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raction of
one ASA) and thus are not greatly affected by radiation
effects. However this means that under CW illumination
absolute stability of the holographic camera must be
assured with tolerances of the order of few wavelengths
of light. This is unlikely to be possible within a civil
engineering structure and therefore a high intensity
pulsed laser must be used to make the recording. When
playing back the recording under CW illumination the
wavelength of the illumination must match that of the
recording pulse in order for successful reconstruction.
Trials by nuclear utilities indicate that that difficulties in
arranging either the imaging stability or the
correspondence of the wavelengths of the different
illumination sources may effectively limit the potential
applications of the technique for the measurement of
large (>1m”) volumes.
Laser scanning devices have been widely employed in
close range industrial inspection applications.
Measurement is generally by intersection and is often
termed active triangulation (photogrammetry being
described as passive triangulation). Typically a point or
line of laser radiation is scanned over the object to be
measured and specular reflection from the object's
surface is imaged on a linear or array CCD sensor. The
geometry of the intersection depends upon the base-line
distance between the laser source and sensor and the
angular measurements determined from the laser
scanning and imaging optics. Alternative techniques
may employ active ranging in which polar
measurements are determined from angular observations
coupled with direct distance measurement from time of
flight or phase based electromagnetic distance
measurement (EDM) systems.
Much recent research has been directed towards the
effective exploitation of such range images. Hardware
developments include laser triangulation systems such
as the SOISIC laser triangulation system from MENSI
(D'Aligny, 1991) which are capable very high
precisions(t 1mm over 5m) at the expense of relatively
long acquisition times or the LETI Advanced Laser
Imaging System (ALIS) which is capable of capturing a
low precision (t 1% of range) range image' of 120 x
150 pixels at 4 frames per second Such systems yield
substantial amounts of data which must be structured if
it is to be integrated into a CAD description. Automatic
segmentation of such data continues to be of
considerable interest to researchers in the machine vision
community (e.g. Besl 1988, Bhanu 1987, Hoffman
1987, Kasvand 1988) but have not yet been widely
applied to the highly complex scenes typical of a
nuclear installation .
2.0 THE HAZMAP MEASUREMENT
SYSTEM
The system developed in this project capitalises on
several developments mentioned in section 1. In
particular the availability of relatively inexpensive
motorised theodolites, CCD cameras, image digitisation
and high powered graphics workstations have enabled
the rapid development of a hybrid imaging system
which can be modelled within a specialised softcopy
workstation developed at UCL.
Figure 3. Laser range image with CCD image
Courtesy of Electricité de France
The proposed system contains three modules which are
represented diagrammatically in Figure 2 and comprise:
* the remote measurement unit;
e the photogrammetric module; and
* the CAD interface.
The remote measurement unit.
Both CCD and vidicon cameras are used frequently by
nuclear engineers for inspection of radioactive plant.
Their environmental tolerance is well established
