How Does a Rangefinder Work?

There’s a whole load of reasons why you might want to use a rangefinder. If you’re shooting either arrows or bullets over any distance you’re going to have to adjust your aim to be able hit your target.

Why? Well gravity pulls both bullets and arrows towards the ground the moment they’re shot from something. 

The longer the distance to target, the longer the drop.

Some people judge distance by eye and through practice but with the technology on offer today the easiest and quickest way of assessing the distance to a target is by using a rangefinder.

So just how does a rangefinder work?

There’s a few different ways you can range find, but the most popular and common nowadays and the one you’ve likely come across is the laser rangefinder.

How Laser Rangefinders Work

A laser rangefinder has a pretty simple principle. It shoots a laser beam from an emitter at the target and measures the time it takes for the beam to be reflected back to a receiver on the finder.

Because the laser travels at the speed of light and the speed of light is a known speed it can be used alongside the time taken to calculate the distance to the target object.

Beam Divergence

The laser beam fired from a rangefinder is usually very narrow but due to the effects of air in the atmosphere the beam will diverge and spread out over long distances.

This means when it reaches a distant target the spread of the laser beam may well be wide enough to cover the target and be reflected back from other things as well as the target.

A laser beam diverging

Laser beams diverge and spread the further they travel

Reflection and Deflection

Some objects are harder to measure than others.

Rangefinders won’t work correctly on all objects. Here’s a few examples…

When the beam strikes a pane of glass, almost all of it passes through and isn’t reflected. So a reading is difficult to achieve.

Let’s also assume the beam strikes a mirror (or another object) that is angled so that all the light is perfectly deflected away and not back to the receiver. This object will also be difficult to range.

A soap bubble reflects light

Even a soap bubble reflects some light (if it didn’t you wouldn’t see it)

In fact any object that’s angled away from the rangefinder will deflect some of the beam away but every surface will reflect some of the available light back, otherwise we’d not be able to see them ourselves. Just how much light comes back determines how easily the rangefinder will be able to take a reading.

Why isn’t a range finder confused by ambient light?

The laser light emitted by the device has a specific wavelength which is different from the wavelength of any normal light that would come from the surroundings. Using that frequency it’s simple to filter out everything from the receiver on the rangefinder except for laser light that’s been reflected from a target. The finder sees only it’s own light. This also helps greatly when a lot of the outgoing light is reflected away by the target, even if the reflected light is a fraction of the original emitted light the finder will be able to pick it out where a human eye couldn’t.

How does a rangefinder choose a reading to display?

Laser rangefinders normally work extremely quickly and fire tens, hundreds or thousands of  pulses at the target object and use this entire sample range to determine which is the correct distance to report.

In all of those readings there will be some from the target itself, and some from other objects and terrain in-front, to the side and behind it.

A rangefinder will take all these readings into consideration, analyze them and use an algorithm to pick the most relevant distance.

Across all the readings, if one distance is more common than others it stands a good chance that this is the object that the user is trying to range. So that is what will be returned.

How Optical Rangefinders Work

Optical rangefinding has it’s benefits. You don’t need a reflective target and optics are never confused by weather, atmospheric conditions or surrounding terrain and the components make them cheap to build. In the video below from Mr Wizard you’ll see how you can accomplish some primitive rangefinding with 2 small mirrors and some wood.

However… optical rangefinding isn’t prevalent today as it once was. You’ll be hard pressed to find a good optical for sale anywhere except an antique shop because laser rangefinders are so cheap and readily available and have been extended with many features that an optical rangefinder just can’t match.

Optical rangefinders can work on the principle of coincidence or stereoscopic rangefinding.

In a coincidence rangefinder images of the target reflected from 2 different sources are shown to an operator who normally looks into the instrument with one eye and must then make adjustments to match their alignment. When the images are aligned this is called placing them into ‘coincidence’ and the amount of adjustment required to get there is used to determine the distance to the target.

Stereoscopic range finding uses both of the eyes of the operator and has them align reference markings inside the reticle to determine a distance.

This is a really great video from Mr Wizard, an 80’s TV show for children that shows the concept of split-image range finding using 2 mirrors and a measurement scale.

Here’s another video from Jimmym40a2 that shows you around a 1942 Barr and Stroud rangefinder and briefly explains how it works.

There’s also a very simple and very cheap type of rangefinder that uses something called a MilDot reticle. That’s simply a marked reticle that allows you to estimate the distance to a target if you know (or can approximate) the size of the target.

Here’s a video from Ted’s HoldOver that takes you through the principles of MilDot reticles.

Other types of Range-finding

While they aren’t applicable to your everyday rangefinding used by target shooters or hunters it’s worth mentioning these other types of range finding equipment and explaining a little about how they work.

RADAR

RADAR stands for Radio Detection And Ranging. RADAR range-finding works similarly to laser range-finding with the exception that instead of a focused laser light beam a pulse of radio signal is sent out in a spread and the time taken for it to be bounced back is measured. As radio waves travel at the speed of light, that speed and the time for them to return from the target can be used to calculate the distance from the radar station to any objects within the spread.

Because RADAR emits over a large area and has a long wavelength it’s better suited to determining the distance and speed of large objects such as aircraft and ships in open space.

RADAR isn’t affected by cloudy weather or ambient light (it works at night or in bright sun) and because the radio waves have a long wavelength it can operate over long distances.

LIDAR

LIDAR works similarly to RADAR but goes back the principle of the laser rangefinder but on a much larger scale. It sends out light pulses over a wide spread instead of radio waves or sound pulses.

LIDAR is much more expensive than RADAR but can provide detection of much small objects.

However LIDAR is affected by weather conditions such as clouds and fog and will only operate over shorter distances than RADAR.

SONAR

Sonar rangefinding uses a sound pulse and measures the time for the sound waves to travel to and back from a target alongside the speed of sound to allow calculation of the distance to a target.

Sonar is used underwater where laser light and radio waves do not travel easily.

Ultrasonic

Ultrasound is a high frequency sound-wave that can’t be heard by the human ear as it’s above the frequency we can hear at (20,000Hz). When these waves strike an object they bound back and if you know the speed of the sound wave (the speed of sound 330 m/s) you can calculate the distance to a target.

Do you have a parking sensor on your car? Chances are it’s working using ultrasonic range finding principles. Ultrasound works in the dark over short distances (something you need on a car) and is harmless to humans.

Whilst it’s great for parking sensors and other applications, ultrasound isn’t good for long range target acquisition purposes.

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