multiple exposures of the same moving line of the pulp
web, effectively increasing the integration time
available to collect incident light (more technical
details can be found in reference [4]).
The mentioned TDI camera is made up of several
parallel CCD lines that shift their electrical charges
from one line to the next one in synchronization with
the inspected-product movement. This shift procedure
is performed in such a way that each product line is
acquired in several positions and the single acquired
images are added to each other by the camera
electronics in order to produce a single line-image that
has several times more dynamic range than a
conventional linear camera, therefore allowing impurity
detection under the appropriate inspection requirements,
where other systems failed.
The web motion is synchronized with the exposures
of the CCD lines in order to ensure that there is no
overlap of the brightness of consecutive lines. This
synchronization is achieved by an encoder, driven by
the web product, that triggers the exposure and the
CCD charge shift. This procedure results in the robust
acquisition of sharp images within an industrial
environment, as can be seen in Fig. 2.
TDI devices progressively combine multiple exposures
of the same line of the pulp into one highresolution
result, dramatically improving sensitivity and
overcoming the above-mentioned lighting problems.