Laser Scanning Jargon

In by Harriet Brewitt

Laser scanning technology may sound complicated but sometimes it’s just about knowing the right terminology. This quick Laser Scanning Jargon guide can help you to understand more about 3D laser scanners, how they work and what you should consider before you make an investment.

A – O
Accuracy– This can be described as the degree of closeness of measured quantity to its true value. Accuracy shouldn’t be confused with precision. Read more on the Accuracy and Precision Explanation page.

Amplitude– The raw measurement of the power strength of the return echo. It is the value of the power of the light that we receive back from the target. Later on, during real-time post processing, we receive amplitude which is defined as the ratio of the actual detected optical amplitude of the echo pulse versus detection threshold of the instrument. Thus, the value of the amplitude reading is a ratio, given in the units of decibel (dB). By introducing amplitude readings in this way we can use it to improve the object classification. Amplitude depends on the distance, further away the scanner is from the target the less power it receives.

Angle of incidence– Assuming a locally flat target (approximated by a plane), the angle of incidence is the angle between laser axis and the plane’s normal vector.

Angular resolution– This is a parameter of the scan mechanism. It corresponds to the minimum possible angular distance between two consecutive laser measurements.

Beam diameter / beam width – The diameter of the laser beam perpendicular to the beam axis. Since beams typically do not have sharp edges, the diameter can be defined in many different ways. RIEGL uses the 1/e² definition, which is in common use for Gaussian beams, (Beams with a Gaussian power density distribution).

Beam divergence (angular width) – An angular measure of the increase in beam diameter with distance from the optical centre from which the laser beam emerges. Angular width is an angle described by the beam at the source.  In RIEGL instruments beam divergence is usually given in mrad for each 100m distance (for instance 0.25 mrad corresponds to 25 mm increase of beam width per 100m).

BDS – (BeiDou Navigation Satellite System), is a Chinese satellite navigation system. It consists of two separate satellite constellations – a limited test system that has been operating since 2000, and a full-scale global navigation system that is currently under construction.

P – Z
Point cloud – A point cloud, sometimes also addressed as cloud of points, is a set of points with coordinate values in a well-defined coordinate system. RIEGL laser scanners provide data at the output, which can be easily converted into a point cloud in the scanner’s own coordinate system (SOCS). Beside the coordinates, each point of the point cloud has valuable additional attributes, such as timestampamplitudereflectance, and pulse shape deviation.

Precision– (also called reproducibility or repeatability) The degree to which further measurements, under unchanged conditions, show the same results. By taking averages over a group of measurements, the precision can be improved, but not the accuracy.

PRR – (Pulse repetition rate) the value indicates the (average) pulse repetition rate or frequency with which the laser of the rangefinder emits pulses. Due to the scan mechanisms employed in RIEGL laser scanners, not all of the rangefinder’s laser pulses can be used for measurements. Thus, the average pulse repetition rate of the emitted laser pulses is significantly lower.

Pulse shape deviation – RIEGL V-Line instruments make use of echo digitization and on-line waveform processing by digital signal processing. Beside target range and amplitude, the pulse shape of the echo signal is compared to the pulse shape representing the so-called system response. The pulse shape deviation is one of the additional attributes to each point of the point cloud. Low values indicate that the echo pulse shape does not deviate significantly from the system response. High values hint to echo signals with a significantly different pulse shape, which may arise from, e.g., merging echo pulses from several targets hit by the laser beam at only slightly different ranges.

Range gate – The difference between the maximum range and the minimum range the laser scanner is capable of performing range measurements is called range gate. In RIEGL V-Line instruments the range gate depends on the measurement program. Instruments with multiple time
around capability have no range gate.

Reflectance– A target property. Refers to the optical power that is reflected by that target at a certain wavelength. RIEGL’s V-Line instruments provide a reflectance reading for each detected target as an additional attribute. The reflectance provided is a ratio of the actual, optical amplitude of that target to the amplitude of a diffuse white flat target at the same rangereading is given in decibel (dB). Negative values indicate diffusely reflecting targets, whereas positive values are usually retro-reflecting targets. Reflectance is distance independent, thus is a perfect attribute for many different classifications and further processing.

 Retro-reflective target – A target with a high directivity of the reflected laser radiation. Examples of retro-reflective targets are reflective foils, corner cube reflectors, and retro-reflective paintings.

If you have enjoyed reading our laser scanning jargon guide, or have any additions you would like to suggest please do contact us.