How Does a Rangefinder Work?

Anthony Cote
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How Laser Rangefinders Work

Like all rangefinders, a laser rangefinder is simply a way of measuring the distance between you and the object you're looking at. Many people choose to use a laser rangefinder simply because they are small and do not clutter up your rangefinder with lense's like the contols do on many rangefinder's.

However, other than being handheld and somewhat smaller, the laser rangefinder works much like the laser rangefinder.

The difference is instead of the laser bouncing off an object and returning to the rangefinder the way it would with your scope, the laser rangefinder can simply shoot a laser beam to the object you are trying to measure the range of, and the point at which the laser hits the object is what the rangefinder reads.

Keep in mind though that because the rangefinder can't see the laser in this instance, this is why the laser rangefinder cannot be used in some rangefinder modes such as pinseeker.

However, in most cases, it is an easy switch from the laser rangefinder on the scope to the laser rangefinder on the laser rangefinder, so it is not too big of a hassle.

Beam Divergence

The purpose of a laser rangefinder is to measure distances. The technology they use to achieve this is very complicated and detailed, but the operation is easy to understand. Here’s a simple breakdown of how it works.

■Transmitted Laser Beam

A laser is a collimated (parallel and highly phased) beam of light. It is created by stimulating an atom, in particular a molecule of Titanium Dioxide (TiO2), which then releases a photon, which is the smallest particle of light and travels in straight line. Depending on the wavelength, the photon can be visible or invisible, but can pass through most transparent materials.

The laser beam is focused by a glass lens – which will also include anti-reflection coating – and it’s amplified by a device known as a gain medium. This is a very elaborate way of saying that the laser beam will be made stronger. There are a variety of gain media, including Nd:YAG (neodymium-doped yttrium aluminium garnet) and frequency-doubling crystal, but inside the rangefinder all you have to know is that the laser beam is more powerful.

■Laser Reflector

The laser beam is shot out of the rangefinder and it hits the reflective target.

Reflection and Deflection

Rangefinders work using reflection or deflection. Both will measure the distance, but with a slight difference in what gives you the distance. The former has you looking into the eyepieces and see a reflection of the target, while the latter has you looking through a lens and see a deflection of the target.

All this difference is really a problem for most hunters. Consider the differences for duck hunting, where you are looking at birds in the sky, and when you are looking at an immense sports stadium for which you are measuring the distance to the end zone. A deflection unit would be most useful for the stadium measurement, while a reflection unit would be more useful for the duck hunting.

Reflection units are best for hunting purposes, though, because you can get a more accurate reading. You can see that the unit has to be in the line of sight for the reflection to happen, so it has greater precision, and you can see it more clearly. Deflection units give you a wider area upon which you look, but they can also be of lower quality in the way they give you the measurements.

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

Rangefinders seem to work well in any kind of light, which might surprise you because most cameras struggle when you send them in to areas with strong contrast. How do they keep that high contrast light from messing with them? The answer is it’s all to do with how it measures the distance.

Rangefinders have a powerful laser built into them. When you press the button, it fires off a beam of light from the laser and measures the time that it takes the light to disappear. If you hold it up in a dark room, this laser light gets sent out and then bounces right back again. Because it’s the same distance, there’s no time difference in the reflection.

However, what happens if you were to take it outside on a sunny day? You’d expect this laser to work the same as it does in the inside, but because the light from the sun is stronger, its reflection may reach your eyes before it makes its way back to the laser. This is an example of the time difference that is produced by a big difference in light levels. That’s why we don’t see rangefinders work properly when we are outside.

How does a rangefinder choose a reading to display?

A laser rangefinder works by measuring the time it takes for the laser beam to hit a distant object and return to the rangefinder. A high-quality device will be able to measure distances in the 0.1-yard to 1,000-yard range with a 0.1-yard accuracy, and they can be used to measure objects up to 32 yards high.

If you're hunting at a range of about 10 yards away, the beam will go out and hit the target and return in about a quarter of a second. The rangefinder receives the beam back and determines the distance to the object by how long it took the beam to make the trip. If the beam took nine-tenths of a second to make the trip, the device displays it as 33 yards away.

