Dear Kyle,
I've heard that the police are now using lasers in addition to radar to catch speeders on the highway. How does a laser measure the speed of a car?
T.N. Eugene
Oregon
This question seems to provoke confusion between two very different ways to measure speed: Doppler shifts (the change in frequency that occurs when a signal hits a moving target) and timing delays (the duration required for a signal to bounce off a target and return to its place of origin). Police radar makes use of Doppler shifts. Laser speed guns rely on timing delays, as explained by James A. Worthey in the Office of Law Enforcement Standards at the National Institute of Standards and Technology in Gaithersburg, Maryland:
"The laser speed gun used by the police contains a pulsed diode laser. When the police officer squeezes the trigger, the laser emits a brief pulse of infrared light, which is focused by a lens so that it travels as a narrow beam. The pulse reflects off the moving car; a small fraction of the original pulse energy is received by a second lens and focused onto a fast, sensitive detector (such as an avalanche photodiode). Electronic timing circuits measure the pulse's round-trip time of flight.
"Multiplying the round trip time by the speed of light in air and dividing by 2 (because of the round trip) gives the distance, or 'range,' of the car. A few milliseconds later, the laser pulses for a new range measurement. The new range will be slightly less or more, depending on whether the car is moving toward the policeman or away. The laser speed gun continues to collect data in this way until dozens of range measurements have been made; the whole process takes only about half a second. The data are then analyzed by a computer in the speed gun.If the range changed steadily during the series of measurements, then the 'slope of the graph'--that is, the change in distance between each pulse--indicates the motorist's speed. If the range data did not change steadily, this is an indication of error.
"If the computer is satisfied that the data are good, the speed gun displays the speed and range (distance) of the car. Of course, the range has only an approximate meaning because the car is moving. The same laser speed guns can be used to make very accurate measurements of the distances to stationary objects.
The level of precision needed to catch a speeding automobile is quite remarkable, as Worthey details. "The speed of light in air is 299,705,663 meters per second," he notes. "If the car is 150 meters (about 500 feet) away, then the round-trip travel time for the laser pulse is about one microsecond, that is, one millionth of a second. To determine speed accurately, the speed gun must measure time with split-nanosecond (billionths of a second) accuracy."
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