5.1 The methods described represent a means for calibration of field radiometers employing standard reference radiometers indoors. Other methods involve the natural sunlight outdoors under clear skies, and various combinations of reference radiometers. Outdoors, these methods are useful for cosine and azimuth correction analyses but may suffer from a lack of available clear skies, foreground view factor and directionality problems. Outdoor transfer of calibrations is covered by standards G167, E816, and E824.

5.2 Several configurations of artificial sources are possible, including:

5.2.1 Point sources (lamps) at a distance, to which the sensors are exposed.

5.2.2 Extended sources (banks of lamps, or lamp(s) behind diffusing or “homogenizing” screens) to which the sensors are exposed.

5.2.3 Various configurations of enclosures (usually spherical or hemispherical) with the interior walls illuminated indirectly with lamps. The sensors are exposed to the radiation emanating from the enclosure walls.

5.3 Traceability of calibration for 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).4 For the purposes of this test method, traceability shall have been established if a parent instrument in the calibration chain can be traced to a reference pyrheliometer which has participated in an International Pyrheliometric Comparison (IPC) conducted at the World Radiation Center, (WRC), Davos, Switzerland.

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

5.3.1.1 An absolute cavity pyrheliometer that participated in a World Meteorological Organization (WMO) sanctioned IPC's (and therefore possesses a WRR reduction factor).

5.3.1.2 An absolute cavity radiometer that has been intercompared (in a local or regional comparison) with an absolute cavity pyrheliometer meeting 5.3.1.1.

5.3.1.3 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). See Zerlaut5 for a discussion of the WRR, the IPC's and their results.

Note 4: Any of the absolute radiometers participating in the above intercomparisons and being within ±0.5 % of the mean of all similar instruments compared in any of those intercomparisons, shall be considered suitable as the primary reference instrument.

5.4 Traceability of calibration of narrow band (for example, Ultraviolet) radiometers is accomplished when employing the method using a reference narrow band radiometer that has been calibrated and is traceable to the National Institute of Standards and Technology (NIST), or other national standards organizations.

5.4.1 The reference narrow band radiometer, regardless of whether it measures total ultraviolet solar radiation, or narrowband UV-A or UV-B radiation, or a defined narrow band segment of ultraviolet radiation, shall have been calibrated by one of the following:

5.4.1.1 By comparison to a standard source of spectral irradiance that is traceable to NIST or to the appropriate national standards organizations of other countries using appropriate filters and filter correction factors [for example, Drummond6].

5.4.1.2 By comparison of the radiometer output to the integrated spectral irradiance in the appropriate wavelength band of a spectroradiometer that has itself been calibrated against such a standard source of spectral irradiance.

5.4.1.3 By comparison to a spectroradiometer that has participated in a regional or national Intercomparison of Spectroradiometers, the results of which are of reference quality.

Note 5: The calibration of reference ultraviolet radiometers using a spectroradiometer, or by direct calibration against standard sources of spectral irradiance (for example, deuterium or 1000 W tungsten-halogen lamps) is the subject of Standard G138.

5.5 The calibration method employed assumes that the accuracy of the values obtained with respect to the calibration source used are applicable to the deployed environment, with additional sources of uncertainty due to logging equipment and environmental effects above and beyond the calibration uncertainty.

5.6 The principal advantages of indoor calibration of radiometers are user convenience, lack of dependence on weather, and user control of test conditions.

5.7 The principal disadvantages of the indoor calibrations are the possible differences between natural environmental influences and the laboratory calibration conditions with respect to the spectral and spatial distribution of the source radiation (sun and sky versus lamps or enclosure walls).

5.8 It is recommended that the reference radiometer be of the same type as the test radiometer, since any difference in spectral sensitivity between instruments will result in erroneous calibrations. However, the calibration of sufficiently broadband detectors (approximately 700 nm or more), such as a silicon photodiode detector with respect to extremely broadband (more than 2000 nm) thermopile radiometers is acceptable, as long as the additional increased uncertainty in the field measurements, due to spectral response and spectral mismatch limitations, is acceptable. The reader is referred to ISO TR 96737 and ISO TR 99018 for discussions of the types of instruments available and their use.

1.1 The method described in this standard applies to the indoor transfer of calibration from reference to field radiometers to be used for measuring and monitoring outdoor radiant exposure levels.

1.2 This test method is applicable to field radiometers regardless of the radiation receptor employed but is limited to radiometers having approximately 180° (2π Steradian), field angles.

1.3 The calibration covered by this test method employs the use of artificial light sources (lamps).

1.4 Calibrations of field radiometers are performed with sensors horizontal (at 0° tilt from the horizontal to the earth). The essential requirement is that the reference radiometer shall have been calibrated at horizontal tilt as employed in the transfer of calibration.

1.5 The primary reference instrument shall not be used as a field instrument and its exposure to sunlight shall be limited to outdoor calibration or intercomparisons.

Note 1: At a laboratory where calibrations are performed regularly it is advisable to maintain a group of two or three reference radiometers that are included in every calibration. These serve as controls to detect any instability or irregularity in the standard reference instrument.

1.6 Reference standard instruments shall be stored in a manner as to not degrade their calibration.

1.7 The method of calibration specified for total solar pyranometers shall be traceable to the World Radiometric Reference (WRR) through the calibration methods of the reference standard instruments (Method G167 and Test Method E816), and the method of calibration specified for narrow- and broad-band ultraviolet radiometers shall be traceable to the National Institute of Standards and Technology (NIST), or other internationally recognized national standards laboratories (Standard G138).

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.