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GENERAL INFORMATION
dome of a pyranometer also has this emission and is cooling down
several degrees below air temperature (the emissivity of glass for the
particular wavelength region is nearly 1). The emitted heat is attracted
from the body (by conduction in the dome), from the air (by wind) and
from the inner dome (through infrared radiation). The inner dome is
cooling down too and will attract heat from the body by conduction
and from the sensor by the net infrared radiation again. The latter heat
flow is opposite to the heat flow from absorbed solar radiation and
causes the well known zero depression at night of approximately -5
2
W/m
. This negative zero offset is also present on a clear day,
however, hidden in the solar radiation signal.
Figure 1 Approximate construction of Kipp & Zonen pyranometer CM 11.
During indoor measurements with a solar simulator, the inner dome
can become warmer than the pyranometer body due to net thermal
radiation from the lamp housing. A positive zero offset A is the result.
The zero offset A can be checked by placing a light and IR reflecting
cap over the pyranometer. A thin aluminium foil cap covering only the
dome seems to be the best solution. The response to solar radiation
8
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