Quantum receivers

Means under the name of receptors in which the particle nature of radiation plays a visible role. The photons, in a manner somewhat similar to that outlined above on the photochemical effect, free electrons, where electrical event. The radiation is converted directly into electricity, we know now very well measured. The wavelength range recorded is generally quite low (0.3 to 0.7 or 0.8 μ), and corresponds approximately to the sensitivity limits of the human eye. The quantum receivers fall into 2 major groups (TERRIEN - 1954):

The photoelectric tubes - In these devices, electrons are driven out of the material, and flowing through a bulb filled with a rarefied gas, where there was a continuing difference of electrical potential,
p 24
p 25

 

gather at the positive electrode. Their flow is an electric current that is measured.

This type of device, expensive and difficult to handle, requiring also an ancillary source of electrical current, has been used rarely in forest photology. Mention however ALEXEYEV (1963), USSR, which used for spectral analysis of light woodland, with a "spectrophotometer rural" developed by KOTZOV & SEMETCHENKO (1960). Mention should also "Spectroradiometer - photons fluxmeter " of ECKARDT, METHY & SAUVERON (1969), employed by the School of Montpellier, to study, very thorough, absorption of radiation by various forest cover. This device, however, quite cumbersome (it is mounted on rails) and is reserved for the measures currently in full development, made in the studies of plant ecology.

The photocells themselves - Their use is very common in the forest, each time you want to measure the intensity of visible radiation, because they are very practical.

The CELL-DEPENDENT RESISTOR - Some bodies, semi-conductors, subject to low light intensity, have their electrical resistance and can therefore decrease if connected to a generator or other, the passage of an electric current of variable intensity . Selenium is the first metal on which this effect has been observed (1873). But its use was impractical, and further research helped to develop cells to lead sulphide, and cadmium sulfide, used in photometers specific (type Metrastar), whose sensitivity can extend far into near infrared. It is noteworthy that these cells require an accessory source of electric current (battery, mains). They may be supplemented by a recording device or odometer.

The PHOTOEMISSION OR SOLAR CELLS - These are by far the most cells currently used in forestry, or ecology. The light falling on a set consists of a thin metal layer, semi-transparent, above a layer of a semiconductor (selenium, or silicon) placed itself on a metal disk driver causes the formation an electrical current that can be measured easily, directly, through a milliammeter, for example. This, without any ancillary source of electrical current, a simple and very attractive. They are portable, instant-read, but can also, if one has an ancillary source of electrical current, combined with totalising or recording devices.


p 25
p 26

 

The electromotive force provided by cells commonly spread is 0.3 to 0.4 volts in the sun for those with selenium (type light meter), and 0.6 to 0.85 volts in the sun for those selenium / silicon (such as solar batteries artificial satellites). Overcast electromotive force falls below 0.1 volts. Note that the exact theory of operation of solar cells is very complex and is far from being fully established.

To use these devices, photometry usual, often using correction filters which reduce the sensitivity of the cell, that little different to the human eye (TARDIEU - 1951). The LAP light meter, using a cell LMT, has been studied from this point of view.

For their use in plant physiology, forestry or ecology, we may, by analogy, bring the other filters (KW ROUSSEL No. 34 - 1953) that alter their sensitivity and closer to that of the charge sheet of chlorophyll ( Fig. 5). Of course, at that time, graduation classic lux no longer corresponds to much, and we can consider that comparative information (Light on "photosynthesis"). The Austrian researchers have also, starting from the same point, developed a filter network which has the same properties as the filter KW No. 34 (DIRMHIRN). In the USSR, often seen in forestry or ecology groups of Phi AP (photosynthetically active radiation), defined as above.

p 26
p 27


FIG. 5 - Interest of KW filter 34 in Forest photology (photocell LMT
on luxmeter LAP) (Roussel 1953).

However, literature photological very rich data provided by solar cells (mainly expressed in the form of illumination on), mentions, almost exclusively, that of normal cells, frequently equipped with neutral gray filters, but without correction.

To cite only some of the first forest researchers who have used this method, one can mention RAMANNA (1911) - LUNDEGARDH (1930) - QUANTIN (1935) - NAEGELI (1940) - ROUSSEL (1946) - PLAISANCE (1955) particular. FAIRBAIRN (1958) has developed an original process of adding information to the photocell, by electrolysis of a solution of silver nitrate, in collaboration with CONNOR.

In Canada, LOGAN & PETERSON (1964) have proposed statistical methods to use the results of many instantaneous measurements to characterize the illumination on average prevailing under cover determined.

Currently, the use of solar cells is so widespread that it seems impossible to count the researchers who use it.