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Why Can You Blame Lasers for Your Last Speeding Ticket?

Laser Speed Detection and LIDAR (LIght Detection and Ranging)

LIDAR measures the vehicle's speed by sending out two laser pulses and calculating the difference in time it takes to detect the pulses of light reflected from the target, your car. Often times people will have radar detectors in their car so they know in advance if a police officer is trying to catch speeder. These work by picking up the radar radio waves that are scattered from other objects. However, it is difficult to detect the use of LIDAR in advance as it is a highly focused laser beam that scatters very little.

LIDAR has the distinct advantage of being able to pick out one vehicle in a cluttered stream of traffic; your vehicle cannot be mistaken for another vehicle unlike radar, which has a broader pulse. It can also be used over a large distance, which makes it even more difficult to spot. License plates are typically the strongest LIDAR target, as they are covered in a retroreflective material that returns light back its source with a bare minimum of scattering.

How LIDAR Works

Light travels from the LIDAR gun at about 3x108meters/second, or 0.3 meters per nanosecond.  After the light hits the target, it is reflected back towards the source. The laser determines distance by calculating how far a returning photon has traveled to and from a target:

Distance = [Speed of Light x Time of Flight] / [2]

  1. Laser gun generates an optical pulse.
  2. The pulse travels from the laser gun to the target and back to the gun.
  3. LIDAR system measures the time of flight from the start pulse to the return pulse.
  4. Time measurement is converted to a distance by using the formula above.

The LIDAR gun then sends out a second pulse and finds the distance again, as well as the time between the two pulses. Using the following formula, it can determine your speed.

Speed = [Change in Distance (Dx)]/ [Change in Time (Dt)]

The LIDAR system then determines your speed by the above formula.

A laser gun can determine your speed very quickly, about 1,000 times per second!


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