How Laser Works
The laser speed gun uses a method that relies on the reflection time of light. You have probably experienced the reflection time of sound waves in the form of an echo. For example, if you shout down a well or across a canyon. The sound takes a noticeable amount of time to reach the bottom of the well and travel back to your ear. Sound travels at approximately 1000 feet (300 metres) per second, so a deep well or wide canyon creates a very apparent round-trip time for sound.
A laser speed gun measures the round-trip time for light to reach a car and reflect back. Light from a laser speed gun moves a lot faster than sound – about 984,000,000 feet per second (300,000,000 metres) or roughly a foot (30 cm) per nanosecond. A laser speed gun shoots a very short burst of infrared laser light and then waits for it to reflect off the vehicle. The gun counts the number of nanoseconds it takes for the round trip, and by dividing it by 2 it can calculate the speed of the car. By taking several hundred samples over the course of a third of a second or so, the accuracy can be very high.
The advantage of a laser speed gun is that the size of the ‘cone' of light that the gun emits is very small, even at a range like 1000 feet (300 metres.) The cone at this distance might be 3 feet (1 metre) in diameter. This allows the gun to target a specific vehicle. A laser gun is also very accurate. The officer has to aim the laser speed gun at a specific target therefore the officer will only target your vehicle if the officer deems that you are speeding.
How Does Police Laser (LIDAR) Work?
Laser is very similar to a torch beam and it has all the same properties in comparison to a standard beam of light. The difference with laser however is that it can send out a tightly focused beam. Radar cannot single out one vehicle in a pack, so the speed reading is usually attributed to the leader. The narrow laser beam reads only the vehicle it strikes.
Laser's narrow beam imposes significant limits on its use however. It must be deliberately and carefully aimed, therefore the operator can't be moving. He must have a clear shot, preferably not through glass. As with radar, you cannot use laser to read speed from the side and there must be oncoming and departing traffic.
However since laser is a narrow beam at a close range the targeting of a car can be accurate enough not to scatter laser light onto other vehicles as the beam is only about a foot wide. This and the fact laser is only emitted when the trigger is pulled can cause problems for receiving suitably advanced warnings of laser speed traps ahead. At a longer range the beam spreads more and provides more scatter to alert other road users.