Speeding ticket: Measuring the speed of car using lasers..How do they do that?

–Karthik Gurumurthy

How many of us have had the fortune of getting speeding tickets? I have gotten it a couple of times..Not proud of that. 

Anyway, let us look at how the laser is used to measure the speed of a car. Laser speed gun used by the cops 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, 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 ( as it is 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 is good, the speed gun displays the speed and range (distance) of the car.

The level of precision needed to catch a speeding automobile is quite remarkable. The speed of the light is 3 X 10^8 meters per second. If the car is 150 meters (about 500 feet) away, then the round trip travel time for the laser pulse is about one microsecond (about 1 millionth of a second). To determine the speed accurately, the speed gun must measure time with split-nanosecond (billionths of a second) accuracy.