25. INSTRUMENTS AND METHODS FOR THE GROUND-LEVEL
REFERENCE MEASUREMENT OF SOLAR RADIATION, ALBEDO
AND NET RADIATION
251
calibrated only in the short-wave region, and an assumption about non
selectivity within the limits about 1–2% is made.
Practically, the requirement of non-selectivity can be fulfilled only for
instruments with open (unprotected by domes) receiving surfaces. On the
other hand, the sensitivity of instruments with open receiving surfaces is
affected by wind speed and air temperature, and the receiving surfaces are
unprotected from precipitation and mechanical damages.
To minimize these disturbing factors, the receiving surfaces of net
radiometers are often protected with domes or covers transparent to
radiation. Such instruments are handy and comfortable to use, but the plastic
domes have also undesirable perturbing effects, which are very often
forgotten. The plastic domes may cause following difficulties:
Selectivity. No optical material is known to be non-selectively
transparent in the spectral region
This means that net
radiometers with plastic domes are much more selective than these with
unprotected surfaces. Optical properties of plastics are quite variable and
depend on the chemical composition, technology used, etc. so that the
characteristics of each instrument must be determined individually.
Since the domes are also sources of thermal radiation, its temperature
regime may cause additional errors or zero-point displacement.
Optical heterogeneity of the domes may cause additional cosine and
azimuthal response.
Optical properties of plastic materials may alter, especially if using
under outdoor conditions. Therefore recurrent sensitivity control
(calibration) is needed.
Since thin plastic domes are mechanically not durable, the instruments
need careful handling and periodical replacement of the domes.
The implication of these problems is that net radiometers with
unprotected receiving surfaces must be preferred. Typically the influence of
the wind speed is about 1–2% per 1 m/sec, and the necessary correction is
not hard to estimate. At the same time the inaccuracy due to the spectral
sensitivity of the instruments with plastic domes may reach 10% or more.
A miniature net radiometer MB-1 with open receiving surfaces was
designed in Tartu Observatory by Jüri Reemann. The device was designed to
measure net radiation inside vegetative cover but it can be used as well for
routine measurements in meteorology, micrometeorology, environmental
science etc. Its specific feature is the particular shape of receiving surfaces
with a firm glossy black coating to protect the receiving surfaces from
damage by contact with the leaves of plants.
REFERENCE MEASUREMENT OF SOLAR RADIATION, ALBEDO
AND NET RADIATION
251
calibrated only in the short-wave region, and an assumption about non
selectivity within the limits about 1–2% is made.
Practically, the requirement of non-selectivity can be fulfilled only for
instruments with open (unprotected by domes) receiving surfaces. On the
other hand, the sensitivity of instruments with open receiving surfaces is
affected by wind speed and air temperature, and the receiving surfaces are
unprotected from precipitation and mechanical damages.
To minimize these disturbing factors, the receiving surfaces of net
radiometers are often protected with domes or covers transparent to
radiation. Such instruments are handy and comfortable to use, but the plastic
domes have also undesirable perturbing effects, which are very often
forgotten. The plastic domes may cause following difficulties:
Selectivity. No optical material is known to be non-selectively
transparent in the spectral region
This means that net
radiometers with plastic domes are much more selective than these with
unprotected surfaces. Optical properties of plastics are quite variable and
depend on the chemical composition, technology used, etc. so that the
characteristics of each instrument must be determined individually.
Since the domes are also sources of thermal radiation, its temperature
regime may cause additional errors or zero-point displacement.
Optical heterogeneity of the domes may cause additional cosine and
azimuthal response.
Optical properties of plastic materials may alter, especially if using
under outdoor conditions. Therefore recurrent sensitivity control
(calibration) is needed.
Since thin plastic domes are mechanically not durable, the instruments
need careful handling and periodical replacement of the domes.
The implication of these problems is that net radiometers with
unprotected receiving surfaces must be preferred. Typically the influence of
the wind speed is about 1–2% per 1 m/sec, and the necessary correction is
not hard to estimate. At the same time the inaccuracy due to the spectral
sensitivity of the instruments with plastic domes may reach 10% or more.
A miniature net radiometer MB-1 with open receiving surfaces was
designed in Tartu Observatory by Jüri Reemann. The device was designed to
measure net radiation inside vegetative cover but it can be used as well for
routine measurements in meteorology, micrometeorology, environmental
science etc. Its specific feature is the particular shape of receiving surfaces
with a firm glossy black coating to protect the receiving surfaces from
damage by contact with the leaves of plants.