Kipp&Zonen CHP 1 Pyrheliometer - Manual (Page 14)

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6.4 Non-stability
This is the percentage change in sensitivity over a period of one year. This effect is mostly due to degradation by UV
radiation of the black absorber paint on the sensing element surface. Kipp & Zonen recommends recalibration every
two years. However, for quality assurance purposes some institutes, companies or networks may require more or
less frequent recalibration. Please read the chapter on the calibration procedure for pyrheliometers for more
information.
6.5 Spectral selectivity
Spectral selectivity is the variation of the window transmittance and absorption coefficient of the black detector
paint with wavelength and is commonly specified as a % of the mean value.