3D Scanning Glossary: Explaining Laser Scanning Jargon

Last Updated on 22nd September 2021

3D laser scanning technology may sound complicated but sometimes it’s just about knowing the right terminology. If you understand the vocabulary around a subject like 3D scanning, you’ll find it easier to grasp the process and how it works.

To help you get to grips with the more technical terminology of laser scanning, we’ve put together this quick guide with the commonly used terms and their definitions.

If you think we’re missing any words from our 3D scanning glossary of terms please leave a comment below.

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3D Scanning

Using devices or systems to collect three-dimensional point cloud data of a physical object. This point cloud data then has a range of uses including as a basis to create CAD models.

A

Accuracy

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 blog.

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 the detection threshold of the instrument. 
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. The 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 the laser axis and the plane’s normal vector.

Angular resolution

This is a parameter of the 3D scanning mechanism. It corresponds to the minimum possible angular distance between two consecutive laser measurements.

B

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.

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.

BDS (BeiDou Navigation Satellite System)

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.

C

Coordinate System

Coordinate systems enable geographic datasets to use common locations for global positioning. A coordinate system is a reference system used to represent the locations of geographic features, imagery and observations.

D

Db (Decibel)

A unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level.

Detection Threshold

Within any well-designed RADAR and LIDAR receiver, not only are echo signals 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 by having 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.

Drift

Data ‘drift’ can occur if the user moves too quickly. Or if there are not enough features for the SLAM.

Dynamic range

The dynamic range is a property of the receiver within the 3D 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.

E

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).

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.

F

Full waveform analysis

This extracts a range of additional attributes on targets from digitized echo signals. Full waveform analysis is carried out offline, on digitized echo signals from a LIDAR instrument, stored during data acquisition on a data recorder. 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.

G

Galileo

This is the name of the global navigation satellite system (GNSS) that is currently being created by the European Union (EU) and the European Space Agency (ESA). It is headquartered in Prague in the Czech Republic, with two ground operations centres, Oberpfaffenhofen near Munich in Germany and Fucino in Italy.

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.

I

Integrated IMU/GNSS system

(Concerns mobile and airborne 3D 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 scan data from IMU, GNSS receiver, and a GNSS base station. 

By mechanically fixing the IMU to the laser scanner, the trajectory also gives 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).

L

Laser class

Lasers are classified by wavelength and maximum output power into four classes and a few subclasses. The classifications categorize lasers according to their ability to produce damage in exposed people, from class 1 (no hazard during normal use) to class 4 (a severe hazard for eyes and skin).

Laser footprint

The beam diameter at the target’s range.

Laser pulse

LIDAR sensors 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 targets.

Laser rangefinder

A laser rangefinder is a device that measures the 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 the atmospheric visibility and sunlight illuminations of the target.

LiDAR

LiDAR scanners use pulses of light to harvest information about buildings and landscapes. It collects a series of points that show the x,y and z coordinates of an area. These are then collated into a point cloud to create a single LiDAR dataset.
For more information about LiDAR, take a look at our articles: What is LiDAR and  What is LiDAR data?

Loop Closure

To start and end a scan in the same place. It is essential that a loop is closed when scanning with a SLAM based system so the sensor can match the features and complete the scan.

M

Measurement rate

This gives the average number of measurements per second and depends on the PRR (pulse repetition rate) of the laser and the actual scan range.

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 the 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.

Multiple-time-around capability

When scanning at long ranges with a high pulse repetition rate, there can be multiple laser pulses in the air simultaneously. In this scenario, a return pulse can be received by the scanner after the subsequent pulse or pulses are emitted, so an ambiguity is introduced. If the scanner assumes the received returns correspond to the immediately preceding emitted pulses, the range to the target feature will be underestimated. It moves points to another zone automatically, calculating the time offsets to assign returns correctly. You can also perform this manually depending on different conditions. The MTA algorithm is employed by RiMTA and has to be supported by the hardware configuration of the laser scanner.

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. Targets situated closer than the multi-target resolution might result in ‘false points’ in between.

P

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. Besides the coordinates, each point of the point cloud has valuable additional attributes, such as timestamp, amplitude, reflectance, 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. Read more on the Accuracy and Precision blog.

PRR (Pulse Repetition Rate)

The value indicates the (average) pulse repetition rate or frequency with which the laser of the rangefinder emits pulses.


Pulse shape deviation

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.

R

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. Instruments with multiple time around capabilities have no range gate.

Reflectance

A target property. Refers to the optical power that is reflected by that target at a certain wavelength. 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 range reading 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.

S

Scan mechanism

This consists of a fast rotating polygon mirror and a slower rotating optical head. The rotating mirror deflects the laser beam into different directions whereas the optical head rotates slowly to cover the full circle of 360 degrees. The optical head carries and moves the whole line scan mechanism.

Scan frequency

The number of scan lines per second.

SOCS (Scanner’s Own Coordinate System)

The coordinate system in which the scanner delivers the raw scan data. The location and orientation of the SOCS with respect to the scanner’s housing is usually defined in the User Manual.

SLAM (Simultaneous Localisation and Mapping)

The computational problem of constructing or updating a map of an unknown environment while simultaneously keeping track of an agent’s location within it. To find out more about SLAM read our article: What is SLAM?

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.

T

TOF (Time of flight)

TOF is the measurement of the time that the 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 the air (propagation medium).

Timestamp

After scan data is processed ( within the scanner) the “timestamp” 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 timestamp 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 3D 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.

U

UTC (Coordinated Universal Time)

The primary time standard by which the world regulates clocks and time.

V

Visibility

In meteorology, visibility is a measure of the distance at which an object or light source can be clearly recognised. To define visibility, a perfectly black object against a perfectly white background is viewed. At the distance equal to the visibility, the contrast ratio between black and white objects is only 2%.

Voxel

A unit on a regular grid in 3D space (a 3D pixel).

Hopefully now, the technical aspects of 3D scanning will feel a little clearer. If you’re ready to learn more about the specifics of our process you can take a look at our guide on the Slam revolution, explanations on what LiDAR data is, and how the technology is used in the construction industry and in mining.

And if you think we’ve missed any definitions from our list, you can leave a comment below.

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