CHAPTER II

Physical study
natural radiation
Devices

The apparatus used to measure the radiation naturally whole, or its only visible part, belong to many types and their classification can be approached in various ways. A frequently used method is to distinguish the thermal receptors and receptors quantum. We will see that this method is too restrictive, and that other means can be envisaged, in particular forest photology. On the other hand, the special requirements for that discipline leads to search, alongside conventional equipment used in meteorology, instruments for easy handling, slim, not too expensive (they must often operate in a number same time), robust (because they are destined to remain more or less time in nature), preferably independent (they are sometimes located very far from any inhabited place), and of course as precise and as sensitive as possible. All these qualities are, we imagine, rather difficult to reconcile in practice.
Suffice it, in what follows, to enumerate and describe briefly what devices are used, either by the forest research stations or by foresters and ecologists in the field. None of them is obviously beyond reproach. But the quantitative information they provide are an element of value undoubtedly superior to the simple and purely subjective assessments were qualitative, so far, the rule in this complex.
Devices sensitive to a wide range of wavelengths were formerly called "actinometer". But, according to currently accepted nomenclature 1a, there are (PERRIN DE BRICHAMBAUT - 1963):

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- The pyranometers measuring radiation from the sun and sky, short wavelength (0.2 to 4μ).

- The pyrgeometers which measure radiation from the earth, long wavelength (4 to 80μ).

- The pyrradiometers which measure the total radiation coming from the sun, sky, atmosphere and land (of about 0.2 to 100μ).

There are more complicated devices such as pyrradiobilanmeters, which are the balance between the total incident energy and the total energy return.

 

 

Thermal receptors


In this type of device, the global radiation (from sun and sky) is converted into heat and this heat produces a specific effect depending on the nature of the equipment used and the extent of this effect can be traced back to the factor which one seeks to measure the intensity in this case, the solar radiation.

The distillation apparatus - - These devices have a value sometimes discussed in meteorology classic. The reason is simple: many of them receiving a spherical body, and they are sensitive to gamma rays from all directions. However, in meteorology, we adopted the rule measure the radiation received on a horizontal surface. The height of the sun above the horizon has a different effect, in both cases. Indeed, the intensity of radiation received on a horizontal surface (i) expressed as a function of radiation received on a surface normal to the main direction of it (I), and the sun's height above horizon (h) by the classical relationship:

i = I sin h

It is noteworthy that some researchers to overcome this difficulty, proposed to adopt a coefficient of equivalence (indications of the unit ball / intensity of radiation on a horizontal surface) varies slightly with the month of observation (GESLIN & GODARD - 1940). For example, according to these authors, 1 cm3 of distilled alcohol in a unit ball (the Bellani) represent, in January: 15.9 cal / cm 2 on a horizontal surface, in February: 16.65 in March: 16 8 April: 17.9 in May: 18.6 in June: 18.05 in July: 17.7 in August: 16.83 in September: 16 October: 15.35 in November: 15.9 and December 14.

Furthermore, biometeorological, the bioclimatology, noted that living things occupy space in 3 dimensions, and a plant, animal or human, located in the natural environment is also subject to radiation Coming from all directions. In sum, they argue that the rule of meteorologists would only be valid for bodies in 2 dimensions (the "larvae" extra plates designed by Sir JAMES JEAN, for example), we can hardly meet in nature.

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The most common of these devices is Bellani spherical pyranometer. It is currently used in photology forest in many parts of the globe (France, Switzerland, East Germany, Czechoslovakia, Belgium, Canada, and some British Commonwealth countries). Its inventor, the Italian canon BELLANI Angelo (1776-1852), had originally designed as a totalizer thermometer (it was a kind of inverted thermometer, comprising a spherical shell at its top, in which heat caused a distillation of a volatile liquid, which accumulated in a long graduated tube below). Enhanced by CANTONI (1887), he was named lucimeter, HENRY, from 1921 to 1926, perfected by showing that if we added a second envelope spherical around the top, and isolating the intermediate part by a high vacuum, we obtained a device totalizer radiation, as good as actinometers of more sophisticated, like KIMBALL, or that of GORCZYNSKI example. At the Meteorological Observatory Davos Physics, headed by Professor MÖRIKOFER, renowned scientist, PROHASKA, in 1947, made sure he gave precise details (± 5%), with the proviso that the distillation is not carried over 2 / 3 of the graduated tube (20 cm3 to 30 to 32 cm3 in total). Finally, in 1954, COURVOISIER & WIERZEJEWSKI devised the spherical pyranometer BELLANI-DAVOS (PBK) can achieve an accuracy of ± 3%. The liquid used was alcohol. It is this device that seems most widely used currently photology forest. (Fig. 3).

