A LiDAR (Laser) speed gun is a device used by the police for speed limit enforcement which uses LiDAR to detect the speed of a vehicle.
About Police Lasers
How does a laser gun work?
- Aim: 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.
- 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 fastest moving vehicle if they choose.
- 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.
- 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). Since the number of pulses must be measured, and in order to determine a velocity and then the variances considered in error checking, it is highly likely that the Police LiDAR gun is capable of getting a reading of target vehicle speed in a time as short as 1/250 second (theoretically).
Where do officers aim LiDAR guns
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.
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 larges 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.
LiDAR Range Calculations
Laser Jammer Legality Map
What is a laser jammer?
How does a laser Jammer work?
Are laser jammers legal?
- Green: Legal
- Red: Illegal
- Yellow: Questionable
Received a ticket due to a police laser?
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