QUESTION #44

# I got a speeding ticket and the cop told me he used a laser gun on me or the car in front of me. How does it work? I would like to fight the ticket.

The police use a principle called the Doppler Effect to calculate how fast you are travelling.

While your car was moving the police fired a ray of light with an accurately known frequency (and therefore wavelength) at you. The frequency will change depending on how fast you were moving relative to the light source.

This is why:
Imagine that your car is moving away from the light source. When the first wave crest reaches the car it will be reflected away normally. When the second wave crest reaches the car the car will be further away so it will take longer to reach the car, and will therefore be reflected off later. This means the reflected wave crest will be further away from the first that the original wave crest. You can see that if this happens with each wave crest then the wavelength of the reflected waves will be larger than the original wavelength. (and there will be a smaller frequency as velocity(which stays constant)=frequency x wavelength).

Now imagine that your car is moving towards the light source. When the first wave crest reaches it will be reflected back normally. However when the next wave crest reaches the car the car will be nearer to the source than it was before. This means that the wave will take a shorter time get there so will be reflected back sooner. This means it will be closer to the wave crest in front of it than it was before. You can see that this would mean that the wavelength would decrease.

There is a computer near the light source and it uses information about the change in wavelength to calculate your speed. If you are too fast it will then send an alert to the police, with your license plate number (which it records) to the police who then track you down. I am afraid this is a very accurate method so there is not much chance of you being able to fight the ticket!
Answered by: Sara Al-Assam, Student, Tiffin Girls' School, Kingston UK

You stated that the police used a laser speed detector, as opposed to a radar. Radar devices do indeed use the Doppler effect to determine speed, as described in the previous answer. However, as an electrical engineer, I have worked on the design of laser-based systems of this type, and they do NOT use the Doppler effect. They instead use 'time-of-flight' measurements of short laser pulses.

The operation is as follows: A semiconductor infrared laser is used to emit a very bright light pulse (usually in the 900 to 950 nanometer range, which the human eye can't see) with a duration of less than 100 nanoseconds. The device is aimed by the operator so that the 'spot' created by the laser will appear on a reflective area of the vehicle. A separate photodetector within the device, focused and aligned so that it 'sees' the laser spot, detects the reflected pulse returning from the vehicle. The interval between the emission of the pulse and the detection of the returning pulse is timed with precise circuitry. Simply multiplying this time delay by the speed of light gives the distance the pulse travelled (twice the actual distance to the vehicle, since the pulse made a round trip). We now have reasonably accurate knowledge of the distance to the vehicle.

The second step is to repeat the measurement. If the vehicle is moving, the second measurement (generally made only a few milliseconds later at most) will result in a different distance than the first. Subtracting these two distances, and dividing by the time between measurements, gives the speed of the vehicle. In fact, the device makes numerous measurements in a short time and averages the results of the speed calculation in order to reduce the effect of electronic and optical noise, operator tremor, and other such error sources. Software in a microcontroller evaluates the consistency of the readings to assure that the result is valid.