However, the eye is used as an instrument
in some indirect methods which will be described:
Photographic paper and photochemical
methods - In these processes, the eye appreciates the degree
of blackening obtained on photographic paper slow, exposed for a
specified time, natural light, undergrowth and fully exposed - or
the time variable with the illumination of the various stations,
made by the photographic paper to take a standard shade. It seems
that HARTIG (1877), a German, who first, has had the idea to use
this method in forest photology. But it was the Austrian who WIESNER,
1890, effected in the forest of many steps, which he published the
most important results in 1907. This author had even tried to determine
the change in spectral composition of light undergrowth, using papers
of different sensitivity (the "B Rodamine Paper" rather
sensitive to yellow-red, and "Normal Paper" reacting to
blue violet). It is generally JULIUS WIESNER as the father of the
forest photology. CIESLAR (1904) is also cited as a forerunner in
this field. LUNDEGARDH (1930), LÉMÉE (1937) also used
this method, which, due to the development of more sophisticated
methods, is now almost deserted. The photographic process is obviously
limited by the sensitivity, rather narrow, normal emulsions, and
the difficulty of accurately assessing the degree of blackening of
the paper. It also requires the constant presence and vigilance of
the observer.
A photochemical process a little different
has been studied and applied in Great Britain (LEYTON - 1950): glass
ampoules containing a mixture of solutions of uranium oxalate and oxalic
acid are placed in different stations. The photochemical reaction undergone
by the fluid causes a change in color and appreciation, delicate thereof,
after a period of time, allows to quantify, a rough way, the amount of
light that they have received. Sometimes used when one does not need
a high accuracy, this method is totaled.
Also noteworthy is the testing in the
USA by BRECHTEL (1967), the natural radiation dose received at a station
is evaluated by changing the polarization rotatory power of a solution
of various sugars exposed to radiation. The recording device itself is
very inexpensive (2 bottles, one white and one black, containing a sugar
solution), but the results are rather difficult to interpret.
The photometer light source constant - This type of device is probably the oldest used in photometry. It is based on the fact that the human eye, unable, it said, to quantify the intensities
light may, for cons, well enjoy equal
beaches bright 2 (one of which is obtained through a standard lamp).
BOUGUER, as early as 1729, had developed such a photometer whose
general principle has been preserved, and which model, very modern,
is currently proposed by JOBIN & YVON (the "nitometer-light
meter). This type of camera, instant-read, must be provided with
a power source in special electricity, which powers a light reference.
It is extremely accurate.
It has been very little used in photology
forest. However, it should be noted the very interesting use has
been made in Switzerland since 1914, by KNUCHEL. The device, manufactured
by SCHWEITZER, was used in forests near Zurich. It was heavy (30
kg), powered by batteries and had to be transported quickly, usually
on a clear sunny, full open at the station studied. 5 wavelengths
were used: red (0.652 μ) - yellow ( 0.589 μ) - green (0.520 μ)
- blue (0.472 μ) - Indigo (0.440 μ). In each station, the proportion
of each color was determined in a cone of observation of low vertical
opening (12 °). Its handling was long and delicate. However,
with this complex apparatus, KNUCHEL could establish some data, some
relationships have not been disproved by research, much more sophisticated,
which have been made recently on this aspect photological.
The aircraft located the cover
image - When one is in the forest, just
look up to realize that the cover consists of a series of luminous
beaches alternating with dark areas. Under coniferous trees,
generally, the distinction between these two kinds of beaches
is very clear. Under the hardwood covered unlike, especially
if they are light, you may receive from other beaches of intermediate
brightness. However, using high-contrast photographic paper can
be obtained with normal goals or more hemispherical images fairly
flat net of these various covered. It thus determines the degree
of transparency, making the relationship between surface bright
beaches, and the total area covered by the lens used. This process
is very fast (time to take a photograph) but obviously a photological
accurate interpretation difficult. It has been used so far by
British and American researchers (EVANS & COOMBE - 1959 BROWN
-- 1962, ANDERSON - 1964 MADGWICK & BRUMFIELD - 1969), in
particular tropical forests, for example, where there was little
opportunity to install permanent equipment or totalizers conventional
recorders.
Another method to approximate the previous one is used in Poland and in some countries of Eastern Europe, by MATUSZ (1960), with
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FIG. 6 - Types of equipment commonly used in forest photology.
Top: Measurement of total solar radiation; pyranometers spherical
DAVOS of BELLANI-installed for calibration.
Bottom: Measure of radiation only visible light meter to LAP photocells
LMT-stick (with filter gear at 1 / 20).
an original device called "azurometru".
A mirror, slightly convex, carried by a tripod, is surmounted by
a hemispherical dome punch many small holes. Located in Forest
Station, this device gives the viewer a picture of the cover, superimposed
on that of the hemispherical dome punch. By counting the total
number of small beaches on the existing light image, it follows,
roughly, the "degree of openness of the canopy. Within minutes
of parking, so we can determine an element that gives undoubtedly
a fairly approximate radiation, or light on reigning in the station
where they operated.
In conclusion of this study, we
may note the extreme diversity of devices recently or currently
used in forest photology. This discipline is very new, we find a
huge variety of instruments, or processes, more or less satisfactory,
while in meteorology conventional types of devices are certainly
allowed fewer. It would be desirable that the Union of Forest Research
Station, for example, advocates a limited number of methods, and
use of certain devices, recognized as the most practical to allow
a better comparison of results published.
However, it seems to emerge from
the forest world literature, as much use now:
1 In the stations fully equipped
and supplied with electric current, the type pyranometers said
thermocouple (like "Sternpyranometer" LINKS, for example) (Fig. 4).
2 In the isolated stations, but
periodic access easy, pyranometers distillation, the type Bellani,
provided that the reference device, placed in full open or near
an inhabited area (Fig. 6 ).
3 In the difficult access stations, or in those where we did not get to travel frequently, the photocell to light meters, which provide only instantaneous information, and whose employment requires certain precautions (Fig. 6).
Can be found at the end of this work, a simple schema of mounting an independent electronic totalizer of light, an accomplishment inexpensive.