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5. FURTHER CONSIDERATIONS 
5.1 General 
Although surveyors tend to see accuracy as a predominant con 
sideration when comparing measuring equipment, for the 
practical use there are numerous other characteristics which 
may be decisive under certain project pre-conditions. This also 
applies to the suitability of 3D scanning equipment for cultural 
heritage recording projects. 
5.2 Speed 
In spite of high data acquisition rates, scanning can become a 
time-consuming process when high point densities are needed 
for high object resolution. 100 points per second can still be 
considered as a slow rate. A rate of about 1000 points per 
second is satisfactory in many cases. It should not be deducted, 
however, that this will complete the same project 10 times 
faster since the additional time needed (e.g. for transportation to 
and between different observation points, for setting up the 
scanning process, for control point measurements, etc.) cannot 
be accelerated and will thus comprise an increasing percentage 
of the whole working time. For the same reason, a boost to tens 
of thousands of measurements each second does not really 
mean a great improvement. 
5.3 Resolution and spot size 
Object resolution is theoretically a function of the size of the 
angular increments the measuring beam can be moved. Of more 
practical evidence is the size of the reflecting area (spot size), 
since this will limit the local resolution. If high resolution is 
needed (which is often the case), it should be carefully checked 
how well the beam is focussed and if an automatic focussing 
procedure is provided for varying ranges. 
5.4 Range limits and influence of interfering radiation 
Range specifications should always be doubted. Possible ranges 
depend highly on the reflectivity of the material itself, on the 
cleanness of the atmosphere and on the additional radiation 
caused by reflected sun or artificial radiation on the object or 
interfering sources near the object. In general, time-of flight in 
struments are relatively robust, whereas phase measurements 
and signal detection on the CCDs of triangulation instruments 
are more sensitive and may demand night-time measurements. 
5.5 Field of view 
Fixed scanners without motorized axes for rotation (camera 
view scanners) have a limited field of view. Typically they can 
scan an area of about 40° by 40°, which amounts to about 0.5 
steradiant (a full sphere amounts to 4n or about 12.6 steradiant). 
Scanners with one axis, like the MENSI SOISIC cover about 
45° by 320° or 4.5 steradiant, whereas instruments with two 
axes (panorama view scanners) can scan anywhere except a 
conical area of about 30° around the nadir, leaving a field of 
view of about 11.7 steradiant. Large fields of view can be of 
great significance in closed rooms where scanners can collect 
large amounts of data from a single observation point without 
any attention by personnel (e.g. over night). 
5.6 Registration devices 
If partial scans from different observation stations have to be 
combined and/or transformed to a common coordinate system 
(registration), it is desirable to have special targets in the object 
space, which can be easily detected by the scanning software. 
Some producers supply special targets (spheres, plane targets 
with high reflectivity) which are adapted to their hard- and 
software. These targets should also be suitable as targets for 
tacheometers and for photogrammetric imaging. 
5.7 Imaging cameras 
Many applications demand for object texture information in 
addition to the geometric definition of the object. If these 
textures are mapped onto the 3D model, a photo-realistic view 
can be achieved. Some scanners record intensity values for the 
returning signal. Usually, this is not sufficient to supply a 
texture information good enough for texture mapping. The same 
applies to the cameras of the triangulation scanners which are 
optimized for spot detection but not for imaging. Some users 
demand the inclusion of high quality (color) cameras into the 
scanning equipment where others would not be ready to pay for 
this extra feature. Adapters to fit a camera to the scanner might 
be a solution. In this case, the relative positions of scanner and 
camera could be calibrated which would facilitate the co- 
location of scanning results and images. 
5.8 Ease of transportation 
Ideally, the scanning equipment should be small and 
lightweight. Most mid- and long-range scanners are still 
relatively bulky. So, for instance, it is not possible to carry them 
as cabin baggage on commercial flights. Especially in cultural 
heritage documentation, where remote locations are common, 
great attention should be paid to the ruggedness of the 
equipment and to the quality of the carrying cases supplied with 
it. 
5.9 Power supply 
Scanners which can be operated by batteries are more versatile 
to use than those needing a power line supply. Portable 
generators may help in the latter case, but it can be necessary to 
run long power cables when working in interiors or caves. Also, 
generators and cables add to the equipment load to be 
transported. 
5.10 Scanning software 
The software needed on site should offer fast and simple 
interfaces for defining scan windows and resolution values. A 
possibility to observe the scanning progress and to estimate the 
remaining scanning time should be available. More 
sophisticated features such as automatic target detection for 
artificial tie and control points and dynamic adjustment of 
resolution are desirable. If large objects are recorded, it should 
be possible to accomplish at least a rough registration of point 
clouds taken from different points of observation in order to 
check the completeness of the scan.