Agriculture Reference
In-Depth Information
or a larger area is required in order to get a suffi cient amount of energy. Thus a balance
between wavelengths and spatial resolutions might be necessary. If a high spatial
resolution is aimed at, using radiation from a short wavelength range in principle
would be advantageous. However, the spatial resolution is not the only criterion
when wavelengths for sensing are selected.
Radiation may come from a natural source or may be artifi cially induced. The most
important natural sources are the sun and the earth. The wavelengths of the radiation
that is emitted by these sources depend on the respective surface temperature. Since
this temperature is much higher on the sun than on the earth, the solar wavelengths are
much shorter than the terrestrial ones. Practically all the solar energy fl ux to the earth
is in the wavelength range between 0.15 and 4.0
m; hence it consists mainly of ultra-
violet-, visible- and some infrared radiation. The maximum energy fl ux of the solar
radiation is in the visible wavelengths. On the other hand, the energy that is emitted
from the surface of the earth is in the region from 3 to 100
μ
m, which is mainly in the
thermal infrared range (Guyot 1998 ). In short, the earth emits long-waved, but it
receives short-waved radiation. There is only a small overlap between emitted and
received wavelengths.
However, this is a rather rough breakup of the energy fl uxes and the wavelengths
involved. It is important that the longer waves do not contribute much energy. A more
precise view is obtained when considering what happens with radiation that is directed
from the sun to the earth or vice versa .
μ
3.2
Emitted, Absorbed, Refl ected and Transmitted Radiation
It is important to distinguish between emitted-, refl ected-, absorbed and transmitted
radiation. Emitted radiation leaves the surface mass of the sun or the earth, as
every body at a temperature above 0 K discharges photons. The higher the tempera-
ture is, the shorter the wavelengths are. Photons that hit a particle en route, rebound
and change the direction. Hence these photons become refl ected radiation . If the
photons are not refl ected, but instead of this provide energy for the matter that was
hit - e.g. for heating or for photosynthesis - absorbed radiation is dealt with.
Finally there is transmitted radiation that was neither refl ected nor absorbed.
Instead of using the absolute values, it is often reasonable to relate the refl ected,
absorbed and transmitted radiation to the initial radiation. These related or normal-
ized signals are denoted as refl ectance , absorbance and transmittance . It should
be noted that the initial radiation - from which the refl ectance, absorbance and
transmittance are obtained - can be but must not be at the stage of emission from the
sun or the earth. The respective initial radiation can also be radiation that was
already refl ected or transmitted at an earlier stage, e.g. on its path from the sun to the
earth. It just depends on what is regarded as the initial radiation.
The sum of the respective refl ectance, absorbance and transmittance in fractions
always adds up to 1 (one). So it suffi ces to measure only two of these radiation
types, the third type can then be calculated. This is important since often the
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