ELDONET – EUROPEAN LIGHT DOSIMETER NETWORK - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 94

Egypt, India, Japan, Argentina, Brazil and New Zealand (Figure 1). Other stations are
welcome to join the network. For this purpose the hardware and installation costs are
comparatively low. In addition to the more than 40 terrestrial stations, seven underwater
stations have been deployed in the North Sea, Baltic Sea, Kattegat, Mediterranean and
Atlantic. Most underwater stations operate in close proximity of terrestrial instruments
so that the attenuation in the water column can be determined independent of
atmospheric variability. Two high altitude stations have been installed in the European
Alps (Zugspitze) and the Spanish Sierra Nevada. All instruments and sensors are
identical in order to warrant intercomparison between the different sites. Only the
entrance configuration of the optical head in the underwater instruments is different to
account for the refractive index of water.
Irradiance at the Earth's surface does not only depend on solar elevation and
ozone content but also on atmospheric transparency and albedo which shows large local
and regional variation 34,37 . The ELDONET allows us to follow local trends and sporadic
occurrences such as mini ozone holes.
The data are transferred to the central server of the network in Pisa on a regular
basis and are available on the internet (http://www.eldonet.org/) free of charge for non
commercial use 41 .
3. Hardware
The entrance optics of the ELDONET instrument consist of a 10 cm integrating
(Ulbricht) sphere with internal BaSO 4 coating 42 covered with a hemispherical quartz
dome 40 (Figure 2). This design warrants a superior cosine response. A baffle blocks the
direct path of the radiation to the detectors. The internal design forces the direct
radiation from the sun always on the inner wall of the sphere and prevents it from
hitting the baffle or the gap between baffle and sphere. One precondition for this
configuration is that the instrument is properly oriented with respect to true North. The
horizontal orientation of the instrument is adjusted by means of a level gauge.
The light rays enter the Si photodiode detectors after multiple reflection through
custom-made filters which closely match the selected wavelength ranges for UV-B,
UV-A and PAR. The energy in the UV-B band in solar emission is rather low (on the
order of up to 2 W m -2 ) as compared to the other wavelength bands or the total radiation,
which exceeds 1000 W m -2 . In addition, the transmission of the UV-B filter is limited
and the spectral sensitivity of the receptors low, so that a high electronic amplification is
required. Si photodiodes have an inherent dependence of their dark current on the
ambient temperature, which together with the other circumstances listed above causes
considerable fluctuations of the background signal which in turn decreases the precision
of the measurement. Therefore a mechanical shutter, powered by a frequency-controlled
servo motor, is inserted before the UV-B channel which can be closed by software
control to determine the dark value at regular intervals in order to