Laser Technology in Traffic Enforcement: Better Late Than Lased
| Publication year | 1992 |
| Pages | 713 |
| Citation | Vol. 21 No. 4 Pg. 713 |
1992, April, Pg. 713. Laser Technology in Traffic Enforcement: Better Late than Lased
November 20, 1991, was the first time that a Colorado motorist received a speeding citation based on a laser speed detection device. The hand-held device weighs 4.5 pounds with dimensions of 3.5" X 5" X 8". It has a speed measurement time of 1/3 second. A report by the Insurance Institute For Highway Safety reveals that speeders stopped in response to laser were four times as likely to have radar detectors as motorists apprehended by officers using conventional radar.(fn1)
This article discusses how laser devices work, why radar detectors are ineffective with laser devices and why the defense in the case of a driver targeted by such devices may require expert testimony.
The word "laser" is actually an acronym for "light amplification by stimulated emission of radiation." A lasing "medium," situated between two mirrors, forms an optical resonator which, in turn, generates laser light. The light bounces back and forth to form a standing wave.
However, merely having the light perform electronic ping-pong is not particularly helpful. To be useful, some of the light must escape. Thus, one of the mirrors is designed to allow a portion of light to hit the targeted vehicle. The type of laser used by the Aurora Police Department in issuing the first speeding citation is an infrared semiconductor laser diode with the following properties:
1. The laser diode emits a narrow cone of radiation from a small area (.17 degrees versus 18 degrees for radar). This allows the light to be transmitted in the narrow beam that gives the laser its pinpoint targeting ability (for example, 3 feet wide at 1,000 feet distance). No diffraction or re-radiation of beam occurs.
2. The laser diode switches on and off quickly, typically in less than one billionth of a second. This gives the laser its superior accuracy.
3. Like all lasers, the laser diode emits only a narrow band of frequencies. No commercially available laser beam detector or jammer has been developed.(fn2) The detector industry must deal with a number of factors, such as the laser's narrow beam, the absence of the beam extending beyond the target vehicle, the short acquisition time and miniaturization of the electronic components.
4. The laser diode emits in the infrared position of the electromagnetic spectrum, so it is invisible to the human eye and cannot be seen by operators of vehicles.
Laser speed detectors determine speed by measuring the time of flight of short pulses of infrared light. Because the speed of light is a constant (approximately 186,000 miles per second), the time it takes the light pulse to reach and return from the target is directly proportional to the distance to the target. By firing two pulses a known time apart, two distances can be calculated. The change in distance, divided by...
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