5.1 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.

5.2 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 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 Pyrheliometer 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 NIST, or other national standards organizations. See Solar Radiation Instrumentation for a discussion of the WRR, the IPC's and their results.

5.2.1 The reference global pyranometer (for example, one measuring hemispherical solar radiation at all wavelengths) shall have been calibrated by the shade/unshade or component summation method against one of the following instruments (see Test Method G167):

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

5.2.1.2 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.

5.2.1.3 A WMO first class pyrheliometer that was calibrated by direct transfer from such an absolute cavity.

5.2.2 Alternatively, the reference pyranometer may have been calibrated by direct transfer from a World Meteorological Organization (WMO) first class pyranometer that was calibrated by shade/unshade against an absolute cavity pyrheliometer possessing a WRR reduction factor, or by direct transfer from a WMO standard pyranometer (see WMO’s Guide No. 8 and ISO 9060:2018 for a discussion of the classification of solar radiometers).

Note 7: 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.

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

5.2.3.1 By comparison to a standard source of spectral irradiance that is traceable to NIST or to another National Measurement Institute (NMI) that are themselves traceable to the BIPM—the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).

Note 8: 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 Test Methods G130 and G138.

5.2.3.2 If it is intended that the UV-A or UV-B, or both radiometers, when calibrated will be used to measure other than solar radiation (for example, natural sunlight). It shall be mandatory that spectral mismatch correction factors be applied to the derived instrument constants in accordance with Appendix X1 of this test method.

5.3 The calibration method employed assumes that the accuracy of the values obtained are independent of time of year within the constraints imposed by the test instrument's temperature compensation (neglecting cosine errors). The method permits the determination of possible tilt effects on the sensitivity of the test instrument's light receptor.

5.4 The principal advantage of outdoor calibration of radiometers is that all types of radiometers are related to a single reference under realistic irradiance conditions.

5.5 The principal disadvantages of the outdoor calibration method are the time required and the fact that the natural environment is not subject to control (but the calibrations therefore include all of the instrumental characteristics of both the reference and test radiometers that are influenced simultaneously by the environment). Environmental circumstances such as ground reflectance or shading, or both, must be minimized and affect both instruments similarly.

5.6 It is preferable that reference UV radiometers be of the same type, model, and manufacture as the test or field radiometer, since a significant difference in spectral sensitivity between instruments will result in erroneous calibrations due to spectral mismatch errors—particularly for UV-B radiometers.

5.7 When the reference UV-A or UV-B radiometer is not of the same type, model, and manufacture as the test or field radiometer—which is often the case, the radiometer spectral mismatch error must be determined in accordance with Appendix X2 of this test method (see 6.1.2). If the radiation source to be measured by the newly-calibrated radiometer (mentioned in 5.2.3.2) is different from the calibration source to be monitored (for example, solar radiation) then the source spectral mismatch error must be computed in accordance with Appendix X1. Failure to do so will likely result in large errors in the measurement of UV.

Note 9: It is essential to conduct a complete spectral mismatch characterization for each UV instrument. This is important because even among a series of UV instruments, particularly UV-B models, there can be notable individual variations.

1.1 This test method covers the transfer of calibration from reference to field pyranometer to be used for measuring and monitoring solar radiation. This standard is applicable to spectrally flat A, B, and C categories as defined in ISO 9060, as well as to silicon photodiode pyranometers. With respect to the overall procedure and the handling of data, this standard has been harmonized with relevant sections of ISO 9847.

Note 1: The calibration results for non-spectrally flat silicon photodiode pyranometers (400 nm to 1100 nm) classified as Class C under ISO 9060 can be affected by several factors, such as spectral mismatch and temperature response. As a result, these calibration outcomes can demonstrate increased uncertainty.

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

1.3 This method is applicable to radiometers with narrow-band spectral response functions, such as filter radiometers for measuring ultraviolet radiation, such as UV-A radiation (315 nm to 400 nm), UV-B radiation (280 nm to 315 nm), and total UV radiation (280 nm to 385 nm, or 400 nm), as well as photosynthetically active radiation (PAR, 400 nm to 700 nm) and photopic radiation (290 nm to 850 nm).

1.4 For filtered radiometers, the methods require the reference and field radiometers to be of the same type, meaning with the same or very similar passband characteristics and spectral response functions (for example, UV-A region, total solar radiation, PAR spectral region, etc.).

1.5 For filtered radiometers, the methods address transfer of calibration from reference narrow-band radiometers to field narrow band radiometers based on two approaches.

1.6 The spectral response function of the reference and field radiometers are known and available as digital/digitized or tabulated data.

1.7 The wavelength limits of the spectral response functions are known, but the spectral response functions are often not known in detail. Thus, the “cut-on” and “cut-off” wavelengths, expressed as wavelengths at which either 10 % or 50 % of the maximum spectral response function for the type of radiometer, is defined. See “passband” in Tables G177.

Note 2: It may be the case that the only available spectral response information is indicated by the “type” of the radiometer, such as the label “UV-A detector,” or “photopic detector.” Radiometer passband wavelength limits must then assigned based on the definition of the spectral region of interest. Users are strongly encouraged to obtain from radiometer manufacturers the greatest amount of spectral response function information possible.

1.8 The calibration covered by this test method employs the use of solar radiation as the source. Calibration performed indoors using lamps as the source of irradiance is not covered in this test method.

1.9 Calibrations of field radiometers may be performed at tilt as well as horizontal (at 0° from the horizontal to the earth).

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

Note 3: 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.

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

1.12 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 (Test Methods G167 and 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 (Test Method G138).

1.13 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.

1.14 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.