Agriculture Reference
In-Depth Information
Fig. 10.2
Refl ectance curves for soil (
fi lled dots
) and for different plant species with the typical
steep incline (red edge) between 680 and 750 nm wavelength (Weis and Sökefeld
2010
)
populations and weed infestations lower to or equal to the economic weed thresh-
olds could only be detected with sensors from a low distance because of the lim-
ited spatial resolution of the sensors and the small area of weed seedlings. Many
scientists therefore mounted the sensors on the tractor. Three groups of optical
sensors have so far been applied for weed detections:
•
spectrometers
•
fl uorescence sensors
•
digital cameras with subsequent image analysis.
10.2.1
Spectrometers
Intact green plants transform the incoming light by their chlorophyll pigments,
which absorb mostly red as well as violet and blue light. Only some part of the
green and most of the near-infrared light is refl ected. The spectral refl ectance of
plants has a minimum in the visible wavelengths of about 650 nm and increases
considerably towards the invisible near infrared above 700 nm (Fig.
10.2
).
The steep part of the curve is called the “red edge” (Guyot et al.
1992
).
Important properties of plants, such as chlorophyll content, leaf-area-index
(LAI), biomass and water status, age, plant health levels can be derived from the
position of the red edge (REP). The spectral curves of different plants have a
similar nonlinear shape, but the soil curve in Fig.
10.2
is linear. The local
extremes of the plant curves are within the green band (550 nm, maximum), the
