In images with a preilluminated reseau (Figure 5), the
measurement is structured as follows:
At first, the mesh to be read is focused by the sensor
and the newly calculated through-light intensity is ad-
justed for the measurement (compare homogenising
the digital partial images). When the partial image is
given into the frame grabber, the sensor is moved to
the following mesh and the next illumination intensity
is adjusted. The time, the scanner requires is used for
initialising the mesh and for measuring the signal.
Figure 5: Picture with preilluminated réseau
Homogenising the Digital Partial Image
To assure a fast and safe signal measurement, it is
necessary to avoid interfering influences by means of
digital image evaluation or adjustment when snapping
the digital partial 1mage. These are the prerequisites
for a fast image identification and for highly accurate
positioning.
For accurate measurement, an even illumination of the
image is essential. An over-exposure would falsify the
objects relative to the background. If, for example, a
black ellipse has to be measured, its edges would
move towards the center in the case of over-exposure
as the bright background would shine into the ellipse.
In another extreme case with an under-exposure of the
image, a black ellipse would not have sufficient con-
trast against the black background and the edges could
not be identified.
The fact that negatives always show an uneven illumi-
nation and the necessity of a perfect illumination of the
image for an accurate measurement results in the de-
mand for an automatic postadjustment of the light used
for snapping the image. This demand was introduced
into MIROS routines as follows:
During the orientation, the through-light intensity, the
user adjusts for measuring réseau crosses is stored. For
any later measurement, this value is taken as a starting
parameter.
The algorithm for post-adjustment of the illumination
intensity does not determine the intensity explicitly
from the grey scale which would lead in the case of er-
roneous measurement to false light values and to the
end of the procedure. The grey scales of the signals
are taken and then, with the example of a white signal,
the correspondence between the grey spectre and the
maximum grey spectre value can be determined.
Should this be the case, the illumination intensity is
decremented or incremented. With this procedure, the
illumination intensity always varies around the optimal
value. If a signal is not measurable, the illumination
intensity 1s not post-adjusted as no reference values
are available.
In contrary to the regulation of different illumination
intensities as described before, only one illumination
intensity is regulated for images with réseau illumina-
tion. In retroimages, the réseau crosses are visible in
the through-light for orienting the sensor and it is
therefore possible to effect the orientation of the sen-
sor and the measurement in the same image. The light
cannot be focused on the crosses or the ellipse to be
measured as this would mean that one of the signals
would always be insufficiently illuminated. For this
reason, the grey scale values of the background are
taken for illumination regulation. After the measure-
ment of the crosses and after the measurement of the
signal targets, a medium grey scale value is chosen
from the area in which only background grey values
are placed. This grey spectre value is regulated on a
value that makes possible both, the measurement of
the white réseau crosses and of the black signal tar-
gets. This procedure assures a sufficient contrast of the
targets and the réseau crosses to the background.
Only three important grey values occur in the retro
images: crosses, signal targets and background. There-
fore, the capacity of the frame grabber to differentiate
between signals and surroundings can be used. From
the original image, two copies are made within the
frame buffer, one for isolating the crosses, one for
isolating the signal targets. In retro images, signal tar-
gets are always black ellipses. This means that the
significant grey values for measuring these signals are
always in the lower grey value spectre. For this reason,
recordings within the screen buffer is always effected
via a threshold Look-Up-Table. Its purpose is to give
identical copies of the grey values from 0-50 and to
take a value of 50 for any others. Thus, bright interfe-
rences are eliminated and darker interferences are re-
duced. The only sharp gradients in the image are then
the edges of the ellipse which can then be found and
measured easily. A similar operation is effected for the
réseau crosses: any grey values below the threshold
value of 150 are set at 150, grey values above this
value are accepted. To react flexibly on the different
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