If you're using the beam to estimate the speed of an oncoming vehicle, it can come in handy. The speed limit may say 55 miles per hour, but you know from experience that the road has a slight curve in a certain spot, so you know that people are driving faster than the speed limit. This is when you pull out your rangefinder so you can travel the road ahead, and it will tell you how far away all of the approaching vehicles are.

If you have a high-end model, it may come with a digital angle compensation feature. This helps the distance you read to be more accurate if you're at an angle to your target.

How Optical Rangefinders Work

Optical rangefinders are exactly as they sound. They use the simple principal of using a small viewfinder combined with magnification that allows you to assess the range of objects. While the optical range finder may be considered to be the most basic and simple form of rangefinder, it is also the most accurate.

The best optical rangefinders use a small lens that is mounted above a small mirror. The target to be assessed is observed through the viewfinder. The person aiming the range finder holds a small button halfway down that locks in the magnification. Once the target is zoomed in on, a small duplex or reticule shows the rangefinder the exact distance to the target. Some of the more sophisticated rangefinders allow the rangefinder to lag behind the zoom. This allows the rangefinder to decipher the distance even when the reticule changes positions.

In order to ensure that the optical rangefinder is able to maintain the same distance, the rangefinder is mounted on a tripod. The tripod prevents the rangefinder from moving around and ensures that the target stays the same distance, no matter which location the rangefinder moves to. This also allows the person aiming the rangefinder to focus on the target without worrying about the distance or the magnification of the target.

Other types of Range-finding

There are a few technologies used in determining range:

  • “Staring” – Maksutov telescopes, which suffer from significant image distortion.
  • “Staring with a moving aperture” (e.g. a spinning wheel) – best-in-class accuracy but with a relatively large device.
  • “Staring with a moving sensor” – Devices similar to optical mouse devices, which have high accuracy and are small.
  • “Moving sensor with a staring aperture” – devices similar to mirror-based laser rangefinders. These suffer from significant image distortions and relatively high locations uncertainty.
  • “Dithering” – devices similar to a mouse with a significant location uncertainty
  • “Moving sensor with a moving aperture” – devices similar to radar, with high accuracy and precision, but typically large devices.

RADAR

RADAR is an acronym for radio detection and ranging. In its basic form, a radar system works due to issuing radio waves into a space. When these waves interact with an object, the signal will bounce back. This reflected signal will then be caught by the radar that can tell the distance and size of the object.

Rangefinders on the other hand, uses only one type of signal, which is light. They operate simply by sending out light waves. These light waves will bounce when it comes in contact with objects, and will then reflect back. Just like in the case of radar, this will then be caught by the rangefinder, and the amount of time it takes for the signal to come forward will tell the rangefinder what the range is.

LIDAR

Surrounds you with invisible laser that is invisible to the human eye, and scans the area around you.

SONAR

Rangefinder hows you were far away from the target object. There are 3 main types of range finders: echo ranging, time of flight, and pulsed laser/radar.

Echo ranging: A continuous wave of (usually) the same frequency as the object's sound wave, is emitted by the device. The emitted signal will bounce back off the object. The device measures the time taken for this echo to return to the device. Based on the travel time, the distance to the object can be calculated. The distance formula to calculate the distance is:

Where:

Frequency: = the frequency at which the device is sending the waves, or the speed of sound.

Wavelength : = the distance the emitted signal travels between the emitter and the receiver

Speed of sound: = the speed of the signal travelling from the emitter to the receiver, multiplied by the length of the path between the two.

Time: = the time taken for the echo to arrive back at the receiver.

Example (based on 560 mph air speed):

In a typical scenario, the time taken between the 'go' signal from the device and the echo is used as input in the distance formula to calculate the distance travelled by the signal, and hence the distance to the object.

Ultrasonic

Rangefinders use an ultrasonic technology to produce a real-time 3D map of objects, people, and terrain. You can purchase models that use either laser or ultrasonic technology.

Laser rangefinders use a light-emitting diode (LED) to project a focused beam of light. The distance is determined by measuring the time in nanoseconds it takes for the light to reflect off an object, to a light sensor. More expensive rangefinders have a larger and more powerful LED array that increases the range of detection. The size of the area you can scan, also depends on the distance you are from the target.

Ultrasonic rangefinders produce a sound wave that reverberates off of objects and returns to a sensor. It is fueled by a lithium-ion rechargeable battery. As with laser rangefinders, the size of the area you can scan, depends on the distance you are from the target.