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Simultaneously, Great Britain, GUNN, KIRK, WATERHOUSE, PEREIRA, MONTEITH and SIECZ (from 1945 to 1960) built the "GUN-BELLANI-RADIATION-INTEGRATOR" whose principle is similar, but that can run either on the distilled water (warm regions) or propyl alcohol (colder). Its operation has been compared to that of thermopile of KIPP, and the results were very satisfactory.

In any case, the fact that Bellani is autonomous and totalizer (at least for one day and discovered open in summer - and for 8 to 15 days in the forest), making it invaluable for measuring solar radiation and its heavenly relatively low cost should in widespread use in forestry and ecology. This, under the important proviso that we should not seek to obtain data on the absolute intensity of natural radiation (for the reasons outlined above), but rather the relationship between the radiation received in the undergrowth and fully exposed. It was used particularly by OUDIN (1932), AUBREVILLE (1937), ROUSSEL (1952-1965), PLAISANCE (1955), VEZINA (1960), KRECMER (1961), VOJT (1965), GALOUX (1967), as well as by many other researchers.

The apparatus dilatation - In this type of device, the heat expands, or a gas (type developed by GOLAY), a blackened metal plate (to absorb heat), as in "Strahlungs-Messgeräte of ROBITZSCH (studied from 1928 to 1951). The receiver, horizontal surface, thus includes a blackened metal plate placed between two metal plates similar, but white. Solar radiation converted into heat, causing an elongation of these two different kinds of blades and this difference, amplified, is recorded by a stylus on a rotating drum. Autonomy is 4 to 7 days, but accuracy, undergrowth and in cloudy conditions is unsatisfactory. Its inertia, too, is large enough. This device has been used in Germany for research photology forest.

Devices using the thermocouple effect - the thermoelectric effect, that is to say the appearance of an electric current in a system consisting of 2 cores of a different nature (copper and constantan - or manganin and constantan, or although pure bismuth and bismuth + 5% tin), welded together, when a weld is at a temperature different from the other, is widely used for the measurement of natural radiation. One can even say that many of the pyranometer, currently in use are of this type (models KIPP-EPPLEY-VOLOCHINE-LINKE particular). These devices are receiving a horizontal surface, and they are therefore on this point, the approval of most experts in meteorology.


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They may be instant-read, but mostly they are coupled with continuous recorders, which require the provision of auxiliary electrical power supply. Their autonomy is practically reduced to stations located in the vicinity of a laboratory measurement, powered by electricity.

In America, GAST (1930) and SHIRLEY (1932), two precursors for photology forest have used thermocouples for measuring radiation, either instantaneous or continuous. Their sensitivity ranges from 0.29 to 2.5 μ about what is sufficient in practice.

In Austria, the forest research station located in the vicinity of Innsbruck, uses the past twenty years, "Sternpyranometer" imagined by LINKE (1934), using the couple: copper / constantan. The elements are arranged in a star on a flat surface, giving it a characteristic appearance and explains its name. Its electromotive force is 2.5 millivolts for a radiation intensity of a calorie per square centimeter per minute. The accuracy of this device, described recently by DIRMHIRN in Vienna, is obviously very satisfying. (Fig. 4).

FIG. 4 - Pyranometer Star - LINKE type (without the device
Registration) (National Weather Photo).

In Germany, BAUMGARTNER (1956), who undertook a series of extensive studies on the vertical distribution of natural radiation in a young plantation of spruce, used a device like this (couple: nickel chromium / constantan) developed Munich by HOFMAN (1952) and improved by POHL (1954).

In Belgium, GALOUX (1967), describing the extensive facilities it has done in the forest of Virelles-Blaimont, said he uses the parallel-Bellani pyranometer of Davos, already described, the thermoelectric battery

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MOLL-GORZYNSKI, is the sole measure of incident radiation at a level determined stand, or for determining the radiation budget (2 aircraft which opposed the sensitivity is increased to a range of wavelengths from 0.3 to 60 ~). There is a mounting pyrradiobilanmeter described above.

In sum, this is a type of device very satisfactory, whose operating principle is accepted by meteorologists, but can only work near a laboratory supplied with electric current, which is connected throughout the observations. This localized, of course, its employment opportunities.

Appliances electrical resistance - They are called, in general, "bolometers. A wire, nickel, platinum, bismuth, or, more recently, a semiconductor body, absorbing radiation, even very low intensity, heats, and then see its resistance changes. This element is introduced into a conventional electrical installation: the WHEASTONE bridge, and unbalance the heating system must be rebalanced. This is the principle of operation of such equipment is extremely sensitive (it can measure the radiation emitted by distant stars). There was little used until now in photology forest, because its cost is high, and his job difficult.

Appliances type thermometer - This device is already old (ARAGO-VIOLLES), based on the observed temperature difference between two thermometers, one white and one black, seems hardly to have been employed in the forest.