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Infrared radiation is one of the electromagnetic radiations that
reach the Earth's surface and are grouped in the Electromagnetic
Spectrum (fig. 1), according to their growing wavelengths.
The infrared radiation is located between the red side of light rays and radio
waves. This range is arbitrarily divided in various ways. A possible
division is provided in fig. 1, another is between "photographic"
infrared (pink strip) and "thermal" infrared (the red
strips in fig. 1 indicate the range used by SW and LW apparatuses),
as the radiation in the two ranges can be respectively highlighted
with special roll films (between 0.75 to about 1µm) or with special
apparatuses - thermal scanners and IR cameras (from about 2.5 µm
onwards). However, the true division is between reflected infrared (from
about 0.75µm to about 2.5µm - the radiation comes from a source
outside the bodies under study, such as the sun or special lamps)
and emitted infrared (from about 2.5µm onwards - the radiation
directly comes from the thermal energy of the bodies under exam). It
is thus possible to view a radiation that is invisible to humans,
like all the radiations that do not belong to the visible radiation
(rainbow strip in fig. 1). Because of gases and substances in the
atmosphere, all wavelengths of the infrared radiation cannot be used
in IR apparatuses, but only transparent ones (atmospheric
transmission windows).
Temperature measurements conducted with thermal cameras rely on the
electromagnetic radiation, or energy, continuously emitted by bodies,
thanks to their thermal content, and according to their surface
temperature: thermal cameras sense the infrared radiation emitted by
bodies. The energy flow also depends on the surface emissivity and
the wavelength range of the radiations emitted. It is now necessary
to briefly introduce some laws governing this phenomenon: for a
complete explanation, please refer to specific texts.
According to
Kirchhoff's law, an ideal emitter (black body) is "a body that
can absorb radiations of any wavelength and re-emit them":
Plank's law applies to this, as it states that the intensity of the
emitted radiation is as follows
where
W(lambda) is the spectral radiant emittance within the spectral range of
1µm, at wavelength λ (lambda)
C1 and C2 are the First and Second radiation
constants, respectively
λ (lambda) is the wavelength in µm
T is the absolute
temperature in Kelvin.
The Stefan-Boltzmann law can be obtained by
integrating this expression for the whole field of wavelengths of
the electromagnetic spectrum. This law states that the total
radiating energy of the black body depends on the fourth power of
the absolute temperature
where
σ (sigma) is the Stefan-Boltzmann constant. In reality, however, no
black bodies exist, but only grey bodies that more or less
significantly differ from the perfect behaviour: this is taken into
consideration by introducing emissivity, epsilon, in the Stefan-Boltzmann
formula to mark the difference between the real body and the ideal
behaviour; the law thus becomes the following
ε emissivity is the ratio of the radiating power of an object to the
energy radiated by a black body at the same temperature and
wavelength, thus emissivity can reach 1, at the maximum. In order to
measure the surface temperature of an object, it is necessary to
know its emissivity. There exists a huge number of emissivity tables
relative to many objects, measured at different temperatures. By
observing these tables, it is clear that values do not depend only
on the nature of the object itself, but also on its surface (smooth,
corrugated, oxidised, polished, etc.).
In general, a value of 0.98 is
assigned to trees: it is not necessary to know surface temperature
values to detect internal decay, but only possible surface
temperature differences, consequently using an emissivity value of
0.98 or 1 doesn't make any difference. To this end, it should be
borne in mind that the first apparatuses used to detect cavities and
internal decay in trees couldn't measure temperatures in any given
point of the thermogram or thermal image (TI), but only showed their
distribution on the surface of the object under study. The
emissivity of barks can be neglected, thus providing a significant
operational simplification: it is clear that the barks of a cedar,
or oak are very different from those of a laurel, pine or palm tree.
Lastly, if we differentiate Plank's law as far as lambda is concerned and a
maximum value is calculated, we obtain Wien's law
that identifies the wavelength at which an object at temperature T
emits the maximum spectral radiant emittance. If we replace T with
the room temperature value (25 oC) in Kelvin (273 + 25 = 298), we thus obtain
that is the maximum spectral radiant emittance falls within the
infrared, and this explains why this radiation is important to study
the environment.
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An Holm oak in an urban park: the TI shows decay in the root system,
particularly on the right side of the tree
Base of a lime tree in a city square: the TI shows decay in the root
system, particularly on the left side of the tree
TI mosaic of an imposing Holm oak in the park of a mansion: the
TI shows decay on the left side of the trunk that goes from
the root system to the big branches on that side. Huge areas of
healthy, reactive tissue on the right side of the trunk and along
the corresponding branches
TI mosaic of another imposing Holm oak in the same park. Decay moves
from the root system to the trunk, especially in the middle, and up
to the branches. Healthy reactive tissue is clearly visible on the
left side of the tree, even if it is less than the previous Holm oak.
The bark at the collar was removed in the past
Aerial TI of an unauthorised landfill taken with an old thermal
scanner
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