Operating Temperature; Field Of View; Directional Response; Maximum Irradiance - Kipp & Zonen CGR 4 Instruction Manual

Pyrgeometer

4.10. Operating temperature

The operating temperature range of the radiometer is determined by the physical properties of the

individual parts. Within the speciﬁed temperature range Kipp & Zonen radiometers can be operated

safely. Outside this temperature range special precautions should be taken to prevent any physical

damage or performance loss of the radiometer. Please contact your distributor for further information

regarding operation in unusually harsh temperature conditions.

The ﬁeld of view is deﬁned as the unobstructed open viewing angle of the radiometer. WMO requires

that a high quality pyrgeometer for the measurement of long-wave atmospheric radiation has a ﬁeld of

view of 180° in all directions (i.e. a hemisphere). CGR 4 achieves this. The inherent ﬁeld of view of the

instrument should not be confused with the clear ﬁeld of view of the installation location.

For less critical applications pyrgeometers having a ﬂat silicon window with a typical ﬁeld of view of

150° are acceptable, such as the CGR 3.

Radiation incident on a ﬂat horizontal surface originating from a point source with a deﬁned zenith

position will have an intensity value proportional to the cosine of the zenith angle of incidence. This is

sometimes called the 'cosine-law' or 'cosine-response' and is very important in the measurement of

short-wave solar radiation. However, the directional response of pyrgeometers is largely irrelevant

since the hemisphere above does not act like a point source.

The maximum irradiance is deﬁned as the total irradiance level beyond which physical damage may

occur to the instrument.

This is the percentage change in sensitivity over a period of one year. This e ect 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 pyrgeometers for more information.

Spectral selectivity is the variation of the dome transmittance and absorption coe cient of the black

detector paint with wavelength and is commonly speciﬁed as % of the mean value.

CGR 4 Manual

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