The laser beam can be used to selectively fight weeds in an early
growth stage. This is the result of a current research project between
the Laser Zentrum Hannover (LZH) and the Biosystems and Horticultural
Engineering (BGT) faculty of the Leibniz Universität Hannover.
More and more, environmentally safe methods are being used to rid
fields used for agricultural and horticultural of unwanted plants, or
weeds. Chemical pesticides can be used selectively and are suitable for
use where conventional, thermal methods such as flaming are either not
exact enough, or are too energy consuming. However, drift and
overdosing often lead to harmful herbicide residues in the top soil
layers, or in surface water. By using an exact, selective laser beam,
the growth of weeds can be impaired by destroying the sensitive growth
centers of the plants, their so-called meristems. Current laboratory
results show that a minimum dose of around 35 Joules is necessary to
kill seedlings, and this laser energy can be exactly and effectively
adapted to the plant species and growth stage.
Researchers from the LZH Department of Materials and Processes, Safety
Technology Group are using a CO2 laser in the infrared range with a
wavelength of 10.6 µm in their current investigations. The laser
radiation has a direct thermal effect on the plants. By using a
galvanometer scanner with a flexible mirror system, the laser beam can
be moved quickly from plant to plant, and can be focused with high
precision on the near-surface meristems. Under laboratory conditions,
an accuracy of <± 1 mm could be achieved, and under
greenhouse conditions, a laser on a rail carriage achieved accuracies
of ± 3.4 mm.
A stereo camera system is used to recognize the plants and optimize the
position of the laser beam. After using a complex processing method,
scientists of the BGT have compared camera images based on threshold
level filtering and edge detection, with Active Shape Models of the
plants. The position of the leaves can be used to determine exactly
where the meristem position is, and once these coordinates are found, a
signal is sent to the laser to “hit the target”.
Scientists at the LZH have been able to determine exactly how much
energy is needed to achieve an optimal effect on the target, making
this method especially efficient. At the moment, different irradiation
times in different weed concentrations are being tested concerning
economic viability. Current knowledge shows that the best results for
large areas can be provided by using autonomous field robots working in
a stop-and-go mode.
The project „Investigations on the Effect of the Laser Beam on
young Plants for Weed Control using Image Processing“ is
subsidized by the German Research Foundation (DFG).
More information at http://www.lzh.de/