Laser gun
A handheld laser measuring device , also known as a laser pistol , or a radar pistol is a device used by the police to monitor speed in road traffic. Its measuring method is based on repeated optical distance measurements with short time intervals.
Measuring principle
A handheld laser measuring device sends out short light pulses. The device calculates the distance from the time difference until the reflected radiation arrives. From a large number of measurements per unit of time, it determines the change in distance and from this the speed. A pulsed light source is infrared - laser .
The measuring principle of a handheld laser measuring device differs from a radar speed measuring device. The speed radar works according to the Doppler principle . It determines the speed on the basis of the beats from the superposition of emitted and reflected radiation, which are caused by the Doppler frequency shift.
The companies LTI, Jenoptik ( Laveg ), Multanova ( Laserpatrol ) or Riegl offer handheld laser measuring devices .
example
A car traveling at 50 km / h covers 14 m in one second. In order to measure the speed within one second with an error of less than 10%, the distance must be known to within 1 m. This requires a time resolution between the transmitted and received pulse better than 3 ns. The light pulses have a duration of 10 to 20 ns.
Handheld laser measuring devices can be used up to a distance of approx. 1000 m with currently six times optical magnification . With a beam expansion of 0.5 °, the diameter of the beam spot is already 0.5 m. It is significantly larger than the license plate of the targeted car, which reflects the majority of the radiation. The clear choice of a car is also becoming more difficult. On the other hand, the angular error of the measuring angle decreases at a great distance.
The measurement angle α is the angle between the direction of travel and the measurement direction. The true distance is reduced by the factor cos (α) . The correction term becomes 1 when the car approaches or moves away from the meter. It follows that measurements, due to the above so-called cosine effect, are always made in favor of the car driver, unless the angle is 0, which, however, rarely occurs. The larger the measurement angle, the more the measured speed deviates from the actual speed.
Speed measuring devices that are used in traffic monitoring are subject to approval and verification in Germany .
Web links
- Brigitte Wartner: Overview of case law on speed measurement with laser measuring devices. In: DAR , 1999, s. 473 ff.
Individual evidence
- ↑ PTB-A 18.11, PTB requirements. Traffic gauges - speed monitors. (PDF; 200 kB) (No longer available online.) Physikalisch-Technische Bundesanstalt , December 2014, archived from the original on October 20, 2015 ; Retrieved December 3, 2015 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
