Laser scanning technology may sound complicated but sometimes it’s just about knowing the right terminology. This quick 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
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.
Detection Threshold- Within any well-designed RADAR and LIDAR receiver, not only echo signals are present, but noise is too. In order to detect a signal as an echo signal, the signals are compared against a threshold within the receiver. For echo signals with an amplitude corresponding to the threshold value, the detection probability is 50 %, half the echo signals will be detected while the other 50 % will be missed.
Diffusely Reflecting Target– This is a target that is characterised with the property to reflect light in many angles rather than at just one angle as in the case of specular reflection. In LIDAR, Lambertian reflection is often used as a model for diffuse reflection. Rough surfaces (roughness in the scale of the laser wavelength), e.g., raw masonry, are well modelled as diffusely reflecting targets.
Dynamic range- The dynamic range is a property of the receiver within the laser scanner that gives information about the range of echo signal amplitudes the receiver can work with. Distant targets with a low laser radar cross section will give echo signal amplitudes near the detection threshold. Nearby retro-reflecting targets will give huge echo signal amplitudes. The ratio of the largest echo signal the receiver can handle to the detection threshold is the dynamic range of the receiver.
Echo digitization- The process of sampling the analogue electrical echo signal and converting the analogue signal into a stream of digitized samples. Echo digitization is carried out by so-called analogue-to-digital converters (ADC). RIEGL devices making use of echo digitization derive all the information from the echo signals, such as target range, amplitude, and reflectance by digital signal processing based on the digitized echo signal.
Echo signal- Similar to acoustics where an echo indicates a reflection of sound from a distant object. The echo signal in laser scanning is the reflection of the emitted laser pulse arriving with a delay, the time of flight, at the laser scanning device. The term echo signal may address the optical signal arriving at the device, but also the electrical signal inside the receiver electronics of the device.
Full waveform analysis – This extracts a range information and additional attributes on targets from digitized echo signals. Full waveform analysis is carried out off line, on digitized echo signals from a LIDAR instrument, stored during data acquisition on a data recorder, e.g., from the RIEGL LMSQ680i. A prominent algorithm for full waveform analysis is Gaussian decomposition based on the underlying assumption of a nearly Gaussian system response. The additional attributes of Gaussian decomposition are amplitude (electrical regime) and pulse width estimate.
GLONASS – (Acronym for Globalnaya navigatsionnaya sputnikovaya sistema). Global Navigation Satellite System, a radio based satellite navigation system operated for the Russian government by the Russian Space Forces.
GNSS – (Global Navigation Satellite System). This refers to a constellation of satellites providing signals from space transmitting positioning and timing data. GNSS provides global coverage. Examples of GNSS might be the USA’s NAVSTAR Global Positioning System (GPS) and Russia’s GLONASS.
GPS – This is the USA’s NAVSTAR ‘Global Positioning System’. It is a space-based global navigation satellite system (GNSS) that provides location and time information in all weather conditions, anywhere on or near the Earth, where there is an unobstructed line of sight to at least four GPS satellites. GPS is maintained by the United States government and is freely accessible with some technical limitations. GPS receivers deliver the position data, without further transformation, in the WGS84 coordinate system.
Integrated IMU/GNSS system – (Concerns mobile and airborne laser scanning) An integrated IMU/GNSS system consists of at least an inertial measurement unit (IMU) and a GNSS receiver, and provides the trajectory of the IMU coordinate system by post-processing raw data from IMU, GNSS receiver, and a GNSS base station. By mechanically fixing the IMU to the laser scanner, the trajectory gives also the position and orientation of the laser scanner over time so that the point cloud provided by the laser scanner can be transformed into the coordinate system the trajectory is specified in (usually WGS84).
Laser footprint – The beam diameter at the target’s range.
Laser pulse – RIEGL laser scanners emit short laser pulses of pulse widths of a few nanoseconds as collimated laser radiation.
Laser radar cross section – Laser radar cross section (LRCS) is a target property. It can be conveniently used to calculate the expected echo signal amplitude when system parameters and the target’s distance are known. The LRCS is the product of three components: the actual area interacting with the laser beam (for targets smaller than the laser footprint), the reflectance of the target, and the directivity of the reflection. The directivity is quite low for diffusely reflecting targets, but very high for retro-reflecting target.
Laser rangefinder – A laser rangefinder is a device that measures distance from the device to a target.
