LASER stands for Light Amplification by Stimulated Emission of Radiation. The Laser Speed Gun uses a process based on the reflection time of light. A laser speed gun measures the round-trip time for light to reach a car and bounce back. Light from a laser velocity gun travels much faster than sound - about 984,000,000 feet per second (300,000,000 meters) or about 1 foot (30 cm) per nanosecond. A Laser Velocity Cannon shoots a very short beam of infrared laser light, then waits for it to bounce off the vehicle. The gun counts the number of nanoseconds it takes to make the round trip and by dividing by 2 it can calculate the distance to the car. If the gun takes 1,000 samples per second, it can compare the change in distance between samples and calculate the car's speed. By taking several hundred samples over the course of about a third of a second, the accuracy can be very high.
The advantage of a laser speed cannon is that the size of the "cone of light" the cannon emits is very small, even at a range of around 300 meters (1,000 feet). The cone at this distance can be 1 meter in diameter. This allows the weapon to target a specific vehicle. A laser gun is also very accurate. The officer must aim the laser speed gun at a specific target, so the officer will only aim your vehicle if the officer thinks you are speeding. 
How does the police laser (LIDAR) work?
LIDAR is an acronym for Light Distance And Ranging. A LIDAR gun emits a highly focused beam of invisible near-infrared light centered at a wavelength of 904 nm and only about 56 cm (22 in) in diameter at 300 m (1000 ft). Unlike RADAR, which directly determines a vehicle's speed by measuring its Doppler shift, LIDAR calculates speed by observing the changing time it takes to "see" reflected pulses of light over a discrete period of time.
Laser cannons can be tripod-mounted, hand-held, or attached to video equipment in or near a vehicle.
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