To work with the digital partial images, two different
frame grabbers can be used at present. Those are the
FG 100 image card and the newer version, the VFG
100 of Image technologies.
The system is completed with the RS1 Scanner of
Rollei Fototechnic (Figure 1) This scanner makes
possible to work with negatives of a format up to 23 x
23 cm and is being updated at present to achieve a
further reduction of the movement intervals from point
to point.
Figure 1: Reseauscanner RS1 with monitor
Basic Principals of the Réseau Measurement
Technique
Its task is to give an exact identification of the position
of ellipse points with high accuracy. This is achieved
by moving a sensor over the image which is able to
take up partial image of 5 square mm in which the sig-
nals are measured later on. The position of the digital
partial image cannot be determined by the actual posi-
tion of the sensor as one would presume. This would
mean that the positioning accuracy of the scanner
would determine the measurement accuracy. For that
reason the position is determined by the réseau orien-
tation. For this purpose, either a glass plate with
réseau crosses is fixed directly above the negative or
such a grid is placed directly in the negative illumina-
tion. The position of the sensor can only be determined
when four crosses (mesh) are measured in the digital
partial image and the accuracy of the scanner is only
relevant for approaching the meshes (DR 91 and GP
88).
MEASUREMENT PREPARATION
Initiating the Mesh System
The first step when measuring in image negatives is to
adjust the scanner so that the information from the
negative is accurate and well illuminated. The user has
the opportunity to move the sensor in vertical direction
to reach perfect focusing. Alternatively, the system is
able to adjust automatically. This is based on the prin-
ciple to reach highest gradients with occurring margins
and therefore, the adjustment received from the system
is better than a manual adjustment. The same is valid
for the regulation of the light intensity which is me-
chanically adjusted with a perfect grey scale spectrum.
As already mentioned, the basic principle of réseau
measurement is the orientation of the sensor via a
mesh. To obtain the correlation between réseau- and
sensor system, a transformation between the two sys-
tems has to be defined. Problems occurring from the
orientation of the réseau system are for example the
different mesh sizes of the initial system, differing fo-
cal lengths of the sensor optics and a possible rotation
of the réseau system in correlation with the sensor
system. To avoid that this proceeding becomes slow
and difficult, an algorithm was implemented which
identifies basic scales and rotations automatically: by
initialising the mesh system on only one mesh in the
center of the scanner, a low error rate to all sides is as-
sured. Given that in many cases the first mesh is cho-
sen incorrectly, the position has to be corrected. When
the sensor focuses at least one mesh, this means that at
least one réseau cross is reflected in the digital partial
image and is therefore measurable. If no crosses are
visible due to extreme interferences, the algorithm sets
a new mesh. When four crosses are found in the first
attempt, the mesh is regarded as being initialised.
Otherwise, the scanner has to be moved in such way
that the cross found is placed into the corner of its
quadrant. For this purpose, the height and width of a
pixel referring to the scanner system must be known in
mm. To receive the necessary scale, the scanner is
moved in x- and y- direction for a quarter of the
known mesh dimension. It is then tried to recognise
the same cross. The viewfinder is limited in such way
Personal computer CCD-sensor
Réseausystem
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