Sensing by Electromagnetic Radiation - Precision in Crop Farming - page 17

Fig. 3.2 Schematic to the effect of the inverse square law. The farer the target area is away from
the point source, the lower is the number of photons per unit area
absorbance is diffi cult to measure directly. Sensing in precision farming mostly
relies on refl ectance from soils or crops. In a few cases, transmittance is used.
In theory, any matter - including constituents of soils and crops - can be identifi ed
either by its refl ected, transmitted, absorbed or emitted radiation. The identifi cation is
a matter of careful analysis of the spectrum - that is the wavelength distribution - of
the respective matter. This might include mathematical processing of spectral data.
The fundamental basis of optical sensing is that in this way for every matter or constituent
of soils and crops an optical fi ngerprint can be derived.
So far the best optical fi ngerprints have been derived from details in the visible-
and/or infrared radiation. These details depend largely on the resolution of the optical
signals and on the range of the spectrum that is used. The optical fi ngerprint might
rely on the whole range of the visible, near-infrared and mid-infrared radiation,
hence might be derived from a full spectrum approach . However, it can also be
based just on a few or even only one narrow wavelength, thus depend on a discrete
waveband approach . For some properties of soils or crops, such optical fi ngerprints
have been well defi ned. There are also cases where research still has to fi nd the best
optical fi ngerprints. Details to this will be dealt with in later chapters.
If electromagnetic radiation emanates from a point source at a constant rate and
the distance to the target (sensor) increases, the photons will spread out over a larger
area. Hence the fewer photons will land on a target area of constant size, the farer
this area is away from the source. This is the effect of the well known inverse
square law , which states that the result per unit of the target area is proportional to
the inverse of the squared distance (Fig. 3.2 ). Since the sensor of the electromag-
netic radiation can be regarded as a target, this can affect the sensing results.
The attenuating result of the inverse square law on radiation is basically indepen-
dent from any effects that are caused by material barriers such as molecules, which