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.
Asked by: Miles Polk
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.
Answered by: Alan Jones