Similar to radar, laser speed measurement works with reflection. The most important differences are explained below using a police laser gun utilized by officers by the side of the street as an example. As you draw closer to the officers, they see your car
and decide to do a measurement of your current speed. The first thing to know starts right here - Just because there are some officers with a speed gun near the road you're driving on, that does not mean they will definitely target you since they can only target one car
at a time. So, for example on roads with multiple lanes, other might cars might seem more promising than your's and even tough you might have been too fast, you won't get a ticket at all.
Given that the officer decides to target your car with his speed gun, he is now pointing the gun at your car - mostly aiming for your license plate since it is the best reflecting part at the front of your car - and pulls the trigger. Only form this very moment the laser gun starts sending light waves (invisible for the human eye). These light waves arrive at your car, get reflected back to the gun and are then absorbed by the sensors inside it. All that happens within a few milliseconds.
For easier explaining, let's assume the police laser is sending out a total of 5 light wave pulses to perform one measurement. As we already know, this happens in the blink of an eye.
As the trigger is pulled, you are 200 meters away, meaning that the first light wave will take the time x to reach your car, get reflected and travel back to the laser gun. Right after that the next light wave is sent, at this point of time you are only 190 meters away
so the light wave won't take time x to do it's journey but a little less than time x because the distance it has to travel is now 10 meters less than before. The same process is repeated with the third, fourth and fifth light wave. Knowing the average difference in running time
between the 5 light waves and also knowing the speed of light, you (the police laser) can now calculate your speed.
The method explained above is called LIDAR, standing for "Light Distance And Ranging". A police laser uses light waves within the infrared spectrum, mostly at a wave length of 904 nm. The light is extremely weak scattered, the scattering angle is (depending on
the model of speed gun) at 2-5 milliradiant. The most precise model currently on the market (and therefore the most difficult to beat with laser jammers) is the "TruSpeed" manufactured by LTI. Below you can find a listing of its specs:
- Scattering angle: 2.5 milliradiant
- Wave length: 905 nm
- Measurement duration: 0.33 seconds
- Max. speed measurable: 320 km/h
- Accuracy: ± 2 km/h
- Min. distance to target vehicle: 15m / 60m with bad weather
- Max. distance to target vehicle: 1220m
- Operating temperature: -30 to +60 degrees celsius
Now, why are these specs, especially those of the TruSpeed, important to us? First, they can be used to make an objection against your ticket, for example if the distance or temperature ranges were not respected. Second, these numbers are important to know how you can protect your self effectively. For example, the extremely weak scattering radius is the reason why any mobile radar detector mounted on your dashboard or windshield can never work as a laser jammer - even tough almost every manufacturer claims that for his devices. Knowing the numbers it's easy to explain why they are wrong: even at a distance of 200 meters the diameter of the laser beam reaching out to your car is no more than 60 cm and since the center of the beam is targeting your license and only hitting the area near it.
Since the signal of a police laser gun is only transmitted at the very beginning of a measurement, you have no chance to get an early warning and reduce your speed. Theoretically you could, but the moment you get the warning, the speed gun is already displaying your speed.
The measurement usually takes 0.3-0.6 seconds. For this reason your laser jammers start sending their own light waves just milliseconds after the first pulse from the speed gun reaches your car and with that also the sensor inside your laser jammer. The timing and pattern
of the jammers signals decides of the jamming succeeds or fails. Mainly there are 2 methods.
Here, your laser jammer simply sends as many pulses as he can, hoping to make it impossible for the police laser to "find" or recognize it's own pulses in the mess of light waves reaching it's sensor. The brute force method is outdated and not uses anymore today because most police lasers can detect it and give an error message letting the officer know you just jammed it. This video explains the brute force method in detail.
The look-up table method uses is a database of all known police lasers to determine the used model of speed gun by measuring the time between the first few pulses. After that, the laser jammer starts so send it's own pulses at times that will definitely not seem legit
to the speed gun resulting in a non-plausible output that could just as well happen if the officer was not aiming right. To the processing unit of the police laser it seems like you are 200 meters away at the first pulse, 150 meters at the second pulse, 180 meters at
the third pulse, 170 meters at the fourth pulse and then again 190 meters at the second pulse. This can never be true since you are obviously always getting closer and you can't move back and forth in just a fraction of a second. This video explains the look-up table
method in detail.
Das Look-Up Table Verfahren wird im nebenstehenden Video sehr anschaulich erklärt.
Given that you are using a modern look-up table laser jammer everything seems normal to the officer. The only difference is that even after multiple seconds of pulling the trigger there is no result displayed. The same exact scenario as if the officer was accidentally
aiming at the traffic sign next to you. Anyway after multiple failed approaches it will seem suspicious which is why most laser jammers stop operating after 4-5 seconds and let the police laser do it's job - but by that time, you already reduced your speed.
Opposite scenario if you use old models with the brute force method - almost every modern police laser recognizes brute force laser jammers instantly and displays a code informing the officer.
If you use modern laser jammers and everything works fine, what the officer will see looks like this:
Important to understand: a police laser can be disturbed, but not detected. (Well, sure you can detect them but then it's already too late.)