What is it and how does a laser gun work?
Laser (or lidar, for light detection and ranging) speed guns use a method that relies on the reflection time from narrow pulses emitted from the laser gun.
You may have experienced the reflection time of sound waves in the form of an echo. If you shouted down a well, the sound takes a noticeable amount of time to reach the bottom of the well and travel back to your ears. Sound travels at about 300 meters per second, so a deep well creates a very apparent round-trip time for the sound.
A laser speed gun measures the round-trip time for light to reach a car and reflect back. Light from a laser speed gun moves a lot faster than sound - 300,000,000 meters per nanosecond. A laser speed gun shoots a very short burst of infrared laser light and then waits for it to reflect off the vehicle.
The gun counts the number of nanoseconds it takes for the round trip, and by dividing by 2 it can calculate the distance to the car. If the gun takes 1,000 samples per second, it can compare the change in distance between the samples and calculate the speed of the moving vehicle.
The advantage of a laser speed gun is that the size of the "cone" of light that the gun emits is very small, even at a range of 300 meters. The cone at this distance might be 1 meter in diameter.
This allows the gun to target a specific vehicle. A laser speed gun is also very accurate. The disadvantage is that the officer has to aim a laser speed gun very steadily where as conventional radar can be used without aiming.
How is speed calculated?
Laser works a lot like ordinary radar, except that it sends out narrow pulses of light rather than broad radio waves. From the moment the pulse is emitted from the the receiver it times, counts and processes the returning light.
Laser depends on knowing the speed of light, approximately 0.3 meters per nanosecond. Using that constant, we can calculate how far a returning light has traveled to and from an object:
Distance = (Speed of Light x Time of Flight) / 2
1. Laser generates an optical pulse.
2. Pulse is reflected off an object and returns to the gun (system receiver).
3. High-speed counter measures the time of flight from the start pulse to the return pulse.
4. Time measurement is converted to a distance by using the formula above.
What does it look like from the operators view?
| For this example I am using the Kustom ProLaser II which is now considered obsolete in Australia and has been replaced by the ProLaser III. Standard Features and Capabilities:
As the vehicle passes I aim the target circle over the vehicle and hold the laser trigger which initiates the laser to the target. By listening to the continual tone made by the laser gun (lidar) you can determine if a surface is reflective enough to get a true travelling speed or if adjustment is needed to get a more reflective surface. After a couple of seconds a ‘beep-beep’ tone is heard to indicate a target speed has been acquired. If the trigger is pressed again, the gun will clear the last recorded speed.
The amount of search results that we receive each month after a current affair show depicts the inaccuracy of these laser measuring devices is astounding. From my experience with the use of the LTI Ultralyte LR100 these lidar units are very creditable and I believe them to be accurate measuring devices. The device operates by using a proprietary algorithm to predict where a vehicle should be and returns an error code if the measurement deviates from this prediction. However under extreme conditions where the device has been used in such a way that is to cause a erroneous reading by way of ‘panning’ or similar methods we have only ever managed to record a erroneous result of 3km/hr. |
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