Finding the Distance: How do Rangefinders Work?

Laser rangefinders are devices that are used to measure the distance of an object from the user. Along with radar and sonar, laser rangefinders are a type of active rangefinder. They are used by hunters, sportsmen, surveyors, soldiers, and many others.

Laser rangefinders produce a beam of photons directed toward an object. The beam is reflected off the object back towards its source, and the rangefinder measures the time it took for the beam to make its journey. Based on the elapsed time and the speed of the beam, the device calculates the distance to the object. This method of distance calculation is based on the “time of flight” principle. In a similar fashion, radar rangefinders use radio waves in place of laser beams, and sonar devices generate sound waves for the same purpose.

Because a laser beam travels very quickly, the amount of time that is required for the beam to return to its source is very small. This makes it extremely difficult to measure, and so laser rangefinders are somewhat limited in their precision of measurement. Still, the least accurate of these devices can accurately calculate distances within about a meter, while the most sophisticated can be trusted to be accurate within a few millimeters.

While the speed of the light beam might be seen as a detriment, it also offers a distinct advantage. A slow-traveling signal such as that provided by radar and sonar is not very good at measuring moving objects, because by the time the signal has returned to the observer, the object will have moved. This problem is compounded by longer distances; and when the direction of the target’s movement is straight toward (or away from) the rangefinder. Since light travels many times faster than radio waves or sound waves, the laser rangefinder is the most responsive active rangefinder for supplying real-time data on a fast-moving object.

The effects of beam scatter, along with atmospheric distortion and interference, can cause variations in the length of time required for even sequential pulses to return to the sending unit. In order to minimize the effects of these errant signals, most laser rangefinders use the average travel time of a series of pulses to improve their accuracy.

The use of sequential laser pulses also lends itself to the coding of the pulses as a way to minimize attempts to jam the device. The efficiency of the pulse is also a critical factor in the rangefinder’s accuracy: more sophisticated systems respond very rapidly to fast input signals, due to a comparatively low pulse rise or fall rate.

An older method of calculating distance, known as triangulation, can be used to simultaneously improve the accuracy of the device and maximize its ability to measure the speed of the target. In order to apply the principles of trigonometry to the process, the rangefinder must establish 3 points of reference instead of two. The first 2 points are established by the device, and can be either dual lasers or dual receivers. Adding the target as a third point, a triangle can be formed. An imaginary line segment, running from the target to the center of the rangefinder, divides the triangle into 2 right triangles, which can be used to mathematically determine the exact distance to the object. When there is a change in the length of time required for each signal to return, the device can calculate the direction of the target’s travel, as well as its speed in relation to the rangefinder.

When a laser beam travels, it is unaffected by the gravity of the earth and its corresponding effect on the inertia of the beam. This makes the measurement impervious to changes in elevation between the target and the device. However, this is not the way that bullets and arrows travel, so rangefinders designed for hunters must also take into account the effects of gravity on the trajectory of the projectile.

When a hunter is positioned above his target, the trajectory of his bullet will be quite different than when he is below his target, and a good hunting rangefinder will compensate for this difference by incorporating an inclinometer and ballistic calculator. The inclinometer provides the ballistic calculator with the required upward or downward angle of the target, and the calculator will then estimate the projectile’s trajectory based on the weapon being used. Some of the best laser rangefinders also incorporate temperature, elevation and barometric pressure into their ballistics calculations.

Because the light from a laser is of a different wavelength than that from ambient light sources, the likelihood of a rangefinder mistakenly identifying natural light instead of the laser light is relatively small. The receiving photodiode is typically filtered to accept only the pulses generated by the device itself.

A common misconception about laser rangefinders is that they will not work as well when the target is at an angle to the light source, but in fact, this is usually not the case. Most of the surfaces present in nature diffuse light very effectively, and provide multitudes of random reflections as a result of the diffusion. The full moon is a perfect example of this, as it appears uniformly bright from the center to the outer edge, more like a flat disk than a sphere. This occurs despite the fact that the light reflected from the outer edge is at a much greater angle to the light source than the center. Smooth, highly reflective surfaces do not diffuse as much light, but these are very rare in most cases when a laser rangefinder would be used. Additionally, the sensitivity of the device is such that very little light is actually needed to tickle the receiver.

The most powerful versions of portable laser rangefinders, such as those in use by the military, are capable of detecting objects at a range of about 10 miles, but most commercially available units are good for between 500 and 1500 yards, which is probably farther than most users would ever need.

Most laser rangefinders are equipped with optical lenses to allow the user to target more effectively, particularly over long distances. These optical subsystems are very similar to a pair of binoculars, and may provide varying degrees of magnification. Some golf rangefinders employ a system that is designed to detect only the nearest object, ignoring anything in the background. By doing this, it is much easier for the golfer to range the flag.

Laser rangefinders typically display their information through an LCD or OLCD screen, much like digital cameras and cell phones. The information displayed may include the target’s distance, speed, degree of inclination, and more. If the rangefinder is equipped with a compass, azimuth information will also be displayed. Rangefinders designed for a specific purpose, such as hunting or golfing, will display any information that is specific to the activity.

Commercially available laser rangefinders sold in the United States are subject to the approval of the Food and Drug Administration (FDA). Because of the potential for eye injury, the lasers used must be considered “eye-safe,” meaning that the beam has to be invisible to the naked human eye, and it must be incapable of causing eye damage. Laser products meeting these requirements are assigned an FDA “Class 1” designation.