Laser rangefinder

A laser rangefinder, also known as a laser telemeter or laser distance meter, is a rangefinder that uses a laser beam to determine the distance to an object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used instead. Laser rangefinders are sometimes classified as type of handheld scannerless lidar.

Pulse

The pulse may be coded to reduce the chance that the rangefinder can be jammed. It is possible to use Doppler effect techniques to judge whether the object is moving towards or away from the rangefinder, and if so, how fast.

Precision

The precision of an instrument is correlated with the rise time, divergence, and power of its laser pulse, as well as the quality of its optics and onboard digital signal processing. Environmental factors can significantly reduce range and accuracy:

Humidity, snow, dust, or other airborne particulates will diffuse the signal. Higher temperature and higher pressure (lower elevation) slightly decrease the speed of light through air. Smaller and less reflective targets return less information. In good conditions, skilled operators using precision laser rangefinders can range a target to within a meter at distances on the order of three kilometers.

Range and range error

Despite the beam being narrow, it will eventually spread over long distances due to the divergence of the laser beam, as well as due to scintillation and beam wander effects, caused by the presence of water droplets in the air acting as lenses ranging in size from microscopic to roughly half the height of the laser beam's path above the earth. These atmospheric distortions coupled with the divergence of the laser itself and with transverse winds that serve to push the atmospheric heat bubbles laterally may combine to make it difficult to get an accurate reading of the distance of an object, say, beneath some trees or behind bushes, or even over long distances of more than 1 km in open and unobscured desert terrain. Some of the laser light might reflect off leaves or branches which are closer than the object, giving an early return and a reading which is too low. Alternatively, over distances longer than 360 m, if the target is in proximity to the earth, it may simply vanish into a mirage, caused by temperature gradients in the air in proximity to the heated surface bending the laser light. All these effects must be considered.

Calculation

The distance between point A and B is given by

D = c t 2 {\displaystyle D={\frac {ct}{2}}}

where c is the speed of light and t is the amount of time for the round-trip between A and B.

t = ϕ ω {\displaystyle t={\frac {\phi }{\omega }}}

where φ is the phase delay made by the light traveling and ω is the angular frequency of optical wave. Then substituting the values in the equation,

D = 1 2 c t = 1 2 c ϕ ω = c 4 π f ( N π + Δ ϕ ) = λ 4 ( N + Δ N ) {\displaystyle D={\frac {1}{2}}ct={\frac {1}{2}}{\frac {c\phi }{\omega }}={\frac {c}{4\pi f}}(N\pi +\Delta \phi )={\frac {\lambda }{4}}(N+\Delta N)}

In this equation, λ is the wavelength ⁠c/f⁠; Δφ is the part of the phase delay that does not fulfill π (that is, φ modulo π); N is the integer number of wave half-cycles of the round-trip and ΔN the remaining fractional part.

Technologies

Time of flight - this measures the time taken for a light pulse to travel to the target and back. With the speed of light known, and an accurate measurement of the time taken, the distance can be calculated. Many pulses are fired sequentially and the average response is most commonly used. This technique requires very accurate sub-nanosecond timing circuitry. Multiple frequency phase-shift - this measures the phase shift of multiple frequencies on reflection then solves some simultaneous equations to give a final measure. Interferometry - the most accurate and most useful technique for measuring changes in distance rather than absolute distances. Light attenuation by atmospheric absorption - The method measures the attenuation of a laser beam caused by the absorption from an atmospheric compound (H2O, CO2, CH4, O2 etc.) to calculate the distance to an object. The light atmospheric absorption attenuation method requires unmodulated incoherent light sources and low-frequency electronics that reduce the complexity of the devices. Due to this, low-cost light sources can be used for range-finding. However, the application of the method is limited to atmospheric measurements or planetary exploration.

Applications

Military

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