This method of scanning makes the production of an
accurate 3-D computer model of the original object possible without
the need to make contact with the object’s surface.
Two types of scanner were tested, the Konica-Minolta
Vivid VI-700 and the Model Maker X made by 3-D scanners Ltd now owned
by Metris. The most successful scanner for these objects was the Model
Maker X, due to the small scale and highly reflective surface of the
With the Model Maker X, the object to be scanned is
placed on a foam support while the laser head moves around it. Instead
of turning the object around a static laser source, the pin-joint arm
enables scanning whilst the object is kept still. The pin-joint arm
contains sensors which automatically locate the laser head in a coordinate
system, as long as neither the tripod nor the object moves.
The scanner head emits a thin strip of low-power laser
light. Light reflected from the surface of the object is recorded by
an off axis CCD-camera. This low power radiation does not harm the object.
The data, which represents an image of the 3-D surface,
is collected as a set of points, in which each point is measured every
35 microns (= 1/1000 mm). This data set describes a 3-D point cloud,
consisting of many millions of points, accurate to ~0.1-0.2 mm.
The results can be accessed immediately on a portable
computer monitor with a software application called Pre-Scan.
Each side of the object is scanned, forming two point-cloud shells,
which are then joined. The resulting virtual object can be turned and
examined from all angles.
Neil Wressell, Conservator,
Explains the scanning of the figures
At this point, if necessary, it is possible to re-scan
certain details if the resolution or accuracy of the original scan is
not sufficient. For example, it was necessary to rescan the undercut
elements, such as crossed legs or facial features, as the dark and hollow
areas of the figures are difficult for the scanner to interpret. This
is because the light from the laser beam is scattered, preventing it
from being accurately detected by the camera chip. The final file size
for a high quality scan of each figure is approximately 10 MB.
Rapid prototyping is a technique developed in the
1980s to enable designers to create models or prototype parts for testing
prior to widespread production.
For the purposes of this project, the following prototyping
techniques were tested at CRDM Ltd in Buckinghamshire to compare different
materials for the replicas of the figures:
SLA – Stereolithography apparatus
SLA was the first established Rapid Prototyping process.
The Stereolithography apparatus consists of a vat of liquid resin, which
solidifies under exposure to an ultra violet laser. The technique can
create models from a range of epoxy or acrylic resins.
The model is built up in layers which map the points
contained in the data file. The photosensitive resin cures on exposure
to the UV light with each layer being created individually, working
from the core of the object to its surface. In the tests carried out
for the figures from Letter to the Censors, the thickness of
each layer was set to 0.1mm. The range for this setting can be chosen
between 0.025 mm up to 0.15 mm.
When the model is complete the excess resin is removed.
The result is a solid translucent resin model, accurate to within 0.1mm
of the original.
The acrylic resin used in this fabrication process is called Envisiontec,
a photopolymer acrylic resin. It is red and translucent in appearance.
It is also tough and provides an extremely high definition surface finish.
SLS – Selective Laser Sintering
The SLS technology makes it possible to use a wider
variety of materials for prototyping. In our tests, the models were
made of white nylon (Polyamide). These prototypes are highly durable
and accurate to ± 0.15 mm. They are also heat resistant up to temperatures
SLS uses a computer operated CO2 laser to fuse or
melt together the Nylon powder in 0.1 mm layers in cross-section. The
system sinters layer after layer until the object is complete. The colour
of the samples can vary depending on the material used.
What is sintering?
Sintering is a method for making objects from powder
by heating the material (below its melting point) until its particles
adhere to each other. Sintering is traditionally used for manufacturing
ceramic objects, and has also found uses in such fields as powder metallurgy.
From these tests SLA was found to be a more effective
technique than SLS for creating the replica figures. This was largely
due to the difference in the surface appearance of the models.
The surface produced by the SLS technique tends to
be either slightly powdery or granular. A powdery surface is produced
if the temperature is slightly too low, this is because during fabrication
some of the particles of the base material fuse together without completely
melting. If the temperature of the uncured SLS powder is too high during
the sintering process, excess fused material can collect on the surface.
Although applying a coating to the SLS models helps
to even out the irregularities in the surface this has the undesirable
effect of reducing the surface detail.
In comparing the two methods, SLA was found to be the more accurate
way of reproducing the fine detail and smooth surface of the figures.
In order to match the colour of the figures the SLA models were painted
using acrylic paints.