Laser scanner maximum range – Maximum range is the maximum achievable target range up to which the laser scanner can perform range measurements. The maximum range depends strongly on the target characteristics (reflectance), but also an atmospheric visibility and sun light illuminations of the target. RIEGL specifies maximum range for different pulse repetition rates, target reflectance assuming perpendicular flat targets larger than the laser footprint, at visibilities of 10 km and 23 km.
Measurement stream – RIEGL V-Line instruments deliver the data in data streams. The so-called measurement contains all the target return detected and processed by the laser scanner, but also additional data like housekeeping data, time synchronization data, and additional messages. The measurement stream can be converted by the RiVLib into a point cloud within the scanner’s own coordinate system (SOCS) with all the additional attributes.
MTA (Multiple Time Around) – This is the situation that might occur while scanning when several laser pulses are emitted before the echo from the previous laser pulse reaches the laser scanner. It means that multiple pulses are ‘in the air’ simultaneously. It depends on range to a target and laser pulse repetition rate. Ranging becomes ambiguous, i.e. assignment of the echoes to their corresponding laser pulses is not possible without additional information. This is often referred to as multiple time around (MTA) problem.
Multi-target capability – By uniquely implementing the online waveform processing, RIEGL hardware is capable of determining the range of all targets that the single laser pulse meets on its way. It is possible to discriminate, as long as the echo signals amplitude is above the detection threshold within the instrument. Instruments providing all these target returns thus exhibit multi-target capability. Phase based laser scanning devices are, fundamentally, not able to provide more than one range reading for each measurement. Some pulsed-laser scanning devices also provide just a single return per laser pulse due to limitations in signal processing. These systems, in contrast to systems with multi-target capability, provide just 2.5 D data and not 3D data.
Multiple time around capability – When scanning at long ranges and also with the high speed it happens that in the air there will be multiple pulses in the same time. First pulse is out and before it is able to return the second one is emitted. Then the return from the first might be understood by the scanner as return from the second one not the first that is actually belongs to. RIEGL developed powerful and stable algorithms for resolving the ambiguity introduced by MTA situations (Multiple Time Around). It moves points to another zone automatically- it calculates the time offsets to assign returns correctly. You can also perform this manually- select points and move them to another zone- depends on the different conditions. These are employed by RiMTA and have to be supported by the hardware configuration of the laser scanner. Such laser scanners provide multiple time around capability.
Multi-target resolution –The correct detection of two following echoes depends on the sampling rate and the time taken for the receiver to ‘reset’ after each echo is received. Within RIEGL V-Line laser scanners, the multi-target resolution is equal 0.8 m. Target situated closer than this value might result in ‘false points’ in between.
Online waveform processing – RIEGL V-Line instruments make use of echo digitization. All subsequent processing is done in the digital regime by means of digital signal processing. The specific signal processing for laser scanning data is called online waveform processing. The result of online waveform processing is a stream of data providing precise information on target range, amplitude, reflectance and pulse shape deviation for each detected target echo. The processing is carried out within the laser scanner in contrast to off-line full waveform analysis.
P – Z
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.
Scan frequency – The number of scan lines per second.
SOCS – (Scanner’s Own Coordinate System) is the coordinate system in which the scanner delivers the raw data. The location and orientation of the SOCS with respect to the scanner’s housing is usually defined in the User Manual.
System response – The system response indicates the shape of the echo signal within the receiver of the laser scanner system after ADC (analogue to digital converter) conversion, resulting from interaction with a flat target at normal incidence.
Time of flight – (TOF) This is the method which RIEGL’s scanners use to perform range measurements. It is the time that laser pulse takes to reach the target surface and comes back to the laser scanner’s receiver divided by two. The range to the target is calculated based on the knowledge of the velocity of the pulses in air (propagation medium).
Time stamp – After data is processed ( within the scanner) the “time stamp” attribute is assigned to each point of the point cloud giving precise information on the emission time of the laser pulse, i.e., the time the range to the target has been measured. Assuming the time information of a GNSS receiver has been provided to the laser scanner (PPS signal and time info datagram), the time stamp is in the domain established by the GNSS receiver, usually UTC or GPS time.
Trajectory – By definition a trajectory is the path that a moving object follows through space as a function of time. The term trajectory describes not just position over time, but position and orientation over time. The trajectory in mobile laser scanning systems and airborne laser scanning systems describe the position and orientation of the IMU over time. The trajectory is the output of the integrated IMU/GNSS system.
UTC – Coordinated Universal Time is the primary time standard by which the world regulates clocks and time.