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Title
New perspectives to save cultural heritage
Author
Altan, M. Orhan

CIPA 2003 XlX' h International Symposium, 30 September - 04 October, 2003, Antalva, Turkey
697
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