CIPA 2003 XlX' h International Symposium, 30 September - 04 October, 2003, Antalva, Turkey 
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of the distance between scanner and object (Boehler, Marbs, 
2002). 
Ranging errors can be observed when known distances in range 
direction are measured with the scanner. If scanners are not 
equipped with a defined reference point (such as forced 
centering) it is only possible to measure range differences 
between targets. Plane, cylindrical or spherical targets may be 
used if their precise positions are surveyed with instruments and 
methods more accurate than the laser scanner. In the case of 
scanners with a field of view larger than 180 degrees, the 
determination of a systematic constant error (zero error) is 
important too, since this error will appear twice when 
measuring a distance between two targets at two opposite sides 
of the scanner (e.g. when scanning all walls from the center of a 
room). 
A very fast and easy check for the noise (accidental error) of 
range measurements can be achieved when a plane target 
perpendicular to the observation direction is scanned and the 
standard deviation of the range differences of the points from an 
intermediate plane through the point cloud is computed. As an 
additional result, this test also detects if range is only provided 
with a certain resolution (e.g. 1 cm) which is the case for some 
instruments (Kern, 2003). 
2.4 Resolution 
The term “resolution” is used in different context when the 
performance of laser scanners is discussed. From a user’s point 
of view, resolution describes the ability to detect small objects 
or object parts in the point cloud. Technically, two different 
laser scanner specifications contribute to this ability, the 
smallest possible increment of the angle between two successive 
points and the size of the laser spot itself on the object. Most 
scanners allow manual settings of the angular increment by the 
user. In relation to the size of the laser spot, some instruments 
allow increments which are so small that they really don’t make 
any sense (as the Cyrax 2500) whereas others have very coarse 
increments that do not allow to accomplish a resolution which 
would actually be possible considering the size of the spot (as 
the vertical increment of the Callidus). 
Since the combined effects of increments and spot size 
determine object resolution, a test object comprising small 
elements or small slots in front of a plane can serve to determine 
application related resolution information. 
2.5 Edge effects 
Even when well focused, the laser spot on the object will have a 
certain size. When the spot hits an object edge, only a part of it 
will be reflected there. The rest may be reflected from the 
adjacent surface, a different surface behind the edge, or not at 
all (when no further object is present within the possible range 
of the scanner). Both, ranging scanners and triangulation 
scanners produce a variety of wrong points in the vicinity of 
edges. The wrong points are usually to be found on the ray from 
the laser deflection point to the edge point, behind the edges 
(when looking from the scanner). The range error may vary 
from just a millimeter to values of several decimeters. 
Obviously, wrong points are inevitable since the laser “spot” 
cannot be focused to point size. It can be assumed that well 
focused lasers will show better results. When using a standard 
target with different types of edges, the performance of different 
types of scanners can be compared. 
A systematic effect can be observed when cylindrical and 
spherical targets are observed from a close distance (Lichti et. 
al. 2002). In this case, at the peripheral parts of the object, the 
center of the reflecting surface area is not identical with the 
center of the center of the transmitted spot. 
2.6 Influence of surface reflectivity 
Laser scanners have to rely on a signal reflected back from the 
object surface to the receiving unit in case of ranging scanners 
and to the camera in case of triangulation scanners. In either 
case, the strength of the returning signal is influenced (among 
other facts such as distance, atmospheric conditions, incidence 
angle) by the reflective abilities of the surface (albedo). White 
surfaces will yield good reflections whereas reflection ls'poor 
from black surfaces. The effects of colored surfaces depend on 
the spectral characteristics of the laser (green, red, near infra 
red). Shiny surfaces usually are not easy to record. 
It has been observed that surfaces of different reflectivity result 
in systematic errors in range. For some materials these errors 
may reach amounts several times larger than the standard 
deviation of a single range measurement. Some scanners which 
provide some type of aperture adjustment show errors in the 
first points after the laser spot has reached an area of a 
reflectivity differing considerably from the previous area, and it 
can be observed that the correct range is achieved only after a 
few points have been measured. For objects consisting of 
different materials or differently painted or coated surfaces, one 
has always to expect serious errors. These can only be avoided 
if the object is temporarily coated with a unique material which, 
of course, is not applicable in most cases. 
If the effect has to be examined and evaluated, one may use 
plane white targets and apply the material in question to the 
center part of the target. When the intermediate planes are 
computed for the colored or coated center part only and then for 
the rest of the (white) target without using the center part, the 
difference between those planes will give an indication of this 
effect. 
2.7 Environmental conditions 
Temperature. Any scanner will only function properly when 
used in a certain temperature range. Even within this range, 
deviations may be observed, however, especially in the distance 
measurement. It should be noted that the temperature inside the 
scanner may be far above the temperature of the surrounding 
atmosphere due to internal heating or heating resulting from 
external radiation (sun). 
Atmosphere. Since short distances only are measured, the 
change of the propagation speed of light due to temperature and 
pressure variations will not seriously affect the results. Many 
users report however that measurements in surroundings where 
dust or steam is present lead to effects similar to the edge effects 
described above. 
Interfering radiation. Lasers operate in a very limited 
frequency band. Therefore filters can be applied in the receiving 
unit allowing only this frequency to reach the receiver resp. the 
camera. If the radiation of the illumination source (sunlight, 
lamps) is strong as compared to the signal, enough of this 
ambient radiation will pass the filter and influence the accuracy 
or prevent any measurements at all. 
2.8 Specifications and considerations besides accuracy 
This article concentrates on accuracy considerations. Of course, 
other scanner specifications influence their applicability as well 
(Boehler, Marbs, 2002). Among these are measuring speed, 
range limits, field of view, registration devices for the combi 
nation of several scans and the transformation to a control