-Karthik Gurumurthy
I’ve always been fascinated by how police laser speed guns work. The technology behind them is both elegant and incredibly precise.
The laser speed gun used by police contains a pulsed diode laser. When an officer squeezes the trigger, the device emits a brief pulse of infrared light, which gets focused by a lens into a narrow beam. This beam travels outward, hits the target vehicle, and a small fraction of the original energy reflects back to the gun, where a second lens focuses it onto a sensitive detector.
The key to measuring speed lies in precise timing. Electronic circuits in the gun measure exactly how long it takes for the light pulse to make its round-trip journey. For a car about 150 meters (roughly 500 feet) away, this round-trip travel time is approximately one microsecond – just one-millionth of a second.
To determine distance, the gun multiplies this round-trip time by the speed of light in air (299,705,663 meters per second) and divides by two (because we’re measuring the round trip). But to measure speed accurately, the device needs split-nanosecond precision – that’s billionths of a second!
What I find clever about the system is how it calculates speed. The gun doesn’t just take one measurement – it takes dozens in rapid succession, all within about half a second. Each pulse gives a slightly different range measurement depending on whether the car is moving toward or away from the officer. By analyzing how the distance changes between pulses – essentially calculating the “slope of the graph” – the onboard computer determines the vehicle’s speed.
If the range data changes in a steady, consistent pattern, the gun displays the calculated speed. But if the measurements are erratic, the system recognizes this as an error and doesn’t give a reading.
The level of precision required for this technology is truly remarkable, especially considering how quickly and reliably it works in everyday law enforcement.
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