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en
2 Physical Principles and Technical Aspects of Remote Sensing
27
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0.5
,
\
2
' "
~ --\
(
-,I
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1.5
2.0
2.5
Wavelength [/lm]
Fig. 2.10. Characteristic spectral reflectivity in the visible and shortwave infrared
for (1) fresh snow, (2) a soybean leave, and (3) wet clay
Visible transmissivity of water ranges from centimeters to tens of meters,
depending on turbidity. Information on phytoplankton and particulate matter
can be derived from the spectral properties of the radiation scattered back
from the liquid volume. The absorption in water increases strongly in the
near infrared.
Main land cover classes can be discriminated due to characteristic spectral
reflectivities. As examples, Fig. 2.10 shows the reflectivity of clean fresh snow
(Grenfell et al., 1981), a green soybean leave (Jacquemoud and Baret, 1990),
and wet clay (Jacquemoud et al., 1992). The strong increase of reflectivity at
0.7 J.Lm is characteristic for vegetation. The near IR reflectivity enables monitoring of vegetation health, the reflectivity is reduced in case of plant diseases
and water stress. Accurate spectral measurements offer excellent capabilities
for a wide range of environmental applications such as monitoring of water
quality, of vegetation type and state, of soil and rock types, and of snow and
ice properties. This was the reason for developing imaging spectrometers.
Microwave Emissivity. Microwaves are able to penetrate into solid material. The penetration depth, dp , represents the distance into the medium so
that the intensity is attenuated to the 0.37 - fold of the value at the surface.
If c~ « c~, as observed for many natural media, the penetration depth can
be approximated by