Most of us have been there: Sitting, quietly fuming, in the driver’s seat of our car. Red and blue lights flash from behind, while a police officer writes a ticket in his cruiser. How did he get me?, you ask. Could he have even gotten my speed from that far away?, you wonder. Or, maybe you’ve gotten lucky this time, and instead cruised by the officer as he continues to look down the barrel of his laser gun at traffic behind you.
Most people do not understand how police laser guns work and how police officers actually target cars for speed detection.
Police laser, formally known as LIDAR stands for LIght Detection And Ranging. Among the LJ community, ‘laser’ is the more frequently used term, but LIDAR is equally acceptable. Police laser guns may seem complicated and daunting, but with a little elementary understanding of math and an example or two, anyone can understand how it works and how to defend against it.
Laser Gun Basics
Let’s first take a look at a laser gun so that we can understand what the different parts are and how they work.
Here is a picture of the front of the police laser UltraLyte LR-B. At the very left are the two laser lenses; this is the end that the officer will point at his target (the car or truck on which he wants a speed reading). One lens is for emitting the light and the other is for receiving the light. At top is the viewfinder, through which the officer will select a target. Below, just like a normal gun, is the trigger. At right is a stock, which the officer can put against his shoulder to steady the gun. There are many different types of laser guns, but most have this ‘gun-like’ setup. The shoulder stock is optional.
Next, you’ll see a picture of the display of the Kustom Signals ProLaser III. Again, all laser guns have somewhat different looking features, but most adhere to the same simple design. Here is a readout where the office can see distance and speed reading for their target. There are also option buttons which allow the officer to configure different settings.
Now that you have an idea of what a police laser gun looks like, you can a good idea of what it’s like to use one. Overall, they are designed to be pretty simple. First, using your eyes only, find a target (usually a fast moving vehicle!). Next, put the viewfinder up to your eye and target the vehicle. When looking through the viewfinder, the laser gun will project a crosshairs or a dot over the center of the scope. Put the crosshairs on top of your target and pull the trigger. If the gun is working normally, you’ll have a speed reading in about a second. The unit will normally give off a beep or chirp and the speed reading will pop up either in the viewfinder, on the display, or both. The display or viewfinder can also provide the range (distance) at which the speed as acquired.
Check out the YouTube video below to see one in action!
Pretty fast, right?
So just how does it attain your speed using light? Essentially, it just measures distance! If you’ve seen someone use a laser rangefinder on the golf course, then you’ve seen the same principles in action.
Principles of LIDAR
When the laser gun’s trigger is pressed, the gun very quickly sends out a beam of infrared light. Infrared light is outside of the visible spectrum of light, so the light that the gun emits is invisible to the human eye, even at night. This infrared beam bounces off of its intended target, a car in this case, and is reflected back at the gun. During this process, the gun’s internal computer is timing how long the beam takes to return. Once it does return, the internal computer calculates the vehicle’s speed using the speed of light.
Overall, the electronics of the police laser gun are very sophisticated. This process of emitting, receiving, and timing the light happens more than a hundred times per second. That’s right - per second. The rate at which this happens is called the pulse rate, and many guns send out light pulses at a rate of 100 pulses per second to 200 pulses per second (pps). Some guns have rates of nearly 400pps!
So what?, you say. How does knowing how far away the car is tell us how fast it is going?
At this point, the gun is measuring the distance of the car at least one hundred times every second. Since the car is moving (let’s say it’s moving toward the officer for this example) towards the officer at a rate of speed, the distance between the car and the officeris getting smaller and smaller as the car barrels toward the officer. The difference in distance between each measurement is miniscule, since the measurement is happening so frequently. However, it is enough to matter.
Distance, Rate, Time
Let’s go back to elementary school math for a minute. We all learned that Distance = Rate * Time. If you’re not great at math, don’t let your eyes glaze over just yet!
We use these forms of measurement all the time – every time we say miles per hour. If you’re traveling 100 miles per hour for an hour, you’ve gone 100 miles. Distance (100 miles) = rate (100mph) * time (1 hr). Using this principle, the gun measures the change in distance of the car, and from that, it gets the speed!
And all of this happens in less than a second! Most guns only need a few distance readings to confirm a speed so, theoretically, even the gun with the slowest pulse rate of 100pps can get the speed of your car in a fraction of a second!
In case you missed it (TL;DR):
- Gun sends out a light pulse and see how long it takes to return
- Gun uses the time the light took to return, and the speed of light, to calculate the distance of the car from the gun.
- Gun does this many times per second and tracks the rate of change of the distance
- Gun then has Distance and Time, so it can solve for Rate (Speed)
Ready to move on? Let’s find out next in How Laser Jammers Work
See Also: What are Laser Jammers?