- 96 - 
i. a long-range scanner for architectural applications 
where an accuracy range of 2-4 mm is admissible; 
ii. a medium-range scanner for close-range architectural 
applications and scanning of artefacts such as statues 
and vases with an accuracy range of better than 1mm 
(in practice an accuracy of up to 300 microns appears 
to have been achieved); 
iii. a short-range scanner with an accuracy rating of better 
than 100 microns for scanning of surface detail of 
items like archaeological small artefacts such as coins; 
iv. a very short-range scanner with an accuracy capable 
of measuring the smallest possible variations - typi 
cally on a paint surface; 
The procurement process resulted in MENSI of France being 
selected for provision of both the long-range and the medium 
range scanners. The choice of the other two systems is still 
under consideration. 
In our initial survey (Stage 1) 3DID used the Soisic LG scan 
ner 8 from Mensi which operates on a plane triangulation princi 
ple. This is a medium range scanner and operates at a range of 
2m up to 20m. This instrument has a quoted standard deviation 
of 0.6mm at 5 meters range, but this value decreases to 9.6mm 
at 20 meters. To ensure maximum accuracy, scans were taken 
8 The laser scanning system records 3 dimensional co-ordinates that 
are presented as thousands and millions of points. Multiple scans 
are taken from different viewpoints creating a 3 dimensional mo 
saic of the object. 
From the cloud of points, a mesh can be created through triangula 
tion. This process gives a solid surface to the points. The mesh can 
be given a photo-real surface through texture mapping. Segmenta 
tion and gathering detailed measurements from the mesh permit the 
creation of sectional elevations and plans. From all three systems 
mentioned here, laser scanning gives the most surface detail, how 
ever, it does not give the necessary data to realise object outlines. 
Furthermore, although it is possible to take digital photographs with 
the integrated camera, the resolution is lower than may be desired 
for texture mapping 
at a distance of 2m to 5m. The scanner was set at vertical and 
horizontal positions in order for the camera and laser beam to 
record the stone surface, and to avoid parts of the temple itself 
‘masking’ the surface being scanned. 
The margin of error of up to 0.6mm of the instrument can result 
in an error propagation, and as a result there is the risk that the 
points gathered from different viewpoints do not consolidate 
into a complete virtual structure. 
At a human level one disadvantage of the Soisic LG is that, 
despite its very high accuracy when compared to most other 
laser scanners, it only scans in low-light conditions, therefore in 
this case the scanning of the temple could only take place at 
night. Scanning with a tight grid is also time-consuming. 
The option of using this scanner was taken in order to 
record as much surface detail of the eroded stone as pos 
sible. However, should the amount of information 
needed about surface texture not have been a priority for the 
aim of the survey, we would have opted to use MCR’s long 
range scanner The NG range scanner from Mensi is a time-of- 
flight technology machine which is many times faster and more 
accurate at a distance than the SOISIC LG. 3DID was fortunate 
to have an NG prototype available for testing purposes even 
before it took delivery of the production version of the NG sys 
tem. 9 
These data acquisition systems systems in the field are backed 
by seven workstations for back-office restitution and other post 
processing within 3DID. Five 1.6GHz Compaq workstations 
equipped with 1Gb RAM, Wildcat video cards and a Sony 24 
9 The long-range scanner presents different problems to the medium 
range machine especially when it comes to aiming at targets which 
can afterwards be used for stitching purposes at the post-processing 
restitution phase. It is not easy to aim at small targets from a dis 
tance. As stated above, the dual-purpose target issue was settled by 
using spheres which integrate prisms which permit the target to be 
used by a theodolite as well as by a laser scanner;