CIP A 2003 xix"' International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
3.3 Range accuracy
Measuring noise. A very simple test to get an indication of the
arbitrary deviations of the range measurements can be per
formed when a plane surface is scanned and modeled. The
resulting deviations of the single points are a reliable source of
information for the relative accuracy of range measurements.
Three different surface paints are used: white, gray and black
with reflectivities of about 80, 40 and 8 %.
Known range differences. Three different experiments are
installed in order to compare known range differences with the
ones measured by the scanners. Spheres were used in either case
for the end points of the distances. In the set-up shown in figure
2, where small range differences can be measured from
distances up to 60 m, the horizontal components are used to
form six independent distances. In addition, the faces of steel
lockers in this long corridor were used to place spheres at well
defined known locations thus allowing the comparison of 4
range differences in mid-range. Finally for close ranges between
3 and 8 m, a sphere is placed on an interferometric comparator
and moved to six positions with 1 m spacing, thus providing
another 3 independent range differences.
Constant (zero) error. As long as range differences at one side
of the scanner are measured only, a zero error is of no rele
vance. If targets are located on both sides, the zero error has to
be known, however. It can be derived from a short distance,
which is measured from in between the two targets as well as
from outside (fig. 5). The zero error can be computed because it
is present only once in the outside measurement, but twice when
measuring from the inside. In order to give reliable results,
groups of three sphere type targets are used (fig. 5). Because
the spheres are observed from short ranges, the systematic
effects mentioned in section 2.5 have an influence on the result.
If the zero error is derived from scans of two parallel planes at a
known distance, different results have to be expected.
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Fig. 5: One method used to detect the zero error.
3.4 Resolution
Since values for increments and spot sizes in the manufacturer’s
specifications do not give much indication about the ability of a
scanner to reach a certain resolution, a practical approach is
chosen in order to achieve resolution information. A box about
300 mm x 300 mm was constructed (fig. 6).
Fig. 6: Target with slots of varying widths for resolution tests.
The front panel has slots which are about 30 mm wide at the
outside becoming smaller towards the center. If a scanner has a
high resolution (small angular increments and a small laser
spot) there should be reflections not only from the front panel
but also from the bottom of the box which is about 55 mm
behind the front panel. If the resolution is very good, these
reflections from the bottom should not only be present in the
outer regions but also near the center. This target can be used to
detect resolution information from different ranges.
3.5 Edge effects
Edges. A board (fig. 7) is used to get an indication how many
points are recorded at wrong locations due to edge effects. The
board is placed against a sky background when scanned. Thus,
the measurement of the outer edges will not be influenced by
objects behind the board whereas the front edges of the attached
smaller board simulate the effect of reflections from two dif
ferent objects. The evaluation is based on a plot of the resulting
point cloud (see fig. 11) and the edge quality is classified as
good, average or poor.
Fig. 7: Board used to study edge effects
Cylinder. A vertical pipe with a diameter of 100 mm is scanned
from a distance of 3 m. A cylinder is modeled from the point
cloud and its diameter compared to the known value.
3.6 Influence of surface reflectivity
Boards showing a wide white frame and a square center part of
different reflectivity (fig. 8) are scanned. Separate planes are
modeled through the frame and the center part (excluding points
at the edges). The range difference between the two planes
indicates the error which has to be expected in similar cases.
The following colors and materials are used:
White dull spray paint, reflectivity 90%. White dull spray paint,
reflectivity 80%. Gray dull spray paint, reflectivity 40%. Black
dull spray paint, reflectivity 8%. Spray paint with metallic
appearance. Polished aluminum foil. Blue retro foil, as used on
CYRAX targets.
Fig. 8: Board with white frame and different surface coatings
