee that the sights were functioning properly during the traffic enforcement period.

Lidar Calibration

Every 6 months all Lidar devices should be sent to the manufacture’s laboratory to be tested for accuracy. Additionally, at the beginning and end of each shift the officer should test the Lidar device in several ways.

First the officer, runs a “self-diagnosis”. The officer will visually verify that all portions of the LCD screen are functioning and that the results of the self-diagnosis are positive. Then the officer measures a known distance with the Lidar device and verifies that it is working accurately. Then the officer tests the HUD to guarantee that the sites are properly aligned. Proper calibration is essential to proper speed enforcement and failure to comply with these calibration requirements is the most common way of defeating a Lidar based reckless driving case.

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