Agriculture Reference
In-Depth Information
decreasing energy per photon in eV or J (without scale)
frequency in MHz
10
14
10
13
10
12
10
11
10
10
10
9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
infrared
microwave
s
radar
x-rays
thermal
r
adi
o
tv
0.001 0.01
0.1
1
10
0.1
1
10
100
0.1
1
10
1
10
wavelength
nanometers
micrometers
centimeters
meters
visible spectrum
1 micrometer (µm) = 0.000001 m
= 1000 nm
blue
green
red
0.4
0.5
0.6
0.7µm
Fig. 3.1
Electromagnetic spectrum on a contiguous wavelength scale. Please note that the wave-
length units change. The exact boundaries between radiation types are not unanimously agreed
upon and thus can vary somewhat. Consequently, the limits between radiation types are approxi-
mates (From Chuvieco and Huete
2010
, redrawn and altered)
or crops. Hence the methods can be classifi ed into contact- or non-contact sensing.
Either of these methods can be used when sampling occurs in conjunction with farm
machines, whereas satellites and aerial platforms rely solely on non- contact sensing.
Since only a few contact sensing methods are available, these are dealt with in
appropriate chapters later. Non-contact methods almost exclusively are based on
sensing by electromagnetic radiation. This chapter concentrates on this.
Electromagnetic sensing is based on
radiation
of photons. This radiation -depending
on its specifi c type - carries energy through space along periodic harmonic waves.
There are many different types of electromagnetic radiation (Fig.
3.1
). An important
criterion is its
wavelength
, which can vary between a tiny fraction of a nanometer and
several meters. The wavelength times the frequency is the speed of the radiation. In a
vacuum and in air, this speed is the same for all types of electromagnetic radiation,
namely 300,000 km per second. Therefore, the shorter the wavelengths, the higher the
frequencies are and
vice versa
.
Another important item is the
energy per photon
. This energy is proportional to
the frequency of the radiation type and consequently inversely proportional to the
wavelength. The shorter the wavelength, the higher is the energy per photon. The
energy of very short wavelengths - ultraviolet radiation and shorter - therefore can
be dangerous to human health. Yet this depends on the particular situation.
The differences in energy per photon also have implications for sensing. For
photons from longer wavelengths, either very sensitive sensing devices are needed
