Significance and Use

The methods described represent the preferable means for calibration of field radiometers employing standard reference radiometers. Other methods involve the employment of an optical bench and essentially a point source of artificial light. While these methods are useful for cosine and azimuth correction analyses, they suffer from foreground view factor and directionality problems. Transfer of calibration indoors using artificial sources is not covered by this test method.

Traceability of calibration of global pyranometers is accomplished when employing the method using a reference global pyranometer that has been calibrated, and is traceable to the World Radiometric Reference (WRR). For the purposes of this test method, traceability shall have been established if a parent instrument in the calibration chain participated in an International Pyrheliometric Comparison (IPC) conducted at the World Radiation Center (WRC) in Davos, Switzerland. Traceability of calibration of narrow- and broad-band radiometers is accomplished when employing the method using a reference ultraviolet radiometer that has been calibrated and is traceable to the National Institute of Standards and Technology (NIST), or other national standards organizations. See Zerlaut for a discussion of the WRR, the IPC's and their results.

The reference global pyranometer (for example, one measuring hemispherical solar radiation at all wavelengths) shall have been calibrated by the shading-disk or component summation method against one of the following instruments:

An absolute cavity pyrheliometer that participated in a WMO sanctioned IPC's (and therefore possesses a WRR reduction factor),

An absolute cavity radiometer that has been intercompared (in a local or regional comparison) with an absolute cavity pyrheliometer meeting the requirements given in 5.2.1.1.

A WMO First Class pyrheliometer that was calibrated by direct transfer from such an absolute cavity.

Alternatively, the reference pyranometer may have been calibrated by direct transfer from a World Meteorological Organization (WMO) First Class pyranometer that was calibrated by the shading-disk method against an absolute cavity pyrheliometer possessing a WRR reduction factor, or by direct transfer from a WMO Standard Pyranometer (see WMO's Guide WMO No. 8 for a discussion of the classification of solar radiometers).

Note 4Any of the absolute radiometers participating in the above intercomparisons and being within