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10
8
6
4
2
[%]
0
0
10
20
30
-2
-4
-6
degrees
Figure 1.4:
The mean cosine response of the pyranometer. With the angle
of incidence on the horizontal axis and the percentage deviation
from ideal cosine behaviour on the vertical axis.
1.2.4 Non-linearity
Non-linearity is the error of the sensitivity variation as a function of
the variation in irradiance. ISO 9060 defines non-linearity of an
instrument as its percentage deviation from the responsitivity at 500
W/m² due to the change in irradiance within 100 W/m² to 1000 W/m².
The linearity however is strongly related to the pyranometer design
and body. Due to a thermal gradient over the hot and cold junctions
(by absorption of radiation) heat convection at the detector surface
causes a non-linearity effect. The CM 4 detector construction has
been designed to keep the thermal gradient very low. Even when the
pyranometer is exposed to a very intense artificial radiating source
the non-linearity of the sensor sensitivity is small. The CM 4 non-
linearity is show in figure 1.5.
1. GENERAL INFORMATION
40
50
60
70
80
min. cosine error
%
typical cosine error
%
max. cosine error
%
15
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