ves of the Photogrammetry, Remote 
    
International Archi 
is somewhat true as long as proper focusing and image 
processing techniques are used wisely (Soucy et al., 1990). 
Nonetheless, one should remember that the underlying 
hypothesis of active optical geometric measurements is that the 
imaged surface is opaque and diffusely reflecting. Hence, not 
all materials can be measured accurately like vapour-blasted 
aluminium (VBAI). Problems arise when trying to measure 
glass, plastics, machined metals, or marble (see Figure 6). As 
reported by Godin et al. 2001, marble departs from this 
hypothesis, and exhibits two important optical properties in this 
context: translucency, and non-homogeneity at the scale of the 
measurement process. 
  
Figure 6. Laser spot projected on a marble surface. 
This structure generates two key effects on the geometric 
measurement: a bias in the distance measurement, as well as an 
increase in noise level, when compared to measuring a 
reference opaque surface like VBAI. They show results for 
triangulation-based laser scanners. With their system, they 
estimated the bias to be about 25-30 um and the range 
uncertainty rises from 10 pm on VBAI to 25-50 um according 
to the spot size (as with many similar commercial systems). 
In another experiment, a flat piece of VBAI and a marble area 
on a pedestal were m sasured conducted using both a FMCW 
system (see Section 2.1.2) and a triangulation laser scanner (see 
Section 2.1.1). In both cases, a plane equation was fitted using 
the same algorithm. The FMCW system gave 14 um on VBAI 
and on marble, 87.6 um (both at a distance of 4 m). The laser 
triangulation system gave 30 um on VBAI and on marble, 49 
wm (at a distance 30 cm). This last system follows the results 
presented in Godin et al., 2001. The FMCW behaved in a 
surprising way! Additionally, the type of feature being 
measured is an important factor affecting the accuracy of a 
machine vision system. The accuracy of active 3D cameras 
drop when measurements are performed on objects with sharp 
discontinuities such as edges, holes, and targets (Soucy et al., 
1990; Wallace et al., 1999; Bochler et al., 2003). This means 
that systems based on only range will not provide sufficient 
data for these applications (El-Hakim et al, 1994). The 
following is a list of concerns encountered with laser scanners 
(and 3D vision system in general): 
e Occlusions/Shadows 
eo Abrupt texture and 
(Buzinski et al., 1992) 
e Laser finite footprint and spread on sloped surfaces 
e Specular reflections (Fisher et al. 1993) 
e Motion: scene, object, ambient vibrations 
Therefore, selecting a vision system for a particular application 
must take into account the ability of the system to measure the 
features of interest with the required accuracy. Many 
applications (like found in cultural heritage) do not allow any 
alterations to the object to suit the vision system, €.g. by 
placing markers or changing the reflectivity of the surface. 
shape variations: Edge curl 
Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
    
3.4 Calibration and standards 
A measurement result has only a meaning if its uncertainty is 
known no matter if it is large or small compared to others. Here 
we give a famous quotation taken from Lord Kelvin: “When 
you can measure what you are speaking about, and express it in 
numbers, you know something about it; but when you cannot 
measure it, when you cannot express it in numbers, your 
knowledge of it is of a meagre and unsatisfactory kind: it may 
be the beginning of knowledge. but you have scarcely, in your 
thoughts, advanced it to the stage of science." - Sir William 
Thompson, Lord Kelvin (1824-1907). It should summarize the 
importance of knowing how a 3D system measures physical 
quantities. 
The statement of uncertainty is usually based on comparisons 
with standards traceable to the national units (SI units). For 
example, standards are available to manufacturers of theodolites 
and CMMs for assessing their measuring systems. A guideline 
called VDI/VDE 2634 has been prepared in Germany for 
particular optical 3D vision systems. It contains acceptance 
testing and monitoring procedures useful for evaluating the 
accuracy of optical 3D measuring systems based on area 
scanning. The guideline applies to optical 3D measuring 
works according to the principle of 
fringe projection, moiré techniques and 
systems, which 
triangulation, e.g. 
photogrammetric/scanning systems. Though no internationally 
recognised standard or certification method exists to evaluate 
the accuracy, the resolution, the repeatability, the measurement 
uncertainty of laser range cameras, the user should devise 
techniques to ensure a confidence level on what is being 
measured. Definitions of terms can be found in the VIM 
standard for metrology (VIM 1993). The user should still 
perform periodic verifications even if the manufacturer provides 
a specification sheet. Studying scientific literature published on 
testing range cameras and attending conferences like this one 
should help in preparing a verification methodology that best 
suit the user's needs. Boehler et al. 2003 and Johansson 2002 
present detailed experimental results on different laser scanners. 
In practice, an object that is distinct from the calibration 
equipment and for which the accuracy is ten times better than 
that of the range camera will be employed in such an 
evaluation. A laboratory can be dedicated to calibration and 
evaluation of machine vision sensors and systems. The main 
objectives could be 
e to perform precise calibration of various types of 
sensors and systems, 
e to monitor sensor stability over time and under 
variations in environmental conditions such as 
temperature and ambient light, 
. to evaluate system geometric measurement accuracy 
on a wide range of specially designed standard 
objects and high-precision positioning devices, and, 
e to validate computer vision algorithms, such as target 
and edge measurement, multi-view registration, 
model-based recognition, and sensor fusion. 
Unfortunately, it is not always possible to own and maintain 
such a facility. Furthermore, bringing à verification object 
(especially if it has to be accurate) to a remote site could be 
difficult. 
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