ANALYSIS OF THE BACKSCATTERED ENERGY 
IN TERRESTRIAL LASER SCANNING DATA 
Norbert Pfeifer a ’ *, Bernhard Hòfle a ’ b , Christian Briese \ Martin Rutzinger c , Alexander Haring a ’ b 
a Institute of Photogrammetry and Remote Sensing, Vienna University of Technology, 1040 Wien, Austria 
b Christian Doppler Laboratory “Spatial Data from Laser Scanning and Remote Sensing” 
c alpS-Centre for Natural Hazard Management, Innsbruck and Institute of Geography, Innsbruck University, Austria - 
(np, bh, cb, ah)@ipf.tuwien.ac.at, martin.rutzinger@uibk.ac.at 
KEY WORDS: Terrestrial Laser Scanning, Calibration, Radiometry, Intensity 
ABSTRACT: 
Terrestrial laser scanning provides a point cloud, but usually also the “intensity” values are available. These values are mainly 
influenced by the distance from sensor to object and by the object’s reflection properties. We demonstrate that it is possible to 
retrieve these reflection properties from the observed range and the intensity value. An experiment with targets of known reflectivity 
behaviour is described. Retrieving object reflectivity is also demonstrated for these targets in another experiment, which was not 
used to determine the functional relationship between range, reflectivity, and intensity. The Lidar equation describes the received 
optical power in terms of the emitted power, range, and target properties. Nonetheless, the intensity values do not follow this 
prescribed behaviour. Therefore, data driven approaches are used, allowing a better prediction of the observed intensity from the 
range and reflectivity of the targets. For a Riegl LMS-Z420i and an Optech ILRIS 3D these experiments were performed. Both 
scanners measure range by the travel time of a pulse. In our experiments, the reflectivity can be estimated from the laser scanning 
data with a standard deviation of 6% or better. This demonstrates the potential for retrieving material properties of natural surfaces, 
too. 
1. INTRODUCTION 
Obtaining geometrical information from terrestrial laser 
scanning (TLS) is an established surveying procedure (Grün 
and Kahmen, 2007, Fritsch, 2007) and used e.g. in cultural 
heritage recording and industrial plant reconstruction. The 
acquired point clouds, i.e. sets of xyz coordinates, are used to 
determine object surfaces by triangulation, surface fitting, or 
primitive instancing. Airborne laser scanning (ALS) is similar 
with respect to the data provided: the point cloud. Calibration 
of the ranging and scanning devices is an issue in the terrestrial 
(Lichti 2007, Nothegger et al., 2007, Reshetyuk, 2006) and the 
airborne case (Kager, 2006 and references therein). This allows 
obtaining high precision, well beyond 1:10000. Laser ranging 
uses energy emitted from the sensor for determining the range 
between sensor and object. It is retrieved by measuring the two- 
way travel time of the signal bounced back at the object. 
Beyond the run-time it is possible to measure the strength of the 
backscattered signal as well. Object properties like specular and 
diffuse reflection behaviour, absorption, and transmission 
influence the strength of this backscatter. The so-called 
“intensity” value is related to the power (amplitude) or energy 
of the returned signal. 1 With calibration it becomes possible to 
convert these intensity values into parameters related to the 
object surface. In ALS methods for radiometric calibration have 
been proposed (Briese et al. 2008, Höfle et al., 2007 and 
references therein). Independent thereof, these intensity values 
have been used in TLS applications, e.g. for orientation (Akca, 
2007), manual inspection of trees (Aschoff et al., 2004), and 
rock face investigation (Rosser et al., 2007). 
1 We use the term “intensity” in this paper, but there is not 
necessarily a unique physical interpretation for these 
intensity values by the different scanner producers. 
In this paper we want to show that a radiometric calibration is 
possible for terrestrial laser scanners as well. It builds on and 
enhances previous work of our group (Pfeifer et al., 2007). The 
next section gives motivations for doing this research. This is 
followed by a section on the theoretical basis, discussing also 
issues of not strictly monostatic laser rangers. Thereafter we 
present our experiments, where a Riegl LMS-Z420i and an 
Optech ILRIS 3D were used 2 . The calibration results and the 
discussion follow in the subsequent chapters. 
2. MOTIVATION 
The overall aim is to extract more information than “only” the 
xyz point cloud from TLS. This becomes possible if influences 
on the loss of emitted energy in comparison to the detected 
energy can be grouped into those depending on the object and 
other influences, e.g. the distance from sensor to object. Not 
only absorption and reflection properties, i.e. the BRDF, but 
also the incidence angle of the measurement are counted in the 
following to the object properties. 
In many monitoring circumstances the objects observed are 
known. One example is TLS for snow monitoring (Prokop, 
2006) for research on and assessment of avalanche risk. In the 
work of Prokop it is also observed that under certain 
meteorological circumstances no range measurements are 
possible, actually referring to an energy level too low to be 
detected. Kaasalainen and Kukko (2007) advance this approach 
with a more physical approach. In (Rees, 2006) it is described 
how the grain size of snow and the snow temperature affect the 
backscatter strength. From the intensity values it should 
therefore, at least theoretically, be possible to reconstruct snow 
2 Much of this paper applies to phase-shift systems as well. 
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