Catena & Thermography
Catena & Thermography
Catena & Thermography
"Treethermography® since 1984"
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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.

Electromagnetic Spectrum

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 1Ám) or with special apparatuses - thermal scanners and IR cameras (from about 2.5 Ám 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

Electromagnetic Spectrum

where
W(lambda) is the spectral radiant emittance within the spectral range of 1µm, at wavelength lambda
C1 and C2 are the First and Second radiation constants, respectively
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

Electromagnetic Spectrum
where
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

Electromagnetic Spectrum
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

Electromagnetic Spectrum
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
Electromagnetic Spectrum
that is the maximum spectral radiant emittance falls within the infrared, and this explains why this radiation is important to study the environment.
Infrared radiation
An Holm oak in an urban park: the TI shows decay in the root system, particularly on the right side of the tree

Infrared radiation
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

Infrared radiation
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

Infrared radiation
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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