International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
Both techniques give points of nearly the same accuracy; and
these are reliable for the reconstruction of the object models'
outer surfaces in a 1:1 scale, though with a significant
difference in cost. Laser scanners using special rotation
mechanisms or mechanical arms cost about 60.000-80.000€,
while optical scanners may have less, but not negligible, cost.
Several different approaches using low cost material have been
proposed by universities’ laboratories and individual
researchers during the recent years. Among the most recent
research efforts presented in the last CIPA conference in 2003
in Antalya must be mentioned two similar low cost approaches
(Yilmaz, U., et. al, 2003, Pavelka, K. and Dolansky, T., 2003).
Common to the above-mentioned approaches is the use of a
laser profiler, which is synchronized to the rotation mechanism
and to the imaging device, in order to provide in several steps
the necessary points to describe the objects’ outer surface.
Simple photogrammetric triangulation processes are then used
to calculate the 3D coordinates of the points lighted by the laser
beam.
The proposal of this paper deals with the construction and use
of an optical scanning system, which has a minimal cost for
both hardware and controlling software combined. Our goal is
to provide an affordable device to institutions and individual
researchers, which routinely suffer from small budgets (e.g.
museums, state archaeological organizations, collectors,
excavators). This device should allow these groups to contribute
large numbers of digital objects inexpensively to the scholarly
world.
3. DESCRIPTION OF THE SYSTEM AND ITS
OPERATION
Almost every scanning system is comprised of three basic
hardware modules, which are also the basic modules of our
system:
1) Rotation unit
2) Structured light or laser pointing unit
3) Imaging unit
All the above-mentioned modules are coordinated under the
control of a dedicated computer, which rotates the disk and
calculates the location of the points (lighted through a laser
beam to produce a vertical line) in every step of the rotation by
using sophisticated photogrammetric processes.
3.1 The Rotation unit
The rotation unit consists of a specially modified turntable. A
stepper motor device (fig. 1.) controls the movement of the
turntable. The stepper motor used in this instance was
manufactured by the AEG Company and provides 24
steps/revolution and is fully controlled through the computer’s
parallel port using a special interface kit (fig. 2). In order to
achieve better angular resolution of the rotation mechanism the
stepper motor is not directly connected to the rotational axis of
the turntable, but the movement is transmitted to the axis
through a special transmission belt, which multiplies the
steps/revolution. As a result the angular resolution of the system
provides 150-600steps/revolution.
The cost of the rotation mechanism is relatively low. The
interface kit includes a Windows and DOS application that can
Fig. 1. The stepper motor
STEPPER-MOTOR-INTERFACE 6 PIN
Kemo® | GERMANY ^ — *Mi08 ] # M108
Fig. 2. The parallel port stepper motor interface kit
be programmed to control the rotation of the motor. However
the incremental steps of the rotation of the system is performed
through the imaging software application, because the rotation,
laser lighting and image-capturing processes must be fully
synchronised in order to achieve the desired images for the
collection of the 3D points.
3.2 Laser Pointing Unit
The laser level used to create the light screen for the
determination of the points to be calculated and to provide the
outer surface of the object is a typical carpenters’ tool used to
create straight cuts. It has been altered to interpose the
switching circuit controlled by the computer, between the
power supply and the laser light source.
3.3 The imaging unit
The imaging unit used is the Fire-i digital Unibrain Camera.
The camera is based on the Sony™ Wfine color 1/4" sensor
CCD using a Built-in 4,65 mm lens with anti-reflective coating.
The camera is connected through the FireWire interface port to
the computer, providing a maximum of 30fps in 640x480
resolution images (pixel format YUV 4:1:1). The quality of the
images adequately meets the needs for producing the 3D points
(fig. 3). Additionally, the camera has no fixed focal length
=
c
giving the opportunity to perform manual focusing to obtain
