NMA Article: What Everyone Should Know About Laser Guns
By James Baxter, NMA President
The use of laser speed estimation devices, sometimes referred to as LIDAR, has been accompanied by a lot of heavy breathing and claims of infallibility on the part of law enforcement agencies. “Pinpoint accuracy, undetectable,” and “tremendous range” are common refrains.
Setting the flim-flam and propaganda aside, there are some things every motorist confronted with a laser ticket should know.
The number one selling point for laser guns is their advertised capability to selectively clock the speed of one vehicle that is traveling amongst other vehicles in moderate to heavy traffic. (Radar cannot reliably be used when there are multiple vehicles in its all encompassing beam.) Its ability to determine a target’s speed in a fraction of a second is another popular feature. Probably, laser’s greatest drawback is that it cannot legitimately be used in a moving mode.
So what’s the real story on laser speed guns? They work; they work if they are properly adjusted, properly used, and used within their limitations.
A quick primer on how laser guns work:
The laser gun has a “sight” where the officer can see the target vehicle and aim the device. When the trigger is pulled a thin beam of invisible infra-red light is emitted in distinct pulses. The beam gradually increases in size and at 1000 feet it is six to eight feet in diameter. When the light beam hits a relatively perpendicular, reflective surface it bounces back toward the laser gun.
When the returning pulses of light are captured by the laser gun the electronics go to work and they have this kind of conversation:
OK, that first pulse took .0015 seconds to get out there and back, that means the target is 1000 feet away. The second pulse took .0014 seconds to get out there and back so now our target is 950 feet away. The third pulse went out and came back at .0013 seconds putting the target 900 feet from the laser gun. It took us one third second to gather these three measurements and that means the vehicle went 100 feet in one third second. Therefore the vehicle is going 49 miles per hour.
In actual practice the laser gun sends out far more than three pulses and measures the targets movements in much smaller increments, but this should give you some sense of the calculating processes the laser device employs.
From this elementary description of how a laser gun works you can begin to understand certain of the critical issues confronting a laser gun operator. First, this is a fairly complicated electronic instrument and it cannot be used to pound nails if a hammer isn’t available. Laser guns are more delicate than radar guns. And, because the aiming function is critical, the adjustment of the sighting mechanism is critical.
An honest and competent laser gun operator is going to take the following steps before he or she begins to use the device for enforcement purposes:
- Check the instruments internal testing and calibration systems.
- Check the sight alignment by picking an object at a reasonable distance (where the beam is still relatively narrow) like a utility pole, and then “sweep the gun past the object in both a vertical and then a horizontal position, with the trigger engaged, to make sure the laser gun records a distance reading at the same time the sight is on the testing target. This assures that the laser beam and the laser sight are coordinated.
- The routine external test is to shoot the laser at an object at a known distance and confirm that the laser distance reading is accurate. If it isn’t the gun needs to be repaired. However, this does not mean the gun is accurately estimating speeds.
- A simple test to check speed accuracy is to drive the patrol car at a fixed speed and then fire the laser at a fixed object, like the flat side of a building. The laser reading should be the same as the vehicle’s speedometer reading. Another option is to shoot a vehicle traveling at a known speed, such as another patrol car.
With a properly aligned sight and properly confirmed instrument operation the officer can use the laser gun with relative confidence that it will yield accurate speed readings, PROVIDED it is used properly and respecting its limitations.
Proper use means a steady rest, no sweeping or other movement of the gun when taking readings, not shooting through glass or in the midst of a rain storm or snow fall.
While it’s possible to clock a target that is 2000 to 3000 feet away the speed reading is of dubious accuracy and highly prone to error. On a clear day with no other traffic in sight a good laser operator can obtain reasonably accurate readings out to 1200, perhaps 1500 feet. However, if there are other vehicles present those distances should be halved.
Remember, at distances in excess of 700-800 feet the laser beam is easily large enough to not only be reflecting off of different parts of the target vehicle (which are simultaneously different distances from the laser gun), but also off of other vehicles, some traveling at different speeds. At distances in excess of 800 feet, the laser operator has no way of knowing what vehicle surfaces or entire vehicles are responsible for the laser speed readings, especially if other vehicles fall within the scope of the laser beam.
Most laser operators prefer to aim at the license plate because it offers a very perpendicular and highly reflective surface.
A vehicle without a front license plate and a low sloping hood, think Corvette, has to be much closer before a good laser reading can be made. However, at distances in excess of 800-900 feet the license plate is indistinguishable from the car as a whole and the laser beam is washing over the entire vehicle.
To put this in another context; think about the challenge of hitting a target the size of a license plate three football fields distant, with a handgun.
In an honest courtroom, any laser reading in excess of 800 feet would not be accepted for evidentiary purposes. The State of New Jersey has set the limit at 1000 feet, which is a step in the right direction. The rest of the country is oblivious to the limitations of this technology, with judges and legislators believing the propaganda, instead of exercising the caution and judgment we have entrusted them to exercise on our behalf.
Feel free to copy this blog and send it to a judge or legislator(s) of your choice.
© 2008 NMA