CIPA 2003 XIX 11 ' International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
Density and accuracy have a very big range in the optical
scanner; therefore it is a flexible system. It is quite obvious that
improvements in accuracy and density are exponentially
expensive in processing time, approaching laser scanner’s
figures if decided to compete. Still there is a certain limit for the
optical scanner in terms of density and accuracy, not to mention
that in laser’s figures of density and accuracy the optical
scanner cannot compete. From the very beginning though, this
was not the task for the optical scanner.
Laser scanner’s processing time is dig mainly because the total
time for a single scan with the optical is much smaller, although
additional processing is necessary. The CNC is moving slowly
in order to achieve the requested density and therefore a single
scan can take from 30 min up to 4 hours. On the other hand
points gathered are many and there is no problem in
discontinuous surfaces. If there are many discontinuities it will
be necessary to rescan the same area with different focusing
settings, hence exploding time. On the opposite side, the optical
scanner cannot accommodate discontinuities due to conceptual
design, no matter how much time will be dedicated in
processing or photography.
Undercuts are the basic problem of both systems, but the optical
scanner is more sensitive because it is based on the assumption
of surface continuity. Therefore only one uninterrupted surface
can be modelled by using a single photograph. This fact poses
limitations on surface complexity.
Therefore, unless it is a very simple geometry, both methods
need filling of small gaps. Therefore it is necessary in most
cases to post process data by a specialist on 3d modelling
(industrial designer in our case). The optical scanner is more
sensitive in occlusions and steep slopes, hence limiting its use.
CNC and laser scanner is out of the question due to limitations
of the scanning area. Laser scanner on a mechanical arm might
be the best solution for such objects, although double scanning
of areas, which is unenviable, can cause serious problems.
Portable laser scanners (in principle the laser head itself
enlarged and modified for such purpose) are in author’s
knowledge the best solution.
In both cases the points gathered form a huge data set, difficult
to handle by any system. NURD modelling (mathematical
representation of the surface) although time consuming and
mostly manual, produces much better results, provided one is
determined to invest in time for correction and conversion in
addition to the inevitable lose of accuracy. Result, though are
far better for continuity of the surface on the reconstructed
model.
It must be noted that certain recent improvements in the optical
scanner have raised its cost, but increased its functionality,
hence increasing functionality, portability and decreasing a bit
processing times and sensitivity to discontinuities.
Further research includes expansion of the technique on smaller
objects such as coins.
References:
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moving camera. IAPRS Vol XXXI, Part B5, Vienna 1996 , pp
220-224.
Kloudas, T., 1995. Three dimensional surface reconstruction
with the Zeiss photogrammetric industrial measurement systme
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Luhmann, T., Wendt, K„ 2000. Recommendations for an
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Pollefeys, M., Koch, R., Vergauwen, M., Van Gool, L„ 1999.
An automatic method for acquiring 3d models from
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Scaioni, M.. Vassena, G., Kludas, T., 1996. Automatic DEM
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Yixuan, Z., Miaozhong, X., Chan. M., T., C., Zhihao, H.. 1999.
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5.1 Acknowledgements.
Work reported, is part of the Eco_marble research project,
financed under the Growth European programme. Participants
were GeoAnalysis S.A. (Thessaloniki, Greece), Fraunhofer
Institute (Bremen, Germany), Polyline S.A. (Thesaloniki,
Greece), FitzWilliam Museum (Cambridge, U.K.), IT'P Gmbh
(Bremen, Germany), Archaeological Receipt Fund (Athens,
Greece), Materialise NV (Leuven. Belgium), Focke Museum
(Bremen, Germany).
