Radar gun

A laser gun ( also erroneously referred to as a radar gun) is the colloquial name for a hand-held laser meter It is one of the speed monitoring devices, which are used for the official traffic monitoring to measure the speed of passing vehicles. The laser hand-held meter is a device for transportation Abeles speed monitoring with fast consecutive optical distance measurements.

Principle of measurement

A hand-held laser meter emits short pulses of light. From the time difference until the arrival of the reflected radiation, the device calculates the distance. From a plurality of measurements per unit of time, it determines the change in the distance and the speed thereof. The light source used is a pulsed infra-red laser.

The measuring principle of a laser handheld instrument differs from a radar speed meter. The speed radar operates according to the Doppler principle. It determines the speed on the basis of the beats from the superposition of radiated and reflected radiation, caused by the Doppler frequency shift.

Provider of laser handheld instruments are for example the company LTI, Jenoptik ( Laveg ) Multanova (laser Patrol ) or Riegl.

Example

A car with a speed of 50 km / h in one second back 14 m. In order to measure an error of less than 10% within a second speed, the distance m 1 must be accurately known. This requires a time resolution between the transmitted and received pulse better than 3 ns. The light pulses have a duration of 10-20 ns.

Laser handheld instruments can be used in the current six -fold optical magnification up to a distance of about 1000 m. With a beam expansion by 0.5 °, the diameter of the beam spot measures already 0.5 m. It is significantly larger than the number plate of the targeted car, which reflects the majority of the radiation. The unique selection of a car is difficult. For this decreases the angular error of the measurement angle at a distance.

The measured angle α is the angle between the speed and the direction of measurement. The true distance is reduced by the factor cos ( α ). The correction term is equal to 1 when the car is traveling towards the meter or away from him. It follows that measurements always be limited by the above, so-called Cosinuseffekt favor the car driver, in addition to the angle would be 0, but this is rare. The greater the angle of measurement, the more deviates from the measured speed down from the actually driven.

Speed ​​measuring devices that are used in the traffic monitoring, are subject to licensing or legal for trade in Germany.