The rangefinder is a tool, originally optical, capable of measuring the distance between the user and any point of the visual field. Currently, the operating system is quite varied, there are optical rangefinders, laser and ultrasound.

Optical rangefinder

Used less and less and replaced by technologies more comfortable, the optical rangefinder is generally formed by a cylinder with two distinct holes at a fixed distance from one another, inside the first hole corresponds to a prism (1) that reflects image on a semi-transparent mirror (2) located inside the second hole. The user acting on the rotation of the prism will ensure that the two images of the object (3) composed by the mirror overlap. Reached the overlap, the degree of rotation of the prism will indicate, via a ladder, the distance of the object. This type of rangefinder is also called “a coincidence” and has been used for a long time inside of the cameras for adjusting the focus, and with a slightly different system, in rangefinders for artillery old generation. Rangefinder cameras, although very rare, are still used by professional photographers instead of SLR in particular situations.

Another type of rangefinder, called “stereoscopic rangefinder” or “stereotelemetro” uses the principle used in nature by many animals, including ‘man, for the evaluation of distances and is based on the phenomenon of parallax. The precision of the instrument is given by the distance of the two observation points, the greater the distance, the greater the accuracy of measurement. This type of rangefinder was used in the past for the determination of distances in the conduct of the bombardment. Some stereotelemetri embarked on the great battleships of the Second World War had openings that normally reached 12 meters on the battleships Yamato, however, there were also units (with dozens of mirrors of varying sizes) of up to 15 m; rangefinder that had an error only ± 1 m of 35 km.

Despite some truly impressive results, it was still of optical instruments, which were affected by weather conditions (fog, clouds, winds raise dust) and camouflage (especially since the ’40s, spread, for vessels, liveries “serrated , “with broken lines and bands that made it difficult to exactly coincide images which arrived from viewers) when they spread the radar (known in Italian Radio Detector Rangefinder, or Radiotelemetro) and laser range finders, optical rangefinders of all types, starting with those with collimation (in decline since the late ’30s) were massively displaced by new technologies.

Laser Rangefinder

The laser rangefinder, currently the most widely used for the low cost of construction, the discrete precision, but especially for the convenience of use has a function of its ancestor radically different: in this case the distance is detected by measuring the time that the ‘laser pulse takes to bounce off the target of measurement and come back. This technology allows measurements of acute short distances (even in the order of centimeters for distances up to 20 meters) and measurements with low visibility (can also be used in the dark), on the other hand it is based on a bright reflection suffers very bright days in of targets and non-perpendicular to the laser beam.

Ultrasonic rangefinder

For the measurement of small distances (up to a few tens of meters) are also used rangefinders that exploit the time taken by a package of acoustic pulses at high frequency (ultrasonic) emitted from the rangefinder to reach the target of which is to measure the distance and return to the rangefinder. The principle of operation is then exactly the same as the ‘depth sounder, with the difference that in this case the transmission medium is the air instead of water.

The ultrasonic rangefinder is a device extremely cheap, but the achievable accuracy is lower than that of other systems, mainly for two reasons: the first is that the propagation speed in the speed of sound in air is not constant, but depends on several factors, including the temperature and the humidity, and the second is that the ultrasonic beam emitted has an opening much greater than that of a beam of light, and this makes it difficult to steer with precision on the point of which is to measure the distance.

Under the best conditions so it is difficult to obtain greater precision of 2%.

Optical instruments

Measuring distances

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